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I. G. Plotzker and G. J. Exarhos
Conclusions Samples of CHFC12,CHFBr2,and CHFIz subjected to argon discharge radiation during condensation with excess argon at 15 K revealed infrared absorptions which were separated into groups by filtered high-pressure mercury arc photolysis. The new absorptions have been assigned to CFXz free radicals, CFX2+ cations, parent cations, (CHFX+)X daughter cations, and two different intramolecular hydogen-bonded parent anions for each precursor. The hydrogen-stretching vibrations for the two anions produced upon electron capture by CHFClz exhibited no carbon-13 shift, which demonstrates proton abstraction from carbon and suggests the F-H--(CC12)- and Cl-H--(CFCl)- arrangements for these two anions. The effect of halogen substitution in both halide and carbon-bonded positions is demonstrated by the observation of similar type I11 hydrogen-bonded species for F-H-(CBr2)-and F-H--(CIz)- with increasing hydrogen bonding strength and by the different type I hydrogen bonded species for Br---HCFBr and I---HCFI with decreasing hydrogen bonding strength.
(6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24)
Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, and the National Science Foundation, Grant CHE 76-11640, for financial support of this research. We also thank Mi-. J. Houston Miller for performing the optical absorption experiments.
(25) (26) (27) (28) (29) (30)
References and Notes (1) Ault, B. S.; Steinback, E.; Pimentel, G. C., J . Phys. Chem. 1975, 79,615. Jacox, M. E.; Milligan, D. E. Chem. Phys. 1976, 16, 195. Jacox, M. E.; Milligan, D. E. Chem. Phys. 1976, 76,381. Andrews, L.; Prochaska, F. T. J. Am. Chem. SOC.1979, 101, 1190. Andrews, L.; Wight, C. A,; Prochaska, F. T.; McDonald, S. A.; Auk,
(31)
(2) (3) (4) (5)
(32) (33) (34)
B. S. J. Mol. Spectrosc. 1978, 73,120. Prochaska, F. T.; Andrews, L. J . Chem. Phys. 1978, 68,5568. Prochaska, F. T.; Andrews, L. J . Phys. Chem. 1978, 82, 1731. Andrews, L.; Prochaska, F. T. J . Phys. Chem. 1979, 83,368. Wight, C. A.; Auk, 8.S.; Andrews, L. J . Chem. Phys. 1976, 65, 1244. Prochaska, F. T.; Andrews, L. J . Chem. Phys. 1977, 67, 1091. Andrews, L.; Willner, H.; Prochaska, F. T. J . Fluorine Chem. 1979, 73, 273. Milliaan. D. E.: Jacox. M. E. J . Mol. Soectrosc. 1973. 46. 460. Milligan; D. E.; Jacox, M. E.; McAuley, j. H.; Smith, C. 'E. J : Mol. Spectrosc. 1973, 45,377. Smith, C. E.; Milligan, D. E.; Jacox, M. E. J. Chem. Phys. 1971, 54, 2780. Andrews, L.; Carver, T. G. J. Chem. Phys. 1969, 49,896. Prochaska, F. T.; Andrews, L., to be submitted for publication. Prochaska, F. T.; Keelan, B. W.; Andrews, L. J . Mol. Spectrosc. 1979, 78, 142. Milligan, D. E.; Jacox, M. E. J . Chem. Phys. 1971, 55, 2550. Mason, M. G.; VonHalle; W. G.; Robinson, D. W. J . Chem. Phys. 1971, 54,3491. Beauchamp, J. L. In "Interactions between Ions and Molecules", Ausloos, P., Ed., Plenum: New York, 1975. Pimentel, G. C.; McCleilan, A. L. "The Hydrogen Bond"; W. H. Freeman: San Francisco, 1960. Andrews, L. J. Chem. Phys. 1988, 48, 979. Andrews, L.; Prochaska, F. T. J . Phys. Chem. 1979, 83, 824. Barnes, A. J.; Hallam, H. E.; Scrimshaw, G. F. Trans. Faraday SOC. 1969, 65,3150. Levi, B. A,; Taft, R. W.; Hehne, W. J. J . Am. Chem. SOC. 1977, 99,8454. Vogt, J.; Beauchamp, J. L. J . Am. Chem. SOC.1975, 97,6682. Jacox, M. E.; Milligan, D. E. J . Chem. Phys. 1989, 50, 3252. Smith, D. W.; Andrews, L. J . Phys. Chem. 1972, 76,2718. Doucet, J.; Sauvageau, P.; Sandorfy, C. J. Chem. Phys. 1973, 58, 3708. Hobrock, D. L.; Kiser, R. W. J . Phys. Chem. 1964, 68, 575. The 12.4-eV ionization potential for CHFCI, reported here is probably too high based on the PES in ref 29 and comparison with CHCI, and CHF,CI. Andrews, L.; Prochaska, F. T.; Auk, 6.S. J. Am. Chem. SOC.1979, 101, 9. Andrews, L.; Tevault, D. E.; Smardzewski, R. R. Appl. Spectrosc. 1978, 32, 157. Gedanken, A.; Raz, B.; Jortner, J. J . Chem. Phys. 1973, 58,1178. Andrews, L.; Keelan, B. W. J. Am. Chem. SOC.1979, 701, 3500.
