Formate stability and carbonate hydrogenation on strained Cu overlayers on Pt(111)

Formate (HCOO) synthesis, decomposition and the hydrogenation of carbonate (CO3) on Cu overlayers deposited on a Pt(111) single crystal are investigated to examine the reactivity of a Cu surface under tensile strain with defects present. Formate is synthesized from a 0.5 bar mixture of 70% CO2 and 30% H2 at varying temperatures, and the evolution is followed with polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS). Subsequent TPD reveals decomposition of the formate species into CO2 and H2 at 420 ± 5 K for strained Cu at sub-monolayer to monolayer coverages. This is a significantly lower decomposition temperature than obtained earlier on pristine Cu(111) (460 K), as well as for thicker Cu layers where we assign an observed decomposition peak at 440 ± 5 K to relaxed, but defect-rich Cu(111). However, the thermal stability of formate on strained and defect-rich Cu is similar to previous results obtained for supported, and lattice-strained, Cu nanoparticles. The hydrogenation of carbonate produced by 0.3 bar CO2 exposure at room temperature was monitored with XPS and TPD showing a significant loss of carbonate when subjected to 0.2 bar H2 at room temperature. However, the presence of formate on the surface, or any other hydrogenation product, could not be established during or after H2 exposure by PM-IRRAS, EELS or TPD. Even so, the results suggest that carbonate and its hydrogenation may constitute a relevant pathway to methanol production.