CHEMISTRY 3030: CATALYSIS COURSE, 2011

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INTRODUCTION

WHAT IS THE DEFINITION OF A CATALYST?

Given the following reaction at equilibrium: A2(g) +2B2(g) 2AB2(g)

Q: What will happen to the equilibrium position in such a reaction, if a small quantity of a catalyst were to

be added? A:

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INTRODUCTION

HENCE: A catalyst is any substance (____________) which, when present in a reaction mixture is directly involved with the reaction sequence (mechanism), and that increases the reaction rate (_________) without altering the position of the thermodynamic equilibrium, but is itself not consumed or altered.

WHAT THEN IS CATALYSIS?

A:

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INTRODUCTION

Reactants

= uncatalysed reaction

Energy

Reaction co-ordinate

Products

Activation Energy

Enthalpy change

HOW DOES A CATALYST DO THIS?

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THREE KEY ISSUES TO CONSIDER WHEN DEVELOPING A CATALYST:

1. ACTIVITY (A)

This a measure of the _________ at which the catalyst is able to transform reactants into products. This speed is related to the rate constant ‘k’ i.e. Rate = –k [reactants]n

The activity (A) of a catalyst is measured by the SI unit: katal (abbreviated to kat).

If the activity of a catalyst is 1 kat : then it ‘enables’ the reaction rate to be___________

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Often it is necessary to disperse (scatter) the catalyst (or ________) on a solid material which has a high surface area. This material is called a _________. Examples of supports are: Al2O3(s), TiO2(s), CaCO3(s), carbon nanotubes (CNTs), etc.

In these cases, the _________ at which the reactions of supported catalysts are measured, have units of molecules converted/ surface area of exposed active phase (in cm2).

If however the number of exposed catalyst sites have been determined experimentally (for later) then the rate has units of molecules converted/ exposed catalyst sites ____________________.

CONT.

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CONT.

2. SELECTIVITY (S)

Multiple products are often formed in a reaction when a catalyst is added. The catalyst thus has an activity for each reaction that leads to a different product.

The catalyst selectivity is then just a ratio of the activity of one product over another (more about this later). The larger the ratio the higher the catalyst selectivity for that product.

Alternatively selectivity can be viewed as the ability of a catalyst to _____________ the rate of _____________of the thermodynamically feasible reactions more than the others.

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CONT.

Consider the following reactions and then place them in decreasing order of the catalyst selectivity:

1) CO(g) + H2(g) → CH3OH(g) = 1.0 x 10-1 Kat

2) CO(g) + 2H2(g) → ½C2H5OH(g) = 2.5 x 10-2 Kat

3) CO(g) + 3H2(g) → CH4(g) + H2O(g) = 9.8 x 10-2 Kat

ORDER: _____________

Cu/Zn/Al2O3

Cu/Zn/Al2O3

Cu/Zn/Al2O3

3. DEACTIVATION

For a variety of reasons catalysts can lose their activity and hence their selectivity ___________.

Some reasons:

1) SINTERING (_______________)

Supported metal catalyst particles are oftentimes more stable when they are ____________or spread out on the support surface.

CONT.

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CoO nanoparticle

Carbon nanotube

___________ occurs at high temperatures when the supported metal catalyst particles spontaneously migrate on the surface. They combine/coalesce with one another to form bigger particles. Hence the metal catalyst ________________.

CONT.

10A.Binder et al. J. Phys. Chem. C 2010, 114, 7816–7821 (Pd on SiO2 or TiO2)

2) POISONING

Some elements/ions (e.g. Cl, S, C, etc.), when they build up in concentration, can block active sites on the surface of a catalyst and hence reduce the activity and selectivity of the catalyst.

For example: ___________, in leaded petrol, can deactivate catalytic converters in cars.

CONT.

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TYPES OF CATALYSIS Several types of catalysis:

1) Homogeneous CatalysisWhen the reactants and the catalyst

are in the ___________:

e.g. O3(g) + A·(g) → O2(g) + AO(g) ….(1)

AO(g) + O3(g) → A·(g) + O2(g)

….(2)Q: Which species is the catalyst and which the intermediate?

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2) Heterogeneous CatalysisWhen the reactants and the catalyst are in

_________________:

For example: The photoreduction of carbon dioxide on titania

Pt/TiO2(s)

CO2(g) CO(g) + O2(g) hν

TYPES OF CATALYSIS

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Examples of heterogeneous catalysis:

1) CO + ½ H2 CO2 (Co, Co-Ni) - Fuel cells. Selec

Ox. of CO in H2

2) CH2CH2 + H2 CH3CH3 (Ni/Pd) Olefin

Hydrogenation – Fuel industry. c.a. 119 million tonnes of C2H4 in 2010!)

3) CHCH + 2H2 CH3CH3 (Pt/Pd) Removal of C2H2

from olefins by hydrogenation)

4) CO + H2 -CH2n+2- (Fe, Co) Fischer-Tropsch.

