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 (E_a^app, 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⁺–V_O) on CeO_2–x readily donated Ce³⁺ 4f¹ electrons to adsorbed O₂ during the reoxidation steps in the redox cycles, giving rise to adsorbed oxygen species like O₂^2– and O^–. These electrophilic O_x^n– species played active roles in the following reduction steps. Benefited from the “shallow” reactive region and therefore multiplied redox cycles of CeO₂, such an “O_x^n–-assisted” Mars–van Krevelen mechanism led to low E_a^app (∼43 kJ/mol) values close to those obtained on platinum catalysts.