CO-Induced Scavenging of Supported Pt Nanoclusters - American

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CO-Induced Scavenging of Supported Pt Nanoclusters: A GISAXS Study Nihed Chaabane, Remi Lazzari, Jacques Jupille, Gilles Renaud, and Edmar Avellar Soares J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 18 Oct 2012 Downloaded from http://pubs.acs.org on October 19, 2012

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The Journal of Physical Chemistry

CO-Induced Scavenging of Supported Pt Nanoclusters: a GISAXS Study Nihed Chaâbane,†,¶ Rémi Lazzari,∗,† Jacques Jupille,† Gilles Renaud,‡ and Edmar Avellar Soares‡,§ Institut des NanoSciences de Paris, Université Pierre et Marie Curie (Paris 6) , CNRS UMR 7588, 4 Place Jussieu, 75252 Paris Cedex 05, France, and Institut NanoSciences et Cryogénie, Commissariat à l’Energie Atomique et aux Energies Alternatives, 17 Avenue des Martyrs, 38054 Grenoble, Cedex 9, France E-mail: [email protected]

∗ To

whom correspondence should be addressed des NanoSciences de Paris ‡ Institut NanoSciences et Cryogénie ¶ INSTN, Commissariat à l’Energie Atomique et aux Energies Alternatives, Saclay, France § Departamento de Física, ICEx-UFMG, Belo Horizonte MG, Brazil † Institut

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Abstract Supported platinum nanoparticles in the presence of carbon monoxide in the 10−6 − 103 Pa CO pressure range were studied by Grazing Incidence Small-Angle X-Ray Scattering. Disruption and aggregation of platinum nanoparticles upon CO exposure are shown to occur in parallel. Particles smaller than a critical size of 1 nm undergo scavenging after disruption in the form of carbonyls at CO pressure as low as 10−1 Pa. Mobile carbonyls then agglomerate to larger clusters. Upon annealing, a CO-driven diffusion and agglomeration of clusters of size ranging between the critical size and 2 nm is observed. These phenomena are discussed in relation to the CO-metal, metal-metal and metal-substrate relative bond strengths. Similar mechanisms are suggested to hold for other supported metal catalysts.

Introduction The efficiency of oxide-supported catalysts frequently relies on the dispersion of the metallic particles 1 whereas the optimization of the proportion of active atoms of the often precious metals involved in catalysts is an economic issue. 2 Beyond the achievement via synthesis processes of the optimum morphology that accounts for the combination of those constraints, a great attention is paid to the phenomena which drive changes in shape, size and structure of the clusters of catalysts in running conditions. Aside the capability to resist high temperature aging, a main concern is the sustainability of catalyst particles upon exposure to reactive atmospheres. 1,3–5 A prototypical case is the effect of CO on transition metals catalysts, of which supported platinum is a thoroughly studied example because it combines a strong practical relevance 2,6–9 with a puzzling stability behavior in the presence of CO. 2,3,6–9 Active in CO oxidation, platinum is a major component of the three-way catalysts of automotive converters 2,10 in which the size of Pt nanoparticles is crucial since the smallest Pt particles on CeO2 -based catalysts show both a higher stability due to Pt-O-Ce bonds 2 and a higher oxygen storage and release capacity. 2,10 Moreover, the turnover rate for CO oxidation is structure-sensitive on oxygen-saturated Pt clusters, although in contrast it poorly depends on the particle dispersion for CO-saturated particle surfaces. 11 2 ACS Paragon Plus Environment

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Regarding the CO-induced changes in morphology, the ability of CO to induce mass transport seems to depend on both the coordination number of Pt sites and the density of crystal faces. High pressure Scanning Tunneling Microscopy (STM) and photemission studies 3 demonstrated a strong restructuring of vicinal Pt surfaces at millibar pressure and room temperature while only an increase in CO coverage was observed on compact Pt(111) plane. 12 The case of nanoparticles with a high density of undercoordinated sites is even more complex. Based on the sensitivity of the vibrational spectrum of CO to sites and charges, the stretching frequency of the adsorbed CO molecules is commonly used to probe adsorption sites and charge transfer with the support and to stress the status of supported Pt nanoparticles. 13 The existence of Pt carbonyls stabilized in zeolite was suggested by infra-red spectroscopy upon exposure of reduced Pt particles at 300 K to CO at 500 mbar. 6 Reduced Pt particles at 300 K were observed to disrupt into small carbonyl clusters when exposed to a mixture containing 1 % CO/ 1 % O2 /He (10 mbar CO partial pressure) at the ignition temperature. 9 Upon admission of CO on Pt supported on titania, alumina and silica in the 0.01-10 torr pressure range, Rasko 14 has assigned the increase in intensity of the 21122106 cm−1 band to the formation of monoatomic Pt0 by disruption of crystallites. The disintegration of titania-supported Pt particles of 1-2 nm in size at 300 K by CO in the 10−3 − 10 mbar range was also reported by STM (Scanning Tunneling Microscopy). 15 By a combination of XANES (Xray Absorption Near-Edge Spectroscopy), EXAFS (Extended X-ray Absorption Fine Structure), EPR (Electron Paramagnetic Resonance) and FTIR (Fourier Transform Infrared Spectroscopy), Pt13 clusters supported in NaY zeolite were shown to give rise to Pt2 (CO)m (m = 4 − 5) clusters by exposure to CO. 16 Evidence that neutral [Pt(CO)2 ]3 carbonyl was formed upon decomposition of Pt nanoparticles involving 4-6 atoms was brought by EXAFS measurements which revealed that the coordination number of Pt decreased from 4.2 to 2.2 and the Pt-Pt distance diminished from 0.274 to 0.267 nm upon CO adsorption. 7,17 Nevertheless, the decomposition of the particles is size-dependent since Pt clusters that are less than five Pt atoms in size were seen to undergo disruption in contrast with bigger clusters which were not affected. 7 Moreover, Pt nanoparticles have



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also been shown to agglomerate under the combined effects of temperature and CO partial pressure. 15 Although commonly observed on Pt and on other metal catalysts such as Ir, 18,19 Rh, 20,21 Ru 22,23 and Ni, 24 the parallel disruption and agglomeration mechanisms of metal nanoparticles in the present of CO are not explained yet.

