To confirm this dissociation of CO2 to CO on the defective Cu(I)/TiO2-x surface, an isotopic carbon-labeling experiment was conducted under the same condition. Fig. 2(b) shows the in situ DRIFTS spectra of 13CO2 interaction with Cu(I)/ TiO2-x, which follows a similar trend to that of 12CO2 in Fig. 2(a). 13CO2– species (1217 cm–1) were quickly formed and then gradually decreased after reaching its maximum at 10 min. A corresponding Cu+–13CO peak at 2064 cm–1 appeared and reached it maximum at 10 min. The result of labeled carbon confirms that the produced CO bound to Cu+ site is indeed derived from CO2 and that CO2 is indeed activated and dissociated on defective Cu(I)/TiO2-x, even in the dark at room temperature. The spontaneous dissociation of CO2 on Rh/TiO2-x and Cu(I)/TiO2-x in the dark is to a large extent associated with the surface VO that provide not only an electronic charge (Ti3+) but also the sites for the adsorption of oxygen atoms from CO2. The formation of Cu+–CO or Rh–CO could be an additional driving force to promote the dissociation of CO2. Furthermore, photoillumination could promote the dissociation of CO2. Fig. 2(c) shows the in situ DRIFTS spectra acquired by sequentially exposing Cu(I)/TiO2-x to CO2 in the dark for 30 min and under photoillumination for another 30 min. After 30 min in the dark, CO2– at 1248 cm–1 and Cu+–CO at 2110 cm–1 were not observed, although they
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