Photocatalytic microreactor for the production of hydrogen

MASTER'S THESIS Master's degree in Chemical Engineering - Smart Chemical Factories PHOTOCATALYTIC MICROREACTOR FOR THE PRODUCTION OF HYDROGEN Report and annexes Author: Tania Martínez Roldán Director: Jordi Llorca Piqué Co-Director: Lluís Soler Turu June 2021 Photocatalytic microreactor for the production of hydrogen 2 Report 3 RESUM L'objectiu d'aquest projecte de màster és la producció d'hidrogen (H2) a partir de mescles que contenen aigua i etanol (relació molar H2O:C2H5OH=9:1) mitjançant un procés fotocatalític amb nanopartícules de metall/TiO2. La reacció heterogènia sòlid-líquid-gas es va dur a terme en un microreactor d'acer inoxidable operat en continu, utilitzant dues plaques catalítiques, una amb 16 microcanals i una altre amb 32 microcanals, i a més una font LED de 365 nm. En el desenvolupament del projecte es van realitzar diferents experiments. En primer lloc, es va utilitzar un bombollejador que contenia aigua i etanol (relació molar H2O:C2H5OH=9:1) per passar fase gas pel microreactor i així poder observar la producció d'hidrogen. En aquest experiment es va utilitzar un cabal de 20 ml/min. A continuació, es va realitzar l'experiment en fase sòlid-líquid-gas, on el líquid (aigua i etanol) va ser introduït per la part de dalt i baixava per acció de la gravetat i les forces capil·lars. En aquest experiment el gas es va posar en contracorrent i es van utilitzar diferents cabals de gas i líquid per poder observar la tendència de la producció d'hidrogen. A més, un cop dut a terme aquests experiments a temperatura ambient, es van realitzar unes altres proves a diferents temperatures (50ºC i 75ºC). D'aquesta forma es va poder observar la tendència de la producció d'hidrogen tant a temperatura ambient com a temperatures més elevades. El treball desenvolupat al laboratori inclou el muntatge experimental, la preparació del mètode per immobilitzar el catalitzador en els microcanals, la preparació per afegir les nanopartícules de metall a la placa catalítica i la realització dels experiments fotocatalítics. Finalment, els millors resultats es van obtenir en la interacció sòlid-líquid-gas fent servir la placa catalítica de 16 microcanals. Es va utilitzar un cabal de líquid de 0,58 ml/min, un cabal de gas de 20 ml/min i es va escalfar el microreactor a 75ºC. En aquest cas la producció màxima d'hidrogen va ser de 157,7 µmol/(min·gcat). Photocatalytic microreactor for the production of hydrogen 4 RESUMEN El objetivo de este proyecto de máster es la producción de hidrógeno (H2) a partir de mezclas que contienen agua y etanol (relación molar H2O:C2H5OH=9:1) mediante un proceso fotocatalítico con nanopartículas de metal/TiO2. La reacción heterogénea sólido-líquido-gas se llevó a cabo en un microreactor de acero inoxidable operado en continuo, utilizando dos placas catalíticas, una con 16 microcanales y otra con 32 microcanales, y además una fuente LED de 365 nm. En el desarrollo del proyecto se realizaron diferentes experimentos. En primer lugar, se usó un burbujeador que contenía agua y etanol (relación molar H2O:C2H5OH=9:1) para pasar fase gas por el microreactor y así poder observar la producción de hidrógeno. En este experimento se utilizó un caudal de 20ml/min. A continuación, se realizó el experimento en fase sólido-líquido-gas, donde el líquido (agua y etanol) fue introducido por la parte superior y bajaba por acción de la gravedad y las fuerzas capilares. En este experimento el gas se puso en contra-corriente y se utilizaron diferentes caudales de gas y líquido para poder observar la tendencia de la producción de hidrógeno. Además, una vez llevado a cabo estos experimentos a temperatura ambiente, se realizaron otras pruebas a diferentes temperaturas (50ºC y 75ºC). De esta forma se pudo observar la tendencia de la producción de hidrógeno tanto a temperatura ambiente como a temperaturas más elevadas. El trabajo desarrollado en el laboratorio incluye el montaje experimental, la preparación del método para inmovilizar el catalizador en los microcanales, la preparación para añadir las nanopartículas de metal en la placa catalítica y la realización de los experimentos fotocatalíticos. Finalmente, los mejores resultados se obtuvieron en la interacción sólido-líquido-gas utilizando la placa catalítica de 16 microcanales. Se utilizó un caudal de líquido de 0,58 ml/min, un caudal de gas de 20 ml/min y se calentó el microreactor a 75ºC. En este caso la producción máxima de hidrógeno fue de 157,7 µmol/(min·gcat). Report 5 ABSTRACT The aim of this master's project is the production of hydrogen (H2) from mixtures containing water and ethanol (molar relation H2O:C2H5OH=9:1) using a photocatalytic process with metal/TiO2 nanoparticles. The solid-liquid-gas heterogeneous reaction was carried out in a stainless-steel falling film microreactor working in continuous mode, using two catalytic plates, one of them containing 16 microchannels and the other containing 32 microchannels, and also a LED source of 365 nm. In the development of the project different experiments were carried out. First of all, a bubbler was used, which contained water and ethanol (molar relation H2O:C2H5OH=9:1) to pass gas phase into the microreactor and to observe the hydrogen production. In this experiment it used a flow rate of 20 ml/min. After this, it realized the experiment in solid-liquid-gas phase, where the liquid (water and ethanol) was fed through the top slit and introduced into the microchannels by gravity and capillary forces. In this experiment the gas was applied in counter-current and I used different flow rates of gas and liquid to observe the trend of hydrogen production. Moreover, after these experiments were carried out at room temperature, further tests were performed at different temperatures (50ºC and 75ºC). In this way it was possible to observe the trend of hydrogen production both at room temperature and at higher temperatures. The work developed in the laboratory includes the experimental set-up, the preparation of the method to immobilized the catalyst in the microchannels, the preparation to add the metal nanoparticles in the reaction plate and the performance of photocatalytic experiments. Finally, the best results were obtained in solid-liquid-gas interaction using the catalytic plate containing 16 microchannels. It used a liquid flow rate of 0,58 ml/min, a gas flow rate of 20 ml/min and the microreactor was heated at 75ºC. In this case the maximum hydrogen production was 157,7 µmol/(min·gcat). Photocatalytic microreactor for the production of hydrogen 6 ACKNOWLEDGEMENTS This project has been realized thanks to the support of a number of people who have contributed their help and effort throughout this time. First of all, I would like to thank the director, Jordi Llorca, for offering me this project and helping me whenever I needed it. I would also like to thank the co-director, Lluís Soler, for guiding me and helping me every day that I requested it. I would also like to thank Isa, for helping me in the laboratory when I needed it. I would not like to forget the people who were sharing the lab with me such as Nacho, Laia, Marina and above all Asier and Yufen who offered me their knowledge and helped me to improve. Finally, I would like to thank my parents, my sister and my partner, for their encouragement and advice during my master's degree. Report 7 GLOSSARY AFCs Alkaline Fuel Cells CB Conduction Band DMFCs Direct Methanol Fuel Cells FFMR Falling Film Microreactor GC Gas Chromatography IEA International Energy Agency LUMO Lower Unoccupied Molecular Orbital Mtoe Million Tonnes of Oil Equivalent MCFCs Molten Carbonate Fuel Cells NPs Nanoparticles PAFCs Phosphoric Acid Fuel Cells PEMFCs Proton Exchange Membrane Fuel Cells SOFCs Solid Oxide Fuel Cells UV Ultraviolet UN United Nations VB Valence Band Photocatalytic microreactor for the production of hydrogen 8 CONTENTS RESUM..................................................................................... 3 RESUMEN ................................................................................ 4 ABSTRACT ................................................................................ 5 ACKNOWLEDGEMENTS ............................................................ 6 GLOSSARY ................................................................................ 7 CONTENTS ............................................................................... 8 1. CHAPTER 1: INTRODUCTION ........................................... 15 1.1. Energy scenario ........................................................................................................... 15 1.2. Hydrogen economy ..................................................................................................... 16 1.2.1. Hydrogen ............................................................................................................. 16 1.2.2. Hydrogen production .......................................................................................... 17 1.2.2.1. Non-renewable Hydrogen ........................................................................... 18 1.2.2.2. Renewable Hydrogen .................................................................................. 19 1.3. Fuel cells ...................................................................................................................... 20 1.3.1. Types of hydrogen-based fuel cells ..................................................................... 20 1.4. Aims of this project ..................................................................................................... 24 1.5. Scope of the project .................................................................................................... 24 1.6. Motivation of the project ............................................................................................ 24 2. CHAPTER 2: THEORETICAL BACKGROUND ....................... 25 2.1. Photocatalysis ............................................................................................................. 25 2.2. Photocatalytic hydrogen production .......................................................................... 25 2.3. Photocatalytic water splitting ..................................................................................... 26 2.4. Strategies to improve photocatalytic reactions .......................................................... 28 2.5. Factors that influence the photocatalyst activity ....................................................... 29 2.6. Photocatalytic reactors ............................................................................................... 31 2.6.1. Slurry photoreactor ............................................................................................. 31 2.6.2. Fluidized photoreactor ........................................................................................ 32 2.6.3. Optical fiber photoreactor .................................................................................. 