Pt-impregnated catalysts on powdery SiC and other commercial supports for the combustion of hydrogen under oxidant conditions

Pt-IMPREGNATED CATALYSTS ON POWDERY SiC AND OTHER COMMERCIAL SUPPORTS FOR THE COMBUSTION OF HYDROGEN UNDER OXIDANT CONDITIONS G.M. Arzac*1, O. Montes 1, A. Fernández*1 1Instituto de Ciencia de Materiales de Sevilla (CSIC-Univ. Sevilla), Avda. Américo Vespucio 49, 41092-Sevilla, Spain. *Corresponding authors: [email protected] (G. Arzac); [email protected] (A. Fernández) Abstract We report the study of the catalytic hydrogen combustion over Ptimpregnated powdery silicon carbide (SiC) using H2PtCl6 as precursor. The reaction was conducted in excess of oxygen. -SiC was selected for the study because of its thermal conductivity, mechanical properties, chemical inertness and surface area. The obtained Pt particles over SiC were medium size (average particle diameter of 5 nm for 0.5wt% Pt). The activity of the Pt-impregnated catalyst over SiC was compared to those obtained in oxidized form over TiO2 and Al2O3 commercial supports (Pt particles very small in size, average particle diameter of 1 nm for 0.5wt% Pt in both cases). The case of a SiO2 support was also discussed. Those Pt/SiC particles were the most active because of their higher contribution of surface Pt0, indicating that partially oxidized surfaces have better activity than those totally oxidized in these conditions. SiC was modified with an acid treatment and thus bigger (average particle diameter of 7 nm for 0.5wt% Pt) and more active Pt particles were obtained. Durability of the SiC and TiO2 supported catalysts was tested upon 5 cycles and both have shown to be durable and even more active than initially. Exposure to the oxidative reaction mixture activates the *Revised Manuscript Click here to view linked References catalysts and the effect is more pronounced for the completely oxidized particles. This is due to the surface oxygen chemisorption which activates catalysts´ surface. Keywords Hydrogen, catalytic combustion, Pt-impregnated catalysts, powdery SiC Introduction It is now generally accepted that in a near future, the world should move towards a greener energetic paradigm. Fossil fuels´ scarcity and the environmental issues related to their extensive use make necessary to think on another energy sources and carriers. In this context, hydrogen appears as a clean energy vector with an attractive energy density (142MJ.kg-1 while for liquid hydrocarbons is 47 MJ.kg-1). [1-3] Once obtained through green resources, hydrogen can produce energy upon generating water as only byproduct. For the implementation of the use of hydrogen, challenges related to production, transportation, storage and combustion should be met first. The catalytic hydrogen combustion (CHC, reaction (1)) is a key reaction in the “hydrogen economy” which can be employed as a means of heat production (cookers, heaters etc) as well as for safety purposes (elimination of undesired hydrogen). [4-17] For heat production applications, high amounts of hydrogen rich mixtures are preferred. Regarding safety purposes, the amount of undesired hydrogen to be eliminated can range from large amounts in the nuclear industry (in case of an accident) to the byproducts of chemical and petrochemical industries or during the operation of various electrochemical processes. Small amounts of hydrogen can be released in the exhaust of fuel cells and its accumulation may constitute a hazard. In this field, the study of CHC in lean hydrogen/air mixtures is especially relevant. For this type of application it is desirable to use catalysts which are active enough to start the reaction even at room temperature. In this sense Pt and Pd oxides are the most appropriate according to previous report. [4] H2 + ½ O2 → H2O -286 kJ.mol-1 (1) Reaction (1) is also interesting from a fundamental point of view. It may contribute to develop fundamental understanding of catalytic oxidation reactions and permits to test metal catalysts in both reducing and oxidizing atmospheres (excess of hydrogen or oxygen respectively). For practical applications, catalyst should be prepared in supported form, which improves dispersion, prevents aggregation and facilitates its use in successive cycles. Most papers report the use of metal oxides such as SiO2, TiO2, CeO2, SnO2, ZrO2, but only recent works