Surface Chemistry and Catalysis - PDF document

Surface Chemistry and Catalysis CHAPTER 2: Surface Chemistry INTRODUCTION Surface Chemistry is closely related to interface and colloidal science. Surface chemistry is important in many criticalchemical processes, such as enzymatic reactions at biological interfaces found in cell walls and membranes, in electronics at the surfaces and interfaces of microchips used in computers, and the heterogeneous catalysts found in the catalytic converter used for cleaning emissions in automobile exhausts. DEFINITION Surface science is the study of chemical phenomena that occur at the interface of two phases (solid–liquid interfaces, solid–gas interfaces, solid–vacuum interfaces, and liquid-gas interfaces). study of chemical re AND CATALYSIS Introduction - Terminologies in surface chemistry - Difference between adsorption and absorption - Types of adsorption - Adsorption isotherm- Freundlich Adsorption Isotherms- Langmuir Adsorption Isotherm - Contact Theory (or) Mechanism of Heterogeneous Catalysis - Kinetics of Surface Reaction - Kinetics of Bimolecular Reaction (Langmuir-Hinshelwood) - Types of Adsorption Isotherm - Application of

Adsorption- Terms - Mechanism of Catalytic Reaction- Criteria (or) Characteristics for Catalyst Types of Catalysis - Homogeneous Catalysis - Heterogeneous Catalysis- Catalytic Poisoning and Promoters Application of Catalysis- Biological Catalyst——Enzymes - Kinetics of Enzyme Catalysed Reaction Or Michaelis and Menten equation - Factors Affecting Enzyme Activity 2.2 T E E RMINOL the su r (Eg.) called d is call e called GIES I N r bate: u r The s u b ent : The p r r face of a sol i O cclusio n gas is ad s r desorption ce: The pla n r Whe y of a solid o i The phe n ed sorption. sion: occlusion. SURFAC E u bstance whi c u bstance on t h r ocess where b i d. Hydrogen g s orbate. removal o f n e which sep a n the molecul e or liquid. Th i n omeno in w adsorption CHEMI S ch gets adso rb h e surface o f b y molecule s g as on Pallad i Ad o n Adsorbent adsorption t a : Adsorption P r f the adsorb e a rates any tw o e s of a substa n i s phenomen o w hic adsorpt i o f gases oc c S TRY d on any s u f which adso r s of gases or l i i u m where p a d sorbent n the surface o

f the solid whe r a kes place r ocess d substanc e o phase is ge n n ce are unifor m on is called b i o n and absor p c urs on the s u u rface is call e r ption takes p i quids adhere a lladiu is a d gas adsorbed f solids from a su r n erally called a m ly distribut e b sorption occur s i u rface of m e Chemistr lace is calle d chemically t o d sorbent an d r face is an throughou t i multaneousl tals it is y . d o d t y Surface Chemistry and Catalysis 2.3 AdsorptionDesorptionAbsorptionFigure 2.2 : Occlusion Process Positive adsorption: When the concentration of solute adsorbed on the solid adsorbent surface is greater than in the bulk it is called Concentrated solution of KCis shaken with blood charcoal, it shows positive adsorption Negative adsorption: When the solvent from the solution may be absorbed by the adsorbent so that the concentration of the solute decreases and the concentration of solution increases than the initial concentration and it is called negative adsorptionDilute solution of KCis shaken with blood charcoal it shows negative adsorption. Amount of heat evolved when 1

mole of an adsorbate gets adsorbed on the surface of an adsorbent is called Molar Heat Molar Enthalpy of AdsorptionDIFFERENCE BETWEEN ADSORPTION AND ABSORPTION Figure 2.3 : Illustration of Absorption and Adsorption Adsorbant Chemisorption Physisorption Absorption dsorption 2.4 T Y 1. It 2. It 3. S u t h 4. 5. E g c h Y PES OF A P d e r o is a bulk phe n is a slow pro c u bstance unif o h roughout the A ttainment of e m e .: Ammon i h arcoal hysical A d Fi ure 2.4 Tem omenon ess rmly distrib u surface quilibrium ta k i a adsorbed I ON : Illustration o f p erature o It is a s u It is a f a u ted Higherspecies es Equilib inEg.: A m Ch Ph y sical and C o f as adsorbe d e b d r s rface pheno m a st process concentratiin the surfac e rium attained m monia adsor b e mical hemical Ads o Tem vs temperatu R PTION enon on of mo e than in the b u easil b ed in wate r o rption re Chemistr lecula u Sur f ace Chemi s N o 1. Cau s van 2. Not s 3. Rev e 4. Mul t on t h 5. Hea a KJ/ 6. No a 7. Dep e Liq 8. Occ u 9. Incr e adso orption o f In adsor p a dsorbe is c s try and

Ca t Physical a d Physis ed by d erwaal’s forc e s pecific in na t e rsible in natu r t imolecular la y h e adsorbed s u a t of adsorpti o m ole) ctivation ene r e nds on natur e u efiable gas e y u rs at low te mp e ase ption in p r Gases on S p tio of gasesalle Solid t sorption or o rption intermole ure ers are form n is less (20 t o r gy is require d e of gas. Easil y e s are ads o mp erature essureinc on solid sur f b . The ext e i g ure 2.6 : A ds d Unim o surfac 40Heat oKJ/m o easeHigh ace, the soli d e nt of adsor p o rption of g as ical adsorp t d by chemical y specific in n a r sible in natur e o lecular layer s e f adsorption i s o le) ctivation ene r ds ateon nat u s es with incre a p ressure is fav o r e does not c a d surface is c a p tion depend s A b s s on solids t ion or Che m bond formati a ture are formme d s large(8 r gy is require d u re of ads o a se in temper a o urable. Decr e a use desorptio n a lle the adso r s on many fa c s orbed Solid isorption on on the 0 to 240 d o rbent and a ture ase in n r bent and th e c tors. e 2.6 Chemistry 2.2 FACTORS AF

