Oxygen vacancy formation as the rate-determining step in the Mars-van Krevelen mechanism

The Mars-van Krevelen mechanism (MvK) is a widely recognized model for describing the role of lattice oxygen in catalysis. Following the MvK mechanism, the formation and conversion of surface oxygen vacancy (VO) are considered as the key steps. CeO2-ZrO2 (CZ) mixed oxides are the typical catalyst support in MvK mechanism. They have the unique property of hosting remarkable amount of VO without significant change in lattice structure, offering O storage and release capability that maintains the required concentration of active O on the catalytic surface. In this regard, the rate of VO formation and conversion directly affect their catalytic performance. In this work, we obtained the VO formation and conversion kinetics by measuring the rate of the Ce4+ reduction and oxidation via operando energy dispersive Extended X-ray Absorption Fine Structure (EDE). The main conclusions are: 1) VO formation is 10 times faster than VO conversion; 2) VO formation rates are comparable with the CO oxidation rates, thereby serving as the rate-determining step in CO oxidation; 3) Pd and Cu serve as catalysts for VO formation by significantly improving its rate by 50 times at 250 C by weakening the metal-O bonding strength, whereas the activation energy have been reduced to 58.4 kJ/mol and 36.5 kJ/mol, respectively. Our method in measuring and analysing partial reaction rates within a turnover is therefore important for all chemical reactions.