- Email: [email protected]
Preparation of Manganese Oxide Catalysts Using Novel NH4MnO4 and Manganese Hydroxide Precursors. Comparison of Unsupported and Alumina Supported Catalysts
G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Editors), Preparation of Catalysts V 01991Elsevier Science Publishers B.V., Amsterdam -Printed in The Netherlands
617
PREPARATION OF MANGANESE OXIDE CATALYSTS USING NOVEL NH4MnO4 AND MANGANESE HYDROXIDE PRECURSORS. COMPARISON OF UNSUPPORTED AND ALUMINA SUPPORTED CATALYSTS A.K.H.
N O H M A N ~ ~D. ~ ,DUPREZ~,c. KAPPENSTEIN~, S.A.A. MANSOUR' AND M.I. ZAKI'
l c h e m i s t r y Department, F a c u l t y o f Science, M i n i a U n i v e r s i t y , El-Minia, EGYPT 2Catalyse en Chimie Organique, Faculte des Sciences de P o i t i e r s , FRANCE.
SUHMARY
Unsupported and a1 umina supported manganese oxide c a t a l y s t s were prepared using manganese n i t r a t e , manganese hydroxide and ammonium permanganate. They were b u l k and surface c h a r a c t e r i z e d by thermal analysis, X-ray d i f f r a c t i o n , d i f f u s e r e f l e c t a n c e , I R and photoelectron spectroscopy, SBET and TPR. Moreover H202 decomposition and CO o x i d a t i o n were used as t e s t r e a c t i o n s . The most a c t i v e supported c a t a l y s t s are t h e manganese hydroxide coated samples which show a rMn 03 phase. For t h e ammonium permanganate-based c a t a l y s t s a s t r o n g i n t e r a c t i o n w i t h t h e c a r r i e r was evidenced.
INTRODUCTION
Studies concerning supported manganese oxides are relatively scarce, despite their potential activity in oxidation reactions. For example manganese oxide catalysts are very active for CO oxidation, particularly when they are promoted with CuO or COO (1). They are also used for methanol oxidation and ethylene hydrogenation (2). These catalysts were first prepared and investigated by Selwood et a1 ( 3 ) by impregnating manganese (11) nitrate onto high surface area alumina and then thermally decomposed. Thereafter, these catalysts were characterized by several techniques (4-9). Baltanas et a1 (2) prepared these catalysts by impregnating manganese nitrate onto alumina and in situ precipitation of manganese hydroxide by ammonia solution. The aim of the present study was to prepare various series of bulk and alumina-supported manganese oxide catalysts. Novel NH4Mn04 and manganese hydroxide precursors as well as the conventional manganese nitrate were used in hope of defining the impacts of the precursors and the support on physico-chemical characteristics and activity of the final catalysts. KMn04 was previously used by Cavallaro et a1 ( 8 ) but with little information. We have taken NH4Mn04 for our investigation to avoid
618
more complications arising from the presence of potassium. By adopting a coating procedure in case of manganese hydroxide, we aimed to have surface layers of manganese oxide on alumina which may be more easily detectable than in the case of the two previous precursors. EXPWIWENTAt
Materials High surface area r-alumina (214 m2 g-l) was obtained by slow gel formation between ammonia and aqueous aluminum nitrate solutions, followed by decantation, drying (12OOC; 4 days) and calcination (450'C: air, 5 h; oxygen, 2 h). Three precursors were used to prepare the various series of unsupported and supported manganese oxide catalysts: (i) manganese (11) nitrate ( W ) ,(ii) ) by a metathetical ammonium permanganate NH4Mn04 ( m synthesized reaction between NH4C1 and KMn04 (lo), and (iii) manganese hydroxide coat (*A) obtained by slow addition of Mn(N03)2 6H2O solution to aqueous ammonia followed by filtration and drying Torr, 25'C). The unsupported catalysts were obtained by calcination of the and &G ! precursors at 150', 300' and 6OO0C for 5 h in air. The products thus obtained are designated by formula like Hn2[1501 or MnC13001. In the case of precursor, crystals of NH4Mn04 were first slowly decomposed at 12OOC for 2 h in air prior to calcination. The product is then calcined as above: the calcination products are denoted like Mn7f1501. In case of manganous nitrate and ammonium permanganate, the supported catalysts were obtained by impregnation from aqueous solutions of various concentrations. The loading level for the cationic adsorption of Mn2+ remains low (0.13, 0.3 and 0.6 wt%-Mn) whereas for MnO4- the impregnation leads to higher values (0.5, 0.9 and 1.7 wt%-Mn). For the manganese hydroxide precipitate, a coating procedure was carried out by formation of the precipitate in presence of the carrier, to provide the loading levels of 0 . 4 , 4.1 and 6.8 wt%-Mn. All these samples were subsequently filtered, dried (25'C and l o q 2 torr) and then calcined (150, 300 or 600-C). They are denoted like v, where x gives the Mn loading.
