J. Phys. Chem. 1987, 91, 869-873 and the finding of no effective quenching by I- is consistent with the previous study. The lack of quenching by I- provides evidence for a third region in the microemulsion in which there must be little or no population of excited pyrene singlets; from the discussion above it appears reasonable that this is the aqueous zone or water pool interior. The difference in the quenching abilities of I- and Cu2+are readily attributable to exclusion of the former from the net negatively charged interface region. The present results indicate that in these water/oil microemulsions consisting largely of reversed micelles there is a facile distribution of the aromatic hydrocarbon between the hydrocarbon and interfacial microphases. The observation of efficient quenching showing simple intensity and lifetime Stern-Volmer relationships for both hydrophilic and hydrophobic quenchers demonstrates that pyrene equilibrates rapidly between domains of quite different micropolarity. That equilibration is complete within the excited-state lifetime of both quenched and unquenched
869
pyrene is indicated by the constancy of the 11/13ratio for a given medium throughout a quenching experiment. It is reasonable to anticipate similar distributions between hydrocarbon and interface for other aromatic hydrocarbons; however, with excited states having shorter lifetimes it should be possible to kinetically separate quenching and exchange processes. The present results emphasize once again the importance of hydrophobic-hydrophilic interfaces as a prominent reaction site in aqueous surfactant media and suggest the utility of reversed micelles as a versatile yet highly organized reaction medium.
Acknowledgment. We are grateful to the U S . National Science Foundation (grant CHE-83 15303) for support of this research. Registry No. Cuz+, 15158-1 1-9; HzO,7732-18-5; NaI, 7681-82-5; CuS04, 7758-98-7; pyrene, 129-00-0; 2,5-dimethyl-2,4-hexadiene, 76413-6; methylviologen2+,4685-14-7; Aerosol OT, 577-1 1-7; heptane, 142-82-5.
CO Adsorption on P t ( l l 1 ) Doped with TiO, FeO, ZnO, and Fe, and Pt Ad-Atoms. Molecular Orbital Study of CO-Dopant Interactions Alfred B. Anderson* and Donald Q. Dowdt Chemistry Department, Case Western Reserve University, Cleveland, Ohio 441 06 (Received: July 16, 1986)
An atom superposition and electron delocalization molecular orbital study shows the oxygen end of CO adsorbed on a cluster model of the Pt(ll1) surface is attracted by A donation to empty d orbitals in Ti and Fe cations and an Fe ad-atom at an adjacent site. This attractive interaction causes tilting from normal and leads to decreases in the CO vibrational frequency of 3-400 cm-'. Adsorbed CO is not attracted toward the closed-shell d'O Zn2+cation or toward the nearly closed-shell Pt ad-atom because the antibonding counterparts to the T donation orbitals are occupied in these cases. These results are related to the strong metal-support interaction literature by way of a brief review of that literature. The predicted tilting with CO frequency decrease supports Sachtler'ssonjecture to this effect, but it is suggested that it is still not certain that such CO bond weakening is a necessary or sufficient condition for Fischer-Tropsch selectivity.
Introduction The strong metalsupport interaction (SMSI) effect discovered by Tauster et al.' refers to the way in which H2and CO adsorption on noble metal catalysts supported by early transition-metal oxides such as TiOz is suppressed in reducing environments. Both a hydrogen pressure or ultrahigh vacuum at moderately high temperatures can provide the necessary reducing conditions.2 It is presently believed that site blocking by reduced TiO, species is responsible for the decreased ad~orption.~ In support of a mobile oxide molecule mechanism for the necessary support oxide mobility to lead to site blocking, recent theoretical work shows RuO, RuOz, R u 0 3 , and R u 0 4 molecules have low diffusion barriers on the Ru(001) ~ u r f a c e . ~In this study the 2+, 4+, and 6+ ruthenium cations as well as the oxygen 2- anions were found to bond to the metal surface. Haller and co-workers5 have EXAFS evidence for similar metal-metal cation bonding between rhodium and titania subjected to reducing conditions. Despite this blocking, SMSI systems show a signitficant enhancement of Fischer-Tropsch methanation.6-" Fischer-Tropsch promotion is thought to be the result of CO dissociation prior to hydrogenation.61' Although the transfer of charge from the reduced oxide to the neighboring metal atoms has been mentioned by Burch and Flambard as possibly important to C O activation,'O an alternative explanation, also suggested by them, has considerable circumstantial evidence to support it. In the latter proposed explanation it was suggested that C O might bridge the metal and support oxide, with C bonded *Address correspondence to this author. 'Present address: Chemistry Department, Allegheny College, Meadville, PA 16335.
