FTIR and TPD Analysis of Surface Species on a TiO2 Photocatalyst Exposed to NO, CO, and NO-CO Mixtures: Effect of UV-Vis Light Irradiation

As a continuation of our previous work on the kinetics of photocatalytic reduction of NO by CO on titanium dioxide, interaction of a Degussa P-25 TiO2 photocatalyst with NO, CO, and NO-CO mixtures at ambient temperature has been studied by FTIR and TPD. Only reversible weak adsorption of NO on surface Ti4+ ions is found to occur in the dark. UV-vis irradiation greatly enhances the NO adsorption on Ti4+ and yields N2O, NO-, NO2-, and NO3- surface species. After irradiation of TiO2 in a CO atmosphere, IR bands of surface CO2- and CO3- species appear in addition to a continuous IR absorption tail towards lower wavenumbers due to free carriers in the reduced semiconductor. When TiO2 is exposed to a equimolar NO-CO mixture, N2O and CO2- are formed without irradiation supposedly by the reaction 2 NO + 2 CO + O-2(-) -> 2 CO2- + N2O. Subsequent light irradiation is accompanied by the accumulation of NO- and Ti-4(+)center dot center dot center dot NO complexes. No TiO2 reduction occurs in this case. FTIR spectra show that NO- produced by the photoinduced adsorption of NO can be eliminated by the following reaction: NO- + CO ->(hv) CO2- + (1/2) N-2. It is believed that this reaction is a key step in the nitrogen production by the photocatalytic process. The data obtained enable us to refine the earlier proposed reaction mechanism and to directly prove some of its key steps.