Adsorption of dichloromethane and its interaction with oxygen on the

Yarw-Nan Wang, Juan A. Marcos, Gary W. Simmons, and Kamil Klier* ... TDS, LEED, and HREELS observations and Monte Carlo simulations suggest a model ...
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J. Phys. Chem. 1990, 94, 7597-7607

7597

Adsorption of Dichloromethane and Its Interaction with Oxygen on the Pd( 100) Surface: Effect of Chlorine Layers on Oxygen Chemisorption and Oxidation of Carbon Residues Yarw-Nan Wang, Juan A. Marcos, Gary W. Simmons, and Kamil Klier* Zettlemoyer Center for Surface Studies and Department of Chemistry, Lehigh University, Bethlehem, Pennsylvania 1801 5 (Received: January 24, 1990) Dichloromethane, CH2CI2,adsorbs dissociatively on a Pd( 100) surface at room temperature with C, CI, and H adsorbing into 4-fold sites while H tends to diffuse underneath the surface layer with increasing CH2C12exposures. Combined AES, TDS, LEED, and HREELS observations and Monte Carlo simulations suggest a model in which C and C1 adsorb at next nearest neighbor (NNN) sites having a Cl-C-C1 angle of 90' and exclude adsorption on all the nearest neighbor (NN) and CI-Cl NNN adsorption sites. Atomic CI desorbs at 1100 K with the activation energy of desorption of ca. 68 kcal/mol. Oxygen-precovered surface with ~ ( 2 x 2 or ) ~ ( 2 x 2structure ) completely inhibits CH2CI2adsorption while CH2C12-precovered surface is very reactive toward oxygen in the temperature range 350-510 K. The selectivity of oxidation of carbon fragments to CO increases with CH2C12preexposures, i.e., CI-C-Cl coverages. A Monte-Carlo simulation of the oxidation of surface carbon by surface oxygen atoms resulted in a semiquantitative account of the observed rates of production of CO and C 0 2 as a function of chlorine coverage. The model explains selective oxidation to C 0 2 on chlorine free palladium and to CO in terms of restricted supply of oxygen to carbon on partially chlorine-covered surfaces. It was also found that two cycles of oxygen adsorption and heating to 900 K can remove all the C and H and leave the CI layer, which originates from CH2C12 adsorption, unchanged. Thermal desorption studies from the surface with coadsorbed 0 and CI showed that the adsorbed CI atoms weaken the Pd-O bonds over several Pd-Pd distances. At saturated coverages, the sum of the concentrations of adsorbed CI and 0 remains approximately constant. 1. Introduction Palladium metal has been reported to perform as a catalyst for partial oxidation of methane to formaldehyde and carbon monoxide in the presence of halogenated hydrocarbons,'V2 in contrast to the well-known behavior of Pd as a catalyst for complete combustion of methane to carbon dioxide and water in the absence of any promoters.' To obtain information on the mechanism of surface reactions occurring in the presence of halogens, we have studied in detail the palladium/oxygen/halogen/carbon system using palladium single crystals as substrates. In an earlier paper we described the results of our studies of the reaction of oxygen with Pd( loo),' and in the present paper we present the results of the interaction of chlorine and dichloromethane with Pd( 100) and concomitant reactions with oxygen. The objective of these studies is the determination of the role that chlorine adatoms have in controlling the activity and selectivity of partial oxidation reactions including methane. It is well known that partial overlayers of oxygen that may leave a portion of the metal surface free for activation of reactants may play an important role in the oxidation reaction^.^.^ Very little has been determined, however, about how the state of surface oxygen could be modified by additives to control selectivity in partial oxidations. Cullis, Keene, and Trim" were one of the first groups who reported that with 0.1-10 mol % of gaseous CH2C12 present in the methane/oxygen feed mixture over palladium sponge and temperature at 650 K, formaldehyde was produced in addition to CO, C 0 2 , and water. Although the concept invoked for utilizing CH2Clzas a promoter in the methane oxidation may have been patterned after the well-developed 1,2-C2H4CI2-promotedepoxidation process,b11 the mechanistic

F Keene, D. E.; Trimm, D. L. J . Catal. 1970, 19, 378. (2) Mann, R. S.;h i , M. K. J. Chem. Technol. Biotechnol. 1979,29,467. (3) Anderson, R. B.;Stein, K. C.; Freeman, J. J.; Hofer, L. J. E. Ind. Eng. Chem. 1961,53, 809. (4) Simmons, G. W.; Wang, Y.-N.; Marcos, J. A.; Klier, K. Submitted. (1) Cullis, C.

( 5 ) Somorjai, G.A. Chemistry in Two Dimensions: Surfaces; Cornell University Press: Ithaca, 1981;Chapter 8. (6) Satterfield, C. N. Heterogeneous Catalysis in Practice; McGraw-Hill: New York, 1980;Chapter 8. (7) Kilty, P.A.; Rol, N. C.; Sachtler, W. M. H. In Catalysis; Hightower, J. W . , Ed.; North-Holland: Amsterdam, 1973;p 929. Kilty, P.A,; Sachtler, W. M.H. Catal. Rev.-Sci. Eng. 1974,10, 1. (8) Meisenheimer, R. G.;Wilson, J. N. J. Coral. 1962,I , 151. (9)Rovida, G.;Pratesi, F. Surf. Sci. 1975,51,270.Rovida, G.;Pratesi, F.; Ferroni, E. J . Carol. 1976,41, 140.

