KINETICS OF REACTIONS CATALYZED BY METALS - ROLE OF SURFACE HYDROCARBON RESIDUES IN CONVERSION OF ALKANES ON PT

It is shown that the total carbon coverage as measured by AES after reaction of alkanes on platinum surfaces can be broken down into three types of carbon atoms: (i) a small carbon coverage of about 4%, present on the metal surface previous to any reaction, thus independent of the reaction studied and attributed to impurities; (ii) carbon atoms existing in the form of chemisorbed radical hydrocarbon active intermediates in the reaction under the catalytic working conditions, and thus reversibly adsorbed under these conditions. This coverage is quite instantaneously attained; (iii) carbon atoms in the form of highly dehydrogenated and irreversibly adsorbed species, the coverage of which increases slowly but continuously with time. The formation of this type of carbon is attributed to a side reaction. It is clearly shown that these carbon atoms act as a poison for the studied reactions and not as a cocatalyst, as previously proposed by Somorjai. This difference in concept is associated with a difference in point of view of the reaction mechanism. Special attention is paid to experimental procedures, some of which induce an apparent irreversibility in the chemisorbed carbonaceous residues. © 1990.