Role of Low-Coordinated Ce in Hydride Formation and Selective Hydrogenation Reactions on CeO₂ Surfaces

Abstract

Catalysts based on ceria exhibit high activity toward selective hydrogenation reactions. There has been much debate on the catalytic mechanisms, especially on the production of hydride (H–) species, which serve as the key species for hydrogenation reactions. Previous studies illustrated that the hydride species are usually formed at oxygen vacancy sites of reduced CeO2 surfaces, and the stoichiometric surfaces are believed to be inactive. In this work, we performed extensive density functional theory calculations corrected by on-site Coulombic interaction (DFT + U) to investigate the mechanisms of H2 dissociation on the various stoichiometric CeO2 surfaces, including the low-index (111) and (100) surfaces and the high-index (221), (223), and (132) ones. We find that the H– species can be generated via H2 heterolytic dissociation on the various CeO2 surfaces, and the stability of the hydride species increases with the decrease of the coordination number of the surface Ce. This is mainly because the repulsive electrostatic interaction between the H– species adsorbed at the low-coordinated Ce species and its surrounding species is much less and it is, therefore, more favorable to occur than the H– species adsorbed at the relatively high-coordinated Ce. In addition, the low-coordinated Ce3+ species can have a relatively high-lying energy level of the localized 4f electron and tend to donate the electron to the adsorbed H to produce a hydride. Moreover, through calculations of the key reaction steps, we showed that the as-formed metastable H– species can regulate the catalytic activity and selectivity for CO2 hydrogenation by preferentially producing HCOO* intermediates.

Acetylene hydrogenation catalyzed by bare and Ni doped CeO2(110): the role of frustrated Lewis pairs

“…On Ni-CeO 2 (110)-O v , hence, our results suggest that H 2 prefers to dissociate via Path I. The barrier (0.41 eV) of this path is 0.29 eV higher than that on CeO 2 (110)-O v , but is about 0.1 eV lower than that (0.50 eV) 23 on Ni-CeO 2 (111)-O v and lower than that of the heterolytic dissociation on the o-terminated step site of CeO 2 (111) (0.48 to 0.73 eV), 45,46 showing the efficiency of Ni-CeO 2 (110)-O v .…”

“…On Ni-CeO 2 (110)-2O v , the barrier is 1.14 eV, again indicating the high stability of the Ce-H hydride. In addition, these barriers (0.73 and 1.14 eV) are much higher than those of the lowcoordinated Ce hydride migration (0 to 0.55 eV) 46 on the step sites of CeO 2 (111). Based on these results, we thus conclude that the hydride formed by the heterolytic dissociation of H 2 is stable and the subsequent C 2 H 2 hydrogenation step on bare and Ni doped CeO 2 (110) proceeds with the hydride.…”

“…[35][36][37][38][39] It is well established that the catalytic activity of ceria is facet dependent. 11,13,14,[40][41][42][43][44][45][46] For example, the heterolytic dissociation of H 2 on stoichiometric CeO 2 (110) is easier than that on the CeO 2 (111) counterpart and the stability of the hydride species is related to the coordination number of Ce on various CeO 2 surfaces such as CeO 2 (221), CeO 2 (223), and CeO 2 (132). 19,[45][46][47] Vile ´et al 13 reported that CeO 2 (111) was more active than CeO 2 (100) for acetylene hydrogenation.…”

“…11,13,14,[40][41][42][43][44][45][46] For example, the heterolytic dissociation of H 2 on stoichiometric CeO 2 (110) is easier than that on the CeO 2 (111) counterpart and the stability of the hydride species is related to the coordination number of Ce on various CeO 2 surfaces such as CeO 2 (221), CeO 2 (223), and CeO 2 (132). 19,[45][46][47] Vile ´et al 13 reported that CeO 2 (111) was more active than CeO 2 (100) for acetylene hydrogenation. Chang and coauthors found that CeO 2 (110) and CeO 2 (100) with O v s are efficient catalysts for the hydrogenation of alkenes and alkynes.…”

Rare‐Earth Single‐Atom Catalysts: A New Frontier in Photo/Electrocatalysis

“…[ 137 ] Concerning RE SACs, coordinatively unsaturated Ce atoms at the surface of stoichiometric CeO 2 have been shown to generate H – species through the heterolytic dissociation of H 2 , thus favoring the reduction of CO 2 to HCOO* species and resulting in high catalytic activity and selectivity. [ 138 ]…”