Effect of zirconium addition on the structure and properties of CuO/CeO2 catalysts for high-temperature water–gas shift in an IGCC system

A series of Ce_xZr_1−xO₂-based Cu catalysts was synthesized by the co-precipitation method. The influences of copper content, zirconium addition, and ratio of ceria to zirconia on the catalytic activity were investigated. BET, N₂O decomposition, XRD, TEM, SEM, EDS, Raman spectroscopy, H₂-TPR, TG/DTA, and XPS were used to characterize the catalysts. The catalytic activity was tested in terms of CO conversion and H₂ selectivity in H₂-rich coal-derived synthesis gas, simulating the actual gas composition of an integrated gasification combined cycle (IGCC) system. The long-term catalyst stability was also examined at 450 °C for 196 h. The addition of zirconium was found to be very important in enhancing catalytic performance. The surface area, copper dispersion, oxygen storage and mobility capacity, reducibility, as well as resistance to sintering all improved after zirconium addition.     

CeO2-promoted Ni/activated carbon catalysts for the water–gas shift (WGS) reaction

The low temperature water–gas shift (WGS) reaction has been studied over carbon-supported nickel catalysts promoted by ceria. To this end, cerium oxide has been dispersed (at different loadings: 10, 20, 30 and 40 wt.%) on the activated carbon surface with the aim of obtaining small ceria particles and a highly available surface area. Furthermore, carbon- and ceria-supported nickel catalysts have also been studied as references. A combination of N₂ adsorption analysis, powder X-ray diffraction, temperature-programmed reduction with H₂, X-ray photoelectron spectroscopy and TEM analysis were used to characterize the Ni–CeO₂ interactions and the CeO₂ dispersion over the activated carbon support. Catalysts were tested in the low temperature WGS reaction with two different feed gas mixtures: the idealized one (with only CO and H₂O) and a slightly harder one (with CO, CO₂, H₂, and H₂O). The obtained results show that there is a clear effect of the ceria loading on the catalytic activity. In both cases, catalysts with 20 and 10 wt.% CeO₂ were the most active materials at low temperature. On the other hand, Ni/C shows a lower activity, this assessing the determinant role of ceria in this reaction. Methane, a product of side reactions, was observed in very low amounts, when CO₂ and H₂ were included in the WGS feed. Nevertheless, our data indicate that the methanation process is mainly due to CO₂, and no CO consumption via methanation takes place at the relevant WGS temperatures. Finally, a stability test was carried out, obtaining CO conversions greater than 40% after 150 h of reaction.     

Promotion effect of Nb for Cu/CeO2 water–gas shift reaction catalyst by generating mobile electronic carriers

A series of CuO/CeO₂ catalysts doping with Nb₂O₅ were fabricated by co-precipitation method. It is found that the introduction of Nb⁵⁺ will result in the substitution of Ce⁴⁺ with Nb⁵⁺, thus creating mobile electronic carriers in the as-prepared catalysts. The characterization results correlating with the catalytic activity evaluation disclose that the catalyst added with 1 wt. % Nb₂O₅ shows the most mobile electronic carries, certain amount of weak, medium basic sites and enhanced reducibility and chemical adsorption of CO, thus the best catalytic activity for water–gas shift reaction. However, excessive Nb₂O₅ addition prevents the incorporation of Cu²⁺ into CeO₂ lattice and partially covers the surface of CuO and CeO₂, resulting in weaken their reducibility and interaction between them, thus leading to inferior catalytic performance.     

Modified precipitation processes and optimized copper content of CuO–CeO2 catalysts for water–gas shift reaction

