2400
(16) (17) (18)
(19)
Pierre Bracconi and Louis-Claude Dufour
N. Mori and T. Mitsui, J. Phys. SOC.Jpn., 26, 1087 (1969). M. Hansen and P. Anderko, "Constitution of Binary Alloys", 2nd ed, McGraw-Hill, New York, N.Y., 1958, p 466. L. N. Larikov and 0. A. Shmatko, Fiz. Met. Metalloved., 30 (6), 1173 11970). F. S. Stone and R. J. D. Tilley, React. Solid, Proc. lnt. Symp., 7th. 1972, 262 (1972). H. Jagodzinski, 2.Kristallogr., 109, 368 (1957). K. P. Sinha and A. P. B. Sinha, J. Phys. Chem., 61, 758 (1957); A. W. Laubengayer and H. W. Mc Cune, J. Am. Chem. Soc., 74,2362 (1952). A. Deschanvres and B. Raveau, Rev. Chim. Miner., 5 , 201 (1958).
(20) G. K. Bansal and A. H. Heuer. Phil. Mag., 30, 709 (1974). (21) R. Sun, J. Chem. Phys., 28, 290 (1958). (22) T. E. Bradburn and G. R. Rigby, Trans. Brit. Ceram. SOC., 52, 417 11953). (23) B. Gillot and P. Barret, C.R. Acad. Sci. Paris, Ser. C,278, 57 (1974). (24) A. L. G. Rees, "Chemistry of the Defect Solid State", Methuen, London, 1954. (25) M. H. Tikkanen, Genie Chim., 92 (3), 57 (1964). (26) G. Grube and M. Flad, 2.flektrochem., 42 (7), 377 (1942). (27) H. Davies and W. W. Meltzer, J. flectrochem. SOC., 121 (4), 543 (1974).
Hydrogen Reduction of Cobalt-Chromium Spinel Oxides. II. CoCr204-Co304 Solid Solutions Pierre Bracconi* and Louis-Claude Dufour Laboratoire de Recherches sur la Reactivite des Solides (associe au Centre National de la Recherche Scientifique),Departement B, Faculte des Sciences Mirande, 21000-Dijon, France (Received June 10, 1974; Revised Manuscript Received June 5, 1975) Publication costs assisted by the Universlty of Dijon
The hydrogen reduction of stoichiometric cobalt chromite (Sl)was investigated in the first part of this series. Here, we are concerned with the reduction of two cobalt chromite-cobalt cobaltite solid solutions, Co2+ [ c r ~ - ~ 3 + c o ~ 3 + ] B owith 4 2 - x = 0.06 in specimen S2 and x = 0.45 in specimen S3; S2 contains, in addition, a large amount of vacancies on the octahedral (B) sites and of associated Cr6+ ions. From the results concerning the reduction of S3 (lattice parameter a = 8.281 A) it is obvious that, below 55OoC,the x COB^+ can be reduced to CO$+, but that, simultaneously, a fraction (x/2) of the tetrahedral C O A ~is+ reduced to the metallic state, x water molecules are formed and the lattice parameter increases continuously; a spinel lattice ( a = 8.320 A) with an abnormal cationic distribution is obtained, which can be formally regarded as an intermediate between CoCr204 and COO.At higher temperatures these COB^+ are reduced to the metallic state and the spinel rearranges into C O ~ + [ C ~ ~ ~ +( a] B ~ ~ A) - which finally reduces to fcc-Co and =O 8.330 a-CrzO3. These two steps have not been separated thermodynamically, but, as they proceed consecutively, it is clear that the reactivity of the cobaltous ion is here related to the type of crystallographic site it occupies. The presence of octahedral vacancies in S2 does not modify the absolute rate of reduction into Co and Cr203 as compared with that measured for S1. Such an observation is in agreement with the conclusion that the rate-determining step of these reactions is a diffusion phenomenon of cobaltous ions through the spinel lattice tetrahedral sites.
