catalyst characterization 0. general overview, and x-ray methods 1. bet, porosimetry, chemisorption...
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Catalyst characterization
0. General overview, and x-ray methods1. BET, porosimetry, chemisorption 2. Temperature programmed methods
Edd A. Blekkan, Dep. of Chemical EngineeringCatalysis and Kinetics GroupNTNU
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IntroductionCatalyst Characterization
• Heterogeneous catalysis: Transformation of molecules at the interface between a solid (catalyst) and the gaseous or liquid phase carrying these molecules
• We need to understand:– What is the composition of the catalyst
• bulk
• surface (catalysis is a surface phenomenon)
– How does it change • chemical reactions
• exchange of atoms between surface and bulk
• sintering and loss
– How is the gas (liquid) phase changed (=kinetics)
– What is the nature of the interface when reaction occurs • adsorbed species, bonding with the surface, intermediates etc..
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General scheme of characterization
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General scheme: Techniques
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X-ray diffraction • X-rays– wavelengths in the Å range
– high energy, can penetrate solids
• Diffraction (= elastic scattering of the photons) pattern can be used to study– identify phases in (crystalline)
bulk solids
– particles, particle size
• Bragg relationn =2dsin ,where
n = 1,2,… (order)
d = lattice spacing
= wavelength
= angle between X-ray beam and the normal to the reflecting plane
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Particle size measurement
• Diffraction line of a perfect, infinite crystal = narrow “spike”• Smaller particles = line broadening• Scherrer formula used to calculate particle size:
where L is the dimension of the particle
is the wavelength
is the peak width
is the angle of reflection
K is a constant, can be assumed to be 1
(This is a simplified analysis)
βcosθ
Kλ=L
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Example 1. Phase identification
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Example 2. Supported metal particles
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Contents part. 1: Techniques applied for studying surfaces
• Adsorption: general background and theory
• Total surface area – BET and other methods
• Pores and pore size distributions
• Specific surfaces (chemisorption methods)– background
– dispersion
– techniques
• Examples of applications
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Fundamentals: adsorption
• Adsorption precedes catalysis
• Definition (Thomas): “the preferential accumulation of material - the adsorbate - at a surface”
• Adsorption is distinguished from absorption– adsorption: gas uptake (at fixed T and P) is proportional to the surface area
– absorption: gas uptake (at fixed T and P) is proportional to the volume of the material:
– not always a clear distinction: • intercalation of species between layers can sometimes generate more internal area (e.g. clay minerals,
graphite)
• highly micro-porous materials with cavities with molecular dimensions (e.g.zeolites)
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General classification of adsorption
Tabell fysikalsk vs. kjemisorpsjonRichardson tab 7.3 side146
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Lennard-Jones diagram
The figure depicts the energies associated with a molecule approaching a surface
Due to physisorbed precursor state activation energy for chemisorption is low non-activated if crossover X
is below potential energy zero
Fig 2.2 T&Tside 67
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T&T fig 2.3.Side 68
Sticking
• Sticking coefficient: the probability of a collision with the surface leading to adsorption
• s can be very low (10-15)
collisionofrate
adsorptionofrates
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Isotherms and isobars
• Equilibrium distribution of adsorbate molecules between surface and gas phase is
– function of temperature– function of gas pressure– function of the nature and area of the
adsorbent– nature of the adsorbate
• Isotherm: amount adsorbed at equilibrium as f(P) at constant T
• Isobar: amount adsorbed at equilibrium as f(T) at constant P
• Isostere: Relation between T and P at equilibrium for a given amount of adsorbate
T&T Fig 2.20s.79
