448
Langmuir 1987, 3, 448-452
solid free volume due to leaving volatile groups (HzO, acetate pyrolysis products) and solid ions overcoming translational barriers at a given temperature. At this point we cannot estimate which of the two factors mentioned in the previous paragraph is prevalent. This question should be answered by further work on this and analogous systems.
Conclusions 1. ESR spectra can be used to monitor the transformation of iron(II1) hydroxide acetate to magnetite, at the early transforming stages. 2. Short heating times lead to an increase of Fe(II1) site diversity, as evidenced by line broadening; this is assigned
to the abstraction of volatile components and to the onset of mobility within the solid, allowing particle coalescence and growth, indicated by a later line-width narrowing. 3. A second event detected by the ESR line intensity steep increase is the onset of magnetic ordering: this is noted under conditions at which the sample still has a low Fe(I1) content and is poorly crystallized. The ferrimagnetic phase can then be observed in its earlier formation stages. 4. There is a temperature threshold for the onset of bulk mass transfer within the amorphous iron(II1) hydroxide acetate. This threshold may be determined either by the temperature required for volatile leaving group abstraction and/or by the solid ions overcoming translational barriers. Registry No. Fe304,1317-61-9.
Structure and Dynamics in Lamellar Liquid Crystals. Effect of Agitation and Aging on Deuterium NMR Line Shapes Frank D. Blum,*t Elias I. Franses,t Kenneth D. Rose,$ Robert G. Bryant,i and Wilmer G. Miller* Departments of Chemistry and Chemical Engineering, University of Minnesota, Minneapolis, Minnesota 55455 Received June 18, 1986. I n Final Form: January 9, 1987 Deuterium NMR has been used to probe the effects of mechanical agitation and aging of lamellar liquid crystals of sodium 4-(1-heptylnony1)benzenesulfonate or sodium bis(2-ethylhexyl) sulfosuccinate with deuterium oxide. Samples prepared by vapor sorption show uniform water uptake and relatively sharp deuterium NMR powder patterns with quadrupole splittings of ca. 2 kHz. The shape of the deuterium NMR pattern was dependent upon the extent of mechanical agitation, with the limit being collapse to a single broad resonance. The NMR spectra of the mechanically agitated samples do not revert to the unagitated form over a period of years if left at room temperature. When liquid water was added directly to the dry surfactant, lamellar liquid crystals were formed which have nonuniform water content. The water becomes uniformly distributed and the deuterium NMR spectra indistinguishable from those prepared by vapor sorption only after aging for many days. The agitation-dependent line shapes can be explained by morphological changes in the liquid crystallites.
Introduction A wide variety of experimental and theoretical studies of the phase behavior of aqueous surfactant systems exist. It is often difficult, however, to determine whether the behavior observed represents that of the equilibrium or some other state which may be unstable or metastable. Liquid-crystalline dispersions may be used in a variety of different applications, including tertiary oil recovery,l drug delivery,2 or model membranes3 where the effectiveness of a lamellar surfactant system may depend on the domain structure and size. Consequently, experimental techniques which can give information on the microstructure of the systems in question can be very important. One such technique is deuterium NMR.4g5 In lamellar liquid crystals the deuterium NMR line shape is found to be an excellent reporter of the microstructure of the material and may 'Present address: Department of Chemistry, University of Missouri-Rolla. Rolla. M O 65401. :Present address: School of Chemical Engineering. Purdue Llniversity, \+'est Laf'ayette. IN 47901. s Present address: Exxon Research and Engineering Co.. Annandale. A'.J 08601. - Present address: Deparrmenr of Biuphysics, University of hc>lcl. Iti,\hc..~cS ~ .S 14G-LL'