I. Introduction The authors studying the reduction of ternary metallic oxides (ref 1-3, see also ref 1-3 and 23 in part I, preceding article in this issue) did not consider the possible relationships between the kinetics and mechanism of such reactions and the particular structure of the initial oxides studied. Dobrovinskii and Balakirev4 observed that some of the successive equilibrium states occurring in the reduction of the spinel CoFe1.75Cr0.2504a t 1000°C and decreasing oxygen partial pressure corresponded to different successive compositions of the spinel phase. On the contrary, the reduction of Co304 by HZ5and by CHd6 is known to yield COOas an intermediate phase. The purpose of this paper is to investigate, in comparison with CoCi-204, the reduction kinetics and mechanism of other cobalt-chromium spinel oxides with different compositions, S2 and S3, in which cobaltic ions replace part of the Cr3+ on the octahedral (B) sites. In addition, one of them, S2, contains octahedral vacancies and associated Cr6+ ions. The Journal of Physical Chemistry, Vol. 79, No. 22, 1975
Speculation about the reduction behavior of such compounds gives rise to interesting questions. For instance, if the CoB3+can be reduced to Co2+,these ions might either migrate on tetrahedral (A) sites (their normal location in the spinel structure) or be expelled in a separate phase such as COO. Concerning S2, the reduction kinetics and mechanism can be, a priori, expected to depend upon the vacancies existence.
11. Experimental Section Techniques. This work was carried out with the same experimental techniques as those already described in part I. Materials. Preparation and Characterization. Specimen 2 (S2) was obtained by decomposition of CoCrz07-4C5H5N in air at 120OOC for 24 hre7Similar to specimen S1 (see part I, section 11) it is crystallized in octahedral grains (see Figure 1 (top)) of average edge size a = 1.5 f 0.5 wm. Its composition is Co~.0~Cr1.~104.00 and its structure of spinel type with a parameter equal to 8.305 f 0.003 A. It is obvious,
tiydr-n
2401
Reduction of Cobait-Chromium Spinel Oxides
I
0
0.5
1
1.5 O(gA1)
I
Figure 2. (a) Accumulated variation of ihs relative BET-surfacs area (rep&+ to ihs maximum value S-0 = 20 m2/g) of an S3 sample successweiy annealed at maximum 0 temperatures reached with a linear temperature rise of 125”CIhr. (b. c, d) Variation with a of the relative BET-surfacearea of S3 samples at different temperatures.
Figure 1. SEM micrographs of (top)S2 initial grains, X 20,000 (bottom) S2 reduction products (at 712% far 42 hr. a = 1.16 gatom). X19.500.
from the above formula, that the number of normally occupied sites defined hy the fcc oxygen lattice exceeds the number of available Cr and Co cations, hut, as S1 (CoCrz04) was prepared from S2 by solid state reaction with chromium oxide, the observed nonstoichiometry must then he due to cationic vacancies, and not to impurities, according to Col.osCrl.sloo.l3Or.oo.The density calculated on that basis, d = 5.11 f 0.01 glcm?, agrees well with the experimental value of d = 5.08 g/cm3. Moreaus prepared specimen 3 (S3) through a method originally used by Adkins and Conofl for copper chromite preparation. The solid obtained a t 45OoC is finely divided; its BET-surface area, after outgassing a t 450°C, reaches the maximum value S = 20 m2/g. Comparison with the results of SEM observations and of X-ray diffraction line broadening measurement suggests that these grains are, as a first approximation, nonporous, monocrystalline, and spherical in shape, with an average diameter d , = 500 A. However that value can be modified substantially above 600OC as shown by the surface area decrease of samples annealed under vacuum (Figure 2a). The duration of thermal equilibration and outgassing of the samples prior to kinetic experiments were then shortened to the minimum. Under vacuum, gases desorb from its surface, up to 450%. in an amount representing 0.65 0.10 wt %, and between 800
+
