Multiscale Modeling of the Water-Gas Shift Reaction on CuAu Nanoparticles

Abstract

The water–gas shift (WGS) reaction is an important step in adjusting H2/CO ratios in syngas and is widely used for pure hydrogen production. Industrial Cu-based catalysts, such as Cu-ZnO-Al2O3, exhibit high activity at low temperatures and are the benchmark for this reaction. In this context, alloying Cu with Au to form CuAu bimetallic nanoparticles has been explored as a strategy to tune adsorption properties, catalyst stability, and activity under reaction conditions. In particular, the Cu: Au ratio can modulate the adsorption strength of key intermediates and the energy barriers of elementary steps. In this work, density functional theory (DFT) combined with kinetic Monte Carlo (KMC) simulations is used to investigate the stability and catalytic performance of Cu, Au, CuAu, Cu3Au, and CuAu3 catalysts towards the WGS and related reactions, in particular the reverse water-gas shift (RWGS) and CO oxidation. Moreover, molecular dynamics (MD) simulations are performed on CuAu nanoparticles to probe how an Ar+ ion-beam treatment modifies the particles and to infer its potential catalytic impact. After obtaining surface energies of CuAu for representative facets and Cu:Au ratios via PBE with and without D3 dispersion corrections, the Wulff constructions indicate that the (111) facet dominates the equilibrium nanoparticle morphology across all evaluated alloy compositions. Adsorption energies and energy barriers for the WGS reaction on the (111) surfaces of Cu and Cu-Au alloys indicate that the associative (via a carboxyl intermediate) mechanism is energetically favored over the direct pathway. Moreover, increasing Au content leads to higher energy barriers, resulting in more favorable energetics for Cu-rich compositions. These trends are further reflected in KMC simulations, which connect atomistic energetics to catalytic performance under realistic reaction conditions. The associative pathway via the COOH intermediate is indeed identified as the dominant pathway to products, with Cu3Au exhibiting the highest activity for the WGS reaction at 300°C and 400°C. Additional kinetic simulations of the RWGS (500°C) and CO oxidation (400°C) reactions reveal that the optimal alloy composition is reaction-dependent, with CuAu and CuAu3 performing best for RWGS and CO oxidation, respectively. Finally, MD simulations of Ar+ ion-beam modification of Cu–Au nanoparticles show preferential sputtering of Cu atoms, leading to surface segregation of Cu and changes in nanoparticle shape and surface morphology. These structural changes affect the availability and nature of active sites, potentially impacting catalytic performance. Overall, the present thesis provides a multiscale framework that combines atomistic, kinetic, and dynamic simulation analysis, linking alloy composition, surface stability, and catalytic behavior in Cu-Au systems.