Catalysis, Chemistry tutorial

Introduction:

Catalysis can be stated as the modification of the speed of a chemical reaction, however the presence of an additional substance, termed as a catalyst that remains chemically unchanged via the reaction. Enzymes, which are among the most powerful catalysts, play a vital role in living organisms, where they accelerate reactions that or else would need temperatures that would demolish most of the organic matter. Here, we shall talk about catalysis in details; physisorption, chemisorptions, the modern techniques of surface studies, kinds of catalyst and the methods of catalytic reactions.

Catalysis:

Catalysis is the change of the speed of a chemical reaction, via the presence of an additional substance, termed as a catalyst. Catalyst remains chemically unchanged via the reaction. A catalyst in a solution with - or in the similar phase as - the reactants is known as a homogeneous catalyst. The catalyst joins by one of the reactants to make an intermediate compound which reacts more readily by the other reactant. The catalyst, though, doesn't affect the equilibrium of the reaction, as the decomposition of the products to the reactants is speeded up to an identical degree. An illustration of homogeneous catalysis is the formation of sulphur trioxide by the reaction of sulphur dioxide by oxygen, in which nitrogen dioxide serves as the catalyst. Under tremendous heat, sulphur dioxide and nitrogen dioxide react to make sulphur trioxide and the intermediate compound nitric oxide, which then reacts by oxygen to re-form the nitrogen dioxide. The similar amount of nitrogen dioxide exists at both the starting and end of the reaction.

Based on the kind of interaction, adsorption is of two-kind-physisorption (that is, physical adsorption) and chemisorption (that is, chemical adsorption).

Both physisorption and chemisorption are usually exothermic process. This must be noted while discussing regarding enthalpy decrease whereas comparing the energies of physisorption and chemisorption.

Physisorption:

Whenever the adsorbate molecules are weakly bounded to the adsorbent, it is categorized as physisorption. It is as well termed as van der Waals adsorption as the forces comprised are of van der Waals type and are of the similar magnitude. Illustration is in the liquefaction of gases. The enthalpy decrease (-DH) related by physisorption is much low (< 40 kJ mol-1) and is of the similar order as the enthalpy of condensation of the adsorbate. Rise of temperature is not favorable to physisorption. Adsorption of gases via charcoal is an illustration of physisorption. The Physisorption is usually independent of the chemical nature of the adsorbent. All the gases show van der Walls adsorption.

Note:

The unsatisfied valencies of the surface might cause breakage of the bonds in the chemisorbed molecules. The fragments which yield in the method are responsible for the increased chemical activity.

Chemisorption:

Whenever the adsorbed molecules react chemically by the surface, we state it chemisorption. The enthalpy decrease related with chemisorption is much high (between 40KJ mol-1 and 400KJ mol-1) and is of the order of bond enthalpies. We shall talk about later how the kind of bonding caused by chemisorption between the adsorbent and the adsorbate finds out the reactivity pattern. Most of the chemisorption processes comprise activation energy as in a chemical reaction. In these cases, the rates of chemisorption and desorption raise with temperature in disparity by the rate of physisorption.

Note

High bond enthalpy of nitrogen (945KJ mol-1) is mostly responsible for its low reactivity. 

Let us notice how the kind of adsorption of nitrogen on iron surface differs by temperature. The studies on the adsorption of nitrogen on iron surface point out that at around 770K (that is, the temperature selected for Haber method), nitrogen is chemisorbed on the iron surface. Chemisorption yields in large discharge of energy. The optimum temperature selected for the reaction and the energy discharged throughout chemisorption are useful in overcoming the large bond enthalpy of nitrogen. Therefore, whenever nitrogen is chemisorbed at 770K, it present as nitrogen atoms and not as molecules.

Note:

One of the procedures followed in the scientific reasoning is to arrive at the similar conclusion through more than one procedure. Take for illustration, the manufacture of ammonia. At Le Chatelier principle, high pressure (200 to 300 atm.) and optimum temperature (670 to 870K) are required for a good yield of ammonia. Such conclusions could be reached from surface studies too. In this part, we have described that approximately 770K, nitrogen is chemisorbed on iron to a huge extent and, this ease the formation of ammonia.

