New 3D Printing Strategy for Structured Carbon Devices Fabrication

Citation: Delgado-Martín, G.; Rodríguez, N.; Domínguez, M.I.; Agámez-Pertuz, Y.Y.; Martínez Tejada, M.; Ruíz-López, E.; Ivanova, S.; Centeno, M.Á. New 3D Printing Strategy for Structured Carbon Devices Fabrication. Catalysts 2023, 13, 1039. https://doi.org/10.3390/ catal13071039 Academic Editor: Helder T. Gomes Received: 11 May 2023 Revised: 11 June 2023 Accepted: 24 June 2023 Published: 26 June 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). catalysts Article New 3D Printing Strategy for Structured Carbon Devices Fabrication Gabriel Delgado-Martín1, Nicolás Rodríguez 2,†, María Isabel Domínguez 1,* , Yazmin Yaneth Agámez-Pertuz 2, Marcela Martínez Tejada 1, Estela Ruíz-López 1, Svetlana Ivanova 1 and Miguel Ángel Centeno 1 1Departamento de Química Inorgánica e Instituto de Ciencia de Materiales de Sevilla, Centro Mixto CSIC—Universidad de Sevilla, 41092 Sevilla, Spain; [email protected] (G.D.-M.); [email protected] (M.M.T.); [email protected] (E.R.-L.); [email protected] (S.I.); [email protected] (M.Á.C.) 2Departamento de Química, Facultad de Ciencias, Universidad Nacional de Colombia, AK 30 No 45-03, Bogotá111321, Colombia; [email protected] (N.R.); [email protected] (Y.Y.A.-P.) *Correspondence: [email protected] † Current address: Departamento de Química, Facultad de Ciencias, Universidad de la Amazonia, Calle 17 Diagonal 17 con Carrera 3F, Florencia 180001, Colombia. Abstract: This work shows a new method for the preparation of 100% carbon-structured devices. The method is based on resorcinol-formaldehyde polymerization, using starch as a binder with the addition of a certain amount of external carbon source before polymerization. Molds obtained by 3D printing are used to shape the structured devices in the desired shape, and the ultimate pyrolysis step consolidates and produces the carbonaceous devices. The proposed method allows obtaining supports with different textural and surface properties varying the carbonaceous source, the solvent, or the pyrolysis conditions, among other factors. The as-obtained devices have demonstrated their usefulness as palladium supports for the gas-phase formic acid dehydrogenation reaction. The monolith shows a high conversion of formic acid (81% according to H 2 production) and a high selectivity towards hydrogen production at mild temperatures (80% at 423 K). Keywords: microreactors; 3D print; carbon microstructured devices; H 2 production; formic acid decomposition 1. Introduction The use of structured and micro-structured catalysts has grown exponentially in recent years as being one of the most efficient solutions to decrease the operational problems and costs related to the mass and heat transport limitations in many industrial reactions. These devices allow high space velocities resulting in process intensification, delocalization and demand-to-manufacture production, continuous processes with higher safety, etc. As outstanding characteristics of the structured catalysts and monolithic porous structures in particular, we can enumerate (i) the high void fraction and large geometrical surface area [ 1 ], providing large contact surfaces between catalyst and reagents, (ii) the low-pressure drops under flow conditions and (iii) the high mechanical stability and dust tolerance. The application of such devices in automobile exhaust (three-way catalyst) [ 2 , 3 ] and NOx emissions removal [ 4 ] is well-known and widespread. However, in the real chemical conversion industry, the application of structured catalysts started becoming an innovative topic [5] in the last few years. The great number of possible catalytic applications requires adapting the nature, morphology, size, and properties of the structured system. Bulk monoliths, monoliths with parallel channels of different geometry (square, circular, etc.), open cell foams, periodic cellular architectures, and monoliths with different shapes (square, cylindrical etc.) must adapt to the fluid dynamics of every particular process. Catalysts 2023,13, 1039. https://doi.org/10.3390/catal13071039 https://www.mdpi.com/journal/catalysts Catalysts 2023,13, 1039 2 of 17 Of special importance is the nature of the substrate forming the structure since it must be selected according to the reaction working conditions: temperature, reaction atmosphere, gas or liquid media, etc. The substrates can be metallic, glass/vitreous, ceramic, or based on carbon materials, the latter being the subject of this study. Although considered unstable in the presence of hydrogen at temperatures above 700 K and in oxygen above 500 K [ 6 ], carbon substrates present several advantages, such as (i) high stability in both acidic and alkaline media [ 7 ], (ii) chemical inertness [ 8 ]; (iii) ease of active phase recovering; (iv) low cost [ 9 ], and (v) tuneable surface chemistry and porosity properties [ 8 ]. It is possible to modify the substrate’s hydrophobic/hydrophilic character, i.e., the chemical nature of the surface and its porous structure, by changing the carbon precursor used during the structure reactors preparation and activation process [6]. Up to now, the fabrication of carbon-based substrates has been essentially based on extrusion after mixing with a certain amount of non-carbonaceous compounds, such as metals. Only in recent years have new strategies for pure carbon monolith production been reported, although their number remains rather scarce. The advances in the production and application of