Environmental Catalysis: Development of bimetallic catalysts for the reduction of nitrates in aqueous solutions

The increasing concentration of nitrates in the ground water together with the quality standards necessary for drinking water generate the urgent need to develop new processes for the removal of nitrates from aqueous solutions. One of the most promising technologies for the nitrate removal from drinking water and from industrial waste water is the liquid phase hydrogenation of nitrate with H 2 or formic acid over noble metal containing bimetallic catalysts. The main advantage of this process is the direct conversion of nitrate to nitrogen, without the generation of nitrate enriched solid or liquid waste streams, while the formation of ammonia as undesired by- product and the high consumption of H2 are the main limitations. Palladium in combination with a transition metal such as copper, indium and tin supported on AI2 O3 was found to be the most active catalyst. However, the structural properties of the metal components and their mutual interaction in the working catalyst were not investigated so far. For a controlled design of highly active and selective catalysts the knowledge about the oxidation state of the metals, the possible formation of inter-metallic phases or alloys under reaction conditions, the size of the metal particles and their interaction with the support is essential. The objective of the project is to establish the relationship between structure and activity/selectivity of the bimetallic catalysts used for the reduction of nitrates in the aqueous phase. This knowledge should serve as a basis for the design of technologically and economically successful catalysts, having a high activity at low temperatures and a high selectivity towards the formation of N2 .

The increasing concentration of nitrates in the ground water together with the quality standards necessary for drinking water generate the urgent need to develop new processes for the removal of nitrates from aqueous solutions. With the increasing rigorous standards of the concentration of nitrogen containing compounds in drinking water it is necessary to develop new technologies for their removal. The legal limits established by the EU are 50, 0.1 and 0.5 ppm for NO 3 - , NO 2 - and NH 4 + respectively. In Austria the average values of nitrates are normally under the legal limits but can reach in certain regions values well above the limits. One promising technology for the nitrate removal from drinking water and from industrial wastewater is the liquid phase hydrogenation of nitrate with hydrogen over bimetallic catalysts. The main advantage of this process is the direct conversion of nitrate to nitrogen, without the generation of nitrate enriched solid or liquid waste streams, while the formation of ammonia, as undesired by-product and the high consumption of hydrogen are the main limitations. Palladium in combination with a transition metal such as copper or tin supported on alumina was found to be the most active catalyst. The transition metal alone is capable to reduce nitrates to nitrites but is oxidized and looses its catalytic activity. In this step Palladium is necessary to activate hydrogen, which reduce the oxidized metal. Therefore a strong interaction between these two metals is necessary. Active catalysts could be prepared by a special synthesis method; furthermore the concentration of the noble metal could be reduced. In a batch reactor total conversion of nitrates was achieved but the concentration of ammonia was too high. In a flow reactor the conversion of nitrates was only about 50% but the limits for ammonia and nitrites could be reached.