J. Phys. Chem. 1982,86,3022-3032
3022
Metal Oxide-Support Interactions in Silica-Supported Iron Oxide Catalysts Probed by Nitric Oxide Adsorption S. Yuen,t Y. Chen,' J. E. Kubsh, J. A. Dumeslc,'* Department of Chemical Engineering, Universlv of Wlsconsin, Madison, Wisconsin 53706
N. Tops0e, and
H. Tops0e
Ifahlor Topsse Research Laboratories, DK-2800 Lyngby, Denmark (Received: December 15, 1981; I n Final Form: March 11, 1982)
Mossbauer spectroscopy, infrared spectroscopy, and volumetric/gravimetric measurements were used to study the adsorption of nitric oxide on silica-supported iron oxide samples (containing 1w t % Fe) reduced in hydrogen at temperatures between 498 and 723 K or in a CO2/CO gas mixture (85mol % COJ at 653 K. Room-temperature Mossbauer spectra were composed of two quadrupole-splitdoublets due to Fe2+in two different coordination sites, and infrared spectra of adsorbed NO showed bands at 1910,1830,1810, and 1750 cm-'. The different reduction treatments of Fe/Si02 led to changes in the intensities of the two Mossbauer spectral doublets and the four IR bands; and it was deduced that the IR bands at 1910,1810,and 1750 cm-' were due to NO adsorbed on Fez+cations of low coordination (e.g. fourfold in the absence of NO), while the IR band at 1830 cm-' was due to NO adsorbed on Fe2+cations of high coordination (e.g., sixfold in the absence of NO). The Fez+cations of low coordination are suggested as being in strong interaction with the silica support (e.g., present as thin iron rafts or as a surface iron silicate phase). These Fez+cations are all accessible to NO and give rise to the IR bands at 1910 and 1810 cm-', which are attributed to dinitrosyl species, and the IR band at 1750 cm-', which is attributed to mononitrosyl species. The Fez+cations of high coordination are proposed to be present in small particles of iron oxide. Cations on the surface of the particles are capable of adsorbing NO in the form of a mononitrosyl species at 1830 cm-l, while cations beneath the surface are inaccessible to NO. As the reduction treatment of Fe/Si02 becomes more severe, Fe2+cations of high coordination are converted into Fez+cations of low coordination, and a greater fraction of the remaining Fez+cations of high coordination become accessible to NO. In contrast to the behavior of the Fez+cations in reduced samples of Fe/Si02, nitric oxide does not absorb to appreciable extent on Fe3+cations produced by oxidizing Fe/Si02 in O2 at 470 K.
Introduction The existence of interactions between highly dispersed metals and the supports on which they are deposited has been well documented in the literat~re.l-~Studies of chemisorption on these supported metals have been instrumental in identifying and probing the metal-support interactions. Such is the case, for example, in the study of group 8 metals supported on titania, where suppression of hydrogen and carbon monoxide chemisorption has been used to indicate the conditions under which strong metal-support interactions take In principle, an analogous situation should exist for the case of supported transition-metal oxides. In fact, the interactions between a transition-metal oxide and the oxidic support on which it is deposited may be stronger than typical metal-upport interactions, since the crystallography and the chemical bonding of the support more closely resemble supported transition-metal oxides than the supported metals. Along these lines, the present paper is a study of such a metal oxide-support interaction using nitric oxide chemisorption as a probe. In particular, the study involves iron oxide supported on silica. That an interaction between iron oxide and silica takes place has already been established in the literature. For example, when a ferric salt is impregnated onto silica at low loadings (ca. 1wt %), the iron cannot be reduced below the divalent state upon hydrogen treatment at 750 K (as reviewed e l s e ~ h e r e ~whereas ? ~ ) , bulk iron oxide is reduced to the metallic state during this treatment. Furthermore, Shell Research and Development Company, Houston, TX. *Visiting Scholar from the Department of Chemistry, Nanking University, Nanking, People's Republic of China. 8 Camille and Henry Dreyfus Foundation Teacher-Scholar. 0022-365418212086-3022$0l.25/0
iron oxide supported on silica does not show the catalytic properties of bulk magnetite (Fe304)for water-gas shiftq7 Spectroscopic information about the nature of iron oxide supported on silica has been provided using Mossbauer spectroscopy. There is general agreement that, following reduction in hydrogen at ca. 750 K, the Mossbauer spectrum of iron on silica at low loadings shows two quadrupole-split doublets.6i"12 The doublet with the larger isomer shift and quadrupole splitting, subsequently denoted the "outer doublet", has been attributed to Fe2+of high coordination (e.g., octahedrally coordinated by oxygen anions); and the doublet with smaller isomer shift and quadrupole splitting, subsequently denoted the "inner doublet", has been attributed either to Fe3+(ref 8) or to Fe2+ of lower coordination.1° Upon exposure of such samples to NH3,10H20,8J3CH30H,10H2,14or C0,14 part (1)J. H. Sinfelt, Catal. Rev. 3, 1975 (1969). (2)R. van Hardeveld and F. Hartog, Adu. Catal., 22, 75 (1972). (3) S. J. Tauster, S. C. Fung, R. T. K. Baker, and J. A. Horsley, Science, 211, 1121 (1981). (4) M. A. Vannice and R. L. Garten, J. Catal., 66, 236 (1979). (5)F.J. Berry, Adu. Inorg. Chem. Radiochem., 21, 255 (1978). (6)H. Topsse, J. A. Dumesic, and S. Msrup in "Applications of Mijssbauer Spectroscopy",Vol. 2,R. L. Cohen, Ed., Academic Press, New York, 1980,p 55. (7)C. R. F.Lund and 3. A. Dumesic, submitted for publication in J. Catal. (8) J. Blomquist, S. Csillag, L. C. Moberg, R. Larsson, and B. Rebenstorf, Acta Chem. Scand., Ser. A , 33, 515 (1979). (9)M. C. Hobson, Jr., and A. D. Campbell, J . Catal. 8, 294 (1967). (10)M. C. Hobson, Jr., and H. M. Gager, J. Colloid Interface Sci., 34, 357 (1970). (11)T. Tachibana and T. Ohya, Bull. Chem. SOC.Jpn., 42, 2180 (1969). (12)B. S. Clausen, Ph.D. Thesis, Department of Applied Physics, Technical University of Denmark, Lyngby, Denmark, 1979.
