TiO2 and ZrO2 supported Ru catalysts for CO mitigation following the water-gas shift reaction

CO oxidation and methanation over Ru-TiO₂ and Ru-ZrO₂ catalysts were investigated for CO removal for applications in proton exchange membrane fuel cells. The catalysts were synthesised by the deposition precipitation method at a pH of 7–7.5 for better interactions between the support and the active Ru metal. Various characterization experiments such as TPR, XPS, FTIR-CO, CO chemisorption and HRTEM were conducted to better understand the physio-chemical properties of Ru on the supports. Both catalysts showed excellent activity for the total oxidation of CO, however, with the addition of H₂, the catalysts activity to CO oxidation decreased significantly. Higher temperatures for the preferential oxidation reaction indicated that the Ru catalysts not only oxidize CO, but hydrogenate it as well. Furthermore, H₂ oxidation was favoured over the catalysts. Hydrogenation of CO over these catalysts gave high CO conversion and selectivity towards CH₄. Both the catalysts showed similar activity across the temperature range screened and gave maximum CO conversions of 99.9% from 240 °C onwards, with 99.9% selectivity towards CH₄. The catalysts also showed good stability in the reaction and the similarities in the catalytic activity of these were attributed to the well-dispersed Ru metal over the supports. The Ru catalysts effectively reduced CO concentrations in the reformate gas to less than 10 ppm, as is required for practical applications.     

Mn-induced Cu/Ce catalysts with improved performance for CO preferential oxidation in H2/CO2-rich streams

A series of xMnCu/Ce catalysts with constant low Cu loading of 1 wt% were prepared by the simple impregnation method. The obtained catalysts were characterized by XRD, BET, H₂-TPR and XPS, and the preferential oxidation of CO was evaluated in CO₂/H₂-rich atmospheres. It was shown that partial Mn and Cu could be incorporated into the Ceria lattice, forming surface ternary Cu–Mn–Ce oxide solid solutions. At Mn/Cu = 0.6, the catalyst presented strong interaction among Cu, Mn and Ce, had more Ce³⁺ and Mn⁴⁺ at the surface and showed the best catalytic performance, making CO conversion increase of 23.57% at 90 °C as compared with the Cu/Ce catalyst. For CO-Prox, the highest CO conversion was 94.7% with an oxidation selectivity of 78.9% at 125 °C. At this temperature, the catalyst revealed stable catalytic performance for a total TOS of 205 h. In addition, with CO/Ar as feed gas, CO conversion was 100%, confirming the negative effects of CO₂/H₂.     

Pt hierarchical structure catalysts on BaTiO3/Ti electrode for methanol and ethanol electrooxidations

Electrooxidations of methanol and ethanol have been investigated on different Pt catalytic titanium-supported electrodes in both acidic and alkaline media using cyclic voltammetry. BaTiO₃ is used for the first time to make a nanoscaled roughness on the surface of Ti foil in order to effectively deposit Pt hierarchical structure and block foulness in solution reactions. The morphology of BaTiO₃ nanocube on Ti foil, Pt catalysts deposited on BaTiO₃/Ti and Ti foil electrodes are characterized by field emission scanning electron microscopy. The results indicate that Pt nanoflowers can be effectively grown on the Ti foil covered with 1 μm layer of BaTiO₃ nanocubes and the catalytic oxidation behaviors to methanol and ethanol are much better than those of the Pt/Ti electrode as Pt nanoparticles can hardly be deposited on the smooth surface of the Ti foil. The Pt/BaTiO₃/Ti electrode could be adopted as excellent catalytic anode in fuel cells.     

Improving temperature uniformity and performance of CO preferential oxidation for hydrogen-rich reformate with a heat pipe

Preferential oxidation (PROX) is an effective, but highly temperature-sensitive, method of CO removal for hydrogen-rich reformates. In a packed-bed catalytic reactor, oxidation is strongest at the inlet side and the local catalyst pellets become over-heated with poor heat conduction. As a result, the enhanced parasitic H₂ oxidation consumes oxygen and suppresses CO conversion. This study applies a heat pipe to improve the temperature uniformity in a tubular one-stage packed-bed reactor by transporting heat downstream and thereby improve CO removal. In the experiments, the fuel mixture containing 2% of CO, 75% of H₂, and 23% of CO₂, further mixed with air at O₂/CO = 0.75, 1.0 or 1.25, is supplied with stepwise increase of feeding rate under a fixed environmental temperature of 99 ± 1 °C. The proposed simple method is found to significantly improve temperature uniformity and CO removal for the present test conditions with O₂/CO = 1.0 and 1.25.     