Fuels, waxes, etc.5) N2 + 3H2 2NH3 (Fe, Ru) Haber Process.

Fertilizers, explosives..6) C6H6 + 3H2 C6H12 (Cu -1925!, Ru,Ni) 90% of

cyclohexane used for Nylon 6 and 66.

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3) Enzyme Catalysis

Enzymes are polymeric molecules which regulate the majority of __________ reactions that take place in living organisms. In the main they are proteins which are made up of amino acid building blocks or ____________.

Enzymes are ___________and have extremely ________________(typically between 10 to 103 molecules converted/enzyme/s).

TYPES OF CATALYSIS

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Q: How does an enzyme catalyse a reaction? A: The reactant molecules that interact with an

enzyme are called ___________.

Each enzyme has a specific site (_________) where only certain shaped substrates can fit into or bind (______________). When the substrate ___________binds to the active site the enzyme changes shape (___________) to form the ___________.

The reaction then takes place, product/s formed and released, enzyme returns to its original shape.

CONT.

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CONT.

http://www.mun.ca/biology/scarr/F09-20bsmc.jpg

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4) Polymer Supported Catalysis

“These are catalyst systems comprising a polymer

support (often based on _________________in the

form of ___________to pack in a reactor) in which

catalytically active species are ___________through

chemical bonds or weaker interactions such as

hydrogen bonds or donor–acceptor interactions

and can be used repeatedly.”

PAC, 2004, 76, 889 (Definitions of terms relating to reactions of polymers and to functional polymeric materials (IUPAC Recommendations 2003)) on page 896

TYPES OF CATALYSIS

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Example: Pd nanoparticles (PdNPs) immobilised on microporous Poly(amidoamine) (PAMAM) dendrimers.

CONT.

Shin Ogasawara and Shinji Kato J. AM. CHEM. SOC. 2010, 132, 4608–4613

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Suzuki-Miyaura reaction in water.

CONT.

Shin Ogasawara and Shinji Kato *4608 9 J. AM. CHEM. SOC. 2010, 132, 4608–4613

Advantages: • Water as a solvent for organic rxn! GREEN rxn!•In bio-active compounds synthesized the catalyst is easily retrievable : No ___________ of the product! •Reduced costs (Pd=$$$); catalyst ___________

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CLASSIFICATION OF HETEROGENEOUS CATALYSTS (BOND pg10)

Catalyst Type: Examples: Reactions:

Metals Ni, Pd, Fe, Pt, Ag HydrogenationDehydrogenationHydrogenolysisOxidation

Semiconducting oxides/sulphides

NiO, ZnO, MnO2, Cr2O3, Bi2O3-MoO3, WS2

DehydrogenationDesulphurisationOxidationHydrogenation

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Catalyst Type: Examples: Reactions:

Insulating oxides Al2O3, SiO2, MgO Dehydration

Acids SiO2-Al2O3, Zeolites

Polymerisation, Isomerisation, cracking, alkylation

CLASSIFICATION OF HETEROGENEOUS CATALYSTS (BOND pg10)

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Catalysts and Surfaces

A reactant must react with the surface atoms of a catalyst. Hence the more atoms on the surface, the more reactants can be transformed into products. Thus the expectation that high surface areas lead to ___________.

Surface Areas fall into three categories:

1. 10 m2 g-1  Small (e.g. _______________)

2. 200 m2 g-1  Normal (e.g. _______________)3. 1200 m2 g-1  Large (e.g. _______________)

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Catalysts and Surfaces

The rate might be expected to be __________ to the number of surface atoms BUT not all surface atoms are the _______. This gives rise to the concept of an _____________(Taylor 1925).

Q: How then can the surface area be maximized?A: _______________

For spherical particles on a hemispherical support, the total surface area is give by

SA (in m2)= ____________________Where M = total mass of catalyst, = density and r = average particle radius, Vpart= volume of particle.

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Catalysts and Surfaces

Q: A batch of hemispherical catalysts (support and active nanoparticles) weighs 1.23 g and has a density of 3.14 g/ml. What is the total surface area of the catalysts if they are loaded with spherical nanoparticles with diameters of 50 nm?

A: SA = _______________

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A2: __________

Factor: Inorganic support

Organic support

Effect of Temperature

Good _______

Heat transfer

Good _______

Chemical reactivity

OK _______

Q: How then can the surface area be maximized?

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A2: __________

Promoters are substances that increase the _______ _______, even though they are not catalysts by themselves. In addition they “allow the active phase to function at its _______________” (Bond pp 76)

e.g. Co or Ni in WS2 catalyst for desulphurisation†

† C. Roukoss et al. / C. R. Chimie 12 (2009) 683-691

There are two types of promoters

1._________2._________

Q: How then can the surface area be maximized?

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1.Structural

A structural promoter ___________by separating the surface ___________.

For e.g. The active phase for the NH3 synthesis

catalyst is Fe, but its promoters may include:_________, ________, _____ and ______*. These inhibit Fe crystallites from coalescing.

