- Email: [email protected]
Ab initio study on the adsorption and dissociation process of methanol on a silver surface
THEO CHEM ELSEVIER
Journal of Molecular Structure (Theochem) 422 (1998) 191- 195
Ab initio study on the adsorption and dissociation process of methanol on a silver surface Kangnian Fan *, Bairong Shen, Wenning Wang, Jingfa Deng Department
ofChemistry,
Fudan
University,
Shanghai
200433,
China
Received 8 November 1996: accepted IO March I997
Abstract The geometries of methanol adsorbed on an oxygen-free silver surface, a promoted silver surface and an oxygen preadsorbed silver surface were optimized at the MP2 level and the energies were calculated at the MP4 level. Our calculations showed that weak physisorption of methanol occurs on the clean silver surface, but stable molecular chemisorption occurs in the other two cases. The adsorption and dissociation process of methanol was postulated to occur via two pathways, i.e. the EleyRideal mode and the Langmuir-Hinshelwood mode. The calculations also showed that the presence of atomic oxygen at a silver surface is essential for the cleavage of the O-H bond in the methanol. The dissociation of methanol in the LangmuirHinshelwood mode has a small energy barrier but has no energy barrier in the Eley-Rideal mode. 0 1998 Elsevier Science B.V. Keywords:
Ab Initio; Methanol; Adsorption
and dissociation;
1. Introduction The partial oxidation of methanol to formaldehyde is an important chemical industrial process. Some studies concerning its mechanism have been reported [l-3]. However, views on its mechanism are divergent. Since quantum chemistry calculation is a powerful tool to investigate microprocesses at the molecular level, we made recourse to it to investigate this reaction. The partial oxidation of ethylene over a silver surface is an important chemical industrial process catalyzed by silver. At present, a few theoretical works about this process can be found in the literature [4,S]. However, no theoretical work about the partial oxidation of methanol to formaldehyde on a silver surface has been found. In this paper, we report an * Corresponding author. 0166.1280/97/$17.00
Silver surface
ab initio calculation for the adsorption and dissociation process of methanol on a silver surface, which is the first step in the partial oxidation of methanol to formaldehyde by a silver catalyst. Theoretical calculations about adsorbate-metal interactions can be done using several models [613] such as the cluster model, the embedded cluster model and the dipped adcluster model, etc. The cluster model method is a common method used effectively in the calculation of catalysis systems [14]. The computational procedures can be fairly rigorous. It can model the metal surface in a cluster containing one or several metal atoms, even if the surface of the cluster has been severely truncated. Important qualitative features can be modeled since the effective interaction radii of adsorbed molecules are in the order of a few ingstriims. In this paper, we consider the reactions on the “surface” of an Agz
0 1998 Elsevier Science B.V. All rights reserved.
PII SOl66-l280(97)00100-0
192
K. Fan et ul./Journal
of MolecularStructure
with a Ag-Ag distance fixed to 2.89 A,, an observed distance in the f.c.c. crystal of silver [21]. The AgAg-0 angle was fixed to 90”, in order to model a high coverage surface by atomic oxygen.
cluster and the “surface” of an Agl-0 cluster, the latter representing the surface with pre-adsorbed oxygen. The catalytic reactions on the above surfaces were modeled in our laboratory. It is reported [ 151 that Ag+ are the active sites on the catalyst surface. It is believed that factors which can stabilize the ionic state of Ag and increase the effective charge on Ag+ could also improve the catalytic properties of silver in the syntheses of HCHO [ 15]a-c. In a real reaction system, the positive charge at the silver surface may be caused by promoters or subsurface oxygen [16]. McKee [4] took Agt as a promoted model to study the interaction of 01 and C2H3 on the silver surface. In this paper, we take Ag; or Agf’ dimer as a model to investigate the effect of promoters in the methanol oxidation process.
