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Langmuir 1991, 7, 140-141
Hydrogen-Assisted Dissociation of CO on a Catalyst Surface G. Blyholder' and M. Lawless Department of Chemistry and Biochemistry, University of Arkansas, Fayetteuille, Arkansas 72701 Received March 5, 1990. In Final Form: July 3, 1990
Molecular orbital calculations for the dissociation of CO on the (100) face of a Feln cluster indicate that the experimentally observed acceleration of CO dissociation on metal catalysts by hydrogen, which has been suggested to occur by several different mechanisms, most likely occurs by the attack of a hydrogen atom on the carbon atom of adsorbed CO. The hydrogenation of CO is an important topic to which recent review articles give an For some metal catalysts the first step has been demostrated to be the dissociation of CO to give a nascent surface C atom, which is subsequently hydrogenated and combined with other surface species to give various products. There is strong supporting evidence for this mechanism of hydrocarbon production over Ru3 and for the methanation reaction on Ni.6 The CO dissociation mechanism is presumed to contribute to reaction on other transition metals as well2 because CO has been shown to readily dissociate on most transition metals in the temperature range from 100 to 300 "C where CO hydrogenation is usually carried out. The dissociation of CO has been shown to occur on iron single crystal surfaces and high area supported iron2s7,8 and has led to the reasonable suggestion that CO hydrogenation is via a CO dissociation mechanism on iron. The presence of hydrogen has been found to have an effect on CO dissociation. For low-temperature adsorption the preadsorption of hydrogen on an iron surface results in a lower amount of dissociated CO being observed in thermal desorption experiments than when hydrogen is not pread~orbed.~ This appears to be due to hydrogen blocking sites for CO adsorption so there is less CO adsorbed and consequently less CO dissociation. For the very different conditions of high-temperature CO hydrogenation, kinetic studies indicate that hydrogen assists CO dissociation. From the kinetics of CO hydrogenation on supported Pd, it has been concluded that H assists CO dissociation with the formation of adsorbed HzO.'O. From the kinetics of CO methanation on Ni/SiO2 it has been suggested that there are CH,O species on the surface and that the mechanism involves formation of a COH surface intermediate with the H bonded to the 0." On Fe/A1203, kinetics of carburization of the surface and formation of CH4 have been used to propose hydrogen-assisted CO dissociation to form adsorbed OH.12 Time-resolved in( 1 ) Anderson, R. B. The Fischer-Tropsch Synthesis;Academic Press: New York, 1984. (2) Rofer-De Poorter, C. K. Chem. Reu. 1981,81, 447. (3) Bell, A. T. Heterogeneous Catalysis; Shapiro, B. L., Ed.; Texas A&M University Press: College Station, TX, 1984. (4) Ponec, V. Catal. Rev.-Sci. Eng. 1978, 18, 151. (5) Biloen, P.; Sachtler, W. M. H. Adu. Catal. 1981, 30, 165. (6) Goodman, D. W. Annu. Rev. Phys. Chem. 1986, 37,425. (7) Broden, G.; Gafner, G.; Bonzel, H. P. Appl. Phys. 1979, 13, 333. (8) Wedler, G.; Colb, K. G.; McElhiney, G.; Heinrich, W. Appl. Surf. Sci. 1978, 2, 30. (9) Benziger, J.; Madix, R. J. Surf. Sci. 1982, 115, 279. (10) Wang, S.-Y.; Moon, S. H.; Vannice, M. A. J. Catal. 1981, 71,167. (11) Ho, S. V.; Harriott, P. J. Catal. 1980, 64, 272. (12) Bianchi, D.; Bennett, C. 0. J. Catal. 1984, 86, 433.
0743-7463/91/2407-0140$02.50/0
frared spectra of CO dissociation on Ru(001) show a 2 orders of magnitude increase in CO dissociation rate effected by hydrogen.13 The mechanism of hydrogen assistance to the decomposition of CO on a metal surface is examined with molecular orbital calculations for the interaction of a H atom both on the carbon end and on the oxygen end of a CO molecule on an Fe(100) surface as the decomposition occurs. A potential energy surface and thus an activation energy for CO dissociation to give CH(a)+ O(a) or C(a) + OH(,) are obtained. The calculations were done with a semiempirical SCF method that is a modification of MIND0 referred to as MINDO/SR. The details of the method as well as its ability to handle a wide variety of compounds including large metal clusters have been reported p r e v i o u ~ l y . ~ ~ - ' ~ The MINDO/SR procedure explicitly includes electronelectron repulsions and is parameterized to give bond energies and lengths for selected reference compounds in agreement with experimental values. The parameters were selected to give reasonable properties to FeH, FeO, Fe(C0)5, HFe(CO)d-, (C0)4FeCHO-, CH3Fe(C0)4-, (C0)4FeC(O)CH3-,HFel2, COFel2, and OFe12. The double-{ basis for the Fe 3d orbitals is from Clementi and Roettilg and the d orbital energy and Slater-Condon parameters are from de Brouckere.20 Spectroscopic terms for the I;D ( k = 0) Slater-Condon parameters, which contribute most significantly to the final result, are used, so these are independent of the orbital exponents used. Some adjustments in these parameters were made toobtain better correlation between calculated and experimental properties for the reference compounds above. The /3 parameters for s, p, and d orbitals are the same. A 12-atom cluster was chosen as being large enough to represent several types of binding sites on a Fe(100) plane and small enough that calculations could be done in a reasonable length of time. The geometric arrangement of the atoms is shown in Figure 1. Calculations were done with the Fe cluster atom positions fixed as in the bulkz1 with a nearest-neighbor distance of 2.48 A and a next nearest distance of 2.86 A. The atoms on the top layer are next nearest neighbors t o each other a n d (13) Hoffmann, F. M.; Robbins, J. L. J. Electron Spectrosc. Relat. Phenom. 1987,45, 421. (14) Blyholder, G.; Head, J.; Ruette, F. Surf. Sci. 1983, 131, 403. (15) Blyholder, G.; Lawless, M. Prog. Surf. Sci. 1987, 26, 181. (16) Blyholder, G.; Head, J.; Ruette, F. Theor. Chim. Acta 1982,60, 429. (17) Blyholder, G.; Head, J.; Ruette, F. Inorg. Chem. 1982,21, 1539. (18) Ruette, F.; Blyholder, G.; Head, J. J.Chem. Phys. 1984,80,2042. (19) Clementi, E.; Roetti, C. Atomic Data and Nuclear Data Tables; Academic Press: New York, 1974; Vol. 14. (20) de Brouckere, G. Theor. Chim. Acta 1970, 19, 310. (21) Interatomic Distances. Spec. Publ.-Chem. SOC.1958, No. 11.
