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Au-based bimetallic nanoparticle catalysts
Though there are a lot of advantages of using gold-based monometallic catalysts, they often showed low activity compared to Pt or Pd-based catalysts and were found not suitable for alcohol oxidation in alkaline conditions. However, bimetallic Au-based systems could overpass such limitations, combining the properties of Au and another metal. Hence, a great enhancement of catalytic properties was found in many reactions when combining gold with another metal through forming new active sites and inducing synergistic effects [1, 2, 6-9].
Bimetallic nanoparticles (NPs) considered here are nanoparticles comprising two different metals inside their structures, such as alloy or core-shell. These NPs show novel optical, electronic and catalytic properties in comparison with monometallic NPs and have hence attracted huge attention. Indeed, NPs composed of two different metal elements possesses not only the combined properties of the two individual metals, but also present sometimes synergetic properties [83, 84]. Hence, the structure of the metallic NPs plays a very important role in the catalytic performance. The two most commonly encountered structures are introduced in the following sections.
Structure of the NPs
In bulk metals, atoms are arranged in various geometries, each metal having its own mode of atom placement. The resulting crystal structure is usually simple and depends on the nature of the metal and on other conditions such as temperature. In the case of metals NPs, the 3D arrangement of atoms may be similar to the crystal structure of the corresponding bulk metal, but in some cases, they may have a rather amorphous structure, depending on preparative conditions. Bimetallic NPs, which are composed of two kinds of metal elements, can have a crystal structure similar to the bulk alloy as well. But, in addition, they can adopt other types of structures. Such structures, defined by the distribution modes of the two elements, include the random alloys, the alloys with an intermetallic compound, and core-shell structures.
Alloy structure
In a bulk, two kinds of metal elements often provide an alloy structure. If the atomic sizes of the two elements are similar, then it will generally be a random alloy. When the atom sizes are quite different and the atomic ratio of the two elements is big enough, then they form an intermetallic compound. In the case of bimetallic NPs, these kinds of alloy structures seem to be more easily produced than in the case of bulk metals. In fact, bimetallic NPs between precious metals and light transition metals have such alloy structures. Mallin and Murphy [85] reported that gold-silver alloy NPs were synthesized via reduction by sodium borohydride of mixtures of HAuCl4 and AgNO3 with different molar ratio in the presence of sodium citrate as a capping agent which was used to inhibit the overgrowth and aggregation of NPs in water. Solution concentrations were adjusted to avoid the precipitation of AgCl during the course of the reaction (Table I-2).
Core-shell nanoparticles
In a core-shell structure, one metal element forms an inner core and the other element surrounds the core to form a shell (Figure I-6). This bimetallic structure is characteristic of poly(N-vinyl-2-pyrrolidone) (PVP) stabilized bimetallic NPs of precious metals. PVP-stabilized Au-Pd and Au-Pt bimetallic NPs were prepared by alcohol co-reduction. The colloidal dispersions of Au-Pd and Au-Pt bimetallic NPs obtained by simultaneous reduction can be examined by UV/VIS spectra and transmission electron microscopy [95-99]. Layered Ag-Au (core-shell) colloids have been described in fewer studies than Ag-Au alloys. In most of these systems, segregation occurs during reduction so that the more noble metal constitutes the core and the less noble metal forms the shell of a cluster [100]. Freeman et al. [101] prepared bimetallic colloids with a gold core and silver shell by consecutive reduction of HAuCl4 and AgNO3 by sodium citrate. Morriss and Collins [102] synthesized this colloid of the same composition by reduction of slightly alkaline HAuCl4 solution with pure phosphorus and successive reduction of Ag2O with NH2OH‚HCl. Reversed colloids, in which the less noble metal, silver, forms the core and the more noble metal, gold, forms the shell, have been reported as well. Mulvaney et al. [103] deposited gold onto radiolytically prepared silver sol by reirradiation of KAu(CN)2 solution, and Treguer et al. [100] prepared a layered colloid by radiolysis of a mixed AuIII/AgI solution. A silver colloid with gold reduced in the surface layer was prepared by Chen and Nickel [104], who mixed a solution of HAuCl4 with a Ag colloid and then added a reducing agent (p-phenylenediamine) [105]. Šloufová et al. used for the preparation of Ag-Au colloids (silver core, gold shell) the modified seed-growth method of Turkevich and Kim [106], who used this method for preparation of gold-plated palladium colloids. A sample of the Ag colloid (mean particle size 9 ± 2 nm) prepared as mentioned above was employed as a source of particle cores for preparation of a series of (Ag)Au bimetallic colloids with the expected core-shell structure and varying thicknesses of Au shells (Table I-3). A variation of the seed-growth procedure described in Kim‘s research and based on the reduction of HAuCl4 with NH2OH‚ HCl was used to grow the Au shell [106].
