Langmuir 1988,4, 646-653
646
nants as is PDPL adsorption. 4. 2,5-Dihydroxy-4-methylbenzylMercaptan (DMBM).Studies by means of polycrystalline Pt thinlayer electrodes have demonstrated that thiophenol derivatives bind to Pt surfaces through the sulfur atom with the aromatic ring perpendicular to the surface.'12 In order to compare the behavior of horizontally oriented adsorbed phenols such as HQ and PL with analogous vertically oriented adsorbates, studies of DMBM were included in this work. Auger and EELS spectra of DMBM appear in Figures 1D and 8D. Packing densities were obtained from C, 0, and S Auger signals by means of eq 26: r c = (Ic/Iopt)/[Bc(1/2
+ 7fc/16 + fc2/16)]
(26)
where IoRwas measured at the positive lobe of the Pt signal at 235 eV to minimize the effect of overlap with other peaks in the spectrum: Bc = 0.848 cm2/nmol and fc = 0.70. Molecular packing density, I', was also determined from attenuation of the positive lobe of the Pt Auger signal at 235 eV:
Ipt/Iopt = (1 - Ksr)(i - 5 ~ ~ 1 3 2
(27)
where Kc = 0.153 and Ks = 0.219 cm2/nmol. Packing densities of 0 and S were obtained from Auger signals due to 0 and S:
r o = (Io/Iopt)/[Bo(l/2
-k fo/4
+ fo2/4)I
rs = (Is/Iopt)/(Bsfs)
nmol/cm2, compared with a theoretical packing density from covalent and van der Waals radii" of 0.399 nmol/cm2 (41.7 A2/molecule). The small peak at 1574 cm-' is an aromatic CC stretch. Although the S-H band is not particularly strong in the IR spectra of solid thiophenols,16 the absence of this band from the EELS spectrum is evidence that the mercaptan hydrogen is removed as a result of the adsorption process:
""a -1 Horn;; ;, 0
t
H2iH
H' + e-
(30)
H28
7577-7Adsorbed DMBM is stable in contact with solution and vacuum: the electrode potential was constant at open circuit, at least on the time scale of the measurements (about 1 h). Furthermore, when the layer was removed from solution, evacuated under UHV for about 1 h, and then transferred from vacuum back into solution, a positive-going scan from the open-circuit potential produced the usual voltammetric peak indicative of adsorbed DMBM?
(28) (29)
where Bo = 1.27 cm2/nmol, Bs = 18.6 cm2/nmol, fo = 0.70, and fs = 0.62. The molecular packing density of DMBM from eq 26 is 0.373 nmol/cm2 and from eq 27 is 0.386
Acknowledgment. Acknowledgment is made to the Air Force Office of Scientific Research for support of this work. Registry No. HQ, 123-31-9; BQ, 106-51-4;PL, 108-95-2; TFHQ, 771-63-1; DMBM, 81753-11-9.
Mossbauer Spectroscopic Studies of Ferrisilicates Zhu Yixiang,? Luis M. Aparicio, and J. A. Dumesic* Department of Chemical Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53706 Received September 10, 1987. In Final Form: December 11,1987 Crystalline and amorphous ferrisilicates were characterized by using Mossbauer spectroscopy to probe the coordination of iron cations in these materials. The trivalent iron originally present in the ferrisilicates could be reduced to divalent iron by treatment with Hz at 700 K. No metallic iron was observed after the reduction pretreatment, except in samples with iron loadings greater than about 5 wt %. In the trivalent and divalent states, coordinatively unsaturated iron accessible to adsorbate gases could be distinguished from coordinatively saturated iron. The fraction of iron found in coordinatively unsaturated sites was found to decrease with iron loading. Nevertheless, it was possible to prepare a sample in which approximately half of the iron was present in low coordination sites at a total iron loading of 3.6 wt %.
Introduction The study of interactions between dispersed metals and supports of metal catalysts is an important area of heterogeneous catalysis. Strong interactions can also be expected, however, between transition-metal oxides and oxidic supports, due to the structural and chemical similarity between these materials. One example of such an interaction has been found for iron oxide supported on silica (Fe/SiOz). Whereas bulk iron oxide is reduced to
* Author to whom correspondence should be addressed. 'Present address: Department of Chemistry, Xiamen University, Xiamen, Fujian, China.
metallic iron by high-temperature treatment with H2, supported iron cannot be reduced below the divalent state by the same treatment.l Supported iron oxide has also been found to be a less active catalyst for water-gas shift as compared to bulk magnetite2 Infrared and Mbssbauer spectra collected after adsorption of NO on Fe/Si02 were interpreted in terms of a strong interaction between ferrous cations and the support, leading to the stabilization of iron cations in sites of low c~ordination.~ (1) Berry, F. J. Adv. Inorg. Chem. Radiochem. 1978,21, 255. (2) Rethwisch, D. G.; Dumesic, J. A. J. Catal. 1986, 101, 35. (3) Yuen, S.; Chen, Y.; Kubsh, J. E.; Dumesic, J. A.; Topme, N.; Topme, H. J. Phys. Chem. 1982,86, 3022.
