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© 2004 Quantachrome Instruments
This presentation is the property of Quantachrome Corporation and has been provided to Johnson-Matthey for its internal use by its bona-fide
employees only. Under no circumstances shall the presentation, or any part of it, be provided to an external or third party without the express prior
written permission of Quantachrome Corporation.
The material contained within is, to the best of Quantachrome’s knowledge, accurate as of the date shown. Nevertheless, it shall be used
at your own risk. Under no circumstances will Quantachrome be responsible for damages or losses arising out of its use.
Unless otherwise stated, all material is Copyright © 2004 Quantachrome Corporation. Copyright notices shall neither be removed nor modified.
By viewing this presentation in part or entirety you agree to the above terms.
QuantachromeI N S T R U M E N T S
© 2004 Quantachrome Instruments
ChemisorptionChemisorption
QuantachromeI N S T R U M E N T S
3
© 2004 Quantachrome Instruments
3. Chemisorption Techniques
3.1 Introduction: Physisorption/Chemisorption
3.2 Classical Models
3.3 Active Metal Area Measurement
3.4 Adsorption Thermodynamics
3.5 Pulse vs. Static
3.6 Temperature Programmed Analyses
© 2004 Quantachrome Instruments
IntroductionIntroduction
QuantachromeI N S T R U M E N T S
3.1
© 2004 Quantachrome Instruments
3.1 Introduction
3.1 Introduction: Physisorption/Chemisorption
3.2 Classical Models
3.3 Active Metal Area Measurement
3.4 Adsorption Thermodynamics
3.5 Pulse vs. Static
3.6 Temperature Programmed Analyses
© 2004 Quantachrome Instruments
The Nature of Gas Sorption at a Surface
• When the interaction between a surface and an adsorbate is relatively weak only physisorption takes place.
• However, surface atoms often possess electrons or electron pairs which are available for chemical bond formation.
• This irreversible adsorption, or chemisorption, is characterized by large interaction potentials which lead to high heats of adsorption.
© 2004 Quantachrome Instruments
Physisorption vs Chemisorption
© 2004 Quantachrome Instruments
On The Nature of Chemisorption
• Chemisorption is often found to occur at temperatures far above the critical temperature of the adsorbate.
• As is true for most chemical reactions, chemisorption is usually associated with an activation energy, which means that adsorbate molecules attracted to a surface must go through an energy barrier before they become strongly bonded to the surface.
© 2004 Quantachrome Instruments
Adsorption PotentialsPo
tent
ial E
nerg
yP
C
ΔHc
ΔHp
A
Potential energy curves for molecular (non-dissociative) adsorption
© 2004 Quantachrome Instruments
Po
ten
tial E
ne
rgy
X + X
ΔHact.
X2
P
C
ΔHdissoc.
A
Adsorption Potentials
Potential energy curves for activated adsorption
© 2004 Quantachrome Instruments
Adsorption Potentials
Potential energy curves for non-activated adsorption
© 2004 Quantachrome Instruments
Isobars
Isobaric variation in quantity adsorbed with temperature. Physisorption isobar (a) represents lower heat of adsorption than chemisorption isobar (b).
Temperature
Qua
ntity
ad
sorb
ed
(a)
(b)
(c)
© 2004 Quantachrome Instruments
On The Nature of Chemisorption
• Because chemisorption involves a chemical bond between adsorbate and adsorbent, unlike physisorption, only a single layer of chemisorbed species can be realized on localized active sites such as those found in heterogeneous catalysts.
• However, further physical adsorption on top of the chemisorbed layer and diffusion of the chemisorbed species into the bulk solid can obscure the fact that chemisorbed material can be only one layer in depth
© 2004 Quantachrome Instruments
Classical ModelsClassical Models
QuantachromeI N S T R U M E N T S
3.2
© 2004 Quantachrome Instruments
3.2 Classical Models
3.2.1 Langmuir
3.2.2 Freundlich
3.2.3 Temkin
© 2004 Quantachrome Instruments
Adsorption Process
Active Sites (Adsorbent)
AdsorbateAdsorptive
© 2004 Quantachrome Instruments
Graduated as a metallurgical engineer from the School of Mines at Columbia University in 1903
1903-1906 M.A. and Ph.D. in 1906 from Göttingen.