At temperatures less than 770K, there is no much of chemisorption of nitrogen on iron surface. At room temperature, iron doesn't adsorb nitrogen at all. However as temperature is lowered and bought close to 80K (that is, the boiling point of liquid nitrogen), iron adsorbs nitrogen gas physically as nitrogen molecules! In short, close to 770K, nitrogen is chemisorbed via iron as nitrogen atoms and close to 80K; it is physisorbed as the nitrogen molecules.

The dissociation of nitrogen molecule on iron surface at around 770 K could facilitate its further reaction like the formation of ammonia via Haber method. However the process of iron catalysis in Haber method is not totally understood, the chemisorption of nitrogen on iron indeed plays a role in it.

Modern methods of surface studies:

The composition of the adsorbent surface, the nature of binding between adsorbent and adsorbate and the degree of surface coverage could be studied by utilizing methods like X-ray or UV photoelectron spectroscopy. Auger spectroscopy and low energy electron diffraction (or LEED). Of such methods, we shall illustrate the principle of X-ray and UV photoelectron spectroscopy.

Note:

Auger effect is the emission of a second electron subsequent to high energy radiation has expelled on the electron. Auger effect is the base of Auger spectroscopy and is much employed in microelectronics industry.

Low energy electron diffraction is the diffraction mainly caused via atoms on the surface by utilizing low energy electrons. The LEED prototype depicts the two dimensional (2-D) structure of a surface. Low energy electrons are employed to make sure the diffraction by atoms on the surface only, however not by atoms in the bulk.

We are familiar in the photoelectric effect, according to which photoelectrons could be expelled via irradiating a metal surface by UV rays. The minimum energy which UV rays should possess for photoelectron emission corresponds to the ionization energy of the valence electrons. If we are interested in the emission of inner electrons, we should make use of X-ray or UV photoelectron. As this photoelectron spectroscopy studies are helpful in acquiring the finger-print of the materials present in a surface of a material, these techniques are termed as electron spectroscopy for the chemical analysis (ESCA). This is possible to recognize the elements present in a specific surface by utilizing X-ray photoelectron spectroscopy since each and every element consists of characteristic inner shell ionization energies. The surface study employing ESCA is made possible by the fact that the ejected electrons can't escape apart from within some nanometers from the surface. The nature of chemisorption between the catalyst surface and reactant molecules could be established through ESCA studies.

Types of Catalysts: 

We are familiar that the rate of a chemical reaction can be increased by increasing the temperature. This raises the fraction of molecules having energies in surplus of some threshold energy (almost equivalent to activation energy). The other way to raise the rate of chemical reaction is to find out an alternating path for a chemical reaction that consists of lower activation energy. The catalyst generates this alternate path for a chemical reaction. The sole function of the catalyst is to lower the activation energy of a reaction. Therefore, a small amount of manganese dioxide raises the rate of decomposition of KClO3; the decomposition of nitrous oxide is accelerated via iodine; in presence of Ni, unsaturated hydrocarbons can be hydrogenated to saturated hydrocarbons. The quantity of a catalyst remains unchanged at the end of a reaction and might be employed repeatedly. A substance that can affect the rate of a chemical reaction however it remains unchanged chemically is known as a catalyst. A catalyst can't begin a chemical reaction that couldn't occur in its absence. A catalyst doesn't modify the position of equilibrium; in another words, it can't change the relative amounts of the reactants and products at the equilibrium. As a result, a catalyst should accelerate equally both the forward and reverse reactions. A catalyst is highly precise in its action, for illustration, MnO2 can catalyze the decomposition of KClO3 however not that of KNO3. In some reactions, one of the products could catalyze the reaction. For example, in the oxidation of oxalic acid via acidified KMnO4, Mn2+ ions formed throughout the reaction increase the rate of the reaction. This kind of phenomenon is known as auto-catalysis. 

