Synthesis, characterization and (photo-)
catalytic performance
Synthesis, characterization and (photo-)
catalytic performance
Proefschrift
ter verkrijging van de graad van doctor aan de Technische Universiteit Delft,
op gezag van de Rector Magnificus prof. dr. ir. J.T. Fokkema, voorzitter van het College voor Promoties,
in het openbaar te verdedigen op dinsdag 6 december 2005 om 10:30 uur
door
Mohamed Sameh Hamdy M. Saad
Master of Science (Physical Chemistry), Universiteit van Helwan, Egypte. geboren te Caïro, Egypte
Prof. dr. J.A. Moulijn Prof. dr. Th. Maschmeyer
Samenstelling promotiecommissie:
Rector Magnificus voorzitter
Prof. dr. J.A. Moulijn Technische Universiteit Delft, The Nederlands, promotor Prof. dr. Th. Maschmeyer Universiteit van Sydney, Australië, promotor
Prof. dr. ir. H. van Bekkum Technische Universiteit Delft, The Nederlands Prof. dr. A.R. Ebaid Universiteit van Helwan, Egypte
Prof. dr. ir. D.E. de Vos Katholieke Universiteit Leuven, België Prof. dr. J.C. Jansen Universiteit van Stellenbosch, Zuid Afrika
Dr. G. Mul Technische Universiteit Delft, The Nederlands, adviseur
Reservelid
Prof. dr. F. Kapteijn Technische Universiteit Delft, The Nederlands
The first two years of the work described in this thesis were financially supported by the Egyptian Government, Ministry of Higher Education, in cooperation with the Laboratory of Applied Organic Chemistry and Catalysis, TU-Delft. The last two years were financially supported by STW.
ISBN 90-8559-112-0
Copyright © by Mohamed Sameh Hamdy M. Saad
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Printed by: Optima Grafische Communicatie, Rotterdam, The Netherlands, 2005
TOCK Renewables Renewables Combinatorial Combinatorial Analysis Analysis Computational Computational Porous Solids Porous Solids CatalysisChiral Chiral TOCK Renewables Renewables Combinatorial Combinatorial Analysis Analysis Computational Computational Porous Solids Porous Solids CatalysisChiral Chiral
HELWAN
To my lovely wife and my naughty son…
To my parents and my sisters…
Chapter 1 Introduction
Mesoporous materials and nano-particles History, synthesis and application
1
Chapter 2 Synthesis and characterization of M-TUD-1 (M=Ti, V, Cr, Mo, Fe, Co and Cu)
Easy formation of isolated catalytic sites and/or nano-particles embedded in mesoporous TUD-1
27
Chapter 3 Selective photo-oxidation of propane
Part I: Nano-particles of TiO2-incorporated TUD-1 as a new
selective photo-catalyst for oxidation of light alkanes
73
Chapter 4 Selective photo-oxidation of propane
Part II: Chromium-incorporated TUD-1 as a visible-light-sensitive photo-catalyst
95
Chapter 5 Unique catalytic activity of Fe-TUD-1 in Friedel-Crafts’ benzylation of benzene
109
Chapter 6 Cobalt-incorporated TUD-1 as an active and highly selective catalyst for liquid phase oxidation of cyclohexane
127
Chapter 7 Laughing gas (N2O) decomposition
Transition elements and noble metals incorporated-TUD-1 as catalysts for N2O decomposition at high temperature
143
Chapter 8 Conclusions and recommendations
TUD-1: a breakthrough or just a new catalyst support?
163
Summary 171
Acknowledgments 175
List of publications 179
1
Introduction
Mesoporous materials and nano-particles
History, synthesis and application
When pursuing the aim of designing materials, and in particular heterogeneous catalysts, for a sustainable society it is important to strive for the highest activity and selectivity of the catalyst. Porosity in heterogeneous catalysts increases the specific surface area which benefits activity per catalyst volume. A hierarchical pore size distribution to optimize mass transfer in the catalyst particle is important. An industrial approach to achieve mesopores in, e.g. microporous materials, is the destructive de-alumination technique. A constructive technique, which is more favoured in academic studies, is an assembly of three catalyst phases with pores at different length scales. Advantages of this technique are that the catalyst phases can be designed and made independently of each other to be assembled later on. Thus, each macro- meso-, and microporous phase might comprise specific catalytic sites and pore sizes.
Ideal porous materials should have certain characteristics, such as high surface area, narrow pore size distribution and pore sizes readily tuneable over a wide range. Zeolites and zeolite-like molecular sieves (zeotypes) often fulfil these requirements in the micropore range. However, despite the many important commercial applications of zeolites, there has been a persistent demand for well-defined mesoporous materials which can be used as catalysts in processes for heavy oil cracking and catalytic conversion of large molecules.
Meso, the Greek prefix, meaning “in between”, has been adopted by IUPAC to define porous materials with pore sizes between 2 and 50 nm [1], along with two other classes of porous solids (defined by pore size), i.e., microporous (< 2 nm) and macroporous (> 50 nm) [2]. Typical macroporous examples include porous glass and hydrogels. Mesopores are present in, e.g., aerogels and pillared clays.
In the early 1990s, Kresge et al. [3] created a new (mesoporous) material by templating silica species with surfactant molecules that lead to the formation of ordered mesoporous silica. These products, known under the group name M41S, dramatically expanded the range of pore sizes in ordered pore systems, which opened new avenues in catalysis. An important member in the M41S family is MCM (Mobil Composition of Matter)-41. These new (alumino) silicate materials possess extremely high surface areas (> 1000 m2 g-1) and narrow pore size distributions in the range of 2-10 nm.
Figure 1. Three structure types observed for silica-surfactant mesophases: (a) hexagonal, MCM-41 (b) cubic, MCM-48 and (c) lamellar, MCM-50.
Mobil scientists employed long chain cationic surfactant molecules as the structure-directing agents for the synthesis of these highly ordered materials. The synthesis procedure is quite versatile and pore size and architecture can be tailored to a large degree. e.g., MCM-41 contains regular arrangements of hexagonal pores in a
honeycomb arrangement [1-4]; the 3-D 48 exhibits cubic symmetry [5-7], and MCM-50 is a layered silicate [8] (Figure 1).
The basis of the synthesis is the application of structural directing agents, the so-called templates. Templating has been defined, in a general sense, as a process in which an organic species functions as a central structure about which oxide moieties organize into a crystalline lattice [9,10]. In other words, a template is a structure (usually organic) around which a material (often inorganic) nucleates and grows, so that upon the removal of the templating structure, its geometric and electronic characteristics are replicated in the (inorganic) materials.
