An interesting route for the valorization of CO2 consists on its photocatalytic conversion into sustainable fuels and/or chemicals in the presence of water and suited photocatalysts [1]; this process is also known as Artificial Photosynthesis (AP).This is a quite challenging process since CO2 is a stable compound and its reduction involves a series of multi-electron reactions. Nevertheless, the process is particularly advantageous considering that reactions can be driven under mild conditions using solar energy and a suitable photocatalysts [1]. Although this process present an interesting route for its valorization, is a quite challenging due to the complexity of the involved multi-step reactions, this process suffers from very low quantum yields and non-selective product distributions.

Extensive efforts are focused on improving the photocatalytic efficiencies. During the last years, a series of innovative materials with versatile properties and multifunctional character, known as hybrid materials, have been developed. Synergistic effects between their components provide these materials with exciting properties for light harvesting and charge separation, fundamental issues in artificial photosynthesis. Therefore, the development of new hybrid multifunctional photocatalysts using sunlight is considered as a cornerstone for CO2 valorisation technologies.

On the other hand, efforts are devoted to shed light on mechanistic aspects of the reaction such as the influence of traces of organic species adsorbed on the catalyst. In order to clarify the effect of series of photocatalytic experiments in combination with operando characterization techniques and theoretical calculations were performed [2].

In this work we report different strategies and modifications photocatalysts to increase process performance. The modification of optoelectronic properties allow controlling the absorption of incident photons, redox capabilities and subsequently the photocatalytic performance.

Results and discussion

AP experiments using bare TiO2, the main products were CO and H2, with low concentrations of CH4 and CH3OH. The use of different spectroscopies reveals that the CO2 reduction mechanism is related to the formation of carbonate/bicarbonate species. On the other hand, the introduction of SP NPs as co-catalyst leads to changes in the conversion and enhanced selectivity to higher demand electron products, such as CH4, while CO and H2 concentrations decrease. Long-lived photohole absorption was observed in SP/TiO2 catalysts, assigned to the accumulation of holes in the valence band of TiO2 after the electron transfer to metal NP. This enhancement in the charge separation is consistent with the change in the selectivity [3].

In the case of organo-inorganic hybrid materials, H2 evolution rates show that the polymer itself exhibit photocatalytic activity higher than bare titania. The reactivity is increasing with the polymer which improves the activity of TiO2. This activity of the hybrid photocatalysis is considerably higher than the sum of the individual components confirming the existence of a synergistic effect. The polymer and hybrid does not show any degradation after H2 production process. This increases is also observed in Photoelectrocatalyic measurements. Hybrid material also show a dramatically reactivity improvement in CO2 photoreduction revealing a synergy effect and promoting selectivity towards hydrogen and high demand electron products such as CH4.

On the other hand, the use of same organic ligands in MOFs synthesis leads to interesting behavior in photo(electro)catalytic H2 production. The mechanistic studies of this reaction confirm the effect as hole transport of this ligand. Best results are obtained with Bi-based MOFs, probably due to a decrease in the e- -h+ recombination rate.