Embedded Rh(1 wt.%)@Al2O3: Effects of high temperature and prolonged aging under methane partial oxidation conditions

Abstract

A simple and efficient synthetic procedure for the production of catalysts stable at high temperature was investigated. Methane partial oxidation was studied over a Rh(1wt.%)@Al2O3 catalyst, composed of Rh nanoparticles embedded in an Al2O3 matrix, and the results were compared with those of a reference Rh(1wt.%)/Al2O3, obtained by incipient wetness impregnation procedure. The embedded catalyst was prepared by precipitation of Al(OH)3 in the presence of stabilised Rh nanoparticles, followed by calcination. H2 chemisorption data confirm the accessibility of most of the protected Rh nanoparticles. Both Rh catalysts are active for MPO, reaching complete CH4 conversion above 1023K. Notably, the embedded catalyst is more resistant to deactivation during consecutive run-up and steady state catalytic experiments. The protection offered by the surrounding layer of porous oxide prevents extensive sintering of the active metal phase, even after high temperature aging, as observed by high resolution TEM. Moreover, with respect to the standard impregnated sample, the embedded Rh(1wt.%)@Al2O3 is more resistant towards the undesirable incorporation of Rh into the alumina under oxidising conditions. At 1023K, stable MPO activity is observed for at least 60h, after which slow deactivation starts, essentially by coke deposition. Treatment with O2 restores the catalytic activity, while brief switches to O2 during reaction prevent deactivation. Increasing the working temperature or the reactant concentration significantly reduces the effects of coke deactivation.