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HNa-Y zeolites
Journal of Molecular Catalysrs, 33 (1985)
345 - 356
PREPARATION AND CHARACTERIZATION TAKAYUKI Department
345
OF Mo/HNa-Y ZEOLITES
KOMATSU, SEITARO NAMBA, TATSUAKI YASHIMA* of Chemurtry, Tokyo Institute of Technology,
KAZUNARI DOMEN and TAKAHARU Research Labomtory Yokohama (Japan)
of Resources
(Received December 31,1984;
Meguro-ku, Tokyo (Japan)
ONISHI
Utilization, Tokyo Institute of Technology,
Midori-ku,
accepted March 12, 1985)
summary Mo/HNa-Y zeolites were prepared by the thermal decomposition of MOM adsorbed on HNa-Y zeolites having various degrees of proton exchange. The maximum content of molybdenum corresponded to two MO atoms per supercage of the Y zeolite, irrespective of the proton concentration of HNa-Y. The surface MO/& ratios obtained by X-ray photoelectron spectroscopy (XPS) were smaller than the bulk Mo/Si ratios. It is suggested that the MO species are uniformly distributed within the crystallites of zeohtes. The oxidation state of the supported molybdenum was investigated by in situ XPS. This technique enabled analysis of the MO species m low oxidation states. The results of the XPS studies qualitatively agreed with the average oxidation numbers. It was found that Moo mainly existed in MO/ Na-Y and that Mo/HNa-Y contamed a certain amount of Mo2+ species in addition to Moo species.
Introduction Certam molybdenum complexes have been used to obtain a new class of supported molybdenum catalysts, compared with the conventional Impregnation catalysts prepared using molybdate solutions. The surface molybdenum species of this class of catalysts have several attractive features. First, high and homogeneous dispersion of surface molybdenum species is easily achieved, due to the chemical interaction between the complex molecule and the surface of the support. Second, it is possible to control the oxidation state of the molybdenum over a wide range. These features are
*Author to whom correspondence 0304-5102/85/$3.30
should be addressed. 0 Elsevier Sequoia/FVmted in The Netherlands
346
advantageous for studying the active sites of molybdenum catalysts for a vanety of reactions. Yermakov and Kuznetzov [l] and Iwasawa et al. [2] have prepared well-defined molybdenum catalysts by using the facile reaction between Mo(n-CsHs), and surface hydroxyl groups of silica or alumma. By oxidation and reduction treatments, they have adJusted the oxidation number of molybdenum to 2, 4 or 6. Bowman and Burwell [ 31 have prepared a small molybdenum cluster from MOM on highly dehydroxylated alumina. They have reported that the average oxidation number of molybdenum in this cluster was less than +l. When zeohte 1s used as a support, one can expect to obtam a molybdenum species which has a higher dispersion and more clearly defined structure than that obtained on amorphous silica or alumina supports, since zeolite has uniform pores in its framework structure. Usually, transition metal cations can be anchored in the framework of zeolite by ion exchange methods with aqueous solutions of the metal salts. However, molybdenum is unstable in catronic form in water, so that ordmary ion exchange methods with aqueous solutions cannot provide zeohte-supported molybdenum. Dai and Lunsford [4] have prepared the molybdenum-containing zeohtes by using a solid-solid exchange reaction of MoOC& with HNa-Y zeolites. They reported that the molybdenum ions in these catalysts were mainly present as Mo6+. In order to obtam the molybdenum species m a low oxidation state, it is preferable to use such molybdenum complexes as MOM. Gallezot et al. [5] have prepared Mo/HNa-Y zeohtes by the decomposition of MOM adsorbed on the HNa-Y. They found that molybdenum was oxidized by the surface hydroxyl groups of HNa-Y, but they did not mention the oxidation state of molybdenum in detail. We have already prepared molybdenum supported HNa-Y zeolites [6, 71 by the modified procedure proposed by Gallezot et al. [ 51. We found that the average oxidation number (AON) of molybdenum varied depending on the dehydration temperature of the zeolites and the decomposition temperature of MOM, and that the molybdenum species of low oxidation state was active for the polymerization of ethylene [6, 71 and the hydrogenation of ethylene [8] . The application of X-ray photoelectron spectroscopy (XPS) to various supported molybdenum catalysts has been reported [2, 4, 9, lo]. These studies revealed the existence of Mo4+, MO’+ and Mo6+ on the supports. Therefore, we expected to determine the oxidation numbers of MO species existmg m the active Mo/HNa-Y catalysts from the XPS spectra. In this paper, we have prepared Mo/HNa-Y by using MOM and HNa-Y zeohtes having various degrees of proton exchange, and attempted to determine the factors which control the amount of supported molybdenum. In addition, we have measured the XPS spectra of Mo/HNa-Y by the in situ method and tried to clarify the MO species in the low oxidation state, taking into account the average oxidation number of molybdenum.
