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Abstract:
We studied adsorption of CO2 on well-ordered thin CaO(001) films, prepared on Mo(001) and Pt(001) single-crystal surfaces in ultrahigh vacuum (UHV) conditions, using infrared reflection-absorption spectroscopy (IRAS) and temperature-programmed desorption (TPD). At low coverages, CO2 adsorbs as monodentate carbonates (CO32-). TPD spectra showed pseudo-first-order desorption kinetics with a maximum shifting from 500 to 470 K with increasing CO2 coverage. However, at further increasing exposures, desorption maximum is shifted to the considerably higher temperatures (570 K), although CO2 uptake remained almost the same. This unusual effect was found to correlate with dissociative adsorption of residual water in the UHV background as observed both by TPD and IRAS. Comparative analysis of spectral evolution on crystalline CaO(001) films and CaO nanoparticles favors the model, where surface hydroxyls only affect adsorption geometry of the carbonates rather than form bicarbonate species. However, hydroxyls show stabilizing effect on CO2 binding to the CaO surface.