FECTING THE EXTENT OF ADSORPTION Nature of Adsorbent The adsorption depends on the type of adsorbents used. When the adsorbent is highly porous the rate of adsorption increases. Activated carbon, metal oxides like aluminum oxide, silica gel and clay are commonly used adsorbents. The rate of adsorption can be increased by activation process. It helps in enhancing the pores in the adsorbent Eg. charcoal adsorbs 0.011 gms of CCat 24°C and activated charcoal adsorbs 1.48 gm of at 24°C. During activation, the adsorbent is heated in steam to about 1500°C. Heating drives out all impurities and leads to a lager free surface for adsorption. It can be done in 3 given ways By making the surface of adsorbent rough. By heating the adsorbent in vacuum so that the water vapour present in pores leave those pores. By increasing the surface area of adsorbent Surface area of adsorbent Increase in surface area of the adsorbent increases the adsorption of gases and the extent of adsorption depends on two factors Greater the surface area greater the adsorption-Increase in surface area increases the number of adsorbing sites. Larger the porosit

y greater the adsorption-Finely divided and highly porous materials acts as good adsorbents. Eg. Charcoal and silica gel (excellent adsorbents). Nature of Gases The amount of gas adsorbed by a solid depends on the nature of the gas. Easily liquefiable gases like H, NHetc., are adsorbed more easily then the permanent gases like H, and Oetc. Physical adsorption is non-specific in nature, so any gas will be adsorbed on the surface under any given conditions of temperature and pressure. Chemisorption is specific in nature so only those gases which forms chemical bonds will be adsorbed. Surface Chemistry and Catalysis 2.7 Critical Temperature (maximum temperature above which the gas cannot be liquefied). Liquefactions of gases depend on critical temperature. When the critical temperature is more the gases will be liquefied and more adsorption occurs. Van der Waal’s forces: Easily liquefiable gases possess greater Vander Waal’s forces than permanent gases, so they are adsorbed more readily. Exothermic Nature Heat of adsorption is defined as the energy liberated when 1 g mol of a gas is adsorbed on a solid surface. Increase i

n temperature increases the kinetic energy of the gas molecules and it results in more number of collisions of gas molecules over the adsorbent surface. (v)When pressure is increased then the rate of adsorption increases initially. The extent of adsorption is expressed as x/m where ‘x’ is amount of adsorbate; ‘m’ is mass of adsorbent when the dynamic equilibrium is established between free gas and the adsorbed gas. But after some time it reaches appoint where no more adsorption occurs and at this point adsorption is independent of pressure. Figure 2.7 : Rate of adsorption ADSORPTION OF SOLUTE FROM SOLUTIONS The process of adsorption of solutes on solid surface can take place from solutions. For example the activated animal charcoal adsorbs colouring matter present in sugar solution and clarifies the sugar solution. It also has the capacity to adsorb acetic acid and oxalic acid from water thereby reducing the concentration of acids in water. There are two (or more) components present in a solution namelysolute solvent. solute may be present in the molecular or ionic form. The extent of adsorption from solution Pressure 2.

8 Chemistry of the solute in the solution, and can be expressed by the log log - is the mass of the solute adsorbed, -is the mass of the solid adsorbent, -is the concentration of the solute in the solution & lue greater than one, -is the proportionality constant. The value of depends upon the nature of solid, its particle size, temperature, and the nature of solute and solvent etc. It the graph is plot between against which gives a straight line which is similar to Freundlich adsorption isotherm. FACTORS AFFECTING ADSORPTION OF SOLUTES FROM Nature of adsorbent Adsorption ofsolute from solution is highly specific. Adsorption depends mainly on nature of adsorbent. Adsorption from solution decreases with rise in temperature. Concentration of solute Adsorption from solution decrease with decrease in concentration of solution.eg charcoal adsorbs water from dilute KCl solution whereas charcoal adsorbs KCl from concentrated KCl solution. ADSORPTION ISOTHERM The process of adsorption is usuallystudied through graphs know as adsorption isotherm. It is the graph between the amounts of adsorbate () adsorbed on the surface of adsor

bent (and pressure (P) at constant temperature. Sur F R ads of a n isot ace Chemi s Ad of the eq u A plot o f s aturation p r R EUNDLI In 1909, H o rptio of a q u o rptio isothe r n adsorbent t h er is given a (or) r e x – is m a m is m a P – is the – are c o s try and Ca t sorbent + A d on isotherm s u ilibriu par t X/m f vs P is p r essure. H ADSO R H erbert Freu n u antity of gas rm is an emp i t o the conce n a s: ss of adsor b a ss of adsor b equilibrium nstants. hows the a m t ial pressure a A dsor ure 2.8 : p lotte to o b R PTION I S n dlic gave a adsorbed by u i rica relation n tratio of th e 1 p log lo b ate, ressure of a d x n m ount of mole a t constant te m p tion Isother m S PS Adsorption Is o b tai adsorpti S OTHER expression u nit mass of s between the e solute in th 1 d cules adsorb e m perature. aturation P r o therm n at consta n M S adsorbe n concentratio e liquid with P on the soli r essure n t temperatu r the isother m n t with press u n of a solute o which it is i n � 1] 2. surface as a r e, Pis calle d m a l variation

o u re.Freundlic n the surfac e n contact. Th e 9 a d e e 2.10 Chemistry At intermediate value of pressure, adsorption is directly proportional to pressure raised To remove proportionality a proportionality constant ‘’ is used which is known as adsorption constant and we get log log The equation is comparable with equation of straight line, where, m represents slope of the line and c represents intercept on y axis. Plotting a graph between log (log , we will get a straight line with value of slope equal to 1/and log as y-axis intercept. x x Surface Chemistry and Catalysis 2.11 ) vs. log p graph Freundlich equation is purely empirical and has no theoretical basis. The equation is valid only upto a certain pressure and invalid at higher pressure. The constants are not temperature independent, they vary with temperature. Frendilich’s adsorption isotherm fails when the concentration of the adsorbate is very high. LANGMUIR ADSORPTION ISOTHERM In 1916, Irving Langmuir proposed another adsorption Isotherm which explained the variation of adsorption with pressure layerFigure 2.10 : Equilibrium between tree

molecule and adsorbed molecules log K (intercepted) A SurfaceSurface 2.12 Chemistry Langmuir proposed his theory by making following assumptions. Surface is energeticallyuniform. Fixed number ofvacant or adsorption sites are available on the surface of the solid. All the vacant sites are of equal size and shape on the surface of adsorbent. Each site can hold maximum of one gaseous molecule and a constant amount of heat energy is released. Heat of adsorption is constant throughout the surface and it ranges from 0 to 1. Dynamic equilibrium exists between adsorbed gaseous molecules and the free gaseous molecules. Adsorption is monolayer or unilayer. Langmuir Equation depicts the relationship between the extent of adsorption and pressure. Langmuir proposed that dynamic equilibrium exists between adsorbed gaseous molecules and the free gaseous molecules. Using the equilibrium equation, equilibrium constant can be calculated. is unadsorbed gaseous molecule is unoccupied metal surface and is adsorbed gaseous molecule Rate of forward reaction = K[A] [B] Rate of backward reaction = Kreaction = KAt equilibrium, Rate of fo[