2. Sol-gel technology
The sol-gel process is a versatile solution process initially used for the preparation of inorganic materials such as glasses and ceramics of high purity and homogeneity. It involves the transition of a system from a liquid “sol” into a solid “gel” phase and has been the subject of several books and reviews [11,12]. Sol-gel processes are one of the most promising research areas in ceramic research. Its ambient processing conditions enable one to encapsulate numerous organic, organometallic, and biological molecules within these sol-gel derived inorganic matrices. Sol-gel is extensively utilized to design and synthesize inorganic-organic hybrid materials with nanometer-scale architecture.
The sol-gel process can usually be divided into the following steps: forming a solution, gelation, aging, drying, and densification. In the preparation of silica materials (e.g. glass), one starts with an appropriate alkoxide, tetramethyl orthosilicate (TMOS) or tetraethyl orthosilicate (TEOS), which is mixed with water and a mutual solvent, such as ethanol or methanol, to form a solution. Hydrolysis leads to the formation of silanol groups Si-OH. These species are only intermediates as they react further, i.e. condense, to form siloxane Si-O-Si groups. As the hydrolysis and condensation reactions continue, viscosity increases until the “sol” ceases to flow and forms a “gel”. The overall reactions leading to the formation of a silica gel starting from TMOS and TEOS are:
(3) (2) (1) H2O H O Si + HO Si condensation hydrolysis Si O Si + ROH + O Si Si alcoholysis condensation O Si H + Si OR ROH + H Si O reesterfication hydrolysis H2O + R O Si
reactions (sol-gel process), including the reactivity of metal alkoxide, water/alkoxide ratio, solution pH, temperature, and nature of the solvent and additives. Furthermore, catalysts are frequently added to help in controlling the rate and the extent of hydrolysis. By varying these processing parameters, materials with different microstructure and surface chemistry can be obtained. Further processing of the "sol" enables one to make ceramic materials in different forms. e.g., thin films can be produced on a piece of substrate by spin-coating or dip-coating.
Figure 2. Sol-gel process and their products
When the "sol" is cast into a mold, a wet "gel" will form. With further drying and heat-treatment, the "gel" is converted into dense ceramic or glass particles. If the liquid in a wet "gel" is removed under supercritical conditions, a highly porous and extremely low density material called "aerogel" is obtained. As the viscosity of a "sol" is adjusted into a proper viscosity range, ceramic fibres can be drawn from the "sol". Ultra-fine and uniform ceramic powders are formed by precipitation, spray pyrolysis, or emulsion techniques. The various processing options in the sol gel procedure are illustrated in Figure 2 [13].
Sol-gel materials synthesized without the addition of templates often are microporous materials when dried in air (denoted as xerogels) [11]. When sol-gel processes are used to prepare mesoporous materials, charged (cationic/anionic) or neutral surfactants are employed as templates, which direct the mesophase formation based on electrostatic interaction and hydrogen-bonding interactions, respectively [14]. Synthesis, stabilization, modification, application, structure characterization, mechanistic studies, structural simulations and computational modeling of the ordered molecular sieves have been extensively reviewed [15].
3. Functionalization of mesoporous materials
In most cases, the siliceous mesoporous materials produced are inert, so there is a need to functionalize these materials to be applied as catalysts. Different kinds of functionalization procedures [16] have been applied including the attachment of:
1- Organic ligands,
2- Organometallic compounds, or 3- Inorganic species.
Here, we will focus on the latest procedure, whether this inorganic species are metal atoms connected to the framework of the siliceous mesoporous material, or nano-particles of metal/metal oxide embedded in the pores of mesoporous material.
4. Nano-science and technology
The discipline nano-science is based on the fact that properties of materials change, and often dramatically, as a function of their physical dimension. Changes typically occur as at least one dimension of the material is reduced near to or into the nano-domain (the scale of 10í9 m), and nano-technology takes advantage of this by applying selected property modifications of this nature to some beneficial endeavour. It turns out that properties change in these confined spaces because the electronic structure of the material is significantly modified when going from the bulk (continuum) to molecular (quantum) domains. In the continuum domain, electronic states near the Fermi level of the material (the electronic states that ultimately control bonding and many material properties) are blurred into continuous bands. In the quantum domain, electrons from the same, but fewer, atoms are in narrow and separated energy states. Suitable control of the properties of nanometer-scale structure can lead to new science as well as new devices and technology. Table 1 lists typical nano-materials with their characteristic dimensions.
Size (approx.) Materials
Nano-crystals and clusters Diam. 1-10 nm Metals, semiconductors, magnetic materials Other nano-particles Diam. 1-10 nm Ceramic oxides
Nano-wires Diam. 1-10 nm Metals, oxides, sulfides, nitrides Nano-porous solids Pore diam. 0.5-10 nm Zeolites, phosphates
2-D arrays (nano-particles) Several nm2-µm2 Metals, semiconductors, magnetic materials Surface and thin films Thickness 1-1000 nm A variety of materials
3-D arrays (super-lattices) Several nm in three dimensions
Metals, semiconductors, magnetic materials
Rao et al. [17] concluded that employing particles, wires and other nano-structures has generated novel chemistry. This includes electrochemical, photochemical, catalytic and other aspects. The authors summarized the immediate objectives of the science and technology of nano-materials as follows:
1- To fully master the synthesis of isolated nano-structure (building blocks) and their assemblies with the desired properties.
2- To explore and establish nano-device concepts and systems architectures. 3- To generate new classes of high performance materials.
4- To connect nano-science to molecular electronics and biology.
5- To improve known tools, while discovering better tools of investigation of nano-structure.
