Modification of acid-base properties of TiO sub(2) by Nb and Mg dopants: Influence on the activity of Pd-Cu/(Mg, Nb)-TiO sub(2) catalysts for nitrate hydrogenation

The influence of the acid-base properties of TiO sub(2)-based supports on the performances of Pd-Cu bimetallic catalysts for the hydrogenation of nitrates in water has been investigated. Nanostructured TiO sub(2) and TiO sub(2) 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 TiO sub(2), all solids exhibiting mesoporosity with average pore sizes smaller than 20 nm. The acid-base properties of TiO sub(2) were strongly modified by doping, Mg introducing a basic character whereas Nb-doped TiO sub(2) becomes acidic. Pd-Cu bimetallic catalysts deposited on TiO sub(2) and Mg- or Nb-doped TiO sub(2) 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.