Evidence for a special formate species adsorbed on the Cu-Zn active site for methanol synthesis

Formate species synthesized by the hydrogenation of CO2 over clean Cu(111) and Zn-deposited Cu(111) surfaces at atmospheric pressure were studied by in-situ infrared reflection absorption spectroscopy (IRAS). The symmetric OCO stretching (nu(s)(OCO)) peak of a bidentate formate species on clean copper was observed at similar to 1330 cm(-1) on both surfaces during the reaction. The initial formation rate of formate species on the clean Cu(111) surface at 353 K and 760 Torr was estimated to be 8.2 x 10(-4) (molecules site(-1) s(-1)). which agreed with results previously obtained by XPS. In contrast, peaks at 1350 and 1585 cm(-1) were observed during the hydrogenation of CO2 only on the Zn-deposited Cu(111) surface. The peak at 1585 cm(-1) agreed with the frequency of the asymmetric OCO stretching (nu(a)(OCO)) of bridging bidentate formate species, thus indicating that an inclined formate species was adsorbed on two different atoms of the Cu-Zn site previously proposed as the active site of methanol synthesis. (C) 1998 Elsevier Science B.V. All rights reserved.