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silica catalysts by temperature-programmed desorption of nitrogen monoxide
Applied Catalysis A: General, 87 (1992) 205-218
205
Elsevier Science Publishers B.V., Amsterdam APCAT
A2308
Characterization of unsupported cupric oxide and cupric oxide/silica catalysts by temperature-programmed desorption of nitrogen monoxide M. Shimokawabe, N. Hatakeyama, K. Shimada, K. Tadokoro and N. Takezawa Department of Chemical Process Engineering, Hokkaido University, Sapporo 060 (Japan) (Received 3 January 1992, revised manuscript received 3 April 1992)
Abstract The thermal desorption of nitrogen monoxide (NO) adsorbed on unsupported CuO and CuO/SiOZ was investigated by means of a temperature-programmed method. In both cases, two NO desorption peaks appeared with maxima in the temperature range of 383-573 and 648-753 K. It was concluded that the former peak was the result of NO adsorbed on copper(I1) sites, and the latter peak, which was accompanied by oxygen desorption, was the result of nitrate type species formed by the reaction of NO with adsorbed oxygen. Over CuO/SiOx, NO was chemisorbed selectively on CuO species formed on the support. The surface area and the percentage exposed (dispersion ) of unsupported CuO were calculated on the basis of the amount of NO desorbed. These values closely agreed with the corresponding ones estimated from the BET surface area. In a similar manner, the dispersion and the average particle size of CuO species formed on silica were calculated. It was shown that these species, which were previously characterized as isolated copper (II) species (or isolated CuO), highly dispersed CuO clusters and bulky CuO, are fine particles of CuO with an average diameter of l&2.5,5-13 and over 15-30 nm, respectively.
Keywords: catalyst characterization (TPD),
NO-TPD, cupric oxide, cupric oxide/silica.
INTRODUCTION
Temperature-programmed desorption (TPD) is one of the most useful methods for the characterization of catalysts, and gives information about the nature and the amount of adsorbed species. The dispersion of the active component supported on carrier is frequently estimated from the amount of chemisorbed species and/or the transmission electron microscopy (TEM) observation [ 11. It has been shown that the dispersion of metallic copper for supported copper catalysts can be estimated from the amount of nitrogen evolved from the reaction with dinitrogen monoxide ( NzO ) [ 2-81, Correspondence to: Dr. M. Shimokawabe, kaido University,
0926-3373/92/$05.00
Department of Chemical Process Engineering, Sapporo 060, Japan. Tel. (+81-11)7162111, fax. (+81-11)7264454.
0 1992 Elsevier Science Publishers B.V. All rights reserved.
Hok-
M. Shimokawabe et al./Applied Catal. A 87 (1992) 205-218
206
0
~“\C~+N,O-Cu~
cu
'Cu+N,
A limited number of papers have been published on the measurement of the dispersions of copper (I) [ 91 and copper (II) oxides [ 10-121. Shelef and coworkers [lo-121 have shown that the active surface area of CuO can be determined by NO chemisorption at isothermal conditions. However, the states of chemisorbed species on CuO were not elucidated. In our previous work [ 13 1, CuO/SiO, catalysts prepared by the ion-exchange method were characterized by means of various analytical methods such as differential thermal analysis (DTA), thermogravimetry (TG), ultraviolet/visible near infrared/spectroscopy (UV/VIS/NIR), X-ray photoelectron spectroscopy (XPS ) , Auger emission spectroscopy ( AES ) and X-ray diffraction (XRD ) . It was found that the precursor states of the catalysts depended qua sensitivity upon the copper loading and the calcination temperature. The copper species formed on the support were classified into three groups; isolated copper (II) ions (or isolated CuO), highly dispersed CuO clusters and bulky CuO. However, the dispersion and the average particle size of these precursor species have not been determined yet. In the present work, the adsorption of NO on CuO and CuO/SiO, were studied by the temperature-programmed desorption method. On the basis of the states of NO chemisorbed and the amount of NO desorbed, the surface area of CuO, and the dispersion and the average particle size of CuO formed on the support were determined as functions of the preparation parameters. EXPERIMENTAL
Catalysts Samples of copper (II) oxide were prepared by decomposition of basic copper (II) carbonate (Kant0 Chemicals, extra pure grade) in air in the temperature range of 573-1173 K for 3 h. CuO/SiO, (Am) was prepared in a similar manner to the catalysts employed in our previous work [ 13,141. Hydroxyl protons on silica gel (Nihon Chromato., 60-80 mesh in granules) were exchanged at pH 11-12 and at 293 K overnight with tetrammine copper (II) cations which had been prepared from copper (II) nitrate ( Wako Pure Chemicals, extra pure grade) and an aqueous ammonia solution. The ion-exchanged silica gel was filtered, washed with distilled water and dried at 383 K for 12 h, and was further calcined in air for 3 h under various conditions covering a temperature range of 573-1073 K. CuO/SiO,(N) and CuO/SiO,(Ac) were prepared by impregnation of silica gel with aqueous solutions of copper (II) nitrate and copper (II) acetate, respectively, in a rotary evaporator at 343 K. The precursors were then dried at 383 K for 12 h and were calcined in air at 773 K for 3 h. The copper
M. Shimokawabe et aLlApplied Catal. A 87 (1992) 205-218
201
loadings were estimated from the amount of copper(I1) ions in the mother solution and in the filtrate. The amount of copper (II) ions were determined by titration with EDTA solution of known concentration by use of l- (2-Pyridylazo) -2-naphthol (PAN) as an indicator. Copper loading varied in the range of 0.5-90 wt.-% copper. Temperature-programmed
desorption (TPD)
Catalyst samples, 0.2 g of CuO or 0.25 to 3.0 g of CuO/Si02, were placed on loosely packed quartz wool in a reactor which was made of quartz tubing of 12 mm in diameter. The samples were pretreated in streams of oxygen diluted with helium at various partial pressures or in helium at a total flow-rate of 50 cm3 min-’ NTP under various conditions covering a temperature range of 573-1073 K for 3 h. Thereafter the reactor was cooled to room temperature and the oxygen stream was switched to a helium stream until no oxygen was detected in the effluent. NO was then adsorbed at room temperature in a flow of 4 vol.-% NO/He at a total flow-rate of 50 cm3 min-’ for 30 min unless otherwise stated. NO in the gas-phase was then flushed with helium until no NO was detected in the effluent. TPD runs were started from room temperature to 823 K at a heating rate of 10 K min-l with a programmable temperature controller (Muto HM-80)) and the NO desorbed was monitored by a thermal conductivity cell attached to a gas chromatograph (Ohkura Electric, Model 701 and Hitachi Model 164). For nitrogen, oxygen and NO, a l-meter molecular sieve 5A column was employed at 343 K; for NzO, a l-meter Porapak Q column was used at 343 K. The concentration of NOa was monitored by an UV/VIS spectrophotometer (Hitachi Model 100-50). The amount of NO held on the catalyst surface was estimated from the peak area of the TPD curves. The infrared spectra of adsorbed NO species were recorded by means of a Fourier transform-infrared (FT-IR) spectrophotometer (Nihon Bunko FTIR 5M) with a diffuse reflectance attachment (Nihon Bunko DR-500/H ). The BET surface area of CuO was determined by nitrogen adsorption at 77 K. RESULTS AND DISCUSSION
TPD of NO adsorbed on unsupported CuO Fig. 1 illustrates the typical TPD curves for CuO calcined at 0.2 atm of oxygen at 773 K for 3 h. Curves (1) and (2) represent the results for CuO on which NO was previously adsorbed in flows of 4 vol.-% NO for 3 and 30 min., respectively. Two desorption peaks of NO appear with maxima at 493-553 K (peak I) and 723-753 K (peak II). The latter peak is always accompanied by oxygen desorption. Nitrogen dioxide (NO,) and NzO were not detected in the whole temperature range studied. In Fig. 2, the amounts of NO desorbed for
M. Shimokawabe et aLlApplied Catal. A 87 (1992) 205-218
208
I
I
373
573
773
Temoerature
/ K
Fig. 1. TPD curves of NO and O2 desorbed from CuO calcined at 773 K. Adsorption time of 4 vol.% NO: (1) 3; (2) 30 min. (p) NO; (----) 0,.