A] [B] = K[AB] A new parameter ‘’ is introduced. Surface Chemistry and Catalysis 2.13 be the number of sites of the surface which are covered with gaseous molecule and (1–è) be the fraction of surface unoccupied by gaseous molecule. Rate of forward direction depends upon two factors, number of sites available on the surface of adsorbent, (1 – ) and pressure, P. P (1 Rate of backward reaction or rate of desorption depends upon number of sites occupied by the gaseous molecules on the surface of adsorbent. At equilibrium, rate of adsorptiThe above equation can be written in terms denominator on RHS by K 2.14 Chemistry Substituting in the above equation we get Langmuir Adsorption EquationLangmuir adsorption equation can be written in an alternate form in terms of volume of gas adsorbed. Let V be volume of gas adsorbed under given sets of conditions of temperature and pressure and Vmono be the adsorbed volume of gas at high pressure conditions so as to cover r of gaseous molecule. mono Substituting the value of KP mono in terms of pressure P we get, Langmuir Adsorption Equation in alternate form. Thus, if we plot a graph

between P/V vs P, we will obtain a straight line with mono and intercept =1/ KVmono.The adsorbed gas has to behave ideally in the vapour phase. Langmuir equation is valid under low pressure only. Langmuir Equation assumes that adsorption is monolayer. But, monolayer formation is possible only under low pressure condition. Under high pressure condition the assumption breaks down as gas molecules attract more and more molecules towards each other. Another assumption is the surface of solid is homogeneous but in real solid surfaces is Sur ( ( Chemi In ads o but th e CONTAC This the o l ysis. Hetero g ( Diffus that in c ( ii) the su r is kno w ( iii) Diffus r s try and Ca t m ui equatio n s ible as wea k o rptio liquef a e value is not z T THEOR Y S IS (OR) A o ry postulat e g eneous cata l Fi ur ion of Reac t s ts. Some of t h c ludes paths a r ption of re a r face of the c a wn as the Sti c i : The re a ng with each o p tio of pro d agai b ion of Prod u r oducts and t h t alysis assumed th a k force of attr a a ctio of gase z ero. (OR) ME A DSORP d by Farad a l ysis has five e 2.11 : Mechan i t ant(s) to

th e h e reactant c r a n d cracks o n a ctants: Bo n a talyst. The a c kin Co-effi c a ctants, whe n o ther, and aft e d : The in t e s available fo u ct(s): The i n h e products a a t molecules a ction exists s taking plac e CHANIS ION THE O a y in 1883. I t steps. sm of hetero ge e Surface h r oss the b arri the externa l n ds are form e a bility for an a c ient. bound to t h er the reactio n t ermediate co m for adsorptio n n termediate c o a re then des o do not inter a even , which resul t M OF HET E O RY explains th e e neous catal y s h e reactants d er and enter t h l surface. ed as the rea c a to or mol e h e surface h a n , they forma n m poun get s n for other m o o mpoun th e o rbe from t h a ct with eac h n molecules o t s in decrease i E ROGENE action of his iffuse to the h e interior e xp c tant(s) are a e cule to stick t a ve a higher p n intermedia t s desorbed fr o o lecules disintegrat e h e surface of t 2.1 other. This i s o f same type. in randomneseterogeneo surface of th e xp ose surfac e dsorbe ont o t o the surfac e p robability o t e compound . o mthe

surfac e e s to form th e t he catalyst. 5 s e e o e e 2.1 C H follo xample, The orig i e nsive iron- b The reac t w in steps: N N H o f eth y lene Ethyl h reaction i nal Habe r – B b ased cataly s t io mechani s N 2 g N 2 g H to ethane Ethylene ne absorbed o e breaking re 2.12 : Conv B osch react i s t, which is s N2 ( g) 3 s m, involving ( adsorbed) ) 2 N (ad s ( adsorbed) A ( B ( N 2 ( 3 H 2 ( 2 N 3 ( A ( g ) B ( g ) A ( a d s B ( a d s d s A B ( k A B ndsersion of eth y l e i on chamber s till used to d H2 ( g) the heteroge n s orbed) A ( a d s B ( a d s d s a s t A B ( e ne to ethane s used osmi u

d ay. 2 (g) n eous cataly s u m as the c a s t, is believed t Chemistr talyst, less t o involve th e y e Surface Chemistry and Catalysis 2.17 2 H (adsorbed)KINETICS OF SURFACE REACTION The kinetics of heterogeneously-catalyzed reactions might vary with the partial pressures of the reactant gases above the catalyst surface which can be predicted by using the Langmuir isotherm. 6.1 KINETICS OF UNIMOLECULAR DECOMPOSITION Examples of unimolecular decomposition Decomposition of NHto Non metal surfaces, Decomposition of Phosphine on glass, Decomposition of Formic acid on glass, P, or TConsider the surface decomposition of a molecule A , i.e. the process A (adsorbed) Pr oduct The decomposition reaction occurs uniformly across the surface sites. Molecule ‘A’ may be adsorbed and is not restricted to a limited number of specific sites. The products are very weakly bound to the surface and, they can be easily deformed. The rate determining step is the surface decomposition step. According to Langmuir adsorption isotherm molecule ‘A’ adsorbed on the surface is in equilibrium with the gas phase and the surface concentrati

on is represented as: P) 2.18 Chemistry k The rate of the surface decomposition is given by an expression : Substituting, The reaction is expressed within two limits: Low pressure limit: P (First order reaction with a first order constant So Rate P is constant. Under low pressure ‘è’ is very small and rate is directly proportional to pressure High pressure limit: P (1 + P and Rate ~ is almost unity. Surface Chemistry and Catalysis 2.19 Rate ~ k (zero order)Rate ~ kbP (first order)Figure 2.13 : Graphical representation of unimolecular surface decomposition 6.2 KINETICS OF BIMOLECULAR REACTION (Langmuir-Hinshelwood) Between molecular adsorbates. Consider the reaction: Assumptionads B( AB(ast AB(The surface reaction between the two adsorbed species is the rate determining step. The rate of the reaction of the two adsorbed molecules for biomolecular surface will be given by: According to Langmuir adsorption isotherm: P P) where two molecules (A & B) are competing for the same adsorption sites then Substituting these into the rate expression gives: A B A A B B 2.20 Chemistry b Reactant Aand B in first orde

r then are very low. Hence, Rate First order in A, but negative first order in B then B B TYPES OF ADSORPTION ISOTHERM Adsorption process is usually studied through graphs known as adsorption isotherm. After saturation pressure P, adsorption does not occur anymore, as there are limited numbers of vacancies on the surface of the adsorbent. At high pressure when all the sites are occupied and further increase in pressure does not cause any difference in adsorption process. At high pressure, adsorption is independent of pressure. There are 5 different types of adsorption isotherms and each of them has specific characteristics. Figure 2.14 : Illustration of different types of Adsorption Isotherm Monolayerpores/capillaries Surface Chemistry and Catalysis 2.21 Type I Adsorption isotherm is for very small pores or microporous adsorbents. Adsorption occurs by filling of micropores and it mainly depicts Monolayer adsorption. Eg. Adsorption of Nitrogen or Hydrogen on charcoal around Figure 2.15 : Type I Type IIAdsorption isotherm shows large deviation from Langmuir modelof adsorption. They are most frequently encountered when

