Research on photocatalytic materials has been a field in continuous expansion in the recent decades, as it is evidenced by the large number of articles published every year. So far, more than 190 different semiconductors have been assayed as suitable photocatalysts. To this figure, it is necessary to add the combinations with other functional materials or between different semiconductors, as well as their morphological modifications. Summing up the outcome of these different preparation strategies eventually leads to the enormous number of photocatalytic systems that have been reported in the scientific literature. Dealing with such an amount of information requires updated and educated guidance to select the most significant realizations, and it also calls for critical assessments on how the expectations are being fulfilled. This perspective article intends to assess the state of the art of photocatalysis with regard to materials and systems, considering the well-established results, but also the emerging aspects, and the envisaged new directions of this technology in the near future. In the first part, the most relevant achievements in this area, some of them already in the market while others still in development, will be reviewed according to the current understanding. The second part of the article is devoted to the most innovative and promising photocatalysts and related systems described in the open literature.

A unique 1D nanostructure of Pt@CeO 2 −BDC was prepared from Pt@CeBDC MOF. The Pt@CeO 2 −BDC was rich in oxygen vacancies (i.e., XPS O β /(O α + O β ) = 39.4%), and on the catalyst, the 2 nm Pt clusters were uniformly deposited on the 1D mesoporous polycrystalline CeO 2 . Toluene oxidation was conducted in a spectroscopic operando Raman−online FTIR reactor to elucidate the reaction mechanism and establish the structure−activity relationship. The reaction proceeds as follows: (I) adsorption of toluene as benzoate intermediates on Pt@CeO 2 −BDC at low temperature by reaction with surface peroxide species; (II) reaction activation and ring-opening involving lattice oxygen with a concomitant change in defect densities indicative of surface rearrangement; (III) complete oxidation to CO 2 and H 2 O by lattice oxygen and reoxidation of the reduced ceria with consumption of adsorbed oxygen species. The Pt clusters, which mainly exist as Pt 2+ with minor amounts of Pt 0 and Pt 4+ on the surface, facilitated the adsorption and reaction activation. The Pt-CeO 2 interface generates reduced ceria sites forming nearby adsorbed peroxide at low temperature that oxidize toluene into benzoate species by a Langmuir−Hinshelwood mechanism. As the reaction temperature increases, the role of lattice oxygen becomes important, producing CO 2 and H 2 O mainly by the Mars-van Krevelen mechanism.

Hybrid structured photocatalysts based on sepiolite, an adsorbent, and TiO2 were prepared by extrusion of ceramic dough and conformed as plates. The influence of the photocatalyst configuration was studied either by including TiO2 in the extrusion process (incorporated materials) or by coating the sepiolite plates with a TiO2 film (coated materials). The influence of the OH- surface concentration in the photocatalytic performance was studied by treating the ceramic plates at different temperatures. The samples were characterized by N2 adsorption-desorption, MIP, SEM, XRD, and UV-vis-NIR spectroscopy and tested in the photocatalytic degradation of trichloroethylene (TCE) as a target VOC molecule. Most of the catalysts presented high photoactivity, but considerable differences were observed when the CO2 selectivity was analyzed. The results demonstrate that there is a significant effect of the catalyst configuration on the selectivity of the process. An intimate contact between the sepiolite fibers and TiO2 particles for incorporated materials with a corncob-like structure favored the migration of nondesirable reaction products such as COCl2 and dichloroacetyl chloride (DCAC) to the adsorbent, reacting with OH- groups of the adsorbent and favoring the TCE mimeralization.

Porous TiO 2-anatase films were prepared by sol-gel route showing higher photocatalytic activity in degradation of trichloroethylene (TCE) in air compared to dense titania films. Thus, titania sols were synthesized with and without a pore generating agent, polyethylene glicol (PEG), to evaluate the effect of porosity in the photocatalytic activity of the coatings. The films were deposited by dipping and sintered at different temperature and time. The characterisation was performed by profilometry, Fourier Transform Infrared Spectroscopy (FTIR), Grazing X-ray Difraction (GXRD) and Field Emision Scan Electron Microscopy (FE-SEM), observing that anatase phase is obtained at temperatures as low as 350 ºC. The maximum specific surface area (S s = 43 m 2 /gr) was obtained for coatings prepared from TiO 2 sol with PEG and sintered at 400 ºC. Porous TiO 2-anatase films present trichloroethylene (TCE) conversion around twenty percent higher than that of dense films. Porous volume, surface area and thickness of the coating play a key role in the photocatalytic activity. On the other side, variation in particle size seems not to be a critical parameter in the studied range.

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