Oxygen Dynamics in Lean Propylene Catalytic Combustion over CeO2 and Pr6O11: Roles and Interplay between Lattice and Adsorbed Oxygen Species

In 1954, Mars and van Krevelen proposed the famous "redox" mechanism to rationalize the oxidation of hydrocarbons (HCs) over vanadium oxide catalysts. According to this mechanism, the reduction of oxide catalysts (hydrogen abstraction, dehydroxylation, and metal–oxygen bond cleavage) are kinetically relevant in most cases, and oxides with high reducibility can be made into catalysts with high activity for HC (deep) oxidation. Such a framework, however, cannot explain the fact that Pr6O11 with the most liable lattice oxygen among lanthanide oxides is a worse low-temperature propylene oxidizer than CeO2. In this article, by comparing the kinetic/isotopic performance and the reduction/reoxidation behavior of rod-like CeO2 and Pr6O11 counterparts during lean propylene catalytic combustion, it was suggested that both these lanthanide oxides ignited propylene via a classical redox mechanism, while the reactive oxygen species involved in their following reactions were quite different. Specifically, the reactions over Pr6O11 were limited by the replenishment of lattice oxygen─the consistent workhorse reactive phase of this catalyst, and could be effectively accelerated at elevated temperature with a drastic dropping in the apparent activation energy (Eaapp, from 75.9 to 60.1 kJ/mol). In contrast, due to the relatively low electrochemical reduction potential of Ce4+/Ce3+ (1.74 eV) than that of Pr4+/Pr3+ (3.2 eV), the propylene-induced defective sites (e.g., Ce3+–VO) on CeO2–x readily donated Ce3+ 4f1 electrons to adsorbed O2 during the reoxidation steps in the redox cycles, giving rise to adsorbed oxygen species like O22– and O–. These electrophilic Oxn– species played active roles in the following reduction steps. Benefited from the "shallow" reactive region and therefore multiplied redox cycles of CeO2, such an "Oxn–-assisted" Mars–van Krevelen mechanism led to low Eaapp (∼43 kJ/mol) values close to those obtained on platinum catalysts.