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Chapter 21
Niobium Oxalate New Precursor for Preparation of Supported Niobium Oxide Catalysts
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Jih-Mirn Jehng and Israel E. Wachs Zettlemoyer Center for Surface Studies, Department of Chemical Engineering, Lehigh University, Bethlehem, PA 18015
The aqueous preparation of supported niobium oxide catalysts was developed by using niobium oxalate as a precursor. The molecular states of aqueous niobium oxalate solutions were investigated by Raman spectroscopy as a function of pH. The results show that two kinds of niobium ionic species exist in solution and their relative concentrations depend on the solution pH and the oxalic acid concentration. The supported niobium oxide catalysts were prepared by the incipient wetness impregnation technique and characterized by Raman, XRD, XPS, and FTIR as a function of niobium oxide coverage and calcination temperature. The Raman studies reveal that two types of surface niobium oxide species exist on the alumina support and their relative concentrations depend on niobium oxide coverage. Raman, XRD, XPS, and FTIR results indicate that a monolayer of surface niobium oxide corresponds to ~ 19% Nb O for an Al O support possessing ~ 180 m /g. The surface niobium oxide phase is found to be stable to high calcination temperatures. 2
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Supported niobium oxide c a t a l y s t s have r e c e n t l y been shown t o be e f f e c t i v e c a t a l y s t s f o r many c a t a l y t i c reactions: pollution abatement, selective oxidation, hydrocarbon conversion, carbon monoxide hydrogénation, e t c . [1] . In a previous study [2] , i t was shown t h a t the presence o f the surface niobium oxide phases r e t a r d s the l o s s i n surface area of the A I 2 O 3 and T 1 O 2 supports and stablizes the V 0 /Ti0 system during high temperature treatments. The surface niobium oxide s p e c i e s on the A I 2 O 3 support was a l s o found t o possess 2
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0097-6156/90/0437-0232$06.00/0 © 1990 American Chemical Society In Novel Materials in Heterogeneous Catalysis; Baker, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
21.
JEHNG AND WACHS
Niobium
Oxalate
s t r o n g Bronsted a c i d i t y [3]. These important p r o p e r t i e s impart the s u r f a c e niobium oxide phase on the AI2O3 support with a high hydrocarbon c r a c k i n g a c t i v i t y at e l e v a t e d temperatures. Niobium ethoxide [Nb(0C H ) ] has t r a d i t i o n a l l y been used as a precursor f o r the p r e p a r a t i o n of supported niobium oxide c a t a l y s t s . T h i s non-aqueous p r e p a r a t i o n method r e q u i r e s a c o n t r o l l e d enviroment and special procedures t o avoid the decomposition of the niobium ethoxide i n the presence of moisture. I t i s well-known that transition metal ions form a stable solution c h e l a t e with oxalate groups, and molybdenum o x a l a t e [4] and vanadium oxalate [5] have been widely used f o r the aqueous p r e p a r a t i o n of supported molybdenum oxide and supported vanadium oxide c a t a l y s t s . In the present study, niobium oxalate [ N b ( H C 0 ) ] was i n v e s t i g a t e d as an aqueous precursor f o r the p r e p a r a t i o n of supported niobium oxide c a t a l y s t s . 2
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EXPERIMENTAL METHODS M a t e r i a l s and P r e p a r a t i o n Methods Niobium oxalate was s u p p l i e d by Niobium Products Company with the f o l l o w i n g chemical a n a l y s i s : 20.5% N b 0 , 790 ppm Fe, 680 ppm S i , and 0.1% insolubles. Niobium oxalate was d i s s o l v e d into a constant c o n c e n t r a t i o n of aqueous o x a l i c a c i d s o l u t i o n , and the pH of the s o l u t i o n was varied from 0.50 to 5.00 by adding ammonium hydroxide. The supported niobium oxide on A1 Û3 catalysts were prepared by the incipient-wetness impregnation method u s i n g the niobium oxalate/oxalic a c i d aqueous s o l u t i o n and A1 0 (Harshaw, 180 m /g) . The samples were d r i e d at 110-120°C f o r 16 hours, and then c a l c i n e d at 500°C under f l o w i n g dry a i r f o r 16 hours. 2
