ABSTRACT
Catalytic naphtha reforming processes are accompanied by side reactions leading to carbonaceous deposits both on metallic sites and on the support of the catalyst, causing catalyst deactivation. The carbonaceous deposits on catalysts generally can be divided into different groups: (1) a constant amount of residual carbon; (2) reversible carbon formed instantaneously during operation at working conditions; and (3) irreversibly adsorbed carbon accumulating during reaction after several hours and eventually forming graphitic-like structures.[1,2] Some of the carbon deposits may be beneficial, whereas most of these materials are harmful, depending on the reactions and catalysts.[3] Under working conditions, there exist several different surface compositions and structures on a Pt/Al2O3-reforming catalyst: predominantly uncovered ensembles of clean Pt sites, twodimensional overlayer carbonaceous deposits, and three-dimensional carbon islands.[ 4a] The presence of coke with different natures, compositions, and structures significantly influences the catalyst activity and frequently alters selectivity and, finally, lifetime. Fortunately, deactivation by coking is usually reversible and the coked catalyst can be refreshed by employing a controlled coke burnoff operation. Thus, the activity and selectivity may be completely or partially restored.
