Increased conversion and selectivity of 4-nitrostyrene hydrogenation to 4-aminostyrene on Pt nanoparticles supported on titanium-tungsten mixed oxides

A catalyst series consisting in platinum nanoparticles photodeposited on pure titania and on W/Ti mixedoxides, these latter prepared by the sol-gel method, were tested in the hydrogenation of 4-nitrostyrene.A remarkable increase in the reaction rate occurred when the catalyst support contained tungsten, with aparallel boosting in the selective reduction of the nitro group. With the selective W-containing catalysts,the reaction proceeded at constant rate (zero order rate law), while the tungsten-free catalyst showed arate-dependence on the 4-nitrostyrene concentration (positive order reaction). The presence of tungstenin the support is beneficial not only because a higher surface area is obtained, thanks to the stabilizationof anatase owing to the presence of tungsten, but also because it allows the photodeposition of smaller,better dispersed platinum particles, on which the adsorption of the aromatic part of 4-nitrostyrene is lessfavored. Tungsten not only substitutes titanium in the titania lattice, as revealed by HAAF-STEM analysis,but it is also present as WOxspecies partly covering the Pt nanoparticles photodeposited on the mixedoxide support, as revealed by an in depth distribution XPS analysis. This accounts for the progressivelylower performance observed with increasing tungsten content in the catalysts, the highest conversionand selective hydrogenation of the 4-nitrostyrene nitro group having been achieved on the catalyst witha 1% W/Ti molar ratio.