The Structure of the Active Pd State During Catalytic Carbon Monoxide Oxidization

Abstract

Using grazing incidence X-rays and X-ray photoelectron spectroscopy during the mass transfer limited catalytic oxidation of CO, the long-range surface structure of Pd(100) was investigated. Under the reaction conditions of 50:4 O 2 to CO, 300 mbar pressure, and temperatures between 200 and 450 °C, the surface structure resulting from oxidation and the subsequent oxide reduction was elucidated. The reduction cycle was halted, and while under reaction conditions, angle-dependent X-ray photoelectron spectroscopy close to the critical angle of Pd and modeling of the data was performed. Two proposed models for the system were compared. The suggestion with the metallic islands formed on top of the oxide island was shown to be consistent with the data.

Bridging the Pressure Gap in CO Oxidation

“…The Pd(100) single crystal was probed with the incoming photon beam at an incidence angle of 0.6° and a photon energy of 4600 eV, an energy for which the critical angle of Pd corresponds to around 0.8°. 39 We estimate the probing depth to approximately 10 Å for the Pd metal. To verify a surface sensitivity of the measurements using a grazing incidence angle, we follow the oxide growth in situ on the crystal.…”

“…At POLARIS, however, the incoming X-rays are impinging on the sample surface at a grazing incident angle, close but below the critical angle of total external reflection, and the measurements can be performed with high surface sensitivity. The Pd(100) single crystal was probed with the incoming photon beam at an incidence angle of 0.6° and a photon energy of 4600 eV, an energy for which the critical angle of Pd corresponds to around 0.8° . We estimate the probing depth to approximately 10 Å for the Pd metal.…”

Concluding remarks: Photoelectron spectroscopy and the future of surface analysis

“…Here grazing incidence XPS (GI-XPS) at different angles during reaction conditions can distinguish the different scenarios. 22 GI-XPS operates on the principle of total external reection, wherein below the critical angle, the light source is reected off the surface with a low penetration depth. At low angles, the X-ray light penetrates less deeply into the material; as the angle is increased, the penetration depth of the light increases.…”

“…By calculating the penetration depth of the light and escape likelihood of the photoelectrons, the different models could be evaluated to determine the structure of the surface. 22 Fig. 7a shows an example of an angle-resolved Pd 3d 5/2 spectrum during CO oxidation at a total pressure of 300 mbar and a gas ratio of 50 : 4 O 2 to CO. 22 The metallic and oxide phases are well-resolved and are determined well through curve tting.…”

“…22 Fig. 7a shows an example of an angle-resolved Pd 3d 5/2 spectrum during CO oxidation at a total pressure of 300 mbar and a gas ratio of 50 : 4 O 2 to CO. 22 The metallic and oxide phases are well-resolved and are determined well through curve tting. Fig.…”