Free Heterogeneous Catalysis & Surface Modification For Wettability Literature Review Example
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Section 1: Catalysis
Catalysis is a process by which chemical reactions are hastened by small amounts of external substances, called catalysts. A suitable catalyst can alter the rate of a thermodynamically attainable reaction, but cannot disturb the position of thermodynamic equilibrium. Most catalysts are solids or fluids, however they may likewise be gasses. Catalysis is a cyclic process. The reactants form a chemical complex with the catalyst, subsequently opening a pathway for their conversion into the respective products, following which the catalyst is released and allows the cycle to continue. Now, catalysts do not have infinite lifetimes. As a result of frequent use, products formed in certain catalytic reactions, environment under which the catalyst is used, etc. can affect the catalyst structure which can lead to its deactivation. Usually, spent catalysts are either reused or replaced.
Types of Catalysis
If the catalyst and the reactants are in the same physical phase, then this kind of catalysis is known as homogenous catalysis. For example, metal salts of organic acids such as CH3COONa, Zn(CH3COO)2, etc. organometallic compounds, carbonyls of Co and Fe, etc. are typical homogenous catalysts. Some examples of homogenously catalysed reactions are the oxidation of toluene to benzoic acid using Co or Mn benzoate as the catalyst, the hydroformylation of olefins to give their respective aldehydes catalysed by the carbonyls of Co or Fe, etc.
When the reaction mixture and the catalysts are in different physical phases, the subsequent catalysis is called heterogeneous catalysis. Some examples of heterogeneous catalysts are metal oxides such as TiO2, ZnO, Pt, Pd, etc. Heterogeneous catalysts can also be organic hydroperoxides, ion exchanges, emzymes, etc. Some examples of heterogeneous catalysis are the synthesis of ammonia from its constituent gasses over solid iron, the hydrogenation of edible oils on Ni-kieselguhr catalysts in the liquid phase, etc. Electrocatalysis is another variant of heterogeneous catalysis where where oxidation or reduction is conducted by the transfer of electrons. For example, catalytically active electrodes are employed in chlor-alkali electrolysis and are widely used in fuel cells.
Photocatalysis is another kind of catalytic process that can be either homogenous or heterogeneous. Here, light is absorbed in the form of photons which result in the formation of electron-hole pairs on semiconductor catalysts which result in the acceleration of the chemical process. For example, oxides of Ti or Zn are used in the decolouration of several organic dyes as well as in cleaning applications. Another kind of catalysis is biocatalysis where enzymes or microorganisms are employed in facilitating certain biochemical processes. Here, the catalysts can be loaded and immobilized on some non-reacting materials like glass, or SiO2. A common biocatalysis example is isomerization of glucose into fructose in the production of soft drinks using the enzyme glucoamylase immobilized on silicon oxide substrates.
The primary principles of catalysis involve the coordination of molecules to central atoms, the ligands of which can be homogeneous or heterogeneous molecular species. Although subtle differences do exist in the different kinds of catalysis, on an elementary molecular or atomic level, there is little to differentiate the difference between the two.
The classical definition of catalyst, that a catalyst is a material that alters the rate of a chemical reaction by keeping its thermodynamic state intact, gives signs that catalysis is indeed a dynamic process. When solid catalysts are used, the reaction conditions vary dramatically. For instance, the reaction temperature can vary as much as between 78K and 1500K and the pressure can vary between 10-9 and 100MPa. The reactants can be aqueous, gaseous or even dissolved in polar or non-polar organic solvents. These reactions can proceed thermodynamically, due to the absorption of photons or even due to electron transfer at a favourable electrode. Even though several theories try to explain all the observations made during a catalytic process, it is close to impossible to formulate a single theory that encompasses all these observations simultaneously. However, in this paper, some basic principles of heterogeneous catalysis will be explained comprehensively.
