On the Role Played By Catalytic Metallic Additives on TiO2 for the Photoreduction of CO2
In this contribution, we will present a comparative analysis among different potential additives such as magnesium, indium. More detailed surface analysis techniques have revealed significant changes at the outer part of the TiO2 material. XPS spectra, especially at the energy zone of oxygen and titanium, and corresponding to the last two nanometers, show significant evolution of the different detected bonds as the concentration of the additives is increased.
All these data have been correlated with the photocatalytic behavior for CO2 reduction as a function of the additive concentration below the limit of their solubility for avoiding segregation effects. A correlation between Ti+3 and adsorbed molecular oxygen concentration found in these outer layers will be discussed as well as their correlation with the total productivity rate, considering all the effective photogenerated electrons for the reduction of CO2 from the GC analysis of the products obtained, including CO, CH4 and C2 hydrocarbons. These findings confirm that the benefit introduced by the catalytic additive is mostly related to the “surface quality” and how it is lost as the concentration of additive is increased. Likewise, it is determined that the overall productivity is kinetically limited by a four-hole chemistry of the water oxidation reaction, like in the photoactivated water splitting process. Nevertheless, in spite of these limitative issues, significant increases, a factor of 3 o 4, can be achieved in the productivity of reduced CO2 if the concentration do not overcome some limits whereas the selectivity of the products becomes determined by the nature of the additive.