Article pubs.acs.org/JPCC
Pt−Zn Clusters on Stoichiometric MgO(100) and TiO2(110): Dramatically Different Sintering Behavior Jonny Dadras,† Lu Shen,† and Anastassia Alexandrova*,†,‡ †
Department of Chemistry & Biochemistry, University of California, Los Angeles, California 90095, United States California NanoSystems Institute, Los Angeles, California 90095, United States
‡
S Supporting Information *
ABSTRACT: Zn was suggested to be a promising additive to Pt in the catalysis of dehydrogenation reactions. In this work, mixed Pt− Zn clusters deposited on two simple oxides, MgO(100) and TiO2(110), were investigated. The stability of these systems against cluster sintering, one of the major mechanisms of catalyst deactivation, is simulated using a Metropolis Monte Carlo scheme under the assumption of the Ostwald ripening mechanism. Particle migration, association to and dissociation from clusters, and evaporation and redeposition of monomers were all included in the simulations. Simulations are done at several high temperatures relevant to reactions of catalytic dehydrogenation. The effect of temperature is included via both the Metropolis algorithm and the Boltzmann-weighted populations of the global and thermally accessible local minima on the density functional theory potential energy surfaces of clusters of all sizes and compositions up to tetramers. On both surfaces, clusters are shown to sinter quite rapidly. However, the resultant compositions of the clusters most resistant to sintering are quite different on the two supports. On TiO2(110), Pt and Zn appear to phase separate, preferentially forming clusters rich in just one or the other metal. On MgO(100), Pt and Zn remain well-mixed and form a range of bimetallic clusters of various compositions that appear relatively stable. However, Zn is more easily lost from MgO through evaporation. These phenomena were rationalized by several means of chemical bonding analysis.
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INTRODUCTION Platinum-based nano and subnanoclusters show great catalytic activity and selectivity for alkane dehydrogenation and cracking.1−4 Catalytic properties of such nanoscale clusters can be superb but strongly depend on cluster size, composition, and the type of support.5−16 Further dehydrogenation to alkynes can occur, as well as methane formation from cracking, and both reactions may result in coke deposits. Coking along with particle sintering are the two main causes for supportedcatalyst deactivation. A popular approach to mitigating coke involves alloying Pt with main group metals and some transition metals.17−22 Also, Gutierrez et al. have shown that coke-induced deactivation of certain Pt-based bimetallic clusters can be reduced by controlling the acidity of the support.23 Sintering of metal nanoparticles is a result of the particles minimizing their surface energy. This leads to populations of small-sized particles “dying off”, directly leading to an increase in the population of large-sized particles. Some of the authors (Dadras and Alexandrova) have previously shown that pure Pd and mixed Pt−Pd clusters deposited on TiO2 readily sinter, by the Ostwald ripening mechanism.24,25 Indeed, Ostwald ripening rather than particle coalescence is expected to be the dominant mode of sintering for Pt-based clusters, at least when supported on TiO2.26 Given that cluster activity and selectivity depend on the number of available edge and vertex atoms, rapid sintering © XXXX American Chemical Society
decreases the catalysts’ useful lifetime. Quantifying supportdependent effects on cluster stability and selectivity becomes a matter of central importance for catalysis. The present work attempts to address the effect of the support on cluster stability, through a fundamental understanding of electronic structure. The supports often play the role of a ligand to tiny (1000 K) this latter effect would lead to the appearance of large nanoparticles that are more Pt-rich than the model predicts. Again, what is more surprising is the predicted “bimodal” distributions on TiO2. For that system, the noted limitations would not have as great an effect based on the aforementioned previous results. By applying reasonable physical-chemical assumptions, that have proven appropriate in prior simulations, clear predictions for the presented systems have been made. These results are in need of experimental validation or falsification. More generally, it is noted that metal−metal covalent bonding between the atoms in the support and the atoms in the deposited clusters may play a significant role in modulating the shapes and properties of deposited clusters. This adds to the already rich and diverse roles that the support can play in impacting the catalytic properties of small surface-mounted clusters. In view of the interest in Pt−Zn clusters for catalytic dehydrogenation, our results suggest the promise to not be very high because of Zn loss via evaporation and, on TiO2, Zn separation from Pt.
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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This material is based in part on work supported by the Air Force Office of Scientific Research (AFOSR) under grant number 10029173-S3 and NSF CAREER Award CHE1351968. Computational resources were provided by the UCLA-IDRE cluster. A portion of the research was performed using EMSL, a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory.
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ASSOCIATED CONTENT
S Supporting Information *
Global and low-energy local isomers in the gas phase, calculated using a number of ab initio methods, plotted for the empirical scaling of the Zn chemical potential used in GCMC simulations. PDOS of Pt and/or Zn for pure and equimolar Pt−Zn clusters. Intracluster Coulomb potentials for tetramers. This material is available free of charge via the Internet at http://pubs.acs.org.
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. G
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DOI: 10.1021/jp512277x J. Phys. Chem. C XXXX, XXX, XXX−XXX
