Atomic Resolution Observation of a Size-Dependent Change in the

Nov 6, 2015 - Identifying the ripening modes of supported metal nanoparticles used in heterogeneous catalysis can provide important insights into the ...
3 downloads 13 Views 7MB Size
Subscriber access provided by Stockholm University Library

Article

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

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Journal of the American Chemical Society is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 8

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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 (