Selective Hydrogenation of Styrene to Ethylbenzene over a Pd/Cu(111) Single-Atom Alloy Surface

Abstract The hydrogenation of styrene (C6H5CH═CH2) to ethylbenzene (C6H5CH2CH3) over a 0.1 ML Pd/Cu(111) single-atom alloy (SAA) surface was investigated under ambient pressure conditions using polarization-dependent reflection absorption infrared spectroscopy (PD-RAIRS). Gas-phase species were monitored using s-polarized light, while the difference between p- and s-polarized spectra revealed the evolution of surface species throughout the reaction. The reaction proceeds via a β-selective hydrogenation pathway forming a benzylic σ-bound intermediate (Ph–CH–CH3*), followed by a second hydrogen addition to yield ethylbenzene. Kinetic measurements show the reaction to be approximately first-order in H2 (1.25 ± 0.17) and near-zero-order in styrene (-0.10 ± 0.20), with Arrhenius analysis revealing two activation energies: 9 ± 2 kJ mol–1 at 340–380 K and 47 ± 2 kJ mol–1 at 320–340 K. The turnover frequency at 380 K is 12 s–1. Time-resolved RAIRS and quantitative gas-phase IR analysis confirmed high selectivity and near-complete conversion under steady-state conditions. Postreaction Auger electron spectroscopy revealed carbon accumulation on the surface; however, catalytic activity persisted over multiple cycles, even in the presence of a carbonaceous overlayer.