steps in which the two halogen-containing species are involved may not necessarily be the same. It is therefore of interest to determine how dichloromethane reacts with the palladium surface and how fragments produced by its chemisorption influence the adsorption and reactivity of the reactants, i.e., oxygen and methane. The adsorption of CH2C12 on evaporated Pd films has been investigated by Anderson and McConkey.I2 This study indicated that the C-Cl bonds are broken before any C-H bond breakage occurred because mass spectroscopic analysis during the reaction revealed the production of CH4 and C2H6. The ratio of CH4 production to that of C2H6 was 2.0 when the adsorption of CH2C12 was carried out a t 273 K, whereas at 373 K, the ratio was 6.0. When H2 was added to CH2C12-preexposedPd film, the gaseous products yielded the ratios of CH4/C2H60.25 at 273 K and 7.0 at 373 K. Similar dissociation reactions of dichloromethane with another transition metal have been reported. Steinbach, Kiss, and Krall13J4studied the adsorption of CH2C12on Co and Ni films at 95 K and identified adsorbed chlorine atoms and CH2species (CH2,sd)with X-ray photoelectron spectroscopy (XPS). When the CH2C12precovered surfaces were heated to 180 K, the CH, fragments further decomposed into CHadand eventually C, with H2 gas evolution as the temperature was increased to 470 K. While the adsorption of CH2C12on metal surfaces appears to be dissociative in nature, a knowledge of the local structure and long-range arrangement of the CH,Cl fragments on the surface is desirable since it may help in the understanding of the elementary mechanistic steps of reactions catalyzed by the Cl-promoted metals. The stability of the m e t a l 4 bond over ranges of temperature typically encounted in oxidation reactions is essential for the effectiveness of adsorbed chlorine as a promoter. Indeed, the metal-C1 bond has been shown to be very strong on several transition-metal For palladium (1 10) and (IO) Marbrow, R. A.; Lambert, R. M. Surf. Sci. 1978,71, 107. (11) Campbell, C. T.; Paffet, M. T. Appl. Surf.Sci. 1984, 19, 28. Campbell, C. T.; Koel, B. E. J . Coral. 1985, 92,272. (12)Anderson, J. R.; McConkey, B. H. J . Catal. 1968, I l l , 54. (13)Steinbach, F.; Kiss, J.; Krall, R. Surf. Sci. 1985,157, 401. (14)Steinbach, F.; Krall, R. J . Catal. 1985,94, 142. (15)Grunze, M.; Dowben, P. A. Appl. Surf.Sci. 1982,IO, 209. (16)Farrell, H.H.In The Chemical Physics ofSolid Surface and Hererogeneous Catalysis; King, D. A,, Woodruff, D. D., Eds.; Elsevier: Amsterdam, 1984;Vol. 3B,p 225.

0022-3654/90/2094-7591$02.50/0 0 1990 American Chemical Society

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( I I I ) single-crystal surfaces, Erley'7,t8and Tysoe and LambertI9 reported that molecular chlorine adsorbs dissociatively on both surfaces. resulting in several ordered phases observed by LEED. Thermal desorption of atomic chlorine was observed from the chlorine-covered Pd surfaces at temperatures 800-IO00 K. corresponding to an activation energy of desorption of 42-67 k~al/mol."-'~ We have undertaken an investigation of the chemisorption of CH,CI, and its interactions with oxygen on a well-defined palladium (100) single-crystal surface using Auger electron spectroscopy (AES). low energy electron diffraction (LEED). thermal desorption spectroscopy (TDS). and high resolution energy loss spectroscopy (HREELS). An auxiliary experiment with chlorine gas adsorption was also carried out to calibrate the chlorine overlayers formed during CH,CI, adsorption. Oxygen adsorption on a Pd( 100) surface has been described in detail in a separate paper' and earlier work of other^.^'-^^ This work fucuses on the determination of the state of CH,CI, adsorbed on the Pd( 100) surface. its eNxt on the state of adsorbed oxygen. and on reactions of surface oxygen with surface carbon. 2. Experimental Section The instrumentation for the present Auger analysis. LEED, TDS. and HREELS observations. as well as the palladium crystal cleaning procedures. have been given elsewhere.' Experimental details described here will only include those additional features necessary for the studies of CI, and CHzCIzadsorption and the co-adsorption of oxygen and CH,CI,. A research grade gas bulb of CI, (ICON Services Inc.) and vapor source of CH,CI, (Aldrich. 99.99%) were attached to the vacuum chamber through leak valves. A freerethaw technique was employed to further purify CI, and CH,CI, at liquid nitrogen temperature. In the very beginning of CI, and CH,CI, exposures, the chamber pressure decreased due to a rapid uptake of chlorine by the chamber walls. The quadrupole mass spectra of gases obtained by admission of CI, and CH2CI, at 1.0 X IO" Torr showed H,. CO. and some noble gases as the main background species, in agreement with literature reports?'.z6 Adsorption studies were carried out by exposing the clean and annealed Pd(100) crystal to CI, or CH,CI, at a constant chamber pressure of I X 10%Torr for specific time increments. Gas exposures are expressed in langmuir (1 langmuir = IO+ Torra) in which pressure readings were corrected by using ion gauge sensitivity factors of 1.14 for C l t 7and 3.7 for CH,CI,?* Measurements of LEED. AES, TDS. and HREELS were made after the CI, or CH,C12 was pumped from the system. The extent of CI2or CH2C12chemisorption for a given exposure wasdetermined by following the Auger peak-tmpeak height ratio of chlorine at 176 eV to that of palladium a t 323 eV. Since the palladium pcak was not significantly attenuated or altered in shape at the chlorine coverages studied. it was assumed that coverages were directly proportional to the respective peak ratios. The chlorine AES peaks were calibrated on the assumption that the well-ordered ~ ( 2 x 2 LEED ) patterns obtained after room temperature saturation of the surface with chlorine gas corresponded to 8, = 0.50 monolayer (ML) (cf. section 3.1). The electron beam of the AES or LEED probe was turned off during exposures of (17) Erlev. W.Sur( Sri. 1980. 94. 281 (18) E& su& Sci. 1982; rri:47. (19)Tyrae. W. T.: Lambert. R. M.SwJ Sci. 1988. 199. 1. (20) Logan. M. A,; Ruckcr. T. G.:Gentle. T. M.; Mucttcrtics. E. L.: Somoriai. G . A. I . Phvs.C h o n 1986. ~~.90. 2109. (21iOrcnt. T. W.;Bader. S . D. SwX Sci. 1982. i I 5 . 323. (22)Nybcrg. C.: Tengrtal. C. G . Solid Stare Camm. 1982. 44. 251. Nyberg. C.: Tengaal. C. G. S w J Sci. 1981 i26, 163. (23) Stuvc. E.M.: Madir. R. J.: Brundlc. C. R.Surj.Sci. 1984. 146. 155. (24)Chang. S.-L.: Thiel. P. A. Phys. R m La!. 1987.59. 296. Chang. S.-L.: Thiel. P. A. 3. Chem. Phyi. 1988.88, 2071. Chang. S..L.; Thiel. P. A,: Evans. J. W.SurJ Sci. 1989. ZOJ. 117. (25) Tu. Y.-Y.: Blakely. J. M. J. Voe. Sri. Techno/. 1978. IS. 563. (26)Hcllcr. S.R.; Milne. G.W.A. EPAINIH Moss Sprcrrol Dora Bow: National Bureau of Standards: Washington. DC. 1978. p 33. (27)Shav. M. L. Rn.. Sci. lnrrrum. 1965. 37. 113. (28)Summers. R. L. NASA Tcchnicol ,Sow TN D-5285. Washington.

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Figure 6. Thermal desorption spectra of CH2C12-precoveredPd( 100) surface with various exposures of CH2Cl2.The detection of CO was due to the coadsorption of residual CO, which completely desorbed at ca.473 K.

