Catalysis is a catalyst-based reaction and nanocatalysts are the catalyst of nanoparticles. These nanocatalysts are used to increase the efficiency and to produce of reaction. The study on the role of nanoparticles in catalysis has become the most fascinating area of new science and research to fulfill the demand of the increasing population in the world. The products and compounds that are formed from the nano catalytic activity have great importance in many fields. Nanoparticles are the leading term in nanoscience, especially in catalysis reactions. Nanocatalysts provide alternatives to fossil fuel, toxic drugs, toxic food atoms. Nanocatalysis reactions are pollution-free reactions. All of the nanoparticle catalysts-based reactions depend upon the size, shape, and distribution of nanocatalysts on the surface of substrates. The potentials of nanocatalysts in the different fields of reactions involved in hydrogenation, dehydrogenation, hydroxylation, and many redox catalytic reactions. Nanocatalysts have different structures for their function, these structures are nanoporous surface, nanomaterials, nanowires, and nanoparticles.
Catalysis is a catalyst-based reaction and nanocatalysts are the catalyst of nanoparticles. These nanocatalysts are used to increase the efficiency and product of the reaction. The study on the role of nanoparticles in catalysis has become the most fascinating area of new science and research to fulfill the demand of the increasing population in the world. The products and compounds that are formed from the nano catalytic activity have great importance in many fields. Nanoparticles are the leading term in nanoscience, especially in catalysis reactions. Nanocatalysts provide alternatives to fossil fuel, toxic drugs, toxic food atoms. Nanocatalysis reactions are pollution-free reactions. All of the nanoparticle catalysts-based reactions depend upon the size, shape, and distribution of nanocatalysts on the surface of substrates. The potentials of nanocatalysts in the different fields of reactions involved in hydrogenation, dehydrogenation, hydrosilylation, and in many redox catalytic reactions. Nanocatalysts have different structures for their function, these structures are nanoporous surface, nanomaterials, nanowires, and nanoparticles.
KEY WORDS: Nanocatalysts, catalysis, fuel cells.
Introduction
As the population of the world is increasing, the amount of food and the consumption of energy is also increasing that resulting in the reduction of fossil fuel and environmental problems. In this way, everyone wants to find alternatives to fossil fuel (Gong et al.2009), to produce inexpensive food, to clean the environment, and to protect humans from toxic substances (Ferreira et al.2008), (Graczyk et al.2007). So, to bring all these things we have needed to perform catalysis in which catalysts are of great importance. Particles of the size from 10 to 10^6 atoms are called nanoparticles (Poole et al.2003). Nanoparticles play an important role in catalysis, as they mimic the activation of the surface and catalysis takes place at the nanoscale. The catalysts used in the nanoparticles catalysis are heterogeneous catalysts that are broken into the nanoparticles to increase the process of catalysis. These catalysts are called nanoparticle-based catalysts (Bahrami et al. 2016). The nanoparticles catalysts make ensure the no composition of nanoparticles when mild conditions are provided (Zalesskiy et al.2012). Nanoparticles are very easy to use, spread, and recycle. The structure of nanoparticles catalysts affects the activity and selectivity of both nanocatalyst and substrate. So, colloidal chemistry plays an important role in the control of size, shape, and in composition of nanoparticles (Tao et al.2014). Ni and Rh nanocatalyst have specific sizes in the hydrogenations Nanoparticles were used as curative medicines and to decorate glasses in China and in Egypt for many years. In the 17th century, nanoparticles were first studied as a catalyst (Antonii et al.1618). The first article on nanoparticles as a catalyst was published in 1941 that was alcohol-protected palladium and platinum nanoparticles catalysts (Rampino et al.1941). this research was carried by Paul Sabatier and Toulouse, they discovered nanoparticle nickels as a catalyst when they catalyzed the hydrogenation using fine divided particles nickels. The nanoparticles of metals are very active even the metals of low transition but till now 12 to 20 metal atoms are used as catalysts in the catalysis having sizes of 1 nm or less (Imaoka et al. 2015). The nanoparticles of 1nm are called nanoclusters and nanoparticles of less than 1 nm size are called sub-nanocluster. These nanoparticles and clusters have different states from molecular stat to solid stat. So, to define the nanocluster we use molecular orbital, and to define the larger nanoparticles we use energy band structure. In nanocluster, there are precise polymetallic molecules with ligand but nanoparticle term is used for those mixtures that have polydisperse of nanocluster. All these types are based on the nature of the atom, type of ligand, and dispersity of them. There was a small