0 1982 American Chemical Society
Metal Oxide-Support Interactions
of the inner doublet is converted into outer doublet, consistent with the conversion of iron cations of low coordination into iron cations of higher coordination due to chemisorption on the former. As a further probe of the surface properties of silicasupported iron oxide, nitric oxide adsorption may be useful. Nitric oxide chemisorbs on Fe304to essentially monolayer coverage a t 273 K and pressures near 10 kPa.15J6 This is to be contrasted to CO adsorption on Fe304,whose uptake over a wide range of temperatures is only about 10% of the NO adsorption uptake.l'J* In addition, the extent of nitric oxide adsorption on silica is small at 273 K and subatmospheric pressures.le This is to be contrasted with NH3, H20, and CH,OH, which show appreciable adsorption on silica at room temperatureS2O Thus, NO may actually be a more sensitive adsorption probe of silica-supported iron oxide than these other molecules used previously. The adsorption of nitric oxide on supported iron cations has been the subject of several previous investigations. Jermyn et aL21used electron spin resonance and infrared spectroscopy to study NO adsorbed on Fe2+in Y zeolite, while Segawa et a1.22 studied this same system using Mossbauer spectroscopy, infrared spectroscopy, and gravimetric methods. Blomquist et d.*and RebenstorP used infrared spectroscopy and Mossbauer spectroscopy to probe the adsorption of NO on silica-supported Fe2+, pretreated under vacuum at high temperatures (e.g., 800 K). In both the Fe2+-Y zeolite and Fe2+/Si02systems, Miissbauer spectroscopy showed the presence of outer and inner doublets, although in different proportions. For the Fe2+/Si02 system, it is possible to vary the relative amounts of inner and outer doublets by suitable sample pretreatment.5i9J2 For example, Fe2+/Si02treated in hydrogen or under vacuum to progressively higher temperatures leads to an increased amount of inner doublet. Accordingly, the study of nitric oxide adsorption on samples with different amounts of inner and outer doublets should make it possible to determine differences in the bonding of NO with these two different forms of iron. Furthermore, it will be shown in this paper that the presence of inner and outer doublets for Fe2+/Si02is due to the interaction of iron oxide with silica. Infrared spectroscopy is used to probe the state of adsorbed NO; Miissbauer spectroscopy is used to determine the chemical states of iron on silica, before, during, and after exposure of the samples of NO; and volumetric and gravimetric adsorption studies are used to measure the quantity of NO adsorbed by each sample. Experimental Section The supported iron oxide samples used in this study were made by impregnation of Cab-0-Si1 (Cabot Corp. Grade S-17,400 m2 g-l) with an aqueous solution of ferric nitrate (J. T. Baker Chemical Co.). Approximately 1 cm3 (13)H.M. Gager, J. F. Lefelhocz, and M. C. Hobson, Jr., Chem. Phys. Lett., 23, 386 (1973). (14)B. S.Clausen, S. M~rrup,and H. Topsse, Surf. Sci., 106, 438 (1981). (15)K. Otto and M. Shelef, J. Cat& 18, 184 (1970). (16)C.R. F.Lund, J. J. Schorfheide, and J. A. Dumesic, J.Catal., 57, 105 (1979). (17)B. Viswanathan, K. R. Krishnamurthy, and M. V. C. Sastri, J. Res. Znst. Catal., Hokkaido Uniu., 27,79 (1979). (18)J. E.Kubsh, Y. Chen, and J. A. Dumesic, J. Catal., 71,192 (1981). (19)A. Solbakkgn and L. H. Reyerson, J. Phys. Chem., 63,1622(1959). (20)H.Kn&inger, Adu. Catal., 25, 184 (1976). (21)J. W.Jermyn, T. J. Johnson, E. F. Vansant, and J. H. Lunsford, J.Phys. Chem., 77,2964 (1973). (22)K. Segawa, Y.Chen, J. E. Kubsh, W. N. Delgass, J. A. Dumesic, and W.K.Hall, J. Catal., in press. (23)B. Rebenstorf, Acta Chem. Scand., Ser. A, 31,547 (1977).
The Journal of Physical Chemistty, Vol. 86, No. 15, 1982 3023
of impregnation solution was used per gram of Cab-0-Sil, and the concentration of this solution was chosen such that an iron loading of 1 w t % Fe on Si02 resulted. After impregnation, the samples were calcined at 400 K for 24 h. For Mossbauer spectroscopy, analogous samples were prepared with the only exception that an b7Fe-enriched ferric nitrate solution was used. After calcination, each sample received one of five treatments before exposure to NO: reduction for 4 h in flowing H2 at 498, 653, or 723 K; reduction for 4 h in a flowing CO2/CO gas mixture (containing 15 mol % CO) at 653 K; or oxidation in flowing O2at 470 K from 2 to 12 h following the previous treatment in CO2/CO. The three different hydrogen treatments were chosen to vary the relative amounts of inner and outer doublets, as discussed above; the treatment in CO2/CO was used since bulk Fe203 is converted to Fe304 in this manner; and the oxygen treatment was employed since bulk Fe304is thereby converted to T-Fe20a. Even though silica-supportediron oxide may not behave like bulk iron oxide, these latter two treatments supplement the treatments in hydrogen as a means of varying the chemical state of iron oxide on silica. Upon completion of each of the five above treatments, the sample was evacuated for 1 h at the same temperature used in the gas treatment. Volumetric adsorption measurements were carried out by using the procedures and apparatus discussed by Lund et al.le in their study of NO adsorption on unsupported Fe304. Known quantities of NO were dosed into a cell containing the sample at 273 K, and the resulting change in NO pressure was used to calculate the amount of NO adsorbed. By successive dosing, an NO-adsorption isotherm was generated. The sample was then evacuated for 0.5 h at 273 K, and a second NO-adsorption isotherm was then collected. The difference between these two isotherms represents the amount of NO strongly adsorbed on the sample. Gravimetric studies were conducted by using the microbalance system described elsewhere." The procedures employed to measure the extent of NO adsorption have been discussed previously26with respect to the study of silica-supportedFe301. Briefly, this entailed measurement of the sample weight before exposure to NO, during exposure to 7 kPa of NO at 273 K, and after evacuation at this same temperature. A quadrupole mass spectrometer (UTI, Model lOOC) was attached to this gravimetric apparatus, allowing gas-phase compositional analyses to be carried out. Also, gaseous samples were collected during the above volumetric adsorption measurements and transferred to the mass spectrometer for analysis. For Mossbauer spectroscopy and infrared studies of Fe/Si02, the samples were pressed (at 10000 psi) into wafers 2.54 cm in diameter. Wafers used in Mossbauer spectroscopy averaged 200 mg in weight, while those for infrared studies averaged 100 mg. These Mossbauer and infrared spectroscopy studies were conducted by using the cell described by Phillips et a1.26 It is made of stainless steel, and it allows sample treatments and spectra to be collected at temperatures from 77 to ca. 700 K. For Mossbauer spectroscopy, y-ray-transparent Be windows (containing finite amounts of iron) were used.27 In order to conduct infrared spectroscopy, we replaced the above (24)J. E.Kubsh and J. A. Dumesic, AZChE J.,in press. (25)J. E.Kubsh, C. R. F. Lund, Y. Chen, and J. A. Dumesic, React. Kinet. Catal. Lett., 17, 115 (1981). (26)J. Phillips, B.S.Clausen, and J. A. Dumesic, J.Phys. Chem., 84, 814 (1980). (27)W.M.Shen, J. A. Dumesic, and C. G. Hill, Jr., Rev. Sci. Instrum., 52, 858 (1981).
3024
The Journal of Physical Chemistry, Vol. 86, No. 15, 1982
windows with NaCl windows (2.54-cm diameter, 0.6 cm thick, Precision Cells Inc.). The flanges containing the NaCl windows could be cooled during high-temperature treatments of the sample. Introduction of NO into the above cell was accomplished by using a 250-mL glass bulb containing ca. 40 kPa of NO. This bulb was attached to the cell via a three-way stopcock. A glass appendage was attached to the bulb, which allowed the NO pressure in the cell to be reduced to its vapor pressure at 77 K (ca. 1 Pa) by submerging the appendage in liquid nitrogen. It should be noted that selected infrared experiments were performed in a glass cell, equipped with movable heat shields that could be placed around the sample during high-temperature treatments. These experiments gave results similar to those obtained with the above stainless-steel cell. Thus, only those results obtained with the latter cell will be reported here. Descriptions have been given elsewhere of the Mossbauer spectrometer and the computer program used to fit the resulting Mlsbauer spectra.26 Doppler velocities were calibrated by using a l-mil metallic iron foil for velocities greater than 4 mm s-l and sodium nitroprusside for velocities less than 4 mm s-'. All isomer shifts reported in this study are given relative to metallic iron at room temperature. In the spectra of the Fe/Si02 samples, the spectral component arising from the iron impurities in the beryllium windows was fitted with a quadrupole doublet. The isomer shift and the quadrupole splitting of this doublet were constrained to be equal to those parameters measured without a sample in the cell; and, in all fits of a given sample, the intensity of this doublet was constrained to be constant. Room-temperature infrared spectra were collected by using a Perkin-Elmer 180 double-beam spectrometer. The Fe/Si02 samples, following various treatments, were placed in one beam of the spectrometer and an attenuator in the other beam was adjusted to position the base line at the same level for all spectra. The resolution of the spectrometer was ca. 4-5 cm-' for the highly opaque Fe/Si02 samples. Survey scans were taken for all samples in the region from 1400 to 4000 cm-'. No data could be obtained below 1400 cm-' because of strong infrared absorption by silica. More detailed scans were then taken in the region between 1400 and 2500 cm-l, since all observable bands due to adsorbed NO on the Fe/Si02 samples fell within this region. The infrared spectrum of Fe/Si02 showed bands in the region between 1400 and 2500 cm-', and this spectrum was, therefore, subtracted from the infrared spectra of NO adsorbed on Fe/Si02. This was done manually, by subtracting absorbances at 5-cm-' intervals in the spectra. The CO2/CO gas mixture (15 mol 9% CO) used for sample treatment was obtained premixed from Matheson. Before use, this gas was flowed through a Pyrex trap at 500 K in order to remove any metal carbonyl species present in the gas mixture. Hydrogen was purified by passage through a Deoxo unit (Englehardt), followed by a bed of Drierite at ambient temperature and activated 13X molecular sieves (Davison) at 77 K. Finally, nitric oxide (CP grade; 99.5% pure, minimum) was obtained from Matheson and further purified by the procedure described elsewhere.16 This involved passage over silica gel at 190 K. Results Mossbauer Spectroscopy. As noted earlier, different sample treatments lead to different states of iron on silica. Figure 1shows the room-temperature Mossbauer spectra that result from different sample treatments: O2oxidation
Yuen et ai.
