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

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 Cu° and CO₂ adsorbed dissociatively on Cu⁺. The decomposition of HCHO and HCOOH could produce CO-Cu⁺ and CO-Cu° 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⁺-Cu° couples on the catalyst surface. In CO hydrogenation, H_xCO (x=1,2) are intermediates. In CO₂ hydrogenation, formate, methylenebisoxy, and formaldehyde are precursors for CH₃OH formation.