Three-dimensional simulation and optimization of an isothermal PROX microreactor for fuel cell applications

Three-dimensional numerical simulations of the reacting flow in rectangular micro-channel PROX reactors are performed. To solve the set of governing equations, a finite volume method is applied using an improved SIMPLE algorithm. A three-step surface kinetics for the chemical reactions is utilized that includes hydrogen oxidation, carbon monoxide oxidation, and water–gas shift reaction. The kinetics chosen are for a Pt–Fe/γ-Al₂O₃ catalyst and operating temperatures of about 100 °C. The PROX reactor is expected to remove the carbon monoxide content in a hydrogen-rich stream from about 2% to less than 10 ppm. Effects of the inlet steam content, oxygen to carbon monoxide ratio, reactor wall temperature, aspect ratio of the channel cross section, and the channel hydraulic diameter are investigated. It is found that increasing the steam content, oxygen to carbon monoxide ratio, or wall temperature may improve the performance of the microreactor. It is also shown that the rate of water–gas shift reaction or its reverse is much lower than the oxidation reactions. Finally, it is revealed that based on a modified CO yield definition, the optimum channel geometry is a square shape.     

Oxidative steam reforming of methane to synthesis gas in microchannel reactors

Oxidative steam reforming of methane to synthesis gas (syngas) over an alumina supported bimetallic Pt–Rh catalyst was comparatively investigated in coated and packed microchannel reactors. In the first configuration, thin layers of catalysts are coated on opposite walls of a single microchannel, while the second one is described by particulate catalysts packed into an empty microchannel of dimensions identical with the first one. Both geometries are compared on the basis of methane conversion and CO selectivity measured at different values of parameters, namely reaction temperature (773–923 K), molar steam-to-carbon (S/C = 0–3.0) and oxygen-to-carbon (O₂/C = 0.47–0.63) ratios in the feed, and contact time (0.36–0.71 mg min cm⁻³). Although methane conversions are found to be comparable, the coated catalyst gave significantly higher CO selectivities than the packed counterpart in the whole parameter range. Increase in all of the parameter values led to improvement in methane conversion, while CO selectivity increased only with temperature and contact time. Molar H₂/CO ratios obtained in the coated microchannel reactor are found to vary between 1.0 and 3.0 which are at least three times smaller than those produced in the packed microchannel reactor. Catalyst deactivation is not detected in both configurations. Stable operation up to 72 h over coated microchannel verified mechanical and chemical stability of the Pt–Rh coating that produced syngas with H₂/CO ratio of 2.12 at temperatures lower than employed in industrial reformers. Different flow distribution properties of coated and packed microchannels seem to play roles in affecting the product distribution.     

Optimization of hydrogen production by filtration combustion of natural gas by water addition

The effect of adding steam during filtration combustion of natural gas–air mixtures was studied with the aim to evaluate the optimization of hydrogen production. Temperature, velocity, chemical products of combustion waves, and conversion from fuel to H₂ and CO were evaluated in the range of equivalence ratio (φ) from stoichiometric (φ = 1.0) to φ = 3.0 and steam content in the mixture from 0% to 39%, at filtration velocities from 12 to 25 cm/s. Numerical simulation was carried out using GRI-MECH 3.0. Results suggest that H₂ and CO concentrations, dominant for rich and ultrarich combustion, are products from partial oxidation and steam natural gas reforming processes. Experimental and numerical results show that hydrogen yield increase with an increase of steam content in the natural gas–air mixtures.     

Hydrogen production via steam reforming of methanol over Cu/(Ce,Gd)O2−x catalysts

Hydrogen production via steam reforming of methanol is carried out over Cu/(Ce,Gd)O_2−x catalysts at 210–600 °C. The CO content in reformate is about 1% at 210–270 °C, which are the typical temperature for hydrogen production via steam reforming of methanol. Largest H₂ yield and CO₂ selectivity and smallest CO content are obtained at 240 °C. The formation rate of CO increases with increasing temperature. The average formation rate of CO becomes larger than that of CO₂ at about 450 °C. The H₂ yield, the CO₂ selectivity and the CO content become constant at about 550 °C. At 240 °C, the smallest CO content is obtained with a catalyst weight of 0.5 g and a Cu content of 3 wt%. The H₂ yield, defined as H₂/(CO + CO₂) in formation rates, at 240 °C is always 3 and not affected by the variations of either the catalyst weight or the Cu content.     