The precipitation processes of CuO–CeO₂ catalysts preparation were modified, and their optimized copper content were also studied in detail. As indicated by the experimental results, the WGS catalytic activities of the CuO–CeO₂ catalysts can be ranked as: stepwise precipitation (SP) > deposition–precipitation (DP) > co-precipitation (CP), suggesting that stepwise precipitation (i.e., a modified DP) is a convenient and effective method to prepare CuO–CeO₂ WGS catalysts. The optimized copper contents of CuO–CeO₂–SP and CuO–CeO₂–CP catalysts are 20 wt.% and 25 wt.%, respectively. Their catalytic activities can be strongly correlated with the results from XRD, XPS, Raman, N₂-physisorption, N₂O chemisorption and H₂-TPR. For DP and SP, a certain amount of copper has substitutively incorporated into ceria lattice with multiple Ce³⁺ and oxygen vacancies, which is considered as one relatively strong interaction between copper and ceria support. The substitutional incorporation of copper creates larger lattice distortion, lattice defect (embodying as larger lattice cell contraction of CeO₂, microstrain and Raman shift) and stronger reducibility (i.e., lower reduction temperature of H₂-TPR), as a consequence, higher surface energy and catalytic activity for WGS reaction. While for CP, copper nitrate and cerium nitrate were simultaneously precipitated, resulting in the burial of many copper species in ceria supports, i.e., occupancy incorporation of isolated copper into ceria lattice vacant site, which is considered as another weak interaction between copper and ceria support. Accordingly, combined with N₂O chemisorption results, it is demonstrated that surface copper species of CuO–CeO₂–CP are fewer and less active than those of CuO–CeO₂–DP and CuO–CeO₂–SP. Lastly, there is a direct relationship between the pore volume along with most probable pore size of the as-synthesized catalysts and their corresponding catalytic activities for WGS reaction.     

Water–gas shift reaction in a Pd membrane reactor over Pt/Ce0.6Zr0.4O2 catalyst

A water–gas shift (WGS) Pt/Ce_0.6Zr_0.4O₂ catalyst has been prepared, which exhibits much faster kinetics than conventional high-temperature ferrochrome catalysts in the temperature range most suitable for operation of WGS Pd membrane reactors, i.e. above 623 K. The performance of the Pt catalyst was tested in a reactor furnished with a supported, 1.4 μm thick high-flux Pd membrane using feeds obtained by autothermal reforming of natural gas. CO conversion remained above thermodynamic equilibrium up to feed space velocities of 9100 l kg⁻¹ h⁻¹ at 623 K, P_total = 1.2 MPa and steam-to-carbon ratio S/C = 3, but H₂ recovery decreased from 84.8% at GHSV = 4050 l kg⁻¹ h⁻¹ to 48.7% at the highest space velocity. This rapid decline of separation performance is attributed to slow H₂ diffusion through the catalyst bed, suggesting that external mass flow resistance has a significant impact on the H₂ permeation rate in such membrane reactors. This could be minimized by the development of WGS catalysts with even faster kinetics which would allow further reduction of the catalyst bed height.     

Cu/MnOx–CeO2 and Ni/MnOx–CeO2 catalysts for the water–gas shift reaction: Metal–support interaction

Monometallic copper and nickel catalysts supported on cerium-manganese mixed oxides are prepared, characterized and evaluated for the Water–Gas Shift (WGS) reaction. Active metal loading of 2.5 wt% and 7.5 wt% are used to impregnate MnO_x–CeO₂ supports with 30% and 50% Mn:Ce molar ratio. The structure of the samples strongly depends on both the active metal employed and the manganese content in the mixed support. For both Cu and Ni samples, the best catalytic behavior is found in samples supported on the MnO_x–CeO₂ oxides with 30% Mn:Ce molar ratio, as a result of the presence of Cu_xMn_yO₄ spinel-type phases in the case of copper catalysts and the presence of a NiMnO₃ mixed oxide with defect ilmenite structure in the case of nickel catalysts.     

Effect of pretreatment on Pt–Co/C cathode catalysts for the oxygen reduction reaction

Carbon supported Pt and Pt–Co electrocatalysts for the oxygen reduction reaction in low temperature fuel cells were prepared by the reduction of the metal salts with sodium borohydride and sodium formate. The effect of surface treatment with nitric acid on the carbon surface and Co on the surface of carbon prior to the deposition of Pt was studied. The catalysts where Pt was deposited on treated carbon the ORR reaction preceded more through the two electron pathway and favored peroxide production, while the fresh carbon catalysts proceeded more through the four electron pathway to complete the oxygen reduction reaction. NaCOOH reduced Pt/C catalysts showed higher activity that NaBH₄ reduced Pt/C catalysts. It was determined that the Co addition has a higher impact on catalyst activity and active surface area when used with NaBH₄ as reducing agent as compared to NaCOOH.     

