Evolution and Stabilization of Coordinatively Unsaturated Cr ³⁺ Sites over the Cr - CoO x /ZrO ₂ Surface in a Lattice Oxygen-Mediated MvK Mechanism

The CO 2 -assisted oxidative dehydrogenation of ethane (CO 2 –ODHE) was a promising approach for ethane valorization and CO 2 utilization. In this work, a lattice oxygen-mediated MvK mechanism was proposed as the dominant pathway governing the catalytic cycle: coordinatively unsaturated Cr 3+ sites (Cr C.U.S 3+ ) preferentially underwent selective C–H bond cleavage, while CO 2 activation occurred on Co 3 O 4 . Reactive oxygen species dissociated from activated CO 2 replenished the consumed lattice oxygen at Cr C.U.S 3+ sites, completing the redox cycle. A superior catalytic performance was observed on the 5Cr3CoO x /ZrO 2 catalyst; an ethane conversion of 18.4%, a CO 2 conversion of 14.6%, and an ethylene yield of 12.9% were achieved at 600 °C, and the ethylene STY reached 403.4 μmol·min –1 ·g cat –1 at 650 °C. Combined with in situ FTIR tests and a series of characterizations (e.g., UV–vis, Raman, H 2 -TPR, and XPS), as well as catalytic tests, it is demonstrated that the evolution and stabilization of Cr C.U.S 3+ sites were responsible for its excellent catalytic performance. Active Cr C.U.S 3+ sites were generated through the in situ reduction of Cr 6+ precursor species under reaction conditions, which dominated the subsequent catalytic process. The surface concentration of Cr 6+ species was enhanced by the introduction of Co species on ZrO 2, and in turn, Co species were stabilized as Co 3 O 4 due to the Cr doping. These results provided both macroscopic and microscopic understanding for the design of an efficient CO 2 –ODHE catalyst.