Preferential oxidation of CO in a H2-rich stream over multi-walled carbon nanotubes confined Ru catalysts

Multi-walled carbon nanotubes (MWNTs) confined Ru catalysts were prepared by a modified procedure using ultrasonication-aided capillarity action to deposit Ru nanoparticles onto MWNTs inner surface. The structure properties of MWNTs supports and Ru catalysts were extensively characterized by XRD, TGA, H₂-TPR, XPS, TEM, FTIR and Raman spectra. The catalytic performance in the preferential oxidation of CO in a H₂-rich stream was examined in detail with respect to the influences of Ru loading, MWNTs diameter, various pretreatment conditions, and the presence of CO₂ and H₂O in the feed stream. In contrast with Ru catalysts supported on MWNTs external surface and other carbon materials, the superior activity was observed for the MWNTs-confined Ru catalyst, which was discussed intensively in terms of the confinement effect of carbon nanotubes. The optimized catalyst of 5 wt.% Ru confined in MWNTs with diameter of 8–15 nm can achieve the complete CO conversion in the wider temperature range and the favorable stability at 80 °C under the simulated reformatted gas mixture, which proves a promising catalyst for preferential CO oxidation in H₂-rich stream.     

Preferential oxidation of CO in H2-rich feeds over mesoporous copper manganese oxides synthesized by a redox method

Mesoporous copper manganese oxides with high surface areas (>268 m²/g) were prepared using the redox method and tested in the preferential oxidation of CO. These materials were highly active and selective under typical operating conditions of a proton-exchange membrane fuel cell. The synthesized catalysts preferentially oxidized CO with a stoichiometric amount of oxygen in the feed gas. The presence of CO₂ and H₂O in the feed gas retarded catalytic activity significantly at low (<90 °C) temperatures. The catalysts showed stable activity in long-term (12 h) experiments with realistic feeds. The high catalytic activity was attributed to a combination of factors, including high surface area, low crystallinity, low activation energy for CO oxidation, compositional homogeneity of the copper manganese oxides, and the presence of readily available lattice oxygen for CO oxidation. The high selectivity (100% with stoichiometric reactants) was ascribed to the lower activation energy for CO oxidation compared to the activation energy for H₂ oxidation.     

Effect of the preparation method on the performance of CuO-MnOx-CeO2 catalysts for selective oxidation of CO in H2-rich streams

Selective oxidation of CO in H₂-rich streams is performed over a series of CuO-MnO_x-CeO₂ catalysts prepared by hydrothermal (CuMC-HY), co-precipitation (CuMC-CP), impregnation (CuMC-IM) and citrate sol-gel (CuMC-SG) methods. The catalysts are characterized by N₂ adsorption/desorption, XRD, SEM, HR-TEM, TPR and XPS techniques. The results show that the catalyst prepared by a hydrothermal method exhibits the best catalytic activity, especially at low temperatures. The temperature of 50% CO conversion (T₅₀) is only 74 °C and the temperature window of CO conversions up to 99.0% is about 40 °C wide, from 110 to 140 °C. Moreover, the temperature window is still maintained 20 °C wide even at lower temperatures when there are 15% CO₂ and 7.5% H₂O in the reaction gas. The superior catalytic performance of CuMC-HY is attributed to the formation of Mn-Cu-Ce-O solid solution, the unique pore structure and the existence of more Cu⁺ and Mn⁴⁺ species as well as oxygen vacancies. The sequence of catalytic activity is as follows: CuMC-HY > CuMC-SG > CuMC-IM > CuMC-CP. The worst catalytic activity, obtained from the catalyst prepared by the co-precipitation method, is possibly related to the existence of independent CuO_x and MnO_x oxides, which weakly interact with ceria in the catalyst.     

Performance and stability of La0.5Sr0.5CoO3−δ perovskite as catalyst precursor for syngas production by partial oxidation of methane

The aim of this work was to investigate the performance and stability of the perovskite La_0.5Sr_0.5CoO_3−δ, as a potential catalyst precursor, for the synthesis gas production by partial oxidation of methane. For this purpose, the catalytic activity of La_0.5Sr_0.5CoO_3−δ was studied as a function of the temperature, flow rate and feed composition. In addition, its stability with the time-on-stream and redox cycles was also explored. Before and after testing, the catalyst precursor was characterized by X-ray diffraction, SEM-EDX and specific surface area (BET). The results evidenced a remarkable catalytic activity due to the stability of the cobalt, which is in a highly disperse state, in its reduced state. The CH₄ conversion and the CO and H₂ selectivities were enhanced with the increase of redox cycles. Finally, the precursor was totally regenerated to the initial perovskite structure under a specific thermal treatment.     

