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Stability of modifiers on metal surfaces
The coordination of modifiers on the metal surface creates steric, electronic, and bifunctional effects, which could promote the catalytic properties of metal nanoparticles.
The stability of modifiers which prevent them from being detached during catalysis [3, 118]. is particularly important for practical applications. However, in some cases, the interaction between modifiers and metal nanoparticles are relatively weak. In this case, a weak interaction of the modifier with the active sites which occurs often in competition with the reacting molecules, allows tuning the catalytic properties. Normally, the non-metallic modifiers containing heteroatoms such as oxygen, nitrogen, sulfur, phosphorus, and halogens are grafted on the metal surface through coordination bonds [22]. The interface of surface metal and modifiers could be considered as a layer of coordination compounds. The stability of modifiers on the metal surface is an important issue for the catalyst reusability, and also related to the separation problem. The stability of the modified metal catalysts is affected strongly by the reaction conditions. Some modifiers like N-based ionic or non-ionic surfactants suffer from leaching problems and loss of selectivity in liquid-phase reaction, however, they could be more stable in low-temperature gas-phase reactions. Thus, the use of surface modifiers bearing strong coordination to the catalyst surface or even chelating groups (e.g. thiolates, diamines) is preferred.
Characterizations toolbox
Suitable characterization of the modifiers and modified metal surface is fundamental to understand the structure-performance relationship. However, it is also a challenge, because of the lack of direct tools to see the molecular-level interactions on the metal surface [22]. A lot of characterizations have been used to identify the amount, position, chemical bonds, and electronic effect of modifiers [23, 96]. The most frequently used characterizations in this area include temperature programmed analysis like TGA, TPD, transmission electron microscopy (TEM), infrared spectroscopy (IR), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), electrochemistry. Elemental analysis gives information about stoichiometry and stability of the modifiers. IR and Raman spectroscopies provide the molecular information of the modifiers. IR also gives specific signatures about the interaction of the modifier with metal by using probe molecules such as CO or pyridine. X-ray absorption near edge spectroscopy (XANES) and extended X-ray absorption fine structure spectroscopy (EXAFS) provide information about the oxidation state, geometry, and distances between the metal and close neighbors. Computational chemistry is a complementary tool to obtain energetic profiles that can relate to structural preferences and particular spectroscopic signatures of surface species that may relate to catalytic activity. The computational methods can confirm or rule out possible structures and allow a deeper understanding of the relation between structure and reactivity. The information obtained by combining complementary techniques in the characterization toolbox allows an atomic-level description of surface sites. This isan essential step toward building structure-activity relationships of the modified metal catalysts.
Modification strategies
There are diverse strategies to anchor non-metallic modifiers on the surface of metal nanoparticles. Generally speaking, the preparation strategies could be summarized in two ways. The first one is direct-preparation, in which the non-metallic modifiers are mixed with the metal precursors in a typical solvent and then after reduction of the metal precursors, the modifiers are grated directly on the surface of metal nanoparticle [35, 36, 38, 41]. The second strategy is post-modification. The non-modified metal catalysts are prepared in advance. Then, the non-metallic modifiers are anchored on the metal surface by post-treatment [33, 42 82, 98, 101].
Direct preparation
Direct-preparation is applied usually for the preparation of ligand-protected colloidal metal nanocrystals (MNCs), where the modifiers normally act as capping or protecting agents to confine the over-growth and aggregation of MNCs. Generally, MNCs with capping agents exbibit lower activity than the bared metal nanoparticles. This phenomenon is mainly due to the partial blockage of active sites by capping agents. However, it was recently found that the capping agent endowed some abnormally.
For example, oAm-capped Pt3Co bimetallic nanocrystals were prepared by thermal decomposition of Pt(acac)2 and Co(acac)2 in the oAm [63]. The mixture was kept stirring at 240 oC with CO flow for 40 min, followed by precipitation and washing with the cyclohexane/ethanol solvent. As shown in Figure 1.3, the as-synthesized oAm-capped Pt3Co NCs were uniform in size and nearly all have a truncated octahedral shape with a particle size around 8 nm. The use of amines as the capping agent is important to obtain the monodisperse Pt3Co nanocrystals and to stabilize the particles against aggregation.
