J . Phys. Chem. 1989, 93, 4594-4598
4594
Structure and Dissociation Mechanisms of Methanol in ZSM-5 Zeolite R. Vetrivel,* C . R. A. Catlow, Department of Chemistry, University of Keele, Keele, Staffordshire. ST5 5BG. U.K.
and E. A. Colbourn Wilton Materials Research Centre, I.C.I. PIC., Wilton, Middlesbrough, Cleveland, TS6 8JE, U.K. (Receioed: October 6 , 1988; In Final Form: December 12, 1988)
We use a combination of lattice simulation calculations and ab initio quantum mechanical techniques to study the adsorption behavior of methanol in ZSM-5. We locate the low-energy adsorption sites for the methanol molecule inside the pores of the ZSM-5 zeolite using lattice simulation techniques. Electronic structure calculationsare carried out on the cluster representing the adsorption complex, while a n array of point charges are included to represent the long-range electrostatic effects. We propose a dissociation mechanism for methanol at the Bransted acid site of this zeolite.
Introduction This paper reports the results of identifying the adsorption sites for methanol in ZSM-5 and deducing the mechanism of its initial reaction. The conversion of relatively abundant natural gas and synthesis gas to convenient transportable gasoline via methanol has recently been achieved.l This was made possible by the discovery2 of the ZSM-5 zeolite, a high-silica aluminosilicate zeolite that converts methanol to gasoline (MTG) with high activity and selectivity to hydrocarbons with The MTG process and also other related processes, such as methylation of phe1-101,~ ber17tne.~t ~ l u e n c and , ~ thiophene,6 which occur over ZSM-5 catalysts, have methanol as the primary reactant. It is therefore important to understand the adsorption and reaction of methanol in the pores of ZSM-5. The catalytic properties of zeolites are crucially dependent on their acidity.' The Bransted acid sites in ZSM-5 zeolites have been confirmed as the active site for the initial adsorption and further conversion of methanol to hydrocarbons in the gasoline range.* It is difficult to determine the most favorable adsorption site for a polar organic molecule such as methanol inside ZSM-5, which has a complicated pore structure. Unambiguous information from experimental sources9 has not been obtained. The formation of a I : I complex of methanol with the Br~nstedacid site has been reported;1° for the particular A1 concentration in the reference cited, this corresponds to 1.25 molecules adsorbed per unit cell in ZSM-5. However, high levels of adsorption, of nearly I O molecules per unit cell, have been reported" even in the silicalite lattice where there are only low levels of Bransted acid sites. I n d previous we showed that the combination of quantum mechanical with energy minimization techniques is a reliable procedure for predicting the acidic properties of zeolites including the proton affinity, the effect of aluminum substitution, ( I) M e i s e l . S. L. Philos. Trans. R. Soc. London 1981, A300, 151. ( 2 ) Meisel. S . L., McCullough. J . P.: Lechthaler. C H.; Weisz, P. B. C H E M T E C H 1976, 86. ( 3 ) Rcnaud. M.: Chantal, P. D : Kaliaguine, S. Can. J. Chem. Eng. 1986, 6 4 . 787 ( 4 ) Chu. C. C.; Kaeding. W. W. Australian Pat. 546331, 1985. ( 5 ) Ishida, H.;Nakajima, H . German Offen. 3522727, 1986. ( 6 ) Salinas. Y: Monaci. R.; Longu. G.: Forni. L. Acta Phys. Chem. 1985, 3 / , 291. ( 7 ) Dwqer. J S t u d . Sur/. S r i . Caral. 1988, 37, 333. ( X i Anderson. J . R.: Foger. K . ; Mole. T.; Rajadhyaksha, R. A , ; Sanders, J . \ J . C'utul. 1979, 58, 114.
and the vibrational frequency of the 0-H bond. The present paper applies the same powerful techniques to understanding the behavior of methanol at the Br~nstedacid sites of ZSM-5 zeolite. We find that the initial sorption of methanol occurs at the Br~nsted site, through the interaction between the oxygen atom of the methanol and the hydroxy group of the ZSM-5. The structure of the adsorbed molecule then optimizes to a minimum-energy configuration, in which the C-H bond cleavage may occur leading to the formation of highly reactive species.
