- Email: [email protected]
A theoretical investigation of methane dissociation on Rh(111)
14 March 1997
CHEMICAL PHYSICS LETTERS ELSEVIER
Chemical
Physics Letters 267 ( 1997) 44-50
A theoretical investigation of methane dissociation on Rh( 111) Chak Tong Au, Meng-Sheng
Liao I, Ching Fai Ng
Department of Chemistry, Hong Kong Baptist University, Kowloon Tong, Hong Kong Received 29 October
1996; in final form 16 December
1996
Abstract Methane dissociation (CH4 -+ CH, + (4 - x) H -+ C + 4H) over a cluster model of the Rh( 111) surface has been investigated using a quasi-relativistic density functional method. The main aim is to evaluate the dissociation energies which
may determine catalytic activity of the metal in the partial oxidation of methane to syngas. Dissociative adsorption of methane can be initiated on the Rh surface. However, the abstraction of H from CH4 to produce gas-phase CHs,, radical is energetically unfavorable. This would explain the absence of CZ products. The presence of adsorbed oxygen at an on-top site promotes methane dehydrogenation while that at a hollow site exerts the opposite effect. All calculated dissociation energies of CH,,s are compatible with those obtained with the bond-order conservation Morse-potential approach. The combination and desorption of surface species, which are critical to the formation of syngas, are also discussed. @ 1997 Elsevier Science B.V.
1. Introduction The catalytic oxidation of methane to syngas (denoted as OMS hereafter) has attracted increased interest in recent years. Oxide-supported Rh has been shown to be an effective catalyst for OMS [ l-101. Some of the first mechanistic studies were done by Hickman and Schmidt [l-3]. The dissociation of methane was regarded as an initial step for syngas formation, and the methane pyrolysis mechanism [see reactions ( 1) -( 5) below] was suggested based on experiments over monolith-supported Rh at short contact time. Later, Buyevskaya et al. [4, 51 studied reaction steps in OMS over AlzOs-supported Rh by using transient pulse technique and in situ DRIFTS (diffuse reflectance infrared Fourier transform spectroscopy). A temporal analysis of the prod-
ucts indicated a direct oxidation initiated by methane dissociation, reaction ( 1) , although the authors suggested a different step for the formation of CO (i.e. C, + CO2,s --+ 2CO). Recently, we investigated the reaction mechanisms of OMS over reduced Rh/SiO;! and oxidized Rh( 0) /SiO2 catalysts over a pulse microreactor [ 61. It was demonstrated that the partial oxidation reaction of CH4 follows the methane pyrolysis mechanism. It was concluded that: (1) Metallic rhodium is active for methane decomposition; (2) Chemisorbed oxygen species from rhodium oxide and SiO2 have participated in the methane oxidation reaction; (3) Methane dissociation is a key reaction step. The possible reaction steps over the reduced Rh catalyst are as follows (g = gas, s = surface) : CH++site-+CH,,,+(4-x)H,+C,+4H,, (1)
’On leave from: Department Xiamen, Fujian 361005,
of Chemistry,
Xiamen University,
China.
0009.2614/97/$17.00 @ 1997 Elsevier Science B.V. All rights reserved. fIISOOO9-2614(97)00076-6
c, +o,
--+ co,
+ co,,
(2)
CT
Hs + Hs --+ H2.s
+
H2z3.
Au et al. /Chemical
Physics Letters
(3)
By-products (COT, H20) may be generated according to the reactions: co,
+ 0, ---t co2.q-t
co2,,,
(4)
0, + 2H,-+ H20,+
H20,.
(5)
The same mechanism has also been applied to explain results of pulse experiments obtained over reduced Ni/AlzOj [ 11, 121 and Ni/LazOj [ 13-151 catalysts. According to reaction ( 1) , the breakage of a C-H bond in methane is the first step in the OMS reaction. This is similar to the OCM reaction in which the methyl radical formation has been reckoned as the initial step for C2 hydrocarbon generation. In the OMS reaction, however, the methyl radicals remain chemisorbed on the surface and subsequently undergo further hydrogen abstractions. In other words, CO and HZ are formed via a surface intermediate species CH,,,. In this letter, we attempt to perform a quasirelativistic density functional (DF) investigation of methane dissociation on Rh( 111) following the mechanism given above. Different mechanisms exist in the literature [ 7-101. It should be pointed out that the OMS reaction steps may depend on other factors (choice of metal, nature of support, reaction conditions, reactor or contact time), but they are not the topics of the present concern. Previously, the activation and dissociation energies for methane dissociation on Rh( 111) with and without the involvement of surface oxygens were obtained by means of the semiempirical bond-order conservation Morse-potential (BOC-MP) approach [ 61. In the investigation, different surface oxygens at on-top, bridge and hollow sites were examined. These BOC-MP results are compared to the ‘pure’ theoretical results based on a cluster model. Because it is difficult to locate a transition state H, on the metal for the reaction CH,,, -+ CH,_t,,+ surface, we focus mainly on an evaluation of the dissociation energies for the CH, species (X = 4,3,2, 1). The BOC-MP results revealed a certain correlation between the activation and dissociation energies for reaction ( 1) , viz. a more negative dissociation energy corresponds to a lower activation energy. In order to have some ideas about barrier heights, we evaluated activation energies for some simple cases. The related
267 (I 997) 44-50
45
combining and desorption properties corresponding to reactions (4) and (5) are also discussed. There have been some theoretical studies of methane activation or methyl adsorption on transition metals (Ni [ 16-191, Fe, Pt [ 161, Ti, Cr, Co [20]). To our knowledge, no similar theoretical studies on Rh have been carried out. So far, no full theoretical treatment of OMS on metal surfaces has been reported in the literature.
