The Effects of P-Atoms on the Selective Dehydrogenation of C₆H₁₀over Model Ru Surfaces

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.