Methodology Computer simulation techniques are ideally suited for finding the minimum-energy configuration of molecules in zeolites. In this paper, our approach is to use lattice simulation calculations to predict adsorption configurations for methanol in ZSM-5 pores for both an all-silicon framework and an aluminum-substituted framework. The configurations so obtained are then studied by using high-level quantum chemical calculations. Ab initio Hartree-Fockself-consistent-fieldcalculations are performed on clusters representing the ZSM-5:CH30H complex. These calculations are carried out with both minimal (STO-3G)14 and extended (SV-3-21G)I5 basis sets. The GAMESS (General Atomic and Molecular Electronic Structure Systems) code developed by Guest and Kendrick16 available at the CRAY-XMP at Rutherford was used for the present calculations. The quantum chemical cluster includes the methanol molecule and three S O 4 units of the zeolite framework which are in the vicinity of the methanol molecule. More distant ions of the zeolite framework are treated as point charges to describe the long-range effects. We found this to be an effective way of fixing the boundary for the quantum chemical cluster, since the electronic effects influence only the neighbor and next-neighbor atoms, while the electrostatic effects extend to much further distances. We consider that the inclusion of the effects of these long-range forces is important for realistic theoretical studies of these systems. In particular, it is important to reproduce accurately the variation in the gradient of forces within the quantum chemical cluster. The detailed procedure for generation of a suitable cluster for such realistic simulation of long-range forces is described e1~ewhere.l~ These calculations yield the minimum-energy geometry, the adsorption energy, and the electronic structure of the adsorbed methanol molecule. This information provides important clues for the way in which the methanol adsorbs and dissociates over the Brmsted
( 9 ) Chang. C. D. Hydrocarboos f r o m Methanol: Marcel Dekker: New
l d r k . 1983 i 10) Ison.
Gorte, R J. J . C'arc~l.1984. 89, 10. t i Zeolite.\ 1987. 341, 7 . R . Cutlw. C R. 4.: Colbourn. E. A S t d Surf: Sci Caral. u . C-. R . A : Cg)lbourn. E. A I'roc
H. S o < .London
,
(14) Hehre, W. J.: Stewart, R. F.; Pople, J. A. J . Chem. Phys. 1969, 5 1 ,
2657. ( 1 5 ) Gordon, M. S.; Binkley, J. S.; Pople. J . A,; Pietro, W. J.: Hehre, W. J . J. Am. Chem. Soc. 1982, 104, 2797. (16) Guest, M. F.: Kendrick, J. GAMESS User Manual: Introductory Guide lo GAMESS: University of Manchester Computer Centre, Manchester, 1986.
c3 1989 American Chemical Society
The Journal of Physical Chemistry, Vol. 93, No. I I, I989 4595
Adsorption Behavior of Methanol in ZSM-5
ci 0
0
a
0
-
Figure 2. Minimum-energy configuration I for methanol inside point charge cluster of ZSM-5 zeolite (view along b axis). The geometry of the methanol in relation to the nearest framework atoms is shown as inset a (view along a axis).
..
Figure 1. Methanol molecule at the center of the cavity formed by the
'-4
AL
intersection of the straight and sinusoidal pores in ZSM-5 zeolite. Molecular axis of methanol lies perpendicular to the straight pore (view along b axis).
a
SI
acid site and allows us to identify the initial reactive products formed.