Both the p(2X2)-0 and c(2X2)-0 LEED patterns remained clear and bright at all exposures to CH2CI2. Auger spectra taken immediately after CHZCl2exposures did not show any chlorine or carbon signals. Thermal desorption from these surfaces gave rise to O2 ( m / e = 32) spectra identical with those from the p(2X2)-0 and c(2X2)-0 phases on a clean Pd(100) ~ u r f a c e . ~ 3.4. Oxygen Adsorption on Pd( 100) wirh Preadsorbed CHzC12. The CHzC12-precovered surfaces were prepared by room temperature exposures of 0.3,0.5, 2.4,4.9, and 20 langmuirs, which gave coverages in terms in chlorine of 0.05, 0.08,0.15, 0.18, and 0.22 ML, respectively. The CH2CI2-precoveredsurface was first ramped to 473 K in order to desorb any adsorbed residual C O

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Figure 7. Dependence of saturated oxygen and chlorine uptakes on the room temperature preexposures of Pd(100) to CH2Cl2. The coverages of oxygen and chlorine were obtained by calibrating the Auger peakto-peak ratios of O(507 eV)/Pd(323 eV) and Cl(176 eV)/Pd(323 eV) to that of c(2X2)-0/Pd(lOO) phase and c(2x2)-Cl/Pd(lOO) phase, respectively.

and then exposed to 90 langmuirs of oxygen at room temperature. Figure 7 shows that the amount of oxygen adsorption decreased with the amount of preadsorbed CH2C12. After the 90-langmuir oxygen exposure following a 5.4-langmuir CH2Clzpreexposure, the HREELS spectra showed the appearance of a 365 cm-' loss peak corresponding to the Pd-0 stretch (see Figure 5c) and the disappearance of the loss peak at 475 cm-' attributed to Pd-H stretch. These results indicate that oxygen atoms were adsorbed in the 4-fold sites left vacant in the CH2C12-precoveredsurface and that hydrogen was removed from the surface by oxygen. Thermal desorption spectra obtained immediately after oxygen adsorption on the CH2Clz-precovered surfaces gave rise to mass spectra of C02, CO, and C1 (Figure 8). An additional small and broad peak of O2at 735 K was detected from the 0.3- and 0.5langmuir CH2ClZ-precoveredsurface. Atomic chlorine ( m / e = 35) desorption peaked at 1100 K was observed in all cases. Hydrogen-containing compounds such as HC1, HzO, HCHO, and C H 3 0 H were not found during the TDS measurements, which is further evidence that hydrogen was removed from the surface at an early stage of exposure to oxygen. This is likely to take place as the reaction of hydrogen and oxygen to form H 2 0on Pd( 100) was readily observed at room temperature.22 The relative amounts of C O and COz that resulted from the reaction of oxygen with the carbon fragments was found to be a function of the CH2C12preexposure. For the surfaces with lower CHzC12preexposures (0.3 and 0.5 langmuir), C02was the major gaseous product of oxidation of the surface carbon and evolved at ca. 470 K. Most of the CO signal at mass 28 in this case is attributed to the fragmentation pattern of C02 in the mass spectrometer (Le., the ratio of mass signals 28/44 = 0.12 is expected for COz). At higher CH2Clzpreexposures (2.4 langmuirs and up), the C 0 2 and CO products each peaked at two temperatures: 365 and 470 K for COz and 400 and 510 K for CO. A shift away from C 0 2 and toward CO production is now evident since the peak temperatures for COz and CO are different and the mass ratio 28/44 exceeds that of the fragmentation pattern for C 0 2 . The relative amounts of CO and C 0 2 formed by the surface oxidation of carbon fragments have been calculated as a function of carbon coverage arising from the CH2C12dissociation. Details of the method used are summarized as follows: (i) the amount of CO, equivalent to the coverage of carbon species that was oxidized and evolved as CO, was determined by calibrating the areas under the CO desorption curves by that obtained from the room temperature saturation of CO on the Pd( 100) surface where

The Journal of Physical Chemistry, Vol. 94, No. 19, 1990 7601

CH2C12-Oxygen Interaction on Pd( 100) Surface TDS

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Temperature, K Figure 8. Thermal desorption spectra after first dose of 9 0 langmuirs of O2on various amounts of CH2C12-precoveredPd( 100) surface. Note that CO, C02,and most of the O2could be desorbed, while leaving the surface chlorine if the ramp was stopped at 900 K. No HCI desorption was observed. a ( 2 4 2 X ~ ' / 2 ) R 4 5LEED ~ pattern corresponding to 0.50 ML of CO was ~bserved,'~ (ii) the amount of desorbed C 0 2 , equivalent to the coverage of surface carbon species that was oxidized and evolved as C 0 2 ,was calculated based on the assumption that the amount C 0 2 evolved from the 0.3-langmuir CH2C12-preexposed surface was equal to the total amount of adsorbed carbon species, Le., 0.025 ML, and (iii) corrections were made in the calibration of the evolved C O and C 0 2 to account for the fragmentation pattern of C 0 2 that yielded mass 28 at 12% of mass 44 and the falling base line in the TDS due to the increase of the pumping speed during each thermal desorption run. The amounts of CO and C 0 2 produced are presented in terms of carbon coverages and chlorine coverages in Figure 9. The selectivity to CO is seen to increase with increasing CI coverage. It is worth noting that the carbon coverages, given by the sum of C O and C 0 2 evolution, amount to half that of the chlorine coverages. The results shown in Figure 9 then represent a correct stoichiometry between the C and C1 surface species from the CH2C12 dissociation. A Pd( 100) surface covered only by chlorine from dissociative adsorption of CH2CI2can be prepared by oxidatively removing all the carbon and hydrogen species. The CH2C12-precovered surfaces were exposed to a dose of 90 langmuirs of oxygen at room temperature followed by immediately ramping the temperature to 900 K and cooling to room temperature. Most of the oxygen reacted with carbon and desorbed as C02 and CO, and the remainder desorbed as 02,while the chlorine remained on the surface. Two of these "dose oxygen and ramp to 900 K" cycles completely removed all the surface carbon since a third cycle yielded only O2in the thermal desorption spectrum with no indication of CO or C 0 2 (see Figures 10 and 11). The surface concentrations and the desorption temperature of chlorine were found to be unchanged after several "dose oxygen and ramp to 900 K" cycles as shown by AES and TDS, and (32) Tracy, J. C.; Palmberg. P. W. J . Chcm. Ph s. 1969, 51, 4852. Bradshaw A. M.;Hoffmann, F. M.Sur/. Sei. 1978, 711 513. Behm, R.J.; Christmann, K.;Ertl, G.; Van Hove, M.A. J . Chcm. Phys. 1980, 73, 2984, and ref 23.