Amount
of NO dosed
I cm3
Fig. 2. Relation between number of NO desorbed and amount of NO dosed. (@) peak I; (0 ) peak II; (0) total.
peaks I and II and the total of these peaks are plotted against the amount of NO dosed on the catalysts. The amounts of NO desorbed increase with an increase in the amount of NO dosed, leveling off when the amount of NO dosed exceeds 60 cm3 NTP. The total amount of NO desorbed attained 1.7.10zo molecules g CuO-’ above 60 cm3 irrespective of the contact time and the vol.-% of NO in the feed. Hence, we concluded that the surface of CuO was saturated with NO adsorbed at the dosing amount of 60 cm3. In the following experiments, the adsorption of NO was carried out in a flow of 4 vol.-% NO for 30 min prior to the TPD runs, so that the amount of dosed NO reached 60 cm3. In the FT-IR spectra of adsorbed NO, absorptions appeared at 1880,X500, 1300 and 1030 cm-‘. They were ascribed to NO coordinated to copper (II) sites ( 1880 cm-‘) and unidentate surface nitrate (1500,1300, and 1030 cm- ’ ), respectively [ 15-211. The former NO species desorbed up to 600 K, whereas the latter species desorbed simultaneously with oxygen above 600 K. Hence, we concluded that peak I arose from the desorption of NO coordinated to copper (II) sites,
M. Shimokawabe et aLlApplied Catal. A 87 (1992) 205-218
209
I Cd* + NO I
I
Cd’-NO + I
whereas peak II arose from the composition
of unidentate
surface nitrate,
I
I
2 Cu”-0-N02+
2 Cu” + NO + 0, I
I
Fig. 3 shows the TPD curves of NO desorbed from CuO which were calcined at various partial pressures of oxygen at 773 K for 3 h prior to the adsorption of NO. The results for CuO calcined at 1,0.3,0.2 or 0 atm of oxygen are shown by curves(I), (2), (3), and (4), respectively. The area of peaks I and II vary markedly with the oxygen partial pressure. These individual peaks are further split into peak couples with increasing oxygen partial pressure. This suggests that two types of coordinatively adsorbed NO and surface nitrate species are formed over CuO. However, the fine structures of these species were unresolved by IR observations. In Fig. 4, the numbers of NO molecules desorbed at each peak are plotted along with oxygen molecules desorbed against the partial pressure of oxygen. It shows that the number of NO molecules for peak II increases with the partial pressure of oxygen at the expense of those for peak I up to 0.3 atm. The total amount of NO desorbed for peaks I and II are substantially constant over the whole range of oxygen partial pressure. The amount of oxygen desorbed is
I
I
373
573
173
Temperature / K
Fig. 3. TPD curves of NO and 0, desorbed from CuO calcined in oxygen with various partial pressures. Partial pressure of oxygen: (1) 1; (2) 0.3; (3) 0.2; (4) 0 atm. (p) NO; (---) 0,.
M. Shimokawabe et al./Applied Catal. A 87 (1992) 205-218
210
-,, 0:: 1-
__r_______._____-I__
71
,*=;-
’: d
I‘, _--________ - AQ ;,' 0 (, I' I I 0.2 0.4 0.6 0
0
0.8
1
O2 partial pressure / atm
Fig. 4. Relation between the number of NO molecules desorbed from CuO and the oxygen partial pressure of the pretreatment atmosphere. ((3 ) peak I; (0 ) peak II; (0 ) total; (q )O2 (at peak II).
practically the same as that of NO desorbed at temperatures where peak II appears. On the basis of these findings, we concluded that surface nitrate species are formed via a step 0 0 \ / N 0 0 0 NO I I I -cu-o-cu-o-- cu -o-cu-oin which adsorbed oxygen species are involved, whereas NO is molecularly coordinated to copper (II) sites. According to this adsorption mechanism, oxygen atoms inhibit the direct chemisorption of NO. However, when two oxygen atoms were present, one nitrate species was formed and one copper (II) site was recovered. Hence, the intensity of peak I decreased with the increase in that of peak II in a one-to-one ratio, and thereby the total peak intensity was kept constant. Determination of the surface area of unsupported CuO from TPD curve In the preceding section, we concluded that peak I and peak II arose from molecularly adsorbed NO and nitrate type species, respectively. In this section, the surface area of CuO catalysts was calculated on the basis of the amount of NO desorbed, and the validity of the calculation is examined by comparison with the surface area determined by the BET method. Table 1 shows the surface area derived from the BET and the TPD method for CuO catalysts calcined at 773 K at various partial pressures of oxygen. In this table, SBET rep-
M. Shimokawabe
et aLlApplied
Catal. A 87 (1992) 205-218
211
TABLE 1 Surface area of CuO (C) calcined at 773 K in various partial pressures of oxygen Partial pressure of oxygen (atm)
Surface area (m* g-‘) S BET
S TPD
0 0.2 0.3 1
14.7 14.7 14.7 14.7
12.6 15.2 14.0 14.0
Temperature
/ K
Fig. 5. TPD curves of CuO calcined at various temperatures. Calcination temperatures of CuO: (1) 573 K; (2) 773 K; (3) 973 K; (4) 1173 K. (---) NO; (---) 0,.