2. Computational
method and models
The calculations were carried out using the Amsterdam density functional method including relativistic effects [ 211. Triple-l ST0 basis sets with one additional polarization function were used throughout. The [ Ar3d] and [He] core definitions are used for Rh and C/O, respectively. Hence the valence set on Rh includes the (n- 1) s and (n- 1 )p shells. Among the several exchange-correlation potentials implemented, the density parametrization form of Vosko, Wilk and Nusair (VWN) [ 221 plus Becke’s [ 231 and additionally Perdew’s [ 241 gradient corrections to the exchangecorrelation potential (VWN + B + P) are used in the calculations. It has been shown that the VWN-B-P functional can give accurate binding energies for both main-group and transition-metal systems [ 25, 261. Relativistic effects on the electronic structures were evaluated using the quasi-relativistic method [ 271. In the present one-component approach, spin-orbit (SO) coupling is not taken into account. The SO contribution to the energy of atomic Rh would be large because the atom has an open d*s’ shell configuration. Here we concentrate mainly on the bonding interaction between a large metal cluster and an adsorbate. The SO effects are expected to be unimportant since the SO coupling would be much smaller in a large cluster than in an atom [ 281. Calculations of open-shell systems were based on the UHF spin-density functional approach. To assess relativistic effects, nonrelativistic calculations were also performed in some cases. Concerning the metal surface modeling, a R7 ( 1,6) cluster was used to simulate the Rh( 111) surface which contains one Rh atom in the first layer and six in the second (the Rh metal crystallizes in cubic close packed structure). It is reasonable to consider that the metal atoms constitute the reaction center. Actually, a
C.7: Au et al. /Chemical
Physics Letters 267 (1997) 44-SO Table I Calculated adsorption energies (in eV, where I eV = 23.06 kcal/mol) based on the Rh4 and Rh7 clusters: g = gas; s = surface. (Values in brackets are the nonrelativistic results)
CH4,, (b)
+
CH4,s
CH3.s --t
CH2.s
CH2,,
CH2,s
-
CH, + CH, c, + c, H, -
HS
0,
0s
-+
1OP
0, --t 0:’ H, + 0;’ + HO, H, + OF’ + HO,
-
Rh,(l,6)I
73*
’‘81
Cd)
CH3,
+ 0, ‘Op +
CH3,g
+ 0, ho’ +
CH4,$
+ 0, ‘Op -
H3C0, H3C0, H3CH0,
On Rh4
On Rh7
-0.07 -1.81 -2.87 -3.76 -5.56 -2.20
-0.08 -2.79 -3.20 -4.14 -6.33 -2.13
(-0.05) (-1.55) (-2.75) (-3.86) (-5.86) (-1.92)
-5.26 -5.03
-5.32 -4.97
(-4.81) (-4.77)
-3.20 -1.42
(-3.22) (-1.44)
-1.71 -0.44 -0.01
,I 15,
Rb (I.@73
for (a), (b), (e) and (f) are restricted to CsV and for the others the C, symmetry was maintained for geometry optimization. The cluster geometry was held fixed in the calculations, and the Rh-Rh distance was based on the bulk crystal data (Rh-Rh = 2.68 A).
3. Results and discussion Rh,61,1
(h, 0 at hollow)
,I 11,
3.1. Adsorption
,I 53)
Fig. I, Optimized structures for the free CH, species and the species adsorbed on the cluster model. Values in brackets are the nonrelativistic results.
smaller tetrahedral Rh4( 1,3) was also tested in order to examine the influence of cluster size on the calculated results. According to the pulse studies [ 61, there were at least two possible pathways for methane dissociation, viz. direct dissociation and oxygen-assisted dissociation, Our calculations considered two types of adsorbed oxygen species (0,) which are located at on-top and hollow sites. The models for the CH, (x = 4,. . . ,O) and H adsorbed on the metal surface are shown in Fig. 1. The C atom in the CHs, CH2 and CH species was placed to point towards the metal. The alternative Hend collinear adsorptions were found to be energetically unfavored. The molecular parts above the metal surface were fully optimized, where the symmetries
Let us first consider the adsorptions of CH,, 0, H species on the metal surface. Their adsorption properties can play an important role in methane dissociation. Panas et al. [ 291 suggested a rule for calculating chemisorption energies of (+ bonded adsorbates. It was argued that the cluster should be prepared for bonding. The preparation of the cluster means that there should be a singly occupied orbital on the cluster of the same symmetry as the u bonding orbital of the adsorbates. The Rh7 cluster has a ground state of 2A2. This state fulfills the requirement for the adsorptions of H and CH3. The calculated adsorption energies are given in Table 1 (here we define adsorption energy equal to the adsorption heat). Figs. 1 (a)-(d) show the changes in bond length and angle of CH, due to adsorption. From CH3 to C, the calculated adsorption energies are sensitive to cluster size: the energies obtained with the smaller Rb cluster are systematically smaller
C.I: Au et al. /Chemical
Physics Letters 267 (1997) 44-50
than those obtained with the Rh7 one. Hence we reckoned that a relatively large cluster is required in order to obtain reliable results. Relativistic contributions to the adsorption energies are generally significant, especially for CH3. However, relativistic changes in the surface-adsorbate distance are small (by at most 0.03 A). Relativistic effects are negligible on the bonds which are not connected to the surface. The results show only weak interaction between CH4 and the metal surface for the adsorption model shown in Fig. 1 (a). We found also that when CH4 approaches from a large distance to the on-top Rh in a Hend collinear fashion, the energy decreases (slightly) until a local minimum is reached, i.e. no activation energy is needed for this approach. The actual situation is slightly different. According to the molecular beam experiment of Stewart and Ehrlich [ 301, a small activation energy of ca. 7 kcal/mol (0.30 eV) was determined for methane dissociation on a highly perfect rhodium surface. Experiments examining CH4 adsorption on Rh films gave an activation energy of about 5 kcal/mol (N 0.22 eV) [ 311. The adsorption energy of H is calculated to be 2.13 eV. This value is much smaller than the energy required to break the tetrahedral CH bond (4.85 eVcalc, 4.5 1 eVexr) . Hence the generation of a gas-phase CHs radical via H-abstraction of CH4 on Rh is unlikely. The calculated adsorption energy of CH3 is large (-2.79 eV = -64.3 kcal/mol), as expected. The strong binding of CH3 on the metal surface also inhibits CH3 desorption. On the other hand, the coupling reaction CH2,, $ CH2,, -+ C2H6.g is also endothermic (AE = +1.52 eV). All these results may account for the fact that no (or nearly no) C2+ products were generated during the catalytic OMS processes on Rh. This is in contrast to metal oxide catalysts, over which both experiments [ 321 and calculations [32, 331 showed that the reaction for the formation of a surface hydroxyl and gas-phase methyl radical is slightly exothermic. Furthermore, the adsorption of CH3 on metal oxide surfaces are relatively weak according to the semiempirical ASEO-MO calculations of Anderson et al. [ 331 ( 1 .O- 1.6 eV, depending on coordinatively unsaturated sites). Therefore OCM takes place rather readily over oxide catalysts. Similar to CH3, the CH, species (x = 2,1,0) are also strongly adsorbed on the metal. The large metal-C binding may be due to donation and back-bonding interac-
4-l
tions involving C. The calculated adsorption energies follow the ordering CHs < CH;! < CH < C. The increase in the adsorption energies corresponds to the decrease in the Rh-C bond lengths along the series. The calculations also show large adsorption energies for 0. So 0 can be strongly trapped on the metal surface. This agrees with various experimental evidences. It is interesting that atomic oxygen is more strongly bound at an on-top site than at a hollow site. Therefore compared to the on-top site oxygen, the hollow site oxygen is actually more weakly held. However, the difference in adsorption energies between Otor and Oh” is insignificant. The calculated Rh-Otor distance of 1.71 8, is remarkably smaller than the sum = 1.25 A, R$‘” = of Pauling’s covalent radii (R$ 0.73 A). The very short bond length is indicative of a strong covalent bond between surface and 0. Adsorption of H on surface 0, results in the formation of surface HO,. The surface-Otor and surfaceOho’ distances are expanded by 0.16 and 0.37 A, respectively after H adsorption. The calculated adsorption energies of H depend strongly on the position at which the 0 is trapped. The O”r-adsorbed hydrogen is about 1.8 eV (41.5 kcal/mol) more stable than the Oh”‘-adsorbed one. So the oxygen adsorbed at a metal on-top site behaves more like a radical species in its ability to form a single bond than that at a hollow site. The adsorption energy of H on the metal surface is intermediate between those of H on Opp and Or’. The CH3 radical produced in methane dissociation may react with 0, to form HsCO,. We hence calculated the adsorption energies of CH3 on 0,. It is found that the product H3COFP is 1.1 eV less stable than HsC,. The order of decrease in binding strengths is HsC - Rh > H3C - Opp > H3C - 0:‘. The adsorption energy of CH4 on adsorbed 0, is also evaluated. Only a very small adsorption energy is obtained and the equilibrium 0. . . H distance is very long (2.36 A). On the other hand, the calculation shows a zero activation energy for a vertical linear C-H-O approach. For a reaction of CH4 with free 0 species, our calculation gives an activation energy of 0.82 eV ( 18.9 kcal/mol), but the complex OHCHs is found to be metastable, unbound by 0.16 eV with respect to 0 + CH4. The O-H distance at the local minimum is calculated as 1.12 A. The transition state is reached when the 0.. .H distance is 1.41 A.
