ERT 208/4 REACTION ENGINEERING: Catalysis & Catalytic Reaction Mechanism Part A: Heterogeneous Reaction By; Mrs Haf iza Bint i Shu kor ERT 208/4 REACTION ENGINEERING SEM 2 (2009/2010)

ERT 208/4 REACTION ENGINEERING SEM 2 (2009/2010) Students should be able to; DISCUSS the steps in a heterogeneous reaction (adsorption, surface reaction, and desorption) DESCRIBE the rate limiting step. EXPLAIN different types of surface reaction (single site, dual site, and Eley-Rideal) ANALYZE data to DEVELOP a rate law for Langmuir- Hinshelwood kinetics.

Catalysts?? ERT 208/4 REACTION ENGINEERING SEM 2 (2009/2010) Definition : Substance that effects the rate of reaction but emerge from the process unchanged. Its changes a reaction rate by promoting a different molecular path (mechanism) for the reaction with lower energy barrier (E). Eg; Gas (slow) Catalyst (fast) Catalyst (fast) Gas (slow) Energy Reaction Coordinate E

Catalysis?? ERT 208/4 REACTION ENGINEERING SEM 2 (2009/2010) Definition : is the occurrence, study, and use of catalyst & catalytic processes (action of catalyst) Homogeneous Catalysis Heterogeneous Catalysis 2 types ;

ERT 208/4 REACTION ENGINEERING SEM 2 (2009/2010) A) Homogeneous catalysis Catalyst is in solution with at least one of the reactants eq: Industrial oxo process for manufacturing normal isobutylaldehyde. REACTANT; Propylene CO H 2 CATALYST; Liquid phase cobalt complex B) Heterogeneous catalysis Involves >than 1 phase Usually, Catalyst (solid phase) & Reactant and gaseous phase). Common types (simple & complete separation of the fluid product mixture from the solid catalyst make it economically attractive cz can be reused) Eg; Dehydrogenation of cyclohexane using platinum-on-alumina catalyst.

Type Catalyst.. ERT 208/4 REACTION ENGINEERING SEM 2 (2009/2010) a)Porous  large area resulting from pores.  eg; Raney nickel used in the hydrogenation of vagetable & animal oils. Platinum on alumina used in the reorming of petroleum naphthas b) Molecular Sieves  so small pore- only admit small molecules but prevent large ones from entering (high selectivity)  usually derived from natural substances such as clays & be totally synthetic such as some crystalline aluminosilicates.

Type Catalyst..cont.. ERT 208/4 REACTION ENGINEERING SEM 2 (2009/2010) c) Monolithic  Can be nonporous  Sufficiently active so that no need to create porous catalyst  Normally encountered in process where pressure drop & heat removal are major considerations.  Platinum is a primary catalytic material in the monolith.  eg.; Platinum gauze reactor used in ammonia oxidation portion of nitric acid manufacture Catalytic converters used to oxidize pollutants in automobile exhaust. They can be porous (wire gauze)

Catalyst Properties..cont.. ERT 208/4 REACTION ENGINEERING SEM 2 (2009/2010) d)Supported  Catalyst consists of minute particles of an active material dispersed over a less active substance.  Active material – pure metal alloy (responsible for the principle reaction & chemical activity depends on the active catalyst agent)  Eg; Packed bed catalytic converter automobile Platinum on alumina catalyst used in petroleum reforming Vanadium pentoxide on silica used to oxidize sulfur dioxide in manufacturing sulfuric acid.

Catalyst Properties..cont.. ERT 208/4 REACTION ENGINEERING SEM 2 (2009/2010) e)Unsupported  Catalyst alone.  Eg; Platinum gauze for ammonia oxidation The promoted iron for ammonia synthesis Silica-alumina dehydrogenation catalyst used in butadiene manufacture.