report the use of SiC. [7,14,17,18] Silicon carbide (especially cubic phase, -SiC) is a highly advantageous support because of its relatively high specific surface area, chemical inertness, thermal conductivity and mechanical properties. [19- 22] The use of SiC as catalyst support is a growing field of research and was reported for reactions like n-butane partial oxidation, H2S selective oxidation and methanol dehydration. [23-25] SiC was also recently studied as support for the deposition of Co nanoparticles in the study of the Fischer Tropsh (FTS) reaction. [26-27] The problem with SiC is its inert and hydrophobic surface which suffers from scarcity of reactive sites for the adsorption of the metal ions during catalyst preparation. To circumvent this, several strategies such as thermal or acid treatments can be employed. [19, 28] In this paper, we have studied the CHC under excess of oxygen (lean, 1% v/v H2/ air mixture) on Pt-impregnated catalysts over powdery commercial SiC and compared with those traditionally used supports. We have prepared the Pt catalysts in their oxidized form over SiC, TiO2 , SiO2 and Al2O3 powders and their activity was compared. We have also intended to modify the SiC support with an acid treatment in order to make its surface more reactive to metal ion adsorption for a better dispersion of the active phase. Some catalytic tests with pre-reduced catalysts are also included. Trends in catalytic activity and durability are discussed herein in connection to the nature of the support, surface, particle size and dispersion. Practical conclusions are also obtained regarding the use of SiC as catalyst support for this important but scarcely studied reaction. 2. Experimental Section 2.1. The supports Commercial TiO2 (nanopowder, 21 nm particle size, Sigma Aldrich), Al2O3 (neutral, 50-200 m particle size, Sigma Aldrich) and SiC (nanopowder, less than 100 nm particle size, Sigma Aldrich) were used as received. The supports were first characterized regarding surface area and zeta potential and results are supplied as supporting information (Fig.1S). The SiO2 support was prepared by calcining the commercial SiC at 1000 °C 7 h under static air (4.2 °C/min ramp). [28] Optimization of calcination conditions and characterization of the prepared SiO2 can be also found in the supporting information. Except for the alumina powder, the rest of the supports have comparable surface area according to BET measurements (see Fig.1S). SiC commercial powder was also further modified to improve its surface composition. [19]. It was treated in acidic medium in order to introduce hydrophilic surface Si-O groups and thus make its surface more reactive to metal ion adsorption for a better dispersion of the active phase. Commercial SiC (300mg) was treated with and acidic solution of H2SO4 (10 ml) and HNO3 (10 ml) and 1ml of MilliQ® water. [19] The mixture was put in an ultrasonic bath for 15 min and then heated under reflux for 1.5h. The product was centrifuged (1000 rpm, 10 min) and washed with 20 ml of water five times. The solid was dried at 120 °C overnight. 2.2. Catalysts´ preparation Catalysts were prepared by pore volume impregnation of the support with aqueous H2PtCl6.6H2O (Sigma Aldrich). When using SiC as support the aqueous H2PtCl6.6H2O solution contained 10% v/v ethanol in order to increase support wettability. The Pt loading was calculated for a final amount of 1 and 0.5 wt% as will be indicated along the text (sample will be named indicating Pt load and support, see Table 1). The pH of the impregnating solution was around 0. The H2PtCl6 concentration in the impregnation solutions depends on the desired loading, the nature of the support and its pore volume, and in this work it is in the range of 0.026-0.06M. After impregnation, catalysts were dried at 120°C for 12 h and then calcined in static air at 400 °C during 4h (3 °C/min ramp). These conditions favor the formation of platinum oxides on catalysts´ surface and the degree of oxidation will depend on the particle size. 2.3. Catalysts´characterization For TEM (Transmission Electron Microscopy) studies, samples were dispersed in ethanol by ultrasound and dropped on a copper grid. Measurements were performed on a FEI Tecnai G2 F30 FEG (field emission gun) microscope, equipped with a HAADF (High Angle Annular Dark Field) detector from