347
Experimental Catalysts Na-Y zeolites (Toyo Soda Ind. Co., Lot Y-30) were treated with 0.05 N or 0.5 N NH4C1 solution at 298 K or 336 K to form NH4Na-Y. After calcination at 743 K m au-, H(x)Na-Y was obtained, where x is the percent degree of proton exchange. Na-Y was washed by pure water, followed by drying at 393 K in air. A specific amount of HNa-Y placed in a quartz tube was heated zn uacuo at various temperatures to yield dehydrated HNa-Y. The desired amount of MOM was added to the dehydrated HNa-Y under nitrogen atmosphere. After the nitrogen gas was pumped out for 20 s, the tube was put m a thermostatted oven at 333 K and allowed to stand for 15 h to adsorb MOM on HNa-Y. Mo/HNa-Y was obtained by heating Mo(CO),/ HNa-Y in uacuo at various temperatures. The contents of sodium in the HNa-Y and molybdenum in the MO/ HNa-Y were analyzed using flame emission spectroscopy and atomic absorption spectroscopy, respectively. X-ray photoelectron spectroscopy The Mo/HNa-Y samples for XPS studies were prepared by two different procedures. For studies of the surface molybdenum content, the samples were prepared by using HNa-Y tablets. Then the tablets were placed in the sample holder under air atmosphere. For the studies on the oxidation state of molybdenum, the samples were prepared by an in situ procedure was follows. A small drop of HNa-Y suspended m pure water was put on the tip of the sample holder and then dried to form a very thin layer of HNa-Y. Mo(CO)6 was then adsorbed on the HNa-Y m a Pyrex tube, which was sealed in uacuo at 77 K after the adsorption was completed. The Mo(CO)JHNa-Y thus prepared was placed in the sample holder in a glove box filled with argon. Immediately after the sample was inserted into the analyzer chamber, it was heated under vacuum ( 10q6 torr) to form Mo/HNa-Y. The XPS spectra were measured on the Shimadzu ESCA 750 spectrometer with Mg Kal,z X-rays (1253 eV) used as the exciting source. The values of binding energies were determined from the 4f,,, line of Au (83.7 eV) deposited on the sample after measurements of spectra of other elements. The C 1s line was used as a secondary standard for each spectrum. The reproducibihty of binding energies was k0.2 eV. Average oxidation number of molybdenum The average oxidation number (AON) of molybdenum after the decomposition of Mo(CO), adsorbed on HNa-Y was determined by two different methods as follows. O2 titration Mo/HNa-Y was exposed to 200 torr of oxygen at room temperature. The sample was then heated at 573 K for 30 min. The amount of O2 con-
346
sumed during this oxidation was measured volumetrically. We calculated the AON of MO by assuming that all of the MO species were oxidized to Mo6+. Measurement of the amount of Hz formed during the decomposition It has been reported that molybdenum is oxidized by the surface hydroxyl groups of HNa-Y durmg the decomposition of the adsorbed Mo(CO), [5]. In this oxidation, CO and Hz are formed as follows. MOM
+ n(-OH) -
(-0~),,Mo”+
+ 6C0 + ; Hz
Part of this Hz was consumed to form methane. We measured the amounts of CO, Hz and CH4 evolved during the decomposition and calculated the AON of MO from the relative amounts of these gases. Results and discussion It has been reported that MOM IS adsorbed m the supercage of dehydrated HNa-Y zeolite [5]. Therefore, the maximum amount of adsorption should depend on the number of MOM molecules adsorbed in a supercage. We have already reported [7] that the MOM adsorbed was only one molecule per supercage of H(82)Na-Y, as indicated by Gallezot et al. [5]. In the present paper, m order to determine the effect of proton concentration of HNa-Y on the amount of supported molybdenum, we prepared Mo/HNa-Y by using zeolites of various degrees of proton exchange. Figure 1 shows the relation between the amount of added Mo(C0)6 and that of supported molybdenum after decomposition. From the sohd
Amount of odded Mo(CO)&rolecules
(SuDercoge)~'
Fig. 1. Relation between the amount of added Mo(CO)s and amount of molybdenum supported on Na-Y (A), H(36)Na-Y (0) and H(82)Na-Y (0). Dehydration temperature of zeolites and decomposition temperature of adsorbed Mo(CO)s were both 573 K.
349
lme m Fig. 1, it is clear that all of the added MO was supported on the HNa-Y, up to 1 MO atom per supercage of Y zeolites, irrespective of the degrees of proton exchange. In the case of the H(82)Na-Y support, the content of molybdenum was restricted to this level, while in the case of H(36)Na-Y or Na-Y, the content linearly increased up to 2 MO-atoms/ supercage. When H(14)Na-Y, H(65)Na-Y and H(74)Na-Y were used as supports, the maximum contents of MO were 2.0, 2.0 and 1.9 MO atoms/ supercage, respectively. These results indicated that the amount of supported molybdenum was independent of the concentration of protons on the zeolite, except for the H(82)Na-Y. The surface area of the H(82)Na-Y was certainly small compared with the other HNa-Y, which could be explained by the partial destruction of the H(82)Na-Y zeolite. Therefore, we thought that the decrease in the maximum content of molybdenum resulted partly from a decrease in the number of supercages. After decomposmg MOM to remove bulky CO ligands, the space in the supercages nearly equal to that of the parent HNa-Y could be regenerated. Consequently, it seems possible that additional MOM are adsorbed in this regenerated space. Based on this assumption, we added Mo(CO), to Mo/HNa-Y contaming the maximum amount of MO. The results are represented in Fig. 1 by dotted lines. The number in parentheses represents the number of adsorptiondecomposition cycles repeated on the same sample. It was found that additional molybdenum was indeed supported on MO/ HNa-Y, and that the increase in content permitted by each cycle was nearly equal to the maximum content by the usual procedure. The loading profiles were again the same for Na-Y and HNa-Y. It is confirmed that the content of molybdenum is dependent on the vacant space in the pores which could adsorb MOM, and is independent of the proton concentration of the zeolite supports. XPS spectra can provide the surface composition in the range of dozens of angstrom depth. In the case of molybdenum supported on zeolites, comparison of the surface composition with the bulk composition reveals whether molybdenum is supported on the external surface of the zeohte crystallites or inside the crystallites. Dai and Lunsford [4] have reported for MO/Y zeolites prepared by a solid-solid exchange method that the Mo/Si ratio calculated from XPS data was not greater than that determined from bulk composition. They concluded that this method resulted in the exchange of MO into the zeolite crystallites. Tri et al. [ll] have prepared bimetallic Pt-Ma/Y zeolites and checked the homogeneity of the MO atom distribution as Dai and Lunsford [4]. They concluded that no surface enrichment in molybdenum occurred for P&rich samples, and that large amounts of Mo(CO), were adsorbed and decomposed in the outer layer of zeolites for MO-rich samples. We calculated the MO/& ratio from the XPS spectra of Mo/HNa-Y by eqns. (1) and (2) [12] :
350
(1) fi=l+
$isiri2f3-l)
(2)
where n is the concentration of the element, N is the photoelectron intensity, c is the total photoionization cross-section, f is the angular asymmetry factor, X is the escaping depth, S is the transmission factor of the spectrometer, /3 is the asymmetry parameter and 8 is the angle of photoelectron emission. We used the values of u, /I and X which were reported by Scofield [13], Reilman et al. [ 141 and Penn [15], respectively, and those of S and 0 presented in the manual of the spectrometer. Table 1 shows the external surface MO/% ratios calculated from the peaks of MO 3d and Si 2p in the XPS spectra together with the bulk Mo/Si ratios derived from the bulk composition. The molybdenum contents of these samples corresponded to 1.6 - 1.8 MO atoms/supercage of HNa-Y. It is clear that the surface Mo/Si ratios are smaller than the bulk ratios, regardless of the degree of proton exchange of the zeolites and the preparation conditions. Consequently, we conclude that enrichment of molybdenum at the external surface of the zeolite crystallites does not occur, and that the MO species are uniformly distributed within the cry&Rites. When MOM molecules are adsorbed on the zeolite at the ratio of 2 MO atoms/ supercage, the MOM should be homogeneously dispersed within the zeolite crystallites. Therefore, we can also conclude that MO atoms do not migrate toward the external surface of the zeolite crystalhtes during the decomposition of MOM, oxidation by 9 and reduction by HZ. It has been reported that molybdenum is oxidized by hydroxyl groups on the surface of alumina [ 161 or HNa-Y [5] during the thermal decomposition of adsorbed MOM. We have already reported [6, 71 that the average oxidation number of MO varies with the dehydration temperature of HNa-Y. In this study, we have measured the XPS spectra of Mo/HNa-Y in order to TABLE 1 Bulk and external surface Mo/Sr ratios Support
Dehydration temperature of zeolite (K)
Decomposition temperature of Mo(CO)e (K)
Treatment
MO/S?
