Raquel Portela

Photocatalytic materials: recent achievements and near future trends

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.

Photocatalytic elimination of indoor air biological and chemical pollution in realistic conditions

The photocatalytic elimination of microorganisms from indoor air in realistic conditions and the feasibility of simultaneous elimination of chemical contaminants have been studied at laboratory scale. Transparent polymeric monoliths have been coated with sol-gel TiO(2) films and used as photocatalyst to treat real indoor air in a laboratory-scale single-step annular photocatalytic reactor. The analytical techniques used to characterize the air quality and analyze the results of the photocatalytic tests were: colony counting, microscopy and PCR with subsequent sequencing for microbial quantification and identification; automated thermal desorption coupled to gas chromatography with mass spectrometry detection for chemical analysis. The first experiments performed proved that photocatalysis based on UVA-irradiated TiO(2) for the reduction of the concentration of bacteria in the air could compete with the conventional photolytic treatment with UVC radiation, more expensive and hazardous. Simultaneously to the disinfection, the concentration of volatile organic compounds was greatly reduced, which adds value to this technology for real applications. The fungal colony number was not apparently modified.

Photocatalysis for Continuous Air Purification in Wastewater Treatment Plants: From Lab to Reality

The photocatalytic efficiency of TiO(2)-SiMgO(x) plates to oxidize H(2)S was first evaluated in a flat laboratory reactor with 50 mL min(-1) synthetic air containing 100 ppm H(2)S in the presence of humidity. The use of the photocatalyst-adsorbent hybrid material enhanced the photocatalytic activity in terms of pollutant conversion, selectivity, and catalyst lifetime compared to previous H(2)S tests with pure TiO(2) because total H(2)S elimination was maintained for more than 30 operating hours with SO(2) appearing in the outlet as reaction product only after 18 h. Subsequently, the hybrid material was successfully tested in a photoreactor prototype to treat real polluted air in a wastewater treatment plant. For this purpose, a new tubular photocatalytic reactor that may use solar radiation in combination with artificial radiation was designed; the lamp was turned on when solar UV-A irradiance was below 20 W m(-2), which was observed to be the minimum value to ensure 100% conversion. The efficient distribution of the opaque photocatalyst inside the tubular reactor was achieved by using especially designed star-shaped structures. These structures were employed for the arrangement of groups of eight TiO(2)-SiMgO(x) plates in easy-to-handle channelled units obtaining an adequate flow regime without shading. The prototype continuously removed during one month and under real conditions the H(2)S contained in a 1 L min(-1) air current with a variable inlet concentration in the range of tens of ppmv without release of SO(2).

Nanostructured ZnO/sepiolite monolithic sorbents for H2S removal

ZnO presents favorable thermodynamics, equilibrium constant and stability for H2S chemisorption and sepiolite has excellent sorptive and rheological properties to be employed as a support. Four conformed ZnO/sepiolite composites are prepared, two by impregnation in zinc nitrate solution – one using activated carbon as a dispersing and pore generating agent – and two by dry mixing of the clay with dispersed or hierarchically assembled ZnO nanoparticles. The extrudates obtained with a mixture of sepiolite and nanodispersed ZnO particles are the most active for H2S chemisorption in temperature-programmed tests performed in an inert atmosphere, which can be correlated with the higher and homogeneous ZnO dispersion. ZnO sulfidation is reversible in the presence of O2. Thus, the activity can be regenerated, and these materials act as catalysts for H2S to SO2 oxidation in air.

Shaping up operando spectroscopy: Raman characterization of a working honeycomb monolith

A new monolithic reactor for operando Raman spectroscopy studies of honeycomb-shaped catalysts has been developed to obtain complete information on these systems, considering the effect of conformation on structure–activity relationships; this is illustrated by Raman-GC monitoring of alumina-supported vanadium phosphorous oxide phases during propane ammoxidation with monolithic and powdered beds.