ERT 208/4 REACTION ENGINEERING SEM 2 (2009/2010)  active ingredients added to increase catalyst activity Promoters Catalyst Deactivation  Caused by;  Aging phenomenon.. Gradual change in surface crystal structure by deposit of foreign material on active portion of the catalyst surface. It make decline in a catalysts activity as time progress.  Poisoning Irreversible deposition of a substance on the active site.  Coking… Deposit of other material on the entire surface

ERT 208/4 REACTION ENGINEERING SEM 2 (2009/2010) catalyst Mg + Ni/SiO Support catalyst Catalyst (active metal) promoter

ERT 208/4 REACTION ENGINEERING SEM 2 (2009/2010)  Collisions only result in the reaction if the particles collide with enough energy to get the reaction started.  This minimum energy required is called as “ ACTIVATION ENERGY”.  To increase the rate of reaction, number of successfully collision should be increase.  Therefore, we can use catalyst to DECREASE ACTIVATION ENERGY.  A catalyst accelerate reaction by provides alternative path to produce product. Catalyst & Activation Energy

ERT 208/4 REACTION ENGINEERING SEM 2 (2009/2010) Types Of Reactions & Representative Catalyst REACTION Halogenation- dehalogenation Hydration-dehydration Alkylation-dealkylation Hydrogenation- dehydrogenation Oxidation Isomerization CATALYST CuCl2, AgCl, Pd Al 2 O 3, MgO AlCl 3, Pd, Zeolites Co, Pt, Cr 2 O 3, Ni Cu, Ag, Ni, V 2 O 5 AlCl 3, Pt/Al 2 O 3, Zeolites

ERT 208/4 REACTION ENGINEERING SEM 2 (2009/2010)  Heterogeneous catalyst works in general term;  (1)Adsorption  (2)Surface Reaction  (3)Desorption HETEROGENEOUS CATALYTIC REACTIONS Fig 1: Schematic Diagram of Tubular Reactor Packed with Catalytic Pellets

ERT 208/4 REACTION ENGINEERING SEM 2 (2009/2010) Steps in Catalytic Reaction; HETEROGENEOUS CATALYTIC REACTIONS… cont…. 1)Mass transfer (diffusion) of the reactant(s)(eg. Species A) 2)Diffusion of the reactant from the pore mouth through the catalyst pores to the immediate vicinity of the internal catalytic surface. 3)Adsorption of reactant A onto the catalyst surface 4)Reaction on the surface of the catalyst (eg; A B) 5)Desorption of the products (eg. B) from the surface 6)Diffusion of the products from the interior of the pellet to the pore mouth at the external surface 7)Mass transfer of the products from the external pellet surface to the bulk fluid.

ERT 208/4 REACTION ENGINEERING SEM 2 (2009/2010) HETEROGENEOUS CATALYTIC REACTIONS… cont…. A B AB External Diffusion Internal Diffusion Catalytic Surface Very fast step Slowest step

ERT 208/4 REACTION ENGINEERING SEM 2 (2009/2010) Info ; Overall rate of reaction is equal to the rate of the slowest step in the mechanism. If diffusion steps (1,2,6 &7) > fast than reaction steps (3,4 &5) – conc in the immediate vicinity of the active sites are indistinguishable from those in the bulk fluid. - the transport / diffusion steps not effect the overall rate of the reaction. If reaction steps (3,4&5) > fast than diffusion steps; - mass transport does effect the reaction rate HETEROGENEOUS CATALYTIC REACTIONS… cont….

ERT 208/4 REACTION ENGINEERING SEM 2 (2009/2010) In system where diffusion from the bulk liquid to catalyst surface or to the mouths of catalyst pores effects the rate, changing the flow conditions past the catalyst should change the overall reaction rate. For porous catalyst, diffusion within the catalyst pores may limit the rate of reaction. The overall rate will be unaffected by external flow conditions even though diffusion effects the overall reaction rate. HETEROGENEOUS CATALYTIC REACTIONS… cont….