The Sabatier’s principle suggests the formation of an unstable reaction intermediate between the catalyst surface and the reactants. The intermediate formed must be stable enough to form in sufficient quantities and also unstable enough to easily decompose into the final products. The Sabatier’s principle is related to linear free energy relationships (Ertl, et al 2008). These equations deal with the heat of a reaction Q and the energy of activation, EA. Using an empirical parameter, a, where 0<a<1, Bronsted’s equation can be written as Δ EA=a ΔQ, where ΔEA is the change in activation energy corresponding to a change in the heat of the reaction. It can be easily seen that when the heat of the reaction increases, the activation energy decreases, which gives the reacting molecules an easier path through which the reaction can proceed, thereby accelerating the rate of the reaction. The Bronsted equation bridges thermodynamics and kinetics and along with the Sabatier principle allows a comprehensive interpretation of the volcano plots. Volcano plots are a result of the rate of the reaction plotted against a parameter that gives the stability of the intermediate (Baladin 1969). This parameter can be the heat of adsorption of one of the reactants, or the heat of formation of a product relative to the surface compound, or it could simply be the position of the catalyst along the periodic table.
Principle of Active Adsorption Sites
For Sabatier’s principle to hold, there must be sufficient chemical bonding between the surface of the catalyst and the reactants, preferably between atoms or functional groups of the reactants and the surface atoms. This theory, proposed by Langmuir forms the basis of the principle of active sites. For instance, in the chemisorption on metal surfaces, there are energetically identical non interacting sites on the metal surface that adsorb just one free molecule in the gaseous phase. This theory resulted in the Langmuir adsorption isotherm and these sites are considered as the active sites.
This principle is not restricted to metal catalysts alone. Active sites for adsorption can include metal cations, anions, anionic groups, acid-base pairs, organometallic compounds, immobilized enzymes etc. A necessary prerequisite for any site to be active is that they should be accessible for chemisorption in the fluid phase. The amount of surface that is active for catalysis can be determined by the catalytic reaction. This means that the surface of the catalyst alters itself to satisfy the reaction conditions. This particular alteration in the catalyst surface is carried out so that the change in the free surface energy is minimum.
Modifiers and Promoters
The performance of heterogeneous catalysts is adjusted by certain materials called modifiers or additives. These modifiers are also called promoters when they increase the rate of the catalytic reaction in terms of reaction rate per available site. Modifiers sometimes deteriorate the catalyst’s performance, in which case it is referred to as a poison. However, it is difficult to distinguish between a promoter and a poison in cases where more than one product are formed of which just one is the desired one. Here, not just high activity but high selectivity becomes an important factor in distinguishing the type of modifier. For instance, in exothermic reactions, high temperatures created locally can lead to the formation of unwanted products such as CO2, or NO2. As a result, sometimes modifiers are required to slow down the rate of the reaction so that the temperature and state of the reaction can be maintained. Even though the modifier behaves like a poison in this case, it is still a promoter since it possesses high selectivity and results in higher yields.
Modifiers can alter the binding energy, the bond strength, etc. of an active site that was discussed earlier and also disrupt its atomic arrangement. This usually occurs when there is alloying between an active and an inactive metal catalyst. For example, in the synthesis of ammonia by Haber’s process over iron catalyst, Al2O3 and K2O promote catalysis. Al2O3 prevents iron from rapidly sintering and consequently stabilizes more sites on iron that favours catalysis. K2O, however, facilitates in the dissociation of N2 and the binding energy of nitrogen on subsequent iron sites.
The Catalytic Cycle
The defining factor that determines whether or not a given material is a catalyst is the catalytic cycle. According to Boudart, a catalyst is a material that converts the reactants into products through uninterrupted and repeated cycles of elementary steps where the catalyst is transformed into several reaction intermediates, which, in the final step recombine to give the catalyst in its original form. In certain cases, it is possible that the active sites may not be present on the catalyst initially. However, these sites are created as a part of the several intermediates that are formed in the cycle. The number of cycles determines the life of the catalyst (Mizuno & Misono 1998). This should be greater than one or else the catalyst would be a reagent and the reaction will proceed stoichiometrically rather than catalytically. Kinetically, the reaction intermediates are treated as quasi-steady state systems.
