CO Induced Adatom Sintering in a Model Catalyst: Pd/Fe3O4

TL;DR

This study uses STM to investigate how CO gas accelerates Pd adatom coalescence on Fe3O4 surfaces, revealing the formation of mobile Pd-carbonyl species and the impact of surface hydroxyls on mobility, elucidating gas-induced sintering mechanisms.

Contribution

It demonstrates the role of CO in forming mobile Pd-carbonyl species that promote coalescence, and shows how surface hydroxyls inhibit this process, providing detailed mechanistic insights.

Findings

CO increases Pd adatom mobility via skyhook effect

Pd-carbonyl species temporarily trap adatoms and promote cluster formation

Surface hydroxyls prevent Pd adatom mobility and coalescence

Abstract

The coarsening of catalytically-active metal clusters is often accelerated by the presence of gases through the formation of mobile intermediates, though the exact mechanism through which this happens is often subject to debate. We use scanning tunneling microscopy (STM) to follow the CO induced coalescence of Pd adatoms supported on the Fe3O4(001) surface at room temperature. We show that highly-mobile Pd-carbonyl species, formed via the so-called skyhook effect, are temporarily trapped at other Pd adatoms. Once these reach a critical density, clusters nucleate; subsequent coarsening occurs through cluster diffusion and coalescence. While CO increases the mobility in the Pd/Fe3O4 system, surface hydroxyls have the opposite effect. Pd atoms transported to surface OH groups are no longer susceptible to the skyhook effect and remain isolated. Following the evolution from well-dispersed metal adatoms into clusters, atom-by-atom, allows identification of the key processes that underlie gas-induced mass transport.