Search query: Nanoparticle sintering in atomic layer deposition of supported catalysts: Kinetic modeling of the size distribution

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abstract: Abstract In industrial catalysis, the sintering of supported nanoparticles (NPs) is often associated with the loss of catalyst activity and thus with periodic plant downtime and economic burdens. Yet, sintering mechanisms are at play also during the synthesis of the catalyst itself. They can, in fact, determine the size distribution of the NPs, and thus the activity and the stability of the catalyst. Here, we examine the role of nanoparticle sintering in a technique borrowed from the semiconductor industry that promises to reconcile atomic scale precision with scalability: atomic layer deposition. By modeling the cyclic influx of single atoms in concomitance with NP sintering via either dynamic coalescence or Ostwald ripening, we establish the "signature" of different growth regimes: the size distribution. In contrast, we show that integral quantities such as the mean diameter, the number of NPs per unit area, and the material loading are poor indicators of the underlying growth mechanism. In particular, a constant number of NPs cannot be interpreted as a sign of no sintering. Finally, we argue that NP sintering, if properly understood, can open up new avenues for the control over the size distribution of NPs, and thus over their catalytic activity and stability.
author_names: Fabio Grillo

Jacob A Moulijn

Michiel T Kreutzer

J Ruud van Ommen
title: Nanoparticle sintering in atomic layer deposition of supported catalysts: Kinetic modeling of the size distribution
year: 2018

abstract: Calorimetric measurements of metal adsorption energies directly provide the energies of metal atoms in supported metal nanoparticles. As the metal coverage increases, the particles grow, revealing the dependence of this energy on particle size, which is found to be much stronger than predicted with the usual Gibbs Thompson relation. It is shown that this knowledge is crucial to accurately model long term sintering rates of metal nanoparticles in catalysts.
author_names: Charles T Campbell

Stephen C Parker

David E Starr
title: The Effect of Size Dependent Nanoparticle Energetics on Catalyst Sintering
venue: Science
year: 2002

abstract: This dissertation examines growth of platinum nanoparticles from vapor deposition on SrTiO 3 using a characterization approach that combines imaging techniques and X ray methods. The primary suite of characterization probes includes atomic force microscopy (AFM) grazing incidence small angle X ray scattering (GISAXS) X ray fluorescence (XRF) scanning electron microscopy (SEM) and X ray absorption spectroscopy (XAS) The vapor deposition techniques include physical vapor deposition (PVD) by evaporation and atomic layer deposition (ALD) For the PVD platinum study, AFM/XRF showed ~10 nm nanoparticles separated by an average of 100 nm. The combination of AFM, GISAXS, and XRF indicated that the nanoparticles observed with AFM were actually comprised of closely spaced, smaller nanoparticles. These conclusions were supported by high resolution SEM. The unusual behavior of platinum nanoparticles to aggregate without coalescence or sintering was observed previously by other researchers using transmissision electron microscopy (TEM) Platinum nanoparticle growth was also investigated on SrTiO 3 (001) single crystals using ALD to nucleate nanoparticles that subsequently grew and coalesced into granular films as the ALD progresses. The expected growth rate for the early stages of ALD showed a twofold increase which was attributed to the platinum deposition occurring faster on the bare substrate. Once the nanoparticles had coalesced into a film, steady state ALD growth proceeded. The formation of nanoparticles was attributed to the atomic diffusion of platinum atoms on iv the surface in addition to direct growth from the ALD precursor gases. The platinum ALD nanoparticles were also studied on SrTiO 3 nanocube powders. The SrTiO 3 nanocubes average 60 nm on a side and the cube faces have a {001} orientation. The ALD proceeded in a similar fashion as on the single crystal substrates where the deposition rate was twice as fast as the steady state growth rate. The Pt nanoparticle size increased linearly starting at ~0.7 nm for 1 ALD cycle to ~3 nm for 5 ALD cycles. The platinum chemical state was also investigated using X ray absorption spectroscopy. Platinum nanoparticles ~1 nm or smaller tended to be oxidized. For larger nanoparticles, the platinum state systematically approached that of bulk platinum metal as the size (number of ALD cycles) increased. The platinum loading was exceptionally low, 10 3 mg cm 2. v ACKNOWLEDGEMENT The counsel to " do something difficult for my posterity " was once given to me. This work would not have been possible without the help from many collaborators, friends, and loved ones. I will
author_names: Steven T Christensen
title: Nanoscale investigation of platinum nanoparticles on SrTiO3 grown via physical vapor deposition and atomic layer deposition
year: 2008

abstract: This paper discusses the mechanisms of surface area loss of supported platinum (Pt) electrocatalysts in low temperature fuel cells. It is argued that submonolayer dissolution of Pt nanoparticles governs the surface area loss at high voltages by increasing the loss of Pt from carbon and coarsening of Pt nanoparticles on carbon.
author_names: Yang Shao-Horn