1906-1909 Instructor in Chemistry at Stevens Institute of Technology, Hoboken, New Jersey.
1909 –1950 General Electric Company at Schenectady where he eventually became Associate Director
1913 -Invented the gas filled, coiled tungsten filament incandescent lamp.
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
Irving Langmuir (1881-1957)
© 2004 Quantachrome Instruments
1927 Coined the use of the term "plasma" for an ionized gas.
1932 The Nobel Prize in Chemistry "for his discoveries and investigations in surface chemistry"
1935-1937 With Katherine Blodgett studied thin films.
1948-1953 With Vincent Schaefer discovered that the introduction of dry ice and iodide into a sufficiently moist cloud of low temperature could induce precipitation.
Irving Langmuir (1881-1957)
© 2004 Quantachrome Instruments
3.2.1 Langmuir’s “Kinetic” Approach
rate of adsorption = ka P(1-)
where is the fraction of the surface already covered with adsorbate, i.e., = V/Vm
rate of desorption = kd
Suggests a dynamic equilibrium. Is it?
© 2004 Quantachrome Instruments
Langmuir (continued…)At equilibrium (any pressure)
ka P(1-) = kd
from which = V/Vm = KP/(1+KP)
where K = ka / kd.
In its linear form, the above equation can be expressed as:
1/V = 1/Vm + 1/(VmKP)
© 2004 Quantachrome Instruments
Confining adsorption to a monolayer, the Langmuir equation can be written
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
KPKP
VV
m
1
mm V
P
KVV
P
1
Or, if you prefer…
© 2004 Quantachrome Instruments
Langmuir Plot
1/P
1/V
Slope = 1/(VmK)
Intercept = 1/Vm
1/V = 1/Vm + 1/(VmKcP1/s)
© 2004 Quantachrome Instruments
Temperature Dependent Models
generally
K = Ko exp(q/RT) where Ko is a constant, R is the universal gas constant, T is the
adsorption temperature and q is the heat of adsorption
• Langmuir:K is constant;q is constant at all • Temkin: assumed that q decreases linearly with
increasing coverage• Freundlich: assumed that q decreases
exponentially with increasing coverage
© 2004 Quantachrome Instruments
TemkinTemkin assumed that q decreases linearly with increasing coverage,that is,
Q=qo(1- )
Where qo is a constant equal to the heat
of adsorption at zero coverage ( = 0) and is a proportionality constant.
© 2004 Quantachrome Instruments
Temkin = A ln P + B
or, since = V/Vm
V = Vm A lnP + VmB
Where A = RT/qo and B = A ln Ko + 1/
© 2004 Quantachrome Instruments
Temkin Plot
Ln(P)
V
Slope = VmA
Intercept = VmB
V = Vm A lnP + VmB
© 2004 Quantachrome Instruments
Multiple Temkin Plots to find
Ln(P)
V
Temp H Temp M Temp L
*mV
* denotes “temperature invariant” or “thermally irreversible” quantity
experimental extrapolated
© 2004 Quantachrome Instruments
FreundlichTemkin assumed that q decreases
exponentially with increasing coverage, that is,
Q = -qm ln
Where qm is a constant equal to the heat
of adsorption at = 0.3679
© 2004 Quantachrome Instruments
Freundlich
ln = C lnP + D
or, since = V/Vm
ln(V/Vm) = C lnP + D
Where C=RT/ qm and D = C lnKo
© 2004 Quantachrome Instruments
Freundlich (continued…)
Ln(P)
Ln(V
)
Slope = C
Intercept = D + ln(Vm)
Ln(V/Vm) = C lnP + D
© 2004 Quantachrome Instruments
Multiple Temkin Plots to find
Ln(P)
Ln(V
)
Temp H Temp M Temp L
*mV
* denotes “temperature invariant” or “thermally irreversible” quantity
experimental extrapolated
© 2004 Quantachrome Instruments
Active Metal AreaActive Metal Area
QuantachromeI N S T R U M E N T S
3.3
© 2004 Quantachrome Instruments
3.3 Active Metal Area
3.3.1 Principles of Calculation
3.3.2 Choice of Adsorbate
3.3.3 Active Site Size Calculation
3.3.4 Metal Dispersion
3.3.5 Accessible vs non-accessible sites
© 2004 Quantachrome Instruments
Active Site Quantification
• Because the formation of a chemical bond takes place between an adsorbate molecule and a localized, or specific, site on the surface of the adsorbent, the number of active sites on catalysts can be determined simply by measuring the quantity of chemisorbed gas
© 2004 Quantachrome Instruments
Active Site on a Catalyst
• Metal on support.
