Microcalorimetric and IR-spectroscopic study of the room temperature adsorption of carbon dioxide on pure and sulfated t-zirconia

The room temperature adsorption of CO2 on pure and sulphated tetragonal ZrO2 (t-ZrO2) was studied, in order to assess how and to what extent the acid–base properties of ZrO2 are modified by the presence of surface sulphate groups. The formation of different species, as well as their stability upon outgassing, was monitored by IR spectroscopy (2000–1000 and 2400–2300 cm−1 regions), whereas the population and the distribution of sites of different kind was quantitatively and energetically characterised by adsorption microcalorimetry. The formation of carbonate-like species was found to be a highly energetic (qads>100 kJ/mol) and irreversible process, occurring only on the (at least partially) dehydrated surface of pure t-ZrO2. Bicarbonate-like (hydrogen-carbonate) species (either of stable type) were found to form on both pure and sulphated t-ZrO2 specimens, and to be accompanied by a heat of adsorption variable in the 100–40 kJ/mol range. The presence at the surface of sulphate groups causes the basicity of the t-ZrO2 matrix to be dramatically depressed, the most basic OH− and/or coordinatively unsaturated (CUS) O2− ions being preferentially consumed by the sulphation process. Linear coordinated CO2 complexes were also found to form on CUS Zr4+ cations, the acidic strength of which increases with the extent of surface dehydration and as a consequence of the presence of charge-withdrawing surface sulphate groups.