Adsorbate-induced alloy phase separation: a direct view by high-pressure scanning tunneling microscopy

Area of Science:

• Surface science
• Materials science
• Physical chemistry

Background:

• Surface alloys, such as gold on nickel (Au/Ni(111)), are crucial in catalysis.
• Understanding their stability under various conditions, including high pressures of reactive gases, is essential for optimizing catalytic processes.

Purpose of the Study:

• To investigate the effect of high carbon monoxide (CO) pressures on the stability of the Au/Ni(111) surface alloy.
• To elucidate the mechanism and driving forces behind CO-induced structural changes in the surface alloy.

Main Methods:

• High-pressure scanning tunneling microscopy (HP-STM) was employed to observe surface structural changes in real-time.
• Time-lapsed STM movies were used to track the dynamics of atomic rearrangement.
• Density functional theory (DFT) calculations were performed to understand the thermodynamics and atomistics of the observed phenomena.

Main Results:

• High CO pressures induce phase separation of the Au/Ni(111) surface alloy.
• Nickel (Ni) atoms are removed from the surface layer during this CO-induced phase separation.
• DFT calculations identified Au-induced compression of the CO overlayer, leading to CO-CO repulsion, as the thermodynamic driving force.
• The atomistic mechanism involves kink-site carbonyl formation and evaporation, which is enhanced by the presence of Au.

Conclusions:

• The Au/Ni(111) surface alloy is unstable at high CO pressures, undergoing a phase separation.
• The presence of gold significantly influences the CO adsorption behavior and the stability of the surface alloy.
• The findings provide fundamental insights into the surface chemistry and dynamics of metal alloys under reactive gas environments.