Enhanced oxidation activity from modified ceria: MnOx–ceria, CrOx–ceria and Mg doped VOx–ceria

• DFT + U studies of MnOx- and CrOx-modified ceria, and VO2-modified Mg-doped ceria.
• The most stable structures are MnO2- and CrO2-modified CeO2(111).
• Oxygen vacancy and H adsorption energies more favourable than unmodified CeO2.
• Cr behaves similarly to V, but Mn reduction competes with Ce reduction.
• VO2 modified Mg-doped CeO2 is favourable for oxygen vacancy formation and H adsorption.

Ceria is an important component of catalysts for oxidation reactions that proceed through the Mars-van Krevelen mechanism, promoting activity. A paradigm example of this is the VOx–CeO2 system for oxidative dehydrogenation reactions, where vanadium oxide species are supported on ceria and a special synergy between them is behind the enhanced activity: reduction of the catalyst is promoted by ceria undergoing reduction. This leads to favourable oxygen vacancy formation and hydrogen adsorption energies—useful descriptors for the oxidation activity of VOx–CeO2 catalysts. In this paper, we examine if this promoting effect on ceria-based catalysts holds for other metal oxide modifiers and we investigate MnOn– and CrOn–CeO2(111) (n = 0 − 4) as examples. We show, combining density functional theory calculations and statistical thermodynamics that similarly to the vanadia modifier, the stable species in each case is MnO2– and CrO2–CeO2. Both show favourable energetics for oxygen vacancy formation and hydrogen adsorption, indicating that VO2–CeO2 is not the only system of this type that can have an enhanced activity for oxidation reactions. However, the mechanism involved in each case is different: CrO2–CeO2 shows similar properties to VO2–CeO2 with ceria reduction upon oxygen removal stabilising the 5+ oxidation state of Cr. In contrast, with MnO2–CeO2, Mn is preferentially reduced. Finally, a model system of VO2–Mg:CeO2 is explored that shows a synergy between VO2 modification and Mg doping. These results shed light on the factors involved in active oxidation catalysts based on supported metal oxides on ceria that should be taken into consideration in a rational design of such catalysts.

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