2. Computational
(Them-hem) 422 11998) 191-195
3. Results and discussion 3.1. Adsorption suq%ce
The 36 core electrons of Ag were replaced by a relativistic effective core potential generated by Hay and Wadt 1171. A (3s 3p 4d) primitive Gaussian basis set contracted to (2s 2p 2d) was used to represent the valance shell of the Ag atom. A (9s 5p/3s 2p) basis set was used for C and 0 with a single set of polarization functions (fC., = 0.75, {O.d= 0.85) and a (4~12s) basis set was used for H [ 181. All calculations were carried out at post HF levels by the use of the programs GAMES [ 191 and GAUSSIAN 92 for Windows [20]. The optimized geometries were obtained at the MP2 level and total energies were calculated at the MP4 level. Considering the real situation of the surface, optimization was carried out
ita5
44
(A) Fig. 1.Models of methanol approaching promoted silver surface Agp.
the oxygen-free
silver
Models of methanol approaching the oxygen-free silver surface with and without promoter are sketched in Fig. 1. The optimized geometries and adsorption energies are listed in Table 1. The calculated results of the geometry of CH30H in Table 1 show that the result at the MP2 level agrees with the experimental data better than that at the HF level. It also implies that calculations at the MP2 level are effective enough for the optimization of the geometries in our system. Data from Table 1 indicate the following. (1) On an oxygen-free silver surface without promoters, methanol is weakly adsorbed in the form of physisorption. In this situation, the Ag-0 distance is 2.591 A at the MP2 level and the adsorption energy is 5.8 kcal mol-’ at the MP4 level. This agrees with the experimental results [ 1,231. (2) When the silver surface is positively charged by a promoter, the Ag-0 distance decreases and the adsorption energy increases depending on the magnitude of the charge. This indicates that the positively charged silver surface can react with methanol via a Lewis acidbase interaction, which increases the overall adsorption probability for methanol on silver surface.
details
f%
of methanol on the oxygen-free
tQ5
ps-
4
0% silver surface: (A) unpromoted
+I.0
+I.0
4-
4 69
silver surface; (B 1) promoted silver surface Agt: (B2)
of Molecular Structure (Theochem) 422 (1998) 191-195
K. Fun et al./Journal Table I Optimized
geometries
and adsorption
energies of methanol adsorbed on oxygen-free
CH,OH
iR(Ag-0) R(O-H) R(C-0) R(C-H) L(OCH) L(AgOC) I(COH) E, (hlP2) E,’ (MP4)
HF
MP2
exp. [22]
0.946 1.404 I .082- I.088 107.1-111.8
0.97 I 1.43 I 1.094-1.101 106.2-I 12.0
0.967 1.421 1.093 107.0
109.7
108.0
108.0
’ Bond lengths in bngstrams,
193
silver surface ’
A
BI
B2
MP2
MP2
MP2
2.59 I 0.977 I .439 I .090 109.7 162.8 109.0 5.6 5.8
2.366 0.974 I .462 1.094 108.8 130.7 107.8 19.9 20.4
2.21 I 0.976 I .498 1.092 107.8 119.0 106.6 47.8 48.3
bond angles in degrees, energies in kcal mol.’
3.2. Adsorption and dissociation of methanol on the oxygen pre-adsorbed silver su$ace The adsorption and dissociation process of methanol on an oxygen pre-adsorbed surface was investigated with two different dissociation modes sketched in Fig. 2. In one mode, the methanol in a gas phase or in a Van der Waals layer on the catalyst reacts directly with the oxygen adsorbed on the silver surface. This pathway is called the Eley-Rideal (E-R) mode. The other mode we investigated is a
surface reaction in which the oxygen adsorbed on the silver surface attacks the methanol adsorbed adjacent to it on the surface. This mode is called the Langmuir-Hinshelwood (L-H) mode. Optimized geometries, adsorption energies of methanol and its dissociated products adsorbed on an oxygen preadsorbed silver surface are listed in Table 2. The potential energy diagram of the dissociation process of methanol over a silver surface is depicted in Fig. 3. Two reaction mechanisms are shown in Fig. 3. The first is the L-H mechanism, in which methanol is in a
