Hydrogen production from propane in Rh-impregnated metallic microchannel reactors and alumina foams

Rh-impregnated alumina foams and metallic microchannel reactors have been studied for production of hydrogen-rich syngas through short contact time catalytic partial oxidation (POX) and oxidative steam reforming (OSR) of propane. Effects of temperature and residence time have been compared for the two catalytic systems. Temperature profiles obtained along the central axis were valuable in understanding the different behaviour of the reactor systems. Gas phase ignition occurs in front of the metallic monolith at furnace temperatures above 700 °C, leading to lower hydrogen selectivity. Lowering the residence time below 10 ms for the microchannel monolith increases the syngas selectivity. This probably due to quenching of the gas phase reactions at high linear gas velocity, and suggests that microchannel reactors have potential for isolating kinetic effects and minimising gas phase contributions. The Rh/Al₂O₃ foam systems show higher initial syngas selectivity than the Rh-impregnated microchannel reactors, but deactivate rapidly upon temperature cycling, especially when steam is added as a reactant.     

Production of hydrogen for fuel cells by steam reforming of ethanol over supported noble metal catalysts

The catalytic performance of supported noble metal catalysts for the steam reforming (SR) of ethanol has been investigated in the temperature range of 600–850 °C with respect to the nature of the active metallic phase (Rh, Ru, Pt, Pd), the nature of the support (Al₂O₃, MgO, TiO₂) and the metal loading (0–5 wt.%). It is found that for low-loaded catalysts, Rh is significantly more active and selective toward hydrogen formation compared to Ru, Pt and Pd, which show a similar behavior. The catalytic performance of Rh and, particularly, Ru is significantly improved with increasing metal loading, leading to higher ethanol conversions and hydrogen selectivities at given reaction temperatures. The catalytic activity and selectivity of high-loaded Ru catalysts is comparable to that of Rh and, therefore, ruthenium was further investigated as a less costly alternative. It was found that, under certain reaction conditions, the 5% Ru/Al₂O₃ catalyst is able to completely convert ethanol with selectivities toward hydrogen above 95%, the only byproduct being methane. Long-term tests conducted under severe conditions showed that the catalyst is acceptably stable and could be a good candidate for the production of hydrogen by steam reforming of ethanol for fuel cell applications.     

Resistance to sulfur poisoning of hot gas cleaning catalysts for the removal of tar from the pyrolysis of cedar wood

In the partial oxidation of tar derived from the pyrolysis of cedar wood, the effect of H₂S addition was investigated over non-catalyst, steam reforming Ni catalyst, and Rh/CeO₂/SiO₂ using a fluidized bed reactor. In the non-catalytic gasification, the product distribution was not influenced by the presence of H₂S. Steam reforming Ni catalyst was effective for the tar removal without H₂S addition, however, the addition of H₂S deactivated drastically. In contrast, Rh/CeO₂/SiO₂ exhibited higher and more stable activity than the Ni catalyst even under the presence of high concentration of H₂S (280 ppm). On the Ni catalyst, the adsorption of sulfur was observed by XPS and Ni species was oxidized during the partial oxidation of tar. In the case of Rh/CeO₂/SiO₂, the adsorption of sulfur was below the detection limit of XPS. This can be related to the self-cleaning of catalyst surface during the circulation in the fluidized bed reactor for the partial oxidation of tar derived from cedar pyrolysis.     

Production of hydrogen by short contact time partial oxidation and oxidative steam reforming of propane

Partial oxidation (POX) and oxidative steam reforming (OSR) of propane have been studied over Rh-impregnated alumina foams in the short contact time regime. The experiments were performed over a wide temperature range (300–1000 °C) at close to atmospheric pressure. It was found that a furnace temperature of 700 °C is optimal for the production of hydrogen for both reactions (POX and OSR) in this system. Variations in the total flow rate revealed an effect of residence time on the product distribution during both POX and OSR. The production of hydrogen was hardly affected by the residence time, but an influence of the residence time on the selectivity to all other products was observed. Hydrocarbon byproducts were increasingly formed at shorter residence times while formation of partial and complete oxidation products increased with longer residence times. The Rh foam catalyst also showed promising stability under strong oxidation conditions.     