Ce–K-promoted Co–Mo/Al2O3 catalysts for the water gas shift reaction

The effect of doping traditional Co–Mo/Al₂O₃ catalysts with Ce and K on their catalytic activity for the water gas shift reaction in coke oven gas was investigated. Doped Co–Mo/Al₂O₃ catalysts were prepared by adding different amounts of Ce and K (CeO₂, K₂O, and CeO₂–K₂O ∼10 wt%) by a wetness impregnation method and characterized by BET specific surface area measurements and scanning electron microscopy (SEM). The characterization results reveal that CeO₂ addition mainly produced an electronic effect and aided to disperse the active ingredient. At the same time, the synergistic effect between Ce and K contributed to the catalytic activity. Activity tests showed that Ce–K-promoted Co–Mo/Al₂O₃ catalysts exhibited greater activity and selectivity than Co–Mo–Ce/Al₂O₃ catalysts and Co–Mo–K/Al₂O₃ catalysts. The maximum promotion of the water gas shift reaction was observed when 3.0 wt% CeO₂ and 6.0 wt% K₂O were added.     

Effect of H2S on the performance of La0.7Ce0.2FeO3 perovskite catalyst for high temperature water–gas shift reaction

The effect of H₂S on the performance of La_0.7Ce_0.2FeO₃ perovskite catalyst was investigated for the production of hydrogen from simulated coal-derived syngas via the water–gas shift reaction at 600 °C and 1 atm. The results show that the catalyst activity decreases with increasing concentrations of H₂S up to 1100 ppm, but the negative effect of H₂S on its activity is reversible. However, even at the high H₂S concentrations catalyst activity is still greater than that measured with sour shift catalyst. Overall, the results indicate that La_0.7Ce_0.2FeO₃ perovskite catalyst has a high degree of H₂S tolerance, particularly in the low H₂S concentration regime.     

H2 production from a single stage water–gas shift reaction over Pt/CeO2, Pt/ZrO2, and Pt/Ce(1−x)Zr(x)O2 catalysts

To develop a single stage water–gas shift reaction (WGS) catalyst for compact reformers, Pt/CeO₂, Pt/ZrO₂, and Pt/Ce_(1−x)Zr_(x)O₂ catalysts have been applied for the target reaction. The CeO₂/ZrO₂ ratio was systematically varied to optimize Pt/Ce_(1−x)Zr_(x)O₂ catalysts. Pt/CeO₂ showed the highest turnover frequency (TOF) and the lowest activation energy (E_a) among the catalysts tested in this study. It has been found that the reduction property of the catalyst is more important than the dispersion for a single stage WGS. Pt/CeO₂ catalyst also showed stable catalytic performance. Thus, Pt/CeO₂ can be a promising catalyst for a single stage WGS for compact reformers.     

Highly efficient Au/ZrO2 catalysts for low-temperature water–gas shift reaction: Effect of pre-calcination temperature of ZrO2

A series of Au/ZrO₂ catalysts with low content of gold (<1 wt.%) were prepared by deposition–precipitation method and evaluated in the low-temperature water–gas shift reaction (WGSR) under H₂-rich reformate atmosphere. The effect of pre-calcination temperature of ZrO₂ on the structural and surface properties of the freshly reduced Au/ZrO₂ catalysts was investigated by XRD, N₂-physisorption, HRTEM, UV–Vis DRS, AAS, EPR and XPS characterizations. The catalytic evaluation results reveal that the one supported on ZrO₂ calcined at 350 °C shows the best catalytic performance. Correlating to the characterization results, it is found that the catalytic performance of Au/ZrO₂ catalysts strongly depends on the concentration of F-center defects on ZrO₂ surface. EPR and XPS results disclose that electrons can be transferred from F-center to the supported Au, resulting in the formation of activated electron-rich Au cluster, which is responsible for the high catalytic activity of the Au/ZrO₂ catalysts. Moreover, the electron transfer leads to the firm anchoring of Au cluster by the F-center, thus favoring a high catalytic stability. It is proposed that the active site for WGSR on the Au/ZrO₂ catalysts can be expressed as Au^δ−[V_o⁻]Zr³⁺, where [V_o⁻] represents an F-center.     

