Radiotracer study of adsorption during catalysis. 14C-chlorobenzene hydrogenation over palladium on silica

Abstract

This paper deals with the simultaneous observation of adsorption and catalysis using radio-actively labelled species. The occupancy principle was used together with time-dependent tracer flow methods. The reaction studied was hydrogenolysis of ¹⁴C-chlorobenzene over a Pd/SiO₂ catalyst in continuous and pulsed-flow modes. Surface radioactivity was monitored by G.–M. counting. Adsorptions and desorptions of ¹⁴C-chlorobenzene and ¹⁴C-benzene were also examined in nitrogen flows. The temperature range was 193 to 448 K and overall pressures atmospheric.

Typical conversions of chlorobenzene were 40% and coverage of the catalyst by chlorobenzene was between half and one monolayer of the total area, i.e. metal plus silica. It appeared that the support acted as a reservoir for molecules which reacted on the Pd particles. Desorption of chloro-benzene from silica turned out to be a slow process compared with the desorptions occurring during conversion to benzene on a supported catalyst.