bulk carbon monoliths [ 10 , 11 ], cell foams [ 12 ], or parallel channel monoliths [13–15] are summarized in Table 1. Table 1. Preparation methods and applications of different monolithic carbon structures. Material Preparation Application Reference (Bulk) monolithic carbon aerogels Sol-gel polymerization of phenolic resin within melamine foam (MF), ambient pressure drying, and co-carbonization. Temperature thermal insulators Organics absorption [10] (Bulk) monolithic carbon xerogels Sol-gel polymerization of resorcinol and formaldehyde in water, CsCO3 as the polymerization catalyst. Crude oil removal from oil-saltwater emulsions [11] (Nickel-nitrogen-doped) carbon foam After curation, the organic gel is immersed in acetone, microwave drying, and carbonization in a nitrogen flow at 900 ◦C. Electrode for CO2electroreduction [12] Monolithic biochar Impregnation of a polyurethane foam template with a solution of coal tar pitch in tetrahydrofuran. Drying at room temperature, oxidation in air at 350 ◦C, and carbonization at 950 ◦C in argon. Catalytic conversion of toluene [13] Monolithic cobalt-nitrogen-carbon frameworks Hydrothermal treatment of wood cylinders in Co-metal solution (200 ◦C). Drying at 105 ◦C and pyrolysis at 800 ◦C in an N2 atmosphere. Electrochemical synthesis of hydrogen peroxide in acidic media [14] Hierarchical monolithic carbon Wood carbonization at 1000 ◦C in an N2atmosphere. Hydrogen evolution reaction [15] Among the various synthetic routes, carbon gels are preferred because of their high purity, homogeneity, and properties tune possibility (morphology, porosity, and surface chemistry). Their versatility is based on the possibility of adapting the precursor concentrations and gel synthesis parameters to obtain materials with targeted properties for a given application [ 16 ]. For example, the pH of the precursor solution governs the size of the final meso/macropores [ 17 ], but their distribution in the final carbon xerogels is controlled by combining different synthesis conditions and chemical activation with KOH [18]. Once prepared, the carbon precursors must be consolidated in carbon monoliths of sufficient mechanical strength for possible future industrial applications. This strength is Catalysts 2023,13, 1039 3 of 17 highly dependent on the starting carbon material and preparation process. Compressive strength of 10.1 MPa has been described for a monolith with 9 channels/cm 2 and a wall thickness of 900 µ m. An increase of the channels to 62 channels/cm 2 and a decrease in the wall thickness to 165 µ m resulted in lower compressive strength of 4.4 MPa per monolith [ 8 ]. Carbon monoliths produced from activated carbon powder are mechanically weak [ 19 ], while the use of phenol-formaldehyde resins results in integral carbon structures with superior mechanical properties [ 20 ]. These resins are also important for the 3D printing strategy of preparation of carbon-structured substrates recently patented by the authors of this study [ 21 ]. This method allows the preparation of integral three-dimensional carbon structures using carbonaceous materials of a very different nature. The novelty of the fabrication process resides in the use of resorcinol-formaldehyde polymer mixed with a natural polymeric binder such as starch [ 22 ] and a carbonaceous source of different natures to obtain carbon monoliths with fully defined geometries and retaining the properties of the used carbon source. The process consists of five steps: (i) preparation and stabilization of organic solution precursor for resorcinol-formaldehyde polymerization; (ii) grinding and sieving of the additional carbonaceous source; (iii) mixing of the latter with the organic solution and (iv) packing in a 3D printed mold and (v) polymer curing and pyrolysis to obtain the final carbon monolith [21]. During the process of fabrication, various parameters can be modified, e.g., the nature of solvent or aldehyde in the polymerization step, the nature and quantity of additional catalysts or binders, the nature of the added carbon (commercial activated carbon, charcoal, biochar or nanotubes, etc.) or the organic solution/carbon (mL/g) ratio. The same is available for the curing and pyrolysis conditions. Parameters such as time, temperature and type of vessel during the curation process or atmosphere, temperature, physical/chemical activation, and heating rate of pyrolysis, and also treatments for monolith demolding can be adjusted. All these factors affect the characteristics of the finally obtained structured carbon materials in terms of mechanical, textural, and surface properties. In previous work, Rodríguez et al. [ 22 ] studied the influence of starch addition as a binder in the resulting carbon xero- and aerogels by resorcinol-formaldehyde polycondensation. They found that starch allowed carbon xerogels and aerogels surface and textural properties modulation, achieving structural properties similar to those of aerogels with a faster and less expensive process of fabrication (convective vs. supercritical drying). In this line, the present work devotes to the study of the influence of the solvent nature in the polymerization stage, the heating rate in the pyrolysis stage, and the initial carbon source on the final properties of structured carbonaceous devices. The utility of the prepared, structured systems is tested in the gas phase hydrogen production via formic acid dehydrogenation using a low charge of Pd metal as the active site. 2. Results and Discussion As indicated in the introduction, the carbon monoliths are prepared in a 5-step process using pre-fabricated polylactic acid (PLA) molds that result in carbon devices exemplified in Figure 1. A detailed description of the preparation procedure is described in Section 3. Catalysts 2023, 13, 1039 4 of 17 Figure 1. Examples of prepared PLA molds (left) and carbon monolithic devices (right). As shown in Figure 1, the method of preparation allows the fabrication of pieces with different sizes, morphology, and number of channels. The final quality of the carbon devices can be influenced at every step of the process. Herein only the nature of the solvent used in the polymerization stage and the effect of the heating rate in the pyrolysis stage will be discussed. 