Transcript Properties

Surface Area
Surface Area
Sur (French “above”)
Face (French “face”)
Area (Latin “open empty space”)
Surface Area – what is it?
“Surface Area is the means through which a
solid interacts with its surroundings, especially
liquids and gases.”
Surface area is created by division of
particles (size reduction) and the generation
of porosity.
Surface area is destroyed by sintering
(exceeding Tg), melting and Ostwald ripening.
How to Create Area
• Size Reduction
– Grinding,
– milling,
– nanoscale preparation
• Make pores
– Partial decomposition
– Leach
– Gel then lyophilize
How to destroy area
• Surface area is destroyed by
– melting
– sintering (exceeding Tg), and
– Ostwald ripening
Surface Area – Importance?
Remember that surface area is the means
through which a solid interacts with its
surroundings.
Consider the following three interactions:
Solid-Solid: autohesiveness (cohesiveness) eg
flow, compactibility etc.
Solid-Liquid: wetting, non-wetting, adsorption
capacity etc.
Solid gas: adsorption, catalysis, etc.
Solid-Solid Interaction
Static angle of repose
What happens next?
Liquid-solid interaction
Wicking: adhesive forces exceed
cohesive forces… liquid wax is
drawn up through fibers of wick to
the exterior where it evaporates,
mixes with air and burns upon
ignition from the hot gases above.
Gas-Solid interaction
Measuring
Accessible
Area
Suitable methods of
determination
• Gas adsorption allows probing of entire surface
including irregularities and pore interiors.
•
The amount adsorbed is a function of temperature,
pressure and the strength of attraction or interaction
potential.
•
Physisorption is generally weak and reversible. The
solid must be cooled and a method used to estimate
the monolayer coverage from which surface area can
be calculated.
The gas sorption process intermolecular forces
4E-07
2E-07
0
8 10 12 14 16 18 20 22 24
-2E-07
-4E-07
Lennard-Jones potential function
Series 1
The gas sorption process
• Langmuir[i] described the kinetic behavior of the
adsorption process. He postulated that at
equilibrium, the rate of arrival of adsorptive
(adsorption) and the rate of evaporation of
adsorbate (desorption) were equal. Furthermore,
the heat of adsorption was taken to be constant
and unchanging with the degree of coverage, θ.
[i] I. Langmuir, J. Amer. Chem. Soc., 40, 1368 (1918)
Irving Langmuir (1881-1957)
Graduated
as the
a metallurgical
engineer
fromfor
thean
1927 Coined
use of the term
"plasma"
School
Mines at Columbia University in 1903
ionized of
gas.
1903-1906
M.A. and
Ph.D. in
1906 from
Göttingen.
1935-1937 With
Katherine
Blodgett
studied
thin films.
1906-1909
Instructor
in Chemistry
at Stevens that the
1948-1953 With
Vincent
Schaefer discovered
Institute
of Technology,
Hoboken,
Newa Jersey.
introduction
of dry ice and
iodide into
sufficiently
moist
cloud of
low temperature
could induce
1909 –1950
General
Electric Company
at
precipitation.
