Mechanistic studies of CO and CO2 hydrogenation to methanol over a 50Cu/45Zn/5Al catalyst by in-situ FT-IR, chemical trapping and isotopelabelling methods

Abstract

Cu-Zn-Al (50/45/5) has been prepared and employed for methanol synthesis from CO2/H2 and CO2/CO/H2. It was found that CO adsorbed molecularly on Cu0 and CO2 adsorbed dissociatively on Cu+. The decomposition of HCHO and HCOOH could produce CO-Cu+ and CO-Cu0 on H2-reduced Cu-Zn-Al, respectively. In methanol synthesis, CO is hydrogenated to formyl and subsequently to methoxy. As for CO2, it hydrogenates first to formate and gradually to methoxy via H2COO and H2CO. CO2 hydrogenation can also follow the pathway of CO hydrogenation via the RWGS reaction. We suggest that the addition of CO2 to the syngas would (i) produce more H2CO and (ii) sustain a sufficient amount of CU+-Cu0 couples on the catalyst surface. In CO hydrogenation, HxCO (x=1,2) are intermediates. In CO2 hydrogenation, formate methylenebisoxy, and formaldehyde are precursors for CH3OH formation.