even be used to determine the phase behavior of a sysIn this study, the use of deuterium NMR to examine the effects of agitation and aging in lamellar liquid crystals consisting of sodium 4-(1-heptylnonyl) benzenesulfonate (SHBS)or sodium bis(Bethylhexy1) sulfosuccinate (aerosol OT or AOT) with deuterium oxide is reported. SHBS is a double-tailed surfactant with a relatively simple primary structure. It is known to form l h e l l a r liquid crystals (L,) (1)Puig, J. E.; Franses, E. I.; Talmon, Y.; Davis, H. T.; Miller, W. G.; Scriven, L. E. SOC.Pet. Eng. J. 1982, 22, 37. (2) Szoka, F.; Padahadjopoulos, D. Annu. Rev. Bcoeng. 1980, 9, 467. (3) Lee, A. G . Prog. Biophys. Mol. B i d . 1975, 29, 3. (4) Mantsch, H.; Saito, H.; Smith, I. P. C. h o g . NMR Spectrosc. 1977, 1 1 , 211. (5) Seelig, J. Q.Reu. Biophys. 1977, 10, 353. (6) Persson, N. 0.;Fontell, K.; Lindman, B. J. Colloid Interface S i 1975, 53, 461. (7) Ulmis, J.; Wennerstrom, H.; lindblom, G.; Arvidson, G. Biochemistry 1977, 16, 5742. (8) Kahn, A.; Soderman, 0.;Lindblom, G. J . Colloid Interface Sci. 1980, 78, 217. (9) Kahn, A.; Fontell, K.; Lindblom, G.; Lindman, B. J. Phys. Chem. 1982,86, 4266. (10) Ulmis, J.; Lindblom, G.; Wennerstrom, H.; Johansson, L. B. A.; Fontell, Soderman, 0.;Arvidson, G. Biochemistry 1982, 21, 1553. (11) Kahn, A,; Fontell, K.; Lindblom, G. J. Phys. Chem. 1982,86, 383.
074S-7463/87/2403-0448$01.50/0 0 1987 American Chemical Society
Langmuir, Vol. 3, No. 4 , 1987 449
Structure and Dynamics in Lamellar Liquid Crystals
at room temperature and very low surfactant concentrat i o n ~ , ' ~ *and ' ~ ) vesicle structures form upon s~nication.'~ The thermal behavior15 and the d y n a m i ~ s ' ~ofJ ~the surfactant and the self-diffusion of the water17 in the liquid crystals have also been studied. AOT is also well-known to form a lamellar phase with water.ls The behavior of these synthetic surfactant liquid crystals also mimics the behavior of phospholipid-water systems. Preparation of phospholipid-water samples for NMR and other studies frequently includes "homogenization" by agitation, which may be in the form of hand or mechanical hak king'^^^^ or repeated centrifugation through a constriction in the sample t ~ b e . ~Consequently, ~~' we believe that the behavior reported here may also be applicable to phospholipid systems with respect to sample preparation. Experimental Section The surfactant sodium 4-(1-heptylnony1)benzenesulfonatewas purchased from the University of Texas. Its purity and characterization have been discussed previously.12$22 Sodium bis(2ethylhexyl)sulfosuccinate was obtained from Americal Cyanamid. Deuterium oxide (99% D) was used as received from Aldrich. The lamellar liquid crystals were prepared by vapor sorption or by direct addition of liquid DzOonto the dry surfactant in a 12-mm NMR tube at room temperature. The vapor sorption procedure was done overnight in a partially evacuated desiccator and produced a nearly equilibrium hydrated liquid crysta1.12*13r22*23 The samples in the present study had compositions of around 76 wt % SHBS and 24 wt % DzO. This correspanded to a composition which was near the phase boundary of the two-phase, liquid crystal-isotropic solution region.12J3This composition was chosen so that virtually all of the water in the sample was in the liquid-crystallinephase and the deuterium NMR spectrum was not dominated by a large isotropic water peak. Typically, even in vapor-sorbed samples, a small amount of isotropic solution was often found. However, in the single-phase liquid-crystalline sample, the central resonance should not be observed.23 The central resonance has often been truncated in the spectra shown in the figures which exhibit quadrupole splitting,so that the details of the powder pattern could be observed. The vapor-sorbed samples appear to be heterogeneous by eye, but the deuterium NMR spectra from the sample were independent of the sample position in the probe. The NMR spectra were taken on a Varian XL-100-15NMR spectrometer operating at 15.358 MHz for deuterium. The multinuclear accessory was home built. The spectra were taken by using 8K data points, a sweep width of 8 kHz, and 90° (50 gs) pulses. Typically on the order of 1000 scans were needed to give adequate signal to noise, and no recycle delay was used because of the shortness of