-c\o __.---
H
I-I .____._
I
“‘0
I
Agl
Ag2 -H
(C)
y
\ H-
y : /”
PH \
“‘0
H---C ‘0. ““‘H.,,,
/../ b
I
Agl
Ag2 (D) Fig. 2. Mode1 for L-H and E-R modes
I+‘2
K. Fan et al.Nournal of Molecular Structure (Theochem) 422 (1998) 191-195
194
Table 2 Optimized
geometries
and adsorption
energies of methanol adsorbed on oxygen pre-adsorbed
C(L-H)
TS(L-H)
MP2
2.25 1 1.896 1.037 1.903 1.435 1.097 109.4 155.8 110.5 36.8 33.1
WV-Q R(Ag’-0’) R(O-H) R(O’-H) R(C-0) R(C-H) Z(OCH) L(AgOC) L(COH) E, E,’ (MP4) ’ Bond lengths in angstroms,
silver surface ’
MP2
P(L-H
2.144 I.996 1.425 1.481 1.422 1.100 110.4 151.9 115.7 29.0 23.9
and E-R) MP2
2.065 2.006 1.947 0.996 1.416 1.105 lH.5 127.7 125.7 52.0 41.8
bond angles in degrees, energies in kcal mol-’
stable chemisorption situation on the oxygen preadsorbed silver surface with a Ag-0 distance of 2.251 A at the MP2 level and an adsorption energy of 33.1 kcal mol-’ at the MP4 level. It is a precursor in this dissociation mechanism and the adsorbed methanol can react with the pre-adsorbed oxygen,
dissociating into methoxide and hydroxyl group via a small energy barrier of about 9.2 kcal mol-‘. The total dissociation adsorption energy is about 41.8 kcal mol-’ at the MP4 level. According to the second mechanism, which is the E-R mechanism, methanol can react directly with pre-adsorbed oxygen
Ag
MP4 ( 56.6 )
HJC
‘?HH __--
Agz+CHsOH A$ Ag . A g 2 0 + C H 3 0 H ‘,==-..._ +MP4(-B.B)__.------. . A -. .\ -. -. . -. . . -. . ---.,E
L 4-l:‘:.
64 (-3 3.1 ) MP2 (-36.8)
---
___-----H3C \O_--H-._O I
Ag - ’
-R -.
.* ** \. ,. ** \x ,t ,* ,t ‘\
_.-_.--__-*
AL
MP4 (-23.9) :.TS ‘. ** :* -. MP2 ‘.;*, .* _I. T--a? :o ) y,, . -_-_-I--- - \ -. _‘\ ‘\ -\ -.*\ ‘. .. . -,. * \. ‘\\ \ \ . \ \
-.
..
-.
)
MP4 (-4 1 .8)
\,
MP2 (-52.0) P
Fig. 3. Energy diagram of adsorption
and dissociation
of methanol over silver surface at MP4 level
K. Fan et al./Journal
qf Molecular
Structure
on the silver surface, dissociating into methoxide and hydroxyl without any precursor. These mechanisms have the same dissociation products and dissociation adsorption energy. In the AgzO model system, the calculated charge on the silver atom (Agl) with no adsorbed oxygen is +0.25. When the molecular methanol in gas phase approaches the silver surface, the electron-deficient silver atom (Ag*‘) reacts with the oxygen atom of the methanol molecule while a surface oxygen atom may react with a hydrogen atom of the hydroxyl of the methanol molecule. The former is a kind of Lewis acid-base interaction. It causes the adsorption of methanol while the latter is a kind of Bronsted acid-base interaction [24] and leads to the dissociation of the O-H bond of methanol. In the L-H mode, the adsorption of methanol and the cleavage of O-H bond take place through two consecutive steps, but they are synchronous steps in the E-R mode. Both dissociation processes of E-R mode and L-H mode may occur on the real reaction system [ 1,251. It is very difficult to distinguish between the two modes by current experiments, since the reaction energy barrier of 9.2 kcal mall’ for the L-H mechanism is very small. Our calculation also showed that it is very difficult for methanol to be dissociated on an oxygen-free surface. The calculated dissociation adsorption energy in our model system is - 56.8 kcal mall’ at the MP4 level (see Fig. 3). However, methanol is very easily dissociated into methoxide and hydrogen on the oxygen pre-adsorbed silver surface. In the latter process, the pre-adsorbed atomic oxygen itself is a Bronsted base center. It converts a neighboring silver atom (Ag2) into a Lewis acid center. Therefore, the preadsorbed atomic oxygen is essential for O-H bond cleavage. This result agrees with experiment [23].