adsorption occurs on nonporous powders or macroporous adsorbents with unrestricted monolayer -multilayer adsorption. Theintermediateflatregiontheisoth correspondsmonolayerformation.Whenr37 oo0m Cte mp. 2H 2 O 2 N o r ea ct io n2 H 2 O 2 Ptb lac k 2 H2 OR C O O R H O2 ⎯⎯→ R C OO H R O 2 C H O n O C H O C H12 2 O2 11 H O2 C H12 2 O2 11V V ma [Sx ] [E S ] K [E ] [0 K [ S] 1 (K 2 K )3[ E t] [S] [E ] [S] [ ES ] ( K 2 K 3 )/ K [1 E S] ([ S] K X [S ]( V ) V ma x[ S ]mV ax [S ] 1( V ) [ S] [S ] 2 V m ax1 K [ S]R a te ( V [S ] V [ S]m ax m ax[ ES ] ] { Et ][S ] /([ S ] K K { E t][ S ][ ES ] [S ] /K (K 2 K )3[ ES ] [E ] [S ]/ K est er 22 K M O +4 5 H C2 O2 4 + 3H 2 SO 4 2 Mca Staly Ost 4 + K S2 O 4 8 H O2 + 1 0 CO2A2 3OC 2 H5 O H C H 2et hen Ce 2 S O 2 O 2 2 S O 3A OF2 3/ /// /2 H 2 + O 2 2 H O2themonolayerformationtheadsorbedmolecano lesarecomplete,multilayerf

ormationstartsc1a2 ne sug22 ar1 1 2 6g 1luco 2se 6 6 fruc 12tos 6e et hyl ace tate or OH ac etic aci d etha nol2 N a S2 O 3 O 2 A lcoh ol 2 N a S2 O 44 N O + 4 N H 4 N 6 H O3 2 2 22 N O 2 4 N H 3 2 3 N 2 6 H O2N H 2 N O2 N O2 H O2N H 2 N O2 O H H O2 N HN O 2CInt oemrm pleedi xateN H NO 2 N 2O O H 2 A H3 2 As 3H 2ca taly sttake place corresponding to the of the isotherms. )) adsorbed at 1950°C on silica gel.f Figure 2.16 : Type II Type III Adsorption Isotherm also shows large deviation from Langmuir model. This isotherm explains the formation of multilayer. They are characterized principally by heats of adsorption which are less than the adsorbate heat of liquefaction. XPsP XPs 2.22 Chemistry Eg: Bromine (Br) at 790°C on silica gel or Iodine (I) at 790°C on silica gel. Surface Chemistry and Catalysis 2.23 Figure 2.17 : Type III Type IV Adsorption Isotherm occur on porous adsorbents possessing pores in the range of approximately 15-1000 angstroms (A

). At lower pressure region of graph is quite similar to Type II. This explains formation of monolayer followed by multilayer. The intermediate flat region in the isotherm corresponds to monolayer formation. The saturation level reaches at a pressure below the saturation vapor pressure. This can be explained on the basis of a possibility of gases getting condensed in the tiny capillary pores of adsorbent at pressure below the saturation pressure (P) of the gas. Eg. Adsorption of Benzene on Iron Oxide (F) at 500°C and adsorption of Benzene on silica gel at 500°C. Figure 2.18 : Type IV Type V Adsorption Isotherm results from small adsorbate-adsorbent interaction potentials similar to the Type III isotherms. However, Type V isotherms are also associated with pores in the same range as those of the Type IV isotherms. Eg: Adsorption of Water (vapors) at 1000°C on charcoal. XPs XPs 2.2 A (i) P (ii) Type III a o rbate – adso r T y pe I V V olume of s adsorbed A PPLICA In Dewa r r in into the a In Gas ma s Activate etc.) and p u a nd Type V r bate interac t V and V sho ION OF A e applicatio n n o high va fl

asks activ a a nnula spac e s ks carbon is us e u rifies air for b X m r isotherms d o t ions than ad s w s phenome 2.20 : Differen t A DSORPT of adsorpt i cuum charcoa l e gets adsorb e ed in gas ma s b reathing. e 2.19 : T y pe not have th e s orbate-adso non of capilIII pes of adso r I ON on is been l l is placed b e t e ks to adsorb V e ‘sharp kne e r bent interac t lary conden s IV0 r ption isother m isted below t ween the w a poisonous g a Ps e shape imp ly t ion. ation of ga s V P0 lls of the fla s a ses (e.g. oxi d Chemistr stronge r s . 0 s k so that ga s d e of sulphur , y r s , Surface Chemistry and Catalysis 2.25 ous gases using activated charcoal (iii)In desiccation or dehumidification Certain substances can be used to remove water vapour or moisture present in the air. Silica gel and alumina are used for dehumidification in electronic equipment. (iv)In clarification of sugar Sugar is decolorized bytreating sugar solution with animalcharcoalpowder which removes the colour producing substances. (v)In paint industry The paint should not contain dissolved gases as it inhibits the a

dherence capacity of paint to the surface to be coated. The dissolved gases are therefore, removed by suitable adsorbents. This is done by adding suitable liquids which adsorbs these films. Such liquids are called wetting agents. Eg. Use of spirit as wetting agent in furniture painting.. (vi)Adsorption chromatography Analytical method, in which molecules are separated according to their adsorptive properties, where a mobile fluid phase is passed over an immobile solid adsorptive stationary phase. (vii)In adsorption indicators Various dyes which possess adsorption property have been introduced as indicators mainly in precipitation titrations. For example KBr is titrated with AgNOusing eosin as an indicator gas molecules A ctivated Charcoal 2.26 Chemistry (viii)Heterogeneous Catalysis In heterogeneous catalytic reactions adsorption of gaseous reactants on solid catalyst occurs. The adsorption mechanism is responsible for the greater efficiency of the catalyst in the finely divided state and helps us to understand the action of catalyst promoters and poisons. eg, Finely powdered nickel is used for the hydrogenation of oils.