the deuteron Tz.The deuterium NMR line shape was found to be sensitive to agitation by mechanical means (a vortex mixer) or even by hand. The simulated spectra were made by digitizing the original vapor sorbed and "fully" shaken samples. These were then used as a basis from which the simulated spectra were made as simple linear combinations of the two. The best fits were estimated by comparing the calculated and experimental spectra by eye. (12) Franses, E. I.; Davis, H. T.; Miller, W. G.; Scriven, L. E. ACS Symp. Ser. 1979, No. 91, 35. (13) Franses, E. I.; Puig, J. E.; Scriven, L. E.; Davis, H. T.; Miller, W. G. J. Phys. Chem. 1980,84, 1547. (14) Franses, E. I.; Talmon, Y.;Scriven, L. E.; Davis, H. T.; Miller, W. G. J . Collioid Interface Sci. 1982, 82, 449. (15) Blum, F. D.; Miller, W. G . J . Phys. Chem. 1982, 86, 1729. (16)Franses, E. I.; Miller, W. G. J . Colloid Interface Sci. 1984, 101, 500. (17) Blum, F. D.; Padmanabhan, A. S.; Mohebbi, R. Langmuir 1985, I, 127. (18) Fontell, K. J . Colloid Interface Sci. 1973, 44, 318. (19) Keough, K. M.; Oldfield, E.; Chapman, D. Chem. Phys. Lipids 1973, IO, 37. (20) Lancee-Hermkens, I. W.; DeKruijff, B. Biochem. Biophys. Acta 1977, 470, 141. (21) Persson, N. 0.;Lindman, B. Mol. Cryst. Liq. Cryst. 1977,38, 327. (22) Franses, E. I. Ph.D. Thesis, University of Minnesota, 1979. (23) Blum, F. D. Ph.D. Thesis, University of Minnesota, 1981.
2 H 15.4MHr
A
1)
SHAKEN
+I
+\
olillel ( k t i r )
Figure 1. Deuterium NMR spectra of 76 wt % SHBS-24 wt % D20 prepared by vapor sorption: (A) initial and (B) fully shaken
in a vortex mixer.
Results Shown in Figure 1A is the deuterium NMR spectrum of a 76 wt % SHBS-D20 sample which was prepared by vapor sorption. The central peak has been truncated to allow the details of the quadrupole powder (Pake) pattern to be examined. The spectra of samples prepared in this manner were independent of time a t room temperature. The spectra exhibit a quadrupole splitting of ca. 1800 Hz between the two singularities. Shown in Figure 1B is the spectrum of the same material after about 50 min of agitation. The quadrupole powder pattern was no longer apparent, instead a single broadened resonance was found. This spectrum is defined as that of a "fully shaken" sample. The spectra taken from the fully shaken sample were independent of time (for at least 5 years), provided the sample was left at room temperature. Qualitatively similar results were found for samples with higher water contents, but the presence of the large isotropic water peaks make it difficult to study the powder pattern in detail. Precisely at the phase boundary, the center line seen in the deuterium NMR spectra of unshaken samples was not visible, suggesting the absence of any sizable fraction of water in an isotropic environment. Qualitatively similar results also found for the AOT system. Shown in Figure 2 are the experimental spectra observed after various degrees of agitation. Comparison of spectra A to F shows that the amount of powder pattern material sequentially diminishes and the intensity was incorporated into the broad central resonance while the maximum splitting observed remains relatively constant. In addition, the powder pattern itself seemed to have broader shoulders from A to F. Also shown in Figure 2 are spectra simulated by various linear combinations of spectra F and A. The similarity of the two sets of spectra shows that the experimental spectra consisted mostly of two types of resonances represented by the fully shaken and "unshaken" spectra. One small difference is that the simulated spectra seem to have sharper features than the experimental spectra. It was possible to conclude that other components were also present which give rise to the broadening seen in the experimental spectra. Shown in Figure 3 are deuterium NMR spectra of a 77 wt % SHBS-D20 sample to which the deuterium oxide was added directly to the dry surfactant in the NMR tube. The spectrum in Figure 3A was recorded 1 day after preparation and that in Figure 3B after 85 days at room temperature. The 85-day spectrum (Figure 3B) shows a sharp deuterium powder pattern with a quadrupole
450 Langmuir, Vol. 3, No. 4 , 1987
Blum et al.