0743-7463f 88f 2404-0646$01.50f 0 0 1988 American Chemical Society
Langmuir, Vol. 4, No. 3, 1988 647
Mdssbauer Spectroscopy of Ferrisilicates To understand the nature of such metal oxide/support interactions, it is useful to compare the properties of materials in which the same transition metal is bonded to oxygen in a variety of coordination environments. For example, insight into these interactions can be obtained by comparing the properties of Fe/Si02 with those of iron-exchanged zeolites. In these zeolites, the iron is present as isolated cations located in well-defined sites whose geometries are known from X-ray diffraction studies. Mbsbauer spectroscopy studies of ferrisilicates were undertaken in the present study to complement the available data on Fe/ Si02 and iron-exchanged zeolites. The goal was to observe what changes in the redox and chemisorptive properties of iron could arise from changes in its coordination environment. A second reason for undertaking the present studies was to probe whether ferrisilicates may have potentially useful catalytic properties. Iron cations in Fe/SiOz have been found to act as adsorption centers for H20, NH,, CH30H,4 NO>5 and pyridine.6 In a similar way, iron cations exchanged into Y-zeolite have been found to act as adsorption centers for HzO, NH3, CH30H,” CO,8 and N0.9J0 Furthermore, iron-exchanged Y-zeolite (Fe-Y) has been found to catalyze the oxidation of CO by 02,NO, or N20 and the decomposition of N20 into ita elements.l1-l3 It is reasonable to expect that iron in ferrisilicates may have similar catalytic and chemisorptive properties. One advantage of developing ferrisilicate catalysh would be that samples with high iron loadings could be prepared without the formation of metallic iron under reducing conditions. In Fe/Si02, metallic iron forms when the loadings are higher than ca. 3 w t % .14J5 Mossbauer spectra obtained in our laboratory have shown that metallic iron is formed in Fe-Y when the loading is greater than ca. 7 w t %. Experimental Section Four different ferrisilicate samples were investigated. The preparation of each sample is described separately below. The first sample studied was a crystalline molecular sieve ferrisilicate analogue of the zeolite ZSM-5. This sample was provided by Professor Szostak of Georgia Tech University. The preparation of this ferrisilicate analogue of ZSM-5 has been described elsewhere.16 Chemical analysis of this sample, done by Galbraith Laboratories, gave an iron loading of 1.73% by weight and a SiOz/Fez03ratio of 90 on a molar basis. A second sample was prepared in an attempt to synthesize a ferrisiicate analogue of ZSM-5 in our laboratory. To prepare this sample, the following solutions were made: solution A 25 e N-brand silicate 25 HZO solution B 1.34 g FeC13.7H20 2.65 e H,SO, 42.5; Hi0 solution C 3.25 g tetrapropylammonium bromide 5.00 g HzO ~
(4) Hobson, M. C., Jr.; Gager, H. M. J.Colloid Interface Sci. 1970,34, 357. (5) Rsthwisch, D. G.; Dumesic, J. A. J. Phys. Chem. 1986,90, 1625. (6) Connell, G.; Dumesic, J. A. J. Catal. 1986, 101, 103. (7) Delgass, W. N.; Garten, R. L.; Boudart, M. J. Phys. Chem. 1969, 73,2970.
(8)Aparicio, L. M.; Dumesic, J. A.; Fang, S. M.; Long, M. A.; Ulla, M. A.: Millman.W.S.: Hall. W. K. J. Catal. 1987. 104. 381. ‘(9) Segawa, K.; Chen, Y.; Kubsh, J. E.; Delgk, W: N.; Dumesic, J. A.; Hall. W.K. J. Catal. 1982. 76. 112. (io) Aparicio, L. M.; Fkg,’S. M.; Millman, W. S.; Hall, W. K.; Dumesic, J. A. J. Catal., submitted for publication. (11) Fu, C. M.; Deeba, M.; Hall, W. K. Znd. Eng. Chem. Prod. Res. n p-t J . 1980 - - - -, -79 - , 2~ -- -. (12) Petunchi, J. 0.;Hall, W. K. J. Catal. 1982, 78,327. (13) Fu, C. M.; Korchak, V. N.; Hall, W. K. J. Catal. 1981, 68,166. (14) Dumesic, J. A.; Topsoe, H. Adv. Catal. 1977,26, 121. (15) Yoshiol(a, T.; Koezuka, J.; Ikoma, H. J. Catal. 1970, 16, 264. (16)Szostak, R.;Thomas, T. L. J. Catal. 1986,100,555.
