7/29/2019 Processes at Solid Surfaces

1/18

PROCESSES AT SOLIDSURFACES

7/29/2019 Processes at Solid Surfaces

2/18

Introduction

Processes at solid surfaces govern the viability ofindustry both constructively, as in catalysis, anddestructively, as in corrosion.

Chemical reactions at solid surfaces may differsharply from reactions in the bulk, for reactionpathways of much lower activation energymay beprovided, and hence result in catalysis.

The concept of a solid surface has been extended inrecent years with the availability ofmicro porousmaterials as catalysts.

7/29/2019 Processes at Solid Surfaces

3/18

The Growth and Structureof Solid Surface

The attachment of particles to a surface iscalled adsorption.

The substance that adsorbs is the adsorbate

The underlying material that we areconcerned with in this section is the

adsorbentor substrate.

The reverse of adsorption is desorption.

7/29/2019 Processes at Solid Surfaces

4/18

Perfect Crystal Surface

A simple picture

of a perfectcrystal surface

is as a tray of

oranges in a

grocery store

7/29/2019 Processes at Solid Surfaces

5/18

Surfaces Defect

7/29/2019 Processes at Solid Surfaces

6/18

7/29/2019 Processes at Solid Surfaces

7/18

The fractional coverage is often expressed in termsof the volume of adsorbate adsorbed by

where V~ is the volume of adsorbatecorresponding to complete monolayer coverage.

The rate of adsorption, d/dt, is the rate of changeof surface coverage, and can be determined byobserving the change of fractional coverage withtime.

=V

V

7/29/2019 Processes at Solid Surfaces

8/18

Technique of adsorption

One commonly used technique is therefore tomonitor the rates of flow of gas into and out of thesystem: the difference is the rate of gas uptake by

the sample. Integration of this rate then gives thefractional coverage at any stage.

7/29/2019 Processes at Solid Surfaces

9/18

Technique of Desorption

In flash desorption the sample is suddenly heated(electrically) and the resulting rise of pressure isinterpreted in terms of the amount of adsorbate

originally on the sample.

Gravimetryinvoleves weighing of the sampleduring the desorption

7/29/2019 Processes at Solid Surfaces

10/18

Physisorption and Chemisorption

Physisorption ChemisorptionVan der walls interaction Covalent bond

Enthalpy of adsorption less

than about 40 kJ/mol

Enthalpy of adsorption

greater than about 80 kJ/mol

No appreciable activationenergy involved in

adsorption process

Activation energy may beinvolved in adsorption

processMultilayer adsorption occurs Adsorption leads to a

monolayer , at most

Exothermic Exothermic

7/29/2019 Processes at Solid Surfaces

11/18

Enthalphy of adsorption

7/29/2019 Processes at Solid Surfaces

12/18

Adsorption Isotherms

The free gas and the adsorbed gas are in dynamicequilibrium, and the fractional coverage of thesurface depends on the pressure of the overlying

gas. The variation of

with pressure at a chosentemperature is called the adsorption isotherm.

7/29/2019 Processes at Solid Surfaces

13/18

Langmuir Isotherms

The simplest physically plausible isotherm is basedon three assumptions:

1 Adsorption cannot proceed beyond monolayer

coverage. 2 All sites are equivalent and the surface is

uniform

3 The ability of a molecule to adsorb at a givensite is independent of the occupation ofneighbouring sites (that is, there are nointeractions between adsorbed molecules).

7/29/2019 Processes at Solid Surfaces

14/18

7/29/2019 Processes at Solid Surfaces

15/18

Example

7/29/2019 Processes at Solid Surfaces

16/18

7/29/2019 Processes at Solid Surfaces

17/18

Graph of example 25.1

7/29/2019 Processes at Solid Surfaces

18/18

Self Test