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Non-alloyed Bimetallic Nanoparticles
Generally, the formation of non-alloyed bimetallic particles is governed by the nucleation and growth rates of the metals involved. When two metals exist in a system, the energy required for nucleation of the metals to occur is minimized when compared to a system that contains only one metal. This is illustrated in Figure 2.3 which relates the free energy of a system containing either a type of metal (a homogeneous system) or more than one metal (a heterogeneous system) with the average particle size of the metal particles formed.
Various structures of non-alloyed bimetallic particles have been reported. Among them are those that exhibit core-shell, random, separated as well as cluster in cluster morphologies. Some of these structures are illustrated in Figure 2.4.
The formation of either of these structures depends on several factors such as the preparation method and metals involved. Typically, bimetallic particles which exhibit core-shell morphologies consist of a metal forming a core with another metal element surrounding the core to form a shell. A variety of factors influence its mechanism of formation. Among them are the reduction potential of the metals involved [7] as well as the method in which the metals are introduced into a system. Even so, a general mechanism of formation of this bimetallic structure takes place in two steps. In the first stage, metal ions are reduced to form atoms, which subsequently aggregate into small clusters and grow at the expense of other metal ions. When metal ions of another metal are available, these ions will adsorb onto the surface of the first metal. Successive reduction in a suitable environment will result in a shell surrounding the first metal [8]. A schematic diagram of the growth mechanism is shown in Figure 2.5. Various techniques have been used to prepare bimetallic particles with these structures. Among them are the deposition technique used to prepare RePd and NiPt particles [9] and the chemical reduction technique to synthesize PtFe2O3 particles [10].
Alloyed Bimetallic Nanoparticles
Alloys have been identified as a metallic system consisting of two or more components irrespective of the nature in which the metals are mixed [11, 12]. The formation of these bimetallic particles can best be described in terms of thermodynamics. A simplified explanation is based on the understanding that alloys occur when the excess of free energy, of the metals involved (ΔG), is negative. Therefore, the enthalpy and entropy of mixing for an alloy can take various values. Typically, most alloys occur spontaneously upon mixing of the metals. In this case, the system has a negative enthalpy of mixing (ΔH < 0) and a positive entropy of mixing (ΔS > 0) [6, 13]. In contrast when the mixing of metals is endothermic, in other words the enthalpy of mixing is positive (ΔH > 0), alloys are formed only at high temperatures as this will lead to a higher contribution from the entropy when compared to the enthalpy of mixing [6, 13]. At low temperatures, the contribution of the entropy of mixing is lower than that of the enthalpy of mixing. Hence, the excess of free energy of mixing is positive. Under these conditions, alloys are not formed as the metals are immiscible [6,13].
The difference in the volume free energy of monometals and alloys formed from these monometals as a function of particle radius gives a good description of the effect of alloying. As shown in Figure 2.6, the volume free energy of alloys is lower than that of monometals.
Ensemble and Ligand Effects of Bimetallic Particles
As catalysts, the formation of bimetallic particles has generated a lot of attention due to the benefits they present. Apart from the enhanced reactivity it can exhibit when compared to their respective monometallic catalysts [15], these particles may also demonstrate better selectivity [16] as well as resistance towards deactivation. Much interest has been given to understand how bimetallic nanoparticles influence these properties. Hence, the ensemble and ligand effects have been put forward to better explain these phenomenons.
The ligand effect which is also termed as the electronic effect refers to the modification in electronic interactions that occur between metals in bimetallic systems [17]. This involves electrons in the d band of transition metals. The ensemble effect (also known as the geometric effects) on the other hand explains the enhanced catalytic properties of alloys, in terms of the number of active sites on a surface that is necessary for a reaction to occur [17]. In other words, the selectivity of a reaction can be altered by blocking certain active sites that favor another reaction [14]. Some researches have expressed this as the dilution of actives sites in a metal with a second inert metal [18]. Several authors have attempted to ascribe the enhanced activity and selectivity of catalysts using the ligand and ensemble effects [19]. However, it has been established that the two effects correlate with each other [11, 20]. As an example, when an atom or several atoms of a metal A is placed in an environment of another metal B, such an occurrence cannot be considered solely as an ensemble effect as either metal may have an electronic effect on the other metal due to its close proximity [11].
