DESIGNING SINTER RESISTANT MONOMETALLIC AND BIMETALLIC NANOMATERIALS FOR CATALYSIS
Date
2020-06-29
Authors
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Journal ISSN
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ORCID
Type
Thesis
Degree Level
Doctoral
Abstract
A major concern associated with the use of noble metal nanoparticles and clusters for catalysis is stability. Stabilizing ligands are used to prevent the agglomeration of nanoparticles synthesized under ambient conditions. However, these ligands may block the active sites on the metal surface. Typically, high-temperature heat treatment (550 ºC-650 ºC) is required for the complete removal of ligands from catalysts, which leads to the sintering of particles to form larger particles. Sintering occurs during heat treatment processes because of the increased mobility of the nanomaterials and can lead to catalyst deactivation as the surface area of the catalytic nanoparticles decreases. Many studies have focused on improving the thermal stability of nanoparticle catalysts by protecting them with metal oxide shells. However, protective shells can also make it harder for substrates to diffuse to the surface and react, thus creating mass-transfer issues. The main focus of this thesis is synthesizing sinter-resistant metallic and bimetallic catalysts.
In Chapter 1, a detailed description of activation processes and methods for enhancing the thermal stability of nanomaterials are provided. Chapter 2 details how a protective silica shell with a thickness of 40 nm enhances the thermal stability of Au25(MUA)18 clusters (MUA=mercaptoundecanoic acid). The morphology of the resulting Au catalysts before and after calcination at temperatures up to 650 ºC was analyzed by TEM and Extended X-ray Absorption Fine Structure Spectroscopy (EXAFS) analyses, which showed that the Au catalysts are much more stable to sintering compared to un-encapsulated clusters. However, mass transfer issues associated with the silica shell were also observed. Chapter 3 shows that Atomic Layer Deposition can be an effective method to control the shell thickness of alumina overlayers using Au25(MUA)18 and Au25(DDT)18 (DDT=dodecanethiol) clusters. TEM and EXAFS analysis were used to study the structural changes before and after thermal treatment. 20 cycles of alumina coating are required to improve the thermal stability of Au catalysts made from Au25(MUA)18 clusters. In Chapter 4, the sintering behavior of Au25(MPTS)18 (MPTS=3-mercaptopropyl)trimethoxysilane) clusters on mesoporous silica supports under oxygen atmosphere (1 Pa) was studied by an in situ TEM technique. Particle migration and coalescence was found to be the more dominant mechanism for the sintering of monodisperse Au25(MPTS)18 clusters on the mesoporous silica support. In situ TEM studies showed that the mobility of the particles increases as the calcination temperature increases. Further TEOS treatment helped to reduce the mobility of Au nanoparticles by forming silica overcoats and the resulting materials showed excellent sinter-resistance up to 550 ˚C. The stability of TEOS treated Au25(MPTS)18/mesoporous silica catalysts were also confirmed by EXAFS analysis. In Chapter 5, the galvanic replacement reaction was employed for synthesizing bimetallic catalysts from silica encapsulated Ag nanoparticles. During the activation process at 650 ºC, Ag nanoparticles were fragmented into smaller particles with an average size of 2.2 ± 1.0 nm and well dispersed in a silica matrix. These activated Ag clusters were then used as a sacrificial template for galvanic replacement reactions using Pd salts. Liquid cell in situ X-ray absorption analysis was utilized to monitor the galvanic replacement reaction of Ag@silica with Pd precursors. Finally, Chapter 6 focuses on the synthesis of the encapsulation of atom-precise Pd clusters in silica for use as catalysts for methane combustion reactions. Catalytic activity studies for methane combustion reactions using encapsulated and non-encapsulated Pd clusters show that encapsulated clusters were much more active for methane oxidation than their non-encapsulated counterparts.
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Keywords
Metal clusters, Catalysis, Sintering, Extended X-ray Absorption Fine Structure Spectroscopy: in situ TEM
Citation
Degree
Doctor of Philosophy (Ph.D.)
Department
Chemistry
Program
Chemistry