*I. Siminiceanu, I. Lazau, Z. Ecsedi, L. Lupa*, C. Burciag.Chem. Bull. "POLITEHNICA" Univ.

(Timisoara) Volume 53(67), 1-2, 2008.

Types of promoters:

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2.Electronic

Electronic promoters are effective due to their ______________. The most widely used electronic promoters belong to _______, _______ and the _______.

For Groups 1A and 2A, their ability to promote is inversely proportional to their electronegativity. Examples: Cs > K > Na (Group 1A)+

Ba > Ca > Mg (Group 2A)++

+ S. Murata, K. Aika, T. Onishi, Chem. Lett., 1990, p. 1067.

++ S. R.Tennison, in: J.R. Jennings (Ed.), Catalytic Ammonia Synthesis, Fundamentals and Practice, Plenum

Press, New York, 1991, p. 303.

Types of promoters:

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Electronic and structural promoter?

Y.V. Larichev. Effect of Cs+ Promoter in Ru/MgO Catalysts. J. Phys. Chem. C 2011, 115, 631–635

When Cs is added: •No Caesium ruthenates form. •The electronic properties change. •RuO2 clusters reduced to metallic Ru. Increased Activ.

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Given the following data for the lanthanides, place them in order of increasing ability to act as electronic promoters:

______________

Problem

Element Pauling’s Electronegativity value

Samarium 1.198

Lutetium 1.201

Lanthanum 1.101

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A3: ______/ _______ maximization

Q: How then can the surface area be maximized?

TEOS = Tetraethyl orthsilicate

Polymers include: polyvinylalcohol and polyethylene glycol with different molec.masses

Sol-gel process

S SATO, T MURAKATA, T SUZUKI,T OHGAWARA. JOURNAL OF MATERIALS SCIENCE 25 (1990) 4880-4885

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A3: _______/ _______maximization

Q: How then can the surface area be maximized?

S SATO, T MURAKATA, T SUZUKI,T OHGAWARA. JOURNAL OF MATERIALS SCIENCE 25 (1990) 4880-4885

No polymer

Polymer of low molec. mass

Polymer of higher molec. mass

Mean pore sizes increase from 3 nm to 7 nm

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ATOM SURFACE CONCENTRATION

The atom surface concentration can be determined by the _____________

Assume that the bulk density is 1 g/cm3

then the molecular density will be 5 x 1022 molecules per cm3. The surface concentration (molecules per cm2) is proportional to _______if one assumes cube like packing. This gives a value of _______molecules per cm2.

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DISPERSION

The fraction of the atoms on the surface is referred to as _______. Mathematically, dispersion (D) is the ratio of the number of surface atoms (NS) to the TOTAL no. of atoms (NT): i.e. _______

For very small particles D = 1However, as the particle grows the number of surface atoms will _______. For a cube of 100 Å, D = 10-3 !!!!

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DISPERSION

0 1000 2000 3000

1.0

Total number of atoms

Dis

pers

ion

(D)

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Surface Atoms

Consider a cube of metal (or metal oxide). The surface atoms rest on the bulk atoms and so must reflect this situation. Previously you learned about how atoms can pack and the way in which structures were built up. Example: _______

http://www.ndt-ed.org/EducationResources/CommunityCollege/Materials/Structure/metallic_structures.htm

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Surface Atoms

Consider a cube of atoms with the _____structure. We can take slices through this structure and this will yield different faces with Miller Indices: (100), (111), (100), etc.

http://www.diracdelta.co.uk/science/source/m/i/miller%20indices/image001.gifM Bowker, The Basis and Applications of HeterogeneousCatalysis, Oxford University Press, 1998., pp 12.

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Surface Atoms

It is quite difficult to get surfaces with only one type of face. Most surfaces have many faces and contain _______, _______, _______ and _______. Thus surfaces are generally not _______. This has implications for the reactant molecules (see example with ammonia synthesis that follows).

Different arrangements of surface atoms have different _______ _______. Generally, surfaces with _______ coordination number have the _______ surface free energy (are the most reactive).

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StepKink edge

Adatom

Terrace A. N. Chaika, S. I. Bozhko, A. M. Ionov, A. N. Myagkov, and N. V. Abrosimov. Semiconductors, 2007, Vol. 41, No. 4, pp. 431–435

http://www.oup.com/uk/orc/bin/9780199236176/lecturer/figures/ch26f19.jpg

41Spectroscopy in Catalysis: An Introduction, Third Edition, J. W. NiemantsverdrietCopyright 8 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, ISBN: 978-3-527-31651-9

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Thus reactivity order is: _______ > _______ > _______ > _______

Atom type Top plane co-ordinationAdatom 0Kink edge 3Step 4Terrace 6

M Bowker, The Basis and Applications of Heterogeneous Catalysis, Oxford University Press, 1998., pp 13.

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Single crystal faces of Fe for NH3 synthesis

G.A. Somorjai, N. Materer / Surface structures in ammonia synthesis. Topics in Catalysis 1 (1994) 215-231

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Molecular Heterogeneous Catalysis

Metal cluster chemistry can assist as an important model to illustrate the interconnectedness between _______ chemistry and _______ chemistry.

