# A Density Functional Theory-Based Particle Swarm Optimization Investigation of Metal Sulfide Phases for Ni-Based Catalysts

## TL;DR

This paper uses advanced computational methods to study how sulfur interacts with nickel surfaces, revealing stable sulfide structures and their formation mechanisms.

## Contribution

The study introduces a novel combination of DFT and PSO to determine stable sulfide phases on Ni(111) surfaces under high sulfur coverage.

## Key findings

- Each pair of sulfur atoms can sulfurize up to three Ni atoms in a single layer (Ni:S = 3:2).
- Ni3S2 is identified as the most stable sulfide phase under typical desulfurization conditions.
- Unsaturated phases like Ni3S and Ni2S are proposed as intermediate states in sulfide formation.

## Abstract

Nickel (Ni) catalysts have numerous applications in the chemical industry, but they are susceptible to sulfurization, with their sulfurized structures and underlying formation mechanisms remaining unclear. Herein, density functional theory (DFT) combined with the particle swarm optimization (PSO) algorithm is employed to investigate the low-energy structures and formation mechanisms of sulfide phases on Ni(111) surfaces, especially under high-sulfur-coverage conditions where traditional DFT calculations fail to reach convergence. Using (3×3
) Ni(111) slab models, we identify a sulfurization limit, finding that each pair of deposited sulfur atoms can sulfurize one layer of three Ni atoms at most (Ni:S = 3:2), with additional sulfur atoms penetrating deeper layers until saturation. Under typical reactive adsorption desulfurization conditions, the ab initio thermodynamics analysis indicates that Ni3S2 is the most stable sulfide phase, consistent with sulfur K-edge XANES data. Unsaturated phases, including Ni3S, Ni2S, and Ni9S5, represent intermediate states towards saturation, potentially explaining the diverse Ni sulfide compositions observed in experiments.