Schenectady where he eventually became Associate
Director
1913
-Invented
gas in
filled,
coiled tungsten
1932 The
Nobelthe
Prize
Chemistry
"for his
filament
incandescent
lamp. in surface chemistry"
discoveries
and investigations
http://public.lanl.gov/alp/plasma/history.html
1919 to 1921, his interest turned to an examination of
atomic theory, and he published his "concentric
theory of atomic structure" . In it he proposed that all
atoms try to complete an outer electron shell of eight
electrons
Langmuirian behavior
Confining adsorption to a monolayer, the Langmuir
equation can be written
V
KP

Vm 1  KP
where V is the volume of gas adsorbed at pressure P,
Vm is the monolayer capacity (i.e. θ=1) expressed as
the volume of gas at STP and K is a constant for any
given gas-solid pair. Rearranging in the form of a
straight line (y=ab+x) gives
P
1
P


V KVm Vm
Adsorption Process
Adsorbate
Adsorptive
Adsorbent
The Isotherm
• The amount of gas adsorbed is a function of
– The strength of interaction between gas and
solid (intrinsic)
– Temperature (fixed)
– Pressure (controlled variable)
Physisorption Process
4)
2
Very Low pressure behavior
(micropore filling)
Low pressure behavior
(monolayer)
The “knee”
Medium pressure behavior
(multilayer)
High pressure behavior
(capillary condensation)
Choice of gas and
temperature
• Gases
–
–
–
–
–
Nitrogen
Argon
Krypton
Carbon dioxide
Others
• Temperatures
–
–
–
–
–
Liquid Nitrogen
Liquid Argon
Dry ice/acetone
Water/ice
Others
Apparatus
Measurement Method
Classical vacuum, volumetric.
Requires that adsorbate be adsorbed by the
sample, at some reduced temperature, as a
function of pressure of pure adsorptive.
Vacuum-Volumetric
• P/Po values are achieved by creating
•
conditions of partial vacuum.
High precision and accurate pressure
transducers monitor pressure changes
due to the adsorption process.
Vacuum-Volumetric
Po, p0, psat
Po value can be
• measured in a dedicated cell
– with or
– without dedicated transducer
• calculated from atmospheric pressure
• input manually (eg Kr 2.63mmHg at 77.4K)
• measured in cell over sample
Vacuum-Volumetric
Working Equation
PV = nRT
nads = ndosed - nvoid
nads = (PV/RT)man. - (PV/RT)cell
Sample
Preparation
Sample preparation
The adsorbate has to “see” the real surface.
•
•
•
•
Whilst a little pre-adsorbed moisture wouldn't affect the
monolayer capacity, it would affect the strength with which
it is adsorbed, so the monolayer would be formed at a
different pressure than on a clean surface.
Pores can be easily blocked by moisture. Water undergoes
capillary condensation at humidities well below bulk
saturation in the confines of the pore (just as nitrogen does
as we will see later).
Micropores can be completely filled, not just blocked!
We always quote surface area per unit of dry mass.
Outgassing of Surface
Sample is cleaned of adsorbed contaminants,
mainly moisture, by the application of vacuum
or flow of dry inert gas and preferably some
heat.
How much heat?
Outgassing of Surface
How much heat?
What is the proper temperature for sample
preparation?
•Should be high enough to promote rapid removal of
surface adsorbed species without changing the surface
texture.
•Obviously not high enough to melt the solid, nor hot
enough to exceed the glass transition point, Tg. Estimate Tg
as melting pt  0.7 in kelvin (allow safety margin).
•Or no more than melting point  0.5 (kelvin) – Tammann
temperature.
•Example:
Magnesium Stearate monograph: 40 °C (for 2 hours)
Vacuum or flow
• A flow is good at removing large quantities
of weakly bonded “wet” water by
displacement of the vapor from external
surfaces. However in the depths of a pore,
water must diffuse out… and in doing so
must battle past a much higher
concentration of purge gas. Only when it is
out of the pore will it be physically swept
away.
Vacuum or flow
• Vacuum is not so good at removing large
quantities of weakly bonded “wet” water since
once it has left the sample it must diffuse towards
the pump. (A “random walk” will make the actual distance
needed to travel MUCH further than it looks to us! It will spend
much of its time wandering back towards the sample!).
However in the depths of a pore, water must
diffuse out… and in doing so does not have to
battle past much - certainly not any purge gas.
Traps
Traps are used on vacuum outgassing systems
to prevent:
• Oil backstreaming from pump (use foreline
trap of activated alumina or cold trap)
(Or use a dry pump system!)
• Evolved species off the sample from
migrating to the pump and
– reducing pump efficiency,
– effectively “pausing” the outgassing at the
vapor pressure of the evolved contaminants.
