Thermodynamics of hydride formation and decomposition in supported sub-10 nm Pd nanoparticles of different sizes

Hydrogen storage properties of supported Pd nanoparticles with average sizes in the range 2.7-7.6 nm were studied using indirect nanoplasmonic sensing. For each particle size, a series of isotherms was measured and, through Van't Hoff analysis, the changes in enthalpy upon hydride formation/decomposition were determined. Contrary to the expected decrease of the enthalpy, due to increasing importance of surface tension in smaller particles, we observe a very weak size dependence in the size range under consideration. We attribute this to a compensation effect due to an increased fraction of hydrogen atoms occupying energetically favorable subsurface sites in smaller nanoparticles

On the performance of Ag/Al2O3 as a HC-SCR catalyst – influence of silver loading, morphology and nature of the reductant

This study focuses on the performance of Ag/Al2O3 catalysts for hydrocarbon selective catalytic reduction (HC-SCR) of NOx under lean conditions, using complex hydrocarbons as reductants. The aim is to elucidate the correlation towards the silver loading and morphology, with respect to the nature of the reductant. Ag/Al2O3 samples with either 2 or 6 wt% silver loading were prepared, using a sol–gel method including freeze-drying. The catalytic performance of the samples was evaluated by flow reactor experiments, with paraffins, olefins and aromatics of different nature as reductants. The physiochemical properties of the samples were characterized by scanning electron microscopy/energy dispersive X-ray spectroscopy, scanning transmission electron microscopy/high angle annular dark field imaging, X-ray photoelectron spectroscopy and N2-physisorption. The 2 wt% Ag/Al2O3 sample was found to be the most active catalyst in terms of NOx reduction. However, the results from the activity studies revealed that the decisive factor for high activity at low temperatures is not only connected to the silver loading per se. There is also a strong correlation between the silver loading and morphology (i.e. the ratio between low- and high- coordinated silver atoms) and the nature of the hydrocarbon, on the activity for NOx reduction. Calculated reaction rates over the low-coordinated step and high- coordinated terrace sites showed that the morphology of silver has a significant role in the HC-SCR reaction. For applications which include complex hydrocarbons as reductants (e.g. diesel), these issues need to be considered when designing highly active catalysts

Tessellation-based stochastic modelling of 3D coating structures imaged with FIB-SEM tomography

To facilitate printing, coatings are typically applied to paperboard used for packaging to provide a good surface for application. To optimise the performance of the coating, it is important to understand the relationship between the microstructure of the material and its mass transport properties. In this work, three samples of paperboard coating are imaged using combined focused ion beam and scanning electron microscope (FIB-SEM) tomography data appropriately segmented to characterise the internal microstructure. These images are used to inform a parametric, tessellation-based stochastic three-dimensional model intended to mimic the irregular geometry of the particles that can be seen in the coating. Parameters for the model are estimated from the FIB-SEM image data, and we demonstrate good agreement between the real and virtual structures both in terms of geometrical measures and mass transport properties. The development of this model facilitates exploration of the relationship between the structure and its properties

Influence of atomic site-specific strain on catalytic activity of supported nanoparticles

Heterogeneous catalysis is an enabling technology that utilises transition metal nanoparticles (NPs) supported on oxides to promote chemical reactions. Structural mismatch at theNP–support interface generates lattice strain that could affect catalytic properties. However, detailed knowledge about strain in supported NPs remains elusive. We experimentallymeasure the strain at interfaces, surfaces and defects in Pt NPs supported on alumina and ceria with atomic resolution using high-precision scanning transmission electron microscopy.The largest strains are observed at the interfaces and are predominantly compressive. Atomic models of Pt NPs with experimentally measured strain distributions are used for firstprincipleskinetic Monte Carlo simulations of the CO oxidation reaction. The presence of only a fraction of strained surface atoms is found to affect the turnover frequency. These resultsprovide a quantitative understanding of the relationship between strain and catalytic function and demonstrate that strain engineering can potentially be used for catalyst design

Methane oxidation over Pd supported on ceria–alumina under rich/lean cycling conditions

