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A nanoporous material is a type of porous material which has a pore diameter of about 1 to 100 nanometers. The presence of these pores makes the bulk material useful by rendering other physical properties (Schmid 2010). They have high surface to volume ratio making them porous (Amabilino 2009).
Classification of nanoporous materials are based on the chemical composition. Nanoporous materials may be polymeric, carbon, glass, alumina-silicate, oxides or metals. All these types have varying technical properties such as pore size, porosity, permeability, strength, thermal stability, chemical stability, cost and life span (Zhao 2004).
"Nano" is Greek word meaning dwarf, and is denoted as the one billionth or minus ninth of the base ten (Hosokawa 2007). Nanoparticles are particles which have a size ranging from 1 to 100 nm and have been found to be invisible to light, because technically, the size of these particles are less than the wavelength of light. They are characterised to have a particle surface of approximately 100 m2/g (Sepeur 2008).
Nanoparticles can be produced by different methods, the most common of which is the top-down method and the constructive synthesis or bottom-up method (Sepeur 2008).
In the top-down method, a ball mill or grinder, which may be hard ceramic material or zirconium dioxide, is used to crush the microparticles into nanoparticles by applying a very large amount of force. The resulting nanoparticles have different characteristics which are dependent on the weight and size of the ball, and the efficiency of the grinding process (Sepeur 2008).
The bottom-up method can be further subdivided into gas condensation method, chemical precipitation, aerosol method, sol-gel process method and the micro-emulsion method. All these methods have been found to be of great value in the production of nanoparticles, and the type method to be used is dependent on the chemical property desired of the nanoparticles (Sepeur 2008).
Nanoparticles may be found as spheres, rods or plates, but the most common are spherical in shape. They can be either hollow or solid, and are made up of layers, each of which has different roles (Clark, 2009). Different properties can be controlled simply by changing the forms of the nanoparticles and these changes may provide catalytic or optical properties (Sepeur 2008).
What are their uses?
The vast properties of nanoparticles has made it a very good raw material for almost all types of applications like cosmetics, biotechnology, petroleum industry, pigments and catalysts (Hokosawa 2007). Application of nanoparticles in fields where the products are colloidal in nature and with minimal solid content is much easier (Hokosawa 2007).
The first use of nanoparticles was in the 1950s, where attempts to cost reduction in the large scale manufacturing has begun, and this led to the development of metal catalysts having dimensions between 1 to 100 nm. Mobil Oil Co. was able to synthesise zeolites by controlling the structure of the catalyst at the atomic level (Zhou 2005).
These zeolite catalysts reduced the need for lead and benzene in the manufacturing of gasoline, consequently minimizing the detrimental effects of these harmful chemicals to humans and the environment (Zhou 2005).
Nanoparticles are also used in the core-shell technology using platinum as electrode catalyst, however platinum production seems to be very costly and efforts have been made to minimise its use. Due to their conductivity, carbon nanoparticles (collectively known as carbon black), were studied and has shown to be a useful alternative support to platinum for the Polymeric Electrolyte Fuel Cell (PEFC). Development of the new carbon nanoparticles for use in the PEFC has lowered the demand for platinum and has reduces the cost of the core material (Tai 2010).
Ruthenium particles have been used as catalysts in the hydrogenation processes in chemical, petroleum and energy industries (Berger 2007).
Nanoparticles possess unique features that are very significant to biomedical applications. The detection levels on imaging can be increased since the signals can be amplified, cell uptake can be enhanced and energy manuipulation can be done in the case of the quantum dots (Bulte 2008).
In bioimaging, the luminescence and optical properties of the nanoparticles are taken into consideration. The use of iron oxide or cobalt core particles have improved the imaging in MRI because the nanoparticles can enter the cells due to their very small size (Clark, 2009).
An example of this application is in the tool such as the scanning tunnelling microscopy which allows reactions at the atomic level to be changed by initiating certain processes (H-E. Schaefer 2010).
Nanoparticles also find potential use on drug delivery by being able to send the drugs to target organs and being specific in terms of its reactivity. Tumor cells may be removed using this type of biotechnology, and may help in fighting cancer cells (Zhang 2008).
The very small size of nanoparticles allow them to penetrate even through the human cell, and their stability in various temperature and pH conditions make them efficient tools in cell labelling. Variations in the shell thickness will provide different colors which is useful in marking at the cellular level (Zhang 2008).
Nanoparticles have also made its way to consumer products. "Scratch-proof eyeglasses, transparent sunscreens, stain-repellent fabrics and ceramic coatings for solar cells are some of these products" (SCENIHR 2006).
Because of the large active surfaces and sensitivity to environmental changes, nanoparticles and nanoporous materials have also been used as materials for sensors and actuators. Sensitivity of gas sensors depend on the surface area, and this property led to the developments which uses metal oxides as detectors for combustible gases and volatile organic compounds like ethanol and hydrocarbons (Zhao 2004).