0022-3654/87/2091-0869$01.50/0
to the metal and 0 bonded to a cation of the support oxide. The observation of low CO vibrational frequencies in a SMSI system has recently led Sachtler to conclude that the bridging structure indeed forms.I2-'* His conclusion is based on comparing the observed frequencies in Fischer-Tropsch promoted systems with those of bridging carbonyl ligands in transition-metal cluster compounds of known structure. There is evidence supporting the idea that low C O vibrational frequencies are associated with relatively easy CO dissociation. According to and
(1) (a) Tauster, S. J.; Fung, S . C.; Garten, R. L. J . Am. Chem. SOC.1978, 100, 170. (b) Tauster, S.J.; Fung, S . C. J . C a r d . 1978, 55, 29. (c) Tauster, S.J.; Fung, S. C.; Baker, R. T. K.; Horsley, J. A. Science 1981, 211, 1221. (2) Sadeghi, H. R.;Henrich, V. E. J . Catal. 1984, 87, 279. (3) Dwyer, D. J.; Cameron, S. D.; Gland, J. Surf. Sci. 1985, 159, 430.
(4) Anderson, A. B.; Awad, Md. K. Surf. Sci., in press. ( 5 ) Sakellson, S.; McMillan, M.; Haller, G. L. J . Phys. Chem. 1986, 90, 1733. (6) Vannice, M.A,; Garten, R. L. J . Catal. 1979, 56, 236. (7) Vannice, M.A,; Garten, R. L. J . Catal. 1980, 66, 242. (8) Bartholomew, C. H.; Pannell, R. B.; Butler, J. L . J . Catal. 1980, 65, 335. (9) Kao, C.-C.; Tsai, S.-C.; Chung, Y . - W .J . Catal. 1982, 73, 136. (10) Burch, R.;Flambard, A. R. J . Catal. 1982, 78,389. (11) Vannice, M.A. J . Catal. 1982, 74, 199. (12) Sachtler, W.M.H. Proc. Int. Congr. Catal., 8th 1984, 1 , 151. (13) Sachtler, W.M.H.; Shriver, D. F.; Hollenberg, W. B.; Lang, A. F. J . Catal. 1985, 92, 429. (14) Sachtler, W.M.H. Iberia-American Conference on Catalysis, July, 1986. (15) Shinn, N. D.; Madey, T. E . Phys. Rev. Lett. 1984, 53, 2481. (16) Shinn, N.D.; Madey, T. E. J . Chem. Phys. 1985, 83, 5928.
0 1987 American Chemical Society
Anderson and Dowd
870 The Journal of Physical Chemistry, Vol. 91, No. 4, 1987 TABLE I: Atomic Parameters Used in the Calculation" P
S
atom
n
IP
r
Pt Ti (0) Fe ( 0 ) Zn ( 0 ) 0 (Ti) 0 (Fe, Zn) Fe c (0) 0 (C)
6 4 4 4 2 2 4 2 2
10.50 8.12 9.77 12.19 26.98 26.98 937 15.09 26.98
2.550 1.500 1.700 1.900 1.846 1.946 1.700 1.658 2.146
n 6 4 4 4 2 2 4 2 2
I?
r
6.46 6.15 7.34 8.18 12.12 12.12 6.94 9.76 12.12
2.250 1.200 1.400 1.600 1.827 1.927 1.400 1.618 2.127
d
n
IP
(1
CI
(2
c2
5 3 3 3
11.1 9.3 10.9 14.0
6.013 4.550 5.350 6.150
0.6562 0.4206 0.5366 0.5951
2.396 1.400 1.800 2.200
0.5711 0.7839 0.6678 0.5951
3
10.5
5.35
0.5366
1.800
0.6678
"Principal quantum number, n, ionization potential, 1P (ev), Slater orbital exponents, ( (au), and linear coefficients, c, for double-( d functions. The text explains their sources.
Top View
SideView
Figure 1. Lying down of CO on Pt3Ti.
binding site'
\central Pt
Figure 3. Cluster of 22 Pt atoms used to model the (1 11) surface.