32 2.6.4. Monolith photoreactor ....................................................................................... 33 Report 9 2.6.5. Fixed bed photoreactor ....................................................................................... 34 2.6.6. Advantages and limitations ................................................................................. 34 2.6.7. Falling film microreactor (FFMR) ......................................................................... 35 2.6.7.1. Catalytic plates ............................................................................................ 37 2.7. Catalyst used (TiO2) in FFMR ....................................................................................... 38 2.8. Methods for immobilizing the catalyst in the microchannels of the FFMR ................ 40 3. CHAPTER 3: EXPERIMENTAL SECTION ............................. 48 3.1. Immobilization of TiO2 ................................................................................................. 48 3.1.1. Heat attachment method .................................................................................... 48 3.1.2. Sol-gel method .................................................................................................... 51 3.2. Add metal nanoparticles (NPs) into catalytic plate ..................................................... 53 3.3. Previous checkpoint tests ........................................................................................... 56 3.3.1. Check water flow using a peristaltic pump ......................................................... 56 3.3.2. Check gas flow using a mass flow........................................................................ 56 3.3.3. Check gas leaks in the microreactor .................................................................... 58 3.4. Photocatalytic tests ..................................................................................................... 59 3.4.1. Gas-phase test ..................................................................................................... 59 3.4.2. Falling film test with gas in counter-current ....................................................... 61 4. CHAPTER 4: RESULTS AND DISCUSSION ........................... 64 4.1. Effect of gas flow rate in S-G interaction .................................................................... 64 4.1.1. Catalytic plate containing 16 microchannels ...................................................... 64 4.1.1.1. At room temperature .................................................................................. 64 4.1.1.2. At 75ºC ........................................................................................................ 65 4.1.2. Catalytic plate containing 32 microchannels ...................................................... 66 4.2. Effect of liquid flow rate in S-L-G interaction .............................................................. 67 4.2.1. Catalytic plate containing 16 microchannels ...................................................... 67 4.2.1.1. At 50ºC ........................................................................................................ 67 4.2.1.2. At 75ºC ........................................................................................................ 68 4.2.2. Catalytic plate containing 32 microchannels ...................................................... 69 4.3. Effect of temperature in S-G interaction..................................................................... 70 4.4. Effect of temperature in S-L-G interaction .................................................................. 72 4.5. Comparison between the two types of catalytic plates.............................................. 74 Photocatalytic microreactor for the production of hydrogen 16 1.2. Hydrogen economy First of all, the concept of hydrogen economy starts in 1970 by the electrochemist John Bockris. [2] As it is well-known, hydrogen has several important applications in the chemical industry such as ammonia and methanol synthesis, oil refining and several petrochemical processes. However, the objective now is a new energy system where human civilization is powered by hydrogen. [2] Moreover, hydrogen is not an alternative energy source, it is actually an energy carrier that is produced consuming primary energy. So, more specifically, is an energy vector such as gasoline or electricity. [2] 1.2.1. Hydrogen Hydrogen is the simplest element that exists and the most abundant element in the universe. Each atom of hydrogen has only one proton and one electron. In normal conditions, pure hydrogen is found in its molecular form (H2) and in gaseous state. Nevertheless, Earth’s atmosphere only contains 0.00005% of hydrogen. Most of the hydrogen is found in chemical compounds. If it is combined with oxygen the product formed is water (H2O) and if it is combined with carbon different compounds or hydrocarbons found in, natural gas, coal and petroleum, and in the biosphere forming organic matter. [4] This element has many different uses. The main ones are listed below [4], [5]:  Energy purposes  Chemical reagent in industrial processes  Reactant in hydrogenation processes  Remove traces of oxygen  Fuel in rocket engines  As a coolant in electrical generators Report 17 Figure 2: Hydrogen use in industry. Extracted from: [4]. 1.2.2. Hydrogen production As explained in the section 1.2, hydrogen is an energy carrier, so its sustainable production is the first step of its possible contribution to the energy system [2]. Moreover, the current annual world production of hydrogen is about 60 million tons that are used for ammonia synthesis (51%), oil refining (35%) and methanol synthesis (8%). This implies that by 2035, the global production capacity of hydrogen would have to be between two and three times the current one [2]. Furthermore, there are several pathways of hydrogen production, one of them is through non renewable sources and the other way is through renewable energy sources [2]. Photocatalytic microreactor for the production of hydrogen 18 1.2.2.1. Non-renewable Hydrogen The main routes for non-renewable hydrogen (grey hydrogen) are schematized in Figure 3. Figure 3: Main hydrogen production pathways from fossil fuels and nuclear power. Extracted from: [2]. As you can observe in this figure, nuclear power has been introduced because the fuel reserves of nuclear energy are limited. Moreover, these routes are used to produce over 96% of the total hydrogen, for this reason they have a great importance. About 48% is obtained from methane (mainly from steam reforming), 30% from petroleum fractions in refineries (through steam reforming and partial oxidation) and 18% from coal gasification. [2] When considering fossil fuels, the hydrogen production is conducted through a series of thermochemical processes involving different catalytic steps. However, the minor pathway for hydrogen production is water electrolysis which only contributes 4% of renewables, and it is capable to produce about 600 hydrogen tons per year. [2] Electrolysis is a process that splits hydrogen from water using an electric current. The electricity for electrolysis can come from renewable and non-renewable sources. So, if the electricity for electrolysis is only produced from fossil fuel (coal, natural gas and petroleum) or biomass combustion, grey hydrogen is produced, and the related environmental effects and carbon dioxide emissions are indirectly associated with this process. [6] The main drawbacks for water electrolysis are [2]:  Economic Report 19  The operating costs are higher than thermochemical routes. 1.2.2.2. Renewable Hydrogen Figure 4 shows the main pathways for renewable hydrogen (green hydrogen). Figure 4: Main hydrogen production pathways from renewables. Extracted from: [2]. Biomass can be used for hydrogen production through thermochemical processes. Observing the conversion technologies of biomass, gasification is expected to be commercially available in the future with attractive hydrogen costs. Another technique for this first group (biomass) of conversion technology is anaerobic fermentation because this technology allows the valorization of organic wastes, although one of the main challenges of this process is the low production rate. [2] The primary energies of wind, hydroelectric, geothermal, wave and tidal are grouped because their contribution to hydrogen production is basically based on water electrolysis, so they are in the 4% of renewables that mentioned above (chapter 1.2.2.1). Also, solar photovoltaic energy can be used to obtain hydrogen from this same method (water electrolysis). [2] One of the most promising forms of renewable hydrogen production is the use of solar energy. It has several conversion technologies like water thermolysis in which the process is unfavored thermodynamically at temperatures lower than 2200 and a maximum water dissociation that can be achieved is around 25%. Another technique for this second group (solar) is photocatalytic water splitting, which allows to obtain hydrogen from the irradiation of water Photocatalytic microreactor for the production of hydrogen 20 with sunlight in the presence of a suitable catalyst that reduces the high activation energy of the decomposition reaction. [2] Other technologies for this same group (solar) are biophotolysis, which consists in the production of hydrogen from water by sunlight energy using biological systems, and photoelectrolysis in which both sunlight and electricity are used to produce hydrogen. 1.3. Fuel cells A fuel cell is like a battery but it uses the chemical energy of hydrogen or another fuel to cleanly and efficiently produce electricity. When hydrogen is employed, the only products are electricity, water and heat. So, this device combines hydrogen and oxygen to produce electricity, with water and heat as by-products. [7] All fuel cells are based in the use of two electrodes separated by a solid or liquid electrolyte that carries electrically charged particles between them. Normally, to accelerate the reactions at the electrodes, a catalyst is used. [7] 1.3.1. Types of hydrogen-based fuel cells There are different types of hydrogen-based fuel cells: 1. Solid Oxide Fuel Cells (SOFCs) This type of fuel cell use a hard, non-porous ceramic compound as the electrolyte. In that case, the cells do not have to be constructed in the plate like configuration typical of other fuel cell types because the electrolyte is solid. [7] SOFCs operate at very high temperatures, normally between 500 and 1000 , and when the cell is in this range of temperature, SOFCs do not require expensive platinum catalyst material. [7] Report 21 Figure 5: Solid Oxide Fuel Cell. Extracted from: [7]. 