Catalysis might be of homogenous or heterogeneous kind. In homogenous catalysis, the catalyst makes a single phase by the reactants and products, while in heterogeneous catalysis, it comprises a separate phase.        

There is the other kind of catalysis termed as enzyme catalysis. Enzymes encompass high relative molecular masses and are protein molecules. The enzymes catalyze a variety of chemical reactions in the living organisms. The enzyme reaction medium is strictly colloidal in nature; enzyme catalysis doesn't fall in homogenous or heterogeneous catalysis.

The enzymes are precise in catalyzing only a specific set of reactions. Enzyme activity is mainly based on the pH of medium.  

Illustrations of all the three kinds of catalysis are given in the table shown below:

Table: Three types of Catalysis

Types                       Illustrative reaction                          Catalyst

A) Homogeneous   1) 2SO2 (g) + O2 (g) → 2SO3 (g)               NO (g)

catalysis                 2) CH3COOC2H5 (l) + H2O (l)                  H3O+ (aq)

                              → CH3COOH (l) + C2H5OH (l)      

B) Heterogeneous   1) HCOOH (g) → H2O (g) + CO (g)         A12O3(s)

catalysis                  2) 2SO2 (g) + O2 (g) → 2SO3 (g)             Pt(s)    

C) Enzyme               1) NH2CONH2 + H2O → 2NH3 + CO2      Urease

catalysis                  2) C6H12O6 → 2C2H5OH + 2CO2            Zymase

It will be noted that in enzyme catalysis, we haven't specified the states of the substances.

Mechanisms of Catalytic Reactions:

In homogenous or enzyme catalysis, the reaction intermediate is made between the reactant and catalyst or the enzyme. The intermediate compound then decomposes to provide the product. The reaction series can be symbolized as follows:

Step (a) Formation of the intermediate compound. S + C → SC

Step (b) Decomposition of the intermediate compound. SC → P + C

Here, S and P are the reactant and the product correspondingly and C is the catalyst or the enzyme; SC is the intermediate compound. The role of catalyst or the enzyme is to lower the activation energies of the forward and reverse reactions. In figure shown below, EC is the activation energy for the conversion of a reactant to a product in the presence of the catalyst and Euc is the activation energy for the similar reaction in the absence of catalyst. We can observe that Ec < Euc. The similar is true for the reverse reaction.

This is interesting to note that most of the biological reactions are catalyzed via enzymes. This is facilitated via the fact that the enzyme catalyzed reactions have much lower activation energies than systems having chemical catalyst. The table illustrated below points out the activation energies for the decomposition of hydrogen peroxide under various conditions.

11_Relative activation Energies for Catalysed and Uncatalysed Reactions.jpg

Fig: Relative activation Energies for catalyzed and uncatalyzed Reactions

Table: Activation Energies for the decomposition of Hydrogen Peroxide solution

Catalyst                      Activation energy/kJ mol    Relative rate of reaction

None                                       75.3                                 1

1-(aq)(homogenous)               56.5                            2.0 x 103

Pt (s)(heterogeneous              49.0                            4.1 x 104

Catalase (enzyme)                     8                              6.3 x 1011 

Note:

We have an idea regarding the significance of enzyme reactions from the fact that ammonia prepared from nitrogen by nitrogenase enzyme is 10 times more than that prepared by Haber method. Moreover, the enzyme provides good results of ammonia at room temperature and pressure. Compare this by the experimental conditions required for Haber method (200 - 300 atmospheric pressure and 670 to 870K). 

In heterogeneous catalysis, the function of the catalyst surface is to bring down the activation energies of the reactions. This occurs because of the chemisorption that is identical to intermediate compound formation in the homogenous catalysis. The capability of a surface to encompass chemisorption of the reactant molecules based on the chemical nature of the surface. ESCA studies are useful in deciding the nature of chemisorption between the surface and reactant molecules. The dissimilarity in the nature of chemisorption could lead to dissimilar products even from the similar reactant.