To understand the origin of those phenomena, a quantitative in situ description of the Pt particles as a function of their size is needed. An appropriate method is Grazing Incidence Small Angle X-ray Scattering (GISAXS) which allows the characterization of the morphology of supported metal particles down to 1 nm in size in any conditions of gazeous environment. 25 The method is of great relevance to analyse in situ supported nanoparticles either during growth 26 or in reactive conditions. 4,27–29 In particular, it has been shown operando that catalytic Au/TiO2 (110) particles are tridimensional- which favors reactivity at corners and edges of gold particles - and that the catalytic activity sharply peaks for particles of 2.1 ± 0.3 nm in size. 29 The present work foresees to track by GISAXS the changes in shape and in distribution of platinum particles supported on MgO(001) as a function of the CO pressure. MgO has been chosen as an archetype of non-reducible support that gives rise to abrupt interfaces with platinum. 30

Experiments and GISAXS analysis framework The X-ray scattering experiments were performed at the European Synchrotron Radiation Facility (ESRF, France) on the ultra-high vacuum (UHV) set-up 31 of the BM32 beamline (base pressure of 10−8 Pa). For X-ray measurements, the samples were mounted on the head of the six-circles z-axis goniometer holding the main chamber. 25,31 The beamline delivered a monochromatic beam (wavelength λ = 0.0689 nm; E = 18 keV) sagittally focused on the sample with a horizontal H = 0.4 mm and vertical V = 0.3 mm sizes (Full Width at Half Maximum), corresponding to divergences of δH = 1 mrad and δV = 0.13 mrad, respectively. The incident angle αi was chosen close to critical value of the MgO substrate αi = 0.12◦ in order to minimize the scattering


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background arising from bulk and to increase the signal from the tiny Pt nanoparticles. 25,32 For GISAXS measurements, the scattered intensity was collected as a function of the in-plane 2θ f and out-of-plane α f angles on a 16-bits x-ray charge coupled device (CCD) detector (1242 × 1152 pixels, pixel size of 56.25 × 56.25µ m2 ) placed at a distance of 920 mm or 1680 mm downstream the sample. Dedicated systems of in-vacuum guard slits and beam stop 25,33 allowed reducing the diffuse background coming from upstream slits and beryllium windows. To avoid absorption and scattering from air, an evacuated pipe was mounted between the output beryllium window of the chamber and the CCD camera. Grazing Incidence X-Ray Diffraction (GIXD) scans were acquired with a NaI scintillator placed behind slits defining a vertical acceptance of ∼ 0.2◦ for in-plane scans. In-plane scans at α f = αi were performed to determine the crystallographic orientation of the Pt nanoparticles. At wide angle, a reciprocal space point is defined herein by its (h, k, ℓ) coordinates in the reciprocal unit cell of MgO while, at small angles, the GISAXS patterns are indexed with the components of the wavevector transfer parallel qy ∼ 2θ f and perpendicular qz ∼ α f to the substrate. 25,32,34

GISAXS data were analyzed in the Disordered Wave Born Approximation (DWBA). 25,32,35 This standard semi-dynamical treatment of scattering allows accounting for multiple scattering effects induced by the grazing geometry and multiple reflections on the flat substrate. For deposits with particle average size lower than ∼ 2 nm (see below), the measurement conditions are not sensitive enough to precisely fit an estimate of the standard deviation of the size distribution. The scattering from these tiny clusters is so spread over reciprocal space that the Porod’s information 25,32 is hidden in the background. Therefore, except for deposits with thee largest average particle size for which a normal size distribution with standard deviation of σD ≃ 0.25 was used (see example of Figure 2 of supporting information - T ≥ 900 K), the analysis was restricted on purpose to a monodisperse object for which the scattering cross section is proportional to the product of the DWBA particle form factor and of the interference function. A gaussian one-dimensional paracrystal 25,32,36,37 was chosen for the latter to account for random disorder in the collection of clusters. It



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is defined by the average distance L between scatterers and its standard deviation σL . For the sake of simplicity, since no special in-plane anisotropy was observed in the scattering patterns, 30 the particles were modeled by a cylinder with a basis of diameter D (hereafter also refereed as the size of the particles) and height H. Notably, a few fits that were performed by using the more demanding truncated sphere shapes led to similar quantitative conclusions since the signal/noise ratio in the Porod’s high-q scattering regime combined with the small particle size 25 does not allow discussing the particle shape. The particle density was estimated through ρ ≃ 1/L2 and the calculated film thickness through t = π D2 H ρ /4. Intensity profiles were extracted from GISAXS patterns and simultaneously fitted using a Levenberg-Marquadt χ 2 criterion minimization. Perpendicular (or qz ) cut at the maximum of the correlation peak and parallel (or qy ) cut taken slightly above the Yoneda’s peak were used for the analysis. In some cases, parallel cuts well above the Yoneda were also included. The analysis was performed with the IsGISAXS software. 34,38 Tabulated values of the complex refraction index (n = 1 − δ − iβ ) of the substrate MgO (δ = 2.310−6 , β = 5.010−9 ) and of the platinum deposit (δ = 1.110−5 , β = 1.110−6 ) at E = 18 keV were used. 39