resents the BET surface area, and S rPD represents the surface area calculated from the total amount of NO desorbed in the TPD run. For the calculation of S TPD,the surface density of copper (II) ions was assumed to be l.l-lO’g atoms rn-’ by averaging those of the three low index planes as was done by Yao and Shelef [lo]. The table shows that the STPDvalues agree well with the SsET values. Fig. 5 illustrates the TPD curves of NO and oxygen desorbed from CuO calcined at 573,773,973 and 1173 K in a stream of 20 vol.-% of oxygen diluted by helium. Table 2 lists the surface area and the dispersion of these catalysts calculated on the basis of the amount of NO desorbed together with the BET surface area. It shows that the surface area calculated from the TPD method agrees well with the BET surface area, except for CuO (573). Hence, TPD of NO is available for the measurements of active surface area of unsupported CuO catalysts.
M. Shimokawabe et al./Applied Catal. A 87 (1992) 205-218
212 TABLE 2
Surface area and dispersion of CuO (C) calcined at various temperatures Catalyst
Surface area
WY
b” g-‘)
cue cue cue cue
(573) (773) (973) (1173)
Dispersion (% ) TPD
S BET
s TPD
55.4 14.7 7.6 0.36
38.8 15.2 8.8 0.41
2.70 1.57 0.61 0.031
“Calcination temperature.
I
I
373
I
Temperature
I
773
573 / K
Fig. 6. TPD curves of NO adsorbed on CuO/SiOz prepared from various copper materials. (1) CuO/SiO,(Am, 8.6 wt.-%); (2) CuO/SiOz(N, 10 wt.-%); (3) CuO/SiO,(Ac, 10 wt.-%).
TPD of NO adsorbed on CuO/SiO, and the determination of dispersion of CuO formed on SiO, Fig. 6 illustrates TPD curves obtained over three CuO/SiO, catalysts, which were prepared from various copper(I1) compounds by ion exchange or by an impregnation method and which were calcined at 773 K in 20 vol.-% oxygen diluted with helium. Two NO desorption peaks are seen at 383-423 and 64% 673 K. As observed on unsupported CuO, the second peak is accompanied by oxygen desorption. The intensity of the peak between 648 and 673 K is very
213
M. Shimokawabe et aLlApplied Catal. A 87 (1992) 205-218
weak compared with that of peak I. Over SiO, supports, no NO peak is discernible. Hence, we concluded that chemisorbed NO was predominantly located on the sites of CuO species formed on the support. In the FT-IR spectra of chemisorbed NO over CuO/SiOz(Am), the absorptions were seen at 1880, 1610, 1580, and 1300 cm-l. The absorption at 1880 cm-l diminished upon heating up to 513 K. The other three absorptions began to decrease above 513 K. According to the literature [ 15-211 the absorption at 1880 cm-’ was ascribed to NO species coordinated to surface copper (II) ions whereas those at 1610,1580, and 1300 cm-’ were ascribed to bidentate surface nitrate species. On the basis of these findings, we concluded that peak I between 383-423 K resulted from the desorption of NO species chemisorbed on copper (II) sites of CuO species whereas the peak between 648-673 K resulted from that of bidentate surface nitrate species. As shown in the previous section, NO reacted with chemisorbed oxygen species, forming surface nitrate. Hence, it is highly probable that two chemisorbed oxygen species were involved in the formation of one bidentate surface nitrate species. Since the oxygen species covered surface copper (II) sites, the number of copper (II) sites originally present was estimated from the sum of the number of NO molecules coordinated to surface copper (II ) ions and was twice that of the surface nitrate. On the basis of the estimated number of the copper (II) sites and the total number of copper (II) ions loaded on SiOa, the dispersion of CuO species formed on the support was calculated. The average particle size of CuO species was also evaluated on the basis of the number of the copper (II) sites and the average surface density (l.l*lO” atoms m-‘) of CuO [lo]. The dispersion and the average particle size of CuO species were calculated within errors of t 5%. Table 3 lists the estimated values of the dispersion and the average particle size of CuO species formed on CuO/SiO, (Am), CuO/SiOZ (N) and CuO/ SiO, (AC). The values for CuO/SiO, (Am) are practically the same as those for CuO/SiO, (N). On the other hand, CuO species for CuO/SiO, ( AC) is poorly TABLE 3 Dispersion and particle size of various CuO/SiO, catalysts CuO/SiO, (wt.-%)
Starting material (loading method)
Dispersion (%)
Particle size (nm)
Am (8.6)
Tetrammine (ion exchange) Nitrate (impregnation) Acetate (impregnation)
10.6
12.9
12.1
11.3
3.9
34.9
N (10) AC (lo)
M. Shimokawabe et aLlApplied Catal. A 87 (1992) 205-218
214
dispersed. The dispersion for CuO/SiO,(Ac) CuO/SiOZ (Am) and CuO/SiO, (N).
is about one third of those for
Effects of loading amounts and calcination temperature on the average particle size of CuO/Si02 Fig. 7 illustrates the TPD curves of NO for CuO/SiO, (N) with various copper loadings. At copper loadings below 60 wt.-%, the desorption of NO becomes appreciable. Table 4 summarizes the dispersion and the average particle size of CuO species for CuO/SiO,(N) and CuO/SiO,(Ac). The dispersion decreases with the increase of the copper loading. Table 5 shows the dispersion and the average particle size of CuO formed on various CuO/SiOz(Am) which were calcined at 773 K. The dispersion increases with the decrease in copper loadings to 1 wt.-%. However at copper loading of 0.5 wt.-%, the dispersion decreases appreciably. As van Dillen et al. suggested [22], CuO species probably reacted with SiOZ at 773 K, forming copper (II) silicate. This would result in a lowering of the amount of NO adsorbed. Table 6 shows how the dispersion and the average particle size of CuO species change with the calcination temperature for CuO/SiOa (Am). The dispersion and the average particle size are greatly affected by the calcination temperature, in particular at copper loadings above 4.6 wt.-%.
