Presentation is loading. Please wait.

Presentation is loading. Please wait.

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

Similar presentations


Presentation on theme: "Quantitative aspects of IR spectroscopy as applied to adsorbed species Edoardo Garrone Dipartimento di Scienze dei Materiali ed Ingegneria Chimica, Politecnico."— Presentation transcript:

1 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

2 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]

3 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)

4 A puzzling case concerning ethylene dissociative adsorption with extended metal surfaces: ethylidene species  C-CH 3 (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)

5 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

6 Much less developed the quantitative use of IR spectroscopy of surface species!

7 Modalities of measurement

8 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)

9 In the present survey, only transmission measurements are considered!

10 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

11 To vacuum line thermal treatments KBr windows Cell with small optical path, working at RT

12 Sample holder (Cu) KBr windows Thermal treatments Liquid N 2 To vacuum line IR cell working at low temperature ca. 100 K

13 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)

14 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)

15 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)

16 Commercial: also for T higher than ambient http://www.aabspec.com

17 For simplicity, from now on an adsorbate showing only one band will be considered

18 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

19 Frequency (peak position) most readily measured quantity

20 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.

21 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)

22 - 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)

23 Two examples: - CO adsorbed on cations - H-bonding

24 CO adsorbed on cations either non d, d 0 or d 10, (non classical carbonyls, with stretching frequencies higher than the isolated molecule) Linear dependence between calorimetrically measured heats of adsorption and the hypsochromic shift:

25 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.

26 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)

27 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).

28 - 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)

29 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)

30 - 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)

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

32 Inagaki, S.; Guan, S.; Ohsuna, T.; Terasaki, O. Nature 2002, 416, 304. 1,4diphenylene PMO

33 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

34 N2N2 CO C6H6C6H6 Propene Ammonia Acetone Cyclohexene Mesitylene Increasing basicity

35 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

36 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!

37 SAPO-40 ZSM-5 MCM-22 THETA SAPO-40 Benzene and mesytilene show deviations with respect to the BHW plot

38 H-ZSM-5 Benzene and toluene show deviations

39 C3H6C3H6 C6H6C6H6 (CH 3 ) 2 CO C2H4C2H4 Hindrance of the interaction by the surroundings free hindered

40 - 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, intensity)

41 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

42 N2N2 CO C6H6C6H6 Propene Ammonia Acetone Cyclohexene Mesitylene

43 Intensity More troublesome quantity

44 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

45 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 sample ε = molar extinction coefficient

46 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)

47 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)

48 T =  [1 - R  2 ] 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 = A app  sd + kd + (s/k) 2 [1 – kd]

49 k depends on the concentration c, s does not. k = k 0 + ε N/S s = s 0 (k 0 = 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

50 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!

51 Teflon Cell PREPARATION Silicon piece (1.1x1.1cm) The electrochemical cell, in figure, is made of Teflon, resisting to HF. The cathode is a platinum rod, whereas the anode is the silicon itself. The electrolyte is an ethanoic HF solution.

52 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) STRUCTURE 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 (Si x H y species) CHEMICAL COMPOSITION

53 Morphology of Porous Silicon p+

54 1.Loss of conductivity due to etching process  nearly insulating material 2.IR TRANSPARENT 3.Electrical reactivation in presence of NO 2 TRACES

55 Characteristic vibrations of Si-H x bonds Curve a: PSi as such Curve b: in contact with.25 mbar NO 2

56 Two effects: - marked increase in the background - decrease in the intensity of Si-H bands (not their location) Cause: injection of charge carriers Affects 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)

57 Same phenomenon observed with reducible oxides (ZnO, SnO 2 ) Reduction converts an insulator oxide into a semiconductor!

58 In conclusion, is LBL valid?

59 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.

60 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 10 6 cm/mol, b = 0.050.