Ethyl diamine (EDA) modified Pt nanowires have been prepared by using EDA as a capping agent [47]. Typically, Pt(acac)2 and EDA were mixed in dimethylformamide (DMF) to get a homogeneous solution, and then the solution was heated up to 150oC for 5 h with 0.2 MPa CO inside. The resulting colloidal products were collected by centrifugation and washed with ethanol several times. The diameter of ultrathin Pt NWs averaged to 1.1 nm; approximately 6 ~ 7 atoms thick. High-resolution STEM revealed that the as-prepared Pd NWs were highly crystalline, with two types of clearly observed lattice fringes having interplanar spacings of 0.227 and 0.196 nm, ascribed to the (111) and (200) planes of fcc Pt (Figure 1.4). PVP stabilized Au clusters have been obtained by mixing an aqueous solution of HAuCl4/PVP and a strong reducing agent NaBH4 at 0 oC. By tuning the concentration of PVP and HAuCl4, the average size of the obtained Au cluster could be adjusted to 1.1 ~ 3.1 nm [41].
Table of contents :
Acknowledgements
ABSTRACT.
Résumé
1. Literature Review
ABSTRACT.
1.1. Introduction
1.1.1. General introduction
1.1.2. Different promoters
1.1.3. Stability of modifiers on metal surfaces
1.1.4. Characterizations toolbox
1.2. Modification strategies
1.2.1. Direct preparation
1.2.2. Post-modifications
1.3. Identification of the role of promoters
1.3.1. Steric effects
1.3.2. Electronic effects
1.3.3. Bifunctionality
1.4. Catalytic hydrogenation over modified metal catalysts
1.4.1. Partial hydrogenation of alkynes to alkenes
1.4.2. Selective hydrogenation of unsaturated aldehydes to unsaturated alcohols
1.4.3. Hydrogenation of nitroarenes to N-hydroxylanilines
1.4.4. Hydrogen peroxide production
1.5. Goals and strategies of the thesis
1.6. Thesis structure
Reference
2. Experiment Section
2.1. Catalyst preparation
2.1.1. Chemicals
2.1.2. Preparation of I modified Pd catalyst
2.1.3. Preparation of Br modified Pd catalyst
2.1.4. Preparation of Br modified Ru catalyst
2.1.5. Preparation of Pd/SiO2 catalyst
2.1.6. Preparation of imprinted Pd/SiO2 catalyst
2.2. Catalyst Characterization
2.2.1. X-ray diffraction (XRD)
2.2.2. Transmission electron microscopy (TEM)
2.2.3. Chemisorption
2.2.4. Thermogravimetric analysis (TGA)
2.2.5. X-ray photoelectron spectrometry (XPS)
2.2.6. Fourier transform Infrared spectroscopy (IR)
2.2.7. X-ray florescence analysis (XRF)
2.3. Evaluation of catalytic reactions
2.3.1. Reductive etherification
2.3.2. Hydrodeoxygenation
2.3.3. Hydrogenolysis of diphenyl ether
2.3.4. Aromatics hydrogenation
2.3.5. Selective benzene removal
3. Modification of Pd Catalyst with Iodide: In-situ Generation of Brönsted Acidity for Selective Reductive Etherification
3.1. Introduction
3.2. Results and discussion
3.2.1. Etherification over the Pd-I catalysts
3.2.2. Characterization of the catalyst
3.2.3. Mechanism of the reaction
3.2.4. Other substrates
3.3. Conclusion
4. Dual Metal-Acid Pd-Br Catalyst for Selective Hydrodeoxygenation of 5-Hydroxymethylfurfural (HMF) to 2, 5-Dimethylfuran at Ambient Temperature.
4.1. Introduction
4.2. Results and discussions
4.2.1. HMF deoxygenation over the Pd-Br catalyst at 60°C
4.2.2. HMF deoxygenation at ambient temperature
4.2.3. Other promoting agents
4.3. Characterization of the Pd-Br catalysts
4.4. Model reactions
4.5. Discussion
4.6. Conclusion
5. Cleavage of Diphenyl Ether Lignin Model Compounds to Single-ring Aromatics over Ru-Br Catalyst
5.1. Introduction
5.2. Results and discussions
5.2.1. Catalytic conversion of DPE
5.2.2. Characterizations
5.2.3. Model reactions
5.3. Conclusion
Reference
6. Molecular Imprinting on the Supported Metal Catalysts for Size-dependent Selective Hydrogenation Reactions
6.1. Introductions
6.2. Results and discussion
6.2.1. Poisoning of supported Pd/SiO2 catalysts by DMAPA
6.2.2. Investigations for imprinting process
6.2.3. Model reactions over imprinted Pd/SiO2 catalyst
6.2.4. Selective removal of benzene from the mixture
6.3. Conclusions
Reference
7. General Conclusions and Perspectives
7.1. General Conclusion
7.1.1. In-situ generation of acid sites over metal nanoparticles
7.1.2. Selective blocking active sites
7.1.3. Molecular imprinting over heterogeneous catalyst
7.2. Perspective