Wenchao Sheng

Shen Chen

Paulo J T Ferreira

Edward F Holby

D Morgan
title: Instability of Supported Platinum Nanoparticles in Low Temperature Fuel Cells
year: 2007

abstract: We review here our studies of the reactivity and sintering kinetics of model catalysts consisting of gold nanoparticles dispersed on TiO2(110) First, the nucleation and growth of vapor deposited gold on this surface was experimentally examined using x ray photoelectron spectroscopy and low energy ion scattering. Gold initially grows as two dimensional islands up to a critical coverage, thcr, after which 3D gold nanoparticles grow. The results at different temperatures are fitted well with a kinetic model, which includes various energetic parameters for Au adatom migration. Oxygen was dosed onto the resulting gold nanoparticles using a hot filament technique. The desorption energy of Oa was examined using temperature programmed desorption (TPD) The Oa is bonded ~40% more strongly to smaller (thinner) Au islands. Gaseous CO reacts rapidly with this Oa to make CO2, probably via adsorbed CO. The reactivity of Oa with CO increases with increasing particle size, as expected based on Bronsted relations. Propene adsorption leads to TPD peaks for three different molecularly adsorbed states on Au/TiO2(110) corresponding to propene adsorbed on gold islands, to Ti sites on the substrate, and to the perimeter of gold islands, with adsorption energies of 40, 52 and 73 kJ/mol, respectively. Thermal sintering of the gold nanoparticles was explored using temperature programmed low energy ion scattering. These sintering rates for a range of Au loadings at temperatures from 200 to 700 K were well fitted by a theoretical model which takes into consideration the dramatic effect of particle size on metal chemical potential using a modified bond additivity model. When extrapolated to simulate isothermal sintering at 700 K for 1 year, the resulting particle size distribution becomes very narrow. These results question claims that the shape of particle size distributions reveal their sintering mechanisms. They also suggest why the growth of colloidal nanoparticles in liquid solutions can result in very narrow particle size distributions.
author_names: Stephen C Parker

Charles T Campbell
title: Reactivity and sintering kinetics of Au/TiO2(110) model catalysts: particle size effects
year: 2007

abstract: A kinetic model for the sintering of metal particles on oxide or other support surfaces is derived and applied to simulate experimental measurements of the sintering of a model gold catalyst: gold nanoparticles supported on \mathrm{TiO}}_{2}(110) It follows the pioneering work of Wynblatt and Gjostein (WG) Progress in Solid State Chemistry (1975, p. 21) but removes several important assumptions that create dramatic errors in sintering rates for particles smaller than $6\phantom{\rule{0.3em}{0ex}\mathrm{nm} in diameter, including (1) use of the Gibbs Thomson relation assuming that the surface free energy of metal particles is independent of size, and (2) neglect of all but the first order terms in a Taylor series expansion. Recent microcalorimetry measurements have shown these assumptions to be untrue in that metal particles smaller than $6\phantom{\rule{0.3em}{0ex}\mathrm{nm} have much higher surface free energies than large particles. A modified bond additivity model more accurately estimates particle energy versus size. This estimate was incorporated into the kinetic model of WG and applied to simulate the sintering of Au particles on \mathrm{TiO}}_{2}(110) as measured by temperature programmed low energy ion scattering. Our model reproduces well the broad temperature range over which sintering typically occurs in such experiments. This is analogous to accurate modeling of long term sintering kinetics of metal nanoparticles under the isothermal conditions of real catalysis. These results also highlight problems with classical methods for determining the sintering mechanism based solely upon the shape of the sintered particle size distribution.
author_names: Stephen C Parker

Charles T Campbell
title: Kinetic model for sintering of supported metal particles with improved size dependent energetics and applications to Au on TiO 2 110
year: 2007

abstract: Recently, small particles have been shown to exhibit a melting temperature which depends on the particle size. The various possible experimental methods have been compared and measurements of the melting points of small gold particles have been made using a scanning electron diffraction technique. This method was applied to particles having diameters down to 20 \AA{ Consideration of the size distribution over an entire sample makes it necessary to carry out a careful analysis of the experimental results in order to deduce the melting temperature of particles having a well defined diameter. The experimental results are quantitatively in good agreement with two phenomenological models. The first model describes the equilibrium condition for a system formed by a solid particle, a liquid particle having the same mass, and their saturating vapor phase. The second model assumes the preexistence of a liquid layer surrounding the solid particle and describes the equilibrium of such a system in the presence of the vapor phase. In order to permit a better comparison between both models, a new expression for the thermodynamic equilibrium condition has been derived in the present work. In the case of the first model, the agreement was obtained using only the physical constants of massive gold. In applying the second model, however, one is compelled to assume the existence of a liquid layer having a thickness of about 6 \AA{
author_names: Philippe A Buffat

