Synthesis and Characterization of Metal Nanoparticles

For various application metallic nanoparticles are functionalized, they are more stable in solutions than non-functionalized metallic nanoparticles.  Non-functionalized nanoparticles forms aggregate in the solution, which resulted in the low surface area and decreases the catalytic activity. Ligands are used to prevent aggregation, but they alter the functionalities of the catalyst. Polymer and oligomers are another options to functionalize the nanocatalysts. Usually catalytic nanoparticles were used with the polymer matrices or they immobilized on solid supports (5). Supported metal catalysts are widely used in petrochemical industry, supported Pt catalyst specially used in naphtha reforming. But some new methods are developed to create a stable catalytic aggregate of the desired shape by using electrostatic interaction forces (6). By changing the adsorption of metal ions (Ag⁺, Pt²⁺) form multi-layer polyelectrolyte film (7).

Synthesis and Characterization of Metal Nanoparticles:

Catalysts play a vital role in chemical reactions in the chemical industry. Heterogeneous catalysts at high temperature are used with an inorganic support and sometimes homogenous catalyst at the lower temperature also used. Almost all important of nanoparticles is due to properties of small size, synthesis of nanoparticles with required quality is an important issue in green catalytic nanotechnology. Different kinds of nanoparticles successfully synthesized by using conventional chemical and physical nanoparticles preparation methods.

Synthesis of metallic nanoparticles is divided into two types of methods, physical methods and chemical methods. In physical methods, bulk metal is crushed into nanoscale by using mechanical crushing and milling techniques. High energy ball milling is used to produce 10 nm size particles and according to study of Hamid Reza Ghorbani (2014) nanocatalysts of Aluminum produced by using ball milling technique are more reactive than standard nanocatalyst of aluminum (8). They are very combustible in air and usually, hexane is used to keep away from the combustion (9). Ball milling is a solid state production of nanocatalysts, this equipment can be divide into high energy and low energy category. This division based on the transformation of mechanical energy to breakdown energy (10). High energy ball milling is a most common method for the production of metallic nanoparticles. Ball mill is loaded with metal powder and heavy balls, high rotation speed provides mechanical energy to balls and that energy is used to convert solid particles into nanoscale (11). A blend of different materials also used in ball milling to improve chemical properties of the nanoparticles (12).

The most ancient and widely used chemical method for synthesis of nanoparticles is the reduction of metallic ions in solution. In ion reduction method metal ions are reduced by providing some extra energy and using the different type of chemical reductants. The provided energy is used to decompose the material, and usually, photo energy, electricity or thermal energy used. It is most frequent chemical method used for the production of stable metallic nanoparticles. Turkevich used this method to produce spherical nanoparticles of gold by reducing the gold hydrochlorate solution with sodium tris citrate  (13). Nanoparticles of gold also produced by reducing the solution of chloroauric acid, this reaction reduced Au⁺³  ion to Au⁺ and further into Au⁰ . Brust method and Perrault method also used to produce metallic nanoparticles, the main theme of both these method is same as in  Turkevich method with different reducing and anti-coagulant agents. In 2010, Martin and Eah produced ultra-small gold nanoparticles of size 3 to 6 nm by precisely controlling ratio of solution and reducing agent with heat.

In last decade, scientist invented many other methods for generation of nanoparticles.

Vapour synthesis:

In this method, the material in the vapour phase are brought in hot wall reactor and variable factors (pressure and temperature) are adjusted in such way that condition favours the nucleation of particles. This method can be used for bimetallic and multicomponent nanoparticles.  Recently Schmechel successfully generated nanoparticles of europium doped yttria from organometallic yttrium and europium by using this process (14).

Sputtering:

In this method material is vapourised from the solid surface by using high-velocity ion of an inert gas. The bombardment of the gas ion causes ejection of atoms from the solid surface. Sputtering is usually done under vacuum. Electrons can also be used instead of ions (15).The main advantage of sputtering is that the produced nanoparticles will have the same composition as the targeted material (16).

Laser Reactors:

In this process substrate is heated by using a laser beam in the presence of an inert gas. The vapour phase of the substrate is directly heated by absorbing laser beam energy. In this technique inert or carrier is not directly heated, but due to collision with the vapour of substrate the temperature rises. And the temperature of inert gas dropped which is resulted into super saturation, nanoparticles induced on the wall of reactor (17).