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Brunauer classification of adsorption isotherms
• Empirical observation: 5 types of isotherms
• Most systems are “Type I”
T&T fig 2-21 s. 80 el. tilsv
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Adsorption isotherms
• Equations describing isotherms are available
• Many can be derived theoretically (e.g. BET, Freundlich, Temkin) using assumptions about the heat of adsorption
T&T Tab. 2-1s. 80-klipp vekk eq. nr
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Heat of adsorption from isotherms
• At true adsorption-desorption equilibrium the heat of adsorption -Ha at a given coverage can be obtained from isotherms measured at different temperatures using the Clausius-Clapeyron equation:
R
H
const)
T1
(d
plnd a
T&T fig2.22aside 82
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Heat of adsorption can be a function of surface coverage
• Major effect: Strongest adsorption sites are filled first
• On single crystal faces
– At high coverage dipole-dipole interactions comes into effect
– Overlapping molecular orbitals contribute
– long range interactions
T&T fig 2.42 s.118
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Some definitions
Handboook tab 1 s 428portrait
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Physical adsorption IUPAC classification of isotherms
Handbook fig 1 s 428
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The BET isotherm
00a
0mm0a
p
psI
ppv
p
or
p
p
Cv
1C
Cv
1
ppv
p
• Theoretical development based on several assumptions:
– multimolecular adsorption
– 1st layer with fixed heat of adsorption H1
– following layers with heat of adsorption constant (= latent heat of condensation)
– constant surface (i.e. no capillary condensation) gives
OT fig1.3
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The BET isotherm, cont.
00a p
psI
ppv
p
• Plot of left side vs. p/p0 should give straight line with slope s and intercept I
• Reorganizing gives
• Knowledge of S0 (specific area for a volume of gas then allows the calculation of the specific surface area Sg:
where mp is the mass of the sample
I
sICand
Is
1vm
OT fig1.5
p
0mg m
SvS
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BET cont’d
• BET method useful, but has limitations– microporous materials: mono - multilayer adsorption cannot occur, (although BET
surface areas are reported routinely)
– assumption about constant packing of N2 molecules not always correct?
– theoretical development dubious (recent molecular simulation studies, statistical mechanics) - value of C is indication o f the shape of the isotherm, but not necessarily related to heat of adsorption
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Simplified method
• 1-point method– simplefied BET assuming value of C 100 (usually the case), gives
– usually choose p/p0 0,15
– method underestimates the surface area by approx. 5%.
0
0a'm
0'm0mm0a
p
ppvv
pv
p
p
p
Cv
1C
Cv
1
ppv
p
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Adsorbates• An adsorbate molecule covers an area , calculated assuming dense packing of the
molecules in the multilayer. The corresponding area per volume gas is S0:
Gas Temp.[K]
σ[Å2/molecule]
S0
[m2/cm3 gas (STP)]N2 77,5 16,2 4,36Kr 77,5 19,5 5,24Ar 77,5 14,6 3,92
H2O 298 10,8 2,90C2H6 90 22,5 6,05CO2 195 19,5 5,24
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Porosity and pore size
• The pore structure (porosity, pore diameter, pore shape) is important for the catalytic properties
– pore diffusion may influence rates
– pores may be too small for large molecules to diffuse into
• Measurement techniques:– Hg penetration
– interpretation of the adsorption - desorption isotherms
– electron microscopy techniques
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Hg penetration
• Based on measuring the volume of a non-wetting liquid forced into the pores by pressure (typically mercury)
• Surface tension will hinder the filling of the pores, at a given pressure an equilibrium between the force due to pressure and the surface tension is established:
where P = pressure of Hg, is surface tension and is the angle of wetting
• Common values used: = 480 dyn/cm and = 140° give average pore radius
valid in the range 50 - 50000Å
cosr2rP 2
Å]cm/kp[P
75000r
2
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Pore size distribution
• If the Hg-volume is recorded as a function of pressure and this curve is differentiated we can find the pore size distribution function V(r)=dV/dr
OT fig 2.3.