Selective CO oxidation in H2 streams on CuO/CexZr1−xO2 catalysts: Correlation between activity and low temperature reducibility

Catalysts synthesized by incorporating CuO (7 wt.% of Cu) on six commercial Ce_xZr_1−xO₂ mixed oxides (x = 1, 0.8, 0.68, 0.5, 0.15, 0) have been prepared by conventional wetness impregnation method. These catalysts have been screened for CO oxidation in hydrogen streams (CO-PROX) and characterized by means of XRD, BET, Raman, XPS and H₂-TPR experiments. Activity towards CO oxidation in hydrogen streams has been discussed and correlated with the properties of the catalysts. XRD and Raman analysis of the supports show an increase of oxygen defect as Zr content increase. Below 150 °C the catalysts reducibility measured by H₂-TPR correlates with ceria content in the support, although an increase of Zr content in the support increases considerably the reduction degree of ceria in the 0–600 °C interval. Activity towards CO oxidation in hydrogen streams also correlates with Ce/Cu molar ratio and low temperature reducibility of copper species. Most of the catalysts give complete CO conversion with high selectivity operating with λ = 2. The most active catalysts is CuO supported on pure ceria, which is able to oxidize completely CO in the interval 96–164 °C, with maximum selectivity of 90%. On the other hand, the operation window becomes narrower as Zr content in the supports increases.     

Catalytic performance of CoAlZn and NiAlZn mixed oxides in hydrogen production by bio-ethanol partial oxidation

CoAlZn and NiAlZn mixed oxides were prepared by sol–gel method and tested in partial oxidation of bio-ethanol (POE). At lower temperatures, CoAlZn showed higher ethanol conversion and higher selectivity to H₂ and CO than NiAlZn. At higher temperatures, ethanol conversion on both catalysts reached 100%, while selectivity (S) to H₂ and CO became higher on NiAlZn. At 750 °C, NiAlZn showed S(H₂) of 95%, S(CO) of 90%, while for CoAlZn these values were 90% and 83% respectively. Both catalysts were resistant to coking, but the amount of carbon deposits was still lower on NiAlZn. During 50 h on-stream, ethanol conversion and selectivity to H₂ and CO on NiAlZn remained unchanged demonstrating stable performance of the catalyst.     

Renewable hydrogen production from steam reforming of glycerol by Ni–Cu–Al,Ni–Cu–Mg,Ni–Mg catalysts

H₂ production from glycerol steam reforming by the Ni–Cu–Al, Ni–Cu–Mg, Ni–Mg catalysts was evaluated experimentally in a continuous flow fixed-bed reactor under atmospheric pressure within a temperature range from 450 to 650 °C. The catalysts were synthesized by the co-precipitation methods, and characterized by the elemental analysis, BET, XRD and SEM. The GC and FTIR were applied to analyze the products from steam reforming of glycerol. The coke deposited on the catalysts was measured by TGA experiments during medium temperature oxidation. The results showed that glycerol conversion and H₂ production were increased with increasing temperatures, and glycerol decomposition was favored over its steam reforming at low temperatures. The Ni–Cu–Al catalyst containing NiO of 29.2 wt%, CuO of 31.1 wt%, Al₂O₃ of 39.7 wt% performed high catalytic activity, and the H₂ selectivity was found to be 92.9% and conversion of glycerol was up to 90.9% at 650 °C. The deactivation of catalysts due to the formation and deposition of coke was observed. An improved iterative Coats–Redfern method was used to evaluate the non-isothermal kinetic parameters of coke removal from catalysts, and the results showed the reaction order of n = 1 and 2 in the Fn nth order reaction model predicted accurately the main phase in the coke removal for the regeneration of Ni–Mg and Ni–Cu–Al catalysts, respectively.     