4.1. Nano-particles in catalysis
Although the term nano-science was not generally used, in industrial catalysis, nano-particles were involved from the dawn of the 20thcentury. The idea that small metal particles would serve as superior catalysts was obvious to many scientists. The reasons are clear and simple to explain. In a macroscopic bulk solid, surface atoms form only a small fraction of the total number of metal atoms, but for nano-particles (size ranges of 1-10 nm) most of the atoms are surface atoms. Such atoms have lower co-ordination numbers than in the bulk and as a consequence are expected to exhibit greatly enhanced activity to all types of substrates. Thus, there is a combined effect. First, greater accessibility to all constituent (surface) atoms and secondly, enhanced activity because of the low coordination number they exhibit. There are numerous examples of using the nano-particles in industrial catalysis. Purification of crude terephthalic acid to give PTA (Purified Terephthalic Acid) is done catalytically by using 0.5% Pd nano-particles on active carbon granules. Another example, in the traditional ammonia synthesis, in which the
catalyst consists of magnetite promoted with small amounts of irreducible oxides (Al2O3,
CaO, K2O), which after several processes, form nano-particles of iron aluminate in Į-iron
[18]. Moreover, in the production of formaldehyde, Fe-Mo oxide is widely used for the oxidation of methanol to formaldehyde at full methanol conversion. The catalyst consists of nano-particles of MoO3 in the structure of Fe2(MoO4)3 [19]. The last two examples are
non-supported nano-particles systems.
4.2. Nano-particles in mesoporous systems
The size control of metal nano-particles is of the utmost importance for the performance of the catalyst based on supported metals. Such control is currently exerted through a keen manipulation of several different variables, such as, the architecture of the pore space, the nature of the precursor, the chemical nature of the surface, the metal concentration and distribution, dispersion of promoters and possible hydrothermal/thermal treatments. Some of these variables are interconnected, making it difficult to evaluate their effect independently.
If a mesoporous material, including particular types of metal oxide nano-particles, is designed for magnetic applications, possible embedding of the nano-particles in the mesopore walls is of no concern, as the magnetic properties are mainly dependent on the particle size and its distribution. The latter should be narrow. However, a narrow particle size distribution is difficult to achieve during uncontrolled particle growth within the silica body, in particular during the calcination step. If the optical properties of the material are the target, the interaction of particle oxide surface with the mesoporous host will strongly influence the properties. Here the position on the nano-particles determines the size of the contact area between the particle surface and that of the mesoporous support.
In the case of catalytic applications, embedding the nano-particles in silica, or other mesoporous oxide walls, will inhibit a significant fraction of the nano-particles activity, as part of the particle surface will be inaccessible to reacting molecules.
5.1. Framework metal ions and/or metal oxide nano-particles
5.1.1. Direct hydrothermal treatment (Sol-gel technique)
A very simple synthetic method is based on the addition of inorganic compounds (metal salts or alkoxides) in the sol-gel mixture. In this method the metal atoms will be introduced in the mesoporous materials’ body, and at high metal loading, nano-particles/particles will form (but not necessarily placed in the pores) [20-22]. However, addition of high metal contents will have a negative influence on the ordering of the mesoporous material, and to avoid such effect, the metal compound loading is kept low.
In addition, a simple way to tune nano-size metal particles consists of thermal treatments under suitable atmospheres, as in [23] when rhodium oxide in MCM-41 mesoporous material was synthesized; at low temperature with long period of time (373 K for 10 days) nano-particles of 6 nm were formed, while at high temperature for shorter time (423 K for 48 hours) nano-particles of the range of 3 nm were produced. Another example was the formation of Pt nano-particles in mesoporous silica by adding H4PtCl4 in
the synthesis mixture [24].
This method is used to prepare nano-particles within the mesoporous solids for catalytic applications has a number of disadvantages but it works well when silica or another oxide should be modified (doped) with suitable ions without nano-particles formation [25].
5.1.2. Impregnation with metal compounds
Impregnation is extensively used in the preparation of metal-functionalized mesoporous materials. The process includes direct impregnation of preformed mesoporous solids with solutions of the desired metal compound. The process is normally followed by subsequent reduction, thermal decomposition, UV-irradiation, or ultrasonic treatment. The formed metal atoms are randomly distributed over the mesoporous surface, and /or particles are formed in the pores, without any structural order.
To form the nano-particle by wet impregnation, several authors used consecutive impregnations (four to five times), drying the material in between, to insure that the mesopores are completely filled with metal precursor [26]. In this case, the amount of precursor is fixed and determined by the pore size and volume; the recipe is well reproducible as the final particle size is controlled by the precursor amount and in some cases by the pore size.
It is remarkable that repetitive impregnation with tetramineplatinum(II) nitrate [Pt(NH3)4(NO3)2] followed by four drying-reduction cycles allowed the authors in [27] to
prepare Pt nano-wires in mesoporous oxides SBA-15 and MCM-41. As found in [28,29], depending on the type of metal compound precursor and conditions of its incorporation into the silica and further transformation, either nano-particles or nano-wires can be synthesized even after a single wet impregnation. However, it is not yet entirely understood when nano-wires and when nano-particles are formed. In a typical impregnation example, Fe2O3 nano-particles within mesoporous MCM-48 silica have been
synthesized by wet impregnation with Fe(NO3)3followed by drying and calcination at 673
K. [30]
Normally, impregnation provides efficient incorporation of metal compounds inside the pores, but the particle growth is not controlled (particle size distribution is broad and particles are located statistically) if no special conditions are applied (see above). On the other hand, particle size is often restricted by the pore size. Nonetheless, the process is laborious.
5.1.3. Template ion exchange with transition metal cations
Another interesting method of inorganic functionalization of mesoporous solids is based on the replacement of the surfactant by transition metal cations. In this case, the metal cations (Mn2+ [31], Co3+ [32], Cd2+ [33], Zn2+ Mn2+ Al3+ [34]) are located at the interior pore surface, replacing the cationic surfactant.
The ion-exchange reaction is normally driven by replacement of monovalent cations (surfactant) with divalent (or trivalent) metal cations, and thus entropy is responsible for an efficient ion exchange. A unique feature of this ion-exchange technique is that the metal ions are transported solely inside the pores, while the external surface can be capped with inert hydrophobic groups.
This method seems to be superior to direct impregnation or chemical vapour deposition methods (see 5.2.1), as no precipitation on outer surface takes place (if outer surface is capped). Its use has been very limited (only a few papers have been published so far) [31-34].
5.1.4. Chemical interaction of metal compounds with functional groups of the mesoporous surface
Here, functionalization of the mesoporous walls is necessary (functionalization with organic groups was widely explored by many authors [35]), and chemical interaction
pores.
This interaction can be realized in two ways, e.g. for Si-compounds (i) during sol-gel reaction when one of the silica precursors bears such groups [35] and (ii) as a post-synthesis via interaction of various compounds with silanol groups [36-38]. In the prepared mesoporous materials, the removal of the organic template should be performed under mild conditions, e.g., by extraction instead of calcination, so that the silanol groups on the pore walls are preserved allowing successful functionalization of the silica walls [39-41]. As functional groups for interaction with inorganic compounds, thiol [42,43] and amine [44] are mostly used.
