Understanding how heterogeneous catalysts change size, shape and structure during chemical reactions is limited by the paucity of methods for studying catalytic ensembles in working state, that is, in operando conditions. Here by a correlated use of synchrotron X-ray absorption spectroscopy and scanning transmission electron microscopy in operando conditions, we quantitatively describe the complex structural dynamics of supported Pt catalysts exhibited during an exemplary catalytic reaction—ethylene hydrogenation. This work exploits a microfabricated catalytic reactor compatible with both probes. The results demonstrate dynamic transformations of the ensemble of Pt clusters that spans a broad size range throughout changing reaction conditions. This method is generalizable to quantitative operando studies of complex systems using a wide variety of X-ray and electron-based experimental probes.

X-ray reflectometry reveals atomic layering at a liquid-liquid interface--mercury in a 0.01 M NaF solution. The interface width exceeds capillary wave theory predictions and displays an anomalous dependence on the voltage applied across it, displaying a minimum positive of the potential of zero charge. The latter is explained by electrocapillary effects and an additional intrinsic broadening of the interface profile, tentatively assigned to polarization of the conduction electrons due to the electric field of the electrochemical double layer at the interface.

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Crystal nucleation and growth at a liquid-liquid interface is studied on the atomic scale by in situ Å-resolution X-ray scattering methods for the case of liquid Hg and an electrochemical dilute electrolyte containing Pb 2+ , F − , and Br − ions. In the regime negative of the Pb amalgamation potential Φ rp = − 0:70 V, no change is observed from the surface-layered structure of pure Hg. Upon potential-induced release of Pb 2+ from the Hg bulk at Φ > Φ rp , the formation of an intriguing interface structure is observed, comprising a well-defined 7.6-Å-thick adlayer, decorated with structurally related 3D crystallites. Both are identified by their diffraction peaks as PbFBr, preferentially aligned with theirc axis along the interface normal. X-ray reflectivity shows the adlayer to consist of a stack of five ionic layers, forming a single-unit-cellthick crystalline PbFBr precursor film, which acts as a template for the subsequent quasiepitaxial 3D crystal growth. This growth behavior is assigned to the combined action of electrostatic and shortrange chemical interactions.electrochemistry | liquid metal L iquid-liquid and liquid-gas interfaces provide exciting new possibilities for material synthesis (1, 2). Contrary to solid interfaces, which exhibit strain and stress, heterogeneities, and defects such as steps, which all strongly affect growth processes, fluid systems provide soft, defect-and stress-free interfaces. The high mobility of reagents, products, and deposited particles in liquid phases facilitates the growth process as well as the selfassembly of ordered particle arrays at the interface.A large variety of materials has been prepared via deposition at liquid-liquid interfaces, such as metals (1), oxides (3, 4), chalcogenides (5, 6), polymers (7), plasmonic materials (2), and nanoparticle catalysts of ceria (3), Pd (8, 9), and Pt (10). As demonstrated by Carim et al., deposition at liquid-liquid interfaces even allows the synthesis of group IV semiconductors such as Ge from oxide materials via a simple one-step, room-temperature electrochemical process (11). Different methods for nanoparticle manufacturing, such as deposition by reduction of metal ions (12) or electrochemical deposition (11), are available at the liquidliquid interface, allowing for particle modification and growth control via adjustment of concentration or interfacial potential.Despite the absence of long-range order, liquid interfaces provide the possibility to control the crystallinity, shape, and orientation of deposits. Examples are the growth of single-crystalline CuO and CuS films (4), the surfactant-induced oriented growth of calcite crystals (13), and the formation of pyramidal PbS crystallites with defined, high surface area facets (5). These phenomena were rationalized by energetic effects, such as the interface energies, surface charges, and specific chemical interactions, as well as by the growth kinetics. However, detailed insight into the phase formation mechanisms is generally precluded by lack of atomicscale data on the in...

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 ...

The study of liquid-liquid interfaces with X-ray scattering methods requires special instrumental considerations. A dedicated liquid surface diffractometer employing a tilting double-crystal monochromator in Bragg geometry has been designed. This diffractometer allows reflectivity and grazing-incidence scattering measurements of an immobile mechanically completely decoupled liquid sample, providing high mechanical stability. The available energy range is from 6.4 to 29.4 keV, covering many important absorption edges. The instrument provides access in momentum space out to 2.54 Å(-1) in the surface normal and out to 14.8 Å(-1) in the in-plane direction at 29.4 keV. Owing to its modular design the diffractometer is also suitable for heavy apparatus such as vacuum chambers. The instrument performance is described and examples of X-ray reflectivity studies performed under in situ electrochemical control and on biochemical model systems are given.

The atomic-scale structure of the mercury-electrolyte (0.01 M NaF) interface was studied as a function of temperature and potential by x-ray reflectivity and x-ray diffuse scattering measurements. The capillary wave contribution is determined and removed from the data, giving access to the intrinsic surface-normal electron density profile at the interface, especially to the surface layering in the Hg phase. A temperature dependent roughness anomaly known from the Hg-air interface is found to persist also at the Hg-electrolyte interface. Additionally, a temperature dependence of the layering period was discovered. The increase in the layer spacing with increasing temperature is approximately four times lager than the increase expected from thermal expansion. Finally, the interface is found to broaden towards the electrolyte side as the potential becomes more negative, in agreement with the Schmickler-Henderson theory. Our results favor a model for the interface structure, which is different to the model formerly used in comparable studies.

This review article takes a new look at the problem of characterization of structural properties and reaction dynamics of supported metal catalysts. Such catalysts exhibit an inherent complexity, particularly due to interactions with the support and the adsorbate molecules, which can be highly sensitive to environmental conditions such as pressure and temperature. Recent reports demonstrate that finite size effects such as negative thermal expansion and large bond length disorder are directly caused by these complex interactions. To uncover the atomistic features underlying the reaction mechanisms and kinetics of metal catalysts, experimental characterization must accommodate the challenging operation conditions of catalytic processes and provide insights into system attributes. The combined application of x-ray absorption spectroscopy (XAS) and transmission electron microscopy (TEM) for this type of investigations will be examined, and the individual strengths and limitations of these methods will be discussed. Furthermore, spatial and temporal heterogeneities that describe real catalytic systems and can hinder their investigation by either averaging (such as XAS) or local (such as TEM) techniques alone will be addressed by conjoined, multiscale, ab initio density functional theory/molecular dynamics modeling of metal catalysts that can both support and guide experimental studies. When taken together, a new analysis scheme emerges, in which different forms of structure and dynamics can be fully characterized by combining information obtained experimentally by in situ XAS and electron microscopy as well as theoretically via modeling. V