Catalysts with highly dispersed palladium on alumina, alumina doped with 20 wt% ceria and ceria have been prepared, characterized and examined for net-lean methane oxidation. In particular, the activity and selectivity were investigated during rich/lean cycling of the feed. The ceria content is found to influence both the general and the instantaneous activity responses. The results indicate that the active phase of palladium changes between reduced and oxidised Pd during the rich/lean cycling, and that the process is influenced by the presence of ceria

Quantitative studies of changes in microstructure and activity during ageing of supported catalysts

Catalytic converters have substantially contributed to the improvement of air quality in populated areas in the last decades by abatement of the emission of hazardous components such as CO, unburned hydrocarbons and NOx. However, due to the exposure to high temperature and different components in the exhaust gas, the performance of the catalysts degrades with time. An understanding of the microstructural and chemical changes of the catalysts during different operating conditions is required in order to develop strategies to reduce the degradation. In the present work, a bimetallic Pt-Pd nanoparticle catalyst supported on gamma-alumina was investigated using high resolution electron microscopy and spectroscopy. The studies included both fresh and aged catalyst systems. In order to allow a quantitative evaluation of the particle size distributions and locations on the support, a new TEM specimen preparation method was developed. The new method allows for monitoring the evolution of the particle size distribution as well as the spatial difference in the porous oxide support. These microstructural observations were complemented by investigations of the chemical composition and alloying of the nanoparticles. Finally, CO oxidation experiments were performed to correlate the observed microstructural and compositional changes to the catalytic activity

Quantitative Electron Microscopy Studies of Metal Nanoparticle Catalysts: Nanostructure, Support Interaction and Ageing Effects

Heterogeneous catalysis plays a major role in modern society, for example in chemical production, sustainable energy production and emission control technologies. Metal nanoparticles (NPs) supported on oxide materials are frequent catalytic systems in this field. Although used and investigated for decades, open questions about the structure of supported catalysts and correlation with their catalytic properties remain. Some of these questions involve the three-dimensional structures of the catalysts, which become increasingly accessible by modern characterisation techniques, as well as the nanoscale structures down to the atomic level.In this work, we focused on both of these aspects. We developed a specimen preparation method to reveal the three-dimensional structures of supported NP catalysts using transmission electron microscopy (TEM). We also refined the imaging of the catalysts’ structures in the size range of a few nanometres down to individual atoms by using high-resolution dark-field scanning TEM (STEM) imaging, reaching a precision of 2 pm. Structural aspects that were investigated included sintering (e.g. coalescence) of NPs in realistic catalysts at different temperatures and in different gas atmospheres, as well as sintering of NPs on model systems to investigate the effect of support surface corrugation. We used the developed specimen preparation method to study the three-dimensional distribution of NPs on the oxide support in a realistic catalyst as a function of ageing temperature. The structural properties were correlated to the catalytic activity, which was evaluated using a continuous flow reactor and simulations. The interaction at the interface between NPs and different support materials was studied by STEM imaging. The high spatial precision of 2 pm enabled the measurement of strain distributions within supported NPs and at external interfaces. This work has given new insights into the detailed three-dimensional nanoscale structure of some of the most commonly used supported catalysts and improved the understanding of the relation between their structural properties and catalytic activity. The observation of interfacial strain indicates the possibility to tailor the catalytic activity by tuning the NP-support interaction

The effect gas composition during thermal aging on the dispersion and NO oxidation activity over Pt/Al2O3 catalysts

