Pt Nanoparticle Sintering and Redispersion on a Heterogeneous

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Pt Nanoparticle Sintering and Redispersion on a Heterogeneous Nanostructured Support Pooya Tabib Zadeh Adibi, Torben Pingel, Eva Olsson, Henrik Grönbeck, and Christoph Langhammer J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b03874 • Publication Date (Web): 16 Jun 2016 Downloaded from http://pubs.acs.org on June 19, 2016

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The Journal of Physical Chemistry

Pt Nanoparticle Sintering and Redispersion on a Heterogeneous Nanostructured Support Pooya Tabib Zadeh Adibi1,2, Torben Pingel1,2, Eva Olsson1,2, Henrik Grönbeck1,2 and Christoph Langhammer1* 1

Department of Physics and 2Competence Centre for Catalysis, Chalmers University of

Technology, 412 96 Göteborg, Sweden

ACS Paragon Plus Environment

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ABSTRACT Understanding how nanostructure and atomic-scale defects of the support affect metal catalyst nanoparticle sintering is of crucial importance to minimize thermal deactivation, as well as to understand the origin of widely observed but still unexplained phenomena, such as transient multimodal particle size distributions, and nanoparticle redispersion. To shed light on these issues we present a generic experimental approach that relies on nanofabrication to introduce controlled structural heterogeneity in a chemically homogeneous model catalyst support. This is achieved by fabricating arrays of nanocone structures separated by flat areas, where both are homogeneously sputter-coated with a thin amorphous alumina layer. Using ex situ aberrationcorrected scanning transmission electron microscopy (STEM) to analyze Pt model catalyst nanoparticles on such nanostructured supports prior and after exposure to 4% O2 in Ar carrier gas at 600 °C, we find that the initial particle size distributions and their time evolution during sintering to be different on the cones and the flat areas. On the cones, redispersion of Pt into highly abundant particles of about 1 nm occurs very rapidly. In contrast, particle shrinkage and growth combined with redispersion occurs on the flat areas, leading to a broader and bimodal size distribution. These processes are amplified and efficiently demonstrated by the nanostructured surface thanks to (i) higher support defect density on the nanocones compared to the flat surfaces in between, and (ii) initially different Pt particle size distributions on the cones and on the flat surfaces. Hence, the nanostructured surface facilitates the clear identification of catalyst redispersion in oxidizing conditions, and experimentally identifies a mechanism that gives rise to (transient) bi- or multimodal particle size distributions during sintering.


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TEXT Introduction

Noble metal nanoparticles dispersed inside a mesoporous oxide matrix with high specific surface area are widely used in heterogeneous catalysis, where the automotive three-way catalytic converter only is one example. The mesoporous oxide matrix is characterized by structural and chemical heterogeneity from the nano- down to the atomic scale1. Such heterogeneity originates from, for instance, structural defects2–5, local curvature6 and inhomogeneous distribution of reaction promoters such as ceria and lanthanum7,8. Consequently, the local environment for the dispersed noble metal nanoparticles may vary considerably across the support, both globally and locally. This is likely to be critical for several aspects of a catalytic process, and it is thus important to understand the role of support heterogeneity in detail.

One process where the role of support heterogeneity is not well understood is catalyst nanoparticle sintering or ripening9. At high temperatures, small nanoparticles tend to coalesce and form larger ones to reduce the total surface energy of the system, with a loss of active surface area and activity as the consequence. Two distinct sintering mechanisms have been proposed and experimentally verified: (i) Ostwald ripening, where monomers (metal atoms or metal-adsorbate species) detach from smaller particles and diffuse over the support (or via the gas phase), followed by attachment to larger particles. (ii) Particle migration, in which entire particles move over the support and coalesce into neighboring particles. During particle migration, particle motion is realized by diffusion of atoms on the particle surface without detachment. Transient particle size distributions during sintering are often complex and bi- or


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multimodal shapes have widely been observed, however, without clearly identifying the mechanism behind their creation10–14.