• Island-like crystallites
• Not all metal atoms exposed.
• Adsorption technique perfectly suited.
(cf Chemical analysis of entire metal content )
© 2004 Quantachrome Instruments
3.3.1 Principles of Calculation
Monolayer Volume, Vm= volume of gas chemisorbed in a monomolecular layer
© 2004 Quantachrome Instruments
Methods to Determine Vm
•Extrapolation
• Bracketing
• Langmuir
• Temkin
• Freundlich
= volume of gas chemisorbed in a monomolecular layer
© 2004 Quantachrome Instruments
Vm
Vol
ume
Ads
orbe
d
Pressure (mm Hg)
Extrapolation method
First (only?)isotherm
© 2004 Quantachrome Instruments
Vol
ume
Ads
orbe
d
Pressure (mm Hg)
The second isotherm
combined
Weak only
© 2004 Quantachrome Instruments
Vol
ume
Ads
orbe
d
Pressure (mm Hg)
The difference isotherm
combined
Weak only
Strong
© 2004 Quantachrome Instruments
Vm from Pulse Titration
… will be covered in 3.5.2
© 2004 Quantachrome Instruments
Metal Area Calculation
To Calculate Metal Surface Area:
A = (Vm) x (MXSA) x (S) x 6.03 x 10-3 (units m2/g)
where MXSA = metal cross sectional area (Å2)
and S = stoichiometry = metal atoms per gas molecule
To calculate metal area per gram of metal, Am:
Am = A x l00/L
where L = metal loading (%) = known value from chemical analysis
© 2004 Quantachrome Instruments
Stoichiometry
The gas-sorption stoichiometry is defined as the number of metal atoms with which each gas molecule reacts.
Since, in the gas adsorption experiment to determine the quantity of active sites in a catalyst sample, it is the quantity of adsorbed gas which is actually measured, the knowledge of (or at least a reasonably sound assumption of) the stoichiometry involved is essential in meaningful active site determinations (area, size, dispersion).
© 2004 Quantachrome Instruments
3.3.2 Choice of Adsorbate
Chemisorption
• CO or H2 on Pt, Pd
at 40 oC
• CO or H2 on Ni
For metal-only area
(& dispersion etc)
Physisorption
• N2 at 77K
• Ar at 87K
• Kr at 77K
• CO2 at 273K
For total surface area
and pore size
© 2004 Quantachrome Instruments
3.3.3 Active Site Size Calculation
To calculate average crystallite size:
d = (L x 100 x f )/AD (units Å)
where f = shape factor = 6ρ = density of metal (g/ml)
© 2004 Quantachrome Instruments
Shape Factor & Crystallite Size
The default shape factor of 6 is for assumed cubic geometry.
Consider a cube of six sides (faces) each of length l.
then the total surface area, A = 6l2.
The volume of the cube is given by l3 or, in terms of total area, substitute A /6 for l2 to give
V= lA/6
For a cube whose mass is unit mass, its volume is given by 1/ (where is the density of the material).