61 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 α v  (CO) = 4  3  v  2

62 Data concerning the vibrational polarizability α ν of CO adsorbed at regular faces of microcrystalline oxides or halides α ν proportional to ε α ν measured through the shifts with coverages (Hammaker equation) (D. Scarano et al. Adv. Catal. Vol 64) Non- classical carbonyls π*-d backdonation

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

64 In the following: cases of quantitative use of IR spectra, relying on LBL, not requiring the knowledge of ε.

65 Simple case of one species characterised by one band of intensity A. If the maximum intensity A M is known, the value of θ results: θ = A/A M The equation of state for the adsorbed species is: F (A, T, p) = 0 or F (θ, T, p) = 0

66 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

67 - the isostere p = p(T) at constant A - the isotherm A = A(p) at constant T - the isobar A = A(T) at constant p - the isochore at constant V

68 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 = -q iso /RT 2 or [  ln p/  (1/T)] A = q iso /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

69 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!

70 - the isostere p = p (T) at constant A - the isotherm A = A (p) at constant T - the isobar A = A (T) at constant p - the isochore at constant V

71 Two cases: - Ideal adsorption (Langmuir model) - non-ideal (UNILAN, Temkin model)

72 Ideal (Langmuir) model: sites all alike and non-interacting Langmuir equation: A / A M = θ = K(T) p/[1 + K(T) p] K(T) = exp [ΔS°/R] exp [-ΔH°/RT] (van’t Hoff equation)

73 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.

74 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

75 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

76 - 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

77 Mechanism of interaction of NO 2 with PSi (hole injection)

78 Each adsorbed molecule injects a carrier (hole)  the background of IR spectrum rises  increase in volume concentration of carriers (Drude formula)  adsorbed amount  isotherm

79 Spectra at increasing equilibrium pressures of NO2

80 The increase in the baseline is due to absorption of free carriers (holes): the absorption, in which A  2, is describable by Drude model. Ω P = the Plasma Frequency; Ω τ is the Damping Constant.

81  P and   are related to free carriers concentration (n) and their mobility (  ), respectively. e is the charge and m the effective mass of the charge carriers. For p-doped silicon the effective mass of holes is 0.37 times, while for electrons is 0.26 times the mass of the electron at rest.

82 DATA FOLLOW THE LANGMUIR MODEL 2. REVERSIBLE CHEMISORPTION PROCESS 2. Carriers increase describable by a REVERSIBLE CHEMISORPTION PROCESS 3. ADSORPTION ISOTHERM 3. ADSORPTION ISOTHERM applicable 1. REGENERATED CARRIERS ≈ [B] 1. REGENERATED CARRIERS ≈ [B] (3X10 19 atoms/cm 3 )  almost all carriers have been reactivated

83 Sites for NO 2 adsorption (surface B atoms): isolated non interacting because of the low concentration of B atoms in the pristine sample  Langmuir conditions

84 - 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

85 O-H band at 3750 cm -1 : - very thin, all sites equivalent - low density (ca. 1 OH/100 Å 2 ) 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)

86 Langmuir isotherm Check of the isotherm and evaluation of K Absence of solvation ammonia

87 Deviation from ideality because of solvation Check of the isotherm and evaluation of K Benzene, methylcyclohexene, acetone, etc.

88 - 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

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

90 Reversible formation of ammonium species: R-COOH + NH 3 (g)  R-COO - + NH 4 + Carbonyl, ammonium and carboxylate modes all present in the IR spectrum! Equilibrium constant: K = θ/ [ (1 – θ) p] as in Langmuir adsorption

91 Deviations because of solvation of ammonium species by molecular ammonia Check of the isotherm: equilibrium constant evaluated!

92 - 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

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

94 RT adsorption of CO on Na-ZSM-5 C-bonded adduct O-bonded adduct

95 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!

96 Non-ideal case: the Temkin (UNILAN) model

97 Temkin isotherm: N a = K 1 ln[1 + K 2 p] a variant of the UNILAN model: rectangular distribution of energies between two values E 1 and E 2, with E 2 –E 1 = 2 s. K h = equilibrium constant for E = (E 1 + E 2 ) / 2. The Temkin isotherm generally assumed for structural heterogeneity: valid also for induced heterogeneity in a regular array of adsorption sites, all structurally equal.

