Research Article pubs.acs.org/acscatalysis
Metal-Free Low-Temperature Water−Gas Shift Catalysis over Small, Hydroxylated Ceria Nanoparticles Xing Huang*,† and Matthew J. Beck*,†,‡ †
Department of Chemical & Materials Engineering, University of Kentucky, Lexington, Kentucky 40506, United States Center for Computational Sciences, University of Kentucky, Lexington, Kentucky 40506, United States
‡
ABSTRACT: The water−gas shift (WGS) reaction is an important process for the production of H2 either for industrial use as, e.g., an ammonia precursor or to produce low-CO concentration H2 gas streams for use in fuel cells. Hybrid metal/metal oxide catalysts have permitted low-temperature WGS reaction by promoting associative reaction mechanisms in which -OH groups adsorbed at the metal oxide surface combine with reactant CO to form metastable intermediates. Here we show that sufficiently small, hydroxylated ceria nanoparticles (CNPs) can directly catalyze the WGS reaction without requiring the presence of a metal cocatalyst. Hydroxyl groups intrinsically present at the surfaces of such CNPs allow associative mechanisms with activation barriers on the order of 0.5 eV, low enough to allow metal-free low-temperature catalysis of the WGS reaction. KEYWORDS: low-temperature water−gas shift catalysis, metal-free, hydroxylated ceria nanoparticles, density functional theory, associative mechanism, formate, carboxyl
1. INTRODUCTION Hydrogen gas (H2) is a critical fuel for use in proton exchange/ polymer electrolyte membrane fuel cells (PEMFCs) for the environmentally friendly generation of electrical energy. Conventionally produced H2 gas contains concentrations of CO that are sufficient to rapidly poison existing PEMFCs, dramatically degrading their performance.1 To meet reasonable operating requirements, CO is typically removed from H2 gas streams destined for use in PEMFCs through various processes, including the water−gas shift (WGS) reaction. Typical WGS processes yield H2 outputs with CO concentrations of 2 eV (see Figure 2)] and will poison the CNP catalyst. With that in mind, the dramatically larger activation energies for formate formation (vs carboxyl formation) imply that low-temperature WGS processes should suppress formate poisoning in favor of carboxyl formation (and eventual dissociation into CO2 and H2). A recent joint experimental and computational study by Carrasco et al. has shown that Ni/CeO2(111) hybrid catalysts exhibit rapid dissociation of H2O with an activation barrier of 0.9 eV. The relatively low barrier to water splitting for Ni/ CeO2(111) was found to be due to the weakening of O−H bonds in water at Ni2+ sites that are stabilized by the ceria support.34 This effect, enhanced water splitting on Ni, was shown to lead to the high activity of the hybrid Ni/CeO2(111) system for the WGS reaction. The connection of high WGS activity to low water splitting activation energies directly parallels our finding that the presence of intrinsic surface -OH groups is critical to achieving low WGS activation energies in the absence of metal cocatalysts. Another recent study demonstrates that commercially available CNPs can be used, without a metal cocatalyst, to catalyze the partial hydrogenation of alkynes.52 The reported reaction invokes the dissociative adsorption of H2 on ceria surfaces leading to the formation two surface -OH groups. These findings suggest that OH-terminated CNPs studied here could also be active catalysts for similar hydrogenation reactions.
existing hybrid metal/metal oxide WGS catalysts. First, small, hydroxylated CNPs do not require metal cocatalysts, reducing the cost, weight, and processing complexity of low-temperature WGS catalyst materials, and second, the density of catalytically active sites on hydroxylated CNPs (effectively, one active site for every surface lattice oxygen site) will be at least an order of magnitude higher than the density of active sites on catalyst materials that require the thermally induced formation of energetically unfavorable lattice oxygen vacancies. The density of surface -OH groups on OH-terminated CNPs is also important as it reduces various activation energies by permitting interactions among neighboring adsorbed -OH groups. For instance, during the transformation of an adsorbed carboxyl group into a desorbed CO2 and an adsorbed H-, the carboxyl group interacts with a neighboring adsorbed -OH group, forming, in effect, an unstable H2O−CO2− complex. This complex represents the transition state for dissociation of the carboxyl group, but it is a complex that requires the presence of neighboring -OH groups. It should be noted that effective catalysis of the WGS reaction at low temperatures (
