J. Phys. Chem. 1988, 92, 4401-4405
4401
Ammonia Adsorption on a Model P t ( l l 1 ) Surface: A Molecular Orbital Approach Cristian Fierro Case Center for Electrochemical Sciences and The Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44106 (Received: June 22, 1987)
Molecular orbital calculations have been performed to gain insight into ammonia chemisorption on Pt(ll1). With 10- and 21-atom cluster models of the (111) surface it is concluded that ammonia binds most stably on the onefold atop site, with the nitrogen end down. It is also found that ammonia is a donor of electrons to Pt( 111) and that the Pt surface atoms, to which the ammonia chemisorption takes place, become depletd in d electrons. The results complement previous experimental and theoretical studies presented in the literature.
Introduction
Ammonia adsorption on metal surfaces has been a widely studied system in surface science and catalysis.'-3 The decomposition pathways of NH3 via N-H activation and its synthesis on metals may have important implicatioqj in catalysis as well as in surface processes of technological importance. The experimental information obtai ed in this system has involved thermal desorption spectroscopy ( DS), electron energy loss spectroscopy (EELS), ultraviolet photoemission spectroscopy (UPS), X-ray photoemission spectroscopy (XPS), work function measurements, and, more recently, metastable quenching spectroscopy (MQS). Nevertheless, theoretical studies of NH3 adsorption on metal substrates and its surface reactions have not been pursued with the same intensity. In one case, a theoretical treatment originally used to study chemisorption processes of saturated hydrocarbons has been extended to study chemisorbed molecules with a lone electron pair. The theory3eproposed a comprehensive model to explain, among other properties, work function changes, corebinding energy shifts, and heat of chemisorption. Among the salient features it is worth mentioning (1) the increased importance of antibonding molecular orbitals of the chemisorbed molecule and (2) the adsorbateinduced surface polarization mechanism. The antibonding orbitals of saturated hydrocarbons were found to be responsible for the acceptor character necessary for the C-H activation and dissociation, while the surface polarization mechanism was proposed to explain both the decrease in work function, and the decrease in the d electron density for the surface atoms to which chemisorption occurs. The model was able to explain several trends observed for hydrocarbon adsorption on metal surfaces and to propose future experiments. The theoretical treatment was also extended to other systems such as N H 3 on Pt(l11) but was unable to predict the preferred site for chemisorption. One objective of the present work is to model the ammonia-metal surface interaction and to study the electron-donor character of NH,. Experimental results obtained by using surface core-level photoemission spectroscopy strongly indicated the electron-acceptor character of NH3 in the same manner as found for saturated hydrocarbons chemisorbed on metal surfaces. This information was based on the 4f7/2core-level energy shift for the Pt surface atoms which were covered with NH3. This surprising shift (by 0.7eV) to higher binding energies with respect to the surface atoms of clean strongly indicated the electron-donor character of the surface metal atoms. On the basis of surface core-level photoemission, the acceptor state involved
I
(1) Baetzold, R. C.; Apai, G.; Shustorovich, E. Appl. Surf.sci. 1984, 19, 353. (2) Apai, G.; Baetzold, R. C.; Jupiter, P. J.; V i m , A. J.; Landau, I. Surj Sci. 1983, 132, 122. (3) (a) Shustorovich, E.; Baetzold, R. C.; Muetterties, E. L. J. Phys. Chem. 1983, 87, 1100. (b) Baetzold, R. C. Phys. Rev. B 1984, 29, 4211. (c) Baetzold, R. C. J . Am. Chem. soc. 1983, 105, 4271. (d) Baetzold, R. C. J . Phys. Chem. 1983,87, 3858. ( e ) Shustorovich, E.; Baetzold, R. C. Science (Washington, D E . ) 1985, 227, 876.