Effect of gas-phase reaction on catalytic reaction for H2/O2 mixture in micro combustor

The catalytic reaction characteristics of the hydrogen and oxygen mixture in the catalytic micro-combustor were detected by thermocouple, infrared thermal imager and OH-PLIF. The equivalence ratio of transition stage from coupling reaction (coupled homogeneous-heterogeneous combustion) to pure catalytic reaction (heterogeneous combustion) was obtained. The three-dimensional pure catalytic reaction model with detailed catalytic reaction mechanism was established in a rectangular catalytic micro-combustor. The simulation model was validated with experimental data. The effects of the intermediate and final products from gas-phase reactions on the pure catalytic reaction were discussed as well as the effects of gas-phase reactions on the catalytic reaction in the process of coupling reactions were studied. The intermediate product OH radical can improve the hydrogen conversion of the surface reaction, and the effect of O radical is not obvious. The final product H₂O has an inhibitory effect on the surface reaction. Since the mass fraction of H₂O is much higher than other gas-phase reaction products, the dominant effect of the gas-phase reaction on the catalytic reaction is suppression. In the coupling reactions, the fuel consumed by the gas-phase reaction weakened the catalytic reaction.     

Autothermal reforming of JP8 on a Pt/Rh catalyst: Catalyst durability studies and effects of sulfur

Autothermal reforming (ATR) of commercial grade JP8 was performed on a Pt/Rh catalyst deposited on a monolith. This study investigated catalyst performance under three test conditions: (i) 120 startup and shutdown cycles, (ii) 80 h of continuous operation with sulfur-free fuel, and (iii) 370 h of testing with JP8 containing 125 ppm of sulfur. Axial reactor temperature profiles and gas composition data showed that startup and shutdown cycling had no impact on catalyst performance. When durability testing was done with fuel containing 125 ppm of sulfur, the catalyst deactivated initially, which was reflected by a decrease in H₂ concentration and decrease in fuel conversion. However, after 250 h of operation the activity stabilized at 66% fuel conversion and product concentrations were constant for the remaining 120 h of testing. The presence of sulfur resulted in higher CO selectivity, lower H₂ concentrations, and lower fuel conversions compared to data with sulfur-free fuel. The data suggests that the presence of sulfur primarily affects steam reforming reactions, and CO oxidation. Regeneration was attempted with air and with fuel-lean combustion but initial H₂ yields and carbon selectivity were not achieved.     

Flame characteristics in H2/CO synthetic gas diffusion flames diluted with CO2: Effects of radiative heat loss and mixture composition

Numerical study is conducted to understand the impact of fuel composition and flame radiation in flame structure and their oxidation process in H₂/CO synthetic gas diffusion flame with and without CO₂ dilution. The models of Sun et al. and David et al., which have been well known to be best-fitted for H₂/CO synthetic mixture flames, are evaluated for H₂/CO synthetic mixture flames diluted with CO₂. Effects of radiative heat loss to flame characteristics are also examined in terms of syngas mixture composition. Importantly contributing reaction steps to heat release rate are compared for the synthetic gas mixture flames of high contents of H₂ and CO, individually, with and without CO₂ dilution. The modification of the oxidation pathways is also addressed.     

Design of a novel hydrogen–syngas catalytic mesh combustor

A small-scale wire-mesh catalytic combustor is developed and evaluated for hydrogen–syngas combustion in domestic power/heating generator. The single- and double-layer wire-mesh catalysts are tested to verify their performance on CO conversions. Experimental results indicate that the double-layer wire-mesh catalytic combustor yields a higher CO conversion ratio (>90%) than that (<40%) of the single-layer wire-mesh catalyst in the range of fuel concentrations, fuel compositions, and flow velocities studied. In order to maintain a stable heterogeneous/homogeneous reaction at the second stage of wire-mesh catalyst, a minimum of 4% hydrogen in syngas and at least 200 °C of preheating temperature on the second wire-mesh catalyst are suggested. The advantages of the wire-mesh combustor are its compactness and ease of assembly and cleaning.     

Experimental studies on syngas catalytic combustion on Pt/Al2O3 in a microreactor

Synthesis gas (a mixture of CO and H₂) oxidation is studied over a supported Pt/Al₂O₃ catalyst in a novel microreactor fabricated for studying the intrinsic chemical kinetics of highly exothermic reactions. CO was found to significantly inhibit H₂ oxidation. In contrast, H₂ addition promotes CO oxidation at low mole fractions but has a small promoting effect at high hydrogen mole fractions. As a result, the apparent reaction order of H₂ changes from positive to zero. The change in hydrogen reaction order is associated with hysteresis. Possible mechanisms for the observed behavior are discussed.     