Preparation of Catalysts
The preparation of supported metal catalysts can generally be prepared using one of two methods. These methods are widely known as the classical and non-classical methods. In both methods, the techniques in which the metal salts are introduced onto the supports are similar. The variation between the methods lies more in the reduction stage of the metal salts.
Classical Methods
The classical method is frequently used for its simplicity. It also has the advantage in eliminating chemical compounds such as sulfur and chlorine available in the metal salts, which can be poisonous in the final catalytic reaction. Generally, this method is a three step procedure that involves the incorporation of metal salts onto a support, calcinations and finally the reduction of the metal oxides to metal particles in a flow of hydrogen at elevated temperatures. Each step plays a significant role in determining the final properties of the catalyst. As an example, though the aim of the calcinations step is to oxidize the metal ions and to remove poisonous compounds, the temperature at which calcination is conducted can induce changes in the metal particle size as well as the support. In terms of the support, it has been reported that γ-Al2O3 changes to α-Al2O3 at calcination temperatures of 1300 K [21]. The reduction step can also influence the support. This particularly occurs when reducible oxides such as TiO2 and CeO2 are used as supports. For instance, CeO2 can be reduced to Ce2O3 [22]. Even though the calcination and reduction steps are important when preparing catalysts via classical methods, it is no doubt that the technique in which the metal salts are introduced onto the support also plays a crucial role. Basically, three types of techniques, the deposition, impregnation and precipitation are often employed.
Precipitation Technique
This technique entails the formation of an insoluble metal hydroxide or carbonate precipitate upon the addition of a solvent or an acid or base to a metal salt solution. The precipitate can then later be converted to metal oxides via calcinations [23]. One or more metals can be precipitated in a system. When two or more metals are involved, sequential or co- precipitation can occur. Sequential precipitation often arises as a result of a large difference in the solubility of the products formed from the original components concerned. Hence, co-precipitation requires a good solubility between the components. Though various metal salt solutions are mixed with the intention of carrying out co-precipitation, sequential precipitation may also occur [24]. This often leads to an inhomogeneity of the metal phase at a macroscopic level [24], which is the main disadvantage of the co-precipitation technique. Contrary to this, if co-precipitation is achieved, very small particles which are beneficial in catalytic reactions can be formed [23].
Impregnation Technique
The impregnation of metal salts into a support is typically carried out by mixing an excess of a metal salt solution with a support. The main objective is to occlude the solution into the pores or to allow it to adsorb onto the pore surface of the support [23]. After mixing for a certain duration of time, the catalysts are then dried and subsequently calcined.
The incorporation of large amounts of metal salts is preferable. However, several factors such as the mixing time, temperature, concentration of the metal salt solution and type of metal salt employed during impregnation can influence the final amount of metal salt incorporated into or onto the support. These preparation parameters have been studied extensively by various research groups.
Frequently, longer durations of impregnation time can increase the amount of impregnated metal salt. Though increasing the mixing time is effective, a major drawback of this technique is its extremely long preparation procedure. To overcome this, other researchers have carried out impregnation in several stages. In this technique, a metal salt solution is mixed with the support for a certain time before drying. This procedure is then repeated several times to increase the amount of impregnated metal salts [25]. Subsequently, to further optimize the amount of impregnated metal salt, higher temperatures have also been used during the mixing stage. Gayen et al. [26] used this technique in order to increase the solubility of the metal salt. As a result, they found that higher amounts of Ni nitrate can be impregnated indirectly reducing the number of impregnation stages required.