ERT 208/4 REACTION ENGINEERING SEM 2 (2009/2010) (1) ADSORPTION ISOTHERMS

ERT 208/4 REACTION ENGINEERING SEM 2 (2009/2010) Chemisorptions is usually a necessary part of a catalytic process. (1) ADSORPTION ISOTHERMS Where, S = active sites (alone will donate a vacant site with no atom, molecule/complex adsorp on it) A.S = mean one unit of “A” will be adsorbed on the site “S”.

ERT 208/4 REACTION ENGINEERING SEM 2 (2009/2010) Further definitions; (1)ADSORPTION ISOTHERMS… continue… Total molar concentration,Ct = (mol/g.cat) No. of active sites perunit mass Avogadro No. Molar concentration = of vacant sites,Cv No. of vacant sites perunit mass of catalyst Avogadro No. Pi = partial pressure of species “i” in the gas phase (atm) Ci.s= surface concentration of sites occupied by species “i” (g.mol/g.cat)

ERT 208/4 REACTION ENGINEERING SEM 2 (2009/2010) Site Balance; Total concentration of sites,Ct = (mol/g.cat) C v + C A.S. + C B.S Vacant Site Occupied Site catalyst A B C A.S. C B.S CvCv CvCv (1)ADSORPTION ISOTHERMS… continue…

ERT 208/4 REACTION ENGINEERING SEM 2 (2009/2010) By considering the adsorption of non reacting gas onto the surface of catalyst… Adsorption data are frequently reported in the form of “Adsorption Isotherm” Isotherm ??– portray the amount of a gas adsorbed on a solid at a different pressure but at one temperature. (1)ADSORPTION ISOTHERMS… continue…

ERT 208/4 REACTION ENGINEERING SEM 2 (2009/2010) Steps to Describe Isotherm?? 1)Proposed model system 2)Isotherm model is compared with the experimental data shown on the curve. 3)If model predicted the curve (agree with exp)-the model reasonably describe what is occurring physically in the real system. (1)ADSORPTION ISOTHERMS… continue…

ERT 208/4 REACTION ENGINEERING SEM 2 (2009/2010) Example: Adsorption of Carbon Monoxide on Metal 1 st Step: Proposed model system – 2 model proposed: a)As molecule (nondissosiated) - CO b) As atom (dissociative) - C and O (1)ADSORPTION ISOTHERMS… continue…

ERT 208/4 REACTION ENGINEERING SEM 2 (2009/2010) MODEL 1: As molecule (nondissosiated) – CO (MOLECULAR ADSORPTION) Rate of adsorption: Rate of desorption: Net rate of adsorption: (1)ADSORPTION ISOTHERMS… continue…

ERT 208/4 REACTION ENGINEERING SEM 2 (2009/2010) The rate of attachment (adsorption): Rate of adsorption for “CO” molecules to the surface directly proportional ;  to no. of collision that these molecules will make with the surface per second.  To the product of partial pressure of “CO” & conc of vacant sites. The rate of detachment (desorption): Rate of detachment for “CO” molecules to the surface directly proportional ;  Conc of sites occupied by molecules, C CO.S (1)ADSORPTION ISOTHERMS… continue…

ERT 208/4 REACTION ENGINEERING SEM 2 (2009/2010) k A independent of Temp k.A exponentially when Temp K A exponentially when Temp Is adsorption equilibrium constant (1)ADSORPTION ISOTHERMS… continue…

ERT 208/4 REACTION ENGINEERING SEM 2 (2009/2010) (1)ADSORPTION ISOTHERMS… continue… At equilibrium; Because of CO is the only material adsorbed on the catalyst, the site balance gives;

ERT 208/4 REACTION ENGINEERING SEM 2 (2009/2010) (1) ADSORPTION ISOTHERMS… continue… (1) In (2); (1) (2)

ERT 208/4 REACTION ENGINEERING SEM 2 (2009/2010) (1) ADSORPTION ISOTHERMS… continue… From, Rearranged; Equation of ADSORPTION ISOTHERM (Langmuir Isotherm) because Cco.s as a function of the partial pressure of CO