and 9OooC, a very small weight loss is still observed, which may he due to the decomposition of remaining precursor or c0304 traces. Then the spinel phase composition in S3, calculated from the corrected concentrations of the elements, is C01.45Cr1.5504.00. If only Cr3+ ions are assumed in S2, the ionic formula accounting for the lattice electroneutrality involves C O ~ ions + on A sites, hut the following results, which have been collected with the aim of establishing, experimentally, the S2 and S3 ionic structures, show that the nonstoichiometry in S2 is more probably due to the presence of C$+ ions. (a) As shown in Figure 3, the lattice parameter of our specimens decreases proportionally with chromium to cohalt ratio as expected for CoCrzO4-CosO4 solid solutions from the works of Makkonen’O and of Hanck and Laitinen.” The vacancies in S2 may explain why its parameter is slightly smaller than that of the normal solid solution having the same CrICo ratio. But Makkonen’s conclusions on the ionic structures of these compounds may he considered as erroneous; it is clear now that CoaO4 is, as CoCrzO4, a normal spinel, and according to the sites “preference” rules in the spinel s t r u c t ~ r e it ~ ~may - ~ ~be predicted that the Cr3+ will only locate on the B sites, while the Co2+and the Co3+ will respectively prefer the A and the B sites. Concerning the cation vacancies, in the spinels y-Fep03, yAI&, yCr203, and y-Mnz0316 as well as in other nonstoichiometric spinel oxides’? they are distributed (orderly or randomly) on B sites. Hence a similar location can he reasonably assumed for S2. More simply, S3 is likely to have the following structure: Col.~2+~Crl.s53+C~.453+lB~4.00z-
i (b) Experimental evidence supporting these conclusions was initially sought by means of X-ray diffractometry. Unfortunately, the atomic scattering factors of the various cations are nearly equal and the problem is further complicated for S3 by the diffraction line broadening due to the grain size. The presence of vacancies in S2 either on A or B sites was hoped to modify sufficiently the relative intensities of certain diffraction lines compared to the stoichiometric chromite S1 of same grain size. These were calculated with the formula 1+cos228 PJsin2 8 cos e
I = ,
Pierre Bracconi and Louis-Claude Dufour
2402 2
1.5
1.0
cr/cO
0.5
u
0
0.5
1
Figure 3. Experimental lattice parameter vs. CrKo ratios for S1, S2. and S3 (0)compared to the results of Makkonen (0)and of Hanck and Laitinen (A)for Co1+,Cr2-,04 solid solutions.
where the structure factors are given by Bertaut.18 However the differences so calculated and experimentally observed for the reflections (ill), (220), (311), (222), and (400) are very weak, and we will only conclude that octahedral location of the vacancies is more probable. (c) On the other hand, the atomic magnetic moments of C?+, Co3+, and Co2+ have fairly different values. In particular, octahedral Co3+ are almost always in a low spin state, and have a zero moment instead of 5.4 pug in as in c0304,~' the high spin state. The moment of tetrahedral Co2+ in c0304 is ~ ( C O A ~ =+4.75 ) Le., slightly higher than the . Cr3+ have a value we measured in S1: 4.58 f 0.11 p ~ The spin only magnetic moment p(Cr3+) = 3.87 pug. Each specimen Curie constant is the average of four values calculated by least-squares analysis from the susceptibilities measured between room temperature and 400°C under argon pressure and with four different magnetic field values. We calculated that the diamagnetic contribution of the oxygen ions is negligible. The Curie constant is related to the atomic moments by
where ni is the number of gram atoms of i-type cation, and pi its magnetic moment (in Bohr magneton, p ~ ) N , is Avogadro's number, and k the Boltzmann constant. For S3, Cexpt = 0.0247 f 0.0003 "K cm3/g; the same value can be calculated from the above relation with p(cOB3+) = 0 and confirming the expected ionic structure ~ ( C O A ~=+4.75 ) (i). For S2, Cexpt = 0.0277 f 0.0004 "K cm3/g; with ~ ( C O A ~=+4.58 ) and p ( C o ~ ~=+ 0, ) a satisfactory value can be calculated for the tetrahedral Co3+ moment with the assumption of tetrahedral vacancies; p(CoA3+)= 5.45 instead of 4.83 with the assumption of octahedral vacancies. However, the hypothesis of Cr6+ also gives a C value, C = 0.0275 "K cm3/g, consistent with the experimental one. Therefore, the presence in our specimens of octahedral cobaltic ions in