Transient Surface Oxides Form on Pt(111) - But Vanish During Ammonia Oxidation

TL;DR

This study shows that platinum surfaces do not form stable oxides during ammonia oxidation; instead, transient structures appear and vanish, with reaction pathways influenced by adsorbed species and oxygen pressure.

Contribution

The paper provides direct operando evidence that Pt(111) does not develop stable surface oxides during ammonia oxidation, clarifying the role of transient structures in the process.

Findings

Transient surface structures form under oxygen-rich conditions.

Stable surface oxides do not develop during ammonia oxidation.

Reaction proceeds via a Langmuir-Hinshelwood mechanism influenced by adsorbed species.

Abstract

Ammonia oxidation on platinum catalysts is pivotal for industrial nitric acid production and environmental abatement, yet the role of surface oxides in this process remains debated. Using operando surface X-ray diffraction (SXRD), crystal truncation rod (CTR) analysis, and near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS), we reveal that Pt(111) does not form stable surface oxides under ammonia oxidation conditions. Instead, transient hexagonal monolayers and a Pt(111)-(8x8) superstructure emerge under oxygen-rich atmospheres and above the catalyst light-off temperature, but vanish upon ammonia exposure. Real-time mass spectrometry and NAP-XPS demonstrate that the reaction proceeds via a Langmuir-Hinshelwood mechanism, where adsorbed NHx and O species availability dictate selectivity toward NO or N2. Reducing the oxygen pressure by an order of magnitude slows the kinetics of oxide growth, only detected after 24 hr, and facilitated by transient and precursor structures.