Pd-Modified Cu–Zn Catalysts for Methanol Synthesis from CO2/H2 Mixtures: Catalytic Structures and Performance

The effect of palladium incorporation on the performance of a CuO–ZnO catalyst for methanol synthesis by hydrogenation of carbon dioxide is studied. Three different catalysts are prepared: the reference CuO-ZnO (CZ), and two Pd-based CuO–ZnO catalysts, PCZ-CP and PCZ-SP, which are prepared by co-precipitation and sequential precipitation, respectively. The PCZ-CP system appears to be almost inactive for methanol synthesis, whereas the PCZ-SP catalyst increases the methanol yield [mol MeOH/(h kgcat)] with respect to the base CZ catalyst. The marked decrease in the catalytic activity of the PCZ-CP catalyst is explained in terms of the loss in copper surface area. The effect of Pd is not capable of overcoming this significant loss in copper surface area. The enhanced methanol yield for PCZ-SP is not due to additional active palladium sites, but to a synergetic effect of Pd on the active Cu sites. The increase in the amount of hydrogen consumed (H2/M) during copper oxide reduction for PCZ-SP suggests that an H2-spillover mechanism is responsible for the increase in the methanol yield. This research reveals the importance of the methodology used for the incorporation of palladium. The results show that the precipitation order has a remarkable influence on the properties of the active phases and, consequently, on the catalytic performance for the hydrogenation of CO2 to methanol.