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Surface chemistry Trieu Tuan Anh

Objectives After studying this chapter, students should be able to  Define important principles  Describe the two types of adsorption, physisorption and chemisorption  Explain how a unimolecular layer is formed  Develop the equations necessary to derive the Langmuir isotherm and plot the fraction of surface covered against concentration for adsorption with and without dissociation  Explain the conditions under which the BET isotherm occurs  Calculate surface area of a solid sample using Langmuir and/or BET equations

Contents  Introduction  Basic

principles

 Adsorption  Adsorption  Surface

Isotherm

tension and capillarity

Introduction

Introduction

Introduction

Introduction Many industrial processes using heterogeneous catalyst such as sulfuric acid, ammonia, and nitric acid, petroleum industrial, synthesis methanol from carbon monoxide and hydrogen(*)

Sinfelt JH (2002) Role of surface science in catalysis. Surf Sci 500:923–946.

Introduction Not just in industrial processes, heterogeneous catalyst also be used in pollution control and prevention, reduces the pollutant emission from automobiles.

Annual average of toxic mobile emissions in Los Angeles County http://www.arb.ca.gov/app/emsinv/emssumcat.php,

Introduction To synthesis clean energy sources: biodiesel from waste cooking - oil, animal fat, jatropha oils, microalgae,… scientists also use heterogeneous catalyst

http://www.arb.ca.gov/app/emsinv/emssumcat.php,

Introduction Conversion chemical energy to electric energy, heterogeneous catalysts were used in both cathode and anode site.

http://americanhistory.si.edu/fuelcells/basics.htm

Introduction Another approach to produce green energy source is using photocatalyst to convert water and CO2 into fuel

http://spie.org/newsroom/5838-solar-fuel-production-for-sustainable-energy-supply

Introduction Semiconductor based technology is very important in modern world. Advances in epitaxial growth of silicon thin films, chemical vapor deposition, and surface etching techniques

http://www.arb.ca.gov/app/emsinv/emssumcat.php,

Introduction Surface chemistry processes for implant biomaterials

http://rsif.royalsocietypublishing.org/content/11/95/20140169

Introduction Surface chemistry processes for chromatography separations,

http://rsif.royalsocietypublishing.org/content/11/95/20140169

Introduction Surface chemistry processes for biosensors SPR-based biosensor for lung cancer detection using dendrimers as surface enhancement polymers

https://www.researchgate.net/figure/263016557_fig1_Fig-3-SPR-based-biosensor-for-lung-cancer-detection-usingdendrimers-as-surface

Surface & interface A boundary that separates two phases is known as a surface or an interface.

Laidler, 4ed, 931

Surface free energy (a) the net force acting on will essentially be zero, (b)The net forces acting on a unit at the surface will be unbalanced in the direction of the bulk phase.

Laidler, 4ed, 931

Absorption & Adsorption

Laidler, 4ed, 931-933

Absorption & Adsorption Absorption Definition Assimilation of molecular species throughout the bulk of the solid or liquid is termed as absorption. Phenomenon It is a bulk phenomenon Heat exchange Endothermic process Temperature It is not affected by temperature

Rate of It occurs at a uniform rate. reaction Concentration It is same throughout the material. Laidler, 4ed, 931-933

Adsorption Accumulation of the molecular species at the surface rather than in the bulk of the solid or liquid is termed as adsorption. It is a surface phenomenon. Exothermic process It is favoured by low temperature

It steadily increases and reach to equilibrium Concentration on the surface of adsorbent is different from that in the bulk

Adsorption - physisorption 

Physisorption: the forces are of a physical nature and the adsorption is relatively weak ( van der Waals forces)



Heat ~ 20kJ/mol

Laidler, 4ed, 931-933

Adsorption - physisorption In “Physisorption of Hydrogen on Microporous Carbon and Carbon Nanotubes” papers on J. Phys. Chem. B 1998, 102, 10894-10898, M. Rzepka*, P. Lamp and M. A. de la Casa-Lillo investigated the storage capability of microporous carbon materials for gaseous hydrogen both theoretically and experimentally and for a storage pressure of 10 MPa a maximum adsorbed hydrogen density of 14kg/m3 can be reached http://pubs.acs.org/doi/abs/10.1021/jp9829602

