Influence of nanoscale surface arrangements on the oxygen transfer ability of ceria-zirconia mixed oxide

Ceria-based materials, and particularly CeO2-ZrO2 (CZ) solid solutions are key ingredient in catalyst formulations for several applications due to the ability of ceria to easily cycling its oxidation state between Ce4+ and Ce3+. Ceria-based catalysts have a great soot oxidation potential and the mechanism deeply relies on the degree of contact between CeO2 and carbon. In this study, carbon soot has been used as solid reductant to better understand the oxygen transfer ability of ceria-zirconia at low temperatures; the effect of different atmosphere and contact conditions has been investigated. The difference in the contact morphology between carbon soot and CZ particles is shown to strongly affect the oxygen transfer ability of ceria; in particular, increasing the carbon-ceria interfacial area, the reactivity of CZ lattice oxygen is significantly improved. In addition, with a higher degree of contact, the soot oxidation is less affected by the presence of NOx. The NO oxidation over CZ in the presence of soot has also been analyzed. The existence of a core/shell structure strongly enhances reactivity of interfacial oxygen species while affecting negatively NO oxidation characteristics. These findings are significant in the understanding of the redox chemistry of substituted ceria and help determining the role of active species in soot oxidation reaction as a function of the degree of contact between ceria and carbon

Exsolution-enhanced reverse water-gas shift chemical looping activity of Sr2FeMo0.6Ni0.4O6-δ double perovskite

This study investigates the structural evolution and redox characteristics of the double perovskite Sr2FeMo0.6Ni0.4O6-δ (SFMN) during hydrogen (H2) and carbon dioxide (CO2) redox cycles and explores the material performance in the Reverse Water-Gas Shift Chemical Looping (RWGS-CL) reaction. In-situ and ex-situ X-Ray Diffraction (XRD) and High-Resolution Transmission Electron Microscopy (HRTEM) studies reveal that H2 reduction at temperatures above 800 °C leads to the exsolution of bimetallic Ni-Fe alloy particles and the formation of a Ruddlesden-Popper (RP) phase. A core–shell structure with Ni-Fe core and a perovskite oxide shell is formed with subsequent redox cycles, and the resulting material exhibits better performance and high stability in the RWGS-CL process. Thermogravimetric (TGA) and Temperature Programmed Reduction (TPR) and Oxidation (TPO) analyses show that the optimal reduction and oxidation temperatures for maximizing the CO yield are around 850 °C and 750 °C respectively, and that the cycled material is able to work steadily under isothermal conditions at 850 °C

A Temperature Programmed and Transient Kinetic Study of CO2 Activation and Methanation over CeO2 supported Noble Metals

The interaction of CO2 with noble metals deposited on ceria surfaces have been studied by temperature-programmed techniques. A transient kinetic study of CO2 activation and methanation over M/CeO2 has also been conducted. The mechanism of interaction between M/CeO2 and CO2 and its activation in the presence of Hz to CH4 is strongly influenced by the reduction temperature, regardless of the metal employed. It is suggested that, by increasing the reduction temperature, a progressive reduction of bulk CeO2 takes place, which is not promoted by the presence of the metal. The interaction mechanism suggested involves activation of CO2 on a surface Ce3+ site with formation of CO followed by oxidation of Ce3+ to Ce4+. The presence of oxygen bulk vacancies will create the additional driving force for the reduction of CO2 to CO and/or surface carbonaceous species which then rapidly hydrogenate to CH4 over the supported metal

CO and CO2 Hydrogenation in Transient Conditions over Rh/CeO2: Novel Positive Effect of Metal-Support Interaction on Catalytic Activity and Selectivity

The high temperature reduction at 773 K of Rh-CeO2 catalysts induced a transient Rh-CeO2 interaction which enhances the rate of CO and CO2 hydrogenation; however, the activity dropped rapidly as a result of formation of water, which reoxidised CeO2 - x, restoring the normal behaviour