373
573 Tenlperature
773 I K
Fig. 7. TPD curves of NO adsorbed on CuO/SiO,(N) Loadingamountsofcopper:
with various loading amounts of copper. (1) 70; (2) 56; (3) 50; (4) 10; (5) 1 wt.-%.
215
M. Shimokawabe et al./Applied Catal. A 87 (1992) 205-218 TABLE 4 Dispersion and particle size of CuO supported on SiO, Catalysts (wt.-%)
Dispersion (%)
Particle size (nm)
CuO/SiO,(N)
1 10 50 56 70 90 100
15.1 12.1 2.9 0.18 ND” ND ND
9.03 11.3 47.0 7.5*102 ND ND ND
CuO/SiO, (AC)
1 10 50 70 80 90 100
27.9 3.9 1.03 0.063 0.005 0.001 ND
4.89 34.9 132 2.2.103 2.7. lo4 1.4.105 ND
“ND: not determined. TABLE 5 Dispersion and particle size of CuO/SiOZ (Am) catalysts with various copper loadings Copper loading (wt.-%)
Dispersion (%)
Particle size (nm)
0.5 0.7 1.0 1.4 2.0 2.9 4.6 8.6 19.7 30.0
55.4 75.2 53.4 62.8 44.0 20.4 24.3 10.6 4.8 1.5
2.46 1.81 2.55 2.17 3.10 6.68 5.61 12.9 28.4 91.0
For the catalysts with 0.5 wt.-% of copper calcined at 573 K, the dispersion attains 92.37%, suggesting that CuO species are dispersed practically on an atomic scale. In a previous paper [ 131, we showed that the precursor states of CuO/SiO, (Am) catalysts were greatly affected by the copper loading and the calcination temperature. Isolated copper (II) species, highly dispersed CuO clusters and bulky CuO exist depending upon these preparation conditions. Fig. 8 shows the estimated particle sizes of CuO species against calcination
M. Shimokawabe et aLlApplied Catal. A 87 (1992) 205-218
216 TABLE 6
Dispersion and particle size of CuO/SiO,
(Am) catalysts calcined at various temperatures
Dispersion (% )
Copper loading (wt.-%)
0.5 1.0 4.6 8.6
Particle size (nm)
573”
773”
1073”
573”
773”
1073”
92.3 61.1 38.2 29.6
55.4 53.4 24.3 10.6
36.3 44.5 10.2 4.3
1.48 2.23 3.57 4.60
2.46 2.55 5.61 12.9
3.75 3.06 13.4 31.7
“Calcination temperature (K )
t
Copper
loading,
wt.-%
Fig. 8. Relation between structure and particle size of copper catalysts, calcination temperature and copper loading. I: isolated copper (II) diammine; II: clustered copper (II) diammine; III: isolated copper (II) (or CuO ) ; IV: highly dispersed CuO; V: bulky CuO.
temperature and the loading amounts of copper. It shows that the precursor species which were previously characterized as isolated copper (II) species (or isolated CuO ), highly dispersed CuO clusters and bulky CuO, are fine particles of CuO with an average diameter of 1.5-2.5, 5-13, and over 15-30 nm, respectively. CONCLUSIONS
The thermal desorption of NO adsorbed on unsupported CuO and CuO/SiO, catalysts was investigated by means of a temperature-programmed method. In both cases, two NO desorption peaks appeared with maxima in the temperature range of 383-573 and 648-753 K. The latter peak was always accompanied by the desorption of oxygen. With the help of FT-IR spectroscopy, we con-
M. Shimokawabe et aLlApplied Catal. A 87 (1992) 205-218
217
eluded that the peak between 383-573 K resulted from NO adsorbed on copper(I1) sites, and the peak between 648-753 K resulted from surface nitrate species formed by the reaction of NO with adsorbed oxygen. Unidentate surface nitrate was formed on unsupported CuO, whereas bidentate surface nitrate was formed on CuO/SiO,. The surface area and the percentage exposed (dispersion) of unsupported CuO were calculated on the basis of the amount of NO desorbed. They agreed closely with the corresponding values estimated from the BET surface area. Over CuO/SiOa, NO was chemisorbed selectively on CuO species formed on the support. On the basis of the amount of NO desorbed, the average particle sizes of CuO species previously assigned to isolated copper (II ) ions (or isolated CuO), highly dispersed CuO clusters and bulky CuO, were estimated to be 1.52.5,5-13 and over 15-30 nm, respectively. ACKNOWLEDGEMENT
The authors thank Mr. S. Fujita of Hokkaido University for his helpful advice with the IR measurements. NOTE
The present work was presented at the Chemical Society of Japan Meeting, April 1, 1985, Tokyo, the symposium of the Society of Calorimetry and Thermal Analysis, Japan, September 25, 1985, Sapporo, the Catalysis Society Meeting, October 7,1985, Kanazawa and the Chemical Society of Japan Meeting, April 1, 1986, Kyoto.
REFERENCES
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J.R Anderson, Structure of Metallic Catalysts, Academic Press, New York (1975). R.M. Dell, F.S. Stone and P.F. Tiley, Trans. Faraday Sot., 49 (1952) 159. J.J.F. Scholten and J.A. Konvalinka, Trans. Faraday Sot., 65 (1969) 2645. Th.J. Osinga, B.G. Linsen and W.P. van Beet, J. Catal., 47 (1979) 277. G. Sengpta, D.K. Gupta, M.L. Kunden and S.P. Sen, J. Catal., 67 (1981) 223. K. Narita, N. Takezawa, H. Kobayashi and I. Toyoshima, React. Kinet. Catal. Lett., 19 (1982) 91. M.J. Luys, P.M. van Oeffelt, W.G.J. Brouwer, A.P. Pijpers and J.J.F. Scholten, Appl. Catal., 46 (1989) 161. M.J. Luys, P.M. van Oeffelt, P. Pieter and R. Ter Veen, Catal. Today, 10 (1991) 283. N. Takezawa, H. Kobayashi, Y. Kamegai and M. Shimokawabe, Appl. Catal., 3 (1982) 381. H.C. Yao and M. Shelef, Catalytic Chemistry of Nitrogen Oxides, Plenum Press, London, 1975, p. 45 M. Shelef, Catal. Rev. Sci. Tech., 11 (1975) 1. H.S. Gandhi and M. Shelef, J. Catal., 28 (1973) 1.