V=1/
© 2004 Quantachrome Instruments
Shape Factor & Crystallite Size
For the same cube of unit mass, the area is then the area per unit mass A and l is rewritten d (crystallite size), the length required to give a cube whose mass is unity. Equating both terms for volume:
dA/6=1/ or
d=6/A
For a supported metal, the loading, L, must be taken into consideration.
d=L6/A
Other geometries can be treated in a similar fashion. For example, a rectangular particle whose length is three times its width has a shape factor of 14/3.
© 2004 Quantachrome Instruments
Supported metalsIt is most likely that the catalyst exists as a
collection of metal atoms distributed over an inert, often refractory, support material such as alumina.
At the atomic level it is normal that these atoms are assembled into island-like crystallites on the surface of the support.
3.3 Metal Dispersion
© 2004 Quantachrome Instruments
3.3 Metal Dispersion• In the case of supported metal catalysts, it is
important to know what fraction of the active metal atoms is exposed and available to catalyze a surface reaction.
• Those atoms that are located inside metal particles do not participate in surface reactions, and are therefore wasted.
© 2004 Quantachrome Instruments
Exposed metal atomsSince these islands vary in size due to both the intrinsic
nature of the metal and the support beneath, plus the method of manufacture more or less of the metal atoms in the whole sample are actually exposed at the surface. It is evident therefore that the method of gas adsorption is perfectly suited to the determination of exposed active sites.
support
Exposed active sitesAdsorbed gas
© 2004 Quantachrome Instruments
Metal Dispersion
• Dispersion is defined as the percentage of all metal atoms in the sample that are exposed.
• The total amount of metal in the sample is termed the loading, χ , as a percentage of the total sample mass, and is known from chemical analysis of the sample.
© 2004 Quantachrome Instruments
Metal Dispersion
• The dispersion, δ, is calculated from:
• Where M is the molecular weight of the metal, Na is the number of exposed metal atoms found by adsorption and WS is the mass of the sample.
%WL
NM
SAv
a 100100
© 2004 Quantachrome Instruments
3.3.5 Accessible vs. Non-accessible Sites1. Adventitious moisture2. Reducing gas accessibility3. Diffusion4. Purge5. Physisorption blocks6. Bulk hydride7. Spillover8. Stoichiometry9. Characterization gas vs. Process gas
© 2004 Quantachrome Instruments
Spatial Ordering
There may exist a number of different adsorption sites that involve different numbers of metal atoms per adsorbate molecule.
© 2004 Quantachrome Instruments
Adsorption Adsorption ThermodynamicsThermodynamics
QuantachromeI N S T R U M E N T S
3.4
© 2004 Quantachrome Instruments
3.4 Adsorption Thermodynamics
3.4.1 Isosteric Heats from Isotherms
See also activation energy under 3.6.1
© 2004 Quantachrome Instruments
3.4.1 Heats of Adsorption
• Whenever a gas molecule adsorbs on a surface, heat is (generally) released, i.e. the process of adsorption is exothermic.
• This heat comes mostly from the loss of molecular motion associated with the change from a 3-dimensional gas phase to a 2-dimensional adsorbed phase.
• Heats of adsorption provide information about the chemical affinity and the heterogeneity of a surface, with larger amounts of heat denoting stronger adsorbate-adsorbent bonds.
• There are at least two ways to quantify the amount of heat released upon adsorption: in terms of (i) differential heats, q, and (ii) integral heat, Q.
© 2004 Quantachrome Instruments
Differential Heats of Adsorption• q, is defined as the heat released upon adding
a small increment of adsorbate to the surface. • Its value depends on (i) the strength of the
bonds formed and (ii) the degree to which surface is already covered.
• i.e a plot of q vs. θ provides a curve illustrating the energetic heterogeneity of the surface.
• Use it to fingerprint surface energetics and to test of the validity of any Vm evaluation method used (see earlier) since each method assumes a different relationship between q and θ.
© 2004 Quantachrome Instruments
Differential Heats of Adsorption• Since q can, and most often does, vary with θ,
it is convenient to express it as an isosteric heat of adsorption, that is, at equal surface coverage for different temperatures.
• Thus, obtain two or more isotherms at different temperatures.
• Determine pressures corresponding to equal coverage at different temperatures.
