Surface layer composition of titania under ambient air
Доклады на конференциях
The produced titania is often stored under ambient air condition. However, the storage of a powdered TiO2 in air may cause the formation of adsorbed layer on its surface. The composition of this layer should be dependent on the air composition which contains a number of major (O2, H2O, CO2) and minor chemical compounds (CO, CH4, H2, etc.) and the adsorption properties of the TiO2 surface in relation to these species.
TiO2 is often treated at high temperature in order to clear the surface out of water and other adsorbates before experiments. Such treatment may cause the changes not only of the adsorbed layer composition but also in the surface TiO2 lattice layer. This may lead to a complete deleting of the adsorbed layer. On the one hand, such state of titania surface is not typical of the ordinary conditions during the heterogeneous photocatalytic processes with gas phase and, moreover, with liquids. On the other hand, the composition of the adsorbed layer of the catalyst should significantly affect the experimental conditions and thus may distort the results. So, there is a practical need for monitoring of the adsorption layer state, especially in the case of a powdered sample with a high specific surface area.
This work deals with the investigation of TiO2 samples of various preparation methods and specific surface area values in order to identify the adsorption layer composition after the TiO2 storage ambient air.
Three different TiO2 samples were investigated in the dark, UV-, and visible light desorption of surface species: TiO2 prepared by dispersion of the titania mono crystal, TiO2 prepared by combustion of a pyrotechnic mixture in air, and commercial TiO2 P25. The composition of the adsorbed layer was identified in the dark and under the UV- and visible light irradiation.
The composition of the adsorbed layer for all investigated samples of titanium dioxide includes mainly CO2. Its amount is high due to presence in the atmospheric air and high adsorption ability of the TiO2 surface with respect to CO2 molecules. The presence of two other gases, CO and NO, on the surface of the samples studied is not so prevalent and the actual amount depends on the nature of TiO2.
Carbon dioxide photodesorption occurs under illumination of titania with UV-light. The kinetics of desorption may be described like a fast CO2 desorption at the initial period (3-5 min) with the subsequent slow desorption at a much lower rate. For the first step, photodesorption is obviously associated with the classical desorption of CO2 under illumination by quanta with the energy higher than TiO2 band gap. The second step of desorption may be described as a photocatalytic oxidation of carbon-containing compounds (e.g. CO) by the lattice oxygen followed by the reduction of TiO2 surface layer.
The composition of desorption products showed the dependence of the adsorption layer state on the TiO2 nature. The methane photodesorption was detected only for the commercial TiO2 P25. The possible reasons for that methane formation include the adsorption of the complete molecules during the TiO2 production process and (or) photocatalytic hydrogenation of CO2 under UV-light.
A water molecule may become the reduction agent for TiO2 surface under the light flux with the energy sufficient to overcome the TiO2 band gap . The reaction proceeds with the formation of hydrogen peroxide and the anion oxygen vacancy:
H2O + [O-]s → H2O2 + [ ]-s
, where [O-]s is the lattice oxygen of TiO2 surface, and [ ]-s denotes the anion oxygen vacancy of TiO2 surface. In this case, the reduction of CO2 by the anion vacancy leads to the formation of CO.
CO2 + [ ]-s → CO + [O-]s
Such mechanism of CO2 reduction over the TiO2 surface was proposed in work 
The methane photodesorption from the sample surface could be a result of the TiO2 origin. Thus, the methane was possibly adsorbed during the synthesis of the commercial P25 or during the long-term storage (the atmospheric methane concentration is about 2 ppm). In this case, the TiO2 illumination causes the methane desorption from the surface.
The methane formation is also likely to be a result of photocatalytic CO2 hydrogenation by hydrogen from the air (atmospheric concentration of hydrogen is about 1 ppm). The photocatalytic reaction of CO2 and H2 with a high yield of CH4 was observed in work :
СО2 + 4Н2 → СН4 + Н2О
Hydrogen can be also produced as a result of uncontrolled photocatalytic dehydrogenation of organic compounds adsorbed on TiO2 surface .
Consequently, a number of reactions proceed on the surface of the commercial P25 in presence of CO2 and H2O:
• The reduction of TiO2 surface by H2O under light;
• The reduction of CO2 with the formation of CO;
• Photocatalytic hydrogenation of CO2 up to CH4 by hydrogen of an unexplained origin.
This work was supported by Russian Academy of Sciences and Federal Agency of Scientific Organizations (project No. 0303-2015-0005).
 Zakharenko V.S., Daybova E.B. High Energy Chemistry (Russia). 48 (2014) 123 -126.
 Raupp G.B., Dumesic J.A. J. Phys. Chem. 89 (1985) 5240 -5245.
 Zakharenko V.S., Parmon V.N. Physical Chemistry Journal (Russia). 73 (1999) 124 -128.