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M. Shimokawabe et al./Applied Catal. A 87 (1992) 205-218 M. Shimokawabe, N. Takezawa and H. Kobayashi, Appl. Catal., 2 (1982) 379. M. Shimokawabe, N. Takezawa and H. Kobayashi, Bull. Chem. Sot. Jpn., 56 (1983) 1337. B.M. Gatehouse, S.E. Livingstone and R.S. Nyholm, J. Chem. Sot., (1957) 4222. N.F. Curtis and Y.M. Curtis, Inorg. Chem., 4 (1965) 804. L.H.Little, Infrared Spectra of Adsorbed Species, Academic Press, New York, 1966, p. 84. J.W. London and A.T. Bell, J. Catal., 31 (1973) 32,96. Yu.A. Loklov and A.A. Davydov, Kinet. Catal., 20 (1979) 1235,1498. A.A. Davydov and A.A. Budneva, React. Kinet. Catal. Lett., 25 (1984) 121. G. Blyholder and M.C. Allen, J. Phys. Chem., 70 (1966) 352. J.A. van Dillen, J.W. Geus, L.A.M. Hermans and J. van der Meijden, in G.C. Bond, P.B. Wells and F.C. Tompkins (Editors), Proc. 6th Internat. Congress on Catal., Vol. 2, The Chemical Society, London, 1976, p. 677.
205
Elsevier Science Publishers B.V., Amsterdam APCAT
A2308
Characterization of unsupported cupric oxide and cupric oxide/silica catalysts by temperature-programmed desorption of nitrogen monoxide M. Shimokawabe, N. Hatakeyama, K. Shimada, K. Tadokoro and N. Takezawa Department of Chemical Process Engineering, Hokkaido University, Sapporo 060 (Japan) (Received 3 January 1992, revised manuscript received 3 April 1992)
Abstract The thermal desorption of nitrogen monoxide (NO) adsorbed on unsupported CuO and CuO/SiOZ was investigated by means of a temperature-programmed method. In both cases, two NO desorption peaks appeared with maxima in the temperature range of 383-573 and 648-753 K. It was concluded that the former peak was the result of NO adsorbed on copper(I1) sites, and the latter peak, which was accompanied by oxygen desorption, was the result of nitrate type species formed by the reaction of NO with adsorbed oxygen. Over CuO/SiOx, NO was chemisorbed selectively on CuO species formed on the support. The surface area and the percentage exposed (dispersion ) of unsupported CuO were calculated on the basis of the amount of NO desorbed. These values closely agreed with the corresponding ones estimated from the BET surface area. In a similar manner, the dispersion and the average particle size of CuO species formed on silica were calculated. It was shown that these species, which were previously characterized as isolated copper (II) species (or isolated CuO), highly dispersed CuO clusters and bulky CuO, are fine particles of CuO with an average diameter of l&2.5,5-13 and over 15-30 nm, respectively.
Keywords: catalyst characterization (TPD),
NO-TPD, cupric oxide, cupric oxide/silica.
INTRODUCTION
Temperature-programmed desorption (TPD) is one of the most useful methods for the characterization of catalysts, and gives information about the nature and the amount of adsorbed species. The dispersion of the active component supported on carrier is frequently estimated from the amount of chemisorbed species and/or the transmission electron microscopy (TEM) observation [ 11. It has been shown that the dispersion of metallic copper for supported copper catalysts can be estimated from the amount of nitrogen evolved from the reaction with dinitrogen monoxide ( NzO ) [ 2-81, Correspondence to: Dr. M. Shimokawabe, kaido University,
0926-3373/92/$05.00
Department of Chemical Process Engineering, Sapporo 060, Japan. Tel. (+81-11)7162111, fax. (+81-11)7264454.
0 1992 Elsevier Science Publishers B.V. All rights reserved.
Hok-
M. Shimokawabe et al./Applied Catal. A 87 (1992) 205-218
206
0
~“\C~+N,O-Cu~
cu
'Cu+N,
A limited number of papers have been published on the measurement of the dispersions of copper (I) [ 91 and copper (II) oxides [ 10-121. Shelef and coworkers [lo-121 have shown that the active surface area of CuO can be determined by NO chemisorption at isothermal conditions. However, the states of chemisorbed species on CuO were not elucidated. In our previous work [ 13 1, CuO/SiO, catalysts prepared by the ion-exchange method were characterized by means of various analytical methods such as differential thermal analysis (DTA), thermogravimetry (TG), ultraviolet/visible near infrared/spectroscopy (UV/VIS/NIR), X-ray photoelectron spectroscopy (XPS ) , Auger emission spectroscopy ( AES ) and X-ray diffraction (XRD ) . It was found that the precursor states of the catalysts depended qua sensitivity upon the copper loading and the calcination temperature. The copper species formed on the support were classified into three groups; isolated copper (II) ions (or isolated CuO), highly dispersed CuO clusters and bulky CuO. However, the dispersion and the average particle size of these precursor species have not been determined yet. In the present work, the adsorption of NO on CuO and CuO/SiO, were studied by the temperature-programmed desorption method. On the basis of the states of NO chemisorbed and the amount of NO desorbed, the surface area of CuO, and the dispersion and the average particle size of CuO formed on the support were determined as functions of the preparation parameters. EXPERIMENTAL
Catalysts Samples of copper (II) oxide were prepared by decomposition of basic copper (II) carbonate (Kant0 Chemicals, extra pure grade) in air in the temperature range of 573-1173 K for 3 h. CuO/SiO, (Am) was prepared in a similar manner to the catalysts employed in our previous work [ 13,141. Hydroxyl protons on silica gel (Nihon Chromato., 60-80 mesh in granules) were exchanged at pH 11-12 and at 293 K overnight with tetrammine copper (II) cations which had been prepared from copper (II) nitrate ( Wako Pure Chemicals, extra pure grade) and an aqueous ammonia solution. The ion-exchanged silica gel was filtered, washed with distilled water and dried at 383 K for 12 h, and was further calcined in air for 3 h under various conditions covering a temperature range of 573-1073 K. CuO/SiO,(N) and CuO/SiO,(Ac) were prepared by impregnation of silica gel with aqueous solutions of copper (II) nitrate and copper (II) acetate, respectively, in a rotary evaporator at 343 K. The precursors were then dried at 383 K for 12 h and were calcined in air at 773 K for 3 h. The copper