• Construct an Arrhenius plot of (lnP) versus (1/T). Values for q at any given coverage, θ, can be calculated from the Arrhenius slopes, m.
© 2004 Quantachrome Instruments
Slopes of (lnP) vs. (1/T).
mRq where
m = d lnP/d(1/T) and R is the universal gas constant.
© 2004 Quantachrome Instruments
Integral Heat of Adsorption• This is simply defined as the total
amount of heat released, Q, when one gram of adsorbent takes up X grams of adsorbate. It is equivalent to the sum, or integral, of q over the adsorption range considered, that is:
where Vm is expressed in mL at STP, and θ ideally ranges from
θmin = 0 to θmax = maximum coverage attained experimentally.
max
min
qV
Q m d22414
© 2004 Quantachrome Instruments
Experimental ApproachesExperimental Approaches
QuantachromeI N S T R U M E N T S
3.5
© 2004 Quantachrome Instruments
3.5 Experimental Approaches
3.5.1 Pulse
3.5.2 Static
© 2004 Quantachrome Instruments
Preparation Techniques
• Sample is heated under inert flow to
remove adsorbed moisture. Whilst
reduction step creates moisture, we don’t
ant the reducing gas to compete for diffusion
to surface.
• Reduce with H2: can be pure hydrogen or
diluted with nitrogen or argon. Higher
concentrations give higher space velocities
for the same volumetric flow rate.
© 2004 Quantachrome Instruments
Preparation Techniques (continued…)
• Purging with inert gas (normally helium) strips
excess reducing gas quickly. Can shorten prep
time and/or give more reproducible data since
hydrogen is difficult to pump.
• Cooling is done under vacuum/flow to ensure
continued removal of residual reducing gas…
though it is the hot removal step (above) which is
critical. That is, don’t cool before removing as
much reducing gas as possible.
© 2004 Quantachrome Instruments
Chemisorption Techniques
• Vacuum method
• Flow methods
© 2004 Quantachrome Instruments
Vacuum Technique
• Sample is heated under inert flow
• Reduced with H2
• Purged with inert, cooled under vacuum/flow
• Adsorbate dosed to obtain isotherm
• Calculate the amount adsorbed
© 2004 Quantachrome Instruments
Static (volumetric) Setup
furnace
manifold
adsorptives
vent
diaphragm pump
Turbo-molecular
(drag) pump
Flow “U” cell
© 2004 Quantachrome Instruments
Setup
Filler rod goes here
Quartz wool
sample capillary
© 2004 Quantachrome Instruments
3.5.2Flow (Pulse) Chemisorption
© 2004 Quantachrome Instruments
Flow Types of Analysis
TPR TPO TPD Monolayer by Titration BET
support
active sites
A flow system permits multi-functional catalyst characterization :
© 2004 Quantachrome Instruments
OverviewAnalysis is done by detecting changes in gas
composition downstream of sample.
• Detector senses – abstraction of reactive species during
adsorption – evolution of previously adsorbed species during
desorption– decomposition products
• Signal detection– Standard: thermal conductivity detector– Optional: mass spectrometer
© 2004 Quantachrome Instruments
ChemBET™ 3000 TPR
© 2004 Quantachrome Instruments
Flow Diagram
AB
1
2
3
4
A
IN
OUT
CLICK FOR BYPASS & LONGPATH
CLICK FOR BYPASS & LONGPATH
CLICK FOR BYPASS & LONGPATH
© 2004 Quantachrome Instruments
Flow/Static (FloStat™) Flow Diagram
12
3
4
5
to mass spec
(optional)
to vent
B
A
oil-free high vacuum
vapor source
(optional)
heater
Schematic representation only. Some vacuum volumetric components omitted for clarity.
heated zone (vapor option)
© 2004 Quantachrome Instruments
TPRWin™ Software
Data Acquisition
© 2004 Quantachrome Instruments
Overview• Quartz flow-through
cell allows – high-temperature (up
to 1100 degC) – in-cell temperature
monitoring– Two t/c’s if necessary,
one to DAQ, one to MassSpec.