0 1991 American Chemical Society
Langmuir, Vol. 7, No. 1, 1991 141
Dissociation of CO on a Catalyst Surface
f
nn
H
\c------o
Fe3
I
QfJ
Figure 1. Feln cluster. Table 1. Activation Energies for Three Mechanisms of CO Dissociation activation enerev. kcal/mol 53 54 32
unassisted H on 0 assisted H on C assisted
nearest neighbors to bottom layer atoms. The iron atoms are approximately in a s1d7 configuration with the s electrons spin paired in bonding orbitals and the d electrons largely localized. A state with multiplicity 39 was found to give the lowest energy. The multiplicity 39 corresponds to a d7 configuration with three unpaired electrons per atom. No magnetic data for small iron clusters are available. In view of the atomic magnetic moment of bulk Fe being 2.22 p ~a state , of ~ high ~ multiplicity is expected. Potential energy curves for CO dissociation on many different sites of the (100) surface of the Felz cluster have been c a l c ~ l a t e d .The ~ ~ dissociation pathway giving the lowest activation energy has the CO molecule dissociating symmetrically over the center of a 4-fold site. Because this was the lowest energy CO dissociation process found, the effect on the dissociation of CO by a H atom in a concerted process at this site was examined. In separate calculations the potential energies along reaction paths when the H interacts with the C atom and when it interacts with the 0 atom of CO were calculated. The activation energies along the reaction paths for the three processes, unassisted CO dissociation, H on 0 assisted CO dissociation, and H on C assisted CO dissociation, were obtained from the potential energy curves and are shown in Table I. The energy scale is set to zero for CO and H separately adsorbed on the cluster. The C, 0, and H atoms are constrained to be on a plane perpendicular to the surace and bisecting the iron 7-8 and the iron 9-10 bonds. ~~
~~
~~
Fe
2.86 A
L/ Figure 2. Structure of activated complex for H on C assisted dissociation of CO.
Otherwise these atoms move to energy minimum positions as the C and 0 atoms are separated. For H on C assisted CO dissociation the H is initially placed above and to the left of the C atom in Figure 2, while for H on 0 assisted dissociation the H is initially above and to the right of the 0 atom. For each C and 0 separation along the reaction path, all three atoms were allowed to move to energy minimizing positions subject only to the constraint of staying in the plane perpendicular to the surface. The structure of the activated complex is shown in Figure 2, where the C-0 interatomic distance of 2.2 A indicates that the C-0 bond is almost completely broken in the transition state. The H on C assisted CO dissociation path is seen in Table I to have a considerably reduced activation energy. This indicates that hydrogen would have a large effect in assisting CO dissociation via this mechanism of a concerted H atom reaction with the C atom during CO dissociation. Kinetic measurements have shown that the presence of hydrogen accelerates the rate of CO dissociation on metal s u r f a c e ~ . ' ~ JPrevious ~ mechanistic proposals include attack on both the carbon and the oxygen end of the adsorbed CO. The calculations here clearly favor attack on the carbon atom. This suggests the inclusion of the following reaction as a mechanistic step in CO hydrogenation:
-
Coa& + HadB [H-C-Oadsl H-Cad, + Oads The production of CHads is consistent with the experimental evidence for reactive CH on the surface. On Fe/ A1203 isotope data have led to a conclusion that reactive CH, species are on the surface during CO hydrogenati~n.~~ By use of the empirical bond-order-conservationmethod, a similar conclusion was reached for Pt and Pd that the dissociation of CO is assisted by H attachment to C before dissociati~n.~~
Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research and to the University of Arkansas for a computing time grant.
~~
(22) Goodenough, J. B. Magnetism and the Chemical Bond; Interscience: New York, 1963. (23) Lawless, M. Ph.D. Dissertation, University of Arkansas, 1988.
(24) Stockwell,D. M.; Bianchi, D.; Bennett, C. 0. J . Catal. 1988,223, 13. (25) Shustorovich,E.; Bell, A. T. J. Catal. 1988, 223,341.