Exhaust-gas reforming using precious metal catalysts

Rh-only and Rh bimetallic catalysts have been screened for exhaust-gas reforming, under conditions that mimic the output of an autoignition gasoline engine. Propane has been used as a model fuel, with simulated exhaust-gas providing the co-reactants (O₂ and H₂O) needed to generate hydrogen. Based on oxygen-conversion as a measure of light-off, Pt–Rh on ceria–zirconia shows the highest activity. In the presence of SO₂, adsorbed sulphur species do not inhibit the oxidation reactions that induce light-off, but suppress the major pathway to hydrogen (steam reforming). By excluding platinum and using silica-enriched alumina as the underlying support, light-off is delayed, but the steam reforming reaction becomes much more insensitive to the presence of sulphur. The Pt–Rh catalyst is most suited to exhaust-gas reforming systems in which the engine runs on a sulphur-free fuel, whereas the Rh-only catalyst is the better choice when the fuel is conventional gasoline.     

Fuel cell grade hydrogen from methanol on a commercial Cu/ZnO/Al2O3 catalyst

This paper presents the results of experiments of the methanol decomposition reaction catalyzed by a commercial Cu/ZnO/Al₂O₃ in the absence and presence of water. Methanol decomposition of 100% in the absence of water was obtained at 290 °C and a space velocity of 2 cm³/h g cat. At these conditions, the hydrogen yield was 1.9–2.0. Water addition to the feed increased the yield of hydrogen and reduced the formation of: dimethyl ether; methyl formate and methane. The variation of the catalyst’s activity and selectivity with time, temperature and feed composition was consistent with previous studies of methanol–steam reforming and water–gas shift reaction, however, this appears to be the first study over the same catalyst of methanol decomposition and methanol–steam reforming. XPS analysis of used catalyst samples and time on-stream data showed that the Cu²⁺ oxidation state of copper favors methanol decomposition, and we propose that the deactivation of the catalyst is mainly caused by the change in the oxidation state of copper.     

Production of hydrogen over bimetallic Pt–Ni/δ-Al2O3: I. Indirect partial oxidation of propane

Indirect partial oxidation (IPOX) of propane was studied over bimetallic 0.2 wt.% Pt–15 wt.% Ni/δ-Al₂O₃ catalyst in the 623–743 K temperature range. The unreduced and reduced forms of the catalyst were characterized by ESEM–EDAX and X-ray diffraction (XRD). In the IPOX tests, the effects of steam to carbon ratio (S/C), carbon to oxygen ratio (C/O₂) and residence time (W/F (g_cat h/mol HC)) on the hydrogen production activity, selectivity and product distribution were studied in detail. The effect of temperature program applied (increasing from 623 to 743 K, ITP; decreasing from 743 to 623 K, DTP) during reaction was also tested. The results showed that the Pt–Ni bimetallic system has superior performance characteristics compared to the monometallic catalysts reported in literature. The reason is thought to be the utilization of the catalyst particles as micro heat exchangers during IPOX; the heat generated by Pt sites during exothermic total oxidation (TOX) being readily transferred through the catalyst particles acting as micro heat exchangers to the Ni sites, which promote endothermic steam reforming (SR). The optimal conditions were found as S/C = 3, C/O₂ = 2.70 and W/F = 0.51 g_cat h/mol HC for IPOX of propane on the basis of high hydrogen productivity and selectivity between 623 and 748 K for the experimental conditions tested. The thermo-neutral points obtained showed the sustainability of reaction in terms of energy.     

Production of hydrogen via partial oxidation of methanol over Au/TiO2 catalysts

Selective production of hydrogen by partial oxidation of methanol (CH₃OH + (1/2)O₂ → 2H₂ + CO₂) over Au/TiO₂ catalysts, prepared by a deposition–precipitation method, was studied. The catalysts were characterized by XRD, TEM, and XPS analyses. TEM observations show that the Au/TiO₂ catalysts exhibit hemispherical gold particles, which are strongly attached to the metal oxide support at their flat planes. The size of the gold particles decreases from 3.5 to 1.9 nm during preparation of the catalysts with the rise in pH from 6 to 9 and increases from 2.9 to 4.3 nm with the rise in calcination temperature up to 673 K. XPS analyses demonstrate that in uncalcined catalysts gold existed in three different states: i.e., metallic gold (Au⁰), non-metallic gold (Au^δ+) and Au₂O₃, and in catalysts calcined at 573 K only in metallic state. The catalytic activity is strongly dependent on the gold particle size. The catalyst precipitated at pH 8 and uncalcined catalysts show the highest activity for hydrogen generation. The partial pressure of oxygen plays an important role in determining the product distribution. There is no carbon monoxide detected when the O₂/CH₃OH molar ratio in the feed is 0.3. Both hydrogen selectivity and methanol conversion increase with increasing the reaction temperature. The reaction pathway is suggested to consist of consecutive methanol combustion, partial oxidation and steam reforming.     