C.I: Au et d/Chemical
48
Physics Letters 267 (I 997) 44-50
Table 2 Calculated dissociation energies D e.s, as defined in scheme 6, for direct dissociation of the CH, species on the Rh( I I1 ) surface (x = 4,3,2, I). Values obtained by the BOC-MP approach are given for comparison. (All energies are in eV. g = gas, s = surface. Values in brackets are the nonrelativistic results)
CH4.s -
CH2.s + Hs
CHz,, -t CH2.s + Hs CHz,, --t CH, + H, CH, -+ C, + H,
-AEl
AEz
AEH
4.g
0.08 2.79 3.20 4.14
-2.19 -3.20 -4.14 -6.33
-2.13 -2.13 -2.13 -2.13
4.85 5.13 4.93 3.72
DB’~ es
D&S
0.01 2.59
1.86 -0.60
(1.43) (2.01) ( 1.90) (-0.20)
0.10 0.61 0.33 -1.51
3.2. Dissociation On the basis of the adsorption energies given in Table 1 and the calculated CH bond strengths in the gasphase CH,,g, one can obtain the dissociation energy D,,,s for CH,,, dissociation on the metal surface. The scheme is shown below and the results are given in Table 2.
-
(a) and (a’) CH,,, + oha’+
CH,,,
“” + CH,-I,,+
Hs
CH,,s + CX,,
T AE2
T AEl CK,,
&+,,
CK-
T AEH (6)
I,~ +H,,
+&,,.
+AE~(ads)
+ OH,
+ Hs
(c) and (c’)
-100
1
CH *a + atop + CH,.,,,
+ OH,
Fig. 2. Schematic illustration of the dissociation energies De for the CH, dissociation on the Rh( 111) surface (see Tables 2 and 3). The calculated dissociation energies for the gas-phase CH,., (top curve) are given for comparison (g = gas, s = surface).
where D,,,=-AEl(ads)
CH,.,,,
(b) and (b’)
+AEH(ads) (7)
In Eq. (7)) the adsorption energies AE( ads) for the CH, species and for H were obtained at the same site on the same cluster. According to this equation, the dissociation energy D,,, can be expressed as a sum of the four energy terms. AEH and D,,, are the adsorption energy of H and dissociation energy of gas-phase CH?,, respectively. They are independent of the adsoption property of CH, on the surface. For each case, -AE is significantly smaller than D,,,. Therefore the relative adsorption energy A E2 (CH,_ 1) - A El (CH,) plays an important role in determining the dissociation mobility of the surface CH,,, species. Both cluster-model and BOC-MP results show that the dissociation of methane on the pure metal is nearly thermoneutral, i.e. first hydrogen abstraction from CH4 on the metal surface requires nearly no additional energy. The subsequent dissociations of CH3 and CH:! are seen to be slightly endothermic. The DF
energies based on the cluster model may be slightly positive compared to the BOC-MP values. The calculated dissociation energy for CH, is about 0.9 eV higher than the BOC-MP estimate. Fig. 2 shows that there are similar variations in the DF and BOC-MP dissociation energies with x in spite of the quantitative discrepancies. We see that only dissociation of CH, leads to stable products C, and H,. Therefore, the C species are easily formed on the metal surface. This has been evidenced by transient pulse experiments on a supported Ni catalyst [ 131. The relativistic effects are found to strongly support the dissociation of CH4,, + CH2,, + Hs, owing to the large relativistic stabilization for the metal-CH3 bonding. Because the relativistic stabilization for the metal-CH2 bonding is less pronounced than for the metal-CH3 bonding, the stability balance of CH2,, versus CHZ,~ is shifted in favor of CH2,, by the relativistic effects. The same is true for CH2,, --f CH, + H,. (But the relativistic
C.I: Au et al. /Chemical
Physics Letters 267 (I 997) 44-50
49
Table 3 Calculated energies (D,,,, in eV) for dissociation of the CH, species (x = 4,3,2,1) with the involvement of chemisorbed oxygen on the Rh( 1 I1 ) surface (s = surface). P is energy difference between the values with and without the involvement of chemisorbed oxygens &S (on-top)
I .06
DBK e.s (on-top)
CH4,, + 0s -
CHz,, + OH,
-
CH2,,
‘32.5
CH,+O,-Cs+OHs
.52 0.79 - I.67
-1.24 -0.74 -1.01 -2.86
A
-I .07
-1.34
+ 0s
-
I
+ 0%
CH2, + 0, -) CH, + OH,
Table 4 Calulated combining = gas, s = surface)
H., + HS c,+o,~co,~co,
H2.s
-
(
1) and desorption (2) energies (in eV) (g
H2+3
co, + OS + coz,, co2.g co, + OS + coz3 H, + HOs - H20s - H20, OS + OS + 02,s - 02.g
(I)
(2)
-0.67 - I .63 -1.15 0.28 -0.41 2.52
0.10 I .43 0.02 0.25 I .56
effects are very small for the latter case.) Table 3 shows the dissociation energies calculated for CH, dissociation on Rh( 111) with the involvement of atomic oxygen at on-top and hollow sites. The initial-stage dissociation reaction of CHd,, + 0:” --f CHz,, + HO, is exothermic. The adsorption energy of H is increased by about 1.1 eV when H adsorbs on an on-top site 0,. This value agrees well with the BOC-MP estimate (- 1.3 eV). On the other hand, an adsorption of H on hollow site 0, would result in a decrease of 0.7 eV in adsorption energy. This value also appears to be comparable to the BOC-MP value (0.4 eV) . The DF results give the same conclusion as the BOC-MP results: oxygens adsorbed at metal ontop sites promote dehydrogenation and those at hollow sites exert the opposite effect. 3.3. Combination and desorption of sur$ace species According to reactions (2)-( 5)) the produced fragments will combine to form a number of molecules (products) on the surface followed by desorption. The calculated energies for both cases are shown in Table 4. The combining reaction of H, + H, is slightly
DW (hollow) 0.72 3.30 2.57 0.11 +0.71
DBW w (hollow) 0.53 I .04 0.76 - 1.09 +0.43
exothermic and the subsequent desorption is also easy. The same is true for CO, + 0, and H, + HO,. So the formation of by-products Hz0 and CO2 is inevitable, in agreement with experimental observation. The surface-OH bond strength is calculated to be 3.37 eV. Hence, with first hydrogen adsorption on OS, the surface-0 bond strength is reduced by 1.95 eV. CO formation from C, + 0, is easy, but the desorption of CO, is slightly endothermic. This means that there exists a relatively strong binding between metal surface and CO. This phenomenon has been observed on various transition metal surfaces. With 0 ending down on the surface, however, the calculations show no bonding interaction between CO and the metal cluster (no minimum was found for the RhT-OC bond). We see that adsorbed 0, still possesses a strong ability to oxidize gas CO, to CO2. Therefore the CO2 selectivity would be dependent on the amount of adsorbed oxygen present on the surface. It was observed [3] that as the Rh surface becomes more and more depleted of 0, species, the rate of CO2 formation decreases relative to the rate of CO formation. If there is insufficient adsorbed OS, the formation of CO:! is unlikely. It is shown that the combination of 0, + 0, cannot take place because of the large reaction endothermicity. On the other hand, molecular 02 can be strongly adsorbed on the metal surface. This may be attributed to the open-shell character of OZ. Therefore adsorbed atomic oxygen species can also be produced easily by dissociative adsorption of gas-phase 02.