In this procedure, the functional groups can play a dual role (i) being an anchor for metal compounds and/or (ii) reacting with the surface of the growing nano-particle, as in [43] when Au particles were formed in mesoporous silica (SBA-15) functionalized with thiol substituent groups. Thiol groups firstly anchored the gold precursor and secondly governed the chemical reduction.
In several cases silanol groups of silica walls can be used for direct functionalization with inorganic compounds followed by the corresponding treatment (reduction, calcination, interaction with H2S, etc.) [45]. MCM-48 mesoporous silica was
functionalized with tungsten and molybdenum metal centers by the anhydrous reaction of metal alkoxides with surface silanol groups [46]. Calcination resulted in metal oxide clusters which were attached through covalent M-O-Si bonds.
This procedure avoids the growth of nano-particles in the outer pore space which is a major drawback of the impregnation/calcination method. In addition, the metal particle size is not influenced by the type of reducing agent, and in fact a very efficient mechanism of size restriction is observed. Presumably the metal nano-particles are forced to nucleate in the mesopore entries, but restricted to grow beyond their cavity. Finally, this method is very simple and robust and allows synthesizing of well-reproducible materials.
5.2. Additional methods to produce metal nano-particles/mesoporous system
5.2.1. Chemical vapour deposition
Another way to control the nano-particle growth and particle spreading is the chemical vapour deposition (CVD) method. However, this method is restricted to thin films or small particles (not suitable for e.g. monolithic samples) to prevent uneven distribution
of the metal compounds within the material. Another limitation of CVD is that under many conditions too large particles are formed.
A good example is the following: Iron oxide nano-clusters were synthesized within mesoporous MCM-41 doped with aluminum using evaporation-condensation of volatile Fe(CO)5 [47]. Subsequent calcination in an O2 flow resulted in amorphous Ȗ-Fe2O3
particles with diameters of 2-3 nm, evenly distributed through the well-defined hexagonally packed cylindrical pores. These results were confirmed by a combination of MĘssbauer spectroscopy, XRD, TEM, and SEM.
Nevertheless, the lack of specific interactions between the silica pore walls and the volatile compounds does not always exclude growth outside the pores. In a number of cases it has been reported that particle sizes exceed the pore diameter (ca. 3-4 nm) and the exact particle location and particle size distribution remain unclear. Probably, high temperature treatment (often up to 900°C) is a major reason for the observed large particle sizes.
5.2.2. Incorporation of prefabricated nano-particles in mesoporous solids
This method is based on the introduction of prefabricated nano-particles in the mesoporous materials; two different procedures are followed. The first is direct incorporation of particles in the sol-gel mixture, as was recently reported in [48]. This method requires good compatibility of nano-particles with the components of the sol-gel reaction. A limitation of this method is the particles, although encapsulated within the mesoporous material, do not necessarily reside in the pores: this limitation is especially crucial for catalytic applications.
An alternative approach was reported when CdS [49,50] and Au (Ag) [51] nano-particles were synthesized in reverse micelles and then incorporated into mesoporous silica (MCM-41 and MCM-48), modified with thiol groups (3-mercaptopropyl-trimethoxysilane was used as one of the silica precursors). The authors [49] observed a sieving effect, i.e. discrimination by particle size: (i) only particles of a certain size (i.e. smaller than the pore diameter) could be incorporated and (ii) the smaller the particles, the easier their incorporation inside the pores occurred.
A clear advantage of this method is the opportunity to use well-developed procedures to control particle size and particle size distribution of the nano-particles.
This method is based on the presence of metal nano-particles in block copolymer micelle cores [52] or in microgels [53]. These polymeric systems are used as templates for silica casting, with which pore size and metal particle growth control can be governed by a template.
Cupric oxide (CuO) nano-particles have been prepared in an amorphous SiO2
matrix, using a complex of Cu2+ with poly(vinyl alcohol) (CuPVA) as template [54]. The authors report the formation of CuO particles within the porous silica after calcination. In this case, the template was not ordered and was well compatible with forming silica, so the material formed must be fully disordered. Moreover, the copper oxide particles can be present, both, in the pores and in the silica body; the latter will obscure particle surface from participation in catalytic reactions which authors consider being a suitable application.
6. Metal-substitution in mesoporous materials
6.1. Titanium
Titanium-incorporated mesoporous materials play an important role as oxidation-, and photo-catalysts. Ti was successfully framework-incorporated in different ways. In post-synthesis procedures, the grafting of a titanium precursor such as titanocene dichloride [55,56], TiCl4 [57], Ti(OEt)4 [58] and Ti-isopropoxide [59] was carried out,
followed by calcination. Alternatively, titanium substituted SiO4-centres were formed
directly by sol-gel techniques, during the synthesis of MCM-41 by controlled hydrolysis of titanium isopropoxide in the presence of ethanol and nitric acid [60].
It has been found that the maximum molar fraction of titanium in a TS-1 framework is around 0.025 (corresponding to a Si/Ti ratio of 39). Above this value, the excess is extra-framework, usually present as anatase particles [61]. Impregnation by titanium isoproxide in ethanol [62], or grafting with Ti6 (hexanuclear titanium oxo carboxylato alkoxide cluster [Ti6(µ3-O)6(µ-O2CC6H4OPh)6(OEt)6]) clusters [63] results in titanium
dioxide anatase particles in SBA-15. Well-dispersed, size-controlled, isolated nano-crystalline TiO2 (6-10 nm) supported on MCM-41 and SBA-15 was formed via sol-gel
6.2. Chromium
Highly dispersed supported chromium oxides are used as commercial catalysts for polymerization, dehydrogenation and selective catalytic reduction of NOx. The presence of
isolated Cr atoms in the silica mesoporous frameworks of MCM-41 and -48 was reported by Sakthivel et al. [65] in which a sol-gel technique was applied by adding chromium nitrate (Cr(NO3)3.9H2O) during the synthesis. Chromium nitrate was also used in sol-gel
synthesis by Anpo et al. [66] to insert variable chromium loading in the HMS framework. In another study, Zhu et al. discussed the immobilization of chromium oxides in MCM-41 [67]. The solids were prepared by incipient wetness impregnation with aqueous Cr2O3 at a
loading of 2 wt% Cr. It was found that Cr interacted strongly with the surface Ti centers of Ti-MCM-41 so that Cr6+ and Cr5+ were highly dispersed on this support whereas Cr2O3
appeared in (Zr, Al, Si)MCM-41, [67].
