DFT study of formaldehyde oxidation on silver cluster by active oxygen and hydroxyl groups: Mechanism comparison and synergistic effect

Abstract

• Promotion effect of active oxygen or hydroxyl on HCHO oxidation is revealed. • Synergic effect of active oxygen and hydroxyl on HCHO oxidation is proposed. • Hydroxyl promotes formate specie formation step. • active oxygen promotes formate oxidation step. Density functional theory is used to study the formaldehyde (HCHO) oxidization mechanism with or without the assist of active oxygen and hydroxyl over silver cluster. We elaborate all possible intermediates adsorption structures and energies of HCHO oxidation on the Ag 19 cluster, and propose the pathways for the active oxygen, hydroxyl and their synergistic function assisted reaction processes. It is found that the direct HCHO dehydrogenation proceeds along HCHO → CHO → CO and the HCHO dehydrogenation is the rate-determining step with a barrier energy ( E b ) of 1.922 eV. With the assist of active oxygen, a reaction pathway of HCHO (+O) → HCHO 2 →CHO 2 (+O) → CO 2 with the direct dehydrogenation of methylenedioxy being the rate-determining step and a E b value of 1.119 eV is demonstrated. While the reaction pathway of HCHO (+OH) → HCHOOH → HCOOH (+OH) → CHO 2 → CO 2 is favorable in the presence of hydroxyl, in which the direct formate dehydrogenation is the rate-determining step with an E b value of 1.424 eV. Combining the promotion effect of active oxygen and hydroxyl together, the preferable reaction route is HCHO (+OH) → HCHOOH → HCOOH (+OH) → CHO 2 (+O) → CO 2 with the HCOOH oxidation step as the controlling step and a low E b value of 0.837 eV. These calculation results give us a clear view of the effect of active oxygen and hydroxyl in HCHO oxidation, and offer a reasonable explanation for the experimental findings in the literature.