Lec 1 Catalysis and Catalytic Reactionseacademic.ju.edu.jo/z.hamamre/Material/Advance Chemical... · 2012. 8. 29. · 1 Dr.-Eng. Zayed Al-Hamamre Advance Chemical Reaction Engineering

Chemical Engineering Department | University of Jordan | Amman 11942, JordanTel. +962 6 535 5000 | 22888

1

Dr.-Eng. Zayed Al-Hamamre

Advance Chemical Reaction Engineering

Catalysis and Catalytic Reactions


2

Content

Catalysts

Catalyst Structure and Properties

Adsorption to the Catalyst Surface

Catalyst Active Site

Steps in a Catalytic Reaction

Concentration Profiles

Adsorption Isotherms

Adsorption rate

Surface Reaction


3

The first observed uses of catalysts were in the making of wine, cheese, and bread.

Catalysis is a term coined to describe the property of substances that affects the rate of a chemical reactions without being consumed in them.

Catalysts provide an alternate path for the reaction to occur

Catalysts

Catalyst can provide a lower barrier for the desired reaction, leaving the undesired reaction rate unchanged

The catalyst provides a lower energy barrier path


4

Catalysts can strongly regulate reactions because they are not consumed as the reaction proceeds.

A catalyst changes only the rate of a reaction; it does not affect the equilibrium.

Very small amounts of catalysts can have a profound effect on rates and selectivity's.

However, catalysts can undergo changes in activity and selectivity as the process proceeds.

They are subject to deactivation, which refers to the decline in a catalyst’s activity as time progresses.

Catalyst deactivation may be caused by

An aging phenomenon, such as a gradual change in surface crystal structure, or by

The deposit of a foreign material on active portions of the catalyst surface (poisoning or fouling of the catalyst).

Catalysts


5

Homogeneous catalysis concerns processes in which a catalyst is in solution with at least one of the reactants

Catalysts can be

Catalysts

A heterogeneous catalytic process involves more than one phase, usually the catalyst is a solid and the reactants and products are in liquid or gaseous form


6

Catalyst Structure and Properties Most solid catalysts are supplied as cylindrical

pellets with lengths and diameters in the range of 2–10 mm.

More complex shapes and monoliths can be used when it is important to minimize pressure drop.

The catalyst is micro-porous with pores ranging in diameter from a few angstroms to a few microns.

The pores may have a bimodal distribution of sizes

The internal surface area, accessible through the pores, is enormous, up to 2000m2 per gram of catalyst

A catalyst may consist of minute particles of an active material dispersed over a less active substance called a support

The active material is frequently a pure metal or metal alloy.


7

Adsorption to the Catalyst Surface For gas-phase reactions catalyzed by solid surfaces, reactants must become attached

(adsorbed) to the surface.

This adsorption takes place by physical adsorption or chemisorption.

Physical adsorption is similar to condensation.

The process is exothermic,

the heat of adsorption is relatively small, being on the order of 1 to 15 kcal/g mol.

The forces of attraction between the gas molecules and the solid surface are weak.

The amount of gas physically adsorbed decreases rapidly with increasing temperature,

Above its critical temperature only very small amounts of a substance are physically adsorbed.

Physical adsorption


8

This type of adsorption affects the rate of a chemical reaction.

The adsorbed atoms or molecules are held to the surface by valence forces of the same type as those that occur between bonded atoms in molecules.

As a result the electronic structure of the chemisorbed molecule is perturbed significantly, causing it to be extremely reactive

Chemisorption

Chemisorption is an exothermic process,

The heats of adsorption are generally of the same magnitude as the heat of a chemical reaction (i.e., 10 to 100 kcal/g mol).

If a catalytic reaction involves chemisorption, it must be carried out within the temperature range where chemisorption of the reactants is appreciable

Interaction with the catalyst causes bonds of the adsorbed reactant to be stretched making them easier to break



9



10

A reaction is not catalyzed over the entire solid surface but only at certain active sites or centers.


An active site as a point on the catalyst surface that can form strong chemical bonds with an adsorbed atom or molecule

One parameter used to quantify the activity of a catalyst is the turnover frequency, N.

It is the number of molecules reacting per active site per second at the conditions of the experiment.

When a metal catalyst such as platinum is deposited on a support, the metal atoms are considered active sites. The dispersion, D, of the catalyst is the fraction of the metal atoms deposited that are on the surface.


11



12



13


Turnover frequency f (TOR) an expression used to quantify the activity of a catalyst.

It is the number of molecules reacting per active site per second at the conditions of the experiment

When a metal catalyst such as platinum is deposited on a support, the metal atoms are considered active sites.