Heterogeneous catalysis is the preferred method in several commercially and environmentally important processes. Commercial processes such as the manufacture of NH3 from its constituent gasses, enzyme catalysed fermentation reactions, hydro-processing reactions, etc. rely heavily on catalysts in a different phase. Environmental remediation processes such as removal of oil from oil spills, automobile exhaust catalysts, catalytic oxidation of toxic gasses, etc. use metal catalysts in the solid form. Thus, this form of catalysis finds significant relevance both research and in industry with several improvisations being made in currently existing catalysts to improve their performance.
Section 2: Wettability
Wetting is a naturally occurring process responsible for many everyday observations. The forces of wetting are responsible for water droplets to bead up and roll off a duck’s back, or a newly waxed car. They are also responsible for binding grains of sand together in a sandcastle. The process that determines a solid’s preference to holding a particular fluid for wetting is known as wettability. The measure of the contact angle between the fluid and the solid surface gives the wettability of that surface to the fluid. A wetting fluid will occupy a large contact angle between the fluid and solid surface and can even displace a less preferred fluid from the surface. In extreme cases, the contact angle can be 1800. Alternatively, a non-wetting fluid will possess a much smaller contact angle. Calculations are made on a direct optical method by measuring the contact angle as the deciding data to determine the degree of wettability. The contact angle measurement also gives valuable insights into other parameters such as the solid surface tension that defines the wetting properties of the solid. Figure 1 shows the wetting of a solid surface by drops of different fluids.
Theory of Surface Wettability – Surface Tension and Young’s Equation
Consider figure 1. The contact angle is defined as the angle between the liquid solid interface and the liquid vapour interface. This is usually measured by measuring the angle at the point of intersection between the tangents drawn to both these interfaces. As can be seen, when the liquid spreads over the surface, i.e. when liquid “wets” the surface, the contact angle is small. When the contact angle is < 900, the fluid is usually considered as a wetting fluid and the wetting of the solid surface is favoured. Such fluids are called hydrophilic fluids. A non-wetting hydrophobic fluid will minimize the surface contact and consequently increase the contact angle. Superhydrophobic fluids have contact angles much greater than 1500 showing that there is almost no contact between the fluid and solid surface giving rise to the “lotus effect” (Lafauma & Quere 2003).
The surface tension of the liquid determines the shape of the liquid drop. In a pure liquid, every fluid molecule attracts every other fluid molecule cohesively in every direction resulting in a net force of zero. However, this balancing out of forces is not present on the surface of the fluid and molecules on the surface experience a net inward pull. Consequently, the liquid surface contracts itself to attain the energetically favourable lowest surface area configuration. This intermolecular force of attraction is called the surface tension of the fluid and is responsible for giving liquid drops their shape. However, external forces like gravity tend to deform the liquid shape. As a result, the shape of a liquid drop is due to the consolidated effect of surface tension and other external forces such as gravity.
As described by Thomas Young, the contact angle of a liquid drop on a solid surface is derived from the mechanical equilibrium of the drop under the action of all forces acting on it as shown in figure 1. Mathematically, this is given by:
γlv cosY= γsv- γsl
whereY is the contact angle and γlv, γsv and γsl are the liquid – vapour, the solid – vapour, the solid – liquid interfacial tensions respectively. This equation is generally known as Young’s equation.
Surface Modification to Tune Surface Wettability
The hydrophobicity or hydrophilicity of the surface depends heavily on the morphology of the solid – liquid interface. The surface morphology controls the molecular interactions between the solid and the liquid. The need for several sophisticated and biocompatible surfaces has resulted in extensive research in modifying surface properties such as roughness to tune wettability. Surface modification is necessary to facilitate fluid transport across surfaces and to help in the adsorption of biomolecules on biocompatible surfaces. Thus, tuning the wettability of solids finds immense applications in areas of microfluidics, biomaterials, protein adsorption, etc. (Synytska et al. 2004).
In recent years, techniques such as self assembled monolayers (SAMs), plasma polymerization, and laser patterning, soft lithography, etc. are employed to modify the surface to obtain the desired wettability profile. However, in certain biochemical applications such as protein immobilization, one should realize that electrostatic effects, pH of the environment and the isoelectric point of the surface play important roles in achieving the objective. One method of tailoring the surface wettability is to chemically alter the surface by adding some chemical compounds to the surface. Usually, for flat surfaces, the maximum contact angle is around 1200. To achieve contact angles greater than this, the lotus effect needs to be induced on the surface which involves increasing the surface roughness. The lotus effect is so called because the surface morphology of lotus leaves is superhydrophobic and has contact angles greater than 1500 and low slide off angles.