and even large cluster of metal-metal bonds found in the catalysis that clear they catalysis is independent of structural integrity (Chini, 1980). these days most people are in the interest of using nanoparticles as heterogeneous catalysts in catalysis, even the catalysis work at high temperature in the presence of nanoparticles catalyst (Astruc et al.2005). The heterogeneous catalysts are used with zeolites, microstructures ( silica, alumina), and other oxides that stabilize the nanoclusters in the catalysis (Polshettiwar et al.2010). The heterogeneous catalyst not only stabilizes the nanoparticles but also acts on the surface of nanoparticles to activate the substrates by comparing the positive energy of these nanoparticles and substrates (Sankar et al,2012). Complex nanoparticles are also used in catalysis, for example, MoS based nanoparticle complex used as a cofactor of nitrogenase enzymes that catalyze the fixation of N2. These catalysts are useful in the catalysis of H2O, N2 and CO2 (Kornienko et al.2019). In recent attentions the scientists are in trying to decrease the size of nanocatalyst from clusters to a single atom catalyst (Qiao et al.2011). Single metal catalysts find their use better in the catalysis as they are less selective and less plydispersed compare to sub nanocatalysts. In oxidation nanoparticles especially metal nanoparticles have general catalytic activity. Nanoparticles form metal-organic framework by atomic layer deposition that offer confined spaces with regular dimensions in which nanoparticles catalyst become stable (Kumar et al.2016). the oxidizing and reducing agent are activated by nanoparticles. For example hydrazine, NaBH4 and aminoborane are activated by nanoparticles in the reduction. In the recent time the gold become one of the most attentive nanoparticle with oxide and carbide support because among the transition metals it is less reactive. (Haruta, Ctal ,1997). Bulk metallic gold is not of great concern in the activation of surface of substrates but the single crystals of the gold are very chemical active due to transition state. The concept of nanocatalysis is developed from heterogeneous and homogeneous catalysis, material science, surface science, bioinorganic and from catalysts. Nanocatalysis will become more benefit and useful when it become green-nanocatalysis mean free from toxic elements. In all the fields the nanoparticles are in most use than other types of particles, So, scientists are in trying to reduce the size further of nanoparticles by controlling the morphology, face orientation of nanoparticles, stability of NPs without any cause to activity. Scientists also try to replace the noble and critical metals with the metals of other transition state that are most abundant in the nature and less toxic to organisms.
Preparation of NPs catalysts
For good performance in the catalysis the size, shape and distribution of NPs should be proper. This show that the size of particles influence the selectivity and reactivity of catalysts during the reaction. So, to prepare catalyst the study of catalytic properties is importance (Coq and Figueras, 2003). There are many methods to design a catalysts of nanoparticles which are divided as a (i) physical method and (ii) chemical method. In physical method ion implantation, pulse laser deposition, evaporation that include (thermal and electron beam) and sputtering are involve. (Moser, 1996). In chemical method sol-gel, chemical vapour synthesis, impregnation, hydrothermal and precipitation is involved.
preparation of nanoparticles catalyst by sputtering
In sputtering we make a bombardment of positive ions rays of high energy on the surface of solid cathode which is our target. These sputtered atoms leave the target surface with many electronvolts energy (Othring,1992). In sputtering we add metallic and semiconductors of nanoparticles in the dielectric which should be transparent (Xu et al.2009). This transparent dielectric is used in the sputtering of nano metals to make a metal fraction and to distribute the metal particles. This chemical method of sputtering is of low cost and use in the preparation of Ag: SiO2 catalyst and in the preparation of Au-Ag: SiO2 catalyst.
preparation of nanocatalyst by sol-gel method
Sol-gel method is used to prepare the nanoparticles of noble metals in the presence of silica and titania matrics (Brinker and Scherer, 1990). In this method we require low temperature, homogeneity and well composition of metal that result in the adjustable concentration to prepare nanoparticles. To make useful product of particles we also add oxidizing agent and reducing agent in the concentration (Garcia et al.1999). We can prepare Au, Cu, Pt and Ag catalysts by sol-gel method in the glassy matrics.
Potential Application of NPs in Catalysis
NPs catalysts have several applications in the production of fuel, food, sensor and to produce fuel cells. NPs catalyst are used to generate, conserve and to store the energy. All these product produce by a catalysis reaction that include hydrogenation reactions, Dehydrogenation reactions, Hydrosilylation reactions, organic redox reactions and C-C coupling reactions etc.