In manufacture of sulphuric acid finely divided vanadium pentaoxide (V) is used in the contact process. diffusion of reactants to surface; adsorption of reactants to surface; reaction on the surface; desorption of products from surface; (v)diffusion of products away from the Figure 2.22 : Process of Heterogeneous Catalysis (ix)In adsorption indicators In many precipitation titrations many dyes are used as indicators which work on the principle of adsorption. In curing diseases Some pharmaceutical drugs have the capacity to adsorb the germs on them and kill them and protect us from diseases. (xi)Lake test for aluminium It is based on adsorption of litmus paper byA(OH)precipitate diffusionadsorptiondiffusion Surface Chemistry and Catalysis 2.27 (xii)Separation of inert gases Due to the difference in degree of adsorption of gases by charcoal, a mixture of inert gases can be separated by adsorption on coconut charcoal at different low temperatures. (xiii)In softening of hard water The use of ion exchangers for softening of hard water is based upon the principle of adsorption chromatography. The ion exchange resins helps t

o remove hardness causing ions from water and make it useful for industrial and domestic applications. (xiv)Arsenic Poisoning Colloidal ferric hydroxide is administered which adsorbs arsenic and removes it from body by vomiting (xv)Formation of stable emulsions in cosmetics and syrups etc. (xvi)Froth floatation method (xvii)In cleaning action of soaps and detergents Figure 2.23 : Cleaning actions of soaps and detergents Fat or Oil Stain dsorbed Surfactant lowers the interfacial tension between the fabric and the stainfabric substratefabric substrateMechanical fabric substrateStain does not desorb spontaneouslyMechanical Fat or Oil GlobuleFat or Oil Globulefabric substrateClean Fabric + adsorbed surfactant prevents re-adsorption of fat globulefabric substrate 2.28 Chemistry (xviii)Application of adsorbents on pollution abatement Many pollutants, both natural and synthetic, are gaseous in nature and it need to be effectively removed from the exhaust. Gaseous industrial pollutants include HCS, SO, Ethylene, Benzene, Ethanol, and HAP’s. Adsorption is a mass transfer process in which a porous solid comes in contact with a liq

uid or gaseous stream to selectively remove pollutants or contaminates by adsorbing them onto the solid. The most common adsorbents used in industry are activated carbon, silica gel, activated alumina (alumina oxide), and zeolite. Activated carbon is the most common non-polar adsorbent. Polar adsorbents have a great attraction to absorb moisture. Most industrial exhaust streams contain moisture the use of polar adsorbents is significantly limited for air pollution control systems. There are two main types of adsorption systems; fixed bed or continuous. [1]Fixed Bed or Packed Bed Systems These are quite simple devices. The fixed bed or packed bed reactors are most commonly used for study of solid catalyst. Afixed bed reactor usually consists of a cylindrical vessel packed with the adsorbent material (eg. activated carbon) and it contains more surface area for adsorption. The contaminated or the polluted air enters the fixed bed system at the side, where there is an exhaust distributor. The exhausted air exits the fixed bed adsorber clean of pollutants or contaminates. Once the adsorbent is fully saturated with adsorbate th

e system requires change- out of the spent materials, which is then packed with new adsorbent material. The spent adsorbent will be thermally cleaned. Ideal plug flow behavior Lower maintenance cost 1. Plugging of bed due to coke deposition which results in high pressure drop. Sur [2] ads Chemi o Chlorin lex and pro v o rbates from o rbate can be s try and Ca t bbing ure 2.2 s Flow Rea c o us-flow rea c v ide continu o the adsorbe n e condensed, t alysis Recirculating CoolerRecirculating 5 : Schematic r c tor S y stem tors are alm o o us operatio n n t. This can b collected, a n un ixed Bed Reac t Caustic M a Blowdown Stream an d Optional epresentation o st always o p n s. These sy s b e done wit h nd re-used i n products and r eacted materi a to separationcatalyst on support t or Device a ke-up e Fixed Bed Rea c d of fixed bed re a p erate at st e s tems provi d h superheate d n the proces s a ls E f or r e a ctor ady state. T h d e in-situ des o d or saturate s . There are t 2. iquid drain e cycle) h ese are mor e o rption of th e d steam. Th e t wo adsorbe r 2 9 e e e r 2.3

[3]flo A cat a itself 3.1 of s u and 3.1 in the syste m h steam. The e ficia purpos e Special T yp A hydro p w s. An integr a i te. They hav e a lyst is defi r emaining c d catalysis. INTROD The wor d b Berzelius ( 1 u bstances, dis c reate the p r Catalysts xpedite the b v ersio of sta r .1 DEFIN c s umed or alt e m . The gases exhaust fro m e . e of Contin u p hobic zeolit e a te thermal e wide accep t n ed as a s u c hemically CTION “catalyst” w 1 779-1848) sev e r esent syste m are of imme n b iochemica r r c h to glucos e ITION ess by whi c e red in the p are being ad s m the desorb e Stirre Rate ure 2.26 : C o u ous S y ste is designedoxidizer is u t ance in indu s CHAPT u w as introdu c w ho also dete rm e ra elements i m of writing c h n se importan c r eactions ne c e . h a substan c p rocess. orbe in on e ed bed can be r O u ntinuous Flo w m (Zeolite Co n in a monolit h se to provi d s tria air poll u E R 3: Cat a w hich alters c e d into scie n rm ine the at o i ncludin sel e h emica sym b c e in chemist r c essary for li f c e speeds u p e unit as the o t e

condensedLiquid Surfa tput Rate w Reactor n centrator, ic rotor in w d e desorptio n u tio control a a lysis c n ce by the gr e o mic and mol e e nium, first is o b ols and reac t r y and b iolog e. (eg) Enzy m p a chemic a t he unit is be for solvent r ce r Rotary C o w hic the co n n of the solv e a pplications. c tion. The p e at Swedish e cula weight s o late silico n t ions. . All enzym e m es in saliva a a l reaction w Chemistr desorbe d r euse or othe r o ncentrator ai r e nts from th e l reaction, p chemist Jon s s of thousan d n and titaniu m e s are catalys t a ccelerate th e w ithout bein g y d r ) r e s d g 2.3 1. Eg. 0 T ERMS s out itself be t is the s r ing in any c Positive a n Catalyst yst.eg.M O e the capacit y o hol retards t h Auto CataIf the pro d A A Cat & B = P t is a substa n i ng change mall amou n c hange in m a n d negativ e h elps in alteri n O acts as cat a y to retard t h h e conversio n lysis ucts of a rea c RCOReactants Reactant s = Product ure 3 . n ce that dec r d at the end o n t of substa n a ss and com p e catalyst n