K
donation '\ stabilization
closed-shell repulsion
Figure 2. Orbital energy level correlations for CO adsorbed lying down on transition-metal surfaces.
theory,I7 at low coverages CO adsorbs lying down on the (1 10) Cr surface, has a large decrease in vibrational frequency, and has a severely reduced dissociation barrier of about 0.4 eV. In other CO is found to lie recent experimental'* and t h e ~ r e t i c a lstudies '~ down on surfaces of the Pt3Ti alloy. This case is relevant to Ti02 promotion in that in the alloy Ti6+centers form because of charge donation to the more electronegative Pt centers.lg CO is predicted to adsorb with the carbon end bonded to Pt and the oxygen end bonded to Ti (Figure 1) and, as on Cr( 1lo), the CO dissociation energy is calculated to be very low, 0.7 eV on the (1 11) surface and 0.9 eV on the (100) surface. The calculations predict a decrease in adsorbed CO vibrational frequency of about 400 cm-l. The ability of CO to adsorb bridging or lying down on Cr( 110) and in association with Ti centers on Pt3Ti has been explained theoretically in terms of the relatively high electron accepting ability of the d electron deficient early transition metal^.'^^'^^^^ This makes donation from the occupied a orbitals of CO to the metal d orbitals a stabilizing possibility because for such metals there are insufficient d electrons to fully occupy the antibonding counterpart orbitals. Figure 2 gives the general bonding picture. Because of the large overlap between CO molecular orbitals and metal surface orbitals when CO lies down so that both atoms contact the surface, not only a donation but also u donation and metal backbonding to CO T * orbital stabilizations are all greater than for CO bonded perpendicular to the surface. The way to view the difference between early and late transition metals vis ii vis CO adsorption is to say that it is closed-shell repulsions between the filled a orbitals and filled metal d band orbitals that (17) Mehandru, S.P.; Anderson, A. B. Surf. Sci. 1986, 169, L281. (18) Bardi, U.; Dahlgren, D.; Ross, P. N. J . Cutul. 1986, 100, 196. (19) Mehandru, S. P.; Anderson, A. B.: Ross, P. N. J . Curd. 1986, 100, 210.
(20) Anderson, A. B.; Onwood, D. P. Surf. Sci. 1985, 154, L261
cause CO to stand up on surfaces of the late transition metals. It is the early transition-metal oxide dopants, including La203, ZrO,, TiO,, Nb205,and MnOz, that are known to activate the formation of Fischer-Tropsch products, that is, those hydrocarbons and higher alcohols and aldehydes formed from dissociated CO.I4 In the presence of ZnO, MgO, and CaO, methanol forms rather than Fischer-Tropsch p r 0 d ~ c t s . lIt~ is reasonable to ask if the early transition-metal cation centers will attract the oxygen end of CO as postulated on the basis of reduced CO vibrational frequencies and as could be anticipated based on the theoretical finding that CO forms stable bridging structures because of a donation to empty d orbitals in Cr( 110) and in Ti of the Pt3Ti alloy. Furthermore, it may be asked if the absence of empty d orbitals makes Zn2+, Mg2+,and Ca2+ inactive toward CO bond scission. It is these questions that we address in the present theoretical study.
Theoretical Model We use the atom superposition and electron delocalization molecular orbital (ASED-MO) method used in the above-mentioned studies of CO chemisorption on metal s ~ r f a c e s . ' ~ , ~ ~ , ~ ~ Dopant oxides of titanium, iron, and zinc on platinum are modeled by placing diatomic TiO, FeO, and ZnO molecules on the 22-atom cluster 2-layer thick model of the Pt( 111) surface shown in Figure 3. A platinum surface is chosen because of considerable experience with the theory of CO adsorption on platinum surfaces in this l a b ~ r a t o r y land ~ * ~because ~ titania-supported platinum is a selective Fischer-Tropsch catalyst" and exhibits the other SMSI proper tie^.^^^^^^-^^ The Pt atom parameters, given in Table I, are Ray, N. K.; Anderson, A. B. Surf. Sci. 1982, 119, 35. Ray, N . K.; Anderson, A. B. J . Phys. Chem. 1982, 86, 4851. Ray, N. K.; Anderson, A. B. Surf.Scir 1983, 125, 803. Anderson, A. B.; Awad, Md. K. J . Am. Chem. SOC.1985,107,7854. Anderson, A. B.; Grimes, R. W.; Hong, S. Y . ,submitted for publication. (26) Anderson, A. B.; Mehandru, S. P.; Smialek, J. L. J . Elecrrochem. SOC. 1985, 132, 1695. (27) Cairns, J. A.; Baglin, J. E. E.; Clark, G. J.; Ziegler, J. F. J . Cutol. 1983, 83, 301. (28) Vannice, M. A,; Sudhakar, C. J . Phys. Chem. 1984, 88, 2429. (29) Belton, D. N.; Sun, Y.-M.; White, J. M. J . Phys. Chem. 1984, 88, 1690. (30) Ocal, C.; Ferrer, S. J . Phys. Chem. 1986, 84, 6474.
