Metal surface interaction with Pi-acceptor molecules: PF3 adsorption

Metal Surface Interaction with Pi-Acceptor Molecules: PF~ Adsorption G. B L Y H O L D E R

A N D R. S H E E T S

i

Department of Chemistry, University of Arkansas, Fayetteville, Arkansas 72701

Received December 18, 1972; accepted August 31, 1973 In a continuing effort to characterize the chemical properties of metal surfaces, infrared spectra of PFa adsorbed on vanadium, iron, nickel, palladium, copper and aluminum evaporated into an oil matrix have been obtained. The spectra indicate adsorption without dissociation to give structures similar to PF3 coordination complexes with symmetric and asymmetric P-F stretching frequencies in the 800 cm-1 region and a P-F bending frequency in the 500 cm-I region. As in the homogeneous complexes CO and PF~ can replace each other although on the surface CO is more tightly bound than PFa. The molecular orbital view of donation and ~r back bonding which has been successful with chemisorbed CO is shown to be useful also in understanding strengths of PF~ adsorption on different metals and shifts in infrared frequencies from metal to metal, with changing surface coverage and as PF8 is replaced by CO. INTRODUCTION One area of surface chemistry which is insufficiently explored with modern techniques is the chemical characterization of metal surfaces b y their interaction with different classes of molecules t h a t have evolved from studies of coordination complexes. If one takes the point of view t h a t a metal a t o m in a surface can be regarded as a metal a t o m in a coordination complex, one becomes i m m e d i a t e l y aware of several possibilities. Adsorbates or ligands m a y be classed as Lewis bases, Lewis acids, or ~ donors and ~r acceptors. Before chemical characterization studies can be undertaken, however, it is necessary to have available a number of different molecules whose detailed structures as adsorbed surface species are known. As yet, such structures are known for only a few molecules. W e have already demon1 Present Address. Department of Chemistry, Southwest Missouri State University, Springfield, MO. 65802.

s t r a t e d pure metal base t r a c t i o n occurs b y a chemisorption s t u d y of BF~ (1). To be useful as surface ligands in infrared studies, molecules should adsorb without dissociation. If d i s s o d a t i o n occurs, the surface becomes covered with fragments of v a r y i n g structures, and i n t e r p r e t a t i o n becomes ext r e m e l y difficult. T h e r e do not a p p e a r to be m a n y chemical compounds which meet this condition. Transition metals possess the characteristic ability to form complexes with a n u m b e r of neutral molecules including carbon monoxide, isocyanides, phosphorus trifluoride, substit u t e d phosphines, nitric oxide, pyridine, arsines, and stibines. M a n y of these complexes contain the metal a t o m in a zero oxidation state. The ligands have lone-pair electrons which can be used to form sigma bonds to the m e t a l atoms, and v a c a n t orbitals which can be used to form p i - t y p e bonds b y accepting electron density from filled m e t a l orbitals. Of the molecules belonging to this group only carbon monoxide

380 Journal of Colloid and Interface Science, Vol. 46, No. 3. March 1974

Copyright ~ 1974by AcademicPress. Inc. All rights of reproductionin any form reserved.