Solution A was titratad into a well-mixed plastic flask containing solution B. The color of the solution changed from green to brown as the titration proceeded, and it became pale brown when the titration was nearly complete. At this point solution C was poured into the flask, followed by the remaining 1-2 mL of solution A. After solution C was added, the entire solution gelatinized. The plastic flask containing the gelatinous material was then kept at 368 K for 2 weeks in an oven. Finally, the product was filtered, dried in air at 373 K, and ground to a yellowish powder. X-ray diffraction analysis showed that this material was amorphous. It is possible that the gelatinous material did not crystallize because ita iron content was too high or because temperature control in the oven was inadequate for crystallization. Chemical analysis of the sample showed that it contained 3.60% Fe by weight, and it had a Si02/Fe203ratio of 48. A third sample was prepared without the addition of a crystal-directing agent. To make this sample, the following solutions were prepared solution A solution B
25.0 g 25.0 g 3.40 g 2.61 g 42.5 g
N-brand silicate HZO Fe(N03)3.9H20 &So4
HZ0
Solution A was titrated into a well-mixed plastic flask containing solution B. During this process, a solution of KOH was added whenever the green solution started to turn brown, and the final product was a green gelatinous material. This product was heated for 5 days at 368 K, filtered, washed with distilled water, dried in air at 373 K, and fiially ground to a white powder. Chemical analysis showed that this sample had 5.41 wt % Fe and a Si02/Fe203molar ratio of 30. In subsequent experiments, the amount of Fe(N03),.9Hz0 used waa increased from 3.40 to 3.82 g. Yellow powders were obtained in all cases, suggesting that some Fez03 had formed. Thus, we believe that 5.4% is near the maximum Fe loading that can be incorporated into silica without the formation of Fez03 by using the above procedure. A fourth sample was prepared to study ferrisilicateswith higher iron loadings. For this sample, the following solutions were made: solution A solution B
25.07 25.0 g1.87 g
2.62 R 42.5
solution C
N-brand silicate NZO Fe(N03)3.9H20 H,SOd Hi0 . KOH + H20
Solution A was titrated into solution B, forming a green gelatinous material. This gel was held at 368 K for 24 h, after which a suitable amount of solution C was added to dissolve the gelatinous material, forming a green solution. This solution was maintained at 368 K until a white precipitate formed. This material contained 15.39 wt % Fe, and it had a SiOz/Fe203molar ratio of 8. Samples for Mossbauer spectroscopy studies were made by pressing the ferrisilicates into wafers that were 1.8cm in diameter and weighed ca.230 mg. The exception was the untreated ZSMd analogue for which these values were 3.8 cm and 600 mg. The wafers were placed in a Mossbauer cell described previously, which allowed sample treatments in flowing gases from room temperature to 700 K and collection of Mossbauer spectra at temperaturea from 77 to 700 K and pressures from c a 1FSto Id Pa8 The spectra of this study were collected at room temperature. All isomer shifts reported in this study are relative to metallic iron at room temperature. The effects on the Mossbauer spectra of NO adsorption were studied for several samples. Introduction of NO into the above cell was accomplished by using a 200-mL glass bulb containing ca. 2.5 kPa of NO. This bulb was attached to the cell via a glass vacuum system.
Results Crystalline Ferrisilicate (1.73 wt % Fe). Room temperature Mossbauer spectra of the ZSM-5 ferrisilicate analogue after various treatments are shown in Figure 1.
648 Langmuir, Vol. 4, No. 3, 1988
-2
-4
Yixiang et al.
2
0
4
VELOCITY ( m m / s )
Figure 2. Mijssbauer spectra of amorphous ferrisilicate (3.60 wt % Fe) after dehydration and oxidation: (a) untreated, (b) after evacuation at 420 K, (c) after treatment with O2at 620 K. Bar shows 0.2% absorption.
Table I. Room Temperature Mossbauer Parameters for Spectra of ZSM-5 Zeolite Analogue (1.73 wt % Fe) in IS,
treatment
mm/s
untreated
0.26 0.32 0.29 0.31
0.40 0.90 1.89 1.09
1.13 0.68
1.67 0.93
700 K,0
2
mm/s Fe3+
mm/s
relative areas, %
0.36 0.78 0.70 1.01
8.0 92.0 37.0 63.0
0.56 0.68
41.4 58.6
The spectrum in Figure IC was obtained following treatment in H2 at 700 K for 3 h. This spectrum could be fit with two doublets characteristic of Fez+,indicating that the iron in the ZSM-5 ferrisilicate analogue could be reduced to Fe2+ by high-temperature treatment with H2. The adsorption of NO on this sample in its oxidized state was also studied. A spectrum was collected after an oxidized sample was evacuated to Pa at room temperature, followed by exposure to 660 Pa of NO. Although the spectrum did not change dramatically upon adsorption of NO, computer fits summarized in Table I1 suggest that the doublet with the larger QS (denoted as the outer doublet) was more perturbed during the adsorption process than the doublet with the smaller QS (denoted as the inner doublet). Therefore, the spectra suggest that the outer doublet represents ferric cations that are accessible to the gas phase. This is the same conclusion reached above with respect to interaction of the sample with water. The inner doublet, on the other hand, probably represents cations that are inaccessible to gaseous molecules. Amorphous Ferrisilicate (3.60 wt % Fe). Figure 2 shows spectra collected for this sample after various dehydration pretreatments. Figure 2a is the spectrum of the untreated sample. Spectrum 2b was obtained following dehydration at 420 K for 5 h under vacuum, and spectrum 2c was obtained following subsequent treatment at 620 K for 6 h in 02. These spectra have been fit with two
Fez+ 700 K,Hz
The corresponding Mossbauer parameters are listed in Table I. Figure l a is a spectrum of the untreated sample. This spectrum was fit with two Fe3+ doublets. The spectrum in Figure l b was obtained after dehydration of the sample under vaccum at 520 K for 1h, 620 K for 0.5 h, and 700 K for 0.5 h, followed by treatment with O2 at 700 K for 1h. This spectrum was also fit with two Fe3+ doublets. I t is interesting to note that after dehydration the quadrupole splitting (QS) of the doublet that has the lower isomer shift (IS) increased dramatically; in contrast, the QS of the other doublet showed little change. This suggests that the doublet with the lower IS represents iron cations that are accessible to HzO. The large QS of this doublet after dehydration indicates that the cations giving rise to this doublet are in sites of low symmetry.14
Table 11. Room Temperature Mdssbauer Parameters for Spectra of Oxidized ZSM-5 Zeolite Analogue and after NO Adsorption IS,
atmosphere 1 atm O2 5 Torr NO
Fe3+outer doublet line width,
QS,
mm/s
mm/s
mm/s
0.29 0.29
1.89 1.89
0.70 0.52
area,
%
2
0
VELOCITY ("1s)
Figure 1. Mbssbauer spectra of crystalline ferrisilicate (1.73 wt % Fe): (a) untreated, (b) after treatment with O2at 700 K, (c) after treatment with H2at 700 K. Bar shows 0.4% absorption.
