Ostwald ripening microkinetic simulation of Au clusters on MgO(0 0 1)

Samantha Francis,Alexandre Boucher,Glenn Jones,Alberto Roldan

doi:10.1016/j.apsusc.2021.151317

Samantha Francis, Alexandre Boucher + Show 2 more

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https://doi.org/10.1016/j.apsusc.2021.151317

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Abstract

Sintering is one of the most common processes responsible for the loss of supported metal nanoparticle catalysts' activity. We have combined ab-initio calculations with microkinetic simulations to investigate the digestion and growth mechanism on Au clusters supported on MgO(001) following a bottom-up approach. The energy barrier for diffusing a single gold atom on the clean MgO surface was found to be 0.29 eV in full agreement with previous reports. Additionally, and as an extension to the entire energy profile related to Ostwald mechanisms, we found all of the activation energies to be below 1.05 eV in the cases investigated. An odd–even cluster trend was observed during ripening, attributed to the stability of pairing the unpaired electrons associated with the single gold atoms. Microkinetic analyses showed that Au single atoms are present on the surface of magnesia up to a temperature of 160 K. At higher temperatures, the system has enough energy for the single atom to diffuse across the surface and attach to other atoms or clusters. At temperatures akin to room temperature, the cluster undergoes ripening to form larger particles in order to achieve a more stable equilibrium.

Highlights

Heterogeneous catalysts provide an alternative, more favourable, reaction pathway for many chemical industry processes such as manufacturing and energy conversion.[1]
From the calculated adsorption, binding and cohesion energies and charge transfer analyses, we found that Au clus ters’ growth leads to the increasing importance of inter-metallic in teractions compared to interactions between the metal cluster and the surface
Significant stability was found in the largest cluster, the energies present an odd–even trend caused by the spin magnetization of odd-numbered clusters and single atoms, and the activation barriers for an Au atom to coalescence are not larger than 1.05 eV

Summary

Introduction

Heterogeneous catalysts provide an alternative, more favourable, reaction pathway for many chemical industry processes such as manufacturing and energy conversion.[1]. One of the most common processes that affect NP’s size and shape is sintering This process has been shown to have a dramatic effect on catalytic ac tivity[8] as it influences the number of active sites and their electronic structure.[9] Sintering is driven by reducing the surface-bulk ratio[10] and can be exacerbated by heat treatment during catalyst synthesis methods and reaction conditions.[11,12,13,14] Two principal mechanisms have been established to contribute to the enlargement of NPs: coales cence and Ostwald ripening.[15] During coalescence,[16] a whole cluster will migrate, collide with another particle and merge to form a single larger nanoparticle, known as Smoluchowski ripening. Through a better un derstanding of sintering rates and its mechanisms, deactivating pro cesses can be minimized or possibly reversed by redispersion of the metal on the catalyst surface.[18]

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