Consider the _____________ reaction of formic acid catalyzed on different metal surfaces: H-COO-H (aq) CO2(g) + H2(g)

MetalPlotting the _______ of each metal surface versus the standard enthalpy of formation (ΔHo

f) of each metal formate, gives what is called a ‘______________’ :

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Molecular Heterogeneous Catalysis

_______ ______________Speed of formation of surface intermediate is low.

_____________ _____________ Speed of decomposition of surface intermediate is low

http://www.oup.com/uk/orc/bin/9780199236176/lecturer/figures/ch26f19.jpg

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Molecular Heterogeneous Catalysis

The shape of the ‘_______’ plot is consistent with the notion that surface intermediates closely resemble bulk intermediates (formates in this case). This is based on the _______ _______. This implies that:Reaction rate = f(_______, _______). Thus the optimum catalytic performance does not relate to a specific _______ _______ but to a balance of interaction & desorption. These are the elementary steps in _______ _______.

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Molecular Heterogeneous Catalysis

Wolfgang Sachtler showed that when reacting molecules _______ onto a surface they form _______ _______. Furthermore, these complexes result in the partial destruction of metal-metal bonds and lead to a ‘______________’ of the surface. This was later termed ‘_______ _______’.

Somorjai & Muetterties showed that the _______ _______ in a catalytic reaction, as well as the surface complexes are similar to homogeneous _______ complexes and reactions.

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Molecular Heterogeneous Catalysis

Comparison of some Somorjai &Muetterties surface complexes with known organic complexes:

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ADSORPTION

Molecular and/or atomic species have essentially two ways in which they can attach or adsorb onto a surface: _______ or _______.1. Physical Adsorption or ___________

Physisorption often occurs in any liquid/solid or gas/solid system where the molecular/ atomic species attach to the solid surface through _______ _______ _______ (van der Waals forces).

The elementary step in physisorption from a gas phase does not involve an _______ _______.

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1. Physisorption (cont.)The typical binding energy of these

physisorbed species on a surface is between ___________. (No chemical specificity).

The process is _______ _______ _______, with little energy. See example of helium gas on metal surfaces afterwards.

The adsorption enthalpy can range between _______ and _______.

For physisorption, under appropriate conditions, gas phase molecules can form _______ adsorption.

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E. Zaremba and W. Kohn (1977). "Theory of helium adsorption on simple and noble-metal surfaces". Phys. Rev. B 15 (4): 1769. doi:10.1103/PhysRevB.15.1769. Retrieved from "http://en.wikipedia.org/wiki/Physisorption"

Example: The physisorption profiles of He on various metal surfaces.

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2. Chemical Adsorption or _____________Chemisorption adsorption occurs when

molecular/atomic species chemically attach to a surface through _______ _______. New species are formed. _______ _______ _______ _______

The typical binding energy of chemisorbed species on a surface is between _______. Binding is usually chemically specific.

The process is less easy to reverse and often takes a lot of energy to do so. Sometimes the process is _______ i.e. due to _______ of the species. Example: _______ _______ _______ _______ _______ _______ _______ _______ _______.

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2. Chemisorption (cont.)The elementary step in a chemisorption

process from the gas phase often involves an _______ _______. (Recall dissociation of oxygen on metal.)

In chemisorption, because the molecular/atomic species are adsorbed on the surface by covalent bonds, they often only form a single or _______ adsorption.

In chemisorption, the adsorption enthalpy can range between _______ and _______.

Q: Why are the adsorption enthalpies of physisorption and chemisorption negative?

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ADSORPTION ISOTHERMS

Q: Why is it important to obtain a relationship between the quantity of a substance adsorbed on the surface of a solid and its gas phase pressure?

1.The _______ of gas coverage on thesurface can be _______ determined.2.The _______ of the adsorption of the

molecules can be quantitatively determined (i.e. chemi or physisorption).

3.The _______ _______ of the solid can be quantitatively determined.

4.The _______ of the catalytic system can be modelled.

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ADSORPTION ISOTHERMS

Q: What then is an adsorption isotherm?A: “The relationship between the _______ of

gas adsorbed on a surface and the _______ with which it is in equilibrium, at

a _______ _______, is called an adsorption isotherm” (Bond pg 15).

Suppose the maximum surface that could be covered was ____ and that it was covered by an amount __, then the ratio or fraction (___) that the surface is covered is: _______ _______, then an adsorption isotherm, based on various assumptions can be developed.

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LANGMUIR ISOTHERM

Suppose that a solid surface has a gas (of a certain pressure) _______ _______ on its

surface and at dynamic equilibrium then:

1 2

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2 3

57

If the ratio or fraction that the surface is covered is θ, then Langmuir made the following assumptions:

1. Adsorption isotherms do not exceed a _______ _______. Thus molecular/ atomic species can only maximally fill the surface, then no further adsorption

occurs i.e. _______ _______ _______

2. All sites on the surface are _______ and _______. This implies that the surface is _______ _______ and that the _______ _______ would be equivalent

throughout the surface.

