Understanding All-Solid Frustrated-Lewis-Pair Sites on CeO₂ from Theoretical Perspectives

Abstract

The development of heterogeneous frustrated-Lewis-pair (FLP) catalysts from homogeneous FLP conception is of great promise in practical applications. While our recent discovery has shown that all-solid FLPs can be created on ceria via surface oxygen vacancy regulation ( Zhang Zhang Nat. Commun.2017815266), a sound understanding of the intrinsic property and reactivity of the solid FLPs is still expected. Here we present a comprehensive theoretical study on the FLPs (Ce···O) constructed on CeO2(110) and (100) surfaces by using density functional theory calculations. We find that the creation of surface oxygen vacancy can enhance both the acidity of FLP-acid site and the basicity of FLP-base site. The enhanced acidity and basicity of Lewis sites together with the elongated distance of Lewis pairs (Ce···O) contribute to the high activity of solid FLPs. The dissociative activation of H2 on FLPs experiences a heterolytic pathway (H2 → Hδ+ + Hδ−) with a low activation energy of 0.07 eV on CeO2(110) and 0.08 eV on CeO2(100). Unlike the phenomenon on stoichiometric CeO2 surfaces that the dissociated hydride (Hδ−) adsorbed at Ce sites is prone to transfer to more stable O sites, the hydride on FLPs can be stabilized at Ce sites and thus benefits the hydrogenation of acetylene via an easier pathway. The rate-determining barriers of acetylene hydrogenation on FLP-CeO2(110) and FLP-CeO2(100) are calculated to be 0.58 and 0.88 eV, respectively. These results could help to understand the nature of solid FLPs and pave the way for rational design of heterogeneous FLP catalysts.

Synthesis of Nickel‐Doped Ceria Catalysts for Selective Acetylene Hydrogenation

“…This is followed by the adsorption and stepwise hydrogenation of acetylene. This mechanism, which is similar to those proposed by Huang et al., provides a much more plausible explanation of the efficient hydrogenation of alkynes than those proposed earlier ,. Interestingly, for Ni‐doped CeO 2 , the adsorption state of an H atom on the Ni dopant is unstable since Ni is saturated in the square planar configuration.…”

Ceria-Based Materials in Hydrogenation and Reforming Reactions for CO2 Valorization

“…In this respect, an interesting and very recent approach is suggested by the work of Huang and coworkers who have demonstrated the possibility of creating series of solid frustrated Lewis pair (FLPs) sites on the surface of ceria (Zhang et al, 2017; Huang et al, 2018). FLPs are constituted by Lewis acids and bases which are sterically prevented from interacting with each other (thus called “frustrated”).…”

H2 Dissociation and Oxygen Vacancy Formation on Ce2O3 Surfaces

“…However, the homolytic product reminds systematically the most stable one, disfavoring the presence of hydride species at the surface. 33,34,36,38 Previous work of Vecchietti et al 39 report the formation of Ce-H hydride species as a key step in the semi-hydrogenation of alkyne. Moreover, Vilé et al 28 and Carrasco et al 29 have shown that the presence of oxygen vacancy was detrimental to the alkyne hydrogenation.…”

“…Recently, ceria has been successfully used in hydrogenation processes [28][29][30][31] with high activity and selectivity, thus opening new avenues in hydrogenation catalysis application in the absence of noble metals. [31][32][33][34][35][36][37][38] The mechanism of the H 2 dissociation on the (111)-CeO 2 surface was proposed by García-Melchor et al 33 and Fernández-Torre et al 34 : the H 2 molecule dissociates according to a heterolytic pathway forming a hydride/proton pair (Ce-H/OH) which is 0.7-0.8 eV endothermic, although the homolytic final product Ce 3+ -OH/Ce 3+ -OH is energetically very favorable, 2-3 eV exothermic with respect to the reactants initial energy. Very recently, H 2 dissociation was also investigated on other CeO 2 surfaces 36,38 and it was found that the surface termination plays a key-role on both the activation energy and the stabilization of the heterolytic product.…”