The dispersion, D, of the catalyst is the fraction of the metal atoms deposited that are on the surface.


14

Example

The number of molecules reacting per active site per second

The dispersion, D, i.e., the fraction of the metal atoms deposited that are on the surface.


15

Example Cont.


16

Range of turnover frequencies as a function for different reactions and temperatures.



17

For H2 chemisorption on Pt

Dihydrogen is perhaps the most common probe molecule to measure the fraction of exposed metal atoms



18

Catalyst Active Site The dispersion of Pt, or the fraction of exposed metal atoms,

As a rule of thumb for spherical particles the Pt particle diameter is


19

Length scales in the reactor


20

Possible pore structures


21

Various forms of catalyst particles


22



23


The overall rate of reaction is equal to the rate of the slowest step in the mechanism.


24


Concentration profile of a reacting species in the vicinity of a porous catalyst particle. Distances are not to scale.


25



26



27

Reactant concentration profiles in x-direction perpendicular to the flow direction z expected for flow over porous catalyst pellets. External mass transfer and pore diffusion produce the reactant concentration profiles shown



28


Reactant concentration profiles around and within a porous catalyst pellet for the cases of reaction control, external mass transfer control, and pore diffusion control. Each of these situations leads to different reaction rate expressions.


29

Diffusion from the Bulk to the External Transport

: The diffusion coefficient


30

The external resistance decreases as

The velocity across the pellet is Increased,

The particle size is decreased.

The boundary layer becomes smaller and the mass transfer coefficient (mass transfer rate) increases,

Diffusion from the Bulk to the External Transport


31

Reactant Concentration Profiles

krkr

kr

Reactant concentration profiles around a catalyst pellet for reaction control and for external mass transfer control


32

Internal Diffusion

For large pellets, it takes a long time for the reactant A to diffuse into interior compared to the time it takes for the reaction to occur on the interior surface

The reactant is only consumed n the exterior surface of the pellet and the catalyst near the center of the pellet wasted catalyst

For very small pellets it takes very little time lo diffuse into and out of the pellet interior and, as a result, internal (fusion no Longer limits the rate of reaction.

The rare of reaction


33

Reactant concentration profiles within a porous catalyst pellet for situations where surface reaction controls and where pore diffusion controls the reactions.

Reactant Concentration Profiles


34


The adsorption of A on a site S is represented by

The total molar concentration of active sites per unit mass of catalyst is equal to the number of active sites per unit mass divided by Avogadro's number

The molar concentration of vacant sites, is the number of vacant sites per unit mass of catalyst divided by Avogadro's number

In the absence of catalyst deactivation, we assume that the total concentration of active sites remains constant

Define

Adsorption data are frequently reported in the form of adsorption isotherms.

Isotherms portray the amount of a gas adsorbed on a solid at different pressures but at one temperature


35


The site balance

Adsorption of carbon monoxide

As molecules

Example

molecular or nondissociated adsorption


36

As oxygen and carbon atoms

Example Cont.

dissociative adsorption

Adsorption rate

i. Adsorption of carbon monoxide as molecules

Considered as an elementary reaction

The rate of attachment of to the active site on the surface is proportional to the number of collisions that these molecules make with a surface active site per sec.

The collision rate is, in turn, directly-proportional to the carbon monoxide partial pressure


37

Adsorption Rate

The net rate of adsorption

or


38

At equilibrium, the net rate of adsorption equals zero

Adsorption Rate

But the site balance

Then


39

Adsorption Rate

Langmuir isotherm for adsorption of molecular carbon monoxide


40

Adsorption Rate

ii. The isotherm for carbon monoxide adsorbing as atoms is derived

where


41

Adsorption Rate

the site balance

Then


42

Adsorption Rate

The adsorption isotherm of A in the presence of adsorbate B is given by

Show this


43

Surface Reaction1. The surface reaction may be a single-site mechanism in which1 only the site on which the

reactant is adsorbed is involved in the reaction.

Because in each step the reaction mechanism is elementary


44

2. The surface reaction may be a dual-site mechanism in which the adsorbed reactant interacts with another site (either unoccupied or occupied) to form the product

Surface Reaction


45

3. Eley-Rideal: the reaction take place between an adsorbed molecule and a molecule in the gas phase

Surface Reaction


46

Desorption

For the desorption of a species

The desorption step for C is just the reverse of the adsorption step for C

And


47

The Rate-Limiting Step

At steady state,

i.e. the rates of each of the three reaction steps in series (adsorption, surface reaction, and desorption) are equal.

The slowest step in the reaction mechanism represents the rate limiting or rate-controlling step.