Sample Preparation
Degassing Parameters
• Method
• Temperature
• Time
• Test
• Backfill
Is it ready?
Test (vacuum)
• A sample still outgassing will cause a
pressure rise when isolated from the
vacuum system
• Hot samples outgas faster than cool ones.
• Generally less than 50 microns/min hot, 20
microns/min cool will indicate readiness.
• (background rise of empty degas station?)
Is it ready?
Backfill (vacuum)
• Cell should be backfilled to ambient pressure to
allow easy removal and to prevent ingress of
atmosphere.
• Avoid inconsistent use of helium as backfill gas –
buoyancy errors.
• He buoyancy error approx 1 mg/ml of cell
volume.
• Backfilled cells cool much faster.
• An evacuated cell barely warm to the touch can be
hot enough to cause burns when backfilled.
Elutriation
Elutriation: The process of separating the lighter
particles of a powder from the heavier ones by
means of an upward directed stream of fluid (gas
or liquid).
IUPAC Compendium of Chemical Terminology 2nd
Edition (1997)
Elutriation
Its how a vacuum cleaner works!
Remember, a vacuum
doesn’t suck!
Filters, pro’s and cons
The ability of a filter to trap particles
depends on its fineness and its tortuosity.
It serves to both sieve out particles and to
slow the rate of evacuation, hence to reduce
gas flow velocity.
The obstruction to flow can limit effective
vacuum.
Filters can become blocked!


Filters, pro’s and cons (continued…)
• What effect might a contaminated filter
•
•
•
have on resultant data?
Isotherms might appear “open”.
Isotherms may be “shifted” slightly.
Surface area values might be lower than
expected.
Sample Cells
Analysis Hardware
Sample cells
“What are the characteristics of a sample cell?”
• Overall length.
• Volume of “bulb”.
• Diameter of stem.
“How do I select the right one?”
Sample cells
• Overall length.
– Sample must be completely immersed under coolant, as should
shoulder part of bulb. Cells are long to allow for the evaporation of
cryogenic coolant. A coolant which does not evaporate can be used
with a “short” cell.
• Volume of “bulb”.
– Should be large enough to accommodate sufficient sample for
reasonable accuracy (e.g. at least 1 m2) but never more than 2/3 full.
Small sample quantities should be accommodated in small bulbs (see
void volume control).
• Diameter of stem.
– Large enough to easily accommodate particle size, but small enough to
improve overall sensitivity.
“How do I select the right one?”
Void volume control
“What is void volume and why bother?”
• Quantity of gas adsorbed is determined by (i)
waiting for minimal/zero pressure change and (ii)
subtracting quantity of gas in internal volume of
equilibration “zone” from total quantity “dosed”
into cell.
• The total volume in which the final equilibrium
pressure is monitored is the void volume.
Void volume control
“Therefore it should be minimized”
“How?”
• Close off the manifold after the dose.
• Use a small manifold if the valve must stay open.
• Use a sample cell with small internal volume.
• Cool as little of the cell as is absolutely necessary.
• Work at low pressure.
Void volume control
•
•
“Close off the manifold after the dose”
A pressure transducer on the analysis station
monitors pressure drop due to adsorption (rise due
to desorption) in the station volume only.
For a given quantity of gas being adsorbed, a
smaller volume gives rise to a larger (and more
easily and accurately measured) pressure change.
n =  PV/RT
i.e. greater sensitivity
This is the standard configuration for all Autosorb
gas sorption analyzers.
Void volume control
“Use a small manifold if the valve must stay open”
• Large manifold volumes can deliver large
•
quantities of gas, but are less sensitive.
Small manifolds can more easily deliver small
quantities of gas (for accurately targeting and
approaching data point pressures).
The standard configuration for Autosorbs and
NOVAs is 18-20 mL, 40mL for a Hydrosorb.
