In situ imaging of gold nanocrystals during the CO oxidation reaction studied by Bragg Coherent Diffraction Imaging
Carregando...
Data
2019-03-26
Autores
Suzana, Ana Flávia [UNESP]
Título da Revista
ISSN da Revista
Título de Volume
Editor
Universidade Estadual Paulista (Unesp)
Resumo
O principal objetivo da catálise heterogênea é entender os mecanismos de reação a nível nanométrico e planejar/sintetizar catalisadores com os sítios ativos desejados. A este respeito, a caracterização de defeitos in situ/operando é crucial, pois esses são sítios catalíticos preferenciais para a ocorrência da reação. A parte principal desta tese foi a investigação da morfologia e evolução estrutural de nano-catalisadores de ouro suportados em dióxido de titânio. Trabalhamos com diferentes métodos de preparação (crescimento de “sementes”, uso de líquidos iônicos como agentes estabilizantes) para sintetizar catalisadores de ouro com tamanhos controlados. Esses catalisadores foram testados para a reação modelo de oxidação de CO, escolhida por sua relevância ambiental e "simplicidade" para ser reproduzível em nosso estudo de imageamento por raios-X. Utilizamos a técnica Imageamento por Difração Coerente de Bragg para acompanhar in situ as alterações da morfologia em 3D nas condições da reação catalítica. Um reator que permite aquecimento, dedicado a experimentos in situ, foi construído. Correlacionamos o strain e displacement field em 3D das nanopartículas às propriedades catalíticas do material. Para uma nanopartícula de 120 nm em particular, nós quantificamos sob condições de funcionamento o stress superficial induzido pelo processo de adsorção no nanocristal, o que leva à reestruturação da nanopartícula e defeitos identificados como uma rede nanotwin. Finalmente, usando nanodifração de raios-X para caracterizar nanorods de ouro individuais de algumas dezenas de nanômetros, pudemos obter um mapa de strain para os cristais de ouro bem orientados na condição de Bragg. Esta caracterização revelou a presença de um padrão de deformação codificado na estrutura dos nanorods, consistente com a chamada Instabilidade de Rayleigh. Esse tipo de estudo provou ser muito importante, pois a presença de instabilidade afeta as propriedades dos nanorods.
The fundamental aim of heterogeneous catalysis research is to understand mechanisms at the nanoparticle level, and then to design and synthesise catalysts with desired active sites. In this regard, the in situ/operando characterisation of defects is crucial as they are preferential catalytic sites for the reaction occurrence. The main part of this thesis was the investigation of the morphology and structure evolution of gold nano-catalysts supported on titanium dioxide. We worked with different methods of preparation (seed-growth, ionic liquid stabilizer) to synthesise gold catalysts with controlled sizes. Those catalytic materials were evaluated for the model CO oxidation reaction, chosen for its environmental relevance and “simplicity” to be reproducible within our X-ray imaging study. We used the Bragg Coherent Diffraction Imaging technique to follow in situ the 3D morphology changes under catalytic reaction conditions. A dedicated in situ heating reactor has been built. We correlated the 3D displacement field and strain distribution of the gold nanoparticles to the catalytic properties of the material. In particular, for a 120 nm gold nanoparticle, we quantified under working conditions the adsorbate-induced surface stress on the gold nanocrystal, which leads to restructuration and defects identified as a nanotwin network. Finally, by using scanning X-ray nanodiffraction to characterise few tenths of nanometers single gold nanorods, we were able to obtain a strain map over the Bragg well oriented gold crystals. This characterisation revealed the presence of a strain pattern encoded in the nanorods structure consistent with the so-called Rayleigh Instability. This kind of study proved to be very important as the presence of instability does affect the properties of nanorods.
The fundamental aim of heterogeneous catalysis research is to understand mechanisms at the nanoparticle level, and then to design and synthesise catalysts with desired active sites. In this regard, the in situ/operando characterisation of defects is crucial as they are preferential catalytic sites for the reaction occurrence. The main part of this thesis was the investigation of the morphology and structure evolution of gold nano-catalysts supported on titanium dioxide. We worked with different methods of preparation (seed-growth, ionic liquid stabilizer) to synthesise gold catalysts with controlled sizes. Those catalytic materials were evaluated for the model CO oxidation reaction, chosen for its environmental relevance and “simplicity” to be reproducible within our X-ray imaging study. We used the Bragg Coherent Diffraction Imaging technique to follow in situ the 3D morphology changes under catalytic reaction conditions. A dedicated in situ heating reactor has been built. We correlated the 3D displacement field and strain distribution of the gold nanoparticles to the catalytic properties of the material. In particular, for a 120 nm gold nanoparticle, we quantified under working conditions the adsorbate-induced surface stress on the gold nanocrystal, which leads to restructuration and defects identified as a nanotwin network. Finally, by using scanning X-ray nanodiffraction to characterise few tenths of nanometers single gold nanorods, we were able to obtain a strain map over the Bragg well oriented gold crystals. This characterisation revealed the presence of a strain pattern encoded in the nanorods structure consistent with the so-called Rayleigh Instability. This kind of study proved to be very important as the presence of instability does affect the properties of nanorods.
Descrição
Palavras-chave
CO oxidation reaction, In situ Bragg CDI, Gold nanocrystals, Strain, Twin defects, Nanodiffraction