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CHEMISTRY 3030: CATALYSIS COURSE, 2011. INTRODUCTION WHAT IS THE DEFINITION OF A CATALYST? Given the following reaction at equilibrium: A 2 (g) +2B 2.

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Presentation on theme: "CHEMISTRY 3030: CATALYSIS COURSE, 2011. INTRODUCTION WHAT IS THE DEFINITION OF A CATALYST? Given the following reaction at equilibrium: A 2 (g) +2B 2."— Presentation transcript:

1 CHEMISTRY 3030: CATALYSIS COURSE, 2011

2 INTRODUCTION WHAT IS THE DEFINITION OF A CATALYST? Given the following reaction at equilibrium: A 2 (g) +2B 2 (g)  2AB 2 (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: 2

3 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: 3

4 INTRODUCTION Reactants = uncatalysed reaction Energy Reaction co-ordinate Products Activation Energy Enthalpy change HOW DOES A CATALYST DO THIS? 4

5 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 katal ( kat) The activity (A) of a catalyst is measured by the SI unit: katal (abbreviated to kat). 1 kat ___________ If the activity of a catalyst is 1 kat : then it ‘enables’ the reaction rate to be ___________ 5

6 ________ _________ 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: Al 2 O 3 (s), TiO 2 (s), CaCO 3 (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 cm 2 ). ____________________. 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 ____________________. 6 CONT.

7 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. 7

8 CONT. decreasing order Consider the following reactions and then place them in decreasing order of the catalyst selectivity: 1) CO(g) + H 2 (g) → CH 3 OH(g) = 1.0 x Kat 2) CO(g) + 2H 2 (g) → ½C 2 H 5 OH(g) = 2.5 x Kat 3) CO(g) + 3H 2 (g) → CH 4 (g) + H 2 O(g) = 9.8 x Kat _____________ ORDER: _____________ Cu/Zn/Al 2 O 3 8

9 3.DEACTIVATION ___________ For a variety of reasons catalysts can lose their activity and hence their selectivity ___________. Some reasons: 1) SINTERING (_______________) ____________ Supported m etal catalyst particles are oftentimes more stable when they are ____________or spread out on the support surface. CONT. 9 CoO nanoparticle Carbon nanotube

10 ___________ ________________ ___________ 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. 10 A.Binder et al. J. Phys. Chem. C 2010, 114, 7816–7821 (Pd on SiO 2 or TiO 2 )

11 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. 11

12 TYPES OF CATALYSIS Several types of catalysis: 1)Homogeneous Catalysis ___________ When the reactants and the catalyst are in the ___________: e.g. O 3 (g) + A · (g) → O 2 (g) + AO(g) ….(1) AO(g) + O 3 (g) → A · (g) + O 2 (g) ….(2) Q:Which species is the catalyst and which the intermediate? 12

13 2)Heterogeneous Catalysis _________________ When the reactants and the catalyst are in _________________: For example: The photoreduction of carbon dioxide on titania Pt/TiO 2 (s) CO 2 (g) CO(g) + O 2 (g) hν 13 TYPES OF CATALYSIS

14 Examples of heterogeneous catalysis: Selec Ox 1)CO + ½ H 2  CO 2 (Co, Co-Ni) - Fuel cells. Selec Ox. of CO in H 2 2)CH 2 CH 2 + H 2  CH 3 CH 3 (Ni/Pd) Olefin Hydrogenation – Fuel industry. c.a. 119 million tonnes of C 2 H 4 in 2010! ) 3)CHCH + 2H 2  CH 3 CH 3 (Pt/Pd) Removal of C 2 H 2 from olefins by hydrogenation ) 4)CO + H 2  -CH 2n+2 - (Fe, Co) Fischer- Tropsch. Fuels, waxes, etc. 5)N 2 + 3H 2  2NH 3 (Fe, Ru) Haber Process. Fertilizers, explosives.. 6)C 6 H 6 + 3H 2  C 6 H 12 (Cu -1925!, Ru,Ni) 90% of cyclohexane used for Nylon 6 and

15 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 10 3 molecules converted/enzyme/s). 15 TYPES OF CATALYSIS

16 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. 16 CONT.

17 17 CONT.

