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.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films