0022-3654/88/2092-4401$01.50/0
in the process was tentatively assigned to N H 3 adsorbed in a hollow. Theoretical work3 concerning hydrocarbon adsorption on transition-metal films, such as CH4 decomposition, or cyclohexane dehydrogenation reaction, was in agreement with a partial population of the antibonding molecular orbital (MO) of an adsorbate. Earlier, F i ~ h e rhowever, ,~ using XPS found an increase in the core-level binding energy of the N ( 1s) electron of N H 3 adsorbed on Pt( 11 1). This shift was interpreted as NH, acting mostly as a donor of electrons toward the metal surface rather than accepting charge through the 4al MO. Thus the main objective of the present work is to gain insight into the donor-acceptor properties of NH3 at the Pt(ll1) surface. At the same time the realization that the adsorption site for NH, on this surface was not certain was another factor in the importance of studying NH3 adsorbed on Pt(ll1). On the basis of experiments performed with NH3 adsorbed on Ir( 111),5 it has been commonly believed that N H 3 resides in a hollow site.
Method of Calculation The NH3-Pt10 system has been studied by using the atom superposition and electron delocalization molecular orbital (ASED-MO) theory.6 In this semiempirical model the total energy, Et, is partitioned into two contributions: a repulsive energy of interaction, E,, between the positive charge of a nucleus as it approaches another atom surrounded by its electrons, and a bonding contribution, Eb, due to electron delocalization and bond formation. E, can be calculated by integrating the HellmannFeynman force when the rigid atoms are superimposed in a molecular configuration. The second term, Eb, however, cannot be calculated by a precise equation, and it is approximated by a one-electron Hamiltonian similar in form to the extended Hiickel Hamiltonian:
Et = E,
+ Eb
The advantage of this theory is the incorporation of a repulsive term that allows the calculations of bond lengths and structural stabilities, retaining the simplicity of the arguments based on perturbation theory. A similar approach has also been taken by the use of the extended Hiickel method.' The parameters used io the calculations are the same as those used in previous studies and are indicated in Table I. The first section of this work will present a qualitative description of the NH3 MOs followed by a theoretical analysis in (4) Fisher, G. B. Chem. Phys. Let!. 1982, 79, 452. (5) Purteli, R. J.; Merrill, R. P.; Seabury, C. W.; Rhodin, T. N. Phys. Reu. Lett. 1980, 44, 1279. (6) (a) Anderson, A. B. J . Am. Chem. SOC.1978, 100, 1153. (b) Anderson, A. B. Surj Sci. 1981,105, 159. (c) Ray, N. K.; Anderson, A. B. Surf. Sci. 1982, 119, 35. (d) Anderson, A . B.; Kotz, R.; Yeager, E. B. Chem. Phys. Lett. 1981, 82, 130. (e) Fierro, C.; Scherson, D.; Anderson, A. B.; Yeager, E. B., to be submitted for publication. (0 Mehandru, S. P.; Anderson, A. B. Appl. Surf.Sci. 1984, 19, 116. (7) (a) Hoffmann, R. Acc. Chem. Res. 1971, 4 , 1. (b) Fujimoto, H.; Fukui, K. In Chemical Reactivity and Reaction Paths; Klopman, G . , Ed.; Wiley: New York, 1974; p 23.
0 1988 American Chemical Society
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The Journal of Physical Chemistry, Vol. 92, No. 15, 1988
Fierro
TABLE I: ASED Parameters Used in the Calculations: Principal Quantum Number (o),Ionization Potential (IP), Slater Exponents ( T ) , and Coefficients (C) for Double-( d Functions S P d atom n IP c n IP r. n IP ll Cl c2 c2 Pt" 6 11.5 2.55 6 6.96 2.25 5 11.6 6.01 0.6567 2.39 0.5715 Nb
2
H'
1
18.33 11.6
1.9237
2
12.54
1.9170
1.2
'The platinum parameters were taken from previous work.6b The IPSwere increased by 1 eV (2 eV total) to approximate self-consistency. bBased on ref 6d,e. The IPS were decreased by 2 eV to approximate self-consistency. EBasedon ref 6a. The IP was decreased by 2 eV to approximate
self-consistency.
-9
->
a-11
I
>
0
[L
w
2-l3 w
++
"
a ,/
-15
-1 7
Figure 1. Ten- and 21-atom cluster model of the Pt(ll1) surface. The arrows indicate the three adsorption sites considered in the calculations. Dashed lines represents atoms below the plane of atoms drawn with solid lines.
P'10
,
Pt gNH
NH3
light of the experimental data reported in the literature.