a low spin state is clearly demonstrated; but the existence of either tetrahedral Co3+ or Cr6+, in S2, is not definitively established; although the first possibility is the least convenient since it requires the vacancies to be on A sites (in opposition to many examples of nonstoichiometric spinels) in order to fit the above magnetic data. (d) An attempt to detect the species CoA3+in S2 by EPlZ was unsuccessful, but this also cannot be regarded as a definitive conclusion. On one hand, at 77"K, S2 (as Sl) is antiferromagnetic and the method is not valid; on the other hand, at 150°K and above (temperatures at which no antiThe Journal of Physical Chemistry, Vol. 79, No. 22, 1975
ferromagnetic interactions between C O A ~and + CrB3+ ions occur any longer), no characteristic EPR spectra of either C O A ~or+ CoA3+ions could be observed. However, Cr6+ assumption has been finally selected, as no example of a cobaltic compound in the structure of which Co3+ is normally tetrahedrally coordinated with oxygen is known yet. As a conclusion, the ionic structure of S2 will be
111. Results
As expected from part I, fcc-Co and a-CrzO3 are the final reaction products obtained below 750°C. Owing to its high surface area, S3 is very reactive; its reduction is investigated in a temperature range (500-700°C) where the reduction of a-CrzO3 does not interfere. Kinetics. In Figures 4 and 5, a represents the weight loss by reduction, at P H =~ 50 Torr, of S3 and S2 samples as a number of oxygen gram atoms per formula i and ii. The hydrogen pressure has a very weak effect on the reaction rates; for instance, for S3 and in the range 10 Torr < PH%< 60 Torr, they vary within experimental error limits. For S2 (Figure 4), the a ( t ) curves are very similar to that observed for S1.They differ, however, on the two following points. (a) The limit corresponding to complete reduction into fcc-Co and a-Cr2O3 ( a = 1.281) is hardly attainable below 750°C. The reaction rate becomes generally very low for a values (poorly reproducible) between this upper limit and that corresponding to the reduction alone of the cobalt ions ( a = 1.095). (b) An initial deformation of the curves up to approximately a = 0.05 g-atom can be observed especially a t the lowest temperatures. The activation energy over the whole temperature range and for 0.1 < a < 1.0 is 40 kcal/mol. The maximum reaction rates at given temperatures are equal to those measured, with the same conditions for S1 reduction (see Figure 3 in part I). All the experimental observations that were explained in part I a5 due to the reduction of a-Cr203 were observed here again and will not be given any further attention. For S3, the curves daldt vs. a , derived from a ( t ) curves recorded at PH*= 50 Torr, are shown in Figure 5. In the first part of the reaction up to a = x (where x represents the number of cobaltic ions on B sites) the rate is constantly decreasing and can even become zero at T < 550°C and a = J C .Up to this limit, a is proportional to a fractional power o f t : CUT= (kTt)l/n with n E 3. The activation energy calculated from the constants kT is found to be E = 30 kcal/mol. A t higher a values and for T > 550"C, this energy increases; it is 35 kcal/mol at a = 0.6 and 0.7 and finally reaches the characteristic value for the CoCrzO4 reduction E = 40 kcal/mol. Structural and Magnetic Analysis. S3 lattice parameter increases during the reduction. This is revealed by X-ray diffractograms of partially reduced samples and Debye and Scherrer photographs on which all the diffraction lines can be indexed in the lattices of a spinel matrix (of variable parameter) and of fcc-Co. It is clear from Figure 6 that the lattice parameter initially increases continuously from a = 8.281 A at a = 0 to 8.320 A at a = x (the corresponding solid phase will be called +), and then slowly reaches the CoCrz04 characteristic value a = 8.330 A. Similar observations are made with S2, the lattice pa-
2403
Hydrogen Reduction of Cobalt-Chromium Spinel Oxides
0
5
n ) o
t ( h l
1
Figure 4. Isothermal thermogravimetric curves for the reduction (at PHz= 50 Torr) of S2. a represents the weight loss in oxygen gram atoms per mole of C O ~ . O ~ C ~ ~ . ~ I O ~ .
