H2 Dissociation and Oxygen Vacancy Formation on Ce2O3 Surfaces

Abstract

H 2 dissociation on ceria (CeO 2) has attracted much attention in the last years because of its potential application in catalysis for hydrogenation reactions, as well as for the stabilization of hydride bulk and surface species. The ability of ceria to split hydrogen is strongly dependent on the surface morphology. However, to the best of our knowledge, the reactivity of the cerium sesquioxide Ce 2 O 3 Atype (hexagonal structure with space group 3 1) has not been previously addressed. In the present study, we investigate (i) the formation of oxygen vacancies in ACe 2 O 3 bulk and (ii) the effect of the surface topology in the H 2 dissociation and in the oxygen vacancy formation for the four most stable surfaces of ACe 2 O 3 : (0001), (01-11), (11-20) and (11-21). Our results indicate a significant decrease of the energetic barrier for the hydrogen dissociation compared to stoichiometric CeO 2 , with an activation energy of ~0.1 eV. Interestingly, Ce 2 O 3 surfaces lead to a heterolytic product with hydride species more stable than the homolytic product, which is the opposite behavior found in CeO 2. These results suggest a better performance of Ce 2 O 3 than CeO 2 for H 2 dissociation and provide insight in the nature of hydride-Ce 2 O 3 interfaces that could be important intermediates in the formation of CeH x phases from cerium oxide.

Influence of Mixed Valence on the Formation of Oxygen Vacancy in Cerium Oxides

“…The lower oxygen vacancy formation energy of 5.4 eV was selected in the following discussions. Our calculated results are consistent with the non-reducible character of Ce2O3, requiring the large energy (> 5.0 eV) to form an oxygen vacancy [24]. In the mixed valence state model, there exist five possible inequivalent oxygen-vacancy sites, as shown in Figure 1.…”

“…We performed non-spin polarized calculations for these systems. The optimized structure parameters are listed in Table 1, together with the experimental and other theoretical values [14,[20][21][22][23][24][25]…”

Raman Scattering from Cerium Hydride Growth Centers Overlayered by Hydrogen-Incorporated Oxide

“…After discussing the hydride growth center and the overlayered hydrogen-incorporated layer we turn to the mutual effect of the two constituents. In terms of catalytic properties, the accessibility of hydrogens was shown to significantly affect the catalytic performance of hydrides. − Can we exploit the unique spectroscopic signature of hydrogen incorporation into the overlayer oxide as a probe to processes that occur underneath it?…”

“…A major driving force to the study of the interaction between H 2 and cerium oxides lies in elucidating their catalytic properties. − ,− The role of oxygen stoichiometry in cerium oxides is expressed in the better-calculated performance of Ce 2 O 3 than that of CeO 2 for H 2 dissociation, while providing insight into the nature of hydride–Ce 2 O 3 interfaces . Hydrogen stoichiometry in cerium hydrides, CeH 2+ x , is also a key factor in their catalytic activity on the hydrogenation of ethylene, as revealed by the essential presence of high-stoichiometry hydrogens (beyond those of the “inactive” dihydrides).…”

“…11−15,20−22 The role of oxygen stoichiometry in cerium oxides is expressed in the better-calculated performance of Ce 2 O 3 than that of CeO 2 for H 2 dissociation, while providing insight into the nature of hydride−Ce 2 O 3 interfaces. 23 Hydrogen stoichiometry in cerium hydrides, CeH 2+x , is also a key factor in their catalytic activity on the hydrogenation of ethylene, 24 as revealed by the essential presence of high-stoichiometry hydrogens (beyond those of the "inactive" dihydrides 24 ). Hence, the essential existence of a hydrogen reservoir in the vicinity of the active site encouraged efforts of exploring "symbiotic CeH 2+x /CeO 2 catalysts" 25 by combining ab initio calculations and electron microscopy.…”