Gold particle size effects in the gas-phase hydrogenation of m-dinitrobenzene over Au/TiO 2

Abstract

The effect of Au particle size, supported on TiO 2, on the gas-phase hydrogenation of m-dinitrobenzene has been considered. The catalysts (Au loading = 0.1 and 1 mol%) were prepared by impregnation with HAuCl 4 and a range of Au particle sizes (3.4–10.0 nm) was generated by temperature-programmed reduction over the interval 603 K ⩽ T ⩽ 1273 K. A thermal treatment of TiO 2 at T ⩾ 873 K was required for the allotropic change from anatase to rutile but the presence of Au lowered the requisite temperature for complete transformation by up to 400 K. m-Dinitrobenzene hydrogenation exhibited a particle size sensitivity where higher specific rates were obtained with smaller Au particles, irrespective of the support composition ( i.e. anatase:rutile ratio). The reaction over each Au/TiO 2 catalyst generated m-phenylenediamine (reduction of both –NO 2 groups) and/or m-nitroaniline (reduction of one –NO 2 group). A parallel/consecutive kinetic model has been applied to quantify the catalytic selectivity where Au particles <5 nm favoured m-nitroaniline production. The dependence of hydrogenation performance on Au particle size is accounted for in terms of a modification to Au electronic character, which impacts on m-dinitrobenzene adsorption/activation.