Results Lattice Simulation Calculations. The calculations employ energy minimization techniques using effective potentials. The crystal is simulated by using the Born model in which we include both the long-range electrostatic terms and the short-range repulsive interionic forces. The partially covalen't nature of the zeolite is represented by the incorporation of a three-body bond-bending term for 0-Si-0 bonds using potentials developed by Sanders et The atomic positions of the ZSM-5 lattice were taken from the single-crystal X-ray diffraction studies by Olson et a1.,18 and they were relaxed to obtain a zero strain lattice. The observed relaxations were small. The sorbate molecule is then introduced and the energy minimized with respect to its position and orientation and with relaxation of surrounding lattice ions being allowed; a spherical region containing 120 atoms is relaxed to equilibrium. These calculations are carried out using the CASCADE (Cray Automatic System for -Calculation of Difect Energies) package developed by Le~1ie.l~ The following features of the calculation should also be noted. (i) The geometry and the net charges of the atoms in the methanol molecule were obtained from the results of ab initio quantum mechanical calculations. (ii) During the relaxation of methanol to locate the adsorption site in ZSM-5, the methanol molecule was constrained to be rigid; i.e., no internal distortion of the molecule was allowed at this stage. The variation in the geometry of methanol molecule inside the zeolite is studied later by performing quantum chemical calculations. (iii) The calculations were performed for the all-silicon (silicalite) structure and for the Al-substituted compound (ZSM-5). To represent AI substitution, we replace one Si4+ion (at the T2 site of the secondary building unit reported by Olson et a1.18) by an AI3+ ion. The T2 site was chosen for aluminum substitution since it lies in the intersection of straight and sinusoidal pores,
Figure 3. Minimum-energy configuration I1 for methanol inside point charge cluster of silicalite (view along b axis). The geometry of the methanol in relation to the nearest framework atoms is shown as inset
a (view
a axis)*
which has been suggested as a probable framework site for AI substitution from NMR studies.20 The 02-ion connecting the adjacent Si4+ion (at the T8site) is replaced by an OH- ion. The protonated oxygen compensates the effective charge of the AI substituent and is responsible for the acidity of the zeolite. The methanol molecule was then introduced at the intersection point of the straight and sinusoidal pores of the ZSM-5 zeolite. The molecular axis of methanol lies perpendicular to the straight pore as shown in Figure 1. Other starting configurations of methanol in the zeolite with the molecular axis lying parallel to the straight channel were also tried, and in all cases the methanol was found to relax to the same final configuration. As mentioned earlier, the interatomic potentials used for the zeolite lattice have been reported by Sanders et aI.l7 The zeolite-methanol interatomic potentials used are the atom-atom parameters reported by Kiselev et aL2I Adsorption Sites for Methanol. The attractive Coulombic forces between the hvdrogen atoms of the methanol and the zeolite framework cause the methanol to relax toward the framework. Two different energy minima were obtained for both the aluminum-substituted framework and the completely siliceous .
(17) Sanders, M. J.; Leslie, M.; Catlow, C. R. A. J . Chem. &IC., Chem. Commun. 1984, I27 1. (1 8) Olson, D. H.; Kokotailo, G. T.; Lawton, S. L.; Meier, W. M. J. Phys. Chem. 1981,85, 2238. (19) Leslie, M. Technical Memorandum No. DL/SCI/TM31T, SERC Daresbury Laboratory, 1982.
I
-
(20) Derouane, E. G.; Hubert, R. A. Chem. Phys. Lett. 1986, 132, 315. (21) Kiselev, A. V.; Lopatkin, A. A.; Shulga, A. A. Zeolites 1985,5, 261.
Vetrivel et al.