METAL SURFACE INTERACTION WITH ~r-ACCEPTOR MOLECULES and nitric oxide have been widely studied as metal-chemisorbed species (2, 3). In the case of NO, some decomposition has been shown to occur upon adsorption (4). While a ~ donor and ~'-acceptor molecular orbital view of adsorbed CO has been presented (5, 6) a wider range of applicability needs to be demonstrated before this orbital view and the underlying comparison of heterogeneous adsorption and homogeneous coordination chemistry can be regarded as firmly established and useful. Phosphorus trifluoride is a prime candidate for nondissociative chemisorption. Phosphorus trifluoride complexes with the transition metal in the zero oxidation state have been prepared using chromium, molybdenum, tungsten, iron, ruthenium, osmium, cobalt, nickel, palladium, and platinum, among others (7). Phosphorus trifluoride, like carbon monoxide, complexes with the hemoglobin of human blood (8). It can replace carbon monoxide in transition metal complexes and its pi-acceptor power is thought to be greater than that of any of the other neutral ligands except, possibly, carbon monoxide (7, 9). The choice of metals for studying chemisorbed PF3 was made on the following basis. Iron, palladium and nickel were chosen because of their known activity in heterogeneous catalysis. Each of these three metals has several valence-shell d-electrons ill the ground state configuration (6, 8, and 8, respectively). Vanadium, with 3 d-electrons, was selected as representative of transition metals with fewer d-electrons and copper was chosen for study because its valence d-orbitals are filled in the ground state. Finally, aluminum, which is one of the few nontransition metals on which carbon monoxide chemisorbs (10), was chosen. More information about the character of chemisorbed PF, was obtained by observing the effects of foreign gases on the chemisorbed PF3 spectrum. Carbon monoxide was particularly useful in this respect, both as a preadsorbed species and as a foreign gas, admitted after chemisorption of PF.~. When CO and a second adsorbate are simultaneously

381

chemisorbed on a metal film, information about electron withdrawal or donation by this latter adsorbate may be obtained by observing the shift it causes in the CO stretching frequencies. It has been shown that the presence of a second, electron-donating ligand causes the carbon monoxide stretching bands to appear at lower frequencies (11). This is presumably because the excess electrons go into a CO antibonding orbital, weakening the C-O bond and decreasing the C-O stretching frequency (7). Electron-withdrawing ligands, on the other hand, remove electrons from the antibonding orbitals, causing the vibrations to occur at higher frequencies. The information obtained in this manner is helpful in determining the structure of the second chemisorbed species. EXPERIMENTAL

METHODS

The wide spectral range experimental technique, which has been described in detail elsewhere (12), consists of evaporating metals from an electrically heated tungsten filament in the presence of a small pressure of helium. The metal particles formed in the gas phase deposit in a hydrocarbon oil film on the salt windows of an infrared cell. The gas to be studied is then admitted to the cell, and the spectrum of the chemisorbed species is obtained. Spectra are recorded before and after admission of the gas to the cell. Five minutes of pumping has been found sufficient to remove all spectra due to gas-phase molecules. For three- and four-carbon-atom molecules 30 rain of pumping may be required to remove molecules dissolved in the oil film. Care was exercised to make sure that all bands which are reported to be due to chemisorbed species are indeed due to such species and not morely to gases trapped in the oil layer. All bands due to chemisorb species reported in this study were still present after the cell had been evacuated at 10-8 Tort for 16 hr or longer. The spectra were obtained using PerkinEhner Models 21, 337, and 457 spectrometers of which the latter two are grating instru-

Journal of Colloid and Interface Science, VoL 46, No. 3, March 1974

382

SHEETS

BLYHOLDER[AND

ments. The Model 21 is equipped with NaC1 optics and an ordinate scale expander. The iron, nickel, palladium, tungsten and copper were obtained in the form of high purity wire from A. D. McKay, Inc. The vanadium was obtained as 5 mil foil, 99.8% pure, from United Mineral and Chemical Corp. The CO was obtained as reagent grade from Matheson Scientific, Inc. and purified by passage through a charcoal trap immersed in liquid air. The PF3 was obtained from Penninsular Chem-research, Inc. and subjected to alternate freeze-thaw cycles with pumping before use. This experimental technique has the advantage that a wide infrared spectral region is available for study. It has the disadvantage that the metal surface is covered with oil. This oil is apparently only weakly adsorbed since many gases have been found to chemisorb readily on the metal. Essentially the oil is regarded as a solvent which has weak interactions with the systems of interest. Having a wide spectral range available aids greatly in attempts to determine structure.