Simulated
Experimental
Offset (kHz1
Figure 2. Simulated (left) and experimental (right) spectra at intermediate amounts of agitation. The experimental spectra correspond to samples A-F with successive degrees of shaking. Simulated spectra correspond to (A) 0%, (B) IO%, (C) 20%, (D) 50%, (E) 80%, and (F) 100% of experimental spectrum F added to experimental spectrum A.
--
2H 15.4MHz
77 wt% SHBS 23 wt% D f l
Direct
Addition
I
A
-L
1 Day
\/
1
,4
/'
\
1
B a5 Days
+--+
i ,
-2
-1
I 0
y--LA
1
2
OFFSET(kHz1
Figure 3. Deuterium NMR spectra of 77 wt % SHBS-23 wt % D20 prepared by direct addition after (A) 1 day and (B) 85 days. The center lines have been truncated.
splitting of about 1800 Hz and is practically identical with that of vapor-sorbed samples (see Figure 1A). The spectrum recorded after only 1day, however, was quite broad and the powder pattern poorly defined. No significant change in the spectra was found with time beyond 85 days. The details of the time dependence of the direct addition samples were not studied. Discussion The spectra shown illustrate that deuterium NMR can be a sensitive probe of the structure of the liquid crystals. The collapse of the deuterium quadrupole powder pattern with agitation, in Figure 1, is dramatic. This effect is of importance because methods of preparation of liquidcrystalline dispersions often involve shaking, centrifugation (often back and forth through a constriction), or other agitation such as s o n i c a t i ~ n . ~ ~ In ~ ~many ' " ~ ~cases ~ ~ ~the effect of sample preparation has been noted in NMR (24) Tyrrell, D. A,; Heath, T. D.; Colley, C. M.; Ryman, B. E. Biochern. Biophys. Acta 1976, 437, 259.
studies on synthetic and biological liquid crystals. Sears has notedz5 that carbon-13 spectra and resonance line widths were sensitive to the method of preparation for liquid crystals of 75% egg phosphatidylcholine and water. He found that the line widths followed a general pattern with those for vapor-sorbed samples being greater than those for unsonicated samples (direct addition of water), which were greater than the line widths for sonicated samples. He attributed the differences to different amounts of ordering in the liquid crystal but did not propose a specific mechanism to account for the differences. The results were also consistent with earlier work on the SHBS-water system15where the effects of sample preparation were apparent in the I3C spectra. The effect of sample preparation has also been noted in previous deuterium NMR studies on the lecithin-D20 and sodium phosphatidylserine-D20 systems.26 In these systems centrifugation of the material back and forth through a constriction yields *H NMR spectra which show signal intensities from a center resonance being transferred to quadrupole powder patterns with time. For samples stored at -20 "C, this process can take as long as 2 months. Unfortunately, even at -20 "C the lipids showed signs of decomposition after 3 months and decomposed rapidly at 70 0C.26In some systems there appeared to be negligible or no apparent time dependence of the ~ p e c t r a .However, ~ it is possible that the agitation during sample preparation may change the morphology of the material to a form which may not be the equilibrium form even though it appears to have kinetic stability. In contrast to previous studies, after 5 years at room temperature, the agitated SHBS-water systems did not revert to their previous structures before agitation. They also did not show any signs of decomposition at room temperature and at higher temperatures the samples may even anneal back to their orginal form.n,28 Observance of this behavior is impossible (25) Sears, B. J. Membr. Biol. 1975, 20, 59. (26) Finer, E. G.; Darke, A. Chern. Phys. Lipids 1974, 12, 1. (27) Miller, W. G.; Blum, F. D.; Davis, H. T.; Franses, E. I.; Kaler, E. W.; Kilpatrick, P. K.; Nietering, K. E.; Puig, J. E.; Scriven, L. E. In Surfactants in Solution; Mittal, K. L., Lindman, B., Eds.; Plenum: New York, 1984; Vol. 1. (28) Blum, F. D.; Nietering, K. E.; Russo, P. S.; Miller, W. G., Manuscript in preparation.
