Quantitative aspects of IR spectroscopy as applied to adsorbed species Edoardo Garrone Dipartimento di Scienze dei Materiali ed Ingegneria Chimica, Politecnico

Quantitative aspects of IR spectroscopy as applied to adsorbed

species

Edoardo Garrone

Dipartimento di Scienze dei Materiali ed Ingegneria Chimica, Politecnico di Torino, Corso Duca degli

Abruzzi 24, 10129 Torino Italy

IR spectroscopy: mainly a qualitative technique, useful for recognising species

Group frequencies:

- carbonylic groups C=O at 1700-1750 cm-1,

- Si-H groups around 2200 cm-1, etc.

[e.g. G. Socrates, Infrared and Raman characteristic group frequencies: tables and charts. (2001) Wiley, Chichester, United Kingdom]

Examples of qualitative use of IR Spectroscopy concerning adsorbed species

- carbon dioxide on basic oxides: carbonate species may be formed, and/or species molecularly adsorbed onto cations

(G. Ramis, G. Busca, V. Lorenzelli, Mater. Chem. Phys. 29 (1991) 425)

- pyridine (or ammonia) adsorption: Brønsted or

Lewis sites revealed by the formation of pyridinium (ammonium) species, or molecularly bound species

(H. Knözinger, Adv. Catal. 25 (1976) 184)

A puzzling case concerning ethylene dissociative adsorption with extended metal surfaces:

ethylidene species C-CH3

(C. E. Anson, N. Sheppard, D. B. Powell, J. R. Norton, W. Fischer, R. L. Keiter, B.F.G.Johnson, J.Lewis, A.K. Bhattacharrya, S. A. R.

Knox, M. L. Turner, J. Am. Chem. Soc. 116 (1994) 3058)

IR Spectroscopy also yields information on the local symmetry and the types of bonds

- carbonate species (unidentate, bidentate, etc)

- carbonylic species (CO on metals): “on- top”, bidentate, tridentate CO species, with different C-O bond order and frequencies, all below 2143 cm-1 (isolated molecule)

- adsorption of CO on cations: C-O stretch with a frequency usually > 2143 cm-1

Much less developed the quantitative use of IR

spectroscopy of surface species!

Modalities of measurement

Most common measurement type in the case of adsorbed species is transmission.

Also available:

• Diffuse Reflectance (DRIFT) techniques

• Attenuated Total Reflection (ATR) or Grazing Angle

• with metal ideal surfaces, the appropriate version of vibrational spectroscopy is the Electron Energy Loss Spectroscopy (EELS)

In the present survey, only transmission measurements are

considered!

Types of transmission cells: a few categories, according to the working temperature (T), and the its control.

• Those working at room T. The actual temperature of the sample is slightly higher than the ambient (heating effect of the IR beam), and not precisely known. If pressure p changes, the temperature of the sample is also not strictly constant.

• Those working at fixed low T (coolant bath, usually liquid nitrogen at the NBP). The temperature is nominally 77 K: the actual temperature is higher (typically ca. 100 K), and not strictly constant with pressure

To vacuum line

thermal treatments

KBr windows

Cell with small optical path, working at RT

Sample holder (Cu)

KBr windows

Thermal treatments

Liquid N2

To vacuum line

IR cell working at low temperature ca. 100 K

• A few cells work at variable T

A) A.A. Tsyganenko: allows accurate measurement of T and p, but not their control. It works in a T range below ambient. Changes in T and p are slow enough so that equilibrium phenomena can be followed

B) A. Zecchina and co-workers: equipped with a cryostat: strict control over T. It works below ambient T and may operate down to 4 K.

C) A commercial cell (AABSPEC) working at controlled T and p also in a range above room T. Limited dead volume (few cubic centimetres)

Tsiganenko cell: T variable, but not controlled

(C.Otero Areán, O.V. Manoilova, A.A. Tsyganenko, G. Turnes Palomino, M. Peñarroya Mentruit, G. Geobaldo, E. Garrone, Eur. J. Inorg. Chem. (2001) 1739)

Zecchina cell: cryostat down to liquid He

(G. Spoto, E. N. Gribov, G. Ricchiardi, A. Damin, D. Scarano, S. Bordiga, C. Lamberti, A. Zecchina, Prog. Surf. Sci. 76 (2004) 71)