(MO 3d)/(Si 2~)~
Na-Y H(14)Na-Y H(65)Na-Y H(74)Na-Y
773 573 673 673
673 573 713 573
573 K in 02 773 Kin Hz
0.10 0.089 0.088 0.092
0.046 0.051 0.048 0.026
‘Values derrved from bulk composition. bValues calculated from KPS spectra.
361
determine more precisely the oxidation state of the surface MO species. The MO species m low oxidation states were easily oxidized by oxygen at room temperature [l, 31. As the MO species m Mo/HNa-Y has an AON below +4 [ 71, we should protect the MO species from this oxidation. We placed the Mo/HNa-Y sample m the sample holder under argon atmosphere using a glove box. From the XPS spectra of the sample thus prepared, it was found that the oxidation state of MO was rather high compared with the AON of the sample. Therefore, we thought that the MO species were oxidized by traces of oxygen and water m the glove box. The possibility of a similar oxidation when using a glove box was discussed by Leclercq et al. [17] for Pt” m Pt-Ma/Y zeohtes. For the purpose of avoiding this oxidation, Mo/HNa-Y was prepared in the analyzer chamber of the XPS instruments. Heating the sample was achieved by an electric heater placed in the sample holder. Therefore, m order to heat the upper side of the sample, we used a thin layer of HNa-Y. Figure 2 shows the MO 3d XPS spectra of MOM adsorbed on H(36)Na-Y. We refer to the peak of MO 3d,,, since its intensity is higher than that of MO 3d,,,. In the first scan (a: scanning time was 68 s), a large peak was observed at a binding energy of 228.5 eV. However, this peak decreased in the second scan (b: 68 s) and third scan (c: 204 s), while the peaks m the high binding energy region increased in intensity. This result indicated that X-ray irradiation caused the decomposition of MOM and the consequent oxidation of the MO species. Figure 3 shows MO 3d XPS spectra of some Mo/HNa-Y after the thermal decomposition. In the case of Na-Y support (a), the spectrum exhibited a peak at 228.4 eV. This spectrum did not change significantly on repeated scans, so that we can exclude a change in the oxidation state caused by X-ray irradiation on the thermally decomposed products. In the case of H(65)Na-Y support (b), a broad split peak at -229 eV was observed.
m C
b
a
240
228 5
235
230
225
Blnding energy/eV
Fig. 2. MO 3d XPS spectra of Mo(CO)e adsorbed on H(36)Na-Y dehydrated at 673 K: a, first scan, scanning time 68 s; b, second scan, scanning time 68 s; c, third scan, scanning time 204 s.
362
I 240
I
1
235
230
I 225
Blnding energy/e\,
3. Mo 3d XpS spectra of molybdenum supported on Y zeolites: a, Mo/Na-Y* zi ‘i!ydration temperature of zeolite 773 K, decomposition temperature of Mo(CO)d 713’ K; b, Mo/H(65)Na-Y, preparation conditions were same aa a, c, sample b exposed to air at room temperature, d, sample c exposed to air at 523 K.
It was clear that the oxidation state was certainly heterogeneous compared with Mo/Na-Y (a). This broad peak consisted of a peak at 229.5 eV m addition to a peak at 228.6 eV, which was nearly equal to the binding energy of the peak in spectrum a. Spectrum c was obtamed after the sample giving spectrum b was oxidized by air at room temperature. A peak at 230.5 eV was observed, but the oxidation state was rather heterogeneous. When this sample was further oxidized by an at 523 K, spectrum d was obtained. This spectrum, having a peak at 233.0 eV, demonstrated the existence of the MO species m a homogeneous oxidation state. In order to determine the oxidation number to which each peak in Fig. 3 corresponds, we have measured the XPS spectra of MO metal and MO compounds. The results are presented in Table 2 with the data reported by others [18 - 201. In the case of data taken from [18 - 201, corrections have been made to the published MO 3d,,, binding energies, taking into account the different referencing procedures (see footnotes a, b and c of Table 2). The binding energies used as reference standards in [18 - 201 were 83.0 eV (Au 4f,,,), 284.0 eV (C 1s) and 83.8 eV (Au 4f,,,), respectively, while those used in this work were 83.7 eV (Au 4f,,,) and 284.5 eV (C 1s). From Table 2, the peak at 228.4 eV observed for Mo/Na-Y apparently corresponded to the molybdenum with an oxidation number between 0 and +2. Here we attribute this peak to Moo for the following reasons.