1.000
-
0.950
-
0.900
5;
1.000
-
0.975
.-
0.950
1.000 0.975 0.950 0.925
1.000 0.975 0.950 0.925
0.900
1 i-
-
-
-
4 -4
-3
-7
-1
0
1
7
3
4
V E L O C I T Y ( m m/ s ) Flgure 1. Roomtemperature Wssbauer spectra of reduced and oxidized 1 % FelSiO,: (a) oxidized in 0,at 470 K, (b) reduced in H, at 498 K, (c) reduced in a 15 % CO-85 % CO, mixture at 653 K, and (d) reduced in H, at 653 K. (Iron-free, Teflon-coated Kapon windows (Du Pont) were used on the Mossbauer spectroscopy cell, instead of beryllium windows, for this series of spectra.)
at 470 K, H2 reduction at 498 K, COz/CO reduction at 653 K, and Hz reduction at 653 K. All spectra are composed of superimposing doublets, rather than the characteristic 6- or 12-line spectra associated with bulk metallic iron or iron oxides such as Fe20Bor Fe30k The broad peaks of the oxidized sample were fitted with two overlapping doublets, each with parameters typical of Fe3+as seen in Table I. The spectra of the three reduced samples were fitted according to Hobson and Gager'O with two partially overlapping doublets. (The negative-most peaks of each doublet overlap near zero velocity, while the positive-most peaks of each doublet are clearly resolved.) The computer-fitted Mossbauer parameters for these spectra are summarized in Table I. The outer doublet decreases in intensity as the reduction treatment becomes more severe, while the inner doublet shows a corresponding increase in
The Journal of phvsical Chemistry, Vol. 86, No. 15, 1982 3025
Metal Oxide-Support Interactions
r
TABLE I: Mossbauer Parameters for Oxidized and Reduced 1%Fe/SiO, outer doublet
IS: sample pretreatment
mm
oxidized in 0, at 470 K H, reduced at 498 K COJCOreduced at 653 K H, reducedat 653 K a Isomer shift.
0.28
inner doublet
relaQS,b tive IS,a mm area, mm
relaQSlb tive mm area,
%
s-l
s-l
%
2.14
52
0.33
1.37
48
1.04
1.83
74
0.80
0.96
26
1.00
1.84
58
0.78
0.91
42
1.01
1.85
49
0.81 0.91
51
s-l
5-l
Quadrupole splitting.
intensity. This is the anticipated result since the outer and inner doublets have previously been attributed to Fe2+ cations of high and low coordination, respectively.10 The temperature dependence of the Miissbauer spectrum for the 1% Fe/Si02 sample reduced in CO2/CO at 653 K was determined. Following this pretreatment, MBssbauer spectra were collected under vacuum at 77,292,480, and 675 K. The sample was then returned to room temperature, at which time another spectrum was collected. This room-temperature spectrum was identical with that of the freshly reduced sample, indicating that no changes occurred to the sample during the temperature study. Increasing the measurement temperature from 292 to 675 K resulted in a greater decrease in the spectral area of the outer doublet compared to that decrease in spectral area of the inner doublet. Accordingly, the effective Debye temperature for the outer-doublet Fe2+is lower than that for the inner doublet. The quadrupole splitting of the outer doublet also showed a strong dependence on temperature decreasing from 1.86 mm s-l at 292 K to 1.34 mm s-l at 675 K. In contrast, the quadrupole splitting of the inner doublet only decreased from 0.90 to 0.83 mm s-l over the same range in temperature. Exposure of the oxidized Fe/Si02 sample to ca. 13 kPa of NO at 273 K resulted in only slight changes in the MBssbauer spectra, reflected as small changes in the quadrupole splittings of both doublets (by ca. 0.1 mm s-l) with no observable changes in isomer shift. Subsequent room-temperature evacuation of the NO atmosphere over the sample resulted in only slight perturbations of the Mossbauer spectrum. In contrast, the interaction of NO with the reduced samples was more dramatic. Figure 2 shows a series of Mossbauer spectra for the 1%Fe/Si02 sample reduced in CO2/CO at 653 K. Analogous results were obtained upon exposure of NO to samples reduced in H2at 498 or 653 K, and, as such, only those spectra for the sample reduced in CO2/CO will be presented in detail. Figure 2a is the room-temperature spectrum following the CO2/CO treatment and 1-h evacuation at 653 K; Figure 2b was collected at 293 K following subsequent exposure of the sample to NO at 13 kPa; Figure 2c was obtained after evacuating the NO from the cell at room temperature; and Figure 2d-f are room-temperature spectra collected after evacuation of the sample for 0.5 h at the progressively higher temperatures of 390, 510, and 650 K. Two spectral changes are predominant in comparing the Mijssbauer spectrum before exposure to NO with that spectrum collected in the presence of NO: the negativemost peak in Figure 2a shifts to a more negative velocity, and the two more positive peaks become more poorly resolved upon exposure to NO. Upon evacuation of the NO at sequentially higher temperatures, the features of the
11
0.025
i?'
- 4 0 - 3 0 - 2 0 -10 0.0
IO
20
3 0 40
VELOCITY ( m r r / s )
Figwe 2. Roomtemperatwe Miissbauer spectra of 1% Fe/SIo, after exposure to NO and evacuation at progressively hlgher temperatures: (a) reduced in a 15% CO-85% CO, mixture at 653 K, (b) exposed to NO at room temperature (PNo = 13 kPa), (c) evacuated at room temperature, (d) heated under vacuum for 0.5 h at 390 K, (e) heated under vacuum for 0.5 h at 510 K, and (f) heated under vacuum for 0.5 h at 650 K.
original spectrum (Figure 2a) are progressively restored. All of the Mbbauer spectra can be consistently intepreted in terms of contributions from five different quadrupolesplit doublets. These can be assigned to (i) Fe2+of the outer doublet, (ii) Fe2+of the inner doublet, (iii) Fe3+,(iv) NO associated with Fez+of the outer doublet, and (v) NO associated with Fe2+of the inner doublet. Since NO does not significantly alter the Mossbauer parameters of Fe3+, a doublet due to NO associated with Fe3+need not be considered. The experimental strategy used to computer fit the spectra was therefore to assign fixed isomer shifts and quadrupole splittings for each of the five doublets and to let the computer determine the relative intensities of these doublets from each spectrum. The choice of Mijssbauer parameters for these doublets was accomplished in the following manner. The spectrum in Figure 2a was used to determine the Mijssbauer parameters of the Fe2+contributing to the outer doublet, Fe2+of the inner doublet, and the small amount of Fe3+ present after treatment in CO2/CO. The parameters for the two new doublets, due to NO associated with Fe2+of the outer and inner doublets, were determined such that they best fit all spectra with adsorbed NO. The results of this procedure are summarized in Table 11. To test the validity of this fitting procedure, we allowed the computer to vary the
Yuen et al.
