Adsorption of H20 and NH3 on Dehydrated Silica dering information is available on any current masthead page. References and Notes (1) .G. S. Manning in "Polyelectrolytes", E. Seiegny, Ed., Reidel Publishing,
Dordrecht, Holland, 1974, p 9. (2) D. Dolar and D. Kozak in "Abstracts of the IUPAC Symposium on Macromolecules, Leiden", Voi. 1. inter Scientias, The Hague, 1970, p 363. (3) D. Dolar in ref 1, p 97. (4) J. Skerjanc, J. Phys. Chem., 79, 2185 (1975). (5) D. Dolar and J. Skerjanc, J. Chem. Phys., 61, 4106 (1974). (6) J. Skerjanc and D. Dolar. J. Chem. Phys., 63, 515 (1975). (7) J. C. T. Kwak, J. Phys. Chem., 77, 2790 (1973). (8) J. C.T. Kwak, M. C. O'Brien, and D.A. MacLean, J. Phys. Chem., 79, 2381 ('1975). (9) F. Oosawa, "Polyelectrolytes", Marcel Dekker, New York, N.Y., 1971. (IO) M. Rinaudo and M. Milas. Eur. Palym. J., 8, 737 (1972). (11) A. Minakata, N. Imai, and F. Oosawa, Biopolymers, 11, 347 (1972). (12) M. Mandei and F. Van der Touw in ref 1, p 285. (13) G. Muller, F. Van der Touw, S.Zwolle, and M. Mandel, Biophys. Chem., 2, 242 (1974). (14) J. J. Van der Klink, D.Y . H. Prins, S.Pwoile, F. Van der Touw, and J. C. Leyte, Chem. Phys. Lett., 32, 287 (1975).
2761 (15) G. S.Manning, Biopolymers, 11, 951 (1972). (16) N. DeMarky and G. S. Manning, Biopolymers. 14, 1407 (1975). (17) G. S. Manning, J. Chem. Phys., 51, 924 (1969). (18) J. D. Wells, Proc. R. SOC. London, Ser. B, 183, 399 (1973). (19) J. D. Wells, Biopolymers, 12, 223 (1973). (20) M. Rinaudo and M. Milas, Chem. Phys. Lett., 41, 456 (1976). (21) K. lwasaand J. C. T. Kwak, J. Phys. Chem., 80, 215 (1976). (22) R. A. Robinson and R . H. Stokes, "Electrolyte Solutions", Butterworths, London, 1959. (23) 2. Aiexandrowicz, J. Polym. Sci., 56, 115 (1962). (24) R. G. Bates and M. Alfenaar, Natl. Bur. Stand. U.S., Spec. Publ., No. 314, 207 (1969). (25) J. N. Butler, Natl. Bur. Stand. U.S., Spec. Publ., No. 314, 164 (1969). (26) R. Huston and J. N. Butler, Anal. Chem., 41, 200 (1969). (27) J. N. Butler and R. Huston, J. Phys. Chem., 71, 4479 (1967), (28) R. D. Lanier, J. Phys. Chem., 69, 3992 (1965). (29) R. A. Robinson and V. E. Bower, J. Res. Natl. Bur. Stand. U.S., Sect. A, 70, 313 (1966). (30) See paragraphat end of text regarding supplementary material. (31) T. Ueda and Y. Kobatake, J. Phys. Chem., 77, 2995 (1973). (32) Y. M, Joshi and J. C. T. Kwak, to be submitted for publication. (33) D. Dolar and K. Juznic. Presented at Int. Conf. Calor. Thermodyn., Ist, Warsaw, Aug 1969 (as quoted in ref 1). (34) H. Krakauer, Biopolymers, 11, 781 (1972). (35) K. Iwasa, J. Chem. Phys., 64,3679 (1976). (36) K. Iwasa, unpublished work.
Infrared Studies of Reactions on Oxide Surfaces. 7. Mechanism of the Adsorption of Water and Ammonia on DehydroxylatedSilica B. A. Morrow," 1. A. Cody, and Lydia S. M. Lee Department of Chemistry, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada (Received April 30, 1976) Publication costs assisted by the National Research Council of Canada
The surface of silica is modified in several ways when it has been dehydroxylated by heating under vacuum at temperatures up to 1200 "C. A highly reactive site is generated which is capable of facilitating the dissociative chemisorption of H2O and "3. This site is assumed to be an unsymmetrical siloxane bridge containing an electron deficient silicon atom which can act as a Lewis acid center. Although this site reacts instantaneously with,water to yield two SiOH groups which do not hydrogen bond [v(OH) = 3741 cm-l], the reaction is slow with NH3 because of the competition between the tendency to coordinate or to dissociate yielding SiOH and SiNH2 (type I). Symmetrical less reactive siloxane sites are also generated which only react with H20 to give isolated pairs of hydrogen bonded SiOH groups with infrared bands at 3720 and 3520 cm-l. Finally, the residual isolated hydroxyl groups [v(OH) = 3748 cm-l] are modified so that at 20 "C they exchange with H2180 or they react to a limited extent with NH3 to give SiNH2 (type 11) and H2O. Neither process occurs without the prior dehydroxylation. The new hydroxyl species which are generated by the chemisorption of water in both types of siloxane bridge hydrogen bond with excess water and the present results are discussed in terms of previous models for the rehydration of hydrophobic silica.
