Letter pubs.acs.org/NanoLett
Multiscale Modeling Reveals Poisoning Mechanisms of MgOSupported Au Clusters in CO Oxidation Michail Stamatakis,† Matthew A. Christiansen,† Dionisios G. Vlachos,*,† and Giannis Mpourmpakis*,†,‡ †
Department of Chemical and Biomolecular Engineering and Center for Catalytic Science and Technology, University of Delaware, Newark, Delaware 19716, United States ‡ Institute of Electronic Structure and Laser, FORTH, Heraklion 71110, Crete, Greece S Supporting Information *
ABSTRACT: Catalyst deactivation mechanisms on MgO-supported Au6 clusters are studied for the CO oxidation reaction via first-principle kinetic Monte Carlo simulations and shown to depend on support vacancies. In defect-poor MgO or in the presence of a Mg vacancy, O2 does not bind to the clusters and the catalyst is poisoned by CO. On Au clusters interacting with O vacancies of the support, O2 can be chemisorbed and transient activity is observed. In this case, an unexpected catalyst “breathing” mechanism (restructuring) leads to carbonate formation and catalyst deactivation, rationalizing several experimental observations. Our study underscores the importance of the cluster’s charge state and dynamics on catalytic activity. KEYWORDS: Au, CO oxidation, metal oxide, DFT, kinetic Monte Carlo, charge
G
Au clusters with an even number of Au atoms are expected to show increased activity because, in addition to having lowcoordinated sites, they also exhibit a preferred electronic configuration for O2 adsorption and activation. However, this is not the case in the aforementioned experiments21 (e.g., Au4 and Au6 were almost inert). Depending on the catalyst and reaction conditions, carbonate-like species (i.e., carbonate, bicarbonate, carboxylate, formate) are observed as reaction intermediates in both the CO oxidation24−33 and water gas shift34 reactions on Au. These species are sometimes spectators24 during reaction but can also be responsible for deactivating the catalyst. For example, a carbonate-like species on MgO-supported Au clusters (average diameter of 104 trajectories reveal that the latter poisoned state is twice as probable as the former. This observation can be explained by a geometry/symmetry argument: suppose that the first carbonate formed occupies sites 1 and 2 as shown in Figure 5b. Subsequently, there are three possible arrangements for the second carbonate marked as (i), (ii), and (iii) in Figure 5b. Both configurations (i) and (ii) will eventually give rise to a poisoned state with three carbonates, whereas configuration (iii) is the poisoned state with two carbonates. Thus, the latter poisoned state is half as probable as the former with three CO3 molecules. Finally, Figure 5c portrays the probability for the deactivation times, namely the times to reach either one of the poisoned states. The distribution of these times can be fit to a Gamma distribution with mean time equal to 17 h and a standard deviation equal to 14 h. These times depend of course on conditions but provide an indication that, unlike other charge states, it may be possible to see some transient activity on Au6− during experimental lab time scales, depending on experimental details (startup, temperature, composition, etc.). Conclusions. In conclusion, we investigated the CO oxidation mechanism on MgO-supported Au6 clusters by combining for the first time DFT calculations with graphtheoretical KMC simulations. The CO oxidation exhibits complex behavior depending on support-induced charge states that are controlled by support vacancies. On a defect-poor MgO support or in the presence of a Mg-vacancy, the Au clusters are neutral and positively charged, respectively, and O2 does not bind (sufficiently) strongly on the metal. As a result, the catalyst is poisoned by CO. On the other hand, O2 chemisorbs on negatively charged Au clusters due to charge gained from O-vacancies on the MgO support. In this case, the catalyst is transiently active but eventually deactivates due to accumulation of CO3 (carbonate) via a rather unexpected catalyst restructuring via a “breathing” mechanism. The dependence of deactivation on charge state of Au catalysts is the first of its kind and explains several experimental observations21,24−34 of subnanometer, MgO-supported Au clusters, including the Au6. Our results reveal that catalyst dynamics (restructuring) is too important to suppress and should be subject to further studies possibly with ab initio molecular dynamics.
ASSOCIATED CONTENT
S Supporting Information *
Computational details, binding energy graphs, and carbonate formation pathways. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (G.M.),
[email protected] (D.G.V.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The research was partially supported by Grant DE-FG0205ER25702 from the Department of Energy. G.M. was supported by a Marie Curie International Outgoing Fellowship within the seventh European Community Framework Programme. The KMC simulations of this work were carried out at the Center for Functional Nanomaterials, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract DEAC02-98CH10886.
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