18 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.” 18 PAC, 2004, 76, 889PAC, 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

19 Example: Pd nanoparticles (PdNPs) immobilised on microporous Poly(amidoamine) (PAMAM) dendrimers. 19 CONT. Shin Ogasawara and Shinji Kato J. AM. CHEM. SOC. 2010, 132, 4608–4613

20 Suzuki-Miyaura reaction in water. 20 CONT. Shin Ogasawara and Shinji Kato * 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 ___________

21 CLASSIFICATION OF HETEROGENEOUS CATALYSTS (BOND pg10) 21 Catalyst Type:Examples:Reactions: MetalsNi, Pd, Fe, Pt, AgHydrogenation Dehydrogenation Hydrogenolysis Oxidation Semiconducting oxides /sulphides NiO, ZnO, MnO 2, Cr 2 O 3, Bi 2 O 3 -MoO 3, WS 2 Dehydrogenation Desulphurisation Oxidation Hydrogenation

22 22 Catalyst Type:Examples:Reactions: Insulating oxidesAl 2 O 3, SiO 2, MgODehydration AcidsSiO 2 -Al 2 O 3, Zeolites Polymerisation, Isomerisation, cracking, alkylation CLASSIFICATION OF HETEROGENEOUS CATALYSTS (BOND pg10)

23 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 m 2 g -1 Small (e.g. _______________) _______________ m 2 g -1 Normal (e.g. _______________) _______________ m 2 g -1 Large (e.g. _______________) 23

24 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 m 2 )= ____________________ Where M = total mass of catalyst,  = density and r = average particle radius, V part = volume of particle. 24

25 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 = _______________ 25

26 __________ A2: __________ 26 Factor:Inorganic support Organic support Effect of Temperature Good_______ Heat transfer Good_______ Chemical reactivity OK_______ Q: How then can the surface area be maximized?

27 __________ 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 WS 2 catalyst for desulphurisation † † C. Roukoss et al. / C. R. Chimie 12 (2009) There are two types of promoters 1._________ 2._________ 27 Q: How then can the surface area be maximized?

28 1.Structural ___________ ___________ A structural promoter ___________by separating the surface ___________. For e.g. The active phase for the NH 3 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, Types of promoters:

29 2.Electronic ______________ _______, _______ _______ Electronic promoters are effective due to their ______________. The most widely used electronic promoters belong to _______, _______ and the _______. Cs > K > Na 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 S. R.Tennison, in: J.R. Jennings (Ed.), Catalytic Ammonia Synthesis, Fundamentals and Practice, Plenum Press, New York, 1991, p Types of promoters:

30 30 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. RuO 2 clusters reduced to metallic Ru. Increased Activ.

31 Given the following data for the lanthanides, place them in order of increasing ability to act as electronic promoters: ______________ ______________ 31 Problem Element Pauling’s Electronegativity value Samarium1.198 Lutetium1.201 Lanthanum1.101

32 ______ / _______ A3: ______ / _______ maximization 32 Q: How then can the surface area be maximized? TEOS = Tetraethyl orthsilicate Polymers include: polyvinyl alcohol and polyethylene glycol with different molec.masses Sol-gel process S SATO, T MURAKATA, T SUZUKI, T OHGAWARA. JOURNAL OF MATERIALS SCIENCE 25 (1990)

33 _______ / _______ A3: _______ / _______ maximization 33 Q: How then can the surface area be maximized? S SATO, T MURAKATA, T SUZUKI, T OHGAWARA. JOURNAL OF MATERIALS SCIENCE 25 (1990) No polymer Polymer of low molec. mass Polymer of higher molec. mass Mean pore sizes increase from 3 nm to 7 nm

34 ATOM SURFACE CONCENTRATION _____________ The atom surface concentration can be determined by the _____________ _______ _______ Assume that the bulk density is 1 g/cm 3 then the molecular density will be 5 x molecules per cm 3. The surface concentration (molecules per cm 2 ) is proportional to _______if one assumes cube like packing. This gives a value of _______molecules per cm 2. 34

35 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 (N S ) to the TOTAL no. of atoms (N T ): i.e. _______ For very small particles D = 1 _______ However, as the particle grows the number of surface atoms will _______. For a cube of 100 Å, D = !!!! 35

36 DISPERSION Total number of atoms Dispersion (D)

37 Surface Atoms 37 _______ 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: _______ metallic_structures.htm

38 Surface Atoms 38 _____ 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. M Bowker, The Basis and Applications of Heterogeneous Catalysis, Oxford University Press, 1998., pp 12.

