L a n g m u i r 1988, 4, 1216-1219
1216
Liquid c r y s t a l l i n e
Figure 3. Illustrative representation for t h e reversible change of liquid crystal alignment mode induced b y the photoisomerization of azobenzene units attached t o a quartz surface.
parallel with the substrate surfaces. Interestingly, the reversible photoinduced LC alignment change took place more quickly for the cell constructed from plate 2 than that from plate 1 on alternate exposure to 365- and to 440-nm light. The transmittance change of plate 2 at 633-nm light (He-Ne laser) is shown in Figure 2. The fact that the LC alignment is governed by the azobenzene-modified surface was simply confirmed by pressing the imagewise exposed cell with the fingers to break down the LC image, which was recovered immediately after the stress was released. T h e reversible change in the LC alignment can be interpreted as illustrated in Figure 3. It is likely that the azobenzene molecules in the trans form stand perpendicular to the substrate surface, and the LC molecules are oriented so that their long axis is parallel to that of the azobenzene. This molecular information of the alignment may be transferred from molecule to molecule to form the homeotropic alignment. When the cell is exposed to 365-nm light to induce the isomerization to the bent cis
form, the upper part of the photochromic unit becomes nearly parallel to the surface. The LC molecules surrounding the azobenzene units change their position consequently so that the LC molecules also become parallel to the glass surface. The information of this change in the alignment may be transferred throughout the LC layer to form the parallel alignment. There are about 1 2 X 1017molecules/cm2 of the LC of MW = ca. 400 in our cell, while ca. 8 X 1013molecules/cm2 of the azobenzene are attached to plate 2. This corresponds to the fact that two azobenzene units command about 15000 LC molecules to change the alignment. In this respect, we propose to refer the present system to a “command surface”, which may offer a new route to effective amplification of optical information. In other words, the information storage in molecular level can be visualized by using the self-assemblageof LC since the cell is essentially transparent to the actinic light; the absorbance of the monolayered chromophore is less than ca. 0.01. Our finding may give a clue to understanding LC alignment on a molecular level, and further studies are in progress to elucidate the characterization of the azobenzene monolayer and the relationship between the structure of the photoisomerizable unit and the extent of photoregulation of LC alignment in order to improve the efficiency of the command surface. Registry No. D O N 103, 115288-48-7; CsAzOC5, 115271-04-0; C6AzOClo, 115271-05-1; quartz, 14808-60-7.
Notes Exploitation of Lateral Effects. 2. A Lewis Base Induced Surface Ensemble on Nickel Steven S. Miller* a n d Brian M. Daviest
United States Department of Energy, Morgantown Energy Technology Center, P.O. Box 880, Collins Ferry Road, Morgantown, West Virginia 26507-0880 Received December 30, 1987. I n Final Form: April 5, 1988
Introduction Recent trends in the interpretation of interactions by coadsorbed species on metallic surfaces have been moving steadily toward the acceptance of short-range (nearestneighbor) e f f e c t P and away from the long-range (global) effects attributable to electron transfer between the adsorbates and the conduction band of the substrateg10 as the predominant mode of intrasurface interaction. Simulations of the extent to which an electronic influence is exerted by an absorbate on sites other than its own point of attachment indicated that perturbations in the conduction band and Fermi level did not extend to sites significantly beyond the nearest neighbor^.^,^ Experimental observations comparing the influence of C1, S, and P on Ni(l o o ) , although holding to the popular ”d-band” theories as a guideline for interpretation, concluded that the effect of P is much less than that predicted by electronegativity