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3. When molecules adsorb onto a surface, they _______ _______ or _______ one

another or incoming molecules that will adsorb. i.e. molecules will adsorb onto a site ___________ from whether or not a site next to them is _______ or _______.

In the light of what you have already learned, take some time to critically analyze Langmuir’s assumptions.

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DERRIVATION OF LANGMUIR ISOTHERM

1) Mono/single site adsorption

Consider a gas A that can reversibly _______and ______ on an active site *, then:

Where kA = ________________________Where kD = ________________________

the number of molecules colliding with thesurface in unit time is proportional to the____________of gas A i.e. ____________

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Suppose the surface has a total of _____ ________and at some time the ________________ is , then:

Fraction of unoccupied sites = ________Number of unoccupied sites = ________

Rate of adsorption PA But:Rate of adsorption no of unoccupied sites

________So:Rate of adsorption PA × N(1-)Rate of adsorption = _____________, (where kA = _____________________)

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But, the rate of desorption amount of adsorbed gas

________Rate of desorption = ________(where kD = _____________________)

At equilibrium:

Rate ___________ = Rate ___________thus: kA× PA × N(1-) = kD × N

Where___ and ___ are the equilibrium valuesof pressure and surface coverage.

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Cancelling like terms and rearranging gives: (kA× PA ) – ( × kA× PA ) = kD × So: = (kA× PA ) (kD + kA× PA )

Then let _______ or the _______________________ then you obtain the following:

L ___

This is the ______________for _____ siteadsorption.

63

Thus:________________________

Recall:b = kA/kD

________

Langmuir adsorption isotherm

64

If V = volume actually coveredAnd Vm = monolayer coverage then:

θ = ________ then since

___

Then rearranging and substituting gives:

This is in the form Y= mX + C , where Y = ____m = ____ , X = __ C = ____.This is allows themonolayer coverage to be calculated

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Since ________and kA = AA×e–{E

A/RT} with kD = AD×e–{E

D/RT}

____

Where ________

Thus b is a function of ________ and________ at temperature ________.When b is ________ then ________ is _______bonded, conversely when b is ________ it is________ bonded.

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2) Dual site adsorption

Consider a gas A that must strike the surfaceat a location where there are _____ adjacentactive sites *, then:

Up to fairly high fractional coverage () it canbe assumed that the adsorption of ______________ will depend on the fraction of vacant sites or ____, and that the adsorptionof the other fragment will also depend onthis ____________.

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2) Dual site adsorption (cont.)

Then: Rate of adsorption = _____________Rate of desorption = ________At equilibrium: Rate of ads= Rate of desorp, So: k’APA[N(1 - )]2 = k’D(N)2

If b = k’A/k’D, Then by subsitution andRearrangement: = (bPA)½ /(1 + (bPA)½ )

For the monolayer coverage (Vm), again, let = V/Vm, then This is in the form Y= mX + C , where Y =______, m = ____ , X = ___ C = _______. Thus a plot of _______ against ___ will give astraight line.

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Measured amounts adsorbed of the pure gases CH4 ( ),CO2 ( ), and N2 ( ) on AC Norit R1 at T D 298 K. Simultaneousfit of all data with the generalized dual-site Langmuir isotherm (—)

F. DREISBACH, R. STAUDT AND J.U. KELLER Adsorption 5, 215–227 (1999)

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3) Non-competitive adsorption

It is possible that ___________(gas A, and gas B), may be in the same container andwill adsorb on different sites. The adsorptionis _________________. Then the isotherm for each gas is simply aLangmuir isotherm for ________ gas. i.e.

= bPA/(1 + bPA) for gas A, and = bPB/(1 + bPB) for gas B

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4) Adsorption of more than one species on the same surface

Consider the reaction of _______ gases on a surface (i.e. the adsorption is ____________).

with rate constants ka(A) and kd(A)

with rate constants ka(B) and kd(B)

Then A = bAPA/(1 + bAPA + bBPB)

And B = bBPB/(1 + bAPA + bBPB)

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5) The General expression

We can work with 1,2,3,4, ….. up to i gases.

Each gas can be expressed by a Langmuir Isotherm:

A = bAPA/(1 +biPi)

Other Non-Langmuir Isotherms i) ____________ Isotherm

This assumes that a __________ decrease of the enthalpy of ____________ occurs with fractional coverage. ( = kP1/n where k and n are constants with n > 1)

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L Zhang, S Hong, J He, F Gan, Y-S Ho. Clean – Soil, Air, Water 2010, 38 (9), 831–836. Freundlich isotherms obtained using linear and nonlinear regression methods for the adsorption of phosphorus onto Al2O3 at temperature of 308 K.

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ii) __________ Isotherm

Assumption here is that the ________ heat of adsorption falls off _________ with coverage:θ= k’ln(k”bPA)

where b and PA have been defined previously.

