CHEM 156 Study Guide - Final Guide: Microporous Material, Physisorption, Inflection Point
Part of gas/ vapor phase remained attached to the solid surface
•
Physisorption (physical adsorption): weak vanderwaals attraction btwn
adsorbate and solid surface
•
Characterization of porous materials allows determination of specific surface
area, pore size distribution and pore volume
•
Adsorption:
Low heats of adsorption, no violent or disruptive structural changes
•
Can involve multiple layers of adsorbate, thus allowing for pore
measurements
•
High temp tend to inhibit physical adsorption
•
Adsprotion equilibrium is achieved quickly since no activation energy is
generally required
•
Physical adsorption is fully reversible, allowing adsorbate to fully adsorv and
desorb
•
Characteristics of physical adsorption:
Desorption isotherms: measuring gas removed as pressure is reduced
•
Type 1 isotherm: pores typically microporous with exposed surface residing
almost exclusively inside micropores, which once filled with adsorbate, leave
little or no external surface for furthur adsorption
•
Type 2: most frequently found when adsorption occurs in nonporous
powders or powders with diameters exceeding micropores
•
Inflection point occurs near completion of first adsorbed monolayer
Type 3: heats of adsorption less than adsorbate heat of liquification,
adsorption proceeds as adsorbate interaction with adsorbed layer is greater
than interaction with adsorbent surface
•
Type 4: occur on porous adsorbents with pores in range of 1.5 - 100 nm;
slope shows increased uptake of adsorbate as pore become filled at higher
temp
•
Adsorption isotherms: obtained by measuring amount of gas adsorbed across
wide range of relative pressures at constant temp
Type 5: observed where theres small adsorbate-adsorbent interactin
potentials (similar to type 3), and also associated with pores in 1.5 - 100 nm
range
•
Inflection point typically occurs near completion of first monolayer
Most common: nitrogen
•
Adsorbate:
BET equation: (Brunauer, Emmett and Teller) used to describe specific surface
area
Adsorption
Thursday, June 14, 2018
2:21 AM
Chem 156 Page 1
Document Summary
Part of gas/ vapor phase remained attached to the solid surface. Physisorption (physical adsorption): weak vanderwaals attraction btwn adsorbate and solid surface. Characterization of porous materials allows determination of specific surface area, pore size distribution and pore volume. Low heats of adsorption, no violent or disruptive structural changes. Can involve multiple layers of adsorbate, thus allowing for pore measurements. Adsprotion equilibrium is achieved quickly since no activation energy is generally required. Physical adsorption is fully reversible, allowing adsorbate to fully adsorv and desorb. Adsorption isotherms: obtained by measuring amount of gas adsorbed across wide range of relative pressures at constant temp. Desorption isotherms: measuring gas removed as pressure is reduced. Type 1 isotherm: pores typically microporous with exposed surface residing almost exclusively inside micropores, which once filled with adsorbate, leave little or no external surface for furthur adsorption. Type 2: most frequently found when adsorption occurs in nonporous powders or powders with diameters exceeding micropores.
