(679b) Long-Range Catalytic Effects Uncovered By Atomically Precise Active Site Control in Pd-Zn Intermetallic Catalysts

Intermetallic compounds, composed of two or more metals, possess long-range crystallographic order (i.e., well defined site occupancy) with distinct differences in catalytic behavior from their parent metals. Relating the observed catalytic kinetics to active site structure and composition is a significant challenge in the field. We have recently shown Pd-Zn γ-brass (Pd_8+xZn_44-x, 0 ≤ x ≤ 2) has tunable active site nuclearity, exposing monomers and/or trimers of Pd in a Zn matrix, with the ratio of monomers to trimers set by stoichiometry. The limited co-adsorption configurations available on monomer and trimer sites allow DFT to fully enumerate all possible coverage scenarios. Catalytic testing demonstrates monomers are inactive and trimers are active for ethylene hydrogenation. For stoichiometries, such as Pd₉Zn₄₃ which expose both monomers and trimers, the hydrogen order decreases and ethylene order increases with increasing temperature. A microkinetic model informed by both DFT calculations and experimental results reveals hydrogen adsorption and dissociation on the central atom of the trimer as the rate determining step. The model also uncovers the role of monomer sites in supplying H atoms to the trimer sites via surface diffusion, leading to the temperature dependence of the reaction orders. In systems with no monomers present (Pd₁₀Zn₄₂), reaction orders show no dependence on temperature over the range tested. The γ-brass structure can also accommodate coinage metals by replacing the central Pd atom of the trimer, creating Pd-M-Pd sites. When M = Cu, the increased barrier of hydrogen dissociation on copper relative to Pd causes non-monotonic dependence of the reaction orders on temperature, revealing a delicate interplay between the short-range (Cu in trimer) and long-range (Pd monomer) H-atom supply. This work demonstrates the power of DFT and microkinetic models to uncover subtle details in reaction mechanisms on well-defined, isolated reaction sites, when informed by rigorous experimental measurements.