Spectroscopic Studies of Ammonia Reduction of Amorphous AgP03 Irene G. Plotzker and Gregory J. Exarhos* Department of Chemistry, Harvard University, Cambridge, Massachusetts 02 138 (Received April 16, 1979) Publication costs assisted by the National Science Foundation
Silver metaphosphate glasses undergo surface oxidation-reduction reactions with a variety of gaseous reducing agents (H2,Hg, K) at moderate temperatures. Anhydrous ammonia has been found to be an effective reducing agent as well, leading to formation of atomic silver in the glass, followed by atom clustering,and eventual surface metal film growth. Vibrational spectra were measured during surface reduction of amorphous AgP03 under a variety of conditions. Spectroscopic results are used to formulate a model for the gas-solid reduction phenomenon.
Introduction A variety of gaseous reducing agents have been shown to be effective in reducing ionic silver in certain metal oxide glasses of the metaphosphate composition.l Vibrational investigations of such glasses reduced by hydrogen under relatively mild conditions ( T < 200 "C) reveal incorporation of H+ into the glass structure, metal cation reduction, and atom clustering followed by surface metal film growth without alteration of the basic metaphosphate backbone structure. One of the more intriguing reducing agents examined is anhydrous NH3 which reduces Ag+ in 0022-3654/79/2083-2496$01 .OO/O
amorphous Ag20.P205with surprising ease under the mild conditions of 60 "C and a partial pressure of ammonia initially equal to 85 torr. The facility of this reaction and curiosity as to the products formed prompted this spectroscopic investigation. Experimental Section Sodium-free silver metaphosphate glass films of ca. 5 pm thickness were blown from a slowly cooling melt of the stoichiometric crystal (AgPO,). The crystalline isomorph from which the glass was prepared was grown from a melt 0 1979 American
Chemical Society
The Journal of Physical Chemistty, Vol. 83, No. 19, 1979 2497
Ammonia Reduction of Amorphous AgP03
1
I
4000 4000
3000
2000
1500
1000
500
Figure 1. Infrared transmission spectra of a thin film of amorphous AgP03 taken at several intervals during reduction with NH3.
