The conversion of CO<sub>2</sub>
to fuels is of significant importance in enabling the production of sustainable
fuels, contributing to alleviating greenhouse gas emissions. While there are a
number of key steps required to convert CO<sub>2</sub>, the initial step of
adsorption and activation by the catalyst is critical. Well-known metal oxides
such as oxidised TiO<sub>2</sub> or CeO<sub>2</sub> are unable to promote this
step. In addressing this difficult problem, recent experimental work shows the potential
for bismuth-containing materials to activate and convert CO<sub>2</sub>, but
the origin of this activity is not yet clear. Additionally, nanostructures can
show enhanced activity towards CO<sub>2</sub>. In this paper we present density
functional theory (DFT) simulations of CO<sub>2</sub> activation on heterostructured
materials composed of extended rutile and anatase TiO<sub>2</sub> surfaces modified
with nanoclusters with Bi<sub>2</sub>O<sub>3</sub> stoichiometry. These heterostructures
show low coordinated Bi sites in the nanoclusters and a valence band edge that
is dominated by Bi-O states. These two factors mean that supported Bi<sub>2</sub>O<sub>3</sub>
nanoclusters are able to adsorb and activate CO<sub>2</sub>. Computed
adsorption energies lie in the range of -0.54 eV to -1.01 eV. In these strong
adsorption modes, CO<sub>2</sub> is activated, in which the molecule bends giving
O-C-O angles of 126 - 130<sup>o</sup> and elongation of C-O distances up to
1.28 Å, with no carbonate formation. The electronic properties show a strong CO<sub>2</sub>-Bi-oxygen
interaction that drives the interaction of CO<sub>2</sub> to induce the structural
distortions. Bi<sub>2</sub>O<sub>3</sub>-TiO<sub>2</sub> heterostructures can
be reduced to form Bi<sup>2+</sup> and Ti<sup>3+</sup> species. The interaction
of CO<sub>2</sub> with this electron-rich, reduced system can produce CO
directly, reoxidising the heterostructure or form an activated carboxyl species
(CO<sub>2</sub><sup>-</sup>) through electron transfer from the heterostructure
to CO<sub>2</sub>. These results highlight that a semiconducting metal oxide
modified with suitable metal oxide nanoclusters can activate CO<sub>2</sub>, thus
overcoming the difficulties associated with the difficult first step in CO<sub>2</sub>
conversion.
Peralkalinity in peraluminous granitic pegmatites. II. Evidence from experiments on carbonate formation in spodumene-bearing assemblages
