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

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

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

Chemical poisoning by zinc and phosphorous of Pt/Ba/Al2O3 NOx storage catalysts

The effects of phosphorous and zinc on the performance of Pt/Ba/Al2O3 catalysts were investigated through wet impregnation of ammonium phosphate and zinc acetate aqueous solutions. Six different sample combinations were studied; 1 wt-% P, 1 wt-% Zn, 1 wt-% P with 1 wt-% Zn, 2 wt-% P, 2 wt-% Zn, 2 wt-% P with 2 wt-% Zn. NOx storage and reduction (NSR) activity and NO2 temperature programmed desorption (TPD) profiles were measured before and after impregnation of P and Zn. Samples containing P performed significantly worse than samples only containing Zn in both NSR activity and TPD measurements. The increased NOx slip during lean phase in activity measurements for P-poisoned samples is mainly related to an increased slip of NO2. This was found for both NO and NO2 in the gas feed during lean phase and suggests that it is mainly the storage component that is poisoned and not the noble metal. Furthermore, the combination (1 wt-% P and 1 wt-% Zn) proved to result in slightly worse performance than only 1 wt-% P, however this was not the case for samples containing 2 wt-%, where the addition of zinc reduced the negative effect of phosphorous. Measurements from NO2-TPD experiments showed that NOx release at low temperature was not affected by the addition of P, while desorption in the temperature range 425–475 \ub0C was significantly reduced. It can therefore be concluded that the poisoning mainly is related to barium NOx storage sites and not to alumina sites. Moreover, X-ray diffraction measurements indicate that some of the barium species are affected by phosphorous. Images from scanning transmission electron microscopy (STEM) and energy-dispersive X-ray spectroscopy (EDX) mapping were in line with the results seen in both the activity tests and NO2-TPD experiments. Phosphorous was concentrated at the same position as barium in the observed images, whereas zinc was more evenly distributed over the surface. For the sample with both 2 wt-% P and 2 wt-% Zn, X-ray photoelectron spectroscopy measurements indicate that Zn and P have a low interaction and this suggests that most of the zinc and phosphorous are separated. However, STEM-EDX showed agglomerates of some zinc and phosphorous, which could be zinc phosphates. This is a plausible explanation of the decreased deactivation observed after introducing 2 wt-% Zn compared to the sample only containing 2 wt-% P