Temperature Programmed Reduction (TPR)
Temperature programmed reduction is a dynamic flow technique that provides information on the reducibility of metal oxides or metal ions in catalysts which are prepared via classical methods. It also gives an insight on the extent of reduction of the metal phase for catalysts synthesized using non-classical methods. The basic principle of this technique involves flowing diluted H2 through a sample, while increasing its temperature at a predetermined rate. Metal oxides or metal ions available in the catalysts will consume hydrogen forming metal nanoparticles in a reduced state. Generally, the position of the peak at which H2 consumption occurs, gives information on the temperature at which the metals can be reduced, the state in which the metals exist whether as ions or oxides, the strength of metal-support interaction as well as the size of the metal particles. For bimetallic supported catalysts, this surface characterization technique indirectly demonstrates how the addition of a second metal to a metal supported catalyst, influences these characteristics. For instance, a study on silica-alumina (ASA) supported nickel catalysts demonstrates that the metal-support interaction decreases with increasing Ni content in the sample or as a result of the addition of Pd in the catalyst. This is elucidated by the decrease in temperature at which H2 consumption for the NiO occurs [57]. Further, upon the addition of Li to the Ni/ASA catalyst, different metal species are observed. Peaks arising at 670, 720 and 820 K are indicative of various nickel oxides while the peak at 944 K corresponds to nickel in a cationic form [57]. Other research groups have also described the use of this characterization technique to comprehend similar characteristics of various catalysts [58, 59].
Table of contents :
