Carbon supported gold and silver: Application in the gas phase hydrogenation of m-dinitrobenzene

We have studied the gas phase continuous hydrogenation of m-dinitrobenzene (m-DNB) over acid treated activated carbon (AC) supported Au and Ag prepared by deposition-precipitation. Temperature programmed reduction of a 1%wt. metal loading generated a broad distribution of Au (mean = 45 nm) and Ag (mean= 13 nm) nanoparticles. A decrease in metal content to 0.1%wt. served to increase dispersion (Au/AC (mean= 4 nm) and Ag/AC (mean =3 nm)) where both catalysts exhibited an electron enriched (from XPS) metal phase. m-DNB hydrogenation activity was stable with time on-stream where partial hydrogenation (to m-nitroaniline (m-NAN)) was favoured at low conversion with a switch to full hydrogenation (to m-phenylenediamine (m-PDM)) at higher conversions consistent with a stepwise reaction mechanism. Reaction over 0.1%wt. Au/AC delivered a greater than 5-fold higher turnover frequency (TOP) relative to Ag/AC that can be attributed to a higher H-2 chemisorption capacity under reaction conditions. Activation energy for m-DNB -> m-NAN was the same (110 kJ mol(-1)) for both catalysts. We have established differences in the conversion/selectivity response where m-NAN formation was enhanced over Ag/AC at the same conversion, which is ascribed to differences in m-DNB adsorption/activation. (C) 2015 Elsevier B.V. All rights reserved.