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Pd(100) in terms of carbon and chlorine coverages. The experimental distributions were derived from thermal desorption yields obtained after the first oxygen dose on the CH2C12precovered surface. The simulated distributions were obtained via the Monte Carlo method by statistically counting the oxygen atoms that reside adjacent to each carbon atom (see text for detail).

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Temperature, K Figure IO. Thermal desorption spectra after a second dose of 9 0 langmuirs of O2on the previous "dose of 9 0 langmuirs of O2and ramp to 900 K" CH2C12-precoveredPd(100) surfaces. These spectra show that chlorine coverage remained the same whereas carbon species is being burned off.

HREELS measurements demonstrated that chlorine atoms were still adsorbed in the 4-fold sites (see Figure Sd). The LEED pattern for the maximum chlorine coverage of 0.22 ML, shown in Figure 12a, exhibiting faint streaks in the 0) and (0, *'/J positions, also remained unchanged after repeated cycles of "dose oxygen and ramp to 900 K". It appears that the bonding of the chlorine species that originated from the CH2C12adsorption was not influenced by adsorbed oxygen atoms or by the presence or absence of carbon. 3.5. Effect of Chlorine Adsorbate on CH2C12and 4 Adsorption. When the chlorine-saturated surface prepared by the

Wang et al.

7602 The Journal of Physical Chemistry. Vol. 94. No. 19. 1990

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tained by removing carbon from the simulated CI-C-CI saturated Pd(100) surface. "dose oxygen and ramp to 900 K" cycles specified in section 3.4 was exposed to additional CH1C12. no further adsorption was detected by either an increase in the CI Auger signal or thermal desorption of CO and CO, after a dose of oxygen. When the chlorine-covered surfaces were exposed to 90 langmuirs of oxygen at rmm temperature, oxygen adsorption readily occurred. The LEED patterns of oxygen-saturated. chlorineprecovered Pd(100) surface were all similar in that sharp ~ ( 2 x 2 ) spots mixed with weak ~ ( 2 x 2 spots ) were observed. Oxygen coverages, determined from the area under the oxygen TDS pcaks, decreased with increasing chlorine coverages as shown in Figure 13. It was also found that the desorprion peak temperarures of oxygen decreased with increasing chlorine coverage (Figure I I). The shapes of these desorption spectra, however, exhibited the same features as those with So? 0.35 ML from the clean Pd( 100)

surface.' The activation energies of oxygen desorption from the chlorine-precovered surfaces have been calculated by following the same procedure as in ref 4. The activation energies for the low temperature peak are obtained by analyzing the desorption rate in the 4-5% leading edge of each desorption spectrum. The activation energies for the high tempcrature peak. however, was determined by Redhead analysislS for Sa = 0.03.0.05, and 0.15 ML. I n each of these cases, single-peak desorption spectra and So of oxygen were experimentally set at 5 4 values. The activation energies obtained for both the low and high temperature peaks decreased as the amount of chlorine on the surface increased, Figure 14. At low CI surface concentrations. Bc1 = 0.05-0.15 ML,the saturated oxygen overlayers with the c(2XZ)+p(ZXZ) structurm were found to undergo phase transformations to [p(5XS)+p(2X 2 ) ] - 0 and [ ( ~ 5 X ~ 5 ) R 2 7 ° + p ( 2 X Z ) ] -structures 0 upon a mild heating to 350 K and 450 K, respectively. These phase transformations were similar to those taking place in the ~ ( 2 x 2oxygen ) adsorbed phase on a clean surface.' For high CI surface concentrations. 0, > 0.1 5 ML,a mild heating of the saturated oxygen

CH2C12-OxygenInteraction on Pd( 100) Surface overlayers to 350 K resulted in the complete disappearance of the [c(2X2)+~(2X2)]-0structure and an appearance of a featureless bright background. 4. Discussion 4.1. C12Adsorption. The result that chlorine desorbs as atoms suggests that chlorine is adsorbed dissociatively on the Pd( 100) surface as has been reported for several transition-metal surfaces.i@11Js-2055333 The absence of molecular chlorine desorption from the Pd( 100) surface has been attributed by Erley17-i8 to a larger m e t a l 4 bonding energy than the CI2 dissociation energy. The surface chlorine seemed to nucleate in ~ ( 2 x 2 patches, ) forming small anti-phase domains that did not coalesce to a long range ordered structure until the coverage reached about half saturation, i.e., 0.20 ML. The ~ ( 2 x 2 LEED ) spots were initially streaked while the integral order spots remained sharp, indicating that CI was not adsorbed on bridge sites. Similar results have been obtained with CI2 absorption on Ag( and Cu( where atomic CI was also found to adsorb in 4-fold sites. Atomic chlorine desorbed from the c(2X2)-CI/Pd( 100) surface at 1100 K as a very broad TDS peak. Using the Redhead's desorption rate equation35

and assuming first-order kinetics n = 1 with u = lOI3 s-I, the activation energy of desorption was calculated to be 68 kcal/mol, compared with 60 kcal/mol on Pd(ll1) and 67 kcal/mol on Pd( 1 10) reported by Erley.I7vi8 The determination of E, from temperature variation of the leading edge of the desorption curve, which is normally considered more reliable than the use of a specific desorption rate was burdened by a significant error because of the large peakwidth and low signal-to-noise ratio. The E, was nevertheless also determined from the leading edge and a value of ca. 58 kcal/mol was obtained. 4.2. CH2C12Adsorption. The observed frequencies of Pd-C (190 cm-I), Pd-Cl(225 cm-I), and Pd-H (centered around 475 cm-') stretching vibrations detected by HREELS indicated that CH2CI2completely dissociated at room temperature with C, C1, and H chemisorbed in 4-fold hollow sites. The fact that the Pd-H stretch was detected by HREELS at low CH2CI2exposures (0.5 langmuir) while it was not seen at higher CH2C12exposures (19 langmuirs) was probably due to either the displacement of hydrogen adatoms by the oncoming C or CI atoms during the increasing CH2CI2exposures or the diffusion of hydrogen adatoms into positions not observable by specular HREELS. Although the displacement of surface hydrogen by other adsorbing gases has been known to take place on many metal surfaces (for example, the displacement of hydrogen from Ni( 100) by CO"), the results of the present studies suggest that hydrogen is absorbed into the metal surface. The TDS results from CH2C12-precoveredsurface showed that the amount of desorbed HCI as well as of residual CI increased with increasing CH2C12 exposures. A careful comparison between the thermal desorption spectra of HCI and that of CI in Figure 3 and Figure 6 revealed that the area ratio of HCl/CI for CH2CI2-coveredsurface was an order of magnitude higher than that for c(2X2)-CI/Pd( 100). This suggests that adsorbed CH2CI2 is a source of hydrogen although the hydrogen was not detected by HREELS when the CHlC12 exposure was greater than 19 L. It is hence proposed that CH2C12initially dissociates to C, CI, an H adatoms with C and CI remaining immobile while the H atoms diffuse into the (33) Zanazzi, E.;Jona, F.; Jepsen, D. W.; Marcus, P. M. Phys. Reu. B

1976. 14. 432.