g the veloci t a lyst in deco m h e chemical r e n of chlorofo r c tio act as a c OR H2O t Cycle C a t alysis c C a s 1 : Catal y sis C eases the ac t o f the chemi c (or) ce which a l p osition at t h t y of the reac t m positio of K e action and t rm to phosge n c atalyst for th e RCO r y ivation ene r c a l reaction l ters the vel o h e end of th e t io and the c K ClO hey are call e n e. reaction, it i s O H R oduct + Cata l ive r gy of a che m o city of rea c e reaction atalyst is cal l K Cand O2. S as negati v s referred to a s l ysts Chemistr reactio n c tion withou l e d as positiv e S ome cataly s v e catalyst .e g s autocatalysi s y n s s Sur M ( ( f Chemi asing energy A cata in mas s mang A sma l Some that is On th e amou anhydr an est e s try and Ca t S M OF C A ga dsorption of ga s a ctive sites. The m bonde Adsorption as s e chemically bon d St A (OR) C H lyst remains on qualitative a s ofchemical n an ese dioxide a te is left as a ll quantity of t imes a trace n ts. For exa m needed to c e other hand, n t to be effect i C us aluminiu m of 30 p

er ce n er t TALYTIC molecule molecules on to m olecules are ch e d to the catalyst parate atoms, in d d ed to catalyst ac t e pwise reacti o g ure 3.2 : Mec h H ARACT unchanged i n a n d quantitat i n ature. How e (MO2) use d a fine powd e catalyst is geof a metal c m ple, one ten - atalyse the d there are ca t i ve. Thus in F 5Cl m chloride f u n t of the mas s 6 ive sites n on catalyst s h anism of catal y E RISTICS mass and c i ve analysis s h e ver, it may u n d as a catalys t e r at the end nerally need e c atalyst is re q - millionth of i d ecompositi alysts whic h F riede AlCl3 C nctions as a c s of b enzene. 2 5 O urface y tic reaction FOR CAT A c hemical co m h ow that a ca t n dergo a phy s t in the therm a to the react i ed to produc e q uire to aff e i ts mass of fi n o n of hydro g h need to be p s reaction, H C H C c atalyst effe c For the aci d g a adsorbed position at t t alyst underg o s icalchange. ldecomposi on. almost unli m e ct very larg n ely divided p g en peroxid e p resent in re l C l c tively when p d and alkaline 2.3 s moleculedesorption of produc

tproduct t he end of th e o es no chang e T hus granula r n g of potassi u m ite reactio n e amounts o p latinu is al l e . atively larg e p resent to th e hydrolysis o 1 e e r n e e 2.32 Chemistry H ORCOOHesterthe rate of reaction is proportional to the concentration of the catalyst (HA catalyst is more effective when finely divided In heterogeneous catalysis, the solid catalyst is more effective when in a state of fine subdivision than it is used in bulk. Thus a lump of platinum will have much less catalytic activity than colloidal or platinised asbestos. Finely divided nickel is a better catalyst than lumps of solid nickel. A catalyst is specific in its action While a particular catalyst works for one reaction, it will not necessarily work for another reaction. Different catalysts, moreover, can bring about completely different reactions for the same substance. For example, ethanol ) when passed over hot aluminium oxide, C H OH CH O 2 5 2 2 2 ethene CHO) CHO2 5 3 2 ethanol A catalyst cannot, in general, initiate a reaction In most cases a catalyst speeds up a reaction already in progres

s and does not initiate (or start) the reaction. But there are certain reactions where the reactants do not combine for very long period (perhaps years). For example, a mixture of hydrogen and oxygen, which remains unchanged almost indefinitely at room temperature, can be brought to reaction by the catalyst platinum black in a few seconds. oomtem No reactionblaThus it is now considered that the catalyst can initiate a reaction. According to this view, the reacting molecules (in the absence of catalyst) do not possess minimum kinetic energies for successful collisions. The molecules rebound from collision without reacting at all A catalyst should remain unchanged in mass and chemical composition during end of the reaction. Surface Chemistry and Catalysis 2.33 Catalyst can alter only the speed of the reaction but it should not affect the equof the reaction. Catalysts are more active at its optimum temperature. Change of temperature alters the rate of a catalytic reaction as it would do for the same reaction without a catalyst. The catalytic activity can be altered by adding a small amount of foreign substance. Such subst

ances which catalyse the catalyst are called as promoters and the substance which inhibits the reaction are called as catalytic poisons or anti-catalyst. TYPES OF CATALYSIS When the reactants and the catalyst are in the same phase (i.e. solid, liquid or gas) it is said to be The depletion of ozone (O) in the ozone layer of the Earth’s atmosphere by chlorine free radicals (C) is a an example where the reactant and product exist in gaseous phase. Slow breakdown of manmade chlorofluorohydrocarbons (CFCs), release chlorine free radical into the atmosphere, which converts gaseous ozone to gaseous oxygen (O). Fischer esterification: Reaction of carboxylic acid with an alcohol involves the use of sulfuric acid as the catalyst and is an example where everything is contained in a liquid phase. HETEROGENEOUS CATALYSIS The catalytic process in which the reactants and the catalyst are in different phases is The catalytic converters in automobiles convert exhaust gases such as carbon monoxide (CO) and nitrogen oxides (NO) into more harmless gases like carbon dioxide (CO) and nitrogen (N). Metals (solids) like platinum (P), palladium

(P) and rhodium (R) are used as the catalyst. 2.34 Chemistry Manufacturing of sulfuric acid (H) involve solid vanadium pentoxide (V) as the catalyst to convert gaseous sulfur dioxide (SO) into gaseous sulfur trioxide (SO). Catalytic hydrogenation of liquid Unsaturated hydrocarbons (alkenes) reacts with gaseous hydrogen (H) to produce liquid saturated hydrocarbons (alkanes) where metals like platinum (P) and palladium (P) as the catalyst.. Haber Process The catalyst is porous iron prepared by reducing magnetite, F, with potassium hydroxide (KOH) added as a promoter. Positive Catalysis: When the rate of the reaction is accelerated by the foreign substance, it is said to be a positive catalyst and phenomenon as positive catalysis. Examples of positive catalysis are given below. Decomposition of KC(S) Negative Catalysis: There are certain, substance which, when added to the reaction mixture, ret ard the reaction rat e instead of increasing it. These are called catalyst and the phenomenon is known as negative catalysisSome examples are as follows. Oxidation of sodium sulphite lcohTetra Ethyl Lead (TEL) is added to petrol to r

etard the ignition of petrol vapours on compression in an internal combustion engine and thus minimize the knocking effect. Surface Chemistry and Catalysis 2.35 When one of the products of reaction itself acts as a catalyst for that reaction In autocatalysis the initial rate of the reaction rises as the catalytic product is formed, instead of decreasing steadily (Figure). The curve plotted between reaction rate and time shows a maximum when the reaction is complete Figure 3.3 : Rate of autocatalytic reaction A chemical reaction is said to have undergone autocatalysis, or be autocatalytic, if the reaction product is itself the catalyst for that reaction. Examples of Autocatalysis Hydrolysis of an Ester. The hydrolysis of ethyl acetate forms acetic acid (CHand ethanol. Of these products, acetic acid acts as a catalyst for the reaction. COOCCOOcatalyst Oxidation of Oxalic acid. Whenoxalic acid is oxidised by acidified potassium permanganate, manganous sulphate produced during the reaction acts as a catalyst for the reaction. + 3H 2 Mcatalyst Decomposition of Arsine. The free arsenic produced by the decomposition of arsine