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The Kelvin equation
0
_
ln
2
pp
RT
Vrk
• If adsorbent is mesoporous we get Type IV isotherm
• Deviation upwards is due to filling of mesopores by capillary condensation - curved liquid meniscus in narrow pores with radius rk:
V is molar volume of the liquid, minus sign introduced since in the actual range of measurement 0 < p/p0 <1
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The Kelvin equation
• Since capillary condensation is preceeded by multilayer adsorption on the wall the value is corrected with t, the thickness of this layer:
Cylindrical pores: rp = rk + t
Parallell sided slits: dp = rk + 2t
Value of t determined from measurements without capillary condensation
• Practical experience, typical values give for circular pores:
• Values for t have been found to be a function of rk, e.g. for rk > 20Å:
][
ln
547,9
0
Å
pp
rk
Årt k61,2ln429,0
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Adsorption-desorption hysteresis
• Hysteresis is classified by IUPAC (see fig.)
• Traditionally desorption branch used for calculation
• H1: narrow distribution of mesopores
• H2: complex pore structure, network effects, analysis of desorption loop misleading
– H2: typical for activated carbons
• H3 & 4: no plateau, hence no well-defined mesopore structure, analysis difficult
– H3: typical for clays
Handbookfig 2 s 431
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Chemisorption and dispersion
• Supported metals: metal particle size and dispersion are very important parameters
• A wide range of techniques available for assessment of particle sizes– electron microscopy (direct observation)
– XRD (line broadening analysis)
– SAXS (small angle x-ray scattering)
– XPS (ratio between surface concentration of support component (e.g. Si in SiO2) and active metal)
– Magnetic methods
– Chemisorption of probe molecules
• Methods have different strengths and drawbacks, combinations of 2 or more methods will give best understanding of a system
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Dispersion - Particle size - Surface area
• Dispersion: Fraction of surface atoms of a metal in a catalyst: D = NS/NT
• Chemisorption can give direct measurement of NS, knowledge of NT allows direct calculation of D
• Assumptions about metal structure, particle shape and exposure of crystal planes allows the calculation of D from relationships with particle size
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Particle size
• Particles usually have a range of sizes - particle size distribution– can be narrow (e.g. metals in zeolite cages)
– can be broad with one or more maxima
• Particles also have a range of shapes - not necessarily nice geometries
• A collection of ni spherical particles of
have mean particle sizes based on length or volume (or weight):
6,
32 i
iiii
dVvolumeanddAareasurfaceddiameter
2
3
::ii
iiVA
i
iiLN dn
dndareavolumeor
n
dndnumberLength
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v
Relationships
VA
m
m
VAisp
ii
ii
isp
ii
iii
iisp
VA
mA
msp
AA
m
d
av
D
DdispersionandsizeparticlemeanbetweeniprelationshtheFinally
dS
giving
dn
dnS
givesVandAforngSubstituti
Vn
AnS
isdsizeparticlemeanandareasurfacespecificbetweeniprelationshThe
surfacellinepolycrystaaonatomanbyoccupiedareasurfacetheisawhereDM
NaS
dispersionwithlinkedisareasurfaceSpecific
numberAvogadrotheisNanddensityismassatomicisMwhereN
Mv
ismetalaofbulktheinatomanbyoccupiedVolume
6
:,
6
6
:
:
,
:
3
2
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Plotted relationships Pt Pd Ni (spherical particles)
Handbook fig. 2 & 3 side 441
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Gas Chemisorption
• Selective chemisorption of a gas:– formation of (or estimate of the amount of gas in) a monolayer of
adsorbed gas
– array of experimental techniques available, including commercial equipment
• Static methods: volumetric or gravimetric
• Dynamic methods: – Flow technique (frontal chromatography)
– Pulse technique
• Desorption method (TPD)
– A range of possible adsorbate gases available• H2,CO,O2, commonly used
• N2O,NO,N2,H2S,CS2,hydrocarbons used for special applications
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Handbook fig 4side 443
Example: CO on EuroPt-1 pt/SiO2
• Monolayer amount vm found by extrapolation of flat part of isotherm
• Specific metal surface A and dispersion can be calculated:
where vm is in cm3 (STP), n is the chemisorption stoichiometry, m is the sample mass (g) and wt% is the meal loading
