MNDO theoretical study of ethanol decomposition process on SnO2 surfaces

MNDO theoretical study of ethanol decomposition process on SnO2 surfaces

THEO CHEM Journal of Molecular Structure (Theochem) 357 (1995) 153-159 MNDO theoretical study of ethanol decomposition process on Sn02 surfaces S.R.M...

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THEO CHEM Journal of Molecular Structure (Theochem) 357 (1995) 153-159

MNDO theoretical study of ethanol decomposition process on Sn02 surfaces S.R.M. Antunesa, J.D. Santosa, A.C. Antmesa, E. Longoa’*, J.A. Varelab “Universidade Federal de Srio Carlos, Departamento de Quimica, C.P. 676. 13565-905, SLio Carlos. SP, Brazil blJniversidaa’e Estadual Paulista, Institute de Quimica, C.P. 355, 14800-900. Araraquara, SP, Brazil

Received 6 March 1995; accepted 28 March 1995

Abstract An MNDO study has been carried out to analyze the decomposition process of the ethanol molecule on a SnOz surface. A (SnOz)-, (110) model has been selected to represent the surface. The decomposition process has been monitored by selection of a hydrogen-a-carbon distance of the ethanol molecule as reaction coordinate. This minimum energy profile shows a maximum of 186 kJ mol-‘, and in the transition state there is a transfer of hydrogen-a-carbon to the Sn02 surface. There is also the interaction between the alcohol hydroxyls and the two oxygens of the oxide.

1. Introduction A semiconductor system as gas sensor has been an active research area in materials science since Brattain and Bardeen [l] demonstrated in 1952 that the resistivity of semiconductors is very sensitive to gas adsorption. The physical chemistry of the interactions between the substance to be measured and the oxide determines how the sensor

works [2-51. To control the sensor properties it is important to understand the surface processes as a function of the conductivity. The first complete work of chemisorption was developed by Wolkenstein [6]who proposed a theory aiming to establish a correlation between the catalytic properties of the semiconductor and its electronic structure. Stetter [7j and Hangen et al. [8] have suggested that the catalytic activity is a necessary condition for high sensitivity and low time response. * Corresponding author.

Heiland and Kohl [9] analyzed the interaction of ethanol with the SnOz single crystal, covered by Pd, concluding that the products of the decomposition process are acetaldehyde, ethylene, and water. In this paper an MNDO study of the ethanol decomposition process on a Sn02 surface has been carried out to analyze this molecular mechanism. The theoretical results are compared with experimental data. In Section 2 the Method and the Model are presented. In Section 3 the results are analyzed and discussed. A short summary of conclusions closes the paper.

2. Method and model The MNDO semiempirical method [lo] included in the MOPAC (version 5.0) program package [l l] was used. The MNDO method has been recently used to study large silicon clusters (Si), with n > 10 proving to be applicable to these systems [12].

0166-1280/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0166-1280(95)04287-3

154

S.R.M. Antunes et al./Journal of Molecular Structure (Theochem)

357 (1995) 153-159

Fig. 1. Crystal model for tin dioxide, (SnO&, interacting with an ethanol molecule.

.r: C

0

Fig. 2. Model of the (110) SnO, plane in which the displacement of the hydrogen bonded to the a-carbon (transition state) occurs

S.R.M. Antunes ef al./Journal of Molecular Structure {Theochem) 357 (1995) 153-159

155

Fig. 3. Model of the (110) Sn02 plane in which the hydrogen bonded to the a-carbon interacts with the crystal oxygen until the hydroxyl forms.

Fig. 4. Model of the (I 10) SnOz plane in which the displacement of the hydroxyl proton in the direction to the crystal occurs.

S.R.M. Antunes et al./Journal of Molecular Structure (Theochem)

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357 (1995) 153-159

Fig. 5. Model of the (110) SnO, plane in which the formation of acetaldehyde occurs.

-5.589

-5.590

8

-5.591

lzl

!

