Density Functional Theory Study of the Mechanism of Formaldehyde Oxidation on Mn-Doped Ceria

Abstract

Formaldehyde is an indoor pollutant, whose removal under mild conditions is of growing importance. Mn-doped CeO 2 is a promising catalyst for the oxidation of formaldehyde to water and carbon dioxide. We have theoretically investigated the origin of the high activity of Mn-doped ceria as compared with ceria. DFT+ U calculations were used to identify adsorption modes and compare different reaction mechanisms. The reaction mechanism involves HCHO adsorption, two C–H bond cleavage steps involving reactive O atoms (either structural O atoms of the support or adsorbed O 2 ), H 2 O formation, and H 2 O and CO 2 desorption. On the stoichiometric surface, a Mars–Van Krevelen mechanism occurs, which involves ceria surface O atoms. The lower coordination number of these O atoms in the stoichiometric Mn-doped ceria results in decreased barriers for C–H bond cleavage. In the presence of defects which will be ubiquitous in the Mn-doped surface, a Langmuir–Hinshelwood mechanism becomes feasible, as O 2 can strongly adsorb on the oxygen vacancy next to Mn where HCHO adsorbs. The adsorbed O 2 molecule is strongly activated by the reduced ceria surface. The barriers for C–H cleavage are lowest for reactions involving adsorbed O 2 . We predict that the HCHO oxidation reaction proceeds with the lowest overall barrier on the defective Mn-doped CeO 2 surface.