S OLID S TATE AND S URFACE C HEMISTRY ( LECTURE 10) Dr. Saeda Rwede Al-Mhyawi Assistant professor in physical chemistry Contact Info: Office hours: S,M 10-11,12-1, W 10-1

O BJECTIVES By the end of this section you should: Know the surface science terminology. be able to identify the types of adsorption, and distinguish between them. Know the definition of isotherms

2. SURFACE CHEMISTRY INTRODUCTION 1934, the English chemist Michael Faraday suggested that the first step in catalyzed reaction was the sticking of reactant molecule to the solid surface 1916 American chemist Irving Langmuir introduced type of adsorption called chemisorption. Surface Science is the study of physical and chemical phenomena that occur at the interface of two phases such as solid-liquid interfaces. A d sorption the process by which a molecule or atom adsorb onto a surface of substrate. Desorption is the removal of bound molecules from the surface (opposite of a d sorption)

A DSORPTION VERSUS A BSORPTION Adsorption Surface process Absorption bulk process Adsorption is the physical adherence or bonding of ions and molecules onto the surface of another phase. Absorption is the incorporation of a substance in one state into another of a different state.

N OMENCLATURE Substrate or adsorbent: surface onto which adsorption can occur. example: catalyst surface, activated carbon, alumina Adsorbate: molecules or atoms that adsorb onto the substrate. example: nitrogen, hydrogen, carbon monoxide, water Sorption: adsorption and absorption phenomena occur simultaneously. Coverage: a measure of the extent of adsorption of a specie onto a surface Exposure: a measure of the amount of gas the surface had been exposed to ( 1 Langmuir = torr s)

T YPES OF A DSORPTION M ODES Two types of adsorption Physical adsorption : generally a molecule is physisorbed if the bond energy is less than 10 kcal/mole. Weak, Long range bonding Van der Waals interactions (e.g. Londondispersion, dipole-dipole).. NOT SURFACE SPECIFIC Physisorption takes place between all molecules on any surface providing the temperature is low enough. ∆H ads = 5 ….. 50 kJ mol -1 Non activated with equilibrium achieved relatively quickly. Increasing temperaturea lways reduces surface coverage. No surface reactions

C HEMICAL ADSORPTION o chemisorbed if bond energy is more than 10 kcal/mole Strong, short range bonding Chemical bonding involving orbital overlap and charge transfer SURFACE SPECIFIC E.g. Chemisorption of hydrogen takes place on transition metals but not on gold or mercury. ∆H ads = 50 … kJ mol -1 Can be activated, in which case equilibrium can be slow and increasing temperature can favour adsorption. Surface reactions may take place:- Dissociation, reconstruction, catalysis.

Physical adsorption MULTILAYER ADSORPTION BET Isotherm used to model adsorption equilibrium. Chemical adsorption MONOLAYER ADSORPTION Langmuir Isotherm used to model adsorption equilibrium..

I SOTHERM Isotherm is a plot of surface coverage as a function of gas pressure at fixed (constant) temperature. Langmuir: for monolayer adsorption BET : for multilayer Adsorption equilibrium If the adsorbent and adsorbate are contacted long enough an equilibrium will be established between the amount of adsorbate adsorbed and the amount of adsorbate in solution. The equilibrium relationship is described by isotherms. Adsorption ⇄ desorption  The adsorption isotherm is used to  Determine equilibrium constant, enthalpy of adsorption, concentration of surface sites available for adsorption.