Combined CO2 reforming and partial oxidation of n-heptane on noble metal zirconia catalysts

The combined CO₂ reforming and partial oxidation (POX) of n-heptane was studied on various noble metal zirconia catalysts between 700 and 900 °C. The activity order of the metals was Rh > Pd > Ir > Pt. Selectivity to syngas increased with the activity of the catalysts but the H₂ to CO molar ratio decreased. The activity and selectivity of the 0.25 wt% Rh/ZrO₂ catalyst were close to the performance of a commercial 15 wt% NiO/Al₂O₃ catalyst. The conversions and product compositions were compared to the calculated thermodynamic equilibria.     

Transient modeling of combined catalytic combustion/CH4 steam reforming

This paper presents an investigation into the complex interactions between catalytic combustion and CH₄ steam reforming in a co-flow heat exchanger where the surface combustion drives the endothermic steam reforming on opposite sides of separating plates in alternating channel flows. To this end, a simplified transient model was established to assess the stability of a system combining H₂ or CH₄ combustion over a supported Pd catalyst and CH₄ steam reforming over a supported Rh catalyst. The model uses previously reported detailed surface chemistry mechanisms, and results compared favorably with experiments using a flat-plate reactor with simultaneous H₂ combustion over a γ-Al₂O₃-supported Pd catalyst and CH₄ steam reforming over a γ-Al₂O₃-supported Rh catalyst. Results indicate that stable reactor operation is achievable at relatively low inlet temperatures (400 °C) with H₂ combustion. Model results for a reactor with CH₄ combustion indicated that stable reactor operation with reforming fuel conversion to H₂ requires higher inlet temperatures. The results indicate that slow transient decay of conversion, on the order of minutes, can arise due to loss of combustion activity from high-temperature reduction of the Pd catalyst near the reactor entrance. However, model results also show that under preferred conditions, the endothermic reforming can be sustained with adequate conversion to maintain combustion catalyst temperatures within the range where activity is high. A parametric study of combustion inlet stoichiometry, temperature, and velocity reveals that higher combustion fuel/air ratios are preferred with lower inlet temperatures (≤500 °C) while lower fuel/air ratios are necessary at higher inlet temperatures (600 °C).     

Fuel-rich catalytic combustion of methane in zero emissions power generation processes

A novel catalytic combustion concept for zero emissions power generation has been investigated. Catalysts consisting of Rh supported on ZrO₂, Ce-ZrO₂ or -Al₂O₃ were prepared and tested under fuel-rich conditions, i.e. for catalytic partial oxidation (CPO) of methane. The experiments were performed in a subscale gas-turbine reactor operating at 5 bar with exhaust gas-diluted feed mixtures.

The catalyst support material was found to influence the light-off temperature significantly, which increased in the following order Rh/Ce-ZrO₂ < Rh/ZrO₂ < Rh/-Al₂O₃. The Rh loading, however, only had a minor influence. The high activity of Rh/Ce-ZrO₂ is probably related to the high dispersion of Rh on Ce-ZrO₂ and the high oxygen mobility of this support compared to pure ZrO₂. The formation of hydrogen was also found to increase over the catalyst containing ceria in the support material.     

Mixed reforming of heptane to syngas in the Ba0.5Sr0.5Co0.8Fe0.2O3 membrane reactor

Fuel cells are recognized as the most promising new power generation technology, but hydrogen supply is still a problem. In our previous work, we have developed a LiLaNiO/γ-Al₂O₃ catalyst, which is excellent not only for partial oxidation of hydrocarbons, but also for steam reforming and autothermal reforming. However, the reaction needs pure oxygen or air as oxidant. We have developed a dense oxygen permeable membrane Ba_0.5Sr_0.5Co_0.8Fe_0.2O₃ which has an oxygen permeation flux around 11.5 ml/cm² min at reaction conditions. Therefore, this work is to combine the oxygen permeable membrane with the catalyst LiLaNiO/γ-Al₂O₃ in a membrane reactor for hydrogen production by mixed reforming of heptane. Under optimized reaction conditions, a heptane conversion of 100%, a CO selectivity of 91–93% and a H₂ selectivity of 95–97% have been achieved.     

