Chasing PtO species in ceria supported platinum during CO oxidation extinction with correlative operando spectroscopic techniques

Interrogating site dependent kinetics over SiO2-supported Pt nanoparticles

A detailed knowledge of reaction kinetics is key to the development of new more efficient heterogeneous catalytic processes. However, the ability to resolve site dependent kinetics has been largely limited to surface science experiments on model systems. Herein, we can bypass the pressure, materials, and temperature gaps, resolving and quantifying two distinct pathways for CO oxidation over SiO2-supported 2 nm Pt nanoparticles using transient pressure pulse experiments. We find that the pathway distribution directly correlates with the distribution of well-coordinated (e.g., terrace) and under-coordinated (e.g., edge, vertex) CO adsorption sites on the 2 nm Pt nanoparticles as measured by in situ DRIFTS. We conclude that well-coordinated sites follow classic Langmuir-Hinshelwood kinetics, but under-coordinated sites follow non-standard kinetics with CO oxidation being barrierless but conversely also slow. This fundamental method of kinetic site deconvolution is broadly applicable to other catalytic systems, affording bridging of the complexity gap in heterogeneous catalysis.

Synthesis and Characterization of Catalytically Active Au Core─Pd Shell Nanoparticles Supported on Alumina

A two-step seeded-growth method was refined to synthesize Au@Pd core@shell nanoparticles with thin Pd shells, which were then deposited onto alumina to obtain a supported Au@Pd/Al₂O₃ catalyst active for prototypical CO oxidation. By the strict control of temperature and Pd/Au molar ratio and the use of l-ascorbic acid for making both Au cores and Pd shells, a 1.5 nm Pd layer is formed around the Au core, as evidenced by transmission electron microscopy and energy-dispersive spectroscopy. The core@shell structure and the Pd shell remain intact upon deposition onto alumina and after being used for CO oxidation, as revealed by additional X-ray diffraction and X-ray photoemission spectroscopy before and after the reaction. The Pd shell surface was characterized with in situ infrared (IR) spectroscopy using CO as a chemical probe during CO adsorption-desorption. The IR bands for CO ad-species on the Pd shell suggest that the shell exposes mostly low-index surfaces, likely Pd(111) as the majority facet. Generally, the IR bands are blue-shifted as compared to conventional Pd/alumina catalysts, which may be due to the different support materials for Pd, Au versus Al₂O₃, and/or less strain of the Pd shell. Frequencies obtained from density functional calculations suggest the latter to be significant. Further, the catalytic CO oxidation ignition-extinction processes were followed by in situ IR, which shows the common CO poisoning and kinetic behavior associated with competitive adsorption of CO and O₂ that is typically observed for noble metal catalysts.

Stability of Pt-Adsorbed CO on Catalysts for Room Temperature-Oxidation of CO

A large signal of gas-phase CO overlapping with those of adsorbates is often present when investigating catalysts by operando diffuse reflectance FT-IR spectroscopy. Physically removing CO(g) from the IR cell may lead to a fast decay of adsorbate signals. Our work shows that carbonyls adsorbed on metallic Pt sites fully vanished in less than 10 min at 30 °C upon removing CO(g) when redox supports were used. In contrast, a broad band assigned to CO adsorbed on oxidized Pt sites was stable. It was concluded that physically removing CO(g) at room temperature during IR analyses will most likely lead to changes in the distribution of CO(ads) and a misrepresentation of the Pt site speciation, misguiding the development of efficient low-temperature CO oxidation catalysts. A tentative representation of the nature of the Pt phases present depending on the feed composition is also proposed.