It is clear from this brief description of the main sintering mechanisms, that the interaction of the metal particles with the support – and thus support heterogeneity – is of central importance for sintering processes, as recently also have been discussed theoretically15,16,23. However, to elucidate experimentally the role of support heterogeneity in sintering processes is challenging due to the structural complexity of the systems at hand, especially considering real catalysts including the three-dimensional mesoporous matrix. These complications are most likely one of the main reasons for the contradictory results reported in the literature concerning sintering processes of metal particles supported in a porous matrix17–21.

To address these issues we introduce a strategy, which employs nanofabrication to introduce controlled three-dimensional structural support heterogeneity from the atomic to the nanometer/tens-of-nanometer length scales on globally flat and chemically homogeneous model systems, compatible with both in situ and ex situ TEM analysis of catalyst nanoparticle sintering. Specifically, arrays of alumina nanocones are fabricated on the surface of flat TEM membranes to introduce surface curvature similar to what is found in a mesoporous washcoat matrix, as well as atomic-level structural heterogeneity in terms of nanoscale surface corrugation obtained on the nanocone walls. By evaporating small amounts of Pt onto the nanostructured support to form nanoparticles, a system with initially different mean particle sizes (as well as size distributions) is obtained on the flat areas of the sample and on the nanocone walls, respectively. In this way, we create controlled local differences in nanoparticle size distribution as the starting point of our


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sintering experiments, as well as a surface that is structurally different at both the atomic and the nanometer scale.

This new platform is used in combination with ex situ STEM analysis to investigate the sintering of Pt nanoparticles in oxidizing environment at atmospheric pressure. We observe significant redispersion into small and surprisingly stable nanoparticles on the nanocone walls, while both redispersion (however less pronounced than on the nanocones) and growth and shrinkage of particles in agreement with the Ostwald ripening mechanism are evident on the flat areas. This gives rise to the appearance of a bimodal size distribution, as observed in numerous other studies10–14, however, without unambiguously identifying the reason. Thus, our findings stress the role of support heterogeneity at different lengths scales in sintering/redispersion of supported metal particles and demonstrate the usefulness of nanofabricated model surfaces with controlled structural (or chemical) heterogeneities for the investigation of support-effects in catalyst nanoparticle sintering and redispersion.

Background Experimental efforts to address the role of the support in sintering processes are numerous in the literature. For example, surface science approaches using well-defined surfaces with controlled amount of impurities or surface defects such as steps and vacancies have been used to mimic support heterogeneity1,22. To this end, studies using scanning tunneling microscopy (STM) of Pt particles supported on TiO2(110)23 and TiO2(771)24 show higher population of particles along the step edges after vacuum annealing. However, this approach is limited to sintering in ultra-high vacuum (UHV) conditions or in the low adsorbate pressure regime3,25.


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Consequently, important aspects like the influence of gas pressure on the coarsening of particles cannot be addressed.

Another common way to study supported catalyst sintering is transmission electron microscopy (TEM),26 where metal particles can be visualized either during the sintering (in situ, within a limited pressure range)27 or after specific time intervals spent in a catalytic reactor where the sintering takes place at high pressure (ex situ). To this end, TEM studies have been carried out both on electron-transparent thin, flat and often amorphous supports with no active control of support surface homo- or heterogeneity14,28–30, as well as on realistic systems based on a mesoporous matrix, however without characterization/specification of support heterogeneity31– 33

. The latter may limit the generality of the results due to both the local character of TEM

measurements and the possibility of locally and globally varying support heterogeneity over the oxide support. In an attempt to overcome the limitations of the approaches summarized above, we introduce and discuss an approach where controlled 3D surface heterogeneity is introduced on a flat model system by means of nanofabrication.