Usually, ruby laser and CO₂ type laser are used for laser pyrolysis. The main advantage of this process that substrate is directly heated by the laser beam, so there is no need for the heating wall. The Hot wall can cause of product contamination. As there no contamination in this process so, it gives good efficiency as compared sputtering techniques. The only disadvantages that this technique is costly due to the laser (18).

Flame Reactors:

Flame is also used for production of nanoparticles. The flame heat start the chemical reaction which produce condensable monomers, they are usually in the form of agglomerates. This is very economical method (19).

Nanoparticles of TiO₂ can be produced by using flame reactors. TiCl₄ is heated and oxidized in the presence of flame. These reactor with a small modification can be also used for complex and free agglomerates product (20).

Plasma reactors:

Plasma can also be used as energy source to start the chemical reaction for the production of nanoparticles. As the temperature of plasma is so high that it disassociate the substrate into atoms and radicals. Nanoparticles are formed on cooling of these gasses. Different type of plasma can be used but usually dc arc plasma, dc plasma jet, and rf induction plasma are preferred. Plasma reactor can be used for multicomponent nanoparticles (21, 22).

Sometime vapours formed by plasma are mixed with cold gas to pace up the production but it destroy the uniformity of nanoparticles (22).

Wire electrical explosion:

In this method the metallic wire is connected to a high current source, as current flow through the wire, it become hot and explode in vapours. This technique is used for such kind of metals (Si, C) that have high boiling point, which cannot be easily vaporized in the furnace (23).

In 1993 Saunders et al. used this technique for the production of Si nanoparticles, he used Ar inert gas for carrier purpose and he get the nanoparticles of size 2-4 nm. Binary metallic, and multicomponent nanoparticles reactive gasses is used for reaction and for carrier purpose (24).

Expansion-cooling:

Condensed gas is allowed to pass through a nozzle, the gas expands, temperature decreases and this sudden expansion and cooling produced homogeneous nucleation. With the change in pressure, the size of particles can be adjusted. High pressure gives bigger sizes, according to the simulation of expansion by Turner et al. (1988) 2 bar pressure for N2 gas in subsonic nozzle give the particles of size 100 nm. But when pressure is decreased to 0.75 bar and other conditions remain same, the produced particles size is in the range of 5 nm (15).

Electrospray Systems:

The simplest way to produce nanoparticles is to evaporate the small sized droplet of dilute solution. Create a diluted solution of desired matter, take this solution in the small droplets (1 μm) and instance drying will give nanoparticles. But these nanoparticles will have some impurities which come from the solvent. So it does not give pure nanoparticles, and 2nd issue is to achieve the droplet in very small size (25).

Characterization plays an important role in controlling over synthesis of nanoparticles, for the complete characterization of the colloidal dispersion of metal, nanoparticles require clarifying the three kinds of structures, which are the structure of stabilizer, structure of dispersion and structure of metallic part.  There are many techniques are developed to characterize nanoparticles. Transmission electron microscopy, scanning electron microscopy are very common methods. Atomic force microscopy, x-ray photoelectron spectroscopy, powder x-ray diffractometry, UV spectroscopy and dynamite light scattering also used. For nanoparticles of gold and palladium, x-ray diffraction (XDR), UV spectroscopy and Fourier transformed infrared spectroscopy (FTIR) techniques are used for characterization (26, 27).  Transmission electron microscopy (TEM), selected area diffraction pattern (SAED), high-resolution TEM are used to characterized to nanoparticles of silver (28). For magnetite, Au, Ag, zinc nanoparticles, x-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) are used (28-31).

As the main reason of the importance of nanoparticles is their size, and this one is also the most important factor of catalytic properties. The total surface area of metal particles is inversely proportional to the square of the diameter of nanoparticles, it means the surface area will increase with decreasing size of particles. All the chemical reactions occur on the surface of the catalyst so with decreasing size of particles resulted in high catalytic activity (32, 33). For some chemical reaction, the rate of reaction increased with the certain size of catalyst, further decrement in the size of nano-catalysts resulted in a slower reaction. In photochemical hydrogen generation by using nanocatalysts of Pt, the critical size of catalysts is 3 nm, above and below size will slow down the chemical reaction (34). The size also effects on the selectivity of catalysis. The selectivity for production of monoene in the partial hydrogenation of cyclopentadiene requires size less than 2 nm. Size greater than 2 nm will change the selectivity of the reaction and the main reason for this selectivity change is the steric effect of Pt nanoparticles (35).