Commercial: also for T higher than ambient

http://www.aabspec.com

http://www.aabspec.com/

For simplicity, from now on an adsorbate showing only one

band will be considered

Measurable quantities for an IR band:

i) frequency (peak position)

ii) intensity (either at the peak or integrated intensity)

iii) half-width

iv) other parameters entering the analytical representation of the band: e.g., fraction of Lorentzian and Gaussian functions

Frequency

(peak position)

most readily measured quantity

Information on the adsorbing centre comes from the perturbation of a significant IR mode (e.g., stretching mode of CO) from a reference value (unperturbed molecule) Usually, the stronger the interaction, the larger the perturbation.

In simple cases, when considering a set of similar systems, the extent of perturbation has a quantitative meaning.

Correlations of the frequency (more commonly, the shift) with:

- the adsorption enthalpy, measured independently

- another frequency of the same system

- the frequency of another (similar) system

- another feature of the same IR band (e.g. halfwidth)





Two examples:

- CO adsorbed on cations

- H-bonding

CO adsorbed on cations

either non d, d0 or d10, (non classical carbonyls, with stretching frequencies higher than the isolated molecule)

Linear dependence between calorimetrically measured heats of adsorption and the hypsochromic shift:

Correlation between CO shift and heat of adsorption for non-d carbonyls

Shift is positive with respect to 2143 cm-1

Non d cations

Cu carbonyls

V. Bolis, A. Barbaglia, S. Bordiga, C. Lamberti, A. Zecchina, J. Phys. Chem. B 108 (2004) 9970.

Only electrostatics involved, no proper chemical bond

If double interactions take place with both ends of the CO molecule as in Al-rich zeolites, the linear relationship does not hold lower frequency and larger interaction enthalpy

(C. Otero Areán, M. Rodriguez Delgado, C. Lopez Bauçà, L. Vrbka, P. Nachtigall, Phys. Chem. Chem. Phys. 9 (2007) 457)

H-bonding: the shift of the O-H stretch Δν(O-H) measures the strength of H-bond

A few formulas proposed: electrostatics basically involved!

Classical work by N. Sheppard and G.C. Pimentel.

(N. Sheppard, in Hydrogen Bonding, ed. D. Hadzi, Pergamon Press, London, 1959, p. 85.

G. C. Pimentel and A. L. McClellan, in The Hydrogen Bond, W. H. Freeman and Co., San Francisco, 1960).





Correlation between two different modes of the same type of adduct.

CO H-bonded to different acidic hydroxyls: the C-O frequency linearly correlated to Δν (O-H)

(O. Cairon, T. Chevreau, J.C. Lavalley, J. Chem. Soc. Faraday Trans. 94 (1998)

3039)





Example:

the silanol in phenylene Periodic Mesoporous Organosilica (PMO) H-bonded to molecules with increasingly basic character

C

H

O

Si

b

a

C

H

O

Si

b

a

Figure 1

Inagaki, S.; Guan, S.; Ohsuna, T.; Terasaki, O. Nature 2002, 416, 304.

1,4diphenylene PMO

Onida, B.; Borello, L.; Busco, C.; Ugliengo, P.; Goto, Y.; Inagaki, S.; Garrone, E. J.Phys. Chem. B, 109 (2005) 11961

Computer models

periodic

cluster

3800 3600 3400 3200 3000 2800

0.5 A.u.

Ab

sorb

an

ce

wavenumbers (cm-1)

N2

CO

C6H6

Propene

Ammonia

Acetone

Cyclohexene

Mesitylene

Increasing basicity

Bellamy-Hallam-William plot

Comparison of the shifts suffered by the O-H stretch of silanols in:

- phenylene PMO

- amorphous silica

Proportionality constant: a measure of the relative acidity of the two O-H species

Proportionality constant ca. 0.96 Silanol in PMO slightly less acidic than in silica

For bridged OH species in zeolites proportionality constant ca. 3 (much more acidic!)

Deviations may occur!