%22414
100
]/%
1001
224142
wtm
MnvD
metalgmwt
am
nNv
A
m
mAm
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Not always straight forward
• Hydrogen adsorption on Pt/Al2O3 at 333 K (Top)
– no flat part of isotherm
– can be fitted to Langmuir isotherm(dissociative) to obtain vm
• CO on Fe (bottom) at 90 K– a) Total adsorption
– b) Second isotherm after evacuation = physical adsorption
– c) Difference is chemisorbed CO
– But: all adsorption is in principle reversible: pumping efficiency and evauation time can generate similar differences
– Common practice to distinguish between “weak” and “strong” adsorption
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Hydrogen chemisorption
• Hydrogen adsorbs dissociatively on metals:H2 + 2M 2M-H
• Stoichiometry: 1 H-atom per metal surface atom valid for a number of transition metals
• Pt much studied, recommended value now (?) 1,1 H atoms per metal surface atom (Boudart & Benson)
• Standardized methods available (ASTM)– evacuation, oxidation, reduction, evacuation
– followed by adsorption at 298 K, equilibrium times of 30 - 60 min.
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H2-O2 titration
• Sensitive and simple method for supported Pt:– Pt + ½H2Pt-H HC; hydrogen chemisorption
– Pt + ½O2Pt-O OC; oxygen chemisorption
– Pt-O + 3/2H2Pt-H+ H2O HT; hydrogen titration
– 2Pt-H + 3/2O22Pt-O+ H2O OT; oxygen titration
– Stoichiometries: HC : OC : HT : OT = 1 : 1 : 3 : 3
Sensitivity 3-fold enhanced, but care must be taken, accepted procedures followed
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Hydrogen chemisorption: Sources of error
• Spillover of H-atoms to the support - can give H : M > 1
• SMSI-effect (decoration of metal particles by reduced support species) reduces hydrogen uptake (TiO2)
• Absorption of hydrogen, hydride formation (Pd, usually avoided by keeping T low < 373 K, titration method)
• Presence of impurities like Cl, S, C, water, metals can alter uptake
• General concern about stoichiometry
• “All chemisorption is a research project in its own right”
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General guidelines for choice of adsorbate
Tabell gammel bok
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Dynamic method: Flow method (frontal chromatography)
• Quick method, but isotherm not easily available.
• Here performed in transient kinetic apparatus
Fig. Fra Bariås
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Pulse technique
• Simple experiment
• Can be combined with desorption experiment
• Pulse time (exposure to adsorbate) is short - kinetics of adsorption can influence the results– cobalt: adsorption slow - pulse technique with hydrogen unsuited
• Time between pulses important parameter: desorption kinetics can also influence the result
• Purity of carrier gas important (e.g. small trace of oxygen will titrate surface)
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Effect of time between pulses
Chromatograms of pulsed hydrogen adsorption on Pt/Al2O3.
From Gervasini and Flego, Appl. Catal., 1991, 72, 153.
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Chemisorption - summary
• Attractive method - gives catalytically relevant data
• Several possibilities of making errors or introducing artifacts– choice of technique
– choice of adsorbate
– choice of conditions
– assumptions made for calculations
• Should be combined with other methods available– several physical measurement principles applied reduces the
danger of errors
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Catalyst characterisation
2. Temperature programmed methods
Edd A. Blekkan, Dep. of Chemical EngineeringCatalysis and Kinetics GroupNTNU
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Temperature Programmed methods
• Thermal analysis (TGA, DSC, DTA etc.)– standard techniques in solid state chemistry, used for characterisation of properties and
reactivities of solid materials
– involves the measurement of the response (e.g. mass change, energy exchange etc. with change (usually a linear ramp) in the temperature)
– also applicable for studies of catalyst preparation - decomposition of salts and precursors
– not a topic today
• TP-methods in catalysis– TPx, where x can be
• Reduction TPR• Oxidation TPO• Desorption TPD• Sulfidation TPS• Reaction Spectroscopy TPRS (or TPR or TPRx)• For model systems in vacuum: TDS: Thermal Desorption Spectrsocopy
allows study of adsorption -desorption processes, kinetic steps, energetics etc.