353 TABLE 2 Binding energies of molybdenum metal and various molybdenum compounds Sample
MO metal
Oxidation number of MO
0
Moe ‘312
MoC13
+2 +3
Moo2
+4
MoC14 MoCls
+4 +5
Moos
+6
(NH~)~Mo,O~-~H~O
+6
Binding energy of MO 3dsp (eV)
Reference
227.9 227.7’ 229.2b 229.6 229.7a 229.5 231.7’ 230.3a 231.7 230.7’ 232.7 232.4a 232.4’ 232.4 232.4’
this work
1181 I191
this work
1181
this work
[I81 WI
this work
I181
this work
I181 [201
this work
1181
aValues corrected by +0.7 eV (see text). bValue corrected by +0.5 eV (see text). ‘Value corrected by -0.1 eV (see text).
(1) It has been reported [21] that the MO 3d,,, binding energy of MOM was higher by 0.5 eV than that of MO metal. This indicates that the bmdmg energy is high if Moo has no metal bond. As shown m Fig. 2, Mo(C0)6/HNa-Y had a peak at 228.5 eV for the first scan. This seems to represent the peak corresponding to Moo in MOM. (2) It has been reported [22] for Pt/Y zeolite that when Pt” was atomically dispersed, the binding energy of Pt” was higher by 1.3 eV than that m metal film and by 0.6 eV than that in a particle of 10 - 20 A in diameter. This result was explained by the change m the extra-atomic relaxation. A similar difference in binding energies (cu. 1.0 eV) depending on the particle size was observed for Rue in Y zeolites [23]. In the case of Mo/HNa-Y, the dispersion of molybdenum was certainly high [7]. We can, therefore, expect a similar increase in the binding energy of molybdenum in the zeolite. It is clear that the peak at 233.0 eV (Fig. 3, d) corresponds to Mo6+, because the binding energies of Mo6+ in some compounds (Table 2) were below 233.0 eV. This assignment was confirmed by the fact that the color of the sample (Fig. 3, d) was white. From Fig. 2 and Table 2, the difference in the Mo6+ binding energies between Mo/HNa-Y and MO compounds was 0.3 - 0.6 eV. Assummg that the same difference exists m the case of Mo4+, we can attribute the peak at 230.5 eV (Fig. 3, c) to Mo4+. From these assignments, the peak at -229.5 eV (Fig. 3, b) clearly corresponds to the MO species whose oxidation number was +1 - +3. From Table 2, the binding energies of
354
Mo3+ and MO’+ m molybdenum chlorides were lower than that of Mo4+ in MoCl, by 0.6 - 0.7 eV and 1.1 eV, respectively. The peak at 229.5 eV was lower by 1.0 eV than the peak of Mo4+ m Mo/HNa-Y. Therefore, we can attribute the peak at 229.5 eV to MO’+. In these assignments, each binding energy is rather high compared with that in the correspondmg compound. Similar positive shifts of binding energies have been reported for metal cations in zeohtes. Pedersen and Lunsford [23] have reported that the Ru 3d,,, binding energy for Ru02 particles within the zeolite cage was higher by 0.9 eV than that on the external surface of the zeohte. They have attributed this change in binding energy to the metal-support interaction and the matrix effects of the zeohte, such as differences in the crystal field potential energy. In the case of NiNaX [24] and NiHZSM-5 [25], the positive shifts m Ni 2p3,2 bmdmg energies were 1.1 eV and 0.7 eV, respectively, compared with Ni2+ compounds. Taking into account these investigations, we conclude that our assignments of the MO 3d,,, binding energies are reasonable. From the above assignments, it is concluded that Mo/Na-Y (Fig. 3, a) contams a considerable amount of MO’, and that Mo/H(65)Na-Y (Fig. 3, b) contains some MO species whose oxidation numbers are 0 and +2. It is clear that the hydroxyl groups of H(65)Na-Y zeolite oxidize some molybdenum to increase their oxidation number from 0 to +2. After oxidation at room temperature (Fig. 3, c), Mo4+ is mainly formed, and further oxidation at 523 K converts all the MO species to Mo6+. An XPS study of Mo/Na-Y and Pt-Ma/Y prepared using MOM has been carried out by Tri et al. [ll]. They reported that only Mo4+, MO’+ and Mo6+ were detected, even in the Pt-containing samples. Because their samples were exposed to air and subsequently reduced by H2 before the XPS measurements, the oxidation state of the sample without exposure to air, however, seems to be lower than Mo4+. Table 3 shows the average oxidation number (AON) of molybdenum in various Mo/HNa-Y determined by two different methods. From the XPS spectra, it was clear that all the MO species existmg m the oxidized MO/ HNa-Y for the O2 titration studies were Mo6+. The AON calculated from the amount of H2 and CH4 formed during the decomposition of MOM roughly agreed with that determined from the O2 titration. These two methods revealed that Moo surely existed in Mo/Na-Y and Mo/HNa-Y. The AON of the samples a, b and c m Fig. 3 were 0.25, 0.61 and 2.7, respectively. These values were somewhat lower than those estimated from the XPS spectra. However, the same tendencies were observed, that is, Moo existed m Mo/Na-Y and some MO species m Mo/HNa-Y had higher oxidation states than those m Mo/Na-Y. We conclude from our an srtu XPS studies that the dommant MO species in Mo/HNa-Y prepared from MOM and HNa-Y zeolites have oxidation numbers of 0 and +2. When Na-Y is used as a support, the lack of surface hydroxyl groups in Na-Y results in the formation of nearly homogeneous Moo species.
355 TABLE 3 Average oxidation numbers of molybdenum
determined by two different methods
Dehydration temperature of zeolite (K)
Decomposition temperature of Mo( CO), (K)
Average oxidation number 02 titration
Measurement of the amount of HZ
Mo/Na-Y
673 773
573 713
0.36 0.31
0.21 0.25’
Mo/H( 65)Na-Y
673 773 773
573 713 713
0.94 0.68 2.60’
0.63 0.61b -
Catalyst
‘Sample corresponded to Fig. 3, a. bSample corresponded to Fig. 3, b. ‘Oxidized by 0s at room temperature; sample corresponded to Fig. 3, c.