0 30 60 90 120 150 1800

100

200

300

400

500

1,3,5-TMB

C3H

6

C6H

6

C2H

4

CO

N2

S

i(O

H)A

l SA

PO

-40 c

m-1

SiOHAerosil

(cm)

SAPO-40

Benzene and mesytilene show deviations with respect to the BHW plot

0 20 40 60 80 100 120 140 1600

100

200

300

400

500

600

C3H

6

C7H

8

C6H

6

C2H

4

CO

N2

S

i(O

H)A

l ZS

M-5 (

cm-1)

SiOHAerosil

(cm-1)

H-ZSM-5

Benzene and toluene show deviations

C3H6

C6H6 (CH3)2CO

C2H4

Hindrance of the interaction by the surroundings

freehindered




- another feature of the same IR band (e.g. halfwidth, intensity)

In H-bonding, the larger the shift, the wider and the more intense the band of the stretching mode of the O-H species engaged

Quantitative relationships are known

3800 3600 3400 3200 3000 2800

0.5 A.u.

Ab

sorb

an

ce

wavenumbers (cm-1)

N2

CO

C6H6

Propene

Ammonia

Acetone

Cyclohexene

Mesitylene

Intensity

More troublesome quantity

IR transmission experiment concerning solutions: measurement of the population of absorbing centres through the classical

Lambert-Beer law (LBL)

A = absorbance; k = absorption coefficient; c = concentration of absorbing centres; d = thickness of the sample; ε = molar extinction coefficient,

LBL: A = k d = ε c d

IRS measurements concerning a pellet:

A = ε N/ S

S = geometrical surface of the pellet N = number of moles of adsorbing centres in the whole sampleε = molar extinction coefficient

Absorbance measures the number of moles in the sample! quantitative aspects!

Two reasons could impede the applicability of LBL:

i) the presence of scattering because of the

powder structure of the samples;

ii) a change in the environment of the absorbing centres due to a change in pressure, in reversible adsorptions (not really important)

Treatment of scattering: Schuster-Kubelka-Munk model (the same for Diffuse Reflectance)

A forward flux I and a backward flux J:

-dI/dx = (k + s) I – s J

+ dJ/dx = -s I + (k + s) J

s = scattering coefficient, k = absorption coefficient

(G. Kortum, Reflectance spectroscopy : principles, methods, applications. (1969), Sprinter-Verlag, New York)

T = [1 - R2] exp [- b s d] / 1 - R

2 exp [- 2 b s d]

b = [(1 + k/s)2 - 1]1/2; d = sample thickness, R = reflectance of the sample at infinite thickness

R = 1 + k/s - [(k/s)2 + 2 k/s] 1/2

In case of moderate scattering (s < 10% k),

-ln T = Aapp sd + kd + (s/k) 2 [1 – kd]

k depends on the concentration c, s does not.

k = k0 + ε N/S s = s0

(k0 = absorption of the solid alone): k is growing with coverage

Result: in case of moderate scattering (s < 10% k), LBL holds (small offset, the term sd). For larger values of s/k, deviations may occur.

Note: scatter of radiation is more often due to voids in the sample than to the actual particles. Silica samples, white when powdered, tend to become transparent when pelleted. The condition s << k is more readily fulfilled for pelleted samples than for loose powders

Example showing that the intensity of a band has to be considered with care:

Porous silicon

Also shows a peculiar way of making a quantitative use of IR spectroscopy!

Teflon CellTeflon Cell

PREPARATIONPREPARATION

Silicon piece (1.1x1.1cm)Silicon piece (1.1x1.1cm)

The The electrochemical electrochemical cell, in figure, is cell, in figure, is made of Teflon, made of Teflon, resisting to HF. The resisting to HF. The cathode is a cathode is a platinum rod, platinum rod, whereas the anode whereas the anode is the silicon itself.is the silicon itself.The electrolyte is The electrolyte is an ethanoic HF an ethanoic HF solution.solution.

Etching parameters (HF concentration,

current density and etching time) define

PS morphology, porosity, and specific

surface area

MICRO-, MESO-, MACRO-POROUS

SILICON

TEM Image of p+ Porous Silicon

by CNR LAMEL (Bologna)

STRUCTURESTRUCTURE

SEM Image of p+ Porous Silicon

by IUT-Lannion (France)

1. etching process does not remove

the doping atoms

2. the surface is passivated by

hydrogen (SixHy species)

CHEMICAL COMPOSITIONCHEMICAL COMPOSITION

Morphology of Porous Silicon p+

1. Loss of conductivity due to etching process nearly

insulating material

2. IR TRANSPARENT

3. Electrical reactivation in presence of

NONO2 2 TRACESTRACES

Characteristic vibrations of Si-HCharacteristic vibrations of Si-Hx x bondsbonds

Curve a: PSi as such

Curve b: in contact with .25 mbar NO2

Two effects:

- marked increase in the background

- decrease in the intensity of Si-H bands (not their location)

Cause: injection of charge carriersAffects intensity, not frequency!

