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Atomic resolution observation of a size-dependent change in the ripening modes of mass-selected Au nanoclusters involved in CO oxidation Kuo-Juei Hu, Simon R. Plant, Peter R. Ellis, Christopher M. Brown, Peter T Bishop, and Richard E. Palmer J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.5b08720 • Publication Date (Web): 06 Nov 2015 Downloaded from http://pubs.acs.org on November 18, 2015
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Journal of the American Chemical Society
Atomic resolution observation of a size-dependent change in the ripening modes of mass-selected Au nanoclusters involved in CO oxidation Kuo-Juei Hu,† Simon R. Plant,† Peter R. Ellis,‡ Christopher M. Brown,‡ Peter T. Bishop,‡ and Richard E. Palmer∗,† Nanoscale Physics Research Laboratory, School of Physics and Astronomy, University of Birmingham, Birmingham, B15 2TT, UK, and Johnson Matthey Technology Centre, Blounts Court Road, Sonning Common, Reading, RG4 9NH, UK Received October 15, 2015; E-mail:
[email protected] Abstract: Identifying the ripening modes of supported metal nanoparticles used in heterogenous catalysis can provide important insights into the mechanisms that lead to sintering. We report the observation of a cross-over from Smoluchowski to Ostwald ripening, under realistic reaction conditions, for monomodal populations of precisely-defined gold particles in the nanometer size range, as a function decreasing particle size. We study the effects of the CO oxidation reaction on the size distributions and atomic structures of mass-selected Au561±13 , Au923±20 and Au2057±45 clusters supported on amorphous carbon films. Under identical conditions, Au561±13 and Au923±20 clusters are found to exhibit Ostwald ripening, whereas Au2057±45 ripens through cluster diffusion and coalescence only (Smolouchowski ripening). The Ostwald ripening is not activated by thermal annealing or heating in O2 alone.
Introduction The inhibition of sintering remains a central challenge for heterogenous catalysis using ultrafine metal particles. 1,2 Sintering leads to the coarsening of the particles, and hence the irreversible deactivation of their catalytic activity as the average particle size increases over time. 3 This places limitations on the efficiency and longevity of supported metal particle catalysts. The size and morphology of small catalyst particles is integral to their activity, as these parameters determine the number of active sites (e.g. facet and edge sites), and other size-dependent electronic effects and quantum effects. 4 Particle ripening is commonly ascribed to one of two modes: Ostwald ripening, 5,6 in which large particles grow at the expense of smaller particles through the migration of single atoms or small atomic clusters, and Smoluchowski ripening, 7 in which whole particles are free to diffuse and coalesce with neighbouring particles. Nanoparticle ripening modes have been investigated in situ with environmental TEM, 8–11 however, the challenge is to replicate realistic reaction conditions whilst also de-coupling the effects of any damage induced through exposure to the electron beam. In general, the imaging of small clusters at the atomic level, even in vacuo, remains challenging. 4,12 It has long been known that supported metal clusters exhibit size-dependent catalytic properties, 13 although it has only been recently that catalytically-active, size-selected PtN (N = 22, 68) clusters on oxide supports have been used to provide evidence of †
University of Birmingham
‡
Johnson Matthey Technology Centre
Ostwald ripening suppression in truly monomodal populations. 14 Particle size has long been associated with catalyst sintering kinetics, 15 but detailed studies of truly monodisperse nanoparticles at atomic resolution have proved elusive. Using amorphous carbon supports in the present work, we begin with stable populations of monodisperse, massselected particles centred at the magic number sizes of AuN (N = 561, 923, 2057). Au nanoparticles of this size range are found to be active in catalysing CO oxidation., 4,16–21 at low-temperatures (≥ 273 K) in particular. 17 Indeed, heterogenous catalysis using Au nanoparticles or clusters (