2.1. Influence of the Nature of Solvent in the Polymerization Stage on the Carbon Structured Device Different solvents have been tested (water, 96% ethanol, polyethylene glycol (PEG), or PEG + 1% polyvinyl alcohol (PVA). The rest of the variables of the process and added carbonaceous source (activated charcoal Darco Sigma-Aldrich) are kept constant. The monoliths have been prepared, as explained in Section 3, using PVA molds. The final carbon monolith measurements are 1.5 cm in diameter, 1 cm high, and 28 entire channels, resulting in a device with 21.5 channels/cm 2 (Figure 2). The nomenclature of the obtained monoliths is resumed in Table 2. Table 2. Nomenclature of the monoliths prepared with different solvents. Solvent Nomenclature Water D_H 2 O Ethanol D_EtOH Polyethylene glycol (PEG) D_PEG PEG + 1% Polyvinyl alcohol (PVA)l D_PVA Figure 2. PVA mold (left) and monolithic carbon device (right) [21]. Figure 3 presents the diffractograms obtained for the commercial activated charcoal and the monoliths made with different solvents. They all exhibit the characteristic features of amorphous carbon materials: broad signals at 25° and 44° 2θ attributed to the (002) and Figure 1. Examples of prepared PLA molds (left) and carbon monolithic devices (right). Catalysts 2023,13, 1039 4 of 17 As shown in Figure 1, the method of preparation allows the fabrication of pieces with different sizes, morphology, and number of channels. The final quality of the carbon devices can be influenced at every step of the process. Herein only the nature of the solvent used in the polymerization stage and the effect of the heating rate in the pyrolysis stage will be discussed. 2.1. Influence of the Nature of Solvent in the Polymerization Stage on the Carbon Structured Device Different solvents have been tested (water, 96% ethanol, polyethylene glycol (PEG), or PEG + 1% polyvinyl alcohol (PVA). The rest of the variables of the process and added carbonaceous source (activated charcoal Darco Sigma-Aldrich) are kept constant. The monoliths have been prepared, as explained in Section 3, using PVA molds. The final carbon monolith measurements are 1.5 cm in diameter, 1 cm high, and 28 entire channels, resulting in a device with 21.5 channels/cm 2 (Figure 2). The nomenclature of the obtained monoliths is resumed in Table 2. Catalysts 2023, 13, 1039 4 of 17 Figure 1. Examples of prepared PLA molds (left) and carbon monolithic devices (right). As shown in Figure 1, the method of preparation allows the fabrication of pieces with different sizes, morphology, and number of channels. The final quality of the carbon devices can be influenced at every step of the process. Herein only the nature of the solvent used in the polymerization stage and the effect of the heating rate in the pyrolysis stage will be discussed. 2.1. Influence of the Nature of Solvent in the Polymerization Stage on the Carbon Structured Device Different solvents have been tested (water, 96% ethanol, polyethylene glycol (PEG), or PEG + 1% polyvinyl alcohol (PVA). The rest of the variables of the process and added carbonaceous source (activated charcoal Darco Sigma-Aldrich) are kept constant. The monoliths have been prepared, as explained in Section 3, using PVA molds. The final carbon monolith measurements are 1.5 cm in diameter, 1 cm high, and 28 entire channels, resulting in a device with 21.5 channels/cm 2 (Figure 2). The nomenclature of the obtained monoliths is resumed in Table 2. Table 2. Nomenclature of the monoliths prepared with different solvents. Solvent Nomenclature Water D_H 2 O Ethanol D_EtOH Polyethylene glycol (PEG) D_PEG PEG + 1% Polyvinyl alcohol (PVA)l D_PVA Figure 2. PVA mold (left) and monolithic carbon device (right) [21]. Figure 3 presents the diffractograms obtained for the commercial activated charcoal and the monoliths made with different solvents. They all exhibit the characteristic features of amorphous carbon materials: broad signals at 25° and 44° 2θ attributed to the (002) and Figure 2. PVA mold (left) and monolithic carbon device (right) [21]. Table 2. Nomenclature of the monoliths prepared with different solvents. Solvent Nomenclature Water D_H2O Ethanol D_EtOH Polyethylene glycol (PEG) D_PEG PEG + 1% Polyvinyl alcohol (PVA)l D_PVA Figure 3presents the diffractograms obtained for the commercial activated charcoal and the monoliths made with different solvents. They all exhibit the characteristic features of amorphous carbon materials: broad signals at 25 ◦ and 44 ◦ 2 θ attributed to the (002) and (100) planes, respectively. The first one is due to aromatic ring ordering and the second one is related to the degree of aromatic ring condensation. The sharp peaks located at 21 ◦ , 23 ◦ , 27 ◦ , 36 ◦ , and 51 ◦ 2 θ correspond to