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Raman Spectroscopy Raman s p e c t r a were obtained with a Spex Triplemate spectrometer(Model 1877) coupled t o an EG&G i n t e n s i f i e d photodiode a r r a y d e t e c t o r cooled t h e r m o e l e c t r i c a l l y t o -40°C, and i n t e r f a c e d with an EGfcG 0 Μ Α III O p t i c a l Multichannel Analyzer(Model 1463). The samples were e x c i t e d with the 514.5nm Ar+ laser. The beam was focused on the sample i l l u m i n a t o r where the sample typically s p i n s at about 2000 rpm t o avoid local heating. The Raman s c a t t e r i n g was c o l l e c t e d by the spectrometer, and analyzed with an 0 Μ Α III b u i l t - i n software package. The o v e r a l l s p e c t r a l r e s o l u t i o n of the s p e c t r a i s about 2 cm" . 1
X-Ray Powder D i f f r a c t i o n
(XRD)
The c r y s t a l l i n e Nb 0s phase i n the supported niobium oxide c a t a l y s t s was detected by an APD 3600 automated X2
In Novel Materials in Heterogeneous Catalysis; Baker, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
234
NOVEL MATERIALS IN HETEROGENEOUS CATALYSIS
ray powdered diffractometer using Cu K (45KV, 30MA) radiation. The ND2O5/AI2O3 samples were c a l c i n e d at 700°C t o increase the ND2O5 p a r t i c l e s i z e and enhance the XRD signals. Q
X-Ray Photoelectron Spectroscopy
(XPS)
XPS experiments were performed on a P h y s i c a l E l e c t r o n i c Instruments ESCA/AUGER system. The samples were placed on the sample holder a t a 45° angle t o the entrance o f a n a l y z e r and the system was evacuated t o 10~ - 10" Torr. The XPS s p e c t r a were c a l i b r a t e d against the Au 4 f / peak using the Mg K l i n e as the X-ray e x c i t i n g r a d i a t i o n .
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a
C0
2
Chemisorption
The CO2 uptake o f supported niobium oxide on AI2O3 a t d i f f e r e n t ND2O5 loadings was measured with a Quantasorb BET apparatus using a 1:9 r a t i o o f C0 /He mixture gases. The samples were degassed a t 250°C f o r 2 hours under f l o w i n g He, and the C0 chemisorpt ion was performed a t room temperature. 2
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REgVLTg AND DISCUSSION Niobium oxide reference compounds The Raman s p e c t r a o f s e v e r a l niobium oxide compounds, with t h e i r corresponding symmetry and c o o r d i n a t i o n , are shown i n Figure 1. The Nb 0 ~ unit i s a wellcharacterized structure which consists o f three d i f f e r e n t types o f Nb-0 bonds a t each niobium center: a short Nb=0 terminal double bond, a longer Nb-O-Nb b r i d g i n g bond, and a very long and weak Nb 0 single bond connected t o the center o f the c a g e - l i k e octahedral s t r u c t u r e [6-8] . From t h e known s t r u c t u r e o f K Nb 0 the main f r e q u e n c i e s o f t h e K Nb 0 Raman spectrum i n Figure 1 can be assigned: Nb=0 t e r m i n a l s t r e t c h i n g mode (903, 879, and 831 cm ), corner o r edge-shared octahedral Nb0 s t r e t c h i n g mode (734, 537, and 463 cm-1) , Nb=0 bending mode (289 cm" ) , and Nb-O-Nb bending mode (223 cm ) . The m u l t i p l e t e r m i n a l s t r e t c h i n g modes present i n t h e high wavenumber region a r e due t o d i s t o r t i o n s present i n t h e K Nb 0 structure. Niobium pentoxide, Nb 0 , possesses a more order octahedral s t r u c t u r e with no Nb=0 terminal bonds, and a major band appears a t 690 cm which i s characteristic o f an octahedral Nb0 s t r e t c h i n g mode as well as Nb-0 and NbO-Nb bending modes a t ~300 cm-1 and ~230 cm-1, respectively. For the niobium oxalate precursor a sharp and s t r o n g Raman band i s present a t 958 cm due t o a Nb=0 t e r m i n a l bond and the a s s o c i a t e d bending modes appear i n the 200-400 cnr* region. The Raman band a t 8
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In Novel Materials in Heterogeneous Catalysis; Baker, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
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21.