factors“ and refers to reduced 3d electron density in the vicinity of the preadsorbed atoms. Other models, as one might expect, call both phenomena into play,12J3 depending substantially upon the electronic nature of the adsorbate under scrutiny. Experimental work, taking advantage of the most modern techniques, indicated changes in adsorbate states which could only be attributable to localized coadsorbate intera~tion.’~J*Simulation of proposed nearest-neighbor interactions indicated that these could be the general behavior of coadsorbed species:~~but techniques typically used to interrogate surface (1)Andersen, N. T.; Topse, F.; Alstrup, I.; Rostrup-Nielsen, J. R. J. Catal. 1987,104,454-465. (2)Alstrup, I.; Andersen, N. T. J. Catal. 1987,104,466-479. (3)Norskov, J. K.; Holloway, S.; Lang,N. D. Surf. Sci. 1984,137, fi5-7A -(4)Miller, S. S.Langmuir 1986,2,599-602. ( 5 ) Reynolds, A. E.; Foord, J. S.; Tildesley, D. J. Surf. Sei. 1986,166, 19-28. (6)Wexler, R. M.: Muetterties, E. L. J. Phys. Chem. 1984, 88, 4037-4041. (7)Hollowav. S.:Norskov, J. K.: Lana N. D. J.Chem. SOC., Faraday Trans. 1 (Farahay Symposium 21) 1987;83,1935-43. (8)Goodman, D. W.; Kiskinova, M. Surf. Sci. 1981,105,L265-LZ70. (9)Houston, J. E.;Rogers, J. W., Jr.; Goodman, D. W.; Belton, D. N. J . Vue. Sci.Technol., A 1984,2,882-883. (10)Dwyer, D. J.; Hardenbergh, J. H. Appl. Surf. Sci.1984,19,14-27. (11)Kiskinova, M.; Goodman, D. W. Surf. Sci. 1981,108, 64-76. (12)Feibelman, P. J.; Hamann, D. R. Surf. Sci. 1985, 149,48-66. (13)Goodman, D. W.Appl. Surf. Sci.1984,19,1-13. (14)Trenary, M.;Uram, K. J.; Yates, J. T., Jr. Surf. Sei. 1985,157, 512-538. (15) Uram, K. J.; Ng, L.;Folman, M.; Yates, J. T., Jr. J. Chem. Phys. 1986,84,2891-2895. ~~
Current address: Department of Physics, University of Texas a t El Paso, El Paso, TX 79968.
This article not subject to U.S. Copyright. P u b l i s h e d 1988 b y the American Chemical Society
Langmuir, Vol. 4, No. 5, 1988 1217
Notes interactions were not sensitive to the e f f e ~ t Reports .~ that implied16 or described14negative ensembles show the importance of the technique(s) chosen to pursue the problem. Models suggest that these effects may be used to advantage for ensemble contr01.',~*~ In a preliminary r e p ~ r tit, was ~ proposed that one could induce t h e formation of surface ensembles and perhaps take advantage of these ensembles for the sake of novel catalytic processes. This second report exhibits evidence indicating successful ensemble formation with the donor/acceptor pair P(OCH,),/CO. Several other related donor species were examined without success due t o the instability of the donor species on the metal substrate.
96.25
1
Experimental Section Spectra were taken as specular reflection from the sample surface, using a Perkin-Elmer 1750 FTIR system fitted with a DRIFTS accessory equipped with a heatable evacuable sample stage (Spectra-Tech)and KBr windows (Spectra-Tech). Hydrogen (Matheson Research Grade, 99.995%), oxygen (Matheson Research Grade, 99.99%), CO (Matheson Research Grade, 99.99%), and phosphine (Matheson Electronic, 99.999%) were used as received. Trimethylphosphine, tri-tert-butylphosphine, and trimethyl phosphite (Strem) were stored in a liquid nitrogen boil-off-filledVacuum Atmospheresglovebox and transferred into glass hydro1 tubes (Kontes) fitted with Teflon valves. Samples were degassed through at least three freeze-evacuate-thaw cycles prior to use. Ni foil (Alfa, 99.998%) was initially degreased with methylene chloride and installed in the DRIFTS cell. In-cell preparation consisted of 24-48 h of exposureof 100Torr of oxygen at 600 "C followed by 24-48 h of exposure to 100Torr of hydrogen, also at 600 "C. This was repeated immediately prior to each experiment. Samples found to be "permanently" passivated were removed from the system and chemically etched. Experimental results on etched samples appeared identical with those of samples not previously etched. Reactant gases were prepared in a 5-L stainless steel cylinder (approximately 1000 times the volume of the DRIFTS cell) equipped with a 50 L/s turbomolecular pump (Balzer) and capacitance manometers (MKS) of 1.000- and 100.0-Torr