inorganic phases (quartz and cristobalite SiO 2 ) present in the commercial activated carbon [23]. Figure 4shows the isotherms obtained for the commercial activated carbon and the four monoliths. It should be noted that, in order to obtain the adsorption-desorption isotherms, it was necessary to crush and sieve the material to a homogeneous particle size of less than 500 µ m. All the carbons present type IV isotherms according to the IUPAC classification, denoting the presence of mesopores [ 24 ]. However, Darco and D_H 2 O isotherms are slightly different and can be classified as a combination of type I and IV isotherms suggesting the coexistence of micro and mesopores. The hysteresis cycles for commercial activated charcoal (Darco), and D_H 2 O can be classified as type H4, typical of activated carbons with flexible slit-type pores. On the other hand, an important change is evidenced in the isotherms of D_PVA, D_EtOH, and D_PEG with respect to the hysteresis cycles, classified as type H2 and corresponding to pores formed by parallel lamellae. This Catalysts 2023,13, 1039 5 of 17 type of hysteresis is characteristic of carbonaceous materials with a high proportion of graphite in their structure [24]. Catalysts 2023, 13, 1039 5 of 17 (100) planes, respectively. The first one is due to aromatic ring ordering and the second one is related to the degree of aromatic ring condensation. The sharp peaks located at 21°, 23°, 27°, 36°, and 51° 2θ correspond to inorganic phases (quartz and cristobalite SiO2) present in the commercial activated carbon [23]. Figure 3. The X-ray diffraction patterns for the commercial activated charcoal Darco and the monoliths made with different solvents. Figure 4 shows the isotherms obtained for the commercial activated carbon and the four monoliths. It should be noted that, in order to obtain the adsorption-desorption isotherms, it was necessary to crush and sieve the material to a homogeneous particle size of less than 500 µm. All the carbons present type IV isotherms according to the IUPAC classification, denoting the presence of mesopores [24]. However, Darco and D_H2O isotherms are slightly different and can be classified as a combination of type I and IV isotherms suggesting the coexistence of micro and mesopores. The hysteresis cycles for commercial activated charcoal (Darco), and D_H2O can be classified as type H4, typical of activated carbons with flexible slit-type pores. On the other hand, an important change is evidenced in the isotherms of D_PVA, D_EtOH, and D_PEG with respect to the hysteresis cycles, classified as type H2 and corresponding to pores formed by parallel lamellae. This type of hysteresis is characteristic of carbonaceous materials with a high proportion of graphite in their structure [24]. 10 15 20 25 30 35 40 45 50 55 60 D_PVA D_PEG D_EtOH D_H 2 O Darco Intensity (a.u.) 2 θ º Figure 3. The X-ray diffraction patterns for the commercial activated charcoal Darco and the monoliths made with different solvents. Catalysts 2023, 13, 1039 6 of 17 0.0 0.2 0.4 0.6 0.8 1.0 0 100 200 300 400 500 600 0 50 100 150 200 0 50 100 150 0 40 80 120 0 100 200 300 400 500 P/P 0 cm 3 /g STP D_PVA D_PEG D_EtOH D_H 2 O Darco Figure 4. N2 adsorption and desorption isotherms at 77 K for the carbonaceous materials. A summary of the structural parameters obtained from X-ray diffraction and the textural properties of the monoliths prepared using different solvents and compared to the properties of the starting activated carbon is shown in Table 3. The value of the interlayer spacing (d002) of the materials remains constant. Only in D_EtOH is a significant change in the average crystallite height (along the c axis, Lc = 1.8 nm) observed in comparison to the values calculated for commercial activated carbon and the other monoliths. The average crystallite width (along the a axis, La) values show either reduction with respect to the commercial activated carbon for the D_EtOH monolith or an increment for the D_PVA monolith. Table 3. Textural properties and structural parameters obtained for commercial activated carbon monoliths varying the type of solvent. Sample SBET (m2/g) V mp (cm3/g) S mp (m2/g) S ext (m2/g) mp % APD DFT (nm) Vp (cm3/g) d002 (nm) Lc (nm) La (nm) AC Darco 892 0.215 470 422 52.7 0.8 0.73 0.37 1.2 4.4 D_H2O 375 0.138 297 78 79.2 0.7 0.21 0.37 1.2 3.1 D_EtOH 301 0.078 171 130 56.8 0.5 0.40 0.37 1.8 2.9 D_PEG 213 0.065 132 81 62.0 0.7 0.24 0.37 1.4 4.4 D_PVA 381 0.103 223 166 58.5 0.5 0.39 0.37 1.2 5.8 mp: micropore; ext: external; APDDFT: average pore diameter by density functional theory. In terms of specific surface area, all the monoliths show lower values than the starting Darco and a higher percentage of microporosity, 62 or 79% for the monoliths prepared with PEG and water, respectively. As indicated above, adding the starch as a binder increases the microporosity of the obtained carbon and is related to the polymeric structure formed by starch during the gelation step and remaining after the pyrolysis [22,25]. The Figure 4. N2adsorption and desorption isotherms at 77 K for the carbonaceous materials. A summary of the structural parameters obtained from X-ray diffraction and the textural properties of the monoliths prepared using different solvents and compared to the properties of the starting activated carbon is shown in Table 3. The value of the inter- Catalysts 2023,13, 1039 6 of 17 layer spacing (d 002 ) of the materials remains constant. Only in D_EtOH is a significant change in the average crystallite