3026 The Journal of Physlcal Chemlstry, Vol. 86, No. 15, 1982
TABLE 11: Mossbauer Parameters for the Quadrupole-Split Doublets Used in Fitting the Spectra of Figure 2, Including Isomer Shift (IS)," Quadrupole Splitting (QS)," Line Width (I'),b and Spectral Area (A)' NO evacuated adsorbed reduced a t 293 K at 293 K a t 390 K at 510 K at 650 K Fez+inner doublet, IS = 0.78, QS = 0.91
r A
0.54 6.61 0.68 9.20
A
0.0
A
0.0 0.34 0.40 16.21
A Fez+ outer doublet, IS = 1.00, QS = 1.84
NO adsorbed on inner doublet, IS = 0.46, QS = 1.37 NO adsorbed on outer doublet, IS = 0.45, QS = 1.87 Fe3++, IS = 0.38, QS = 2.23 total area, % x m m s-l a
Units: mm
s-l.
r r r r
A
0.26 0.11 0.83 5.01 0.92 8.32 0.58 2.72 0.39 0.29 16.45
Full width at half-maximum intensity, mm s-l.
% X mm
0.0 0.83 5.13 0.91 8.67 0.47 2.51 0.35 0.41 16.72
0.34 2.16 0.50 6.94 0.99 5.77 0.49 2.16 0.34 0.23 17.26
0.66 4.93 0.66 6.52 0.82 4.56 1.51 1.13
0.50 5.78 0.65 6.48 1.40 3.33 3.38 0.59 0.45 0.62 16.80
0.0 17.14
s-l.
TABLE 111: Mossbauer Parameters for the Quadrupole-Split Doublets Used in Fitting the Spectra of Figure 4, Including and Spectral Area (A)' Isomer Shift (IS)? Quadrupole Splitting (QS)," Line Width reduced Fea+inner doublet, IS = 0.80, QS = 0.92
r A
0.55 8.92 0.71 9.41
A
0.0
A
0.0
A
0.0
A
0.0 18.33
A F e z +outer doublet, IS = 1.01, QS = 1.84
NO adsorbed on inner doublet, IS = 0.47, QS = 1.28 NO adsorbed on outer doublet, IS = 0.47, QS = 1.93 Fe3+(sites of higher distortion), IS = 0.25, QS = 2.01 Fe3+(sites of lower distortion), IS = 0.34, QS = 1.16 total area, % x m m s'' a Units: m m s-],
r
r r r r
Full width a half-maximum intensity, m m
Mijssbauer parameters of all five doublets in the fits of each spectrum in Figure 2. The new isomer shifts and quadrupole splittinga typically differed by less than 0.1 mm s-l from those values given in Table II;and the new relative spectral areas typically differed by leas than 5% from those values of Table 11. This indicates that the constrained fits are, in fact, good fits of the experimental data. Moreover, the strategy employed in arriving at these constrained fits gives a physical interpretation to each of the five doublets used. The changes in spectral area accompanying exposure of Fe/Si02 to NO and evacuation of the NO at progressively higher temperatures are shown graphically in Figure 3. It can be seen therein that the amount of Fe3+in all spectra is small and constant. Ekposure of the sample to NO leads to the complete loss of the inner doublet and a partial loss of the outer doublet. This is accompanied by the appearance of doublets due to NO associated with Fe2+. Evacuation at room temperature does not lead to significant changes in the Miisabauer spectrum. However, at progressively higher evacuation temperatures, the Fe2+of the inner and outer doublets increases in intensity at the expense of the doublets due to NO associated with Fe2+. In fact, it seems that the Fe2+ of the outer doublet is restored to a constant value at a lower temperature (390 K)than is the Fe2+of the inner doublet (which continues to grow upon evacuation a t temperatures up to 650 K). The interaction of NO with a reduced sample was further studied by investigating the effect of treating the sample in an NO atmosphere at progressively higher temperatures. Figure 4, a and b, depict the room-temperature spectra collected following C02/C0reduction at 653 K and exposure to ca.13 kPa of NO at 293 K, respectively. Figure
s-l.
NO adsorbed at 293 K
at 470 K
0.26 0.36 0.73 5.38 0.75 6.50 0.52 5.60 0.23 0.20 1.76 0.08 18.12
0.47 0.04 0.79 2.90 0.82 5.75 0.63 7.22 0.30 0.77 0.63 2.00 18.68
heated in NO a t 570 K a t 670 K 0.0 0.64 0.98 0.72 1.71 0.58 5.89 0.55 5.89 0.58 5.20 19.67
0.0 0.0 0.50 0.72 0.49 3.18 0.66 11.02 0.55 5.90 20.82
% X m m s-l.
0.0
n x/
0 REDUCED
NO 293K
fVACUATE 293K
"
-
DESORB 390K
DESORB 510K
DESORB 6506
Flguro 3. Variation with sample treatment in spectral areas of the quabupolespyt dwblets used In computer fitting the spectra of Figure 2: (0) outerdoublet Fe2+,( 0 )innerdoublet Fe2+,(0)Fe3+,(0)NO adsorbed on outerdoublet Fe2+,(A)NO adsorbed on innerdoublet Fe2+.
4c-e illustrate the room-temperature spectra collected following treatment in the same NO atmosphere for 0.5 h at 470, 570, and 670 K, respectively. The spectra of Figure 4 were fitted with the same five doublets used in the constrained fits of Figure 2 with one exception. The Fe3+doublets used in Figure 2 were replaced by two different Fe3+doublets in order to adequately fit the spectra of Figure 4 (especiallythe spectrum of Figure 4e). These two Fe3+doublets have similar isomer shifts but different quadrupole splittings, consistent with ferric ions present in sites of differing symmetry. The Fe3+doublet with the larger quadrupole splitting is assigned to ferric ions in sites of higher distortion (from cubic symmetry), while the smaller quadrupole-split Fe3+doublet is associated with
The Journal of Physical Chemistry, Vol. 86, No. 15, 1982 3027
Metal Oxide-Support Interactlons I
TABLE IV: NO Adsorption Uptakes for Reduced 1%Fe/SiO, irreversible NO uptake, pmolof iron NO/g of dispersion, reduction % treatment sample H, at 498 K 32 18 CO/CO, at 653 K 125 70 H, at 723 K 165 93
1
to Fe3+after treatment in NO at 670 K, while a significant
i-
u
-4.0 -3.0 -2.0 -1.0 0.0 VELOCITY
1.0 2.0 3.0
(mm/s)
m e 4. Room-temperature h46ssbauer spectra of 1% Fe/SIOz after exposure to NO and heating In NO at progressively higher temperatures: (a) reduced In a 15% CO-85% COP mixture at 653 K, (b) exposed to NO at room temperature (PNo 13 kPa), (c) heated In NO for 0.5 h at 470 K, (d) heated in NO for 0.5 h at 570 K, and (e) heated in NO for 0.5 h at 670 K.
a
W
n a
-
1 a 4.0n
+ 0
W
a m n ".-n -
REDUCED
NO
293 K
NO 4 7 0 ~
NO
570 K
h0 670K
5. Variation wtth sample treatment In the spectral areas of the q"b@t doublets used in computer fitting the spectra of Flgure 4: (0)outerdoublet Fez+,( 0 ) Innerdoublet Fez+, (0)Fe3+at sites of lower dlstortion, (0)Fe3+ at sltes of hlgher dlstortion, ( 0 ) NO adsorbed on outerdoublet Fez+, (A)NO adsorbed on Innerdoublet Fe2+.
