Operando DRIFTS and DFT Study of Propane Dehydrogenation over Solid- and Liquid-Supported Ga_xPt_y Catalysts

Abstract

Supported catalytically active liquid metal solutions (SCALMS) represent a class of catalytic materials that have only recently been developed, but have already proven to be highly active, e.g., for dehydrogenation reactions. Previous studies attributed the catalytic activity to isolated noble metal atoms at the surface of a liquid and inert Ga matrix. In this study, we apply diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) with CO as a probe molecule to Ga/Al2O3, Pt/Al2O3, and Ga37Pt/Al2O3 catalysts, to investigate in detail the nature of the active Pt species. Comparison of CO adsorption on Pt/Al2O3 and Ga37Pt/Al2O3 shows that isolated Pt atoms are, indeed, present at the surface of the liquid SCALMS. Combining DRIFTS with online gas chromatography (GC), we investigated the Ga/Al2O3, Pt/Al2O3, and Ga37Pt/Al2O3 systems under operando conditions during propane dehydrogenation in CO/propane and in Ar/propane. We find that the Pt/Al2O3 sample is rapidly poisoned by CO adsorption and coke, whereas propane dehydrogenation over Ga37Pt/Al2O3 SCALMS leads to higher conversion with no indication of poisoning effects. We show under operando conditions that isolated Pt atoms are present at the surface of SCALMS during the dehydrogenation reaction. IR spectra and density-functional theory (DFT) suggest that both the Ga matrix and the presence of coadsorbates alter the electronic properties of the surface Pt species.

Ultrathin ionic liquid films on metal surfaces: adsorption, growth, stability and exchange phenomena

“…Replacement effects of this kind are further related to stability concerns in IL-based catalysts. One example could be the poisoning of a SCILL system, where the adsorption of large molecular reactants might block the reactive sites on the surface, irreversibly lowering the catalytic activity over time, similar to poisoning of Pt catalysts by sulfur or other nonreactive species [60,[195][196][197][198]. Another example would concern SILP systems based on large molecules (like metalloporphyrin derivates) as catalytic centers [171].…”

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.…”

Coke Formation during Propane Dehydrogenation over Ga−Rh Supported Catalytically Active Liquid Metal Solutions

“…In contrast to conventional supported liquid phase catalysis, the catalytic reaction in SCALMS occurs exclusively at the highly dynamic liquid metal/gas interface, as the liquid metal does not provide any relevant reactant solubility. Recently, the use of Ga‐rich supported bimetallic mixtures with Pd, Rh, or Pt (atomic Ga/Me ratio >10) has been reported for the highly endothermal dehydrogenation of n‐butane and propane . Remarkably, these SCALMS materials outperform, under some reaction conditions, commercial dehydrogenation catalysts (Pt/Al 2 O 3 , Cr 2 O 3 /Al 2 O 3 ) without extensive material or process optimisation .…”

“…As explanation for this unexpected behaviour, angle‐resolved X‐ray photon spectroscopy (XPS) experiments and molecular dynamics simulations suggest depletion of the topmost layer in Ga‐rich alloys regarding the catalytically active metal, while the layer directly underneath is enriched . The atomic distribution at the liquid catalyst surface is strongly affected by substrates approaching the surface and coming into contact with the active metal atoms –. Dehydrogenation of propane is suggested to require only a single active metal atom .…”

“…Dehydrogenation of propane is suggested to require only a single active metal atom . In turn, side reactions requiring a second active site are strongly reduced in the SCALMS approach due to the low probability of neighbouring active metal atoms at the surface of the liquid alloy– in combination with the generally low activation energy for desorption of propene . When compared to the dehydrogenation of butane, propane dehydrogenation (PDH) is expected to be rather vulnerable for the formation of coke.…”