Comparative in Operando Studies in Heterogeneous Catalysis: Atomic and Electronic Structural Features in the Hydrogenation of Ethylene over Supported Pd and Pt Catalysts

Abstract

There exists an emerging opportunity, engendered by advances made in experimental methods of research, to address long-standing questions about the nature of the molecular mechanisms that are operative in important heterogeneous catalytic processes, as well as the nature of the complex atomic and electronic structural features that mediate them. Of particular interest in this regard is the understanding of the dynamical attributes of catalytic processesan understanding that might allow design principles to be applied to optimize the atomic and electronic structure of heterogeneous catalysts to sustain their performance in essentially any operating process condition. The current work explores these ideashighlighting capabilities of in operando methods of spectroscopic characterization as applied to an exemplary heterogeneous catalytic process, olefin hydrogenation. No heterogeneous catalytic process has been studied more intensively than olefin hydrogenation. The extensive literature available establishes important features by which metal catalysts activate and efficiently transform the bonding of the hydrogen and alkene reactants to generate a product alkane. Even so, many important mechanistic questions remain poorly understood due to the inherent multiscale complexity associated with heterogeneous catalytic transformations, as well as the paucity of methods suitable for their characterization in operando. The recent literature documents the development of new capabilities for characterization afforded by in situ and in operando methods. Of these, X-ray absorption spectroscopy (XAS) has become a particularly important technique for studying the mechanisms of catalytic reactions due to its capabilities for elucidating the nature of the atomic and electronic structural features of operating catalysts. Many important questions can now be addressed, in particular those that follow from the unique dynamical impacts and patterns of reactivity that occur in higher pressure (non-UHV) environments. In this Perspective, we examine important structure−property correlations for an exemplary model reactionethylene hydrogenationas elucidated in operando for two efficient catalyst materialsnanoscale Pd and Pt clusters supported on SiO 2 . The examined features include the following: the structural dynamics of the metal clusters and their sensitivity to the composition of the reactant feed; the role of hydrogen, and metal-and/ or support-bonded forms of adsorbates more generally, in forming intermediates and products; the influences of adsorbate bonding states (e.g., hydrogen) on reactivity; the role played by carbonaceous deposits (and the mechanisms of their formation); the quantitative nature of the atomistic features that exist within the structure−sensitivity correlations of this catalytic reaction; and mechanisms that mediate the sintering of catalysts operating in high-pressure ambient environment. Here we present a comparative overview of the hydrogenation of ethylene over ≈1 nm-sized Pd and Pt catalysts supported ...

Generation of subnanometric platinum with high stability during transformation of a 2D zeolite into 3D

“…The assignation of weaker peak is too long to be ascribed Pt-O bonds from PtO2 (1.65 Å, see Figure S12), but according to literature data it is compatible with Pt-O bonds between Pt and the zeolite frameworks. 27 The presence of Pt-O bonds in Pt@MCM-22 sample has also been confirmed by the XANES spectrum of Pt@MCM-22. As it can be seen in Figure 3b, the curve shape of Pt@MCM-22 sample looks like Pt(0) (fist derivative and oscillations around 11580 and 11595 eV) but white line intensity is higher, indicating the presence of Pt-O bonds.…”

Surface‐supported cluster catalysis: Ensembles of metastable states run the show

“…Determination of intrinsic reaction kinetics of catalyzed reactions is of importance in the safe and economical design and control of industrial chemical and environmental processes, and in the development of new and improved catalysts . Echoing all the assertions in this review, operando studies have shown that catalysts may undergo dynamic structural transformations upon small changes in the reaction conditions and such transformations have a strong impact on the performance of the catalysts . Strictly speaking, the active state of a catalyst only exists during the catalysis.…”

“…[123][124][125][126][127][128] Echoing all the assertions in this review, operando studies have shown that catalysts may undergo dynamic structural transformations upon small changes in the reaction conditions and such transformations have a strong impact on the performance of the catalysts. 129,130 Strictly speaking, the active state of a catalyst only exists during the catalysis. Many crucial mechanistic questions remain poorly understood thanks to the inherent multiscale complexity of heterogeneous catalytic transformations.…”

Complex structural dynamics of nanocatalysts revealed in Operando conditions by correlated imaging and spectroscopy probes

“…The work specifically investigates the active structures of silica-supported Pt clusters mediating the process of ethylene hydrogenation. Previous studies by operando XAFS have uncovered irreversible structural transformations in this system when it is cycled between hydrogen- and ethylene-rich environments 20 . Ex situ scanning transmission electron microscopy (STEM) studies showed that the average particle size is approximately the same after the reaction, but the measured variation was much smaller than the precipitous drop in the metal-metal coordination number measured by operando extended XAFS (EXAFS) 20 .…”