Fischione Instruments, and operated in STEM (scanning TEM) mode at 300 kV. The Pt particle size distributions were evaluated from the TEM micrographs. The images were processed with an image processing software (Adobe Photoshop) to identify the particles as well-defined dark contrast areas which were then analyzed with the ImageJ software. X-ray diffraction measurements were performed using the Cu Kα radiation in a Siemens D5000 diffractometer in a Bragg-Brentano configuration in the 2θ angle range of 10-90 degrees. XPS spectra were recorded with a Leybold Heraus LH electron spectrometer using Al Ka radiation with 40 eV pass energy at normal emission take off angle. The alumina and titania supported materials´ spectra were calibrated with the signal of C 1s at 284.6 eV (contamination). Those SiC and SiO2 supported materials´ spectra where calibrated by setting the signal corresponding to Si-C or Si(IV)-O in the Si 2p level at 100.6eV or 103.5 eV, depending on the sample, in accordance to literature. [29] For comparison, the spectra were normalized to the same area. For some samples, deconvolution of the Pt 4f spectra was carried out to evaluate the contributions of the individual oxidation states. Peaks were adjusted using CASA XPS software ® under the following constraints: peak positions, half width at full maxima (FWHM), distance (3.3eV) and relative area ratios (4/3) of the 4f7/2-4f5/2 doublet, according to literature. [16, 30] The Pt 4 f7/2 peak positions were set as follows: metallic Pt: 70-71 eV, PtII: 72- 72.3eV and PtIV: 73.8-74 eV and the FWHM were set with ±0.2eV at Pt: 1.2 eV, PtII: 1.7 eV and PtIV: 1.9 eV. [16, 30-31] The area of the individual peaks signified the amount of Pt in different oxidation states. Monte Carlo error analysis was performed for error estimates also using CASA XPS software ®. 2.4. Catalytic tests Catalytic tests were performed in a fixed bed quartz reactor (20 mm diameter, 500 mm length). The activity of the empty reactor, the quartz wool and the bare supports was first tested and has shown around 30% conversion at 450 °C and 15% at 150°C. Prior to each experiment, the reactor was loaded with 10 mg of supported catalyst plus 90 mg of bare support (both catalyst and support previously mortar mixed) over quartz wool, to ensure covering the whole reactor section. The catalyst amount was first optimized to achieve a sigmoid conversion vs temperature curve. These conditions are essential to evaluate catalytic properties (activation energy, T50, etc). Unless indicated the contrary, catalysts were tested with no previous conditioning treatment. The reactor was heated from outside using an electric furnace and temperature was controlled using a PID controller. Temperature was measured by a K-type thermocouple placed at the center of the packed bed. Reactions were carried out with a feed mixture consisting of 200ml.min-1 of 1% v/v mixture of H2 in air. Such lean hydrogen mixture was selected for catalysts´ comparison purposes as reported before. [4] Working gases (H2 99.999% and air) were measured through Bronkhorst ® mass flow controllers and premixed in a chamber before feeding the reactor. High gas flow rate and low catalyst weight limits H2 conversion and thus one can obtain the kinetics of the process controlled by the reaction at nearly isothermal conditions. A HP 5890 chromatograph was employed to measure H2 amount, with a ShinCarbon ® packed column and a thermal conductivity detector. N2 was used as the carrier gas. Catalytic tests were first done at RT and then the temperature was increased to 420 °C or 200 °C depending on the experiment. After the 420 degrees were reached, the temperature was maintained during 1h and then conversion vs temperature curve was measured during cooling down to RT. For those experiments heated to 200 °C, the conversion vs temperature curve was measured immediately during cooling down to RT Some catalytic experiments were conducted with previous reduction of the catalyst (in situ, 100 ml.min-1N2, 50 ml.min-1 H2, 400°C 4h). After reduction, the reactor was cooled down under nitrogen to 250 °C and the catalysts were exposed to the 1% H2/air mixture for some activation. The conversion vs temperature curve was measured from 200 °C to RT. 3. Results and Discussion 3.1. Catalysts and supports Figure 1.a shows XRD measurements performed on the 