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

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Abstract It has recently been shown that the dynamic behavior of surface‐supported nanocluster catalysts in realistic reaction conditions defies conventional models used in catalysis. This opens new doors in catalysis by giving more leverage in catalyst design, but also requires a major revision of the understanding of how dynamic heterogeneous catalytic interfaces operate, as well as of the computational approaches of catalyst modeling, and experimental methods of catalyst characterization. Major aspects of the new paradigm include the collective action of many catalyst states that form a statistical ensemble in reaction conditions, the catalytic activity and selectivity being driven by rare and metastable catalyst states, reaction thermodynamics and kinetics being controlled by different states of the catalyst, broken scaling relationships, non‐Arrhenius behaviors, and catalyst dynamic restructuring being an essential part of the reaction mechanism. For computation, this complexity means the departure from the standard density functional theory calculations of reaction mechanisms on a single catalyst structure. For experiment, it calls for the development of operando characterization tools with the per‐site resolution and the ability to find the minority sites that govern the catalytic activity. For catalyst design, the goal becomes the creation of the catalyst state (geometric and electronic) that might not be present in the as‐prepared catalyst, but would develop in the reaction conditions and would have the desired activity then. While cluster catalysts are the most dramatic in their dynamic fluxionality, other amorphous interfaces also exhibit some of it, and thus are also subject to similar paradigm revision. This article is categorized under: Structure and Mechanism > Reaction Mechanisms and Catalysis
Optimized geometries of isolated and CeO2‐supported Pt13 cluster in vacuum, aqueous phase, and deposited on CeO2 after 10 ps AIMD simulations. Ce (light yellow), O (red), Pt (cyan), and H (white). Adapted with permission from Reference . Copyright 2018 American Chemical Society
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Computed correlations of the binding energies of (a) O and OH, (b) O and OOH, and (c) OH and OOH on Ptn (n = 1–6) clusters in the gas phase, in the presence of one or two adsorbates. Blue—PBE results; red—PBE0 results. Poor correlations are observed due to clusters changing shapes and adsorbate binding sites. Adapted with permission from Reference . Copyright 2019 American Chemical Society
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Side and top view of stable CO2 adsorption configurations on the perfect anatase TiO2(101) surface in the presence of Pt hexamers (PH) (O in red, C in black, Ti in blue, and Pt in green). The numbers indicate the bond lengths in Å. Adapted with permission from Reference . Copyright 2015 Royal Society of Chemistry
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Three local minima structures of Pt13H26 that can catalyze methane activation. The activation energies (Ea [eV]) and the relative contribution to the reaction rate (rectangle bars) are shown for these three structures. The weighted reaction rate constant is given with a reference of 1 × 102 at 400°C. Adapted with permission from Reference . Copyright 2018 American Chemical Society
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(Top) Experimental temperature programmed desorption spectra for dehydrogenation of deuterated ethylene on size‐selected Ptn clusters on Al2O3: The difference in activity is explained on the basis of accessibility of highly active metastable states of Pt7, not characteristic of Pt8 that has a highly dominant global minimum. P300—Population of the given isomer at 300 K. ΔQ—Charge transferred from the support to the cluster (Bader charge scheme). The blue arrow represents the effect of cluster boration on the catalytic activity (smaller activity means less coking), measured experimentally. (Bottom) Theoretical modeling of coking reproduces the reduction of activity and coking upon boration, but only at high T, when the ensemble of cluster states is expanded toward metastable states. Adapted with permission from References 6 and 46. Copyright 2018 American Chemical Society
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(a–c) Optimized structures of gas‐phase Pd8 and Pd8 deposited on the stoichiometric and defective ceria surfaces. (d) Metastable structure of Pd8 on the defective ceria. (e, f) Structures of Pd8Ox/CeO2 (x = 12 and 6) obtained by GCMC‐DFT at 300 K with oxygen atmospheres of 1 atm and 10−20 atm, respectively. Color coding: Cyan, red, white, and small yellow spheres represent Pd, O, Ce4+, and Ce3+ atoms, respectively; the purple spheres in defective ceria represent O atoms adjacent to O vacancy sites. Adapted with permission from Reference . Copyright 2018 American Chemical Society
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(a) Phase diagram of Pd oxidation states in the Pd7Ox/Ce33O66(111) model obtained from ab initio calculations. The solid shaded regions separated by solid lines indicate stable regions in (T,P) space. The striped region and dashed line indicate the region where the supported Pd7O8 cluster is thermodynamically stable, yet methane activation is kinetically preferred over the Pd7O9 cluster. (b, c) DFT optimized structure of the embedded Pd7O9/Ce33O66(111) cluster model (b) and the Pd7O8/Ce33O66(111) cluster model (c). Hydrogen abstraction sites during methane activation are indicated by arrows. Adapted with permission from Reference . Copyright 2016 American Chemical Society
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