(Aautocatalyses the reaction. catalyst Completion of reactionSigmoid Curveercentagereaction 2.36 Chemistry CATALYTIC POISONING AND PROMOTERS Promoters The activity of a catalyst can often be increased by addition of a small quantity of a second material. This second substance is either not a catalyst itself for the reaction or it may be a feeble catalyst. A substance which, though itself not a catalyst, promotes the activity of a catalyst is called a promoter. They are substances when added in small concentration can increase the activity of a catalyst. Molybdenum (M) or aluminium oxide (A) promotes the activity of iron catalyst in the Haber synthesis for the manufacture of ammonia. In some reactions, mixtures ofcatalysts are used to obtain the maximum catalytic efficiency. For example, in the synthesis of methanol (CHOH) from carbon monoxide and hydrogen, a mixture of zinc and chromium oxide is used as a catalyst. Explanation of Promotion ActionThe theory of promotion of a catalyst is not clearly understood. Presumably: Change of Lattice Spacing. The lattice spacing of the catalyst is changed thus enhancing the spaces

between the catalyst particles. The absorbed molecules of the reactant (say Hare further weakened and cleaved. This makes are reaction go faster. Increase of Peaks and Cracks. The presence of the promoter increases the peaks and cracks on the catalyst surface. This increases the concentration of the reactant molecules and hence the rate of reaction. feature of heterogeneous catalysis. Surface Chemistry and Catalysis 2.37 Distance between catalyst particlesCovalent bond much weakened and cleaves readilyFigure 3.4 : How the change of crystal lattice spacing of catalyst makes the reaction go faster. CATALYTIC POISONS Small amounts ofsubstances can reduce the activity of catalyst. If the reduction in activity is reversible, the substances are called inhibitors. Inhibitors are sometimes used to increase the selectivity of a catalyst by retarding undesirable reactions. A substance which destroys the activity of the catalyst to accelerate a reaction is called a poison and the process is called Catalytic Poisoning. The platinum catalyst used in the oxidation of sulphur dioxide (Contact Process), is poisoned by arsenic oxide (A

The iron catalyst used in the synthesis of ammonia (Haber Process) is poisoned by HThe platinum catalyst used in the oxidation of hydrogen is poisoned by carbon monoxide 2 2.38 Chemistry Temporary Poisoning Catalyst regains its activity when thPermanent Poisoning Catalyst cannot regain its activity even if the catalytic poison is removed. Eg.ASpoisons catalyst Pt permanently in manufacturing of SOThe poison is adsorbed on the catalyst surface in preference to the reactants. Even a monomolecular layer renders the surface unavailable for further adsorption of the reactants. The poisoning byAor CO appears to be of this kind. O O OFigure 3.5 : Poisoning of platinum catalyst by carbon monoxide The catalyst may combine chemically with the impurity. The poisoning of iron catalyst S falls in this class ACID AND BASE CATALYSIS A number of homogeneous catalytic reactions are known which are catalysed by acids or bases, or both acids and bases. These are often referred to asAcid-Base catalysts. Arrhenius pointed out that acid catalysis was, in fact, brought about by Hions supplied by strong acids, while base catalysis was caused b

y OHions supplied by strong bases. Sur ( ( ( ( I c m f Chemi Many re a o n transfer a r d ifferent wa y m ple Acid-s occur Base- ples o f A Inversio Decomp hanism of A I n acid cata l c omplex wit m echanis o s try and Ca t a ctions are c a r e esterificat i y s (Specific c a p ecific (acid c s in presence o s pecific (bas e e s in the pres e c id-Base C a n of Cane Su g C H 12 2 cane l Tautomeri s o sitio of Nit r NH is of an Este r -Base C a ly sis, the H + h the reacta n f keto-enol t a t alysis by b i o n and aldo l a talysis and g c atalysis) — D o f sulfuric ac i e catalysis) — e nce of sodiu m a talysis O H 2 2 11 2 s uga ofAceton e r amide : N O2 r H+ C OOC H 3 2 5 h yl acetate a talysis + (or a proto n n t, which the a utomeris o b oth acids an d l reaction. C a eneral cataly s D ecompositi — Addition o m hydroxide . + C H 6 1 gluc : N2 O H+ C or OH3 donated by n reacts to g o f acetone is: d bases. Typ i a talysis by ei t s is). of Sucros e o f hydrogen c . C H O 2 6 6 12 6 o se fructos e O C tic acid e y Bronsted a c g ive b

ack the i ca reaction s t he acid or b e into glucos e c yanide to a l O e 5 e thanol id) forms a n proton. For 2.3 catalysed by b ase can be i n e and fructos e l dehydes an d n intermediat e example, th e 9 y n e d e e 2.4 I t ( C A con It co n carb I n base cat a t o form an i n ( or Bronste d C COO– i o A PPLICA Catalytic A device i v ertin pollut a Catalytic erters are us e n verts three h on dioxide, n r ocarbons (c a Catalytic ic beads or d iu are use d C onsider th e Carbon e they get a d desorbed. y sis, the O H n termediate c o d base). For o ns may be e x NH ION OF C C onverters nt gases int o converters within inte r h armfu subs t n itrogen oxi d a use smog an d converters c o honeycomb . Alumina, C e reaction onoxide an d d sorbe to ca t c converters H – ion (or an y o mplex whic h example, t h x plaine as fo N O NHNO C in the exhau s o less harmful a re used for m r na combusti o t ances into h a d es (cause a c d respirator y o nsist of a sta i c oate with c C eri can als o Nitrogen m t alyst and res u can be affe c y Bronst

ed ba h then reacts h e decompo s fo llows: H 2 O t system of a ones. itigatin engines f u a rmless ones c c id rain and y problems) i n i nless steelb o c atalysts (Ex p o be used wh e 2 CO2 onoxide wil l u lt in formati o c ted by catal y a se) accepts or decompo s ition of nit r O H Intermediate Complex a motor vehic l u tomobile ex h u ele by eithe r c arbo mono x smog) into n n to carbon di o o x attached t o p ensive meta l en combined w N l be adsorbe d on of carbon d y tic poison. a proton fro m ses to regen e r amide by O l e, containin g h aust emissi o r petrol (gaso l x ide (a poiso n n itrogen an d o xide and w a o the muffler a l s like platinu m w it expensi v d on the surf a d ioxide and n Chemistr m the reacta n e rate the OH – O H– ions an d g a catalyst fo r o ns. Catalyti c l ine) or diese l n ous gas) int o d oxygen, an d a ter. d containin g m , Palladiu m v e metals.) a ce of cataly s n itroge whic h y n t d r c l d g m Surface Chemistry and Catalysis 2.41 Eg. Lead is a very good example for catalytic poisoning. It gets adsorbed to the honey co