Acknowledgements Financial support for this work by the Asahi Glass Foundation Industrial Technology 1s gratefully acknowledged.
for
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345 - 356
PREPARATION AND CHARACTERIZATION TAKAYUKI Department
345
OF Mo/HNa-Y ZEOLITES
KOMATSU, SEITARO NAMBA, TATSUAKI YASHIMA* of Chemurtry, Tokyo Institute of Technology,
KAZUNARI DOMEN and TAKAHARU Research Labomtory Yokohama (Japan)
of Resources
(Received December 31,1984;
Meguro-ku, Tokyo (Japan)
ONISHI
Utilization, Tokyo Institute of Technology,
Midori-ku,
accepted March 12, 1985)
summary Mo/HNa-Y zeolites were prepared by the thermal decomposition of MOM adsorbed on HNa-Y zeolites having various degrees of proton exchange. The maximum content of molybdenum corresponded to two MO atoms per supercage of the Y zeolite, irrespective of the proton concentration of HNa-Y. The surface MO/& ratios obtained by X-ray photoelectron spectroscopy (XPS) were smaller than the bulk Mo/Si ratios. It is suggested that the MO species are uniformly distributed within the crystallites of zeohtes. The oxidation state of the supported molybdenum was investigated by in situ XPS. This technique enabled analysis of the MO species m low oxidation states. The results of the XPS studies qualitatively agreed with the average oxidation numbers. It was found that Moo mainly existed in MO/ Na-Y and that Mo/HNa-Y contamed a certain amount of Mo2+ species in addition to Moo species.
Introduction Certam molybdenum complexes have been used to obtain a new class of supported molybdenum catalysts, compared with the conventional Impregnation catalysts prepared using molybdate solutions. The surface molybdenum species of this class of catalysts have several attractive features. First, high and homogeneous dispersion of surface molybdenum species is easily achieved, due to the chemical interaction between the complex molecule and the surface of the support. Second, it is possible to control the oxidation state of the molybdenum over a wide range. These features are
*Author to whom correspondence 0304-5102/85/$3.30
should be addressed. 0 Elsevier Sequoia/FVmted in The Netherlands
346
advantageous for studying the active sites of molybdenum catalysts for a vanety of reactions. Yermakov and Kuznetzov [l] and Iwasawa et al. [2] have prepared well-defined molybdenum catalysts by using the facile reaction between Mo(n-CsHs), and surface hydroxyl groups of silica or alumma. By oxidation and reduction treatments, they have adJusted the oxidation number of molybdenum to 2, 4 or 6. Bowman and Burwell [ 31 have prepared a small molybdenum cluster from MOM on highly dehydroxylated alumina. They have reported that the average oxidation number of molybdenum in this cluster was less than +l. When zeohte 1s used as a support, one can expect to obtam a molybdenum species which has a higher dispersion and more clearly defined structure than that obtained on amorphous silica or alumina supports, since zeolite has uniform pores in its framework structure. Usually, transition metal cations can be anchored in the framework of zeolite by ion exchange methods with aqueous solutions of the metal salts. However, molybdenum is unstable in catronic form in water, so that ordmary ion exchange methods with aqueous solutions cannot provide zeohte-supported molybdenum. Dai and Lunsford [4] have prepared the molybdenum-containing zeohtes by using a solid-solid exchange reaction of MoOC& with HNa-Y zeolites. They reported that the molybdenum ions in these catalysts were mainly present as Mo6+. In order to obtam the molybdenum species m a low oxidation state, it is preferable to use such molybdenum complexes as MOM. Gallezot et al. [5] have prepared Mo/HNa-Y zeohtes by the decomposition of MOM adsorbed on the HNa-Y. They found that molybdenum was oxidized by the surface hydroxyl groups of HNa-Y, but they did not mention the oxidation state of molybdenum in detail. We have already prepared molybdenum supported HNa-Y zeolites [6, 71 by the modified procedure proposed by Gallezot et al. [ 51. We found that the average oxidation number (AON) of molybdenum varied depending on the dehydration temperature of the zeolites and the decomposition temperature of MOM, and that the molybdenum species of low oxidation state was active for the polymerization of ethylene [6, 71 and the hydrogenation of ethylene [8] . The application of X-ray photoelectron spectroscopy (XPS) to various supported molybdenum catalysts has been reported [2, 4, 9, lo]. These studies revealed the existence of Mo4+, MO’+ and Mo6+ on the supports. Therefore, we expected to determine the oxidation numbers of MO species existmg m the active Mo/HNa-Y catalysts from the XPS spectra. In this paper, we have prepared Mo/HNa-Y by using MOM and HNa-Y zeohtes having various degrees of proton exchange, and attempted to determine the factors which control the amount of supported molybdenum. In addition, we have measured the XPS spectra of Mo/HNa-Y by the in situ method and tried to clarify the MO species in the low oxidation state, taking into account the average oxidation number of molybdenum.