Synthesis of highly-dispersed CuO–CeO2 catalyst through a chemisorption-hydrolysis route for CO preferential oxidation in H2-rich stream

The CuO–CeO₂ catalyst (CuO loading: 15 wt%) was prepared by a novel chemisorption-hydrolysis method, and employed for the preferential oxidation of CO (CO PROX) in H₂-rich stream. For comparison, several other conventional methods such as impregnation, co-precipitation and deposition-precipitation were also used to prepare the catalyst. It is found that the CuO–CeO₂ catalyst prepared by chemisorption-hydrolysis method exhibits the best catalytic performance, giving not only the widest temperature window (120–170 °C) for CO complete conversion, but also the highest oxygen to CO₂ selectivity of 99.9% at 120 °C. The results of XRD, N₂O chemisorption and in-situ FT-IR conformably indicate that this catalyst possesses the highest dispersion of Cu species, which facilitates the formation of Cu⁺ carbonyl species, and simultaneously prevents the adsorption and oxidation of H₂. With the increase of reaction temperature, Cu⁺ is gradually reduced to Cu⁰, enhancing the adsorption and oxidation of H₂, as a result, the selectivity of oxygen towards CO₂ is lowered obviously. The presence of CO₂ and H₂O exhibits negative effects on the catalytic performance, shortening the activity window to 150–170 °C region and decreasing the CO₂ selectivity to 87% at the temperature for the initial 100% conversion of CO. Based on the above study, a potential reaction pathway for CO PROX over the CuO–CeO₂ catalyst is proposed.     

Optimal design of parallel channel patterns in a micro methanol steam reformer

This study describes the performance of micro methanol steam reformers with channel widths optimized using the simplified conjugate gradient method (SCGM), which uses a minimum objective function of the H₂ mass fraction standard deviation in channels. A three-dimensional numerical model and optimal simplified conjugate gradient algorithm were built to predict and search for the effects of channel widths and flow rate on the performance of chemical reactions. Furthermore, this simulation model was compared to; and corresponded well with existing experimental data. Distributions of velocity, temperature, and gas concentrations (CH₃OH, CO, H₂, and CO₂) were predicted, and the methanol conversion ratio was also evaluated. The mole fraction of CO contained in the reformed gas, which is essential to preventing poisoning of the catalyst layers of fuel cells, is also investigated. In the optimization search process, the governing equations use the continuity, momentum, heat transfer, and species equations to evaluate the performance of the steam reformer. The results show that channel width optimization can not only increase the methanol conversion ratio and hydrogen production rate but also decrease the concentration of carbon monoxide. The velocity and mixture gas density distributions in channels are discussed and plotted at various locations for an inlet liquid flow rate of 0.3 cc min⁻¹. Full development is not obtained in the downstream channel flow, the velocity in channel is increased from 1.28 m s⁻¹ to 2.36 m s⁻¹ at location Y = 1 mm–32 mm, respectively. This can be attributed to a continuous increase in the lightweight H₂ species as a result of chemical reactions in the channels.     

Thermodynamic equilibrium calculations of hydrogen production from the combined processes of dimethyl ether steam reforming and partial oxidation