Figure 2. Energy-level correlation diagram for ammonia chemisorption on top of a Pt atom (see Figure I ) , at 2.05 8, from the surface. The height was optimized in 0.05-8, increments. The cross-hatched bands
Molecular Orbitals of Ammonia
correspond to single occupied levels, (eight unpaired electrons).
The important NH, MOs are schematically shown in 1. The
hydrogen atoms. It is this particular orbital, relatively high in energy, that may be responsible for the electron-acceptor properties of NH3. Ammonia Adsorption on the On-Top Site of Pt( 11 1)
le
4a I
1
shape and energy of these orbitals have been previously analyzed by several authors.8 The highest occupied MO (HOMO), the 3al lone pair, has a small N-H bonding character and is ideally polarized for a c interaction at the nitrogen end of the molecule. As may be seen in 1, the nitrogen-down geometry of N H 3 can form a strong u overlap with appropriate orbitals of the metal. Next, at lower energies, is located the l e set, which contain most of the N-H bonding character. It has a high contribution at the nitrogen end of the molecule, and thus, it is well suited for T interactions with the d,, dyzmetal orbitals. The lowest unoccupied M O (LUMO 4a,) is N-H antibonding and more localized at the (8) (a) Gimarc, B. M. J . Am. Cbem. SOC.1971,93,593. (b) Gimarc, B.
M. Acc. Chem. Res. 1974, 7, 384. (c) Gimarc, B. M.In Molecular Struture and Bonding; Academic: New York, 1979. (d) Burdett, J. K. In Molecular
Shapes; Wiley: New York, 1981. (e) Albright, T. A.; Burdett, J. K.; Whangbo, M.-H. In Orbital Interactions in Chemistry; Wiley-Interscience: New York, 1985.
As a model for the Pt( 111) surface, a Ptlo and a PtZl cluster used in previous studies&$(see Figure 1) were chosen. The smaller cluster provides a satisfactory model for the calculations. However, for some adsorption modes the use of the bigger cluster was also considered. The different sites of N H 3 chemisorption on this surface are indicated in the figure. The NH3 geometry was taken from the gas-phase ammonia without further geometry optimization. Thus the work is in the spirit of an extended Hiickel analysis of bonding and geometry predictions with emphasis on the qualitative trends rather than absolute values. The basic relations that translate the numbers obtained in the calculations into manageable concepts include: (1) the overlap between the important MOs, (2) the match in energy between them, and (3) symmetry considerations. As discussed elsewhere' these relations are based on second-order perturbation theory and will be used throughout this work to interpret the calculations. The energy correlation diagram for N H 3 directly above a surface atom is shown in Figure 2. The most stable Pt-N distance of 2.05 A was optimized to the nearest 0.05 A. The unoccupied s-p and occupied s-d bands are located on the left side of the diagram with the cross-hatched areas indicating single occupied levels. On the right side of the figure the ammonia MOs are shown before the interaction with the surface. It is important to point out that ASED-MO theory and the pt cluster chosen for this study
The Journal of Physical Chemistry, Vol. 92, No. 15, 1988 4403
Ammonia Adsorption on Pt(II1) TABLE II: Contributions to the Different MOs (percent) Obtained for Ammonia Chemisorbed on the &-Top Site, at 2.05 A, on the Pt Cluster (See Figure 2) 3% a, 3al* lone pair (NH3) 38.93 24.85 5.19
Pt,' Pt9"
31.92 29.15
25.10
50.05
Pt, -0.722 -0.032 +0.045 -0.011 -0.826 -0.087 +0.189 Pt, +1.226 +0.387 -0.092 +0.312 +0.153 +0.105 +0.361 N
40.42 54.39
"Pt, and Pt, state for the Pt atom underneath the chemisorbed ammonia and the extra Pt atoms from the cluster, respectively. preclude any quantitative interpretation of either bulk- or surface-sensitive spectroscopy measurements such as work function or d-band width. As stated earlier, however, the emphasis of this work is on qualitative trends rather than absolute values. Upon adsorption two Pt-N u bonding combinations (3al, a,) appeared beneath the bottom of the s-d band. Correlation lines are not drawn to the Ptlo cluster because it is difficult to retrace the resulting MOs to a particular state or set of states from the metal cluster. However, a Miilliken population analysis will show (vide infra), that the lower Pt-N bonding combination (3al) originates predominantly from the interaction