Spectroscopic Observation and Molecular Dynamics Simulation of Ga Surface Segregation in Liquid Pd–Ga Alloys

Abstract

Liquid binary Pd-Ga alloys with low Pd contents of 0.8, 1.8, and 4.7 at % of Pd were examined as a function of sample temperature in ultra-high vacuum by using angle-resolved XPS. Upon cooling from 750 to 400 K, a pronounced temperature-dependence of the Pd concentration in the liquid phase was observed, which was explained by the transition from the pure liquid phase to a two-phase system, consisting of a solid Ga Pd phase and a Pd-depleted liquid Pd-Ga alloy. In the liquid Pd-Ga alloy, Pd is always depleted from the topmost interface layer, as deduced from angle-resolved XPS at 0 and 80° emission, independent of temperature and Pd concentration. This observation is interpreted as an inhomogeneous depth distribution function of Pd, that is, the segregation of Ga to the surface of the liquid phase. The results of a DFT-based molecular dynamics simulation (MD) independently show interfacial stratification of Ga and an inhomogeneous Pd distribution along the surface normal. The evaluation of the experimental data with a rigid layer model based on the MD calculations leads to excellent agreement with the simulation.

GaPt Supported Catalytically Active Liquid Metal Solution Catalysis for Propane Dehydrogenation–Support Influence and Coking Studies

“… 1 , 3 In short, the outer layer of the liquid alloy droplets is typically depleted in atoms of the catalytically active metal. 10 In the presence of the reactants, however, the catalytically active metal atoms appear as single atoms at the gas–liquid metal interface to provide their catalytic reactivity to the system. Upon desorption of the formed products, the active metal atoms dive back into the bulk of the Ga-rich droplets resulting in a self-regenerating process of the catalytically active interface.…”

Capturing spatially resolved kinetic data and coking of Ga–Pt supported catalytically active liquid metal solutions during propane dehydrogenationin situ

“…Ga. [10][11][12]19 The catalytic reaction in SCALMS occurs exclusively at the liquid metal/gas interface. 10,12,[19][20][21][22][23][24][25] Contrary to conventional SLP catalysis, the reactants and products are insoluble in the liquid phase. In addition, the liquid metal/gas interface is highly dynamic on an atomic scale.…”

“…The topmost layer of the Ga-rich alloy droplets is depleted of active metal atoms in the absence of substrates. 10,12,20,21 Nevertheless, the active metal atoms may diffuse to the surface of the droplets and interact with substrates via adsorption, which is a prerequisite for the catalytic activity of SCALMS. In the case of PDH over Ga-Rh SCALMS, ab initio molecular dynamics simulations suggest that the presence of a propane molecule at the liquid metal/gas interface may trigger the diffusion of Rh atoms to the surface of the liquid alloy droplet.…”

Oxidation induced restructuring of Rh–Ga SCALMS model catalyst systems

“…: Ga 21 Rh 4 , Ga 16 Rh 3 in Rh-Ga model SCALMS particles). [1][2][3][4][5] Temperature-dependent xray photoelectron spectroscopy (XPS) revealed that for macroscopic alloy droplets the crystallization of the intermetallic compounds (IMCs) occurs inside the bulk of the droplets, resulting in a transition-metal-depleted surface and near-surface region. Upon heating above the liquidus temperature, the IMCs dissolve in the liquid Ga phase, yielding a fully liquid alloy.…”

“…Upon heating above the liquidus temperature, the IMCs dissolve in the liquid Ga phase, yielding a fully liquid alloy. [3][4][5] Abinitio molecular dynamics (AIMD) simulations showed that the near-surface region of liquid Pd-Ga, Pt-Ga, and Rh-Ga SCALMS systems has a layered structure along the surface normal: The first atomic layer is transition-metal-depleted, and the second layer is transition-metal-enriched with the expected bulk density approached over the next few layers. These results have been confirmed by angle-dependent XPS for the Pd-Ga system.…”