ranges, allowing the ready preparation of precise reactant mixtures. The chamber was typically passivated with the appropriate phosphine derivative immediately prior to preparation of the reactant mixture. The pressure during passivation was approximatelyequal to the partial pressure of the material in the final reactant composition, minimizing changes in sample composition as a result of adsorption to, or desorption from, the storage vessel. Thereafter, the chamber was evacuated, and the appropriate pressure of the Lewis base (0.010-0.250 Torr) was allowed into the vessel. This was immediately diluted with CO to a final pressure of 10-30 Torr. Prior to the passivation procedure, the hydrogen atmosphere was removed from the sample cell, and it was allowed to cool to ambient temperature (19-21 "C) whilst the spectrometer was purged with liquid nitrogen boil-off. Serial 25-scan reference spectra were acquired. Upon stabilization of the background (viz-a-viz background H20and COz), a 50-scan background spectrum was acquired for use. Thereafter, the sample was exposed to a flow of the reactant gas mixture for ca. a 0.2-Torr reduction in pressure for the system, whereupon the exit valve of the DRIFTS cell was closed and the mixture allowed to reach equilibrium pressure with the 5-L reservoir. The spectral acquisition program was commenced during the flow sequence and continued until the experiment was considered done (such as if the sample were poisoned against further CO adsorption, vide infra). The first two spectral runs were of 5 scans each, and subsequent runs were each of 25 scans, spaced by 65 and 250 s, respectively, intrinsic to the FTIR operation. The instrument is capable of 2-cm-' resolution, and data points were taken at 1-cm-' intervals in the range 4000-600 cm-'. Following the acquisition of typically 6-8 such data sets, the cell was evacuated for a complete 25-scan cycle and refilled with (16)Moon, D. W.; Dwyer, D. J.; Bernasek, S. L. Surf. Sci.1985, 163, 215-229.
85.89
I
2057
I
Energy (cm-1)
Figure 1. Reflectance spectrum of CO adsorbed on polycrystalline Ni foil at 27 "C, showing both the CO rotational band structure centered about 2143 cm-' (11 Torr of CO) and the linearly ad-
sorbed CO at 2057 cm-'. the reactant mixture. For an experiment involving pure CO, the spectrum scon replicated those taken prior to the evacuation cycle. For experiments with gas mixtures, the results were observed to be markedly different. This was attributed to the difference in the rates of adsorption/desorption of the different sample components. The CO was found to adsorb and desorb most rapidly, with saturation coverage being reached in several minutes. Desorption of CO was instantaneous on the time scale of the experimental setup. Trimethyl phosphite, on the other hand, was a minority component of the system and apparently could not readily displace" the CO(ads). Upon evacuation of the binary mixture from the cell, however, one could see the residual phosphite on the surface for many minutes, whilst CO(ads) was no longer observable.
Results and Discussion In order to ensure a mutual affinity for the binary systems to be investigated, the choice was that of donor/acceptor pairs. As a n acceptor species, the simple, well-studied n-acid, CO, was chosen. It is easily detected by infrared spectroscopy, and t h e strength of the surface-adsorbate interaction may be inferred via the C-0 stretching frequency. The donor species were taken from the phosphine family. The large number of substituted phosphines available commercially allowed a variety of ligand properties to be explored. P(CH3)3(a-donor), P(t-C4HJ3 (a-donor/n-donor), and P(OCH3)3(a-donor/nacceptor) were initially chosen as representative species.18 PH3 was used as a base-line species for determination of Ni behavior poisoned by phosphide." (17) Shananhan, K. L.; Muetterties, E. L. 1996-2003.
J. Phys. Chem. 1984, 88
(18) Golovin, M. N.; Rahman,M. M.; Belmonte, J. E.; Giering, W. P. Organometallics 1985, 4,1981-1991.
1218 Langmuir, Vol. 4 , No. 5, 1988
Notes
0.02 r
2051
99.59 I
I
CO (ads)
99.38
99.17 0
C
.-e
5 c!