347
Experimental Catalysts Na-Y zeolites (Toyo Soda Ind. Co., Lot Y-30) were treated with 0.05 N or 0.5 N NH4C1 solution at 298 K or 336 K to form NH4Na-Y. After calcination at 743 K m au-, H(x)Na-Y was obtained, where x is the percent degree of proton exchange. Na-Y was washed by pure water, followed by drying at 393 K in air. A specific amount of HNa-Y placed in a quartz tube was heated zn uacuo at various temperatures to yield dehydrated HNa-Y. The desired amount of MOM was added to the dehydrated HNa-Y under nitrogen atmosphere. After the nitrogen gas was pumped out for 20 s, the tube was put m a thermostatted oven at 333 K and allowed to stand for 15 h to adsorb MOM on HNa-Y. Mo/HNa-Y was obtained by heating Mo(CO),/ HNa-Y in uacuo at various temperatures. The contents of sodium in the HNa-Y and molybdenum in the MO/ HNa-Y were analyzed using flame emission spectroscopy and atomic absorption spectroscopy, respectively. X-ray photoelectron spectroscopy The Mo/HNa-Y samples for XPS studies were prepared by two different procedures. For studies of the surface molybdenum content, the samples were prepared by using HNa-Y tablets. Then the tablets were placed in the sample holder under air atmosphere. For the studies on the oxidation state of molybdenum, the samples were prepared by an in situ procedure was follows. A small drop of HNa-Y suspended m pure water was put on the tip of the sample holder and then dried to form a very thin layer of HNa-Y. Mo(CO)6 was then adsorbed on the HNa-Y m a Pyrex tube, which was sealed in uacuo at 77 K after the adsorption was completed. The Mo(CO)JHNa-Y thus prepared was placed in the sample holder in a glove box filled with argon. Immediately after the sample was inserted into the analyzer chamber, it was heated under vacuum ( 10q6 torr) to form Mo/HNa-Y. The XPS spectra were measured on the Shimadzu ESCA 750 spectrometer with Mg Kal,z X-rays (1253 eV) used as the exciting source. The values of binding energies were determined from the 4f,,, line of Au (83.7 eV) deposited on the sample after measurements of spectra of other elements. The C 1s line was used as a secondary standard for each spectrum. The reproducibihty of binding energies was k0.2 eV. Average oxidation number of molybdenum The average oxidation number (AON) of molybdenum after the decomposition of Mo(CO), adsorbed on HNa-Y was determined by two different methods as follows. O2 titration Mo/HNa-Y was exposed to 200 torr of oxygen at room temperature. The sample was then heated at 573 K for 30 min. The amount of O2 con-
346
sumed during this oxidation was measured volumetrically. We calculated the AON of MO by assuming that all of the MO species were oxidized to Mo6+. Measurement of the amount of Hz formed during the decomposition It has been reported that molybdenum is oxidized by the surface hydroxyl groups of HNa-Y durmg the decomposition of the adsorbed Mo(CO), [5]. In this oxidation, CO and Hz are formed as follows. MOM
+ n(-OH) -
(-0~),,Mo”+
+ 6C0 + ; Hz
Part of this Hz was consumed to form methane. We measured the amounts of CO, Hz and CH4 evolved during the decomposition and calculated the AON of MO from the relative amounts of these gases. Results and discussion It has been reported that MOM IS adsorbed m the supercage of dehydrated HNa-Y zeolite [5]. Therefore, the maximum amount of adsorption should depend on the number of MOM molecules adsorbed in a supercage. We have already reported [7] that the MOM adsorbed was only one molecule per supercage of H(82)Na-Y, as indicated by Gallezot et al. [5]. In the present paper, m order to determine the effect of proton concentration of HNa-Y on the amount of supported molybdenum, we prepared Mo/HNa-Y by using zeolites of various degrees of proton exchange. Figure 1 shows the relation between the amount of added Mo(C0)6 and that of supported molybdenum after decomposition. From the sohd
Amount of odded Mo(CO)&rolecules
(SuDercoge)~'
Fig. 1. Relation between the amount of added Mo(CO)s and amount of molybdenum supported on Na-Y (A), H(36)Na-Y (0) and H(82)Na-Y (0). Dehydration temperature of zeolites and decomposition temperature of adsorbed Mo(CO)s were both 573 K.
349
lme m Fig. 1, it is clear that all of the added MO was supported on the HNa-Y, up to 1 MO atom per supercage of Y zeolites, irrespective of the degrees of proton exchange. In the case of the H(82)Na-Y support, the content of molybdenum was restricted to this level, while in the case of H(36)Na-Y or Na-Y, the content linearly increased up to 2 MO-atoms/ supercage. When H(14)Na-Y, H(65)Na-Y and H(74)Na-Y were used as supports, the maximum contents of MO were 2.0, 2.0 and 1.9 MO atoms/ supercage, respectively. These results indicated that the amount of supported molybdenum was independent of the concentration of protons on the zeolite, except for the H(82)Na-Y. The surface area of the H(82)Na-Y was certainly small compared with the other HNa-Y, which could be explained by the partial destruction of the H(82)Na-Y zeolite. Therefore, we thought that the decrease in the maximum content of molybdenum resulted partly from a decrease in the number of supercages. After decomposmg MOM to remove bulky CO ligands, the space in the supercages nearly equal to that of the parent HNa-Y could be regenerated. Consequently, it seems possible that additional MOM are adsorbed in this regenerated space. Based on this assumption, we added Mo(CO), to Mo/HNa-Y contaming the maximum amount of MO. The results are represented in Fig. 1 by dotted lines. The number in parentheses represents the number of adsorptiondecomposition cycles repeated on the same sample. It was found that additional molybdenum was indeed supported on MO/ HNa-Y, and that the increase in content permitted by each cycle was nearly equal to the maximum content by the usual procedure. The loading profiles were again the same for Na-Y and HNa-Y. It is confirmed that the content of molybdenum is dependent on the vacant space in the pores which could adsorb MOM, and is independent of the proton concentration of the zeolite supports. XPS spectra can provide the surface composition in the range of dozens of angstrom depth. In the case of molybdenum supported on zeolites, comparison of the surface composition with the bulk composition reveals whether molybdenum is supported on the external surface of the zeohte crystallites or inside the crystallites. Dai and Lunsford [4] have reported for MO/Y zeolites prepared by a solid-solid exchange method that the Mo/Si ratio calculated from XPS data was not greater than that determined from bulk composition. They concluded that this method resulted in the exchange of MO into the zeolite crystallites. Tri et al. [ll] have prepared bimetallic Pt-Ma/Y zeolites and checked the homogeneity of the MO atom distribution as Dai and Lunsford [4]. They concluded that no surface enrichment in molybdenum occurred for P&rich samples, and that large amounts of Mo(CO), were adsorbed and decomposed in the outer layer of zeolites for MO-rich samples. We calculated the MO/& ratio from the XPS spectra of Mo/HNa-Y by eqns. (1) and (2) [12] :
350
(1) fi=l+
$isiri2f3-l)
(2)
where n is the concentration of the element, N is the photoelectron intensity, c is the total photoionization cross-section, f is the angular asymmetry factor, X is the escaping depth, S is the transmission factor of the spectrometer, /3 is the asymmetry parameter and 8 is the angle of photoelectron emission. We used the values of u, /I and X which were reported by Scofield [13], Reilman et al. [ 141 and Penn [15], respectively, and those of S and 0 presented in the manual of the spectrometer. Table 1 shows the external surface MO/% ratios calculated from the peaks of MO 3d and Si 2p in the XPS spectra together with the bulk Mo/Si ratios derived from the bulk composition. The molybdenum contents of these samples corresponded to 1.6 - 1.8 MO atoms/supercage of HNa-Y. It is clear that the surface Mo/Si ratios are smaller than the bulk ratios, regardless of the degree of proton exchange of the zeolites and the preparation conditions. Consequently, we conclude that enrichment of molybdenum at the external surface of the zeolite crystallites does not occur, and that the MO species are uniformly distributed within the cry&Rites. When MOM molecules are adsorbed on the zeolite at the ratio of 2 MO atoms/ supercage, the MOM should be homogeneously dispersed within the zeolite crystallites. Therefore, we can also conclude that MO atoms do not migrate toward the external surface of the zeolite crystalhtes during the decomposition of MOM, oxidation by 9 and reduction by HZ. It has been reported that molybdenum is oxidized by hydroxyl groups on the surface of alumina [ 161 or HNa-Y [5] during the thermal decomposition of adsorbed MOM. We have already reported [6, 71 that the average oxidation number of MO varies with the dehydration temperature of HNa-Y. In this study, we have measured the XPS spectra of Mo/HNa-Y in order to TABLE 1 Bulk and external surface Mo/Sr ratios Support
Dehydration temperature of zeolite (K)
Decomposition temperature of Mo(CO)e (K)
Treatment
MO/S?
