A PM-IRRAS study on the interaction of methanol vapor with the Cu(111) surface under ambient conditions

The interaction of methanol with copper surfaces has a great importance for various catalytic processes such as methanol stream reforming [1], methanol oxidation for hydrogen generation [2], anhydrous formaldehyde formation, and CO/CO₂ + H₂ conversion to methanol [3]. These processes usually involve various intermediates which formed on the Cu surface during the reaction, including methoxy (CH₃O^*), hydroxymethoxy (CH₃O₂^*), formate (HCOO^*), carboxyl (COOH^*), etc. The effect of surface structure and reaction conditions on the dynamics between adsorbate species may provide valuable insights to the molecular level understanding of the surface processes during the above-mentioned catalytic reactions.

In the present work, methanol adsorption on the Cu(111) surface was studied by means of polarization modulation‐infrared reflection‐adsorption spectroscopy (PM-IRRAS) under 1 mbar methanol pressure at a temperature range of 25-100 °C. In addition, methanol adsorption on a sputtered Cu(111) was measured (only at room temperate) as reference. The PM-IRRAS spectra of methanol adsorption on both flat and sputtered Cu(111) surfaces indicate the formation of both methoxy (CH₃O^*) and formate (HCOO^*) species. At room temperature, the degree of formate formation is found to be higher on the sputtered (rough) Cu(111) compared to the flat (annealed) morphology, indicating that formate can be stabilized on active sites on the sputtered surface such as step sites and low-coordinated Cu atoms. The C-O stretch vibration peak (~1020-1065 cm^-1) shows a significant time dependence for the various temperatures examined. The peak position is found to shift towards low wavenumbers and its intensity decreases with time after methanol introduction. The kinetics of this process is very slow, and currently we associate it with the dehydrogenation process of methanol to methoxy on the Cu(111) surface.

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