The catalytic activity also affected by the material composition of nanocatalysts, small quantity of any other metal can improve catalytic activity. This effect attributed to the ligand effect and ensemble effect. Core-shell structure of nanocatalysts also effect the reaction and this effect is due to the redox potential of metals (36).   For example, the highest catalytic activity for selective partial hydrogenation of 1,3-cyclooctadiene to cyclooctene achieved at the ratio of nanoparticles of Pd/Pt = 4. But the decreasing ratio will also decrease the catalytic activity of this bimetallic nanoparticles. These bimetallic catalysts have high activity than Pd catalyst, this is due ligands effect of the presence of Pt at the core of particles (37).

The stability of catalyst is very important in the chemical reaction, and in the case of nanocatalysts, this is done by using different kind of polymers (38). Some polymers are used to stabilize the metallic nanocatalysts in both aqueous and non-aqueous solutions. The polystyrene-b-poly-4-vinylpyridine is used to stabilize the nanocatalysis of Pd for coupling reaction of styrene and 4-bromoacetophenone in Heck reaction (39).

Green Catalytic Application of Metal Nanoparticles

Still most of the catalysts are made by mixing and shaking some components, their surface structures are not well controlled. For any catalyst, we want to achieve long life cycle, 100% selectivity and low energy consumptions, and these properties can be only gained by controlling their size, shape, electronic structure and chemical and thermal stability of catalyst particles (40). In the case of nanocatalysts, selectivity ratio can be adjusted by precisely controlling the size of nanoparticles. So precious metallic catalysts are usually used in the form of nanoparticles to improve their selectivity.

Nanocatalysts are used in chemical process Industry due to their lower energy consumption, environmental friendly behavior with an improved economy.  Nanocatalysts of NiO with novel aluminum oxide support are used in the production syngas and bio-oil from pyrolysis of biomass. Nanocatalysts improved the quality of syngas by decreasing the fraction of carbon monoxide in the syngas, it also reduced the fraction of tar in products (41, 42).

Biomass energy is fulfilling about 15 % of total energy requirement of the world, the broad range of organic materials included woods, crops, organic wastes (animal dungs, municipal wastes) are used for the production of biomass energy. And mostly these materials produced from the process of photosynthesis, according to the study of Luo and Zhou (2012) approximately 720 tons biomass produced per year (43, 44). There are three main types of reaction for the conversion of biomass into fuel energy, it include combustion, pyrolysis, and gasification.  Pyrolysis technique is preferred on other techniques due to its product named as pyrolytic oil or bio-oil. High moisture content, long chain hydrocarbon and low hydrogen-carbon ratio make it difficult to burn. So, mainly biofuel is not directly used as fuel. Aravind and Jong (2012) study the effect of nanocatalysts on the processing of bio-oil into syngas and concluded his study that nanoparticles with high surface area are the best alternatives of conventional catalysts to improve the yield of syngas  (45, 46). Tar is by and undesirable product in syngas production, due to its high boiling point it block the filter and pipes, tar also poisoned to the catalyst (47). There are two methods to control the production of tar, one is treatment within gasifier and the second one is hot gasses treatment after the gasifier. The second method is most economical and that is why the second method is only used in industry. Nanocatalysts of sodium, potassium and calcium are used for hot gasses treatment of effluent to reduce the amount of tar (48).  Catalytic reforming of tar by using metallic nanocatalysts are preferred because there is no need for any additional energy and that is the reason why nanocatalyst are called green catalysts. This catalytic reaction also includes steam reforming, hydrocracking, and hydro-reforming (49, 50).