Direct comparison of the infraredspectra of CO chemisorbed on metals using this oil matrix technique (5, 6, 12, 13) can be made to those obtained by evaporating a metal onto a bare salt plate for Ni, Co, Fe, Mn, Cr, Pd, and Pt (14-18) and to CO adsorbed on Armatrix isolated Ni (19). In all cases the oil matrix spectra for chemisorbed CO are quite similar to those obtained by the other methods. Direct comparison of the interaction of alcohols with oil matrix metals and silica supported metals shows no fundamental difference in the adsorption processes on Fe, Co, and Ni (20). Even though saturated hydrocarbons only adsorb to the extent of covering a small fraction of a metal surface, a few of the most active specia! adsorption sites are undoubtedly lost in the oil matrix technique due to oilmetal interactions but for most of the surface in the case of a strongly adsorbed molecule this technique is satisfactory. The interaction of CO with Cu is an illustration of the limitations of this technique since CO is so weakly adsorbed on Cu that whereas CO adsorption is detectable on bare Cu, CO is unable to displace

TABLE

I

.FREQUENCIES (cnl-1), INTENSITIES a AND BAND ASSIGNMENTS FOR ~IIOSPttORUS TRIFLUORIDE AND CARBON MONOXIDE CHEMISORBED ON IRON AND PALLADIUM AT 2 5 ° C PF~ on palladium

CO and PF~ on palladium

PF8 on iron

CO and PF~ on iron

PF~ (gas) (15)

Pd(PF~)* (16)

8 9 0 sh, v s

8 7 0 sh, w

8 7 0 sh, vs

850 sh, w

892 vs

9 1 0 vs

8 7 0 vs

845 w

845 vs

825 w

848 v s

860 v

520 s

520 w

515 vs

515 w

532 vs

--

--

4 8 7 vs

--

--

--

--

2 0 5 0 rn 1930 vs "Abbreviations broad.

--

--

1950 vs 589 m

Fe(PF0~ (17) 915 901 873 864 851 586 572 543 469 405 374 339 314 292 --

vs vs w w vs rn rn vs vs w vs sh w w

Assignment

P-F stretch (sym.) P - F s t r e t c h (asyrn)

P-F bend

P-F bend

Added CO

u s e d in t h i s a n d T a b l e I I : vs, v e r y s t r o n g ; s, s t r o n g ; m , m e d i u m ; w , w e a k ; sh, s h o u l d e r ; b,

Journal of Colloid and Interface Science, Vol. 46, No. 3, March 1974

METAL SURFACE INTERACTION WITH rr-ACCEPTOR MOLECULES

383

t00 9¢ 8O o

70

/,,,'

,~ 60

,s

//

5O

"J/ 40

%. i #

30 I

2t00

1900

I

1700

t,~

I

~ l Ic~00 9O0 8OO Frequency (crn-t )

I

700

I

600

r

500

FIG. 1. Carbon monoxideand PFa on palladium: (A) chemisorbed PFz; (B) CO added and evacuated; (C) PF, added and evacuated; (*) background oil band. weakly interacting oil from the surface of Cu in oil. The technique of evaporating metals onto a bare salt plate was not used for the work reported here because that method gives bands for chemisorbed CO which are barely detectable so that adsorbates other than CO, which has an extremely high extinction coefficient, are likely to be unobservable. RESULT~ Evaporated palladium films were exposed to gaseous PF3 at pressures of 30 Torr for 30 rain periods. The cells were evacuated for 1 hr and infrared spectra were taken in the 4000-400 cm-~ region. Table I shows the resulting bands, their relative intensities and tentative assignments (21-23). Figure 1 shows representative spectra of PF3 chemisorbed on palladium. Evaporated iron films gave similar results which are recorded in Table I also. The intensities of the bands on iron were around one-third of the intensities for palladium. After spectra had been taken of PF~ chemisorbed on an evaporated palladium film, carbon monoxide at a pressure of 30 Torr was admitted to the cell. The resulting spectrum showed the presence of both gaseous carbon monoxide and gaseous phosphorus trifluoride as well as various chemisorbed species. The cell was evacuated after 30 rain and spectra