4596 The Journal of Physical Chemistry, Vol. 93, No. 11 1989 I
TABLE I: Results of ab Initio Calculation for Configuration I
clusters final optimized configuration of the complex HSi2AIOIa:CH30Hin 82-ion
total energy, au STO-3G SV-3-21G basis set basis set -1688.1853 -1703.0091
point charge cluster final optimized configuration of the zeolite -1574.6267 -1588.5918 cluster (HSi2A101a)in 82-ion point charge cluster -1 13.5460 -1 14.3957 free methanol molecule -0.0126 -0.0216 adsorption energy of methanol (AEad) framework. Different starting configurations were tried for the methanol molecule to ensure that no other minimum-energy sites were missed. The geometries of the methanol in the two configurations are quite distinct and are shown in Figures 2 and 3. The geometry of the molecule as viewed from a perpendicular direction, in relation to the nearest framework atoms, is shown in Figures 2a and 3a (as insets). In the purely siliceous system, the configuration shown in Figure 3 has greater stability, whereas in the AI-substituted zeolite the alternative configuration has a higher binding energy, although the other configuration is also stable. We shall see in the next section how the quantum mechanical studies reverse the relative orders of the calculated stabilities for the AI-substituted system. The values of adsorption energies are -0.64 and -0.96 eV for configurations I and 11, respectively. These values are strongly dependent on the charges on the atoms of methanol molecule. More reliable estimates of this energy are given by the quantum chemical study. Indeed, the methods used in the calculations described above are inherently approximate but are efficient and inexpensive ways of locating the geometries of the sorbed species. In the next section we use nonempirical quantum chemical methods to obtain details of the minimum-energy configuration and the electronic structure and reaction mechanisms of the sorbed species. Hartree-Fock-Self Consistent-Field Calculations. We represent the adsorption complex in the ZSM-5 zeolite, namely, the sorbed methanol and the neighboring framework atoms, by clusters (Figures 2a and 3a) containing three “SiO,” units. One of the Si4+ions is replaced by an AI3+ion, and the 02-ion bridging the silicon and aluminum is replaced by the OH- ion. Hence, the HSi2AlOIocluster represents the active site in the zeolite. For the first configuration (I) (Figure 2a), the tetrahedral groups at the T2, T8, and T7 sites are considered, while for the second configuration (11) (Figure 3a), the tetrahedral groups at T,, T,, and T3 sites are considered, since these are the atoms that interact with the methanol molecule. In both the clusters, the aluminum is at the T2 site and the proton is attached to the oxygen bridging the T2 and T8 sites. As mentioned earlier, the quantum chemical clusters were embedded in an array of point charges representing the long-range forces in the ZSM-5 lattice. The point-charge cluster containing 82 ions (shown in Figures 2 and 3) is able to generate the effect of the infinite lattice on the quantum chemical cluster including the methanol molecule in terms of the Madelung energy and the gradient of forces. Charges of 2+ and I- are assigned to the points representing Si and 0 atoms in this region, in view of the partly covalent nature of the zeolites. The geometry of the methanol molecule as well as the neighboring atoms of the zeolite framework was optimized to obtain an equilibrium configuration. The adsorption energy (LEad)of methanol over ZSM-5 may then be calculated as the difference between the V B des of the total energy for the optimized configurations of the adsorption complex and the sum of the energies for the zeolite cluster and the isolated methanol molecule, both of which were in turn geometrically optimized. Thus, for AE,, we write AEad = [TE(HSi2AlOIo:CH3OH)][TE(HSi2A10,0)+ TE(CH30H)] where TE is the total energy
TABLE 11: Results of ab Initio Calculations for Configuration I1
clusters final optimized configuration of the
total energy, au STO-3G SV-3-21G basis set basis set -1688.0728 -1702.7320
complex HSi2A1010:CH,0Hin 82-ion point charge cluster final optimized configuration of the zeolite -1 574.4609 -1 588.3344 cluster (HSi2A101a)in 82-ion point charge cluster -1 13.5460 -1 14.3957 free methanol molecule -0.0659 -0.0019 adsorption energy of methanol (AEad)
0
s 1
Q “
0 .
Figure 4. Starting geometry of configuration 11.