height (along the c axis, Lc = 1.8 nm) observed in comparison to the values calculated for commercial activated carbon and the other monoliths. The average crystallite width (along the a axis, La) values show either reduction with respect to the commercial activated carbon for the D_EtOH monolith or an increment for the D_PVA monolith. Table 3. Textural properties and structural parameters obtained for commercial activated carbon monoliths varying the type of solvent. Sample SBET (m2/g) V mp (cm3/g) S mp (m2/g) S ext (m2/g) mp % APDDFT (nm) Vp (cm3/g) d002 (nm) Lc (nm) La (nm) AC Darco 892 0.215 470 422 52.7 0.8 0.73 0.37 1.2 4.4 D_H2O 375 0.138 297 78 79.2 0.7 0.21 0.37 1.2 3.1 D_EtOH 301 0.078 171 130 56.8 0.5 0.40 0.37 1.8 2.9 D_PEG 213 0.065 132 81 62.0 0.7 0.24 0.37 1.4 4.4 D_PVA 381 0.103 223 166 58.5 0.5 0.39 0.37 1.2 5.8 mp: micropore; ext: external; APDDFT: average pore diameter by density functional theory. In terms of specific surface area, all the monoliths show lower values than the starting Darco and a higher percentage of microporosity, 62 or 79% for the monoliths prepared with PEG and water, respectively. As indicated above, adding the starch as a binder increases the microporosity of the obtained carbon and is related to the polymeric structure formed by starch during the gelation step and remaining after the pyrolysis [ 22 , 25 ]. The APD DFT values calculated with a non-local function show the presence of micropores ranging from 0.5 to 0.8 nm for all solids. From these results, it can be deduced that in all materials, both the microporous and mesoporous fractions contribute to the specific surface area (SBET). Thus, it is possible to obtain monoliths with completely defined pore volumes, surface areas, and crystallite sizes and different from the starting carbonaceous material only by using different solvents. The possibility of using different solvents provides the advantage of being able to work with carbonaceous materials with hydrophilic, hydrophobic, or amphoteric characteristics for the production of monoliths. 2.2. Influence of the Heating Rate in Pyrolysis Step on the Carbon Structured Device Different heating rates were studied by preparing pellet-type monoliths using water as a solvent and carbon xerogel (CXG) as a carbon source. The heating rate used ranges from 2 to 20 K/min, with all other variables constant. The full methodology for monolith preparation is given in Section 3. The obtained monoliths have been named according to the heating rate, as shown in Table 4. Table 4. Nomenclature of monoliths prepared with different carbonization heating rates. Heating Rate (K/min) Nomenclature 2 2 K/min 5 5 K/min 10 10 K/min 15 15 K/min 20 20 K/min Figure 5shows the diffractograms of the pellet-type monoliths obtained by varying the heating rate in the pyrolysis stage. All the diffractograms are typical of amorphous carbonaceous materials with characteristic peaks corresponding to the (002) and (100) planes. The total absence of any other type of signal is noted, thus indicating the production of all-organic integral monoliths with high carbon content [26]. Catalysts 2023,13, 1039 7 of 17 Catalysts 2023, 13, 1039 7 of 17 APDDFT values calculated with a non-local function show the presence of micropores ranging from 0.5 to 0.8 nm for all solids. From these results, it can be deduced that in all materials, both the microporous and mesoporous fractions contribute to the specific surface area (SBET). Thus, it is possible to obtain monoliths with completely defined pore volumes, surface areas, and crystallite sizes and different from the starting carbonaceous material only by using different solvents. The possibility of using different solvents provides the advantage of being able to work with carbonaceous materials with hydrophilic, hydrophobic, or amphoteric characteristics for the production of monoliths. 2.2. Influence of the Heating Rate in Pyrolysis Step on the Carbon Structured Device Different heating rates were studied by preparing pellet-type monoliths using water as a solvent and carbon xerogel (CXG) as a carbon source. The heating rate used ranges from 2 to 20 K/min, with all other variables constant. The full methodology for monolith preparation is given in Section 3. The obtained monoliths have been named according to the heating rate, as shown in Table 4. Table 4. Nomenclature of monoliths prepared with different carbonization heating rates. Heating Rate (K/min) Nomenclature 2 2 K/min 5 5 K/min 10 10 K/min 15 15 K/min 20 20 K/min Figure 5 shows the diffractograms of the pellet-type monoliths obtained by varying the heating rate in the pyrolysis stage. All the diffractograms are typical of amorphous carbonaceous materials with characteristic peaks corresponding to the (002) and (100) planes. The total absence of any other type of signal is noted, thus indicating the production of all-organic integral monoliths with high carbon content [26]. 