again were taken. Finally PF3 at a pressure of 30 Torr was again admitted to the cell, left for 30 min, evacuated and spectra were again taken. Figure 1 shows the effect of carbon monoxide on PF3 chemisorption on palladium. Band assignments are listed in Table I. An evaporated iron film with preadsorbed PF3 was exposed to CO in the same manner as that described above. The spectra obtained were quite similar to the palladium spectra. Table I lists the spectral band assignments. Evaporated nickel films were exposed to PF3 at pressures of 20-50 Torr for periods of 15 min to 1 hr and the cells were evacuated for periods of up to 17 hr. A spectrum representative of those taken in the 1000-400 cm-1 region is shown in Fig. 2. The appearance of a second band in the 500 cm-1 region, at 560 cm-1 here, was peculiar to nickel. The bands and assignments are listed in Table II. Also in Fig. 2 are spectra obtained by exposing an evaporated nickel film first to PF3, then to CO, and, filally, to PF3 again. In each instance the pressure of the gas used was 30 Torr, exposure was for 30 rain and the cell was evacuated for 30 rain before a spectrum was taken. Band assignments are listed in Table IL The band at 560 cm-1 is seen to be affected by the addition of CO in a different manner from the 505 cm-1 band. An evaporated nickel film on which PF3 had previously been adsorbed was

Journal of Colloid and Interface Science, Vol. 46, ]No. 3, M a r c h t 9 7 4

384

BLYtIOLDER AND SHEETS

9c f ~

/._

.._,

, .

•

..

70

'

I

A/

1

t /

*

J 50 ~

~

1

2200

I

2000

1800

~ ~

1600

'1000

I

[

I

I

I

900

800

700

600

500

Frequency (crn"I)

Fro. 2. Carbon monoxide and PF3 on nickel: (A) chemisorbed PF3; (13) CO added and evacuated; (C)

PF~ added and evacuated; (*) backgroundoll band. exposed to hydrogen at a pressure of 60 Torr for 2 hr. After evacuation spectra were taken in the 4000-700 cm-1 region. There was no observable effect on the spectrum. It was necessary to expose the vanadium films for relatively long periods of time in order to obtain useful spectra. The spectrum shown in Fig. 3 was obtained by exposing a vanadium film to PF3 at a pressure of 80 Tort for 5 hr. The spectrum was made using an ordinate expansion of 5X. Thekbands and assignments are listed in Table II. An evaporated vanadium film on which PF3 had been adsorbed was exposed to CO at a pressure of 30 Torr for 15 rain. Bands due to gaseous PF3 were observed in the spectrum. Spectra taken after evacuation showed that all bands due to

chemisorbed PF3 had disappeared and that a band due to chemisorbed CO had appeared at 1920 cm-1. Evaporated aluminum and copper films were exposed to PF3 at pressures o f 70 Torr for 4 hr. The bands for PF3 chemisorbed on aluminum and copper were similar to those for Pd but at a much lower intensity and are listed in Table II. A spectrum of PF3 chemisorbed on an evaporated aluminum or copper film was taken and the film was exposed to CO at a pressure of 30 Torr while a second spectrum was taken. After 1 hr, the cells were evacuated and a third spectrum was taken. No differences were observed in the spectra taken before and after exposure to CO either on aluminum or copper.

TABLE

II

F R E Q U E N C I E S (cm -1), INTENSITIES A N D B A N D ASSIGNMENTS F O R P H O S P H O R U S TRI]~LUORIDE A N D

CARBON MONOXIDE Ct/lgN[ISOI~BEDON COPPER~ ALUHI~UH, VANADIU~ AND NICKEL AT 25°C PF8 on

CO and PF3 on nickel

Ni(PFs)~ (15)

Assignment

Copper

Aluminura

Vanadium

Nickel

8 3 0 sh, m

850 m

865 w

8 8 0 sh, v s

860 m

898 v s

P-F

stretch (sym)

810 m

810 m

805 w

850 vs

825 s

859 vs

P-F

stretch (asym)

--

--

--

560 m

560 m

--

P-F

bend

550 m

--

--

505 v s

505 s

503 v s

P-F

bend

386 vs

I'-F bend

--

Added CO

. .