Figure 1 QS, line width,
-2
-4
xmm/s 0.51 0.21
IS,
(1.73 wt % Fe) before
FeS+inner doublet line width,
QS,
mm/s
mm/s
mm/s
0.31 0.31
1.09 1.09
1.01 0.87
area,
% X mm/s 0.87 0.85
Langmuir, Vol. 4, No. 3, 1988 649
M6ssbauer Spectroscopy of Ferrisilicates
Table 111. Room Temperature Massbauer Parameters for Spectra of Ferrisilicate (3.60 wt % Fe) in Figure 2 treatment species IS, mm/s QS, mm/s line width, mm/s relative areas, % untreated 420 K,vacuum 670 K,0 2
Fe3+(outer) Fe3+(inner) Fe3+(outer) Fe3+ (inner) Fe3+(outer) Fe3+ (inner)
0.32 0.17 0.31 0.28 0.31 0.28
0.91 0.36 0.88 0.59 0.74 0.61
0.85 0.33 1.20 0.68 1.49 2.19
92.0 8.0 68.2 31.8 51.4 43.6
Table IV. Room TemDerature Mossbauer Parameters for SDectra of Ferrisilicate (3.60 wt % Fe) in Figures 3 and 4 cycle 1st
2nd
3rd
4th
treatment 670 K,6 h in O2
Figure 3a
670 K,5.5 h in H2
4a
420 K,2 h in O2
3b
670 K, 4 h in H2
4b
420 K,3 h in O2
3c
670 K, 5 h in H2
4c
670 K,5 h in O2
3d
670 K, 2 h in H2
4d
species Fe3+ (outer) Fe3+(inner) Fe3+(outer) Fe3+(inner) Fez+ (outer) Fez+(inner) Fe3+ (outer) Fe3+ (inner) Fe3+(outer) Fea+ (inner) Fe2+(outer) Fe2+(inner) Fe3+ (outer) Fe3+ (inner) Fe3+ (outer) Fe3+ (inner) Fe2+(outer) Fe2+(inner) Fe3+(outer) Fe3+ (inner) Fe3+ (outer) Fe3+ (inner) Fe2+(outer) Fe2+(inner)
IS, mm/s 0.28 0.31 0.28 0.31 1.02 0.75 0.28 0.31 0.28 0.31 1.02 0.75 0.28 0.31 0.28 0.31 1.02 0.75 0.28 0.31 0.28 0.31 1.02 0.75
line width, mm/s 0.61 0.74 0.55 0.65 0.71 0.51 0.61 0.76 0.53 0.67 0.70 0.49 0.56 0.73 0.51 0.63 0.72 0.54 0.55 0.72 0.75 0.85 0.69 0.48
QS, mm/s 2.19 1.49 2.19 1.49 1.80 0.89 2.19 1.49 2.19 1.49 1.80 0.89 2.19 1.49 2.19 1.49 1.80 0.89 2.19 1.49 2.19 1.49 1.80 0.89
relative areas, % 43.6 56.4 9.6 7.6 45.2 37.6 39.4 60.6 6.8 9.2 52.6 31.4 32.2 67.8 7.2 7.8 53.6 31.6 27.6 72.4 3.0 3.8 64.8 28.4
doublets, the parameters of which are given in Table 111.