Thermodynamic analyses of producing a hydrogen-rich fuel-cell feed from the combined processes of dimethyl ether (DME) partial oxidation and steam reforming were investigated as a function of oxygen-to-carbon ratio (0.00–2.80), steam-to-carbon ratio (0.00–4.00), temperature (100 °C–600 °C), pressure (1–5 atm) and product species.Thermodynamically, dimethyl ether processed with air and steam generates hydrogen-rich fuel-cell feeds; however, the hydrogen concentration is less than that for pure DME steam reforming. Results of the thermodynamic processing of dimethyl ether indicate the complete conversion of dimethyl ether to hydrogen, carbon monoxide and carbon dioxide for temperatures greater than 200 °C, oxygen-to-carbon ratios greater than 0.00 and steam-to-carbon ratios greater than 1.25 at atmospheric pressure (P = 1 atm). Increasing the operating pressure has negligible effects on the hydrogen content. Thermodynamically, dimethyl ether can produce concentrations of hydrogen and carbon monoxide of 52% and 2.2%, respectively, at a temperature of 300 °C, and oxygen-to-carbon ratio of 0.40, a pressure of 1 atm and a steam-to-carbon ratio of 1.50. The order of thermodynamically stable products (excluding H₂, CO, CO₂, DME, NH₃ and H₂O) in decreasing mole fraction is methane, ethane, isopropyl alcohol, acetone, n-propanol, ethylene, ethanol and methyl-ethyl ether; trace amounts of formaldehyde, formic acid and methanol are observed.Ammonia and hydrogen cyanide are also thermodynamically favored products. Ammonia is favored at low temperatures in the range of oxygen-to-carbon ratios of 0.40–2.50 regardless of the steam-to-carbon ratio employed. The maximum ammonia content (i.e., 40%) occurs at an oxygen-to-carbon ratio of 0.40, a steam-to-carbon ratio of 1.00 and a temperature of 100 °C. Hydrogen cyanide is favored at high temperatures and low oxygen-to-carbon ratios with a maximum of 3.18% occurring at an oxygen-to-carbon ratio of 0.40 and a steam-to-carbon ratio of 0.00 in the temperature range of 400 °C–500 °C. Increasing the system pressure shifts the equilibrium toward ammonia and hydrogen cyanide.     

Ni/Ce-MCM-41 mesostructured catalysts for simultaneous production of hydrogen and nanocarbon via methane decomposition

For the first time, simultaneous production of hydrogen and nanocarbon via catalytic decomposition of methane over Ni-loaded mesoporous Ce-MCM-41 catalysts was investigated. The catalytic performance of the Ni/Ce-MCM-41 catalysts is very stable and the reaction activity remained almost unchanged during 1400 min steam on time at temperatures 540, 560 and 580 °C, respectively. The methane conversion level over these catalysts reached 60–75% with a 100% selectivity towards hydrogen. TEM observations revealed that most of the Ni particles located on the tip of the carbon nanofibers/nanotubes in the used catalysts, keeping their exposed surface clean during the test and thus remaining active for continuous reaction without obvious deactivation. Two kinds of carbon materials, graphitic carbon (C_g) as major and amorphous carbon (C_A) as minor were produced in the reaction, as confirmed by XRD analysis and TEM observations. Carbon nanofibers/nanotubes had an average diameter of approximately 30–50 nm and tens micrometers in length, depending on the reaction temperature, reaction time and Ni particle diameter. Four types of carbon nanofibers/nanotubes were detected and their formations greatly depend on the reaction temperature, time on steam and degree of the interaction between the metallic Ni and support. The respective mechanisms of the formation of nanocarbons were postulated and discussed.     

Syngas production from catalytic gasification of waste polyethylene: Influence of temperature on gas yield and composition

The catalytic steam gasification of waste polyethylene (PE) from municipal solid waste (MSW) to produce syngas (H₂ + CO) with NiO/γ-Al₂O₃ as catalyst in a bench-scale downstream fixed bed reactor was investigated. The influence of the reactor temperature on the gas yield, gas composition, steam decomposition, low heating value (LHV), cold gas efficiency and carbon conversion efficiency was investigated at the temperature range of 700–900 °C, with a steam to waste polyethylene ratio of 1.33. Over the ranges of experimental conditions examined, NiO/γ-Al₂O₃ catalyst revealed better catalytic performance as a view of increasing product gas yield and of decreasing char and liquid yields in the presence of steam. Higher temperature resulted in more H₂ and CO production, higher carbon conversion efficiency and product gas yield. The highest syngas (H₂ + CO) content of 64.35 mol%, the highest H₂ content of 36.98 mol%, and the highest CO content of 27.37 mol%, were achieved at the highest temperature level of 900 °C. Syngas produced with a H₂/CO molar ratio in the range of 0.83–1.35, was highly desirable as feedstock for Fischer–Tropsch synthesis for the production of transportation fuels.     

