The effect of O-vacancies on intermediates stability and electron delocalization over MgO modified Ru/ZrO2: Spectroscopic insights during CO2 methanation

O-vacancies are defects commonly correlated with the production of CH4 by CO2 hydrogenation on reducible catalysts. Their effect on the stability of active COads and its origin is, however, not commonly explored for low Ru-loaded catalysts (Ru < 0.5 wt%). Herein, we investigated two catalysts containing a comparable amount of Ru (≈ 0.4 wt%) on two different zirconia supports: ZrO2 and ZrO2-MgO (2.6 wt%-MgO). The rate of CH4 formation on Ru/ZrO2-MgO is ca. 3-fold higher than on Ru/ZrO2. Combined results from various ex-situ and in-situ characterization techniques provide evidence supporting the correlation between methane evolution and the extended Ru-support perimeter as well as changes in the bond strength of intermediates modulated by O-vacancies. This latter effect represents the main difference between the two catalysts. Since the replacement of Zr4+ by Mg2+ ions in the ZrO2 lattice promotes the generation of O-vacancies, facilitating electron transfer from them to interfacial Ru sites. This not only enhances H2 activation but also weakens the stability of adsorbed bidentate formate (b-HCOO-ads), which subsequently decomposes to COads and then hydrogenates to CH4 at the Ru-support perimeter.