Oxygen Dynamics in Lean Propylene Catalytic Combustion over CeO<sub>2</sub> and Pr<sub>6</sub>O<sub>11</sub>: Roles and Interplay between Lattice and Adsorbed Oxygen Species
Xiwei Gao, L. Li, Yuquan Liu, Changlong Zheng, Wei Liu, Min Li, Xiaodong Wu, Shuang Liu
Abstract
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 Pr 6 O 11 with the most liable lattice oxygen among lanthanide oxides is a worse low-temperature propylene oxidizer than CeO 2 . In this article, by comparing the kinetic/isotopic performance and the reduction/reoxidation behavior of rod-like CeO 2 and Pr 6 O 11 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 Pr 6 O 11 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 Ce 4+ /Ce 3+ (1.74 eV) than that of Pr 4+ /Pr 3+ (3.2 eV), the propylene-induced defective sites (e.g., Ce 3+ –V O ) on CeO 2– x readily donated Ce 3+ 4f 1 electrons to adsorbed O 2 during the reoxidation steps in the redox cycles, giving rise to adsorbed oxygen species like O 2 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 CeO 2, 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.