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SUMMARY OF LANGMUIR ISOTHERMS

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POROUS MATERIALS

Many materials are _______. Information about the pore ______, pore ________, pore ________ as well as the _________ can be obtained from two different types of adsorption experiments:

1) Multilayer gas adsorption Here we use an _________ _________ or _______e.g. N2, Ar, Kr to physisorb onto the material.

2) Mercury PorosimetryLiquid mercury is forced _______ into the pores of the material (e.g. _______ gives information on ______pore radius)

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PORE SHAPES/TYPES

Uniform/cylindrical

Funnel shaped

Ink bottle shaped

Blind pore

Closed pore

Through pore

Porous network

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POROUS MATERIALS

Pore SizePores are classified according to size_________ < 2 nm _________ 2 nm < x < 20 nm_________ > 20 nm

 Yun Wu, Xianfeng Du, Honghua Ge and Zhen Lv Starch/Stärke 2011, 00, 1–9 DOI 10.1002/star.201000036

Microprous starch

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IUPAC Classification of porous solids

There are _________ of adsorption isotherms that have been observed. Each gives information about the types of pores contained in a solid as well as the capacity of the solid to adsorb a gas. 

79

IUPAC Classification of porous solids

I II

III

IV V VI

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In the previous figures the _________ of the curve gives information about the solid – its _________. Let us examine these a bit closer:

Microporous solids (see I)At ______pressure: adsorption in _________ firstAt _________ pressure: then coverage of _________ surface takes place.

Mesoporous solids (see IV)At _____ pressure: ________coverage (plateaus)At _________ pressure: adsorption in _________. After the pores are filled adsorption occurs on the external surface.

IUPAC Classification of porous solids (Cont.)

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Macroporous solids (see II)

At _____ pressure: __________ coverageAt _______ pressure: _______ coverage until condensation occurs. There tends to be overlap between the 2 regions Uniform ultra-microporous solids (see VI)

If all sites the same: _________ coverageIf not: _________ isotherm for _________Example: Zeolites Range of pore sizes

Zeolite (small size range)

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HYSTERESIS LOOPS

 

Evaporation from a pore takes place at a _________ than condensation thus the path of _________ differs from _________. Four types of Hystereses have been identified and classified (IUPAC)

P/P* 0 0.25 0.5 0.75 1.0

Adso

rbed

vol

ume

P/P* 0 0.25 0.5 0.75 1.0

Adso

rbed

vol

ume

P/P* 0 0.25 0.5 0.75 1.0

Adso

rbed

vol

ume

P/P* 0 0.25 0.5 0.75 1.0

Adso

rbed

vol

ume

The four _________ shapes of adsorption isotherms typically associated with N2 adsorption

Type H1 Type H2 Type H3 Type H4

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H1/H2 TYPEParticles with _________ or aggregates of _________ particlesH1: uniform size/shapeH2: non-uniform size/shape i.e. different size pore mouth and pore body e.g. ink bottle type pores H3/H4 TYPEAggregates with _________ poresH4: uniform size/shape H3: non-uniform size/shape e.g. zeolites, carbons 

HYSTERESIS LOOPS (cont.)

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Lord Kelvin noted that the evaporation of condensed gas molecules from a surface with very fine pores is more difficult than their condensation. This is because there is a greater probability, as compared to a _________ _________, that the molecules which evaporate from a ____________ meniscus will _________.

Using the Kelvin equation, it is possible to measure a pore radius at a given P/P*: 

_________ _________, where V = molar volume of liquid, = surface tensionr = pore radius, = contact angle (usually = zero)R=gas constant, T = temperature

KELVIN EQUATION

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 Consider a zeolite material onto which nitrogen was adsorbed and desorbed. If the P/P* was 0.25 at –183.15oC and the surface tension of nitrogen was 1.0 x 10-1 Nm-1 (and the contact angle was zero), then use the graph to calculate what the pore radius of the zeolite was.

PROBLEM 0

10.0

2

0.0

30.

0

P/P* 0 0.25 0.5 0.75 1.0

Adso

rbed

vol

ume

ml/

mol

86

Porous Materials

I) Internal surface area

If the pores are _______, _________ uniform cylinders then:

r = radius of pore, S = internal surface area  II) Total Surface Area This is given by: S = nmLm, where:nm = moles gas adsorbed in __________m = area of ________ adsorbed moleculeL (or N) = Avagadro constant 

SURFACE AREA

S = 2Vp/rVp =pore volume

87

The Brunauer, Emmett and Teller (BET) Method(Used on type II adsorption curves for multiple layer physisorption - see IUPAC classification of porous solids)

Here:

Which can be rearranged to give the ____________:

Vm = volume of gas, monolayer coverage,C = _________ _________ constant

SURFACE AREA

V = θ = CPVm (P* –P) 1 + (C–1)P P*

XP PP* = 1 + (C-1) P*Vm 1 – P C.Vm C. Vm

P*Y Y – intercept. Thus calculate Vm

Slope

88

0 0.1 0.2 0.3 0.4 0.5

0

4

8

1

2

16

20

P/P*

P/P*

/ 1

0-1 c

m-3

Vm

(1-P

/P*)

_________ _________ _________

89

Single point methodThis is a _________ method. It arises because the slope > intercept (which tends to zero when P/P* is in the 0.2 to 0.3 region see (type II)).