(F. Geobaldo, P. Rivolo, S. Borini, L. Boarino, G. Amato, M. Chiesa and E. Garrone J. Phys. Chem. B 108 (2004) 18306)

Same phenomenon observed with reducible oxides (ZnO, SnO2)

Reduction converts an insulator oxide into a semiconductor!

In conclusion, is LBL valid?

LBL validated a posteriori (several examples in this talk)

• LBL probably holds in the vast majority of cases.

• The problem with LBL, though, is that determination of ε is difficult, because very seldom A and N are simultaneously determined.

• More often, A and N are measured in separate experiments, one spectroscopic, one volumetric: identity of temperature not assured.

Example of the uncertainties on molar extinction coefficients: non classical carbonyls on cationic centres.

Generally believed that ε increases with the frequency, though moderately, e.g.

ε = 0.7 + b (ν -2143)

when ε is given in 106 cm/mol, b = 0.050.

A.A. Tsyganenko has recently proposed a decreasing behaviour of ε with frequency for a set of carbonyls in zeolitic cationic centres

E.V. Kondriateva, O.V. Manoilova and A.A. Tsyganenko, Kinetics

and Catalysis 49 (2008) 451.

Entirely different measurement of ε through the vibrational polarizability αv

(CO) = 4 3 v2

Data concerning the vibrational polarizability αν of CO adsorbed at regular faces of microcrystalline oxides or halides

αν proportional to ε

αν measured through the shifts with coverages (Hammaker equation)

(D. Scarano et al. Adv. Catal. Vol 64)

Non-classical carbonyls

π*-d backdonation

Concerning ε, CO adsorbed on zeolitic isolated cationic centres is different from CO adsorbed at regular faces of oxides?

In the following: cases of quantitative use of IR spectra,

relying on LBL, not requiring the knowledge of ε.

Simple case of one species characterised by one band of intensity A. If the maximum intensity AM is known, the value of θ results:

θ = A/AM

The equation of state for the adsorbed species is:

F (A, T, p) = 0 or F (θ, T, p) = 0

By keeping constant one observable at a time, one obtains:

- the isostere p = p(T) at constant A

- the isotherm A = A(p) at constant T

- the isobar A = A(T) at constant p

Also possible:

- the isochore at constant overall V volume




- the isochore at constant V

The isosteric heat is related to the change in the pressure yielding a certain value of A with temperature, through a Clausius-Clapeyron-like relationship:

[ ln p/ T]A = -qiso/RT2 or

[ ln p/ (1/T)]A = qiso/R

This procedure does not require the assumption of any model, and the procedure may be repeated at different coverages (intensities).

E. A. Paukshtis, R. I. Soltanov, E. N. Yurchenko, React. Kin. Catal. Lett. 23 (1983) 339

In principle:

in the presence of several species, isosteric heat may be calculated for each species: advantage over direct calorimetry!

Same for isotherm, etc…

Separation into several contributions!

- the isostere p = p (T) at constant A

- the isotherm A = A (p) at constant T

- the isobar A = A (T) at constant p


Two cases:

- Ideal adsorption (Langmuir model)

- non-ideal (UNILAN, Temkin model)

Ideal (Langmuir) model: sites all alike and non-interacting

Langmuir equation:

A / AM = θ = K(T) p/[1 + K(T) p]

K(T) = exp [ΔS°/R] exp [-ΔH°/RT]

(van’t Hoff equation)

IRS provides a priori indications on the ideal nature of the adsorption

Related IR band is expected: - to be narrow - to have a Lorentzian shape- not to shift with coverage.

Definite evidence comes from a constant heat of adsorption, as measured independently.

Identity among sites is provided by the structure in some cases.

