Kinetic investigations of the deactivation by coking of a noble metal catalyst in the catalytic hydrogenation of nitrobenzene using a catalytic wall reactor

The vapour phase hydrogenation of nitrobenzene to aniline is a highly exoth ermic reaction deactivated by coking of the palladium catalyst supported on cc-alumina carrier. For studying the deactivation of the catalyst a cataly tic wall reactor was used in order to ensure isothermal reaction conditions for the kinetic measurements. Furthermore, the catalytic wall reactor allo wed the determination of axial coke profiles by total carbon analysis of di fferent wall segments. On the assumption that the main reaction and the dea ctivation of the catalyst can be assumed separable both the steady state an d the unsteady state kinetics were studied. Nitrobenzene was identified as the relevant coke precursor whereas aniline has neither an influence on the main reaction nor on the deactivation. It could be shown that the hydrogen ation of nitrobenzene to aniline follows a Langmuir-Hinshelwood mechanism c onsidering the surface reaction of the adsorbed nitrobenzene molecule and o ne adsorbed hydrogen atom as the rate determining step. The differentiation of coke on the active sites and coke on the support must be taken into acc ount to model the kinetics of coke formation with sufficient accuracy. (C) 1999 Elsevier Science S.A. All rights reserved.