The approach in determining catalytic and heterogeneous mechanisms is usually termed the Lungmuir-Hinshelwood approach


48

Synthesizing a Rate Law, Mechanism, and Rate-Limiting Step The decomposition of cumene to form benzene and propylene.

Sequence of steps in reaction-limited catalytic reaction


49

The decomposition of cumene is represented by a series of elementary reactions

Synthesizing a Rate Law, Mechanism, and Rate-Limiting Step

1.


50


Where

2.


51


Propylene is not adsorbed on the surface

3.

But


52

At steady state, there is no accumulation of reacting species on the surface the rates of each step in the sequence are all equal:


If the Adsorption of Cumene Rate-Limiting

The reaction rate constant of this step (in this case kA) is small with respect to the specific rates of the other steps (in this case kS and kD), i.e.


53


But

But


54


At equilibrium

Overall partial pressure equilibrium constant


55


And

: The change in the Gibbs free energy

The concentration of vacant sites, CV can be eliminated


56


Initially


57

Synthesizing a Rate Law, Mechanism, and Rate-Limiting StepIf the Surface Reaction Rate-Limiting

The rate of surface reaction is

kA is large by comparison when surface reaction is controlling


58


The concentration of vacant sites, CV can be eliminated

The initial rate is


59


At low partial pressures of curnene

At high partial pressures


60

If the Desorption of Benzene Rate-Limiting


Initially

the initial rate would be independent of the initial partial pressure of cumene


61

Synthesizing a Rate Law, Mechanism, and Rate-Limiting Step The experimental observations

The rate law derived by assuming that the surface reaction is rate-limiting agrees with the data

Actual initial rate as a function of partial pressure of cumene


62

If the feed contains inert, then it would not participate in the reaction but would occupy sites on the catalyst surface


The site balance becomes

Since the adsorption of the inert is at equilibrium, the concentration of sites occupied by the inert is

Then, the rate of reaction is


63

Temperature Dependence of the Rate Law

The specific reaction rate, k, will usually follow an Arrhenius temperature dependence and increase exponentially with temperature

The adsorption of all species on the surface is exothermic.

o The higher the temperature, the smaller the adsorption equilibrium constant. o As the temperature increases, KA and KB decrease resulting in less coverage of the surface

by A and B

o At high temperatures

For a surface-reaction-limited irreversible isomerization


64

Temperature Dependence of the Rate Law

Initial rare versus total pressure for various rate controlling steps


65

Design of Reactors for Gas-Solid Reactions For an ideal batch reactor, the differential form of the design equation for a heteriogeneous

reaction is

For a packed-bed reactor, the differential form of the design equation for a heterogeneous reaction is

The design equation for a perfectly mixed "fluidized" catalytic reactor can be replaced by that of a CSTR


66

Heterogeneous Data Analysis for Reactor Design

Data obtained form differential reactor for the Hydrogen and toluene reaction over a solid mineral catalyst containing clinoptilolite (a crystalline silica-alumina)

To design the PBR, the rate law must be determined


67

Dependence on the product methane


If the methane were adsorbed on the surface, the partial pressure of methane would appear in the denominator of the rate expression and the rate would vary inversely with methane concentration:

From runs 1 and 2


68


Dependence on the product benzene

In runs 3 and 4,


69

The rate decreases with increasing concentration of benzene. A rate expression in which the benzene partial pressure appears in the denominator could explain this dependency:


Dependence on toluene

Runs 10 and 11 and runs 14 and 15


70

Heterogeneous Data Analysis for Reactor Design At low concentrations of toluene (runs 10 and 1 l), the rate increases with increasing partial

pressure of toluene, while at high toluene concentrations (runs 14 and 15), the rate is essentially independent of the toluene partial pressure. A form of the rate expression that would describe this behavior is

The rate law may be of the form


71

Dependence on hydrogen


Runs 7, 8, and 9

The rate increases linearly with increasing hydrogen concentration and we conclude that the reaction is first-order in H2

hydrogen is either not adsorbed on the surface or

it’s coverage of the surface is extremely low


72


Combining


73

Evaluation of the Rate Law Parameters

The multiple regression techniques can be used to evaluate the rate parameters.

One can use the linearized least squares analysis to obtain initial estimates of the parameters k, KT, KB , in order to obtain convergence in nonlinear regression.

Selecting specific experiments to evaluate the rate parameters.


74

Example


75

Example Cont.


76

Example Cont.

Documents

Lec 1 Catalysis and Catalytic Reactionseacademic.ju.edu.jo/z.hamamre/Material/Advance Chemical... · 2012. 8. 29. · 1 Dr.-Eng. Zayed Al-Hamamre Advance Chemical Reaction Engineering