A Novel Pd-decorated Carbon Nanotubes-promoted Pd-ZnO Catalyst for CO2 Hydrogenation to Methanol

Abstract

A novel metallic palladium-decorated CNTs-promoted Pd-ZnO catalyst was developed, with excellent performance for hydrogenation of CO2 to methanol. Over a composition-optimized catalyst, Pd0.1Zn1-10 %(5.0 %Pd/CNTs), under reaction conditions of 5.0 MPa, 543 K, V(H2)/V(CO2)/V(N2) = 69/23/8, GHSV = 15,000 mL g−1 h−1, the conversion of CO2 hydrogenation and the corresponding space–time-yield of methanol (STY(CH3OH)) reached 6.98 % and 343 mg g−1 h−1, respectively. This STY(CH3OH) value was 1.7 times that (202 mg g−1 h−1) of the corresponding (5.0 %Pd/CNTs)-free counterpart, Pd0.1Zn1, under the same reaction conditions. Addition of a minor amount of the Pd-decorated CNTs into the Pd-ZnO host catalyst caused little change in the apparent activation energy for the CO2 hydrogenation. The Pd-decorated CNTs or the simple CNTs as promoter function through their excellent performance of adsorbing/activating H2 to generate a surface micro-environment with higher stationary-state concentration of H-adspecies at the surface of functioning catalyst. This resulted in a marked increase of surface concentration of the Pd0-species in the form of PdZn alloys, a kind of catalytically active Pd0-species closely associated with the formation of methanol. On the other hand, those active H-species adsorbed on the CNTs (or Pd-decorated CNTs) could be readily transferred to Pd i Zn j active sites via the CNTs-assisted hydrogen spillover. The aforementioned two factors both were conducive to increasing the rate of hydrogenation-conversion of CO2. Hydrogenation of CO2 to methanol over Pd-decorated CNT-promoted Pd-ZnO catalyst