M-MCM-41 molecular sieves (M = Ce or Cr) were prepared by a hydrothermal method and impregnated with TiO 2 . The materials were characterized by XRD, N 2 adsorption-desorption, DRS and XPS. Their potential application to photooxidize H 2 S in a wet gas stream was tested in a continuous flow reactor operating at a flow rate of 110 mL·min -1 at 50% relative humidity and using 30 ppm v of the pollutant. The photocatalytic efficiency using UV-A and visible-light was compared to the activity of TiO 2 /MCM-41 without heteroatoms incorporated into the MCM-41 structure. It was found that the incorporation of Ce did not improve the performance of TiO 2 /MCM-41, but Cr-containing samples presented higher initial efficiency and were able to photooxidize H 2 S without formation of SO 2 as a by-product, in contrast to the other prepared samples and to Degussa P-25 TiO 2 . Moreover, no other gaseous by-product was detected. The isomorphic incorporation of Cr into the structure of MCM-41 followed by TiO 2 incorporation produced photocatalysts that presented good adsorption capacity and were much more active under visible-light than under UV-light. This performance represents an important advantage for solar applications. Their photoactivity depended on the concentration of chromium; the highest efficiency was attained with samples with a Si/Cr ratio of 50. Finally, deactivation was observed as a consequence of sulfate accumulation on the surface of the catalyst and reduction of Cr 6+ .

a b s t r a c tA novel versatile tubular reactor that may use both types of radiation, solar and/or artificial, and different types of suspended or immobilized photocatalysts is proposed. The photocatalytic reactor was evaluated for air treatment at laboratory scale and semi-pilot-plant scale. UV-A transparent immobilized photocatalysts were employed, which allowed an efficient use of radiation. Two different types of photocatalytic modules were tested: (a) TiO 2 -coated PET monoliths and (b) TiO 2 -coated glass slides, arranged in monolith-like units with the help of especially designed star-shaped polygonal structures. Both types of units were easy to handle and assured the adequate distribution of the photocatalyst inside the tubular reactor. The efficiency of the photocatalytic system with both solar and artificial radiation to oxidize the H 2 S contained in an air stream was demonstrated at the laboratory roof and in the treatment of real air of a wastewater treatment plant located in Madrid (Spain). As a consequence of the chemical nature of the pollutant, the photocatalytic activity decayed over time due to the accumulation of sulfate on the surface, but easy regeneration of the exhausted photocatalyst was achieved by washing with water.

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).