Compositional changes of Pd-Au bimetallic nanoclusters upon hydrogenation

Vece, Marcel Di; Bals, Sara; Verbeeck, Johan; Lievens, Peter; Tendeloo, G. Van

doi:10.1103/physrevb.80.125420

Cited by 32 publications

(22 citation statements)

References 22 publications

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“…106,107 Pd 0.7 Au 0.30 single crystal surface after vacuum annealing is strongly enriched by gold. 106 Di Vece et al 108 observed changes in the size distribution and composition of bimetallic PdAu nanoparticles that have been exposed to hydrogen at RT. The effect was identified to be due to Ostwald ripening of Pd after Pd has segregated to the surface.…”

Section: Hydrogenation Reactionsmentioning

confidence: 99%

Alloys in catalysis: phase separation and surface segregation phenomena in response to the reactive environment

Zafeiratos

Piccinin

Teschner

2012

Catal. Sci. Technol.

214

182

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Alloys play a crucial role in several heterogeneous catalytic processes, and their applications are expected to rise rapidly. This is essentially related to the vast number of configurations and type of surface sites that multi-component materials can afford. It is well established that the chemical composition at the surface of an alloy usually differs from that in the bulk. This phenomenon, referred to as surface segregation, is largely controlled by the surface free energy. However, surface energy alone cannot safely predict the active surface state of a solid catalyst, since the contribution of other parameters such as size and support effects, as well as influence of the adsorbates, play a major role. This can lead to numerous surface configurations as for example over the length of a catalytic reactor, as the chemical potential of the gas phase changes continuously over the catalyst bed and hence different reactions may prevail at different catalyst bed segments. Thanks to the rapid improvement of the analytical surface science characterization techniques and theoretical methodologies, the potential effects induced by alloyed catalysts are better understood. For catalysis, the relevance of measurements performed on well-defined surfaces under idealized ultrahigh vacuum conditions has been questioned and studies in environments that closely resemble conditions of working alloy catalysts are needed. In this review we focus on experimental and theoretical studies related to in situ (operando) observations of surface segregation and phase separation phenomena taking place on the outermost surface layers of alloy catalysts. The combination of first principles theoretical treatment and in situ observation opens up new perspectives of designing alloy catalysts with tailored properties

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Section: Hydrogenation Reactionsmentioning

confidence: 99%

Alloys in catalysis: phase separation and surface segregation phenomena in response to the reactive environment

Zafeiratos

Piccinin

Teschner

2012

Catal. Sci. Technol.

214

182

View full text Add to dashboard Cite

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“…This chemical sensitivity through the Z dependence allows compositional effects to be studied, such as core-shell effects (e.g. Garcia-Gutierrez et al 2004) or quantitative elemental analysis (Di Vece et al 2009;Epicier et al 2012).…”

Section: Introductionmentioning

confidence: 99%

HAADF‐STEM characterization and simulation of nanoparticle distributions in an inhomogeneous matrix

Liu

Épicier

Lefkir

et al. 2017

Journal of Microscopy

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Measuring with a high accuracy the size distribution of small metallic nanoparticles loaded in a mesoporous metal oxide matrix is of particular interest for many studies related to new generations of interesting metamaterials. Transmission electron microscopy (TEM) is a powerful tool to determine the nature and morphology of very small particles, but their reliable and automatic identification in an inhomogeneous environment where the nanoparticle/background contrast locally varies is not straightforward. Here, we present how a quantitative analysis of high-angle annular dark field scanning TEM (HAADF STEM) images, accounting for the chemical sensitivity of the technique, can improve the accuracy of semiautomatic segmentation methods based on morphological processing to calculate size histograms. The paper also provides an estimate of the reliability of this method through the analysis of numerically synthesized images. The latter are based on the simulation of HAADF STEM projections of a volume filled with titania, pores and silver particles, whose morphological features, such as dimensions, shapes and densities are evaluated from experimental measurements of real samples. The results obtained with synthesized images prove the performances of the quantitative analysis to suppress nonsilver nanoparticles from the statistics and allow to infer empirical rules to determine imaging parameters that ensure a good reliability of histograms.

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“…16 While there is no general consensus on this topic, a wide range of values of α (1.3-2) has been used in the literature, [17][18][19][20][21][22][23][24][25][26][27] often without a full evaluation of microscope conditions. This situation hinders wider applications of quantitative STEM in nanomaterials characterization.…”

mentioning

confidence: 99%

Quantitative Z-contrast imaging in the scanning transmission electron microscope with size-selected clusters

Wang

Park

et al. 2011

Phys. Rev. B

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This paper describes a new approach of quantification of annular-dark-field or Z-contrast image intensity as a function of inner acceptance angle of the detector in a scanning transmission electron microscope. By using size-selected nanoclusters of Pd (Z = 46) and Au (Z = 79), it is shown experimentally that the exponent in the power law I ∼ Z α varies strongly between 1.2 and 1.8 as the collection angle changes from 14 to 103 mrad. The result is discussed in line with existing theoretical models. Factors, such as cluster size, structure, and orientation as well as the detector geometry, are also discussed for potential use of the work. PACS number(s): 36.40.−c With today's developments in nanoscience and technology, there is an increasing demand to correlate, in a quantitative way, chemical composition, structure, and property of materials with reduced dimension and size. For example, Serpell et al. showed recently that the design, synthesis, and understanding of industrial catalysts with improved selectivity could benefit from knowing internal structures and composition of core-shell bimetallic nanoparticles. 1 Similarly, examples have been seen in nanoalloyed AuAg particles with potential use of tunable optical properties. 2 Here, Z-contrast imaging in scanning transmission electron microscopy (STEM) are applied in obtaining chemical composition within nanoparticles.The so-called Z-contrast imaging method originates from the strong dependence of STEM image intensity on atomic numbers of elements when the signals are collected by a highangle annular dark-field (HAADF) detector. [3][4][5] In this case, as proposed by Howie in 1979, the signals are predominantly thermal scattered electrons, and coherence is largely reduced. 6 Since then, there have been many studies on the capabilities and limitation of Z-contrast imaging. 7-14 While there are a number of specimen-related factors that contribute to the image contrast, including not only atomic number, but also the specimen thickness, the crystal structure, and the crystal orientation, the geometry of annular detector also plays an important role. The dependence of Z contrast on the detector geometry was investigated theoretically by Hartel et al., who pointed out that, for thin objects, an analytical expression for the Z dependence of image intensity can be approximated by an exponential function of the form I ∼ Z α , where α is smaller than 2 and in the range 1.6-1.9 for most cases. 11 This power law has been widely accepted by the materials and microscopy community, however, without being experimentally verified. For effective quantitative characterizations using Z-contrast imaging, it is advantageous to know the critical exponent α as a function of detector angles.Zhu et al. pointed out from their combination of simulation and experimental work on SrTiO 3 crystals (∼10-50 nm thickness) that the dependence of the imaging contrast on collection angle varies with sample thickness, 15 while Klenov and Stemmer show experimentally for a 40-nm-thick PdTiO 3 film on a...

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Compositional changes of Pd-Au bimetallic nanoclusters upon hydrogenation

Cited by 32 publications

References 22 publications

Alloys in catalysis: phase separation and surface segregation phenomena in response to the reactive environment

Alloys in catalysis: phase separation and surface segregation phenomena in response to the reactive environment

HAADF‐STEM characterization and simulation of nanoparticle distributions in an inhomogeneous matrix

Quantitative Z-contrast imaging in the scanning transmission electron microscope with size-selected clusters

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