Experimental Fabrication of nanostructured support The nanostructured catalyst support surfaces were nanofabricated directly onto 40 nm thick Si3N4 TEM-“window” membranes34 by use of hole-mask colloidal lithography.35 This method is based on the electrostatic self-assembly of polystyrene (PS) colloidal particles (80 nm mean diameter) as the basis for the fabrication of an evaporation mask. The diameter of the PS particles determines the size of the holes in the evaporation mask and, thus, also of the final


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nanostructure on the surface. To achieve a low nanocone density suitable for our experiments, we pipetted a 0.02 wt.% PS particle suspension in deionized water on the sample, using a residence time of one minute before rinsing and blow-drying. The nanocones were fabricated by evaporating 120 nm aluminum oxide (rate of 1 Å/s) through the hole-mask using an AVAC HVC-600 e-beam evaporator operated at a base pressure of 3×10-6 mbar. The cone-formation is a consequence of the deposition of aluminum oxide also on the rims of the holes in the mask, resulting in a continuous shrinking of the holes during deposition and thus the formation of tapered nanostructures.36 After nanocone growth and subsequent lift-off of the mask, the entire sample was sputter-coated with a 10 nm thick amorphous layer of aluminum oxide (FHR MS 150), in order to create a nominally chemically homogeneous surface that is identical on the cones and on the flat areas separating the cones. Importantly, however, we note that both types of areas by no means have to be structurally homogeneous – in fact, as discussed in our previous study, structural heterogeneity in terms of patches with different interface energies may also be present on the flat areas14. As the last step of the support fabrication, the samples where heattreated for 36 h at 615 °C in air for stabilization. Finally, Pt nanoparticles were grown on the support by e-beam evaporation of an ultra-thin Pt film with nominal thickness of 5 Å, at a rate of 0.5 Å/s onto the water-cooled support. A sketch together with representative TEM images of the obtained arrangement is shown in Figure 1. Note the different initial average particle size and particle size distribution on the cone wall and on the flat areas (Figure 1c). Compared to the flat surface, a much higher particle density and smaller average particle sizes are observed on the cones. This effect is a consequence of the different growth of the Pt particles on the slanted and flat surfaces, respectively. This is most likely due to the higher density of defects (i.e. nucleation


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sites for nanoparticles during growth) on the cone walls compared to the flat surface, yielding more but smaller particles on the cones, as discussed in detail below.

Sintering experiments The samples were inserted in a gas flow reactor and exposed to 4 % O2 in Ar carrier gas using a flow rate of 100 ml/min. The upstream gas temperature was measured by a thermocouple and controlled by a Eurotherm temperature controller. The samples were heated in Ar to 600 °C with a heating rate of 10 °C/min, followed by a 10 min dwell for temperature stabilization. Thereafter, the samples were exposed to 4% O2 in Ar for 4, 10, 30 and 60 minutes, respectively. Finally, the samples were cooled to room temperature in pure Ar to ensure that no further sintering took place.

TEM analysis TEM imaging was performed using an FEI Titan 80-300 operated at 300 kV accelerating voltage. The instrument is equipped with a field-emission electron gun and a probe spherical aberration corrector. To better resolve the metal particles on the thick nanocone, and to achieve high Z-contrast, imaging was performed in STEM mode where the diffracted electrons are collected by a high-angle annular dark field (HAADF) detector. To derive histograms of the particle size distribution, the Pt particles in the acquired images were outlined manually and analyzed using Image J37. The particle diameters were determined from their projected areas based on a circular approximation. Thus, the particles on the cones were analyzed based on their apparent projected diameter, which is slightly smaller than the real projected diameter obtained on a flat surface (see Figure S1 in the Supporting Information (SI) and the corresponding


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discussion). However, this small inconsistency between derived particle diameters on the cone and flat surfaces is not important for the conclusions drawn from our analysis, as only trends and not absolute numbers are compared. To get good statistics in the PSD analysis, two images at the same magnification for each sample type were analyzed, which typically means a total number of around 250 and 70 particles analyzed on the cone and the flat surface, respectively, for each image.

Results and discussion The main results of our study are summarized in Figure 2 and Figure 3. They display a series of HAADF-STEM images of flat areas and single cones from the same sample, together with the corresponding particle size distribution histograms for samples sintered in O2 for different time intervals. The initial PSDs (t0) are almost Gaussian and have a mean particle diameter of 3.3 ± 1.0 nm and 1.9 ± 0.6 nmon flat and cone surfaces, respectively.