10 15 20 25 30 35 40 45 50 55 60 20K/min 15K/min 10K/min 5K/min 2K/min Intensity (a.u.) 2θº Figure 5. The X-ray diffraction patterns for pellet-type monoliths obtained at different heating rates. Table 5 shows the structural parameters obtained for the monoliths compared to those of the xerogel used as a carbon source. The interlayer spacing (d002) oscillates around the same Figure 5. The X-ray diffraction patterns for pellet-type monoliths obtained at different heating rates. Table 5shows the structural parameters obtained for the monoliths compared to those of the xerogel used as a carbon source. The interlayer spacing (d 002 ) oscillates around the same value, 0.38 nm. The crystallite height (Lc) shows values between 1.3 and 1.5 nm, higher than those of the xerogel, and the crystallite size varies between 2.4 and 3.1 nm, with the xerogel showing an intermediate value. Although no clear relationship between the heating rate and these parameters is found, the largest crystallites are obtained using the highest heating rate. Table 5. Structural and textural parameters calculated for pellet-type monoliths obtained by varying the heating rate in the pyrolysis stage and for xerogel used as a carbonaceous source. Sample SBET (m2/g) V mp (cm3/g) S mp (m2/g) S ext (m2/g) mp % Vp (cm3/g) d002 (nm) Lc (nm) La (nm) CXG 596 0.12 271 325 45 0.59 0.4 1.1 2.8 2 K/min 524 0.16 257 266 49 0.59 0.38 1.3 2.5 5 K/min 560 0.15 430 130 77 1.35 0.38 1.5 2.7 10 K/min 590 0.13 289 301 49 1.14 0.40 1.3 2.7 15 K/min 558 0.12 300 258 54 0.66 0.39 1.5 2.4 20 K/min 432 0.12 309 123 72 0.67 0.38 1.5 3.1 mp: micropore; ext: external. Figure 6shows the isotherms of the monoliths obtained at different heating rates. According to IUPAC, all isotherms present a characteristic type IV and I shape, indicating mesoporous materials with high microporosity contribution confirmed by the adsorbed volumes at very low relative pressures. All hysteresis cycles have similar shapes and correspond to type H4 hysteresis, associated with flexible slit-like pores [24]. A summary of the textural properties of the monoliths obtained at different heating rates during the pyrolysis stage is given in Table 5compared to those of the xerogel used as a carbonaceous source. The BET surface area values vary between 432 and 590 m 2 /g. The micropore volume presents values between 0.12 and 0.16 cm 3 /g, and it is observed how this volume decreases with increasing the heating rate. The micropore area percentages of the monolith remain between 50 and 70%, being the manufactured monolith with a heating rate of 5 K/min being the one with the highest microporosity percentage. The external area, mainly due to the existence of mesopores and macropores in the monoliths, varies between 130 and 301 m 2 /g, and the pore volume between 0.59 and 1.35 cm 3 /g. The Catalysts 2023,13, 1039 8 of 17 material obtained at 10 K/min is the one that presents the greatest similarity in textural properties to the starting carbon material, differing from the latter only in pore volume, which is bigger for the monolith. It must be noted that both are obtained with the same heating rate. If the xerogel is compared with the other four monoliths, more significant differences are found. It should also be emphasized that these conclusions are valid when water is used as a solvent. Similar studies would be necessary when other solvents are used to see if the trend is the same. Catalysts 2023, 13, 1039 9 of 17 0.0 0.2 0.4 0.6 0.8 1.0 0 85 170 255 340 0 55 110 165 220 0 110 220 330 440 0 83 166 249 0 45 90 135 180 10K/min 5K/min 2K/min P/P 0 20K/min 15K/min cm 3 /g STP Figure 6. N2 adsorption and desorption isotherms at 77 K for the carbonaceous monoliths obtained at different carbonization rates. Finally, we should emphasize that this method of preparation produces materials with well-defined surface characteristics such as micro and mesoporosity, very attractive characteristics for different catalytic processes that prefer inhomogeneous small/large pore distributions in the same support. 2.3. Catalytic Study Formic acid dehydrogenation reaction has been chosen to test a representative sample of the structured carbon devices. Formic acid has proven to be a good liquid organic hydrogen carrier (LOHC), presenting a safer option for energy storage due to its high volumetric hydrogen capacity (53 g H2/L), low toxicity, and flammability under ambient conditions [27,28]. Moreover, being liquid at room temperature, its handling is comparable to that of diesel and petrol, making it easy to transport and refuel [29]. Another relevant aspect of using formic acid as a hydrogen storage material is that the liberated CO2 after dehydrogenation can be hydrogenated subsequently again to formic acid resulting in a carbon-free process for hydrogen production [30,31]. Formic acid decomposition (FAD) implies two thermodynamically stable reactions: the dehydrogenation reaction (Equation (1)) and the dehydration reaction (Equation (2)). Figure 6. N 2 adsorption and desorption isotherms at 77 K for the carbonaceous monoliths obtained at different carbonization rates. When comparing the monoliths prepared under identical conditions but with diverse carbonaceous sources (Darco vs. CXG: D_H 2 O vs. 10 K/min), the differences in textural properties are evident. When the added carbon is xerogel (obtained by a procedure similar to that used for the manufacture of the monolith), the properties of the final monolith do not differ significantly from those of the starting carbon. The contrary is observed for the monoliths prepared with Darco as a carbon source. The latter suggests that the properties of the monolith are greatly influenced by the added carbonaceous source in the first place and by the organic solution and pyrolysis afterward. Finally, we should emphasize that this method of preparation produces materials with well-defined surface characteristics such as micro and mesoporosity, very attractive Catalysts 2023,13, 1039 9 of 17 characteristics for different catalytic processes that prefer inhomogeneous small/large pore distributions in the same support. 