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Membrane reactors applied to catalytic reactions are currently being studied in many places world-wide. Significant developments in membrane science and the vision of process intensification by multifunctional reactors have stimulated a lot of academic and industrial research, which is impressively demonstrated by more than 100 scientific papers on catalytic membrane reactors being published per year. Palladium as a noble metal with exceptional hydrogen permeation properties and, at the same time, broad applicability as a catalyst, first of all for hydrogenation, is part of many of these developments. This paper discusses two different membrane reactor concepts which both rely on supported palladium, on the one hand as a permselective membrane material, and on the other hand as base component of a membrane-type hydrogenation catalyst. Dense palladium composite membranes can be used for hydrogen separation from packed-bed catalysts in gas-phase hydrocarbon dehydrogenation reactions. Mesoporous membranes containing dispersed bimetallic Pd/X-clusters can be employed as so-called catalytic diffusers for liquid-phase hydrogenation, e.g. of nitrate and nitrite in water. The principles of both concepts are introduced, recently obtained experimental data are evaluated in connection with literature results, and the perspectives for further development are highlighted.

A catalytic membrane contactor for selective hydrogenation of nitrate in water to nitrogen is discussed as a promising new approach to develop a technically feasible catalytic process for nitrate reduction from ground and surface water. Metal organic chemical vapour deposition (MOCVD) was used to place catalytically active metals, i.e., palladium and tin, inside the porous top-layer of asymmetric ceramic membranes (e.g. alumina) with different pore sizes and thicknesses of the top-layer. The influence of the membrane properties, i.e., the material, pore size and thickness of the catalytic layer, and the influence of the MOCVD process parameters on the deposition of the metals was studied and catalytic membranes with different palladium loading and palladium/tin ratio were prepared. These membranes were characterised with respect to the pore structure and the distribution of the active metals. Their catalytic performance in the hydrogenation of nitrate in water was investigated in a laboratory stirred tank membrane reactor under continuous flow conditions to explore the influence of important process parameters. The results show that the catalytic membranes offer a high activity and a fair selectivity to nitrogen which may be further improved by an optimisation of the preparation procedure.

Hydrogenation of nitrate using supported bimetallic Pd-Cu or Pd-Sn catalysts provides a promising new alternative for denitrification of drinking water with several advantages over established technologies such as reverse osmosis or biological denitrification (Sell et al., 1992). One of the major problems of this technology is the formation of undesired side products, i.e. ammonium and nitrite, both subject to a limiting value far below the admissible nitrate content. Previous studies have shown that the particle size, among other effects, is a key parameter determining the selectivity of the catalyst, pointing to a detrimental effect of pore diffusion limitation (Hahnlein et al., 1998). Moreover, activity and selectivity depend on the concentration of dissolved hydrogen, i.e. high concentration means high activity, but at the same time favoring of ammonium formation due to further hydrogenation. In this paper a porous catalytic membrane acting as a hydrogen diffuser is proposed as a means to solve these problems by creating an efficient three-phase contact between the catalytic surface, dissolved nitrate, and hydrogen gas. Such a design offers the potential to control the activity and selectivity of the process through controlled dosage of hydrogen. Experimental data on the hydrogenation of aqueous nitrate using commercial ceramic membranes modified by insertion of palladium and copper or tin into the pore-structure through impregnation and MOCVD (metalorganic chemical vapor deposition) are presented and discussed together with characterization results obtained by SEM/EDAX and XPS.