ABSTRACT
      Nitrates contamination in natural water is a problem that affects many agricultural and livestock areas. To reduce the nitrates present in that water some techniques have been proposed being the best, from an environmental point of view, the catalytic reduction of nitrates to nitrogen.
      
      In this thesis, that technique has been studied to remove the nitrates in natural contaminated waters, using a continuous reactor. The reaction conditions have been optimized, stating that the best results are obtained with a pH around 6,5; room temperature; a stirring velocity of 900 rpm; a water flow of 5mL/min; 3grams of catalyst and atmospheric pressure.
      
      The catalyst composition and its preparation have been optimized. Different metal combinations have been studied, achieving the best results using a Pd-Sn catalyst with a noble metal/no noble metal ratio of 2. Maintaining this ratio and working in the optimal conditions it is possible to treat a natural water contaminated with nitrates with a catalyst containing 1%Pd and 0,5%Sn. 
      
      The influence of the support in the catalyst activity has been studied. Some catalysts supported on oxides, carbons and zeolites have been used, and the best activity and selectivity to nitrogen has been obtained with the catalyst based in alumina or in a mesoporous material such as MCM-41, since both have low electrical conductivity, have no micropores and have adequate specific area. From these results it was concluded that the best catalyst to remove nitrates in a continuous reactor is the Sn/Pd catalyst supported on alumina. The best results are obtained with a catalyst prepared by wetness impregnation adding first the Sn salt and then the Pd salt and activating the catalyst at temperatures below 200°C.
      
      The catalyst lifetime and the deactivation of a catalyst with 2,5%Sn and 5%Pd supported on alumina have been studied and it was found that 3 grams of the catalyst can treat during 17 days 5 mL/min of natural water containing 100 ppm of nitrates. Moreover, it is possible to reuse several times the catalyst after its regeneration. The regeneration was made with distilled water because the catalyst deactivation is mainly related to the deposition of ionic species on the support surface. However, it must be stressed that it is impossible to avoid during the nitrates reduction the production of ammonium, therefore some options have been studied to remove this by-product. It has been determined that the best way to remove ammonium from the media is using a natural zeolite (clinoptilolite) as anionic exchanger.
      
      The catalyst Sn/Pd/Al2O3 has been characterized using different techniques, before and after reaction, to determine the active phases, the textural properties, the metal particle size and the metal dispersion. The results obtained shows that the catalyst is stable during the reaction and there are not modifications of its structure or of its textural properties.
      
      The comparison of the results obtained in the catalyst activity study with the results obtained in the characterization study show that first the nitrates are reduce to nitrites, and then the nitrites are reduced to nitrogen or ammonium. The active phase in the reduction of the nitrates to nitrites is the Pd-Sn combination, while the noble metal is able to reduce the nitrites to nitrogen or ammonium. Also, it has been determined that in order to obtain an active catalyst the palladium has to be reduced and highly dispersed, while the tin has not to be as metallic tin or forming PdxSny species. 
      
Finally the activity of new Pd/(Cu or Sn) catalysts supported on other materials such as hydrotalcites or delaminated zeolites has been determined. The conclusion is that the best activity and selectivity is obtained with the Pd-Sn catalyst supported on delaminated ITQ-2 zeolite. This is probably due to its high external surface area that decreases the diffusional problems. This opens new possibilities for the design of new active and selective catalysts in this reaction.