4. Conclusions The main conclusions follows:
drawn from the results are as
50
C.i? Au et al. /Chemical
Physics Letters 267 (I 997) 44-50
(i) Free CH4 is only weakly bound to the metal surface. However, dissociative adsorption of CH4 on the surface is possible. The first dehydrogenation step reaction CH4,, ---f CHz,, + H, (s = surface), which is nearly thermoneutral, takes place readily. (ii) The calculated adsorption energy of H is much lower than the calculated C-H bond strength in CH4. Thus no hydrogen abstraction with subsequent release of methyl radical into the gas-phase is expected over the metal surface. Therefore metal Rh catalyst does not mediate the formation of higher carbon products. This result provides an explanation for the absence of C2 products in the OMS reaction over Rh catalysts. (iii) The adsorption of CH, on Rh formed by H abstraction is strong. However, the subsequent dissociation of surface CH,,, depends also on the relative adsorption energies. Thus, the dissociation reactions are found to be slightly endothermic for CHs and CH2. This means that a sufficiently high temperature will be required for further dissociations of CHs and CH2. The trend in dissociation energies calculated with the cluster model is similar to that obtained in the BOCMP approach. (iv) Oxygen species adsorbed at metal on-top and hollow sites exhibit different behavior towards methane dissociation. The calculated dissociation energies in both cases confirm the conclusion that oxygens adsorbed at the metal on-top sites promote methane dedydrogenation while those at hollow sites are not beneficial to methane dissociation. A similar deduction was drawn from the semi-empirical BOC-MP results. (v) Adsorbed fragments O,, C,, H, and OH, can combine easily to form CO, Hz, Hz0 and CO2. Therefore the selectivity of CO2 is dependent on the amount of adsorbed oxygen present on the surface. (vi) Relativistic effects contribute significantly to the adsorption energies and so have a great influence on the methane dissociation on the Rh surface. This work was supported by the Faculty Research Grant (FRG/94-95/11-53) of the Hong Kong Baptist University. References I I ] D. A. Hickman and L. D. Schmidt, J. Catal. 138 ( 1992) 267. [ 21 D. A. Hickman and L. D. Schmidt, Science 259 ( 1993) 343.
[3] D. A. Hickman and E. A. Haupfear and L. D. Schmidt, Catal. Len. 17 (1993) 223. [4] 0. V. Buyevskaya, D. Wolf and M. Baems, Catal. Len. 29 ( 1994) 249. [5] K. Walter, 0. V. Buyevskaya, D. Wolf and M. Baems, Catal. Len. 29 (1994) 261. 161 C. T. Au and H. Y. Wang, submitted to J. Catal. [7] P D. E Vernon, M. L. H. Green, A. K. Cheetham and A. T. Ashcroft, Catal. Lett. 6 (1990) 181. [8] A. Erdiihelyi, J. Csemnyi and F. Solymosi, J. Catal. 141 (1993) 287. [9] Y. Boucouvalas, 2. L. Zhang and X. E. Verykios, Catal. Len. 27 (1994) 131. lo] Z. L. Zhang, A. Kladi and X. E. Verykios, 111 121 [ 131
[ 141 [ 151 [ 161
J. Catal.
148
(1994) 737. C. T. Au, Y. H. Hu and H. L. Wan, Catal. Lett. 27 ( 1994) 199. C. T. Au, Y. H. Hu and H. L. Wan, Catal. Lett. 36 ( 1996) 159. Y. H. Hu and E. Ruckenstein, J. Catal. 158 (1996) 260. Y. H. Hu and E. Ruckenstein, Catal. Lea. 34 ( 1995) 41. E. Ruckenstein and Y. H. Hu, Catal. Lett. 35 ( 1995) 265. A. B. Anderson and J. J. Maloney, J. Phys. Chem. 92 ( 1988) 809.
I171 0. Swang, K. Faegri Jr, 0. Gropen, U. Wahlgren and P Siegbahn, Chem. Phys. 156 (1991) 379. 1181 T. H. Upton, J. Vat. Sci. Technol. 20 ( 1982) 527. [191 J. Schiile, P Siegbahn and U. Wahlgren, J. Chem. Phys. 89 (1988) 6982. 1201 C. Zheng, Y. Apeloig and R. Hoffmann, J. Am. Chem. Sot.
110 (1988) 749. 1211 E. J. Baerends, D. E. Ellis and P Ros, ADF program package, version 2.0.1: Chem. Phys. 2 ( 1973) 41; G. te Velde and E. J. Baerends, J. Comp. Phys. 99 ( 1992) 84. t221 S. H. Vosko, L. Wilk and M. Nusair, Can. J. Phys. 58 (1980) 1200. [231 A. D. Becke, Phys. Rev. A 38 (1988) 3098. [241 J. P Perdew, Phys. Rev. B 33 (1986) 8822. 1251 B. G. Johnson, P M. W. Gill and J. A. Pople, J. Chem. Phys. 98 (1993) 5612. [261 I. Li, G. Schreckenbach and T. Ziegler, J. Am. Chem. Sot. 117 (1995). ~271 T. Ziegler, V. Tschinke, E. J. Baerends, J. G. Snijders and W. Ravenek, J. Phys. Chem. 93 (1989) 3050. [281 M. -S. Liao, Ph.D. Thesis, University of Siegen, Siegen, Germany (1993) t291 1. Panas, J. Schiile, P Siegbahn and U. Wahlgren, Chem. Phys. Lett. 149 (1988) 265. [301 C. N. Stewart and G. Ehrlich, J. Chem. Phys. 62 (1975) 4672. 1311 S. G. Brass and G. Ehrlich, Surf. Sci. 187 (1987) 21. [321 K. J. Borve and L. G. M. Pettersson, J. Phys. Chem. 95 (1991) 3214. 1331 S. P Mehandru, A. B. Anderson and J. E Brazdil, J. Am. Chem. Sot. 110 (1988) 1715.