6.3. Molybdenum
Isolated molybdenum atoms incorporated in the framework of SBA-15 mesoporous material by impregnation or by sol-gel techniques were reported by Dai et al. [68]. The authors used ammonium molybdate solutions as molybdenum source.
Morey et al. [46] reported a detailed study of molybdenum oxide supported on MCM-48, produced by the reaction of surface silanols with Mo2(OEt)10, followed by
calcination. The maximum loading before appearance of bulk metal oxide clusters is 2.1 mol% (0.15 atom nm-2), being much lower for Mo than for Ti and V deposited using similar approaches. In MCM-41, Molybdenum oxide particles were also incorporated by wet impregnation [69], using molybdena (AR) as a molybdenum source, or sodium molybdate during the synthesis (sol-gel technique) [70].
6.4. Iron
The synthesis of Fe3+-substituted mesoporous materials (e.g. MCM-41 [71-72], MCM-48 [73], and HMS [74]) has been reported. Fe-MCM-41 was prepared by both direct hydrothermal treatment, and by template-ion exchange methods [75]. Fe2O3
nano-particles were also formed in mesoporous systems by different techniques, in [76] direct hydrothermal treatment and incipient wet methods were applied. The former method was also applied in [77] to form Fe2O3 nano-particles in MCM-41. In MCM-48 [78] the wet
impregnation technique was applied. Basically, iron(III) nitrate [Fe(NO3)3.9H2O] is reported
VOx centers were produced at the surface of silica MCM-48 by gas phase deposition or liquid phase impregnation of vanadyl acetylacetonate [79,80] and on MCM-41 by both template ion exchange and direct hyrothermal treatment methods (by using vanadyl oxalate as vanadium precursor) [81], or through impregnation with vanadyl sulfate [82,83].
In addition, the formation of VOx particles within the mesoporous materials was reported. Microparticles of VOx were impregnated on SBA-15 by using an aqueous solution of NH4VO3 [84], or by direct hydrothermal treatment method as in [85] by using
vanadium (V) oxide as a vanadium precursor. Also for MCM-41, vanadium oxide particles were prepared by impregnation, using vanadyl acetylacetonate [86].
6.6. Cobalt
Only in a few papers, the formation of cobalt (II) in the mesoporous framework as isolated sites has been discussed, as in [87] when isolated Co2+ was incorporated in the framework of MCM-41 by direct hydrothermal treatment, using cobalt sulfate [CoSO4.xH2O] [87] or cobalt nitrate [Co(NO3)2.6H2O] [88].
Nano-particles of cobalt oxide were formed as well in the mesoporous matrix of MCM-41 [89] by direct hydrothermal treatment methods when CoCl2 was used as cobalt
precursor. Impregnation techniques were also applied [90]. Cobalt nitrate was used as a cobalt source [91]. In an interesting study, different cobalt sources were applied to prepare cobalt oxide nano-particles in MCM-41 [92] and it was shown that cobalt nitrate was the best precursor for preparing the cobalt oxide nano-particles in MCM-41.
7. Catalytic properties generated in mesoporous materials by atomic substitution
7.1. The mesoporous surface
7.1.1. Acidic properties
Siliceous mesoporous materials exhibit a very low acidity due to the silanol groups on the surface. Just like in zeolites, substitution of silica by trivalent atoms (Al3+, B3+, Ga3+ or Fe3+) in the inorganic framework of mesoporous molecular sieves was performed to create acidity.
Insertion of aluminium induced the highest acidity with formation of both Brønsted and Lewis sites. In any case aluminium represents the best candidate for incorporation in
the silica framework. For the substitution of silicon atoms by aluminium, the protonic acid form is obtained by exchange of the calcined HMS with ammonium salts followed by calcination or by direct synthesis for calcined amine-templated HMS [93]. The state of aluminium may be observed by 27Al MAS NMR, which evidences tetra-coordinated framework species (Al(IV) observed at 52 ppm), generating Brønsted acid sites, and hexa-coordinated extra-framework species (Al(VI) observed at 0 ppm), generating Lewis acidity. In general, the number of acid sites increases with the aluminium content, but the acid strength is very low and often similar to that of amorphous silica-alumina. Boron-substituted MCM-41, was synthesized [94,95], and presented a high activity for the Prins condensation of isobutylene with formaldehyde to isoprene with 100% selectivity [96].
Many studies have reported the activity in acid catalysis of Al-, Ga- and Fe-MCM-41 [97,98] or -MCM-48 [99] and indicate that the acid strength and catalytic activity decrease in the order Al>Ga>Fe. It was shown that Fe-MCM-41 presented weak acidity, and is essentially composed by Lewis acid sites. Similarly, Lewis acid sites were observed upon introduction of iron in MCM-48 [100] (also for V-MCM-41 [101]).
7.1.2. Basic properties
Basic heterogeneous mesoporous materials were less discussed than acidic ones. Mesoporous materials can gain basic properties by: (i) cation-exchange (Na+, Cs+, …) [102,104]; (ii) impregnation [105,106]; (iii) functionalization with organic molecules (like amines) by post-synthesis treatment [107,108], or direct introduction in the synthesis gel [109,110].
As discussed, introducing aluminium in the mesoporous framework induces negative charges, which generate acidity when compensated by a proton. Then a simple cationic exchange with alkali salts permits to create basic properties. However, the incorporation of basic species by cationic exchange is limited by the exchange capacity of the support. Increasing the aluminium framework content results in an increase of the exchange level and this goes with an increase of basic strength, a phenomenon well known in zeolites [111,112].
Brunel et al. [107] were the first who presented results on the functionalization of mesoporous surfaces by alkoxysilanes, particularly amine-terminated. The basic phase was bound covalently to the inorganic surface. The functionalization process consists of a grafting of the organic basic molecule over the calcined mesoporous material in an anhydrous refluxing solvent (toluene) under an inert atmosphere. Hydrolysis and condensation of alkoxysilanes with silanol surface groups result in a covalent linkage between the organic and inorganic phase yielding the formation of hybrid materials. The
synthesis gel. This kind of one-pot synthesis was first proposed by Macquarrie [109].