Hence assume the BET plot passes through the origin. i.e. assume Y-intercept at the origin.

Then slope = (C-1)/C.Vm and thus Vm can be calculated. An error bar of 5% is acceptable in these experiments.

BET SURFACE AREA (cont).

90

Single point method (cont.)

Alternatively, if a simple extrapolation is made from the _________ of a set of data, then the molar volume can be obtained. This method delivers a rough estimate of the molar volume of within 10% (Bowker pp 57).

P/P*

V

91

The constant C in the BET equation is related to the following equation:

C = e[(Ha

– H1

)]/RT, where Ha = enthalpy of _________ of the _________ and H1 = enthalpy liberated from the second and subsequent layers (similar to the _________ of the gas). Thus C helps give an estimate of ΔHads and it influences the ______ of the adsorption isotherm:

BET SURFACE AREA (cont).

V

0

1

P/P*

C=1

C=2

C=10

C=10 000

92

Gases used for analyses: Gas Area/10-20m2 Saturation P (torr)N2 16.2 760 at 77KAr 13.8 220 at 77KO2 14.3 760 at 77 K There is a limit on using nitrogen: _______ pore corresponds to 5 molecule width. Hence we use Ar or Kr for low surface area measurements (_________ ). 

SURFACE AREA

93

Kinetic measurements and the interpretation of the kinetic data lies at the heart of catalysis. Kinetic analysis allows for: Reactor design Correlating and rationalising catalytic activity Mechanism determination

Rates of reaction, Order, Effect of Temperature Consider the reaction

__A + __B __CRate (r) given by (rate of form = rate of consump):r = – (1/__)dA/dt = – (1/__)dB/dt = +(1/__)dC/dtUnits of r = _________

KINETICS

94

In a heterogeneous reaction the rate will depend on the _____________area available to the reactants. This is expressed as the ____________ (TOF). TOF = no of molecules converted per unit of time _______________ .

Gas Phase Reaction (Homogeneous reactions)

We know that the reaction rate can be expressed in terms of _________ _________ .Reaction : aA + bB cCRate = k(PA)a(PB)b(PC)c ….This is referred to as

a ________________

KINETICS

95

Given: Rate = k(PA)a(PB)b(PC)c ,

a,b,c = orders of the reactants (don’t have to =integers).k = rate constantRate can be expressed per _________ :

r = mrm, where m = mass of catalyst ORIf the total surface area (S) of the catalyst hasbeen determined, then rate can be expressed______________ : r = Srs

Recall the Arrhenius equation: k = Acatexp(–Ecat/RT)

Where Ecat = Activation Energy

And Acat = pre-exponential factor

96

ADSORPTION MECHANISMS

We will now look at two different mechanisms proposed for adsorption:

(A) Langmuir-Hinshelwood (B) Eley-Rideal

97

I) LANGMUIR-HINSHELWOOD (L-H) MODEL

Three general assumptions to this model:

1. Adsorption is _____ and _________from the gas phase

2. The reaction of the adsorbed molecules is the rate determining step (RDS) i.e. the surface chemical reaction = RDS i.e. k very small

3. The _________ of an _________ species is determined by the appropriate Langmuir Isotherm.

Two types to consider:

(A) L-H model for unimolecular reactions

Example: E(g) E (Ads) → C(g)

If bE or PE are _______, then: Rate= k bE PE …i.e. __________

)1(EE

EECE

Pb

Pk

dt

dP

dt

dPRate b

If bE or PE are _______, then: Rate → k …..i.e. ____________

θE

fast Surface rxn

=RDS

Molecule E at PE

Product Molecule C

fast

E

E C

C

Ek

dt

dPRate E

99

1. This type of kinetics is not specific to catalysis

2. Mainly applies in: 3 step reactions Pre-equilibrium systems

e.g. Michaelis Menton equation for enzymatic catalysis in Biochemistry

Things to note about L-H model for unimolecular reactions

(B) L-H model for bimolecular reactions

A(g) A (Ads)

B(g) B (Ads)

A(Ads) + B(Ads) → AB(Ads) → AB(g) Surface rxn

=RDS

Fast

And

Two extra assumptions to this model:

1. Molecules A and B are adsorbed on _______ sites with _____________

2. Product molecule AB is very ___________and comes off the surface fast.

)PbPb1(

Pb

BBAA

AA

A

)PbPb1(

Pb

BBAA

BB

B

ABkRate

2)1(BBAA

AABB

PbPb

PbPkbRate

.. Langmuir-Hinshelwood

Equation

θA

Molecule B at PB

fast

RDS

Molecule A at PA

Product molecule BA

θB

B

B B

BA

A

A

A

fast

102

1. If molecules A and B are _______ adsorbed, bA and bB are _______ , then:

Rate = k’ PAPB ….i.e. _______, (Where k’= kbA bB)

2. If molecule ____is _______ adsorbed and molecule B is strongly adsorbed i.e. _____________then: Rate = k’’ PA/ PB , (Where k’’= kbA/bB). This is an indication that B poisons the surface.