CHEMICAL PHYSICS LETTERS ELSEVIER
Chemical
Physics Letters 267 ( 1997) 44-50
A theoretical investigation of methane dissociation on Rh( 111) Chak Tong Au, Meng-Sheng
Liao I, Ching Fai Ng
Department of Chemistry, Hong Kong Baptist University, Kowloon Tong, Hong Kong Received 29 October
1996; in final form 16 December
1996
Abstract Methane dissociation (CH4 -+ CH, + (4 - x) H -+ C + 4H) over a cluster model of the Rh( 111) surface has been investigated using a quasi-relativistic density functional method. The main aim is to evaluate the dissociation energies which
may determine catalytic activity of the metal in the partial oxidation of methane to syngas. Dissociative adsorption of methane can be initiated on the Rh surface. However, the abstraction of H from CH4 to produce gas-phase CHs,, radical is energetically unfavorable. This would explain the absence of CZ products. The presence of adsorbed oxygen at an on-top site promotes methane dehydrogenation while that at a hollow site exerts the opposite effect. All calculated dissociation energies of CH,,s are compatible with those obtained with the bond-order conservation Morse-potential approach. The combination and desorption of surface species, which are critical to the formation of syngas, are also discussed. @ 1997 Elsevier Science B.V.
1. Introduction The catalytic oxidation of methane to syngas (denoted as OMS hereafter) has attracted increased interest in recent years. Oxide-supported Rh has been shown to be an effective catalyst for OMS [ l-101. Some of the first mechanistic studies were done by Hickman and Schmidt [l-3]. The dissociation of methane was regarded as an initial step for syngas formation, and the methane pyrolysis mechanism [see reactions ( 1) -( 5) below] was suggested based on experiments over monolith-supported Rh at short contact time. Later, Buyevskaya et al. [4, 51 studied reaction steps in OMS over AlzOs-supported Rh by using transient pulse technique and in situ DRIFTS (diffuse reflectance infrared Fourier transform spectroscopy). A temporal analysis of the prod-
ucts indicated a direct oxidation initiated by methane dissociation, reaction ( 1) , although the authors suggested a different step for the formation of CO (i.e. C, + CO2,s --+ 2CO). Recently, we investigated the reaction mechanisms of OMS over reduced Rh/SiO;! and oxidized Rh( 0) /SiO2 catalysts over a pulse microreactor [ 61. It was demonstrated that the partial oxidation reaction of CH4 follows the methane pyrolysis mechanism. It was concluded that: (1) Metallic rhodium is active for methane decomposition; (2) Chemisorbed oxygen species from rhodium oxide and SiO2 have participated in the methane oxidation reaction; (3) Methane dissociation is a key reaction step. The possible reaction steps over the reduced Rh catalyst are as follows (g = gas, s = surface) : CH++site-+CH,,,+(4-x)H,+C,+4H,, (1)
’On leave from: Department Xiamen, Fujian 361005,
of Chemistry,
Xiamen University,
China.
0009.2614/97/$17.00 @ 1997 Elsevier Science B.V. All rights reserved. fIISOOO9-2614(97)00076-6
c, +o,
--+ co,
+ co,,
(2)
CT
Hs + Hs --+ H2.s
+
H2z3.
Au et al. /Chemical
Physics Letters
(3)
By-products (COT, H20) may be generated according to the reactions: co,
+ 0, ---t co2.q-t
co2,,,
(4)
0, + 2H,-+ H20,+
H20,.
(5)
The same mechanism has also been applied to explain results of pulse experiments obtained over reduced Ni/AlzOj [ 11, 121 and Ni/LazOj [ 13-151 catalysts. According to reaction ( 1) , the breakage of a C-H bond in methane is the first step in the OMS reaction. This is similar to the OCM reaction in which the methyl radical formation has been reckoned as the initial step for C2 hydrocarbon generation. In the OMS reaction, however, the methyl radicals remain chemisorbed on the surface and subsequently undergo further hydrogen abstractions. In other words, CO and HZ are formed via a surface intermediate species CH,,,. In this letter, we attempt to perform a quasirelativistic density functional (DF) investigation of methane dissociation on Rh( 111) following the mechanism given above. Different mechanisms exist in the literature [ 7-101. It should be pointed out that the OMS reaction steps may depend on other factors (choice of metal, nature of support, reaction conditions, reactor or contact time), but they are not the topics of the present concern. Previously, the activation and dissociation energies for methane dissociation on Rh( 111) with and without the involvement of surface oxygens were obtained by means of the semiempirical bond-order conservation Morse-potential (BOC-MP) approach [ 61. In the investigation, different surface oxygens at on-top, bridge and hollow sites were examined. These BOC-MP results are compared to the ‘pure’ theoretical results based on a cluster model. Because it is difficult to locate a transition state H, on the metal for the reaction CH,,, -+ CH,_t,,+ surface, we focus mainly on an evaluation of the dissociation energies for the CH, species (X = 4,3,2, 1). The BOC-MP results revealed a certain correlation between the activation and dissociation energies for reaction ( 1) , viz. a more negative dissociation energy corresponds to a lower activation energy. In order to have some ideas about barrier heights, we evaluated activation energies for some simple cases. The related
267 (I 997) 44-50
45
combining and desorption properties corresponding to reactions (4) and (5) are also discussed. There have been some theoretical studies of methane activation or methyl adsorption on transition metals (Ni [ 16-191, Fe, Pt [ 161, Ti, Cr, Co [20]). To our knowledge, no similar theoretical studies on Rh have been carried out. So far, no full theoretical treatment of OMS on metal surfaces has been reported in the literature.