ferric ions in sites of lower distortion. The isomer shifts and quadrupole splittinge of these six doublets, as well as the spectral area associated with each doublet in the spectra of Figure 4, are summarized in Table 111. The change in spectral area for each of the six doublets following treatment in NO is shown in Figure 5. As before, exposure of the COz/CO-reduced sample to NO at room temperature causes a decrease in the area of the outer doublet and the disappearance of the inner doublet, with the appearance of doublets associated with NO adsorbed on iron associated originally with both the inner and outer doublets. Treatment in NO at elevated temperatures leads to the oxidation of Fe2+to Fe3+. Figure 5 also shows that essentially all of the inner-doublet Fe2+has been converted
amount of outer-doublet Fe2+(associated with adsorbed NO) remains after this treatment. Accordingly, after heating in NO at 670 K the amount of Fe3+ in sites of higher distortion approaches the amount of inner-doublet Fez+originally present in the sample, while the amount of Fe3+in sites of lower distortion remains less than the amount of outer-doublet Fe2+ originally present in the sample. These results point to the relative ease of oxidizing low-coordination Fe2+to Fe3+as compared to high-coordination Fe2+in an NO atmosphere. Volumetric and Gravimetric Adsorption Measurements. Volumetric adsorption measurements at 273 K were conducted on Fe/Si02samples having received three different reduction treatments: H2 reduction at 498 K, CO2/CO reduction at 653 K, and Hzreduction at 723 K. In addition, gas-phase compositional analyses were conducted mass-spectroscopically after exposure of the NO to the showed only the presence Fe/Si02 sample. These analof NO, with the absence of peaks due to either NzO or Na. Since N2and N20 do not adsorb to a significant extent on iron oxide,16the absence of these species in the gas phase indicates that NO has not been reduced to Nz or N20 by Fe/SiOa. Thus, Fe/Si02 is not oxidized by NO at 273 K, and the volumetric adsorption measurements correspond to NO adsorbed on Fe2+. For NO pressures greater than ca.13 P a , the adsorption isotherms for all samples were essentially parallel. This included the first and second isotherms for each Fe/SiOz sample (these isotherms representing the total and the reversible NO adsorption, respectively, as discussed earlier) and the isotherm for NO on Cab-0-Si1 alone. Table IV contains a summary of these adsorption data. The irreversible NO uptakes, reported as micromoles of NO adsorbed per gram of sample, were calculated by subtracting the fmt and second NO adsorption isotherms at pressures greater than 13 P a . These data indicate that the amount of irreversible NO uptake increases as the reduction of Fe/SiOz becomes more severe. If it is assumed that each coordinatively unsaturated iron cation adsorbs one molecule of NO irreversibly (this will be examined at length in the Discussion section of this paper), then the iron dispersions shown in Table IV are obtained. These are simply the irreversible NO uptakes divided by the total number of iron cations in the sample. Gravimetric studies were conducted to study the adsorption of NO on the Fe/Si02 sample which had been oxidized at 470 K. The extent of irreverisble NO adsorption on this sample was small, amounting to 9 pmol 8'. This is consistent with the above observation that the Mhsbauer spectrum of oxidized Fe/Si02 was only slightly perturbed by exposure of the sample to NO. Infrared Spectroscopy. The above volumetric and Mihbauer spectroscopy results showing that NO does not adsorb irreversibly on Cab-0-Si1 were confirmed by infrared spectroscopy. Only those bands associated with gaseous NO were detectable when Cab-0-Si1was exposed to NO (at ca. 13 kPa). Moreover, evacuation of the NO
3028
The Journal of Physical Chemistry, Vol. 86, No. 15, 1982
Yuen et ai.
tL- L L L d A l d e
1600
1700
1800
1900
2000
1'5)
1600
WAVENUMBER (cm-' )
Figwe 6. Roomtemperature infrared spectra of 1% Fe/Si02 reduced in CO2/CO at 653 K, exposed to NO and evacuated at progressively higher temperatures: (a) exposed to NO at room temperature (PNo 13 kPa), with the NO pressure reduced by condensation at 77 K; (b) evacuated at room temperature; (c) heated under vacuum for 0.5 h at 373 K (d) heated under vacuum for 0.5 h at 498 K; and (e) heated under vacuum for 0.5 h at 053 K.
at room temperature resulted in the disappearance of all NO bands, leaving only the IR spectrum associated with the silica itself. For reduced Fe/Si02, however, strong IR bands due to adsorbed NO were observed upon exposure of the sample to NO. This is shown in Figure 6 for the Fe/Si02 sample that had been reduced in CO2/COat 653 K. Only the wavenumber range from 2000 to 1600 cm-' is shown since all observable bands resulting from adsorbed NO were found within these limits. Spectrum a was collected after exposure of the sample to ca. 13 kPa of NO, followed by condensation of the NO at 77 K in the previously described glass appendage. This procedure allows IR spectra to be collected with a low, but reproducible, pressure of NO in the gas phase, thereby minimizing contributions of the gas phase to the observed spectrum. Spectrum a contains a very broad, intense band centered at ca. 1810 cm-l and a second, less intense band at ca. 1910 cm-'. The glass appendage was then isolated from the cell, and room-temperature evacuation of the cell commenced. The resulting spectrum (spectrum b) shows that the broad 1810-cm-' band has decreased in intensity, revealing two ban&. a band at 1750 cm-' and another band at 1830 cm-'. The intensity of the band at 1750 cm-' actually increased upon evacuation, while the intensities of the bands at 1910 and 1810 cm-I decreased during this treatment. Subsequent reexposure of the sample to NO resulted in the reappearance of spectrum a. In fact, this transformation from spectrum a to spectrum b and back to spectrum a was completely reversible. After room-temperature evacuation, the sample was subsequently evacuated for periods of 0.5 h at each of the progressively higher temperatures of 373,498, and 653 K. The corresponding infrared spectra collected after cooling to room temperature are shown in spectra c-e of Figure 3. Desorption at the reduction temperature of 653 K removed all bands associated with adsorbed NO. This latter finding is in qualitative agreement with the result that the Mossbauer spectrum obtained after exposure of Fe/Si02 to NO followed by evacuation at 653 K was very similar to that spectrum of Fe/SiOz obtained before exposure of the sample to NO. In summary, the results of Figure 6 can be interpreted in terms of four bands: namely, bands at 1910,1830, 1810, and 1750 cm-'. In the presence of NO
1700
1800
1900
2001
WAVENUMBER (cm-')
Flgwe 7. Roomtemperatwe infrared spectra of 1% Fe/Si02 reduced in H, at 498 K, exposed to NO, and evacuated at progressively higher temperatures: (a) exposed to NO at room temperature (PNo N 13 kPa), with the NO pressure reduced by condensation at 77 K; (b) evacuated at room temperature; (c) heated under vacuum for 0.5 h at 323 K (d) heated under vacuum for 0.5 h at 373 K and (e) heated under vacuum for 0.5 h at 498 K.
W
0 2
b
0
C
a m a v)
m
a
u 600
1700
1800
WAVENUMBER
1900
2001
(cm-')
F W 8. Roomtemperature infrared spectra of 1 % Fe/SiO, reduced in H2at 653 K, exposed to NO, and evacuated at progresstvely higher temperatures: (a) exposed to NO at room temperature (PNo E 13 kPa), with the NO pressure reduced by condensatlon at 77 K; (b) evacuated at room temperature; (c) heated under vacuum for 0.5 h at 373 K; (d) heated under vacuum for 0.5 h at 498 K and (e) heated under vacuum for 0.5 h at 653 K.