Niobium
JEHNGANDWACHS
235
Oxalate
-1
~570 cm a r i s e s from the bidentate oxalate 1igands coordinated t o t h e niobium [9,10]. The Raman f r e q u e n c i e s o f the reference compounds are t a b u l a t e d i n Table 1.
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Niobium Oxalate Aqueous S o l u t i o n s
Niobium oxalate has a low s o l u b i l i t y i n aqueous solutions, but i t s s o l u b i l i t y can be d r a m a t i c a l l y increased by the a d d i t i o n o f o x a l i c a c i d t o the aqueous solutions. At high o x a l i c a c i d c o n c e n t r a t i o n s , however, the niobium oxalate and o x a l i c a c i d p r e c i p i t a t e from solution. The s o l u b i l i t y curve o f niobium oxalate i n aqueous s o l u t i o n s i s shown i n Figure 2 as a f u n c t i o n o f the o x a l i c a c i d c o n c e n t r a t i o n s . Figure 3 shows a s e r i e s of Raman s p e c t r a o f t h e niobium oxalate i n aqueous o x a l i c a c i d s o l u t i o n s with v a r y i n g pH (0.50 t o 5.00). At low pH (5.00), two new Raman bands form a t ~670 cm and ~220 cm which i n d i c a t e t h e formation o f hydrated Nb 0 . 1
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It i s known t h a t t h e niobium oxide complexes i n oxalic a c i d aqueous s o l u t i o n s d i s p l a y an e q u i l i b r i a between two i o n i c s p e c i e s c o n t a i n i n g 2 o r 3 oxalate groups which depend on the s o l u t i o n pH and t h e o x a l i c a c i d c o n c e n t r a t i o n [11,12]. Thus, the two Nb=0 t e r m i n a l bonds appearing i n the aqueous Raman s p e c t r a are assigned t o the two d i f f e r e n t niobium oxalate ionic s p e c i e s present in the s o l u t i o n . The Raman s p e c t r a a l s o show t h a t the r e l a t i v e i n t e n s i t y o f two Nb=0 bands, ~910 cm and ~930 cm , changes with i n c r e a s i n g pH. When ammonium hydroxide i s added t o the s o l u t i o n , the niobium oxalate species with 3 oxalate groups starts to hydro lyze t o 2 oxalate groups as one o f t h e oxalate groups i s replaced by OH groups. T h i s r e s u l t s i n an increase i n i n t e n s i t y o f the ~910 cm Nb=0 band with i n c r e a s i n g s o l u t i o n pH. Increasing the pH t o about 5.00 by f u r t h e r a d d i t i o n o f ammonium hydroxide causes the niobium oxalate species t o hydrolyze and coagulate t o a hydrated Nb 0 precipitate. The aqueous solution chemistry o f niobium oxalate i s shown below: -1
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In Novel Materials in Heterogeneous Catalysis; Baker, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
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NOVEL MATERIALS IN HETEROGENEOUS CATALYSIS
T a b l e 1: R a m a n compounds
frequencies of bulk
niobium
oxide
Wavenumber (cm~l) Vibrational Modes
K Nb 0 8
Nb 0 2
1 9
Nb(HC 0 )
5
2
-
958
734 537 463
690
-
-
-
572
i(Nb-O)
289
302
284
j(Nb-O-Nb)
223
238
243
*(Nb=0)
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e
903,879,831
v(Nb0 ) 6
„(Nb0 C ) 2
2
j-φΛ Ο 0
χ
c
K Nb 0 8
5
A /
i u
\
f\
Ο
I/O-.
0
4
I/
Nb
Nb
ο
ô
0
/
ΝV
Nb. 0 2
0
(500°C)
/ /
0 X\
x
J ι
1200
ι
1000
I
Il/X
r
x Nb(HC 0 ) 2
4
(X: H C . O J
ι
1
800
1
/
5
A^J I
600
ι
ι
400
ι
ι
200
Raman Shift (crrr*) F i g u r e 1: T h e s o l u b i l i t y o f n i o i b u m o x a l a t e as a f u n c t i o n o f o x a l i c a c i d added.
in solution
In Novel Materials in Heterogeneous Catalysis; Baker, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
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21.