The results for the two configurations will now be described in detail. Configuration I. For the configuration shown in Figure 2a, the calculated values of the energies for the complex, the zeolite cluster, and the methanol molecule as well as the adsorption energy are given in Table I. The final optimized configurations obtained by using both basis sets were very similar. The adsorption energy calculation was carried out for the final geometry using the SV6-21G basis set, and a value (-0.021 au = -0.58 eV) close to that was obtained by using the SV-3-21G basis set. This value is encouragingly similar to that obtained by the lattice simulation calculations. The values of AEadindicate that this is a physically adsorbed configuration, and there were no significant changes in the bond lengths and bond angles obtained from the earlier CASCADE calculations. Calculations were performed to investigate whether the framework proton could be transferred to the methanol molecule. Such a reaction was found to be energetically unfavorable. Furthermore, no distortions were found in the positions of the framework atoms and the methanol molecule. The activation energies for breaking the C-0 bond and 0 - H bond were calculated by stretching these bonds in the adsorbed configuration. The values of the activation energy barrier were exactly the same as calculated for a free methanol molecule, indicating that the dissociation of the methanol molecule is not facilitated in this configuration. Configuration IZ. The same calculation procedure was repeated for configuration I1 (shown in Figure 3a) in the ZSM-5 lattice. The results of the calculations performed with different basis sets are given in Table 11. Very different behavior was observed for the methanol molecule during the energy minimization procedure for this configuration. Calculations with the STO-3G basis set predict a large adsorption energy, indicating chemisorption. The detailed geometries for the initial configuration and the final minimum-energy configurations are shown in Figures 4 and 5, respectively. The net charges on the various atoms of the cluster in the initial and final configuration calculated by a Mulliken population analysis are given in Table IV. The analysis of the geometry (Table 111) indicated a strong interaction between the framework and a hydrogen atom in the methanol molecule. There
The Journal of Physical Chemistry, Vol. 93, No. 11, 1989 4597
Adsorption Behavior of Methanol in ZSM-5
TABLE IV: Net Charges on the Various Atoms of the Quantum Chemical Cluster Representing the Adsorption Complex
0 0
0 0 Figure 5. Final geometry of configuration 11, after optimization. TABLE 111: Variations in the Interatomic Distances of the HSi2AIOIo:CH30HCluster in Configuration I1 during Optimization
atoms C(methano1)-H(methano1) 02(zeolite)-H(methanol) O13(zeolite)-Al(zeolite)
OI3(zeolite)-H(zeolite) H(zeolite)-O(methano1)
interatomic distances, 8, initial final 1.08 1.97 2.23 1 .oo 1.60 1.77 0.98 0.98 1.77
3.21
is a considerable amount of distortion in the positions of the framework atoms, indicating the influence of the polar guest molecules such as methanol on the local environment inside the zeolite host lattice. In the final configuration (Figure 5), one of the hydrogens of the methyl group has essentially dissociated and rebonded to a framework oxygen. The calculations show no activation energy barrier to the process. This result is somewhat surprising, and it may be that there is a small barrier which is not reproduced by our calculations. Calculations performed with a higher quality basis set, namely, SV-3-21G basis, also predicted a similar final configuration. However, the adsorption energy predicted is now surprisingly small. We found that the formal charges calculated on the atoms of the methanol molecule using the Mulliken population analysis have larger absolute values for the SV-3-21G basis than for the STO-3G basis set. These larger formal charges may give rise to greater repulsive forces between the electrons and the point charges present to simulate the zeolite lattice. However, the most significant result obtained here is the geometry of the final configuration rather than the absolute value of the adsorption energy. In the final configuration (Figure 5) the distance of one of the hydrogens of the methyl group from carbon and an oxygen in the framework indicates the abstraction of a hydrogen atom from methanol by the zeolite framework, confirming the calculation with the smaller basis set. The formal charges calculated also suggest the formation of a [ C H 2 0 H ] species. Discussion
There is little information available in the literature regarding the exact location and geometry of methanol in ZSM-5 zeolite. I n their study of the adsorption of methanol over ZSM-5 in the temperature range 273-425 K using quasi-elastic neutron scattering spectroscopy techniques, Jobic et a1.22characterized a methanol species bound to the zeolite via hydrogen bonds in addition to another free diffusing species inside the pores. Though they could not arrive at a unique model for the adsorbed species from the variation of the elastic incoherent structure factor with temperature, the most probable model proposed by them is con(22) Jobic, H.; Renouprez, A,; Bee, M.; Poinsignon, C. J . Phys. Chem. 1986, 90, 1059.
