Reported studies showed that the
peak at 2171 cm−1 can be ascribed to carbonyl adsorbed on 5-fold
coordinated Ti4+ sites (_-sites) in the absence of adsorbed water
[35,36]. In addition, the calculated _(CO) frequencies for (1 0 1),
(1 0 0) and (1 1 2) of TiO2 surfaces are all grouped in a narrow interval
of 2185–2175 cm−1 [37–39]. The present results obtained in the
CO adsorption measurements revealed that CO coordinates on Ti4+
sites present on the different faces (2173–2167 cm−1). Therefore,
it is possible to assume that CO adsorbs at the TiO2 _-sites and
related surfaces. In addition, the presence of characteristic peaks
(2117–2115 cm−1), in all materials, was assigned to the adsorption
at the Ti3+ ions [40].