2. Proton Exchange Membrane Fuel Cells (PEMFCs) Also known as Polymer Electrolyte Fuel Cells or PEMFC, they provide with high power density and have different advantages related to its low weight and volume. [7] This type of fuel cell uses a polymeric membrane as an electrolyte and porous carbon electrodes containing a platinum catalyst. To operate, PEMFCs only need hydrogen and oxygen from the air, and their operation do not involve corrosive fluids. Also, the temperature of operation is only around 80 . [7] The operation at low temperatures have an important advantage but also have some drawbacks. The advantage is that this fuel cell can quickly reach the operation temperatures starting from ambient temperature. The drawbacks are that they need the platinum-based catalyst which increase costs, and this type of catalyst is very sensitive to CO poisoning, making it compulsory to use an additional reactor to reduce CO in the fuel gas if the hydrogen comes from alcohol or hydrocarbon reforming. [7] Figure 6: Proton Exchange Membrane Fuel Cell. Extracted from: [7]. Photocatalytic microreactor for the production of hydrogen 22 3. Alkaline Fuel Cells (AFCs) This type of fuel cell use an aqueous solution of potassium hydroxide as the electrolyte and can use a diversity of non-precious metals as a catalyst at the anode and cathode. [7] High temperature AFCs operate at temperatures between 100 and 250 , but the new designs of AFCs operate at lower temperatures, approximately 70 . [7] The efficiency obtained on pure hydrogen from this type of fuel cell is approximately 60%. [7] The advantage in these type of fuel cells is that they are currently the cheapest fuel cells, because the materials used as catalyst on their electrodes are inexpensive compared to other fuel cells like PEMFC. But AFCs have one limitation that is that they are sensitive to CO2, which may be present in the fuel or air. [7] Figure 7: Alkaline Fuel Cell. Extracted from: [7]. 4. Phosphoric Acid Fuel Cells (PAFCs) PAFCs use liquid phosphoric acid as an electrolyte and porous carbon electrodes which contain a platinum catalyst. This type of fuel cell operate between 150 and 200 and the steam obtained from the operation can be used for heating. The efficiency increases up to 70%. [7] At lower temperatures phosphoric acid is a poor ionic conductor, and CO poisoning of the platinum catalyst in the anode becomes significant. The advantage of PAFCs is that they are less sensitive to CO than PEMFCs and AFCs. [7] Moreover, PACs admit fuels that contain CO and can tolerate a CO concentration of about 1.5%, which increases the range of fuels that can be used [7]. Report 23 Figure 8: Phosphoric Acid Fuel Cell. Extracted from: [7]. 5. Molten Carbonate Fuel Cells (MCFCs) MCFCs operate at high temperature and use an electrolyte composed of a molten carbonate salt mixture suspended in a porous, chemically inert ceramic lithium aluminum oxide matrix. Since MCFCs operate at high temperatures (650 and above), catalysts do not need to be precious metals such as platinum, and can be operated directly with natural gas (internal reforming). [7] The efficiency obtained in MCFCs is around 60% when they produce electricity, and 85% if they are used in cogeneration. [7] The advantages of high temperature operation is that there is an increment of efficiencies and allows the use of catalysts that are less expensive. And the disadvantages is that they have a low power density and an aggressive electrolyte. [7] Figure 9: Molten Carbonate Fuel Cell. Extracted from: [7]. Photocatalytic microreactor for the production of hydrogen 24 1.4. Aims of this project The main objective of this project is the production of hydrogen using mixtures containing water (H2O) and ethanol (EtOH) by means of a heterogeneous solid-liquid-gas photocatalytic process in a falling film microreactor that operates in continuous mode. In this project, the hydrogen production in different situations will be studied. The reaction will be carried out in solid-gas phase as well as in solid-liquid-gas configuration. A stainless-steel catalytic plate equipped with microchannels will be coated with TiO2 anatase and functionalized with Au nanoparticles, and a complete laboratory bench reaction system will be designed and tested. 1.5. Scope of the project To achieve the general objective said before, different objectives or activities are defined: 1. To search different methods for immobilizing the catalyst support (TiO2). 2. To add metallic nanoparticles (Au, Pt) in the catalytic plate containing titanium dioxide to observe the hydrogen production. 3. To study the photocatalytic reaction in terms of hydrogen production under ultraviolet light using gas chromatography analysis. 4. To study the optimum conditions to produce more hydrogen (gas flow rate, liquid flow rate and temperature). 1.6. Motivation of the project The microreactor used in this project, falling film microreactor (FFMR), is normally used in organic reactions where the product of interest is in the liquid phase [[8],[9], [10]]. The motivation and main novelty of this project with respect to previous works, will be based on the analysis of the gas phase where we will find the product of interest (hydrogen). Report 25 2. CHAPTER 2: THEORETICAL BACKGROUND 2.1. Photocatalysis As IUPAC says, photocatalysis can be defined as the change in the rate of a chemical reaction or its initiation under the action of ultraviolet, visible, or infrared radiation in the presence of a substance (the photocatalyst) that absorbs light and is involved in the chemical transformation of the reaction partners [11]. When the photocatalyst absorbs light is considered to be in an excited state. There are two types of photocatalysis [12]:  Homogeneous photocatalysis: occurs when the reactant and photocatalyst exist in the same phase (solid, liquid or gaseous).  Heterogeneous photocatalysis: occurs when the reactant and photocatalyst exist in different phases. The photocatalysis starts with light (with an adequate wavelength) in order to provide energy equal or higher than the band gap of the photocatalyst, which is the separation between the valence band (VB) and the conduction band (CB). If this occurs, an electron in VB is excited and then goes to the CB, resulting the separation of an electron (e-)-hole(h+) pair. The electrons generated are involved for reduction processes and the holes in VB are consumed in oxidation processes [13]. 2.2. Photocatalytic hydrogen production Hydrogen can be produced by photocatalytic water splitting (Figure 10), or by photocatalytic reforming of organics. In the first method, water undergoes redox reaction with electron and holes. In both cases, the proton ions are converted to hydrogen by the involvement of electrons over the photocatalyst. [13] The production of hydrogen using photocatalysis was demonstrated the first time with a ntype TiO2 semiconductor electrode that was exposed to UV light. When the surface of the titanium dioxide electrode is irradiated with UV light with wavelengths shorter than 415 nm, the photocatalysis reaction takes place. [14] The overall reaction is [14]: Photocatalytic microreactor for the production of hydrogen 32 2.6.2. Fluidized photoreactor A fluidized reactor is a combination of the two most common configurations, stirred tank and packed bed continuous flow reactors. It has excellent heat and mass transfer. [17] In this type of reactor, due to the dragging force of the upward flow, the photocatalyst is maintained in suspension. [17] Figure 14: Fluidized reactor. Extracted from: [16]. 2.6.3. Optical fiber photoreactor These reactors use optical fibers to transmit light from the light source to the photocatalyst. [13] The light is reflected and transmitted along the fiber, which is used to radiate the light uniformly inside the photoreactor. [13] Report 33 Figure 15: Optical fiber reactor. Extracted from: [13]. 2.6.4. Monolith photoreactor These reactors contain uniform honeycomb blocks with parallel channels prepared by extrusion into different shapes and sizes. [13] Monoliths can provide better mass transfer, better porosity, good coating adherence, low pressure drop, good mechanical strength and thermal stability over catalyst powders or pellets. [13] Figure 16: Monolith reactor. Extracted from: [13]. Photocatalytic microreactor for the production of hydrogen 34 2.6.5. Fixed bed photoreactor Fixed bed reactors are the simplest type of reactor to design, and it consists of solid catalyst particles being loaded and packed into the bed. [18] However, this reactor has several problems such as poor mass transfer and low thermal conductivity. [18] Figure 17: Fixed bed reactor. Extracted from: [16]. 2.6.6. Advantages and limitations Table 1: Advantages and limitations of the different main types of photoreactor. Extracted from: [13]. Reactor design Advantages Limitations Slurry photoreactor - They can be operated in fixed bed mode or continuous flow patterns. - They use a combination of gas-liquid-solid phase. - They use of entire external surface illumination during the reaction. - They use a high catalyst loading. - Continuous stirring causes the additional cost. - Active contact surface for reaction is low. - Separation of catalyst particles from mixture is difficult. Fluidized photoreactor - They have a high photocatalytic activity. - They have an efficient heat and mass transfer rate by vigorous agitation of solid. - Abrasion of particles and attrition of the catalyst causes erosion at reactor. - Catalyst is difficult to be separated from mixture. Optical fiber photoreactor - Surface area is larger. - Efficiency of light utilization is higher. - Deactivation catalyst at high temperature. Report 35 -Efficient processing capacities of the catalyst. - Low costs of operation. - Maximum reactor volume cannot be applying. - Uniform coating of fibers is complex. Monolith photoreactor - Ratio of surface to volume is higher. - Pressure drop is low. - Flow rate is higher - Light efficiency is low. - Catalyst adhesion on wall is lower. Fixed bed photoreactor - Surface area is large. - Reaction time is faster. - Conversion rate per unit mass of catalyst is high. - Low costs of operation. - Temperature gradient between gas and solid surface is common. - Exposure of catalyst to light is low. - Conversion and yield rate is low. 2.6.7. Falling film microreactor (FFMR) A microreactor is a reactor with reaction channels of the order of micrometers. The most popular type of gas-liquid contactor is the falling film microreactor (FFMR) [19]. As it can be observed in some articles [[8],[9], [10]] and as mentioned above in motivation of the project, the FFMR is normally used in organic reactions where the product of interest is in the liquid phase. In the case of this project, the product of interest is in the gas phase (hydrogen), so a specific setup will have to be implemented. Different types of falling film microreactor exists, i.e. cylinder and plate as it can be seen in Figure 18. [10] Figure 18: Left and middle: types of falling film microreactor. Right: schematic view of the structured reaction plate and the continuous contacting between the gas-liquid phases. Extracted from: [10]. The FFMR has specific connections for the photocatalytic reactions which can be observed in Figure 19. [20] Photocatalytic microreactor for the production of hydrogen 36 Figure 19: Connections of the FFMR. Extracted from: [20]. Also, the FFMR is composed of a back plate carrying the heat exchanging unit, and the inlet and outlet for the reaction solution (see Figures 18 'plate' and 19). It is followed by the catalytic plate as the first element of the microreactor. On the front side of the reaction plate, microchannels with different geometries for each plate have been incorporated by wet chemical etching (see the Figures in 2.6.7.1. Catalytic plates). [8] This method is suitable for mass production and allows fabrication of wide range of channels depths. [10] Figure 20: Falling film microreactor with a magnetically attached heat sink carrying exchangeable LED arrays. Extracted from: [8]. Moreover, in this type of microreactor the reaction solution is fed through the top slit and pulled into the microchannels by gravity and capillary forces. The catalytic plate is fixed with a mask and sealing gasket followed by the top plate of the reactor housing. This last part contains a window made of borosilicate glass (visible to irradiation: length lirr = 54 mm, width wirr = 29 mm, thickness = 12 mm). For illumination, an aluminium body was designed as heat Report 37 sink carrying two LEDs (right of Figure 20). These LEDs can be fixed magnetically in front of the window of the FFMR. [8] Furthermore, the falling film microreactor has demonstrated its successful implementation for several gas-liquid reactions, i.e. fluorination, sulfonation, chlorination and hydrogenation. [10] Nevertheless, plate falling film microreactor presents important parameters to take into account in the photocatalytic reactions. These parameters are summarized in table 2. Table 2: Important parameters of FFMR. Extracted from: [20]. Maximum operating pressure 10 bar Maximum operating temperature Of the FFMR: 180 Of the magnets: 80 Maximum flow rate of the catalytic plates Containing 16 microchannels: 25 ml/min Containing 32 microchannels: 8,3 ml/min Typical gas flow rate In the range of 8,33 ml/min up to 133,3 ml/min 2.6.7.1. Catalytic plates Falling film microreactor has two different types of reaction plates available: (i) reaction plates with 600 x 200 x 7,8 cm length containing 32 microchannels, and (ii) reaction plate with 1200 x 400 x 7,8 cm length containing 16 microchannels (Figure 21). Figure 21: Left: reaction plate containing 16 microchannels; Right: reaction plate containing 32 microchannels. Photocatalytic microreactor for the production of hydrogen 38 2.7. Catalyst used (TiO2) in FFMR Titanium dioxide is a solid which absorbs ultraviolet light, making it an important ingredient in many sunscreen products. TiO2 is a commercial product which is available in two forms: TiO2 P25 and TiO2 P90, the main difference being the size of the nanoparticles and, consequently, the exposed surface area. In this project, the commercial product Aeroxide TiO2 P90 produced by Evonik Industries is used. It is a white powder with an average particle size of 14 nm and a surface area approximately of 90 20 m2/g. It has a 80-90% by weight of anatase and the rest is rutile. The mixture of anatase and rutile is beneficial in terms of higher activity, because the rutile-anatase interface acts as catalytic hot spots. [21] Figure 22: Titanium dioxide. Extracted from: [22]. TiO2 exists in three different crystalline polymorphs. These are anatase, brookite and rutile (Figure 23). Anatase exhibits the highest overall photocatalytic activity. In each structure there is a TiO6 octahedron, in which a central titanium atom is surrounded by 6 oxygen atoms. The main difference is the way these octahedral are arranged. In rutile, there are chains of TiO6 octahedral connected by sharing an edge with the c-axis and then interlinked by sharing corner oxygen atoms to form a three-dimensional lattice. In anatase structure, the three-dimensional lattice is formed only by sharing edges between TiO6 octahedral. It means that the octahedral in anatase share four edges and are arranged in zig-zag chains whithin the octahedral. In brookite both edges and corners are shared and an orthorhombic structure is formed. [23] Report 39 Figure 23: Crystalline structure of (a) anatase, (b) brookite and (c) rutile. Extracted from: [23]. The properties of anatase, brookite and rutile are shown in the following table: Table 3: Properties of anatase, brookite and rutile. Extracted from: [23], [24]. Anatase Brookite Rutile Crystal structure Tetragonal Rhombohedral Tetragonal Density (g/cm3) 3,83 4,17 4,24 Band gap (eV) 3,20 3,13 3,02 Photocatalytic microreactor for the production of hydrogen 40 2.8. Methods for immobilizing the catalyst in the microchannels of the FFMR In other final project [21] two methods were investigated for immobilizing the catalyst into a catalytic surface. One of them was slurry washcoat where the solution containing the catalyst passed through all the microchannels where the reaction took place. The other method was the deposition in the support which microchannels’ walls were coated with the photocatalyst, therefore immobilizing it and ensuring enhanced mechanical and chemical stability. The results showed that deposition obtained better results than the slurry washcoat. For this reason, this project is centered in methods using deposition. Once the best method is known for immobilizing the catalyst, it is also important to know the material of the surface where the catalyst should be immobilized. In this case, it is made by stainless steel, but also there exist other supports which also use this technique, like glass or ceramic. Table 4 shows different methods for immobilizing the catalyst and the explanation for each one, and table 5 shows different sol-gel or dip-coating methods for different supports. Report 41 Table 4: Different methods for TiO2 immobilization. Methods Explanation Reference Heat attachment method - A suspension containing 5 g/L TiO2-P25 in deionized water was prepared and then sonicated for 15 min by using ultrasonic irradiation in order to improve the dispersion of catalyst particles in deionized water. The obtained TiO2-P25 suspension was poured on microchannels and then placed in an oven at 100 C. After drying, the excess catalyst particles on the plate was removed and then washed with deionized water for the removal of weakly attached catalyst particles. [25] Coating process - Technique developed by Netherlands Energy Research Foundation (ECN) researchers for deposition of TiO2 particles on a quartz sheet of solar cells. - A suspension of TiO2 with 1% solid content of TiO2 in mass was prepared by dispersing 15 g of P-25 Degussa into a mixture of 1125 g of absolute ethanol and 375 g of nitric acid (2M). - Existence of nitric acid in the suspension guarantees an adequate dispersion of stabilized TiO2 colloids. - Before this process, the suspension was ultrasonicated for 15 min. - The loading and discharging (at a constant rate of 10cm/min) processes were repeated 5 times in order to obtain a uniform thin layer of TiO2 over the surface of the glass tubes. [26] Microwave-assisted and microwave assisted sol-gel method - The microwave-assisted method is a rapid, energy-saving and a high yielding method for the preparation of functional nanomaterials. Microwave radiation frequencies range from 300 MHz to 300 GHz. When microwaves interact with matter, they transfer energy directly, producing a fast and homogeneous heating process. - The microwave-assisted sol-gel method has been reported as a versatile procedure for preparing metal or non-metal doped titanium dioxide. [27] Photocatalytic microreactor for the production of hydrogen 48 3. CHAPTER 3: EXPERIMENTAL SECTION 3.1. Immobilization of TiO2 In this section I used two different possible methods to observe and to compare which method is better for the immobilization of titanium dioxide. 3.1.1. Heat attachment method First of all, I made a solution of 0,5M HCl 37%. Once the solution was prepared, I got 25 mL and I added 5g/L of TiO2 P-90. After that, I put the solution into ultrasonic for 15min. After this, I impregnated the catalytic plate (1200 µm x 400 µm) using a needle of 330 µm and when the catalytic plate was impregnated, I inserted this into the oven at 100 for 4h. Finally, I repeated this procedure 3 times more, but instead putting the catalytic plate into the oven at 100 for 1h. After this, I inserted the catalytic plate into the furnace at 500 for 2h using a ramp of 5 . Moreover, I weighed the catalytic plate at the final of every impregnation and the results obtained are shown in the following table. Table 6: Weight in each impregnation in heat attachment method. Number of impregnations Weight (g) 1st impregnation (4h in oven at 100 ) 22,1874 2n impregnation (1h in oven at 100 ) 22,1886 3r impregnation (1h in oven at 100 ) 22,1898 4r impregnation (1h in oven at 100 ) 22,1873 The weight for each impregnation is practically equal, so I decided that the weight is not a good way to observe if the nanoparticles of titanium dioxide are remove after cleaning the catalytic plate or not. Report 49 Once this procedure was finished, I observed in the microscopy if there were some nanoparticles of titanium dioxide adhered on the catalytic plate. Apparently, there were not much nanoparticles, so I decided to repeat this same procedure but with some differences: 1) Adding 20g/L in the same reaction plate. 2) Only doing 2 impregnations and each impregnation inserted in the oven during 1h. Finally, I observed the reaction plate on the RAMAN microscopy and I observed the characteristic peaks of titanium dioxide (anatase). The aspect of reaction plate before and after cleaning with water can be observed in Figure 24. Now, in the following graphic it can be observed the spectrum of this procedure before and after cleaning the reaction plate with water, to observe if the TiO2 P-90 nanoparticles were removed or not. Figure 25: Spectrum of reaction plate using heat attachment method before and after cleaning with water. 0 200 400 600 800 1000 1200 1400 Intensity (a.u.) Raman Shift (cm-1) Reaction plate using heat attachment method before and after cleaning Before After Figure 24. Aspect of reaction plate using heat attachment method. From left to right, reaction plate before cleaning, reaction plate after cleaning. Photocatalytic microreactor for the production of hydrogen 50 Figure 26: Spectrum of anatase TiO2. Extracted from: [44]. If the Figures 25 and 26 are compared, it can be appreciated that there is titanium dioxide adhered on the catalytic plate, because in spectrum made by RAMAN microscopy it is obtained the peaks of titanium dioxide at the same Raman shift. Also, it can be said that practically it has an anatase structure because when calcining at 500 , more percentage of anatase than rutile is obtained [45] and this can be checked by comparing the spectrum of anatase TiO2 and the spectrum obtained in the Raman microscopy (Fig.25). Report 51 3.1.2. Sol-gel method First of all, I prepared all materials in the extractor hood. After that, I opened the argon and nitrogen gases in order to avoid that titanium isopropoxide and the recipient to do the sol caught moisture. The procedure to do the sol was the following: 1. Add 5g of titanium isopropoxide. 