4. Conclusions Our calculations at MP2 and MP4 levels indicate the following. (1) On a clean silver surface, methanol is weakly absorbed in the form of physisorption. (2) If a silver surface preadsorbs oxygen atoms or is positively charged by promoter, methanol can undergo a stable form of molecular chemisorption. (3) The dissociation of methanol can occur via two
(Theochem)
422
(I 998)
191-l
95
195
pathways. One is the E-R mode without a energy barrier, the other is the L-H mode which has a small energy barrier. (4) The presence of atomic oxygen is essential for O-H bond cleavage.
Acknowledgements This work was supported by the National Science Foundation of P. R. China.
Natural
References [I] I.E.Wachs, R-l. Madix, Surf. Sci. 76 (1978) 531. [2] X. Bao, J. Deng, .I. Catal. 99 (1986) 391. [3] X. Bao, J. Deng, R. Zhai, D. Wang, X. Guo, Catal. Lett. 4 (1990) 25. [4] M.L. McKee, J. Chem. Phys. 87 (1987) 3143. [5] E.A. Carter, W.A. Goddard III, Surf. Sci. 209 (1989) 243. [6] J.W. Gadznk, Surf. Sci. 43 (1974) 44. [7] D. Post, E.J. Baerends, Surf. Sci. 109 (1981) 167. [8] M. Simonetta, Int. J. Quantum Chem. 29 (1986) 1555. [9] C.W. Bauschlicher, Chem. Phys. Lett. 129 (1986) 586. [IO] R.P. Messmer, Surf. Sci. I58 ( 1985) 40. [I I] P.V. Madhaven, J.L. Whitten, Surf. Sci. I I2 (1981) 38. [I21 H. Nakatsuji, H. Nakai, Can. J. Chem. 70 (1992) 404. 1131 H. Nakatsuji, H. Nakai, Y. Fukumishi. J. Chem. Phys. 95 (1991) 640. 1141 J. Sauer, Chem. Rev. 89 (1989) 199. [ 151 (a) A.N. Pestryakov, A.A. Davydov, Kinet. Katal 35 (1994) 278: (h) W. Dai, Y. Cao, H. Li, J. Deng, Chem. Lett.. 3 (1997) 197; (c) Y. Cao, W. Dai, J. Deng, Appl. Catal. A: General, in press. [ 161 H.A. Engelhardt, D. Menzel, Surf. Sci. 57 (1976) 591. [17] P.J. Hay, W.R. Wadt, J. Chem. Phya. 82 (1985) 270. [ 181 T.H.Dunning, Jr., P.J. Hay, in: H.F. Schaefer, III (ed.), Modern Theoretical Chemistry, vol. 3, Plenum, New York, 1977. [I91 M.W. Schimdt, K.K. Baldridge, J.A. Boatr, J.H. Jesen, S. Koseki, M.S. Gordon, K.A. Nguyen, T.L. Windus. S.T. Elhert. New version of GAMES QCPE Bull. IO (1990) 52. [20] M.J. Frish, G.W. Wong, J.B. Foreman, M.A. Rohh, M. HeadGordon. E.S. Replogle, R. Gomperts, J.L. Andrews, K. Raghavachari. J.S. Binkly, C. Gonzalez. R.L. Martin, D.J. Defrees, J. Baker, J.J.P. Stewart, J.A. Pople (eds.), GAUSSIAN ~Z/DFT. Revision F.2. Gaussian, Pittsburgh, PA. 1993. [2l] A. Taylor, B.J. Kagel, Crystallographic Data in Mate1 and Alloy Stucture, Dover Publications, New York, 1963. [22] D.J. Defrees, B.A. Levi, S.K. Pollack, W.J. Hehre, J.S. Binkly, J.A. Pople, J. Am. ChemSoc. IO1 (1979) 4085. [23] T.E. Felter, W.H. Weinberg. G.Y. Lastushkina. P.A. Boreskol, J. Appl. Surf. Sci. I6 (1983) 351. [24] M.A. Barteau. R.J. Madix, Surf. Sci. 120 (1982) 262. [25] C.T. Rettner. D.J. Auerbach, Science 263 (1994) 365.