MgO(001) substrates (size ∼ 1 cm2 , miscut lower than 0.1◦ ) were prepared in air at 1800 K to improve bulk cristalline mosaicity (rocking scan width of the (220) reflection of MgO : ∆ω = 0.056◦ ). The surface was then ion bombarded at 1800 K 40 in an ancillary vacuum chamber to yield large terraces while removing segregating Ca contamination, quenched in O2 partial pressure and capped by ∼ 100 nm thick Ag films. Once the MgO crystal had been introduced in the beamline chamber, the protective Ag film was completely desorbed by annealing above 900 K. Pt was evaporated from a high purity rod heated by electron bombardment (EFM4/Omicron evaporator). The evaporation rate was regulated through a feedback on the current due to the fraction of ionized Pt; it was estimated in the range of ∼ 0.2 nm/hour by a quartz microbalance calibration. Several Pt deposits of monolayer thickness (1MLPt (001) = 0.196 nm) were synthesized at substrate temperatures ranging from 300 K to 1100 K and then stored in the UHV lock-load before gas exposure in the main chamber where high purity (∼ 1 ppm of impurities) CO and O2 were admitted without


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further purification from baked-out and bled gas lines. On the basis of the data of Inouye and De Van, 41 it was estimated that the rate of formation of spurious carbonyl was marginal in working conditions that were used herein. To insure that the experiment has not been perturbed, the absence of Ni, Fe, Cr was checked by x-ray photoemission spectroscopy in an ancillary system. Low pressure gas exposure was performed in dynamical pumping while static back-filling of the chamber was used at pressure higher than 10−2 Pa. Depending on the conditions, pressure was measured by either Bayard-Alpert or Pirani gauges.

Results and discussion To explore the effect of the CO partial pressure on platinum particles, it has been chosen to focus on supported clusters of 1 to 2 nm in size on the basis of STM measurements which show that aggregates of that size are strongly affected by CO above a partial pressure of 10−1 Pa. 15 Such particle size, which is in the low limit of the metal clusters size that can be analyzed by GISAXS on the beamline, was obtained in the previous Pt/MgO(001) study of Olander et al. 30 at 600 K, 800 K and 1000 K for an average Pt thickness close to 0.3 nm, i.e. in the monolayer range. Therefore, Pt/MgO(001) deposits were prepared at 300 K, 700 K, 800 K, 900 K and 1100 K at a similar coverage. In-plane reciprocal diffraction scans from the as-deposited samples (Figure 1) were found in agreement with previous results. 30 Samples prepared at temperature above 900 K are characterized by a [100](001)Pt ∥ [100](001)MgO epitaxy with satellite diffraction peaks at (2.16,0,0) and (2.16,2.16,0). Only a faint signal is observed for the 800 K growth. The Pt deposit at 300 K did not give rise to a clear diffraction signal. The [110](111)Pt ∥ [110](001)MgO expected orientation 30 with a peak below the (220)MgO reflection is hardly visible probably because of the small particle size. All attempts made to modify the morphology of platinum deposits by exposure to oxygen failed, whatever the deposition temperature. Upon exposure to CO up to 102 Pa, no effect could be seen on the deposits prepared at 700 K and above (see Figures 1-2 of supporting information). In contrast, strong changes occurred for the poorly crystallized Pt particles of the 300 K sample.



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Their description is the objective of the present work.

The GISAXS pattern recorded in UHV conditions on that deposit is shown in Figure 2-a. The diffuse scattering leads to the usual reciprocal space representation in form of two lobes that are symmetric with respect to the plane of incidence of the x-ray beam. 25 The deposit kept at 300 K was exposed step by step to partial pressures of CO ranging between 10−6 and 10 Pa. In the 10−6 − 10−1 Pa CO pressure range, the cluster morphology was stable even when exposed for hours. At a pressure of 10−1 Pa, a shrinking of the scattering lobes towards the centre of the reciprocal space was indicative of an increase of all the characteristic dimensions (D, H, L) of the average Pt particle in direct space (Figure 2-b), but further increase in the CO partial pressure did not produce any visible perturbation (Figure 2-c). Then, changes in GISAXS patterns were observed by visual inspection upon increasing the temperature to 470 K in vacuum after evacuation of CO (Figure 2-d,e) which is just below the desorption temperature of CO (∼ 500 K) from Pt nanoparticles. 13,42 Such temperature was chosen to observe the effect of annealing on CO-covered Pt particles. Slight changes were then observed by annealing at 650 K in vacuum. Additional treatments by exposure to CO and annealing had no observable effects (Figure 2-f) even under a reactive mixture of CO and O2 in contrast with what was observed for Au/TiO2 (110) nanoparticles. 28,29 All treatments are detailed in the next section.

