The activation of CO2 is the key first step in its conversion to more useful chemicals. This is of great interest as CO2 is a difficult molecule to activate . Thus, finding materials that can activate CO2 in the initial step of adsorption is a problem of deep significance. On metal oxides, the reduction of the oxide by e.g. temperature or hydrogen can produce electron-rich reduced species that may activate CO2, the control of these process is difficult and there arises also the question of having suitable adsorption sites for the CO2 molecule. To address this difficult problem of CO2 activation on metal oxides, we have investigated the interaction of CO2 at TiO2 surfaces that are modified with sub-nm metal oxide nanoclusters; the metal oxides we use are CrOx, CuOx and CeOx supported on rutile and anatase TiO2 surfaces. We find that these composite oxide-TiO2 systems are highly reducible, easily losing oxygen to produce electron-rich species and also display many non-bulk like features such as low coordinated oxygen and metal sites. Particularly important is that CuO nanoclusters are easily reduced so that the ground state contains Cu+ and Cu2+ sites. These off-stoichiometric systems are able to adsorb and activate CO2 with exothermic adsorption energies of around 0.8 - 1 eV. The CO2 molecule bends with O-C-O angles of around 130o and elongated C-O distance. Computed vibrational modes are consistent with adsorbed, activated CO2. On CrOx the CO2 can either be adsorbed (adsorption energy 3 eV) and activated or exothermically form CO. Finally, on CeOx-rutile (110), the ceria clusters are reduced and are able to adsorb CO2 and cause the molecule to bend and activate. The results of this work show that reducible metal oxide nanoclusters supported on TiO2 surfaces are capable of activating CO2 and thus overcoming difficulties associated with the difficult first step in CO2 conversion.

Activating carbon dioxide on metal oxide (CrOx, CuOx, CeOx) nanocluster modified TiO2

255th National Meeting (2018)

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