2. Add 5g of ethanol 100%. 3. Add a dry fly. 4. Shake well and add 1mL of hydrochloric acid during the agitation (slowly). 5. Stirring continuously, add 2mL of water (very slowly). 6. Shake 10 min with inert. 7. Dilute 7 times the final mixture with ethanol, stirring continuously. In that case, I have 18 mL of solution, so I need to add 125 mL of ethanol. 8. Shake 2min more and cover up the solution. Once I prepared the sol, I impregnated two times the reaction plate using a 330µm thick syringe and needle. After each impregnation, I inserted the reaction plate into the oven at 100°C for 15min. Finally, I inserted the reaction plate into the furnace at 500 °C for 2h using a ramp of 5 °C/min. Once this procedure was finished, I observed in the microscopy if there was titanium dioxide on the reaction plate. Apparently, there was not much titanium dioxide, so I decided to repeat this same procedure but adding 5g more of titanium isopropoxide. Finally, I observed the reaction plate on the RAMAN microscopy and I observed more titanium dioxide adhered on the reaction plate. The aspect of reaction plate before and after cleaning with water can be observed in the following figure. Photocatalytic microreactor for the production of hydrogen 52 Moreover, in the following graphic it can be observed the spectrum of this procedure before and after cleaning the reaction plate with water, to observe if the titanium dioxide adhered was removed or not. Figure 28: Spectrum of reaction plate using sol-gel method before and after cleaning with water. If the Figures 26 and 28 are compared, it can be appreciated that there is titanium dioxide adhered on the reaction plate, but comparing with heat attachment method, in this case (solgel method) there is more titanium dioxide adhered on reaction plate, because the arbitrary units indicate more counts. As explained before, practically it has an anatase structure because when calcining at 500 , more percentage of anatase than rutile is obtained [45] and this can be checked by comparing the spectrum of anatase TiO2 (Fig.26) and the spectrum obtained in the Raman microscopy (Fig.28). 0 200 400 600 800 1000 1200 1400 Intensity (a.u.) Raman Shift (cm-1) Reaction plate using sol-gel method before and after cleaning Before After Figure 27: Aspect of reaction plate using sol-gel method. From left to right, before cleaning with water, after cleaning with water. Report 53 3.2. Add metal nanoparticles (NPs) into catalytic plate First of all, the photodeposition was done using platinum nanoparticles. The photodeposition for this metal consists in these following steps: 1. Calculate the amount of platinum that I need to put to have 1% of the total amount of titanium dioxide. In this case, the catalytic plate has 1,2mm length, 0,4mm width and 0,02mm thickness, so it has 0,0096mm3. Taking into account that the density of titanium dioxide is 4,23 g/cm3, it has 0,0406mg in each microchannel. Now, if I multiply this amount by 16, that is the total microchannels in this reaction plate, I obtain 0,6497 mg of titanium dioxide. So, the 1% of this amount is 0,006497 mg Pt, but I need to calculate the amount necessary of K2PtCl6 doing: Then, if I weight 1,6 mg of K2PtCl6, I dissolve this amount in 100 mL of water and after that I catch 1 mL of this solution, I have the amount needed of platinum (1% of total amount of TiO2). 2. Add 59mL of water in a bowl and 1mL of the solution of platinum prepared before. 3. Shake during 15-20min. 4. Add the catalytic plate (treated with the sol-gel method) and also add 60mL of ethanol. 5. Place the light at a distance close enough to the bowl. In this case it has been placed at approximately 2,8cm from the bowl. 6. Program the light for 12 hours, cover the area around the light and switch it on. This process to add the platinum NPs is called photodeposition. In the following figure it can be appreciated the photodeposition process before switch on the light. Photocatalytic microreactor for the production of hydrogen 54 Figure 29: Photodeposition process. When photodeposition was finished, I inserted the catalytic plate in the oven during 2 hours, approximately. In the experiments realized I observed that probably platinum nanoparticles did not adhere correctly. So, I decided to repeat the photodeposition but depositing gold nanoparticles instead platinum nanoparticles. The procedure to add gold nanoparticles was: 1. Calculate the amount of gold that I need to put to have 1% of the total amount of titanium dioxide. In this case, the catalytic plate has 1,2mm length, 0,4mm width and 0,02mm thickness, so it has 0,0096mm3. Taking into account that the density of titanium dioxide is 4,23 g/cm3, it has 0,0406mg in each microchannel. Now, if I multiply this amount by 16, that is the total microchannels in this reaction plate, I obtain 0,6497 mg of titanium dioxide. So, the 1% of this amount is 0,006497 mg Au. The solution of gold is HAuCl4 with a molecular weight of 339,8 g/mol, and the concentration of this solution is 5·10-4 M. So, I need a volume of 38,3 µL of this solution. 2. Add 175mL of water in a bowl and 38,3 µL of HAuCl4. 3. Shake during 15-20min. Report 55 4. Add the catalytic plate (treated with the sol-gel method) and also add 175mL of ethanol. 5. Place the light at a distance close enough to the bowl. In this case it has been placed at approximately 2,8cm from the bowl. 6. Program the light for 12 hours, cover the area around the light and switch it on. The set-up of the process to add gold nanoparticles is the same process than platinum nanoparticles seen in Figure 29. When photodeposition was finished, I inserted the catalytic plate in the oven during 2 hours, approximately. To observe if the gold nanoparticles were attached, a violet color must be visible on the surface of the microchannels. In this case it was not very visible, so I decided to add 6%wt. of the gold solution (5·10-4 M HAuCl4) manually with subsequent calcination at 300°C for 3h. Photocatalytic microreactor for the production of hydrogen 56 3.3. Previous checkpoint tests 3.3.1. Check water flow using a peristaltic pump First of all, I needed to check if the flow that I specified in the peristaltic pump was correct. To check this, I cleaned the tubes and after clean it I switched on the pump. I used two beakers, one of them filled with water so that the pump could absorb the water, and the other I put it on the scale to calculate the amount of water in a given time. In this way, I was able to determine whether the water flow rate specified on the pump was correct or not. Now, in the following figure it could be seen the peristaltic pump used in the development of the experiments. Figure 30: Peristaltic pump. 3.3.2. Check gas flow using a mass flow The following test that I had done is to check the gas flow using a mass flow controller. A mass flow controller is a device for specify the gas flow. In this case the inlet was connected to a gas (argon) and the outlet was connected to a test tube containing at the end a mixture of water and soap (see Figure 32), in this way by squeezing a bubble came out and by calculating the time it took to reach a certain volume I was able to know the gas flow rate at the outlet. When it was carried out the test, the real gas flow at the outlet was higher than the theoretical flow, for this reason it was made the following calibration line that show what flow should be put in the mass flow so that the outlet has the flow that is wanted. (see annex: CHAPTER 10: GAS CALIBRATION CURVE) Report 57 Finally, in the following figures it could be observed the mass flow controller used and the bubbler meter used. Figure 31: Mass flow controller. Figure 32: Bubbler meter. Photocatalytic microreactor for the production of hydrogen 64 4. CHAPTER 4: RESULTS AND DISCUSSION In this chapter it will be discussed about the effect of three parameters: gas flow rate, liquid flow rate and temperature, in order to observe the trend of hydrogen production. 4.1. Effect of gas flow rate in S-G interaction In this sub-chapter it will be compared the effect of gas flow rate using three values, 20 ml/min, 40 ml/min and 60 ml/min, at room temperature and at 75ºC. 4.1.1. Catalytic plate containing 16 microchannels 4.1.1.1. At room temperature Table 8: Data obtained in S-G interaction at room temperature using the catalytic plate containing 16 microchannels. Gas flow rate (ml/min) Hydrogen production (µmol/min*gcat.) Acetaldehyde production (µmol/min*gcat.) 20 3,8 17,7 40 - 10,1 60 - 7,6 Figure 38: Acetaldehyde production vs gas flow rate at room temperature using the catalytic plate containing 16 microchannels. As the hydrogen peak is practically not seen, this graph is centred in the production of acetaldehyde, because acetaldehyde and hydrogen are produced in the same magnitude. In the following reaction you can observe that 1 mol of ethanol produce 1 mol of acetaldehyde and 1 mol of hydrogen. 0 5 10 15 20 0 20 40 60 80 Acetaldehyde production (µmol/min*gcat.) Gas flow rate (ml/min) Acetaldehyde production vs gas flow rate Acetaldehyde production vs gas flow rate Report 65 4.1.1.2. At 75ºC Table 9: Data obtained in S-G interaction at 75ºC using the catalytic plate containing 16 microchannels. Gas flow rate (ml/min) Hydrogen production (µmol/min*gcat.) 20 20,2 40 17,7 60 15,1 Figure 39: Hydrogen production vs gas flow rate at 75ºC using the catalytic plate containing 16 microchannels. In both cases, it is observed that as the gas increases the hydrogen production decreases. This is due to the fact that as more gas passes through the microreactor, the hydrogen produced is dissolved in it, and as there is more gas in the same volume this hydrogen decreases, as the GC detects more gas (argon) than hydrogen, although the amount of hydrogen produced is always the same. 0 5 10 15 20 25 0 20 40 60 80 Hydrogen production (µmol/min*gcat.) Gas flow rate (ml/min) Hydrogen production vs gas flow rate Hydrogen production vs gas flow rate Photocatalytic microreactor for the production of hydrogen 66 4.1.2. Catalytic plate containing 32 microchannels For the catalytic plate containing 32 microchannels, this experiment was carried out directly at 75ºC, as this is where more hydrogen production was observed in the catalytic plate containing 16 microchannels. Table 10: Data obtained in S-G interaction at 75ºC using the catalytic plate containing 32 microchannels. Gas flow rate (ml/min) Hydrogen production (µmol/min*gcat.) 20 75,7 40 75,7 60 75,7 Figure 40: Hydrogen production vs gas flow rate at 75ºC using the catalytic plate containing 32 microchannels. In this case, no variation in hydrogen production was observed if the gas flow was varied. 