Ambient pressure phase transitions over Ir(1 1 1): at the onset of CO oxidation

In this study we report on the adsorbate structures on an Ir(1 1 1) surface during the phase transition from the inactive to the active state during CO oxidation. The CO oxidation over Pt(1 1 1) is used as a reference case. Where Pt(1 1 1) either is inactive and CO covered or active and O covered, Ir(1 1 1) exhibits a transition state with co-existing chemisorbed O and CO. The observed structural differences are explained in terms of DFT-calculated adsorption energies. For Pt(1 1 1) the repulsive CO–O interaction makes co-existing chemisorbed CO and O unfavourable, while for Ir(1 1 1) the stronger O and CO adsorption allows for overcoming the repulsive interaction. At the onset of CO oxidation over Ir(1 1 1), a CO structure containing defects forms, which enables O2 to dissociatively adsorb on the Ir(1 1 1) surface, thus enabling the CO oxidation reaction. At the mass transfer limit, the Ir(1 1 1) surface is covered by a chemisorbed O structure with defects; hence, the active surface is predominately chemisorbed O covered at a total pressure of 0.5 mbar and no oxide formation is observed

Active Surface Oxygen for Catalytic CO Oxidation on Pd(100) Proceeding under Near Ambient Pressure Conditions

Catalytic CO oxidation reaction on a Pd(100) single-crystal surface under several hundred mTorr pressure conditions has been studied by ambient pressure X-ray photoelectron spectroscopy and mass spectroscopy. In-situ observation of the reaction reveals that two reaction pathways switch over alternatively depending on the surface temperature. At lower temperatures, the Pd(100) surface is covered by CO molecules and the CO₂ formation rate is low, indicating CO poisoning. At higher temperatures above 190 °C, an O–Pd–O trilayer surface oxide phase is formed on the surface and the CO₂ formation rate drastically increases. It is likely that the enhanced rate of CO₂ formation is associated with an active oxygen species that is located at the surface of the trilayer oxide

High-Pressure NO-Induced Mixed Phase on Rh(111): Chemically Driven Replacement

The interaction between nitric oxide (NO) and Rh(111) surface has been investigated by a combination of near-ambient-pressure X-ray photoelectron spectroscopy, low energy electron diffraction, and density functional theory calculations. Under low-temperature and ultrahigh vacuum conditions, our experimental and computational results are consistent with the previous reports for NO adsorption phases on Rh(111). While at room temperature and upon exposure to gaseous NO of 100 mTorr, NO molecules partially dissociate followed by chemical removal of atomic nitrogen by NO from the surface, and the remaining atomic oxygen and NO form a NO/O mixed phase. Interestingly, this mixed phase is stable even after NO evacuation and shows a well-ordered (2 × 2) periodicity. These observations provide a new insight into the NO/Rh(111) system under near-ambient-pressure condition

In Situ Ambient Pressure XPS Study of CO Oxidation Reaction on Pd(111) Surfaces

The CO oxidation reaction on the Pd(111) model catalyst at various temperatures (200–400 °C) under hundreds mTorr pressure conditions has been monitored by in situ ambient pressure X-ray photoelectron spectroscopy and mass spectroscopy. In situ observation of the reaction revealed that the Pd(111) surface is covered by CO molecules at a lower temperature (200 °C), while at higher temperatures (300–400 °C) several oxide phases are formed on the surface. We found that the reactivity is enhanced in the presence of a surface oxide and significantly suppressed by formation of a cluster oxide and the PdO bulk oxide. CO titration experiments suggest that less coordinated oxygen atoms are more reactive for CO oxidation