Requirements for non-interaction among sites:

- the solid constitutes an insulator matrix

- a low density of sites

Examples of ideal adsorbing systems:

- Boron atoms at the surface of PSi

- isolated silanols on amorphous silica (Aerosil, MCM- 41, etc.)

- carboxylic groups on functionalized silica

- cationic sites on zeolites



- carboxylic groups on functionalized silica - cationic sites on zeolites

Mechanism of interaction of NO2 with PSi(hole injection)

Each adsorbed molecule injects a carrier (hole)

the background of IR spectrum rises increase in volume concentration of

carriers (Drude formula) adsorbed amount isotherm

Spectra at increasing equilibrium pressures of NO2

0 100 200 300 400

1/p

- p

0 (n

um

be

r o

f ca

rrie

rs/c

m3 )

1/PNO

2

(mbar)

2

2

1

max

NO

NOadsads KP

KPNN

2

/1/1/1/1 maxmaxNOadsadsads PKNNN

DATA FOLLOW THE LANGMUIR MODELDATA FOLLOW THE LANGMUIR MODEL

2. 2. Carriers increase describable by a REVERSIBLEREVERSIBLE

CHEMISORPTION PROCESSCHEMISORPTION PROCESS

3. ADSORPTION ISOTHERM 3. ADSORPTION ISOTHERM applicable

1. REGENERATED CARRIERS1. REGENERATED CARRIERS ≈≈ [[B]B] (3X1019

atoms/cm3) almost all carriers have been

reactivated

Sites for NO2 adsorption (surface B atoms):

• isolated

• non interacting

because of the low concentration of B atoms in the pristine sample

Langmuir conditions




O-H band at 3750 cm-1:

- very thin, all sites equivalent

- low density (ca. 1 OH/100 Å2)

The coverage θ is calculated from the ratio between the actual intensity and the maximum intensity

Methylcyclohexene on Silica (B. Onida, M. Allian, E. Borello, P. Ugliengo, E. Garrone Langmuir 13 (1997) 5107)

Langmuir isotherm

Check of the isotherm and evaluation of K

Absence of solvation

ammonia

Deviation from ideality because of solvation

Check of the isotherm and evaluation of K

Benzene, methylcyclohexene, acetone, etc.




SBA-15-COOH deg 200°C: adsorption of NH3

2000 1800 1600 1400-0.2

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

A

wavenumber (cm -1)

deg 200 6.2 11.6 20.7 32.1 44.2 72.0 87.7 105.7 124.4 145.8

2000 1900 1800 1700 1600 1500 1400 1300-0.4

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

SBA-15-COOH spettri differenza invio NH3

A

wavenumber (cm -1)

Reversible formation of ammonium species:

R-COOH + NH3(g) R-COO- + NH4+

Carbonyl, ammonium and carboxylate modes all present in the IR spectrum!

Equilibrium constant:

K = θ/ [ (1 – θ) p]

as in Langmuir adsorption

0 20 40 60 80 100 120 140 160

0

2

4

6

8

10

12Parameter Value Error------------------------------------------------------------A -0.09641 0.02328B 0.04011 6.08931E-4------------------------------------------------------------

pNH3

Deviations because of solvation of ammonium species by molecular ammonia

Check of the isotherm: equilibrium constant evaluated!




CO/Na-ZSM-5 blue: Na; green: C; red: O

RT adsorption of CO on Na-ZSM-5

C-bonded adduct

O-bonded adduct

Three types of isotherms:

- volumetric

- calorimetric

- optical (sum of the intensities of C-down and O-down bands)

Optical isotherm as good as the others!

Non-ideal case: the Temkin (UNILAN) model

Temkin isotherm: Na = K1 ln[1 + K2p]

a variant of the UNILAN model: rectangular distribution of energies between two values E1 and E2, with E2 –E1 = 2 s.

Kh = equilibrium constant for E = (E1 + E2) / 2.

The Temkin isotherm generally assumed for structural heterogeneity: valid also for induced heterogeneity in a regular array of adsorption sites, all structurally equal.

Example: CO adsorption on TiO2 and ZrO2.

One to three CO species present. ZrO2 outgassed at 500°C: only one band is present.

Frequency not constant: proportionality between the shift and the overall intensity of the band.