(34) Cit&, H.; Hammann, D. R.; Mattheis, L. F.; Rowe, J. E. Phys. Reu. Lett. 1982, 19, 1712. (35) Redhead, P. A. Vacuum 1962, 12. 203. (36) Habenschaden, E.; Kuppers, J. Surf. Sci. 1984,138, 1147. Vollmer, M.; Tragcr, F. Surf. Sci. 1987, 187, 445. (37) Conrad, H.; Ertl, G.; Kuppen, J.; Latta, E. E.Proceedings ojthe 6th International Congress of Catalysis; London, 1976; p 427.

The Journal of Physical Chemistry, Vol. 94, No. 19, 1990 7603 bulk of the palladium crystal. The hydrogen atoms, however, migrate to the surface during the thermal desorption and react with CI atoms forming HCI at 610 K. Desorption of atomic chlorine at 1100 K from the CH2C12covered surface (Figure 6) resembles that of c(2X2)-Cl/Pd( 100) (see Figure 3). This suggests that chlorine atoms from both dissociated CH2C12and CI2 have identical binding states that are not influenced by the presence of other species such as CH, fragments from CH2CI,. 4.3. Structures in Chemisorbed CH2Clz Luyers. LEED studies of adsorption of chloro-substituted alkenes on the Pt(100) surface had been made by Clarke, Gay, and Mason.3s Faint and streaky spots at 0) and (0, *1/2) were observed and attributed to be the out-of-registry ~ ( 2 x 2 )domains formed by dissociative adsorption of the halogenated alkene molecules.38 This faint and streaky LEED structure, however, was ascribed by Grunze and DowbenI5to the out-of-registry 4 2 x 2 ) domains caused solely by surface chlorine adatoms. A precise determination of the surface structure requires a detailed intensity analysis of the diffraction beams utilizing dynamical theory. In the course of CH2C12adsorption on the Pd(100) surface, the LEED patterns observed at different stages of the surface reaction exhibited weak and streaky features and indicated an imperfect order. Such weak and streaky LEED patterns render the surface structure analysis using the dynamical theory impractical. Nevertheless, knowing the dissociative nature of CH2C12adsorption and the saturation coverages, reasonable surface structures may be estimated by employing Monte Carlo simulation and kinematic approximation. In the present study, it was assumed that the C and C1 species formed by dissociative chemisorption of CH2C12are immobile. This assumption is supported by the LEED observation of fuzzy patterns that did not change upon oxidation and thermal treatments at temperatures up to 800 K (cf. Results section). The faint and streaky LEED patterns were initially suggested to be due to partially disordered fragments containing carbon, chlorine, and hydrogen. Considering that hydrogen may diffuse into the sublayers and play no role in determining the surface structure, the residual C and CI atoms would then constitute a specific surface structure. The partially disordered C and C1 fragment arrays would give rise to a large amount of antiphase domains, which would generate a streaky LEED pattern and an unique saturation coverage by chlorine. As a consequence, when C fragments had been removed by oxidation cycles, the residual chlorine atoms were expected to remain in their original sites and also give rise to a faint streaky LEED structure. Several types of CC12 overlayers structures were simulated by Monte Carlo techniques utilizing random distributions subject to specific rules such as site blocking, nearest neighbor exclusion, and an elementary site structure that accommodated one carbon and two chlorine atoms. In these models, the kinematic calculations of the CHzCl2-coveredsurfaces were carried out only using the two-dimensional lattices of C and Cl atoms without considering the palladium layer. The criteria that applied to determine whether a result of a simulation was acceptable were the following: (i) ,,e for CH2CI2-coveredPd(100) must be accounted for by the Monte Carlo simulation, (ii) e,, and eo for coadsorbed CH2CI2 and oxygen must be accounted for, and (iii) the Fourier transform of the Monte Carlo simulated structure must agree with the LEED patterns of (a) CH2C12on Pd( loo), (b) oxygen on CH2C12-covered Pd( loo), (c) CI overlayers originating from CH2CI2by burning off carbon, and (d) oxygen on C1 overlayers prepared as in c. The details of the Monte Carlo and the Fourier transform techniques are described in Appendix I. Twelve simulated CCI2 structures that were tested against the experiment are summarized in Appendix 11. The most acceptable structure that satisfied all criteria i-iii above consisted of a random distribution of Cl-C-CI fragments having CI and C adsorbed at next nearest neighboring (NNN) sites with the CI-C-CI angle of 90° that excluded all the nearest (38) Clarke, T. A.; Gay, I. D.; Mason, R. Chem. Phys. Lett. 1974,27,562.

7604 The Journal of Physical Chemistry, Vol. 94, No. 19, I990

....... : '_1TJ:. :. . ... ; .. . .... *

Wang et al.

BEST FIT FOR CC12 ADSORPTION

-"-*

.y+

1. C C l p blocks all NN adsorption

2. No CI-CI NNN adsorption

(a)

Saturated CI Coveraae Monte CarlQ 0.19 ML

Exoerimental 0.22 M L

Figure 15. Best fit of the Monte Carlo simulation for CH2C12dissociative adsorption on the Pd(100) surface. Only carbon and chlorine atoms were used in the simulation. The dots represent the Pd atoms, while the full small circles represent C atoms and the larger circles represent CI atoms. Representation a shows the CCI2 grouping that was used in the simulation. The solid lines indicate that all the NN adsorption sites are blocked, while the dashed lines indicate that CI-CI NNN adsorption is also blocked. A 50 X 50 square array (b) was saturated with these ensemble groupings with surface coverage O,, = 0. I9 ML.