. .

. .

. .

. 2060 vs 1920 v s

Journa~ of Colloid and Interface Science, Vol. 46, No. 3, March 1974

METAL SURFACE INTERACTION WITH ~r-ACCEPTOR MOLECULES

385

100

96

\.,

94

xXX

2 v-- 92 o~

\\\~'~x x

90

X

x\ %%

I000

I

950

r

909

q

I

850

I

800

750

700

Frequency (cm -1 )

FIO. 3. Phosphorous trifluoride on vanadium : ( The relative strengths of the CO-metal bonds and the PF~-metal bonds for the chemisorbed species were investigated by the following procedure. A mixture of equal volumes of the two gases at a total pressure of 100 Torr was admitted to a clean glass cell and a spectrmn was taken. Only PF~ and CO gas bands were observed. Heating the mixture for 1 hr at 100°C had no effect on the spectrum. Next an evaporated nickel film was exposed to such a gas mixture for 30 rain. The resulting spectrum showed strong bands due to chemisorbed CO but no bands which could be attributed to chemisorbed PF3. These experiments indicated that for nickel, iron, vanadium, and palladium, the CO-metal bonds are more stable than the PF3-metal bonds. DISCUSSION Chemisorption of PF~ appears to occur without dissociation, thus confirming its predicted usefulness in chemical characterization of metal surface studies and in comparing heterogeneous to homogeneous processes. When films of iron, nickel, palladium, and vanadium containing preadsorbed PF3 were exposed to carbon nlonoxide, CO was chemisorbed and PF3 was desorbed as neutral PF3 molecules. If dissociation has occurred, to M-PF2 and M - F species, for example, one would have expected the M - F band to be strong enough so that reformation and desorption of PF3

) background; (-- --) chemisorbed PF~. molecules would not readily take place. Further, no ir bands were found which could be identified as metal-fluorine bands. The similarities between spectra of chemisorbed PF3 and corresponding zero-valent metal complexes also very strongly support this interpretation. For PF3 chemisorbed Oil iron, palladium and nickel infrared band assignments were made by comparing their positions with those of the analogous zero-valent metal complexes as listed in Tables I and II. The vibrational frequencies and physical properties of these complexes are well known (7). The iron and nickel complexes are stable at room temperature but the palladium complex decomposes above --20°C. Similar PF3 complexes with vanadium, copper and aluminum have not been reported. Band assignments for chemisorbed PF3 on these metals were made by comparison with the nickel, iron and palladium spectra. Bands lying between 800-900 cm -j are assigned to P - F stretching vibrations. Two bands are observed in this region--one due to the P - F symmetric stretching vibration and one due to the P - F asymmetric stretching vibration. Bands at 500-520 cm-1 are assigned to one of the P - F bending vibrations. The other bending frequency lies beyond the range of the instruments used. A medium intensity band at 560 cm -1 is