0 10 20 30 40 50 60 70 80 0 20 40 60 80 Hydrogen production (µmol/min*gcat.) Gas flow rate (ml/min) Hydrogen production vs gas flow rate Hydrogen production vs gas flow rate Report 67 4.2. Effect of liquid flow rate in S-L-G interaction 4.2.1. Catalytic plate containing 16 microchannels The results obtained at 50ºC and at 75ºC will be shown where the optimum conditions for hydrogen production can be seen. 4.2.1.1. At 50ºC Table 11: Data obtained in S-L-G interaction at 50ºC using the catalytic plate containing 16 microchannels. Liquid flow rate (ml/min) Gas flow rate (ml/min) Hydrogen production (µmol/min*gcat.) 0,29 20 83,3 0,58 20 87,1 1,16 20 66,9 2,61 20 47,9 5,22 20 32,8 10,16 20 29,0 Figure 41: Hydrogen production vs liquid flow rate at 50ºC using the catalytic plate containing 16 microchannels. 0 10 20 30 40 50 60 70 80 90 100 0 2 4 6 8 10 12 Hydrogen production (µmol/min*gcat.) Liquid flow rate (ml/min) Hydrogen production vs liquid flow rate at 50ºC Hydrogen production vs liquid flow rate Photocatalytic microreactor for the production of hydrogen 68 4.2.1.2. At 75ºC In this case, as the temperature increases, the microreactor allows less liquid flow rate. For this reason at 75 I varied the liquid flow rate from 0,29 ml/min to 5,22 ml/min, to avoid that the reactor was flooding. Table 12: Data obtained in S-L-G interaction at 75ºC using the catalytic plate containing 16 microchannels. Liquid flow rate (ml/min) Gas flow rate (ml/min) Hydrogen production (µmol/min*gcat.) 0,29 20 131,2 0,58 20 157,7 1,16 20 140,1 2,61 20 127,4 5,22 20 108,5 Figure 42: Hydrogen production vs liquid flow rate at 75ºC using the catalytic plate containing 16 microchannels. As you can observe, the optimum conditions for the hydrogen production using the catalytic plate containing 16 microchannels are: 0,58 ml/min of liquid flow rate, 20 ml/min of gas and heat the microreactor to 75ºC. 0 20 40 60 80 100 120 140 160 180 0 1 2 3 4 5 6 Hydrogen production (µmol/min*gcat.) Liquid flow rate (ml/min) Hydrogen production vs liquid flow rate at 75ºC Hydrogen production vs liquid flow rate Report 69 4.2.2. Catalytic plate containing 32 microchannels This experiment was carried out directly at 75ºC, as this is where more hydrogen production was observed in the catalytic plate containing 16 microchannels. Table 13: Data obtained in S-L-G interaction at 75ºC using the catalytic plate containing 32 microchannels. Liquid flow rate (ml/min) Gas flow rate (ml/min) Hydrogen production (µmol/min*gcat.) 0,29 20 113,5 0,58 20 100,9 1,16(*) 20 105,9 * Result not accurate because the microreactor was flooding. Figure 43: Hydrogen production vs liquid flow rate at 75ºC using the catalytic plate containing 32 microchannels. For this S-L-G interaction using the catalytic plate containing 32 microchannels, the optimum conditions are not the same than the other catalytic plate (16 microchannels). In this case, the microchannels are narrower and therefore the reactor needs less liquid to pass through to avoid flooding. Moreover, the peristaltic pump used does not allow working at liquid flow rates below 0,29 ml/min, so it was not possible to find the optimum conditions for this catalytic plate. 100 102 104 106 108 110 112 114 116 0 0.5 1 1.5 Hydrogen production (µmol/min*gcat.) Liquid flow rate (ml/min) Hydrogen production vs liquid flow rate at 75ºC Hydrogen production vs liquid flow rate at 75ºC Photocatalytic microreactor for the production of hydrogen 70 4.3. Effect of temperature in S-G interaction In this sub-chapter, I will try to calculate the activation energy (the minimum energy required for a reaction to take place) for S-G interaction using Au/TiO2 photocatalyst from the respective graphs. Table 14: Effect of temperature in S-G interaction using the catalytic plate containing 32 microchannels. Temperature (ºC) Gas flow rate (ml/min) Hydrogen production (µmol/min*gcat.) 20 20 5,0 50 20 45,4 75 20 75,7 Figure 44: Comparison of gas flow rate of 20 ml/min at different temperatures. As you know, the activation energy for this interaction (S-G) can be calculated using the Arrhenius equation. [46] Where: - K is the rate constant (frequency of collisions resulting in a reaction). - A is the pre-exponential factor, a constant for each chemical reaction. 0 10 20 30 40 50 60 70 80 0 20 40 60 80 Hydrogen production (µmol/min*gcat.) Temperature (ºC) Hydrogen production vs temperature (S-G) Hydrogen production vs temperature (S-G) Report 71 - T is the absolute temperature (in Kelvin). - Ea is the activation energy for the reaction (in the same units as RT). - R is the universal gas constant. [46] Table 15: Data for the calculation of activation energy for the S-G interaction. Temperature (ºC) K (µmol/min*gcat.) ln(K) T (K) 1/T (K-1) 20 5,0 1,62 293 0,0034 50 45,4 3,82 323 0,0031 75 75,7 4,33 348 0,0029 If I plot ln(K) vs 1/T and I present the equation on the graph, with the value of the slope the activation energy can be calculated, since: Figure 45: Ln(K) vs 1/T for S-G interaction. So, the activation energy is: This value is practically the same with the values founded by other authors for the same reaction over Au/TiO2 photocatalyst, 30,3 kJ/mol [47]. y = -5150.4x + 19.362 R² = 0.9415 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 0.0028 0.0029 0.003 0.0031 0.0032 0.0033 0.0034 0.0035 Ln (K) (µmol/min*gcat.) 1/T (K-1) Ln(K) vs 1/T Ln(K) vs 1/T Lineal (Ln(K) vs 1/T ) Photocatalytic microreactor for the production of hydrogen 72 4.4. Effect of temperature in S-L-G interaction As the maximum hydrogen production is in 0,58 ml/min of liquid flow rate and 20 ml/min of gas flow rate, it was tried at different temperature to observe the effect of this. So, in this sub-chapter, I will try to calculate the activation energy for S-L-G interaction using Au/TiO2 photocatalyst from the respective graphs. Table 16: Effect of temperature in S-L-G interaction using the catalytic plate containing 16 microchannels. Temperature (ºC) Liquid flow rate (ml/min) Gas flow rate (ml/min) Hydrogen production (µmol/min*gcat.) 20 0,58 20 20,2 50 0,58 20 87,1 75 0,58 20 157,7 Figure 46: Comparison of the optimum condition for maximum hydrogen production at different temperatures. Now, with the same equation shown above (Arrhenius) and with the same procedure followed, the activation energy for the S-L-G interaction will be calculated. 0 50 100 150 200 0 20 40 60 80 Hydrogen production (µmol/min*gcat.) Temperature (ºC) Hydrogen production vs temperature (S-L-G) Hydrogen production vs temperature (S-L-G) Report 73 Table 17: Data for the calculation of activation energy for the S-L-G interaction. Temperature (ºC) K (µmol/min*gcat.) ln(K) T (K) 1/T (K-1) 20 20,2 3,01 293 0,0034 50 87,1 4,47 323 0,0031 75 157,7 5,06 348 0,0029 Figure 47: Ln(K) vs 1/T for S-L-G interaction. So, the activation energy is: In the S-L-G interaction as in the S-G interaction, the value obtained for the activation energy is practically the same with the values founded by other authors for the same reaction over Au/TiO2 photocatalyst, 30,3 kJ/mol [47]. y = -3865.5x + 16.267 R² = 0.9811 0 1 2 3 4 5 6 0.0028 0.0029 0.003 0.0031 0.0032 0.0033 0.0034 0.0035 Ln (K) (µmol/min*gcat.) 1/T (K-1) Ln(K) vs 1/T Ln(K) vs 1/T Photocatalytic microreactor for the production of hydrogen 80 cm-1, 612 cm-1 and 826 cm-1. So, observing the graphs above the phase representative is anatase because the graphs has the approximately the same peaks than the peaks of anatase phase in the literature cited. 4.6.2. UV-vis This graph could only be obtained after the reaction, as the equipment was not working before the reaction took place. In Figure 55, what it wants to observe is whether the gold plasmon is visible in order to clarify that there is gold adhered into the catalytic plate. Moreover, this equipment shows the gold qualitatively, so it will not be known how much gold is adhered. Figure 55: Comparison of the UV-vis between both catalytic plates. As you can see, there is a broad and very little highlighted peak with a maximum in 554nm, which corresponds to the plasmon absorption of gold (see Figure 55). Report 81 5. CHAPTER 5: CONCLUSIONS In this project it has studied different methods of immobilization of titanium dioxide on a 316Ti stainless steel catalytic plate, and it has also studied the way in which metallic nanoparticles (Au, Pt...) can be attached. At this point, it is recommended to use the sol-gel immobilization method and the manual impregnation followed by calcination as a technique to adhere metallic NPs, as this is where the best results have been obtained. In this project two different experiments were carried out. First of all, the experiments were realized in the solid-gas interaction and after that, they were realized in the solid-liquid-gas interaction, to compare at the end the effect of both interactions. The effect of gas flow, the effect of liquid flow and the effect of temperature were studied for both catalytic plates (16 microchannels and 32 microchannels). It was observed that the temperature favored the reaction, obtaining more hydrogen production, since the kinetics of the reaction improves as the temperature increases. Furthermore, it was possible to determine the optimal conditions for the catalytic plate containing 16 microchannels, where a hydrogen production of 157,7 µmol/(min·gcat) was obtained using a liquid flow rate of 0,58 ml/min, a gas flow rate of 20 ml/min and heating the microreactor to 75ºC. For the 32 microchannel catalytic plate, the optimal conditions could not be obtained because the available peristaltic pump did not allow liquid flow rates below 0,29 ml/min. Finally, with the RAMAN and UV-vis techniques it was possible to observe that the peaks of the spectra obtained were of the anatase phase, and it was obtained a small broad peak at 554 nm which corresponded to the plasmon absorption of gold. Photocatalytic microreactor for the production of hydrogen 82 6. CHAPTER 6: PLANNING 6.1. Timeline Table 20: Timeline CODIFICATION ACTIVITY DESCRIPTION TIME SPENT A Project acceptance Acceptance of the experimental project to be carried out. 2h B Determination of the objectives Identification of the objectives. 12h C Impregnation of the catalytic plates Search different methods for impregnation on the catalytic plates and try heat attachment and sol gel methods. 206h D Previous checkpoint tests Check water flow in the peristaltic pump, check gas flow rate using a mass flow controller, and check leaks in the microreactor. 55h E Photodeposition Prepare the procedure to add metal NPs in the catalytic plate containing titanium dioxide. 44h F Tests using gas chromatography It has done 2 different tests using GC. First in S-G interaction, and second in S-L-G interaction putting the gas in countercurrent and the liquid go down by gravity and capillary forces. 142h G Writing of the TFM report Writing of the final report of the TFM. 120h G.1. Writing of the theoretical report Writing of the theoretical part of the project. 30h G.2. Writing of the experimental report Writing of the experimental part of the project. 62h G.3. Budget Project price calculation. 12h G.4. Writing of the annexes Additional project information 16h H Report delivery Uploading the document on the intranet. 