Scavenging scattered Pt adatoms and small clusters by CO at 300 K To explore in more details the change in morphology of the 300 K deposit upon exposure to CO, the intensity profiles recorded in directions perpendicular and parallel to the surface (qz ∼ α f and qy ∼ 2θ f ) were fitted with model clusters of cylindrical shape (Figure 3). Changes in particle height, size and interparticle distance are shown in Figure 4. Aspect ratio and distribution of particles are presented in Figure 5 where the apparent thickness is derived from the size, shape and density determined by GISAXS analysis. In both figures, the successive treatments that were per8 ACS Paragon Plus Environment

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formed are shown on the bottom axis. Changes in GISAXS patterns upon pressure and temperature variations were observed to occur within about a few minutes. Deposit morphologies were stable afterwards. Notably, the GISAXS pattern recorded after the annealing at 650 K (Figure 2-d) did not show any visible evolution after hours of further exposure to CO environment and annealing. A key observation is the increase in the average thickness of the deposit as determined by GISAXS, from ∼ 0.16 nm on the as grown deposit, to ∼ 0.27 nm after the series of treatments, a final value close to the average thickness of 0.3 nm measured by the quartz balance (Figure 5).

After exposure to partial pressure of CO of 10−1 Pa, 1 Pa and 103 Pa of the 300 K deposit, Pt particles increase in height and size whereas the interparticle distance only slightly increases (Figure 4). Indeed, both the increase by 25 % in the particle size determined by GISAXS (Figure 4) and the parallel increase by 50 % in the average film thickness (Figure 5) indicate an agglomeration to large clusters of particles too small to be analyzed by GISAXS. The suggestion of a scavenging of such small entities is supported by the evolution of the specular rod (Figure 6) of which increase in intensity and decrease in width conveys an increase in the X-ray reflection coefficient of the surface. 25,43 The presence of very small clusters in a Pt deposit grown at 300 K is consistent with the height of the diffusion barrier for isolated atoms which order of magnitude is around ∼ 1/3 of the adsorption energy (1.36-2.35 eV). 30,44–46 Indeed, supported Pt7−10 clusters were observed to be stable up to 600 K in vacuum or under reductive conditions. 27,47 The morphology of the as-deposited Pt film is reminiscent of the bimodal size distribution featured in a previous simulation of the growth of Ag/MgO(001) in which the low size tail of the distribution was shown to involve a large fraction of the total amount deposited, 48,49 all the more because the cristallinity of the particles is poor. Finally, to assess that this low size tail indeed escapes the analysis, GISAXS was modeled on a q-range and with parameters similar to those of Figure 3 with monomodal and bimodal distributions (see inset of Figure 7). The latter being chosen such that the total mass in the small (D ∼ 0.6 nm) and large particles (D ∼ 1.2 nm) is the same, the difference between the two curves is within experimental uncertainties (see Figure 3) for the reason that the scattered intensity



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scales with the square of the volume.

Because the scavenging of the Pt particles does not occurs in vacuum but only upon exposure to CO, and then quite quickly, it is attributed to the often demonstrated 6,14,16 formation of mobile Ptx COy moieties that diffuse and agglomerate into bigger particles. Irreversible changes in size and density upon the successive exposures to 0.1 Pa, 1 Pa and 103 Pa CO mostly occurred within the first minutes. Further exposures at the same pressure had hardly any effect, but a new diffusion accompanies each increase in pressure, as if CO under a given pressure could react with specific family of sites in a similar way as in zeolites where large pressures are required to make isolated Pt0 mobile. 6

Critical size for disruption of Pt clusters by CO The exposure of the 300 K sample at room temperature to 10−1 Pa of CO results in a decrease in particle density which is directly observable in Figure 2-b in which the two GISAXS lobes shift toward the center of the reciprocal space. This decrease is estimated to ∼ 15 % with respect to the initial distribution (Figure 5). Higher CO pressure have no more effect of that sort, as evidenced by the absence of visible change in the GISAXS pattern (Figure 2-c) and by the associated fit (Figure 3). The ∼ 15 % of particles that disappear from the observed distribution likely involve the smallest particles. Assuming a normal size distribution, this means that those particles are outside the confidence interval of 70 % with sizes lower than Dlimit = D − 1.1σD , where D and σD are the mean size and the standard deviation of the distribution, respectively. With D = 1.2 nm (as deposited film, Figure 4) and σD ≃ 0.25D a typical value found for supported metallic particles, 25,30 Dlimit = 0.725D ≃ 0.9 nm which corresponds to ∼ 25 − 30 Pt atoms per cluster. On the normal size distribution basis (see Fig. 7), the clusters that disappear upon exposure to CO (D < Dlimit ) involve less than 4 % of the initial total mass analyzed by GISAXS which is an order of magnitude smaller than the increase in "visible" mass of ∼ 50 % that results from the above scavenging pro10 ACS Paragon Plus Environment

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cess. Nevertheless, although the phenomenon is marginal, its mechanism has to be understood to fully describe the CO-induced change in morphology of the 300 K Pt deposit.