neighbor (NN) adsorption and CI-CI NNN adsorption [see Figure 1sa]. The maximum coverage of chlorine obtained by the array of these CCI, fragments [Figure 15b] was ~ 0 . 1 9ML. It can be seen that many sites were left unoccupied even after the surface had been saturated with the CCI, fragments. The simulated CI coverage of 0.19 is considered satisfactorily close to the experimental 0.22 in view of the limited size of the modeled area in Figure 15b. The Fourier transform of this CC12-saturated surface exhibited broad and fuzzy spots a t &I/,) (Figure 4b), in good agreement with the experimental LEED pattern (Figure 4a). Since the C1 atoms were strongly adsorbed and unaffected by the oxidation of carbon residues, the structures of the C1 overlayers that originated from the Cl-C-CI fragments could be determined by using the procedure of Appendix I, but with the carbon atoms of the CI-C-CI fragments removed. The simulated LEED pattern for the saturated coverage BCl = 0.19 ML is shown in Figure 12b where the streaky features at (&I/,, 0) and (0, &I/,) are in a very good agreement with experimentally observed patterns, Figure 12a. The simulation of the chlorine-saturated surface was further shown to make unique ensembles that effectively excluded any additional adsorption of CCI, fragments, in agreement with the experimental observation. The lack of CH2CIzadsorption into the CI layer also demonstrates that the CI layer is atomically dispersed and does not form dense patches and large free metal areas. This residual CI-covered surface still allows oxygen adsorption, however, indicating that dissociative oxygen adsorption requires a smaller area of free metal sites than CH2CI2. 4.4. Effect of Cl on Adsorbed Oxygen. The results of consecutive adsorption of dichloromethane and oxygen have demonstrated two effects that have often been considered in catalysis involving electronegative adsorbates: (i) a long-range repulsion effect and (ii) a local or ensemble (site blocking) effect. In Figure 13, it is clearly seen that the oxygen uptake was reduced by increasing the precoverage by chlorine, indicating that chlorine atoms effectively blocked sites available for oxygen adsorption. Furthermore, the total coverages of co-adsorbed oxygen and chlorine were all in the range of 0.40 ML < Bo + B,-, I 0.50

ML, indicating that each chlorine atom has blocked at least five sites. However, oxygen could still adsorb into the vacant area left by the five-site ensembles of chlorine and form patches of saturated ~ ( 2 x 2 phase. ) For the oxygen-saturated Cl/Pd(100) surfaces with Bc, = 0.05-0.08 ML, the metal surface left free by chlorine appeared to be large enough so that reconstruction of surface Pd atoms induced by adsorbed oxygen4 proceeded as on the CI-free surface. This is supported by the fact that surface phase transformation from [ c ( 2 ~ 2 ) + ~ ( 2 ~ 2 )to ] - [0( d / s ~ v ' / s ) R 2 7 ~ + p ( 2 x 2 ) ]was -0 observed by LEED during mild heating to 450 K in vacuum and further evidenced by the lower temperature (ca.600 K) sharp peak detected in the oxygen thermal desorption spectra. For the oxygen-saturated Cl/Pd(100) surfaces with BcI 1 0.15 ML, the disordered CI layer has effectively reduced the size of free metal surface, thereby inhibiting the formation of the [p(2X2)+ ( ~ ' 5 X d 5 ) R 2 7 ~ ] -phases 0 via the c(2X2)-0 phase transformation. Further understanding of the effect of chlorine on adsorbed oxygen is obtained by examining the thermal desorption spectra of consecutively adsorbed CH2CI2and oxygen (see Figure 11). The activation energies of oxygen desorption from the chlorineprecovered surfaces smoothly decreased with increasing residual-Cl coverage, indicating that the 0-Pd bond strength was indeed weakened by the presence of chlorine (Figure 14). The drop in activation energy pertains to a sharp low temperature peak associated with the dense adsorbed oxygen phase, shown to be 4 kcal/mol at chlorine coverage as low as 0.05 ML, indicating that surface chlorine reduced the Pd-O bond strength at an average C1-0 distance of some 3-4 Pd-Pd lattice constants. A still larger decrease of desorption energies accompanies the higher temperature peak associated with the rare phase. The initial activation energy drop was ca. 10 kcal/mol at the chlorine coverage as low as 0.03 ML, indicating that the Pd-O bond strength in the rare adsorbed oxygen phase was reduced even more than that of the dense phase a t large CI-0 distances. 4.5. Coadsorption of 0 and CHzC12and Oxidation of Carbon Fragments. The oxygen and CH,CI, adsorption studies demonstrated that the formation of coadsorbed layers depends on the

CH2CI2-Oxygen Interaction on Pd( 100) Surface

Oxygen-Rich Phase

ec, = 0.05 ML eox= 0.33 ML

The Journal of Physical Chemistry, Vol. 94, No. 19, 1990 7605

Oxygen-Deficient Phase

ec, = 0.19

ML

eox= 0.14

ML

Figure 16. (a) Oxygen-rich O/CHzCIz/Pd(100) phase and (b) oxygen-deficient O/CH2Cl2/Pd(100) phase obtained from Monte Carlo simulations with the (a) 0.025 ML and (b) 0.10 ML, respectively, CIC-CI fragments precovered Pd(100) saturated with 0 atoms. The dots represent the Pd atoms; full small circles, the C atoms; medium circles, the 0 atoms; and large circles, the CI atoms.

sequence in which oxygen and CH2ClZare introduced. The well-ordered ~ ( 2 x 2 or ) ~ ( 2 x 2 oxygen ) overlayers exclude any further dissociative adsorption of CHzCl2since there is no area available for the CI-C-CI fragments. The reverse, however, is not true, since ensembles of free Pd atoms generated by random dissociative adsorption of CHZCIzare available for oxygen adsorption. It also appears that oxygen adsorption into the vacant areas left by CHzClzpreadsorption, particularly at low dClxxl, is similar to that on clean Pd(100) and reaches saturation with a well-ordered ~ ( 2 x 2 phase ) in the areas between the CI-C-Cl fragments. The reaction between oxygen and carbon atoms that were generated by dissociative adsorption of CH2Clz may be of significance for catalysis as CO is p r o d u d with increasing selectivity at the expense of the C02 production upon increasing preadsorption of CH2ClZ. A series of Monte Carlo simulations were carried out to subject the carbon oxidation and its selectivity to CO and C 0 2 to a quantitative test. The objective was to simulate various oxygen and CHZCl2co-adsorbed overlayers and then to determine the amount of C O and C 0 2 molecules by counting the oxygen atoms that reside adjacent to the neighboring carbon atoms. A program and exclusion rules identical with those used to simulate the CI-C-CI adsorption in Appendix I were employed. A 50 X 50 square array of Cl-C-Cl fragments were randomly generated at the chlorine coverages, del, of 0.025,0.05,0.075,0.10, 0.125, 0.150, and 0.185 ML. The surface was then filled with oxygen atoms into an ordered ~ ( 2 x 2 structure. ) Of the simulated outputs, the two extremes, i.e., the oxygen-rich O/CCl2 surface and the oxygen-deficient O/CCl2 surface, are presented in Figures 16a and 16b, respectively. An oxygen-rich environment in the simulated array of Figure 16a would favor the complete oxidation of carbon forming COz. An oxygen-deficient environment shown in Figure 16b, on the other hand, will favor the partial oxidation of carbon to CO at the expense of COz. It is herein proposed that CO and COz are formed in parallel surface reactions between the carbon and oxygen atoms and that selectivity of carbon oxidation is determined by the availability of surface oxygen, which is effectively controlled by the concentration and distribution of the CI-C-CI fragments. In a specific

