Enhanced Surface Charge Localization Over Nitrogen-Doped In<sub>2</sub>O<sub>3</sub> for CO<sub>2</sub> Hydrogenation to Methanol with Improved Stability
Yuxiang Yang, Chenyang Shen, Kaihang Sun, Donghai Mei, Changjun Liu
Abstract
Indium oxide (In 2 O 3 ) is active and promising for selective hydrogenation of CO 2 to methanol. However, it suffers from over-reduction at elevated temperatures, causing deactivation. Herein, a nitrogen-doped In 2 O 3 (N–In 2 O 3 ) catalyst was prepared using a plasma-intensified nitrogen-doping technology. It is confirmed that nitrogen doping is effective for the stabilization of In 2 O 3 . The doped nitrogen enhances the surface charge localization, which inhibits the over-reduction on the oxide surface and limits the generation of excessive surface oxygen vacancies. The doped nitrogen also serves as the active site, synergistically with surface oxygen vacancy, which leads to an enhanced dissociation of CO 2 to adsorbed CO* intermediates. The electron-rich nitrogen causes a strong adsorption of CO on N–In 2 O 3 and inhibits the formation of free CO. A significantly improved methanol selectivity with a higher turnover frequency (TOF) is thus achieved on N–In 2 O 3, compared to the un-doped In 2 O 3 . For example, at 21,000 cm 3 h –1 g cat –1, 300 °C, and 5 MPa, the TOF of N–In 2 O 3 reaches 37.0 h –1 with methanol selectivity of 75.1%, while the TOF of the un-doped In 2 O 3 is only 16.0 h –1 with methanol selectivity of 62.3%. Different from pristine In 2 O 3, N–In 2 O 3 takes the CO hydrogenation route for CO 2 hydrogenation to methanol. This explains the reason why the N–In 2 O 3 catalyst possesses improved selectivity for CO 2 hydrogenation to methanol.