Environmental Applications of Photocatalysis

Extensive research continues to optimize and widen the spectrum of potential applications of semiconductor photocatalysis. The applications are based on photooxidation, photoreduction, photosterilization and photoinduced superhydrophilicity phenomena or a combination thereof, and related with air and water treatment, active surfaces, green chemistry and energy conversion. The commercial potential of photocatalysis is impressive, including hygiene and sanitation; architecture and construction; automotive, food, chemical and textile industries; or environmental protection.

Influence of Catalyst Properties and Reactor Configuration on the Photocatalytic Degradation of Trichloroethylene Under Sunlight Irradiation

In this work, the influence of the reactor configuration and the characteristics of the catalysts on the photodegradation of trichloroethylene (TCE) vapors are studied under sunlight illumination. The photocatalytic activity tests were carried out using two types of continuous flow reactors: (i) a compound parabolic collector (CPC) and (ii) a simple flat reactor. Three different photocatalysts based on TiO 2 were utilized: (i) commercial powders calcined at 500°C (ii) a Ti0 2-x N x sample synthesized by treating the commercial sample at 500°C in an NH 3 gas flow, and (iii) TiO 2 thin film coatings on differently shaped borosilicate glass supports prepared by a sol-gel procedure. The obtained data reveal that the photonic efficiency for the removal of TCE is quite high but slightly decreases with increasing the light intensity. The commercial TiO 2 sample presents the highest efficiency while nitrogen doping seems to be slightly detrimental for photoactivity, despite the fact that certain photoresponse in the visible can be envisaged. In contrast, transparent sol-gel TiO 2 coatings present the highest TCE degradation rate per mass of catalyst. Regarding the type of reactor, it is found that the use of CPCs can be advantageous, especially when dealing with high volumes of effluent and elevated concentration of TCE, although flat reactor also shows a considerable efficiency.

Photocatalytic Oxidation of H2S on TiO2 and TiO2-ZrO2 Thin Films

Abstract Hydrogen sulfide is a widespread pollutant responsible for foul odors and severe damage to health and materials. However, current removal technologies have serious drawbacks. Photocatalysis seems to be a good alternative technology for eliminating H2S gas. Destruction efficiencies up to 100% have been achieved, and although the catalyst is deactivated, its regeneration by simple rinsing with water has been demonstrated. Experiments with different initial concentrations and residence times were carried out to define the dependence of conversion on these parameters. The effect of relative humidity, which noticeably influences the photocatalytic process, was also tested. Glass “Raschig” rings with different numbers of TiO2-ZrO2 and TiO2 layers deposited by a dip-coating procedure were used as the photocatalyst. The reaction rate increased with the number of oxide layers in the coating but no benefit from adding ZrO2 was found.

Non-metal Doping for Band-Gap Engineering

Non-metal doping appeared in 2001 as a promising alternative strategy to red-shift the absorption edge of TiO2 in photocatalytic applications. Since then, nearly all non-metals have been explored in single-element doping or codoping with other non-metal, a metal, or even a metalloid. The chemical state, amount, and position of the non-metal depend on the dopant selection and the synthesis method, which determine the success of the doping strategy. A big amount of theoretical and experimental research is underway, being the achievement of higher activity under visible than under ultraviolet light the main challenge.

Solar Photocatalysis for the Elimination of Trichloroethylene in the Gas Phase

In the present work, we have studied the photocatalytic degradation of trichloroethylene (TCE) under sunlight illumination with the aim of determining the feasibility of using this technology for gas purification. For these experiments, a continuous flow reactor was employed. This system is basically constituted by a Pyrex glass tube located in the focus of a compound parabolic collector made of anodized aluminum. Raschig rings of borosilicate glass coated with TiO 2 and randomly packed within the reactor tube were used as photocatalyts. Results obtained using sunlight illumination indicate that this continuous flow solar reactor can achieve complete TCE degradation. Experimentally, it is found that for moderate conversions, the TCE degradation rate increases linearly with solar irradiance, but a dependence of lower order is observed when removal of the pollutant is almost complete. On the other hand, photonic efficiency decreases linearly with solar irradiance, and it is higher when the molar flow fed to the photoreactor increases. In contrast, the selectivity toward the different partial degradation products (dichloroacetyl chloride, COCl 2 , etc.) is basically insensitive to solar irradiance.