Nanoparticle size evaluation of catalysts by EXAFS: Advantages and limitations (original) (raw)
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Determination of Single- and Multi-Component Nanoparticle Sizes by X-ray Absorption Spectroscopy
Journal of The Electrochemical Society, 2018
An approach to estimate the size of quasi-spherical nanoparticle systems composed of atoms crystalizing in face centered cubic lattice using the data from X-ray absorption spectroscopy is introduced. For monoatomic clusters, the number of atomic layers surrounding the X-ray-absorbing atom m is evaluated from the coordination numbers, and the size of the particle calculated as the diameter of a sphere having the same volume as the cuboctahedron with the edge length of m atoms, assuming complete atomic layer structure. The particle size estimated by the approach agrees well with the experimental data obtained by other methods for clusters up to 5 nm. For bimetallic systems, coordination numbers of one component to the other in various inner-structured mixtures were derived. The interatomic distance of the solid solution is approximated to the weighted mean of the two constituents' lattice parameters and these mean values were used to estimate the particle size. The method could be applied to core-shell structures of bimetallic systems, as well as to three-component particles using reasonable approximations, and the particle sizes are estimated in a similar manner. The diameters of polyatomic clusters estimated by this approach were shown to closely follow the experimental data obtained by independent techniques.
physica status solidi (b), 2018
In the present work, the effect of grain size distribution on the diffraction profile shape is inspected via analysis of the mutual ratio of Lorentz and Gauss components in pseudo-Voigt function which is used for simulating X-ray profiles of nanoparticles. As established from the plotted dependences, the error in the average Pt nanoparticles size determination reaches 56% and the discrepancy between calculated Pt nanoparticle surface areas attains 60%. Furthermore, the determination error becomes greater with increasing the Lorentz contribution to pseudo-Voigt function, or, in fact, with enlarging particle size distribution. The empirically found electrochemical surface area of Pt/C electrocatalyst is compared with that evaluated from XRD data using the Scherrer formula and particle size distribution data analysis.
The Journal of Physical Chemistry C, 2014
Nanoparticles research represents one of the most active fields in science due to the importance of nanosized materials in a wide variety of applications. Their characterization needs the comparison of data coming from different experimental techniques, but the peculiar properties of the nanosystem that each technique points out are not always properly taken into account and misleading results have been often reported. In this work, we generated transmission electron microscopy like (TEM-like) data to predict the extended X-ray absorption fine structure (EXAFS) and chemisorption-like typical outputs as the average coordination numbers up to fourth shell of the particles distribution and the surface area. The aim of the simulations is to explore the dependence of the calculated average coordination number (ACN) and average dispersion (AD) values from each parameter characterizing a particle size distribution (PSD), as the mean diameter, the width, the shape, and the profile, and shows that a range of distributions is compatible with given values of ACN and AD. In this way, we have established a general method to properly take into account the above-mentioned parameters and to allow for an accurate analysis and comparison of results. Furthermore, it will be shown that unfavorable distribution shape makes the comparison among techniques critical and potentially misleading if performed with an oversimplified model of the PSD such as those using the average diameter only.
Size, Shape, Composition and Chemical state effects in nanocatalysis
2016
The field of nanocatalysis has gained significant attention in the last decades due to the numerous industrial applications of nanosized catalysts. Size, shape, structure, and composition of the nanoparticles (NPs) are the parameters that can affect the reactivity, selectivity and stability of nanocatalysts. Therefore, understanding how these parameters affect the catalytic properties of these systems is required in order to engineer them with a given desired performance. It is also important to gain insight into the structural evolution of the NP catalysts under different reaction conditions to design catalysts with long durability under reaction condition. In this dissertation a synergistic combination of in situ, ex situ and operando state-of-the art techniques have allowed me to explore a variety of parameters and phenomena relevant to nanocatalysts by systematically tuning the NP size, chemical state, composition and chemical environment. environments as long as oxygen species were present. In the presence of oxygen, an enhanced Ni surface segregation was observed at all temperatures. In contrast, in hydrogen and vacuum, the Ni outward segregation occurs only at low temperature (<200-270°C), while PtOx species are still present. At higher temperatures, the reduction of the Pt oxide species results in Pt diffusion towards the NP surface and the formation of a Ni-Pt alloy. A consistent correlation between the NP surface composition and its electrocatalytic CO oxidation activity was established. In Chapter 9 the chemical and morphological stability of size-and shape-selected octahedral PtNi NPs was investigated after different annealing treatments up to a maximum temperature of 700°C in vacuum and under 1 bar of CO. AFM was used to examine the mobility of the NPs and their stability against coarsening, and XPS to investigate the surface composition, chemical state of Pt and Ni in the NPs and thermally and CO-induced atomic segregation trends. Exposing the samples to 1 bar of CO at room temperature before annealing in vacuum was found to be effective at enhancing the stability of the NPs against coarsening. In contrast, significant coarsening was vii observed when the sample was annealed in 1 bar of CO, most likely as a result of Ni(CO)4 formation. Sample exposure to CO at room temperature prior to annealing lead to the segregation of Pt to the NP surface. Nevertheless, oxidic PtOx and NiOx species still remained at the NP surface, and, irrespective of the initial sample pretreatment, Ni surface segregation was observed upon annealing in vacuum at moderate temperature (T<300°C). Interestingly, a distinct atomic segregation trend was detected between 300°C-500°C for the sample pre-exposed to CO, namely, Ni surface segregation was partially hindered. This might be attributed to the higher bonding energy of CO to Pt as compared to Ni. Annealing in the presence of 1-bar CO results in the occupation of the NP surface by Ni atoms at 400°C as a result of Ni(CO)x formation. Above 500°C, and regardless of the sample pretreatment, the diffusion of Pt atoms to the NP surface and the formation of a Ni-Pt alloy is observed. Chapter 10 contains the summary and outlook of the thesis. viii This dissertation is dedicated to my parents and my wife who were extremely patient and supportive with me during all these years. ix ACKNOWLEDGMENT My deepest thanks is to my advisor Prof. Beatriz Roldan Cuenya for her support and guidance during my academic and personal Phd life. Prof. Roldan gave me the freedom to approach different project in my own way, which gave me the opportunity of tackling problems independently, while at the same time her guidance in critical moments wouldn't allow me to get lost in the passage. Her broad mindedness toward diverse ideas taught me the team working and tolerance in a research group. Her hard working, honesty and commitment to scientific values would be my example of a successful researcher. I am also thankful to her for carefully reading, revising and commenting my manuscripts. I want to also thank my group members Dr.