rameter also increasing during the reaction. However, as the specimens are reduced in the temperature range 70085OoC,the fractional reaction (up to a = x = 0.063) expected by analogy with S3 cannot be clearly distinguished. The amount of ferromagnetic cobalt present in S3 samples partially reduced to the a values a = 0.28, 0.40, and 0.45 was calculated from the ratio of these samples saturation magnetization to that of a totally reduced one. This amount expressed in gram atoms of cobalt is proportional to a: n(Co g-atom) = (0.54 f 0.05) a , Le., n N a/2. Unfortunately the presence of the ferromagnetic phase does not allow the magnetic susceptibility and Curie constant of the spinel matrix to be measured and, hence, its ionic structure to be interpreted. Morphology. From this view-point S1 and S2 reductions are shown to be perfectly similar by SEM (see Figure 1) which allows the same conclusions to be drawn in both cases (see part I, section 111). The reduction of S3 was investigated by measurement of the BET-surface area of initial and partially reduced samples (Figure 2). A t low temperatures (curve b) the area remains practically constant up to a = x , indicating (provided that the influence of the metallic phase already present is negligible) that each grain of S3 yields one grain of the intermediate spinel phase (6).At higher temperatures a process of heterogeneous sintering of Co with Cr2O3 rather than an independent sintering of Co alone can explain both the observed decrease of surface area (curves c and d) and the abnormally easy reducibility of Cr2O3 above 750'C (see part I, section IVb). Moreover, we observed that, first, the reoxidation of the cobalt (in reduced samples) was very easy; under oxygen pressure Po2 = 10 Torr and at increasing temperature (150°C/hr) it is completely oxidized into Cos04 at 550OC; second, S3 spinel synthesis from the Co304Cr2O3 mixture so obtained is completed within 5 hr only at 63OoC, indicating that C0304-Cr203 interface and hence Co-Cr203 interface in the reduced samples is very large.
IV. Discussion A correct interpretation of the intermediate reduction of I S3, C02+[Cr2-,3+C0,3+]B042- where x = 0.45, must ac0.5 1 L5 a(gA') count for the three fundamental observations: (a) continuous variation of the spinel matrix lattice parameter up to a Flgure 5. Variation with a of the reaction rate daldt derived from the isothermal and isobaric a(t)curves for the reduction of S3. a is = 8.320 A; (b) production of x/2 gram atoms of metallic co~ ~ and O ~ (c) . formation of x water molecules. Then, the two the weight loss in oxygen gram atoms per mole of C O ~ . ~ ~ C ~ ~ .balt; following assumptions which do not satisfy all these conditions, have to be discarded. (1) As occurs in the reduction of c 0 3 0 4 , ~cobaltous ~~ oxide would form according to S3
y I
0.20 8.26
/
I
/ 1.0 05 Ci IgAt)
Figure 6. Variation with a of the lattice parameter of the spinel matrix in S2 and S3 samples partially reduced at the temperatures indicated on the figure and PH? = 50 Torr; (0H) values from photographs by the powder method; (0)values from diffractograms; (A) value calculated from the (311) reflection line only on a diffractogram. The open signs refer to S3,the black squares to 52.
