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

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..

General scheme of characterization

General scheme: Techniques

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

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

Example 1. Phase identification

Example 2. Supported metal particles

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

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)

General classification of adsorption

Tabell fysikalsk vs. kjemisorpsjonRichardson tab 7.3 side146

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

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

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

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

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

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

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

Some definitions

Handboook tab 1 s 428portrait

Physical adsorption IUPAC classification of isotherms

Handbook fig 1 s 428

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

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

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

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

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

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

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

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.

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

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

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

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

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

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

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

Plotted relationships Pt Pd Ni (spherical particles)

Handbook fig. 2 & 3 side 441

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

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

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

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.

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

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”

General guidelines for choice of adsorbate

Tabell gammel bok

Dynamic method: Flow method (frontal chromatography)

• Quick method, but isotherm not easily available.

• Here performed in transient kinetic apparatus

Fig. Fra Bariås

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)

Effect of time between pulses

Chromatograms of pulsed hydrogen adsorption on Pt/Al2O3.

From Gervasini and Flego, Appl. Catal., 1991, 72, 153.

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

Catalyst characterisation

2. Temperature programmed methods

Edd A. Blekkan, Dep. of Chemical EngineeringCatalysis and Kinetics GroupNTNU

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.

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

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)

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

Example 1: Bimetallic catalyst• Prestvik (NTNU, Thesis 1995) studied Pt-Re/Al2O3 reforming catalysts using TPR after different

drying:

Hydrogen consumption

• Differences in peak temperatures and hydrogen consumption due to changes in reduction mechanism

Pt-Re reduction mechanism

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

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

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)

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.