Hydrogen production from water gas shift reaction in a high gravity (Higee) environment using a rotating packed bed

Hydrogen production via the water gas shift reaction (WGSR) was investigated in a high gravity environment. A rotating packed bed (RPB) reactor containing a Cu–Zn catalyst and spinning in the range of 0–1800 rpm was used to create high centrifugal force. The reaction temperature and the steam/CO ratio ranged from 250 to 350 °C and 2 to 8, respectively. A dimensionless parameter, the G number, was derived to account for the effect of centrifugal force on the enhancement of the WGSR. With the rotor speed of 1800 rpm, the induced centrifugal force acting on the reactants was as high as 234 g on average in the RPB. As a result, the CO conversion from the WGSR was increased up to 70% compared to that without rotation. This clearly revealed that the centrifugal force was conducive to hydrogen production, resulting from intensifying mass transfer and elongating the path of the reactants in the catalyst bed. From Le Chatelier’s principle, a higher reaction temperature or a lower steam/CO ratio disfavors CO conversion; however, under such a situation the enhancement of the centrifugal force on hydrogen production from the WGSR tended to become more significant. Accordingly, a correlation between the enhancement of CO conversion and the G number was established. As a whole, the higher the reaction temperature and the lower the steam/CO ratio, the higher the exponent of the G number function and the better the centrifugal force on the WGSR.     

Preferential oxidation of CO in H2 stream on Au/ZnO–TiO2 catalysts

A series of gold catalysts supported on ZnO–TiO₂ with various ZnO contents were prepared. ZnO–TiO₂ was prepared by incipient-wetness impregnation using aqueous solution of Zn(NO₃)₂ onto TiO₂. Gold catalysts with nominal gold loading of 1 wt. % were prepared by deposition-precipitation (DP) method. Various preparation parameters, such as pH value and Zn/Ti ratio on the characteristics of the catalysts were investigated. The catalysts were characterized by inductively-coupled plasma–mass spectrometry, X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy and high-resolution transmission electron microscopy. The preferential oxidation of CO in H₂ stream (PROX) on these catalysts was carried out in a fixed bed micro-reactor with a feed of CO: O₂: H₂: He = 1: 1: 49: 49 (volume ratios) and a space velocity of 30,000 ml/g h. Limited amount of oxygen was used in the feed. A high gold dispersion and narrow gold particle size distribution was obtained. Au/ZnO–TiO₂ with Zn/Ti atomic ratio of 5/95 showed the highest CO conversion at room temperature. The conversion increased with increasing temperature even in the presence of limited amount of oxygen, showing suppression in H₂ oxidation. Au/ZnO–TiO₂ prepared at pH 6 had a higher CO conversion and higher selectivity of CO oxidation than those prepared at other pH values. The addition of ZnO on TiO₂ resulted in higher dispersion of gold particles and narrow particle size distribution. The stronger the Au–Zn(OH)₂ interaction, the finer the supported Au nanoparticles, and the better the catalytic performance of the catalyst for PROX reaction. Part of Au was in Au⁺ state due to the interaction with Zn(OH)₂ and nano Au size. The oxidation state of gold species played an important role in determining its CO conversion and selectivity of CO oxidation in hydrogen stream. The catalysts were stable at 80 °C for more than 80 h.     

