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Tim Mays > Adsorption > Flashcards

Adsorption Flashcards

1
Q

What is the definition of adsorption

A

Adsorption is the enrichment of a component i in a fluid at its interface with a solid adsorbent

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2
Q

What is sub and super critical adsorption

A

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

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3
Q

Why does adsorption occur

A

Mainly driven by attractive intermolecular interactions

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4
Q

What key adsorbate interactions are there

A

Adsorbate - adsorbate
Adsorbate - adsorbent
Adsorbate - adsorptive

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5
Q

Outline the differences between physisorption and chemisoprtion

A

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
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6
Q

What is an issue with the classification of the IUPAC

A
  • Not consistent with SI
  • Biased towards small pores
  • only three ranges
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7
Q

Draw type I, II, II isotherms

A

slide 35

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8
Q

What is the langmuir equation

define all components

A 
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

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9
Q

How does langmuir freundlich differ from langmuir

A

bP is taken to the power 1/n

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10
Q

how does toth differ from langmuir

A

Bottom term bP is taken to the power t and the whole bottom is taken to 1/t

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11
Q

what is the BET equation

A

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

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12
Q

Re-arange the BET equation to linearise it

A

notes

0.05 and 0.3

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13
Q

How do you aquaria nm from a graph of

A

at the knee of the graph n = nm

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14
Q

What is the surface area of BET equation

A

A = (sigma)Nanm

nm is capacity of mono layer
sigma is adsorbent area 0.162nm2

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15
Q

Write the kelvin equation

A

notes

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16
Q

What is the dubinin analysis

A

notes

17
Q

What are the assumptions of the BET equations

A
  • 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
18
Q

Draw the type VII Isotherm

A

notes

19
Q

how is the excess adsorbed defined

A

integral of the difference in densities

20
Q

What pores are seen in type I and II

A

Micro in type I and micro and meso in type II

21
Q

Describe why the shape of type I is as it is

A
  • 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
22
Q

Describe why the shape of type II is as it is

A
  • 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
23
Q

Describe why the shape of type III is as it is

A
  • weak interaction forces
  • water carbon
  • high pressure forces particles to adsorb
24
Q

Describe the reasons behind shape of type VIII

A
  • 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

Tim Mays (3 decks)

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