Studies of Catalytic Materials with In Situ Transmission Electron Microscopy

Abstract

In conclusion, the work in this thesis explores the use of liquid phase transmission electron microscopy for studying dynamic processes taking place in solid heterogeneous catalysts in liquids at the nanoscale. It was clear that the main challenge for successfully studying these materials in LP-TEM is the effect of the electron beam interacting with the liquid and solid, causing degradation of several important oxides, such as SiO2, Al2O3 and MgO. Possible strategies to mitigate this were found and include the use of lower electron dose rates, the use of more stable oxides (TiO2, ZrO2 and Nb2O5 in this work) and in the case of SiO2, the use of radical scavengers to lower the concentration of reducing radicals the oxide is exposed to. Despite great challenges, using gold nanoparticles supported on TiO2 as a model catalyst proved successful to study liquid phase Ostwald ripening and to study the structural changes of this catalyst relevant for the selective oxidation of HMF. Through the use of LP-TEM combined with laboratory experiments, novel insights in both of these processes was obtained. In the former, it was found that although particle ensemble behavior matched ex situ results and OR predictions very well, at the individual nanoparticle level, the process was found to differ significantly. Most importantly, no clear correlation between the particle size and growth or shrinkage behavior could be found, in contrast to predictions of classical Ostwald ripening models. In addition, the existence of a significant fraction of inert particles that neither grew, nor shrank, was observed. LP-TEM also showed that particle shrinkage was sudden and seemed to be a stochastic process while particle growth by monomer attachment was slow and likely the rate-determining step for sintering in this system. In the latter, it was found that extensive gold particle detachment and subsequent particle coalescence and growth occurred under typical reaction conditions during HMF oxidation, predominantly as a result of the high pH required in this reaction. It was proposed that the origin of this detachment is the negative charge present on both the gold nanoparticles and the TiO2¬ support when the pH is higher than the point of zero charge (PZC) of TiO2. In addition, this thesis also provides new strategies and clues for other studies on synthesis and evolution of solid heterogeneous catalysts in liquid phase.