As was observed for the ZSM-5 ferrisilicate analogue, the QS of the doublet with the lower IS increases significantly upon dehydration. This indicates that this doublet represents iron cations that are accessible to H20. Figures 3 and 4 show spectra collected after repeated oxidation-reduction treatments of this ferrisilicate sample. In particular, Figure 3 shows spectra collected after successive oxidation treatments, while Figure 4 shows the correspondingspectra following reduction treatments. The computer-fitted Mossbauer parameters for these spectra and the treatment conditions during these oxidation-reduction studies are summarized in Table IV. The spectra of Figure 3 were fit with two Fe3+ doublets, while the spectra of Figure 4 were fit with these same doublets plus two doublets due to Fe2+. The doublet with the smaller QS in each case is denoted as the inner doublet, while the doublet with the larger QS is denoted as the outer doublet. It is interesting to note that the relative area of the Fe3+ outer doublet in Figure 3 (i.e., the doublet due to iron cations that are accessible to the gas phase) decreases with successive oxidation-reduction cycles, and this is matched by a corresponding decrease in the relative area of the Fe2+ inner doublet in Figure 4. This suggests that the Fe3+outer doublet and the Fez+inner doublet represent the same iron site. A similar relationship exists between the Fe3+inner doublet and the Fez+ outer doublet. The Fe3+ outer doublet and the Fez+ inner doublet both have a low IS, suggesting that they represent iron cations in sites of low coordination.'J' Various authors have discussed the relationship between the QS of iron cations and the symmetries of the iron sites.14 In general, the Q S of Fe2+ increases with increasing lattice symmetry, whereas the (17)Heller-Kallai, L.; Rozeson, I. Phys. Chem. Miner. 1981, 7, 223.
>
t
v)
z W c
4 w
2c a
-I
w
a
-4
-2
0
2
4
VELOCITY ( m m / s )
Figure 3. M6ssbauer spectra of oxidized amorphous ferrisiiicate (3.60wt % Fe) after repeated redox cycles: (a) after first cycle, (b) after second cycle, (c) after third cycle, (d) after fourth cycle.
Bar shows 0.4% absorption.
QS decreases with increasing symmetry for Fe3+. Accordingly, the results of the present study suggest that iron
650 Langmuir, Vol. 4, No. 3, 1988
Yixiang et al.
Table V. Room Temperature MBssbauer Parameters for Spectra of Oxidized Ferrisilicate (3.60 wt % Fe) before and after Treatment with Haat 420 K pretreatment 670 K, 6 h in O2 420 K, 2 h in H2
species Fe3+(outer) Fe3+(inner) Fe3+(outer) Fe3+(inner) Fez+(outer) Fez+ (inner)
IS, mm/s
QS, mm/s
line width, mm/s
relative areas, %
0.28 0.31 0.28 0.31 1.02
2.19 1.49 2.19 1.49 1.80 0.89
0.61 0.76 0.36 0.81 0.86 0.46
39.4 60.6 6.2 81.6 10.8 1.6
0.75
Table VI. Room Temperature Mossbauer Parameters for Spectra of Ferrisilicate (5.41 wt % Fe) in Figure 5 treatment 700 K, 0, 420 K, Hi 500 K, H2
600 K,
700 K,
Hz
H2
Figure 5a
5b 5c
5d
5e
species Fe3+ Fe3+ Fe3+(outer) Fez+ (outer 1)" Fez+(outer 2) Fez+(inner) Fe3+(inner) Fez+(outer 1) Fez+(outer 2) Fez+(inner) Fe3+ Fez+(outer 1) Fez+(outer 2) Fez+(inner) Fe3+
IS, mm/s
QS, mm/s
line width, mm/s
relative areas, %
0.24 0.24 0.24 1.04 1.02 0.56 0.39 1.04 1.02
1.06 1.06 1.06 1.93 1.46 1.14 0.32 1.39 1.46 1.14 0.32 1.93 1.46 1.14 0.32
0.66 0.65 0.59 0.64 0.45 0.36 0.30 0.51 0.40 0.44 0.30 0.53 0.40 0.40 0.30
100 100 70.4 15.0 3.8 7.0 3.2 58.1 20.4 16.6 4.8 59.6 20.4 15.6 4.2
0.56 0.39 1.04 1.02 0.56 0.39
"This notation refers to outer doublet 1; outer 2 refers to outer doublet 2.
cations of low symmetry and coordination are converted to cations of higher symmetry and coordination by successive oxidation-reduction cycles. The sites of low symmetry and coordination are probably located on the surface of the ferrisilicate. Evidence for this can be seen in Table V, which summarizes Mbssbauer spectroscopy parameters obtained before and after exposure of an oxidized sample to Hzfor 2 h at 420 K. Comparison of the spectral areas contributed by each doublet shows that the iron cations represented by the Fe3+outer doublet are reduced more easily to Fe2+during this moderate treatment in H2 Gravimetric measurements of the extent of pyridine adsorption on this sample are consistent with the Mhsbauer Spectroscopydata. They confirm the existence of surface iron cations. Specifically, it was measured that the oxidized ferrisilicate adsorbs approximately 2 X 1020 pyridine molecules/g of sample (ca. 1pyridine molecule/2 Fe cations), whereas the pyridine uptake by silica alone is negligible under these experimental conditions. Amorphous Ferrisilicate (5.41 wt % Fe). This sample was initially treated a t 700 K for 8 h under vacuum Pa), oxidized for 3 h in 02,and reduced for 3.5 h in H2 at this same temperature. After this initial pretreatment, the sample was oxidized again at 700 K in O2 for 3 h and cooled to room temperature, and the spectrum shown in Figure 5a was collected. Mossbauer spectra were then collected following a series of reduction treatments. Parts b-e of Figure 5 are the spectra of the sample after reduction at 420 K for 3 h, at 495 K for 4 h, at 595 K for 3 h, and a t 700 K for 3 h, respectively. The Mossbauer parameters for these spectra are listed in Table VI. The spectrum in Figure 5a was fit with a single doublet. The low IS and QS of this doublet suggest that the iron in this sample has a different coordination environment than the iron in the ZSM-5 ferrisilicate analogue and the 3.6 wt % Fe amorphous ferrisilicate. Such a low IS is characteristic of Fe3+cations in sites that are of low coordination or sites where the Fe-O bonds have a high covalent ~ h a r a d e r . ' ~ J ~ (18) Kai, A. T.; Annersten, H.; Ericsson, T. Phys. Chem. Miner. 1980, 5 , 343.