619
Characterization techniaues The various samples of unsupported and supported manganese oxide catalysts were subjected to a range of physical and chemical characterization methods, so as to examine their surface as well as bulk properties, and hence the effect of the preparation variables. For the bulk properties the following techniques were used : thermogravimetric and differential thermal analysis (TGA and DTA), Shimadzu apparatus type DT-30 H, heating rate 10'C min-l, reference a-A1 203; X-ray diffraction (XRD), Siemens D 500 diffractometer with microcomputer attachment, Cu Ka radiation (1.5418 A ) ; infrared spectroscopy (IR), Perkin-Elmer recording spectrophotometer (Model 580 B), KBr pellets : and temperature programmed reduction (TPR) in H2: pulses of H2 (0.285 cm3) being injected every other minute from ambient temperature to 500'C (4 C min-l ) On the other hand, the following methods were employed as surface characterizing techniques: - surface area measurements (BET) by low temperature nitrogen adsorption method; - diffuse reflectance spectroscopy (DRS), Beckmann 5240 spectrometer equipped with an integrating sphere and coupled to an HP 9816 microcomputer; dehydrated BaS04 was used as a standard for all the spectral regions (250 - 2500 nm); - X-ray photoelectron spectroscopy (XPS), Riber spectrometer , A1 Ka source (1488.6 eV), reference C l s at 285 eV.. Moreover, and in order to reveal the effect of preparation variables on the redox activity of these catalysts, two model reactions were studied: (i) H202 decomposition in aqueous solution and (ii) CO oxidation in transient flow, carried out with the same chromatographic apparatus as for TPR measurements. The latter technique leads to the determination of the oxygen storage capacity (OSC) of the catalyst: pulses of CO were injected every other minute at 3OO0C on a sample predosed with O2 pulses at 300'C .
.
RESULTS AND DISCUSSION
Bulk characterization A part of the catalysts used are listed in Table 1. TGA and DTA results of precursor indicated that this material commences decomposition at 80-C to give Mn02 which leads to
620
a-Mn2O3 upon calcination at 600'C, in agreement with previous results (11). XRD data and IR findings confirmed these results catalysts, the thermal behavior (table 1). For the supported was not the same, indicating a probable interaction of manganese nitrate with r-A1203 (6) due to the very low load of the samples. No detectable thermal events have been evidenced, indicating that the surface species do not change upon heating, also reflected by the pale brown color exhibited by all the samples. However TPR profiles are similar for calcination temperature 150 and 30OOC but different for 6OO0C. (Fig.1). The first peak of the TPR curves (= 350'C) in the case of the samples calcined at 150 and 300'C can be attributed to the reduction of adsorbed nitrate ions. It was shown previously that NO3- ions adsorbed on A1203 reduced quantitatively into N2 during TPR, thus requiring 5H/N03- for their reduction to be completed (12). However even at 15OOC the content of residual nitrate is low, typically of the order of 30% of the initial loading associated with Mn. At higher calcination temperature, these ions are decomposed. TABLE 1: crystalline phases, surface area, oxygen storage capacity
and kinetic rate (9-1 catalyst)
.
constant
for
the
decomposition
of
H20i
ISC, 300'C lcrnol 0 9-3 Mn2(150)
NS
(300) NS (600) NS
0.6Mn2(150) (300) (600)
MnC(150)
S
S S
NS
( 3 0 0 ) NS ( 6 0 0 ) NS S S
S
NS NS NS S S S
€5-Mn02 + few a-Mn2Og R-Mn02 + few a-Mn203 a-Mn203 Only r-A1203j Only r-A1203, Only r-A1203;
10 11 12
4.3
3.2
2.4
139 149
0.03:
0.10 0.13
180
r-Mn203 22 r - ~ n ~ + o~ r ~ -1 ? ~ 0 ~ 28 a-Mn2O.3 24
178 86
10.7 11.3 10.2
r-Mn203 + F - A ~ ~ o ~174 1833 r-Mn2O3 + ~ ~ 1 ~ 0 1883 r - ~ n ~+ o~ ~ ~ 1 ~ 0
115 83 39
17.6
--
11.0
a-Mn203 +few MnOl.88 MnOle88 + a-Mn203 MnOl 88 + a-Mn20 more * crystallize2 Only I'-Al2O3{ Only r-A1203, Only r-A1203;
a) NS: non supported:
S:
supported.
60
131
82
149 130 147
79
907 480
-25
0
18.3 10.8
24.5 22.2
10.2
2.1
0.52
621
UNSUPPORTED
200 I
I
400 I
I
600 T("C) I
I
-
Fig.1: TPR p r o f i l e s . Surface area i n mmol H 4 - l
C L1
t 1
C
t 1
0
li
._ s 0
w
F i g . 2 : thermal a n a l y s i s o f some sarrlpl e s .
