NN ELSEVIER Journal of Photochemistry. and Photobiology A: Chemistry 89 (1995) 67-68 ,lmarnalof AND A: CHF~MIS'r I~" Photocatalytic reduction of nitrite and nitrate ions on Ru/TiO2 catalysts K.T. Ranjit, T.K. Varadarajan, B. Viswanathan* Department of Chemistry, Indian Institute of Technolog3; Madras 600 036, India Received 27 October 1994; accepted 13 December 1994 to ammonia Abstract Metals which have high over potential can stabilized H,as species. Thus, Ru loaded TiOz catalysts can be advantageously used for hydrogenation/reduction reactions. This idea has been extended for the photocatalytic reaction of nitrite and nitrate ions to ammonia and a scheme is suggested for the reduction process. Keywords: Photocatalytic reduction; Nitrate and nitrate anions; Ru/TiO2 1. Introduction The reduction of nitrite and nitrate anions has been studied electrochemically [1-3], but reports on the photocatalytic reduction of nitrite and nitrate ions are scarce [4,5]. Kudo et al. have reported the photocatalytic reduction of nitrate ions to nitrite and ammonia over Pt-based semiconductors [6]. The present authors [5,7] have observed that the photocatalytic reduction of nitrite and nitrate ions is dependent on various experimental factors such as the irradiation time, the pH of the medium, the nature of the sacrificial agent and the nature of the metal loaded on the semiconductor. Noble-metal-loaded catalysts of the type MFFiO2 have been employed in photocatalytic reactions mainly be- cause (i) they have high electron affinity and hence increase the lifetime of the excitons, thus slowing the recombination of the charge carriers, i.e. electrons and holes, and (ii) they are good hydrogenation catalysts since they promote the dissociative adsorption of hy- drogen as Ha~s. Thus high rates of hydrogen evolution can be expected for metals with low H2 overpotential (e.g. Pt) and similarly one can expect that metals with high overpotential (e.g. Ru) will stabilize Haos. Thus by depositing Ru on TiO2, one could advantageously use Hads for hydrogenation reduction reactions. This idea has not been examined so far. Herein we report the photocatalytic reduction of nitrite and nitrate ions to ammonia on Ru/TiO2 systems and a scheme is * Corresponding author. 1010-6030/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSDI 1010-6030(94)04029-X suggested for the photocatalytic reduction of nitrite and nitrate ions to ammonia. 2. Experimental details The Ru-loaded semiconductor was prepared by an impregnation method. The procedure employed for studying the photocatalytic reaction is described else- where [7]. 3. Results and discussion The results of the photocatalytic reduction of nitrite and nitrate ions on Ru/TiO2 are summarized in Table 1. At high concentrations of nitrite and nitrate (0.1 M) no ammonia was detected. This is because at these concentrations part of the light is absorbed by the Table 1 Photocatalytic reduction of nitrite and nitrate to ammonia on Ru/ TiO: catalysts Catalyst Solution Yield of NH 3 (gmol) TiO2 100 ppm NO3- - TiO2 100 ppm NO2- - RuFFiO2 100 ppm NO2- 0.53 Ru/'I'iO2 10 ppm NO2 1.06