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

Formaldehyde is an indoor pollutant, whose removal under mild conditions is of growing importance. Mn-doped CeO2 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), H2O formation, and H2O and CO2 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 CeO2 surface.