Carbon deposition on catalysts

The deposition of carbonaceous material on catalysts is categorized in specific and non-specific forms, referring to the location of the carbonaceous deposits in respect with the catalytically active sites. Complete loss of hexane cracking activity of silica-alumina catalysts is established by a relatively small amount of carbonaceous material, which indicates that the deposition blocks the active sites of the catalysts. A different deposition route is followed on alumina-supported metal catalysts. Alumina-supported cobalt and nickel show the formation of filamentary carbon, which causes an initial increase in hexane conversion followed by complete deactivation. The hexane conversion reaches a steady-state level for alumina-supported molybdenum. Non-specific deposition of coke is found on used hydrotreating catalysts, which is reflected in the high "coke tolerance". The coke on these catalysts consists of a three-dimensional carbonaceous structure.