Two alternative processes can be invoked for the disappearance from the observed distribution of the Pt clusters of less than ∼ 25 − 30 atoms in size, either coalescence with larger clusters upon CO adsorption or after CO-driven diffusion or disruption into smaller entities. The decrease in cluster density is almost completed after exposure to 10−1 Pa CO with no effect of further exposures to 1 Pa and 103 Pa CO (Figure 5). Therefore, a CO-driven diffusion of aggregates is unlikely since it should be enhanced by an increase in pressure. Moreover, the aspect ratio of the Pt clusters remains constant (Figure 5), which discards a coalescence mechanism similar to that observed at 470 K (see below). In a similar way, static coalescence upon steric effects due to the adsorbed CO layer covering the particles can be ruled out as a major process since the gap between visible particles (∼ 1.4 nm) is much larger than two times the size of the carbonyl group (0.11 nm) plus a typical Pt-CO bond length. Conversely, there exists a series of reasons that favor the disruption of small clusters in the presence of CO. First, the energy per atom is expected to decrease in small clusters. Baetzold has found a minimum for aggregates involving 5 − 10 atoms. 50 The average cohesion energy of 255 kJ.mol−1 found in Pt10 is twice as small as that determined for bulk Pt (528 kJ.mol−1 ). 51 Moreover, at undercoordinated surface sites of which density is increasing as Pt particles become smaller, the shift towards the Fermi level of the d band centre results in an increase of the CO chemisorption energy 8,13,42,52 due to the coupling between the lowest unoccupied 2π ∗ CO molecular orbital and the metal d states; 53 as a result, in a way similar to stepped platinum surfaces which exhibit twofold desorption spectra with peaks around 415 K and 500 K, 42,54 the desorption temperature of CO, which amounts to 415 K on wide terraces of clusters, 13 increase to more than 500 K on small clusters. 13,42 In addition, the undercoordinated sites of stepped surfaces tend to populate first 42,54 which is confirmed in the case of small Pt particles by the increase of the 2π ∗ -like Fermi level local density of states observed by NMR (Nuclear Magnetic Resonance) which correlates with the red shift in stretching frequency due to back-bonding 55 that characterizes



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edge- and kink-bound CO adsorbate. 55,56

Indeed, CO has been often seen to disrupt small Pt particles. 9,14,15 But the switch between that disruption and the agglomeration such as that described in the previous section is not clarified yet. The apparent contradiction between the two phenomena can be lifted under the assumption that the disruption requires that the Pt-CO adsorption energy is stronger than the Pt-Pt cohesive energy. 6,7,52 Conversely, clusters bigger than the critical size are stable in the presence of CO, as the balance between Pt-CO adsorption energy and Pt-Pt cohesive energy is reversed. Therefore, the above limit of ∼ 25 − 30 Pt atoms per cluster defines a critical size for disintegration, which is consistent with the estimate by EXAFS analysis (between 4-6 atoms and 15-50 atoms 7,17 ) of the size-dependent decomposition of Pt agreggates in the presence of CO. It is assumed herein that clusters smaller than the critical size, no matter they belong to the low size tail of the GISAXS distribution or to the collection of particles that escape GISAXS analysis, give rise to Ptx COy carbonyl moieties of which size is governed by the Pt-CO interaction. 6,14,16 The transfer of electrons from the metal to the anti-bonding orbital of the adsorbed CO molecules results in a reduction of the adsorption energy of Pt clusters on the MgO surface. 53 This makes the Ptx COy species more mobile than corresponding pure Pt clusters and, consequently, enhances the probability for these species to coalesce with Pt clusters larger than the critical size for which the Pt-Pt bond energy is stronger than the CO-Pt attachment. 6 This explains both the disruption/scavenging of Pt particles smaller than the critical size and the agglomeration of these particles to larger clusters otherwise not affected by the presence of CO. A similar contrast is oberved between the strong restructuring undergone by the undercoordinated Pt(557) surface in the presence of CO and the inertness of the dense Pt(111) surface. 3


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CO-driven coalescence The density of the Pt/MgO(001) particles deposited herein at 300 K is very close to those observed for deposits of the same thickness (0.3 nm) that were grown at 600 K and 800 K. 30 The similarity of the aspect ratios found herein at 300 K (1.4 − 1.45, Figure 5) and previously at 600 K, 800 K and 1000 K 30 (Figure 5) indicates that deposition at any temperature in the 300-1000 K range of small Pt particles of 1-2 nm in size leads to particle shapes close to equilibrium. This makes meaningful the change in morphology undergone by Pt clusters by annealing the 300 K deposit at 470 K in vacuum after exposure to 103 Pa of CO (Figures 2-d, 4, 5), with simultaneous increases in aspect ratio, to ∼ 1.9 (Figure 5), and size, by more than 30 %, at nearly constant height (Figure 4). By annealing at 470 K and 650 K in vacuum, the cluster density decreases by about 45 % to reach a value not too far from that previously observed upon deposition at 1000 K. Upon annealing at 650 K and further treatments, the aspect ratio decreases progressively to reach a value of 1.6 (Figure 5) closer to the equilibrium value.

The increase in aspect ratio associated with a constant height indicates an out of equilibrium process in which Pt particles diffuse and coalesce at a temperature too low to allow a quick diffusion within particles. A simple effect of temperature is unlikely since the particle density reached at 470 K is half that observed previously by direct deposition of the same coverage at a temperature as high as 800 K. 30 It is only by deposition at 1000 K that such density could be achieved. 30 Consistently, Pt clusters of similar size dispersed on SiO2 were found to be resistant to sintering under vacuum or reductive atmosphere up to ∼ 600 K. 4,27,47 Much higher temperatures (∼ 700 K) are needed to induce sizeable oxygen promoted ripening of alumina supported nanoparticles of 2-4 nm in size. 57,58 The strong bonding of CO to Pt clusters suggests another mechanism. Since the annealing temperature of 470 K is below the desorption temperature of CO from small clusters (>500 K 13,42 ), the weakening of the bonding to the substrate of the CO-covered clusters combined to the annealing results in a diffusion of the particles and then to coalescence. This diffusion is only observed for clusters small enough to entail a strong Pt-CO attachment. Annealing from 470 K to 13 ACS Paragon Plus Environment


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650 K, through the desorption of CO from the Pt, results in some more mobility of the supported clusters, with a small decrease in aspect ratio and in density. Further treatments lead to a slight decrease in aspect ratio (toward equilibrium) with no sizeable change in size and density of the Pt clusters (Figure 5).

