The environment is affected directly or indirectly by various industrial outputs, such as the discharge of toxic effluents in water. Applications of nanocatalysts enable better yield of products by using less energy and minimizing waste production, protecting the environment from various harmful aspects of industrial processes. This article discusses the various types of nanocatalysts and their environmental applications.
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Catalysis and Nanocatalysis
In a chemical reaction, a catalyst is a substance that enhances a reaction rate without being consumed. For instance, enzymes are naturally occurring biocatalysts that take part in innumerable biochemical reactions. Besides increasing the reaction rate, catalytic reagents can also reduce the reaction temperature, decrease the reagent-based waste, and promote optimum reactions to obtain a high-quality product.
Typically, perfectly designed catalysts are preferred over stoichiometric reagents. This is because stoichiometric reagents are used in large quantities to perform a single reaction, whereas a small amount of perfectly created catalytic reagents are capable of performing a single reaction multiple times.
Catalysis can be categorized as homogeneous, heterogeneous, or enzymes based.
Catalysis is one of the most critical applications of nanoparticles. Scientists have designed nanocatalysts based on homogeneous and heterogeneous catalytic frameworks. The application of nanocatalysts enables rapid chemical conversion with a higher yield and simplistic catalyst separation.
In a homogeneous nanocatalysis reaction, the nanosized catalysts remain in maximum contact with reactants; however, in the case of heterogeneous nanocatalysts, the reactive solvents are insoluble.
Several nanomaterials of aluminum, iron, titanium dioxide, and silica are utilized as catalysts. The large surface-to-volume ratio, small size, and varied shapes of nanomaterials are highly beneficial to catalytic reactions.
Applications of Nanocatalysts
As stated above, owing to the unique size, shapes, and properties of nanomaterials, scientists have designed many nanocatalysts with wide-ranging functions. Some of the applications of nanocatalysts in protecting the environment directly or indirectly are discussed below:
Photocatalytic Decolorization
Large amounts of textile dye (e.g., malachite green dye, methylene blue, azo dyes, etc.) effluents are often discharged into water bodies without processing, negatively affecting aquatic ecosystems.
For example, marine organisms are affected by the coloration of the dye effluents, the toxic substances present in the dye-containing wastewater, enhancement of the chemical oxygen demand (COD), and biochemical oxygen demand (BOD) level. Coloration in the water bodies inhibits the penetration of the sun’s rays and, hence, the photosynthetic activity of aquatic flora is affected.
Many conventional processes, such as ozonation, incineration, and adsorption on solid phases, which have been introduced to process dye effluents, have certain limitations. However, a new method called heterogeneous photocatalysis, which utilizes nanocatalysts (e.g., ZnO, Nb2O5, and TiO2), is a powerful alternative that can degrade harmful organic dye contaminants.
Among the mentioned nanocatalysts, TiO2 nanotubes are most favorable because of their high catalytic efficiency, high chemical stability, cost-effectiveness, and toxicity. Another advantage of this nanotube is that it can be recycled and reapplied in many photodegradation cycles. According to new research, TiO2 nanotubes could maintain 80% activity even after ten cycles of reaction.
Production of Biofuel
The combustion of fossil fuels produces many harmful gases that negatively affect the environment. Additionally, the reduction in petroleum reserves has prompted scientists to search for alternative renewable fuels.
Biofuel is the monoalkyl esters of fatty acids, regarded to be a promising alternative renewable fuel. Many countries such as Germany, Japan, France, Italy, and the USA are currently replacing petroleum fuel with biofuel. Recently, biofuels have been prepared using the ‘‘green” method based on heterogeneous catalysts.
Heterogeneous catalytic methods have certain disadvantages: they are typically resistant to mass transfer, time-consuming, and inefficient. These shortcomings can be overcome with the development of nanocatalysts, which possess high specific surfaces and have greater catalysis abilities.
Scientists have reported that the use of solid base nanocatalyst KF/CaO in biodiesel production yielded more than 96% of biofuel. An X-ray diffraction (XRD) analysis revealed that this nanocatalyst has a large specific surface area and pore size that effectively improved transesterification efficiency.
Hydrogen Energy Carrier
Carbon nanotubes (CNTs) have been applied in many fields, such as Li-ion secondary batteries, molecular sieves, electric nanoconductors, electric double-layer capacitors, and fuel cells. Due to its high porosity, light weight, structural stability, and non-expensive development, CNTs have recently been used to adsorb hydrogen.
Such a development is impactful as hydrogen has high energy content and no detrimental effect on the environment. The tubular structure of CNT makes it an ideal candidate for a hydrogen energy carrier. Scientists predict that using CNT as a hydrogen storage material could be widely used in the future.
Nanocatalyst in Solid Rocket Propellants
Recently, the use of aluminum nanoparticles, of approximately 50nm size, in the combustion of composite solid fuels, based on ammonium perchlorate and hydroxyl-terminated poly-butadiene binder has gained much attention. This is because aluminum nanoparticles could plateau the burning rate trends. Scientists have stated that replacing micro-aluminum with nano-aluminum has enhanced the propellant burning rate by around 100%
Water Purification
The use of silver (Ag) nanocatalysts in water purification is a popular strategy as this catalyst is highly efficient in controlling microbes in water. Additionally, Ag-supported catalysts are reusable.
Besides Ag nanocatalyst, the combination of Al2O3 and carbon is also used for water purification. This combination of nanomaterials possesses low acidity, high mechanical strength and contains mesopores, which are essential for the water purification process.
Wastewater Treatment
Halogenated organic compounds (HOCs) are one of the most dominating candidates of water pollution in industrialized countries. HOCs are toxic compounds that may cause serious health problems such as cancer or mutagenic damage. Therefore, during water treatment, the destruction of these compounds is mandatory. Nanosized palladium catalysts are used in wastewater treatment, either cyclic batch or continuous flow-through reactor.
Continue reading: Improving Catalytic Activities Through Nanoparticle Observation.
References and Further Reading
Incheon National University (2021) Biopolymer-coated nanocatalyst can help realize a hydrogen fuel-driven future. ScienceDaily. [Online] Available at: www.sciencedaily.com/releases/2021/02/210223121648.htm
Somwanshi, B.S. et al (2020) Nanocatalyst: A Brief Review on Synthesis to Applications. Journal of Physics: Conference Series. 1644 012046. Available at: https://doi.org/10.1088/1742-6596/1644/1/012046
Chaturvedi, S. et al (2012) Applications of nanocatalyst in new era. Journal of Saudi Chemical Society. 16 (3). pp. 307-325. Available at: https://doi.org/10.1016/j.jscs.2011.01.015
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