0.5 wt. % Pt catalysts over SiC, TiO2 , SiO2 and Al2O3. For the 0.5Pt/SiC catalyst, broad diffraction peaks corresponding to the Pt metallic phase (ICDD 00-004-0802) were found. For the TiO2, SiO2 and Al2O3 supported catalysts, no diffraction peaks corresponding to metallic platinum or platinum oxide were found, indicating the presence of very small nanoparticles. Figure 2 shows the representative STEM/HAADF images of the 0.5 wt. % Pt catalysts over SiC, TiO2 , SiO2 and Al2O3. Their corresponding histograms are also shown. The SiC supported catalyst shows medium size particles (5±4) nm while those TiO2 , SiO2 and Al2O3 supported catalysts exhibit very small particles (around 1-2 nm, depending of the sample as shown in Table1), in accordance to XRD measurements. The case of the 0.5Pt/SiO2 sample is peculiar, because not only very small and disperse nanoparticles were obtained but also many big aggregates were found. The obtention of small and monodisperse particles for the TiO2 and Al2O3 supports can be understood in terms of the interaction of the positively charged surface of the supports (Fig. 1S, supplied as supporting info.) at the impregnation pH with the [PtCl6]2-, [PtCl5(H2O)]- species. These species where reported before as the most abundant in our impregnating conditions (pH and H2PtCl6 concentration). [32] The medium size Pt particles for the SiC support and its broad size range can be understood by taking account the presence of Si-C bonds on surface. The surface charge of the SiC is positive and relatively high at the impregnation pH as shown by zeta potential measurements, (Fig. 1S supplied as supporting info.) probably due to the presence of surface acidic Si-O-C and Si-O-Si groups. However, XPS measurements on the Si 2p level (Fig. 2S, supplied as supporting info.) show a high contribution of Si-C which explains that interactions of metal-complex anions with the support are not well favored. In addition, the obtention of big aggregates for the SiO2 supported sample is related to its low zeta potential at the impregnating pH (Fig. 1S). With the idea of increasing the dispersion of the active phase on the SiC support, chemical treatment was employed in order to introduce hydrophilic surface Si-O groups. The SiC support was treated with a mixture of H2SO4 and HNO3 during 1.5h at 60°C (SiC sulfo 1.5h). Figure 3S (supplied as supporting information) shows the XRD difractogram of the as received SiC in comparison to the acid treated sample. The SiC shows diffraction peaks at 35.6, 41.3, 60, and 71.7 degrees (2) which correspond to the -SiC (ICDD 00-029-1129). The acid treated SiC (SiC sulfo 1.5h) does not show any contribution of SiO2 in the XRD diffractogram and shows only a slight increase in the intensity of the Si-O-Si IR signal respect to the SiC (Fig 3S). This shows that the acidic treatment has not been very successful in introducing many surface Si-O groups. Nevertheless acid treated SiC supports were impregnated with 0.5 wt.% Pt and compared with the 1 and 0.5 wt% Pt catalysts supported on powdery SiC as received. XRD measurements are shown in Fig.1.b For the catalysts supported on as received SiC and acidic treated SiC, the broad peak at 39.76 degrees (2) assigned to (111) plane of Pt is clearly observed although does not permit to calculate crystal size accurately. STEM/HAADF images are shown in Fig 3 for the 0.5 wt% Pt impregnated catalysts on the as received SiC in comparison to the acid modified SiC powder. Their corresponding histograms are shown. Particle size for the as received SiC supported 3.2.5. Comparison with literature The comparison of catalysts with literature is difficult since it is hard to find papers in which materials were tested in exactly the same conditions as ours. Activity for CHC depends on catalysts´ history, H2 and O2 concentration, total gas flowrate, amount of catalyst, nature and state of the metal (metal substituted more active than impregnated), type of support (reducible more active than inert), metal loading, etc. [15-16, 34-36] Haruta and Sano screened metal oxides (300mg) for CHC in air (1%H2) and obtained a Volcano plot in which bulk PtO2 has similar T50 to our SiC supported catalyst. [4] From that plot it can also be derived that our TiO2 supported catalyst has a T50 similar to bulk Ag2O. [4] Our SiC sulfo supported catalyst has similar