between occupied states of the cluster and the NH3 lone pair. The 3al Pt-NH, MO has 70% contribution from both Ptl (where Pt, states for the Pt atom directly interacting with NH,; see Table 11),and the lone pair of ammonia. Because of the high 3al contribution to this level, it is formally assigned to the lone pair of chemisorbed NH,. The bonding combination (a,) is mainly metal in character and has 50% contribution from the Pt atoms (Pt,)that are not directly interacting with NH,. The strong N H 3 adsorption found in this geometry can be rationalized by considering the second-order perturbation to the energy in the following expression7 E,(z)= lHi,I2/(E,- E,)
TABLE 111: Electron Density Change for a Pt Atom Having NH3 on It (Pt,) for the Other Pt Atoms (Ptg) and Nitrogen (N)'** Ap As Apu APT Adu Adr Ad6
-0.562
0.000 -0.563
+0.001
'The total change (Ap), in specified orbitals is expressed relative to the uncovered surface layer and nonchemisorbed ammonia. * A negative number indicates that electron density is lost. change in electron density is separated according to the symmetry type orbital involved in the interaction (As; Apu = p,; Apa = p,, py; Ada = dzz; Ada = d,,, dy,; Ad6 = d,, d,z-y2). The changes are taken relative to the free Pt cluster and the nitrogen atom of nonchemisorbed ammonia; a negative number indicates loss of electron density. The transfer of 0.72 electrons from Ptl correlates with the depletion of 0.82 electrons from the dZzorbital involved in the u interaction. This loss of 0.82 electrons from Pt, is then redistributed into the other nine Pt atoms and onto the d6 orbitals. The resulting large increase in charge density for Pt9 of 1.22 electrons should then be associated with the charge redistribution mentioned above in addition to the electron-donating properties of NH,. This phenomena is not new and has been presented previously for the adsorption of CO, hydrogen, and C2H, fragments on metal s ~ r f a c e s . ~ The rehybridization mentioned in ref 1 has also been found in this work. As the predominantly d9 orbital of Pt, is destabilized, it moves up in energy and strongly mixes with the p band as depicted in 2 and Figure 2. Similar rehybridization has been found
(1)
where E , and Ej are the orbital energies before interaction, H,, is the interacting Hamiltonian, and E t 2 )is a measure of orbital stabilization (bonding) or destabilization (antibonding) after the bond is formed. A strong chemisorbed bond will be favored when the energy gap ( E , - E, = AE) between the interacting orbitals is small as indicated in the denominator of eq 1. However, a weak interaction can be enhanced substantially by a strong overlap (S,) between the distant orbitals involved since S, is implicitly included in the numerator of eq 1 through the Wofhberg-Helmholz formula:
H i . = yzkSij(Hii+ Hjj) Thus the adsorption of NH, may be explained qualitatively by the application of the above two equations. For N H 3 on top of a Pt atom, the 3al lone pair not only is close in energy to the s-d band (small AE) but also is directed toward the dg orbital of the metal for a strong u overlap. Such a situation is considered to be ideal for the formation of a strong chemisorbed bond. A dynamic process could be envisioned to better rationalize the NH3-Pt bond formation. As NH3 approaches the metal surface, as an inverted umbrella toward Pt,,the lone pair will be stabilized by the u interaction with the occupied states of the metal while the Pt-N antibonding interaction (with a high metal d9 character) will be pushed up in energy (the 3al* MO). The latter MO, as the NH3-Pt antibonding M O is pushed up in energy, may drop its electrons to lower available metal states. Thus a substantial electron drift will be generated toward the metal (Pt,), via the Pt, surface atom which is involved in bonding with NH,. Therefore the destabilization of the d z orbital results in a Pt atom (mainly Pt,) that has been electron depleted (see Table 111) as found experimentally by surface core-level photoemission.' This interpretation is in agreement not only with conventional chemical reasoning, which formally assigned a donor character to NH,, but also the 4f7/2 core-level energy shift for the surface Pt atom bonded to NH,. Mulliken population analysis may further clarify the NH3-Ptl u interaction. As shown in Table I11 each contribution to the total