ERT 208/4 REACTION ENGINEERING SEM 2 (2009/2010) MODEL 2: As atom (dissosiated) – C and O (ATOM ADSORPTION) (1) ADSORPTION ISOTHERMS… continue… Rate of adsorption: Rate of desorption: Net rate of adsorption:

ERT 208/4 REACTION ENGINEERING SEM 2 (2009/2010) (1) ADSORPTION ISOTHERMS… continue… The rate of attachment (adsorption): Rate of adsorption for “C and O” atoms to the surface directly proportional ;  Pressure of CO in the system cz this rate governs the no. of gaseous collision with the surface  2 adjacent vacant site are required (C.S and O.S)  2 vacant site occuring adjacent to one another is proportional to square of conc of vacant sites.  To the product of CO partial pressure & square of vacant The rate of detachment (desorption): Rate of detachment for “C and O” atoms to the surface directly proportional ;  2 occupied sites must be adjacent  To the product of occupied site conc (C.S and O.S)

ERT 208/4 REACTION ENGINEERING SEM 2 (2009/2010) (1) ADSORPTION ISOTHERMS… continue… k A and k.A exponentially when Temp K A exponentially when Temp Is adsorption equilibrium constant

ERT 208/4 REACTION ENGINEERING SEM 2 (2009/2010) (1) ADSORPTION ISOTHERMS… continue… At equilibrium; Because of C and O is the only material adsorbed on the catalyst, the site balance gives;

ERT 208/4 REACTION ENGINEERING SEM 2 (2009/2010) (1) ADSORPTION ISOTHERMS… continue… For ; From,

ERT 208/4 REACTION ENGINEERING SEM 2 (2009/2010) (1) ADSORPTION ISOTHERMS… continue… Substitute in site balance; From site balance, Solving for Cv;

ERT 208/4 REACTION ENGINEERING SEM 2 (2009/2010) (1) ADSORPTION ISOTHERMS… continue… From; and Solving for Co.s ;

ERT 208/4 REACTION ENGINEERING SEM 2 (2009/2010) (1) ADSORPTION ISOTHERMS… continue… Taking inverse for both sites of equations,,,,then multiplying by ; From ;

ERT 208/4 REACTION ENGINEERING SEM 2 (2009/2010) 2 nd Step - Isotherm model is compared with the experimental data shown on the curve. To checking whether a model (molecular VS dissociative adsorption) predicts the behavior of the experimental data is to linearize the models equation and then plot. 3 rd Step - If model predicted the curve (agree with exp)-the model reasonably describe what is occurring physically in the real system. If linear – we can accept the model (1) ADSORPTION ISOTHERMS… continue…

ERT 208/4 REACTION ENGINEERING SEM 2 (2009/2010) 1)Assume sequence step in reaction (such a mechanism – OR site reaction) 2)Assume reversible step 3)Rate limiting step is postulate 4)Step are not limiting is use to ……….coverage dependent terms. Lang Muir Hinshelwood Approach …

ERT 208/4 REACTION ENGINEERING SEM 2 (2009/2010) (2) SURFACE REACTIONS

ERT 208/4 REACTION ENGINEERING SEM 2 (2009/2010) The rate of adsorption of species A onto solid SURFACE; (2) SURFACE REACTIONS… … Rate of adsorption: Rate of desorption: Net rate of adsorption:

ERT 208/4 REACTION ENGINEERING SEM 2 (2009/2010) (2) SURFACE REACTIONS… …cont.. Is adsorption equilibrium constant

ERT 208/4 REACTION ENGINEERING SEM 2 (2009/2010) (2) SURFACE REACTIONS… …cont.. Once the reactant has been adsorbed onto the surface, it is capable of reacting in a number of ways to form the reaction product. 3 ways……. a)Single site mechanism b)Dual site mechanism c)3 rd mechanism Langmuir Hinshelwood kinetic i)Reactant interact with another site. ii)Reaction between 2 adsorb species iii)2 species adsorb on different types of site (S and S’)

