Supported bimetallic palladium catalysts for water-phase nitrate reduction

Technologies for the removal of nitrate from drinking water and waste water will be required in the near future, and the catalytic nitrate reduction is one of the most promising methods. So far. the understanding of nitrate reducing catalysts has been very poor. Experimental trends in nitrate removal activity and selectivity for various pH values, and for different bimetallic catalysts, as well as results described in literature were used to set up a mechanistic model for the reaction. The nitrate reduction activity is determined by bimetallic ensembles, at which nitrate is adsorbed and reduced to nitrite by hydrogen, which is supposed to spillover from palladium sites to the bimetallic sites. Formic acid, on the other hand, reacts with nitrate by a transfer hydrogenation mechanism from neighboring palladium sites. The extent of positive charging of the bimetallic sites is supposed to influence the activity at different pH values. At a high pH, strongly adsorbing oxygenated species block bimetallic nitrate adsorption sites as well as palladium sites. The selectivity is determined by the ratio of nitrogen species to reductant species at monometallic palladium sites. At these sites, the reduction of nitrite and other intermediates take place, finally leading to the end products. If this ratio of nitrogen to reductant species changes, the selectivity changes as well, e.g. at different ratios of the two metals. The trends in the experimental data are well described by this model. (C) 2001 Elsevier Science B.V. All rights reserved.