The differential heat of adsorption, calorimetrically determined, has a nearly linear decrease

(V. Bolis, B. Fubini, E. Garrone, C. Morterra, J. Chem. Soc.,

Faraday Trans. 85 (1989) 1383)





Rather rare in the literature on oxides. Example:adsorption of CO at the (100) face of MgO followed

at temperatures below 60 K

(G. Spoto, E. Gribov, A. Damin, G. Ricchiardi, A. Zecchina, Surf. Sci. Lett. 540 (2003) 605.).

As the adsorption has ideal features, the elaboration is straightforward: the equation (see below)

Ln [θ / p (1 – θ)] = ln [A / p (AM – A)] = ΔS° / R – ΔH° / RT

is used dropping the constant term in pressure. Information on ΔS° is lost!

Spectroscopic determination of thermodynamic features of CO adsorption on metal particles (substantial electronic effects): Bianchi and associates

Constancy of pressure is obtained by flowing the adsorbate gas in a dynamic system at a constant pressure.

Temkin model adopted

(A. Bourane, O. Dulaurent, D. Bianchi J. Catal. 196 (2000) 115)

Three peaks, coverage dependent:

• “on-top” • bridged• “hidden” species.

Each species follows a Temkin equation.

Analysis of the behaviour of the intensities with temperature through the Temkin equation allows the determination of E1

and E2.





Constancy of volume: obtained by closing the cell after gas admission, then varying T and, consequently, p.

Desorption counterbalanced by the increase in pressure

Design of the cell: A.A. Tsiganenko

Methods: E. Garrone, C.O. Arean

(E. Garrone and C. Otero Areán, Chem. Soc. Rev. 34 (2005) 1)

Acronym: VTIR

Three types of process followed so far:

i) Langmuir-type adsorption on a cationic site (or hydroxyl species);

ii) isomerism between two forms of adsorbate (e.g. carbonyl/isocarbonyl);

iii) formation of dicarbonyls from monocarbonyls.

Langmuir-type adsorption on a cationic site (or hydroxyl species)

isomerism between two forms of adsorbate (e.g. carbonyl/isocarbonyl);

formation of dicarbonyls from monocarbonyls.

Suppose only one band is present, of absorbance A.

The three quantities A, T and p are related by a mass balance equation, e.g. of the type:

Nt = p Vg/RT + A/(ε S)

if the gas phase has an ideal behaviour (Nt = total number of moles and Vg = volume of the cell).

It is, however, convenient to treat T and p as independent variables, and to study the function

A = A(T, p).

Van’t Hoff equation, under the assumption of entropy and enthalpy of adsorption constant with temperature:

θ = A/AM = exp [S°/R] exp[-H°/RT] p / {1 + exp [S°/R]exp[-H°/RT] p}

AM = a parameter to be determined

More conveniently: ln [θ / p (1 – θ)] = ln [A / p (AM – A)] = ΔS° / R – ΔH° / RT

At very low coverages it results:

Ln [A / T)] = const – ΔH° / RT

does not require AM !

Two examples:

CO on protonic zeolites

H2 on cationic zeolites

Two examples:



Coverage measured directly from the ratio of intensities

HY zeolite, interaction with CO

Results: ΔH° = -25.6 kJ/mol and ΔS° = -161 J mol-1K-1

Excellent agreement with the calorimetric value for H-ZSM-5 of - 27 kJ/mol (more acidic!)

(S. Savitz, A.L. Myers and J.R. Gorte J. Phys. Chem. B 103 (1999) 3687)

Two examples:



VTIR spectra around 100 K of dihydrogen on Na-FER

Increasing T

H0 = - 6.0 kJ/mol

S0 = -78 J/(mol K)

Result not readily obtained in other ways! Low T calorimetry is difficult, volumetric isosteric methods as alternative

The study of H2 adsorption on several zeolitic systems has shown that ΔS0 values are correlated to the corresponding ΔH0 values.

Compensation effect.

Result relevant in storage problem

-10 0 10 20 30 40 50 60 70 80 90 100

0

5

10

15

20

25

Carbons

?

Ca-Y

(Mg,Na)-Y

K-FER

Na-MFIMg-X

Li-MFILi-FER

-H

0 (kJ

mol

-1)

-S0 (J mol-1 K-1)

Vertical segment represents carbons.

