On the Role Played By Catalytic Metallic Additives on TiO2 for the Photoreduction of CO2

In photocatalytic processes, it is not only need to generate an electron-hole pair, but the electronic transfer must be efficient in the reaction process, involving multi-electronic redox reactions, occurring at the near vicinity at the surface of the semiconductor. All these factors need the surface engineering of the semiconductor to increase the overall efficiency of the process. The surface properties of TiO₂ can be modified by the addition of some elements which give rise to a local lattice modification, introduce new potential catalysts issues and alter or vary the composition and bond ending of the last two nanometers layer being it essential and outstanding for the final photocatalytic properties.

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.