2. Computational
method and models
The calculations were carried out using the Amsterdam density functional method including relativistic effects [ 211. Triple-l ST0 basis sets with one additional polarization function were used throughout. The [ Ar3d] and [He] core definitions are used for Rh and C/O, respectively. Hence the valence set on Rh includes the (n- 1) s and (n- 1 )p shells. Among the several exchange-correlation potentials implemented, the density parametrization form of Vosko, Wilk and Nusair (VWN) [ 221 plus Becke’s [ 231 and additionally Perdew’s [ 241 gradient corrections to the exchangecorrelation potential (VWN + B + P) are used in the calculations. It has been shown that the VWN-B-P functional can give accurate binding energies for both main-group and transition-metal systems [ 25, 261. Relativistic effects on the electronic structures were evaluated using the quasi-relativistic method [ 271. In the present one-component approach, spin-orbit (SO) coupling is not taken into account. The SO contribution to the energy of atomic Rh would be large because the atom has an open d*s’ shell configuration. Here we concentrate mainly on the bonding interaction between a large metal cluster and an adsorbate. The SO effects are expected to be unimportant since the SO coupling would be much smaller in a large cluster than in an atom [ 281. Calculations of open-shell systems were based on the UHF spin-density functional approach. To assess relativistic effects, nonrelativistic calculations were also performed in some cases. Concerning the metal surface modeling, a R7 ( 1,6) cluster was used to simulate the Rh( 111) surface which contains one Rh atom in the first layer and six in the second (the Rh metal crystallizes in cubic close packed structure). It is reasonable to consider that the metal atoms constitute the reaction center. Actually, a
C.7: Au et al. /Chemical
Physics Letters 267 (1997) 44-SO Table I Calculated adsorption energies (in eV, where I eV = 23.06 kcal/mol) based on the Rh4 and Rh7 clusters: g = gas; s = surface. (Values in brackets are the nonrelativistic results)
CH4,, (b)
+
CH4,s
CH3.s --t
CH2.s
CH2,,
CH2,s
-
CH, + CH, c, + c, H, -
HS
0,
0s
-+
1OP
0, --t 0:’ H, + 0;’ + HO, H, + OF’ + HO,
-
Rh,(l,6)I
73*
’‘81
Cd)
CH3,
+ 0, ‘Op +
CH3,g
+ 0, ho’ +
CH4,$
+ 0, ‘Op -
H3C0, H3C0, H3CH0,
On Rh4
On Rh7
-0.07 -1.81 -2.87 -3.76 -5.56 -2.20
-0.08 -2.79 -3.20 -4.14 -6.33 -2.13
(-0.05) (-1.55) (-2.75) (-3.86) (-5.86) (-1.92)
-5.26 -5.03
-5.32 -4.97
(-4.81) (-4.77)
-3.20 -1.42
(-3.22) (-1.44)
-1.71 -0.44 -0.01
,I 15,
Rb (I.@73
for (a), (b), (e) and (f) are restricted to CsV and for the others the C, symmetry was maintained for geometry optimization. The cluster geometry was held fixed in the calculations, and the Rh-Rh distance was based on the bulk crystal data (Rh-Rh = 2.68 A).
3. Results and discussion Rh,61,1
(h, 0 at hollow)
,I 11,
3.1. Adsorption
,I 53)
Fig. I, Optimized structures for the free CH, species and the species adsorbed on the cluster model. Values in brackets are the nonrelativistic results.
smaller tetrahedral Rh4( 1,3) was also tested in order to examine the influence of cluster size on the calculated results. According to the pulse studies [ 61, there were at least two possible pathways for methane dissociation, viz. direct dissociation and oxygen-assisted dissociation, Our calculations considered two types of adsorbed oxygen species (0,) which are located at on-top and hollow sites. The models for the CH, (x = 4,. . . ,O) and H adsorbed on the metal surface are shown in Fig. 1. The C atom in the CHs, CH2 and CH species was placed to point towards the metal. The alternative Hend collinear adsorptions were found to be energetically unfavored. The molecular parts above the metal surface were fully optimized, where the symmetries
Let us first consider the adsorptions of CH,, 0, H species on the metal surface. Their adsorption properties can play an important role in methane dissociation. Panas et al. [ 291 suggested a rule for calculating chemisorption energies of (+ bonded adsorbates. It was argued that the cluster should be prepared for bonding. The preparation of the cluster means that there should be a singly occupied orbital on the cluster of the same symmetry as the u bonding orbital of the adsorbates. The Rh7 cluster has a ground state of 2A2. This state fulfills the requirement for the adsorptions of H and CH3. The calculated adsorption energies are given in Table 1 (here we define adsorption energy equal to the adsorption heat). Figs. 1 (a)-(d) show the changes in bond length and angle of CH, due to adsorption. From CH3 to C, the calculated adsorption energies are sensitive to cluster size: the energies obtained with the smaller Rb cluster are systematically smaller
C.I: Au et al. /Chemical
Physics Letters 267 (1997) 44-50
than those obtained with the Rh7 one. Hence we reckoned that a relatively large cluster is required in order to obtain reliable results. Relativistic contributions to the adsorption energies are generally significant, especially for CH3. However, relativistic changes in the surface-adsorbate distance are small (by at most 0.03 A). Relativistic effects are negligible on the bonds which are not connected to the surface. The results show only weak interaction between CH4 and the metal surface for the adsorption model shown in Fig. 1 (a). We found also that when CH4 approaches from a large distance to the on-top Rh in a Hend collinear fashion, the energy decreases (slightly) until a local minimum is reached, i.e. no activation energy is needed for this approach. The actual situation is slightly different. According to the molecular beam experiment of Stewart and Ehrlich [ 301, a small activation energy of ca. 7 kcal/mol (0.30 eV) was determined for methane dissociation on a highly perfect rhodium surface. Experiments examining CH4 adsorption on Rh films gave an activation energy of about 5 kcal/mol (N 0.22 eV) [ 311. The adsorption energy of H is calculated to be 2.13 eV. This value is much smaller than the energy required to break the tetrahedral CH bond (4.85 eVcalc, 4.5 1 eVexr) . Hence the generation of a gas-phase CHs radical via H-abstraction of CH4 on Rh is unlikely. The calculated adsorption energy of CH3 is large (-2.79 eV = -64.3 kcal/mol), as expected. The strong binding of CH3 on the metal surface also inhibits CH3 desorption. On the other hand, the coupling reaction CH2,, $ CH2,, -+ C2H6.g is also endothermic (AE = +1.52 eV). All these results may account for the fact that no (or nearly no) C2+ products were generated during the catalytic OMS processes on Rh. This is in contrast to metal oxide catalysts, over which both experiments [ 321 and calculations [32, 331 showed that the reaction for the formation of a surface hydroxyl and gas-phase methyl radical is slightly exothermic. Furthermore, the adsorption of CH3 on metal oxide surfaces are relatively weak according to the semiempirical ASEO-MO calculations of Anderson et al. [ 331 ( 1 .O- 1.6 eV, depending on coordinatively unsaturated sites). Therefore OCM takes place rather readily over oxide catalysts. Similar to CH3, the CH, species (x = 2,1,0) are also strongly adsorbed on the metal. The large metal-C binding may be due to donation and back-bonding interac-
4-l
tions involving C. The calculated adsorption energies follow the ordering CHs < CH;! < CH < C. The increase in the adsorption energies corresponds to the decrease in the Rh-C bond lengths along the series. The calculations also show large adsorption energies for 0. So 0 can be strongly trapped on the metal surface. This agrees with various experimental evidences. It is interesting that atomic oxygen is more strongly bound at an on-top site than at a hollow site. Therefore compared to the on-top site oxygen, the hollow site oxygen is actually more weakly held. However, the difference in adsorption energies between Otor and Oh” is insignificant. The calculated Rh-Otor distance of 1.71 8, is remarkably smaller than the sum = 1.25 A, R$‘” = of Pauling’s covalent radii (R$ 0.73 A). The very short bond length is indicative of a strong covalent bond between surface and 0. Adsorption of H on surface 0, results in the formation of surface HO,. The surface-Otor and surfaceOho’ distances are expanded by 0.16 and 0.37 A, respectively after H adsorption. The calculated adsorption energies of H depend strongly on the position at which the 0 is trapped. The O”r-adsorbed hydrogen is about 1.8 eV (41.5 kcal/mol) more stable than the Oh”‘-adsorbed one. So the oxygen adsorbed at a metal on-top site behaves more like a radical species in its ability to form a single bond than that at a hollow site. The adsorption energy of H on the metal surface is intermediate between those of H on Opp and Or’. The CH3 radical produced in methane dissociation may react with 0, to form HsCO,. We hence calculated the adsorption energies of CH3 on 0,. It is found that the product H3COFP is 1.1 eV less stable than HsC,. The order of decrease in binding strengths is HsC - Rh > H3C - Opp > H3C - 0:‘. The adsorption energy of CH4 on adsorbed 0, is also evaluated. Only a very small adsorption energy is obtained and the equilibrium 0. . . H distance is very long (2.36 A). On the other hand, the calculation shows a zero activation energy for a vertical linear C-H-O approach. For a reaction of CH4 with free 0 species, our calculation gives an activation energy of 0.82 eV ( 18.9 kcal/mol), but the complex OHCHs is found to be metastable, unbound by 0.16 eV with respect to 0 + CH4. The O-H distance at the local minimum is calculated as 1.12 A. The transition state is reached when the 0.. .H distance is 1.41 A.