I
M
>
tv) z
U
I-
f W
2I-
< 4 w
p:
Y i d -4
-2
0
2
4
VELOCITY ( m m / s )
Figure 4. M6ssbauer spectra of reduced amorphous ferrisilicate (3.60 wt % Fe) after repeated redox cycles: (a) after first cycle, (b) after second cycle, (c) after third cycle, (d) after fourth cycle. Bar shows 0.5% absorption.
It can be seen in Figure 5b-d that the ferric cations in the 5.41 wt 9% sample were not reduced to the divalent state in H2at 420 K, while they were partially reduced at 495 K and completely reduced at 595 K. The spectrum of Figure 5d has been fit with four doublets. One doublet is due to Fe3+s t i l l present in the sample after the reduction pretreatment. Two of the remaining doublets have parameters similar to the Fe2+outer doublets of Fe/Si023*6 and Fe-Y In fact, these outer Fez+ doublets,
Mossbauer Spectroscopy of Ferrisilicates
Langmuir, Vol. 4 , No. 3, 1988 651
Table VII. Room Temperature MBesbauer Parameters for Swctra of Fe Shown in Figure 6 treatment before NO adsorp
(I
Figure 6a
NO adsorp for 1 min
6b
NO adsorp for many hours
6c
degas at 300 K
6d
degas at 500 K
6e
degas at 700 K
6f
species Fez+(outer 1)" Fez+(outer 2) Fez+ (inner) Fe3+ Fez+(outer 1) Feat (outer 2) Feat (inner) Fe3+ Feat (outer 1) Fez+(outer 2) Fez+ (inner) Fe3+ Fez+(outer 1) Fez+ (outer 2) Fez+ (inner) Fes+ Fe2+(outer 1) Feat (outer 2) Fez+(inner) Fe3+ Fez+ (outer 1) Fez+(outer 2) Fez+(inner) Fe3+
IS, mm/s 1.03 1.03 0.49 0.40 1.03 1.03 0.49 0.40 1.03 1.03 0.49 0.40 1.03 1.03 0.49 0.40 1.03 1.03 0.49 0.40 1.03 1.03 0.49 0.40
QS, mm/s 1.95 1.39 1.10 0.33 1.95 1.39 1.10 0.33 1.95 1.39 1.10 0.33 1.95 1.39 1.10 0.33 1.95 1.39 1.10 0.33 1.95 1.39 1.10 0.33
line width, mm/s 0.54 0.41 0.61 0.26 0.59 0.44 0.78 0.26 0.60 0.41 0.79 0.26 0.59 0.34 0.85 0.26 0.52 0.33 0.69 0.23 0.52 0.33 0.67 0.26
relative areas, % 51.4 15.8 27.4 5.2 54.0 12.6 30.6 2.6 53.4 10.8 33.4 2.4 49.0 8.8 39.4 2.8 46.4 11.2 39.4 5.4 46.6 12.6 34.6 6.4
This notation refers to outer doublet 1;outer 2 refers to outer doublet 2.
>
t
v)
z w c
f W
2
c a 4
w K
-4
-2
0
2
4
VELOCITY ( m m / s )
Figure 5. MWbauer spectra of oxidized and reduced amorphous ferrisilicate (5.41 w t % Fe): (a) after treatment with O2 at 700 K, (b) after treatment with H2 at 420 K, (c) after treatment with H, a t 495 K, (d) after treatment with H Z at 595 K, (e) after treatment with H2 a t 700 K. Bar shows 1.0% absorption.