net charges on various atoms initial final configurationb configurationC Zeolite Framework A12 +0.80 +0.87 0 1 -1.26 -1.23 0 2 -0.78 -0.60 0 6 -1.26 -1.19 O13 -0.69 -0.59 Si8 + 1.08 +1.10 012 -1.19 -1.16 os -1.22 -1.19 07 -1.16 -1.13 Si, +0.90 +0.92 0 3 -1.20 -1.14 O19 -0.99 -0.94 020 -1.25 -1.19 H (attached to OI3) +0.22 +O. 18 atoms numbering"
H (attached to 0) 0 C H (nearer to framework) H H
Methanol +0.18 -0.28 -0.07
+0.06 +0.06 +0.06
+0.02 -0.34 -0.30 +0.23 -0.14 -0.13
"Reference 18. bAs in Figure 4. cAs in Figure 5. sistent with the configuration predicted by the present calculations for the ZSM-5 lattice. From the infrared spectroscopy study of the methanol molecule adsorbed on H-ZSM-5, Ison and Gorte'O concluded that the adsorbed methanol remains unreacted and desorbs without undergoing any reaction. However, a thermal desorption study of methanol by Novakova et aLZ3reports the production of gaseous products ranging from CH4, HCHO, to Cz-s aliphatic compounds. Hence, there is no conclusive evidence available from the experimental studies. The importance of C-H bond weakening was also discussed in the paper of Novakova et al., although they argued that the dissociation of methanol proceeded through the breaking of the C-0 bond in methanol, with the C H 3 fragment then binding to a framework oxygen. However, the formation of gaseous products observed by Novakova et al. could be explained by the intermediate predicted by our calculations. From this study, we may be able to answer some critical questions regarding the initial mode of adsorption and dissociation mechanism of methanol in the ZSM-5 zeolite. In the initial configuration (Figure 4), the zeolite proton and the oxygen of the methanol are within a hydrogen-bonding distance as can be seen from Table 111. However, during the geometry optimization of the quantum chemical cluster the bridging oxygen (0,)between AI at the T, site and Si at the site stretches into the pores toward one of the hydrogens of the methyl group. This causes a repulsive force between the 0,(zeolite) and the O(methanol), and hence in the final configuration (Figure 5), there is no interaction between the zeolite proton and the oxygen of the methanol molecule. This mechanism indicates that the methanol activates at the Bransted acid site, dissociates probably to a C H 2 0 H species, and leaves the Brransted acid site free for other reactants. This partly explains the experimentally observed phenomena of high turnover frequency of the methanol conversion over ZSM-5 zeolite in spite of the low concentration of the active sites. An additional factor may be the reaction of the methanol from the gas phase directly with the growing hydrocarbon chain. It is difficult at this stage to comment further on the fate of the [ C H 2 0 H ] species in the reaction pathway. The conversion of methanol to dimethyl ether in ZSM-5 occurs readily, and ethylene has also been identified as a primary product under a wide range of reaction conditions. We speculate that the [C-
r3
(23) Novakova, J.; Kubelkova, L.; Dolejsek, Z. J. Coral. 1987, 108, 208.
4598
J. Phys. Chem. 1989, 93, 4598-4603
H,OH] species may dissociate to form a C H 2 radical which is a precursor for the formation of ethene; alternatively, the [CH,OH] may react with other methanol molecules to form surface bonded oxonium methylide as proposed by Hutchings et al.24 Indeed, although our calculations predict the abstraction of hydrogen from the methyl group of methanol, the abstraction of hydrogen may occur from an oxonium methylide intermediate, since the rate of formation of this intermediate may be faster than the dissociation of methanol. Thus, the key feature of our results is the suggestion of C-H bond weakening which may occur in (24) Hutchings, G. J.: Gottschalk, F.; Hall, M. V . M.; Hunter, Soc., Faraday Trans. 1 1987, 83, 571
R. J. Chem.
species other than methanol. Our future work will focus on the study of the adsorption behavior of ethene and dimethyl ether in ZSM-5 zeolite to improve the understanding of the subsequent steps in the reaction pathway. It is of course also possible that sorption at different sites characterized by different AI substitutions may give rise to alternative dissociation mechanisms. Future studies will examine these possibilities. The present study demonstrates, we believe, the power of the combination of modeling with quantum chemical techniques in the study of these catalysts. Acknowledgment. We thank IC1 PIC for financial support to R.V. (under the Joint Research Scheme arrangement). Registry No. MeOH, 67-56- I .