1h Total 582h Report 83 6.2. GANTT CHART Codification Activity January February March April May June A Project acceptance B Determination of the objectives C Impregnation of the catalytic plates D Previous checkpoint tests E Photodeposition F Tests using gas chromatography G Writing of the TFM report G.1. Writing of the theoretical report G.2. Writing of the experimental report G.3. Budget G.4. Writing of the annexes H Report delivery Photocatalytic microreactor for the production of hydrogen 84 7. CHAPTER 7: ECONOMIC ANALYSIS The materials and resources used to carry out this project have been supplied by the Catalysis and Energy laboratory. Therefore, this budget is an idea of the cost that would be involved in carrying out this project if these materials and resources were not available. 7.1. Cost of materials and reagents In tables 21 and 22 are shown the materials and the reagents used in the development of this project. Table 21: Materials used in the development of the project. Material Price/unit (€) Amount Total price (€) Petri dish 4,6 3 13,8 Crucible 10,5 3 31,5 Volumetric flask 100 mL 10,8 2 21,6 Beaker 10 mL 10,4 3 31,2 Beaker 50 mL 16,6 1 16,6 Beaker 100 mL 14,2 1 14,2 Beaker 250 mL 23,7 1 23,7 Syringe 0,2 3 0,6 Needle 330µm 0,4 3 1,2 Fly for magnetic stirrer 2,1 3 6,3 Wash bottle 6,8 2 13,6 Spanner 19,5 4 78,0 TOTAL 252,3€ Table 22: Reagents used in the development of the project. Reagent Distributor Price Amount Total price (€) Hydrochloric acid 37% Panreac 20,6 €/L 11,4 mL 0,2 TiO2 P-90 Evonik Aeroxide 44,6 €/kg 1.2 g 0,1 Titanium isopropoxide Sigma - Aldrich 5,5 €/mL 20 g (density: 937 g/L) 117,4 Ethanol Scharlau 142,1 €/L 535 mL 76,0 K2PtCl6 Sigma - Aldrich 31,3 €/g 1,6 mg 0,1 HAuCl4 Sigma - Aldrich 160,8 €/g 38,3 · 10-3 mL (density: 3900 g/L) 24,0 TOTAL 217,8€ Report 85 7.2. Personnel cost In order to be able to determine the personnel costs, the development of the research, carried out by a chemical engineer, and the management of the project have been taken into account. Table 23 shows the costs according to the hours worked: Table 23: Personnel cost. Role Category Price per hour Persons Hours worked Total price (€) Research development Chemical engineer 12 1 582 6.984 Project management Doctor of chemistry 45 2 55 4.950 TOTAL 11.934€ 7.3. Equipment cost This cost is determined by the expenditure generated by the different equipment used in the development of the project. Table 24: Equipments used in the development of the project taking into account the depreciation. Equipment Useful life (year) Time used Price (€) Depreciation cost (€) Oven 10 3 months 5.200 130 Furnace 10 3 months 3.500 87,5 Gas chromatography 5 2 months 30.000 1.000 Microreactor and power source 10 3 months 17.460 436,5 Analytic balance 10 3 months 3.000 75 Magnetic stirrer 10 3 months 300 7,5 Peristaltic pump 10 2 months 530 8,8 Mass flow controller 8 2 months 800 16,6 Photodeposition light (Xenon light) 10 2 months 3.000 50 Ultrasound 10 1 month 650 5,4 Optical microscope 10 1 month 2.000 16,6 RAMAN microscopy 10 3 days 140.000 116,7 TOTAL 1.950,6€ The depreciation of the equipment has to be taken into account and is calculated according to the following formula: Photocatalytic microreactor for the production of hydrogen 86 The equipment used for the realization of this project has a residual value of 0. The depreciation of the oven shall be calculated as an example. 7.4. Total cost The total cost is determined by the sum of the costs calculated above. Table 25: Total cost of the final master's project Cost type Total price (€) Material 252,3 Reagents 217,8 Personnel 11.934 Equipment (depreciation cost) 1.950,6 TOTAL 14.354,7€ Report 87 8. CHAPTER 8: FUTURE PROSPECTS 8.1. Environmental impact The environmental impact is defined as any change to the environment, whether adverse or beneficial, resulting from a facility’s activities, products, or services. In other words it is the effect that people's actions have on the environment [48]. The aim of this project is to somehow replace the fuels that are used today, which in some way pollute and damage the environment, with hydrogen. By means of photocatalysis, the solution which passed through the microreactor is ethanol and water, which do not pollute, and hydrogen is obtained, a gas that can also be used as fuel and does not harm the environment. Another contaminants which can be considered in this project are: noise pollution, water and electricity consumption. The first one is caused especially by the gas chromatograph. The second one is important to be highlighted because the consumption of water is little only if the solution is recirculated, and the third is also important because the electricity needed is a little high due to thermostatic bath at 75ºC. 8.2. Proposals for future projects Based on the results obtained in this project, the following is recommended for possible future work: 1. Use a liquid pump that allows flow rates below 0,29 ml/min in the catalytic plate containing 32 microchannels, to obtain the optimal conditions in this. 2. Use a different metal NPs, for example platinum, and compare the results obtained with the gold NPs results. 3. Perform the metal photodeposition continuously, i.e. by means of a peristaltic pump, where the liquid flows continuously through the microchannels. 4. Realize the tests with the gas in co-current and compare the results with the gas in counter-current. Photocatalytic microreactor for the production of hydrogen 88 9. CHAPTER 9: REFERENCES [1] N. Jiménez Divins, “Catalytic hydrogen production over RhPd / CeO2 catalysts and CO purification over Au / TiO2 catalysts,” 2015. [2] L. M. Gandía, G. Arzamendi, and P. M. Diéguez, Renewable Hydrogen Technologies: Production, Purification, Storage, Applications and Safety. Elsevier, 2013. [3] IEA, “Key World Energy Statistics 2020 – Analysis,” IEA OECD, vol. 33, no. August, p. 4649, 2020. [4] J. Llorca, El hidrógeno y nuestro futuro energético, UPC. Barcelona, 2010. [5] D. Das and T. N. Veziroǧlu, “Hydrogen production by biological processes: A survey of literature,” Int. J. Hydrogen Energy, vol. 26, no. 1, pp. 13–28, 2001. [6] EIA, “Hydrogen explained: Production of hydrogen,” U.S. Energy Information Administration, 2021. [Online]. Available: https://www.eia.gov/energyexplained/hydrogen/production-of-hydrogen.php. Accessed in: February 2021. [7] J. H. R. Enslin, R. Ramakumar, T. R. Mancini, R. Messenger, J. Ventre, and G. Hoogers, “Alternative power systems and devices,” Systems, Controls, Embedded Systems, Energy, and Machines, 2017. [Online]. Available: https://hydrogeneurope.eu/fuel-cells. Accessed in: February 2021. [8] T. H. Rehm, S. Gros, P. Löb, and A. Renken, “Photonic contacting of gas-liquid phases in a falling film microreactor for continuous-flow photochemical catalysis with visible light,” React. Chem. Eng., vol. 1, no. 6, pp. 636–648, 2016. [9] D. C. Fabry et al., “Blue light mediated C-H arylation of heteroarenes using TiO2 as an immobilized photocatalyst in a continuous-flow microreactor,” Green Chem., vol. 19, no. 8, pp. 1911–1918, 2017. [10] K. Mevissen and V. Hessel, “Pseudo 3-D simulation of a falling film microreactor based on realistic channel and film profiles,” Elsevier, vol. 63, pp. 5149–5159, 2008. [11] S. E. Braslavsky, “Glossary of terms used in photochemistry 3rd edition: (IUPAC Recommendations 2006),” Pure Appl. Chem., vol. 79, no. 3, pp. 293–465, 2007. [12] S. C. Ameta, R.; Ameta, Photocatalysis: Principles and Applications, CRC Press. Boca Ratón, 2016. [13] N. Fajrina and M. Tahir, “A critical review in strategies to improve photocatalytic water splitting towards hydrogen production,” Int. J. Hydrogen Energy, vol. 44, no. 2, pp. 540– 577, 2019. Report 89 [14] S. Sébastien, “Les soignants face à la mort,” Rev. Infirm., vol. 1, no. 180, pp. 39–41, 2012. [15] M. M. Mahlambi, C. J. Ngila, and B. B. Mamba, “Recent Developments in Environmental Photocatalytic Degradation of Organic Pollutants: The Case of Titanium Dioxide Nanoparticles-A Review,” J. Nanomater., vol. 2015, 2015. [16] S. U. N. Jiulong, “Development of Inorganic-Organic Hybrid Materials for Waste Water Treatment,” no. 1, pp. 36–37, 2014. [17] Q. M. Mao and O. E. Potter, “Fluidized Bed Reactor Modelling.,” Annu. Meet. - Am. Inst. Chem. Eng., vol. 80, no. 241, pp. 65–71, 1984. [18] J. C. Jubin, “Fixed bed reactor system,” Sci. Direct, pp. 1–13, 1988. [19] D. Lokhat, A. K. Domah, K. Padayachee, A. Baboolal, and D. Ramjugernath, “Gas–liquid mass transfer in a falling film microreactor: Effect of reactor orientation on liquid-side mass transfer coefficient,” Chem. Eng. Sci., vol. 155, pp. 38–44, 2016. [20] S. Version, “Assembly Instructions for the Falling Film Micro Reactor with LED arrays,” no. April, pp. 1–19, 2016. [21] V. C. Calbet, “Hydrogen photochemical production in a microreactor with Au / TiO2 nanoparticles,” no. June, 2015. [22] ALIBABA, “De cerámica pinturas de las propiedades químicas de TiO2,” Alibaba. [Online]. Available: https://spanish.alibaba.com/product-detail/ceramic-paintschemical-properties-of-tio2-60285406351.html. Accessed in: April 2021. [23] J. Zhang, B. Tian, L. Wang, and M. Xing, Photocatalysis. Shanghai: Springer, 2018. [24] R. Lance and J. Tate, “Optical Analysis of Titania: Band Gaps of Brookite, Rutile and Anatase,” Bachelor Thesis Dep. Phys. Oregon State Univ., p. 24, 2018. [25] H. Eskandarloo, A. Badiei, M. A. Behnajady, and G. M. Ziarani, “UV-LEDs assisted preparation of silver deposited TiO2 catalyst bed inside microchannels as a high efficiency microphotoreactor for cleaning polluted water,” Chem. Eng. J., vol. 270, pp. 158–167, 2015. [26] M. Mehrjouei, S. Müller, and D. Möller, “Design and characterization of a multi-phase annular falling-film reactor for water treatment using advanced oxidation processes,” J. Environ. Manage., vol. 120, pp. 68–74, 2013. [27] R. Hernández et al., “Au-TiO2 synthesized by a microwave-and sonochemistry-assisted sol-gel method: Characterization and application as photocatalyst,” Catalysts, vol. 10, no. 9, pp. 1–18, 2020. Photocatalytic microreactor for the production of hydrogen 96 Figure 59: Sol-gel method before reaction without gold NPs. Figure 60: Sol-gel method after reaction with gold NPs. 12.2. Catalytic plate containing 32 microchannels Figure 61: Sol-gel method after reaction with gold NPs. Report 97 13. CHAPTER 13: RAMAN SPECTROSCOPY 13.1. What is RAMAN Spectroscopy? RAMAN Spectroscopy is a molecular spectroscopic technique which uses the interaction of light with matter to gain insight into a materials make up or characteristics. The information provided by this technique, results from a light scattering process. [1] Moreover, is a non-destructive chemical analysis technique which provides detailed information about [2]:  Chemical structure and identity.  Phase and polymorphism.  Intrinsic stress/strain.  Contamination and impurity. 13.2. The RAMAN Spectroscopy principle RAMAN is a light scattering technique, whereby a molecule scatters incident light from a high intensity laser light source. Depending if the scattered light is at the same wavelength or at different wavelength has different name. If the scattered light is at the same wavelength as the laser source, this is called Rayleigh Scatter, and if the scattered light is at different wavelengths (or colors), which depend on the chemical structure of the analyte, this is called Raman Scatter. [2] Photocatalytic microreactor for the production of hydrogen 98 Figure 62: RAMAN Principle. Extracted from: [2]. 