Pt–Ga Model SCALMS on Modified HOPG: Thermal Behavior and Stability in UHV and under Near-Ambient Conditions

Abstract

We have prepared model systems for Pt–Ga supported catalytically active liquid metal solutions (SCALMS) by physical vapor deposition of Pt and Ga onto highly oriented pyrolytic graphite (HOPG). Prior to metal deposition, the HOPG support was modified by Ar+ bombardment. In this work, we focus on the stability of the Pt–Ga deposits toward agglomeration, which is a critical challenge in application. Specifically, we study the behavior of the model catalysts in ultrahigh vacuum (UHV) and under near ambient pressure (NAP) conditions at different temperatures. We use CO as a probe molecule to examine changes in the surface composition of the Pt–Ga deposits by in situ infrared reflection absorption spectroscopy (IRAS). In specific, we combine IRAS and ex situ atomic force microscopy (AFM) to study changes induced by annealing. We find that sintering effects are strongly dependent on the thickness of the deposited Pt–Ga layers. Our data suggest that particle ripening leads to a separation of the system into disperse Pt-rich and large Ga-rich particles. In the NAP regime, we performed simultaneous measurements with polarization modulation IRAS (PM-IRAS) and near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS). The alloying of Pt and Ga is accompanied by a shift of Pt 4f and Ga 3d photoelectron lines to higher binding energies. Although partial oxidation of Ga is observed upon CO exposure at pressures in the millibar regime, PM-IRAS shows adsorbed CO at a frequency typical for Pt ensembles in a Ga matrix.

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

“…Therefore, more subtle effects may cause the observed decline in performance. Recently, investigation using model GaPt SCALMS on highly oriented pyrolytic graphite (HOPG) evidenced that the liquid droplet can separate into Pt-rich and Ga-rich domains as the droplet migrates over the surface of the support . It is possible that a similar phenomenon may also be at play on the surface of the supports tested in this study.…”

“…Recently, investigation using model GaPt SCALMS on highly oriented pyrolytic graphite (HOPG) evidenced that the liquid droplet can separate into Pt-rich and Ga-rich domains as the droplet migrates over the surface of the support. 31 It is possible that a similar phenomenon may also be at play on the surface of the supports tested in this study. This phase separation process may generate droplets of liquid alloy depleted in Pt and Pt-rich droplets that are presumably solid under the applied reaction conditions.…”

Oxidation induced restructuring of Rh–Ga SCALMS model catalyst systems

“…[1,[3][4][5] Highly mobile, single transition metal atoms dynamically appearing as active centers at the transition-metal-depleted gas/liquid interface have been proposed as an explanation for the superior performance of SCALMS-based catalysts. [1,[3][4][5][6][7][8] For the application of such catalysts, the formation of oxides upon exposure to ambient conditions and necessary activation processes to obtain the active liquid metal catalyst can play an important role. Using spatially resolved time-of-flight secondary ion mass spectrometry (TOF-SIMS), Chabala studied the oxidation of pure liquid Ga at 27.7°C, where liquid Ga is slightly supercooled.…”

Stable and Selective Dehydrogenation of Methylcyclohexane using Supported Catalytically Active Liquid Metal Solutions – Ga52Pt/SiO2 SCALMS

“…Second, Pt comes into contact with the substrate MCH only when it appears dynamically at the alloy/ gas interface. [7][8][9] After product desorption, Pt dives back into statistically preferred Ga matrix, [7] while hydrocarbon compounds do not dissolve in liquid gallium. Lastly, the dynamics of the liquid gallium surface prevents the blocking of the active Pt atoms via agglomeration of coke precursors.…”

“…This particular nature of the active sites has been demonstrated by some of us together with collaboration partners using XPS, DFT, and operando DRIFTS investigations. [7][8] Concerning catalytic applications, we have applied these SCALMS systems for butane (GaPd on porous glass) and propane (GaRh on Al 2 O 3 , GaPt on Al 2 O 3 , GaPt on SiO 2 ) dehydrogenation reactions already. [7,9] In the present work, we extend the application of SCALMS systems to the dehydrogenation of cyclic alkanes.…”