Size effects in the catalytic activity of unsupported metallic nanoparticles
Journal of Nanoparticle Research, 2003
The influence of the size of nanoparticles on their catalytic activity was investigated for two systems on unsupported, i.e. gasborne nanoparticles. For the oxidation of hydrogen on Pt nanoparticle agglomerates, transport processes had to be taken into account to extract the real nanoparticle size effects. The results indicate an optimum particle size for the catalytic activity below 5 nm which points clearly toward a real volume effect. In the case of the methanation reaction on gasborne Ni nanoparticles, no transport limitations were observed and the product concentration was directly proportional to the activity of the primary particles. We found an activity maximum for particles of about 19 nm in diameter. This size is too large to be attributed to a real nanoparticle size effect induced by the electronic band structure. Therefore, we concluded that the particle size influences the adsorption behavior of the carbon monoxide molecules. In fact, it is known that intermediate adsorption enthalpies may favor dissociation processes, which is an essential step for the reaction, as manifested in the so called volcano-shaped curve. Then, in addition to the material dependence of the adsorption, we would also encounter a direct size dependence in the case of methanation on gasborne Ni nanoparticles.
Atomic structure of nanomaterials: combined X-ray diffraction and EXAFS studies
Journal of Alloys and Compounds, 2004
The structures of Pd nanoparticles and layered MoS 2 -based nanocomposites prepared by wet routes under mild chemical conditions were studied by a combination of EXAFS spectroscopy and powder X-ray diffraction (XRD). Experimental diffractograms were compared with the patterns calculated for model clusters approximating the structure of the nanomaterials. The calculations were performed using a specially designed program, which simulates the XRD patterns of atomic aggregates based on the Debye formula. This combination "XAFS + XRD + modeling" proves to be an efficient tool for a characterization of poorly ordered nanomaterials.
Catalysis from First Principles: Towards Accounting for the Effects of Nanostructuring
Topics in Catalysis, 2013
The article deals with an issue of density-functional description of heterogeneous catalysts by nanoparticle models instead of still commonly employed slab models. Typically, active (metal) components are present in catalysts as nano-aggregates formed of many thousands atoms, remaining due to their size inaccessible even for modern first-principles computations. However, such species could be rather realistically represented by notably smaller computationally tractable model nanoparticles, whose surface sites only marginally change the reactivity with increasing particle size. Herein presented results are mainly related to methane dehydrogenation on Pt catalysts, methanol decomposition on Pd catalysts and the composition of active sites on Pt/ceria catalysts. They document feasibility of taking nanostructuring effects into account in density-functional modeling (at least for transition metals) and, more importantly, demonstrate that ignoring the nanoeffects in these systems leads to misrepresentation of their catalytic properties.
2012
This work investigates the current theories that have been used to explain the particle size effect on electrocatalytic activity. In particular, this work focuses on electronic structure effects, geometric shape effects, crystallographic orientation effects, support structure effects, and the concept of territory theory. To investigate these theories, gold and nickel-phosphorus nanowire arrays have been used as model systems for electrocatalytic testing of the hydrogen evolution reaction in basic medium. In this study, the current particle size effect theories cannot adequately account for the observed structure-sensitive trends that are exhibited in the aforementioned systems. This work systematically looks at each of these different theories and discusses new possible factors that could serve as a foundation for a new theory on structure-sensitive catalysis.
X-Ray Characterization of Platinum Group Metal Catalysts
2015
Platinum group metals (PGMs) are used extensively as catalysts, employed in several sectors of the world energy economy. Fuel cells employing PGM catalysts show promise as power sources in the proposed hydrogen economy, using alcohols as hydrogen storage media. Currently, the most economically important application for PGMs is for the mitigation of emissions from internal combustion engines via catalytic converters. In all applications, efficient use of these expensive metals to fabricate robust catalysts is of the utmost importance. Understanding the catalyst structure/property relationship is the key to the improvement of existing catalysts and the discovery of new catalysts. For example, catalyst particle size can have profound effects on catalyst activity, as in the case of gold nanoparticles. Catalyst particle size control and stability is also important for the efficient use of PGM metals and catalyst deactivation prevention. The challenge is to identify and characterize structural features and determine if and how these features may relate to catalytic properties. The ultimate goal is to simultaneously measure catalyst structural