39 Surface Atoms 39 ______________, ______________ _______ 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).

40 40 Step Kink edge AdatomTerrace A. N. Chaika, S. I. Bozhko, A. M. Ionov, A. N. Myagkov, and N. V. Abrosimov. Semiconductors, 2007, Vol. 41, No. 4, pp. 431– /lecturer/figures/ch26f19.jpg

41 41 Spectroscopy in Catalysis: An Introduction, Third Edition, J. W. Niemantsverdriet Copyright WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, ISBN:

42 42 ______________ ______________ Thus reactivity order is: _______ > _______ > _______ > _______ Atom typeTop plane co-ordination Adatom0 Kink edge3 Step4 Terrace6 M Bowker, The Basis and Applications of Heterogeneous Catalysis, Oxford University Press, 1998., pp 13.

43 Single crystal faces of Fe for NH 3 synthesis 43 G.A. Somorjai, N. Materer / Surface structures in ammonia synthesis. Topics in Catalysis 1 (1994)

44 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)  CO 2 (g) + H 2 (g) Metal _______ ______________ Plotting the _______ of each metal surface versus the standard enthalpy of formation (ΔH o f ) of each metal formate, gives what is called a ‘______________’ : 44

45 Molecular Heterogeneous Catalysis 45 _______ ______________ _______ ______________ Speed of formation of surface intermediate is low. _____________ _____________ Speed of decomposition of surface intermediate is low

46 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 _______ _______. 46

47 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. 47

48 Molecular Heterogeneous Catalysis surface complexesknown organic complexes Comparison of some Somorjai &Muetterties surface complexes with known organic complexes: 48

49 ADSORPTION 49 ______________ 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 _______ _______.

50 50 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.

51 51 E. Zaremba and W. Kohn (1977). "Theory of helium adsorption on simple and noble-metal surfaces". Phys. Rev. B 15 (4): doi: /PhysRevB doi /PhysRevB Retrieved from "http://en.wikipedia.org/wiki/Physisorption"http://en.wikipedia.org/wiki/Physisorption Example: The physisorption profiles of He on various metal surfaces.

52 52 _____________ 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: _______ _______ _______ _______ _______ _______ _______ _______ _______.

53 53 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?

54 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 the _______ surface can be _______ determined. _______ 1.The _______ of the adsorption of the molecules can be quantitatively determined (i.e. chemi or physisorption). _______ _______ 2.The _______ _______ of the solid can be quantitatively determined. _______ 3.The _______ of the catalytic system can be modelled. 54

55 ADSORPTION ISOTHERMS Q: What then is an adsorption isotherm? _______ A:“The relationship between the _______ of gas adsorbed on a surface and the _______ _______ _______ Bond pg 15 _______ 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. 55

56 LANGMUIR ISOTHERM _______ _______ Suppose that a solid surface has a gas (of a certain pressure) _______ _______ on its surface and at dynamic equilibrium then: 56 

57 assumptions 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. 57

58 _______ _______ _______ ___________ ______________. 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. 58

59 DERRIVATION OF LANGMUIR ISOTHERM 1) Mono/single site adsorption _______ Consider a gas A that can reversibly _______ ______ and ______ on an active site *, then: ________________________ Where k A = ________________________ ________________________ Where k D = ________________________ the number of molecules colliding with the surface in unit time is proportional to the ________________________ ____________of gas A i.e. ____________ 59

60 _____ Suppose the surface has a total of _____ ________the ________ ________and at some time the ________ ________ ________ is , then: ________ Fraction of unoccupied sites = ________ ________ Number of unoccupied sites = ________ Rate of adsorption  P A But: Rate of adsorption  no of unoccupied sites ________  ________ So: Rate of adsorption  P A × N(1-  ) _____________ Rate of adsorption = _____________, _____________________ (where k A = _____________________) 60

61 But, the rate of desorption  amount of adsorbed gas ________  ________ ________ Rate of desorption = ________ _____________________ (where k D = _____________________) At equilibrium: ___________ ___________ Rate ___________ = Rate ___________ P A thus: k A × P A × N(1-  ) = k D × N  ___ ___ Where___ and ___ are the equilibrium values of pressure and surface coverage. 61