CuO/ZrO2 catalysts for water–gas shift reaction: Nature of catalytically active copper species

A series of CuO/ZrO₂ catalysts with different Cu loadings (4.1, 6.1 and 8.4 wt.%) were synthesized by a deposition-precipitation method and evaluated with the water–gas shift (WGS) reaction. In order to distinguish the different supported copper oxide species, (NH₄)₂CO₃-leaching process was conducted on the three CuO/ZrO₂ catalysts. The parent and leached catalysts were characterized by ICP-OES, XPS, XRD, N₂-physisorption, N₂O titration, UV–Vis DRS, H₂-TPR and CO-TPR. The results reveal that three types of copper oxide species are present on the parent CuO/ZrO₂ catalysts: (α) highly dispersed CuO that is weakly bound with ZrO₂; (β) strongly bound Cu-[O]-Zr species, which can not be leached by (NH₄)₂CO₃ solution and is possibly associated with the surface oxygen vacancy of ZrO₂; (γ) crystalline CuO. The XRD and TEM results of the freshly reduced catalysts disclose that the three types of CuO species are transformed into their corresponding metallic states after the H₂-pretreatment. It is found that the reaction rate correlates well with the amount of Cu-[O]-Zr species, indicating the metallic Cu derived from this species should be the catalytically active copper species for the WGS reaction. Moreover, CO-TPR results disclose that the Cu-[O]-Zr species play a significant role in promoting the reactivity of the surface hydroxyl groups, as is thought to be responsible for the high WGS activity of the CuO/ZrO₂ catalysts.     

Cu/ZnO/Al2O3 water–gas shift catalysts for practical fuel cell applications: the performance in shut-down/start-up operation

The Cu/ZnO/Al₂O₃ catalysts were prepared by the coprecipitation method, and were evaluated in the water–gas shift (WGS) reaction. The effects of the calcination temperature on the BET surface area and crystallite size were characterized. In WGS reaction, the Cu/ZnO/Al₂O₃ catalysts suffered from continuous deactivation in shut-down/start-up operation – the daily requirement for mobile and residential fuel cell systems. Among them, the Cu/ZnO/Al₂O₃ catalyst prepared at the calcination temperature of 450 °C showed the best activity and stability, with the decrement of the CO conversion of only 12.8% after three shut-down/start-up cycles. Deactivation of the Cu/ZnO/Al₂O₃ catalysts is attributed to the blocking or deterioration of the active sites by Zn₆Al₂(OH)₁₆CO₃·4H₂O resulting from the degeneration of the oxides under cyclic operations. Removal of the hydroxycarbonate species by calcination in air followed by re-reduction could restore the steady-state WGS activity; however, the regenerated catalyst underwent much severe deactivation in subsequent shut-down/start-up operation.     

The optimization of Nb loading amount over Cu–Nb–CeO2 catalysts for hydrogen production via the low-temperature water gas shift reaction

The effect of Nb promotion over a Cu–CeO₂ catalyst was investigated in the low-temperature water gas shift reaction. The Nb loading amount was systematically varied from 0 to 5 wt% for the Cu–Nb–CeO₂ catalyst, and the 1 wt% Nb promoted Cu–Nb–CeO₂ catalyst exhibited the highest catalytic performance even at extremely high GHSV of 72,152 h⁻¹. The catalysts were characterized through various techniques such as Brunauer-Emmet-Teller measurements, X-ray diffraction, N₂O-chemisorption, H₂-temperature programmed reduction, X-ray photoelectron spectroscopy, and transmission electron microscopy. It was found that the superior performance of the 1 wt% Nb promoted Cu–Nb–CeO₂ catalyst was due to its enhanced reducibility, high BET surface area, small metallic Cu crystallite size, and high number of oxygen vacancies.     

