The Role of Heterogeneous Catalysts in the Chemical Industry
Catalysts are elements, compounds or substances that, ideally, very active, highly selective in their action and can remain useful for a long time. Catalysts participate in transformative reactions or a reaction for synthesis without being used in these reactions. Therefore, catalysis is the ability of a substance to facilitate the reaction of other elements without being consumed in the same. Catalysis is an important technical principle that combines both economic and ecological values. Many of the manufactured products and chemicals in use have undergone a catalytic process at some point. According to (Das, et al., 2011, p. 1), catalytic reactions/processes contribute up to 38% of the global GDP. Many industries, ranging from petroleum, automotive industry, plastics manufacture and others employ the use of a catalyst at some point (Deutschmann, et al., 2009, p. 2). According to (Heveling, 2012, p. 1530), over 85% of all manufactured chemical products are produced with the aid of a catalyst. Catalysts are used in environmental abatement processes and environmental conservation processes such as removal of harmful gases from vehicle exhausts all of which have no economic benefit (Heveling, 2012, p. 1530).
The main objective of using catalysts is to increase the rate of the reaction to make the process economically viable. In an equilibrium system, the rate of reaction between the reactants will increase but the equilibrium composition will be sustained. This is because the catalyst catalyses both the forward and reverse reactions. Therefore, catalysis is not a thermodynamic process but a kinetic process. A catalyst can occur in all three states: liquid, solid or gas, but are typically used as either solids or liquids. Catalysts work by creating complex structures between their surfaces and the reacting reagents. This opens them up to then react and form the final product or products by providing an alternative path with a lower activation energy. This inevitably increases the rate of reaction (Deutschmann, et al., 2009, p. 2).
Catalysts can be categorized depending on the phases of the products or reactants and the catalysts in which the reaction occurs. They can be categorized as homogenous or heterogeneous catalysts. If the reactants and the catalyst occur in the same physical phase, the catalyst is known as a homogeneous catalyst. For example, cobalt and manganese benzoates catalyse the formation of benzoic acid via the oxidation of toluene. On the other hand, if the reactants and the catalyst are in a different physical state then the catalyst used is a heterogeneous catalyst. A reaction involving liquids or gases, such as the formation of ammonia through the Haber process, can be catalysed by a solid catalyst which is pure iron in this case. This forms the biggest advantage of heterogeneous catalyts since the products of the reaction and the catalyst can be easily seperated. Bilological reactions are also catalysed by enzymes in process known as biocatalysis. Enzymes are, therefore, biocatalysts. (Chorkendorff & Niemantsverdriet, 2007, p. 5).
To comprehend the functioning of heterogeneous catalysts and their influence in the chemical industry
To elaborate the methods by which heterogeneous catalysts are produced.
To study the effect of a heterogeneous catalyst on a specific chemical reaction.
To study the advantages of heterogeneous catalysts that make them suitable for industrial processes.
Heterogeneous catalysts can further be categorised depending on how they are used. They can be categorised as metals alone whereby a pure metal catalyses a reaction. They can be in the form of metal powder or gauze. Another category is metals plus other components. Here a metal can be used as a metal oxide, sulphide or salt. If the heterogeneous catalyst is supported by another structure, it is known as a supported catalyst. These can be supported metals or metals plus other components. The support used is also a type of heterogeneous catalyst since it can participate in the reaction (Chorkendorff & Niemantsverdriet, 2007, p. 6).
Figure 1 shows the cyclic nature of heterogeneous reactions. Reactants, in this case CO and O2, get adsorbed, react and get desorbed continuously on the same active site.
Figure 1: Reaction cycle for the catalytic oxidation of CO.
Catalysed reactions usually occur on the surface of the catalyst. In heterogeneous catalyst, it is believed that the reaction does not occur on the entire surface of the catalyst but on specific places known as active sites. Irregularities on the surface can cause the occurrence of unsaturated atoms which then form the active sites. Atoms can also possess chemical properties that enable the reaction to take place. The number of active sites available for reaction is directly proportional to the activity of the catalyst. The activity of the catalyst is expressed in terms of turnover frequency (Das, et al., 2011, p. 3). Active sites can be occupied by elements or compounds through adsorption thus blocking reactions from taking place. These substances that block active sites are known as poisons. A novel advantage of heterogeneous catalysts is that they work despite being exposed to several poisons (Deutschmann, et al., 2009, p. 4).
