Carbonaceous substances, catalyst

Carbonaceous substances and cokes cover the catalyst surface. 

Aromatic compounds larger than naphthalene are thermally and catalyti-cally labile toward condensation through their oligomers and carbonaceous derivatives (133). Hence, the organic substances, which stay on the catalyst for extended periods of time, become carbonaceous, irreversibly covering the catalyst surface. Such carbonaceous substances may also trap minerals, thus increasing the volume of coating materials. 

Which is the best catalyst for accelerating the reaction depends on the nature of the working materials. For the reaction of hydrogen or oxygen in potassium hydroxide solution, nickel or silver is suitable for carbonaceous fuels as well as for the reaction of oxygen in acid electrolytes platinum metals were up to the middle 60s, the only known catalysts. Precious metals are ruled out by price for wide application in fuel cells, and the search for cheaper catalysts has been actively pursued in many research laboratories. Many classes of inorganic substances (carbides, nitrides, oxides, sulfides, phosphides, etc.) have been investigated and, in particular, several chelates. 

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