Journal of Colloid and Interface Science, Vol. 46, No. 3, March 1974

386

BLYHOLDERAND SHEETS

observed in spectra of PF3 chemisorbed on nickel. This band is not affected by addition of CO, although all other chemisorbed PFa bands on nickel are changed. No band is observed in this region in spectra of PF3 chemisorbed on any of the other metals studied. The band could be due to a decomposition product of PF3, possibly a N i - F species, but this seems unlikely since spectra of matrixisolated NiF2 show absorption bands only in the 780-700 cm-~ region (24). The fact that the 505 cm-I band has disappeared completely after CO addition while the 850 cm-~ band still remains at about one-third of its original intensity with the band maxima shifted to a lower frequency as shown in Fig. 2, suggests that the 560 cm-1 band is due to PF3 adsorbed without dissociation on different surface sites from the other PFa. PF, was chosen for study because its characteristics as a ligand in coordination chemistry were similar to those of CO thereby suggesting it would be a good test of the validity of using as a framework for considering adsorption on metals the molecular orbital view which has been successful in understanding CO adsorption. The ability of the orbital view to rationalize the data for PF3 adsorption is now considered. Studies have been made which show that the 7r-acceptor strength of PF8 is comparable to that of CO and greater than that of any of the other usual ligands (7, 9). The approximate equality of ~r-acceptor strength of CO and PF3 as adsorbed species is shown by the C-O stretching band position being unchanged when CO and PF3 are simultaneously adsorbed. That the band position for chemisorbed CO is sensitive to electronic factors is shown by the shifting of the C-O band to lower frequencies when NH3 is coadsorbed (25) and to higher frequencies when BF~ is coadsorbed (1). Comparison of the P - F stretching frequencies through the series V, Fe, and Ni reveals a continuous increase in frequency. In the orbital view of chemisorbed CO it was noted (6) that the valence state ionization potentials increase across this series so the

extent of donation of ~r-electrons to the adsorbate decreases. In the orbital view of ligand PF3, less back donation of z-electron charge results in a higher P - F frequency. Thus the frequencies for adsorbed PF~ are in accord with expectation based on coordination complexes. As might be expected Cu with a filled d band and A1 with its d orbitals being much higher in energy than the valence shell s and p orbitals do not fit into this series. The orbital view of PF, also indicates a ~r-electron effect that correlates decreasing P - F frequency with increasing M - P bond strength. For the chemisorbed species this leads to an expectation that the Fe-P bond is stronger than the Ni-P bond. Kruck (7) indicates that for coordination complexes the Fe-P bond strength is greater than that of Ni-P, so here again the heterogeneous and homogeneous systems agree. As judged by infrared band intensities PF3 adsorbed most readily on Fe, Ni, and Pd but with greater difficulty on Cu, V, and A1. There is no ready correlation of the 7r-bonding system between these diverse groups so the differences here probably reflect a difference in the basic stability of the ~-bonding framework. Kruck (7) reports that PF8 and CO can replace each other unrestrictedly and reversibly in zero-valent metal complexes. Experiments to test the ability of PF3 and CO to replace each other as chemisorbed species were carried out. As shown in Figs. 1 and 2 and noted in the Results section on iron, vanadium, nickel, and palladium, CO readily replaced PFa, causing a small shift iabout 25 cm-1) to lower frequencies of the PF3 stretching frequencies as PF~ surface coverage decreased. This same dependence of the P - F stretching frequency on the amount of surface coverage was observed on iron by exposing a fresh film to successive small doses of PF3 gas. Exposure to CO at a pressure of 100 Torr was sufficient to completely remove all bands due to chemisorbed PF3. On these metals PF3 also replaced CO but not nearly to the same extent. The intensities of the chemisorbed CO bands decreased by only a few percent upon chemisorption of PFs. Even prolonged exposure of

Journal of Collo{d and Interfac* Science, Vol. 46, No. 3, March.1974

METAL SURFACE INTERACTION WITH ~r-ACCEPTOR MOLECULES

the films to gaseous PF3 had little effect on the CO band intensities, although PF~ bands of moderate size appeared. The CO band positions were not affected by the chemisorption of PF~. No chemisorption of CO on copper or aluminum was observed, either on freshly evaporated metal films or on films containing preadsorbed PFa. Exposure of the latter films to CO caused no changes in the infrared spectra. Several of the results reported here have a bearing on the possible mechanisms of the displacement and adsorption reactions. All of the adsorbed species whose spectra are reported here are stable to evacuation of the cell to 10-6 Torr for several days. Therefore the displacement reactions cannot be occurring by a dissociative mechanism as has been found to be operative in some carbonyl displacement reactions (26). The fact that the P - F stretching frequency is a function of coverage regardless of whether coadsorbed CO is present or not indicates that the P - F frequency is a function of the site and not changing electronic factors caused by a changing amount of adsorption. Using the previously mentioned correlation of P - F frequency and M - P bond strength, the CO is now seen to displace the more weakly held PFa first and the PFa adsorption on a fresh surface occurs first on the most tightly binding sites. While quite reasonable this type of behavior is b y no means a priori necessary. In summary PF3 has been found to adsorb without dissociation, a close correlation of homogeneous and heterogeneous behavior has been found, and the simple molecular orbital view with z donation and ~r back bonding has been found in the case of PF3 as well as CO to be a useful way to understand the experimental data. ACKNOWLED GMENT This investigation was supported in part by Research Grant No. AP 00818 from the Air Pollution Control Office, Environmental Protection Agency.