13.3. Type of samples analyzed with RAMAN This technique can be used for different type of samples, but today is not suitable to use in metals and their alloys. Normally it is used for [2]:  Solids, powders, liquids, gels, slurries and gases.  Inorganic, organic and biological materials.  Pure chemicals, mixtures and solutions.  Metallic oxides and corrosion. Report 99 14. CHAPTER 14: GAS CHROMATOGRAPHY 14.1. What's gas chromatography? Gas chromatography is an analytical technique used to separate the chemical components of a sample mixture. Then this technique detect each component and determine if there are some amount of some component and if there are, how much is present. [3] For the satisfactory performance of gas chromatography in analysis, it is needed that the components of the sample mixture are volatile, usually with a molecular weight below 1250 Da and thermally stable to avoid degradation on the GC system. [3] 14.2. How does it work? Gas chromatography uses a carrier gas in the separation to transport the sample molecules through the GC system. [3] First of all, the sample is introduced into the gas chromatography, either with a syringe or transferred from an autosampler. The sample is injected into the GC inlet through a septum which enables the injection of the sample mixture without losing the mobile phase, which contains the carrier gas said before. After that, the sample arrives to the analytical column that is long, narrow fused silica or metal tube which contains the stationary phase coated on the inside walls. The analytical column is held in the column oven which is heated during the analysis to elute the less volatile components. [3] After this, the outlet of this analytical column is inserted into the detector which emit a signal when a chemical component is eluted. The signal is recorded by a software on a computer to produce a chromatogram. So, in the chromatogram you can observe all the chemical components eluted in specific time. [3] Finally, in the following figures you can appreciate the diagram of a gas chromatography and a gas chromatography chromatogram, respectively. [3] Photocatalytic microreactor for the production of hydrogen 100 Figure 63: Gas chromatography diagram: (1) carrier gas, (2) autosampler, (3) inlet, (4) analytical column, (5) detector and (6) computer. Extracted from: [3]. When you are analyzing some sample, the analysis is finished when you observe a straight line with no peaks indicating that an element is present. Figure 64: Gas chromatography chromatogram. Extracted from: [3]. As you can observe, the x axis is the retention time (tR) which indicates the time in which each chemical element is eluted through analytical column. Components that have a greater affinity for the stationary phase spend more time in the column and thus elute later and have a longer retention time. Other thing that you can see in the chromatogram figure (Figure 64) is the Report 101 baseline that shows the signal from the detector when no analyte is eluting from the column or it is below the detection limit. The response of the baseline is a mixture of electrical noise and chemical noise, like impurities in the carrier gas, column stationary phase bleed and system contamination. If the baseline is higher than it should be it indicates that there is a problem or that maintenance is required. [3] Moreover, various measurements can be taken from the peak like peak width, width at half height, peak height and area, but the area is the measurement used for quantification as it is less affected by band broadening. [3] Finally, if it centers in the peaks, narrow peaks give better sensitivity (signal to noise ratio) and a better resolution. But sometimes, there exists peak tailing (the right side of the peak is wider) that indicates activity or a dead volume in the system, and also there exists peak fronting (the left side of the peak is wider) that indicates the column is overloaded. [3] 14.3. Advantages In the following points you have defined different advantages for the gas chromatography. [4][5]  The use of longer columns and higher velocity of carrier gas allows the fast separation in a matter of a few minutes.  Higher working temperatures up to 5000 and the possibility of converting any material into a volatile component make gas chromatography one of the most versatile technique.  GC is favored for non-polar molecules.  GC is usually used in applications where small, volatile molecules are detected and with non-aqueous solution.  Accurate quantification.  Small sample needed. Photocatalytic microreactor for the production of hydrogen 102 14.4. Limitations In the following points you have defined different disadvantages or limitations for the gas chromatography. [4][5]  Compound to be analyzed should be stable under GC operation conditions.  The samples are also required to be salt-free. They should not contain ions.  Limited to volatile sample.  During injection of the gaseous sample, proper attention is required.  Samples be soluble and don't react with the column.  Not suitable for thermally labile samples. 15. CHAPTER 15: ULTRASOUND 15.1. The sound and ultrasounds Ultrasound is associated with a number of frequency harmonics within the hearing range. Therefore, when studying the health effects of ultrasound, this set of high-pitched and very high-pitched sounds (generally from a frequency above 10 kHz) should also be included. [6] In addition, ultrasound has essentially the same physical properties as sound waves, but due to its higher frequency and therefore shorter wavelength it diffracts less than audible sound waves. However, they are transmitted through substances that exhibit elastic properties. [6] The propagation speed of ultrasound in air at ambient temperature is equal to that of audible sound (343 m/s), while the propagation speed in a liquid medium, such as water, is higher and can reach a value of 1500 m/s. [6] The sources of ultrasound generation are classified according to their frequency, so three groups can be established [6]:  Low frequency (between 10 and 100 kHz). Used in cleaning, drilling, welding, chemical processes, etc.  Medium frequency (range from 100 kHz to 10 MHz). For therapeutic uses.  High frequency (from 1 to 10 MHz). Their main applications are in medicine and nondestructive control devices (e.g. measurement of liquid or gas flow in pipes). In the following picture can be observed the ultrasonic bath used in the laboratory. Report 103 Figure 65: Ultrasonic bath. 15.2. The cavitation phenomenon When an acoustic wave propagates through a liquid, zones of compression and expansion (rarefaction) are created, because the vibrational motion produced by the emitting source is communicated to the molecules of the medium, each of which transmits this motion to adjacent particles before returning to the initial position. [6] This continuous movement of particles generates in certain areas of the liquid changes in pressure (a positive pressure change is called a compression zone, and a negative pressure change is called a rarefaction zone). If a negative pressure Pc (Pc = Pacustic - Phidrostatic) is large enough so that the distance between molecules exceeds the critical molecular distance necessary to keep the liquid intact, voids will be generated inside the liquid, i.e. bubbles will form (cavitation bubbles). When the Pc equals the vapor pressure (Pv) of the liquid, cavitation bubbles will be produced. These bubbles, filled with solvent vapor in the gaseous state, are unstable and disappear in an instant, at about 10-6 s, due to the compression half-period. The collapse of the bubbles, caused by the compression period of the ultrasonic waves, forms powerful shock waves, called microcurrents. The most important consequence is the release of energy in the collapse zone of the bubble, where peak temperatures of the order of several thousand degrees K and peak pressures of the order of kilobars are reached during very short periods of time. Because of the large magnitude of these values, the interpretation of sonochemical phenomena is known as the Hot Spot Theory. [6] Photocatalytic microreactor for the production of hydrogen 104 Thus, in the laboratory, ultrasound is basically used in two processes [6]:  Sonochemistry  Cleaning of glassware Figure 66: Representative diagram of the cavitation phenomenon. Extracted from: [6]. 15.3. Sonochemistry The application of ultrasound in chemistry is known as sonochemistry. In certain chemical processes, ultrasound is used to accelerate certain chemical reactions. The cavitation phenomenon, and consequently the efficiency of the sonochemical reaction, will depend on the following four factors. [6] 15.3.1. The wave frequency In principle, any supplied frequency capable of inducing cavitation can be chemically active. However, an increases in the frequency of the wave decreases the production of cavitation. Consequently, the higher the frequency, the more power must be supplied. [6] 15.3.2. The solvent As the solvent is the energy carrier, its physical properties play an important role. Solvents with a high vapor pressure produce lower cavitation effects. In the case of liquids, these are Report 105 very viscous and optimal cavitation will be difficult to achieve due to the high cohesive forces between solvent molecules. [6] On the other hand, cavitation performance can be increased if the liquid is degassed or ultrafiltrated, as the presence of gas nuclei (air) or impurities induces the formation of stable bubbles. [6] 15.3.3. The temperature As the reaction temperature increases, the vapor pressure of the solvent increases and the cavitational effect decreases. For this reason, in order to accelerate the sonochemical process, it is advisable to keep the temperature low. [6] 15.3.4. Location of the reactor relative to the ultrasonic generator The energy distribution in an ultrasonic bath is uneven. In contrast, in ultrasonic generators of the breaker-probe and whistling-probe type, the energy distribution is uniform. For this reason, it is very difficult to quantify exactly the amount of power delivered to the reactor in an ultrasonic bath, as this depends on the dimensions and shape of the bath, and also on the thickness and position of the reactor in the ultrasonic bath. [6] 15.4. Cleaning of glassware Ultrasound has two synergistic effects: it loosens the dirt from the glass material and improves the dispersion of organic solvents in aqueous detergents, thus achieving greater cleanliness. The cleaning process of the glass material is achieved, on the one hand, due to the collapse of cavitation bubbles during the compression period of the ultrasonic waves, and on the other hand, due to the formation of shock waves that promote the process of cleaning, dispersion and erosion of solid substances. [6]