Modification of acid-base properties of TiO2 by Nb and Mg dopants: Influence on the activity of Pd-Ca/(Mg, Nb)-TiO2 catalysts for nitrate hydrogenation
Résumé
The influence of the acid-base properties of TiO2-based supports on the performances of Pd-Cu bimetallic catalysts for the hydrogenation of nitrates in water has been investigated. Nanostructured TiO2 and TiO2 doped with various amounts of Mg and Nb (1-7 wt%) were prepared by co-hydrolysis of metal alkoxides. The surface area increased and the crystallinity decreased for Mg- and Nb-doped TiO2, all solids exhibiting mesoporosity with average pore sizes smaller than 20 nm. The acid-base properties of TiO2 were strongly modified by doping, Mg introducing a basic character whereas Nb-doped TiO2 becomes acidic. Pd-Cu bimetallic catalysts deposited on TiO2 and Mg- or Nb-doped TiO2 were found very active for the catalytic hydrogenation of nitrates, but the selectivity to ammonium was high. Nb-doping slightly decreased the reactivity of nitrates and Mg-doping slightly improved the reactivity of nitrites, whereas the selectivity to ammoniums was only marginally modified. The activity and products selectivity were very moderately affected by the acid or basic character of the support, which suggests that this property is not relevant to the reaction mechanism. (C) 2013 Elsevier B.V. All rights reserved.