The aging of a model 1 wt.% Pt/Al2O3 catalyst was performed stepwise under different reactive atmosphere to study the evolution of metal dispersion and NO oxidation activity. After each aging step the dispersion was evaluated by CO chemisorptions and the activity of the catalyst for NO oxidation was measured using 500 ppm NO and 8%O-2 diluted in Ar. After a degreening step at 500 degrees C, aging was performed at 600, 700,800 and 900 degrees C. Five wash-coated cordierite monoliths were aged in Ar, 10% O-2, 1% H-2 30 ppm SO2 and 30 ppm SO2 + 10% O-2, respectively. The general trend showed a linear decrease in dispersion when increasing the aging temperature for the lower aging temperatures and for the highest ones the dispersion levels off. When the platinum dispersion decreased the NO oxidation activity increased, due to that the reaction is structure sensitive. H-2 seemed to hinder sintering at low aging temperature. Interestingly, after aging in 10% oxygen at 600 degrees C the NO oxidation activity was significantly higher compared to the Ar aged sample, although the dispersions were similar. Aging in oxygen at higher temperatures resulted in a decrease of dispersion and a slightly decreasing NO oxidation activity. Moreover lower dispersion limit was reached with oxygen aging. Aging in SO2 provoked a severe dispersion drop at low aging temperature meanwhile the activity increased only moderately. However, activity kept increasing with further treatments at higher temperature. The combination of O-2 and SO2 enabled to decrease rapidly the dispersion and to greatly enhance the catalytic NO oxidation activity after the first aging step at only 600 degrees C. The best overall conversion was obtained for the catalyst treated with this mixture after aging at 800 degrees C. A 22-h aging at 250 degrees C in a mixture containing 500 ppm NO, 10% O-2 and 30 ppm SO2 led to a significant decrease of Pt dispersion, which shows the ability of SO2 to promote platinum sintering already 250 degrees C. The low temperature sintering was confirmed with STEM measurements. Several larger particles were observed, but also many small particles remained. Thus the SO2 + O-2 induced low temperature sintering results in a large variation of particle sizes. This treatment resulted in an increase of the maximum NO conversion (after reduction of the sample) from 45% to 76%. The different aging experiments show that it is beneficial to add SO2 during aging and the reason is the increased particle size, but also a clear chemical effect was observed

Revealing local variations in nanoparticle size distributions in supported catalysts: a generic TEM specimen preparation method

The specimen preparation method is crucial for how much information can be gained from transmission electron microscopy (TEM) studies of supported nanoparticle catalysts. The aim of this work is to develop a method that allows for observation of size and location of nanoparticles deposited on a porous oxide support material. A bimetallic Pt-Pd/Al2O3 catalyst in powder form was embedded in acrylic resin and lift-out specimens were extracted using combined focused ion beam/scanning electron microscopy (FIB/SEM). These specimens allow for a cross-section view across individual oxide support particles, including the unaltered near surface region of these particles. A site-dependent size distribution of Pt-Pd nanoparticles was revealed along the radial direction of the support particles by scanning transmission electron microscopy (STEM) imaging. The developed specimen preparation method enables obtaining information about the spatial distribution of nanoparticles in complex support structures which commonly is a challenge in heterogeneous catalysis. Lay Description Catalysis is important in our everyday lives, whether it is for the production of chemicals and food or for cleaning the exhaust gas of cars, ships and power plants. Therefore it is crucial to get a better understanding of the structure and properties of the catalysts involved, which often are in the form of nanoparticles. Electron microscopy has proven to be a powerful tool to investigate these nanoparticles, and especially transmission electron microscopy (TEM) has been used to investigate the smallest structures, down to single atoms. Although the TEM is very capable of imaging even the smallest structures, the specimens in question need to be very thin, of the order 100 nanometres, which can make the specimen preparation demanding, since the nanoparticles are mostly supported on much larger porous alumina particles. We have developed a new specimen preparation method using a focused ion beam, which allows obtaining TEM specimens of these catalysts in a very controlled manner, compared to previous procedures which involved crushing the catalyst powder in a mortar to obtain small pieces. With these specimens, we could in the TEM analysis investigate the spatial distribution of nanoparticles within the alumina support structure, and we observed a different nanoparticle size distribution for different parts of the support, namely the outer edge and the interior. This was never before observed using specimens obtained from the conventional crushing method

Pt Nanoparticle Sintering and Redispersion on a Heterogeneous Nanostructured Support

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 aberration-corrected scanning transmission electron microscopy (STEM) to analyze Pt model catalyst nanoparticles on such nanostructured supports prior and after exposure to 4% O-2 in Ar carrier gas at 600 degrees 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 occur on the flat areas, leading to a broader and bimodal size distribution. These processes are amplified and efficiently demonstrated by the nanostructured surface because of (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 dear 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