Observation of Cyanate and Isocyanate Surface Species during the Reaction of Ammonia and Carbon Monoxide over Supported Rhodium D. K. Paul, M. L. McKee, S. D. Worley,* Department of Chemistry, Auburn University, Auburn, Alabama 36849
N. W. Hoffman, Department of Chemistry, University of South Alabama, Mobile, Alabama 36688
D. H. Ash, and J. Gautney National Fertilizer Development Center, Tennessee Valley Authority, Muscle Shoals, Alabama 35660 (Received: October 7 , 1988; In Final Form: January 27, 1989)
The reaction of CO with NH3 over preoxidized Rh/Si02 has been studied by infrared spectrwcopy to detect surface intermediates. Two of the surface species having infrared bands at 2172 and 2225 cm-l have been identified as RhNCO and RhOCN, respectively. This is likely the first observation of these two surface species simultaneously on a supported transition-metal catalyst. High-quality ab initio computations have been employed to aid in the band assignments.
Introduction More efficient synthetic routes to dicyandiamide, a potent nitrification inhibitor that significantly enhances nitrogen fertilizer efficiency and has potential for reducing ground water pollution by nitrates, are being explored by the fertilizer industry. Cyanamide, which is an intermediate in the production of dicyandiamide, could be a product in the catalytic reaction of ammonia with C O or CO,. We have been exploring the CO/NH, reaction over a variety of supported transition-metal catalysts. Among our efforts have been studies of adsorbed species on these catalysts using infrared spectroscopy in work similar to that reported earlier for catalytic methanati~nl-~ and the decomposition of small organic molecule^.^ Specifically, in this investigation surface species formed during the C O / N H 3 reaction over preoxidized Rh/Si02 in an infrared-cell reactor were monitored and high-quality ab initio computations for the suspected rhodium cyanate and isocyanate species were performed. Experiment and Theory Supported Rh/Si02 films (2.2 wt % Rh) were prepared by spraying a slurry of RhC1,.3H20 (Alfa Products, Inc.), SiO,
(Cabosil M5, 200 m2 g-l, Cabot Corp.), acetone, and water onto a heated 20-mm CaF2 infrared plate. The IR plate containing the adhered RhCI3.3H2O/SiO2was placed in an IR cell reactor similar in design to that employed previously in these laborat o r i e ~ . I -A ~ chromel-alumel thermocouple was used to monitor the temperature of the film during reaction. The samples were outgassed at 298 K for 15-20 h, heated at 10” Torr for 1 h, and subjected to 5-, 5-, IO-, and 20-min cycles of exposure to 78 Torr of H2 or O2at 523 K (each cycle followed by evacuation to Torr) and 1 h of evacuation to 10” Torr at 533 K. Then the cell was dosed with a reactant gas (2:l CO/NH3, HNCO, HCN, or formamide) with the pressure measured by an MKS Baratron capacitance manometer, and the infrared spectra were monitored (IBM FTIR 44 Fourier-transform IR spectrometer operating at a resolution of 4 cm-I) as a function of temperature and/or reaction time. The HNCO gas was prepared by reaction of aqueous KOCN with 95% H3P0, at 283 K; H C N was generated from reaction of KCN with H 2 S 0 4at 298 K. Both were purified by trap-to-trap distillation. Formamide was purchased from Aldrich Chemical Co. and purified by vacuum distillation. The theoretical computations employed in this work were of the ab initio type (GAUSSIAN M).’ The 36 core electrons of Rh
( 1 ) Henderson. M. A.; Worley, S. D.
J. Phys. Chem. 1M15, 89, 1417. ( 2 ) Worley, S. D.; Mattson, G . A,; Caudill, R . J . Phys. Chem. 1983, 87,
1671. (3) Dai. C . H.; Worley, S. D. J . Phys. Chem. 1986, 90, 4219. ( 4 ) Dai, C . H . ; Worley, S. D. Langmuir 1988, 4, 326.
0022-3654/89/2093-4598$01 .Sol0 , , ,
( 5 ) GAUSSIAN 86: Frisch, M. J.; et al. Carnegie-Mellon Quantum Chemistry Publishing Unit, Carnegie-Mellon University: Pittsburgh, PA. The ECP code was obtained from the Theoretical Division, Los Alamos National Laboratory, Los Alamos, N M .
Q 1989 American Chemical Society