activity to bulk PdO. [4] Madras et al tested Pt and Pd substituted ceria based catalysts for CHC in air (3%H2, 10mg catalyst) and found that Ce0.98Pd0.02O2-Pd-substituted) and Pd/CeO2 (Pd impregnated) had similar activity at 50°C to our 0.5Pt/SiC sulfo in oxidized form and our 0.5Pt/TiO2 with previous in-situ reduction (around 200mlH2.min-1gcatalyst-1all). [15] However Ce0.83Ti0.15Pd0.02O2- and Ce0.83Ti0.15Pt0.02O2-catalysts showed higher activity at 50°C than any of the catalysts presented herein (around 550 mlH2.min-1gcatalyst-1). [15] In our previous work, we tested a commercial catalyst (0.27wt % Pt/SiC-washcoat) which consisted of 2-20nm Pt particles dispersed on alumina washcoat on monolithic SiC (mashed into granules) in similar conditions to herein. [18] We found a T50 around 34°C which is comparable to that obtained for our 0.5Pt/SiC (with higher Pt loading). However, activities show that the 0.27wt % Pt/SiC-washcoat (38 mlH2.min-1gcatalyst-1) is less efficient than our 0.5Pt/SiC catalyst at 50°C. 3.2.6. Durability upon cycling Durability of the 0.5 wt % Pt catalysts over TiO2 and SiC was tested upon cycling (5 cycles) and results are shown in Figure 8. For both materials, the activity has shown an increase upon 5 cycles. However, the 0.5Pt/TiO2 sample has shown a 20°C decrease in T50, while the 0.5Pt/SiC has shown a 10°C decrease, indicating different sensitivity to the cycles. For a comprehension of this effect, both used samples were characterized by TEM and XPS. As described in the experimental section, for catalytic tests, samples are diluted 1/10 with bare support. This makes difficult the detection of Pt levels through XPS and also it is hard to find small particles by TEM for the used catalysts. Despite these difficulties, some data were obtained and results are shown in Fig. 9. Regarding the 0.5Pt/SiC sample, neither Pt 4f nor Pt 5d levels were detected by XPS. As we mentioned before due to lower dispersion the Pt signals were already difficult to detect before dilution. A series of STEM/HAADF images were studies and a representative one is depicted in Figure 9.a for the 0.5Pt/SiC sample together with its corresponding particle size distribution plot. The comparison of the particle size of the Pt particles after 5 cycles (5±3) nm with the fresh catalyst (5±4) nm shows no evidence of particle aggregation. Also, no evidence of aggregation or growth was found for the 0.5Pt/TiO2 sample used in 5 cycles (Fig 9.b) respect to the initial fresh sample. In our conditions, particle aggregation seems not to be favored and does not explain the activation of both catalysts upon cycling. XPS measurements on the Pt 4f level of the 0.5Pt/TiO2 sample used 5 cycles in comparison to the fresh sample shows an increase in the contribution of PtIV (Fig 9.c). This result is connected with the results presented in section 3.2.2., regarding catalyst´s history. Exposure of catalysts to the reaction mixture produces an active oxide layer which activates catalysts. For the 0.5Pt/TiO2 sample, the enhancement is higher because the reaction occurs by weak H2 adsorption over strongly adsorbed oxygen, while for the 0.5Pt/SiC (partially oxidized) reaction occurs by a mixture of this mechanism and dissociative adsorption of H2 and oxygen, as explained in section 3.2.3. From a practical point of view, this finding is very interesting, since catalysts do not suffer from deactivation upon use. In practical applications in which the amounts of lean hydrogen mixtures are high, the heat generated by the reaction would increase the efficiency of the catalysts during operation. 4. Conclusions In this work, Pt particles were prepared on commercial SiC (-SiC) powder by pore volume impregnation with CPA and tested as catalysts for CHC under excess of oxygen. The activity was compared with those particles obtained impregnating CPA on traditional powdery supports such as titania, silica and alumina. Thinking of a practical application, catalysts were tested in their oxidized (or partially oxidized) form. The activity of the medium size Pt particles obtained on SiC was higher than those smaller obtained in other supports and increased upon exposure to the reaction mixture at high temperatures. No significant change in the activity of the SiC supported particles was found with previous in-situ pre-reduction of the catalyst. All