– mass spectrometer sampling port.
T/C #1T/C #2
Modified cell holder
Capillary to mass spec.
Gas flow
© 2004 Quantachrome Instruments
Pulse Titration • Metal area, dispersion and crystallite size are
calculated from the amount of analysis (reactive) gas adsorbed.
• Variable volumes of analysis gas are injected into the inert carrier gas stream, which continuously flows over the sample.
• Detector measures the volume of gas that remains unadsorbed by the sample. Subtraction from the total amount injected gives the total amount adsorbed to within 1uL accuracy.
© 2004 Quantachrome Instruments
Titration Pulse Titration of Active Sites
H2 or CO titration
N2 and He carrier respectively Constant temperature (room temp?) Multiple injections until saturation
M M MMH
H H H H
H2 CO
CO CO CO
N2He
© 2004 Quantachrome Instruments
Titration
Data Acquisition
© 2004 Quantachrome Instruments
Titration
injections
sig
nal
LOAD INJECT
© 2004 Quantachrome Instruments
Titration Calculations
1. Calculate total nominal volume of reactive gas adsorbed by comparison with calibration injection or average of last n (three) peaks
(note: peak area represents gas not adsorbed!)
Total vol adsorbed =
(Peak Avg - Peak1) + (Peak Avg - Peak2) + (Peak Avg - Peak3) etc
x nominal injection volume = Vnom (units l)
© 2004 Quantachrome Instruments
Titration Calculations
2. Convert to STP:
(Vnom) x (273/rt) x (Pamb/760) = Vstp (units l)
3. Convert to specific volume adsorbed:
Vstp /sample wt = Vsv (units l/g)
4. Convert to micromoles per gram (weight as supplied ):
Vsv / 22.4 = Vm (units mole/g)
© 2004 Quantachrome Instruments
Requirements for Different Analysis Types
Long cell
Short cell
Std. cell
5% H2
100% H2
5% O2
100% N2
100% He
30% N2
Inj. Furnace
Mantle Dewar Long path
TPR ()
TPO TPD Metal Area* () () * *
BET ()
* Using H2 active gas. If using CO, substitute 100% CO for 100% H2 & 100% He for 100% N2.
L
© 2004 Quantachrome Instruments
Temperature Programmed (TP) Temperature Programmed (TP) ExperimentsExperiments
QuantachromeI N S T R U M E N T S
3.5
© 2004 Quantachrome Instruments
3.6 Temperature Programmed (TP) Experiments
3.6.1 TP-Reduction
3.6.2 TP-Oxidation
3.6.3 TP-Desorption
3.6.4 TP-Reaction
© 2004 Quantachrome Instruments
3.6.1 TP-Reduction
• Metal oxides are readily characterized by their ease of reduction.
CeO2 CeO2-x + x/2O2
• TPR profiles represent that ease of reduction as reduction rate as a function of increasing temperature.
2CeO2 + H2 Ce2O3 + H2O
© 2004 Quantachrome Instruments
Temperature Programmed Reduction
• A low concentration of pre-mixed hydrogen (e.g.5%) in nitrogen or argon (or other reducing gas for custom research applications) flows over the sample as it is heated during a linear increase (ramp) in temperature.
• Peak reduction temperature is also a function of heating rate and may be used to calculate activation energy for the reduction process.
