Study of Bulk and Surface Reduction by Hydrogen of CexZr1-xO2 Mixed Oxides Followed by FTIR Spectroscopy and Magnetic Balance

Abstract

The CeO2 and CexZr1-xO2 reduction by hydrogen was studied using IR spectroscopy to follow the evolution of the ν(OH) vibrational modes and a magnetic balance to estimate the global reduction percentage from the magnetic susceptibility. H2 was introduced between 298 and 873 K on activated samples. On ceria, different OH groups exist depending on cerium unsaturation. In particular, the band due to OH type II shifts toward higher frequencies when ceria is reduced. It is therefore possible to monitor the surface oxidation state of ceria or mixed oxides through the ν(OH) band wavenumber. For ceria, the surface reduction begins at around 573 K. That leads to the formation of OH(I) species and adsorbed H2O which are observed at the beginning of the reduction. Their elimination leads to the creation of surface O-vacancies. At higher temperatures, there is a surface/subsurface reorganization through the reverse migration of O-vacancies and O-ions from the bulk. However, the adsorption sites are conserved during the evacuation step, the bonded OH, OH(II−B), and OH(II-A) species disappearing by evacuation in the range 773−873 K through H2 (and not H2O) evolution. For mixed CexZr1-xO2 compounds, the most interesting OH species are those adsorbed on cerium ions. The results lead to the conclusion that the mechanism of reduction is the same as in the case of pure ceria but that an increased mobility of bulk oxygen (in relation with the Zr content) for mixed compounds allows surface oxygen vacancies to be faster filled from O migration in subsurface underlayers. The hydrogen reduction was also followed in a magnetic balance in the case of Ce0.5Zr0.5O2 mixed oxide. The Ce3+ content obtained at different temperatures confirms that the surface reduction is easier for the mixed oxide, the hydrogen chemisorption occurring for T > 373 K and the reduction of Ce4+ into Ce3+ ions beginning at 473 K. Moreover, the better reducibility of the bulk that is observed (76% of Ce3+ at 873 K for the mixed oxide instead of 17% for ceria) evidences the higher oxygen mobility in the bulk, in good agreement with the FTIR conclusions.