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20 Cards in this Set
- Front
- Back
Absorption
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gas/liquid enters a material and is taken up by the bulk phase
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Adsorption
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gas/liquide adheres (chemically binds) to the surface
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Coverage =
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number of filled adsorption sites/ number of sites availible
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Internal surface
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Area wich surrounds the closed pores, fissures and cracks which penetrate deeply into the interior of the absorbent. deeper>wide
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External surface
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Protrusions and superficial cracks. Wider>deep
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Describe surfaces of solids? (4)
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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 |
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Tarnishing rate, collision flux (z) =
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P/ sqroot(2(pi)mk(boltz)T)
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Physisorption
Ea, enthalpy change, layers, specificity, reversible |
Ea - none
enthalpy - -10 to -20 kJmol-1 multilayers not specific reversible |
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Chemisorption
Ea, enthalpy change, layers, specificity, reversible |
Ea - small but finite
enthalpy - 40 to -800 kJmol-1 monolayer highly specific usually irreversible |
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Volumetric technique for measuring adsorption? (3 steps, 2 pros, 3 cons)
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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 |
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Gravimetric technique? (1 pro, 1 cons)
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Weigh amount adsorbed
Pro - can be followed dynamically Cons - very small masses involved Used for reversible systems i.e. physisorption |
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Dynamic method for measuring adsorbed amounts? (5 steps)
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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 |
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coverage =
in therms of K and Pa |
KPa/(1+KPa)
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Assumptions of Langmuir Isotherm? (4)
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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) |
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Coverage =
in term of V, Vmon |
V/Vmon (Vol adsorbed to give monolayer coverage)
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Assumptions of BET isotherm? (5)
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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 |
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Limits of BET?
x<P/P0<y |
0.05<P/P0<0.3
underestimates at low P and over estimates at high P |
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Good catalyst properties? (6)
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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 |
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Mechanism of catalysis? (6 steps)
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1)Diffusion to surface
2)Adsorption to surface 3)Surface diffusion 4)Reaction of surface species 5)Desorption 6)Diffusion away from surface (fast) |
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What happens first chemisorption or physisorptions?
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Physisorption
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