The first step in a heterogeneously catalysed reaction is whereby a molecule arrives on the surface of the catalyst and gets adsorbed onto the surface in a process known as chemisorption. Chemisorption is characterised by an enthalpy change of approximately 80 kJ mol-1. The reactant gets diffused from the pore mouth of the active site onto the internal catalytic surface. A very strong bond is formed between the reactants and the catalytic surface. Due to the immense strength of this chemical bond, heterogeneous catalysts are very specific to the reactions they can catalyse (Farnetti, et al., 2010, p. 5). The reactants then react on the surface to form the products. Lastly, the product is desorbed from the surface of the catalyst. The product gets diffused from the internal surface, through the pore mouth, to the external surface. The slowest reaction rate in this chain of reaction forms the overall rate of reaction of the system (Chorkendorff & Niemantsverdriet, 2007, p. 7).
Figure 2: Steps in solid catalytic reactions.
High cost metals are often used as solid catalysts. In many industrial processes where a catalysed is used, it is paramount to reduce the costs and maximize returns. However, the bulk of atoms occur in the lattice of a metal than on the surface therefore, using the catalyst in bulk would raise the operation costs and reduce efficiency. In order to reduce the cost, the catalyst is used as a powder which has a much higher surface area to volume ratio. The powder or granulated metal can be placed on a support (Deutschmann, et al., 2009, p. 9).
Catalysts break the bond holding molecules together. This is their most significant contribution. Once the bonds are broken, a favourable thermodynamic regime is created allowing the reaction to occur with lower activation energies. Had the reaction taken place without the catalyst, a huge amount of energy would be required (Chorkendorff & Niemantsverdriet, 2007, p. 9).
Applications of Heterogeneous catalysts.
Considering the Haber process for the formation of ammonia, the active sites on the catalyst (iron) are denoted by “*”. Nitrogen molecules and Hydrogen molecules are chemisorbed onto the surface of the iron. The two gas molecules dissociate into their respective atoms as follows:
〖3H〗_2+6^* □(↔┴ ) 〖6H〗^*
N_2+ 2^* □(↔┴ ) 〖2N〗^*
Nitrogen and hydrogen then proceed to react and form ammonia. Since ammonia is relatively stable, it is desorbed from the surface of the iron (Notheisz & Smith, 2009, p. 50). As the ammonia gets desorbed, the active site gets freed up to react with other molecules.
〖2N〗^*+〖6H〗^* □(↔┴ ) 〖NH〗_3+8^*
Another very important application of heterogeneous catalysts is the hydrogenation of organic functional groups. The differences noted in the use of hydrogen saturated or non-saturated catalysts is dependent on the metal used to catalyse the reaction (Notheisz & Smith, 2009, p. 54).
Production of Heterogeneous Catalysts
The reaction characteristic properties of solid catalysts are greatly influenced by the method of preparation, the underlying conditions of the preparation and the quality of the source materials. In order to ensure maximum quality of product, therefore, it is essential to monitor and control the physical, chemical and mechanical properties as well as the conditions in each production step. Advanced tools such as SPC (Statistical Process Control) and CAE (Computer Aided Engineering) are used to facilitate this. The method of production is dependent on the type of catalyst i.e. supported and unsupported catalysts (Deutschmann, et al., 2009, p. 44).
Unsupported catalysts are those whereby the catalysts is exposed to the reactant as itself and not on another substrate. One method used to manufacture them is known as mechanical treatment. In order to stabilize the structure or pore forming agents, the active materials or their precursors are kneaded, mixed or milled together with agents or promoters. Another method used is known as precipitation and coprecipitation. In this method, concentrated solutions of different salts are formed and then crystallised after undergoing an ageing process. Unsupported catalysts have lower activity and selectivity (Deutschmann, et al., 2009, p. 48).
Supported catalysts are those in which the active substrate is placed on the surface of a support structure and forms a small part of the entire catalyst. The support may be passive or may take part in the reaction. Adsorption is one method used to prepare the catalysts. In this method, particles or powders adsorb equilibrium quantities of salt ions when exposed to their solutions. Other methods are such as precipitation of the material onto the support, ion exchange or mechanical treatment.
Catalysts are widely used in almost all chemical production industries. They can be categorised as homogeneous or heterogeneous depending on the phase of operation. Heterogeneous catalysts are the most widely used since they can be easily separated from the reactants and re-used or treated. Disintegrating the catalyst from a large bulky structure into a powder increases the number of active sites and therefore, rate of the reaction.
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