Principal Investigator Yang Shao-Horn
Identifying a precious-metal-free catalyst design principle that links material properties to the catalytic activity can accelerate the search for highly active and abundant catalysts to replace platinum. Although Sabatier’s principle provides a qualitative argument for tuning catalytic activity by varying the bond strength between the catalyst surface and the reactant/product (neither too strong nor too weak, leading to maximum activity at moderate bond strength), it has no predictive power to find catalysts with enhanced activity. We have recently identified unique catalyst properties (“activity descriptors”) that govern the metal-oxygen bond strength and the ORR activity of transition-metal-oxide-based catalysts. Using the oxide-based ORR activity descriptors -- the eg orbital occupancy and B-site transition-metal bond covalency, we can quantitatively account for more than four orders of magnitude in tthe ORR activity trend for perovskite transition oxide catalysts (formula: A1–xA'xByB'1–yO3, where A or A' is a rare-earth or alkaline-earth metal and B or B' is a transition metal). We explain our finding using a molecular orbital framework, which reflects the influences of the eg orbital and metal–oxygen covalency on the competition between O22–/OH– displacement and OH– regeneration on surface transition-metal ions as the rate-limiting steps of the ORR, and thus highlight the importance of electronic structure in controlling oxide catalytic activity.