Water–gas shift reaction over Pt and Pt–CeOx supported on CexZr1−xO2

The water–gas shift (WGS) reaction was examined over Pt and Pt–CeO_x catalysts supported on Ce_xZr_1−xO₂ (Ce_0.05Zr_0.95O₂, Ce_0.2Zr_0.8O₂, Ce_0.4Zr_0.6O₂, Ce_0.6Zr_0.4O₂, Ce_0.7Zr_0.3O₂ and Ce_0.8Zr_0.2O₂) under severe reaction conditions, viz. 6.7 mol% CO, 6.7 mol% CO₂, and 33.2 mol% H₂O in H₂. The catalysts were characterized with several techniques, including X-ray diffraction (XRD), CO chemisorption, temperature-programmed reduction (TPR) with H₂, temperature-programmed oxidation (TPO), inductively coupled plasma-atomic emission spectroscopy (ICP-AES) and bright-field transmission electron microscopy (TEM). Among the supported Pt catalysts tested, Pt/Ce_0.4Zr_0.6O₂ showed the highest WGS activity in all temperature ranges. An improvement in the WGS activity was observed when CeO_x was added with Pt on Ce_xZr_1−xO₂ supports (x = 0.05 and 0.2) due to intimate contact between Pt and CeO_x species. Based on CO chemisorptions and TPR profiles, it has been found that the interaction between Pt species and surface ceria-zirconia species is beneficial to the WGS reaction. A gradual decrease in the catalytic activity with time-on-stream was found over Pt and Pt–CeO_x catalysts supported on Ce_xZr_1−xO₂, which can be explained by a decrease in the Pt dispersion. The participation of surface carbonate species on deactivation appeared to be minor because no improvement in the catalytic activity was found after the regeneration step where the aged catalyst was calcined in 10 mol% O₂ in He at 773 K and subsequently reduced in H₂ at 673 K.     

Effect of Pt–Pd/C coupled catalyst loading and polybenzimidazole ionomer binder on oxygen reduction reaction in high-temperature PEMFC

Polybenzimidazole (PBI) was studied as an ionomer binder at varying ratios (1–7) in a 20–40 wt% Pt–Pd/C cathode-coupled catalyst layer for the oxygen reduction reaction (ORR) in a high-temperature proton exchange membrane fuel cell (HT-PEMFC). Catalytic activity was examined by CV and LSV, while the properties of the catalysts were characterized by FESEM-EDX, N₂ adsorption–desorption, XRD and FTIR. The results showed that the distribution of metals on the carbon surface, carbon wall thickness and the interaction between ionomer and coupled catalysts affected the ORR performance. The fabricated membrane electrode assembly with 5:95 PBI: 30 wt% Pt–Pd/C catalyst ratio exhibited the best performance and highest durability for HT-PEMFC at 170 °C, yielding a power density of 1.30 Wcm⁻² with 0.02 mg_Pt/cm Pt loading. This performance of ultra-low metal loading of coupled Pt–Pd/C electrocatalyst with PBI binder was comparable to those reported by other studies, highlighting a promising catalyst for fuel cell application.     

Effect of MgO/Al2O3 ratio in the support of mesoporous Ni/MgO–Al2O3 catalysts for CO2 utilization via reverse water gas shift reaction

2 and 5 wt.% nickel was supported on different MgO to Al₂O₃ (M/A) ratios (0.5, 1 and 1.5) and evaluated in reverse water gas shift (RWGS) reaction. The catalysts were prepared by impregnation method and the nanocrystalline supports were synthesized by simple surfactant (CTAB) assisted precipitation technique. The following catalytic activity was observed for 2% & 5% Ni supported on different M/A ratios; M/A = 1 > M/A = 1.5 > M/A = 0.5. The perceived order was related to difference in the structural properties of supports and catalysts. The BET results revealed decrease of specific surface area with increase in M/A ratio, mesoporous structure for M/A = 0.5 and 1 and meso-macroporous structure for M/A = 1.5. The effect of nickel loading on the support with M/A = 1 was also investigated. 1.5% Ni showed high CO₂ conversion of 39.2% at 700 °C and CO selectivity higher than 90% at all temperatures. Increase of nickel loading higher than 1.5% was in favor of CH₄ formation. The TEM images of 1.5% Ni on M/A = 1 revealed uniform distribution of Ni particles with average size of 4.9 nm. The H₂-TPR analysis displayed shifting of maximum temperature of the main peak (γ) to higher temperatures with increase of M/A ratio in the support, indicating harder reducibility of catalysts with higher MgO content. The 1.5% Ni supported on M/A = 1 (MgAl₂O₄) showed great catalytic stability and CO selectivity (>98%) after 15 h on stream.     

