Characterization of oxide-supported Cu by infrared measurements on adsorbed CO

Infrared spectroscopy on CO chemisorbed on Raney Cu and materials with Cu dispersed as nanoparticles on oxide supports was used to evaluate support effects on the Cu surface properties. The C-O frequency (ν_C-O) is sensitive to the charge on the adsorption site with ν_C-O being high on Cu⁺, intermediate on Cu⁰, and low on Cu⁻, whereby this method can probe the charging state of the Cu surface. The Raney Cu reference demonstrates the complex analysis of the IR band intensity, which can be susceptible to dipole coupling. This means that the most intense IR bands may be higher frequency bands strengthened by such coupling effects rather than the bands arising from the most abundant sites. The ν_C-O of the major band attributable to CO adsorbed on the metallic surface follows the order: Cu/SiO₂ > Raney Cu > Cu/Al₂O₃ > Cu/TiO₂. Given the charge-frequency relationship these support-dependent frequency shifts are attributed to changes in the charging of the Cu surface caused by support effects. The Cu surface is more electron deficient for Cu/SiO₂ and electron enriched for Cu/TiO₂. For the Cu/ZnO(/Al₂O₃) samples, which are important as industrial methanol synthesis catalysts, band assignments are complicated by a low ν_C-O on Cu⁺ sites connected to the ZnO matrix. However, Cu/ZnO(/Al₂O₃) has a spectral feature at 2065-68 cm⁻¹, which is a lower frequency than observed in the Cu single crystal studies in the literature and thus indicative of a negative charging of the Cu surface in such systems. Experiments with co-adsorption of CO and electron-withdrawing formate on Cu/ZnO and Cu/SiO₂ show that ν_C-O in the adsorbed CO shifts upwards with increasing HCOO coverage. This illustrates that the surface charge is donated to the electron-withdrawing formate adsorbate, and as a result co-adsorbed CO experiences a more charge depleted Cu surface that yields higher ν_C-O. The support-dependent surface charging may thus affect the interaction with adsorbates on the metal surface and thereby impact the catalytic properties of the Cu surface. Dilution of the samples in KBr, which has been used in many studies in the literature, had pronounced effects on the spectra. The presence of KBr leads to an increase in ν_C-O indicative of an electron depleted surface attributed to transfer of electron-withdrawing bromine species from KBr to the sample.