622
For the unsupported samples, the thermal analysis curves (Fig.2) and the XRD indicate that this precursor is most probably changed from the hydrated r-Mn203 to the a-Mn2O3 form above 40OOC. The presence of nitrate ions up to 300'C was evidenced by IR bands at 1385 cm-I (Fig.3) and by the different exothermic peaks of the DTA curve (Fig.2). This is in agreement with the TPR profiles (Fig.1) showing a first reduction peak between 300 and 340'C which disappears for MnCf600). sample. TPR profiles for Mn reduction (>400'C) for MnCf1501 and MnC16001 display a small difference which can be associated with the change from the r to the Q form of Mn2O3. This behavior was modified on coating the carrier, since up to 600'C the surface species of the supported samples remains r-Mn203 (Table 1 and Fig.4). The TPR curves show that the content of NO3is higher for 6.8HnCf150). than for 6.8MnCf6001 whereas the profiles at higher reduction temperature remain the same. Accordingly, the bulk phases of manganese oxide in these supported catalysts are detectable and the calcination temperature does not markedly affect either the crystalline or the chemical nature of these species, whereas the unsupported catalyst calcined at 600'C was markedly affected. Hence the interaction with the support may play an important role. In case of &Q unsupported and supported samples, bulk phases of manganese oxides were expected to be different owing to the different mode of decomposition and subsequent calcination. This was proved to be the case through the different results obtained for these catalysts. No bulk phases of manganese oxides were detectable for the supported catalysts by XRD, due to the low Mn loading. The possibility of strong interaction with the support is evident for the catalysts calcined at 600'C (cf. TPR profiles in fig.1). Surface characterization Surface characteristics of these catalysts are reflected on and their SBET, DRS and XPS results. The surface areas of unsupported samples are lower than the corresponding knx catalysts. This may be attributed to the differences in the porosity despite the similarity of the chemical nature of these different samples, as pointed out in their bulk characterization. On the other hand, the drop of surface area of all the supported catalysts relative to the support (214 m2 g-1), is most likely due
623
1
1050 650 75c F ig . 3 : IR-spectra of MnC f o r d i f f e r e n t c a l c i n a t i o n temperatures. 1450
DIF-KUB
.F
.5 0.6 :ln2(600)
.4
.2
461
0 700
10
\
1000
0. 9 Mn7
5
3 ' : 'r v )3 .
5=a
*a !=
I--!
r e f . : ?-?In 0 2 3
Fig.4: X-ray data f o r alumina, coated sample and d i f f e r e n c e s p ect r u n . The 1 ines correspond t o t h e reference compounds Y-Al 203 and Y-Mn203.
0 Fig.5: OR d i f f e r e n c e spe c tra o f some samples obtained by substra c tion of t h e spectrum of Y - A ~2 3 3 .
624
to the formation of a manganese oxide phase for 6.8MnC samples or to a blockage of the micropores for 0.9Mn7 and 0.6Mn2 samples (13). DRS results can give information on surface species of Mn present in supported samples. The difference spectra (Fig.5) of 0.9Mn7 samples show that the surface species at 600'C are different from those formed at lower calcination temperatures, in accordance with the TPR curves of the corresponding samples (Fig.1). Moreover the surface species at 6OO0C are comparable to those of 0.6Mn21600LI displaying the same band position ( = 460 nm). Thus the surface species are probably the same as in the case of 0.9Mn7f6001 sample,the activities becoming similar for the two catalysts. From the XPS data given in table 2, the variations of the Mn2p3/2 binding energy can be associated with the oxidation state of manganese (9,,14,15). Thus for the mechanical mixture Mn3O4 + A1203, this value (640.4 eV) corresponds to the presence of Mn(I1) and Mn(II1). In the case of 4.1NnC and 0.5Mn7 samples the oxidation state of Mn is higher probably between I11 and IV, and decreases slightly after calcination for 4.lMnC. The Mn/A1 ratio for the mechanical mixture is in agreement with the value calculated from the composition of the mixture (0.042). For 4.lMnC samples this ratio is higher than the calculated Mn/A1 ratio (0.04), reflecting the partial coating of the alumina surface, and for 0.5Mn7 the values correspond to a good dispersion of manganese on the surface of the carrier (calculated Mn/A1 ratio : 0.0047). TABLE 2: XPS data, surface area ratio and kinetic rate constant for the decomposition of H707. I
I
Binding Energy IIEbll eV fO. 2 Samples
Mn
0
1 1 1 1 3P
2P1/2 2P3/2
Is
Area ratio
C, 30°C
;-lg-1 O/Al
Mn3O4 + A1203 651.9 640.4 48.5 531.2 (5 wt%-Mn)
1.76
4.1MnC(RT) 4.1MnC(600)
653.3 641.7 48.7 531.3 652.7 641.2 48.5 531.2
1.79 1.76
0.069 0.060
0.12 0.10
8.1
653.7 642.0 48.7 531.3 653.6 641.9 48.5 531.2
1.77
1.80
0.024 0.025
0.044
0.043
7.0 0.15
0.5Mn7(RT) 0.5Mn7(600)
I
t
I
I
6.2
625
Activity The rate constant values K~~~~ obtained at 3Q°C for the catalyzed decomposition of H202 as well as the values of OSC at 3QO'C are reported in table 1. These values are clearly correlated despite the fact that one of them is performed in aqueous solution, whereas the other is carried out in the gas phase Concerning the supported catalysts, 6.8MnC are the most active samples whereas the activity of the 0.6Mn2 samples for both reaction remains very low. The catalytic activities cannot be correlated with the values of SBETl except for the supported 0.6Mn2 serie. As a rule, when the atomic surface ratio Mn/O increases, the activity of the corresponding catalysts increases (compare 4.1MnC and 0.5Mn7 series, Table 2 ) . Accordingly one may conclude that a samples contain more surface manganese species with more surface active oxygen as indicated from OSC, which can and XNnC series initiate the decomposition of H202. For both the rise of the calcination temperature results in a decrease of the catalytic activity, this being more pronounced €or the former. This can be attributed to the loss of surface hydroxyl groups and probably of surface active oxygen upon calcination although the manganese oxide phase remains the same (6.8MnC series). Similar effects were already stated on Mn02 (16). On the contrary, for 0.6Nn2 supported samples, with lower values of the kinetic rate constant, the catalytic activity This can be increases with the calcination temperature correlated with the increase of the surface area and a possible explanation is the migration of manganese leading to a better dispersion. In the case of the unsupported samples the series displays the highest activity in correlation with higher surface areas. The variation in the activity of these catalysts reflects the role of the stoichiometry and crystalline modification of the manganese oxides.
