Evidence for bicarbonate formation on vacuum annealed TiO2(110) resulting from a precursor-mediated interaction between CO2 and H2O

The reaction of CO2 and H2O to form bicarbonate (HCO3-) was examined on the nearly perfect and vacuum annealed surfaces of TiO2 (110) with temperature programmed desorption (TPD), static secondary ion mass spectrometry (SSIMS) and high resolution electron energy loss spectrometry (HREELS). The vacuum annealed TiO2 (110) surface possesses oxygen vacancy sires that are manifested in electronic EELS by a loss feature at 0.75 V. These oxygen vacancy sites bind CO2 only slightly more strongly (TPD peak at 166 K) than do the five-coordinated Ti4+ sites (TPD peak ar 137 K) typical of the nearly perfect TiO2 (110) surface. Vibrational HREELS indicates that CO2 is linearly bound at the latter sites with a v(a) (OCO) frequency similar to the gas phase value. In contrast, oxygen vacancies dissociate H2O to bridging OH groups which recombine to liberate H2O in TPD at 490 K. No evidence for a reaction between CO2 and H2O is detected on the nearly perfect surface. In sequentially dosed experiments on the vacuum annealed surface at 110 K, CO2 adsorption is blocked by the presence of preadsorbed H2O, adsorbed CO2 is displaced by postdosed H2O, and there is little or no evidence for bicarbonate formation in either case. How ever, when CO2 and H2O are simultaneously dosed. a new CO2 TPD state is observed at 213 K, and the 166 K state associated with CO2 at the vacancies is absent. SSIMS was used to tentatively assign the 213 K CO2 TPD state to a bicarbonate species. The 213 K CO2 TPD state is nor formed if the vacancy sites are filled with OH groups prior to simultaneous CO2+H2O exposure. Sticking coefficient measurements suggest that CO2 adsorption at 110 K is precursor-mediated, as is known to be the case for H2O adsorption on TiO2(110). A model explaining the circumstances under which the proposed bicarbonate species is formed involves the surface catalyzed conversion of a precursor-bound H2O-CO2 van der Waals complex to carbonic acid. which then reacts at unoccupied oxygen vacancies to generate bicarbonate. but falls apart to CO2 and H2O in the absence of these sites. This model is consistent with the conditions under which bicarbonate is formed on powdered TiO2, and is similar to the mechanism by which water catalyzes carbonic acid Formation in aqueous solution. (C) 1998 Elsevier Science B.V.