Oxygen Mobility in Pre-Reduced Nano- and Macro-Ceria with Co Loading: An AP-XPS, In-Situ DRIFTS and TPR Study

The size effect of ceria nanoparticles on surface oxygen mobility and formation of surface oxygen vacancies in ethanol steam reforming was investigated. Higher concentration of Ce3+ surface sites of the ceria nano-particles (similar to 4 nm, NP) was observed compared to the micro-particles (similar to 120 nm, MP). Similarly, studies using ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) exhibited higher intensity of O1s assigned to the hydroxyl group bonding to Ce3+ and significantly lower intensity for lattice oxygen, stemming from an increase in the number of oxygen vacancies and enhanced oxygen mobility in the nano-ceria under ethanol steam reforming conditions. The presence of fully reduced cobalt particles (Co-o) facilitated ceria reduction through hydrogen spillover. The comparison of cerium oxidation states between pre-reduced CeO2 and pre-reduced Co/CeO2 indicated higher extent of reduction of cerium in the case of Co/CeO2 for ethanol steam reforming. These results, together with our previous investigations where higher Ce3+ was observed over CeO2 compared to Co/CeO2 after pre-oxidation treatments, indicate that the initial state of cobalt in Co/CeO2 affects the oxidation state of cerium. Lastly, both nano-ceria and micro-ceria bare supports showed moderate C-C cleavage activities in ethanol steam reforming where better activity was observed over nano-ceria. Formate species were observed predominantly in the DRIFTS spectra of the nano-ceria whereas major species were acetates for micro-ceria. The dissimilarity in the reaction network was attributed to the difference in the number of surface oxygen vacancies. The Co/CeO2-NP catalyst was found more active compared to Co/CeO2-MP with higher hydrogen yield and ethanol conversion.