Tin promoted palladium catalysts for nitrate removal from drinking water

Hydrogenation of nitrate to nitrogen using Pd/Al2O3 catalysts promoted by a second metal offers a promising process for nitrate removal in drinking wa ter treatment. This study was aimed to elucidate the nature and function of promoting tin species in PdSn/Al2O3 catalysts obtained in different prepar ation routes. On one hand, a parent Pd/Al2O3 catalyst was doped via impregn ation with aqueous solutions of SnCl2 of different concentrations. On the o ther hand, the palladium surface of the same parent Pd/Al2O3 catalyst was m odified via controlled surface reaction (CSR) with hexane solutions of Sn(C 4H9)(4). The structure of the different PdSn/Al2O3 catalysts was investigat ed by Sn-119-Mossbauer spectroscopy and by means of various chemisorption t echniques (static and pulse chemisorption of H-2 or CO, measurement of the differential heat of CO chemisorption, FTIR spectroscopy of CO chemisorptio n). Catalytic properties were studied in batch experiments under atmospheri c pressure. Promoting of the Pd/Al2O3 catalyst by CSR resulted in catalysts with a significantly higher activity compared to PdSn/Al2O3 catalysts obta ined via incipient wetness method. Obviously, Sn(II)-species being present in the latter in high portion inhibit the nitrate reduction on bimetallic P dSn ensembles. CO chemisorption reflected a 'palladium site blocking' by tin species in bo th kinds of catalysts and indirectly indicated the generation of palladium- tin ensembles. in case of the CSR preparation the palladium is alloyed by m etallic tin. The palladium surface is diluted by tin atoms, i.e. bimetallic PdSnx ensemb les are generated which are able to adsorb and activate nitrate ions. There is an optimum of tin loading, i.e. the activity decreases and the undesire d ammonium production in a site reaction increases when the surface becomes too tin rich. Sn(II) species, being preferentially present in catalysts ob tained by SnCl2 impregnation also strongly modify the chemisorption propert ies of the palladium surface, but obviously these species inhibit the nitra te reduction on the bimetallic PdSn ensembles, which are also present in th ese catalysts. (C) 2001 Elsevier Science B.V. All rights reserved.