of Ag,O and H3P04 at -400 0C.2 The films were immediately mounted in an evacuable stainless steel reaction vessel, 5 cm on an edge, which was carefully cleaned and baked prior to each use. The reaction vessel was fitted with KBr windows for mid-IR transmission studies and polyethylene windows for far-IR measurements. For mid-IR transmission studies, the sample was reduced ( P N H 3 = 84 torr) at 60 "C for 57 h. Infrared spectra were measured (4000-450 cm-l) during the course of the reaction on a Cary-White Model 90 IR spectrophotometer. Spectra were also taken of the KBr windows before and after each run to ascertain that the changes seen were due to structural changes in the glass sample and not to defect formation in the KBr window material. To prove that anhydrous NH3 does not degrade the glass metaphosphate network, we subjected a thin glass film of NaP03 to an= 81 torr) at 60 "C for 48 h, during hydrous NH, (PNH8 which time several spectra were obtained. Far-infrared spectra of a sodium-free thin glass film before and after reaction were measured with a Nicolet 7199 Fourier transform IR spectrometer with a 6.25-pm beam splitter. This sample was exposed to anhydrous NH3 at a pressure of 85 torr at 60 "C for 14.5 h. Spectra were taken of the sample outside the reactor at room temperature. Spectra were also obtained of a thin glass film of NaP0, before and after it had been subjected to the same conditions for 15.3 h. The Raman spectrum of the reduced sodium-free sample was excited with the 5145-A line of a Spectra Physics 164 Ar+ laser in a back-scattering geometry (15" off normal incidence) and measured on a Cary 82 Raman spectrometer. Low laser power (ca. 75 mW at the sample) was used to prevent sample photolysis. The polycrystalline compound NH4P03was prepared by heating an equimolar mixture of urea [(NH,),CO] and NH4H2PO4at 280 "C for 16 h under an ammonia atmo~ p h e r e .Raman ~ spectra of the reactants and product were obtained to check that the reaction had proceeded and that the product was not mixed with unreacted urea or NH4H2P04. The mid-IR transmission spectrum of the product dispersed in KBr was measured on a Perkin-Elmer 457 grating IR spectrophotometer. Far-IR spectra were obtained of NH4P03mulled in dried petroleum jelly or dispersed in low density polyethylene; no significant differences were observed between the samples. Results Mid-IR Transmission Studies. Several spectra taken during the reaction of AgP03 with NH3 are presented in Figure 1;in Figure 2, the spectrum of an untreated AgPO,
1000
1500
2000
500
Figure 2. Infrared transmission spectra of a thin film of amorphous AgP03 before (a) and after (b) reduction with NHB.
%J
IH
f
P ,"O,
I
I
I
I
4000
3000
2000
1500
,
1000
UJhp-') 500
Figure 3. Transmission infrared spectrum of amorphous ",PO3.
sample is compared with that of the reduced film after residual NH3 had been removed. The sample itself was metallic in appearance at the end of the run and had become somewhat cloudy; the glass had apparently begun to devitrify. The sharpening of certain bands in the spectrum, Le., 710 cm-l, also indicates the presence of polycrystallites. Other dramatic spectral changes occur, such as the appearance of broad, overlapping bands in the N-H, 0-H stretching region (2400-3400 cm-l), the growth of a strong absorption at 1440 cm-', concurrent diminution and shift to lower wavenumber of the band near 1230 cm-l, and the growth of a strong, broad band at 1090 cm-'. These changes occurred very early in the reaction, as can be seen in Figure la. After the first several hours, the spectrum changed more gradually; for example, the 1230-cm-l band did not fully disappear until the end of the run. Other changes may be noted. Weak bands appear at 2200 and 1700 cm-l. Bands have appeared at 955,910, and 525 cm-l, while those at -875 and 770 cm-l are no longer visible. The only change exhibited by the spectrum of NaP03 after exposure to NH, was the appearance of broad, weak features in the 2800-3200-crn-' region, probably caused by NH3 adsorption. The mid-IR spectrum of the sodium-free sample examined in the far-IR and Raman studies below indicated that the reaction had not progressed as rapidly as in the above experiment; the bands at 1230 and 1440 cm-' were of equal intensity after 14.5 h of reaction. Also, the sample was brownish in color, rather than coated with a silver film. This sample had significantly more surface water present than did the conventionally prepared sample and, therefore, much of the adsorbed NH3 reacted with hydroxyl groups rather than reducing the silver ions. The transmission spectrum of NH4P03is presented in Figure 3. The broad, strong feature in the 2800--3500-~m-~ region contains overlapping NH and OH bands, the KBr
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The Journal of Physical Chemistry, Vol. 83, No. 19, 1979
I. G. Plotzker and G.J. Exarhos
r
~
RAMAN
Figure 4. Far-infrared absorption spectra of a thin film of amorphous AgP0, before (a) and after (b) reduction with NH,.
Flgure 6. Raman spectrum of polycrystalline NH4P0,.
