Elucidating the interaction between Ni and CeOx in ethanol steam reforming catalysts: A perspective of recent studies over model and powder systems

Bulk metallic nickel is a poor catalyst for the reforming of oxygenates being deactivated by the deposition of coke. In contrast, Ni-ceria is an active system for the catalytic extraction of H-2 from the ethanol steam reforming reaction (ESR, C2H5OH +3H(2)O <-> 2CO(2) + 6H(2)). Numerous studies, with model (well-defined crystal surfaces) and technical (high surface area powders) catalysts, have been devoted to understand the fundamental role of each catalyst component, the performance of adjacent sites in the metal-oxide interface, and the complex mechanistic steps that convert two oxygenated reactants (ethanol and H2O) into H-2. The size and low loading of Ni on ceria facilitate metal-oxide support interactions that probably enhance the reactivity of the system.To establish the precise role of both Ni and Ce is challenging. However it is clear that both Ni and Ce are associated with the dissociation of H2O (OH+H), while ceria readily adsorbs and partially dissociates ethanol (i.e. ethoxy formation). The most difficult step of C-C bond dissociation likely occurs only on Ni or at the Ni-Ce interface. H2O and OH remain as important agents for the prevention of excess C build up during the C-H/C-C dissociation process. Often, deactivation upon C build up, is a direct result of Ni sintering and decoupling of the Ni-Ce interactions. One strategy to maintain good activity and stability is to protect the Ni-Ce interaction, and this can be achieved through the use of solid solutions (Ce1-xNixO2-y) or by employing stabilizing agents such as W (NixWyCezO2). In this article, we present and discuss the most recent work for the ESR reaction and show the important role of ceria which participates directly in the reaction and also enhances catalytic activity through metal-support interactions. (C) 2016 Elsevier B.V. All rights reserved.