Morphology effect of ceria on the performance of CuO/CeO2 catalysts for hydrogen production by methanol steam reforming

Nano-rod(R), nano-particle(P) and sponginess(S) of ceria samples were used to study catalytic performance of hydrogen production by methanol steam reforming. The samples were prepared by hydrothermal method, precipitation method, and sol-gel method, respectively, and the CuO was supported on the different morpholopy of CeO₂ samples by wet impregnation. SEM, TEM, XRD, XRF, BET, H₂-TPR, XPS and N₂O titration methods were used to study correlation between the structure and the catalytic performance for methanol steam reforming. The results showed that the morphology of the prepared CeO₂ support dramatically influenced the performance of catalysts. Due to the stronger interaction between copper oxide and ceria support, the CuO/CeO₂-R catalyst had exhibited the better catalytic activity than those of the CuO/CeO₂P and CuO/CeO₂S catalysts. Moreover, higher Cu dispersion, lower reduction temperature of CuO, and higher content of active species Cu⁺ were also advantageous to raising catalytic effects. Besides, with the highest content of surface Ce³⁺, the CuO/CeO₂-R had estimated the content of oxygen vacancy on the surface of the catalyst. The existence of surface oxygen vacancy had a positive effect on the methanol steam reforming.     

Dry reforming over mesoporous nanocrystalline 5% Ni/M-MgAl2O4 (M: CeO2, ZrO2, La2O3) catalysts

In this article mesoporous nanocrystalline 5 wt%M-95 wt%MgAl₂O₄ (M: CeO₂, ZrO₂, La₂O₃) powders were prepared by a novel on-step sol-gel process and employed as a support for the synthesis of 5 wt%Ni catalysts for synthesis gas production via dry reforming. The magnesium aluminate spinel prepared with this sol-gel method possessed a high BET area of 264 m² g⁻¹ with a high pore volume of 0.436 cm³ g⁻¹. The results indicated that the addition of promoters (CeO₂, ZrO₂, La₂O₃) to magnesium aluminate improved the BET area and pore volume and also decreased the crystallite size. Among the prepared powders and catalysts, 5 wt%La₂O₃-95 wt%MgAl₂O₄ and 5 wt%Ni/5 wt%CeO₂-95 wt%MgAl₂O₄ exhibited the highest BET area of 306 m² g⁻¹ and 263 m² g⁻¹, respectively. The catalytic results indicated that the 5 wt%Ni/5 wt%CeO₂-95 wt%MgAl₂O₄ catalyst exhibited the highest activity and the lowest carbon formation among the prepared catalysts with the same content of the promoter. The influence of the CeO₂ content on the textural and catalytic performance was also investigated and the results illustrated that the increment in CeO₂ content improved the methane conversion and reduced the amount of deposited carbon, which could be related to the redox properties of the catalyst support.     

Methanation of carbon dioxide over Ni/CeO2 catalysts: Effects of support CeO2 structure

The CeO₂, which were prepared by hard-template method, soft-template method, and precipitation method, were used as support to prepare Ni/CeO₂ catalysts (named as NCT, NCS, and NCP catalysts, respectively). The prepared catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and Brunauer–Emmett–Teller (BET). Hydrogen temperature-programmed reduction (H₂-TPR) was also used to study the reducibility of the support nickel precursors. Moreover, CO₂ catalytic hydrogenation methanation was used to investigate the catalytic properties of the prepared NCT, NCS, and NCP catalysts. H₂-TPR and XRD results showed that the NiO can be reduced by H₂ to produce metal Ni species, and the surface oxygen species existing on the surface of the support CeO₂ can also be reduced by H₂ to form surface oxygen vacancies. Low-angle XRD, TEM, and BET results indicated that the NCT and NCS catalysts had developed mesoporous structure and high specific surface area of 104.7 m² g^?1 and 53.6 m² g^?1, respectively. The NCT catalyst had the highest CO₂ methanation activity among the studied NCT, NCS, and NCP catalysts. The CO₂ conversion and CH₄ selectivity of the NCT catalyst can reach 91.1% and 100% at 360 °C and atmospheric pressure. The NCP catalyst, which had low specific surface area and low porosity, performed less CO₂ conversion and higher CH₄ selectivity than the NCT and NCS catalysts till 400 °C.     

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.     