The Raman spectrum of polycrystalline NH4P03was obtained by using a 90" scattering geometry with about 200 mW of 5145-A excitation. A band at 3070 cm-l is distinguishable above the fluorescence. The strongest features are observed at -1140,685, and 645 cm-l, with other bands present at 1285,1245,390,335,and 280 cm-I and possibly at 500, 445, and 210 cm-l (Figure 6).
-
Flgure 5. Far-infrared absorption spectrum of powdered amorphous NH4P03dispersed in polyethylene.
-
windows almost certainly having picked up some water. Absorption bands appear at 2120,1870 (shallow), 1685, 1430,1290 (shoulder), 1250,1140 (shoulder), 1080,1010, 875,757,760 (shoulder), 555,458,40, and 400 (shoulder) cm-l. Far-IR Absorption Spectra. This sample, as noted above, had not been reduced as fully as those examined in mid-IR experiments. Absorption spectra obtained before and after reaction are presented in Figure 4. The prereduced sample reveals a broad weak feature at 110 cm-l which has been assigned to the localized cation vibration of Agt in its site in the g1a~s.l~ Upon reduction, the intensity of this band is greatly diminished and a strong broad feature centered at -218 cm-' has appeared. The bending mode near 500 cm-' has shifted higher in energy, as was noted in a mid-IR spectrum measured. No changes whatsoever were observed in the NaP03spectrum after exposure to NH3. A far-IR spectrum of NH4P03dispersed in low density polyethylene is presented in Figure 5. A broad feature split into four peaks appears in the region 80-300 cm-l. A t higher frequency, the characteristic phosphate bending modes are exhibited. Raman Spectroscopic Results. The Raman spectrum of the sodium-free AgP03 thin glass film after 14.5 h of NH3 treatment has been obtained. Very high spectrometer sensitivity (150 counts/s full scale) and zero suppression were necessary to measure the weakly scattered light. Peaks are distinguishable at -1120,990,700,565,545,510, and 450 cm-l. Other features are questionable because of the noise level. Possible higher frequency bands are masked by noise and fluorescence.
Discussion Ammonium Metaphosphate. The vibrational spectrum of polycrystalline NH4POBis consistent with the metaphosphate composition. The out-of-chain vB(POz)band appears at -1140 cm-', with va,(P02)at 1245 and 1285 cm-' and v,(POP) at 685 and 645 cm-l. The band at 3070 cm-l is a N-H stretching mode of the ammonium ion. Other modes of NH4' should be exhibited in the Raman spectrum near 1680 and 1400 cm-l and are presumably masked by fluore~cence.~ The low-frequency bands are lattice modes including both phosphate and NH4+bends and torsional motions. The mid-IR transmission spectrum is consistent with these assignments and is similar to those seen in other metaphosphates, with vas(P02)at 1290 and 1250 cm-' and us(POz)at 1140 cm-l. Besides the broad band in the OH, N-H stretching region (2800-3500 cm-'), NH4+ vibrations are manifest in the features at 1685 and 1430 cm-'. The bands below 600 cm-' are most likely bending modes of the phosphate chain. The region between 1100 and 600 cm-l poses some difficulty because this is where N-H rocking and wagging motions and phosphate vibrations are expected. However, if this spectrum is compared with that of untreated AgPO, in Figure 2, a one-to-one match is observed between the bands at 1080,1010, 875, and 760 cm-' in NH4POBand those at 1065,1010,885,and 770 cm-' in AgPO,. These are then the in-chain symmetric and asymmetric P-0-P and 0-P-0 modes of the metaphosphate chain. The one remaining band is at 795 cm-' and may be attributed to a NH3 rocking motiona4 The low-frequency bands centered near 200 cm-I can be regarded as cation motion bands; the splitting resembles that exhibited by the cation motion band in NaP03 upon de~itrification.~ Here, the interaction is not primarily ionic as it is in alkali metaphosphates but involves substantial hydrogen bonding. The cation motion band for NH4+ would be expected to be influenced by significant hydrogen
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Ammonia Reduction of Amorphous AgPO,
The Journal of Physical Chemistty, Vol. 83, No. 19, 1979 2499
P-+!