62 Cancelling like terms and rearranging gives: (k A × P A ) – (  × k A × P A ) = k D ×  So:  = (k A × P A ) (k D + k A × P A ) _______ ______________ Then let _______ or the ______________ _________ _________ then you obtain the following: L ___ ___________________ This is the ______________for _____ site adsorption. 62

63 63 Thus:________________________Recall: b = k A /k D ________ Langmuir adsorption isotherm

64 If V = volume actually covered And V m = 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 the monolayer coverage to be calculated 64

65 ________ Since ________ and k A = A A ×e –{E A /RT} with k D = A D ×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. 65

66 2) Dual site adsorption Consider a gas A that must strike the surface _____ at a location where there are _____ adjacent active sites *, then: Up to fairly high fractional coverage (  ) it can _____ be assumed that the adsorption of _____ _________ _________ will depend on the fraction of ____ vacant sites or ____, and that the adsorption of the other fragment will also depend on ____________ this ____________. 66

67 2) Dual site adsorption (cont.) _____________ Then: Rate of adsorption = _____________ ________ Rate of desorption = ________ At equilibrium: Rate of ads= Rate of desorp, So: k’ A P A [N(1 -  )] 2 = k’ D (N  ) 2 If b = k’ A /k’ D, Then by subsitution and Rearrangement:  = (bP A ) ½ /(1 + (bP A ) ½ ) For the monolayer coverage (V m ), again, let  = V/V m, then This is in the form Y= mX + C, where ______, ______________ Y =______, m = ____, X = ___ C = _______. _______ ___ Thus a plot of _______ against ___ will give a straight line. 67

68 68 Measured amounts adsorbed of the pure gases CH 4 ( ), CO 2 ( ), and N 2 ( ) on AC Norit R1 at T D 298 K. Simultaneous fit of all data with the generalized dual-site Langmuir isotherm (—) F. DREISBACH, R. STAUDT AND J.U. KELLER Adsorption 5, 215–227 (1999)

69 3) Non-competitive adsorption ___________ It is possible that ___________(gas A, and gas B), may be in the same container and will adsorb on different sites. The adsorption _________________ is _________________. Then the isotherm for each gas is simply a ________ Langmuir isotherm for ________ gas. i.e.  = bP A /(1 + bP A ) for gas A, and  = bP B /(1 + bP B ) for gas B 69

70 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 k a (A) and k d (A) with rate constants k a (B) and k d (B) Then  A = b A P A /(1 + b A P A + b B P B ) And  B = b B P B /(1 + b A P A + b B P B ) 70

71 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 = b A P A /(1 +  b i P i ) Other Non-Langmuir Isotherms ____________ i) ____________ Isotherm __________ ____________ This assumes that a __________ decrease of the enthalpy of ____________ occurs with fractional coverage. (  = kP 1/n where k and n are constants with n > 1) 71

72 72 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 Al 2 O 3 at temperature of 308 K.

73 __________ ii) __________ Isotherm ________ _________ Assumption here is that the ________ heat of adsorption falls off _________ with coverage: θ= k’ln(k”bP A ) where b and P A have been defined previously. 73

74 SUMMARY OF LANGMUIR ISOTHERMS 74

75 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. N 2, Ar, Kr to physisorb onto the material. 2) Mercury Porosimetry _______ _______ ______ Liquid mercury is forced _______ into the pores of the material (e.g. _______ gives information on ______pore radius) 75

76 PORE SHAPES/TYPES 76 Uniform/cylindrical Funnel shaped Ink bottle shaped Blind pore Closed pore Through pore Porous network

77 POROUS MATERIALS Pore Size Pores are classified according to size _________ _________ < 2 nm _________ _________ 2 nm < x < 20 nm _________ _________ > 20 nm 77 Yun Wu, Xianfeng Du, Honghua Ge and Zhen Lv Starch/ Stärke 2011, 00, 1–9 DOI /star Microprous starch

78 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. 78

79 IUPAC Classification of porous solids 79 IIIIII IVVVI

80 _________ _________ 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 _________ first _________ _________ At _________ 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. 80 IUPAC Classification of porous solids (Cont.)