Development of a dual functional structured catalyst for partial oxidation of methane to syngas

Rh-LaCoO₃ structured catalysts for the oxidative production of syngas from methane were developed by deposition of the active components on La-γ-Al₂O₃ washcoated honeycomb monoliths. SEM/EDS analysis showed a good adhesion of the washcoat layer and a uniform distribution of La and Co, while Rh was favourably located on the outer shell. Catalytic partial oxidation of methane was tested under both isothermal and pseudo-adiabatic conditions showing that the process can be conducted with high yield and selectivity and stable performance at short contact times over the novel catalysts, characterised by a limited content of noble metal and no need for pre-reduction. Further experiments of CO₂ autothermal reforming indicated the possibility to enhance CO production and to reduce the H₂/CO ratio through secondary endothermic reactions consuming CO₂, which are autothermally self-sustained in a single catalytic reactor operated at short contact time by the heat generated through partial oxidation reactions.     

Hydrogen Production from Partial Oxidation and Steam Reforming of N‐octane Over Alumina‐supported Ni and Ni‐Pd Catalysts

Hydrogen production by partial oxidation and steam reforming (POSR) of n‐octane was investigated over alumina‐supported Ni and Ni‐Pd catalysts. It showed that Ni‐Pd/Al₂O₃ had higher activity and hydrogen selectivity than the nickel catalyst under the experimental conditions, which indicated Ni‐Pd/Al₂O₃ could be an effective catalyst for the production of hydrogen from hydrocarbons.     

Investigation of alumina-supported Ni and Ni-Pd catalysts by partial oxidation and steam reforming of n-octane

Aseries of nickel and nickel-palladium supported upon alumina catalysts were prepared in order to obtain a suitable catalyst that could be used in the process of producing hydrogen by partial oxidation and steam reforming of n-octane. Hydrogen production by partial oxidation and steam reforming (POSR) of n-octane was investigated over alumina-supported Ni and Ni-Pd catalysts. The process occurred by a combination of exothermic partial oxidation and endothermic steam reforming of n-octane. It was found that Ni/Al₂O₃ catalyst activity was high at high temperatures and increased with the Ni loadings. Its activity, however, was not obviously increased when Ni loadings were over 5.0 wt%. Compared with nickel catalyst, the bimetallic catalyst of Ni-Pd/A1₂O₃ showed markedly increased activity and hydrogen selectivity at experimental conditions. The catalytic performance also became more stable when the palladium was added, which indicated that palladium plays an essential role in the catalytic action. The used catalysts of Ni-Pd/A1₂O₃ were regenerated three times by using air at space velocity of 2,000 h⁻¹ to obtain a long duration catalyst. Also, the typical catalyst was characterized by using SEM, BET, TG and ICP methods in detail.     

Partial oxidation of methane over rhodium catalysts for power generation applications

The partial oxidation of methane (POM) to syngas, i.e. H₂ and CO, over supported Rh catalysts was investigated at atmospheric pressure. The influence of support material, Rh loading and the presence of water vapor on the methane conversion efficiency and the product gas composition was studied. The catalysts containing ceria in the support material showed the highest activity and formation of H₂ and CO. By increasing the Rh loading, a decrease of the ignition temperature was obtained. The addition of water vapor to the reactant gas mixture was found to increase the ignition temperature and the formation of hydrogen, which is favorable for combustion applications where the catalytic POM stage is followed by H₂-stabilized homogeneous combustion.     

Reaction and oxygen permeation studies in Sm0.4Ba0.6Fe0.8Co0.2O3 − δ membrane reactor for partial oxidation of methane to syngas

A disk-type Sm_0.4Ba_0.6Co_0.2Fe_0.8O_3 − δ perovskite-type mixed-conducting membrane was applied to a membrane reactor for the partial oxidation of methane to syngas (CO + H₂). The reaction was carried out using Rh (1 wt%)/MgO catalyst by feeding CH₄ diluted with Ar. While CH₄ conversion increased and CO selectivity slightly decreased with increasing temperature, a high level of CH₄ conversion (90%) and a high selectivity to CO (98%) were observed at 1173 K. The oxygen flux was increased under the conditions for the catalytic partial oxidation of CH₄ compared with that measured when Ar was fed to the permeation side. We investigated the reaction pathways in the membrane reactor using different membrane reactor configurations and different kinds of gas. In the membrane reactor without the catalyst, the oxygen flux was not improved even when CH₄ was fed to the permeation side, whereas the oxygen flux was enhanced when CO or H₂ was fed. It is implied that the oxidation of CO and H₂ with the surface oxygen on the permeation side improves the oxygen flux through the membrane, and that CO₂ and H₂O react with CH₄ by reforming reactions to form syngas.     