at low pressures, bands at 1910,1830, and 1810 are present; and, upon evacuation at room temperature, the bands at 1910 and 1810 cm-' decrease in intensity, while the band at 1750 cm-' increases. The infrared spectra of NO adsorbed on Fe/SiOz samples having been reduced in Hzat 498 or 653 K were analogous to those presented in Figure 6, and these are shown in Figures 7 and 8, respectively. All bands discussed above were present after exposure of these two samples to NO, and no new bands appeared as the result of differing reduction conditions. In addition, the behavior described during room-temperature evacuation and desorption of NO at higher temperatures was observed for
Metal OxMe-Support Interactions
all Fe/Si02 samples. That is, the intensities of the bands at 1910 and 1810 cm-' decreased upon evacuation at rcam temperature; the intensity of the band at 1750 cm-' increased during this evacuation; and the effects of roomtemperature evacuation could be reversed by reexposure of the sample to NO. These figures also suggest that the band at 1750 cm-' is more stable against evacuation at higher temperatures than the band a t 1830 cm-l. Comparison of Figures 6-8 shows that the intensities of all IR bands increase with increasing severity of reduction. The infrared spectrum of the oxidized (at 470 K) Fe/ SiO, sample after exposure to NO showed only a weak band at 1820 cm-'. Upon evacuation at room temperature, this band decreased in intensity, and two weak, broad bands appeared in the spectral region between 1700 and 1750 cm-'. These results are consistent with the previous Mossbauer spectroscopy and gravimetric findings: NO adsorbs to a low extent on oxidized Fe/Si02. Discussion General Behavior. It is clear from the volumetric adsorption measurements and from infrared spectroscopy that more sites for NO adsorption are generated by more severe reduction of Fe/SiO,. Mossbauer spectroscopy shows that, in parallel with the above changes, Fe2+of the outer doublet is converted into inner doublet as Fe/Si02 is more severely reduced. Thus, at least some of the adsorbed NO is associated with Fez+ of the inner doublet. Blomquist et al.,B in fact, attributed all of the adsorbed NO on Fe/Si02 to be associated with such sites; and their samples contained only small amounts of outer-doublet Fe2+. The Mossbauer spectra collected in the present study after exposure of Fe/SiO, to NO demonstrate, however, that both the inner and outer doublets are affected by adsorbed NO. Furthermore, infrared spectroscopy shows that a number of different adsorbed NO species are present on reduced Fe/Si02, suggesting that several different types of surface sites exist on the surface of these samples. It is thus tempting to attribute some of the IR bands to NO adsorbed on Fe2+of the inner doublet and the remaining IR bands to NO adsorbed on Fe2+of the outer doublet. Infrared bands due to NO adsorbed on Fe3+need not be considered since only small amounts of Fe3+ are present in the reduced samples and since the results of this study indicate that NO does not adsorb to any appreciable extent on Fe3+. That NO does not adsorb to a significant extent on Fe3+is consistent with the behavior of Fe3+-Y zeoliten and c ~ - F e , 0upon ~ ~ exposure to NO at room temperature. In a related study of NO adsorption on Fe2+ in Y zeolite,,, it has been shown that a pair of bands at 1917 and 1815 cm-l is due to a dinitrosyl species. (The distinction between dinitrosyl species and (NO), dimers is not made in this paper. The term "dinitrosyl" is meant to denote two NO molecules associated with a single adsorption site.) Furthermore, upon room-temperature evacuation, these bands decreased in intensity while a band a t 1767 cm-' increased in intensity. This was due to the removal of one molecule of the dinitrosyl species, leaving a mononitrosyl species (at 1767 cm-'). A comparison of this behavior with that noted in the present study of NO on Fe2+/SiOZleads to the suggestion that the IR bands at 1910 and 1810 cm-' for Fe/SiO, are due to dinitrosyl species and that the band at 1750 cm-l is due to mononitrosyl species on the same Fe2+cations. The formation of dinitrosyl species has also been shown to occur for (28)C. H. Rochester and S. A. Topham, J. Chem. SOC.,Faraday Trans. 1 , 7 5 , 1259 (1979).
The Journal of Physlcal Chemistry, Vol. 86, No. 15, 1982 3029
alumina-supported Mo, W, and Cr by Kazusaka and H ~ w e for , ~ unsupported Cr203by Kugler et aL30 and for CoMo/A1203catalysts by Topsere and Top~ere.~'Steric considerations involved in the adsorption of two NO molecules on a single Fe2+cation suggest that the dinitrosyl species are formed from iron cations that are of low coordination before adsorption of NO. Such cations are Fe2+ of the inner doublet. Consistent with this conclusion is the observation*a of dinitrosyl species on Fe/SiO, samples which showed mainly the Fez+ inner doublet in their Mossbauer spectra. With the assignment of the pair of bands at 1910 and 1810 cm-' to dinitrosyl species and the single band at 1750 cm-' to mononitrosyl species on Fe2+of the inner doublet, it is suggested that the band at 1830 cm-' is due to NO on Fe2+of the outer doublet. This assignment is consistent with the NO stretching frequencies of known transitionmetal mononitrosyls.32 In particular, as the NO stretching frequencies increases, the Fe2+-N-0 bonding becomes more linear. Steric considerations suggest that NO adsorbed on a Fe2+cation of high coordination (such as Fe2+ of the outer doublet) would prefer to be in a linear configuration. In contrast, NO adsorbed on Fez+ of lower coordination (such as Fe2+of the inner doublet) would be free to assume a more bent configuration. This would give rise to an IR band at lower frequency (Le., 1750 cm-') for NO adsorbed on Fez+of the inner doublet. It should be noted that such arguments relating the NO stretching frequency with the coordination of Fe2+have also been used to interpret the IR bands observed upon exposure of Fe2+-Y zeolite to NOSn In addition, a similar correlation of the NO stretching frequency with the coordination of the iron cation was found by Tanabe et al.% in their study of iron supported on silica and tin oxide. Silica-supported samples which contain iron in both fourfold and sixfold coordination sites produced IR bands at 1810-and 1735 cm-' after NO adsorption; and a Sn0,-supported sample, which contains octahedral sites for the iron cations, produced only the IR band at 1810 cm-'. Further comparison between the infrared and the Mossbauer spectra gives additional support to the above assignments of specific IR bands to NO adsorbed on the different Fez+species observed by MBssbauer spectroscopy. After exposure of Fe/Si02 to NO followed by evacuation at progressively higher temperatures, NO is desorbed from the Fez+of the outer doublet at lower temperatures than it is from Fe2+ of the inner doublet. In the analogous infrared experiments, the intensity of the band at 1830 cm-' decreases at lower temperatures than the band at 1750 cm-'. This suggests, as proposed above, that the band at 1830 cm-l is due to NO associated with Fe2+of the outer doublet, while the band at 1750 cm-' is due to NO associated with Fe2+of the inner doublet. Indeed, one would expect the more highly unsaturated cations (i.e., Fe2+of the inner doublet) to more strongly adsorb NO than cations with lower coordinative unsaturation (i.e., Fe2+of the outer doublet). Carley et al.34also report that a highly linear mononitrosyl species (i.e., high-frequency IR band) and a partially bent mononitrosyl species (i.e., lower-frequency IR band) are formed upon exposure of a nickel film (29)A. Kazusaka and R. F. Howe, J. Catal., 63, 447 (1980). (30)E.L. Kugler, A. B. Kadet, and J. W. Gryder, J. Catal., 41, 72 (1976). (31)N.Topsee and H. Topsee, Bull. SOC.Chim. Belg., in press. (32)J. H. Enemark and R. D. Feltham, Coord. Chem. R e a , 138,339 (1974). (33)K. Tanabe, H. Ikeda, T. Iizuka, and H. Hattori, React. Kinet. Catal. Lett., 11, 149 (1979). (34)A. F. Carley, S. Raseias, M. W. Roberta, and T. H. Wang, J . Catal., 60,385 (1979).