J P Borel
title: Size effect on the melting temperature of gold particles
year: 1976

abstract: Exciting new opportunities are emerging in the field of catalysis based on nanotechnology approaches. A new understanding and mastery of catalysis could have broad societal impacts, since about 80% of the processes in the chemical industry depend on catalysts to work efficiently. Efforts in surface science have led to the discovery of new heterogeneous catalysts, however, until recently the only way to develop new or improved catalysts was by empirical testing in trial and error experiments. This time consuming and costly procedure is now rapidly being replaced by rational design methods that utilize fundamental knowledge of catalysts at the nanoscale. The advent of nanoscience and nanotechnology is providing the ability to create controlled structures and geometries to investigate and optimize a broad range of catalytic processes. As a result, researchers are obtaining fundamental insight into key features that influence the activity, selectivity, and lifetime of nanocatalysts. This review article examines several new findings as well as current challenges in the field of nanoparticle based catalysis, including the role played by the particle structure and morphology (size and shape) its chemical composition and oxidation state, and the effect of the cluster support.
author_names: Beatriz Roldan Cuenya
title: Synthesis and catalytic properties of metal nanoparticles: Size, shape, support, composition, and oxidation state effects
year: 2010

abstract: Abstract Chemical Vapour Deposition (CVD) of films and coatings involve the chemical reactions of gaseous reactants on or near the vicinity of a heated substrate surface. This atomistic deposition method can provide highly pure materials with structural control at atomic or nanometer scale level. Moreover, it can produce single layer, multilayer, composite, nanostructured, and functionally graded coating materials with well controlled dimension and unique structure at low processing temperatures. Furthermore, the unique feature of CVD over other deposition techniques such as the non line of sight deposition capability has allowed the coating of complex shape engineering components and the fabrication of nano devices, carbon carbon (C C) composites, ceramic matrix composite (CMCs) free standing shape components. The versatility of CVD had led to rapid growth and it has become one of the main processing methods for the deposition of thin films and coatings for a wide range of applications, including semiconductors (e.g. Si, Ge, Si 1 x Ge x III V, II VI) for microelectronics, optoelectronics, energy conversion devices; dielectrics (e.g. SiO 2 AlN, Si 3 N 4 for microelectronics; refractory ceramic materials (e.g. SiC, TiN, TiB 2 Al 2 O 3 BN, MoSi 2 ZrO 2 used for hard coatings, protection against corrosion, oxidation or as diffusion barriers; metallic films (e.g. W, Mo, Al, Au, Cu, Pt) for microelectronics and for protective coatings; fibre production (e.g. B and SiC monofilament fibres) and fibre coating. This contribution aims to provide a brief overview of CVD of films and coatings. The fundamental aspects of CVD including process principle, deposition mechanism, reaction chemistry, thermodynamics, kinetics and transport phenomena will be presented. In addition, the practical aspects of CVD such as the CVD system and apparatus used, CVD process parameters, process control techniques, range of films synthesized, characterisation and co relationships of structures and properties will be presented. The advantages and limitations of CVD will be discussed, and its applications will be briefly reviewed. The article will also review the development of CVD technologies based on different heating methods, and the type of precursor used which has led to different variants of CVD methods including thermally activated CVD, plasma enhanced CVD, photo assisted CVD, atomic layer epitaxy process, metalorganic assisted CVD. There are also variants such as fluidised bed CVD developed for coating powders; electrochemical vapour deposition for depositing dense films onto porous substrates; chemical vapour infiltration for the fabrication of C C composites and CMCs through the deposition and densification of ceramic layers onto porous fibre preforms. The emerging cost effective CVD based techniques such as electrostatic aerosol assisted CVD and flame assisted CVD will be highlighted. The scientific and technological significance of these different variants of CVD will be discussed and compared with other vapour processing techniques such as Physical Vapour Deposition.
author_names: Kwang-Leong Choy
title: Chemical vapour deposition of coatings
year: 2003

abstract: The terraces, edges, and facets of nanoparticles are all active sites for heterogeneous catalysis. These different active sites may cause the formation of various products during the catalytic reaction. Here we report that the step sites of Pd nanoparticles (NPs) can be covered precisely by the atomic layer deposition (ALD) method, whereas the terrace sites remain as active component for the hydrogenation of furfural. Increasing the thickness of the ALD generated overcoats restricts the adsorption of furfural onto the step sites of Pd NPs and increases the selectivity to furan. Furan selectivities and furfural conversions are linearly correlated for samples with or without an overcoating, though the slopes differ. The ALD technique can tune the selectivity of furfural hydrogenation over Pd NPs and has improved our understanding of the reaction mechanism. The above conclusions are further supported by density functional theory (DFT) calculations.
author_names: Hongbo Zhang

Xiang-Kui Gu

Christian P Canlas

A Jeremy Kropf

Payoli Aich

Jeffrey P Greeley

Jeffrey W Elam

Randall J Meyers

James A Dumesic

Peter C Stair

Christopher L Marshall
title: Atomic layer deposition overcoating: tuning catalyst selectivity for biomass conversion.
venue: Angewandte Chemie
year: 2014