Following the time evolution of the PSDs on the flat and cone surfaces, respectively, we notice distinct differences. On the flat part of the support, we observe growth of some of the nanoparticles and a transition to a bimodal size distribution after 60 minutes. This has been observed previously for similar systems14. The increased abundance of small particles in the PSD in combination with the growth of other particles can be interpreted as the signature of Ostwald ripening, where small particles shrink and large particles grow. However, as discussed below, this is not the complete picture.


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The same analysis for the cone surface reveals a very different behavior. Already after 10 minutes the PSD is completely different with a large number of nanoparticles much smaller compared to the initial stage. This is most likely due to the smaller initial mean particle size on the cone compared to the flat surface, which yields low inherent stability and a strong driving force for mass transport to the flat areas according to the Ostwald-ripening mechanism. Moreover, the total number of particles per area (particle density) has significantly increased compared to the as-grown state of the sample, which indicates that new particles have been formed during the process. Notably, the PSD of these small nanoparticles remains more or less constant also after 1 h. These observations have two key implications. Firstly, on the nanocone, we clearly observe significant redispersion of the catalyst as a new population of small nanoparticles is formed during the initial sintering. Secondly, the redispersed small nanoparticles are surprisingly sintering resistant – most likely (see below) since they are formed and stabilized at specific defect sites abundant on the nanocone surface.

To further quantify the observations from the TEM images, we chose three descriptors of the PSDs, namely the mean particle diameter, , the particle density and the particle surface coverage, which are reported as a function of exposure time to the oxygen atmosphere in Figure 4. During the first 10 minutes, on both flat and cone surfaces decreases. At the same time, the particle density on the cone is drastically increasing, whereas it also increases – but not as drastically - on the flat surface (note that also the absolute values the particle density on the cone are based on the apparent projected area - see Figure S1 in the SI and the corresponding discussion for details). There, it reaches a maximum already after 10 min, whereas it keeps


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increasing on the cone surfaces. This supports the hypothesis of redispersion on the cone that is significantly more pronounced than on the flat surface. Further corroboration comes from the PSDs for both flat surface and cone shown in Figure 2, where it becomes evident that a population of small (< 2 nm) particles is formed in both systems after 10 minutes, which shifts towards smaller values. At first glance, the observed initial decrease in mean particle size on the flat surface contradicts our earlier study14 and another report on a similar system28. However, due to the higher resolving power of the TEM instrument used in the present study compared to earlier works, for the first time, very small particles and clusters are resolved. Since a new population of these small particles is formed during the initial redispersion, it transiently reduces . Therefore, the reason for the apparent discrepancy between the present and previous studies is the fact that particle sizes below 1 nm previously have been overlooked.

For the surface coverage, we observe a large decrease from 33 % to 10 % on the cone surface due to the complete disappearance of the (initially grown) large particles between 4 and 10 min. On the flat surface, the change is much smaller (from 27 % to about 20 %) and can partly be explained by the observed changes in particle shape from more elongated and asymmetric to spherical, and partly by particle sintering where small nanoparticles shrink and disappear. For the later sintering stages, the observed changes are not as pronounced for any of the three descriptors, with the key observations being a virtually constant mean diameter on the cone, indicating high sintering resistance of the redispersed particles. Furthermore, the particle density on the cone continues to increases up to 30 min of sintering, indicating further redispersion and the formation of additional small particles, before a slight decrease is observed at 60 min.


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The evolution of the size distribution on the cone surface is contrasted by an increase of the mean diameter on the flat surface (see also Figure S2 for a corresponding plot using the volume average diameter instead). Consulting the corresponding PSDs reveals that, at 30 min, a new population of larger particles starts to form, giving rise to the observed increase in , and results in a distinct bimodal PSD at 60 min, as also observed for the same system at identical conditions in our previous study14. Notably, the maximum of the second peak in the PSD is clearly shifted to a larger particle size, indicating significant sintering to occur on the flat surface from 30 to 60 min. This is further corroborated by a monotonously decreasing particle density after the maximum at 10 min. At the same time, the population of the really small particles formed by redispersion during the initial phase is more or less unchanged, which indicates that these particles are more resistant to sintering compared to the as-grown ones. This is most likely because these particles, as on the cone surface, nucleate at “stabilizing” defect sites, not present during the initial nucleation and growth of the Pt nanoparticles. One possible reason for the activation of these sites during the sintering treatment may be desorption of water and a related change hydroxyl group coverage, which significantly changes the acidic/basic properties of the alumina38.