2.3. Catalytic Study Formic acid dehydrogenation reaction has been chosen to test a representative sample of the structured carbon devices. Formic acid has proven to be a good liquid organic hydrogen carrier (LOHC), presenting a safer option for energy storage due to its high volumetric hydrogen capacity (53 g H 2 /L), low toxicity, and flammability under ambient conditions [ 27 , 28 ]. Moreover, being liquid at room temperature, its handling is comparable to that of diesel and petrol, making it easy to transport and refuel [ 29 ]. Another relevant aspect of using formic acid as a hydrogen storage material is that the liberated CO 2 after dehydrogenation can be hydrogenated subsequently again to formic acid resulting in a carbon-free process for hydrogen production [30,31]. Formic acid decomposition (FAD) implies two thermodynamically stable reactions: the dehydrogenation reaction (Equation (1)) and the dehydration reaction (Equation (2)). HCOOH →H2+CO2(1) HCOOH →H2O+CO (2) Considering the extremely low CO tolerance of the fuel cell (FC), a complete selectivity to formic acid dehydrogenation is desired for all FAD systems [ 32 ]. For the catalytic study, palladium has been selected as the active phase, a metal proven to be very effective in this reaction, with better performance than other metals [32,33]. Thus, low charge 0.2%Pd/C monoliths with 9 channels, a length of 29 mm, and a diameter of 17 mm have been prepared (Figure 7), as described in Section 3.3. The manipulation and activation in the HNO 3 acid and palladium deposition process do not alter the physical integrity of the monolith. Catalysts 2023, 13, 1039 10 of 17 HCOOH → H+ CO (1) HCOOH → HO+ CO (2) Considering the extremely low CO tolerance of the fuel cell (FC), a complete selectivity to formic acid dehydrogenation is desired for all FAD systems [32]. For the catalytic study, palladium has been selected as the active phase, a metal proven to be very effective in this reaction, with better performance than other metals [32,33]. Thus, low charge 0.2%Pd/C monoliths with 9 channels, a length of 29 mm, and a diameter of 17 mm have been prepared (Figure 7), as described in Section 3.3. The manipulation and activation in the HNO 3 acid and palladium deposition process do not alter the physical integrity of the monolith. Figure 7. Monolith used in the catalytic test. SEM images and EDX analysis (Figure 8) allow us to corroborate the presence of Pd nanoparticles on the monolith carbon surface with a very homogeneous distribution. Figure 8. SEM micrograph and EDX analysis of the 0.2%Pd/C monolith. In order to evaluate not only the catalytic activity but also the stability and resistance of our structured system, we have carried out a long-time experience (30 h in a continuous flow reaction). Figure 9 presents gas-phase FAD activity in terms of H 2 , CO 2 , CO, and CH 4 flows (mol/min) vs. time at a fixed temperature of 423 K. Because of the high exothermicity of the formic acid dehydrogenation reaction and the low thermal conductivity of the carbon substrate; we decided to test the FAD in a long-term study at a temperature with a Figure 7. Monolith used in the catalytic test. SEM images and EDX analysis (Figure 8) allow us to corroborate the presence of Pd nanoparticles on the monolith carbon surface with a very homogeneous distribution. In order to evaluate not only the catalytic activity but also the stability and resistance of our structured system, we have carried out a long-time experience (30 h in a continuous flow reaction). Figure 9presents gas-phase FAD activity in terms of H 2 , CO 2 , CO, and CH 4 flows (mol/min) vs. time at a fixed temperature of 423 K. Because of the high exothermicity of the formic acid dehydrogenation reaction and the low thermal conductivity of the carbon substrate; we decided to test the FAD in a long-term study at a temperature with a high (but not total) formic acid conversion, in order to assure the stabilization of the reaction and to observe catalyst deactivation if it occurs. A previously reported blank experiment with SiC shows that formic acid thermal decomposition starts at 548 K, reaching complete conversion Catalysts 2023,13, 1039 16 of 17 5. Kapteijn, F.; Moulijn, J.A. Structured catalysts and reactors—Perspectives for demanding applications. Catal. Today 2022 , 383, 5–14 . [CrossRef] 6. Rodríguez-Reinoso, F. The role of carbon materials in heterogeneous catalysis. Carbon 1998,36, 159–175. [CrossRef] 7. Xue, D.; Yu, F.; Ying, Q.; Wu, Y.; Lee, W.-J.; Kwon, S.-H.; Yang, Y. Phosphate-assisted dispersion of iron phosphide in carbon nanosheets towards efficient and durable ORR catalysts in acidic and alkaline media. ChemCatChem 2021 ,13, 4431–4441. [CrossRef] 8. Vergunst, T.; Linders, M.J.G.; Kapteijn, F.; Moulijn, J.A. Carbon-based monolithic structures. Catal. Rev.-Sci. Eng. 2001 ,43, 291–314. [CrossRef] 9. Chang, X.; Chen, D.; Jiao, X. Starch-derived carbon aerogels with high-performance for sorption of cationic dyes. Polymer 2010 , 51, 3801–3807. [CrossRef] 10. Wu, K.; Cao, J.; Qian, Z.; Luo, Y.; Niu, B.; Zhang, Y.; Long, D. Monolithic carbon aerogels within foam framework for high temperature thermal insulation and organics absorption. J. Colloid Interface Sci. 2022,618, 259–269. [CrossRef] 11. Medina, O.E.; Galeano-Caro, D.; Ocampo-Pérez, R.; Pérez-Cadenas, A.F.; Carrasco-Marín, F.; Franco, C.A.; Cortés, F.B. Development of a monolithic carbon xerogel-metal composite for crude oil removal from oil in-saltwater emulsions: Evaluation of reuse cycles. Microporous Mesoporous Mater. 