2
previously in transition-metal chemistry1° and qualitatively explained by using the same arguments used in the present study. The electron occupation of the antibonding level 2 will result in a polarization of charge into the bulk Pt atoms as explained by their model, by Baetzold and S h ~ s t o r o v i c h . ~As* ~suggested ~ the resulting surface dipole and the polarization of charge density toward the bulk atoms may account for the work function decrease and the surface core-level shift observed experimentally. On the basis of this work and with the assumption for a moment that NH, on top of a Pt atom is the preferred site for chemisorption, the work function decrease may involve, among other considerations, the N H 3 dipole at the surface in addition to the well-known donating properties of ammonia. The (al) u bonding combination shown in Figure 2 is responsible for a fraction of the extra electrons gained by pts. This MO should be associated to the donor character of NH,, which transfers 0.56 electrons to the metal cluster. As shown in Table 11, this particular M O is predominantly metal in character and delocalized throughout the Pt,. Lower in energy, the N H 3 l e set shows negligible interaction with the metal s-d band. This is to be expected since as depicted in 3 these orbitals have the right
3 (9) (a) Saillard, J.-Y.; Hoffmann, R. J . Am. Chem. SOC.1984, 106, 2006. (b) Sung, S.-S.; Hoffmann, R. J. Am. Chem. SOC.1985, 107, 578. ( c ) Silvestre, J.; Hoffmann, R. Langmuir 1985, I , 621. (10) Elian, E.; Hoffmann, R. Inorg. Chem. 1975, 14, 1058.
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The Journal of Physical Chemistry, Vol. 92, No. 15, 1988
TABLE IV: Energy of Interaction for Ammonia on a Ptlo Cluster Model (See Figure i)',b on top on a bridge on a hollow
eneriv. eV -0.81 1 (-0.94) +0.656 +0.845
height.
A
2.05 2.05 2.05
"The experimental value calculated from TDS is indicated in parentheses. Negative energies indicate bonding interactions. For the onefold bonding to the surface, the Pt-N distance was optimized to the nearest 0.05 A.
symmetry for a P interaction, but the large energy difference with the Pt s-d band, combined with the large bond length for a strong P overlap, precludes a stronger interaction with the metal surface. Finally, the acceptor 4a, M O was found to play a negligible role under this particular adsorption geometry. Again this should come as no surprise since the empty NH, MO is polarized toward the hydrogen end of the molecule and it is localized at a large energy distance from the Pt s-d band. Considering the approximation involved in this work, the adsorption energy of 0.81 eV (see Table IV), by assuming NH, on top of a Pt atom, compared favorably with the experimental 0.94 eV calculated from thermal desorption spectroscopy (TDS) ,4 The use of Pt21yielded an adsorption energy of 0.98 eV. A similar increase in chemisorption energy with cluster size has been attributed to the widening of the platinum d band,6f which will decrease the energy denominator in eq 1. Ammonia Adsorption on a Hollow and on a Bridge Site
Calculations performed for N H 3 adsorption on these sites indicate a repulsive interaction. The combination of the dZ2,d,,, and dj2 orbitals of the Pt atoms are weakly ifiteracting with the 3a, lone pair under these configurations. Without a strong overlap, as in the on-top site, the Pt-N antibonding levels are not destabilized above the Fermi level and consequently remain occupied by electrons leading to a Pt-N closed-shell repulsion. For the purpose of comparison with the on-top site, the values reported in Table IV were set to an arbitrary distance of 2.05 %, from the surface plane. Not surprisingly, the calculations performed by using the Pt2, cluster model yielded similar results. Comparison with Experiments