Journal of Molecular Structure (Theochem) 422 (1998) 191- 195
Ab initio study on the adsorption and dissociation process of methanol on a silver surface Kangnian Fan *, Bairong Shen, Wenning Wang, Jingfa Deng Department
ofChemistry,
Fudan
University,
Shanghai
200433,
China
Received 8 November 1996: accepted IO March I997
Abstract The geometries of methanol adsorbed on an oxygen-free silver surface, a promoted silver surface and an oxygen preadsorbed silver surface were optimized at the MP2 level and the energies were calculated at the MP4 level. Our calculations showed that weak physisorption of methanol occurs on the clean silver surface, but stable molecular chemisorption occurs in the other two cases. The adsorption and dissociation process of methanol was postulated to occur via two pathways, i.e. the EleyRideal mode and the Langmuir-Hinshelwood mode. The calculations also showed that the presence of atomic oxygen at a silver surface is essential for the cleavage of the O-H bond in the methanol. The dissociation of methanol in the LangmuirHinshelwood mode has a small energy barrier but has no energy barrier in the Eley-Rideal mode. 0 1998 Elsevier Science B.V. Keywords:
Ab Initio; Methanol; Adsorption
and dissociation;
1. Introduction The partial oxidation of methanol to formaldehyde is an important chemical industrial process. Some studies concerning its mechanism have been reported [l-3]. However, views on its mechanism are divergent. Since quantum chemistry calculation is a powerful tool to investigate microprocesses at the molecular level, we made recourse to it to investigate this reaction. The partial oxidation of ethylene over a silver surface is an important chemical industrial process catalyzed by silver. At present, a few theoretical works about this process can be found in the literature [4,S]. However, no theoretical work about the partial oxidation of methanol to formaldehyde on a silver surface has been found. In this paper, we report an * Corresponding author. 0166.1280/97/$17.00
Silver surface
ab initio calculation for the adsorption and dissociation process of methanol on a silver surface, which is the first step in the partial oxidation of methanol to formaldehyde by a silver catalyst. Theoretical calculations about adsorbate-metal interactions can be done using several models [613] such as the cluster model, the embedded cluster model and the dipped adcluster model, etc. The cluster model method is a common method used effectively in the calculation of catalysis systems [14]. The computational procedures can be fairly rigorous. It can model the metal surface in a cluster containing one or several metal atoms, even if the surface of the cluster has been severely truncated. Important qualitative features can be modeled since the effective interaction radii of adsorbed molecules are in the order of a few ingstriims. In this paper, we consider the reactions on the “surface” of an Agz
0 1998 Elsevier Science B.V. All rights reserved.
PII SOl66-l280(97)00100-0
192
K. Fan et ul./Journal
of MolecularStructure
with a Ag-Ag distance fixed to 2.89 A,, an observed distance in the f.c.c. crystal of silver [21]. The AgAg-0 angle was fixed to 90”, in order to model a high coverage surface by atomic oxygen.
cluster and the “surface” of an Agl-0 cluster, the latter representing the surface with pre-adsorbed oxygen. The catalytic reactions on the above surfaces were modeled in our laboratory. It is reported [ 151 that Ag+ are the active sites on the catalyst surface. It is believed that factors which can stabilize the ionic state of Ag and increase the effective charge on Ag+ could also improve the catalytic properties of silver in the syntheses of HCHO [ 15]a-c. In a real reaction system, the positive charge at the silver surface may be caused by promoters or subsurface oxygen [16]. McKee [4] took Agt as a promoted model to study the interaction of 01 and C2H3 on the silver surface. In this paper, we take Ag; or Agf’ dimer as a model to investigate the effect of promoters in the methanol oxidation process.
2. Computational
(Them-hem) 422 11998) 191-195
3. Results and discussion 3.1. Adsorption suq%ce
The 36 core electrons of Ag were replaced by a relativistic effective core potential generated by Hay and Wadt 1171. A (3s 3p 4d) primitive Gaussian basis set contracted to (2s 2p 2d) was used to represent the valance shell of the Ag atom. A (9s 5p/3s 2p) basis set was used for C and 0 with a single set of polarization functions (fC., = 0.75, {O.d= 0.85) and a (4~12s) basis set was used for H [ 181. All calculations were carried out at post HF levels by the use of the programs GAMES [ 191 and GAUSSIAN 92 for Windows [20]. The optimized geometries were obtained at the MP2 level and total energies were calculated at the MP4 level. Considering the real situation of the surface, optimization was carried out
ita5
44
(A) Fig. 1.Models of methanol approaching promoted silver surface Agp.