Attempts to change morphology of Pt films either deposited at high temperature or annealed after deposition at 300 K by exposure to gases systematically failed. The 0.3 nm thick film deposited at 700 K with particles of 1.9 nm in size is insensitive to exposure to 10−1 Pa of CO. Indeed, the population of particles of sizes lower than 0.9 nm (∼ 25 − 30 atoms) which are expected to be disrupted by CO is marginal at such growth temperature. The 0.16 nm thick film grown at 900 K which involves clusters of 4.2 nm in size is also insensitive to exposure to 2.102 Pa of CO (see Figure 2 of supporting information) for the same reason.

Comparison with other CO-metal systems Parallel behaviors were observed for late transition metals nanoparticles with similar puzzling questions about the simultaneous aggregation and disruption of clusters. In the presence of CO, the oxidative disruption of Rh, 20,21,59–62 Ru, 22,23,63,64 Ir 18,19,65 and Re 66 nanoparticles at room temperature gives rise to the formation of Mex (CO)y moieties (Me = Rh, Ru, Ir, Re), of which the most common is Me(CO)2 , that have been characterized by infrared spectroscopy, via the analysis of the CO stretching frequency, 18,20,22,23,60,67,68 and by EXAFS. 59,62 Beyond the suggestion that the oxidation of Rh0 , Ir0 and Ru0 into RhI , 68 IrI 18 and RuI 22,23 that follows the disruption of nanoclusters is mediated by surface OH groups, the composition of the Mex (CO)y entities is debated 64,69 as well as the charge of the Me element. In all cases, the disruption of the supported metal nanoclusters is however not complete. Bonding typical of small Rh clusters are seen by EXAFS to persist upon CO exposure 62 and it was shown by STM that, if Rh clusters of 1-2 nm in size are quickly disintegrated, the process is slower for larger particles while particles of 8-10 nm in diameter are not at all affected. 21,61 In a much 14 ACS Paragon Plus Environment

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similar manner, Ir particles involving 8-10 atoms are readily dissociated in the presence of CO, but clusters of 3-4 nm in size undergo only marginal corrosion and larger clusters are not affected. 19,65 Indeed, reductive agglomeration of Mex (CO)y species (assumed to diffuse more easily than Me adatoms 21 ) into Irn , 18,19 Run 23 and Rhn 20,21,60–62 clusters is likely favored by raising the temperature of the Ir, Ru and Rh films, respectively. Notably, in the case of the Pt deposit, it is only after annealing at 500 K that dispersed Pt atoms are completely agglomerated to particles which might be due to the above mentioned difference in mobility of dispersed platinum. 6

Those observations suggest for all the listed metal catalysts a mechanism similar to that was evidenced for MgO-supported Pt to explain the switch between disruption and agglomeration. This mechanism is based on the existence of critical sizes that are specific to each metal since they depend on the coordination number below which CO adsorption disrupts the metal lattice. Therefore, the present findings appear to be consistent with the behavior of other metals.

Conclusion A mechanism that explains the simultaneous disruption/agglomeration of supported Pt particles on the basis of a critical size has been evidenced by GISAXS analysis of Pt/MgO(001) particles upon exposure to CO from 10−6 − 103 Pa: (i) particles below a critical size estimated to ∼ 1 nm are disrupted at room temperature above a CO pressure of 10−1 Pa. The existence of a critical size depends on the coordination number at which the adsorption of CO can disrupt the metal lattice; (ii) disrupted Pt atoms form carbonyl species (of which diffusion is easier than Pt adatoms) that agglomerate to stable clusters (larger than the critical size) in a kind of CO-driven ripening mechanism; the process which is already observed at room temperature, is completed by annealing at 470 K; (iii) CO-covered clusters of size ranging between the critical value and 2 nm, diffuse to coalesce upon annealing. Those data demonstrate that Pt nanoparticles of less than 2 nm in size



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are unstable with respect to CO exposure at partial pressures higher than 10−1 Pa, in the absence of support that can help to stabilize those particles. A similar mechanism is suggested to apply in the case of other metal catalysts such as Ru, Rh and Ir.

Acknowledgement The invaluable support of the technical staff of BM32 beamline is acknowledged.

Supporting Information Available The supporting information shows how stable are the Pt/MgO(001) nanoparticles of sizes above the critical one upon exposure and thermal treatments. GISAXS data from samples grown at 800 and 900 K with average particle sizes of 3.6 and 4.2 nm are provided provided as an illustration. This material is available free of charge via the Internet at http://pubs.acs.org/.

References (1) Henry, C. Surf. Sci. Rep. 1998, 31, 231–325. (2) Nagai, Y.; Dohmae, K.; Ikeda, Y.; Takagi, N.; Hara, N.; Tanabe, T.; Guilera, G.; Pascarelli, S.; Newton, M. A.; Takahashi, N. et al. Catal. Today 2011, 175, 133–140. (3) Tao, F.; Dag, S.; Wang, L. W.; Liu, Z.; Butcher, D. R.; Bluhm, H.; Salmeron, M.; Somorjai, G. A. Science 2010, 327, 850–853. (4) Wyrzgol, S. A.; Schäfer, S.; Lee, S.; Lee, B.; Di Vece, M.; Li, X.; Seifert, S.; Winans, R. E.; Stutzmann, M.; Lercher, J. A. et al. Phys. Chem. Chem. Phys. 2010, 12, 5585–5595. (5) Nolte, P.; Stierle, A.; Jin-Phillipp, N. Y.; Kasper, N.; Schulli, T.; Dosch, H. Science 2008, 321, 1654–1658. (6) Stakheev, A. U.; Shpiro, E. S.; Jaeger, N. I.; Schulz-Ekloff, G. Catal. Lett. 1995, 34, 293–303.