Preferred Sitefor NH, Chemisorption. At low coverages (less than a quarter of a monolayer), Fisher4 found one species denoted as a which, as suggested by its desorption temperature, was strongly bound to the Pt surface atoms. Depending somewhat on coverage, the CY state was found to desorb at 350 B. At lower temperatures and at higher coverages another species denoted as 0 was found to molecularly desorb at 160 K. This work shows that N H 3 adsorbed on top of a Pt atom may account for the experimental results reported for the CY state. The energy of interaction not only is in close agreement with thermal desorption data but also may explain the UPS reported by Fisher. The 1.75-eV stabilization of the 3al lone pair found theoretically is in good agreement with the experimental 2.5-eV shift observed in UPS, although it should be noted that initial-state relaxation effects are not considered. The calculated data should be regarded as mostly qualitative. In particular UPS measurements on Pt surfaces place the 3al and l e set at approximately 7 and 12 eV below the Fermi level, while Figure 2 shows the calculated values at ca. 3.5 and 4.5 eV below the highest occupied level. Similar lone-pair stabilizations have been found for NH, adsorption on iron," n i ~ k e l , ' and ~ ~ 'ruthenium,lg ~ suggesting a (r bonding on top ( I 1) (a) Grunze, M.; Bozso, F.; Ertl, G.; Weiss, M. Appl. Surf.Sci. 1978, I , 241. (b) Grunze, M. Surf.Sci. 1979, 81, 603. (12) Klauber, C.; Alvey, M. D.; Yates, J. T., Jr. Surf.Sci. 1985, 154, 139. (13) Seaburry, C. W.; Rhodin, T. N.; Purtell. R. J.; Merril, R. P. Surf. Sci. 1980, 93, 117. (14) Benndorf, C.; Madey, T. E. Surf. Sci. 1983, 135, 164.
Fierro of a metal atom in analogy to Pt( 111). In the low-coverage UPS spectrum of Fisher: features below the Fermi level were related to changes in the Pt surface density of states. These calculations show that the bonding combination a', mainly metal in character, may be tentatively ascribed to those features beneath the bottom of the s-d band. The localization of charge at the nitrogen end of the molecule, through the 3al lone pair, and the stronger overlap with metal atoms either in discrete transition-metal complexes or on surfaces is the origin for the preferred on top site adsorption of NH,. Previous theoretical work on other systems have shown the validity of these comparisons with organometallic complexes." The authors of an EELS study?' however, presented a different interpretation for the CY species. On the basis of the small shift observed for the N-H vibrational frequencies, as observed in stable platinum amine complexes, the a species was suggested to be weakly bound to the P t ( l l 1 ) surface. It was also observed that the N-H frequencies for the fl species were more perturbed than those of the CY state. The evidence was provided by a low-frequency band at 350 cm-' attributed to a Pt-N stretching for the fl species. Fisher, however, interpreted this low-frequency mode as a torsional vibration about an axis perpendicular to the main molecular axis. This torsional vibration is likely to occur for a second layer of NH, which is not directly chemisorbed on the Pt atoms. Thus a reinterpretation of the fl state may involved a second adsorption layer of NH, on top of chemisorbed ammonia as recently suggested by Benndorf and MadeyI4 for the case of ammonia on Ru(OO1). The hydrogen-bonding network between the molecules in the p state will weaken the N-H bond and should lower the energy for the N-H stretching frequency as found experimentally by EELS. In addition, the negligible contribution of the fl species to the work function change may also be explained by the loss of preferential orientation and lack of charge transfer between the second layer of NH, and the metal surface. Donor-Acceptor Properties. Baetzold et al.,'-3 in a series of papers, have discussed the electron-acceptor properties of saturated hydrocarbons when adsorbed on metal surfaces. The acceptor properties of these molecules have been found to play an important role in catalytic processes related to C-H activation and disso~ i a t i o n . ~ ~As, ~it, is ' ~often found in these molecules, the acceptor state is highly destabilized (AE >> 0), so only a small contribution to the chemisorbed bond may be expected from those states of the adsorbate. The hydrogen atoms, however, which should be identified with the acceptor properties of these molecules may modify the situation through the numerator of eq 1. In fact, in practically any orientation of these species at the surface, at least one hydrogen atom of the adsorbate will point toward the metal surface to generate a good overlap with a metal state (imagine CH4, for example). Thus acting through the numerator of eq 1 the acceptor state is enhanced, overcoming to some extent the large AE found between the metal s-d band and the acceptor MO. As depicted in 1, the ammonia