t Ago
Flgure 7. Model proposed for the surface reduction reaction of Ag+
by
"3.
bonding between the ammonium moiety and the oxygen anionic site. The &PO, + NH, Reaction. Other nitrogen-containing species besides NH4+are produced in this reaction. This is indicated by comparing the spectra taken during and after the reaction, and comparing with the NH4P03results discussed above. It is also noted that the reaction proceeds under mild conditions; any proposed mechanism must take this into account. Such a mechanism will now be considered. In the AgP0, H2 reaction studied previously,l the H2 molecules are assumed to adsorb dissociatively at the oxide glass surface. Electron transfer occurs from the resultant H atoms to Ag+ ions, possibly with formation of an AgH complex. No evidence of such a complex has been seen in these studies, but such species have been postulated in the reduction of C U +and ~ Ag+7 by hydrogen. The hydrogen ions produced form bonds with network oxygen ions to yield hydroxyl groups, while the neutral silver atoms form aggregates at the glass surface. The reduction by ammonia can proceed in a similar manner. The electron-rich nitrogen will preferentially adsorb at the positive surface sites provided by the silver ions. If a substantial amount of water is present, NH4+ groups would be formed instead; this is apparently what happened in the case of the hygroscopic sodium free glass film,in which reduction proceeded more slowly than in the initial experiments. In a reasonably dry film, an Ag+-NH3 complex would be formed. Electron transfer from the nitrogen lone pair would occur and the now neutral silver atom would migrate from its site, leaving the positive ammonia ion to bond to the phosphate chain as illustrated in Figure 7. One hydrogen forms a hydroxyl group with an oxide ion and the N-0 bond is formed. The two free hydrogens would be expected to interact somewhat with oxide ions on a neighboring phosphate chain. The proposed abstraction of a hydrogen atom by the oxide ion is consistent with reported observations of ammonia adsorption as NH, + H on oxide glass surfaces.8 The proposed mechanism is also consistent with the changes exhibited by the infrared spectrum during the course of the reaction. As the partial double bond character of increasing numbers of out-of-chain >PO, groups diminishes, the v,,(P02) band at -1230 cm-l decreases in intensity and moves to lower energy. The band that appears concurrently a t 1090 cm-l can be assigned to an asymmetric stretch of the now singly bonded PO2 group in the product. Other bands may be assigned by analogy with the IR spectrum of a film of hydroxylamine, NH20H.9 The bands at 3200 and 3010 cm-l can then be assigned to NH stretches similar to the 4") modes of 3302, 3245, and 3173 cm-' in ",OH, while the 2850-~m-~ band would be v(OH) in analogy with the 2867-cm-l mode and also of course with the bands observed here in AgPO, and Hg(POJ2 after reduction by hydrogen. The G(HNH) mode at 1515 cm-l in hydroxylamine appears at 1440 cm-l in this sample, still an acceptable frequency for this mode. The band a t 910 cm-l corresponds to v(N0) observed at 912 cm-l in NHZOH, while the new feature at 955 cm-' is
+
analogous to the p,(NH2) mode a t 950 cm-l in hydroxylamine. The 525-cm-l absorption could be either a phosphate bending mode or a torsional mode of NH2, similar to pt(NH2)a t 535 cm-l in NH20H. No band is observed near 1600 cm-l, the region appropriate for NH3 deformation modes, or between 1200 and 1300 cm-l in sufficiently reduced samples, where another NH, bending mode typically occurs in amine complexes. Such bands have been observed in infrared spectra of crystalline [Ag(NH3)2]2S04, Ag(NH3)2N03,and Ag(NH3)2C104.10J1 This is consistent with the proposed abstraction of hydrogen by an oxide ion. Phosphate vibrations a t 1070, 1010, and 710 cm-I are still present after the reaction. The increase in intensity and sharpening of the last of these is characteristic of AgP03 devitrification. The weak band near 2200 cm-l may be an overtone of the band near 1100 cm-l, while that near 1700 cm-l may be a combination of