-,xC00 +
(1 - -3CoCrzO4 + 2 Co + xH2O
condition (a) would not be satisfied and no cobaltous oxide was experimentally observed. (2) The amount of CoB3+ would decrease up to a new value corresponding to an equilibrium concentration in the thermodynamical conditions of the reaction
-
3x
+
C o ~ - ~ ~ ~ z + [ C r ~ - x 3 + C o ~+~~ 3Co + ] ~xH2O O~-xz~ 4 conditions (a) and (b) would not be satisfied. The parameter of this solid solution (x = 0.253) would be 8.305 8, (see Figure 3). We rather think that all the CoB3+is reduced to COB^+ in the matrix; this requires (in accordance with condition (b))
S3
The Journal of Physical Chemistry, Vol. 79, No. 22, 1975
2404
Pierre Bracconi and Louis-Claude Dufour
the fraction x/2 of the C O A ~to+ be simultaneously reduced to the metallic state so as to respect the electroneutrality condition in it, according to
S3
-
+X
s 2 -I-0.17H2
+
C 0 1 + + / 2 ~ + C r 2 - ~ ~ + 0 4 --Co ~ ~ - xH2O 2 Then, the number of normally occupied sites available in the spinel becomes smaller than the number of cations, and the structure must be regarded as abnormal. The following ionic arrangement
4 = [C01-x/22+]A[Cr2-r3+cOx2+]B04-x2with excess Co2+on normally unoccupied B sites and a lack of Co2+ on the A sites, has the formal advantage of being valid for any x value. Indeed, when x = 0, S3 and 4 = CoCrzO4, i.e., no such intermediate phase forms in the reduction of CoCr204. When x = 2, i.e., when the reduction of co2+[co23+]Bo42-is concerned, 4 = [co2+]Bo2-. This last ionic structure is equivalent to cobaltous oxide in which the Co2+ ions occupy all the octahedral sites of the fcc oxygen sublattice. The second step of S3 reduction involves a further increase of the lattice parameter up to the CoCr204 characteristic value a = 8.330 A, and may be interpreted by co1-x/22+[c~2-~3+cox 2+]Bo4-x
-
2-
--*
+
+ xH2O
C 0 1 - ~ / 2 ~ + [ C r 2 - ~ ~ + ] ~ 0 4 - XCO 2~~-
+
+
i.e., 4 (1 - x/2)CoCr204 xCo xH2O. The CoCrzO4 so obtained is finally reduced into Co and Cr2O3 as described in part I, section IVa. This means that the octahedral cobaltous ions are more readily reduced to the metallic state than the tetrahedral ones. However, that reactivity difference related to the site location is very weak. I t is probable that the above three successive reaction steps, whose simpler formulation reduces to xcoB3+ 4 xCOB~+ and
x COB'+ (1 - x/2)coA2+
-
+
X/2coA2+
-
x/2 Coometal (l)
x Coometal
(2)
(1- X/2)CO0metal
(3)
proceed according to the same elementary processes, i.e., a Co2+ outward diffusion and a rearrangement of the other unreduced cations in the spinel matrix; however the cationic rearrangements involved in these three steps differ as follows. In step 1, the rearranged structure (4) extends to the whole bulk of each grain without causing it to split, and there exists temperature conditions (2' < 55OOC) where this reaction can be isolated (from the kinetic viewpoint). In step 2 the rearranged structure (CoCr204) can still extend to a sufficiently large volume of the grains so as to be experimentally in evidence, but reactions 2 and 3 are no longer separable. Step 3 corresponds to CoCrzOs reduction and thus may be accounted for by the reaction model developed in part I for S1. In this case the transient cationic rearrangement (of y-Cr2O3 type) is limited to very small superficial domains of the grains. Thus the different kinetics observed for steps 1 and 3 are explained; they are both regulated by cobaltous ions diffusion (along similar diffusion paths, within different spinel structures) in the first case, through the entire bulk of the grains, and in the second one, through a discontinuous "interfacial layer" of limited thickness. Concerning the reduction of S2, we first emphasize that The Journal of Physical Chemistry, Vol. 79, No. 22, 1975