Langmuir, Vol. 3, No. 4, 1987 451
Structure and Dynamics in Lamellar Liquid Crystals with phospholipids because of their decomposition. Comparison of the experimental and simulated spectra in Figure 2 suggests that the sample has two main components a t intermediate degrees of agitation. The experimental spectra in Figure 1 were reasonably good representations of the two components. To understand the reason for the collapse of the powder pattern, the splittings in the vapor-sorbed sample were examined. The quadrupole splittings observed in the spectra are given for an axially symmetric system by5 Av(O) = ( 3 / 4 ) ( e 2 q Q / h ) ( ( 3cos2 O(t) - 1))
(1)
where (e2qQ/h)is the quadrupole coupling constant which is taken as 220 kHz for liquid heavy O(t) is the angle between the 0-D bond vector and the applied magnetic field, and ( ) denotes the average over the time scale dominated by the reciprocal of the static coupling constant. While interpreting the actual magnitude of the splitting is beyond the scope of this work, a number of approaches have been used to interpret the actual value of the quadrupole splitting in various liquid-crystalline s y ~ t e m . ~ ~ ~ ~ ~ ~ ~ In the vapor-sorbed samples the quadrupole splitting observed was on the order of 1800 Hz. This was reduced by roughly a factor of 100 from the static splitting, suggesting that the motion of water inside the liquid crystal is rapid, but not completely isotropic. The initial state of the vapor-sorbed sample consisted mainly of planar layers as shown by electron m i c r o s ~ o p yand ~ ~ ~deuterium ~~ NMR.23 For water in between the planar layers, the angular part of eq 1can be rewritten in terms of the director (normal to the bilayer) as5v31
( ( 3 COS2 6 - 1))=
y2(
(3 cos2 -1)(3 COS2 p - 1 ) ) (2)
where a and p are the angles between the director and the applied magnetic field axis and the 0-D bond vector axis, respectively. This equation is valid as long as the azimuthal dependence of the 6 term is averaged to zero and axial symmetry e ~ i s t s In . ~large ~ ~ ~planar liquid crystals without defects, the water molecule experiences only one domain and hence, one director over the deuterium NMR time scale. In this case the a term would be a constant and factored out of the average, leaving only the time dependence of to account for the reduced splitting observed in the vapor-sorbed samples. In order to explain the collapse of the quadrupole splitting upon agitation, a mechanism must be proposed to allow additional averaging of the 0-D bond vector relative to the applied magnetic field. There are several possibilities: First, the agitation could alter the liquidcrystalline domains or a t least create defects a t which the water could move i~otropically.~~ These defects might be expected to heal with time, but this type of behavior was not observed. Second, upon agitation, the domain size of the liquid crystal may be reduced, with most of the material found in small-radii concentric shells (liposomes) or in Dupin cyclides, the allowed types of three-dimensional, constant bilayer thickness, closed surfaces.33 Assuming that the local motional properties of the water in the spherical shells are not much different from those in planar layers, the p averaging in eq 2 would be expected to be the (29)Soda, G.;Chiba, T. J. Chem. Phys. 1969,50,439. (30)Finer, E. G.,J. Chem. SOC.,Faraday Trans. 2 1973,69,1590. (31)Petersen, N.0.; Chan, S. I. Biochemistry 1977,16,2657. (32)Zasadzinski, J. A.;Scriven, L. E.; Davis, H.T.Philos. Mag. A 986, 51, 287. (33)Maxwell, J. C. Q. J.Math. 1868,9, 111.
