H2S photodegradation by TiO2/M-MCM-41 (M = Cr or Ce): Deactivation and by-product generation under UV-A and visible light

M-MCM-41 molecular sieves (M = Ce or Cr) were prepared by a hydrothermal method and impregnated with TiO2. The materials were characterized by XRD, N2 adsorption–desorption, DRS and XPS. Their potential application to photooxidize H2S 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 ppmv of the pollutant. The photocatalytic efficiency using UV-A and visible light was compared to the activity of TiO2/MCM-41 without heteroatoms incorporated into the MCM-41 structure. It was found that the incorporation of Ce did not improve the performance of TiO2/MCM-41, but Cr-containing samples presented higher initial efficiency and were able to photooxidize H2S without formation of SO2 as a by-product, in contrast to the other prepared samples and to Degussa P-25 TiO2. Moreover, no other gaseous by-product was detected. The isomorphic incorporation of Cr into the structure of MCM-41 followed by TiO2 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 Cr6+.The authors would like to acknowledge Comunidad de Madrid (DETOX-H2S S-0505/AMB/0406), FAPERJ and Fundación Carolina for the funding and IQ-Unicamp for DRS–UV–vis analysis.Peer reviewe

Utilização e recuperação de sílica gel impregnada com nitrato de prata Use and recycling of silica gel impregnated with silver nitrate

<abstract language="eng">Argentation chromatography is used to increase the selectivity of the chromatographic process, chiefly in the resolution of complex mixtures of nonpolar substances. Although efficient, this technique generates residues containing heavy metal which makes its discarding through common procedures impracticable. In the present work a simple method for recycling of silica, and also silver, from argentation chromatography is described. This procedure uses initially a treatment of H2O2/HNO3, with subsequent treatment with H2O2/H2SO4 , allowing an efficient recycling of both components. This methodology is simple, costless, removes impurities efficiently, and does not modify retention parameters nor specific surface in a significant way