The hydrogenation of 1,3-hexadiene over a Pd/silica catalyst has been investigated using in situ FTIR
spectroscopy. This study shows that the presence of an organic species on the surface of the catalyst can
have a marked effect on the rate of hydrogenation of 1,3-hexadiene. Toluene-d
8 was coadsorbed on the
surface of the catalyst and, depending on the order of introduction of the reactants to the system, has been
shown to act as either a promoter or a poison for the hydrogenation of the diene. Introduction of the
toluene-d
8 prior to the diene has a promoting effect with the toluene-d
8 acting as a H-transfer agent. When
the toluene-d
8 is introduced after the diene it acts as a poison, blocking active sites and covering the
surface. The ability of toluene-d
8 to act as a promoter or poison appears to be dependent on the availability
of active sites at the time of its introduction to the reaction system. To act as a promoter, or H-transfer
agent, the toluene-d
8 must first be able to form σ bonds to the surface by dissociation of the D atoms of
the methyl group. When the site availability is poor, the toluene-d
8 retains its molecular identity upon
adsorption, thus blocking active sites and reducing the rate of hydrogenation.
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