Hydrogen production with a low carbon monoxide content via methanol steam reforming over CuxCeyMgz/Al2O3 catalysts: Optimization and stability

Catalytic activities of Ce–Mg promoted Cu/Al₂O₃ catalysts via methanol steam reforming was investigated in terms of the methanol conversion level, carbon monoxide selectivity and hydrogen yield. The factors chosen were the reaction temperature, copper content, Mg/(Ce + Mg) weight-percentage and steam to carbon ratios. The catalysts were prepared by co-precipitation and characterized by means of XRD, BET, H₂-TPR, and FESEM. The Ce–Mg bi-promoter catalysts gave higher performance due to magnesium penetration into the cerium structure causing oxygen vacancy defects on the ceria. A response-surface-model was then designed to optimize the condition at a 95% confidence interval for complete methanol conversion to a high H₂ yield with a low CO content, and revealed an optimal copper level of 46–50 wt%, Mg/(Ce + Mg) of 16.2–18.0%, temperature of 245–250 °C and S/C ratio of 1.74–1.80. No deactivation of the Cu_0.5Ce_0.25Mg_0.05/Al catalyst was observed during a 72-h stability test.     

Comparative study of CuO supported on CeO2, Ce0.8Zr0.2O2 and Ce0.8Al0.2O2 based catalysts in the CO-PROX reaction

CuO supported on CeO_2, Ce_0.8Zr_0.2O₂ and Ce_0.8Al_0.2O₂ based catalysts (6%wt Cu) were synthesized and tested in the preferential oxidation of CO in a H₂-rich stream (CO-PROX). Nanocrystalline supports, CeO₂ and solid solutions of modified CeO₂ with zirconium and aluminum were prepared by a freeze-drying method. CuO was supported by incipient wetness impregnation and calcination at 400 °C. All catalysts exhibit high activity in the CO-PROX reaction and selectivity to CO₂ at low reaction temperature, being the catalyst supported on CeO₂ the more active and stable. The influence of the presence of CO₂ and H₂O was also studied.     

CO2 methanation over CoNi bimetal-doped ordered mesoporous Al2O3 catalysts with enhanced low-temperature activities

The Ni based catalysts have been considered as potential candidates for the CO₂ methanation owing to the low cost. However, the poor low-temperature catalytic activities limit their large-scale industrial application. In order to address this challenge, a series of CoNi bimetal doped ordered mesoporous Al₂O₃ materials have been designed and fabricated via the one-pot evaporation induced self-assembly strategy and employed as the catalysts for CO₂ methanation. It is found that the large specific surface areas (up to 260.0 m^2/g), big pore volumes (up to 0.59 cm³/g), and narrow pore size distributions of these catalysts have been successfully retained after 700 °C calcination. The Co and Ni species are homogenously distributed among the Al₂O₃ matrix due to the unique advantage of the one-pot synthesis strategy. The strong interaction between metal and mesoporous framework have been formed and the severely thermal sintering of the metallic CoNi active centers can be successfully inhibited during the processes of catalyst reduction and 50 h CO₂ methanation reaction. More importantly, the synergistic effect between Co and Ni can greatly enhance the low-temperature catalytic activity by coordinating the activation of H₂ and CO₂, prominently decreasing the activation energy toward CO₂ methanation. As a result, their low-temperature activities are evidently promoted. Furthermore, the effect of the Co/(Co + Ni) molar percentage ratio on the catalytic property has been also systematically investigated over these catalysts. It is found that only the catalyst with appropriate ratio (20.0%) behaves the optimum catalytic performances. Therefore, the current CoNi based ordered mesoporous materials promise potential catalysts for CO₂ methanation.     

Synthesis of nanocrystalline mesoporous Ni/Al2O3SiO2 catalysts for CO2 methanation reaction

A series of nanocrystalline mesoporous Ni/Al₂O₃SiO₂ catalysts with various SiO₂/Al₂O₃ molar ratios were prepared by the sol-gel method for the carbon dioxide methanation reaction. The synthesized catalysts were evaluated in terms of catalytic performance and stability. The catalysts were studied using XRD, BET, TPR and SEM. The BET results indicated that the specific surface area of the samples with composite oxide support changed from 254 to 163.3 m²/g, and an increase in the nickel crystallite size from 3.53 to 5.14 nm with an increment of Si/Al molar ratio was visible. The TPR results showed a shift towards lower temperatures, indicating a better reducibility and easier reduction of the nickel oxide phase into the nickel metallic phase. Furthermore, the catalyst with SiO₂/Al₂O₃ molar ratio of 0.5 was selected as the optimal catalyst, which showed 82.38% CO₂ conversion and 98.19% CH₄ selectivity at 350 °C, high stability, and resistivity toward sintering. Eventually, the optimal operation conditions were specified by investigating the effect of H₂/CO₂ molar ratio and gas hourly space velocity (GHSV) on the catalytic behavior of the denoted catalyst.