© 2004 Quantachrome Instruments
TPR Temperature Programmed Reduction
Metal oxide to metal 5% hydrogen reactive gas Balance N2 or Ar (not He ! ...unless MS) Ramp rate Activation Energy
H2
MO MO MOMO
H2O
M M MM
© 2004 Quantachrome Instruments
TPR
temperature
sig
nal
tmax
© 2004 Quantachrome Instruments
TPRLinearly ramped
furnace is essential for standard TP
profiles
© 2004 Quantachrome Instruments
time
sig
nal
tmax
tem
pera
ture
TPR Profiles for Different Heating Rates
1
2
3
© 2004 Quantachrome Instruments
TPR Profiles for Different Heating Rates
800 1000
0 20 40 60 80
100 120 140 160 180 3
1
2
Sig
nal
Temperature / K
© 2004 Quantachrome Instruments
TPR Profile
Heating Rate (K-1)
Peak Temperature (Tmax)
1 10 874
2 15 902
3 20 928
Heating Rate & Peak Temperature
© 2004 Quantachrome Instruments
Kissinger (Redhead) Equation
1.08 1.10 1.12 1.14
-11.2
-11.1
-11.0
-10.9
-10.8
-10.7
s lope = -8.6
Ea = 72 kJ mol -1
ln(
Tm
ax-2
)
1000 /Tmax
(K -1)
max
a2max T
1
R
EK
Tln
© 2004 Quantachrome Instruments
3.6.2 TP-Oxidation
• Temperature programmed oxidation (using 2%-5% O2 in He for example) is performed in a manner analogous to TPR.
• TPO can be particularly useful for looking at carbons:– Carbon supports (graphite vs. amorphous)– Carbon deposits from coking– Carbides
© 2004 Quantachrome Instruments
TPO Temperature Programmed Oxidation
Metals and carbon to oxides 2-5% oxygen reactive gas balance He (not N2 !) Ramp rate Activation Energy
O2
C C CC
CO + CO2
M M MM
carbon
© 2004 Quantachrome Instruments
TPO: Signal vs. Temperature
© 2004 Quantachrome Instruments
TPO: Signal & Temp. vs. Time
© 2004 Quantachrome Instruments
Temperature Programmed Oxidation
Zhang and Verykios reported that three types of carbonaceous species designated as C, C, and C were found over Ni/Al2O3 and Ni/CaO±Al2O3 catalysts in the TPO experiments.
Zhang ZL and Verykios XE,. Catal. Today 21 589-595 (1994).
Goula et al identified two kinds of carbon species on Ni/CaO Al2O3 catalysts from TPO experiments. The high-temperature peak was assigned to amorphous and/or graphite forms of carbon. The lower temperature peak suggested a filamentous form.
Goula MA, Lemonidou AA and Efstathiou AM, J Catal 161 626-640 (1996).
© 2004 Quantachrome Instruments
3.6.3 Temperature Programmed Desorption
• The monitoring of desorption processes is equally easy.
• A pure unreactive carrier gas carries evolved species from the sample to the detector as the user-programmable furnace heats the sample.
• This technique is commonly employed to determine the relative-strength distribution of acidic sites by means of ammonia desorption.
© 2004 Quantachrome Instruments
TPD Temperature Programmed Desorption
Remove previously adsorbed species Helium/Nitrogen purge Ramp rate Activation Energy
NH3MO MO MOMO
NH3
© 2004 Quantachrome Instruments
Ammonia TPD
© 2004 Quantachrome Instruments
Pyridine TPD
Physisorbed pyridine is clearly evident in the first sample (low temp.), but absent in the second.
Multiple acid sites revealed by peak deconvolution
© 2004 Quantachrome Instruments
TPD
temperature
sig
nal
tmaxIncreasing mass
© 2004 Quantachrome Instruments
Overview• Quartz flow-through
cell allows – high-temperature (up
to 1100 degC) – in-cell temperature
monitoring– Two t/c’s if necessary,
one to DAQ, one to MassSpec.
– mass spectrometer sampling port.
T/C #1T/C #2
Modified cell holder
Capillary to mass spec.
Gas flow
© 2004 Quantachrome Instruments
With Mass Spectrometer
Capillary or capillary connector to mass spectrometer
Tube endsjust below port connection
In-situ thermocouple
¼” swagelok® compression fitting
T/C #1T/C #2
Modified cell holder
Capillary to mass spec.
Gas flow
© 2004 Quantachrome Instruments
3.6.4 TP-Reaction
• Essentially everything that is not standard TPR or TPO!!
• Can be a single reactive gas, or a mixture of reactants… akin to microreactor work.
• Need not be done over a bare metal surface… might have one reactive species preadsorbed on the surface
e.g. OHCHNiCONi Hn 242
2