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Abstract
The surface chemistry of ethylene, propylene and 2-butenes on clean and oxygen-modified Mo(100) has been studied by TPD UPS and RAIRS to shed some light on the catalytic mechanism of olefin metathesis in heterogeneous phase. The currently accepted mechanism for olefin metathesis in homogeneous phase is the so-called "carbene-metallacyclobuatane". The kinetic studies of small olefin metathesis on molybdenum model catalysts also suggest that a "carbene" mechanism operates in heterogeneous phase.
It is found that olefins adsorb on clean and oxygen-covered Mo(100) predominantly by donation of $\pi$ electrons to the molybdenum surface so that the desorption activation energy increases with increasing oxygen coverage $(\theta$(O)) since surface oxygen lowers the Fermi level of the molybdenum. For identical $\theta$(O), the heat of adsorption varies in the order $\rm C\sb4H\sb8>C\sb3H\sb6>C\sb2H\sb4,$ which is in accord with variations in the ionization energy of the $\pi$ orbitals in ethylene (10.5 eV), propylene (9.7 eV) and 2-butene (9.1 eV). Olefins adsorbed on the four-fold hollow site of the Mo(100) surface dehydrogenate to release hydrogen and adsorb carbon so that the hydrogen yield decreases linearly with $\theta$(O). Two other reaction processes for adsorbed alkenes are C=C double bond cleavage and self-hydrogenation. The double bond dissociation leads to the formation of surface carbenes (for ethylene and propylene) which can subsequently hydrogenate to produce methane, and methyl carbenes (for propylene and 2-butene) which can decompose to yield methane. The methane yield detected from an adsorbed ethylene overlayer shows a maximum when $\theta$(O) is $\sim$0.6 ML and is zero when $\theta$(O) is zero or greater than unity. For 2-butene the methane yield decreases linearly with increasing $\theta$(O). This trend for adsorbed propylene comprises a linear combination of ethylene's and 2-butene's. Self-hydrogenation is blocked by the addition of oxygen on the Mo(100) surface but is much enhanced by pre-covering the surface with hydrogen.
The carbene chemistry on O/Mo(100) is investigated by grafting:CH$\sb3,$ $\rm{\cdot}CH\sb2$ and:CHCH$\sb3$ species onto the surfaces from thermal decomposition of their iodides. Methane is found in all the cases and it is shown that the rate-limiting step in the hydrogenation of carbenes is the addition of the second hydrogen to adsorbed methyl species.