.
CONCLUSION
The most active unsupported catalysts for both model reactions are the permanganate-based samples m ,after calcination at 300 or 60Q'C; these samples display the highest surface area and correspond to the highest oxidation number of manganese. On the contrary, the supported catalyst samples m , prepared with the same precursor, exhibit a drastic drop in the catalytic activity
626
despite an equally high surface area. The TPR measurements showed these supported samples to be difficult to reduce after calcination at 6 0 0 ° C , thus suggesting a strong interaction with the support. For the supported samples the use of the coating technique, with the hydroxide precursor, leads to the most active catalysts and manganese oxide phases were XRD detectable. Moreover, for samples show this precursor, supported 6.8MnC and unsupported comparable activities, in relation with the dispersion effect of the support. In the case of the supported catalysts prepared with manganese (11) nitrate, higher loadings of manganese are recommended, in order to verify the influence of the calcination temperature; this needs to change the impregnation procedure which presently limits the loading level. ACKNOhlLEDGJiXENT
We thank very g r a t e f u l l y P r o f . J.F. Hemidy (Univ. o f Caen) and P r o f . G. Perot (Univ. o f P o i t i e r s ) f o r DRS data, XPS measurements and valuable discussions. A.K.H. Nohman thanks a p p r e c i a t e l y t h e Egyptian Government f o r t h e g r a n t given t o him.
REFWENCES
1 W.B. Innes, i n : P.H. Emmett (Ed.), Catalysts, Vol. 2, Reinhold, New-York, 1955, Ch.1. 2 M.A. Baltanas, A.B. S t i l e s and J.R. Katzer, Appl. Catal., 28 (1986) 13-33. 3 P.W. Selwood, T.E. Moore, M . E l l i s , J. Amer. Chem. SOC., 73 (1949) 693. 4 G.T. P o t t and B.D. McNicol, Discuss. Faraday SOC., 52 (1971) 121-131. 5 M. Lo Jacono, M. S c h i a v e l l o and G. Mercati, Gazz. Chim. I t a l . , 105 (1975) 1165-1176. 6 L. Burlamacchi and P.L. V i l l a , React. K i n e t . Catal. L e t t . , 3 (1975) 199-204. 7 M. Lo Jacono and M. S c h i a v e l l o i n : B. Delmon, P.A. Jacobs and G. Poncelet (Ed.), Preparation o f C a t a l y s t s I, E l s e v i e r , Amsterdam, 1976, pp. 474-487. 8 S. Cavallaro,.N. Bertuccio, P. Antonucci, N. Giordano and G.C. Bart, J. Catal., 73 (1982) 337-348. 9 B.R. Strohmeier and D.M. Hercules, J. Phys. Chem., 88 (1984) 4922. 10 L.L. Bircumshaw and F.M. Taylor, J. Chem. SOC., (1950) 3674. 11 R.D.W. Kemmit, in: J.C. B a i l a r (Ed.), Comprehensive I n o r g a n i c Chemistry, Vol. 3, Pergamon Press, New York, 1973, Ch.37, p.771.
12 0. Duprez and S . Kacimi, personal communication. 13 D. D o l l i m o r e and J. Pearce, Powder Technology, 25 (1980) 71-78. 14 C.D. Wagner, W.M. Riggs, L.E. Davis and J.F. Moulder; "Handbook o f X-Ray Photoelectron Spectroscopy", Eds. G.E. Mullenberg, 1979.. 15 M. Lenglet, A. D'Huysser, J. Kasperek, 3. P. Bonnelle and J. Durr, Mat. Res. B u l l . , 20 (1985) 745-757. 16 S.B. Kanungo, J. C a t a l . , 58 (1979) 419-435.
