CATALYTIC ACTIVITY OF BULK TUNGSTEN CARBIDES FOR ALKANE REFORMING .3.REACTION-MECHANISMS AND THE KINETIC-MODEL

This paper describes in close molecular detail the complex reactions w hich occur when a variety of hydrocarbons are adsorbed on fresh or oxy gen-modified bulk tungsten carbide catalysts. On fresh tungsten carbid e without oxygen, cracking reactions are the only reaction processes o ccurring; the results have been interpreted with a kinetic treatment w hich assumes successive carbon-carbon bond ruptures. The conditions un der which the bulk tungsten carbide powders are treated with oxygen ha ve a determinantal influence on their catalytic properties. If air is introduced at moderate temperature (350 degrees C), an acidic characte r of the catalyst is observed which leads to a bifunctional mechanism. Cracking reactions are the most important processes, and correspondin g mechanisms have been interpreted using the presence of OH surface gr oups leading to alkoxy species as intermediates. If air is introduced in flowing hydrogen at high temperature (700 degrees C), the metallic character of the material is observed, leading to cracking and isomeri zation reactions explained via classical metallacyclobutane and sigma- alkyl species, respectively. (C) 1997 Academic Press.