The nature of the active sites of Pd–Ga catalysts in the hydrogenation of CO₂ to methanol

The hydrogenation of CO2 to methanol is a viable alternative for mitigating greenhouse gas net emissions as well as a route for hydrogen storage and transportation. In this work, we studied the nature of the active sites generated by the promotion of Pd with Ga and the surface species and their reactivity under reaction conditions, in the hydrogenation of CO2 to methanol. SiO2-supported Pd and Pd–Ga catalysts with different Pd/Ga molar ratios were synthesized. Results show that in a narrow interval of Pd/Ga molar ratio (0.5 to 1), the methanol formation rates increased by up to two orders of magnitude compared to that on non-promoted Pd. Interestingly, the CO formation rates were barely changed, which resulted in a high selectivity (60% at 800 kPa and 280 °C). Characterization by quasi in situ XPS, XRD, CO adsorption DRIFTS and TEM-EDS demonstrates that the primary cause for the increase in activity towards methanol synthesis is the formation of intermetallic compounds such as Pd2Ga, rather than a cooperative mechanism between Pd and Ga2O3. The variation of the Pd/Ga ratio defines the surface concentration of Ga2O3, where the deposition of excess Ga in its oxidized form on the mono or bimetallic (Pd or Pd–Ga) phases of the catalysts inhibits their catalytic performance. Additionally, operando DRIFTS characterization showed the formation of bidentate formate species (b-HCOO) on the metallic Pd or Pd–Ga sites acting as intermediates in the hydrogenation of CO2 to methanol. Unlike that of methanol, the CO formation rate is mainly dependent on the amount of surface metallic sites, irrespective of their nature (Pd or Pd–Ga). Results show that the presence of PdGa intermetallic compounds is necessary to improve the methanol formation rate of Pd–Ga/SiO2 catalysts.