I-
98.96
CO (ensemble) 2023
n
98.75
1
0.00
2100
2050
2000
Energy (cm-')
Figure 2. Reflectance spectrum of CO (10.5 Torr) coadsorbed with P(OCH3), (0.10 Torr) on polycrystalline Ni foil. The spectrum exhibits three key features: "normal" linearly adsorbed CO (2057 cm-'), induced linearly adsorbed CO (2023 cm-'), and the persistent (upon evacuation)transition exhibited by P(OCH3)3 (2081 cm-'). Gaseous background transitions (CO,HzO,COJ were subtracted for clarity. For pure CO(ads), the resonance was reproducibly observed at 2057 cm-', with fwhm of 14 cm-' (Figure 1). The CO gas rotovibrational spectrum was well resolved, with the Q-branch centered a t 2143 cm-'. In the region of primary interest, the P(OCH3),(g) spectrum exhibits a doublet a t 2100 and 2066 cm-' (4.7 Torr). At lower pressures (0.1 Torr) only the adsorbed state is observed a t 2081 cm-'. This latter peak appears as a somewhat dubious shoulder on the 2066-cm-l component of the doublet. Upon coexposure to P(OCH3), and CO (1OO:l-500:1), a new transition appears a t 2023 cm-l (Ni foil) and the 2081-cm-' phosphite peak is enhanced (Figure 2). The gas-phase phosphite transition was no longer observed. The existence of undissociated P(OCH3), on the surface is further confirmed by the C-0 stretching frequency a t 1036 cm-l. PH3, P(CH3)3,and P(t-C,H,), were found to decompose on the Ni surface rapidly when compared to the time scale of the experiment, and the resultant phosphide thoroughly poisoned the surface against CO adsorption. The decomposition of PR3 species on Ru(001) has been reported." With the passage of time, it was found that P(OCH3), would gradually disappear from the spectrum. This fact, coupled with the increasing reluctance of the Ni surface to readsorb CO, indicated that P(OCH3)3 itself decomposed on the surface to presumably yield the phosphide.lg When the gas mixture was evacuated from the sample cell, the adsorbed phosphite peaks were ob(19) Experiments that preexposed the surface to phosphite exhibited a strong gas-phase and adsorbed spectrum, with the adsorbed phase disappearing in some tens of minutes. This disappearance, if attributed to the formation of surface phosphide, would explain the poor results achieved when the surface is preexposed to pure phosphite.
I
I
I
I
I
2250
2200
2150
2100
2050
98.54
i DO
Energy (cm.1)
Figure 3. Taken under the same conditions as Figure 2, this is the type of spectrum observed following evacuation and readmission of the donor/acceptor mixture to the sample cell. Only the induced transition at 2023 cm-l is observed. It is presumed that the decomposition of the phosphite to phosphide inhibits adsorption on sites that are not enhanced by the donor base. served to persist for some while after the CO peaks had disappeared from the spectrum. Readmission of the mixture to the cell yielded a spectrum consisting of CO(g) and CO(ads) a t the 2023-cm-' position only (Figure 3). The interpretation of these events is based upon the following kinetic arguments.
Conclusions and Commentary The initial absorbed species is observed to be CO in the unperturbed state (uco = 2057 cm-'). If we suppose that CO adsorbs on Ni a t least as well as the phosphite, then the surface would initially contain 2-3 orders of magnitude more CO than phosphite. We have observe (vide supra) that the phosphite desorbs very much more slowly than does CO, and hence, with time, the phosphite species will become established on the surface a t the expense of CO. The ensembles are then observably formed, yielding the new, more tightly bound state of CO (vc0 = 2023 cm-l). The reexposed surface has an initial supply of preadsorbed phosphite, and, since the phosphite induces a more tightly bound state, the adsorbed CO will migrate rapidly to the ensemble sites. This results in the appearance of the 2023-cm-' peak as the initial peak upon reexposure of the sample. It is further conjectured that the very poor adsorption of CO in the unperturbed state may result from the poisoning of the surface in neighborhoods where phosphite had decomposed to phosphide. If we conjecture that a mutual repulsion exists between adsorbed phosphites (donor-donor repulsion), then, during the evacuation phase of the experiment, the phosphites would be able to spread throughout the surface (with and without subsequent decomposition). The new surface would consist of a combination of enhanced and antienhanced sites
1219
Langmuir 1988, 4 , 1219-1221