C.I: Au et d/Chemical
48
Physics Letters 267 (I 997) 44-50
Table 2 Calculated dissociation energies D e.s, as defined in scheme 6, for direct dissociation of the CH, species on the Rh( I I1 ) surface (x = 4,3,2, I). Values obtained by the BOC-MP approach are given for comparison. (All energies are in eV. g = gas, s = surface. Values in brackets are the nonrelativistic results)
CH4.s -
CH2.s + Hs
CHz,, -t CH2.s + Hs CHz,, --t CH, + H, CH, -+ C, + H,
-AEl
AEz
AEH
4.g
0.08 2.79 3.20 4.14
-2.19 -3.20 -4.14 -6.33
-2.13 -2.13 -2.13 -2.13
4.85 5.13 4.93 3.72
DB’~ es
D&S
0.01 2.59
1.86 -0.60
(1.43) (2.01) ( 1.90) (-0.20)
0.10 0.61 0.33 -1.51
3.2. Dissociation On the basis of the adsorption energies given in Table 1 and the calculated CH bond strengths in the gasphase CH,,g, one can obtain the dissociation energy D,,,s for CH,,, dissociation on the metal surface. The scheme is shown below and the results are given in Table 2.
-
(a) and (a’) CH,,, + oha’+
CH,,,
“” + CH,-I,,+
Hs
CH,,s + CX,,
T AE2
T AEl CK,,
&+,,
CK-
T AEH (6)
I,~ +H,,
+&,,.
+AE~(ads)
+ OH,
+ Hs
(c) and (c’)
-100
1
CH *a + atop + CH,.,,,
+ OH,
Fig. 2. Schematic illustration of the dissociation energies De for the CH, dissociation on the Rh( 111) surface (see Tables 2 and 3). The calculated dissociation energies for the gas-phase CH,., (top curve) are given for comparison (g = gas, s = surface).
where D,,,=-AEl(ads)
CH,.,,,
(b) and (b’)
+AEH(ads) (7)
In Eq. (7)) the adsorption energies AE( ads) for the CH, species and for H were obtained at the same site on the same cluster. According to this equation, the dissociation energy D,,, can be expressed as a sum of the four energy terms. AEH and D,,, are the adsorption energy of H and dissociation energy of gas-phase CH?,, respectively. They are independent of the adsoption property of CH, on the surface. For each case, -AE is significantly smaller than D,,,. Therefore the relative adsorption energy A E2 (CH,_ 1) - A El (CH,) plays an important role in determining the dissociation mobility of the surface CH,,, species. Both cluster-model and BOC-MP results show that the dissociation of methane on the pure metal is nearly thermoneutral, i.e. first hydrogen abstraction from CH4 on the metal surface requires nearly no additional energy. The subsequent dissociations of CH3 and CH:! are seen to be slightly endothermic. The DF
energies based on the cluster model may be slightly positive compared to the BOC-MP values. The calculated dissociation energy for CH, is about 0.9 eV higher than the BOC-MP estimate. Fig. 2 shows that there are similar variations in the DF and BOC-MP dissociation energies with x in spite of the quantitative discrepancies. We see that only dissociation of CH, leads to stable products C, and H,. Therefore, the C species are easily formed on the metal surface. This has been evidenced by transient pulse experiments on a supported Ni catalyst [ 131. The relativistic effects are found to strongly support the dissociation of CH4,, + CH2,, + Hs, owing to the large relativistic stabilization for the metal-CH3 bonding. Because the relativistic stabilization for the metal-CH2 bonding is less pronounced than for the metal-CH3 bonding, the stability balance of CH2,, versus CHZ,~ is shifted in favor of CH2,, by the relativistic effects. The same is true for CH2,, --f CH, + H,. (But the relativistic
C.I: Au et al. /Chemical
Physics Letters 267 (I 997) 44-50
49
Table 3 Calculated energies (D,,,, in eV) for dissociation of the CH, species (x = 4,3,2,1) with the involvement of chemisorbed oxygen on the Rh( 1 I1 ) surface (s = surface). P is energy difference between the values with and without the involvement of chemisorbed oxygens &S (on-top)
I .06
DBK e.s (on-top)
CH4,, + 0s -
CHz,, + OH,
-
CH2,,
‘32.5
CH,+O,-Cs+OHs
.52 0.79 - I.67
-1.24 -0.74 -1.01 -2.86
A
-I .07
-1.34
+ 0s
-
I
+ 0%
CH2, + 0, -) CH, + OH,
Table 4 Calulated combining = gas, s = surface)
H., + HS c,+o,~co,~co,
H2.s
-
(
1) and desorption (2) energies (in eV) (g
H2+3
co, + OS + coz,, co2.g co, + OS + coz3 H, + HOs - H20s - H20, OS + OS + 02,s - 02.g
(I)
(2)
-0.67 - I .63 -1.15 0.28 -0.41 2.52
0.10 I .43 0.02 0.25 I .56
effects are very small for the latter case.) Table 3 shows the dissociation energies calculated for CH, dissociation on Rh( 111) with the involvement of atomic oxygen at on-top and hollow sites. The initial-stage dissociation reaction of CHd,, + 0:” --f CHz,, + HO, is exothermic. The adsorption energy of H is increased by about 1.1 eV when H adsorbs on an on-top site 0,. This value agrees well with the BOC-MP estimate (- 1.3 eV). On the other hand, an adsorption of H on hollow site 0, would result in a decrease of 0.7 eV in adsorption energy. This value also appears to be comparable to the BOC-MP value (0.4 eV) . The DF results give the same conclusion as the BOC-MP results: oxygens adsorbed at metal ontop sites promote dehydrogenation and those at hollow sites exert the opposite effect. 3.3. Combination and desorption of sur$ace species According to reactions (2)-( 5)) the produced fragments will combine to form a number of molecules (products) on the surface followed by desorption. The calculated energies for both cases are shown in Table 4. The combining reaction of H, + H, is slightly
DW (hollow) 0.72 3.30 2.57 0.11 +0.71
DBW w (hollow) 0.53 I .04 0.76 - 1.09 +0.43
exothermic and the subsequent desorption is also easy. The same is true for CO, + 0, and H, + HO,. So the formation of by-products Hz0 and CO2 is inevitable, in agreement with experimental observation. The surface-OH bond strength is calculated to be 3.37 eV. Hence, with first hydrogen adsorption on OS, the surface-0 bond strength is reduced by 1.95 eV. CO formation from C, + 0, is easy, but the desorption of CO, is slightly endothermic. This means that there exists a relatively strong binding between metal surface and CO. This phenomenon has been observed on various transition metal surfaces. With 0 ending down on the surface, however, the calculations show no bonding interaction between CO and the metal cluster (no minimum was found for the RhT-OC bond). We see that adsorbed 0, still possesses a strong ability to oxidize gas CO, to CO2. Therefore the CO2 selectivity would be dependent on the amount of adsorbed oxygen present on the surface. It was observed [3] that as the Rh surface becomes more and more depleted of 0, species, the rate of CO2 formation decreases relative to the rate of CO formation. If there is insufficient adsorbed OS, the formation of CO:! is unlikely. It is shown that the combination of 0, + 0, cannot take place because of the large reaction endothermicity. On the other hand, molecular 02 can be strongly adsorbed on the metal surface. This may be attributed to the open-shell character of OZ. Therefore adsorbed atomic oxygen species can also be produced easily by dissociative adsorption of gas-phase 02.