Production of syngas via autothermal reforming of methane in a fluidized-bed reactor over the combined CeO2–ZrO2/SiO2 supported Ni catalysts

Production of syngas via autothermal reforming of methane (MATR) in a fluidized bed reactor was investigated over a series of combined CeO₂–ZrO₂/SiO₂ supported Ni catalysts. These combined CeO₂–ZrO₂/SiO₂ supports and supported Ni catalysts were characterized by nitrogen adsorption, XRD, NH₃-TPD, CO₂-TPD and H₂-TPR. It was found that the combined supports integrated the advantages of SiO₂ and CeO₂, ZrO₂. That is, they have bigger surface area (about 300 m²/g) than pure CeO₂ and ZrO₂, stronger acidity and alkalescence than that of pure SiO₂, and enhanced the mobility of H adatoms. Ni species dispersed highly on these combined CeO₂–ZrO₂/SiO₂ supports, and became more reducible. Ni catalysts on the combined supports possess higher CO₂ adsorption ability, higher methane activation ability and exhibited higher activity for MATR. H₂/CO ratio in product gas could be controlled successfully in the range of 0.99–2.21 by manipulating the relative concentrations of CO₂ and O₂ in feed.     

SO3 decomposition over CuO–CeO2 based catalysts in the sulfur–iodine cycle for hydrogen production

The effect of several catalyst supports with large specific surface area (such as SiC, Al₂O₃, SiC–Al₂O₃–ball, and SiC–Al₂O₃) on catalytic activity was evaluated in this study. CuO–CeO₂ supported on SiC–Al₂O₃ exhibited high stability and activity, which was considerably close to the thermodynamic equilibrium curve at 625 °C during the stability test for 50 h. The SO₃ decomposition temperature decreased from 750 °C to 625 °C. SiC–Al₂O₃contained numerous micropores and mesopores and had a large specific area, indicating strong adsorption, as determined by transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), and nitrogen adsorption measurement. X-ray photoelectron spectroscopy (XPS) revealed that the surface of SiC–Al₂O₃consisted of Al₂O₃, SiC, and SiO₂ and that the cerium oxide surface had the largest number of defects. Temperature-programmed reduction (H₂-TPR) results indicated that the cerium–copper oxides on the surface of powdered SiC–Al₂O₃ had the strongest redox potential and that CuO had the lowest reduction temperature.     

Development and performance of a K–Pt/γ-Al2O3 catalyst for the preferential oxidation of CO in hydrogen-rich synthesis gas

Proton exchange membrane fuel cells are widely employed in micro combined heat and power cogeneration (micro-CHP) systems, and the feed to them should be essentially free of CO. CO preferential oxidation is an effective method for the thorough removal of CO from synthesis gas. A series of K–Pt/γ-Al₂O₃ catalysts are prepared and tested for their CO cleaning capabilities. The catalyst is prepared from potassium nitrate acid, chloroplatinic acid and γ-Al₂O₃ powder by normal or ultrasonic impregnation. The catalyst performance is investigated in a micro-reactor system. The effects of K loading, Pt loading, ultrasonic processing, space velocity, O₂-to-CO ratio and operation temperature on catalyst performance are studied. A CO concentration of less than 10 ppm is achieved when the CO concentration in the feed gas is 0.45%. It was found that both ultrasonic processing and the addition of K promote the catalyst performance. The 15K1.0Pt/Al–U catalyst exhibits the best performance.