617
PREPARATION OF MANGANESE OXIDE CATALYSTS USING NOVEL NH4MnO4 AND MANGANESE HYDROXIDE PRECURSORS. COMPARISON OF UNSUPPORTED AND ALUMINA SUPPORTED CATALYSTS A.K.H.
N O H M A N ~ ~D. ~ ,DUPREZ~,c. KAPPENSTEIN~, S.A.A. MANSOUR' AND M.I. ZAKI'
l c h e m i s t r y Department, F a c u l t y o f Science, M i n i a U n i v e r s i t y , El-Minia, EGYPT 2Catalyse en Chimie Organique, Faculte des Sciences de P o i t i e r s , FRANCE.
SUHMARY
Unsupported and a1 umina supported manganese oxide c a t a l y s t s were prepared using manganese n i t r a t e , manganese hydroxide and ammonium permanganate. They were b u l k and surface c h a r a c t e r i z e d by thermal analysis, X-ray d i f f r a c t i o n , d i f f u s e r e f l e c t a n c e , I R and photoelectron spectroscopy, SBET and TPR. Moreover H202 decomposition and CO o x i d a t i o n were used as t e s t r e a c t i o n s . The most a c t i v e supported c a t a l y s t s are t h e manganese hydroxide coated samples which show a rMn 03 phase. For t h e ammonium permanganate-based c a t a l y s t s a s t r o n g i n t e r a c t i o n w i t h t h e c a r r i e r was evidenced.
INTRODUCTION
Studies concerning supported manganese oxides are relatively scarce, despite their potential activity in oxidation reactions. For example manganese oxide catalysts are very active for CO oxidation, particularly when they are promoted with CuO or COO (1). They are also used for methanol oxidation and ethylene hydrogenation (2). These catalysts were first prepared and investigated by Selwood et a1 ( 3 ) by impregnating manganese (11) nitrate onto high surface area alumina and then thermally decomposed. Thereafter, these catalysts were characterized by several techniques (4-9). Baltanas et a1 (2) prepared these catalysts by impregnating manganese nitrate onto alumina and in situ precipitation of manganese hydroxide by ammonia solution. The aim of the present study was to prepare various series of bulk and alumina-supported manganese oxide catalysts. Novel NH4Mn04 and manganese hydroxide precursors as well as the conventional manganese nitrate were used in hope of defining the impacts of the precursors and the support on physico-chemical characteristics and activity of the final catalysts. KMn04 was previously used by Cavallaro et a1 ( 8 ) but with little information. We have taken NH4Mn04 for our investigation to avoid
618
more complications arising from the presence of potassium. By adopting a coating procedure in case of manganese hydroxide, we aimed to have surface layers of manganese oxide on alumina which may be more easily detectable than in the case of the two previous precursors. EXPWIWENTAt
Materials High surface area r-alumina (214 m2 g-l) was obtained by slow gel formation between ammonia and aqueous aluminum nitrate solutions, followed by decantation, drying (12OOC; 4 days) and calcination (450'C: air, 5 h; oxygen, 2 h). Three precursors were used to prepare the various series of unsupported and supported manganese oxide catalysts: (i) manganese (11) nitrate ( W ) ,(ii) ) by a metathetical ammonium permanganate NH4Mn04 ( m synthesized reaction between NH4C1 and KMn04 (lo), and (iii) manganese hydroxide coat (*A) obtained by slow addition of Mn(N03)2 6H2O solution to aqueous ammonia followed by filtration and drying Torr, 25'C). The unsupported catalysts were obtained by calcination of the and &G ! precursors at 150', 300' and 6OO0C for 5 h in air. The products thus obtained are designated by formula like Hn2[1501 or MnC13001. In the case of precursor, crystals of NH4Mn04 were first slowly decomposed at 12OOC for 2 h in air prior to calcination. The product is then calcined as above: the calcination products are denoted like Mn7f1501. In case of manganous nitrate and ammonium permanganate, the supported catalysts were obtained by impregnation from aqueous solutions of various concentrations. The loading level for the cationic adsorption of Mn2+ remains low (0.13, 0.3 and 0.6 wt%-Mn) whereas for MnO4- the impregnation leads to higher values (0.5, 0.9 and 1.7 wt%-Mn). For the manganese hydroxide precipitate, a coating procedure was carried out by formation of the precipitate in presence of the carrier, to provide the loading levels of 0 . 4 , 4.1 and 6.8 wt%-Mn. All these samples were subsequently filtered, dried (25'C and l o q 2 torr) and then calcined (150, 300 or 600-C). They are denoted like v, where x gives the Mn loading.
