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.

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Rate measurements in the kinetically controlled regime and equilibrium carbon and oxygen chemical titrations show two distinct mechanistic paths during COx methanation reactions on first‐row transition metal clusters. On Ni and, for a limiting set of conditions, Ni−Co clusters, the reaction occurs via the addition of a hydrogen adatom into CH3* intermediates on clusters partially covered with carbon. On Co and, in a subset of conditions, Ni−Co clusters, it occurs via the donation of hydrogen from OH* to CH3* on clusters partially covered with reactive oxygen adatoms (O*). The [CO]2‐to‐[CO2] and [CO2]‐to‐[CO] operating ratios are the surrogates of carbon and oxygen chemical potentials, respectively, as a consequence of water‐gas shift equilibration. These ratios, together with the carbon and oxygen binding energies, determine the relative surface coverages of carbon and oxygen, the involvement of H* vs. OH* in the kinetically‐relevant step, and in turn, the rate dependencies. Stronger carbon and oxygen binding energies lead to more stabilized transition states of the kinetically relevant steps and larger methanation rates.

Kinetic measurements, microkinetic modeling, and CO and O2 uptake experiments lead to a proposed mechanism for CO oxidation on dispersed Ag cluster surfaces. CO turnovers occur via kinetically relevant reactions between O2* and CO* on Ag cluster surfaces nearly saturated with O* and CO*. The operating pressure ratio of O2 to the square of CO, [O2]-to-[CO]2, dictates the relative O* and CO* coverages and in turn the rate coefficients. Low [O2]-to-[CO]2 ratios lead to Ag cluster surfaces saturated with CO*, during which the first-order rate coefficients (r CO[CO]−1) increase linearly with the pressure ratio. As the [O2]-to-[CO]2 ratio increases, the CO* coverages decrease and O* coverages concomitantly increase, and the rate coefficients become independent of the [O2]-to-[CO]2 ratio. CO* binds to Ag clusters more strongly than O*, and as a result, CO* coverages decrease and O* coverages increase as the reaction temperature rises when comparing at a constant [O2]-to-[CO]2 ratio. The rate coefficients for CO oxidation on CO* covered Ag clusters initially increase with increasing Ag cluster diameter to ∼5 nm, but decrease with a further increase in Ag diameter beyond 5 nm. The former trend reflects more weakly bound and reactive O2* and CO*, and the latter likely reflects the depletion of O2* molecules.