Dehalogenation and hydrogenation
In the hydrogenolysis of C-Cl bonds of polychlorinated biphenyls, the NPs catalyst show their activity (Panahi et al.2017), (Roucoux et al.2002). To synthesize herbicides and pesticides we make hydrogenation of halogenized aromatic amines in the presence of NPs catalyst. This hydrogenation of halogenized amines is also use to produce diesel fuel (Panahi et al.2017). The hydrogenation of C-Cl with H2 in the effect of kinetic isotope to locate the aromatic ring in experiments. Buil et al. prepared rhodium nanoparticles by making a rhodium complex. Theses rhodium nanoparticles are use in the hydrogenation of benzene ring to cyclohexane and in the dehydrogenation of aromatic compounds (Roucoux et al.2002). Nanoparticles catalysts are also use in the hydrogenation of cinnamaldehyde and citronellal (Yu et al.2000), (Yu et al.1998). the ruthenium nanocatalysts are more selective in the hydrogenation of citronellal.
Hydrosilylation reactions
The nanoparticles catalyst use in the hydrosilylation reaction are formed by the reduction of palladium, platinum, nickel, cobalt and gold with the silanes. All these catalysts catalyze the hydrosilylation reactions (Tamura et al. 2003). The nanoparticles of the palladium and the nanoparticles of the gold are use in the hyrosilylation of the styrene in mild conditions. The nanoparticles of palladium are found more active in the catalysis reaction hydrosilylation as compared to the non-nanoparticles of palladium (Tamura et al.2003), (Leeuwen et al.). hydrosilylation reaction can also be catalyzed by nanoparticles consist of two types of metals (Lewis et al.1986).
FIGURE 1; Hydrosilylation reaction
Organic redox reactions
Organic redox reactions Cobalt nanoparticles are used as a catalyst in the oxidation reaction to form a adipic acid (Panahi et al.2017). adipic acid is used in the industry to form nylon 6,6 polymer. Metallic nanoparticles aalso use in the catalysis oxidation reaction of glucose, in oxidation of cyclooctane and in the oxidation of ethane (Panahi et al.2017).
FIGURE 2; Oxidation reaction of cyclohexane to synthesize adipic acid
C-C coupling reactions
C-C coupling reactions are Suzuki coupling reactions, Heck coupling, synthesis of vitamin E and the hydroformylation of olefins. In all of these C-C coupling reactions metallic nanoparticles are used as a catalyst that speed up these reactions (Panahi et al.2017). Palladium nanoparticles are found as a more active catalyzer in the Hack coupling reactions. The catalytic activity of palladium nanocatalyst increase when we increase the electronegativity of ligand in the reaction (Zalesskiy et al.2012).
FIGURE 3; Heck coupling reaction
The source of Pd(0) is the compound Pd2(dba)3 which is used to make the nanoparticles of palladium that is use in the catalysis reactions e.g. in the catalysis cross coupling reaction (Zalesskiy et al.2012). In the past Pd2(dba)3 was considered as a homogeneous type of catalyst but in recent time many research article suggest it as a heterogeneous precursor of catalyst. Pd2(dba)3 is a source of heterogeneous nanocatalyst of palladium.
Alternative fuels
Nanoparticles are also use in the reactions that convert the gases into the liquid fuels that we use as a alternative of fuels. Iron oxide and cobalt are used as a nanocatalyst in the conversion of gases into liquid hydrocarbon like fuel. In the reaction that involve the conversion of carbon monoxide and hydrogen into liquid hydrocarbon, we add iron oxide and cobalt nanoparticles on the surface of active ligand like alumina to covert the gases into liquid fuel by using Fischer Tropsch process (Vengsarkar et al. 2015), (Khodakov et al.2007). Researcher are in trying to increase the effectiveness of nanoparticles catalyst in the fuel catalysis reactions to increase the fuel production, So in this production of fuel the platinum is one the most common nanocatalyst although platinum catalyst is highly expensive. To make a less expensive and alternative of platinum nanocatalyst the nanoscientists are in struggle to make more and more nanoparticles catalyst of other metals with maximixe catalytic activity by shrinking metals in nanoparticles. For example the nanoparticles of gold found to have some catalytic activity in the catalysis reaction, but the bulk metal of gold is inactive in the catalytic reaction, so we convert bulk gold into the nanoparticles to perform catalytic reactions. The nanoparticles of zirconium metals which are stabilized by yitrium are found in increasing the catalytic activity and efficiency of solid fuel cell (Ananikov et al.2007). In the hydrogen stage of fuel production the nanomaterial catalyst ruthenium and platimum are used to purify the hydrogen (Beal and James,2012). To clean the environment from pollution, the CO and NO are oxidized in the catalytic reaction in which organometallic ligands are use to functionalized the nanomaterial catalyst palladium that proceed the oxidation of CO and NO (Moshfegh,2009). Nanoparticles catalysts are use to catalyze the production of carbon nanotube in which nanoparticles catalysts act as cathode and provide support to fuel cell. As discuss earlier that nanocatalyst is also consist of two metals, so bimetallic nanoparticles platimum-cobalt is also use in the growth of carbon nanotube in which paltimum-cobalt act as a electrode in methanol fuel cells (Moshfegh,2009).