(MO 3d)/(Si 2~)~
Na-Y H(14)Na-Y H(65)Na-Y H(74)Na-Y
773 573 673 673
673 573 713 573
573 K in 02 773 Kin Hz
0.10 0.089 0.088 0.092
0.046 0.051 0.048 0.026
‘Values derrved from bulk composition. bValues calculated from KPS spectra.
361
determine more precisely the oxidation state of the surface MO species. The MO species m low oxidation states were easily oxidized by oxygen at room temperature [l, 31. As the MO species m Mo/HNa-Y has an AON below +4 [ 71, we should protect the MO species from this oxidation. We placed the Mo/HNa-Y sample m the sample holder under argon atmosphere using a glove box. From the XPS spectra of the sample thus prepared, it was found that the oxidation state of MO was rather high compared with the AON of the sample. Therefore, we thought that the MO species were oxidized by traces of oxygen and water m the glove box. The possibility of a similar oxidation when using a glove box was discussed by Leclercq et al. [17] for Pt” m Pt-Ma/Y zeohtes. For the purpose of avoiding this oxidation, Mo/HNa-Y was prepared in the analyzer chamber of the XPS instruments. Heating the sample was achieved by an electric heater placed in the sample holder. Therefore, m order to heat the upper side of the sample, we used a thin layer of HNa-Y. Figure 2 shows the MO 3d XPS spectra of MOM adsorbed on H(36)Na-Y. We refer to the peak of MO 3d,,, since its intensity is higher than that of MO 3d,,,. In the first scan (a: scanning time was 68 s), a large peak was observed at a binding energy of 228.5 eV. However, this peak decreased in the second scan (b: 68 s) and third scan (c: 204 s), while the peaks m the high binding energy region increased in intensity. This result indicated that X-ray irradiation caused the decomposition of MOM and the consequent oxidation of the MO species. Figure 3 shows MO 3d XPS spectra of some Mo/HNa-Y after the thermal decomposition. In the case of Na-Y support (a), the spectrum exhibited a peak at 228.4 eV. This spectrum did not change significantly on repeated scans, so that we can exclude a change in the oxidation state caused by X-ray irradiation on the thermally decomposed products. In the case of H(65)Na-Y support (b), a broad split peak at -229 eV was observed.
m C
b
a
240
228 5
235
230
225
Blnding energy/eV
Fig. 2. MO 3d XPS spectra of Mo(CO)e adsorbed on H(36)Na-Y dehydrated at 673 K: a, first scan, scanning time 68 s; b, second scan, scanning time 68 s; c, third scan, scanning time 204 s.
362
I 240
I
1
235
230
I 225
Blnding energy/e\,
3. Mo 3d XpS spectra of molybdenum supported on Y zeolites: a, Mo/Na-Y* zi ‘i!ydration temperature of zeolite 773 K, decomposition temperature of Mo(CO)d 713’ K; b, Mo/H(65)Na-Y, preparation conditions were same aa a, c, sample b exposed to air at room temperature, d, sample c exposed to air at 523 K.
It was clear that the oxidation state was certainly heterogeneous compared with Mo/Na-Y (a). This broad peak consisted of a peak at 229.5 eV m addition to a peak at 228.6 eV, which was nearly equal to the binding energy of the peak in spectrum a. Spectrum c was obtamed after the sample giving spectrum b was oxidized by air at room temperature. A peak at 230.5 eV was observed, but the oxidation state was rather heterogeneous. When this sample was further oxidized by an at 523 K, spectrum d was obtained. This spectrum, having a peak at 233.0 eV, demonstrated the existence of the MO species m a homogeneous oxidation state. In order to determine the oxidation number to which each peak in Fig. 3 corresponds, we have measured the XPS spectra of MO metal and MO compounds. The results are presented in Table 2 with the data reported by others [18 - 201. In the case of data taken from [18 - 201, corrections have been made to the published MO 3d,,, binding energies, taking into account the different referencing procedures (see footnotes a, b and c of Table 2). The binding energies used as reference standards in [18 - 201 were 83.0 eV (Au 4f,,,), 284.0 eV (C 1s) and 83.8 eV (Au 4f,,,), respectively, while those used in this work were 83.7 eV (Au 4f,,,) and 284.5 eV (C 1s). From Table 2, the peak at 228.4 eV observed for Mo/Na-Y apparently corresponded to the molybdenum with an oxidation number between 0 and +2. Here we attribute this peak to Moo for the following reasons.