characteristic of cations in sites of high coordination, are the predominant components of the spectrum. The fourth doublet is an inner doublet that can be assigned to ferrous cations in sites of low coordination. Comparison of parts d and e of Figure 5 shows that essentially no change takes place in the Mossbauer spectrum when the reduction temperature is raised from 595 to 700 K. In contrast, a
fraction of the outer doublet in Fe/Si02 is converted to the inner doublet when the reduction temperature is rai~ed.~~~ Successive oxidation-reduction cycles also had little effect on the spectrum of the 5.41 wt % sample. Mossbauer spectroscopic experiments paralleling those summarized in Figures 3 and 4 showed no conversion of iron cations in sites of low symmetry into cations in sites of high symmetry. Mossbauer spectra collected after exposure of this sample to NO at room temperature are given in Figure 6, and the Mossbauer parameters for these spectra are listed in Table VII. Figure 6a was collected after evacuating the reduced sample shown in Figure 5e for 4 h under vacuum at 700 K. After collection of this spectrum, the sample was exposed to NO at a pressure of 660 P a for 60 s. The cell was then evacuated at room temperature, and the spectrum shown in Figure 6b was collected under vacuum. The sample was subsequently exposed to 660 P a of NO for 1 h, and Figure 6c was collected in this NO atmosphere. Parts d-f of Figure 6 show spectra collected following evacuation a t room temperature for 2 h, at 500 K for 2 h, and at 700 K for 2.5 h. Parts b and c of Figure 6 and Table VI1 show that the adsorption of NO slightly increased the spectral areas of the Fe2+inner doublet and one of the Fe2+outer doublets. The slight increase in area of this outer doublet probably arose from the conversion of a fraction of inner doublet to outer doublet by the formation of mononitrosyl species as has been found for Fe-Y.'O The slight increase in the inner doublet intensity may be due to the formation of dinitrosyl species, since such species have parameters that are similar to those of the inner doublet of this ferrisilicate.1° When compared to the adsorption of NO on Fe/Si023and Fe-Y,9J0these effects are very small. This result shows that the major part of the iron in this ferrisilicate sample is much less accessible to NO than the iron in either Fe/Si02 or Fe-Y. Amorphous Ferrisilicate (15.39 wt 7 ' 0 Fe). This sample was dehydrated for 4 h under vacuum at 700 K and then treated in O2for 7 h at this same temperature. Figure 7a shows the spectrum of the oxidized sample. This
652 Langmuir, Vol. 4, No. 3, 1988
Yixiang et al.
Table VIII. Room Temperature Mclssbauer Parameters of Iron for Spectra of Ferrisilicate (15.39 wt Sample in Figure 7 treatment
Figure
700 K, 12 h in O2 700 K. 3 h in H,
7a 7b
species Fe3+
IS, mm/s
Feo
Fea Fez+(outer 1)' Fez+(outer 2) Fez+(inner)
Fe3+ a
0.26 0.01 0.02 1.02 1.00 0.69 0.45
QS, "1s 0.98 6.11 1.75 2.20 1.69 1.48 0.34
% Fe)
line width, mm/s
area, %
0.59 0.32 0.29 0.44 0.44 0.51 0.26
100.0 9.2 2.8 43.0 30.2 10.4 4.2
relative
This notation refers to doublet 1; outer 2 refers to outer doublet 2. I
I
1
-4
-2
0
2
4
VELOCITY ( m m / s )
Fkure 7.
jssbauer stxctra of amomhous ferrisilicate (1 1.4 w t %%& (a) after treatm'entwith O2a t h K, (b) after treatment with H2 at 700 K. Bar shows 5.0% absorption. of high coordination. It is important to note, however, that the interaction between iron and silica is still sufficiently strong in the 15.39 wt % Fe sample to keep most of the iron in the divalent state after treatment in HP
VELOCITY
("1s)
Figure 6. Mtksbauer spectra of amorphous ferrisilicate (5.41 w t % Fe) after exposure to NO: (a) after treatment with H2 at 700 K and evacuation, (b) after exposure to NO for 60 s and evacuation, (c) after exposure to NO for 1 h, (d) after evacuation at room temperature, (e) after evacuation at 500 K, ( f ) after evacuation at 700 K. Bar shows 1.0% absorption.
spectrum was fit with a single doublet. The low IS of this doublet suggests that the iron cations of this sample are located in sites where low coordination exists or where the Fe-0 bonds are highly covalent. The sample was subsequently treated in H2 for 3 h at 700 K, after which the spectrum of Figure 7b was recorded. This spectrum was fit with one Fe3+ doublet, three Fez+doublets, and four peaks due to metallic iron. The Miissbauer parameters for these spectra are listed in Table VIII. The appearance of metallic iron shows that, as was found for Fe/Si02, the interaction between iron and silica becomes weaker when the iron content of the ferrisilicate increases. Also, the majority of the Fe2+is present as outer doublet, i.e., in sites
Discussion The results described above show that trivalent iron in both crystalline and amorphous ferrisilicates is reduced to the divalent state by high-temperature treatment with H2 Some rearrangement of the structure probably occurs during this reduction, suggesting that the structure of ferrisilicates prepared as described above is not as stable as the aluminosilicate structure of zeolites. The structure of Na-Y zeolite is believed to be stable to temperatures of at least 900 K.I9 Reduction of iron to the metallic state was also seen in the ferrisilicates but only in sdmples with high iron loadings. In this way, the iron in the ferrisilicates behaved like iron in Fe/Si02 and iron-exchanged zeolites. When a comparison is made between the amorphous samples with iron loadings of 3.60% and 5.41%, it is seen that the sample with the lower loading shows more Fe2+ inner doublet in its Mossbauer spectrum after reduction. This Fe2+ inner doublet has been assigned to ferrous cations in sites of low coordination. Hence, the results show that such low-coordination sites exist preferentially in samples with high silica contents. This agrees with the work of Connell and Dumesic,20who found that such sites exist on Fe/Si02 and Fe/Ti02 at low loadings but not on (19) Baker, R. W.; Maher, D. K.; Blazek, J. J. Hydrocarbon Process. 1968, 47, 125. (20) Connell, G.; Dumesic, J. A. J. CataE. 1986, 102, 216.