While studies focus on the tailoring of surfaces to imitate nature and understand the effects of surface topography and chemistry on these surfaces, the mechanisms involving simultaneous topographical and chemical alterations of these surfaces are not clearly understood. Recent studies have supported the modification of surfaces with chemical compositions that vary continuously along a given direction. This kind of chemical gradient in the framework produce surfaces with differing properties in wettability, polymer thickness and other physicochemical viewpoints along that direction. The advantages in such setups are their improved use in controlled movement of droplets on surfaces and selective biomolecule adsorption for biomaterial design. With a combination of suitable analytical methods, these gradients are powerful tools for examining a range of effects on a solid specimen surface.
Advances made in surface chemistry and fabrication methods are made use of in tailoring surfaces with surface morphologies ranging into the nanometer scale. For instance, porous silicon is widely used in the fabrication of nanostructured surfaces that are used in several biological applications. These materials are highly biocompatible in several biologically hostile conditions and are also highly biodegradable. These materials have highly tuneable optical properties and a surface chemistry that promotes hydrosilylation or silanisation. This material is usually synthesized by the electrochemical etching of silicon wafers in the presence of hydrofluoric acid. The porosity, architecture of the microstructures, the thickness of the layer, etc. can be varied by changing the anode current density and time.
There is a direct correlation between the porosity of surface structures and the current density which makes such materials highly effective in studying gradients. It is also possible to fabricate surfaces that have different distribution of pore sizes through electrochemical methods. The current in the electrolyte varies according to the distance between the two electrodes due to a change in the resistance encountered through the solution which results in the current density decreasing as a function of the distance between the two electrodes. This particular feature can be made use of in fabricating surfaces that contain pores having a gradient of sizes ranging from 1μm to 5μm on the same surface. Surfaces modifies through this method find immense use in the manufacture of optical filters and in the study f biomolecule adsorption.
On a flat surface, the key variable on wettability is the effects of the surface science and surface energies. As surface energy decreases, the hydrophobicity and the consequent contact angle increases. At the point when a permeable surface is presented in coordination with necessary surface chemistry, the subsequent contact angle formed will be the net of both variables. Under specific conditions, special kind of wetting such as Cassie-Baxter or Wenzel wetting conduct is observed (Rosario et al 2004). Whilst the Cassie-Baxter model accept that a water droplet lays just on top of the surface of the structures without filling pores, Wenzel's model suggests that the liquid drop penetrates the pores. Tailoring surface topologies to control surface wettability results in surfaces that are of major technological and medical importance. Several new methods of surface modification such as continuous and pulsed laser patterning of PMMA substrates for microfluidic channelling and biomolecule adsorption are currently hot topics of research that have shown promising results.
Ertl, G. (2008). Handbook of heterogeneous catalysis (2nd, completely rev. and enl. ed.). Weinheim: Wiley-VCH ;.
Balandin, A. A. (1969). Modern state of the multiplet theory of heterogeneous catalysis.Adv. Catal. Rel. Subj., 19, 1-210.
Mizuno, N., & Misono, M. (1998). Heterogeneous catalysis. Chemical Reviews,98(1), 199-218.
Lafuma, A., & Quéré, D. (2003). Superhydrophobic states. Nature materials,2(7), 457-460.
Synytska, A., Ionov, L., Minko, S., Motornov, M., Elchorn, K., Stamm, M., & Grundke, K. (2004). Tuning wettability by controlled roughness and surface modification using core-shell particles. Polym Mater Sci Eng, 90, 624-625.
Rosario, R., Gust, D., Garcia, A. A., Hayes, M., Taraci, J. L., Clement, T., & Picraux, S. T. (2004). Lotus effect amplifies light-induced contact angle switching. The Journal of Physical Chemistry B, 108(34), 12640-12642
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