Adsorbate-Induced Alloy Phase Separation: A Direct View by High-Pressure Scanning Tunneling Microscopy

Abstract

The influence of high pressures of carbon monoxide (CO) on the stability of a Au/Ni(111) surface alloy has been studied by high-pressure scanning tunneling microscopy. We show that CO induces a phase separation of the surface alloy at high pressures, and by means of time-lapsed STM movies we find that Ni atoms are removed from the surface layer during the process. Density functional theory calculations reveal the thermodynamic driving force for the phase separation to be the Au-induced compression of the CO overlayer with a resulting CO-CO repulsion. Furthermore, the atomistic mechanism of the process is shown to be kink-site carbonyl formation and evaporation which is found to be enhanced by the presence of Au.

Simulation of Reaction-Induced Phase Separation in Surface Alloy

“…The gradient of Ni coverage time dependence leads to step-flow rate, v, Fig. 1b, which can be compared with experiment [1]. An increase in v with c CO agrees well with the experiment and is rather insensitive to the change of simulation parameters.…”

“…Three types of reactants corresponding to CO, Au, and Ni are considered. Initially lattice is covered with 30% of Au and 70% of Ni atoms randomly as in experiment [1] with two steps created by removing Au and Ni atoms from twelve rows in the middle of the lattice. To meet the observation [2] that Au coverage on step ridges is close to unity, we replace Ni step atoms by Au.…”

“…The Au-Ni surface alloy on Ni(111) is one of such novel catalysts with improved properties in activity and selectivity. However, it was unstable at industrially relevant high CO pressures due to Ni(CO) 4 formation reaction [1].…”

Controlling magnetic interfaces using ordered surface alloys

“…The energetic and kinetic requirements for the formation of surface alloys are met in a significant number of adsorbate-substrate systems. The stability of surface alloys has also received * huang.l@sustc.edu.cn † phaltman@ust.hk particular attention because they are known to form even between many elements that are immiscible in bulk [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32]. Whether for reasons of bulk immiscibility, energetics of the surface alloy configuration, or even kinetic limitations to the formation of more deeply penetrated alloys, alloying is very frequently confined to the topmost atomic layer.…”

“…Practical interest in surface alloys as new materials has been motivated by the fact that their electronic structure and properties can differ substantially from the bulk surfaces of their constituent elements. The unique properties of surface alloys have been studied largely in connection with chemisorption and catalytic behavior in the past [11][12][13][14][15][16][17][18]25]. This outlook is encouraged by the prospect of tailoring properties through a careful selection of alloy constituents.…”