same in both cases. However, translational diffusion in the spherical layers would be expected to change the director as a function of time, resulting in the averaging of the (Y angle and reducing the splitting. These structures could be small enough that the motion of the water molecule may sample so many different bilayer orientations that the residual quadrupole powder pattern could be completely collapsed into a single resonance. Assuming that the collapse of the deuterium powder pattern is due to the formation of small liposomes, it is possible to make a rough estimate of the size of the liposome corresponding to the collapsed powder pattern. In the liposomes, water can travel around a curved surface and further average the angle 6 without any great change in the translational or rotational diffusion of the water molecule. The self-diffusion coefficients for the water in the SHBS-water system have been determined by using pulsed-gradient spin-echo NMR17 to be D = 8 X m2/s. For the water to collapse the powder pattern it must experience approximately ?r/2 radians in a time t , short with respect to the reciprocal of the residual splitting (ca. s). The approximate distance traveled by the water molecule during this time is equal to (2Dt)1/2or about 0.5 pm. If this travel occured in a spherical shell it would correspond to a shell with a radius of about 0.3 pm. This suggests a rough lower size limit of about 1-pm diameter for the size of liposomes which would be expected to show collapsed deuterium NMR line shapes. Spherical liposomes much larger than 1 pm or planar domains would be expected to show quadrupole splitting similar to that in Figure 2A. Intermediate-size liposomes would be expected to show smaller splittings and were probably responsible for the broader shoulders in the experimental spectra as compared to the simulated ones. Liposomes much smaller than 1pm would be expected to show only a single resonance. The change in liposome size with agitation will be documented elsewhere.28 Achievement of sample homogeneity by using direct addition of liquid water was found to be much slower than when vapor-sorption methods were used. However, once the samples were homogeneous, the spectra from them were identical with those obtained from vapor sorption samples. This result indicates that the average structures from these two types of preparations are probably similar. Differential scanning calorimetry studies also yield similar re~u1ts.l~ In addition, the broadening seen in the spectra of the day-old sample was perhaps due to the superposition of quadrupole powder patterns for liquid crystals at various water contents. This assumption is reasonable for this system a t room temperature because the quadrupole splitting actually decreases as the water concentration decreases.23 On the basis of the time independence of the splitting in the agitated samples, this process is probably not due to the extension of smaller liposomes into larger ones.
Conclusions I t is clear that deuterium NMR spectra can be a very powerful tool in probing the structure of liquid-crystalline material. The collapse of the deuterium quadrupole powder pattern with agitation was easily observed and probably due to the transformation of the large planar liquid crystals into smaller concentric liposomes which were on the order of a micrometer or less in diameter. The agitated material does not regain the quadrupole powder pattern with aging for at least 5 years at room temperature. Finally, it was observed that a material whose spectral properties are similar to the vapor-sorbed material can be prepared by direct addition of the water to the surfactant,
452
Langmuir 1987, 3, 452-459
provided sufficient aging time was allowed. We believe that these results will allow new insight into the time dependence of the domain structure in a wide variety of liquid-crystalline systems.
Acknowledgment. We thank R. M. Riddle (currently
of Texaco, Exploration and Production Research, Houston, TX) for the construction of the multinuclear accessory for the XL-100. This work was supported by the Department of Energy and the National Science Foundation. Registry No. SHBS, 67267-95-2; aerosol OT, 577-11-7.
