Surfaces

Front 1

Absorption

Back 1

gas/liquid enters a material and is taken up by the bulk phase

Front 2

Adsorption

Back 2

gas/liquide adheres (chemically binds) to the surface

Front 3

Coverage =

Back 3

number of filled adsorption sites/ number of sites availible

Front 4

Internal surface

Back 4

Area wich surrounds the closed pores, fissures and cracks which penetrate deeply into the interior of the absorbent. deeper>wide

Front 5

External surface

Back 5

Protrusions and superficial cracks. Wider>deep

Front 6

Describe surfaces of solids? (4)

Back 6

1) Inhomogenities e.g. kinks
2) Coordinatly unsaturated (dangling bonds)
3) May have chemical quality e.g. acidic
4) high energy sites are very reactive

Front 7

Tarnishing rate, collision flux (z) =

Back 7

P/ sqroot(2(pi)mk(boltz)T)

Front 8

Physisorption
Ea, enthalpy change, layers, specificity, reversible

Back 8

Ea - none
enthalpy - -10 to -20 kJmol-1
multilayers
not specific
reversible

Front 9

Chemisorption
Ea, enthalpy change, layers, specificity, reversible

Back 9

Ea - small but finite
enthalpy - 40 to -800 kJmol-1
monolayer
highly specific
usually irreversible

Front 10

Volumetric technique for measuring adsorption? (3 steps, 2 pros, 3 cons)

Back 10

1)Measure P, V and T then calculate n
2)Open to adsorbate and let equilibrate
3)Measure new P&V calculate change in n
Pros - easy and simple
Cons - Dead space Vol, Equilibrium, Insensitive at high coverage
For reversible systems i.e. physisorption

Front 11

Gravimetric technique? (1 pro, 1 cons)

Back 11

Weigh amount adsorbed
Pro - can be followed dynamically
Cons - very small masses involved
Used for reversible systems i.e. physisorption

Front 12

Dynamic method for measuring adsorbed amounts? (5 steps)

Back 12

1)Use gas chromatography, flow gas over sample (with carrier molecule)
2)Let equilibriate
3)Cool sample causing adsorption
4)Measure change hence amount adsorbed
5) heat column to desorb gas

Front 13

coverage =
in therms of K and Pa

Back 13

KPa/(1+KPa)

Front 14

Assumptions of Langmuir Isotherm? (4)

Back 14

1)Adsorption cannot proceed monolayer
2)All sites equlivent, uniform surface
3)No interactiosn between adsorped molecules so ability to adsorb independant of coverage
4)Adsorption is localised, molecules don't move (low T)

Front 15

Coverage =
in term of V, Vmon

Back 15

V/Vmon (Vol adsorbed to give monolayer coverage)

Front 16

Assumptions of BET isotherm? (5)

Back 16

1)Adsorption onto clean surface is same as langmuir (equal energy sites, no lateral interaction)
2)second layer forms on first, third on second....
3)When P=P0 (vapP of pure liquid) layer behaves like liquid
4)First layer, change in enthalpy=langmuir value
5)Second, third.. change in enthalpy of adsorption = -change in enthalpy of vapourisation

Front 17

Limits of BET?
x<P/P0<y

Back 17

0.05<P/P0<0.3
underestimates at low P and over estimates at high P

Front 18

Good catalyst properties? (6)

Back 18

1)High and stable activity and selectivity
2)Controlled SA and porosity
3)Good resistance to poisons (irrevesibly bind to catalyst)
4)Good resistance to high T and fluctuation in T
5)High mechanical strength
6)No uncontrollable hazards

Front 19

Mechanism of catalysis? (6 steps)

Back 19

1)Diffusion to surface
2)Adsorption to surface
3)Surface diffusion
4)Reaction of surface species
5)Desorption
6)Diffusion away from surface (fast)

Front 20

What happens first chemisorption or physisorptions?

Back 20

Physisorption