98 Example: CO adsorption on TiO 2 and ZrO 2. One to three CO species present. ZrO 2 outgassed at 500°C: only one band is present. Frequency not constant: proportionality between the shift and the overall intensity of the band.

99 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)

100 - the isostere p = p(T) at constant A - the isotherm A = A(p) at constant T - the isobar A = A(T) at constant p - the isochore at constant V

101 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 (A M – A)] = ΔS° / R – ΔH° / RT is used dropping the constant term in pressure. Information on ΔS° is lost!

102 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)

103 Three peaks, coverage dependent: “on-top” bridged “hidden” species. Each species follows a Temkin equation.

104 Analysis of the behaviour of the intensities with temperature through the Temkin equation allows the determination of E 1 and E 2.

105 - the isostere p = p(T) at constant A - the isotherm A = A(p) at constant T - the isobar A = A(T) at constant p - the isochore at constant V

106 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

107 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.

108 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.

109 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: N t = p V g /RT + A/(ε S) if the gas phase has an ideal behaviour (N t = total number of moles and V g = 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).

110 Van’t Hoff equation, under the assumption of entropy and enthalpy of adsorption constant with temperature: θ = A/A M = exp [  S°/R] exp[-  H°/RT] p / {1 + exp [  S°/R]exp[-  H°/RT] p} A M = a parameter to be determined

111 More conveniently: ln [θ / p (1 – θ)] = ln [A / p (A M – A)] = ΔS° / R – ΔH° / RT At very low coverages it results: Ln [A / T)] = const – ΔH° / RT does not require A M !

112 Two examples: CO on protonic zeolites H 2 on cationic zeolites

113 Two examples: CO on protonic zeolites H 2 on cationic zeolites

114 Coverage measured directly from the ratio of intensities HY zeolite, interaction with CO

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

116 Two examples: CO on protonic zeolites H 2 on cationic zeolites

117 VTIR spectra around 100 K of dihydrogen on Na- FER Increasing T  H 0 = - 6.0 kJ/mol  S 0 = -78 J/(mol K)

118 Result not readily obtained in other ways! Low T calorimetry is difficult, volumetric isosteric methods as alternative The study of H 2 adsorption on several zeolitic systems has shown that ΔS 0 values are correlated to the corresponding ΔH 0 values. Compensation effect. Result relevant in storage problem

119 Limiting value for ΔS°, corresponding to loss of translational modes (E. Garrone, B. Bonelli and C. Otero Areán Chem. Phys. Letters 456 (2008) 68)

120 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.

121 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!

122 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

123 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) = K iso (T)  A(CO)/A(OC) and the van’t Hoff eqn.: K(T) = exp [ΔS°/R] exp [-ΔH°/RT]

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

125 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.

126 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)

127 13 CO-C CO/CaNaY VTIR spectra. Low dose: monocarbonyl at 2198, dicarbonyl with two unresolved modes at 2190 cm -1

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

129 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)

130 Behaviour of peak position with peak intensity monocarbonyl dicarbonyl

131 Opposite ends of the diagram: regions where the monocarbonyl or the dicarbonyl predominate. This allows, through the use of θ = A/A M = 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).

132 Use of: θ(CO)/θ(OC) = K iso (T)  A(CO)/A(OC) K(T) = exp [ΔS°/R] exp [-ΔH°/RT] allows the evaluation of the enthalpy changes related to the isomerisms Sr(CO) ++  Sr(OC) ++ Sr(CO) ++ 2  Sr(CO,OC) ++

133 Enthalpies of formation of the various CO adducts formed with SrY zeolite Fairly complete energetic characterisation of the possible adducts!

134 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!)

135 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!

136 Thanks for your attention


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

Similar presentations


Ads by Google