M. Shimokawabe et aLlApplied Catal. A 87 (1992) 205-218
201
loadings were estimated from the amount of copper(I1) ions in the mother solution and in the filtrate. The amount of copper (II) ions were determined by titration with EDTA solution of known concentration by use of l- (2-Pyridylazo) -2-naphthol (PAN) as an indicator. Copper loading varied in the range of 0.5-90 wt.-% copper. Temperature-programmed
desorption (TPD)
Catalyst samples, 0.2 g of CuO or 0.25 to 3.0 g of CuO/Si02, were placed on loosely packed quartz wool in a reactor which was made of quartz tubing of 12 mm in diameter. The samples were pretreated in streams of oxygen diluted with helium at various partial pressures or in helium at a total flow-rate of 50 cm3 min-’ NTP under various conditions covering a temperature range of 573-1073 K for 3 h. Thereafter the reactor was cooled to room temperature and the oxygen stream was switched to a helium stream until no oxygen was detected in the effluent. NO was then adsorbed at room temperature in a flow of 4 vol.-% NO/He at a total flow-rate of 50 cm3 min-’ for 30 min unless otherwise stated. NO in the gas-phase was then flushed with helium until no NO was detected in the effluent. TPD runs were started from room temperature to 823 K at a heating rate of 10 K min-l with a programmable temperature controller (Muto HM-80)) and the NO desorbed was monitored by a thermal conductivity cell attached to a gas chromatograph (Ohkura Electric, Model 701 and Hitachi Model 164). For nitrogen, oxygen and NO, a l-meter molecular sieve 5A column was employed at 343 K; for NzO, a l-meter Porapak Q column was used at 343 K. The concentration of NOa was monitored by an UV/VIS spectrophotometer (Hitachi Model 100-50). The amount of NO held on the catalyst surface was estimated from the peak area of the TPD curves. The infrared spectra of adsorbed NO species were recorded by means of a Fourier transform-infrared (FT-IR) spectrophotometer (Nihon Bunko FTIR 5M) with a diffuse reflectance attachment (Nihon Bunko DR-500/H ). The BET surface area of CuO was determined by nitrogen adsorption at 77 K. RESULTS AND DISCUSSION
TPD of NO adsorbed on unsupported CuO Fig. 1 illustrates the typical TPD curves for CuO calcined at 0.2 atm of oxygen at 773 K for 3 h. Curves (1) and (2) represent the results for CuO on which NO was previously adsorbed in flows of 4 vol.-% NO for 3 and 30 min., respectively. Two desorption peaks of NO appear with maxima at 493-553 K (peak I) and 723-753 K (peak II). The latter peak is always accompanied by oxygen desorption. Nitrogen dioxide (NO,) and NzO were not detected in the whole temperature range studied. In Fig. 2, the amounts of NO desorbed for
M. Shimokawabe et aLlApplied Catal. A 87 (1992) 205-218
208
I
I
373
573
773
Temoerature
/ K
Fig. 1. TPD curves of NO and O2 desorbed from CuO calcined at 773 K. Adsorption time of 4 vol.% NO: (1) 3; (2) 30 min. (p) NO; (----) 0,.
Amount
of NO dosed
I cm3
Fig. 2. Relation between number of NO desorbed and amount of NO dosed. (@) peak I; (0 ) peak II; (0) total.
peaks I and II and the total of these peaks are plotted against the amount of NO dosed on the catalysts. The amounts of NO desorbed increase with an increase in the amount of NO dosed, leveling off when the amount of NO dosed exceeds 60 cm3 NTP. The total amount of NO desorbed attained 1.7.10zo molecules g CuO-’ above 60 cm3 irrespective of the contact time and the vol.-% of NO in the feed. Hence, we concluded that the surface of CuO was saturated with NO adsorbed at the dosing amount of 60 cm3. In the following experiments, the adsorption of NO was carried out in a flow of 4 vol.-% NO for 30 min prior to the TPD runs, so that the amount of dosed NO reached 60 cm3. In the FT-IR spectra of adsorbed NO, absorptions appeared at 1880,X500, 1300 and 1030 cm-‘. They were ascribed to NO coordinated to copper (II) sites ( 1880 cm-‘) and unidentate surface nitrate (1500,1300, and 1030 cm- ’ ), respectively [ 15-211. The former NO species desorbed up to 600 K, whereas the latter species desorbed simultaneously with oxygen above 600 K. Hence, we concluded that peak I arose from the desorption of NO coordinated to copper (II) sites,
M. Shimokawabe et aLlApplied Catal. A 87 (1992) 205-218
209
I Cd* + NO I
I
Cd’-NO + I
whereas peak II arose from the composition
of unidentate
surface nitrate,
I
I
2 Cu”-0-N02+
2 Cu” + NO + 0, I
I
Fig. 3 shows the TPD curves of NO desorbed from CuO which were calcined at various partial pressures of oxygen at 773 K for 3 h prior to the adsorption of NO. The results for CuO calcined at 1,0.3,0.2 or 0 atm of oxygen are shown by curves(I), (2), (3), and (4), respectively. The area of peaks I and II vary markedly with the oxygen partial pressure. These individual peaks are further split into peak couples with increasing oxygen partial pressure. This suggests that two types of coordinatively adsorbed NO and surface nitrate species are formed over CuO. However, the fine structures of these species were unresolved by IR observations. In Fig. 4, the numbers of NO molecules desorbed at each peak are plotted along with oxygen molecules desorbed against the partial pressure of oxygen. It shows that the number of NO molecules for peak II increases with the partial pressure of oxygen at the expense of those for peak I up to 0.3 atm. The total amount of NO desorbed for peaks I and II are substantially constant over the whole range of oxygen partial pressure. The amount of oxygen desorbed is
I
I
373
573
173
Temperature / K
Fig. 3. TPD curves of NO and 0, desorbed from CuO calcined in oxygen with various partial pressures. Partial pressure of oxygen: (1) 1; (2) 0.3; (3) 0.2; (4) 0 atm. (p) NO; (---) 0,.