For illustration, ethyl alcohol is dehydrogenated on Ni, Pd or Pt catalysts to provide acetaldehyde.

CH3CH2OH + (3/4NI, 3/4Pd or 3/4Pt) → CH3CHO + H2

On the other hand, ethyl alcohol experiences dehydration reaction on alumina.

CH3CH2OH → (3/4 Alumina) CH2 = CH2 + H2O

On Ni, Pd or Pt surfaces, the linkage of ethyl alcohol is via two hydrogen atoms (figure shown below). The strong affinity between Ni and hydrogen accounts for the elimination of two hydrogen atoms from ethyl alcohol.

1085_Dehydrogenation Process on Ni.jpg

Fig: Dehydrogenation process on Ni

On contrary, alumina acts in a different way because of its dissimilar structure. Alumina consists of both oxide groups and hydroxyl groups. The linkage of ethyl alcohol to alumina is via hydrogen and oxygen atoms as illustrated in the figure illustrated below. The elimination of hydrogen and hydroxyl groups from the adjacent carbon atoms leads to the dehydration reaction.

1210_Dehydration Process on Alumina.jpg

Fig: Dehydration process on Alumina

Some substances enhance the activity of a catalyst. These substances are known as promoters. Such substances might not themselves be efficient catalysts. A promoter might raise the number of active sites on a catalyst surface. In the light of this explanation, let us observe the catalysis in Haber method of manufacturing ammonia. The mixture of iron, potassium and aluminium oxides allows this reaction. The hydrogen atmosphere decreases the iron oxide into porous iron which consists of large surface area that acts as the catalyst. The mixture of potassium oxide and aluminium oxide acts as the promoters.

Now consider some applications of catalysts in the chemical industries. 

a) In the formation of edible fats from vegetable and animal fats, controlled partial hydrogenation by a catalyst like nickel assists in eliminating some of the double bonds. In the absence of hydrogenation, such double bonds could be oxidized by air that imparts the oil the rancid odor on storage.

b) Cautious studies of the catalytic surface have been useful in privileged formation of a product beginning from a reactant. Therefore, it is possible to form different oxidation products of ethylene like ethanol, acetaldehyde, vinyl chloride or vinyl acetate via appropriate choice of catalysts and reaction conditions.

c) We might be aware that 'cracking' is the procedure of generating small organic molecules via the breaking of long-chain hydrocarbon molecules. Generally, silica-alumina catalysts are utilized for this purpose. Cracking is needed to generate branched-chain isometric hydrocarbons which encompass more fuel efficiency in the automobile engines.

Inhibition and Poisoning:

We are familiar that the reactants are to be adsorbed on the surface for the chemical reaction to be affected by the surface. For the improvement of the reaction rate, the reactants should be adsorbed more or less to similar degree. Whenever one of the reactants is more strongly adsorbed as compare to the other or whenever a product is adsorbed to a greater degree as compare to the reactants, then the active centers on the catalyst surface wouldn't be fully available for the reaction and the reaction rate reduces. Such a condition is known as inhibition of the catalyst. One of the reactants or the products that gets strongly adsorbed and thus reduces the reaction rate is known as the inhibitor. For illustration, in the decomposition of ammonia on platinum surface, hydrogen (that is, a product) is strongly adsorbed and slows down the reaction.

This is possible that the reaction could be inhibited through a foreign molecule that doesn't take part in a reaction. This kind of inhibition is known as catalytic poisoning. This is noticed that even small quantity of the catalytic poison could be effectual in stopping a reaction. This phenomenon could be described by the fact that the active centers comprise merely a small fraction of the net surface sites on a catalyst and the meager amount of poison could inhabit these positions. This prevents the reactant molecules from occupying these positions. For illustration, in the contact procedure of manufacturing sulphuric acid, a small quantity of arsenic impurity can even poisons the Platonized asbestos catalyst and the reaction approximately stops.  

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