Page 16 of 29

Page 17 of 29

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


(7) Mojet, B.; Miller, J.; Koningsberger, D. J. Phys . Chem. B 1999, 103, 2724–2734. (8) Nakamura, M.; Hanioka, Y.; Ouchida, W.; Yamada, M.; Hoshi, N. Chem. Phys. Chem 2009, 10, 2719–2724. (9) Vicente, B. C.; Nelson, R. C.; Singh, J.; Scott, S. L.; van Bokhoven, J. A. Catal. Today 2011, 160, 137–143. (10) Dong, F.; Tanabe, T.; Suda, A.; Takahashi, N.; Sobukawa, H.; Shinjoh, H. Chem. Eng. Sci. 2008, 63, 5020–5027. (11) Allian, A. D.; Takanabe, K.; Fujdala, K. L.; Hao, X.; Truex, T. J.; Cai, J.; Buda, C.; Neurock, M.; Iglesia, E. J. Am. Chem. Soc. 2011, 133, 4498–4517. (12) Longwitz, S. R.; Schnadt, J.; Vestergaard, E. K.; Vang, R. T.; Lægsgaard, E.; Stensgaard, I.; Brune, H.; Besenbacher, F. J. Phys. Chem. B 2004, 108, 14497–14502. (13) Gan, S.; Liang, Y.; Baer, D.; Sievers, M.; Herman, G.; Peden, C. J. Phys . Chem. B 2001, 105, 2412–2416. (14) Raskó, J. J. Catal. 2003, 217, 478–486. (15) Berkó, A.; Szöokö, J.; Solymosi, J. S. Surf. Sci. 2004, 566-568, 337–342. (16) Akdogan, Y.; Anantharaman, S.; Liu, X.; Lahiri, G. K.; Bertagnolli, H.; Roduner, E. J. Phys. Chem. C 2009, 113, 2352–2359. (17) Mojet, B. L.; Koningsberger, D. C. Catal. Lett. 1996, 39, 191–196. (18) Solymosi, F.; Novák, E.; Molnár, A. J. Phys . Chem. 1990, 94, 7250–7255. (19) Berkó, A.; Solymosi, F. J. Phys . Chem. B 2000, 104, 10215–10221. (20) Solymosi, F.; Pdsztor, M. J. Phys. Chem. 1985, 89, 4789–4793. (21) Berkó, A.; Solymosi, F. J. Catal. 1999, 183, 91–101. 17 ACS Paragon Plus Environment


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

(22) Solymosi, F.; Raskó, J. J. Catal. 1989, 115, 107–119. (23) Mizushima, T.; Tohji, K.; Udagawa, Y.; Ueno, A. J. Phys. Chem. 1990, 94, 4980–4985. (24) Martra, G.; Arena, F.; Baricco, M.; Coluccia, S.; Marchese, L.; Parmaliana, A. Catal. Today 1993, 17, 449–458. (25) Renaud, G.; Lazzari, R.; Leroy, F. Surf. Sci. Rep. 2009, 64, 255–380. (26) Renaud, G.; Lazzari, R.; Revenant, C.; Barbier, A.; Noblet, M.; Ulrich, O.; Leroy, F.; Jupille, J.; Borenstzein, Y.; Henry, C. et al. Science 2003, 300, 1416–1419. (27) Winans, R.; Vajdaa, S.; Ballentinea, G.; Elam, J.; Lee, B.; Pelline, M.; Seifert, S.; Tikhonov, G.; Tomczyk, N. Top. Catal. 2006, 39, 145–149. (28) Saint-Lager, M.; Bailly, A.; Mantilla, M.; Garaudée, S.; Lazzari, R.; Dolle, P.; Robach, O.; Jupille, J.; Laoufi, I.; Taunier, P. Gold. Bull. 2008, 42, 159. (29) Laoufi, I.; Saint-Lager, M.; Lazzari, R.; Jupille, J.; Robach, O.; Garaudée, S.; Cabailh, G.; Dolle, P.; Cruguel, H.; Bailly, A. J. Phys. Chem. C 2011, 115, 4673–4679. (30) Olander, J.; Lazzari, R.; Mangili, B.; Goniakowski, J.; Renaud, G.; Jupille, J. Phys. Rev. B 2007, 76, 075409. (31) Baudoing-Savois, R.; De Santis, M.; Saint-Lager, M.; Dolle, P.; Geaymond, O.; Taunier, P.; Jeantet, P.; Roux, J.; Renaud, G.; Barbier, A. Nucl. Inst. Meth. Phys. Res. B 1999, 149, 213– 227. (32) Lazzari, R. In X-ray and neutron reflectivity: principle and applications; Daillant, J., Gibaud, A., Eds.; Springer, Lectures Notes in Physics, 2009; Chapter Grazing incidence small angle x-ray scattering from nanostrctures, pp 283–342. (33) Renaud, G.; Ducruet, M.; Ulrich, O.; Lazzari, R. Nucl. Inst. Meth. B 2004, 222, 667–680. (34) Lazzari, R. J. Appl. Cryst. 2002, 35, 406–421. 18 ACS Paragon Plus Environment