Homogeneous combustion of fuel-lean H2/O2/N2 mixtures over platinum at elevated pressures and preheats

The gas-phase combustion of H₂/O₂/N₂ mixtures over platinum was investigated experimentally and numerically at fuel-lean equivalence ratios up to 0.30, pressures up to 15 bar and preheats up to 790 K. In situ 1-D spontaneous Raman measurements of major species concentrations and 2-D laser induced fluorescence (LIF) of the OH radical were applied in an optically accessible channel-flow catalytic reactor, leading to the assessment of the underlying heterogeneous (catalytic) and homogeneous (gas-phase) combustion processes. Simulations were carried out with a 2-D elliptic code that included elementary hetero-/homogeneous chemical reaction schemes and detailed transport. Measurements and predictions have shown that as pressure increased above 10 bar the preheat requirements for significant gas-phase hydrogen conversion raised appreciably, and for p = 15 bar (a pressure relevant for gas turbines) even the highest investigated preheats were inadequate to initiate considerable gas-phase conversion. Simulations in channels with practical geometrical confinements of 1 mm indicated that gas-phase combustion was altogether suppressed at atmospheric pressure, wall temperatures as high as 1350 K and preheats up to 773 K. While homogeneous ignition chemistry controlled gaseous combustion at atmospheric pressure, flame propagation characteristics dictated the strength of homogeneous combustion at the highest investigated pressures. The decrease in laminar burning rates for p ? 8 bar led to a push of the gaseous reaction zone close to the channel wall, to a subsequent leakage of hydrogen through the gaseous reaction zone, and finally to catalytic conversion of the escaped fuel at the channel walls. Parametric studies delineated the operating conditions and geometrical confinements under which gas-phase conversion of hydrogen could not be ignored in numerical modeling of catalytic combustion.     

Preferential oxidation of carbon monoxide in simulated reformatted gas over PtAu/CexZnyO2 catalysts

A series of Pt–Au catalysts prepared by co-precipitation (CP) and single step sol-gel (SSG) methods was investigated for selective CO oxidation. The characteristics of the prepared catalysts were determined by XRD, BET surface area, SEM, H₂-TPR, chemisorption analysis, and FTIR. The simulated reformatted gas consisted of 1% CO, 1% O₂, 0% to 10% H₂O, 0–20% CO₂, and 40% H₂ in He balance. The operating temperature range was varied from 50 °C to 190 °C at atmospheric pressure. The experimental results elucidated that the catalytic preparation method had a significant effect on the catalyst characteristics and its activity. The catalytic performance over PtAu/Ce₁Zn₁O₂ prepared by co-precipitation was higher than that of PtAu/CeO₂ and PtAu/ZnO because of the incorporation of Ce⁴⁺ ions and the Zn²⁺ ions in the lattice. To encourage better catalytic performance, the catalysts should be calcined at 500 °C for 5 h and pretreated in a H₂ atmosphere. The CO conversion for the single- and double-stage reaction was reduced when adding water vapor and CO₂ to the feedstream; the water vapor and CO₂ molecules compete for the adsorption with CO on the active sites of the catalysts. During the deactivation test for 60 h, the CO conversion and selectivity are maintained.     

Calcium silicate-based catalytic filters for partial oxidation of methane and biogas mixtures: Preliminary results

Development and testing of catalytic filters for partial oxidation of methane to increase hydrogen production in a biomass gasification process constitute the subject of the present study. Nickel, iron and lanthanum were coated on calcium silicate filters via co-impregnation technique, and catalytic filters were characterized by ICP-MS, XPS, XRD, TEM, TGA, TPR and BET techniques. The influences of varying reaction temperature and addition of Fe or La to Ni-based catalytic filters on methane conversion, and hydrogen selectivity have been investigated in view of preliminary results obtained from reactions with 6% methane-nitrogen mixture, and catalytic filters were tested with model biogas mixtures at optimum reaction temperature of each filter which were 750 °C or 850 °C. Approximately 93% methane conversion was observed with nearly 6% methane-nitrogen mixture, and 97.5% methane conversion was obtained with model biogas containing CH₄ which is 6%, CO₂, CO, and N₂ at 750 °C. These results indicate that calcium silicate provides a suitable base material for catalytic filters for partial oxidation of methane and biogas containing methane.     