Base-free glucose oxidation over Au-based bimetallic catalysts
In this work, the influence of factors such as structure of the bimetallic Au-based NPs, support, composition, preparation method and synergistic effect between both metals on the catalytic performance in base-free oxidation of glucose was studied. Biomass is the most abundant and sustainable carbon source nowadays. Carbohydrates constitute 75% of the annual renewable biomass with glucose as monomer of cellulose being the most abundant monosaccharide [30, 113]. The oxidation of carbohydrates can provide new compounds and materials with interesting physicochemical properties, and this process is the source of a variety of high-added-value chemicals used in foods, cosmetics, detergents, and pharmaceuticals (for example, vitamin C). Biocatalytic and stoichiometric, as well as homogeneous and heterogeneous catalytic methods have been applied for the oxidation of glucose. Efficient conversion of glucose to valuable compounds, so-called platform chemicals, is of great importance and it is a current hot topic of interest in chemistry [114, 115]. Within the glucose platform chemicals, d-gluconic acid obtained by aerobic oxidation of glucose is an important compound (Scheme I-1). It is widely used as environmentally friendly chelating agent and water-soluble cleansing agent in food and pharmaceutical industries with an annual market of about 100,000 tons per year [116]. It is usually produced by fermentation of glucose with microbial species such as Aspergillus niger, Penicillium sp., Zymomonas mobilis, G. oxydans and Gluconobacter sp. [117-119]. However, in the fermentation medium, the accumulation of gluconic acid inhibits the microbes function leading to low yields and slow overall reaction rate [116, 117]. In recent years, due to the complexity of fermentation process, increased research efforts have been devoted to find environmentally friendly alternative, such as technology based on heterogeneous catalysis.
Table of contents :
LIST OF ABBREVIATION
S, ACRONYMS AND SYMB OLS
GENERAL INTRODUCTION
Chapter I Literature Review
1. SUPPORTED AU NANO PARTICLES CATALYSTS
1.1 Nanoparticles
1.2 Metallic nanoparticle
1.3 Monometallic Au supported catalysts
1.4 Methods used for monometallic Au based catalysts preparation
1.4.1 Impregnation
1.4.2 Co precipitation
1.4.3 Deposition Precipitation
1.4.4 Other methods of preparation
1.4.5 Conclusions
1.5 Characterization
2. AU BASED BIMETALLIC NAN OPARTICLE CATALYSTS
2.1 Structure of the NPs
2.1.1 Alloy structure
2.1.2 Core shell nanoparticles
2.2 Preparation methods
2.2.1 Coreduction of mixed ions
2.2.2 Successive reduction of metal ions
3. BASE FREE GLUCOSE OXIDATI ON OVER AU BASED BIMETALLIC CATALYSTS
3.1 Over supported Au based catalysts
3.2 Over bimetallic Au based Catalysts
3.3 Mechanism
3.4 Factors in fluencing activity
3.4.1 Effect of the size of gold particles
3.4.2 Effect of the support
3.4.3 Effect of reaction temperature
3.4.4 Effect of reaction time
3.4.4 Effect of reaction time
3.5 Stability
3.5 Stability
CONCLUSION
Chapter II ExperimentalExperimental
INTRODUCTION
1. SYNTHESIS AND CHARACTERIZATION OF THERACTERIZATION OF THE CATALYSTSCATALYSTS
