Oxygen-coverage effects on molecular dissociations at a Pt metal surface

Area of Science:

• Surface science
• Catalysis
• Chemical kinetics

Background:

• Adsorbate coverage significantly impacts catalytic surface reactions, but these effects are not fully understood.
• Nitrogen oxide (NO) oxidation catalysis involves competing reactions, primarily O2 and NO2 dissociations on metal surfaces.
• Platinum surfaces, particularly Pt(111), are widely studied model systems for heterogeneous catalysis.

Purpose of the Study:

• To investigate and contrast the rates of O2 and NO2 dissociations on a Pt(111) surface as a function of oxygen coverage.
• To elucidate the mechanistic differences governing O2 and NO2 reactions under varying adsorbate coverages.
• To determine the preferred reaction pathway for NO oxidation catalysis under realistic surface conditions.

Main Methods:

• In situ x-ray photoelectron spectroscopy (XPS) was employed to monitor surface reactions and adsorbate coverages in real-time.
• Density-functional theory (DFT) simulations were utilized to model reaction pathways and understand surface interactions at a fundamental level.
• Kinetic analysis was performed to compare the rates of O2 and NO2 dissociation under different oxygen coverages.

Main Results:

• The dissociation rate of NO2 was found to be significantly less sensitive to increasing oxygen coverage compared to O2 dissociation on Pt(111).
• DFT simulations revealed that the NO2 reaction pathway exhibits greater adaptability to a crowded surface environment than O2 dissociation.
• While O2 and NO2 dissociation rates were comparable at low oxygen coverage, NO2 dissociation became orders of magnitude faster at coverages typical for NO oxidation catalysis.

Conclusions:

• NO2 dissociation is a more robust and efficient reaction pathway than O2 dissociation on Pt(111) surfaces, especially under conditions of high oxygen coverage.
• The adaptability of the NO2 reaction mechanism is key to its superior performance in crowded catalytic environments.
• These findings have significant implications for optimizing catalysts for nitrogen oxide oxidation processes, favoring NO2 dissociation pathways.