Characterising the structure and properties of bimetallic nanoparticles

The work presented in this thesis explores the use of aberration corrected scanning transmission electron microscopy (ac-STEM) in characterising the structure and properties of bimetallic nanoparticles. STEM imaging and energy dispersive X-ray spectroscopy (EDX) are used to show the influence that reaction kinetics have over the formation of structure in AuRh and AuPd nanorods. Correlated imaging and electron energy loss spectroscopy (EELS) are used to characterise the localised surface plasmon resonance (LSPR) response of single AuPd and AuRh nanorods, showing how ac-STEM can be used to identify the origins of this response. Finally the full range of ac-STEM techniques (imaging, EDX and EELS) are applied to identify the formation of ordered AuAg oxide structures in AuAg nanoparticles that have been aged in air and to show the impact their structural evolution has on LSPR properties. Through this work we show that the unrivalled spatial resolution and range of elementally sensitive techniques available in ac- STEM is essential to the characterisation of bimetallic nanoparticles. The capabilities of these instruments give the possibility to gain a fundamental understanding of the metalmetal interactions taking place in these systems, and thus form connections to the properties that result from them, which is necessary to their effective use in both catalytic and plasmonic applications

Insights into the influence of the Ag loading on Al2O3 in the H2-assisted C3H6-SCR of NOx

International audienceThe addition of H2 has been reported to promote drastically the selective catalytic reduction of NOx by hydrocarbons (HC-SCR). Yet, the influence of the Ag loading on the H2-promoted HC-SCR has been the subject of a very limited number of investigations. The H2-HC-SCR earlier studies reported mostly on Ag/Al2O3 samples containing about 2 wt% Ag, since this particular loading has been shown to provide optimum catalytic performances in the HC-SCR reaction in the absence of H2. The present study highlights for the first time that the H2-C3H6-SCR catalytic performances of Ag/Al2O3 samples improved in the 150–550 °C temperature domain as the Ag loading (Ag surface density: x (View the MathML sourceAg/nmAl2O32)) decreased well below 2 wt%. A detailed kinetic study of H2-C3H6-SCR was performed in which the reaction orders in NO, C3H6 and H2, and the apparent activation energies were determined for the reduction of NOx to N2 on a Ag(x)/Al2O3 catalysts series, for which Ag was found to be in a highly dispersed state by TEM and HAADF-STEM. Remarkably, changes in these kinetic parameters were found to occur at an Ag surface density close to View the MathML source0.7 Ag/nmAl2O32 (Ag loading of 2.2 wt%) coinciding with the changes observed earlier in the NOx uptakes of the Al2O3 supporting oxide [18]. Interpretation of the activity and kinetic data led us to conclude that the H2-C3H6-SCR reaction proceeds via the activation of H2 and C3H6 on Ag species and their further reaction with NOx adspecies activated on the Al2O3 support. The unexpected higher catalytic performances of the Ag samples with the lower Ag surface densities was attributed to the higher concentration of active sites on the Al2O3 supporting oxide able to chemisorb NOx species, in agreement with the NOx uptake data. The kinetic data obtained for Ag surface densities lower than View the MathML source0.7 Ag/nmAl2O32 also suggest that the interaction between NOx and C3H6 adspecies would be rate determining in the C3H6-SCR process

Observation of compositional domains within individual copper indium sulfide quantum dots

The origin of photoluminescence in copper indium sulfide (CIS) quantum dots (Qdots) has previously been ascribed to a donor-acceptor pair (DAP) recombination, with a crystal lattice defect implicated as the origin of the donor state. In this study, electron energy-loss spectroscopy (EELS) was used to observe defect-rich compositional domains within individual CIS Qdots, supporting a model of defect-state-mediated photoluminescence for these particles, and identifying them as an ideal model system for future study of lattice defects on Qdot properties

Overgrowth of rhodium on gold nanorods

[Image: see text] This study focuses on the deposition and growth mode of rhodium (Rh) on gold (Au) seed nanorods (NRs). Using a combination of scanning transmission electron microscopy imaging, energy-dispersive X-ray spectroscopy, and UV–visible absorption spectroscopy, we show that Rh deposition results in an uneven overlayer morphology on the Au NR seeds, with a tendency for Rh deposition to occur preferentially on the Au NR ends. The results suggest that complex and kinetically driven metal–metal interactions take place in this system

HRTEM and STEM-HAADF characterisation of Au–TiO2 and Au–Al2O3 catalysts for a better understanding of the parameters influencing their properties in CO oxidation

International audienceGold catalysts supported on titania (Au-TiO2) and alumina (Au-Al2O3) were prepared by deposition- precipitation with urea and then activated before characterisation and reaction in CO oxidation, either by calcination in air at 500 1C or reduction under H2 at 300 1C. Gold nanoparticles with average size in the range 2-4 nm were obtained, with calcination leading to larger gold nanoparticles than reduction. For Au-TiO2, high activity was observed in CO oxidation at room temperature, independent of the activation treatment. This high activity could not be correlated to the presence of sub-nanometer gold clusters as reported in the literature, since they could not be detected by atomic-resolution high-angle annular dark-field scanning-transmission electron microscopy (HAADF-STEM). In the case of Au-Al2O3, the performance in CO oxidation was found to strongly depend on the water content in the reaction gas feed and on the activation conditions, with calcination resulting in a poorly active catalyst whereas reduction gave activity of the same order as Au-TiO2. A comparative study of Au-TiO2 and Au-Al2O3 by electron microscopy did not reveal distinct differences in the shapes of the Au nanoparticles, which are mostly flattened through interaction with the substrate in both samples, with side profile shapes varying from rounded hemispherical to well faceted truncated cubo-octahedra. More faceting is found for the samples calcined at 500 1C than reduced at 300 1C. Various possible parameters affecting the catalytic properties of gold in CO oxidation are discussed in the context of the relevant literature