81 Macroporous solids (see II ) _______________ At _____ pressure: __________ coverage ______________ At _______ 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: _________ coverage __________________ If not: _________ isotherm for _________ Example: Zeolites 81 Range of pore sizes Zeolite (small size range)

82 82 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* Adsorbed volume P/P* Adsorbed volume P/P* Adsorbed volume P/P* Adsorbed volume _________ The four _________ shapes of adsorption isotherms typically associated with N 2 adsorption Type H1Type H2Type H3 Type H4

83 H1/H2 TYPE _________ _________ Particles with _________ or aggregates of _________ particles H1: uniform size/shape H2: non-uniform size/shape i.e. different size pore mouth and pore body e.g. ink bottle type pores H3/H4 TYPE _________ Aggregates with _________ pores H4: uniform size/shape H3: non-uniform size/shape e.g. zeolites, carbons 83 HYSTERESIS LOOPS (cont.)

84 _________ _________ ____________ _________ 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 tension r = pore radius,  = contact angle (usually = zero) R=gas constant, T = temperature 84 KELVIN EQUATION

85 Consider a zeolite material onto which nitrogen was adsorbed and desorbed. If the P/P * was 0.25 at – o C and the surface tension of nitrogen was 1.0 x Nm -1 (and the contact angle was zero), then use the graph to calculate what the pore radius of the zeolite was. 85PROBLEM P/P* Adsorbed volume 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 = n m L  m, where: __________ n m = moles gas adsorbed in __________ ________  m = area of ________ adsorbed molecule L (or N) = Avagadro constant 86 SURFACE AREA S = 2V p /r V p =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 ____________: V m = volume of gas, monolayer coverage, _________ _________ C = _________ _________ constant 87 SURFACE AREA V = θ = CP V m (P* –P) 1 + (C–1)P P* X P P* = 1 + (C-1) P* V m 1 – P C.V m C. V m P* Y Y – intercept. Thus calculate V m Slope

88 P/P* P/P* / cm -3 V m (1-P/P*) _________ _________ _________

89 Single point method _________ This 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.V m and thus V m can be calculated. An error bar of 5% is acceptable in these experiments. 89 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). 90 P/P* V

91 The constant C in the BET equation is related to the following equation: _________ _________ _________ ______ C = e [(H a – H 1 )]/RT, where H a = enthalpy of _________ of the _________ and H 1 = enthalpy liberated from the second and subsequent layers (similar to the _________ of the gas). Thus C helps give an estimate of ΔH ads and it influences the ______ of the adsorption isotherm: 91 BET SURFACE AREA (cont). V 01P/P* C=1 C=2 C=10 C=10 000

92 Gases used for analyses: GasArea/ m 2 Saturation P (torr) N at 77K Ar at 77K O 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 (_________ ). 92 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  __C Rate (r) given by (rate of form = rate of consump): ______ r = – (1/__)dA/dt = – (1/__)dB/dt = +(1/__)dC/dt _________ Units of r = _________ 93KINETICS

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  cC ________________ Rate = k(P A ) a (P B ) b (P C ) c ….This is referred to as a ________________ 94KINETICS

95 Given: Rate = k(P A ) a (P B ) b (P C ) c,  a,b,c = orders of the reactants (don’t have to = integers).  k = rate constant _________  Rate can be expressed per _________ : r = mr m, where m = mass of catalyst OR  If the total surface area (S) of the catalyst has been determined, then rate can be expressed ______________ ______________ : r = Sr s Recall the Arrhenius equation: k = A cat exp(–E cat /RT) Where E cat = Activation Energy And A cat = pre-exponential factor 95

96 ADSORPTION MECHANISMS 96 We will now look at two different mechanisms proposed for adsorption: (A) Langmuir-Hinshelwood (B) Eley-Rideal

97 97 I)LANGMUIR-HINSHELWOOD (L-H) MODEL Three general assumptions to this model: ______________ 1.Adsorption is _____ and _________from the gas phase RDSsurface chemical reaction = RDS i.e. k very small 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:

98 (A) L-H model for unimolecular reactions Example: E(g)  E (Ads) → C(g) _________________ If b E or P E are _______, then: Rate= k b E P E …i.e. __________ ___________________ If b E or P E are _______, then: Rate → k …..i.e. ____________ θEθEθEθE fast Surface rxn =RDS Molecule E at P E at P EProduct Molecule C fast E E C C

99 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

100 (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 extra 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.