the 1400- and 218-cm-l modes; a combination band of ~ ~ ( 1 4 0cm-l) 0 and a lattice mode appears a t about 1750 cm-l in the IR spectrum of NH4C1.4 The Raman spectrum of the partially reduced sample is consistent with this model, though assignments must be more tentative since it is probable that additional bands are present but cannot be distinguished because of noise and fluorescence. The v,(P02)band has already decreased over 20 cm-' in frequency to 1120 cm-l. The band at 990 cm-' may be assigned to a POH bend12or an NH2 rocking mode and those at 565 and 545 cm-l may be NH2 torsional modes. The far-IR band at 218 cm-l cannot be assigned unambiguously. It may involve an Ag-N interaction, a bending motion similar to the N-M-N modes observed near this frequency in Ag(NH&+ salts;1° the Ag-N stretching frequency in these compounds is in the 400-500-crn-' regionlOJ1and so in our case would be difficult to distinguish from a phosphate bending mode. Alternatively, the band could be due to a us mode of hydrogen bonds formed with the oxide ions, similar to those in NH4P03and in AgPO, after reduction with H2. Substitution of ND, for NH, in the reaction would prove conclusive, as vu would be much more strongly shifted than Ag-N. Two further arguments should be examined. Is NH3the only reducing agent present? A simple thermodynamic calculation for the equilibrium 2NH3 = N2 3H2indicates that ammonia at 85 torr will eventually decompose to yield on the order of 23 torr of H2 at the temperature used in these experiments. The rate of decomposition can depend on the type of adsorption sites available; McAllister and Hansen13found that the rate of NH3 decomposition over a tungsten single crystal could be described by a bP"32I3, where a and b depended on the specific crystal face exposed. Addition of ammonia to aqueous solutions of silver ion does not produce silver metal but does increase the ease with which silver ions are reduced; this knowledge has been exploited in silvering mirrors for over a century.14 Hydrogen, then, could be the actual reducing agent, producing Ago and NH4+. This reaction, however, can be expected to proceed only to a small extent. First, while reduction of metal ions in oxides and zeolites by hydrogen at high pressures (or flow is essentially independent of PH2 rates) and temperatures, it has been shown to be first order in PH2a t low pressures and temperatures.15J6 Thus, reduction by hydrogen at such low pressure and temperature is expected to proceed very slowly, too slowly to account for the dramatic changes observed in the mid-IR spectrum in the course of a few hours. Also, the uas(POz)band is very much in evidence at 1250 cm-l in NH4P03,and so substitution of NH4+ for Ag+, an essential process for this proposed mechanism, cannot explain the shift and eventual N
+
+
2500
I. G. Plotzker and G. J. Exarhos
The Journal of Physical Chemistry, Vol. 83, No. 79, 1979
disappearance of the vag(P02) band at 1230 cm-l. Other spectral changes, such as the shift of the vB(POz)band down to 1120 cm-l in the Raman spectrum, are also inconsistent with this mechanism. Its appropriateness could be demonstrated conclusively by using NH3/H, mixtures of known composition to reduce Ag+ in AgP0, and recording infrared spectra during the course of the reaction. If increasing the partial pressure of H, does not alter the nature of the changes exhibited by the spectrum but increases the rate at which they appear, then hydrogen is the actual reducing agent. However, in view of the argument outlined above, it is more likely that little change in the reaction rate would occur a t low Hz pressures. As PH2is increased, spectral changes similar to those previously notedl would begin to appear in addition to those observed when only NH, is added to the reactor. In short, two oxidation-reduction reactions would occur instead of just reduction by NHB. One preliminary experiment was performed to verify that indeed NH3 and not H2 is the actual reducing agent in this reaction. A thin glass film of AgPO, was placed in a stainless steel reaction vessel fitted with KBr windows. The vessel was heated