Langmuir 1988,4,653-655 Fe/MgO or Fe/A1203. The results of both studies suggest that one way silica and titania can interact with supported iron is through the stabilization of iron in low-coordination sites. The low-coordination sites in the ferrisilicates of this study were found to be unstable with respect to repeated oxidation-reduction cycles. Such cycles converted the low-coordination iron in the sample with an iron loading of 3.60% to high-coordination iron. Although the same behavior was not observed for the sample with the higher iron loading, most of the iron in that sample was already in sites of high coordination after the first reduction. Corresponding experiments have not yet been tried for Fe/SiOz. Nevertheless, it can be seen from the M h b a u e r spectroscopy spectral areas in Table IV that the ferrisilicate sample with an iron loading of 3.60% had approximately half of its iron in low-coordination sites after the first reduction. The iron loading of this sample was approximately 7 times the loading at which a similar amount of inner doublet was observed for Fe/SiOa6 Hence, ferrisilicatessynthesized as described above provide the opportunity to study and utilize low-coordination iron in samples with relatively high loadings. This may make these materials useful for catalytic reactions where iron cations function as Lewis acid or redox centers. The NO adsorption experiments done on the sample with a loading of 5.41% showed that iron in the high-coordination sites of the ferrisilicates has only a limited
653
accessibility to gas-phase molecules like NO. However, low-coordination iron in Fe/Si02 and iron-exchanged zeolites has been shown before to readily adsorb NO?J’*9J0 In addition, low-coordinationiron in the 3.60% ferrisilicate described above was found to act as a strong adsorption center for pyridine. Therefore, while the ferrisilicate sample with the lower loading (3.60%) has iron in sites that are accessible to the gas phase, the 5.41 w t % ’ sample appears to be a less promising catalytic material. The two samples with the higher iron loadings were found to exhibit doublets with a low IS after treatment with Oa Ferric doublets of low IS are indicative of iron that is covalently bonded.20 Hence, it was found that the higher the silica content the more ionic were the Fe-0 bonds. This suggests that another way silica can interact with iron cations is by making the bonding of iron with oxygen more ionic. This may be one reason why silicasupported metal cations display interesting acidic properties at low cation loadings.6*20
Acknowledgment. We express gratitude to the National Science Foundation for Grant CBT-8414622, which supported this work. In addition, we gratefully acknowledge funding from the Chinese government that allowed Z.Y. to be a visiting scholar at the University of Wisconsin. Registry No. Fe, 7439-89-6; SOz, 7631-86-9;iron silicate, 12673-39-1.
Mean Aggregation Number and Water Vapor Pressure of AOT Reverse Micellar Systems Determined by Controlled Partial Pressure-Vapor Pressure Osmometry (CPP-VPO) M. Uedal and Z. A. Schelly* Department of Chemistry, The University of Texas at Arlington, Arlington, Texas 76019-0065 Received September 28, 1987. In Final Form: December 21,1987 The mean aggregation number it and water vapor pressure pwof 0.1 M Aerosol-OT reverse micelles were determined by controlled partial pressure-vapor pressure osmometry (CPP-VPO)at 37 “C in cyclohexane and isooctane. In the range R 1 [H,O]/[AOT] < 1.5, A is essentially constant, but at 1.5 < R < 18 (in isooctane) and 1.5 < R < 15.5 (in cyclohexane),it increases linearly with R. The water vapor pressure of the solutions rapidly approaches that of pure water, with increasing R. The results are compared with data available in the literature.
Introduction The simplest solutions of surfactants in nonpolar solvents are three-component systems, for water is always present at least in a trace amount which cannot be removed through routine drying procedures. The water present does not behave as a neutral impurity, but rather it promotes the aggregation of the amphiphile to reverse micelles. Part of the water present serves as a “glueing” agent2 between the polar head groups of the surfactant monomers in the cores of the aggregates, and the rest is usually molecularly dispersed in the nonpolar bulk solvent.
“Dry” reverse micelles have only tightly bound structural water in their interior. If more H 2 0 is introduced into the solution, an additional amount of water can be solubilized by the aggregates. This water has properties similar to that of bulk water, constituting the *p00ln3of the reverse micelles. Such systems are sometimes called swollen reverse micelles or, if the diameter of the micelle exceeds 10 nm, water-in-oil (w/o) microemulsions. Because of its thermodynamic properties (e.g., water vapor pressure pw),the pool of a reverse micelle can be viewed as an aqueous microphase, which can solubilize a great variety of polar
(1)R. A. Welch Postdoctoral Fellow. On leave from the Osaka Municipal Technical Research Institute, Japan. (2) Zundel, G. Hydration and Zntermolecuhr Interaction;Academic: New York, 1969.
(3) Menger, F. M.; Donohue, J. A.; Williams, R. F. J. Am. Chem. SOC. 1973,95, 286. (4) Luisi, P. L.; Magid, L. J. CRC Crit. Reu. Biochem. 1986, 20,409.
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substance^.^
0743-~463/88/240~-0653$01.50/0 0 1988 American Chemical Society