4. Conclusions The main conclusions follows:
drawn from the results are as
50
C.i? Au et al. /Chemical
Physics Letters 267 (I 997) 44-50
(i) Free CH4 is only weakly bound to the metal surface. However, dissociative adsorption of CH4 on the surface is possible. The first dehydrogenation step reaction CH4,, ---f CHz,, + H, (s = surface), which is nearly thermoneutral, takes place readily. (ii) The calculated adsorption energy of H is much lower than the calculated C-H bond strength in CH4. Thus no hydrogen abstraction with subsequent release of methyl radical into the gas-phase is expected over the metal surface. Therefore metal Rh catalyst does not mediate the formation of higher carbon products. This result provides an explanation for the absence of C2 products in the OMS reaction over Rh catalysts. (iii) The adsorption of CH, on Rh formed by H abstraction is strong. However, the subsequent dissociation of surface CH,,, depends also on the relative adsorption energies. Thus, the dissociation reactions are found to be slightly endothermic for CHs and CH2. This means that a sufficiently high temperature will be required for further dissociations of CHs and CH2. The trend in dissociation energies calculated with the cluster model is similar to that obtained in the BOCMP approach. (iv) Oxygen species adsorbed at metal on-top and hollow sites exhibit different behavior towards methane dissociation. The calculated dissociation energies in both cases confirm the conclusion that oxygens adsorbed at the metal on-top sites promote methane dedydrogenation while those at hollow sites are not beneficial to methane dissociation. A similar deduction was drawn from the semi-empirical BOC-MP results. (v) Adsorbed fragments O,, C,, H, and OH, can combine easily to form CO, Hz, Hz0 and CO2. Therefore the selectivity of CO2 is dependent on the amount of adsorbed oxygen present on the surface. (vi) Relativistic effects contribute significantly to the adsorption energies and so have a great influence on the methane dissociation on the Rh surface. This work was supported by the Faculty Research Grant (FRG/94-95/11-53) of the Hong Kong Baptist University. References I I ] D. A. Hickman and L. D. Schmidt, J. Catal. 138 ( 1992) 267. [ 21 D. A. Hickman and L. D. Schmidt, Science 259 ( 1993) 343.
[3] D. A. Hickman and E. A. Haupfear and L. D. Schmidt, Catal. Len. 17 (1993) 223. [4] 0. V. Buyevskaya, D. Wolf and M. Baems, Catal. Len. 29 ( 1994) 249. [5] K. Walter, 0. V. Buyevskaya, D. Wolf and M. Baems, Catal. Len. 29 (1994) 261. 161 C. T. Au and H. Y. Wang, submitted to J. Catal. [7] P D. E Vernon, M. L. H. Green, A. K. Cheetham and A. T. Ashcroft, Catal. Lett. 6 (1990) 181. [8] A. Erdiihelyi, J. Csemnyi and F. Solymosi, J. Catal. 141 (1993) 287. [9] Y. Boucouvalas, 2. L. Zhang and X. E. Verykios, Catal. Len. 27 (1994) 131. lo] Z. L. Zhang, A. Kladi and X. E. Verykios, 111 121 [ 131
[ 141 [ 151 [ 161
J. Catal.
148
(1994) 737. C. T. Au, Y. H. Hu and H. L. Wan, Catal. Lett. 27 ( 1994) 199. C. T. Au, Y. H. Hu and H. L. Wan, Catal. Lett. 36 ( 1996) 159. Y. H. Hu and E. Ruckenstein, J. Catal. 158 (1996) 260. Y. H. Hu and E. Ruckenstein, Catal. Lea. 34 ( 1995) 41. E. Ruckenstein and Y. H. Hu, Catal. Lett. 35 ( 1995) 265. A. B. Anderson and J. J. Maloney, J. Phys. Chem. 92 ( 1988) 809.
I171 0. Swang, K. Faegri Jr, 0. Gropen, U. Wahlgren and P Siegbahn, Chem. Phys. 156 (1991) 379. 1181 T. H. Upton, J. Vat. Sci. Technol. 20 ( 1982) 527. [191 J. Schiile, P Siegbahn and U. Wahlgren, J. Chem. Phys. 89 (1988) 6982. 1201 C. Zheng, Y. Apeloig and R. Hoffmann, J. Am. Chem. Sot.
110 (1988) 749. 1211 E. J. Baerends, D. E. Ellis and P Ros, ADF program package, version 2.0.1: Chem. Phys. 2 ( 1973) 41; G. te Velde and E. J. Baerends, J. Comp. Phys. 99 ( 1992) 84. t221 S. H. Vosko, L. Wilk and M. Nusair, Can. J. Phys. 58 (1980) 1200. [231 A. D. Becke, Phys. Rev. A 38 (1988) 3098. [241 J. P Perdew, Phys. Rev. B 33 (1986) 8822. 1251 B. G. Johnson, P M. W. Gill and J. A. Pople, J. Chem. Phys. 98 (1993) 5612. [261 I. Li, G. Schreckenbach and T. Ziegler, J. Am. Chem. Sot. 117 (1995). ~271 T. Ziegler, V. Tschinke, E. J. Baerends, J. G. Snijders and W. Ravenek, J. Phys. Chem. 93 (1989) 3050. [281 M. -S. Liao, Ph.D. Thesis, University of Siegen, Siegen, Germany (1993) t291 1. Panas, J. Schiile, P Siegbahn and U. Wahlgren, Chem. Phys. Lett. 149 (1988) 265. [301 C. N. Stewart and G. Ehrlich, J. Chem. Phys. 62 (1975) 4672. 1311 S. G. Brass and G. Ehrlich, Surf. Sci. 187 (1987) 21. [321 K. J. Borve and L. G. M. Pettersson, J. Phys. Chem. 95 (1991) 3214. 1331 S. P Mehandru, A. B. Anderson and J. E Brazdil, J. Am. Chem. Sot. 110 (1988) 1715.