619
Characterization techniaues The various samples of unsupported and supported manganese oxide catalysts were subjected to a range of physical and chemical characterization methods, so as to examine their surface as well as bulk properties, and hence the effect of the preparation variables. For the bulk properties the following techniques were used : thermogravimetric and differential thermal analysis (TGA and DTA), Shimadzu apparatus type DT-30 H, heating rate 10'C min-l, reference a-A1 203; X-ray diffraction (XRD), Siemens D 500 diffractometer with microcomputer attachment, Cu Ka radiation (1.5418 A ) ; infrared spectroscopy (IR), Perkin-Elmer recording spectrophotometer (Model 580 B), KBr pellets : and temperature programmed reduction (TPR) in H2: pulses of H2 (0.285 cm3) being injected every other minute from ambient temperature to 500'C (4 C min-l ) On the other hand, the following methods were employed as surface characterizing techniques: - surface area measurements (BET) by low temperature nitrogen adsorption method; - diffuse reflectance spectroscopy (DRS), Beckmann 5240 spectrometer equipped with an integrating sphere and coupled to an HP 9816 microcomputer; dehydrated BaS04 was used as a standard for all the spectral regions (250 - 2500 nm); - X-ray photoelectron spectroscopy (XPS), Riber spectrometer , A1 Ka source (1488.6 eV), reference C l s at 285 eV.. Moreover, and in order to reveal the effect of preparation variables on the redox activity of these catalysts, two model reactions were studied: (i) H202 decomposition in aqueous solution and (ii) CO oxidation in transient flow, carried out with the same chromatographic apparatus as for TPR measurements. The latter technique leads to the determination of the oxygen storage capacity (OSC) of the catalyst: pulses of CO were injected every other minute at 3OO0C on a sample predosed with O2 pulses at 300'C .
.
RESULTS AND DISCUSSION
Bulk characterization A part of the catalysts used are listed in Table 1. TGA and DTA results of precursor indicated that this material commences decomposition at 80-C to give Mn02 which leads to
620
a-Mn2O3 upon calcination at 600'C, in agreement with previous results (11). XRD data and IR findings confirmed these results catalysts, the thermal behavior (table 1). For the supported was not the same, indicating a probable interaction of manganese nitrate with r-A1203 (6) due to the very low load of the samples. No detectable thermal events have been evidenced, indicating that the surface species do not change upon heating, also reflected by the pale brown color exhibited by all the samples. However TPR profiles are similar for calcination temperature 150 and 30OOC but different for 6OO0C. (Fig.1). The first peak of the TPR curves (= 350'C) in the case of the samples calcined at 150 and 300'C can be attributed to the reduction of adsorbed nitrate ions. It was shown previously that NO3- ions adsorbed on A1203 reduced quantitatively into N2 during TPR, thus requiring 5H/N03- for their reduction to be completed (12). However even at 15OOC the content of residual nitrate is low, typically of the order of 30% of the initial loading associated with Mn. At higher calcination temperature, these ions are decomposed. TABLE 1: crystalline phases, surface area, oxygen storage capacity
and kinetic rate (9-1 catalyst)
.
constant
for
the
decomposition
of
H20i
ISC, 300'C lcrnol 0 9-3 Mn2(150)
NS
(300) NS (600) NS
0.6Mn2(150) (300) (600)
MnC(150)
S
S S
NS
( 3 0 0 ) NS ( 6 0 0 ) NS S S
S
NS NS NS S S S
€5-Mn02 + few a-Mn2Og R-Mn02 + few a-Mn203 a-Mn203 Only r-A1203j Only r-A1203, Only r-A1203;
10 11 12
4.3
3.2
2.4
139 149
0.03:
0.10 0.13
180
r-Mn203 22 r - ~ n ~ + o~ r ~ -1 ? ~ 0 ~ 28 a-Mn2O.3 24
178 86
10.7 11.3 10.2
r-Mn203 + F - A ~ ~ o ~174 1833 r-Mn2O3 + ~ ~ 1 ~ 0 1883 r - ~ n ~+ o~ ~ ~ 1 ~ 0
115 83 39
17.6
--
11.0
a-Mn203 +few MnOl.88 MnOle88 + a-Mn203 MnOl 88 + a-Mn20 more * crystallize2 Only I'-Al2O3{ Only r-A1203, Only r-A1203;
a) NS: non supported:
S:
supported.
60
131
82
149 130 147
79
907 480
-25
0
18.3 10.8
24.5 22.2
10.2
2.1
0.52
621
UNSUPPORTED
200 I
I
400 I
I
600 T("C) I
I
-
Fig.1: TPR p r o f i l e s . Surface area i n mmol H 4 - l
C L1
t 1
C
t 1
0
li
._ s 0
w
F i g . 2 : thermal a n a l y s i s o f some sarrlpl e s .