In the study by Lee et al (2006), they synthesized dimethyl ether (DME) over Cu/ZnO catalyst using promoters and copolymerization. Their study showed production of dimethyl ether at 1 kg/L-h at 50 bar production.
Why do we care so much about them?
"The most important parameters for nanoporous materials are the pore structures and functionalities within the pores" (Amabilino 2009 p 391). The impact of nanotechnology to the society is in most likelihood to that of semiconductors (Bhushan 2007). Technological researches that focus on the application of nanotechnology has its target applications in the fields of manufacturing, nanoelectronics, biotechnology and energy (Bhushan 2007).
Nanoparticles serve as the building blocks in the design of active materials which may be used in chemical and medical applications (Pradeep 2007).
The applications of nanotechnology is of great importance in the world's endeavour to minimise global warming through researches on more environment-friendly and alternative fuel sources (Zhou 2005) and in the field of biotechnology where the treatment of cancer and other diseases provide future opportunities (Zhang 2008). Despite the abundance of applications of nanotechnology, ethical issues are still present, especially when dealing with biotechnology. The use of the nanoparticles in treatments poses certain societal issues that deal with human safety, as well as environmental concerns especially when waste disposal is considered (Miller 2005).
What is a catalyst?
A catalyst is a substance which plays a temporary role in a chemical reaction (Richardson 1989). Catalysts accelerate the reaction by combining with the reactants, and decreasing the energy of activation of the reaction, then recycled after the reaction has been completed (Poole 2003). In cases where there may be more than one reaction, the catalyst becomes active in only one of these reactions, leading to its property as being selective; and they hasten the reaction through complexation or chemisorptions with either the reactants or products (Richardson 1989).
Catalysts can be used either in the methods to prepare nanotubes and other nanostructures, while some can serve as the catalysts in the subsequent reactions (Owens 2008).
What different types of catalysts are there?
Catalysts can be either homogeneous or heterogeneous (Poole 2003). "Homogeneous catalysts are dispersed in the same phase as the reactants, the dispersal being ordinarily in a gas or liquid solution" (Poole 2003 p. 264); while heterogeneous catalysts are separated from the reactants and are in a different phase. Homogeneous catalysts have limited industrial applications because they do not meet the significant requirement of being separated from the products and reactants, however, there are some chemicals which are manufactured in this manner (Richardson 1989).
Heterogeneous catalysts may be distinguished based on the difference in phase with the reactants (Spivey 2004). Reactions involving gases or liquids used with solid catalysts are the most common form; and because these types of catalysts can be prepared easier, more controllable and give efficient yield, they are more convenient for commercial applications (Richardson 1989).
Catalysts may be classified according to the core-shell structure that they have. They can be Inorganic core-shell, Organic-Inorganic hybrid core-shell and Polymeric core-shell nanoparticles (Zhang 2008).
Inorganic core-shell catalysts are those that have inorganic core or shell, and may be metallic, semiconductors or lanthanides. Metallic core-shell nanoparticles are those that use either metal ions, metal oxides or silica as the material in the catalyst. The most commonly used core-shell nanocomposites are that with gold or silver (Zhang 2008).
Semiconductor nanoparticles are those that have alloys or metal oxides and may exist in binary or tertiary structures. The most common structure having one core and one shell have been commonly called quantum dots, and are composed of CdSe/Cds, ZnSe/ZnS. These nanoparticles have fluorescent properties and are thus, used for imaging in medicine. Quantum dots have versatile mechanical, electronic and magnetic properties, and various combinations of the alloys will give different possible applications (Schmid 2010).
Lanthanide nanoparticles are those that contain lanthanide elements like Ti, Ce, Y and Eu. These nanoparticles can be used in electronics and imaging (Zhang 2008).
Organic-Inorganic hybrid core-shell nanoparticles are made up of organic polymers and inorganic materials. Examples of these nanoparticles are polyethylene/silver, TiO2/cellulose, and CuS/Polyvinylalcohol (Zhang 2008).
Polymeric core-shell nanoparticles are those which have polymeric materials somprising its core and shell. Polymers such as polymethylmethacrylate coated with a metal oxide and compounded with another polymeric compound imparts strength and enhances toughness of materials like those found in PVC's (Zhang 2008).
What are the components of a catalyst?
Catalysts are made up of two parts: the core and the shell. The shell is the outer part which protects the core where the active component may be found (Ghosh 2006).
The core and the shell can be composed of different materials, with the shell material being highly considered so as to minimise the agglomeration of particles (Zhang 2008).
What makes a good catalyst?
Good catalysts must be selective and not easily consumed; must be mass and heat transfer efficient and should be active (Furusaki et al 2002). These properties should be inherent in the catalyst so that its participation in the reaction will be significant. Surface area of catalysts must be high for higher throughput and with minimum particle agglomeration (Zhang 2008).
Active surface and selectivity are the two most important properties of a catalyst. The size of the particle is inversely proportional to its active surface, meaning, as the particle size decreases, the active surface increases and this result to more efficient reactions (Filipponi n.d.). Selectivity of the catalysts is also of great importance and is influenced by the geometric and electronic structures of the nanocatalyst (Filipponi n.d.).
