Physical adsorption in porous glasses

Optica Applicata, Vol. XXXIII, No. I, 2003

Physical adsorption in porous glasses

Volodymyr Ovechko, Andriy Dmytruk, Valentyna Mygashko

Radiophysical Faculty, K yiv National Taras Shevchenko University, Academician Glushkov Prosp., 6, Kyiv, 03127, Ukraine.

We have presented the results o f our investigation concerning adsorption in porous glass (PG). Chemical adsorption o f gas molecules is due to indicator substances which were deposited from solution on the surface of pores. Physical adsorption o f organic molecules is due to OH-adsorption centres which are always arranged on the surface o f PG at room temperature. Adsorption processes were investigated with infrared spectroscopy.

Keywords: porous glass, physical adsorption, vibration spectroscopy.

1. Introduction

The base of PG matrix is composed of oxide-silicon tetrahedrons S i0 4 (d = 0.16 nm, φ = 109°). Amorphous matrix consists o f tetrahedrons that have random orientation. The most stable orientation corresponds to the longest distance between Si atoms. This is the orientation in which neighbouring tetrahedrons have one common oxygen atom. On the surface o f PG there are two types of Si-atoms: single qualifier tie S i - 0 and double one Si < q . After the interaction with H20 molecules these ties form hydroxyl groups: single Si-O H and geminal S i< Q . In the first case a distance betwen hydroxyl groups is about 5

A,

A.

2. Oscillator model of physical adsorption

It is necessary to consider more closely the way molecule forms bonds with OH-center in PG. The energy of physical adsorption is much lover than chemical bounding of OH groups with PG matrix. Molecular complex can be represented as two relatively independent parts which are bonded by weak Van der Waals forces. The energy of nonpeculiar Van der Waals interactions involves orientation, induction and dispersion potentials [4]. Common properties of these interactions are their electromagnetic origin and identical dependences of energy on intermolecular distance. Taking into consideration the additive property o f Van der Waals energy and its correlation nature,

42 V. Ovechko, A. Dmytruk, V. Mygashko

we can write potential energy of complex adsorbate molecule-adsorbent complex (for short adsorbate-adsorbent) in harmonic approximation

2 _ ^ 2 2 V { q x\ q 2\4 3· · · ^ ; Θ η ; 0 u ; 02 i ) = k n q l + + * 1 2 ( ^ 1 “ <h)

/ > 3

+ 9,-cos θι,.)2 + ^ k 2i(qiCOs92i- q 2)2

i * 3 ifc3

where <2^, g2, <7; - stretching coordinates of OH group, adsorbate-adsorbent and

adsorbent, respectively, ktj - force constants, and 6n = (q l ,q2)^ = (9i ><?.)>

θ 2 i = f e f l , · ) .

We place 0 12 = 0° in Eq. (1) because this value corresponds to the maximum o f interaction [5]. Then we have to average Eq. (1) over 6U, Q2i angles because Van der Waals forces are not directional as in case of chemical ones. Then

(U ) = q \

\ f Λ ,

*ιι + Σ * “ + *ΐ2 + ch k n + Y J kn + £ ^ ( * » ' + 2 * u + I * 2*

i > 3 / > 3 i i 3

It may be deduced from the last formula that the molecule-adsorbent interaction results in force constants reduction and formation of a new oscillator. The harmonic approximation potential and kinetic energy have standard expressions:

mqj) - |Σ ^ ·

2

A - 2(*if+ 5*u+ 5*w)· ' s3

(6)

where Cj are kinematic constants.

Relations (2)-(6 ) can be simplified if we assume that the force constants of this intermolecular interaction are much lower than the ones inside the m olecule. Then we can write the approximate formulae for frequency shifts for the adsorbent Δα^, adsorbate molecules Aq)2j and normal vibration of the adsorbate-adsorbent complex ω2

Physical adsorption in porous glass 43

Actfj - ω , - ω 01 - ω01 2k^x ' (7)

* υ + Σ * * Acoj = (or a>y = ω0;· — — ---,

^Kjj ύλ, = '22 λ Ί 1/ 2 *12 + X * 2 y 7 > 3 7 J (8) (9)

where ωοι, coqj are eigenfrequencies o f the adsorbent centers and molecules in free states, and

2k 12k u

ωοι = ' c T ' ω°1 =

From Eqs. (7)-(9) and equilibrium condition k l2 + ^ k2i = 2 ^ k u one can get

1 Ολ, = c ₂₂ Δω, _ Δω,· ^η1/2 — l- 2 k n + 4 \ — ^k·. 11 ^ o>oj JJ j* 3 7 ' ω,01 (10)

The last formula can be written as a function of frequencies and kinematic constants

NH / 2 ω·, = Cu 2 c_C22 ^ Δ ω 1ω0ι + Σ Δ<ΒΛ · 7-3 ( Π )

This formula shows the eigenfrequency ω2 of the complex.

Let us suppose that oscillator ω2 can be represented by Morse potential with the unharmonic coefficient χ 2. Then dissociation potential

ο = § α - * 2 ) 2 . (12)

This value can be presented as potential o f adsorption.

3. Experimental

Theoretical value o f De for an ammonia molecule was calculated taking into consideration two stretch modes for NH3 (ω03 = 3444 cm-1, ω04 = 3337 cm-1) and measured spectral shifts (Δω3 = 34 cm-1, Δω4 = 30 cm-1, Δω, = 675 cm-1) - see the Figure. The substitution of these data into Eq. (11) gives value ω2 = 418 cm-1. We consider Cu /C22 = 17, ω01 = 3750 cm-1 in this calculation. Substitution of ω2 into

44 V. Ovechko, A. Dmytruk, V. Mygashko

Figure. Panoramic absorption spectrum o f adsorbed ammonia (---) and clear PG (--- ).

Eq. (12), assuming £ = 0.022 for H nucleus, leads to the dissociation potential (or adsorptive energy) De = 4560 cm-1 = 54.6 kJ/moll. One can compare this value with the reported ones (52 kJ/moll [5]). A qualitative agreement between the theory and experiment is a reasonable one.

4. Summary

The physical adsorption of molecules NH3 on the surface of PG was studied by means o f IR spectroscopy in order to state connection between frequency shifts o f normal vibration modes and stretch frequency of molecule-adsorbent bond. Oscillator model of physical adsorption, which takes anharmonicity of vibration o f molecule-adsorbent bond into account, enables to make theoretical estimation of adsorption energy. The energy o f adsorption was found in good agreement with known microcalorimetry data for NH3 molecule.

References

[1] OvechkoV .S, DmytrukA.M, MygashkoV.P, MulenkoS.A., Vib. Spectrosc. 22 (2000), 87. [2] BolisV., B usco C., BordigaS., UgliengoP., LambertiC., ZecchinaA., Appl. Surf. Sci. 196 (2002),

56.

[3] KiselevV., KozlovS., ZoteevA., Basic o f Physics o f Solid State Surface, M oscow Univ. Print, 1999

(in Russian).

[4] HobzaP., ZahradnikR., Inter molecular Complexes, Academia, Prague 1988, p. 376.

[5] RoggeroI., CivalleriB., UgliengoP., Chem. Phys. Lett. 341 (2001), 625.