Identification of a Facile Pathway for Dioxymethylene Conversion to Formate Catalyzed by Surface Hydroxyl on TiO2-Based Catalyst

Formaldehyde (HCHO) is either a reactant, intermediate, or product in numerous industrial catalytic reactions; therefore, revealing the full transformation process of HCHO during heterogeneous catalysis is of prime importance in understanding the mechanisms of many HCHO-related reactions and designing more efficient catalysts. As two main intermediates in HCHO transformation, the conversion of dioxymethylene (DOM) to formate is one of the key steps. There are three recognized pathways for this step, including direct dissociation of DOM, DOM combining with surface O, or through a Cannizzaro-type reaction. Herein, we reveal that the catalytic reaction between DOM and the abundant surface OH groups is a facile pathway for the transformation of DOM to formate species on TiO2 and also on TiO2-supported noble metal catalysts. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) results demonstrate that the presence of water vapor is favorable for the conversion of DOM to formate species on TiO2, and the loading of noble metals (Ir/Pt) on TiO2 further promotes the activation of H2O, thereby boosting the transformation of DOM into formate. Density functional theory (DFT) calculations show that HCHO consumes the bridging OH groups to form DOM species first followed by the reaction between DOM and the terminal OH groups on the TiO2 surface to produce formate, and the latter reaction is both thermodynamically and kinetically favorable. These results provide deeper insight into HCHO transformation and demonstrate another role of surface hydroxyl groups in catalytic reaction.