The
incorporation of phosphorus into the Ru(0001) surface increases
the selectivity of cyclohexene (C6H10) dehydrogenation
to benzene (C6H6) by 100-fold when compared
to Ru(0001) under steady-state conditions. We propose a series of
elementary steps for the reactions of C6H10 over
Ru(0001) and P0.4-Ru(0001) based on temperature-programmed
reaction (TPR) of C6H10, 1,3-cyclohexadiene
and reactive molecular beam scattering (RMBS) of C6H10 on Ru(0001) and P0.4-Ru(0001). TPR of 1,3-cyclohexadiene
shows that P atoms alter the kinetically relevant step for C6H10 dehydrogenation from C–H activation in 1,3-cyclohexadiene
on Ru(0001) to C–H activation in 2-cyclohexenyl on P0.4-Ru(0001). During TPR of C6H10, C6H6 forms over P0.4-Ru(0001) with an intrinsic
activation energy that is 40 kJ mol–1 lower than
that for Ru(0001). In addition, the presence of P atoms increases
the apparent activation energy for deactivation by 21 kJ mol–1 during RMBS of C6H10. The increase in the
barrier for deactivation, presumably by C–C bond rupture steps,
significantly reduces the quantity of coke formed by consecutive TPR
of C6H10 and contributes to greater selectivities
for C6H6 formation. These observations suggest
that the addition of P atoms to Ru(0001) introduces both electronic
and geometric effects that alter the metal–adsorbate interactions.
These findings indicate that transition-metal phosphides may be useful
for selective dehydrogenation reactions important for reforming light
hydrocarbons (e.g., ethane, propane, and cycloalkanes) to increase
the yield of valuable alkenes and arenes.