M. Shimokawabe et al./Applied Catal. A 87 (1992) 205-218
210
-,, 0:: 1-
__r_______._____-I__
71
,*=;-
’: d
I‘, _--________ - AQ ;,' 0 (, I' I I 0.2 0.4 0.6 0
0
0.8
1
O2 partial pressure / atm
Fig. 4. Relation between the number of NO molecules desorbed from CuO and the oxygen partial pressure of the pretreatment atmosphere. ((3 ) peak I; (0 ) peak II; (0 ) total; (q )O2 (at peak II).
practically the same as that of NO desorbed at temperatures where peak II appears. On the basis of these findings, we concluded that surface nitrate species are formed via a step 0 0 \ / N 0 0 0 NO I I I -cu-o-cu-o-- cu -o-cu-oin which adsorbed oxygen species are involved, whereas NO is molecularly coordinated to copper (II) sites. According to this adsorption mechanism, oxygen atoms inhibit the direct chemisorption of NO. However, when two oxygen atoms were present, one nitrate species was formed and one copper (II) site was recovered. Hence, the intensity of peak I decreased with the increase in that of peak II in a one-to-one ratio, and thereby the total peak intensity was kept constant. Determination of the surface area of unsupported CuO from TPD curve In the preceding section, we concluded that peak I and peak II arose from molecularly adsorbed NO and nitrate type species, respectively. In this section, the surface area of CuO catalysts was calculated on the basis of the amount of NO desorbed, and the validity of the calculation is examined by comparison with the surface area determined by the BET method. Table 1 shows the surface area derived from the BET and the TPD method for CuO catalysts calcined at 773 K at various partial pressures of oxygen. In this table, SBET rep-
M. Shimokawabe
et aLlApplied
Catal. A 87 (1992) 205-218
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TABLE 1 Surface area of CuO (C) calcined at 773 K in various partial pressures of oxygen Partial pressure of oxygen (atm)
Surface area (m* g-‘) S BET
S TPD
0 0.2 0.3 1
14.7 14.7 14.7 14.7
12.6 15.2 14.0 14.0
Temperature
/ K
Fig. 5. TPD curves of CuO calcined at various temperatures. Calcination temperatures of CuO: (1) 573 K; (2) 773 K; (3) 973 K; (4) 1173 K. (---) NO; (---) 0,.
resents the BET surface area, and S rPD represents the surface area calculated from the total amount of NO desorbed in the TPD run. For the calculation of S TPD,the surface density of copper (II) ions was assumed to be l.l-lO’g atoms rn-’ by averaging those of the three low index planes as was done by Yao and Shelef [lo]. The table shows that the STPDvalues agree well with the SsET values. Fig. 5 illustrates the TPD curves of NO and oxygen desorbed from CuO calcined at 573,773,973 and 1173 K in a stream of 20 vol.-% of oxygen diluted by helium. Table 2 lists the surface area and the dispersion of these catalysts calculated on the basis of the amount of NO desorbed together with the BET surface area. It shows that the surface area calculated from the TPD method agrees well with the BET surface area, except for CuO (573). Hence, TPD of NO is available for the measurements of active surface area of unsupported CuO catalysts.
M. Shimokawabe et al./Applied Catal. A 87 (1992) 205-218
212 TABLE 2
Surface area and dispersion of CuO (C) calcined at various temperatures Catalyst
Surface area
WY
b” g-‘)
cue cue cue cue
(573) (773) (973) (1173)
Dispersion (% ) TPD
S BET
s TPD
55.4 14.7 7.6 0.36
38.8 15.2 8.8 0.41
2.70 1.57 0.61 0.031
“Calcination temperature.
I
I
373
I
Temperature
I
773
573 / K
Fig. 6. TPD curves of NO adsorbed on CuO/SiOz prepared from various copper materials. (1) CuO/SiO,(Am, 8.6 wt.-%); (2) CuO/SiOz(N, 10 wt.-%); (3) CuO/SiO,(Ac, 10 wt.-%).
TPD of NO adsorbed on CuO/SiO, and the determination of dispersion of CuO formed on SiO, Fig. 6 illustrates TPD curves obtained over three CuO/SiO, catalysts, which were prepared from various copper(I1) compounds by ion exchange or by an impregnation method and which were calcined at 773 K in 20 vol.-% oxygen diluted with helium. Two NO desorption peaks are seen at 383-423 and 64% 673 K. As observed on unsupported CuO, the second peak is accompanied by oxygen desorption. The intensity of the peak between 648 and 673 K is very
213
M. Shimokawabe et aLlApplied Catal. A 87 (1992) 205-218
weak compared with that of peak I. Over SiO, supports, no NO peak is discernible. Hence, we concluded that chemisorbed NO was predominantly located on the sites of CuO species formed on the support. In the FT-IR spectra of chemisorbed NO over CuO/SiOz(Am), the absorptions were seen at 1880, 1610, 1580, and 1300 cm-l. The absorption at 1880 cm-l diminished upon heating up to 513 K. The other three absorptions began to decrease above 513 K. According to the literature [ 15-211 the absorption at 1880 cm-’ was ascribed to NO species coordinated to surface copper (II) ions whereas those at 1610,1580, and 1300 cm-’ were ascribed to bidentate surface nitrate species. On the basis of these findings, we concluded that peak I between 383-423 K resulted from the desorption of NO species chemisorbed on copper (II) sites of CuO species whereas the peak between 648-673 K resulted from that of bidentate surface nitrate species. As shown in the previous section, NO reacted with chemisorbed oxygen species, forming surface nitrate. Hence, it is highly probable that two chemisorbed oxygen species were involved in the formation of one bidentate surface nitrate species. Since the oxygen species covered surface copper (II) sites, the number of copper (II) sites originally present was estimated from the sum of the number of NO molecules coordinated to surface copper (II ) ions and was twice that of the surface nitrate. On the basis of the estimated number of the copper (II) sites and the total number of copper (II) ions loaded on SiOa, the dispersion of CuO species formed on the support was calculated. The average particle size of CuO species was also evaluated on the basis of the number of the copper (II) sites and the average surface density (l.l*lO” atoms m-‘) of CuO [lo]. The dispersion and the average particle size of CuO species were calculated within errors of t 5%. Table 3 lists the estimated values of the dispersion and the average particle size of CuO species formed on CuO/SiO, (Am), CuO/SiOZ (N) and CuO/ SiO, (AC). The values for CuO/SiO, (Am) are practically the same as those for CuO/SiO, (N). On the other hand, CuO species for CuO/SiO, ( AC) is poorly TABLE 3 Dispersion and particle size of various CuO/SiO, catalysts CuO/SiO, (wt.-%)
Starting material (loading method)
Dispersion (%)
Particle size (nm)
Am (8.6)
Tetrammine (ion exchange) Nitrate (impregnation) Acetate (impregnation)
10.6
12.9
12.1
11.3
3.9
34.9
N (10) AC (lo)
M. Shimokawabe et aLlApplied Catal. A 87 (1992) 205-218
214
dispersed. The dispersion for CuO/SiO,(Ac) CuO/SiOZ (Am) and CuO/SiO, (N).