‘.O)

(SnOZ)SSnOOH-

+ CH3CHOH+

8

& ii

-5.592

z -5.593 L1:

%

w

2

-5.594

e -5.595

(Sn02)7

+ CHJCHZOH c (SnOZ)BSnOOH-

-5.596 1.0

1.5

2.0

2.5

+ CH3CHOH+

3.0

DISTANCE (A) REACTION COORDINATE Fig. 6. Reaction coordinates for the exit of the hydrogen bonded to the a-carbon, forming a hydroxyl on the crystal surface.

DISTANCE

(A)

Fig. 7. Charge transfer versus displacement distance, of the hydrogen bonded to the a-carbon, to the crystal.

S.R.M. Antunes et al./Journal of Molecular Structure (Theochem)

(SnO2)6%00&

-5.596

357 (1995)

153-159

157

+ CHSCHOH+

5

E -5.599% Y,

6

E -5.599-

is a E

-5.600-

-5.602 1

0.5

(SnO2)6Sn(OH)2

1.0

1.5

2.0

+ CHXOH

2.5

DISTANCE (A) REACTION COORDINATE Fig. 8. Reaction

coordinates

for the exit of the hydroxyl

proton,

Tin oxide crystallizes in the rutile type in which each tin atom is at the center of an octahedron coordinated by six oxygen neighbors [ 131.The geometry of the (110) surface model is taken from the bulk geometry and used in all the calculations of this work. Our first model for the partial geometry optimization of adsorbed ethanol on the Sn02 surface comprises seven SnOz units (SnO&, as shown in Fig. 1. The coordinates for the atoms are: R,(SnO) = 1.990A, R2(SnO) = 2.102A, A(O-Sn-0) = 77.2”, B(O-Sn-0) = 101.9”, and C(Sn-0-Sn) = 130.2”. The ethanol molecuJe has the following coordinates: d(O;H) = 0.96A, d(C-0) = 1.32A, d(C-C) = 1.53 A, d(C-H) =

forming

acetaldehyde

and a second hydroxyl

on the crystal

surface.

1.01 A, and angles (H-O-C) = 108”,(O-C-C) = lOl”, (C-C-H) = 109.2”, and (H-C-C) = 108.4”. The displacement of the hydrogen bonded to the a-carbon is depicted in Fig. 2. The hydrogeno-carbon distance of the ethanol molecule has been selected as the reaction coordinate. Fig. 3 shows the hydroxyl formation in the crystal and Fig. 4 shows the displacement of the hydroxyl proton in the direction of the oxygen of the crystal, and finally Fig. 5 shows the formation of acetaldehyde. This Figure shows the optimized geometry of (SnO& - Sn(OH)* and CH$OH.

158 3.

S.R.M. Antunes et al./Journal of Molecular Structure (Theochem) 357 (1995) 153-159

Results and discussion

1"'

.



“.

‘.

0.9

In Fig. 6 the minimum energy profile for the decomposition process has been depicted. The displacement of the hydrogen-a-carbon of the ethanol molecule occurs in the direction of the crystal (Fig. 2). This process needs 520 kJmol_’ to overcome the energy barrier and it corresponds to an exothermic reaction (Fig. 6). The evolution of charge transfer is depicted in Fig. 7. In the reactants the negative charge is 0.20 a.u., whereas at the top of this barrier (transition structure) 0.89 a.u. of electronic charge has been transferred from ethanol toward the surface. In the product CH&OH+ the total charge transfer is 0.69 a.u. The system suffers a strong polarization due to the formation of hydroxyl and carbocation on the surface. This effect can be used to detect the presence of the ethanol molecule on Sn02, acting as a sensor. According to the sequence represented in Figs. 2 and 5 and after the transition state, the system stabilizes with an energy gain of 149 kJmol_’ (Fig. 6), which is an exothermic reaction. The polarization of the system decreases due to structural rearrangement between the carbocation and the crystalline structure. A new hydrogen transfer has been analyzed by means of the characterization of the minimum energy profile, selecting the hydrogen-a-carbon distance of the carbocation as reaction coordinate (Fig. 4). In Fig. 5 a schematic representation of acetaldehyde formation is given. The energy barrier is 186 kJ/mol and the process corresponds to an exothermic reaction (Fig. 8). The analysis of charge transfer is depicted in Fig. 9, being 0.49a.u. for the total positive charge transfer from the carbocation to the surface in the production of acetaldehyde. This effect can be used to detect the presence of the carbocation. The above theoretical results shown in Figs. 7 and 9 can explain the different signals produced by sensors when several interactions between crystal structure and gas molecules occur. These signals are observed when a chemical interconversion takes place due to opposite charge transfer processes. This signal can be used to detect the presence of the carbocation, and then the crystal acts as sensor for ethanol. The (SnO& surface model acts as a catalyst

