Gas-Solid Interactions Light bulbs, Three way catalysts, Cracking, Corrosion, Electronic Devices etc.

Adsorption-the key step Extent of adsorption usually given by fractional coverage N is often equivalent to number of surface atoms of the substrate Associative (or non-dissociative) adsorption is when a molecule adsorbs without fragmentation Dissociative adsorption is when fragmentation occurs during the adsorption process

Adsorption Isotherms Models describing equilibrium between the gaseous and the adsorbed phases at a given fixed temperature Simplest is that based on Irving Langmuir’s proposals Born in Brooklyn January 31, 1881 Graduated from Columbia University in 1903 Postgraduate under Nernst in Göttingen Post at Stevens Institute of Technology New Jersey 1909 hired by General Electric Company Studies embraced chemistry, physics, and engineering Investigated properties of adsorbed films and the nature of electric discharges in high vacuum and in certain gases at low pressures. http://www.nobel.se/chemistry/laureates/1932/langmuir-bio.html

Langmuir Isotherm Simplifying assumptions Adsorption proceeds to monolayer formation only All sites are equivalent and the surface is uniform Molecule adsorption is independent of occupation of neighbouring sites

Langmuir Isotherms

SA/(mass of substrate) Using the isotherm SA = N x Am Specific surface area SA/(mass of substrate) Atkins & dePaula 8th p. 918 Attard & Barnes p. 4

Dissociative Adsorption

Typical Langmuir isotherms Associative adsorption isotherms Dissociative adsorption isotherms http://www.oup.com/uk/orc/bin/9780199271832/01student/graphs/lg_16_17_18_20.htm

Gas adsorption to a solid is exothermic. Heats of Adsorption Gas adsorption to a solid is exothermic. The magnitude and variation as a function of coverage may reveal information concerning the bonding to the surface. Calorimetric methods determine heat, Q evolved. qi = integral heat of adsorption qD = differential heat of adsorption

Enthalpy of Adsorption Heats of adsorption change as a function of surface coverage differentiate Van’t Hoff equation

Isosteric enthalpy of adsorption Re-arranging Langmuir Differentiate & re-arrange Use van’t Hoff

Measuring isosteric enthalpies Attard & Barnes p. 83 Isosteric HEATS of adsorption sometimes used instead of enthalpies

Measuring isosteric enthalpies Atkins & de Paula, 8th p. 919-920 Note

BET Isotherm When adsorption of a gas can occur over a previously adsorbed monolayer of the gas Brunauer, Emmett & Teller extends the Langmuir isotherm model to multilayer adsorption Assumptions: Adsorption of 1st layer takes place on a surface of uniform energy 2nd layer only adsorbs on 1st, 3rd on 2nd, etc. When p=p*, infinite layers form. At equilibrium, rates of condensation & evapouration are same for each individual layer For layers ≥ 2, ΔH0AD = -Δ H0VAP

BET As before, we can replace N with masses or volumes.

BET “knee” in some isotherms represents monolayer coverage BET underestimates adsorption at low p and overestimates adsorption at high p

Using the BET Atkins & dePaula 8th p. 921 Principle behind the surface area and pore size analyzers on the market. Use nitrogen at 77K as adsorbate. Knowing size of molecule, the surface area and/or pore size can be determined from the isotherm. http://www.beckman.com/products/instrument/partChar/pc_sa3100.asp

IUPAC Classification

Other isotherms When adsorption sites are not equivalent, enthalpy of adsorption changes as a function of coverage Temkin: Assumes enthalpy changes linearly with pressure Freundlich: Assumes enthalpy changes logarithmically with pressure Try example in Attard & Barnes, p.83