Effects of phosphorus addition on selectivity and stability of Pd model catalysts during cyclohexene dehydrogenation

Abstract

Transition metal phosphides are more selective for hydrocarbon dehydrogenation than the corresponding unmodified transition metals, however there are few detailed experiments probing thermodynamics and kinetics of adsorption and reactions of hydrocarbons on these surfaces. Here, we use a combination of temperature programmed reaction (TPR) and reactive molecular beam scattering (RMBS) of cyclohexene (C₆H₁₀) and benzene (C₆H₆) and temperature programmed desorption of CO and H₂ to examine how the addition of P-atoms alters the catalytic properties of Pd(111) for non-oxidative dehydrogenation of C₆H₁₀ to C₆H₆. Quantitative analyses of TPR results demonstrate that a phosphorus modified Pd(111) surface (P_0.34–Pd) dehydrogenates C₆H₁₀ to C₆H₆ with a selectivity approaching 100%, which is ∼3-fold greater than that of the unmodified Pd(111) surface (∼30%). These differences in selectivity correlate with a significantly reduced desorption energy for small molecules (e.g., CO, C₆H₆, C₆H₁₀, H₂) on P_0.34–Pd in comparison to Pd(111). Together these comparisons suggest that adsorbates exchange less charge with P_0.34–Pd than with Pd(111), which agrees with prior computational predictions. Diminished electron exchange between Pd atoms and bound reactive intermediates leads also to greater near steady-state selectivities for dehydrogenation of C₆H₁₀ to C₆H₆, decreased rates of C–C bond rupture, and increased catalyst stability. Apparent activation energies for dehydrogenation of C₆H₁₀ are lower for Pd (−13 kJ mol⁻¹) than P_0.34–Pd (−7 kJ mol⁻¹), while those for C₆H₆ decomposition seem to differ more significantly (∼90 kJ mol⁻¹ on Pd; immeasurable rates and barrier for P_0.34–Pd) as shown by the RMBS of C₆H₁₀ and C₆H₆ respectively. As a consequence, P also increases apparent barriers for catalyst deactivation observed during RMBS of C₆H₁₀ (1 kJ mol⁻¹ on Pd to 38 kJ mol⁻¹ on P_0.25–Pd) due to the decreased tendency to decompose C₆H₆ to more hydrogen-deficient forms of carbon. These results demonstrate that the presence of P-atoms on and within the near surface region of Pd significantly impacts the energetics of C–H and C–C bond rupture pathways in ways that lead to greater selectivities and stabilities for hydrocarbon dehydrogenation reactions.