Deactivation of reforming catalysts by coking - a review

Dehydrogenated species, produced by a metal in bifunctional catalysts, lead to the accumulation of coke on the metal and on the support through polymerization. After the first few minutes of the reaction, the metallic surface attains a coke coverage that remains constant while coking of the support continues. The coke deposited on the metal is less dehydrogenated than the coke deposited on the support. For platinum catalysts small metallic particles are less sensitive to coke formation than larger particles. Coke deposition is a complex reaction that results from both the production of coke precursors and their destruction. The metal plays a prominent part in the reaction, (i) by yielding olefins, which can be polymerized on the acidic sites of the support, (ii) by stabilizing carbonaceous deposits through dehydrogenation by an inverse spill-over effect and (iii) by destroying coke precursors. The modification of Pt by different additives such as Ir or Re enhances this hydrogenolysis reaction but also changes the location and the nature of the coke.