The adsorption of CO on ZnO nanomaterials samples after different pretreatment procedures was investigated with microcalorimetry. The interaction between CO and ZnO nanomaterials became much stronger after the adsorption and partial desorption of CO₂. On carbonate-free ZnO surfaces, the CO equilibrium coverage is less than 0.1 micro mol/m² for p = 100 Pa, and the differential heat of adsorption (q^diff) is only 40 kJ/mol for this coverage (at coverage < 0.05 micro mol/m², q^diff may be extremely high due to the active sites).

After the adsorption sites on ZnO were partly covered by CO₂ (after CO₂ adsorption, the sample is evacuated at room temperature overnight), the equilibrium coverage for CO reaches 0.8 micro mol/m² at p = 100 Pa, and q^diff is higher than 60 kJ/mol. The adsorption rate also became faster. This strong adsorption is due to the existence of surface bidentate carbonates on the mostly exposed ZnO (10-10) faces, which increase the Lewis acidity of Zn²⁺ on this faces, as also shown by TPD and HREELS studies on ZnO (10-10).

Corresponding to similar results for CO adsorption on sulphate-doped TiO₂, it can be concluded that these acid anions generally increase the Lewis acidity of the transition metal ions exposed on metal oxides.

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