the oxygen-free
silver
Models of methanol approaching the oxygen-free silver surface with and without promoter are sketched in Fig. 1. The optimized geometries and adsorption energies are listed in Table 1. The calculated results of the geometry of CH30H in Table 1 show that the result at the MP2 level agrees with the experimental data better than that at the HF level. It also implies that calculations at the MP2 level are effective enough for the optimization of the geometries in our system. Data from Table 1 indicate the following. (1) On an oxygen-free silver surface without promoters, methanol is weakly adsorbed in the form of physisorption. In this situation, the Ag-0 distance is 2.591 A at the MP2 level and the adsorption energy is 5.8 kcal mol-’ at the MP4 level. This agrees with the experimental results [ 1,231. (2) When the silver surface is positively charged by a promoter, the Ag-0 distance decreases and the adsorption energy increases depending on the magnitude of the charge. This indicates that the positively charged silver surface can react with methanol via a Lewis acidbase interaction, which increases the overall adsorption probability for methanol on silver surface.
details
f%
of methanol on the oxygen-free
tQ5
ps-
4
0% silver surface: (A) unpromoted
+I.0
+I.0
4-
4 69
silver surface; (B 1) promoted silver surface Agt: (B2)
of Molecular Structure (Theochem) 422 (1998) 191-195
K. Fun et al./Journal Table I Optimized
geometries
and adsorption
energies of methanol adsorbed on oxygen-free
CH,OH
iR(Ag-0) R(O-H) R(C-0) R(C-H) L(OCH) L(AgOC) I(COH) E, (hlP2) E,’ (MP4)
HF
MP2
exp. [22]
0.946 1.404 I .082- I.088 107.1-111.8
0.97 I 1.43 I 1.094-1.101 106.2-I 12.0
0.967 1.421 1.093 107.0
109.7
108.0
108.0
’ Bond lengths in bngstrams,
193
silver surface ’
A
BI
B2
MP2
MP2
MP2
2.59 I 0.977 I .439 I .090 109.7 162.8 109.0 5.6 5.8
2.366 0.974 I .462 1.094 108.8 130.7 107.8 19.9 20.4
2.21 I 0.976 I .498 1.092 107.8 119.0 106.6 47.8 48.3
bond angles in degrees, energies in kcal mol.’
3.2. Adsorption and dissociation of methanol on the oxygen pre-adsorbed silver su$ace The adsorption and dissociation process of methanol on an oxygen pre-adsorbed surface was investigated with two different dissociation modes sketched in Fig. 2. In one mode, the methanol in a gas phase or in a Van der Waals layer on the catalyst reacts directly with the oxygen adsorbed on the silver surface. This pathway is called the Eley-Rideal (E-R) mode. The other mode we investigated is a
surface reaction in which the oxygen adsorbed on the silver surface attacks the methanol adsorbed adjacent to it on the surface. This mode is called the Langmuir-Hinshelwood (L-H) mode. Optimized geometries, adsorption energies of methanol and its dissociated products adsorbed on an oxygen preadsorbed silver surface are listed in Table 2. The potential energy diagram of the dissociation process of methanol over a silver surface is depicted in Fig. 3. Two reaction mechanisms are shown in Fig. 3. The first is the L-H mechanism, in which methanol is in a
-c\o __.---
H
I-I .____._
I
“‘0
I
Agl
Ag2 -H
(C)
y
\ H-
y : /”
PH \
“‘0
H---C ‘0. ““‘H.,,,
/../ b
I
Agl
Ag2 (D) Fig. 2. Mode1 for L-H and E-R modes
I+‘2
K. Fan et al.Nournal of Molecular Structure (Theochem) 422 (1998) 191-195
194
Table 2 Optimized
geometries
and adsorption
energies of methanol adsorbed on oxygen pre-adsorbed
C(L-H)
TS(L-H)
MP2
2.25 1 1.896 1.037 1.903 1.435 1.097 109.4 155.8 110.5 36.8 33.1
WV-Q R(Ag’-0’) R(O-H) R(O’-H) R(C-0) R(C-H) Z(OCH) L(AgOC) L(COH) E, E,’ (MP4) ’ Bond lengths in angstroms,
silver surface ’
MP2
P(L-H
2.144 I.996 1.425 1.481 1.422 1.100 110.4 151.9 115.7 29.0 23.9
and E-R) MP2
2.065 2.006 1.947 0.996 1.416 1.105 lH.5 127.7 125.7 52.0 41.8
bond angles in degrees, energies in kcal mol-’
stable chemisorption situation on the oxygen preadsorbed silver surface with a Ag-0 distance of 2.251 A at the MP2 level and an adsorption energy of 33.1 kcal mol-’ at the MP4 level. It is a precursor in this dissociation mechanism and the adsorbed methanol can react with the pre-adsorbed oxygen,
dissociating into methoxide and hydroxyl group via a small energy barrier of about 9.2 kcal mol-‘. The total dissociation adsorption energy is about 41.8 kcal mol-’ at the MP4 level. According to the second mechanism, which is the E-R mechanism, methanol can react directly with pre-adsorbed oxygen
Ag
MP4 ( 56.6 )
HJC
‘?HH __--
Agz+CHsOH A$ Ag . A g 2 0 + C H 3 0 H ‘,==-..._ +MP4(-B.B)__.------. . A -. .\ -. -. . -. . . -. . ---.,E
L 4-l:‘:.