surrounding the two types of phosphorus species. This is then observed as what might be a fortuitous binary mechanism for ensemble formation, wherein the Lewis base, by its decomposition, eliminates the Ni sites that would otherwise compete with the ensembles for adsorption of C0.20 In searching for proof of a Lewis base induced surface ensemble, one must first determine precisely how this type of adsorption would manifest itself in an unambiguous manner. Model systems that presume the formation of such ensembles indicate that changes in the relative surface ratio of two species coadsorbed on a surface are an ineffective probe of the process4 and might be construed as ambiguously differentiating localized electronic interactions from steric (local) and global electronic effects. A change in the surface-adsorbate bond strength is in and of itself ambiguous with respect to similar electronic considerations. In particular, if the change is observed to be a gradual change in bond strength, then one might rightly conjecture that the new surface-adsorbate interaction is based upon global electronic effects. What would ideally be observed is the simultaneous existence of both the new adsorbate state and the unperturbed adsorbate state. These can be seen in Figure 2, with resonances at 2023 and 2057 cm-', respectively. A further observation that can be targeted is the relative rate of reformation of the two states in the presence of a surface with a preadsorbed base. In the case of a positive ensemble, wherein the effect of the base is to strengthen the adsorbate-surface bonding, it might be possible to observe the adsorption of the new state prior to the original state (Figure 3). For a negative ensemble, special conditions might be necessary to force adsorption in the new sites.14 Since the purpose of this research is to lead toward preconfiguration of catalytic utility, the positive ensemble was targeted. The next step in this process should be the determination of a donor species sufficiently robust so as to be stable under conditions of catalytic utility. Registry No. CO, 630-08-0;P(OCH3)3, 121-45-9; Ni, 7440-02-0. (20) This donor/acceptor interaction has interesting implications for the formation of (two-dimensional)surface agglomerations (islands). If, for example, an electron-donating adsorbate (H2S,P!OCH,)j, etc.) is a precursor to an electron-withdrawing and relatively immobile product species (S, P), then the precursor/prcduct interaction should tend to form islands of product species. The size of these agglomerations should be related to the stability and surface mobility of the adsorbed precursor. Conversely, a stable adsorbate should result in a chaotic and dispersed surface state.
Determining Template Removal from a Crystalline P e n t a d Zeolite by Xenon NMR Spectroscopy Chihji Tsiao,' Cecil D bowski,**' Darrell Walker,' Vincent Durante, a n d David R. Corbins
7
Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716, Sun Refining and Marketing Co., Applied Research and Development, Marcus Hook, Pennsylvania 19061, and Central Research and Development Department, E . I . du Pont de Nemours and Co., Wilmington, Delaware 19898 Received May 17, 1988. In Final Form: July 6, 1988
In the production of highly crystalline silicoaluminates such as the pentasil zeolites, one often uses a template
r\
/
145
140
135
I
130
125
120
119
CHEMICAL SHIFT (PPM)
Figure 1. lzeXe NMR spectra of xenon adsorbed in silicalite samples treated in a flow of air at 480 "C. The numbers at the left of each spectrum are the treatment times. T h e shift at 12 h is not meaningful.
molecule to promote the growth of structure. After the synthesis is complete, this molecule must be removed by high-temperature combustion to produce the pure silicoaluminate. T h e removal of a template is a protracted process, and, typically, the loss of template is monitored by DTA or some spectroscopicmethod.' In this paper we describe a method for following the kinetics of this process based on the NMR spectroscopy of xenon gas sorbed in the material. The NMR spectral parameters of xenon-129 are quite sensitive to the details of collisions in which it is involved.24 When it is sorbed in microporous materials such as zeolites, the dominant collisional process involves collisions with the structure. Because a collision with the template molecule makes the spectroscopy of xenon in cavities containing them different from the spectroscopy of xenon in cavities without the template, one may infer the percentage of xenon in these environments directly
* Author to whom correspondence should be addressed. University of Delaware. *Sun Refining and Marketing Co. 8 E. I. du Pont de Nemours and Co. Contribution no. 4762. (1)Song, T.; Xu, R.; Li, L.; Ye, Z. R o c . Int. Cor$ Zeolite, 7th (Tokyo) 1986, 201-206. (2) Ito, T.; Fraissard, J. J. Chem. Phys. 1982, 76, 5225. (3) Ripmeester, A.; Davison, D. J . Mol. Struct. 1981, 75, 67. (4) Ito, T.; deMenoval, L.; Guerrier, E.; Fraissard J . Chem. Phys. Lett. 1984, I l l , 271.
0743-7463/88/2404-1219$01.50/0 0 1988 American Chemical Society
