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Open Access Publications from the University of California

Scholarly Works (16 results)

  • Thesis
  • Peer Reviewed
UCLA Electronic Theses and Dissertations (2023)

Realistic modelling of catalysis requires the incorporation of complexity in the form of fluxionality. This means the inclusion of multiple thermally accessible isomers in the starting ensemble, and along potential reaction pathways. Furthermore, the role of fluxionality on fundamental physical behavior of the system must also be accounted for. This work focuses on extending this fluxionality paradigm both along reaction mechanisms to capture the full complexity of experiment, as well as part of the fundamental physical processes of cluster catalyst deactivation such as sintering or poisoning. Fluxionality is incorporated into DFT calculations via global optimization of cluster catalyst structures and relevant reaction intermediates. It essential to account for isomeric diversity for improved interpretation of experimental results, which enables the identification of novel size-dependent sintering behavior that has subsequently been confirmed experimentally. Furthermore, this allows us to go beyond simple interpretation of experimental results to more complex predictions of changes in the structure and composition of the system. For example, this enabled the pre- diction of self-limiting coke formation in Pt4Ge systems, and aided in understanding why Pt4 in contrast rapidly continues to coke.

    Cover page: The Role of Fluxionality in the Deactivation Processes of Supported Cluster Catalyst
    • Article
    • Peer Reviewed
    UCLA Previously Published Works (2022)
    Improvements in operando spectroscopy have enabled the catalysis community to investigate the dynamic nature of catalysts under operating conditions with increasing detail. Still, the highly dynamic nature of some catalysts, such as fluxional supported subnano clusters, presents a formidable challenge even for the most state-of-the-art techniques. The reason is that such fluxional catalytic interfaces contain a variety of thermally accessible states. Operando spectroscopies used in catalysis generally fall into two categories: ensemble-based techniques, which provide spectra containing the signals of the entire ensemble of states of the catalyst and are not necessarily dominated by the most active species, and localized techniques, which provide atomistic-level information about the dynamics of active sites in a very small area, which might not include the most active species. Combining many different kinds of techniques can provide detailed insight; however, we propose that effective utilization of specific computational techniques and approaches within the fluxionality paradigm can fill the gap and enable atomistic characterization of the most relevant catalytic sites.
      Cover page: Theoretical Perspective on Operando Spectroscopy of Fluxional Nanocatalysts
      • Article
      • Peer Reviewed
      UCLA Previously Published Works (2021)
      Size selection was demonstrated to suppress Ostwald ripening of supported catalytic nanoparticles. When the supported clusters are subnanometer in size and highly fluxional, such as Pt clusters on the rutile TiO2 (110) surface, this paradigm breaks down, and the established theory of sintering needs a revision. At temperatures characteristic of catalysis (i.e. 700 K), sub-nano clusters thermally populate many low-energy metastable isomers. As these isomers all have different geometric and electronic structures, and thus, formation and dissociation energies (in lieu of surface energy), Ostwald ripening is not suppressed, despite the size-selection. However, some clusters arise as magic numbers in terms of sintering stability at the ensemble level. Acceleration of sintering by metastable species persists though weakens in polydisperse cluster systems. We propose a competing pathways theory for sintering, which at the atomistic level describes the found size-specific sintering behavior.
        Cover page: When Fluxionality Beats Size Selection: Acceleration of Ostwald Ripening of Sub‐Nano Clusters
        • Article
        • Peer Reviewed
        UCLA Previously Published Works (2024)
          Creative Commons 'BY' version 4.0 license
          • Article
          • Peer Reviewed
          UCLA Previously Published Works (2023)
            • Article
            • Peer Reviewed
            UCLA Previously Published Works (2022)
              Cover page: Interpreting the Operando X‑ray Absorption Near-Edge Structure of Supported Cu and CuPd Clusters in Conditions of Oxidative Dehydrogenation of Propane: Dynamic Changes in Composition and Size
              • Article
              • Peer Reviewed
              UCLA Previously Published Works (2022)
                Cover page: Oxidation Dynamics of Supported Catalytic Cu Clusters: Coupling to Fluxionality
                • Article
                UCLA Previously Published Works (2022)
                Subnano cluster catalysts, while highly promising due to unique activity, selectivity, and atom-efficiency, are limited in wider applications, as they are prone to deactivation via sintering. Even size-selection, which was previously shown to reduce sintering of nanoparticles, cannot reduce the sintering of highly fluxional subnano clusters, due to their inherent isomeric diversity. Here, we use a combination of theory and experiment to show that Pt clusters on Al2O3 exhibit size-dependent sintering resistance. We furthermore show that Pt4/Al2O3 and Pt7/Al2O3 are “magic” sinter-resistant cluster sizes. Their stability is attributed to the greater degree of bulk-like crystallinity of the dominant isomers. In addition, we identify different spatial signatures characteristic of the sintering of clusters with differing sintering stabilities.
                  • Article
                  • Peer Reviewed
                  UCLA Previously Published Works (2022)
                  Subnano cluster catalysts, while highly promising due to unique activity, selectivity, and atom-efficiency, are limited in wider applications, as they are prone to deactivation via sintering. Even size-selection, which was previously shown to reduce sintering of nanoparticles, cannot reduce the sintering of highly fluxional subnano clusters due to their inherent isomeric diversity. Here, we use a combination of theory and experiment to show that Pt clusters on Al2O3 exhibit size-dependent sintering resistance. We furthermore show that Pt4/Al2O3 and Pt7/Al2O3 are "magic" sinter-resistant cluster sizes. Their stability is attributed to the greater degree of bulk-like crystallinity of the dominant isomers. In addition, we identify different spatial signatures characteristic of the sintering of clusters with differing sintering stabilities.
                    • Article
                    UCLA Previously Published Works (2022)
                    Deactivation via coking due to a lack of selectivity is a persistent problem for the longevity of Pt-based dehydrogenation catalysts. Ge as a promoter improves the exper- imental selectivity and stability of subnano Pt clusters. The origin of this improvement is self-limiting coking, to form a Pt4GeC2 cluster which is more stable and selective than the bare Pt4Ge cluster. In this paper we compare the dehydrogenation abilities of Pt4 and Pt4C2 with and Pt4Ge and Pt4GeC2 with DFT calculations in order to explore the origin of self-limiting coking in the presence of Ge. The unique stability of Pt4GeC2 is attributed to electron donation from Ge to the C2 atoms. This prevents the coke from drawing electrons from the Pt, which is the origin of deactivation via coking. Thus, we identify an electronic mechanism for coke deactivation and then use an electronically driven doping strategy to improve catalyst longevity. This differs from the common perception of coke deactivating via steric blocking of active sites.Furthermore, Pt4C2 and Pt4GeC2 show differences in kinetic accessibility of different isomers, which brings us into a new paradigm of sub-ensembles of isomers, where the dominant active sites are determined by kinetic stability under reaction conditions, rather than Boltzmann populations.
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