64 (-3 3.1 ) MP2 (-36.8)
---
___-----H3C \O_--H-._O I
Ag - ’
-R -.
.* ** \. ,. ** \x ,t ,* ,t ‘\
_.-_.--__-*
AL
MP4 (-23.9) :.TS ‘. ** :* -. MP2 ‘.;*, .* _I. T--a? :o ) y,, . -_-_-I--- - \ -. _‘\ ‘\ -\ -.*\ ‘. .. . -,. * \. ‘\\ \ \ . \ \
-.
..
-.
)
MP4 (-4 1 .8)
\,
MP2 (-52.0) P
Fig. 3. Energy diagram of adsorption
and dissociation
of methanol over silver surface at MP4 level
K. Fan et al./Journal
qf Molecular
Structure
on the silver surface, dissociating into methoxide and hydroxyl without any precursor. These mechanisms have the same dissociation products and dissociation adsorption energy. In the AgzO model system, the calculated charge on the silver atom (Agl) with no adsorbed oxygen is +0.25. When the molecular methanol in gas phase approaches the silver surface, the electron-deficient silver atom (Ag*‘) reacts with the oxygen atom of the methanol molecule while a surface oxygen atom may react with a hydrogen atom of the hydroxyl of the methanol molecule. The former is a kind of Lewis acid-base interaction. It causes the adsorption of methanol while the latter is a kind of Bronsted acid-base interaction [24] and leads to the dissociation of the O-H bond of methanol. In the L-H mode, the adsorption of methanol and the cleavage of O-H bond take place through two consecutive steps, but they are synchronous steps in the E-R mode. Both dissociation processes of E-R mode and L-H mode may occur on the real reaction system [ 1,251. It is very difficult to distinguish between the two modes by current experiments, since the reaction energy barrier of 9.2 kcal mall’ for the L-H mechanism is very small. Our calculation also showed that it is very difficult for methanol to be dissociated on an oxygen-free surface. The calculated dissociation adsorption energy in our model system is - 56.8 kcal mall’ at the MP4 level (see Fig. 3). However, methanol is very easily dissociated into methoxide and hydrogen on the oxygen pre-adsorbed silver surface. In the latter process, the pre-adsorbed atomic oxygen itself is a Bronsted base center. It converts a neighboring silver atom (Ag2) into a Lewis acid center. Therefore, the preadsorbed atomic oxygen is essential for O-H bond cleavage. This result agrees with experiment [23].
4. Conclusions Our calculations at MP2 and MP4 levels indicate the following. (1) On a clean silver surface, methanol is weakly absorbed in the form of physisorption. (2) If a silver surface preadsorbs oxygen atoms or is positively charged by promoter, methanol can undergo a stable form of molecular chemisorption. (3) The dissociation of methanol can occur via two
(Theochem)
422
(I 998)
191-l
95
195
pathways. One is the E-R mode without a energy barrier, the other is the L-H mode which has a small energy barrier. (4) The presence of atomic oxygen is essential for O-H bond cleavage.
Acknowledgements This work was supported by the National Science Foundation of P. R. China.
Natural
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