is about one third of those for
Effects of loading amounts and calcination temperature on the average particle size of CuO/Si02 Fig. 7 illustrates the TPD curves of NO for CuO/SiO, (N) with various copper loadings. At copper loadings below 60 wt.-%, the desorption of NO becomes appreciable. Table 4 summarizes the dispersion and the average particle size of CuO species for CuO/SiO,(N) and CuO/SiO,(Ac). The dispersion decreases with the increase of the copper loading. Table 5 shows the dispersion and the average particle size of CuO formed on various CuO/SiOz(Am) which were calcined at 773 K. The dispersion increases with the decrease in copper loadings to 1 wt.-%. However at copper loading of 0.5 wt.-%, the dispersion decreases appreciably. As van Dillen et al. suggested [22], CuO species probably reacted with SiOZ at 773 K, forming copper (II) silicate. This would result in a lowering of the amount of NO adsorbed. Table 6 shows how the dispersion and the average particle size of CuO species change with the calcination temperature for CuO/SiOa (Am). The dispersion and the average particle size are greatly affected by the calcination temperature, in particular at copper loadings above 4.6 wt.-%.
373
573 Tenlperature
773 I K
Fig. 7. TPD curves of NO adsorbed on CuO/SiO,(N) Loadingamountsofcopper:
with various loading amounts of copper. (1) 70; (2) 56; (3) 50; (4) 10; (5) 1 wt.-%.
215
M. Shimokawabe et al./Applied Catal. A 87 (1992) 205-218 TABLE 4 Dispersion and particle size of CuO supported on SiO, Catalysts (wt.-%)
Dispersion (%)
Particle size (nm)
CuO/SiO,(N)
1 10 50 56 70 90 100
15.1 12.1 2.9 0.18 ND” ND ND
9.03 11.3 47.0 7.5*102 ND ND ND
CuO/SiO, (AC)
1 10 50 70 80 90 100
27.9 3.9 1.03 0.063 0.005 0.001 ND
4.89 34.9 132 2.2.103 2.7. lo4 1.4.105 ND
“ND: not determined. TABLE 5 Dispersion and particle size of CuO/SiOZ (Am) catalysts with various copper loadings Copper loading (wt.-%)
Dispersion (%)
Particle size (nm)
0.5 0.7 1.0 1.4 2.0 2.9 4.6 8.6 19.7 30.0
55.4 75.2 53.4 62.8 44.0 20.4 24.3 10.6 4.8 1.5
2.46 1.81 2.55 2.17 3.10 6.68 5.61 12.9 28.4 91.0
For the catalysts with 0.5 wt.-% of copper calcined at 573 K, the dispersion attains 92.37%, suggesting that CuO species are dispersed practically on an atomic scale. In a previous paper [ 131, we showed that the precursor states of CuO/SiO, (Am) catalysts were greatly affected by the copper loading and the calcination temperature. Isolated copper (II) species, highly dispersed CuO clusters and bulky CuO exist depending upon these preparation conditions. Fig. 8 shows the estimated particle sizes of CuO species against calcination
M. Shimokawabe et aLlApplied Catal. A 87 (1992) 205-218
216 TABLE 6
Dispersion and particle size of CuO/SiO,
(Am) catalysts calcined at various temperatures
Dispersion (% )
Copper loading (wt.-%)
0.5 1.0 4.6 8.6
Particle size (nm)
573”
773”
1073”
573”
773”
1073”
92.3 61.1 38.2 29.6
55.4 53.4 24.3 10.6
36.3 44.5 10.2 4.3
1.48 2.23 3.57 4.60
2.46 2.55 5.61 12.9
3.75 3.06 13.4 31.7
“Calcination temperature (K )
t
Copper
loading,
wt.-%
Fig. 8. Relation between structure and particle size of copper catalysts, calcination temperature and copper loading. I: isolated copper (II) diammine; II: clustered copper (II) diammine; III: isolated copper (II) (or CuO ) ; IV: highly dispersed CuO; V: bulky CuO.
temperature and the loading amounts of copper. It shows that the precursor species which were previously characterized as isolated copper (II) species (or isolated CuO ), highly dispersed CuO clusters and bulky CuO, are fine particles of CuO with an average diameter of 1.5-2.5, 5-13, and over 15-30 nm, respectively. CONCLUSIONS
The thermal desorption of NO adsorbed on unsupported CuO and CuO/SiO, catalysts was investigated by means of a temperature-programmed method. In both cases, two NO desorption peaks appeared with maxima in the temperature range of 383-573 and 648-753 K. The latter peak was always accompanied by the desorption of oxygen. With the help of FT-IR spectroscopy, we con-
M. Shimokawabe et aLlApplied Catal. A 87 (1992) 205-218
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eluded that the peak between 383-573 K resulted from NO adsorbed on copper(I1) sites, and the peak between 648-753 K resulted from surface nitrate species formed by the reaction of NO with adsorbed oxygen. Unidentate surface nitrate was formed on unsupported CuO, whereas bidentate surface nitrate was formed on CuO/SiO,. The surface area and the percentage exposed (dispersion) of unsupported CuO were calculated on the basis of the amount of NO desorbed. They agreed closely with the corresponding values estimated from the BET surface area. Over CuO/SiOa, NO was chemisorbed selectively on CuO species formed on the support. On the basis of the amount of NO desorbed, the average particle sizes of CuO species previously assigned to isolated copper (II ) ions (or isolated CuO), highly dispersed CuO clusters and bulky CuO, were estimated to be 1.52.5,5-13 and over 15-30 nm, respectively. ACKNOWLEDGEMENT
The authors thank Mr. S. Fujita of Hokkaido University for his helpful advice with the IR measurements. NOTE
The present work was presented at the Chemical Society of Japan Meeting, April 1, 1985, Tokyo, the symposium of the Society of Calorimetry and Thermal Analysis, Japan, September 25, 1985, Sapporo, the Catalysis Society Meeting, October 7,1985, Kanazawa and the Chemical Society of Japan Meeting, April 1, 1986, Kyoto.
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