622
For the unsupported samples, the thermal analysis curves (Fig.2) and the XRD indicate that this precursor is most probably changed from the hydrated r-Mn203 to the a-Mn2O3 form above 40OOC. The presence of nitrate ions up to 300'C was evidenced by IR bands at 1385 cm-I (Fig.3) and by the different exothermic peaks of the DTA curve (Fig.2). This is in agreement with the TPR profiles (Fig.1) showing a first reduction peak between 300 and 340'C which disappears for MnCf600). sample. TPR profiles for Mn reduction (>400'C) for MnCf1501 and MnC16001 display a small difference which can be associated with the change from the r to the Q form of Mn2O3. This behavior was modified on coating the carrier, since up to 600'C the surface species of the supported samples remains r-Mn203 (Table 1 and Fig.4). The TPR curves show that the content of NO3is higher for 6.8HnCf150). than for 6.8MnCf6001 whereas the profiles at higher reduction temperature remain the same. Accordingly, the bulk phases of manganese oxide in these supported catalysts are detectable and the calcination temperature does not markedly affect either the crystalline or the chemical nature of these species, whereas the unsupported catalyst calcined at 600'C was markedly affected. Hence the interaction with the support may play an important role. In case of &Q unsupported and supported samples, bulk phases of manganese oxides were expected to be different owing to the different mode of decomposition and subsequent calcination. This was proved to be the case through the different results obtained for these catalysts. No bulk phases of manganese oxides were detectable for the supported catalysts by XRD, due to the low Mn loading. The possibility of strong interaction with the support is evident for the catalysts calcined at 600'C (cf. TPR profiles in fig.1). Surface characterization Surface characteristics of these catalysts are reflected on and their SBET, DRS and XPS results. The surface areas of unsupported samples are lower than the corresponding knx catalysts. This may be attributed to the differences in the porosity despite the similarity of the chemical nature of these different samples, as pointed out in their bulk characterization. On the other hand, the drop of surface area of all the supported catalysts relative to the support (214 m2 g-1), is most likely due
623
1
1050 650 75c F ig . 3 : IR-spectra of MnC f o r d i f f e r e n t c a l c i n a t i o n temperatures. 1450
DIF-KUB
.F
.5 0.6 :ln2(600)
.4
.2
461
0 700
10
\
1000
0. 9 Mn7
5
3 ' : 'r v )3 .
5=a
*a !=
I--!
r e f . : ?-?In 0 2 3
Fig.4: X-ray data f o r alumina, coated sample and d i f f e r e n c e s p ect r u n . The 1 ines correspond t o t h e reference compounds Y-Al 203 and Y-Mn203.
0 Fig.5: OR d i f f e r e n c e spe c tra o f some samples obtained by substra c tion of t h e spectrum of Y - A ~2 3 3 .
624
to the formation of a manganese oxide phase for 6.8MnC samples or to a blockage of the micropores for 0.9Mn7 and 0.6Mn2 samples (13). DRS results can give information on surface species of Mn present in supported samples. The difference spectra (Fig.5) of 0.9Mn7 samples show that the surface species at 600'C are different from those formed at lower calcination temperatures, in accordance with the TPR curves of the corresponding samples (Fig.1). Moreover the surface species at 6OO0C are comparable to those of 0.6Mn21600LI displaying the same band position ( = 460 nm). Thus the surface species are probably the same as in the case of 0.9Mn7f6001 sample,the activities becoming similar for the two catalysts. From the XPS data given in table 2, the variations of the Mn2p3/2 binding energy can be associated with the oxidation state of manganese (9,,14,15). Thus for the mechanical mixture Mn3O4 + A1203, this value (640.4 eV) corresponds to the presence of Mn(I1) and Mn(II1). In the case of 4.1NnC and 0.5Mn7 samples the oxidation state of Mn is higher probably between I11 and IV, and decreases slightly after calcination for 4.lMnC. The Mn/A1 ratio for the mechanical mixture is in agreement with the value calculated from the composition of the mixture (0.042). For 4.lMnC samples this ratio is higher than the calculated Mn/A1 ratio (0.04), reflecting the partial coating of the alumina surface, and for 0.5Mn7 the values correspond to a good dispersion of manganese on the surface of the carrier (calculated Mn/A1 ratio : 0.0047). TABLE 2: XPS data, surface area ratio and kinetic rate constant for the decomposition of H707. I
I
Binding Energy IIEbll eV fO. 2 Samples
Mn
0
1 1 1 1 3P
2P1/2 2P3/2
Is
Area ratio
C, 30°C
;-lg-1 O/Al
Mn3O4 + A1203 651.9 640.4 48.5 531.2 (5 wt%-Mn)
1.76
4.1MnC(RT) 4.1MnC(600)
653.3 641.7 48.7 531.3 652.7 641.2 48.5 531.2
1.79 1.76
0.069 0.060
0.12 0.10
8.1
653.7 642.0 48.7 531.3 653.6 641.9 48.5 531.2
1.77
1.80
0.024 0.025
0.044
0.043
7.0 0.15
0.5Mn7(RT) 0.5Mn7(600)
I
t
I
I
6.2
625
Activity The rate constant values K~~~~ obtained at 3Q°C for the catalyzed decomposition of H202 as well as the values of OSC at 3QO'C are reported in table 1. These values are clearly correlated despite the fact that one of them is performed in aqueous solution, whereas the other is carried out in the gas phase Concerning the supported catalysts, 6.8MnC are the most active samples whereas the activity of the 0.6Mn2 samples for both reaction remains very low. The catalytic activities cannot be correlated with the values of SBETl except for the supported 0.6Mn2 serie. As a rule, when the atomic surface ratio Mn/O increases, the activity of the corresponding catalysts increases (compare 4.1MnC and 0.5Mn7 series, Table 2 ) . Accordingly one may conclude that a samples contain more surface manganese species with more surface active oxygen as indicated from OSC, which can and XNnC series initiate the decomposition of H202. For both the rise of the calcination temperature results in a decrease of the catalytic activity, this being more pronounced €or the former. This can be attributed to the loss of surface hydroxyl groups and probably of surface active oxygen upon calcination although the manganese oxide phase remains the same (6.8MnC series). Similar effects were already stated on Mn02 (16). On the contrary, for 0.6Nn2 supported samples, with lower values of the kinetic rate constant, the catalytic activity This can be increases with the calcination temperature correlated with the increase of the surface area and a possible explanation is the migration of manganese leading to a better dispersion. In the case of the unsupported samples the series displays the highest activity in correlation with higher surface areas. The variation in the activity of these catalysts reflects the role of the stoichiometry and crystalline modification of the manganese oxides.