Catalytic combustion over platinum group catalysts: fuel-lean versus fuel-rich operation

Performance data are presented for methane oxidation on alumina-supported Pd, Pt, and Rh catalysts under both fuel-rich and fuel-lean conditions. Catalyst activity was measured in a micro-scale isothermal reactor at temperatures between 300 and 800 °C. Non-isothermal (near adiabatic) temperature and reaction data were obtained in a full-length (non-differential) sub-scale reactor operating at high pressure (0.9 MPa) and constant inlet temperature, simulating actual reactor operation in catalytic combustion applications.

Under fuel-lean conditions, Pd catalyst was the most active, although deactivation occurred above 650 °C, with reactivation upon cooling. Rh catalyst also deactivated above 750 °C, but did not reactivate. Pt catalyst was active above 600 °C. Fuel-lean reaction products were CO₂ and H₂O for all three catalysts.

The same catalysts tested under fuel-rich conditions demonstrated much higher activity. In addition, a ‘lightoff’ temperature was found (between 450 and 600 °C), where a stepwise increase in reaction rate was observed. Following ‘lightoff’ partial oxidation products (CO, H₂) appeared in the mixture, and their concentration increased with increasing temperature. All three catalysts exhibited this behavior.

High-pressure (0.9 MPa) sub-scale reactor and combustor data are shown, demonstrating the benefits of fuel-rich operation over the catalyst for ultra-low emissions combustion.     

Oxidation and reforming reactions of CH4 on a stepped Pt(5 5 7) single crystal

The oxidation and reforming kinetics of methane by O₂, CO₂ and H₂O were studied on a stepped Pt(5 5 7) single crystal from 623 to 1050 K under methane rich conditions. The rate of carbon deposition was followed by ex-situ Auger electron spectroscopy under non-oxidative conditions. The apparent activation energy for methane decomposition was significantly lower than the apparent barriers measured for both total oxidation, CO₂ and H₂O reforming. Total oxidation of methane to CO₂ and H₂O followed by combined dry and steam reforming (combined combustion-reforming) led to CO production rates which were higher than direct CO₂ or H₂O reforming rates. The enhanced rates are most likely due to the ability of adsorbed oxygen to prevent carbon nucleation and/or scavenge carbon enabling the reforming reaction to turnover on a larger fraction of sites. Comparable amounts of carbon were found by Auger electron spectroscopy measurements after both direct dry or steam reforming, while combined oxidation-reforming had considerable less carbon. During direct dry or steam reforming, CO₂ and H₂O serve only to scavenge adsorbed atomic carbon, while in the presence of oxygen, carbon is removed by both combustion and reforming routes.     

Partial oxidation of CH4 and C2H6 over noble metalcoated monoliths

The direct partial oxidation of hydrocarbons offers promising alternatives to chemical synthesis. By replacing endothermic processes such as steam reforming and steam cracking, fast and exothermic oxidation reactions should require much smaller and simpler reactors. However, direct oxidation reactions are much more difficult to manage because of potential heat release in total oxidation and hazardous because of the possibility of homogeneous reactions which are nonselective and can produce flames and explosions. We describe experiments in which monolith catalysts are used for partial oxidation of CH₄ and C₂H₆ to produce synthesis gas or alkenes by direct oxidation at or above atmospheric pressure in pure O₂ in nearly adiabatic reactors operating at 1000°C with very high flowrates (space velocities of 10⁶h⁻¹ and residence times of 10⁻³ s). With methane oxidation we obtain over 90% selectivities to synthesis gas (a 2:1 H₂:CO mixture) with> 90% conversion of the methane and complete conversion of O₂ on Rh coated ceramic monoliths with contact times of 10⁻³ s. With Pt catalysts under the same conditions, the H₂ selectivity drops to 70%; while with Pd, the catalyst rapidly forms carbon. This process appears to be primarily a surface reaction in which CH4 pyrolyzes on the hot Rh surface and the H atoms dimerize and the carbon is oxidized to CO. A model has been constructed which accurately predicts the conversions and selectivities and the variations between Rh and Pt. With higher alkanes, synthesis gas is produced on Rh with comparable selectivities and conversions on metal-coated monoliths. However, with Pt we observe up to 70% selectivity to alkenes with 80% conversion of alkanes at adiabatic temperatures near 1000°C with approximately 5 ms contact times. These results can be explained as occurring by predominantly surface reactions in which the alkane adsorbs to form the alkyl by H abstraction with adsorbed O atoms. Then the adsorbed alkyls undergo primarily β-elimination reactions on Pt to produce alkenes. These products are therefore far from thermodynamic equilibrium at these very short contact times, even though the temperatures are very high. The use of very short contact times and high temperatures promises to provide new routes to production of partial oxidation products with very small adiabatic reactors and thus opens up new types of reactions and reactors for chemical synthesis.