Void volume control
“Use a sample cell with small internal volume.”
• Choose narrowest stem reasonable for use with
•
•
your sample.
Use a filler rod.
Use as small a bulb size as reasonably practicable.
The bulb is usually at greatly depressed
temperature, so volume per volume, the “cold
zone”contains many more gas molecules than the
“warm zone”.
See “coolant level”
Void volume control
“Cool as little of the cell as is absolutely necessary”
• Immersion depth should be limited.
• Additional benefit in that hydrostatic pressure
variation in Po is minimized.
Standard condition in Autosorb is to use thermistor
coolant level control.
Void volume control
•
•
•
•
•
“Work at low pressure”
When adsorption amounts are small, the error associated with
subtracting the quantity of gas in the void volume from the
quantity dosed can be significant.
The quantity of gas in the void volume is a function of pressure
as well as volume, therefore void volume errors continuously
diminish as the pressure decreases.
So, use krypton gas. Po = 2.63 mmHg at 77.4K. (surface area
still uses same P/Po range).
For P/Po = 0.3, krypton measurement has approximately 1/30
the number of gas molecules in the void volume as for nitrogen
analysis.
Additional benefit: pressure measurements for krypton done
using 1 and/or 10 torr full scale transducer (c.f. 100 torr range
transducer for nitrogen BET).
Sample cells
• Stems, outside diameter: 6mm (1/4”), 9mm,
•
•
12mm.
Corresponding internal diameters: 4mm, 7mm,
10mm (1mm wall thickness).
Bulbs: none (bulbless – sample space has same
diameter as stem), small (jelly-bean shape or
“spherical” according to stem diameter and
instrument, large (1’/2.5cm) diameter sphere
Filler rods
• Should be long enough to fill most of stem.
• Should be clean!!
• Don’t try to make it exactly same size as
•
I.d. of stem (glass will jam).
Rods with caps should not close off gas
ingress and egress to/from the cell.
How much sample ?
• For all gas sorption equipment – minimum
amount in cell required !
– > 0.5 m2 total area = minimum
– Use 10X to be sure !
•
•
•
•
Example : we have an approximately 5 m2/g sample
So, 2g X 5 m2/g = 10 m2 total = OK
100mg (0.1g) => 0.5 m2 total = Borderline OK
50mg (0.05g) = 0.25m2 total = TOO LITTLE !
• Upper limit –> volume of cell, time !
Sample Weight
• Recommended procedure :
–
–
–
–
–
–
Weigh Cell empty (9.0g)
Add sample – weight full (9.55g)
Difference = weight before outgassing = 0.55g
Outgass sample
Weigh cell (9.45g)
Difference = .45g (18.2% loss) = weight for
analysis.
• Caution – backfill with N2 to avoid
bouyancy errors.
B.E.T.
Calculation of Specific Surface Area
Principles of BET Surface Area
Measurement
and Calculation
Determine the monolayer capacity Vm from which
the surface area of the solid can be computed.
Adsorbate most commonly used is nitrogen...
• Readily available in high purity
• Appropriate coolant, liquid nitrogen, also
plentiful.
• Gas-solid interaction relatively strong.
• Widely accepted cross sectional area.
?
?
?
Quiz
?
?
?
?
BET: Since when?
A
1938
Brunauer, Emmett & Teller
• Model of adsorption extended to
multilayers. BET ‘C’ constant varies from
solid to solid. Low values represent weak
gas adsorption typical of low surface area
solids, organics and metals in particular.
1
1
C  1 P 


 
V [(P0 P)  1] VmC VmC  P0 
Measurement
Obtain at least three data points in the relative pressure
range
0.025 to 0.30
Plot 1/[VSTP(Po/P)-1] versus P/Po. It should yield a
straight line… if the BET model holds true.