353 TABLE 2 Binding energies of molybdenum metal and various molybdenum compounds Sample
MO metal
Oxidation number of MO
0
Moe ‘312
MoC13
+2 +3
Moo2
+4
MoC14 MoCls
+4 +5
Moos
+6
(NH~)~Mo,O~-~H~O
+6
Binding energy of MO 3dsp (eV)
Reference
227.9 227.7’ 229.2b 229.6 229.7a 229.5 231.7’ 230.3a 231.7 230.7’ 232.7 232.4a 232.4’ 232.4 232.4’
this work
1181 I191
this work
1181
this work
[I81 WI
this work
I181
this work
I181 [201
this work
1181
aValues corrected by +0.7 eV (see text). bValue corrected by +0.5 eV (see text). ‘Value corrected by -0.1 eV (see text).
(1) It has been reported [21] that the MO 3d,,, binding energy of MOM was higher by 0.5 eV than that of MO metal. This indicates that the bmdmg energy is high if Moo has no metal bond. As shown m Fig. 2, Mo(C0)6/HNa-Y had a peak at 228.5 eV for the first scan. This seems to represent the peak corresponding to Moo in MOM. (2) It has been reported [22] for Pt/Y zeolite that when Pt” was atomically dispersed, the binding energy of Pt” was higher by 1.3 eV than that m metal film and by 0.6 eV than that in a particle of 10 - 20 A in diameter. This result was explained by the change m the extra-atomic relaxation. A similar difference in binding energies (cu. 1.0 eV) depending on the particle size was observed for Rue in Y zeolites [23]. In the case of Mo/HNa-Y, the dispersion of molybdenum was certainly high [7]. We can, therefore, expect a similar increase in the binding energy of molybdenum in the zeolite. It is clear that the peak at 233.0 eV (Fig. 3, d) corresponds to Mo6+, because the binding energies of Mo6+ in some compounds (Table 2) were below 233.0 eV. This assignment was confirmed by the fact that the color of the sample (Fig. 3, d) was white. From Fig. 2 and Table 2, the difference in the Mo6+ binding energies between Mo/HNa-Y and MO compounds was 0.3 - 0.6 eV. Assummg that the same difference exists m the case of Mo4+, we can attribute the peak at 230.5 eV (Fig. 3, c) to Mo4+. From these assignments, the peak at -229.5 eV (Fig. 3, b) clearly corresponds to the MO species whose oxidation number was +1 - +3. From Table 2, the binding energies of
354
Mo3+ and MO’+ m molybdenum chlorides were lower than that of Mo4+ in MoCl, by 0.6 - 0.7 eV and 1.1 eV, respectively. The peak at 229.5 eV was lower by 1.0 eV than the peak of Mo4+ m Mo/HNa-Y. Therefore, we can attribute the peak at 229.5 eV to MO’+. In these assignments, each binding energy is rather high compared with that in the correspondmg compound. Similar positive shifts of binding energies have been reported for metal cations in zeohtes. Pedersen and Lunsford [23] have reported that the Ru 3d,,, binding energy for Ru02 particles within the zeolite cage was higher by 0.9 eV than that on the external surface of the zeohte. They have attributed this change in binding energy to the metal-support interaction and the matrix effects of the zeohte, such as differences in the crystal field potential energy. In the case of NiNaX [24] and NiHZSM-5 [25], the positive shifts m Ni 2p3,2 bmdmg energies were 1.1 eV and 0.7 eV, respectively, compared with Ni2+ compounds. Taking into account these investigations, we conclude that our assignments of the MO 3d,,, binding energies are reasonable. From the above assignments, it is concluded that Mo/Na-Y (Fig. 3, a) contams a considerable amount of MO’, and that Mo/H(65)Na-Y (Fig. 3, b) contains some MO species whose oxidation numbers are 0 and +2. It is clear that the hydroxyl groups of H(65)Na-Y zeolite oxidize some molybdenum to increase their oxidation number from 0 to +2. After oxidation at room temperature (Fig. 3, c), Mo4+ is mainly formed, and further oxidation at 523 K converts all the MO species to Mo6+. An XPS study of Mo/Na-Y and Pt-Ma/Y prepared using MOM has been carried out by Tri et al. [ll]. They reported that only Mo4+, MO’+ and Mo6+ were detected, even in the Pt-containing samples. Because their samples were exposed to air and subsequently reduced by H2 before the XPS measurements, the oxidation state of the sample without exposure to air, however, seems to be lower than Mo4+. Table 3 shows the average oxidation number (AON) of molybdenum in various Mo/HNa-Y determined by two different methods. From the XPS spectra, it was clear that all the MO species existmg m the oxidized MO/ HNa-Y for the O2 titration studies were Mo6+. The AON calculated from the amount of H2 and CH4 formed during the decomposition of MOM roughly agreed with that determined from the O2 titration. These two methods revealed that Moo surely existed in Mo/Na-Y and Mo/HNa-Y. The AON of the samples a, b and c m Fig. 3 were 0.25, 0.61 and 2.7, respectively. These values were somewhat lower than those estimated from the XPS spectra. However, the same tendencies were observed, that is, Moo existed m Mo/Na-Y and some MO species m Mo/HNa-Y had higher oxidation states than those m Mo/Na-Y. We conclude from our an srtu XPS studies that the dommant MO species in Mo/HNa-Y prepared from MOM and HNa-Y zeolites have oxidation numbers of 0 and +2. When Na-Y is used as a support, the lack of surface hydroxyl groups in Na-Y results in the formation of nearly homogeneous Moo species.
355 TABLE 3 Average oxidation numbers of molybdenum
determined by two different methods
Dehydration temperature of zeolite (K)
Decomposition temperature of Mo( CO), (K)
Average oxidation number 02 titration
Measurement of the amount of HZ
Mo/Na-Y
673 773
573 713
0.36 0.31
0.21 0.25’
Mo/H( 65)Na-Y
673 773 773
573 713 713
0.94 0.68 2.60’
0.63 0.61b -
Catalyst
‘Sample corresponded to Fig. 3, a. bSample corresponded to Fig. 3, b. ‘Oxidized by 0s at room temperature; sample corresponded to Fig. 3, c.
Acknowledgements Financial support for this work by the Asahi Glass Foundation Industrial Technology 1s gratefully acknowledged.
for
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