Tunable gas phase hydrogenation of m-dinitrobenzene over alumina supported Au and Au-Ni

The gas phase hydrogenation of m-dinitrobenzene (m-DNB) has been investigated over alumina supported Au (1 mol%), Ni (10 mol%) and Au-Ni (Au/Ni mol ratio = 1/10). Au/Al₂O₃ delivered a time invariant exclusive formation of m-nitroaniline (partial reduction), Ni/Al ₂O₃ promoted total nitro-group reduction to give m-phenylenediamine as the sole product and Au-Ni/Al₂O₃ generated both products (partial and complete hydrogenation); Ni/Al ₂O₃ and Au-Ni/Al₂O₃ exhibited a temporal decline in activity. A physical mixture of Au/Al₂O ₃ + Ni/Al₂O₃ delivered a hydrogenation rate and product composition that was equivalent to that obtained with Ni/Al ₂O₃, i.e. Au/Al₂O₃ had a negligible contribution to the catalytic process, which was governed by the Ni component in the mixture. The catalysts have been characterized in terms of temperature-programmed reduction (TPR), H₂ chemisorption/temperature- programmed desorption (TPD), BET area/pore volume, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), diffuse reflectance UV-vis (DRS UV-vis) and HRTEM-EDX measurements. The presence of zero valent Au and Ni is established post-TPR where EDX mapping has revealed a close surface proximity of both metals (mean particle size = 9 nm) in Au-Ni/Al₂O₃ with evidence (from XPS) of electron donation from Ni to Au. While the addition of Au to Ni/Al₂O₃ served to suppress H₂ chemisorption post-TPR, H₂-TPD revealed a more facile release of a greater amount of hydrogen from Au-Ni/Al₂O₃ relative to Ni/Al₂O₃, which we attribute to surface Au-Ni interaction that is also manifest in the hydrogenation activity/selectivity response. Our results establish the feasibility of controlling product distribution through the application of mono- (Au or Ni) and bimetallic (Au-Ni) catalysts. © 2010 Elsevier B.V. All rights reserved.