In Situ Detection of Active Edge Sites in Single-Layer MoS$_2$ Catalysts

TL;DR

This study employs ambient pressure X-ray Photoelectron Spectroscopy and Density Functional Theory to observe and understand the atomic-scale structure and electronic state of active MoS2 edge sites during catalysis, revealing a unique metallic signature.

Contribution

It introduces an in-situ spectroscopic method combined with theoretical modeling to identify and characterize active edge sites of MoS2 catalysts under working conditions.

Findings

In-situ AP-XPS detects MoS2 edge sites during catalysis.

Core level shifts indicate a metallic character of active edges.

The metallic state correlates with catalytic activity.

Abstract

MoS2 nanoparticles are proven catalysts for processes such as hydrodesulphurization and hydrogen evolution, but unravelling their atomic-scale structure under catalytic working conditions has remained significantly challenging. Ambient pressure X-ray Photoelectron Spectroscopy (AP-XPS) allows us to follow in-situ the formation of the catalytically relevant MoS2 edge sites in their active state. The XPS fingerprint is described by independent contributions to the Mo3d core level spectrum whose relative intensity is sensitive to the thermodynamic conditions. Density Functional Theory (DFT) is used to model the triangular MoS2 particles on Au(111) and identify the particular sulphidation state of the edge sites. A consistent picture emerges in which the core level shifts for the edge Mo atoms evolve counter-intuitively towards higher binding energies when the active edges are reduced. The shift is explained by a surprising alteration in the metallic character of the edge sites, which is a distinct spectroscopic signature of the MoS2 edges under working conditions.