to 60 " C and a gas mixture consisting of 20 torr of NH3 and 60 torr of Hz was admitted. Infrared absorption spectra were obtained at 15-min intervals over the course of 2 days. Results support the contention of the previous paragraph, namely, that NH3 is the reducing agent in this reaction. Spectral changes observed in this trial run are identical with those changes seen when only NH3 was employed as the reducing agent. Furthermore, the reaction rate appears to be somewhat slower than when NH3 at a partial pressure of 85 torr was used as the sole reducing agent. Reduction with H2 alone a t this low temperature proceeds much more slowly than with NH3,1 and, on this time scale, infrared changes assignable to H2 reduction products (strong broad bands at 2325 and 2800 cm-l) were not observed. The second question is then, why does reduction proceed at all? If excess NH, is added to a solution containing Ag+ ions, Ag(NH3)z+is formed, but reduction does not occur unless a reducing agent such as sugar or formaldehyde is added. One possible explanation is that the silver ion at the surface of AgP0, glass finds itself in a highly electron-rich environment when the NH, molecule approaches, surrounded by the negative charge of the terminal oxygens on the phosphate chain and by the nitrogen lone pair electrons, This provides increased opportunity for the Ag+ ion to abstract an electron. The NH, molecule would be
drawn particularly close to the silver ion owing to bond formation between a hydrogen atom of ammonia and the nearby oxide ion. This results in a weakening of the N-H bond and formation of an 0-H bond; the moiety left behind is nearly NHz-, an extremely basic entity whose proximity heightens the probability of electron transfer to the silver ion. With such transfer accomplished, the neutral silver atom is no longer bound to its site and is free to diffuse easily, leaving behind the species illustrated in Figure 7.
Conclusions Anhydrous ammonia effectively reduces silver cations in amorphous AgPO, under relatively mild conditions. Spectroscopic evidence indicates that the reduction process directly involves NH3, and that its equilibrium product, H2, has a minor role in the reduction. The rate of NH3 reduction of Ag+ is significantly greater than with H2alone. Acknowledgment. This research has been supported by the Materials Science Program, Harvard University, NSF Grant No. DMR-76-01111, and by an NSF Graduate Fellowship to the first author. Thanks are given to Dr. Barry N. Nelson for his assistance in obtaining the FT-IR spectra. References and Notes (1) I. E. Greenwald and 0. J. Exarhos, J. Non-Clyst. Solid, 26, 259 (1978). (2) Analogous phosphate crystalline phases may be prepared for a variety of metal oxides by this same technique. (3) C. Y. Chen, N. E. Stahlheber, and D. R. Dyroff, J . Am. Chem. Soc., 91, 62 (1969). (4) K. Nakamoto, "Infrared and Raman Spectra of Inorganic and Coordination Compounds", 3rd ed, Wiley, New York, 1978. (5) Y. Haren and B. Vekerk, Phys. Chem. Glasses, 6, 38 (1965). (6) J. Texter, D. H. Strome, R. G. Herman, and K. Klier, J . fhys. Chem., 81, 333 (1977). (7) A. G. Sykes, "Kinetics of Inorganic Reactions", Pergamon Press, New York, 1966, p 214f. (8) H. Knozinger in "The Hydrogen Bond", P. Schuster, G. Zundel, and C. Sandorfy, Ed., North Holland Publishing Co., New York, 1976, Chapter 27. (9) R. E. Nightingale and E. L. Wagner, J. Chem. Phys., 22, 203 (1954). (IO) A. L. Geddes and G. L. Bottger, Inorg. Chem., 8, 802 (1969). (11) M. G. Miles, J. H. Patterson, C. W. Hobbs, M. J. Hmper, J. Overendard, and R. S. Tobias, Inorg. Chem., 7, 1721 (1968). (12) D. E. C. Corbriige and E. J. Lowe, J . Chem. Soc., 493, 4555 (1954). (13) J. McAilister and R. S. Hansen, J . Chem. fhys., 59, 414 (1973). (14) A. M. Setapen in "Silver in Industry", L. Addicks, Ed., Reinhold, New York, 1940, Chapter 11. (15) H. Beyer, P. A. Jacobs, and J. B. Uytterhoeven, J. Chem. Soc., Faraday Trans. 7 , 72, 674 (1976). (16) M. Pospisil, Collect. Czech. Chem. Commun., 42, 1278 (1977). (17) G. J. Exarhos, P. J. Miller, and W. M. Risen, Jr., J . Chem. fhys., 80, 4145 (1974).