Adsorption - physisorption Question! What is the volume of 14kg of hydrogen 1. At STP? 2. At 0oC, 10 MPa

http://pubs.acs.org/doi/abs/10.1021/jp9829602

Adsorption - physisorption In “An overview of technologies for immobilization of enzymes and surface analysis techniques for immobilized enzymes” Mohamad and colleagues confirm: 1. The enzymes being physically adsorbed or attached onto the support material. Adsorption can occur through weak non-specific forces such as van der Waals, hydrophobic interactions and hydrogen bonds. 2. The reversibly immobilized enzymes can be removed from the support under gentle conditions, a method highly attractive as when the enzymatic activity has decayed, the support can be regenerated and reloaded with fresh enzyme. The cost of the support is often a primary factor in the overall cost of immobilized catalysts. http://pubs.acs.org/doi/abs/10.1021/jp9829602

Adsorption - chemisorption 

Chemisorption: the adsorbed molecules are held to the surface by covalent forces of the same general type as those occurring between atoms in molecules.



Heat ~ 100 – 500 kJ/mol



Called activation adsorption

Laidler, 4ed, 931-933

Adsorption - chemisorption The extent of adsorption A increases with increasing pressure p, but eventually approaches a saturation limit when the pressure becomes sufficiently high. This means that the chemisorption process is complete when the surface is covered by a single layer of molecules Laidler, 4ed, 931-933

Adsorption Physisorption

Chemisorption

Weak van der Waals force

Chemical bonds

It occurs at low temperature

It occurs at high temperature

Heat of adsorption ~ 20 kJ/mol

Heat of adsorption 100 – 500 kj/mol

Reversible process

Irreversible process

Multilayer adsorption, several molecules thick

Monolayer adsorption (unimolecular thickness

Laidler, 4ed, 931-933

The Langmuir Isotherm  

The rate of the adsorption ? The rate of the desorption?

Laidler, 4ed, 933-935

The Langmuir Isotherm The rate of the adsorption will then be proportional to the concentration [A] of the molecules in the gas or liquid phase and also the proportional to the fraction of the surface.

va  ka [A](1   )

Laidler, 4ed, 933-935

The Langmuir Isotherm The rate of the desorption vd is proportional only to the number of molecules attached to the surface, which in turn is proportional to the fraction of the surface covered.

vd  kd Laidler, 4ed, 933-935

The Langmuir Isotherm 

At equilibrium

ka [A](1   )  kd kd   [A] 1   ka







1 

 K [A]

K [A]   1  K [A] Laidler, 4ed, 933-935

{K  kd / Ka }

The Langmuir Isotherm – example Benzene adsorbed on graphite is found to obey the Langmuir isotherm to a good approximation. At the pressure of 1.00 Torr the volume of benzene absorbed on a sample of graphite was found to be

4.2mm3 at STP (0oC and 1 atm pressure) anf 3.00 torr it was 8.5 mm3. Assume a benzene molecule to occupy 30Å2 and estimate the surface of the graphite. Laidler, 4ed, 933-935

The Langmuir Isotherm – example 

Using the Langmuir equation



Apply it for each case



Can replace [A] by pressure P



Suppose that the amount of adsorbed when the

K [A]  1  K [A]

surface is saturated is x mm3 

Calculating x value ( two equations, two variables)



Calculation the area of graphite Laidler, 4ed, 933-935

Adsorption with Dissociation

Laidler, 4ed, 935-936

Adsorption with dissociation The process of adsorption is now a reaction between the gas molecule and two adjacent surface sites, and the rate of adsorption is therefore.

va  ka [A](1   )

2

Laidler, 4ed, 935-936

Adsorption with dissociation The desorption process involves reaction between two adsorbed atoms, and the rate is therefore proportional to the square of the fraction of surface covered

vd  kd

Laidler, 4ed, 935-936

2

Adsorption with dissociation At the equilibrium the rates are equal, and therefore, 1/2

 ka    [A]  1    kd 



Laidler, 4ed, 935-936

1

 K [A] 2

1

2

Adsorption with dissociation Above equation can be written as

1



Laidler, 4ed, 935-936

K 2 [A] 1

1

2

1  K 2 [A]

1

2

Competitive adsorption 

The isotherm for two substances adsorbed on the same surface is of importance in connection with inhibition and with kinetics of surface reactions involving two reactants A and B

