Adsorption Flashcards
What is the definition of adsorption
Adsorption is the enrichment of a component i in a fluid at its interface with a solid adsorbent
What is sub and super critical adsorption
Referring to critical temperature, at sub critical temperature an increase in pressure will eventually cause condensation and so a relative pressure to this condensation must be used
at super critical temperature the gas can never be a liquid and standard pressure can be used
Why does adsorption occur
Mainly driven by attractive intermolecular interactions
What key adsorbate interactions are there
Adsorbate - adsorbate
Adsorbate - adsorbent
Adsorbate - adsorptive
Outline the differences between physisorption and chemisoprtion
Physisorption
- Low heat of adsorption
- non specific
- mono or multilayer
- no dissociation of adsorbed species
- only significant at low temperatures
- rapid, non active, reversible
- no electron transfer
Chemisorption
- High heat of adsorption
- highly specific
- mono layer only
- may involve dissociation of adsorbed species
- possible over wide range of temperatures
- activated, irreversible, may be slow
- electron transfer leads to chemical bonds
What is an issue with the classification of the IUPAC
- Not consistent with SI
- Biased towards small pores
- only three ranges
Draw type I, II, II isotherms
slide 35
What is the langmuir equation
define all components
n = abP/(1+bP) n = a when p is very high because 1+bp approximately equals bP
n = abP when P is very low because 1 + bP is approximately 1
b = bo*exp(E/RT) it is affinity for the adsorbent
How does langmuir freundlich differ from langmuir
bP is taken to the power 1/n
how does toth differ from langmuir
Bottom term bP is taken to the power t and the whole bottom is taken to 1/t
what is the BET equation
n/nm = cx/(1-x)(1+(c-1)x)
x is relative pressure = p/po
c = exp(E1 - El/RT)
nm is capacity of mono layer
as c increases the isotherm moves from type III to type I
Re-arange the BET equation to linearise it
notes
0.05 and 0.3
How do you aquaria nm from a graph of
at the knee of the graph n = nm
What is the surface area of BET equation
A = (sigma)Nanm
nm is capacity of mono layer
sigma is adsorbent area 0.162nm2
Write the kelvin equation
notes
What is the dubinin analysis
notes
What are the assumptions of the BET equations
- adsorption sites are energetically equaly
- infinite layers
- langmuir theory can be applied to each layer
- In the second and all higher layers the energy of adsorption has the same value as the liquefaction energy El
- no interactions between layers
Draw the type VII Isotherm
notes
how is the excess adsorbed defined
integral of the difference in densities
What pores are seen in type I and II
Micro in type I and micro and meso in type II
Describe why the shape of type I is as it is
- Strong interactions due to overlapping adsorbent forces
- limited by micropore volume rather than surface area
- levels off due to saturation of pores
- steep slope due to strong forces
- mono layer adsorption
Describe why the shape of type II is as it is
- multi and mono layers
- initial steep slope due to mono layer
- capillary condensation describes the gradual gradient of the graph
- knee corresponds to the point where the mono layer is at capacity
Describe why the shape of type III is as it is
- weak interaction forces
- water carbon
- high pressure forces particles to adsorb
Describe the reasons behind shape of type VIII
- adsorbate adsorbs as expected until saturated
- because of the definition of how the excess is calculated the excess drops
- as the bulk density increases the density in the pores does not and so the excess appears to drop