In Fischer-Tropsch Synthesis of green diesel, nanocatalysts of iron and Co of size 10-15 nm are used in slurry reactors to improve the production of high molecular waxes. These waxes then hydrocracked to generate green diesel (51, 52). Fischer-Tropsch is an important method to convert nanopetroleum feed stock like coal, biomass into clean diesel, which can be used as fuel. Uses of nanocatalysts dramatically decrease the cost of this process (53).

FT syntheses technology produced high-quality ultra clean fuel with low aromatic content and zero sulfur. The product mostly consists of the mixture of linear and branched hydrocarbons (54).  Two type of techniques are adopted for FT synthesis but the slurry bubble column techniques gain more interest due to its low pressure and stable temperature (55). Generally, Fischer–Tropsch synthesis follow the ASF distribution mechanism, but this distribution in unselective in this case (56). Later on the novel Fischer-Tropsch catalysts are developed, novel FT nanocatalyst of Co with porous silica support is more selective toward hydrocarbons of C₁₀ to C₂₀  (diesel) (52). Study of Jincan Kang et al. showed that nanocatalyst of Ru are more favourable in the high moisture content, but the only issue they have low activity. This issue was solved by using carbon nanotube supported ruthenium catalyst, it gives excellent selectivity for diesel hydrocarbons and also high activity of the catalyst (52, 57).

Solid catalysts of Aluminium dodeca-tungsto-phosphate (Al_0.9 H_0.3 PW₁₂ O₄₀) or nanotubes of   Zn_1.2 H_0.6 PW₁₂ O₄₀ are used for esterification of waste cooking oil to produce biodiesel, these catalysts improved yield from 42.6% to 96%. It is due to improved surface area of the nanocatalysts from the bulk conventional catalyst (52). Animal and used cooking oil are an alternative source of diesel fuel to run the vehicles (58). But high water content and the high ratio of free fatty acid make them unfavourable for the production of diesel fuel. Sulfuric acid with a large quantity of methanol at high temperature and pressure reduce the quantity of free fatty acid. High pressure and high temperature increase the diesel fuel production cost (59). According to the study of Li J et al. (2009) shows that ZnPW nanotubes can be used to reduce free fatty acid at lower temperature and pressure.

Nanoparticles also used in fossil fuel sector to improve the reaction, selective hydrogenation, paraffin hydrogenation, naphtha reforming and hydrodesulphurization are some major units where nanocatalysts are used in oil refineries.  Nanoparticles Hexanethiol monolayer protected Palladium of size 1.5 nm are used to improve energy consumption of catalytic combustion of JP-10 activation fuel. These nanoparticles decrease the ignition temperature to 240 ^oC (60, 61).

John Meurig et al. studied the bimetallic nanocatalysts of size range 3-30 nm with nanoporous silica support in single step hydrogenation and find that this catalyst greatly facilitates the separation of product from reactant with easy recycling at low temperature and pressure conditions (62).

Hydrogen gas can be used in fuel cell to generate green energy, now almost 95% of hydrogen produced by using partial oxidation and steam reforming of hydrocarbons. Both oxidation and steam reforming produce carbon dioxide, which is a major constituent of global warming (63). Catalytic reforming of green biofuel like ethanol can be a green way for the production of hydrogen gas, ethanol usually available with high water content, the water present in ethanol and other impurities deactivate the conventional catalysts. Nicolas Bion et al. (2005) worked on the designing of nanocatalysts for the production of hydrogen gas by using ethanol and concluded that nanocatalyst Rhodium metal with alumina support gives the highest yield of hydrogen gas with a better protection from water. Rhodium catalyst produces cleavage between and Carbon and hydrogen, and this is the main reason of C-C bond rupture (64).  Hydrogen in fuel cell gives 50% to 60% efficiency while in diesel engine only 20-25% fuel converted into useful energy (65, 66).

In conclusion, nanoparticles are used to increase the selectivity and activity of catalyst which resulted in less energy consumption and high yield. They are replacement of costly metallic catalysts, by decreasing the size to nanoscale, particles offer much time greater surface area as compared to the bulk catalysts, increase the reaction rate at lower temperature and pressure conditions. Nanocatalysts are energy efficient, they reduce the global warming by the reducing the required energy for any catalytic process with the less poisoning ratio. It also decreases the chemical wastes, these all factors resulted into an improved economy and safer environment.

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