CO Induced Adatom Sintering in a Model Catalyst: Pd/Fe3O4 (1303.0664v1)

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.

• The paper demonstrates that CO forms Pd-carbonyl intermediates, markedly increasing Pd adatom mobility on Fe3O4 at ambient conditions.
• It employs STM and DFT to capture the ‘skyhook effect’ where CO weakens the Pd-substrate bond, triggering rapid cluster nucleation.
• The findings suggest that controlling gas-induced mobility can mitigate sintering, offering strategies to extend catalyst lifespan in heterogeneous systems.

CO-Induced Adatom Sintering in Pd/FeO3 Model Catalyst

The paper investigates the phenomenon of CO-induced coalescence and sintering of palladium (Pd) adatoms on an iron oxide (FeO3) surface, specifically reviewing the key mechanisms by which gas molecules can enhance surface atom mobility, contributing to cluster formation that is crucial in heterogeneous catalysis. Using scanning tunneling microscopy (STM) and density functional theory (DFT) calculations, the paper offers a detailed view of the atom-by-atom transformation from isolated Pd adatoms to clusters, addressing the "skyhook" effect where Pd-carbonyl intermediates form, leading to enhanced mobility on the FeO3 substrate.

Key Findings and Methodology

1. Adatom Mobility: The research identifies the formation of Pd-carbonyl species as a significant facilitator of Pd adatom mobility on the FeO3(001) surface at room temperature. Without CO, most Pd adatoms show negligible mobility. The introduction of CO forms Pd-carbonyl intermediates, enabling adatoms to traverse the surface.
2. Skyhook Effect: This mobility increase ties to prior work outlining the "skyhook effect," enhancing surface atom diffusion through gas adsorption. For Pd on the FeO3 model system, CO acts similarly to previously observed systems such as H-Pt, by weakening Pd's bond to the substrate, thereby facilitating adatom migration.
3. Cluster Formation and Dynamics: The paper carefully tracks the progression from isolated Pd adatoms to larger clusters. A critical density of mobile Pd-CO species is essential for nucleation; cluster nucleation notably occurs when these mobile species interact, leading to a characteristic collapse of adatom dispersion.
4. Role of Surface Hydroxyls: Surface hydroxyls, in contrast to CO, inhibit Pd mobility. Pd atoms forming H-Pd complexes at hydroxyls remain immobilized, suggesting a stabilization mechanism that contradicts the otherwise CO-induced mobility.
5. Sintering Mechanism Implications: The findings emphasize the importance of mobile molecule-metal intermediates in heterogeneous catalysis, specifically offering insights into sintering processes that influence catalyst deactivation. The distinct observation of homogeneous nucleation and rapid cluster diffusion challenges conventional assumptions about heterogeneous nucleation at defects and Ostwald ripening as primary sintering mechanisms.

Implications and Future Directions

The results of this paper possess significant implications for catalyst design, particularly in processes involving precious metal nanoparticles. Understanding the role of gas-induced mobility can inform preventative measures against sintering, thereby extending catalyst lifespan and efficiency. Additionally, the observed dynamics offer potential strategies for tailoring surface properties to control particle growth and stability, with applications potentially extending to other metal/oxide systems.

Future research could delve further into tailoring the chemical environment of the catalyst surface to favor desired sintering behaviors or modify adatom diffusion characteristics. Extending similar investigations to other catalyst systems can elucidate mechanisms of mass transport and cluster formation across different environmental and material contexts. Additionally, the FeO3 support's inherent stability suggests broader experimental utility, potentially serving as a base for diverse studies into basic catalytic processes and material interactions.

Conclusion

This exploration into CO-induced Pd adatom sintering highlights pivotal processes in catalytic systems, leveraging STM and DFT calculations for granular insight. The detailed observation of the transformation from discrete Pd atoms to clusters advances understanding of mass transport, offering critical insights into sinter-resistant strategies within heterogeneous catalysis frameworks. The work sets a foundation for further exploration into gas-adatom interactions across various material systems and catalytic scenarios.