Ni/CeO2(111): a model catalyst for water-gas shift reaction

Abstract

Ceria is an extensively used heterogeneous catalyst carrier due to its unique oxygen storage capabilities associated with the easy conversion between Ce3+ and Ce4+ ions. In addition, strong metal-ceria interactions result in an enhance performance by avoiding sintering of the active phase. One important example that boosted the use of ceria as a catalyst support is the water-gas shift (WGS) reaction for the production of syngas (H2 + CO2), where supported coinage metals (Au, Cu, or Pt) are typically employed as the active phase. Experimental insight revealed that nickel-based catalysts can also be stable, inexpensive and highly active, showing an excellent potential for the WGS reaction at small Ni coverages, whilst catalysing the production of methane from CO and H2 at medium and large coverages. Here we study the electronic structure, geometries, and adsorption of C, CO, and H2O on small Nin (n=1 and 4) particles supported on CeO2(111) using density-functional theory (DFT) with the DFT+U approach and compared to Ni(111) and CeO2(111). We show that the CO bond strength follows the trend: Ni(111) < Ni4/CeO2(111) < Ni1/CeO2(111) [4]. The stronger CO bond found for the smallest particles provides an explanation for the experimentally reported Ni coverage dependence of the CO methanation reaction on Ni/CeO2(111). Ni-ceria interactions are crucial for the interpretation of these results. In addition, we explore the adsorption and dissociation of water on these systems, since a good WGS catalyst should be able to oxidize and remove CO efficiently, but still be active enough to dissociate water. We observe that the molecular adsorption of water on the bare CeO2(111) surface (−0.55 eV) [5] is similar to the adsorption under the presence of supported single Ni atoms (−0.62 eV). Nevertheless, dissociative water adsorption is more favourable on the Ni1/CeO2(111) (−1.00 eV) than on the bare CeO2(111) surface (−0.60 eV) [5]. We have established computational models for Ni/ceria systems for the WGS reaction that are consistent with experimental knowledge for powder catalysts and experimental model catalysts and thus help to bridge the gap between them.