1.1 Materials
1.1.1 Supports
1.1.2 Reagents
1.1.3 Gases
1.2 Catalyst preparation
1.2.1 Sol–immobilization methodimmobilization method
1.2.2 Precipitation–reduction methodreduction method
1.2 Catalyst characterization
1.2 Catalyst characterization
1.2.1 X–ray fluorescence (XRF)ray fluorescence (XRF)
1.2.2 Inductively Coupled Plasma Optical Emission Spectrometry (ICP
1.2.2 Inductively Coupled Plasma Optical Emission Spectrometry (ICP–OES)OES)
1.2.3 N22 adsorption/desorption (BET method)adsorption/desorption (BET method)
1.2.4 X–ray powder diffraction (XRD)ray powder diffraction (XRD)
1.2.5 X–ray photoelectron spectroscopy (XPS)ray photoelectron spectroscopy (XPS)
1.2.6 Transmission electron microscopy (TEM)
2. EXPERIMENTAL SETUPP
2.1 Autoclave reactor
2.2 Screening Pressure Reactor (SPR)
3 HPLC ANALYSIS OF PRODUCTS AND CALCULATRODUCTS AND CALCULATIONION
Chapter III BaseBase–free oxidation of glucose over Aufree oxidation of glucose over Au–Pd catalystsPd catalysts
INTRODUCTION
1. AU–PD NPS SUPPORTED ON PD NPS SUPPORTED ON TITANIUM DIOXIDETITANIUM DIOXIDE
1.1 Sol–immobilization methodimmobilization method
1.1.1 ICP–OES and XRF analysesOES and XRF analyses
1.1.2 XRD
1.1.3 TEM
1.1.4 XPS
1.1.5 Catalytic test
1.1.6 Recyclability testcyclability test
1.2 Precipitation–reduction methodreduction method
1.2.1 ICP–OES and XRF analysesOES and XRF analyses
1.2.2 XRD
1.2.3 TEM
1.2.4 Catalytic test
1.3 Comparison of both preparation methodarison of both preparation method
2. AU–PD SUPPORTED ON ZIRCPD SUPPORTED ON ZIRCONIUM DIOXIDEONIUM DIOXIDE
2.1 Sol–immobilizationimmobilization
2.1.1 ICP–OES and XRF analysesOES and XRF analyses
2.1.2 XRD
2.1.3 TEM
2.1.4 XPS
2.1.5 Catalytic test
2.1.6 Recyclability testcyclability test
2.2 Precipitation–reduction methodreduction method
2.2.1 ICP–OES and XRF analysesOES and XRF analyses
2.2.2 XRD
2.2.3 TEM
2.2.4 Catalytic test
2.3 Comparison of both preparation methodarison of both preparation method
3. DISCUSSION
Chapter IV BaseBase–free glucose oxidation over Aufree glucose oxidation over Au–X (with X= Cu, Bi or X (with X= Cu, Bi or Pt) catalysts and comparison with AuPt) catalysts and comparison with Au–
INTRODUCTION
1. STUDY OF THE AU–CU CATALYSTS SERIESCU CATALYSTS SERIES
1.1 Supported on titanium dioxide
1.1.1 ICP–OES and XRF analysOES and XRF analyseess
1.1.2 XRD
1.1.3 TEM
1.1.4 XPS
1.1.5 Catalytic test
1.2 Supported on zirconium dioxide
1.2.1 ICP–OES and XRF analysesOES and XRF analyses
1.2.2 XRD
1.2.3 Catalytic test
1.3 Comparison of both supports
2. AU–BI SUPPORTED ON ZIRCBI SUPPORTED ON ZIRCONIUM DIOXIDEONIUM DIOXIDE
2.1 ICP–OES and XRF analysesOES and XRF analyses
2. 2 XRD
2.3 Catalytic test
3. AU–PTPT SUPPORTED ON ZIRCONISUPPORTED ON ZIRCONIUM DIOXIDEUM DIOXIDE
3.1 ICP–OES and XRF analysesOES and XRF analyses
3.2 XRD
3.3 Catalytic test
4. DISCUSSIONSSION
4.1. Effect of the second metal:
4.2. Support effect
4.3. Insight into the mechanism of carbohydrates oxidation in baseInsight into the mechanism of carbohydrates oxidation in base–free conditionsfree conditions
Chapter V General conclusion and perspectivesGeneral conclusion and perspectives
REFERENCE
