The selective catalytic reduction of nitrates (NO(3)(-)) in pure water towards N(2) formation by the use of gaseous H(2) and in the presence of O(2) (air) at 1 atm total pressure and 25 degrees C has been investigated over Pd-Cu supported on TiO(2)-Al(2)O(3) pellets (spherical shape). The effects of internal mass transport phenomena, of Pd and Cu loading (wt%), and of reducing feed gas composition on the reaction rate. N(2)-selectivity, and the kinetics of the overall reaction in terms of apparent reaction orders with respect to nitrate concentration (mg/L) and H(2) partial pressure (bar) have been investigated. Apparent reaction orders of 0.32 and 1.04 were estimated, respectively with respect to NO(3)(-) concentration and H(2) partial pressure in the reducing feed gas stream (H(2)/He). However, when 20% air is added in the hydrogen reducing feed gas stream, the apparent reaction order with respect to the H(2) partial pressure was increased to 1.7. It is demonstrated for the first time that the presence of internal mass transport phenomena when large catalytic particles (2-3 mm in size) are used in a semi-batch mode reactor operation results in the reduction of the selectivity of reaction with respect to NH(4)(+) and NO(2)(-) products, and at the same time in the increase of N(2) product selectivity, whereas the opposite is true for the rate of reaction. Moreover, it was found that the reaction rate, the NO(3)(-) conversion (%), and the reaction product selectivity are affected significantly by the presence of oxygen in the reducing feed gas stream, and by the Pd and Cu metal loading used in catalyst's composition. (C) 2010 Elsevier B.V. All rights reserved.