Surface modification of MCr2O4 (M = Mg and Zn) by Cu-Doping: Theoretical prediction and experimental observation of enhanced catalysis for CO oxidation

We theoretically investigated surface modification of MCr2O4 (M = Mg and Zn) by Cu-doping and doping effects on catalytic activity for CO oxidation. DFT + U calculations elucidated that Cu-dopants in MCr2O4(1 1 1) exist on the surface layer, CO oxidation by Cu-doped MCr2O4(1 1 1) occurs through Mars van Krevelen mechanism, ratedetermining step (RDS) is the first CO2 desorption with oxygen vacancy (OV) formation, and the catalytic activity is enhanced very much by Cu-doping. Also, we experimentally observed that CO conversion was dramatically improved to 70% by the Cu-doped ZnCr2O4 from 5.2% by non-doped ZnCr2O4 at 300 degrees C. Bader charge analysis and difference density map show that M atom on the MCr2O4 surface receives electron density from the dissociating O atom in the OV formation step and the Cu-dopant can receive more electron density than Mg and Zn atoms to facilitate the OV formation and accelerate the first CO2 desorption(RDS). This larger capability of Cudopant for receiving electron density results from the facts that the Cu atom has large electronegativity and + I oxidation state besides + II in metal oxide but Mg and Zn have small electronegativity and only + II oxidation state.

Automated summary

Theoretical and experimental investigations reveal that Cu-doping can significantly enhance the catalytic activity of MCr2O4 (M = Mg and Zn) for CO oxidation. Cu-dopants exist on the surface layer of MCr2O4(1 1 1), and CO oxidation occurs through the Mars van Krevelen mechanism with the first CO2 desorption being the rate-determining step. The electron density transfer from the dissociating O atom to the surface M atom is facilitated by Cu-dopant, leading to accelerated OV formation and improved catalytic activity.