ERT 208/4 REACTION ENGINEERING SEM 2 (2009/2010) (2) SURFACE REACTIONS… …cont.. a)Single site mechanism………….  Only the site on which reactant is adsorbed is involved in the reaction. Eg: an adsorbed molecules of A may isomerizes (or perhaps decompose) directly on the sites to which it is attached. catalyst C A.S. C B.S catalyst AB Single site

ERT 208/4 REACTION ENGINEERING SEM 2 (2009/2010) (2) SURFACE REACTIONS… …cont.. Rate of adsorption: Rate of desorption: Net rate of adsorption: Surface reaction equilibrium constant Because in each step the reaction mechanism is ELEMENTARY, the rate law becomes;

ERT 208/4 REACTION ENGINEERING SEM 2 (2009/2010) (2) SURFACE REACTIONS… …cont.. b) Dual site mechanism…………. i) Adsorbed reactant interacts with another site (either occupied) to form product. Eg: adsorbed A may react with an adjacent vacant site to yield a vacant site and a site on which the product is adsorbed. catalyst C A.S. C B.S catalyst AB Dual site S S

ERT 208/4 REACTION ENGINEERING SEM 2 (2009/2010) (2) SURFACE REACTIONS… …cont.. Rate of adsorption: Rate of desorption: Net rate of adsorption: Surface reaction equilibrium constant

ERT 208/4 REACTION ENGINEERING SEM 2 (2009/2010) (2) SURFACE REACTIONS… …cont.. b) Dual site mechanism…………. ii) Reaction between 2 adsorbed species catalyst C A.S. C C.S catalyst AC Dual site C B.S. B C D.S. D

ERT 208/4 REACTION ENGINEERING SEM 2 (2009/2010) (2) SURFACE REACTIONS… …cont.. Rate of adsorption: Rate of desorption: Net rate of adsorption: Surface reaction equilibrium constant

ERT 208/4 REACTION ENGINEERING SEM 2 (2009/2010) (2) SURFACE REACTIONS… …cont.. b) Dual Site mechanism…………. iii) Reaction of 2 species adsorbed on different types of site S and S’ catalyst C A.S. C D.S catalyst CD Dual site BA C B.S’ C C.S’

ERT 208/4 REACTION ENGINEERING SEM 2 (2009/2010) (2) SURFACE REACTIONS… …cont.. Rate of adsorption: Rate of desorption: Net rate of adsorption: Surface reaction equilibrium constant

ERT 208/4 REACTION ENGINEERING SEM 2 (2009/2010) (2) SURFACE REACTIONS… …cont.. c) 3 rd mechanism………….  reaction between an adsorbed molecules & a molecules in the gas phase catalyst C A.S. C C.S catalyst D C site B A

ERT 208/4 REACTION ENGINEERING SEM 2 (2009/2010) (2) SURFACE REACTIONS… …cont.. Rate of adsorption: Rate of desorption: Net rate of adsorption: Surface reaction equilibrium constant

ERT 208/4 REACTION ENGINEERING SEM 2 (2009/2010) (3) DESORPTION

ERT 208/4 REACTION ENGINEERING SEM 2 (2009/2010) (3) DESORPTION… …..  in each the cases before, the products of the surface reaction adsorbed on the surface are subsequently desorbed into the gas phase. catalyst C C.S. S catalyst C Single site C

ERT 208/4 REACTION ENGINEERING SEM 2 (2009/2010) (2) SURFACE REACTIONS… …cont.. Rate of adsorption: Rate of desorption: Net rate of adsorption: Is just the reciprocal of the adsorption equilibrium constant for “C”, Kc Rate of desorption of “C” just a reverse of the rate of adsorption for “C”

ERT 208/4 REACTION ENGINEERING SEM 2 (2009/2010) End of Part A