Structure of Chemisorbed Ammonia on Silica
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(17) J. C. Joubert, G. Berthet, and E. F. Bertaut in "Problems of Nonstoichiometry", A. Rabenau, Ed., North Holland Publishing Co., Amsterdam, 1970, p 179. (18) F. Bertaut, C. R. Acad. Sci., 230, 213 (1950).
(19) P. Cossee, J. horg. Nuci. Chem., 8, 483 (1956). (20) P. Cossee, Reci. Trav. Chim. Pays-Bas, 75, 1089 (1956). (21) F. S. Stone and R. J. D. Tilley, React. Solids, Proc. int. Symp. 7th. 1972, 262 (1972).
Infrared Studies of Reactions on Oxide Surfaces. IV. The Structure of Chemisorbed Ammonia on Silica B. A. Morrow,* 1. A. Cody, and,Lydia S. M. Lee Department of Chemistry, University of Ottawa, Ottawa, Canada, KIN 6N5 (Received May IS, 1975) Publication costs assisted by the National Research Council of Canada
When gaseous ammonia is allowed to react with silica a t 650°C, the same high wave number spectral features (bands a t 3540, 3452, and 1550 cm-l) are produced as when ammonia reacts at 2OoC with a silica which had been previously degassed at 80OoC prior to reaction. With thin samples (4 mg/cm2), an additional band has been observed a t 932 cm-l. Isotopic shift data .have been used in a force constant refinement calculation to show that the chemisorbed species is a surface SiNHz group and that the 932-cm-l band can be assigned to the Si-N stretching mode.
The formation of surface SiNHz groups when ammonia has been allowed to react with highly dehydroxylated silica has been postulated by several workers who have used infrared spectroscopy as a means of detection.1,2 The assignment was based solely on the observation of two bands near 3500 cm-l which were assigned to the symmetric and antisymmetric NH stretching modes, and of a band a t 1550 cm-', which was assigned to the deformation mode. This assignment must be considered tentative since SiONH2 or coordinatively bonded NH3 would give rise to identical spectral features. Low wave number spectral data have been lacking in previous studies because the silica was totally absorbing below about 1300 cm-l. In the earlier work1B2 ammonia was presumed to react with a strained siloxane bridge site as follows NH2 OH Si
+ NH,
-I
si
+ siI
where the reactive bridge site was formed during the dehydroxylation process. In a brief communication3 we reported that a new type of SiOH group (with v(OH) at 3741 cm-l) is produced in this reaction (or after reaction with HzO or CH30H) thus corroborating the above apparent stoichiometry. In the present paper, we have carried out a detailed spectroscopic investigation of all H/D, 15N/14N, and l6O/lSO isotopic variations, and, combined with the observation of the SiN stretching mode, we have been able to prove that the infrared bands observed by others1,2 near 3540, 3450, and 1550 cm-l can be assigned to SiNH2. (Further details of the mechanistic aspects of this work will be published as part IV, but the present spectroscopic proof of the assignment is a necessary precursor to that work.) To the authors belief, this is the first time that low-frequency (