Numerical analysis of a 20-kWe biogas steam reformer in PEMFC applications

This study supports the development of a 20-kW_e biogas reformer for PEMFC applications. Biogas contains a high concentration of carbon dioxide (CO₂) in the feedstock, which can act as a reforming agent but increases the carbon monoxide (CO) concentration in the product. The feedstock composition and the external heat supply impact both the methane (CH₄) conversion and the CO concentration in the product; thus, a numerical model is developed to investigate these impacts. As the CO₂ content in the feedstock or the external heat supply increases, both the CH₄ conversion and the CO concentration in the product increase. A high CO₂ concentration or a high operating temperature stimulates the reforming reactions but also shifts the equilibrium to favor CO production. However, the performance can be improved by controlling the heat supply method. Using a more uniform temperature distribution at approximately 650 °C, the CO concentration decreases while the CH₄ conversion increases. This finding suggests the importance of temperature control in biogas steam reforming.     

The effect of copper on the selective carbon monoxide oxidation over alumina supported gold catalysts

The selective oxidation of CO in the presence of H₂ was investigated on Au catalysts promoted with different amounts of Cu. Au catalysts were prepared by the deposition–precipitation method and exhibited a satisfactory activity at low temperature with adequate selectivity. A considerable improvement in CO conversion was achieved when the O₂/CO ratio was increased from the value of 0.5–1.0. The addition of Cu to Au/Al₂O₃ catalysts caused an increase in the selectivity to CO oxidation due to an interaction between Au and Cu on the surface of the catalysts. However, this beneficial effect was limited to an optimal content of Cu. The catalysts were characterized by temperature programmed reduction and DRS UV–vis spectroscopy, indicating the formation of small bimetallic Au–Cu particles. The presence of water vapor in the feed stream played a positive effect in the CO conversion and selectivity while the CO₂ presence diminished the CO conversion and selectivity. In the case of a realistic reformate, when both H₂O and CO₂ are present, the positive effect of H₂O was able to compensate the negative effect of CO₂ depending on the temperature of reaction.     

Experimental investigation on hydrogen production by methanol steam reforming in a novel multichannel micro packed bed reformer

A novel multichannel micro packed bed reactor with bifurcation inlet manifold and rectangular outlet manifold was developed to improve the methanol steam reforming performance in this study. The commercial CuO/ZnO/Al₂O₃ catalyst particles were directly packed in the reactor. The flow distribution uniformity in the reactor was optimized numerically. Experiments were conducted to study the influences of steam to carbon molar ratio (S/C), weight hourly space velocity (WHSV), reactor operating temperature (T) and catalyst particle size on the methanol conversion rate, H₂ production rate, CO concentration in the reformate, and CO₂ selectivity. The results show that increase of the S/C and T, as well as decrease of the WHSV and catalyst particle size, both enhance the methanol conversion. The CO concentration decreases as the S/C and WHSV increase as well as the T and catalyst particle size decrease. Moreover, T plays a more important role on the methanol steam reforming performance than WHSV and S/C. The impacts on CO concentration become insignificant when the S/C is higher than 1.3, WHSV is larger than 1.34 h⁻¹ and T is lower than 275 °C. A long term stability test of this reactor was also performed for 36 h and achieved high methanol conversion rate above 94.04% and low CO concentration less than 1.05% under specific operating conditions.     

Characteristics of hydrogen production by a plasma–catalyst hybrid converter with energy saving schemes under atmospheric pressure

This paper investigated the production of hydrogen from methane under atmospheric pressure using a plasma–catalyst hybrid converter with emphasis on energy conservation. A spark discharge was used to ionize the hydrocarbon fuel and air mixture with a catalyst to enhance hydrogen production using two energy saving schemes, namely, heat recycling and heat insulation. The experimental results showed that higher methane feeding rate resulted in higher reformate gas temperature and a corresponding increase in methane conversion efficiency. The energy saving systems also enabled the oxygen/carbon ratio to be decreased to reduce oxidation of hydrogen and carbon monoxide and thereby improving the concentrations of hydrogen and carbon monoxide. By heat recycling, a lower methane feeding rate showed an 8.7% improvement in methane conversion efficiency whilst improvement was not apparent with higher methane supply rates due to the already high conversion efficiency. Moreover, it was shown that hydrogen production increased significantly with the reaction from water–gas shifting under the same operation parameters but with high methane selectivity. The best combination resulting in a total thermal efficiency of 77.11% was 10 L/min methane feeding rate and 0.8 O₂/C ratio. With water–gas shifting (S/C ratio=0.5), an 86.26% hydrogen yield, equating to 17.25 L/min hydrogen production rate could be achieved. The equilibrium production rate was calculated using the commercialized HSC Chemistry software (^©ChemSW Software, Inc.). Good correlation was obtained between the calculations and the experimental results.     