version of the model presented here, it was assumed that oxygen atoms can move only along the [01] and [ 101 directions in the Pd( 100) surface. The rules for the CO and C02 formation were made on the basis of the oxygen distribution around each carbon atom as follows: (i) if there are two or more than two oxygen atoms simultaneously present at the third nearest neighboring (3") 4-fold sites to a carbon atom, then a COz is formed, (ii) if there is one or no oxygen atom present at the 3NN sites to a carbon atom, then CO is formed, (iii) if there are two oxygen atoms simultaneously present at the 3" sites of two carbon atoms, then two CO are formed, and (iv) the maximum number of oxygen atoms that could be attached to a carbon atom is two; if there are three oxygen atoms simultaneously present at the 3" sites of two carbon atoms, then a CO and a C 0 2 are formed, and so on. The resulting distributions of CO and C 0 2 formed according to the above rules from the simulated O/Cl-C-Cl overlayers are plotted together with the experimental results in Figure 9 (dashed lines). The simulation of CO and COz formation using the Monte Carlo method appears to be in a favorable agreement with experiment. This indicates that the random distribution of the C1-C-CI fragments, and thereby CI atoms, does provide for the control of oxygen supply to the carbon, which leads to the increasing selectivity of CO against COz. The above-described controlling characteristic of carbon oxidation by chlorine is believed to be an ensemble size effect whereby the chlorine adsorbate restricts the access of oxygen to the surface carbon and thus directs the reaction in favor of the C O product. The long range weakening of the Pd-0 bond due to the presence of the adsorbed chlorine may be attributed to a number of repulsion effects. Although such effects may be expected to influence overall reaction rates, the evidence and model presented here strongly suggest that the ensemble-controlled reaction pattern is dominant in determining the selectivity to CO/CO2. 5. Conclusion Dichloromethane, CHzC12,readily dissociated upon adsorption a t room temperature on Pd( 100) with C, CI, and H adsorbing into 4-fold hollow sites and H apparently diffusing into the bulk

7606 The Journal of Physical Chemistry, Vol. 94, No. 19, 1990

of the palladium crystal upon increasing exposure to CH2Cl2. The CH2CI2adsorption reached saturation after 20-langmuir exposure with faint cross-shaped LEED spots positioned a t and a CI coverge equivalent to 0.22 ML. With use of Monte Carlo simulation, a random distribution of C1-C-C1 fragments CI and C adsorbing at NNN sites and a CI-C-CI angle of 90' were shown to represent the most appropriate model for CH2Clzadsorption. The Pd(100) surface with a well-ordered ~ ( 2 x 2 or ) ~(2x2) structure of adsorbed oxygen completely inhibited CH2CI2adsorption because of insufficient free metal area for CH2C12 dissociation, whereas CH2C12-precovered surface was very reactive toward oxygen. The surface carbon from dissociated CH2CI2 oxidized at temperatures of 365-510 K, forming C O and C 0 2 with the selectivity toward CO increasing with CI-C-CI coverages. The selectivity of carbon oxidation was suggested to be determined primarily by the accessibility of oxygen to carbon fragments, which are actually controlled by the distribution of the CI-C-CI fragments. A satisfactory agreement was obtained between a Monte Carlo simulation model of oxygenarbon-chlorine overlayers and selectivity of carbon oxidation to C O or C02. It was also found that two cycles of oxygen adsorption onto and heating of CH2C12,-precoveredPd( 100) to 900 K can remove all the C and H species, while the structure and concentration of adsorbed chlorine remained unchanged. Such a procedure hence enables CH2CI2to serve as a source for a disordered phase of chlorine, which may be the key step in surface modification for the partial oxidation of methane.ls2 From the AES, LEED, TDS, and HREELS results and Monte Carlo simulations, it is concluded that chlorine exhibits both an ensemble effect and a long range electronic effect upon oxygen reactivity, but the ensemble control dominates the CO/CO2 selectivity pattern in carbon oxidation.

Appendix I The Monte Carlo simulations were carried out by using a program that randomly generated the surface coordinates of the chemisorbed species. Two-dimensional lattices of C and C1 atoms were used in the simulation with the stoichiometry C/Cl = 1/2. A 50 X 50 square array was randomly filled with the CCI2 fragments using various adsorption geometries and exclusion rules (see Appendix 11). The possible overestimate of the saturation coverage caused by edge adsorption was taken into account. When the randomly chosen array element was located on the edge of the array, its neighbor site just outside the array was assumed to have a 50% probability of being occupied. The output of the Monte Carlo simulations was used as a representation of the real space surface lattice. The simulated LEED pattern was then obtained via Fourier transform39 I(h,k) =

.-

= -t 0.7

w

2

r

0.6

0

0

LL

0.5

-

0.4

-

.E 2 0.3 a

-

0.2

-

0)

.-cL

0 0 0 0 0

COS

2r(hx

c

m

0.1

30 60 90 S c a t t e r i n g Angle e ( d e g r e e s ) Figure 17. Atomic scattering factorsf,(@) for carbon and chlorine used in the simulation of LEED patterns for the proposed CH2C12-adsorbed Pd(100) structures.

0

TABLE I no.

CCll structure

0 coverage on LEED weak spots and streaks as in CCI2 precovered Figures 4a and 12a CI coverage surface

I

no

2

no

3

no

4

no

5

no

no

where I(h,k) is the intensity of the diffracted beam in the h,k direction, h and k are integral variables that map reciprocal space, and f, is the scattering factor of the atom n at coordinates x, y. The atomic scattering factors for carbon and chlorine were obtained by interpolating the theoretical values calculated at 25, 50, 75, and 100 eVa to our LEED experimental conditions, 66 eV. The estimated scattering factors of carbon and chlorine are shown in Figure 17 and expressed as functions of the scattering angles ghk as follows:

7

9

no

fC-8, = 0.5371 - 3.782 x 1048h&- 3.852 x i0-%9),k2

IO

Yes

oh&

8

Yes

no

Yes (-0.02 ML)

no (+0.04 ML)

Yes (+0.01 ML)

no (+0.03ML)

no (-0.05 ML)

Yes (+0.02ML)

Yes

no

(+O.OI ML)

(-0.04 ML)

no (-0.08 ML)

no (+0.04 ML)

Yes (+0.01 ML)

(-0.05 ML)

no (-0.05 ML)

(+0.04 ML)

Yes

no

no

(f0.00 ML)

no (-0.04 ML)

no (-0.06 ML)

Yes (+O.OI ML)

Yes

Yes

(-0.03 ML)

(+O.OI ML)

Yes (-0.03 ML)

no (+0.05ML)

Yes (+0.03 ML)

(-0.1 ML)

5 60' 11

(39) Ertl, G.; Kuppers, J. Surf. Sci. 1970, 21, 61. (40) Fink, M.; Yates, A. C. Aromic Dura 1970, I , 385. Fink, M.; Ingram, J. Aromrc Dura 1972, 4, 129. Fink, M. Technical Reports No. AFOSR70I652TR and AFOSR71-1204; Department of Physis, University of Texas,

Austin.