Medicines
Nanomaterials catalysts are also use in the industry of medicines to support catalytic activities of enzymes.
Nanozymes
The nanoparticles with enzymes are termed as a nanozymes (Wei et al.2013). in nanozymes catalyst ,the nanoparticles are use to enhance the action of natural enzyme by mimicking the catalytic activity of enzymes.
Many nanoparticles of metals are used to mimic the catalytic activities of many varieties of enzymes. Peroxidase, catalase, oxidase, SOD, and nuclease etc. are ntural enzymes whose catalytic activity is enhanced by nanoparticles In water treatment, in therapeutics tests, in bioimaging, in biosensing and n many other fields the nanozymes are used.
Nanostructures in the electrocatalysis
Nanomaterials as a catalysts has a great importance in electrocatalysis of fuel cells and electrocatalyzer. There are following nanostructures that play an important role in the catalysis reaction called electrocatalysis
Nanoporous surfaces
In electrocatalysis that take place in the fuel cells, we use the nanoporous materials to make a cathode. The nanomaterials of platinum with nanoporous surface have very reactivity in the electrolysis reaction. But the nanomaterials with nanoporous surface are less stable and have short lifetime (Bae et al.2012).
Nanoparticles
The use of nanoparticles have one problem of accumulation in the electrocatalysis reaction. to overcome this problem we must use appropriate and specific catalyst support. Nanoparticles are the optimal nanostructures which have ability to detect specific molecules, so we can use these nanoparticles as a nanosensors. Palladium can be use as a nanosensor, Pd has ability of electrodeposite on the carbon nanotubes wall. Therefore, palladium use as a catalyst in the cross coupling reactions in which Pd act as a good active nanocatalyst (Radtke and Mariusz,2015).
Nanowires
Nanowires are other nanostructures that are used in electrocatalysis reactions. Nanowires have the greater surface of the reactants to become activated. Therefore, nanowires are easy to control and easy to produce, and mostly used in electrocatalysis reactions (Mistry et al.2016)
Nanomaterials
We use different nanostructure materials in the process of electrocatalysis. The different nanomaterials have different electrocatalysts abilities, so nanomaterials are very important to stabilize the physical and chemical conditions of electrocatalysis reaction. nanomaterials have good conductivity and are less costly (Hu et al.2019). The nanomaterials are made by transition metals. The common transition metals are iron, cobalt, nickel, platinum which are the main sources of nanomaterials. These metals are extracted from ores and from other combined states. Every metal has different properties, so nanomaterials of these metals also have different properties of selectivity and activity. By the modern techniques, expensive and rare metals are also used to make nanometals (Strasser et al.2010). In the future, stable materials with more activity and with low cost will be available because many nanomaterials like metallic glasses and many carbon nitrides are extracted from the metal-organic frameworks. These materials will be used as a catalyst in the future (Wang et al.2019), (Li et al.2018).
Photoctalysis
Nanoparticles are also used in the catalysis reaction of photocatalysis where they act as a co-catalyst with semiconductor quantum dots (QD). To make a photocatalytic reactor for water splitting where we place the water solution in the reactor witch has semiconductor quantum dots coupled with nanomaterial and both act as co-catalyst. In this design of photocatalysis reactor, the quantum dots take electromagnetic radiation and make them excited while nanocatalyst act as a co-catalyst controls the overall reaction in the reactor (Chen et al.2017).
conclusions
Nanoparticles catalysts are important in many technological and industrial fields. As we discussed earlier the use of nanoparticles in a different field but is expected that the nanoparticles catalysts will have great importance in bioscience, in molecular science, in metallic science, and in many other fields that have catalysis reactions. The nanoparticles catalysts will make able to control the catalytic activity of the catalysis reaction, as the nanoparticles catalysts are very specific to their activity and selectivity. By controlling the nanoparticles’ activity we may perform many catalytic reactions according to our demand. We can also improve the performance of the reaction by using nanoparticles catalysts. In future the nanoparticle-based catalysts will be used as the alternative to fuel energy which produces safe byproducts to reduce air pollution. Nanocatalysts will use as a biosensor to identify the diseases and to find default in the cell working. So, focused on these beneficial aspects of nanoparticles catalysts the investor have changed their mind to invest in nanocatalysts to find a new development in the world.
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