sow m JOWMI
of phvsical mmbtry, VOI, 86, NO. 15, 1982
to NO, with the partially bent nitrosyl species being more strongly bound to the surface than the linear species. As Fe/Si02 is more severely reduced, all bands in the IR spectrum increase in intensity. Concomitant with these changes are the MWbauer spectroscopy observations that the inner doublet increases while the outer doublet decreases in intensity. Upon exposure of Fe/Si02 to NO at room temperature, all of the inner doublet in the Miissbauer spectrum is converted into a new doublet (i.e., NO associated with Fe2+of the inner doublet), while only a fraction of the outer doublet is converted into a new doublet (see Figure 3). Thus, essentially all of the Fe2+ cations of the inner doublet are capable of adsorbing NO. This is consistent with previous studies of adsorption on Fe/Si02 using such adsorbates as NH3, H20, and CH30H9J3and H2 and C0.14 It then follows that, as the amount of inner doublet increases in the Mossbauer spectrum before NO adsorption, the IR bands at 1910, 1810, and 1750 cm-I obtained after exposure to NO should increase in intensity. Not all of the Fe2+ of the outer doublet, however, is capable of adsorbing NO, as seen in Figure 3. Yet, as the severity of the reduction treatment increases, the dispersion of the Fe2+contributing to the outer doublet may increase. This can be seen by comparing the Mossbauer spectra of the Fe/Si02 samples before and after exposure to NO at room temperature. The fraction of the outer doublet that is perturbed upon exposure of Fe/Si02 to NO increases from ca.0.70 to 0.90 when the temperature of the H2 reduction is increased from 498 to 653 K. Thus, while the amount of outer doublet decreases with increasing severity of reduction, the dispersion of the Fe2+ cations contributing to the outer doublet increases, thereby explaining the corresponding increase in intensity of the IR band at 1830 cm-I. Interactions between Iron Oxide and Silica. To obtain information about the interactions between iron oxide and silica, it is necessary to interpret more quantitatively the origin ot the inner and outer doublets in the Mossbauer spectrum of reduced Fe/SiOz. In one possible explanation, the inner and outer doublets are attributed to surface and bulk iron cations, respectively, of small particles of iron oxide. This interpretation, however, is not consistent with the conversion of outer doublet into inner doublet as the reduction of Fe/Si02 becomes more severe. This was noted previously by Hobson and Gager,lo Clausen,12and Clausen et al.14 This interpretation is also inconsistent with the observation of the present study that the recoil-free fraction of the inner doublet is greater than that of the outer doublet, since surface atoms are expected to have greater mean square vibrational amplitudes (and hence smaller recoil-free fractions) than bulk atoms. This study also shows that a portion of the Fe2+cations which contribute to the outer doublet are capable of adsorbing NO. This could not be explained if the outer doublet were due to Fe2+cations in the bulk of iron oxide particles. In a second possible interpretation of the MBssbauer spectra, it may be imagined that both the inner and outer doublets are due to Fe2+cations at the surface of small iron oxide particles. This would explain the conversion of outer doublet into inner doublet, as the reduction of Fe/Si02 becomes more severe, in terms of surface dehydroxylation for example. This model is also consistent with the observation that the outer-doublet cations are capable of adsorbing NO. The iron oxide particles of this model, however, must be very small with essentially all iron cations being at the surface, since moat of the outer doublet can be converted to inner doublet by reduction at high temperatures. Remaining unexplained by this model is
Yuen et al.
why only a portion of the outer-doublet cations are capable of adsorbing NO and why the inner doublet has a greater recoil-free fraction than the outer doublet. To remedy these deficiencies of the above models, a third interpretation of the Mossbauer spectra is proposed. The essential feature of this model is the assignment of the inner doublet to Fe2+interacting with the silica support. This is similar to the proposal of Blomquist et a1.: except that these authors interpreted the inner doublet in terms of Fe3+. However, in view of the results that a variety of gases, when chemisorbed on Fe/Si02, convert the inner doublet into outer doublet (the latter being definitely Fe2+),9J2-14 the assignment of the inner doublet to Fe3+seems doubtful. Furthermore, the results of the present study indicate that NO does not adsorb to a significant extent on Fe3+. A strong interaction between Si02 and Fe2+of the inner doublet would explain why this iron shows a higher recoil-free fraction than the outer doublet. Analogously, the lower recoil-free fraction of the outer doublet can be explained by assigning this doublet to Fe2+ cations in small particles of iron oxide. In this way, the observed adsorption of NO on a portion of the Fe2+cations contributing to the outer doublet can be attributed to those cations on the surface of these small iron oxide particles. To explain the conversion of outer doublet into inner doublet during reduction of Fe/Si02, it is proposed that the size of the iron oxide particles decreases with increasing severity of reduction. That is, more severe reductions lead to more extensive interaction between the iron oxide particles and the support. This could be the result of surface dehydroxylation or it could be due to spreading of the iron oxide particles over the support surface. In either case, Fe-0-Si linkages are formed at the expense of Fe-0-Fe linkages, and outer-doublet Fe2+cations are converted into inner-doublet Fe2+cations. As a result of this process, the dispersion of the iron oxide particles would increase, thereby explaining the Mossbauer and IR spectrscopy results which indicate that the dispersion of the outer-doublet Fe2+cations increases as the reduction of Fe/Si02 becomes more severe. The above conversion of outer-doublet Fe2+to inner-doublet Fe2+may be reversed by a oxygen-hydrogen cycle at ca. 500 K. For example, the Mossbauer spectrum of Figure lb, which contains 74% outer doublet, can be obtained by oxidizing a sample which contains 50% outer doublet, followed by H2 reduction at 500 K. It may be questioned why the surface and subsurface iron cations of the aforementioned iron oxide particles cannot apparently be distinguished in the Mossbauer spectra collected before exposure of Fe/Si02 to NO. For this reason, it may be proposed that the iron cations at the surface of these particles are not coordinatively unsaturated, perhaps because of hydroxyl groups or a full oxygen capping layer at the surface. In this way, all of the iron cations of the particles would be highly coordinated (e.g., surrounded by six ligands). Adsorbed nitric oxide, however, is able to bond with iron cations by reconstructing the surface. Precedent for this ability of NO to bind with otherwise inaccessible cations can be found in the studies of nitric oxide adsorption of Fe2+-Y zeolitezzand a series of supported cobalt samples.35 In the former case, NO causes migration of Few from sites in the hexagonal prisms to more accessible sites in the supercages; and in the latter case, NO adsorbs preferentially on Co2+cations which are octahedrally coordinated before interaction with NO. The above model of reduced Fe/Si02 may be briefly by stating that the inner doublet is due to Fez+ (35) N. Topsm and H. Topsm, submitted for publication in J. Catal.
Metal Oxide-Support Interactions
cations strongly interacting with the silica support, while the outer doublet is due to Fe2+cations in small iron oxide particles. Evidence that small iron oxide particles exist on these samples was shown by the appearance of broad peaks in the EPR spectrum due to exchange interactions between neighboring cations.% Furthermore, evidence that these particles are associated with the outer doublet can be found in the work of Clausen.12 This author found that metallic iron particles could be produced by prolonged hydrogen reduction of Fe/Si02 at high temperatures. Moreover, this metallic iron was formed from Fe2+of the outer doublet. It is reasonable to suppose that Fe2+of the inner doublet is not reduced to the metallic state because of the interaction of these cations with the silica support. Instead, metallic iron particles would be formed by reduction of iron oxide particles, thereby explaining why it is the outer doublet that leads to the appearance of metallic iron upon prolonged reduction. It must be remembered, however, that these iron oxide particles are in intimate contact with the silica support, in order to explain their slow reduction to the metallic state upon treatment in hydrogen. In related experiments, Clausen12showed that increased reduction times at constant temperature led to increased amounts of inner-doublet from outer-doublet Fe2+. Thus, the reduction of outer-doublet Fe2+to metallic iron is in competition with the formation of a strong interaction between Fe2+and the silica support, manifested by the production of inner-doublet Fe2+. Also relevant to the present study is the observation by Clausen that the metallic iron particles formed during the early stages of reduction were larger than those particles formed during the later stages of reduction. This is in constrast to nucleation and growth phenomena for which the opposite behavior would have resulted. Instead, the smaller metallic iron particles are formed later during reduction because they stem from reduction of smaller iron oxide particles. These smaller iron oxide particles are in greater contact with the silica support and hence are less reducible than are larger iron oxide particles. Evidence that the inner-doublet iron cations in Fe/Si02 are in iron-rich clusters (in contrast to or in addition to being uniformly dispersed across the silica surface as isolated cations) is found in the volumetric adsorption and IR spectral band intensity changes vs. reduction treatment. In particular, the amount of irreversible NO adsorption increases with increasing reduction severity. In addition, it can be seen in Figures 6-8 that the IR intensities of the dinitrosyl bands (at 1910 and 1810 cm-l) increase slightly with increasing reduction severity, while the intensity of the mononitrosyl band at 1750 cm-l increases markedly with increasing reduction severity (for spectra collected before room-temperature evacuation). The dinitrosyl species and the low-frequency (i.e., 1750 cm-’) mononitrosyl species are both assigned to iron cations of low coordination, and it must be concluded that the latter are favored over the former as the reduction of Fe/Si02 becomes more severe. This could be explained if the iron cations of low coordination (i.e., Fe2+of the inner doublet) were present in thin “rafts”, strongly interacting with the SiOz support. This may correspond, for example, to formation of a surface iron silicate phase, and it does not exclude the possibility that some of the inner-doublet Fe2+ may be present as isolated cations on the support. Evidence for the existence of iron rafts in reduced Fe/Si02 samples has, in fact, been reported by Clausen et al.12 in a study employing transmission electron microscopy. The (36) R. F. Howe, Deptartment of Chemistry, University of Wisconsin-Milwaukee, personal communication.