7.1.3. Redox properties
Titanium-silicalite has been successfully used [113,114] in selective oxidation of paraffins, olefin and alcohols. In order to be able to process larger molecules, efforts have been made to extend the pore sizes of this type of catalyst. For example, Ti-beta-zeolite has been synthesized [115,116], and in 1994 the first synthesis of Ti-MCM-41 was reported [117]. It was shown that Ti-MCM-41 could selectively oxidize olefins to epoxides, using H2O2 or organic hydroperoxides as the oxidizing agent. Other oxidation reactions
have also been performed [118,119]. Furthermore, bulky sulfides have been oxidized to the corresponding sulfoxides and sulfones [120].
A bifunctional acidic oxidation catalyst has also been prepared in order to perform two different reaction steps over the same material [121]. The Ti-Al-MCM-41 contains both tetrahedrally co-ordinated titanium oxidation sites and H+ associated to tetrahedrally coordinated aluminium, as acidic sites. The catalyst was tested with tert-butyl hydroperoxide in the multi-step oxidation of linalool to cyclic furanol and pyran hydroxy ethers. The selectivity was 100% at a ratio of 0.89. This process outperforms the conventional one, involving performic acid.
Owing to the success in the incorporation of Ti and its relatively good behaviour as a selective oxidation catalyst, other transition metal elements with potential activity were attempted to be introduced on the walls of MCM-41. In an interesting study [122], Ti, Fe, Cr, V and Mn were incorporated in MCM-41, and the authors showed the tendencies of these metals to remain in the framework after calcination. Ti was the most stable in the structure, Fe came next, whereas Cr and V showed dispersed oxidic species present at the surface.
A major problem with transition metal-incorporated mesoporous catalysts is the strong decrease in activity and selectivity when the metal content is increased. Another obstacle is alternation of the oxidation state of the metals during calcination and regeneration. Metals are leaching strongly, dependent on the nature of the substrate, solvent, oxidizing agent and reaction condition. Care should be taken in testing these materials and one should check that homogeneous reactions due to leaching of the metal do not occur.
7.2. Nano-particles inside mesopores – Applications
The catalytic properties of mesoporous materials with embedded nano-particles are mainly determined by the nature of the particle. Over the mesoporous solids containing nano-particles, all catalytic reactions which are normally known for the particular metals or alloys, can be carried out in principle. The important advantage of mesoporous oxides is their stability at high temperatures (better than the nano-particles embedded in polymeric systems). As stated before the advantage of mesoporous catalysts is an appropriate use of pores as nano-reactors of optimal size. This can be beneficial for large molecules or to reactions where pore size and shape will influence the reactive path, e.g., in the case of cyclization reactions [123].
The isomerization of n-hexane over Pt nano-particles contained in MCM-41 aluminosilicate was reported [124]. Hydrogenation of various substrates with Pd or Pt (or various bimetallic) nano-particles embedded in mesoporous materials was studied in [125,126]. One of the applications of mesoporous solids with metal nano-particles is hydrocracking of vacuum gasoil [127,128]. Here reaction temperatures exceed 400°C and no polymer or other organic catalysts can be used. Ni-Mo nano-particles were formed in MCM-41 via impregnation of molybdenum and Ni compounds (12 wt% MoO3 and 3 wt%
NiO), followed by calcination. The catalyst showed higher hydrodesulfurization and hydrodenitrogenation activity than the other heterogeneous catalysts [128].
Another important catalytic application is the photo-reduction of Cr6+ to Cr3+ over titania-modified MCM-41 containing Pd [129], in which the activity was much greater than that of corresponding conventional supports.
Pt and Rh nano-wires and nano-particles prepared in FSM-16 displayed interesting catalytic properties in water-gas shift reaction [28] and hydrogenolysis of butane [130]. In hydrogenolysis of butane, Pt wire/ FSM-16 demonstrated higher catalytic activity than Pt particle/FSM-16. A Pt-Rh wire/FSM-16 displayed a high activity in butane isomerization.
Catalytic activity of MCM-41 with rhodium oxide nano-particles prepared by addition of (RhCl3.3H2O) to the sol-gel mixture was studied in the high temperature
reaction of NO and CO [22]. Catalyst containing RhOx nano-particles with diameters less
than 3 nm exhibited a novel promotional effect in the amount of N2 and N2O formation with
8. TUD-1 as a new mesoporous material
In the laboratory of TOCK, at the Delft University of Technology, Dr. Zhiping Shan succeeded in 1999 to prepare a new mesoporous material. The target was the synthesis of a three-dimensional silica framework with high surface area in a one-pot procedure. Most importantly, the synthesis procedure had to be unique and cost-effective compared to other templated mesoporous materials such as MCM-41, MCM-48 or HMS.
In 2001, the first publication about this material appeared [ 131]. This new material was called TUD-1 (the abbreviations of Technische Universiteit Delft). In that paper the authors reported the synthesis of TUD-1 through a new templating method using an inexpensive non-surfactant pathway in a one-pot procedure. The synthesis was mainly related to mixing a silica source (tetraethyl orthosilicate) with diluted triethanolamine to obtain a homogeneous mixture at molecular level, followed by aging, drying and finally calcination. Hydrothermally stable sponge-like mesopore networks with high surface area (>600 m2/g) were thus obtained. The porosity parameters (surface area, pore volume and
pore size) were found to be tuneable. A computer simulation image of a mesoporous material is shown in Figure 3, while a series of more detailed pictures of the 3-D structure of TUD-1 is shown by HR-TEM images at different angles in Figure 4.
70 nm 70 nm
Figure 3. Computer image of a sponge-like mesoporous material, comprising two endless phases of silica and pores.
Following the success of TUD-1 synthesis, titanium has been incorporated in the TUD-1 framework by using titanium n-butoxide [Ti-(OC4H9)4] via direct hydrothermal
treatment and also by impregnation [ 132]. The prepared samples showed high catalytic activity in the epoxidation of cyclohexene [ 133].
A B
C D
Figure 4. 3-D HR-TEM images of a TUD-1 particle [134]. The same particle is shown in angles
A-0o, B-45o, C-90o and D-180o. The size of the particle is 40 nm in height and the mesopores are
The research described in this thesis focussed on studying the synthesis and characterization of functionalized TUD-1 by different transition elements (Ti, V, Cr, Mo, Fe, Co and Cu). The catalytic performance of the prepared materials was evaluated in various catalytic applications.
In chapter 1, a general introduction is given; the history of mesoporous materials, the synthesis procedures, nano-particles, mesoporous catalysts properties, and the thesis outline are briefly discussed.