101 .. Langmuir-Hinshelwood Equation Equation θAθAθAθA Molecule B at P B at P B fast RDS Molecule A at P A at P A Product molecule BA θBθBθBθB B BB B A A A A fast

102 102 _______ _______ 1.If molecules A and B are _______ adsorbed, b A and b B are _______, then: _______, Rate = k’ P A P B ….i.e. _______, (Where k’= kb A b B ) ___________ Bstrongly _____________ B 2.If molecule ____is _______ adsorbed and molecule B is strongly adsorbed i.e. _____________then: Rate = k’’ P A / P B, (Where k’’= kb A /b B ). This is an indication that B poisons the surface. _______ _______AB 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

103 The effect of surface concentration on the rate in bimolecular reactions PAPAPAPA rate For constant P B Rate limited by surface Concentration of A of A θ B >> θ A A A A B BB B B B B B B B Rate limited by surface concentration of B of B θ A >> θ B A A A A A A A A A A A B B B

104 II) Eley-Rideal (E-R) model for bimolecular reactions ______________ RDS 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. PAPAPAPA fast RDS PBPB A AA A A A A B B B B B θAθAθAθA

105 Eley-Rideal bimolecular surface reactions θ A = 1 PAPAPAPA Rate For constant P B _____ Note: For constant P A, the rate is always first order wrt P B b A P A << 1 Thus: k Rate = k b A P A P B …….. first order in A __________________ b A P A >> 1 Thus: k Rate = k P B …zero order in A __________________ _____

106 How can the two bimolecular reaction models be distinguished from one another? ___________________ BUT: Experimentally if the reaction rate is measured as a function of the surface coverage of A i.e. θ A, 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. θ A  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 θ A  1.

107 θAθAθAθA Rate 01 Eley-Rideal model Langmuir-Hinshelwood 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 the reaction: A  C k Rate = –dP A /dt = k  A = kb A P A. ________ BUT in this equation BOTH ___ and _____ are functions of temperature. For instance: _____________________k k = A·exp(- E true /RT) 1.If b A is ________ and _____ is ________, then k will be the only variable that is a function of T. i.e. k = A·exp(- E true /RT) small_____ 2.If b A is small then  tends to _____, then from the van’t Hoff Isochore we have: (dlnb A )/dT =  H θ A /RT 2, then ∫(dlnb A ) = ∫ (  H θ A /RT 2 ) dT 108

109 Thus: b A = C·exp(–  H θ A /RT) lnb A = –  H θ A /RT + C OR b A = C·exp(–  H θ A /RT) (where C = integration constant & –  H θ A is the standard molar enthalpy of adsorption of reactant A) kb A Now we have rate = k  A and  A = b A P A kb A We can substitute into the equation for both k and b A kb A k i.e. Rate = –dP A /dt = k  A = kb A P A = k[C·exp(–  H θ A /RT)](P A ) A·exp(–E true /RT)[C·exp(–  H θ A /RT)] = A·exp(–E true /RT)[C·exp(–  H θ A /RT)](P A ) Re-writing this out: Re-writing this out: = P A ·A·C·exp(–E true –  H θ A )/RT = P A ·A·C·exp(–E app )/RT Where: (–E app ) = (–E true –  H θ A ), henceE true = E app –  H θ a and since  H θ A is always negative, then E true > E app 109

110 Arrhenius plot for a catalysed reaction over a range of temperatures gives: 110 I IIIII Surface coverage 1 1>  >0 ~ 0 Order,n01>n>01 Slope x 2.3R E true ---- E app III II I Log 10 Rate 1/T

111 ________ ____________ Diffusion or mass-transport limited reactions are those in which the transport of the ________ /or ____________ the catalyst influence the rate: vs______________ 1.Rate  [catalyst] n and n< 1 vs ______________ vs____________________ 2.Rate  stirring rate vs ____________________ vs______________ 3.E act about 10 – 15 kJ mol –1 vs ______________ vs_________________ 4.Rate  (temperature) 1/2 vs _________________ An Arrhenius plot showing the onset of diffusion limitation can be drawn: 111 THE EFFECT OF DIFFUSION ON KINETICS

112 112 ____________ ____________ for diffusion limited reactions the RAQ’ curve (purple curve) is detected, because the slower of the two processes controls the rate. R Q’ Q R’ Log 10 Rate 1/T A’A: Surface reaction is rate-limiting rate-limiting B’B: Reaction is diffusion is diffusion limited limited A


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