BACKGROUND Titanium dioxide is the photocatalyst par excellence in environmental applications. Nevertheless, over the years various methods aiming to improve its efficiency have been presented. Herein, we report that TiO2 synthesis in supercritical medium can result in a significant enhancement in the rate of CO2 photocatalytic conversion. RESULTS Specifically, catalysts obtained from two titanium precursors (titanium tetraisopropoxide and diisopropoxititanium bis(acetylacetonate)) and two alcohols (ethanol and isopropyl alcohol) by hydrothermal synthesis in supercritical CO2 are shown to exhibit improved properties in comparison with the standard reference catalyst (Degussa P‐25, Evonik). CONCLUSION In particular, upgraded characteristics are related to reactants adsorption (higher specific surface areas, presence of surface hydroxyl groups), light absorption and excitation (better absorbance in visible range, lower band gap energy), and charge separation (appropriate morphology and crystallinity). Furthermore, when these catalysts are tested in the photocatalytic reduction of CO2, CO and CH4 production rates 3‐ and 15.7‐fold higher than those corresponding to the commercial catalyst have been found. © 2016 Society of Chemical Industry

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BACKGROUND In previous studies the enhanced activity of TiO2‐based catalysts synthesized in supercritical medium for photocatalytic reduction of CO2 was proved. RESULTS In this study, Cu/TiO2 photocatalysts were synthesized by hydrothermal methods in supercritical CO2. Two titanium precursors [titanium tetraisopropoxide and diisopropoxititanium bis(acetylacetonate)], two alcohols (ethanol and isopropyl alcohol), and one metal precursor (Cu (II) acetylacetonate) were used in the synthesis. Catalysts produced showed improved properties in comparison with the commercial reference catalyst (Degussa P‐25, Evonik). CONCLUSIONS Specifically, it has been found that Cu/TiO2 catalysts may yield methane production rates 20 times larger than that of commercial TiO2 catalyst without diminishing CO production rate (about 5 times higher than that of commercial catalyst). This result has been mainly imputed to both the formation of oxygen vacancies during the synthesis in supercritical medium and the high capacity of copper to adsorb and activate CO2 molecules, while preventing CO molecules from reoxidation, and avoiding the competitive reaction of hydrogen formation. © 2017 Society of Chemical Industry

Climate change is one of the most critical issues facing the world. One of the pillars of the fight against this phenomenon is the mitigation of greenhouse gas (GHG) emissions, CO 2 in particular. Although many achievements have already been made in CO 2 capture and storage technologies, promising methods to convert waste streams concentrated in this gas into valuable products are currently under way. This chapter intends to describe one of them, the photocatalytic reduction of CO 2 to fuel products, that is, the reaction to produce fuel between the CO 2 molecules and a reducing agent (usually water), in the presence of a semiconductor material that provides electrons (e À ) and holes (h + ) when it is illuminated by light of appropriate energy. Moreover, if solar energy were employed as light source, this type of energy difficult to store could be transformed into liquid or gaseous fuels, useful in conventional power production systems. In this chapter, firstly, a wide review about photocatalysis fundamentals, photocatalysts, synthesis of particles in supercritical fluids, characterization techniques, photocatalytic reactors, etc., is carried out. Subsequently, previous results from the authors' research on this subject are shown. More specifically, the characteristics of photocatalysts synthesized in a supercritical medium to convert CO 2 into fuels are presented and discussed.