Ij- 0.8

25

E

IL 0.7 UI T OL I- 0.6 z 4 0

0.5

0.4 0.8

1.2

1.6

2.0

DISTANCE

(A)

2.4

2.8

Fig. 9. Charge transfer versus displacement distance, of the proton bonded to an ethanol hydroxyl, to the crystal.

toward the ethanol gas molecules. The two reaction steps studied in this work are described as follows: (SnO&SnOO

+CH3CH20H

+ CH,-CHOH-

+ (SnO-&SnOOH-

DE = - 149 kJ/mol

703

r.

A acetaldehyde I water

450 TEMPERATURE

(“C 1

Fig. 10. Yield of acetaldehyde and water after interaction of the ethanol molecule with the SnO, surface.

S.R.M. Antunes et al./Journal of Molecular Structure (Theochemj

(Sn02)&3n00H-

+ CHs-CHOH

x (Sn0&Sn(OH)2

-+

+ CHs-COH

DE = -440 kJ/mol Fig. 10 shows the experimental yield resulting from the ethanol interaction with the Sn02 surface. The gases resulting from these interactions are acetaldehyde and water, and were detected by using a gas chromatograph attached to a furnace [9]. It is observed in this Figure that the acetaldehyde yield is higher than that for water and reached 65% at 400°C in accordance with theoretical results.

4. Conclusions This work presents the semiempirical MNDO study of ethanol decomposition with water and acetaldehyde formation using (SnO& to model the (110) surface of SnOz. The results of the minimum energy profile study show that: (i) the physical adsorption of the ethanol molecule occurs without charge transfer in the first step of the reaction; (ii) the chemical adsorption with the formation of the carbocation transfers 0.69 a.u. from the ethanol to the Sn02 model; (iii) the

357 (1995) 153-159

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formation of acetaldehyde is followed by a charge transfer of 0.49 a.u. from the SnOz model to the carbocation; (iv) energy barriers of 520 and 186 kJ/mol were determined for the first and second proton transfers, respectively.

References 111W.N. Brattain and J. Bardeen, Bell Syst. Tech. J., 32 (1953) 1. PI T. Maekawa, J. Tamaki, N. Miura, N. Yamazoe and S. Matsushima, Sensors Actuators, B9 (1992) 63. 131G. Chiotti, A. Chiorino, W.X. Pan and L. Marchese. Sensors Actuators, B7 (1992) 691. [41 K.D. Schierbaum, V. Weimar and W. Gopel, Sensors Actuators, B7 (1992) 709. PI S. Matsuhima, T. Maekawa, J. Tamaki, N. Miura and N. Yamazoe, Sensors Actuators, B9 (1992) 71. PI T. Wolkenstein, Adv. Catal., 12 (1960) 189. [71 R.J. Stetter, J. Colloid Interface Sci., 65 (1978) 4321. PI W. Hangen, R.E. Lambrich and J. La Gois, Adv. Solid State Phys., 23 (1983) 259. 191G. Heiland and D. Kohl, Sensors Actuators, 1 (1985) 25. 1101M.J.S. Dewar and W. Thiel, J. Am. Chem. Sot., 99 (1977) 4899. 1111J.J.P. Stewart, Quantum Chem. Prog. Exch. Bull., 3 (1983) 43. WI T. Oshiro, C.K. Lutrus, D.E. Hagen, S. Beth and S.H.S. Salk, Solid State Commun., 87 (1993) 801. u31 R.W.C. Wyckoff, Crystal Structures, Vol. 1, Interscience, New York, 1963.