the assumption from which the Cr6+ would be first reduced into Cr3+ (with vacancies destruction) according to
-
0.96C0~+[Crl.~~~+Coo.ii~+]~O4~-!- 0.17H20
can be discarded. This would yield a solid solution of same nature as S3 which would be reduced with the same mechanism. The corresponding overall kinetics would be completely different from the experimental one and the lattice parameter would also increase more slightly than observed. S2 reduction can be understood on the same basis as s3, the few (0.06) CoB3+ions being first reduced to COB^+ (for a < 0.06) and then to the metallic state (for 0.06 < a < 0.12) according to S2
+ 0.12H2
-
0.97C0~+[Crl.~4~+Cr0.13~+00.~3]~04~+ O.09Co 0.12H20
+
The first step explains the initial deformation of the a ( t ) curves and the associated lattice parameter increase. At this point of the reaction ( a N 0.12) vacancies and the Cr6+ ions are still present in the spinel matrix, the lattice parameter is consequently smaller than that of CoCr204. It may seem a priori to be surprising that the reduction rates of this nonstoichiometric structure and of stoichiometric CoCr204 (Sl) are equal (cf. Figure 3, part I). This is explained a5 follows. First, S1 and S2 have similar initial surface areas, and the first 52 intermediate reaction step does not modify this situation since it does not cause any splitting of the initial octahedral grains. Hence the reaction rates can directly be compared. Second, in the spinel structure, the normally occupied octahedral sites, and thus the vacancies in S2 do not lie on the energetically most favorable diffusion path for C O ~ +Therefore, .~~ they do not migrate and are not involved in the rate-determining step of the reaction. I t seems from the slight inadequacy between the expected and observed gravimetric results below 750°C (see section 111,kinetics (a)) that either not all the Cr6+ ions are reduced into Cr3+ during the spinel lattice destruction or that all of them are reduced to intermediate oxidation states, Cr5+ or Cr4+. In both cases these Cr5+, Cr4+, or remaining Cr6+ would have to be incorporated as excess point defects into the a-CrzO3 lattice. Unfortunately the presence of ferromagnetic cobalt in the reduced samples does not allow this to be checked, by measurement of the a-Cr2O3 magnetic constants, so that no definitive conclusion can be drawn on this particular point.
References and Notes (1) J. Guerassimov, Congr. h t . Chim. Pure Appl., 16th 1957, 227 (1958). (2) F. Colin and J. Thery, Rev. Chim. Miner., 3, 121 (1966). (3) K. Shimakage, T. Ejima, and S. Morioka, Trans. Jpn. lnst. Met., 11, 335 (1970). (4) R. Yu. Dobrovlnskii, V. F. Balakirev, V. I. Dvinin, B. 2 . Kudinov, and G. I. Chufarov. Dokl. Akad. Nauk. SSSR, 176 (5), 1067 (1967). (5) G. I. Chufarov, M. G. Jouravleva, and E. P. Tatievskaiia. Dokl. Akad. Nauk. SSSR, 73 (6), 1209 (1950); L. Visnyovsky. Arch. Eisenhuttenwes., 39, 733 (1968). (6) V. Dorokovitch and V. V. Veselov, lsv. Akad. Nauk. SSSR, 3, 40 (1969). (7) E. Whipple and A. Wold, J. lnorg. Nucl. Chem., 24, 23 (1962). (8) M. Moreau, These, Dijon, 1970. (9) H. Adkins and R. Conor, J. Am. Chem. SOC.,53, 1092 (1931). (IO) R. J. Makkonen, Suomen Kern;. B, 35, 230 (1962). (1 1) K. W. Hanck and H. A. Laitinen, J. lnorg. Nucl. Chem., 33, 63 (1971). (12) D. S. McLure, J. Phys. Chem. Solids, 3, 311 (1957). (13) R. Ward in "Progress in Inorganic Chemistry", North-Holland Publishing Co., Amsterdam, 1967, p 253. (14) P. F. Bongers, Tech. Philips, 5 (1968). (15) L. E. Orgel in "Chimie des Metaux de Transition", Dunod. Paris, 1964, p 83. (16) K. P. Sinha and A. P. B. Sinha, J. Phys. Chem., 61, 758 (1957).
Structure of Chemisorbed Ammonia on Silica
2405
(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 (
