Unusual Redox Behavior of Ceria and Its Interaction with Hydrogen

Abstract

The reaction between ceria and hydrogen has been subject to numerous theoretical and experimental studies due to its importance as a catalytic material. Here we present dynamic and reversible evolution of the cerium oxidation states observed through X-ray Absorption Spectroscopy experiments in addition to the investigation of associated lattice expansion and contraction through X-ray diffraction and PDF methods. Employing a novel calculation of the temperature dependence of the Gibbs free energy through consideration of the relationship between the instantaneous thermal lattice expansion and the rate of change of the cerium oxidation state, the unusual redox chemistry is reported here. This unusual behaviour is interpreted as due to the formation of a metastable cerium oxyhydride as suggested.

Temperature reversible synergistic formation of cerium oxyhydride and Au hydride: a combined XAS and XPDF study

“…From this method the initial proportion of Ce(III) is found to be 1%, whilst at 623 K 26 AE 2% Ce(III) is formed. These results are in line with our previous publication 54 suggesting a stoichiometry of HCe 4 O 8 at 623 K. After cooling in H 2 there is approximately 7% of the Ce(III) remaining. This may be due to remaining a partial extraction of oxygen from the ceria lattice or by remaining surface or subsurface hydroxyl groups.…”

“…Therefore it can be noted that the exposure to hydrogen leads to the formation of Ce(III). However, as with our previous findings on a high surface area ceria material, 54 upon cooling the majority of formed Ce(III) is re-oxidised to Ce(IV) without the presence of an oxidising atmosphere. This effect can be clearly seen by the fact that area of peak E, shaded blue, at 623 K is greater than after cooling to RT.…”

“…While peak fitting examples for the Ce L 3 edge XANES is commonly employed for standard transmission XANES analysis, [61][62][63][64] such an approach has not routinely been employed for analysis of HERFD-XANES. The association of the peaks to Ce(III) and Ce(IV) species was made in comparison to a NIST ceria reference material and previously reported associations and electronic transitions in the literature, 54,[64][65][66] and illustrated in Fig. 1.…”

“…This behaviour has been associated to the formation of Ce(III) ions within the ceria lattice due to the interaction with hydrogen. 54 A non-linear lattice expansion cannot be explained solely by thermal expansion of the ceria lattice. However, the rate of the Ce(III) formation is noted to be somewhat faster for the Au containing sample.…”

“…Our previous work on the formation of a cerium oxyhydride species demonstrated that this species is lost upon cooling. 54 Whilst there have been numerous studies that point towards the potential formation of complex hydride species in Au and ceria containing materials, to date there are very limited examples of studies probing the dynamic structure of such materials. Therefore, to gain further understanding of the reactivity of supported Au catalysts we have used a combined XAS, at the Ce L 3 -edge and X-ray total scattering study.…”