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TPR
• Metal catalysts are prepared via precursors and must be reduced:MOn + nH2 M + nH2O
• Reduction can only occur if thermodynamically allowed:
• The more “noble” the metal - the easier the reduction (from a thermodynamical point of view), higher ratio water : hydrogen allowed
• Gas composition becomes important: hydrogen purity, water removal
• Base metals: Study thermodynamics and kinetics
• “Noble” metals: Study reduction kinetics
• Temperature ramping– allows a more rapid investigation– may resolve different processes
0p
plnRTnGG
2H
O2H0rr
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Handbookfig 1%2side 677
Experiments• Gradients unwanted - use differential
conditions– but must ensure sufficient analytical
precision
• Gas must be pure, without traces of O2 or poisons
• Analysis of hydrogen consumption– TCD
– MS
– can also use TGA/EGA type apparatus
• Usually one of several functions in “multi-purpose” characterisation instrument (TPx, pulse adsorption)
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Interpretation
• Qualitative interpretation– temperature of reduction onset, reduction completion
– comparison of samples, fingerprinting
– simple or multistep reduction
– slow or fast reduction
– effect of promoter, support, metal loading etc.
• Simple quantitative interpretation– calculation of degree of reduction from H2 consumption
– potential problem: stoichiometry of oxide
e.g. supported cobalt: Co3O4 or CoO?
• Quantitative interpretation of kinetic parameters– possible if the process is uniform and clear (no overlapping, interference form other
processes)• particles uniform in size and composition
• no diffusion limitations, heat transfer effects on rates
– usually not suited for practical supported metal catalysts
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Example 1: Bimetallic catalyst• Prestvik (NTNU, Thesis 1995) studied Pt-Re/Al2O3 reforming catalysts using TPR after different
drying:
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Hydrogen consumption
• Differences in peak temperatures and hydrogen consumption due to changes in reduction mechanism
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Pt-Re reduction mechanism
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Example 2: Reduction promoter• Interaction with the support leads to poor reducibility of supported cobalt catalysts
• Addition of easily reducible metal like Pt promotes the reduction, as seen from TPR profiles
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Conventional (isothermal) reduction process can be checked:
• The degree of reduction after a “normal” isothermal reduction can be checked by subsequent TPR - reducible cobalt in TPR indicates incomplete reduction
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Summary TPR
• Simple, cheap routinely applied technique• Suitable for rapid assessment of
– reducibility
– interaction in bimetallic systems
– support effects, promoters
• Caution: – Data from practical supported catalysts usually not suitable for
evaluation of kinetic processes (influence of various other processes like mass and heat transfer)
– Profiles strong function of conditions
– Only gas phase composition monitored - solid state reactions without H2 consumption are not detected (sintering and particle growth, structural changes)
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References and background literature
• Handbook of Heterogeneous Catalysis, ed. By G. Ertl, H. Knözinger and J. Weitkamp, VCH, Weinheim 1997.
• J.M. Thomas and W.J. Thomas, “Principles and Practice of Heterogeneous Catalysis”, VCH, Weinheim 1997.
• J.W. Niemantsverdriet, “Spectroscopy in Catalysis”, VCH, Weinheim 1993.
• O. Tronstad, “Overflate og porefordelingsmålinger, Inst. For industriell kjemi, NTH 1992.
• F. Dellanay (Ed.), “Characterization of Heterogeneous Catalysts”, Dekker, New York 1984.