3. Similarly the rate is affected if the _______ _______of molecule A and B are varied:

Things to note about L-H model for bimolecular reactions

The effect of surface concentration on the rate in bimolecular reactions

PA

rate

For constant PB

Rate limited by surface Concentration of A

θB>>θA

A

A

A

B

B B B B

BB

BB

B

Rate limited bysurface concentration of B

θA >> θB

A A

A

A

A

AA A

AA

A

B

B B

II) Eley-Rideal (E-R) model for bimolecular reactions

Unlike the L-H model, in the E-R model it is proposed that an adsorbed molecule may react _______with an _______ gas molecule by a collisional mechanism. The surface reaction is still the RDS i.e.

PA

fastRDS

PB

AAA A

A

A

AB

BB

BB

θAfast

Eley-Rideal bimolecular surface reactions

θA = 1

PA

Rat

e

For constant PB

_____

Note: For constant PA, the rate is

always first order wrt PB

BA

PkRate)Pb1(

PPkb

AA

ABA

bAPA << 1

Thus:Rate = k bA PA PB …….. first order in A

__________________

bAPA >> 1

Thus:Rate = k PB …zero order in A

__________________

_____

How can the two bimolecular reaction models be distinguished from one another?

Experimentally if the reaction rate is measured as a function of the

surface coverage of A i.e. θA, then the rate will initially increase for

both mechanisms at ___________________. There will be slight to

no difference between the two proposed models observed. BUT:

If the reaction proceeds by the model proposed by Eley-Rideal

then at ___________________(i.e. θA 1) the

_________________until the surface is covered by A.

If the reaction proceeds by the model proposed by Langmuir-

Hinshelwood then the ______________

________________and then gradually decreases to zero as θA 1.

θA

Rat

e

0 1

Eley

-Rid

eal m

odel

Langmuir-Hinshelw

ood

The effect of surface coverage on rate for the Eley-Rideal and Langmuir-Hinshelwood models

Q: Will the rate of E-L model increase ad

infinitum?

108

THE EFFECT OF TEMPERATURE ON KINETICS:CURVATURE IN ARRHENIUS PLOTS

In heterogeneous systems we will still assume for thereaction: A C

Rate = –dPA/dt = kA = kbAPA.

BUT in this equation BOTH ___ and _____ are functions of temperature. For instance:

1. If bA is ________ and _____ is ________, then k will be the

only variable that is a function of T. i.e. k =

A·exp(-Etrue/RT)

2. If bA is small then tends to _____, then from the van’t

Hoff Isochore we have:

(dlnbA)/dT = HθA/RT2 , then ∫(dlnbA) = ∫ (Hθ

A/RT2) dT

Thus:

lnbA = –HθA

/RT + C OR bA = C·exp(–HθA/RT)

(where C = integration constant & –Hθ

A is the standard

molar enthalpy of adsorption of reactant A)

Now we have rate = kA and A = bAPA

We can substitute into the equation for both k and bA

i.e. Rate = –dPA/dt = kA = kbAPA = k[C·exp(–HθA/RT)](PA)

= A·exp(–Etrue/RT)[C·exp(–Hθ

A/RT)](PA)

Re-writing this out: = PA·A·C·exp(–Etrue–HθA)/RT

= PA·A·C·exp(–Eapp)/RT

Where: (–Eapp) = (–Etrue–HθA), hence Etrue = Eapp–Hθ

a

and since HθA

is always negative, then Etrue > Eapp 109

Arrhenius plot for a catalysed reaction over a range of temperatures gives:

110

I II III

Surface coverage 1 1>>0 ~ 0

Order,n 0 1>n>0 1

Slope x 2.3R Etrue ---- Eapp

III

II

I

Lo

g 1

0 R

ate

1/T

111

Diffusion or mass-transport limited reactions are those in which the transport of the ________ /or ____________ the catalyst influence the rate:

1. Rate [catalyst]n and n< 1 vs ______________2. Rate stirring rate vs ____________________3. Eact about 10 – 15 kJ mol–1 vs ______________

4. Rate (temperature)1/2 vs _________________

An Arrhenius plot showing the onset ofdiffusion limitation can be drawn:

THE EFFECT OF DIFFUSION ON KINETICS

112

____________ for diffusion limited reactions the RAQ’ curve (purple curve) is detected, because the slower of the twoprocesses controls the rate.

R

Q’

Q

R’L

og

10

Rat

e

1/T

A’A: Surface reaction is rate-limiting

B’B: Reaction is diffusion limited

A