In chapter 2, the synthesis of different transition elements incorporated in TUD-1 by a relatively easy one-pot procedure, in addition to the synthesis mechanism, are discussed. The characterization results obtained by applying different techniques (physically; by X-ray diffraction, N2 sorption measurements and HR-TEM and chemically;
by29Si NMR, elemental analysis, and UV-VIS- and Raman spectroscopy) are discussed in detail.
Chapters 3 and 4 discuss the first catalytic application, i.e. the photo-catalytic selective oxidation of propane over Ti-TUD-1 and Cr-TUD-1, respectively. In these two chapters the importance of the active site structure is discussed. Chapter 3 shows that the nano-particles of TiO2 are the active sites (and not isolated Ti4+) for the photo-reaction at
360 nm. In chapter 4 the isolated Cr6+ is the active site and the nano-particles/bulk crystals of Cr2O3 are almost inactive at 430 nm.
In chapter 5 the Friedel-Crafts’ type benzylation of benzene is discussed over different M-TUD-1 samples. It will be shown that Fe-TUD-1 is a unique catalyst for this reaction. The results obtained by GC were also compared with an in-situ FT-IR study.
The selective oxidation of cyclohexane is reported in chapter 6. Cr-, Cu- and Co-TUD-1 show the best catalytic performance evaluated by a first screening of all M-Co-TUD-1 catalysts. However, due to leaching of Cr and Cu, we concluded that Co-TUD-1 is the catalyst of choice for this reaction.
In chapter 7 the decomposition of N2O is discussed. Noble metals (Rh and Ru)
incorporated in TUD-1 were prepared specifically for this study; hence the synthesis and characterization of these materials are also given. It will be shown that noble metals in TUD-1 are promising catalysts for N2O decomposition.
In chapter 8, the final conclusions, recommendations and drawbacks of TUD-1 are addressed. Furthermore, the potential of functionalized TUD-1 in various catalytic applications is discussed.
Chapter 1 Introduction
Chapter 2
Synthesis and characterization
Chapter 3 Ti-TUD-1 Chapter 4 Cr-TUD-1 Chapter 5 Fe-TUD-1 Chapter 7 Rh- and Ru-TUD-1 Chapter 6 Co-TUD-1
Photo-catalytic oxidation of propane Friedel-Crafts’ benzene benzylation
Cyclohexane selective oxidation N2O decomposition
Chapter 8
Conclusions and recommendations
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2
Synthesis and characterization of M-TUD-1
(M=Ti, V, Cr, Mo, Fe, Co and Cu)
Easy formation of isolated catalytic sites and/or
nano-particles embedded in mesoporous TUD-1
Abstract
An easy, cost-effective, surfactant-free and one-pot procedure was developed for the synthesis of M-TUD-1 (M = Ti, V, Cr, Mo, Fe, Co and Cu) by using the bi-functional template triethanolamine TEA. The prepared samples were characterized by means of XRD, UV-Vis, N2 sorption measurements, elemental analysis, 29Si NMR, HR-TEM, and
Raman spectroscopy. Results indicate that at low metal loading, highly dispersed isolated metal sites in the TUD-1 silica matrix are obtained, while at high metal loading, homogeneously dispersed nano-sized metal oxide particles are formed inside the mesopores of TUD-1.
The contents of this chapter are patented in:
Z. Shan, M.S. Hamdy, J.C. Jansen, C. Yeh, P. Angevine, Th. Maschmeyer, (Delft University of Technology, ABB Lummus Global Inc.) U.S. Pat. 6930219 (2005) and WO 2004052537 (2004).
The discovery of ordered mesoporous silica by Mobil scientists in 1992 [1,2] opened up a new rich field in the world of catalysis, and it may be justified to state that the discovery of such silicate and aluminosilicate mesoporous materials was one of the most exciting discoveries in the field of materials over the last decade. The importance of such materials comes from the fact that they posses a larger pore size than zeolites, which should allow the relatively large molecules present in crude oil and in production of fine chemicals to react inside the pores. The mesoporous materials have been developed in several directions, and many of mesoporous materials are now well-known in the catalysis world (e.g. MCM-41, MCM-48, HMS-1 and SBA-15). The success achieved in preparing mesoporous silica was the starting point for using the concept to produce materials with potential catalytic applications. Different kinds of functionalization have been applied to mesoporous solids, including those involving attachment of organic ligands and organometallic compounds [3,4]. Numerous metal oxides can be incorporated in the mesoporous silica such as Ti, V, Cr and other transition metals [5,6]. The first attempt was to produce mesoporous acidic materials which could be used for cracking large molecules present in vacuum gas oil. Thus materials such as MCM-41 and MCM-48 with walls of aluminosilicate, where aluminium atoms are tetrahedrally coordinated in the silica framework, have been synthesized [7-9]. For a good review see [10].
When one expects that a periodically ordered mesoporous molecular sieve has been synthesized, a well-established methodology must be followed to demonstrate this. The procedure involves, first, the use of X-ray powder diffraction (XRD), which should be carried out focusing at the low-angle area. The XRD powder diffractogram can provide the relevant d-spacing, e.g. of MCM-41 or MCM-48, that can be indexed on a hexagonal and cubic lattice, respectively [11,12]. XRD combined with other techniques such as HR-TEM have been among the key methods for the characterization of such mesoporous materials and identification of the phase obtained, i.e. cubic (MCM-48), hexagonal (MCM-41), lamellar (MCM-51) or a 3-D sponge-like structure (TUD-1).
Adsorption of molecules has been widely used to map the pore size distribution, surface area, and pore size of the solid catalysts. In this sense the physisorption of gases such as N2, O2 or Ar has been used to characterize the porosity of M41S samples and
more specifically MCM-41 [13]. Mesoporous materials should exhibit type IV N2 sorption
adsorption to determine the pore diameter, one can combine the XRD results with the pore size to calculate the thickness of the mesoporous walls.
Another powerful technique for the characterization of mesoporous materials is NMR spectroscopy. One of its benefits includes the possibility to investigate the mechanism of formation of the material, which was illustrated by Maschmeyer and co-workers [15] for Ti-TUD-1 formation. The bi-functionality of the triethanolamine phase as a mesopore template, and as an anchoring agent of 4-coordinated titanate active sites was demonstrated. In the case of transition metal-substituted mesoporous materials, a combination of several techniques is required to provide the necessary information to prove that these elements are incorporated in the framework of the mesoporous materials. Elemental analysis should be used to provide information about the synthesis efficiency.