Another striking observation, which motivates a more detailed discussion, is the aforementioned stability of the sub-nanometer particles on the cones during aging. In fact, the percentage of particles with diameters about 1 nm does not change significantly after 10 minutes up to 1 h (Figure 2). Since defect sites have been reported to be preferential anchoring sites3,39,40, it is likely that such sites, as already indicated above, are present with higher abundance on the cones than on the flat surfaces and serve two functions.


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Firstly, they act as nucleation sites for the small particles formed during the redispersion, which is driven by Ostwald ripening mass-transport (via surface diffusion) from the cone to the flat support due to the initial difference in particle size on the cone and on the flat parts. Note that in our earlier study14, we showed the Pt mass preservation on the same model catalyst and under similar aging conditions, meaning that we can rule out mass-transport via the gas phase. Secondly, they prevent or significantly slow down the sintering of the small particles formed at these defect sites due to their specific energetics. However, as the kinetics of Ostwald ripening is determined by the detachment energy of monomers (being single atoms of PtxOy species) from the particles, it appears that a larger fraction of the particles is in direct contact with defect sites, due to their apparent higher abundance on the cone surfaces as compared to the flat surfaces.

In order to identify the structural features that induce the support heterogeneity on the nanocones, we performed conventional TEM imaging since local steps on the surface give rise to Fresnel fringes enhancing the contrast from surface steps, while this contrast mechanism is not obtained in the annular dark field STEM mode. Figure 5 shows top and side-view images of an individual cone. The corrugation seen in the top-view image indeed corresponds to step-like features on the cone surface. They are even more evident in the side-view images at high magnification (Figure 5b,c). These steps are likely preferred anchoring sites for the subnanometer particles that are formed by redispersion of the initial particles during the early stages of sintering driven by the initial Pt particle size difference between flat and cone surface, in combination with the possible chemical modifications38 of the support induced by the sintering conditions. This may activate new nucleation sites that were not present during the initial nucleation and growth of the Pt nanoparticles in high vacuum and at room temperature. We also


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note that similar defects most likely also are present on the flat support, however, at lower abundance. However, due to the sample geometry such sites cannot be imaged with TEM.

The role of support heterogeneity in sintering processes has recently been analyzed theoretically15,16. Using these models, the possible role of support heterogeneity in the formation of bimodal size distributions has been discussed in Ref. [14] in connection with Pt sintering on flat silica and alumina supports. The model postulates that the surface consists of two types of areas or “patches” with different metal-support interface energy (which in reality can be caused both by local structural or chemical heterogeneity of the support). In brief, this implies that particles of the same size on two different patches have different chemical potentials, which results in transient bimodal size distributions during sintering.

In the present study, thanks to the higher resolution of the used TEM, we are able to identify one type of “patch” as defects at which small Pt clusters nucleate during an initial redispersion process (driven by Ostwald-ripening mediated mass transport between particles), and where they are significantly stabilized compared to the as-deposited particles. This process is amplified and made visible by the used nanostructured surface, which provides two key effects: (i) There is a higher defect density in the sputtered alumina support on the nanocones compared to the flat surfaces in between. (ii) Owing to the initially different Pt particle size distributions obtained on the cone and on the flat surfaces, Ostwald ripening from the cones towards the flat support is significantly accelerated and thus rapidly “exposes” the small particles formed during initial redispersion, making them much more apparent. Hence, the used nanostructured surface acts as a mediator for the clear identification of an initial catalyst redispersion effect, and for the


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identification of surface heterogeneity that gives rise to (transient) bi- or multimodal particle size distributions observed widely in model system studies10–14.