Thin silica disks (10 mg cm-2) are opaque to infrared radiation between about 1350 and 750 cm-l except for a window of partial transparency between 1000 and 850 cm-l. We have recently shown in parts V and V11s2 (hereafter referred to as V and VI) that two strong bands a t 908 and 888 cm-I and a shoulder near 940 cm-1 appeared if silica was degassed at greater than 400 "C and that the intensity of these bands increased as the degassing teniperature increased, reaching a maximum near 1200 "C when about 90% of the isolated surface silanol groups [u(OH) = 3748 cm-l] had been removed (see Figure 1A). When a dehydroxylated silica was allowed to react with
HZO, all three bands disappeared and a new SiOH group was created [v(OH) = 3741 cm-l]. The bands also disappeared when an excess of NHs was added and SiOH and SiNH2 were formed.2 However, when pyridine or trimethylamine was added1 only the 888- and 940-cm-l bands disappeared and the spectra indicated that these molecules reversibly coordinated on a Lewis acid site. The total site was assumed to be an asymmetric siloxane bridge containing an electron deficient silicon atom. Part VI was mainly concerned with questions of reaction stoichiometry, site concentration, and conditions necessary for formation and regeneration of the active site. The present The Journal of Physical Chemistry, Vol. 80, No. 25, 1976
2762
B. A. Morrow, I. A. Cody, and L. S. M. Lee
paper deals with the question of the reaction mechanism and its relevance to the problem of the rehydration of hydrophobic ~ilica.~-7
0.
Experimental Section The experimental details have been described previous1y.l The silica used was Cab-0-Si1 HS-5 and had a BET (N2) surface area of 320 m2 g-l. The reaction cell had a volume of about 300 ml so that a 1Torr dose of reactant corresponds to about 16 ymol.
Results and Discussion (a) Reaction with "3. The infrared spectrum of a silica sample which had been heated at 1200 "C under vacuum so as to create the active sites responsible for the bands a t 908 and 888 cm-l and a shoulder at 940 cm-l is shown in Figure 1A. All bands disappeared in unison (see VI)2if the silica was titrated with micromole doses of HzO or B F S .However, ~ with NH3 the first small dose (about 2 ymol) caused both of the 908- and 888-cm-1 bands to diminish in unison (Figure 1B) but with the second increment (Figure 1C) the 888-cm-l band diminished substantially, a new relatively sharp band appeared at 932 cm-l and the 908-cm-l band appeared as a shoulder near 913 cm-l. After the spectra were rescanned 5 and 10 min later (Figure lD,E) and 932-cm-l band had broadened considerably and the transmission near 900 cm-l had decreased. No further changes occurred in the next 3 h or following the addition of another 10 pmol dose of "3. If 200 Torr of NH3 were added (Figure 1F) the weak bands at 913/888 cm-l immediately disappeared and the intensity a t 932 cm-l increased. Accompanying these changes was the progressive growth of bands at 1550,3447, and 3525 cm-l due to SiNH2 and a t 3741 cm-l due to SiOH as described in VI (see below for further spectra). The 932-cm-l band is due to the Si-N stretching mode of SiNHz9 and the unusual changes in shape and intensity noted above always occurred during a titration sequence with "3. Indeed, this band was so broad at saturation with NH3 that the transmission at 900 cm-l decreased as SiNHz was formed whereas it always increased when HzO or CH3OH were used. Thus it was difficult to see how the intensity of the 913-cm-' band changed during reaction. The v(SiN) mode of SiND2 is near 882 cm-l and it was possible to observe how the 908-913-cm-l band behaved during a similar sequence. The spectra on the right in Figure 1 show that after the first dose there was again an even decrease in the intensity of both bands but thereafter the 888cm-l band decreased more rapidly until after 1h, most of the sites had been consumed and the 908-cm-l band had shifted to 913 cm-l. When the Lewis bases pyridine or trimethylamine coordinated with an electron deficient site on dehydroxylated silica1 only the 888-cm-l band disappeared and the 908-cm-l band shifted to 913 cm-l. With NH3 it would appear that both chemisorption and coordination occur simultaneously. This is born out upon examination of the NH stretching region. When >20 Torr of NH3 was added in one dose a spectrum such as that shown in Figure 1F was immediately observed and a t higher frequencies there was a strong band at 3447 cm-1 and a weak band at 3525 cm-l which have been assigned2 to the symmetric and antisymmetric v(NH) modes of SiNH2, respectively. However, if a small dose,