.
CONCLUSION
The most active unsupported catalysts for both model reactions are the permanganate-based samples m ,after calcination at 300 or 60Q'C; these samples display the highest surface area and correspond to the highest oxidation number of manganese. On the contrary, the supported catalyst samples m , prepared with the same precursor, exhibit a drastic drop in the catalytic activity
626
despite an equally high surface area. The TPR measurements showed these supported samples to be difficult to reduce after calcination at 6 0 0 ° C , thus suggesting a strong interaction with the support. For the supported samples the use of the coating technique, with the hydroxide precursor, leads to the most active catalysts and manganese oxide phases were XRD detectable. Moreover, for samples show this precursor, supported 6.8MnC and unsupported comparable activities, in relation with the dispersion effect of the support. In the case of the supported catalysts prepared with manganese (11) nitrate, higher loadings of manganese are recommended, in order to verify the influence of the calcination temperature; this needs to change the impregnation procedure which presently limits the loading level. ACKNOhlLEDGJiXENT
We thank very g r a t e f u l l y P r o f . J.F. Hemidy (Univ. o f Caen) and P r o f . G. Perot (Univ. o f P o i t i e r s ) f o r DRS data, XPS measurements and valuable discussions. A.K.H. Nohman thanks a p p r e c i a t e l y t h e Egyptian Government f o r t h e g r a n t given t o him.
REFWENCES
1 W.B. Innes, i n : P.H. Emmett (Ed.), Catalysts, Vol. 2, Reinhold, New-York, 1955, Ch.1. 2 M.A. Baltanas, A.B. S t i l e s and J.R. Katzer, Appl. Catal., 28 (1986) 13-33. 3 P.W. Selwood, T.E. Moore, M . E l l i s , J. Amer. Chem. SOC., 73 (1949) 693. 4 G.T. P o t t and B.D. McNicol, Discuss. Faraday SOC., 52 (1971) 121-131. 5 M. Lo Jacono, M. S c h i a v e l l o and G. Mercati, Gazz. Chim. I t a l . , 105 (1975) 1165-1176. 6 L. Burlamacchi and P.L. V i l l a , React. K i n e t . Catal. L e t t . , 3 (1975) 199-204. 7 M. Lo Jacono and M. S c h i a v e l l o i n : B. Delmon, P.A. Jacobs and G. Poncelet (Ed.), Preparation o f C a t a l y s t s I, E l s e v i e r , Amsterdam, 1976, pp. 474-487. 8 S. Cavallaro,.N. Bertuccio, P. Antonucci, N. Giordano and G.C. Bart, J. Catal., 73 (1982) 337-348. 9 B.R. Strohmeier and D.M. Hercules, J. Phys. Chem., 88 (1984) 4922. 10 L.L. Bircumshaw and F.M. Taylor, J. Chem. SOC., (1950) 3674. 11 R.D.W. Kemmit, in: J.C. B a i l a r (Ed.), Comprehensive I n o r g a n i c Chemistry, Vol. 3, Pergamon Press, New York, 1973, Ch.37, p.771.
12 0. Duprez and S . Kacimi, personal communication. 13 D. D o l l i m o r e and J. Pearce, Powder Technology, 25 (1980) 71-78. 14 C.D. Wagner, W.M. Riggs, L.E. Davis and J.F. Moulder; "Handbook o f X-Ray Photoelectron Spectroscopy", Eds. G.E. Mullenberg, 1979.. 15 M. Lenglet, A. D'Huysser, J. Kasperek, 3. P. Bonnelle and J. Durr, Mat. Res. B u l l . , 20 (1985) 745-757. 16 S.B. Kanungo, J. C a t a l . , 58 (1979) 419-435.