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

摘要

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(H-2)/V(CO2)/V(N-2) = 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 H-2 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 Pd-0-species in the form of PdZn alloys, a kind of catalytically active Pd-0-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.