CHAPTER 1 – INTRODUCTION
1.1 A Brief Overview
1.2 Problem Statements
1.3 Research Objectives
1.4 Scope of Study
1.5 Thesis Layout
1.6 References
CHAPTER 2 – LITERATURE REVIEW
2.1 Nanoparticles
2.2 Bimetallic Nanoparticles
2.2.1 Non-alloyed Bimetallic Nanoparticles
2.2.2 Alloyed Bimetallic Nanoparticles
2.2.3 Ensemble and Ligand Effects of Bimetallic Particles
2.3 Preparation of Catalysts
2.3.1 Classical Method
2.3.1.1 Precipitation Technique
2.3.1.2 Impregnation Technique
2.3.2 Non-classical Methods
2.3.2.1 Chemical Reduction
2.3.2.2 Microwave Reduction
2.3.2.3 Mechanical Attrition
2.4 Supports
2.4.1 Silicon (IV) Dioxide (SiO2)
2.4.2 MCM-41
2.5 Characterization Techniques
2.5.1 Temperature Programmed Reduction
2.5.2 Temperature Programmed Desorption
2.5.3 X-ray Diffraction
2.5.4 X-ray Photoelectron Spectroscopy
2.5.5 Transmission Electron Microscopy
2.6 Application
2.6.1 Energy
2.6.2 Environment
2.6.3 Industries
2.7 Bimetallic PtNi Nanoparticles
2.8 Benzene
2.8.1 Hydrogenation of Benzene
2.9 References
CHAPTER 3 – EXPERIMENTAL
3.1 Materials
3.2 Methods
3.2.1 Preparation of Stock Solutions
3.2.1.1 Pt/Ni Supported Crystalline Silica Catalysts
3.2.1.2 Pt/Ni Stabilized Oleic Acid (Pt/Ni-OA)
3.2.1.3 Pt/Ni –OA Supported Crystalline Silica Catalysts (Pt/Ni-OA/Silica)
3.2.1.4 Pt/Ni Supported MCM-41 Catalysts (Pt/Ni-MCM)
3.2.2 Synthesis of Pt/Ni Supported Crystalline Silica via Co-precipitation.
3.2.3 Synthesis of Pt/Ni Supported Crystalline Silica via
Co-impregnation
3.2.4 Synthesis of Pt/Ni Supported Crystalline Silica via
Step-impregnation
3.2.5 Synthesis of Pt/Ni Stabilized Oleic Acid Particles
3.2.5.1 Effect of Various Concentrations of Oleic Acid
3.2.5.2 Effect of Various Reaction Temperatures
3.2.6 Preparation of Pt/Ni-OA/Silica Catalysts
3.2.7 Preparation of Pt/Ni-MCM Catalysts via Non-classical
3.2.8 Preparation of Pt/Ni-MCM via Classical Methods
3.3 Characterization Techniques
3.3.1 H2-Temperature Reduction (H2-TPR)
3.3.2 H2-Chemisorption and H2-Temperature Desorption (H2-TPD)
3.3.2.1 Non-classical Catalysts
3.3.2.2 Classical Catalysts
3.3.3 Temperature Programmed Surface Reaction (TPSR)
3.3.4 O2-Chemisorption
3.3.5 Transmission Electron Microscopy
3.3.6 Powder X-ray Diffraction
3.3.7 Fourier Transform Infrared (FTIR)
3.3.8 X-ray Photoelectron Spectroscopy (XPS)
3.4 Calculation Methods
3.4.1 Determination of Fractal Dimension
3.4.2 Determination of Metal Dispersion
3.4.2.1 Borodzinski and Banarowska Method
3.4.2.2 H2-Chemisorption Method
3.4.3 Total Surface Area of Metal Phase
3.4.4 Particle Size
3.4.4.1 H2-Chemisorption Method
3.4.4.2 XRD Technique
3.4.5 Degree of Reduction
3.5 Catalytic Reaction
3.6 Kinetic Studies
3.6.1 Determination of Reaction Orders
3.6.2 Determination of Energy of Activation
3.7 References
CHAPTER 4 – THE SYNTHESIS AND CATALYTIC PROPERTIES OF Pt/Ni SUPPORTED SILICA CATALYSTS PREPARED VIA NON-CLASSICAL METHODS
4.1 Introduction
4.2 Structural studies
4.3 Surface Characteristics
4.3.1 H2-TPR Profiles
4.3.2 H2-Chemisorption
4.3.3 H2-TPD Analysis
4.3.4 XPS
4.4 Effect of Borohydride Reduction
4.5 Hydrogenation of Benzene
4.6 Summary
4.7 References
CHAPTER 5 – EFFECT OF IMPREGNATION TECHNIQUE FOR CATALYSTS PREPARED VIA NONCLASSICAL METHODS
5.1 Introduction
5.2 Surface Characteristics
5.2.1 H2-TPR Analysis
5.2.2 H2-Chemisorption
5.2.3 H2-TPD Analysis
5.3 TEM Analysis
5.4 Catalytic Activity
5.5 Characteristics of Pt55Ni45-CI Catalyst
5.6 Summary
5.7 References
CHAPTER 6 – CATALYTIC STUDIES OF Pt/Ni STABILIZED OLEIC ACID BIMETALLIC PARTICLES INCORPORATED ONTO SILICA
6.1 Introduction
6.2 Formation of Pt/Ni Bimetallic Nanoparticles
6.3 Alloying of Pt/Ni Bimetallic Nanoparticles
6.4 Morphology
6.4.1 Effect of Oleic Acid Concentration
6.4.2 Effect of Reaction Temperature
6.5 Pt/Ni Interaction with Oleic Acid
6.6 Oleic Acid Stabilized Pt/Ni Deposited on Silica
6.6.1 Morphology of Active Phase in the Pt/Ni-OA/Silica Catalysts
6.6.2 Surface Characteristics
6.6.2.1 H2-TPR Analysis
6.6.2.2 H2-Chemisorption
6.6.2.3 H2-TPD Analysis
6.6.3 Benzene Hydrogenation
6.7 Summary
6.8 References
CHAPTER 7 – EFFECT OF REDUCTION CONDITIONS
7.1 Introduction
7.2 H2-TPR Profiles
7.3 Effect of Reduction Temperature
7.4 Effect of NaBH4 Concentration
7.5 Effect of Reduction Medium
7.6 Comparison with Monometallic Catalysts at Optimum Conditions
7.7 Summary
7.8 References
CHAPTER 8 – Pt/Ni SUPPORTED MCM-41 CATALYSTS PREPARED VIA CLASSICAL METHODS
8.1 Introduction
8.2 Effect of Activation Conditions
8.3 O2-Chemisorption
8.4 Surface Characteristics
8.4.1 H2-TPR Analysis
8.4.2 H2-Chemisorption Studies
8.4.3 H2-TPD Profiles
8.5 Structural Properties
8.6 Morphological Studies
8.7 Catalytic Activity
8.8 Kinetic Investigations
8.9 Classical vs Non-classical Catalysts
8.10 Summary
8.11 References
CHAPTER 9 – CONCLUSIONS
9.1 Conclusion
9.2 Recommendations for Future Work
LIST OF PUBLICATIONS AND PRESENTATIONS
APPENDIX
