Gas phase chemoselective hydrogenation of p-nitrobenzonitrile over gold: effect of metal particle size, support and the metal-support interface

We report the catalytic gas phase hydrogenation of p-nitrobenzonitrile (p-NBN) to p-aminobenzonitrile (p-ABN) over a series of oxide (CeO₂, Fe₂O₃, Fe₃O₄, TiO₂, ZrO₂ and Al₂O₃) supported (1 mol%) Au catalysts (mean size 3–8 nm from electron microscopy). Hydrogenation rate was structure sensitive with lower turnover frequencies (TOF) over larger Au nanoparticles in the 4–8 nm interval and a decrease in TOF for Au ≤ 3 nm. This size dependence also applies to H₂ chemisorption under reaction conditions. Rate normalised with respect to H₂ uptake showed a dependence on support redox potential where the formation of oxygen vacancies (from O₂ titration) served to stabilise the −NO₂ group, lowering reactivity. Reaction over Au/TiO₂ with modified electronic character (from XPS) deviated from this trend and delivered the highest specific hydrogenation rate, which is attributed to –NO₂ activation at the Au-TiO₂ interface; this effect extends to TiO₂ supported Ag and Pd.