The Journal of Physical Chemistry, Vol. 91, No. 4, 1987 871
M O Study of CO-Dopant Interactions
TABLE II: Binding Energies BE ( e v ) , Overlap Populations, and Charge Transfers for Various Dopants Adsorbed on the 22-Atom Cluster Model of Pt(ll1) ~~
molecule or atom BE Ti0 4.89 FeO 5.15 ZnO Fe
Pt
1.46 5.81 2.98
Figure 4. Calculated electronic structures of TiO, FeO, and ZnO and the results of bonding lying down on the PtZ2cluster. Shaded band
0
1.32 1.29 1.02
iii
regions indicate doubly occupied orbitals. n
Mulliken overlap M-0 gas adsorbed M-Pt 0-Pt 1.30 1.25 0.91
1.46 1.53 0.94 1.99 1.05
charge transferred to Pt
0.21 0.32 0.30
0.18 0.46 -0.54 0.6 1 -0.3 1
ii
-10 h
Q)
-301 TiO/Pt,, Figure 5. Predicted equilibrium structures for CO on doped surfaces. The dopant metal lies at the cation binding site of Figure 3, and C is bonded to the central Pt of Figure 3.
those used in the Pt3Ti study.I9 The Ti, Fe, Zn, and 0 parameters for the adsorbed diatomic molecules are from a recent ASED-MO these are also listed in Table I. We have included in our study the interaction of C O with a lone iron atom on the surface. The parameters used for this atom, given in Table I, are based on the atomic parameters in ref 21 with the same 1.5-eV increase in ionization potentials that is applied to the atomic platinum parameters. The interaction of adsorbed CO with a coadsorbed platinum atom was studied and this atom was assigned the same parameters as those in the Pt2*cluster. Finally, the C O atomic parameters given in Table I are those used in the Pt,Ti study of ref 19. The choice of diatomic molecules to represent the dopant oxides is a simplifying approximation since the actual structures are unknown. A number of T i 0 geometries were explored and perpendicular bonding through the Ti end to the Pt surface was found to be more stable than lying-down orientations by about 0.6 eV. For structural reasons we chose for our surface model a parallel orientation for Ti0 with Ti in twofold bridging site at an optimized height of 1.9 A, determined by using 0.1-8, increments, above the surface plane of nuclei. Calculations indicated that the T i 0 bond would stretch by about 0.03 A in this orientation. It was therefore decided to use the calculated gas-phase Ti0 value of 1.63 A25for the surface studies. The same structures were assumed for the FeO and ZnO surface oxide models and their heights were similarly optimized to be 1.8 and 1.9 A, respectively. The calculated gas-phase bond lengths for FeO and ZnO are 1.61 and 1.67 A,25respectively. We have applied the rule of unpairing electrons so that each d band level holds at kast one electron. The number of unpaired electrons in the PtZ2cluster is therefore 14 and in the energy level diagrams half-filled orbital level bands are cross-hatched and doubly filled ones are black. In the diatomic oxide molecules electrons unpair in the d orbitals as shown in Figure 4 so that the ground states are T i 0 (3A), FeO (5A), and ZnO (IZ). When these molecules are adsorbed on the Pt22model of Pt( 11l), as discussed above and as shown in Figure 5 , there are charge transfers and orbital stabilizations leading to chemisorptive bonding. The number of unpaired electrons is taken as 14 in the chemisorption systems. Adsorption energies are calculated to be high for FeO and T i 0 at about 5 eV (similar to RuO on the Ru(0001) surface4)
CO/