Laidler, 4ed, 936-937

Competitive adsorption 

Suppose that the fraction of surface covered by molecules type A is θa, type B is θb. The fraction bare is 1- θa- θb



A A v  k The rates of adsorption A a a [A](1   a  b )



The rates of adsorption B

vaB  kaB [B](1  a  b )



The rates of desorption

v k  A d

A d A

v k  B d

Laidler, 4ed, 936-937

B d B

Competitive adsorption 

At equilibrium vd = va for both of subtances A and B

KA [A] A  1  KA [A]  KB [B] KB [B] B  1  KA [A]  KB [B]

Laidler, 4ed, 936-937

BET isotherm This is an extension of the Langmuir treatment to allow for the physisorption of additional layers of adsorbed molecules. It was delivered by balancing the rates of adsorption and condensation for the various layers ( only one enthalpy adsorption for the first layer, and the enthalpy of liquefaction applies to the second and subsequent layer)

PPo 1 P   V (Po  P ) V0K Vo Laidler, 4ed, 937-938

Surface tension and capillarity The floating needle

Laidler, 4ed, 948-951

Surface tension and capillarity The floating needle

Laidler, 4ed, 948-951

Surface tension and capillarity Awesomely Perfect and Refreshing Example of Surface Tension

Laidler, 4ed, 948-951

Surface tension and capillarity  

A molecule in the interior of a liquid: no resultant force tending to move it in any direction. A molecule at the surface: a net inward attraction on the surface molecules.

Laidler, 4ed, 948-951

Surface tension and capillarity  

F (N) = γ(N/m)[2l (m)] ɣ: is known as the surface tension.

Laidler, 4ed, 948-951

Surface tension and capillarity 

F = ɣ2π(r+R)

Laidler, 4ed, 948-951

Surface tension and capillarity    

Liquid wets the glass, the level of liquid will rise in the tube Force by surface tension F1=2πrɣ Force due to the weight of liquid F2=πr2hρg If the liquid does not completely wet the glass. There is an angle θ between the meniscus and the surface

rhρh γ 2cosθ Laidler, 4ed, 948-951

Surface tension and capillarity 

Langmuir’s film balance

Laidler, 4ed, 948-951

Surface tension and capillarity Pendant Drop Method

mg = 3a cos()  is the surface tension of liquid  is the contact angle at which a liquid/vapor interface meets the solid surface.

Contact Angle Goniometer. Laidler, 4ed, 948-951

Surface tension and capillarity 

Directly depends on intermolecular forces in the solution



Inversely depends on temperature



 of metallic liquid > ionic liquid > covalent liquid

Surface tension and capillarity Liquid in a Vertical Tube

 ls   la cos Concave

ls = liquid-solid surface tension la = liquid-air surface tension = contact angle

Concave Adhesive>>Cohesive Convex Adhesive P

PPo 1 P   V (Po  P ) V0K Vo Laidler, 4ed, 948-951

KPA  1  KPA

Exercise 6 The following data were obtained for the adsorption of krypton on a 1.21 g sample of a porous solid. If the saturation vapor pressure is 19.0 Torr, estimate a surface area for the solid, assuming that a molecule of krypton occupies an area of 2.1*10-21m2. Pressure/Torr

1.11

3.08

Volume adsorbed/cm3 (STP)

1.48

1.88

PPo 1 P   V (Po  P ) V0K Vo Laidler, 4ed, 948-951

Exercise 7 The decomposition of ammonia on platinum 2NH3 ↔ N2 + 3H2 Is the first order in NH3 and the rate is inversely proportional to the hydrogen concentration (eq.18.39). Write the differential rate equation for the rate of formation of hydrogen, dx/dt, interms of the initial concentration of ammonia, ao, and the concentration x of hydrogen at time t.

Laidler, 4ed, 948-951

Exercise 8 The surface tension of water at 20oC is 7.27*10-2Nm-1 and its density is 0.998 gcm-3. Assuming a contact angle θ of zero, calculate the rise of water at 20oC in a capillar tube of

radius (a) 1 mm and (b) 10-3 cm. take g=9.81 ms-2.

Laidler, 4ed, 948-951

Exercise 9 The density of liquid mecury at 273K is 13,6gcm-3 and the surface tension is 0.47Nm-1. If the contact angle is 140oC, calculate the capillary depression in a tube of 1 mm diameter

Laidler, 4ed, 948-951