On all surfaces the BET model fails to accurately predict
the multilayer adsorption behavior above P/Po = 0.5 (the
onset of capillary condensation which fills pores with
liquid adsorbate)
Calculation
Fit best straight line through BET data set
using least squares regression to find:
C 1
slope s 
VmC
1
intercept i 
Vm C
Calculation (continued)
Solving for Vm
1
Vm 
si
Total surface area, St, is calculated thus
Vm Lav Am
St 
Mv
Lav = 6.022 x 1023
Am = 0.162 nm2
Mv = 22 414 mL
nm2 to m2, x 10-18
?
?
?
Quiz
?
?
?
?
Why L for Avogadro’s
Number?
Loschmidt calculated the
A
number, not Avogadro!
Multi-Point BET Plot
(Interpretation)
•
•
Never use data points too low in relative pressure (P/Po).
Never use data points too high in (P/Po).
Under-equilibrated Data
Under-equilibrated Data
Multi-Point BET Plot
(Interpretation)
•
•
Discard under-equilibrated points (at low P/Po)
Never use less than three, preferably five data points.
Single-point BET Method
•
•
•
Set intercept to zero, i.e ignore ‘C’ (adsorption strength)
Monolayer volume is inverse of slope.
P/Po = 0.3 gives good general agreement with multi-point – the
higher the C value, the better the agreement.
(note: monolayer is formed closer to P/Po = 0.2
Approximate values of C
C = 2 to 50 metals, polymers, organics
C = 50 to 200 oxides, silicates
C = >200 activated carbons, zeolites
Flow
• Required
•
•
P/Po is achieved by diluting nitrogen
(adsorbate) with helium (non-adsorbing).
The sample is cooled with liquid nitrogen, to
cause adsorption.
The adsorption (and subsequent desorption)
process is monitored using a thermal conductivity
detector.
Dynamic Flow
The signal is
calibrated against
a known volume
of pure nitrogen
injected into the
gas stream.
• The method is extremely rapid and ideally suited to
manufacturing processes using the single point method.
• Very dilute concentration of krypton at liquid nitrogen
temperature is used for extremely low surface areas.
Other Uses of the Single-point
Method
•
•
•
To determine appropriate P/Po range for multi-point BET.
Acquire minimum seven data points in the P/Po range 0.05 to
0.3.
Discard objectively from the multipoint surface area calculation
those higher P/P0 values that clearly do not lie on a straight BET
line…
The upper limit of the linear BET range can usually be obtained
by calculating the single-point BET area using each datum point
in turn. Normally, the calculated single-point area will increase
with increasing P/P0 up to some maximum, beyond which the
calculated value will decrease. That maximum indicates the
upper limit for the multi-point range.
Limitations of BET Surface Area
Method
•
In certain cases, the
calculated single–
point value never
goes through a
maximum. Evidenced
by a gradual
decrease in slope in
the BET plot.
•This may or may not be accompanied by a short linear
region at lower relative pressures, If there is no truly linear
region, then it can be said that the BET equation is invalid
for that particular sample…
Enhanced Adsorption
•
•
•
Isotherms of such samples appear to have some type III
character at very low pressure, indicative of very weak
adsorption, yet demonstrate enhanced adsorption within the
normal BET region.
This behavior can be attributed to a cooperative adsorption
process.
Has been described for slit-shaped pores of critical
dimensions; water on carbon (hydrophobic surface) is a
typical example.
Questioning BET data
•
•
•
Caution !
Clue – adsorbed
amounts low. (OK
if sample mass
sufficient)
Sample mass =
1.52 g (OK?)
•
•
•
•
Resulting BET Plot
– looks OK?
r=0.998, surface
area = 0.22m2/g
Problem : 1.52g *
0.22m2/g = 0.33m2
Solution => Use
more sample to
validate result.
Common Problem : Bulk
Condensation at top of isotherm
P > P0 ! Bulk condensation.
Note point spacing, nearly infinite slope !
Data OK, can be recalculated with P0 from
point of pressure rise