Page 18 of 29

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(35) Rauscher, M.; Paniago, R.; Metzger, H.; Kovats, Z.; Domke, J.; Pfannes, H. D.; Schulze, J.; Eisele, I. J. Appl. Phys. 1999, 86, 6763–6769. (36) Hosemann, R.; Bagchi, S. N. Direct analysis of diffraction by matter; North-Holland Publishing Company: Amsterdam, 1962. (37) Leroy, F.; Lazzari, R.; Renaud, G. Acta. Cryst. A 2004, 60, 565–581. (38) Lazzari, R. IsGISAXS can be downloaded with an user guide from:

http://ln-

www.insp.upmc.fr/axe4/Oxydes/IsGISAXS/isgisaxs.htm. (39) Laboratory, L. B. N. Center for X-ray Optics. http://www-cxro.lbl.gov/. (40) Robach, O.; Renaud, G.; Barbier, A. Surf. Sci. 1998, 401, 227–235. (41) Inouye, H.; De Van, J. J. Material for Energy Systems 1979, 1, 52–60. (42) Altman, E. I.; Gorte, R. J. Surf. Sci. 1988, 195, 392–402. (43) Lazzari, R.; Leroy, F.; Renaud, G. Phys. Rev. B 2007, 76, 125411. (44) Yudanov, I.; Pacchioni, G.; Neyman, K.; Rösh, N. J. Phys. Chem. B 1997, 101, 2786. (45) Grönbeck, H.; Broqvist, P. J. Chem. Phys. 2003, 119, 3896. (46) Bogicevic, A.; Jennison, D. R. Surf. Sci. 1991, 437, L741. (47) Winans, R. E.; Vajda, S.; Lee, B.; Riley, S. J.; Seifert, S.; Tikhonov, G. Y.; Tomczyk, N. A. J. Phys. Chem. B 2004, 47, 18105–18107. (48) Lazzari, R.; Renaud, G.; Revenant, C.; Jupille, J.; Borenstzein, Y. Phys. Rev. B 2009, 79, 125428. (49) Revenant, C.; Renaud, G.; Lazzari, R.; Jupille, J. Phys. Rev. B 2009, 79, 235424. (50) Baetzold, R. C. J. Phys. Chem. 1976, 80, 1504–1509. 19 ACS Paragon Plus Environment


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(51) Watwe, R. M.; Spiewak, B. E.; Cortright, R. D.; Dumesic, J. A. Catal. Lett. 1998, 51, 139– 147. (52) Maillard, F.; Eikerling, M.; Cherstiouk, O. V.; Schreier, S.; Savinova, E.; Stimminga, U. Faraday Discuss. 2004, 125, 357–377. (53) Hammer, B.; Nielsen, O. H.; Nørskov, J. K. N. Catal. Lett. 1997, 46, 31–35. (54) Xu, J.; Yates, J. T. Surf. Sci. 1995, 327, 193–201. (55) Tong, Y.; Rice, C.; Wieckowski, A.; Oldfield, E. J. Am. Chem. Soc. 2000, 122, 1123–1129. (56) Park, S.; Wasileski, S. A.; Weaver, M. J. J. Phys. Chem. B 2001, 105, 9719–9725. (57) Simonsen, A. B.; Chorkendorff, I.; Dahl, D.; Skoglundh, M.; Sehested, J.; Helveg, S. J. Am. Chem. Soc 2010, 132, 7968–7975. (58) Simonsen, S. B.; Chorkendorff, I.; Dahl, S.; Skoglundh, M.; Meinander, K.; Jensen, T. N.; Lauritsen, J. V.; Helveg, S. J. Phys . Chem. C 2012, 116, 5646–5653. (59) van‘t Blik, H. F. J.; van Zon, J. B. A. D.; Huizinga, T.; Vis, J. C.; Koningsberger, D. C.; Prins, R. J. Am. Chem. Soc. 1985, 107, 3139–3147. (60) Solymosi, F.; Bánsági, T. J. Phys. Chem. 1993, 97, 10133–10138. (61) Berkó, A.; Ménesi, G.; Solymosi, F. J. Phys. Chem. 1996, 100, 17732–17734. (62) Newton, M. A. Top. Catal. 2009, 52, 1410–1424. (63) Robbins, J. L. J. Catal. 1989, 115, 120–131. (64) Hadjiivanov, K.; Lavalley, J. C.; Lamotte, J.; Maugé, F.; Saint-Just, J.; Chez, M. J. Catal. 1998, 176, 415–425. (65) Berkó, A.; Solymosi, F. Surf. Sci. 1998, 411, L900–L903.


Page 20 of 29

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(66) Solymosi, F.; ; Bánsági, T. J. Phys. Chem. 1992, 96, 1349–1355. (67) Yang, C.; Garl, C. W. J. Phys. Chem. 1957, 61, 1504–1512. (68) Basu, P.; Panayotov, D.; Yates, J. T. J. Am. Chem. Soc. 1988, 110, 2074–2081. (69) Chin, S. Y.; Williams, C. T.; Amiridis, M. D. J. Phys. Chem. B 2006, 110, 871–882.



6

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Figure 1: In-plane GIXD reciprocal scan for the as-deposited samples. Diffracted intensity is plotted as a function of the reciprocal lattice units of MgO along the a) [100] and b) [110] directions of MgO. Bragg peaks of MgO are located at (200) and (220) positions while Pt in (001) epitaxy contributes to the satellite at (2.16, 0, 0) and (2.16, 2.16, 0).


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CO-Induced Scavenging of Supported Pt Nanoclusters - American

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