1

0

+ ky)J2 + [n=O f, sin 2r(hx + ky)J2 (2)

20' 5

Carbon 0 Chlorine

Q)

N- I

fn n-0

S c a t t e r i n g F a c t o r s a t 66 eV

6

N- I

[

Wang et al.

12

no

no

Yes

J . Phys. Chem. 1990, 94, 7607-761 1 and

fcl/A

1.572 - 0.09536hk

+ 4.885 X

10-36hk2- 1.238 X 1048/jk3 1.059 x Iod8hk4 20' 5 8hk 5 52'

+

Appendix I1 The structures of the CC12 fragments that were investigated involved those listed in Table I. For a structure to be considered acceptable, the main features of the LEED patterns, namely, the presence of the weak spots and streaks in Figure 5a of the CClz overlayers and Figure 12a of the C1 overlayers originating from it, the chlorine coverage and the extent of oxygen coverage on the CC12-precovered surfaces had to agree with experiment in the

7607

whole range of C1 surface concentrations studied. In Table I we mark the agreement or disagreement of the model structure with experiment to indicate that the CClZstructure 10 is the most satisfactory. In the column showing the CClZstructures, the Pd atoms are at the intersection of the lines, C atoms are shown as small circles, and C1 atoms as large circles. The numbers in parentheses in the two right columns indicate the deviations between the simulated and experimental results. For the present study, the acceptable deviation for coverages was assumed to be 50.03 ML.

Acknowledgment. This work was supported by U.S.Department of Energy, Office of Basic Energy Sciences, under Grant DE-FG02-86ER13580.

Adsorption of Nitroxide D20 Solutions on X and Y Zeolltes Studied by Electron Spin Resonance and Electron Spin Echo Spectroscopies Ciacomo Martini,* Sandra Ristori, Dipartimento di Chimica, Universitci di Firenre, 501 21 Firenze, Italy

Maurizio Romanelli, Istituto di Chimica, Universitci della Basilicata, 85100 Potenza, Italy

and Larry Kevan* Department of Chemistry, University of Houston, Houston, Texas 77204-5641 (Received: January 22, 1990; In Final Form: April 23, 1990)

Both continuous-wave ESR and pulsed wave ESE experimental patterns of DzOsolutions of a positively charged radical adsorbed on X and Y zeolites were analyzed to get data on the structural and dynamical properties of the adsorbed radical. The analysis was mainly carried out by best-fit computer simulation procedures. The obtained results revealed significant differences in both the adsorption properties and the structure of the radical environment in the supercages of the zeolites X and Y because of different interactions with the surface charge. The effects on the structural and dynamical properties of the exchange of Na+ with Ca2+as the main cocation in the zeolite framework were negligible. A relevant role is played by the nitroxide methyl groups in determining the spin relaxation process at 4 K.

Introduction Electron spin resonance (ESR) spectroscopies, both in continuous-wave and in pulsed electron spin echo (ESE) forms, of transition-metal ions or nitroxide radicals adsorbed as liquid solutions onto porous supports have been largely used to get detailed information on (i) the dynamics of both probe and solvent, (ii) the molecular arrangement around the paramagnetic species, and (iii) the peculiar features of the solid-liquid interface.'-' For instance., the combined use of CW ESR and ESE on a negatively charged nitroxide in water solutions adsorbed on homoporous silica gels has allowed one to establish the structural changes undergone by both the adsorbed probe and the liquid at different distances from the s ~ r f a c e . The ~ pulse technique was used to evaluate the effect of the water and nitroxide proton nuclei on the modulation of the electron spin echo signal, which enabled calculation of the type and the number of nuclei in the immediate environment of the unpaired electron. Similarly, the analysis of the C W ESR (1) Martini, G.; Ottaviani, M. F.; Romanelli, M.; Kevan, L.Colloids Surf. 1989, 41, 149. ( 2 ) Martini, G . Colloids SurJ 1990, 45, 83. (3) Time Domain Electron Spin Resonance; Kevan, L.,Schwartz, R. N., Eds.; Wiley-Interscience: New York, 1979. (4) Pinnavaia, T. A. Aduanced Techniques for Clay Mineral Analysis; Fripiat, J. J., Ed.; Elsevier: New York, 1981; p 139. ( 5 ) Romanelli, M.: Martini, G.: Kevan, L.J . Chem. Phys. 1986.84, 4818.

0022-3654/90/2094-7607$02.50/0

line shape of nitroxides in alcohol solutions adsorbed on X-zeolite showed different liquid mobility with respect to the unadsorbed liquid.68 In these systems the pulsed techniques gave results from which a sketch of the structure of the radical environment was derived for both pure and adsorbed solutions. The information on the dynamical behavior of the spin system is contained in the decay of the spin echo signal^.^-'^ From an analysis of this spectral feature one can elucidate the mechanisms and the related time constants governing the spin relaxation. Very recently Romanelli and KevanI6 proposed a computational procedure to evaluate relevant physical parameters from the decay functions in the two- and three-pulse ESE patterns on the basis (6) Mazzoleni, F.; Ottaviani, M. F.; Romanelli, M.; Martini, G. J . Phys. Chem. 1988, 92, 1953. (7) Romanelli, M.; Martini, G.; Kevan, L.J . Phys. Chem. 1988,92, 1958. (8) Romanelli, M.; Ottaviani. M. F.; Martini, G.:Kevan. L. J . Phvs. Chem. 1989, 93, 317. (9) Herzog, B.; Hahn, E. L.Phys. Reu. 1956, 103, 148. (10) Klauder, J. R.; Anderson, P. W. Phys. Reu. 1962, 125, 912. (11) Mims, W. B. Phys. Reo. 1968, 168, 370. (12) Mims, W. B.; Nassan, K.; McGee, J. L.Phys. Reu. 1961, 123,2059. (13) Hu, P.; Hartmann, S. R. Phys. Reu. B. 1974, 9. 1. (14) Salikhov, K. M.; Tsvetkov, Yu. D. Time Domain Electron Spin

Resonance: Kevan, L., Schwartz, R., Eds.; Wiley-Interscience: New York, 1979; Chapter 7. (15) Brown, I. M. Time Domain Electron Spin Resonance; Kevan, L., Schwartz, R., Eds.; Wiley-Interscience: New York, 1979; Chapter 6. (16) Romanelli, M.: Kevan, L. J . Magn. Reson., in press.

0 1990 American Chemical Society