387

REFERENCES 1. SHEETS,R., ANO BLYHOLDER,G., J. Amer. Chem. Soc. 94, 1434 (1972). 2. LITTLE, L. H., "Infrared Spectra of Adsorbed Species." Academic Press, New York, 1966. 3. HAIR, M. L., "infrared Spectroscopy in Surface Chemistry." Dekker, New York, 1967. 4. BLYHOLDER,G., AND ALLEN, M. C., Y. Phys. Chem. 69, 3998 (1965). 5. BLMHOLDER, G., J. Phys. Chem. 68, 2772 (1964). 6. BLYHOLDER,G., ANDALLEN,M. C., J. Amer. Chem. Soc. 91, 3158 (1969). 7. KRIJCt¢, T., Angew. Chem., Int. Ed. Engl. 6, 53 (1967). 8. WlLIClNSON,G., Nature (London) 168, 514 (1951). 9. STORH~EIER, W., AND MUELLER, F., Chem. Bet. 100, 2812 (1967). 10. TRAPNELL, B., "Chemisorption," p. 173. Butterworths, London, 1955. 11. GIJERRA, C. R., J. Colloid Interface Sci. 29, 229 (1969). 12. BLYI-IOLDER, G., J. Chem. Phys. 36, 2036 (1962). 13. BLYHOLDER, G., AND SHEETS, P~., J. Phys. Chem. 74, 4335 (1970). 14. HARROD, J. F., ROBERTS, R. W., AND RlSSMAN, E. F., Y. Phys. Chem. 71, 343 (1967). 15. BAKER, F. S., ]BRADSHAW,A. M., PRITCHARD, J., AND SYKES, K. W., Surface Sci. 12, 426 (1968). 16. BRADSAI-IW,A. M., AND PRITCtIARD,J., Surface Sci. 17, 372 (1969). 17. BRADSHAW,A. M., AND PRITCHARD,J., Proc. Roy. Soc., Ser. A 316, 169 (9970). 18. BRADSHAW,A. M., ANDVIERLE, O., Bet. Bunse~ges. Phys. Chem. 74, 630 (1970). 19. BLY~IOLDER, G., AND TANAKA, M., J. Colloid Interface Sci. 37, 753 (1971). 20. BL¥tIOLDER~ G., AND WYATT, W. V., J. Phys. Chem. 70, 1745 (1966). 21. SEEL, F., BALLREICH, K., AND SCHMTJTZLEE, R., Chem. Bet. 94, 1173 (1961). 22. KRUCI~, T., AND BAVR, K., Chem. Bet. 364, 192 (1969). 23. KRUCK, T., AND PRASCH, A. Z. Anorg. Allg. Chem. 356, 118 (1968). 24. MILLIGAN, D., .[ACOX, M , AND McKINLEY, J., J. Chem. Phys. 42, 902 (1965). 25. SHEETS, R., AND BLY/clOLDER,G., J. Phys. Chem. 76, 970 (1972). 26. BASOLO, F., AND PEARSON~ R. G., "Mechanisms of Inorganic Reactions," 2nd Ed. Wiley, New York, 1967.

doarnal of Colloid and In~erfaceScienae, Vol. 46, No. 3, March 1974