H₂-D₂ Exchange Activity and Electronic Structure of Ag Pd_1- Alloy Catalysts Spanning Composition Space

Many computational studies of catalytic surface reaction kinetics have demonstrated the existence of linear scaling relationships between physical descriptors of catalysts and reaction barriers on their surfaces. In this work, the relationship between catalyst activity, electronic structure, and alloy composition was investigated experimentally using a Ag Pd_1- Composition Spread Alloy Film (CSAF) and a multichannel reactor array that allows measurement of steady-state reaction kinetics at 100 alloy compositions simultaneously. Steady-state H₂-D₂ exchange kinetics were measured at atmospheric pressure on Ag Pd_1- catalysts over a temperature range of 333-593 K and a range of inlet H₂ and D₂ partial pressures. X-ray photoelectron spectroscopy (XPS) was used to characterize the CSAF by determining the local surface compositions and the valence band electronic structure at each composition. The valence band photoemission spectra showed that the average energy of the valence band, ε̅_v, shifts linearly with composition from -6.2 eV for pure Ag to -3.4 eV for pure Pd. At all reaction conditions, the H₂-D₂ exchange activity was found to be highest on pure Pd and gradually decreased as the alloy was diluted with Ag until no activity was observed for compositions with x _Pd < 0.58. Measured H₂-D₂ exchange rates across the CSAF were fit using the Dual Subsurface Hydrogen (2H') mechanism to extract estimates for the activation energy barriers to dissociative adsorption, ΔE _ads ^‡, associative desorption, ΔE _des ^‡, and the surface-to-subsurface diffusion energy, ΔE _ss, as a function of alloy composition, x _Pd. The 2H' mechanism predicts ΔE _ads ^‡ = 0-10 kJ/mol, ΔE _des ^‡ = 30-65 kJ/mol, and ΔE _ss = 20-30 kJ/mol for all alloy compositions with x _Pd ≥ 0.64, including for the pure Pd catalyst (i.e., x _Pd = 1). For these Pd-rich catalysts, ΔE _des ^‡ and ΔE _ss appeared to increase by ∼5 kJ/mol with decreasing x _Pd. However, due to the coupling of kinetic parameters in the 2H' mechanism, we are unable to exclude the possibility that the kinetic parameters predicted when x _Pd ≥ 0.64 are identical to those predicted for pure Pd. This suggests that H₂-D₂ exchange occurs only on bulk-like Pd domains, presumably due to the strong interactions between H₂ and Pd. In this case, the decrease in catalytic activity with decreasing x _Pd can be explained by a reduction in the availability of surface Pd at high Ag compositions.