Vertical dotted line corresponds to the free gaseous molecule (loss of all degrees of freedom).

ΔS° vs ΔH° for H2 on several cation-exchanged zeolites.

Limiting value for ΔS°, corresponding to loss of translational modes

(E. Garrone, B. Bonelli and C. Otero Areán Chem. Phys. Letters 456 (2008) 68)

Langmuir-type adsorption on a cationic site (or hydroxyl species);



RT adsorption of CO on Na-ZSM-5

C-bonded adduct

O-bonded adduct

At relatively high T, both species are favoured, regardless their energy!

VTIR spectra of CO on Na-ZSM-5

HF band: C-bonded species

LF: O-bonded species

At low T, the more stable C-bonded species is favoured

Low-lying bands: CO species interacting with the cation through the O end

equilibrium between the two species, O-bonded and C-bonded:

M-CO M-OC

to which corresponds the equilibrium constant:

θ(CO)/θ(OC) = Kiso(T) A(CO)/A(OC)

and the van’t Hoff eqn.:


ΔH°iso is 3.8 kJ/mol, i.e. 14% of the enthalpy of formation of the C-bonded adduct.

Langmuir-type adsorption on a cationic site (or hydroxyl species);



Several cations in zeolites may coordinate more than one CO molecule.

Example: CO adsorbed on Ca(Na)Y

Up to tri-carbonyls formed with increasing coverage at 77 K

(K.I. Hadjiivanov, H. Knozinger Chem. Phys. Lett. 303 (1999) 513)

219

0

219

8

221

2

0,0

0,2

0,4

0,6

0,8

1,0

1,2

Abs

orba

nce

2100 2200 Wavenumbers (cm-1)

2149 20

95

2137

0.4

x15x15

21 C

-70 C

-196 C

-120 C

13C O-C

CO/CaNaY VTIR spectra. Low dose: monocarbonyl at 2198, dicarbonyl with two unresolved modes at 2190 cm-1

IR spectra at variable temperature concerning the adsorption of CO at variable temperature on SrY

One major band shifting with decreasing temperature (i.e. increasing coverage) from 2191 cm-1 to 2187 cm-1.

- monocarbonyl Sr(CO) 2191 cm-1

- dicarbonyl Sr(CO)2, two unresolved modes at 2187 cm-1

One minor band shifting from 2098 to 2095 cm-1

(O-bonded complexes)

(E. Garrone, B. Bonelli, A.A. Tsiganenko, M. Rodriguez Delgado, G. Turnes Palomino, O.V. Manoilova, C. Otero Areán J. Phys. Chem. B 107 (2003) 2537)

Behaviour of peak position with peak intensity

monocarbonyl

dicarbonyl

Opposite ends of the diagram: regions where the monocarbonyl or the dicarbonyl predominate. This allows, through the use of

θ = A/AM = exp [ΔS°/R] exp[-ΔH°/RT] p/{1 + exp [ΔS°/R] exp[-ΔH°/RT] p},

to measure the enthalpy changes of:

- monocarbonyl formation (adsorption on the naked cation)- dicarbonyl formation (coordination of a second CO molecule).

Use of:

θ(CO)/θ(OC) = Kiso(T) A(CO)/A(OC)


allows the evaluation of the enthalpy changes related to the isomerisms

Sr(CO)++ Sr(OC)++

Sr(CO)++2 Sr(CO,OC)++

Enthalpies of formation of the various CO adducts formed with SrY zeolite

Fairly complete energetic characterisation of the possible adducts!

Conclusions

Frequency readily measured: quantitative correlations possible with either, other IR features or quantities independently measured

Intensity troublesome: i) validity of LBL not always assured; ii) ε known with poor accuracy (band intensity and adsorbed amounts not measured simultaneously!)

Information may come from measurements relying on LBL but not actually using any ε.

All types of variable temperature IR measurements (including VTIR) yield thermodynamic data on:

i) ideal adsorption; ii) Temkin-like adsorption; iii) isomerism between species; iv) formation of multi-ligand complexes.

Advantage over direct calorimetry: possible separation of concurrent phenomena!

Thanks for your attention

Documents

Quantitative aspects of IR spectroscopy as applied to adsorbed species Edoardo Garrone Dipartimento di Scienze dei Materiali ed Ingegneria Chimica, Politecnico