"The control of catalyst nanoparticle size, composition, dispersion, crystal structure exposure and stability is the key to producing pure, efficient and strong catalysts" (Zhou 2004).
Transition metals or their complexes have been shown to be the most effective type of catalyst (Berger 2007). Transition metal oxides like TiO2 show good catalytic properties, and in its anatase form, it has a good photocatalytic effect (Zhao 2004). According to Wilde (2009), the method used to prepare the catalyst and the type of support used has a strong effect on the stability, size and dispersion of the nanocatalyst.
Which type of catalyst concerns this core/shell work?
One type of a core-shell catalyst is the metal-metal oxide core-shell nanoparticle and has been widely used in different industries (Pradeep 2007).
It is undeniable that metal catalysts are of great commercial importance, and the preparation of samples with consistent size and shape of metal particles has always been a challenge for scientists (Zhou 2007). The dendrimer-assisted method is a new method for the synthesis of metal particles, where the metal ions are complexed with a dendrimer. The strong affinity of metal ions to the amine group of the dendrimer allows very minute differences in the metal/amine ratio, resulting to uniformly-sized metallic particles (Zhou 2007).
Why do nanoporous materials/nanoparticles make good catalysts?
The versatility and abundant surface properties make nanoporous materials good catalysts. They are stable at elevated temperatures, non-reactive or inert and are not cytotoxic (Zhao 2004).
The porous shells in a core-shell nanoparticle allow the movement of electrons through them, which makes them good conductors. Core-shell nanoparticles also possess optical properties due to the uniform layer of spheres that allows light scattering. Furthermore, core-shell nanoparticles have magnetic properties which render them available for conversion to biocompatible materials (Pradeep 2007).
The general properties of nanoporous materials and nanoparticles such as selectivity, adsorption kinetics, mechanical properties and durability are not the only characteristics considered. If the nanoporous material or the nanoparticle is intended to be used as catalyst, the acidity and basicity, as well as the shape must also be considered (Zhao 2004).
Generally, size of the particle will provide greater surface area, and is ideally a better catalyst. However, there are reactions which are dependent on the size of the catalyst involved. An example of this reaction is the synthesis of hydrogen peroxide, where it was observed that when the particle size of the catalyst was lower than 4 nm, water was the product (Zhou 2005).
How much of these catalysts are made every year?
Catalysts are made using various methods and continuous research have been done to search for the most efficient but cost-effective production.
Zeolites play an important role in the petroleum industry. During the 1980s, production of fluid cracking catalysts was about 250,000 tons per year, and about 88% of these catalysts are zeolites (Wallace 1981).
Titanium oxide (TiO2) is used for cleaning gaseous waste from industrial incinerators and other equipments that may emit nitrogen oxides. The demand for Selective Catalytic Reduction catalysts as TiO2 has been estimated to be 10 x 103 tons per year (Buxbaum, 2005).
How much is the industry worth?
In 2005, it was estimated that the market for nanoparticle catalysts is about US $7.4 billion, and a US $8 to US $68 billion projected market growth (Zhou 2005). It was estimated that in 2006, products which were improved or developed through nanotechnology is about US $50 billion and is expected to rise to US $2.6 trillion by 2016 (H-E. Schaefer 2010).
The refinery industry uses nanocatalysts in the "Platforming" of gasoline to increase its octane number, and the catalyst market is estimated to be around US $100 million (Zhou 2005).
The photocatalytic properties of TiO2 has gained much attention in research and the market is estimated to exceed US $5 billion, in Japan alone (Zhao 2004).
What forms do these catalysts take?
Different forms of catalysts are present because of their various applications. They may be oxide-supported, dendrimers, or molecularly-assembled with metallic active sites; and various combinations may provide opportunities in the synthesis of catalysts which have exact sizes and spatial properties (Rotello 2004).
Monometallic catalysts is a type of homogeneous catalyst which has been largely used in experiments to detect the modifications and changes needed to the nanoparticles to be able to get better catalytic properties (Astruc 2008).
Bimetallic catalysts on the other hand provide evidence of the different properties that can be modified by changing the metal-to-metal core or shell composition (Astruc 2008).
Organometallic catalysts have also found their use in the industry. The usual polymer-metal core-shell structures have been used in the fields of material sciences and engineering, because of the versatility of the catalysts, and the nanoparticles can provide better strength and durability of the products when properly controlled (Zhang 2008).
Common types of catalyst particles are pellets, spheres, granules or exudates and their costs differ depending on the particle size (Richardson 1989).
The most abundant, but probably, underrated catalysts are the enzymes. Despite their natural availability, enzymes have been characterised to be in the middle of a molecule and a microparticle, as well as a homogeneous and heterogenous catalyst. Being found to be more efficient than inorganic catalysts, and considering their unparalleled role in life processes which proves their selectivity and reactivity, scientists have become even more interested to incorporate these naturally-occurring catalysts to industrial materials (Richardson 1989).
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