Thermodynamic analysis of propane dry and steam reforming for synthesis gas or hydrogen production

Thermodynamics was applied to investigate propane dry reforming (DR) and steam reforming (SR). Equilibrium calculations employing the Gibbs free energy minimization were performed upon a wide range of pressure (1–5 atm), temperature (700–1100 K), carbon dioxide to propane ratio (CPR, 1–12) and water to propane ratio (WPR, 1–18). From a thermodynamic perspective, it is demonstrated that DR is promising for production of synthesis gas with low hydrogen content, as opposite to SR which favours generation of synthesis gas with high hydrogen content. Complete conversion of propane was obtained for the range of pressure, temperature, CPR and WPR considered in this study. Atmospheric pressure is shown to be preferable for both DR and SR. Approximately 10 mol of synthesis gas can be produced per mole of propane at a temperature greater than 1000 K from DR when CPR is higher than 6. The optimum conditions for synthesis gas production from DR are found to be 975 K (CPR = 3) for a H₂/CO ratio of 1 and 1100 K (CPR = 1) for a H₂/CO ratio of 2. The greatest CO₂ conversion (95%) can be obtained also at 1100 K and CPR = 1. Preferential conditions for hydrogen production from SR are achieved with the temperatures between 925 and 975 K and WPRs of 12–18. The maximum number of moles of hydrogen produced is 9.1 (925 K and WPR = 18). Under conditions that favour hydrogen production, methane and carbon formation can be eliminated to negligible level.     

A comparative study on the performance of mesoporous SBA-15 supported Pd–Zn catalysts in partial oxidation and steam reforming of methanol for hydrogen production

Using mesoporous SBA-15 (Santa Barbara Amorphous No. 15, a mesoporous material) as support, Pd–Zn nanocatalysts with varying Pd and Zn content were tested for hydrogen production from methanol by partial oxidation and steam reforming reactions. The physico-chemical characteristics of the synthesized SBA-15 support were confirmed by XRD, N₂ adsorption, SEM and TEM analyses. The PdZn alloy formation during the reduction of Pd–Zn/SBA-15 was revealed by XRD and DRIFT study of adsorbed CO. Also, the correlation between Pd and Zn loadings and PdZn alloy formation was studied by XRD and TPR analyses. The metallic Pd surface area and total uptakes of CO and H₂ were measured by chemisorption at 35 °C. The metallic Pd surface area values are in linear proportion with the Pd loading. The formation of PdZn alloy during high temperature reduction was confirmed by a shift in absorption frequency of CO on Pd sites to lower frequency due to higher electron density at metal particles resulted from back-donation. The reduced Pd–Zn/SBA-15 catalysts were tested for partial oxidation of methanol at different temperatures and found that catalyst with 4.5 wt% Pd and 6.75 wt% Zn on SBA-15 showed better H₂ selectivity with suppressed CO formation due to the enhanced Pd dispersion as well as larger Pd metallic surface area. The O₂/CH₃OH ratio is found to play a significant role in CH₃OH conversion and H₂ selectivity. The performance of 4.5 wt% Pd–6.75 wt% Zn/SBA-15 catalyst in steam reforming of methanol was also tested. Comparatively, the H₂ selectivity is significantly higher than that in partial oxidation, even though the CH₃OH conversion is less. Finally, the long term stability of the catalyst was tested and the nature of PdZn alloy after the reactions was found to be stable as revealed from the XRD pattern of the spent catalysts.