lone pair completely modifies the reactivity of this molecule with respect to saturated hydrocarbons, since the acceptor character of ammonia will require a reorientation of the molecule on the surface to allow the hydrogen atoms of the 4al MO to overlap with the d orbitals of the metal. Such chemisorption geometry, however, will disrupt the lone-pair interaction with the metal. Thus only a weakly interacting ammonia with a m e t a l surface may allow a tilting of the NH, axis of symmetry to induce some overlap with the hydrogen atoms. As mentioned earlier, the on-top geometry has both a strong overlap (15) (a) Hermann, K.; Bagus, P. S.; Bauschlicher, C. W. Phys. Rev. B 1985,31,6371. (b) Bagus, P. S.;Hermann, K.; Bauschlicher, C. W. J . Chem. Phys. 1984, 81, 1966. ( c ) Bagus, P. S.; Hermann, K.; Bauschlicher, C. W. Ibid. 1984, 80, 4378. (16) (a) Mehandru, S. P.; Anderson, A. B. J . Am. Chem. SOC.1985, 207, 844. (b) Kang, D. B.; Anderson, A. B. Surf. Sci. 1985, 155, 639. (17) Lee, L.; Arias, J.; Hanrahan, C.; Martin, R. M.; Metir, H. Surf.Sci. 1986, 165, L95. (18) Prince, K. C.; Bradshaw, A. M . Surf.Sci. 1983, 126, 49. (19) Benndorf, C.; Madey, T. Chem. Phys. Lett. 1983, 101, 59. (20) (a) Ray, N. K.; Anderson, A. B. Surf. Sci. 1983, 125, 803. (b) Anderson, A. B.; Ray, N. K. J . Phys. Chem. 1982, 86, 488. (21) Sexton, B. A,; Mitchell, G. E. Surf Sci. 1980, 99, 523.
J. Phys. Chem. 1988, 92, 4405-441 1 with a dz2 orbital of the metal and a good match in energy with the s-d band. Experimental evidence, from use of metastable quenching spectroscopic (MQS)," has suggested that a new filled NH3 orbital located a t the Fermi level of the surface may be associated to a Pt-NH, bonding interaction with acceptor states of NH,. Based on these calculations, an alternative explanation for the new filled state may involve a Pt-NH, antibonding interaction with donor states of NH3 which has been partially populated by electrons. A similar partial population of acceptor states has indeed been observed for the case of oxygen on Ag(1 lo).'* From the above analysis a strong (weak) metal-nitrogen interaction, via lone pair, should be the best approach to avoid (enhance) the electron-acceptor properties of ammonia. For a cathodically charged surface, in which the 3al/metal s-d energy gap is increased thus, decreasing the energy of adsorption may activate the N-H bond of chemisorbed ammonia. This has been done by coadsorption of NH3with alkali metals on metal surfaces. Recent ESDIAD experiments of Benndorf and Madeyig have found that ammonia chemisorption on Ru(001) can be dramatically changed upon coadsorption with sodium atoms. Unfortunately experimental evidence for the case of Pt surfaces is not available at the present. Nevertheless a theoretical model to simulate alkali coadsorption effects for C O adsorption in the presence of potassium has been proposed earlier20a by using ASED-MO theory. The potential dependence of water adsorption on an Fe electrode has been studied by using a similar approach.20b Preliminary calculation on the Pt-NH, system indicates that by shifting the d band 0.5 and 1 eV closer to the vacuum, to simulate a decrease in the work function, the energy of N H 3 adsorption decreases to 0.55 and 0.33 eV, respectively, as expected from
4405
second-order perturbation theory. This is in agreement with the experiment of Benndorf and Madey on Ru(001). Summary
The calculations have shown that ammonia is adsorbed on top of a Pt atom via the 3al lone pair on a Pt(ll1) cluster model. In this configuration the strong overlap between the dZ2orbital of the metal with the lone pair can account for the strong interaction reported experimentally for the a state. The lower l e set and the empty 4al M O were found to play a negligible role under this particular configuration. The /3 species observed by TDS most probably corresponds to molecules that are not directly bonded to the surface. Chemisorbed ammonia was found to be a net donor of electrons. This result is in agreement with the d-electron depletion found for those atoms having adsorbed N H 3 on it. As shown by other workers and found in this study, the decrease in charge density for those atoms should be associated with a strong Pt-N bonding interaction which shifts the antibonding combination above the Fermi level, emptying those electrons to lower available states.