Summary and conclusions Utilizing nanolithography on a conventional TEM membrane we introduced a generic experimental approach to study catalyst nanoparticle sintering that relies on nanofabrication, to introduce controlled structural heterogeneity in a chemically homogeneous model catalyst support. This is achieved by fabricating arrays of nanocone structures separated by flat areas, both homogeneously sputter-coated with a thin amorphous alumina layer. Using ex situ aberration-corrected scanning transmission electron microscopy (STEM) we find both the initial particle size distributions of Pt nanoparticles, as well as their time evolution during sintering, to be different on the cones and the flat areas. On the flat areas, particle shrinkage and growth according to the Ostwald ripening mechanism combined with some particle redispersion occurs, leading to a broader and bimodal size distribution. In contrast, on the cones drastic redispersion of Pt into highly abundant particles of about 1 nm occurs very rapidly, driven by the initially (asgrown) smaller mean particle size, which gives rise to a strong driving force for mass transport to the flat areas. We find that these different processes are significantly amplified and made visible by the used nanostructured surface via two mechanisms. Firstly, due to the initially different Pt particle size distributions on the cones and on the flat surfaces, which accelerates mass transport via surface diffusion from the cones to the flat parts of the sample. Secondly, due to a higher support defect density on the nanocones compared to the flat surfaces, which provides an abundance of anchoring sites for stable redispersed nanoparticles in the 1 nm size range. In this way, the nanostructured surface facilitates the clear identification of catalyst redispersion in


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oxidizing conditions, and experimentally identifies a mechanism for the formation of previously observed transient bi- or multimodal particle size distributions during sintering. The results also potentially pave the way to rational design of supported catalysts with higher stability, as well as to strategies towards catalyst redispersion in oxidizing conditions.


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ASSOCIATED CONTENT Supporting Information. Illustration of particles with different contact angle on a nanocone surface and side-view HAADF-STEM image of Pt nanoparticles on the nanocone surface.

AUTHOR INFORMATION Corresponding Author *[email protected], Phone: +46 31 772 33 31 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript Notes The authors declare no competing financial interests. ACKNOWLEDGMENT The Competence Centre for Catalysis (KCK) is hosted by Chalmers University of Technology and is financially supported by the Swedish Energy Agency and the member companies AB Volvo, ECAPS AB, Haldor Topsoe A/S, Scania CV AB, Volvo Car Corporation AB, and Wärtsilä Finland Oy. We acknowledge additional support from the Swedish Research Council (CL), the ERC StG SINCAT (CL), and the Knut and Alice Wallenberg Foundation for their support of the my-fab cleanroom infrastructure in Sweden.


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FIGURES

Figure 1. (a) Schematic illustration of Pt nanoparticles supported on a nanostructured alumina surface on a TEM membrane. (b) TEM image of an array of nanofabricated alumina cones on a flat surface, both homogeneously sputter-coated by a 10 nm thick sputtered amorphous alumina layer to ensure chemical homogeneity. (c) Scanning transmission electron microscopy (STEM) image of a single nanocone and the adjacent flat area decorated with Pt nanoparticles. Note the clear difference in Pt particle size and density on the cone and on the flat area, respectively.


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Figure 2. STEM images and corresponding particle size histograms of alumina-supported Pt particles during sintering in 4% O2 at 600 °C on both flat areas and fabricated nanocones. The yellow square represents the area on top of each cone that was analyzed to obtain the size distributions (bin size=0.5 nm). The scale bar in the TEM images is 10 nm.


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Figure 3. STEM micrographs corresponding to the series shown in Figure 3, but at higher magnification to highlight the redispersion process into small Pt nanoparticles during the aging.


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Figure 4. (a) Mean particle diameter, (b) particle density, and (c) surface coverage as a function of sintering time in 4% oxygen at 600 °C. Red squares and blue circles are values related to the Pt particles on the flat and the cone surfaces, respectively. Error bars indicate the standard deviation of the values obtained from the two analyzed images. Where no error bar is given, it is smaller than the symbol size.


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Figure 5. (a) Top- and (b,c) side-view TEM micrographs of a cone at different magnification revealing structural irregularities on the cone surface that may act as preferred anchoring sites for the redispersed Pt nanoparticles and provide them with significant sintering resistance due to the site-specific energetics. The blue arrows are a guide to the eye indicating the boundary between the nanocone surface and vacuum in the TEM experiment. Surface corrugation can be observed.


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Pt Nanoparticle Sintering and Redispersion on a Heterogeneous

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