mb of expensive metals and inhibits the function of catalyst. Catalytic converter has also forced the removal of lead from petrol. CatalystFigure 3.6 : Basic Catalytic Converter Petroleum Refining Fluid catalytic cracking: Breaking large hydrocarbon into smaller hydrocarbons. Catalytic reforming: Reforming crude oil to produce high quality gasoline component Hydrodesulfurization: Removing sulfur compounds from refinery intermediate products : Breaking large hydrocarbon molecules into smaller ones component Isomerization: Converting pentane into a high-quality gasoline component Chemicals and petrochemicals Haber process for ammonia production Styrene and Butadiene synthesis for use in producing synthetic rubber Contact process for production of sulfuric acid Ostwald process for production of nitric acid 2.42 Chemistry Methanol synthesis Production of different plastics and synthetic fabrics Other Fischer-Tropsch and Coal gasification processes for producing synthetic fuel gases and liquid fuels Various processes for producing many different medicine BIOLOGICAL CATALYST——ENZYMES Numerous organic reactions are taking place

in the body of animals and plants to maintain the life process. These reactions being slow remarkably catalysed by the organic compounds Enzymes. All enzymes have been found to be complex protein molecules. Thus: Enzymes are protein molecules which act as catalysts to speed up organic reactions in living cells. The catalysis brought about by enzymes is known as Enzyme Catalysis. Each enzyme is produced in a particular living cell to catalyse a reaction occurring in that cell. Many enzymes have been identified and obtained in pure crystalline state from the cells to which they belong. However the first enzyme as prepared by synthesis in the laboratory in 1969. Enzymes are substances found in biological systems that act as catalyst for specific biochemical process. Enzymes are usually protein or steroid which is synthesized in the living cells of animals and plants. Enzymes catalyze reactions inside organism. Enzymes possess a incredible capacity to carry out complex chemical reactions like hydrolysis, oxidation, reduction Eg.(i) Amylase is an enzyme which breaks down starch into glucose. (ii) Diastase converts starch to m

altose and maltase converts maltose to glucose 12 11 12 Sur reac f ace Chemi s Enzymes substrates b v e site is the l o t io catalyze d y mes do not Change t h KINET Freeenergy s try and Ca t speed up rea c b ind. This is t o catio on t h d energy barrierreactantscourse o e equilibriu m G for a reac t a nonsponta n I CS OF E N MENTEN the enzym e t alysis tions by lo w t erme as e enzyme su r Uncatalyst Reaction reaction 3.7 : Rate of fr e m constant fo r t io n eous reactio n N ZYME CA E QUATIO catalyzed r w erin activati n duced fit”. E r face where s ener barrireactant e e ener gy vs c o r a reaction n into a spont a TALYSE D N r n energy. M E nzymes ha v s ubstrates n Catalyst Reactio course of reac t o urse of reacti o a neous react i D REACTI O M any enzyme s v e active site s n d, and wher e n products ion o on. N OR MI C 2. s change shap e s . The enzym e e the chemic a C HAELIS e e a 2.4 S S 4 p 1: Format i p It can be r e the terms s tant. Rate of f Rate of b At stead y be temporar y Rearran m K i on o f enz ym osition o f e V m

ax ormation o f b state, the f o y . ing the abo v 1 K m m e substrate n z y me subs t a s follows: V max [S] [S] and th e f ES = k f ES = ( rmation an d k1 [E]×[S [ES] [ES] K and substit u [ES] [E ] complex rate compl ex e re is a const × [S] ) × [ES] d the b reakd = ( x[S] ] x[S] ) / K1 u ting in abo v ] [S] / K m , whi c o wn are equa l × [ES] v e equation a c h is known l . This stead y a nd we get Chemistr Michael state woul d y i s Surface Chemistry and Catalysis 2.45 The total amount of enzyme equals equals T] = [E] + [ES] [E] = [E ] [ES] Substitute the value of E Then [ES] [Et][S] [ES] [S] / K + Km This simplifies to: [ES] [Et] [S] / ([S]Multiplying both sides by the kinetic constant kgives the velocity of the reaction k3×[ET] ×(([S] ([S] + KM ) and substituting Vmax T] leads to the familiar form of the Michaelis Menten Equation: The above equation is called Michaelis –Menton equation. This equation is applicable to enzyme catalysed reaction having a single substrate. Aquantitative estimation of initial rate of reaction, maximum velocity and substrate concentration is c

ombined through a constant called Michaelis constant. First order reaction-When concen++ S then s is neglected and then the equation becomes max (M) Case 2: Zero order reaction-When concentration of substrate is high. X[S]max 2.46 Chemistry 0.150.100.05If K S then Km is neglected and equation becomes max ×[S] | [S] so Vmax = [S] then max [S] ] + [S] 2 max Rate (max [S] Vmax [S] ] Rate (max [S] Vmax [S] The above equation is similar to an equation of straight line . Agraph is plotted between 1/rate and 1/[S] we get a straight line. 0.1 0.0 0.1 0.2 0.30.3 Figure 3.8 where slope = Kmax and Intercept =1/Vmax 1/V Surface Chemistry and Catalysis 2.47 FACTORS AFFECTING ENZYME ACTIVITY Enzyme Concentration If we keep the concentration of the substrate constant and increase the concentration of the enzyme, the rate of reaction increases linearly. (That is if the concentration of enzyme is doubled, the rate doubles.) This is because in practically allenzyme reactions the molar concentration of the enzyme is almost always lower than that of the substrate. If we keep the concentration of the enzyme constant and increase the co

ncentration of the substrate, initially, the rate increases with substrate concentration, but at a certain concentration, the rate levels out and remains constant. So at some point, increasing the substrate concentration does not increase the rate of nd any active sites to attach to. Temperature For enzyme-catalyzed reactions, like all chemical reactions, rate increases with temperature. However, enzymes are proteins, and at higher temperatures proteins become denatured and inactive. Thus, every enzyme has an optimum temperature. Optimum temperature - the temperature at which enzyme activity is highest-usually about 25Effect of pH Small changes in pH can result in enzyme denaturation and loss of catalytic activity. Because the charge on acidic and basic amino acid residues located at the active site depends on pH. Most enzymes only exhibit maximum activity over a very narrow pH Most enzymes have an optimum pH that falls within the physiological range of 7.0- Notable exceptions are the digestive enzymes pepsin and trysin. pepsin (active in the stomach) - optimum pH of 1.5 trypsin (active in the small intestine) - optimum p