these results point out that for practical applications in which lean hydrogen/air mixtures should be eliminated, the best activity is achieved with partially oxidized Pt surfaces as the ones obtained on SiC. Efforts to get high dispersion on SiC seem to be unnecessary. Instead, the higher size Pt particles obtained with previous acid treatment on SiC have shown better activity. The catalyst was activated upon cycling when tested from 200°C in cooling mode. For the design of a practical device, heating under the reaction mixture would be beneficial to the activity of the catalyst, and less dangerous than in-situ reducing in hydrogen atmosphere. However, if the desired application permitted in-situ reducing treatments, washcoating the SiC with TiO2 would lead to the best activity probably because of the synergies between the amount of metallic Pt surface and the contribution of oxygen from the support. Acknowledgements Financial support is acknowledged from the Junta de Andalucía (grant PE2012-TEP862, EU co-financed by FEDER), the Spanish MINECO (grants CTQ2012-32519 and CTQ2015-65918-R, EU co-financed by FEDER) and the CSIC (PIE-201460E018). TEM measurements were performed at the Laboratory for Nanoscopies and Spectroscopies LANE at the ICMS. 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(b) The 0.5Pt/SiC and 1Pt/SiC samples in comparison to the 0.5wt% catalysts supported on previously acid modified SiC. Main peaks positions from reference β-SiC are also indicated at top bar diagram. Figure 2: Representative STEM/HAADF images of the 0.5 wt% Pt catalysts supported on (a) SiC, (b) TiO2 (c) Al2O3 and (d) SiO2 powders. The corresponding particle size histrograms are also shown on the right. Figure 3: Representative STEM/HAADF images of the Pt catalysts on commercial and modified SiC samples: (a) 0.5Pt/SiC (b)0.5Pt/SiC sulfo. The corresponding particle size distribution plots are shown on the right. Figure 4: XPS measurements on the Pt 4f level of the 0.5Pt/SiC sample in comparison to the titania, silica and acid treated SiC supported catalysts. Figure 5: Effect of catalysts´ history. Comparison of the conversion vs temperature plots in cooling mode from 200°C and 450°C for the 0.5 Pt/SiC and the 0.5Pt/TiO2 catalysts. Figure 6: Conversion vs temperature plot for the 0.5Pt/SiC sample in comparison to the alumina, titania and silica supported catalysts. (a)With no previous catalyst conditioning (oxidized or partially oxidized form) (b) With previous in-situ reduction. Figure 7: Conversion vs temperature plots of the 0.5Pt/SiC sample in comparison to the Pt catalysts supported on previously acid modified SiC. Figure 8: Durability upon cycling (5 cycles) of (a) the 0.5Pt/SiC catalyst and (b) the 0.5Pt/TiO2 sample. Figure 9: Representative STEM/HAADF images of (a) the 0.5Pt/SiC and (b) the 0.5Pt/TiO2 catalysts both after 5 cycles. The insets show the particle size distribution plot. (c) XPS measurements of the Pt 4f level on the fresh and used (5 cycles) 0.5Pt/TiO2 sample. Table Legends Table 1: Catalysts´characteristics Figure 6 200 150 100 50 0 0 20 40 60 80 100 Conversion (%) Temperature (C) in situ reduced before reaction (b) 200 150 100 50 0 0 20 40 60 80 100 Conversion (%) Temperature (C) 0.5 Pt/TiO2 0.5 Pt/SiC 0.5 Pt/SiO2 0.5 Pt/Al2O3 (a) calcined Figure Figure 7 200 150 100 50 0 0 20 40 60 80 100 Conversion (%) Temperature (°C) 0.5Pt/SiC 0.5 Pt/SiC sulfo Figure Figure 8 200 150 100 50 0 20 40 60 80 100 Conversion (%) Temperature (C) cycle 1 cycle 2 cycle 3 cycle 4 cycle 5 T50= -10C (a) 200 150 100 50 0 20 40 60 80 100 Conversion (%) Temperature (C) cycle 1 cycle 2 cycle 3 cycle 4 cycle 5 T50= -20C (b) Figure Figure 9 0 5 10 15 0 100 200 300 nr. of particles Diameter (nm) 500 particles 0 5 10 15 20 25 30 0 5 10 15 20 25 30 nr. of particles Diameter (nm) 117 particles 80 78 76 74 72 70 PtIV PtIV PtII PtII Pt0 Intensity (a.u) Eb (eV) 0.5Pt/TiO2 5 cycles 0.5Pt/TiO2 Pt0 a) b) c) (5±3) nm (0.8±0.2)nm Figure *Graphical Abstract Catalytic hydrogen combustion was studied under oxidant conditions Pt particles in oxidized form studied over powdery -SiC and other supports Catalysts activated upon exposure to the oxidant mixture and high temperatures Medium size, partially oxidized Pt particles on SiC show high activity Catalysts have shown a reactivation upon cycling experiments *Highlights (for review)