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to fuels is required to enable the production of sustainable fuels and to
contribute to alleviating CO2 emissions. In considering conversion
of CO2, the initial step of adsorption and activation by the catalyst
is crucial. In addressing this difficult problem, we have examined how
nanoclusters of reducible metal oxides supported on TiO2 can promote
CO2 activation. In this paper we present density functional theory
(DFT) simulations of CO2 activation on heterostructures composed of extended
rutile and anatase TiO2 surfaces modified with chromia nanoclusters.
The heterostructures show non-bulk Cr and O sites in the nanoclusters and an
upshifted valence band edge that is dominated by Cr 3d- O 2p interactions. We
show that the supported chromia nanoclusters can adsorb and activate CO2 and
that activation of CO2 is promoted whether the TiO2
support is oxidised or hydroxylated. Reduced heterostructures, formed by
removal of oxygen from the chromia nanocluster, also promote CO2
activation. In the strong CO2 adsorption modes, the molecule bends
giving O-C-O angles of 127 - 132o and elongation of C-O distances up
to 1.30 Å; no carbonates are formed. The electronic properties show a strong CO2-Cr-O
interaction that drives the interaction of CO2 with the nanocluster
and induces the structural distortions. These results highlight that a metal
oxide support modified with reducible metal oxide nanoclusters can activate CO2,
thus helping to overcome difficulties associated with the difficult first step
in CO2 conversion.
Science Foundation Ireland through the US-Ireland R&D Partnership Program Project SusChem SFI 14/US/E2915 and the M-ERA.net co-fund program, grant number SFI 16/M-ERA/2918, H2020 Grant Agreement 685451