Near-Ambient-Pressure X-ray Photoelectron Spectroscopy for Mechanistic Insights and Applications in Catalysis.

With the rapid development of catalytic science, research has shifted from empirical exploration toward a mechanistic understanding of catalytic processes. In this context, probing surface chemical properties under realistic reaction conditions is essential for uncovering reaction mechanisms and guiding the development of improved catalytic materials. Conventional surface-sensitive techniques, however, are often limited to ultrahigh-vacuum environments and thus cannot fully capture the complexity of practical catalytic systems. Near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS), also commonly referred to as ambient-pressure XPS (AP-XPS), has emerged as a powerful characterization tool that narrows this gap by enabling surface analysis under millibar- to bar-level reaction environments. By directly monitoring elemental composition, chemical states, and electronic structures, NAP-XPS provides unique insights into active site identification, catalyst dynamic reconstruction, reaction intermediate detection, and reaction pathway investigation. Here, we review recent progress in the application of NAP-XPS to heterogeneous catalysis, highlighting its contributions to mechanistic understanding. We also discuss current instrumental limitations, challenges in spectral interpretation, and future opportunities for advancing both the methodology and applications of NAP-XPS in catalytic research.