Unraveling the promotional effects of K-doping on the mobility of surface oxygen species of CoCr2O4 for improved formaldehyde catalytic oxidation: The weakened metal-oxygen bond strength

Abstract

Doping alkali metals is an effective approach to improve the catalytic activity of formaldehyde oxidation for transitional metal oxides, while its effect mechanism on oxygen species has not been well understood. Herein, a series of KxCo1-xCr2O4 catalysts with different potassium doping ratios were prepared, and their structure-performance relationship and reaction mechanism for formaldehyde oxidation were investigated. Characterizations revealed that potassium cations with an appropriate ratio (x ≤ 0.05) substituted the cobalt cations in CoCr2O4 successfully, and led to advantageous changes in physiochemical properties. K0.02Co0.98Cr2O4 showed the best activity with the T90 decreased by 49 °C compared with CoCr2O4, and the reaction rate increased by about 10 times at temperatures below 90 °C. Theoretical calculations verified that K-doping could weaken the strength of metal–oxygen bonds in CoCr2O4, thus promoting the mobility of surface lattice oxygen for better formaldehyde oxidation. Moreover, K-doping restrained the generation of unfavorable carbonates during the reaction and increased the conversion rates of the key intermediates (formate and dioxymethylene). The hydroxyl groups could be produced by water dissociation with the help of surface lattice oxygen, with which formates easily react to form CO2 and H2O. This work provides new insights for developing highly efficient transitional metal oxide catalysts for formaldehyde abatement.