Coadsorbate Interactions: Sulfur and CO on Ni( 100) Marjanne C. Zonnevylle and Roald Hoffmann* Department of Chemistry and Materials Science Center, Cornell University, Ithaca, New York 14853 Received October 31, 1986. In Final Form: January 16, 1987 The influence of a S adlayer on CO adsorption onto Ni(100) is examined. Tight-bindingextended Huckel calculations on a three-layer model slab indicate that the interadsorbate separation distance determines not only the mechanism but also the effect of the interaction. If the C-S distance is short, sulfur induces site blockage of CO chemisorption by means of a direct, repulsive interadsorbate mechanism. If the separation is increased beyond the normal S-C bond range, the sulfur adatoms work indirectly via modification of the electronic structure of the substrate. This is a form of through-bond coupling. It is consistent with the well-documented sulfur poisoning of CO adsorption and its usual explanation via relative electronegativities of adsorbates, but there are some conceptual differences. At longer coadsorbate separations, there is an interesting reversal of the bonding trends, which has some experimental support. The effect of atomic adsorbates on the chemisorption of small molecules and on the rate of certain catalytic reactions is dramatical Electronegative impurities, such as halogens and chalcogens, tend to hinder these processes. In contrast, alkali metals and other electropositive elements can function as promoters. A great variety of experimental and theoretical studies have been conducted with the motivation of understanding the poisoning and enhancement effects of coadsorbates. For example, the adsorption of CO onto sulfided surfaces is often used as a model for the sulfur poisoning of Fisher-Tropsch hydrocarbon catalysis from CO and H,. We will focus on S/CO coadsorbates on nickel surfaces, as the body of experimental work dedicated to these systems is voluminous. As is true of other coadsorbate systems, the basic nature of the interactions between surface species is a matter of controversy. Opinions are substantially polarized between two extremes. The mechanism is described either as dominated by delocalized long-range effects which allow a single impurity adatom to modify many adsorption sites2 or alternatively as mediated by local bonding and shortrange site b l ~ c k a g e . ~Using the army of acronymical methodologies available to surface science, experimental evidence can be found to support either side. We list only a few examples. Goodman and KiskinovaC4and Erley and Wagner5 advocate the long-range theory based on the well-documented nonlinearity of both the CO saturation coverage and CO/H2 catalytic methanation as a function of preadsorbed sulfur on Ni(100) and Ni(ll1). On Ni(100), a one-fifth sulfur monolayer causes an order of magnitude decrease in the rate of methanation. Goodmane concludes (1) For recent reviews on poisoning and promotion, see: (a) Martin, G. A. In Metal Support and Metal-Additiue Effects in Catalysis; Imelik, B., et al., Eds.; Elsevier: Amsterdam, 1982; p 315. (b) Goodman, D. W. "Role of Promoters and Poisons in CO Hydrogenation"In Proc. ZUCCP Symp., TX, 1984. (2) Goodman, D. W.; Kiskinova, M. Surf. Sci. 1981, 108, 64. (3) Johnson, S.; Madix, R. J. Surf. Sci. 1981, 108, 77. (4) Goodman, D. W.; Kiskinova, M. Surf. Sci. 1981, 105,L265. (5) Erley, W.; Wagner, H. J. Catal. 1978, 53, 287. (6) Goodman, D. W. Appl. Surf. Sci. 1983, 19, 1.
0743-7463 I 8 7 12403-0452$01.50 IO
that each sulfur atom affects some 10 Ni surface atoms. Madix et al.3*798implicate local site blockage to explain similar results. Gland et al.g favor the short-range model for Ni(100) in light of high-resolution electron energy loss spectroscopy (HREELS) and temperature-programed desorption (TPD) studies. The infrared reflection-adsorption spectroscopy (IRAS) work of Trenary et al.1° on Ni(ll1) is in good agreement with the latter. Theoretical treatments of adatom poisoning and promotion of CO chemisorption are generally presented in the framework of the Blyholder model." The adsorption geometry is widely accepted to be through the carbon end, exactly or nearly perpendicular to either the clean or preadsorbed nickel surface.12"-d Regardless of the specific adsorption site, the chemisorptive bond in the Blyholder model results from the electron donation from the CO 5a, la, into the empty surface levels and back-donation from the surface into the CO 2 ~ * lb. , As both CO levels are
la
lb
20
2b
(7) Madix, R. J.; Thornburg, M.; Lee, S. B. Surf. Sci. 1983, 133, L447. (8) Madix, R. J.; Lee, S. B.; Thornburg, M. J . Vac. Sci. Technol. A 1983, 1, 1254. (9) Gland, J. L.; Madix, R. J.; McCabe, R. W.; DeMaggio, C. Surf. Sci. 1984, 143, 46. (10) Trenary, M.; Uram, K. J.; Y a h , J. T.,Jr. Surf. Sci. 1985,157,512. (11) Blyholder, G. J. Phys. Chem. 1964, 68, 2772. (12) (a) Behm, R. J.; Ertl, G.; Penka, V. Surf. Sci. 1985,160, 387 and references therein. (b) Allyn, C.; Gustafason,T.; Plummer, E. Solid State Commun. 1978,28,85. (c) Andenson, S.; Pendry, J. B. Phys. Reu. Lett. 1979, 43, 363. (d) Passler, M.; Ignatiev, A.; Jona, F.; Jepsen, D. W.; Marcus, P. M. Phys. Rev. Lett. 1979, 43, 360. 0 1987 American Chemical
Societv