2021,327, 111424. [CrossRef] 12. Zhao, S.; Xiao, N.; Li, H.; Guo, Z.; Bai, J.; Xiao, J.; Guo, H.; Ma, X.; Qiu, J. A nickel-nitrogen-doped carbon foam as monolithic electrode for highly efficient CO2electroreduction. J. CO2 Util. 2021,49, 101549. [CrossRef] 13. Tian, B.; Dong, K.; Guo, F.; Mao, S.; Bai, J.; Shu, R.; Qian, L.; Liu, Q. Catalytic conversion of toluene as a biomass tar model compound using monolithic biochar-based catalysts decorated with carbon nanotubes and graphic carbon covered Co-Ni alloy nanoparticles. Fuel 2022,324, 124585. [CrossRef] 14. Liu, W.; Feng, J.; Yin, R.; Ni, Y.; Zheng, D.; Que, W.; Niu, X.; Dai, X.; Shi, W.; Wu, F.; et al. Tailoring oxygenated groups of monolithic cobalt-nitrogen-carbon frameworks for highly efficient hydrogen peroxide production in acidic media. Chem. Eng. J. 2022,430, 132990. [CrossRef] 15. Lin, Y.; Zhu, W.; Li, Y. Hierarchical monolithic carbon with high transfer performance for hydrogen evolution reaction. J. Energy Chem. 2022,73, 41–48. [CrossRef] 16. Maldonado-Hódar, F.J. Advances in the development of nanostructured catalysts based on carbon gels. Catal. Today 2013 , 218–219, 43–50. [CrossRef] 17. Al-Muhtaseb, S.A.; Ritter, J.A. Preparation and properties of resorcinol–formaldehyde organic and carbon gels. Adv. Mater. 2003 , 15, 101–114. [CrossRef] 18. Zubizarreta, L.; Arenillas, A.; Pirard, J.P.; Pis, J.J.; Job, N. Tailoring the textural properties of activated carbon xerogels by chemical activation with KOH. Microporous Mesoporous Mater. 2008,115, 480–490. [CrossRef] 19. Gadkaree, K.P. Carbon honeycomb structures for adsorption applications. Carbon 1998,36, 981–989. [CrossRef] 20. Tennison, S.R. Phenolic-resin-derived activated carbons. Appl. Catal. A Gen. 1998,173, 289–311. [CrossRef] 21. Centeno, M.A.; Santos, J.L.; Odriozola, J.A.; Rodríguez, N.; Agámez, Y.Y.; Díaz, J.J. Procedimiento de Obtención de Monolitos Integrales de Carbono y Monolitos de Carbono Obtenidos (ES P201931067); CSIC: Madrid, Spain; Universidad de Sevilla: Sevilla, Spain; Universidad Nacional de Colombia: Bogotá, Colombia, 2021. 22. Rodríguez, N.; Agámez-Pertuz, Y.Y.; Romero, E.; Díaz-Velásquez, J.J.; Odriozola, J.A.; Centeno, M.A. Effect of starch as binder in carbon aerogel and carbon xerogel preparation. J. Non-Cryst. Solids 2019,522, 119554. [CrossRef] 23. Santos, J.L.; Megías-Sayago, C.; Ivanova, S.; Centeno, M.A.; Odriozola, J.A. Structure-sensitivity of formic acid dehydrogenation reaction over additive-free Pd NPs supported on activated carbon. J. Chem. Eng. 2020,420, 127641. [CrossRef] 24. Donohue, M.D.; Aranovich, G.L. Classification of Gibbs adsorption isotherms. Adv. Colloid Interface Sci. 1998 ,76–77, 137–152. [CrossRef] 25. Bakierska, M.; Molenda, M.; Majda, D.; Dziembaj, R. Functional starch based carbon aerogels for energy applications. Procedia Eng. 2014,98, 14–19. [CrossRef] 26. Takagi, H.; Maruyama, K.; Yoshizawa, N.; Yamada, Y.; Sato, Y. XRD analysis of carbon stacking structure in coal during heat treatment. Fuel 2004,83, 2427–2433. [CrossRef] 27. Eppinger, J.; Huang, K.-W. Formic acid as a hydrogen energy carrier. ACS Energy Lett. 2017,2, 188–195. [CrossRef] 28. Navlani-García, M.; Mori, K.; Kuwahara, Y.; Yamashita, H. Recent strategies targeting efficient hydrogen production from chemical hydrogen storage materials over carbon-supported catalysts. NPG Asia Mater. 2018,10, 277–292. [CrossRef] 29. Enthaler, S.; Von Langermann, J.; Schmidt, T. Carbon dioxide and formic acid—The couple for environmental-friendly hydrogen storage? Energy Environ. Sci. 2010,3, 1207–1217. [CrossRef] 30. Fellay, C.; Dyson, P.J.; Laurenczy, G. A viable hydrogen-storage system based on selective formic acid decomposition with a ruthenium catalyst. Angew. Chem. Int. Ed. 2008,47, 3966–3968. [CrossRef] 31. Navlani-García, M.; Mori, K.; Salinas-Torres, D.; Kuwahara, Y.; Yamashita, H. New approaches toward the hydrogen production from formic acid dehydrogenation over Pd-based heterogeneous catalysts. Front. Mater. 2019,6, 44. [CrossRef] 32. Ruiz-López, E.; Ribota Peláez, M.; Blasco Ruz, M.; Domínguez Leal, M.I.; Martínez Tejada, M.; Ivanova, S.; Centeno, M.A. Formic acid dehydrogenation over Ru- and Pd-based catalysts: Gasvs. liquid-phase reactions. Materials 2023 ,16, 472. [CrossRef] [PubMed] Catalysts 2023,13, 1039 17 of 17 33. Dong, Z.; Mukhtar, A.; Lin, H. Heterogeneous catalysis on liquid organic hydrogen carriers. Top. Catal. 2021 ,64, 481–508. [CrossRef] 34. Solymosi, F.; Koós, Á.; Liliom, N.; Ugrai, I. Production of CO-free H 2 from formic acid. A comparative study of the catalytic behavior of Pt metals on a carbon support. J. Catal. 2011,279, 213–219. [CrossRef] 35. Job, N.; Sabatier, F.; Pirard, J.-P.; Crine, M.; Léonard, A. Towards the production of carbon xerogel monoliths by optimizing convective drying conditions. Carbon 2006,44, 2534–2542. [CrossRef] 36. Okolo, G.N.; Neomagus, H.W.J.P.; Everson, R.C.; Roberts, M.J.; Bunt, J.R.; Sakurovs, R.; Mathews, J.P. Chemical–structural properties of South African bituminous coals: Insights from wide angle XRD–carbon fraction analysis, ATR–FTIR, solid state 13C NMR, and HRTEM techniques. Fuel 2015,158, 779–792. [CrossRef] 37. Collins, J.; Zheng, D.; Ngo, T.; Qu, D.; Foster, M. Partial graphitization of activated carbon by surface acidification. Carbon 2014 , 79, 500–517. [CrossRef] Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). 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