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

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 VO x -CeO 2 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 VO x -CeO 2 catalysts.In this paper, we examine if this promoting effect on ceria-based catalysts holds for other metal oxide modifiers and we investigate MnO n -and CrO n -CeO 2 (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 MnO 2and CrO 2 -CeO 2 .Both show favourable energetics for oxygen vacancy formation and hydrogen adsorption, indicating that VO 2 -CeO 2 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: CrO 2 -CeO 2 shows similar properties to VO 2 -CeO 2 with ceria reduction upon oxygen removal stabilising the 5+ oxidation state of Cr.In contrast, with MnO 2 -CeO 2 , Mn is preferentially reduced.Finally, a model system of VO 2 -Mg:CeO 2 is explored that shows a synergy between VO 2 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.