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

• Platinum supported on W-doped titania catalyzes selective NO2 hydrogenation.
• Tungsten in the support allows for smaller particle size of titania and platinum.
• Selective nitro group reduction is favored on small Pt nanoparticles.
• Amorphous WOx on the support also influences activity and selectivity.
• Conversion and selective reduction decrease with increasing tungsten content.

A catalyst series consisting in platinum nanoparticles photodeposited on pure titania and on W/Ti mixed oxides, 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 a parallel 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 a rate-dependence on the 4-nitrostyrene concentration (positive order reaction). The presence of tungsten in the support is beneficial not only because a higher surface area is obtained, thanks to the stabilization of 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 less favored. Tungsten not only substitutes titanium in the titania lattice, as revealed by HAAF-STEM analysis, but it is also present as WOx species partly covering the Pt nanoparticles photodeposited on the mixed oxide support, as revealed by an in depth distribution XPS analysis. This accounts for the progressively lower performance observed with increasing tungsten content in the catalysts, the highest conversion and selective hydrogenation of the 4-nitrostyrene nitro group having been achieved on the catalyst with a 1% W/Ti molar ratio.

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