The Journal of Physical Chemistty, Vol. 86, No. 15, 1982 3031
W
F@m 9. Schematic representation of iron oxide on smCa at krcreaslng reductlon 88verity. CrossAatched, dotted, and black regions repregent outerdoublet Fe?+,InnerdoubletFe2+, and metallic Iron, respecthrely.
iron cations at the edges of these rafts would be able to form dinitrosyl species upon exposure of the sample of NO, while those cations on the faces of these rafts would be unable to form these dinitrosyls because of steric interactions between NO molecules adsorbed on neighboring Fe2+cations. As the reduction of Fe/Si02 becomes more severe, outer-doublet Fe2+is converted into inner-doublet Fe2+in the Mossbauer spectrum. It is possible that, as more Fe2+cations of the inner doublet are produced, the size of thin rafts in strong interaction with the support would increase, causing an increase in the relative number of face to edge cations in the rafts. Thus, the observation that increased reduction severity leads to a greater increase in the number of mononitrosyl species, compared to dinitrosyl species, on Fe2+ cations of low coordination is thereby explained. It should be noted that room-temperature evacuation of the Fe/Si02 sample reduced in H2 at 498 K removes nearly all of the dinitrosyl IR bands at 1810 and 1910 cm-’, while these bands persist after roomtemperature evacuation of the Fe/Si02 that had experienced more severe reduction. This behavior is not completely understood at present. (It can be suggested, though, that the dinitrosyl species present on the iron cations at the edges of the above rafts are more strongly adsorbed when the neighboring regions of the silica support are more completely dehydroxylated.) Further evidence for the presence of a highly dispersed Fe2+phase having a high degree of interaction with the silica support and a phase consisting of small iron oxide particles comes from the Mossbauer results depicted in Figure 5 for treatment of reduced 1% Fe/Si02 in an NO atmosphere at elevated temperatures. The ease of oxidizing inner-doublet Fe2+to Fe3+ (in sites of high distortion) as compared to the oxidation of outer-doublet Fe2+ to Fe3+ (in sites of lower distortion) is consistent with a highly accessible inner-doublet phase and a less accessible outer-doublet phase. One possible schematic representation of the above model for the interaction between iron oxide and silica is shown in Figure 9. As one looks down onto the support surface, the cross-hatched regions represent iron oxide particles consisting of outer-doublet Fe2+and the dotted regions represent inner-doublet Fe2+interacting with silica.
J. PhyS. Chem. 1982,86,3032-3038
3032
This inner-doublet Fe2+may be present as isolated cations on the support or it may be clustered into rafta on the silica near the particles of outer-doublet Fe2+. As the severity of reduction increases, outer-doublet Fe2+is converted into inner-doublet Fe2+,and metallic iron particles (represented by the black region) may be formed during severe reduction treatments such as prolonged exposure to hydrogen at 700 K.
Conclusions The combination of Mossbauer spectroscopy, infrared spectroscopy, and volumetric/gravimetric adsorption measurements suggest that two states of Fe2+exist in reduced Fe/Si02 samples: Fe2+strongly interacting with the support and Fe2+ in small particles of iron oxide. The former cations are of low (e.g., fourfold) coordination, giving rise to the inner doublet in the Mossbauer spectra and to bands at 1910, 1810, and 1750 cm-’ in the IR spectrum of adsorbed NO. The bands a t 1910 and 1810 cm-’ are due to dinitrosyl species, and the band a t 1750 cm-’ is due to mononitrosyl species on these iron cations of low coordination. It is suggested that these iron cations are present as thin “rafts” on the silica support (although this does not exclude the possibility that some of the inner-doublet Fe2+ may exist as isolated cations on the support). All of the cations in these rafts are capable of adsorbing NO; however, for steric reasons, dinitrosyl species are formed at the edges and mononitrosyl species are formed on the faces of these rafts. The Fe2+in the small particles of iron oxide are of high (e.g., sixfold) co-
ordination, giving rise to the outer doublet in the Mossbauer spectra and the band at 1830 cm-’ in the IR spectrum of adsorbed NO. Cations on the surface of these particles are capable of adsorbing NO, while those cations beneath the surface do not participate in adsorption. These particles are also in intimate contact with the silica support in order to explain their stability against reduction to the metallic state during hydrogen reduction. In competition with this reduction to metallic iron, these iron oxide particles are converted into the Fe2+rafts in strong interaction with the support. Consequently, the dispersion of the iron oxide particles increases, and a greater fraction of the iron cations remaining in high coordination are capable of adsorbing NO. This increased dispersion, coupled with the production of Fe2+of low coordination in strong interaction with the support, causes the NO adsorption uptake of Fe/Si02 to increase with increased reduction severity.
Acknowledgment. We acknowledge stimulating discussions with R. F. Howe (University of Wisconsin-Milwaukee) and B. S. Clausen (Haldor Topsoe Research Laboratories) regarding nitrosyl complexes and Fe/Si02, respectively. In addition, we thank Terrence Udovic (University of Wisconsin-Madison) for assistance in computer subtraction of IR spectra. This work was supported, in part, by the National Science Foundation through grant ENG-7911130, for which we are grateful. Finally, J.A.D. acknowledges the cooperation and hospitality of the Haldor Topsoe Research Laboratories.
Formation and Properties of Large Aggreqates in Concentrated Aqueous Solutions of Ionic Detergents E. Lemner’ and J. Frahm Mex-plsnck-InsMM fuer blqhyskalbche Chemk 03400 Qoefflngen, West Germny (Received: December 15, 198 1;
I n Finel Form: March 24, 1982)
In the first part of this paper kinetic experiments on concentrated aqueous solutions of ionic detergents are presented. At high concentrations the relaxation amplitudes show a distinct maximum, in quantitative contradiction to the theory. It is suggested that the sudden increase of the amplitudes is attributed to the onset of formation of large nonspherical aggregates. This suggestion is supported by comparison with measurements of the concentration dependence of the mean aggregation number m by other authors, which show a sudden increase of m at the same counterion concentration. In the second part of this paper ‘HNMR experiments on the same systems are presented. Again, the line widths show a sudden increase at the same counterion concentration. The TIand T2relaxation time profiles show three distinct regions: the monomer region, the region of ‘spherical” micelles, and that of large aggregates. The analysis of the profiles leads to the conclusion that for spherical micelles any motions other than rapid local conformation changes must be rather restricted. For the large aggregatea the sudden change of the T2profiles indicates that the rapid motions are superimposed by a slower motion, possibly of several segments of the chains or even of whole molecules. I. Introduction In a recently published paper1 we have suggested that ionic micelles may be considered as charged colloidal particles. At lower counterion concentration these particles are stable with respect to coagulation due to the repulsive electrostatic forces. Consequently, they grow by stepwise incorporation of monomers according to the reaction (1) Leeener, E.; Teubner, M.; Kahlweit, M. J. Phys. Chem. 1981,86 1529, 3167. See also: Kahlweit, M. Pure Appl. Chem. 1981,53, 2069.
0022-3654/82/2086-3032$01.25/0
Ni-1 + N1+ Ni (1.1) With increasing counterion concentration, however, the electric double layer around each particle becomes increasingly compressed, so that the attractive dispersion forces can lead to a reversible coagulation reaction Nk + Ni* Ni k+1=i (1.2) where k and 1 are classes of submicellar aggregates, i.e., of aggregates on the leftrhand side of the minimum of the size distribution, whereas i is a class of the proper micelles, Le., 0 1982 American Chemical Society