Acknowledgment. I express may appreciation to Prof. Gerischer for his kind hospitality and to the Max-Planck-Gesellschaft for a fellowship. Appreciation is express to Prof. Alfred Anderson, Daniel Scherson, and Jiirgen Sass for helpful discussions during the preparation of the manuscript. The computer assistance of Dr. AI Preusser and the useful comments of Dr. Jeffry Eldridge, Dr. Robin Grimes, Dr. Peter Faguy, and Santiago Nasar are also acknowledged. Registry No. NH3, 7664-41-7;Pt, 7440-06-4.
Unusual Mlcellar Properties of a New Class of Fluorinated Nonionic Surfactants B. M. Fung,*st Darryl L. Mamrosh,* Edgar A. O'Rear,* Cheryl Baldwin Frech,t and Jalees Afzalt Department of Chemistry, Department of Chemical Engineering, and the Institute for Applied Surfactant Research, University of Oklahoma, Norman, Oklahoma 73019 (Received: June 25, 1987; In Final Form: January 28, 1988) Some physical properties of monodisperse homologues of the nonionic surfactant series perfluoroalkyl N-polyethoxylated amides (C,F2,+,C(0)NH(CH2CH2O),,,H; n = 3,643; m = 2-4) have been measured. A number of unusual characteristics of these surfactants in aqueous solution have been observed. Above the cmc, the I9F NMR spectra show separate signals due to monomers and micelles, indicating slower exchange between the two species than any other micellar system previously reported. From line width measurements, the mean lifetime was found to be 1.1 i 0.2 s. In concentrated solutions of the 7-2 compound, threadlike structures with approximate diameters of double the molecular length are clearly visible in the freeze-fracture scanning electron micrographs. Aqueous solutions of three of the fluorocarbon surfactants (n-m = 7-2, 7-3,8-2) exhibit non-Newtonian pseudoplastic behavior with very high viscosities, which increase rapidly with the increase of concentration. On the other hand, some of the less hydrophobic fluorasurfactants(n-m = 6-2,7-4,3-2) and the hydrocarbon analogues of this series have normal rheological properties (Le., Newtonian with low viscosities). The non-Newtonianhomologues are much more water soluble than the more hydrophilic homologues, which show Newtonian behavior. The non-Newtonian, viscous behavior is likely due to the formation of rodlike micelles as evidenced by the electron micrographs. The cmc's were also determined for some of the fluorosurfactants;the surface tension at cmc is less than 20 dyn/cm for four of the compounds tested .
Introduction
Monodisperse perfluoroalkyl N-polyethoxylated amides (I) with small numbers of ethoxy groups have been synthesized recently C n b n tC ,I
H0
them as emulsifiers for perfluorochemicals in blood-substitute preparations; fluorinated surfactants tend to form more stable emulsions with perfluorochemicals than their hydrocarbon anal o g u e ~ . ~ The , ~ stability of perfluorodecalin (PFD) emulsions prepared with some of our fluorinated surfactants is discussed
NH(CbCH20)mH I (FEA n - m )
by Gartiser et al.' and by ourselves2 with the intention of using 'Department of Chemistry and Institute for Applied Surfactant Research. 'Department of Chemical Engineering and Institute for Applied Surfactant Research.
0022-365418812092-4405$01.50/0
(1) Gartiser, T. G.; Selve, C.; Mansuy, L.; Robert, A,; Delpeuch, J . J . J . Chem. Res. M 1984, 2612. (2) Afzal, J.; Fung, B. M.; ORear, E. A. J. Fluorine Chem. 1987, 34, 385. (3) Clark, L. C.; Moore, R. E.; Kinnett, D. G.; Inscho, E. I. Prog. Clin. Biol. Res. 1983, 122, 169. (4) Gangoda, M.; Fung, B. M.; ORear, E. A. J. Colloid Interface Sci. 1987, 116, 230.
0 1988 American Chemical Society
