Chemical and Structural Dynamics of Nanostructures in Bimetallic Pt–Pd Catalysts, Their Inhomogeneity, and Their Roles in Methane Oxidation

Abstract

This study unravels the diverse sizes and chemical compositions of various nanostructures, from single atoms to monometallic clusters and bimetallic particles in realistic, supported bimetallic Pt–Pd catalysts. Aberration-corrected scanning transmission electron microscopy, CO infrared spectroscopy, and oxygen uptake-titration studies probe the structural dynamics of these nanocreatures in response to changing gas-phase compositions and oxygen chemical potentials, whereas rate assessments in the kinetically controlled regime under differential fuel-lean conditions at 698–773 K elucidate their catalytic roles in C–H bond activation during methane oxidation catalysis. Reductive treatments on Pt–Pd bimetallic catalysts (0.92–3.67 wt % Pt, 1 wt % Pd) lead to redistributions of the metals as Pt single atoms, small Pt clusters (∼2 nm), and large Pt–Pd alloy clusters (>5 nm), and their relative abundances depend largely on the overall Pt-to-Pd atomic ratio. Treatments in incremental O2 pressures at temperatures relevant to CH4–O2 catalysis redisperse the small Pt clusters, thus increasing the density of Pt single atoms, while the remaining clusters retain their metallic bulk. The large Pt–Pd alloy clusters, however, undergo incipient structural reconstruction, forming a thin PdO shell covering a Pt-rich core, driven by the large, negative free energy of PdO formation and the lower surface free energy of PdO in comparison to Pt. During CH4–O2 catalysis, Pt single atoms and small Pt clusters are largely inactive. In contrast, the core–shell clusters are highly reactive. On these cluster surfaces, the O2– anions are highly nucleophilic, whereas the Pd2+ cations are highly electrophilic, as they are contacted to the underlying Pt-rich core. They form Pd2+–O2– site pairs that catalyze the kinetically relevant C–H bond cleavage of methane at <40 kJ mol–1 via the formation of the highly stabilized four-center transition state (H3Cδ−- -Pd2+- -Hδ+- -O2–)⧧ much more effectively than monometallic O*-covered Pt or PdO clusters. An increase in the Pt-to-Pd atomic ratio results in excess Pt that is present as inactive Pt single atoms or Pt clusters, thus lowering the overall, ensemble average rate constants. The Pt-to-Pd atomic ratio of ∼0.5 is optimal for creating effective Pd2+–O2– site pairs on bimetallic core–shell clusters and minimizing the density of inactive Pt single atoms and clusters for CH4–O2 reactions.