Activated carbon incorporated with first-row transition metals as catalysts in hydrogen production from propane

Transition metals (V, Mn, Fe, Co and Ni) were mixed with bituminous coal and carbonized at 600 °C to obtain activated carbons (AC) with different catalytic and physical properties. A variety of analytical techniques - XRD, SEM, FTIR and BET surface area - were used to characterize the samples. The presence of different proportions of crystalline structures and surface functional groups were observed depending on the metal added in the carbon matrix. The samples surface areas changed from 140.9 to 296.0 m²/g for V- and Ni-AC, with corresponding increase in pore volumes and diameters. There are graphitic and metal-carbon phases as evidenced by XRD. The aromatic stacking appears to decrease from V- to Co-AC. V-AC exhibited fewer occurrences of turbostratic carbon interlayers. In the FTIR spectra, region assigned to aromatic CC stretching and CH-bending have been observed. Their relative proportions diminish based on the nature of the transition metal used. The samples were tested as catalysts at 450-550 °C using a feed mixture of propane and argon - in the presence or absence of CO₂ in the feed stream. Propylene selectivities per unit area increase along the groups of the metals. Ni-AC exhibited the best performance per gram in terms of propane conversions (1.5-17.7%) and selectivities to propylene (39.9-49.0%) in the temperature range. Partial substitution of argon with CO₂ in the feed exhibited general increase in propane conversion with no significant change in propylene selectivities.     

The study of Au/MoS2 anode catalyst for solid oxide fuel cell (SOFC) using H2S-containing syngas fuel

Au/MoS₂ is a promising anode catalyst for conversion of all components of H₂S-containing syngas in solid oxide fuel cell (SOFC). MoS₂-supported nano-Au particles have catalytic activity for conversion of CO when syngas is used as fuel in SOFC systems, thus preventing poisoning of MoS₂ active sites by CO. In contrast to use of MoS₂ as anode catalyst, performance of Au/MoS₂ anode catalyst improves when CO is present in the feed. Current density over 600 mA cm⁻² and maximum power density over 70 mW cm⁻² were obtained at 900 °C, showing that Au/MoS₂ could be potentially used as sulfur-tolerant catalyst in fuel cell applications.     

Highly stable M/NiO–MgO (M = Co,Cu and Fe) catalysts towards CO2 methanation

NiO–MgO nanocomposites are synthesized using solution combustion, sonochemical, and co-precipitation synthesis to understand the catalytic activity of CO₂ methanation. Excellent particle size distribution was noticed with the sonochemical routed synthesis method, and the CO₂ conversions are found to be better with the same synthesis protocol. Surface modifications in NiO–MgO composite were incorporated by doping M (M = Co, Fe, and Cu). The active catalysts are characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to understand physical, structural properties and surface morphology of the nanocomposites. All catalysts showed excellent catalytic activity for the conversion of CO₂ to methane and selectivity towards methane to be higher than 85%. However, 2%Co/NiO–MgO showed the lowest activation energy of about 43 ± 2 kJ mol⁻¹ among other synthesized catalysts. The mechanism of CO₂ methanation was investigated with the inputs from temperature programming reduction with H₂ (H₂-TPR), and temperature programming desorption with CO₂ (CO₂-TPD) studies. Detailed reaction mechanism and kinetics are investigated for all doped catalysts. M/NiO–MgO offered excellent stability up to 50 h reaction time with high CO₂ conversions and CH₄ selectivities.     

LaNiO3@SiO2 core–shell nano-particles for the dry reforming of CH4 in the dielectric barrier discharge plasma

LaNiO₃@SiO₂ core–shell nano-particles were prepared by coating LaNiO₃ nano-particles with SiO₂ and then employed to catalyze the dry reforming of CH₄ to produce syngas (CO + H₂) in a coaxial dielectric barrier discharge (DBD) plasma reactor under ambient conditions. Compared to the traditional Ni-based catalysts (Ni/SiO₂, LaNiO₃/SiO₂ and LaNiO₃), LaNiO₃@SiO₂ exhibited better catalytic performance in the dry reforming of CH₄ in DBD plasma reactor, such as higher reactant conversion and product selectivity, and excellent catalytic stability. The conversions of CH₄ and CO₂ reached 88.31% and 77.76%, and selectivities of CO and H₂ were 92.43% and 83.65%, respectively. Results manifested the core–shell structure endowed LaNiO₃@SiO₂ with excellent catalytic performance because the SiO₂ shell was capable of preventing Ni from sintering and mitigating carbon deposition during the reaction.