Raman spectroscopy can provide useful information about the different phases formed in the as-synthesized samples or in the calcined final product, e.g. the discrimination between anatase and rutile phases of nano-sized TiO2-incorporated in TUD-1 silica matrix
[16]. On the other hand, UV-Vis spectroscopy gives very valuable information on the co-ordination of a transition element in the mesopore framework.
The present study involves the synthesis and characterization of functionalised M-TUD-1 (M = Ti, Cr, Mo, Fe, V and Co) with different metal loading to form either isolated catalytic active sites and/or nano-sized particles of metal oxides embedded in the 3-D TUD-1 framework.
+M
+M
O O O Si O O O Si O O O O Si M M O O O O O O Si O O O O Si O O O Si O O O O O O Si O O O Si O O O O Si O O O O O O Si O O O O Si O O M M O Si O O O O O O Si O O O Si O O O O Si O O O O O O Si O O O O Si O O M M O Si O O OFigure 1. Illustration of the synthesis strategy of metal-incorporated TUD-1 mesoporous material. As a function of increasing metal loading either isolated metal atoms, nano-particles of metal oxide
2. Experimental
2.1. M-TUD-1 synthesis
2.1.1. Materials
Tetraethyl orthosilicate (TEOS, +98% ACROS), triethanolamine (TEA, 97%, ACROS), tetraethylammonium hydroxide (TEAOH, 35% Aldrich) and the different metal sources summarized in Table 1, were used for functionalized TUD-1 synthesis.
Table 1. The different metals source used in the synthesis of M-TUD-1.
Metal Name Chemical formula Source
Titanium Titanium (IV) n-butoxide Ti(O-C4H9)4 ACROS Vanadium Vanadium (V) triisopropoxide oxide VO[CHO(CH3)2]3 Alfa Aesar Chromium Chromium (III) nitrate nonahydrate Cr(NO3)3.9H2O Aldrich Molybdenum Ammonium molybdate tertahydrate (NH4)6Mo7O24.4H2O Fluka Iron Ferric nitrate nonahydrate Fe(NO3)3. 9H2O Aldrich Cobalt Cobalt (II) sulfate heptahydrate CoSO4.7H2O Aldrich Copper Copper (II) nitrate trihydrate Cu(NO3)2.3H2O Aldrich
2.1.2. General procedure of M-TUD-1 synthesis
M-TUD-1 mesoporous materials with different Si/M ratios were synthesized by aging, drying and calcination of a homogeneous synthesis mixture, with a molar ratio composition of 1SiO2: (0.01-0.1)MOy: 0.5TEAOH: 1TEA: 11H2O which basically involves
the following steps:
1. Deionized water was added to triethanolamine and the mixture was shaken by hand for a few minutes until a pale yellow diluted mixture was obtained.
2. The metal salt (cf. Table 1) was dissolved in deionized water (with exception of the Ti-source) and stirred at room temperature until the salt was completely dissolved. If small amounts of impurities remained, the solution had to be filtered.
3. The aqueous metal salt solution was added dropwise into tetraethyl orthosilicate while vigorously stirring for a few minutes.
TEA + H2O
TEOS + M solution
GEL
TEAOH
Aging for 1day at ambient conditions
Hydrothermal treatment Calcination TEA + H2O TEOS + M solution GEL TEAOH
Aging for 1day at ambient conditions
Hydrothermal treatment
Calcination
Figure 2. Schematic diagram illustrating the synthesis procedure of M-TUD-1.
4. The solution obtained from step 1 was added dropwise to the mixture obtained from step 3 and then TEAOH was added dropwise and the overall mixture stirred vigorously for at least two hours until a clear homogeneous mixture (i.e. no precipitate should be formed) or a gel is obtained.
5. The resulting homogeneous mixture was subsequently aged at room temperature for 24 hours, and then dried at 98oC for another 24 h.
6. The obtained solid was gently ground, hydrothermally treated in a 50 ml Teflon-lined stainless steel autoclave at 178oC for 2-24 h.
7. The obtained solid was ground again, and finally calcined at 600oC for 10 h at a heating ramp rate of 1 degree/min in air.
Different sets of M-TUD-1 were prepared (M = Ti, V, Cr, Mo, Fe, Co, and Cu). Basically each set consists of 4 samples with different Si/M ratio (100, 50, 20 and 10) with exception of Ti, where samples of Si/Ti ratio = 5, 3.3, 2.5, 2 and 1.6 were included.
XRD. Powder X-ray diffraction patterns were measured on a Philips PW 1840 diffractometer equipped with a graphite monochromator using CuKĮ radiation (Ȝ = 0.1541 nm). The samples were scanned over a range of 0.1-80° 2Twith steps of 0.02°.
N2 sorption. Nitrogen adsorption/desorption isotherms were recorded on a
QuantaChrome Autosorb-6B at 77 K. The pore size distribution was calculated from the adsorption branch using the Barret-Joyner-Halenda (BJH) model [17]. Samples were previously evacuated at 623 K for 16 h. The BET method was used to calculate the surface area (SBET) of the samples, while the mesopore volume (Vmeso) was determined
with the t-plot method according to Lippens and Boer [18].
UV-Vis. The materials prepared were investigated by diffuse reflectance UV-Vis spectroscopy. Spectra were collected at ambient temperature on a CaryWin 300 spectrometer using BaSO4 as reference. Samples were ground carefully, heated overnight
at 180oC, and then scanned from 190 – 800 nm. The UV-Vis absorption data were converted to Kubelka-Munk units using the following formula:
F(R) = (1-R)2 / 2R
INAA. Instrumental Neutron Activation Analysis (INAA) was used for chemical composition determination (elemental analysis), on the THER nuclear reactor with a thermal power of 2 MW and maximum neutron reflux of 2x1017 m-2s-1. This method was applied because of difficulties in dissolving the samples. The method proceeds in three steps, irradiation of the elements with neutrons in the nuclear reactor, followed by a period of decay, and finally a measurement of the radioactivity resulting from irradiation. The energy of the radiation and the half-life period of radioactivity enable a highly accurate quantitative analysis [19].
HR-TEM. High-Resolution Transmission Electron Microscopy was carried out on a Philips CM30UT electron microscope with a field emission gun as the source of electrons operated at 300 kV. Samples were mounted on a copper-supported carbon polymer grid by placing a few droplets of a suspension of the ground sample in ethanol on the grid, followed by drying at ambient conditions. EDX elemental analysis was performed using a LINK EDX system.