Niobium
JEHNGANDWACHS
0
i
1
0.00
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1
0.05
237
Oxalate
1
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Γ
0.15
Weight Fraction of Oxalic Acid
Figure 2 : Raman s p e c t r a o f bulk niobium compounds.
oxide
Raman Shift ( c m ' l )
Figure 3 : Raman s p e c t r a o f niobium oxalate i n o x a l i c a c i d s o l u t i o n as a f u n c t i o n o f pH from 0 . 5 t o 5 . 0 0 .
In Novel Materials in Heterogeneous Catalysis; Baker, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
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NOVEL MATERIALS IN HETEROGENEOUS CATALYSIS
Equilibrium
V7
+
0=Nb
C 0 2
4
IS Polymerization
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Hydrolysis
20H
20H
0=NbC
Supported
NJ%0«(S)
Niobium Oxide on Alumina,
The Raman s p e c t r a of supported niobium oxide on alumina are shown i n F i g u r e 4 as a f u n c t i o n o f N b 0 l o a d i n g . The nature o f the supported niobium oxide phase i s determined by comparison of the Raman s p e c t r a o f the supported niobium oxide samples with those of niobium oxide r e f e r e n c e compounds. The Raman f e a t u r e s o f 1 - 2 2 % N b 0 / A l 0 3 samples are d i f f e r e n t than the bulk niobium oxide compounds due t o the formation o f a twodimensional s u r f a c e niobium oxide o v e r l a y e r on the alumina support [2]. At low s u r f a c e coverages ( < 8 % N b 0 / A l 0 ) , the weak and broad Raman band i n the 8 9 0 9 1 0 cm region i s present due t o a d i s t o r t e d o c t a h e d r a l (approaching square-pyramidal) s u r f a c e niobium oxide s p e c i e s p o s s e s s i n g Nb=0 bonds, and the mode a t ~ 2 3 0 cnr i s c h a r a c t e r i s t i c of a Nb-O-Nb l i n k a g e . At high s u r f a c e coverages ( > 8 % N b 0 / A l 0 ) , an a d d i t i o n a l Raman band a t ~ 6 3 0 cm* i s a l s o present due t o a s l i g h t l y d i s t o r t e d o c t a h e d r a l s u r f a c e niobium oxide s p e c i e s . The Raman s t u d i e s reveal t h a t two types o f s u r f a c e niobium oxide s p e c i e s e x i s t on the alumina support, and t h a t t h e i r r e l a t i v e c o n c e n t r a t i o n s depend on the s u r f a c e niobium oxide coverage. A series of supported niobium oxide on alumina c a t a l y s t s , 0 - 4 5 % N b 0 / A l 0 , were f u r t h e r c h a r a c t e r i z e d by XRD, XPS, C 0 chemisorption, as w e l l as Raman spectroscopy i n order t o determine the monolayer content of the N b 0 / A l 0 system. The t r a n s i t i o n from a twodimensional metal oxide o v e r l a y e r t o three-dimensional metal oxide p a r t i c l e s can be detected by monitoring the 2
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In Novel Materials in Heterogeneous Catalysis; Baker, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
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21.
JEHNGANDWACHS
*1
1
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Niobium
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1000
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80ff
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Riimun Shift (crrr*)
Figure 4 : Raman s p e c t r a of Nb 05/Al 0 ( 5 0 0 ° C ) f u n c t i o n o f niobium oxide coverage. 2
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as a
(Nb/Al) r a t i o s i n such systems with XPS because o f the v a s t l y d i f f e r e n t XPS c r o s s - s e c t i o n s o f these two phases [ 1 3 ] . The ( N b / A l ) r a t i o s o f t h e niobium oxide on the alumina support were obtained by i n t e g r a t i n g the areas o f the Nb 3 d 3 / 2 , 5 / 2 and t h e A l 2p photoelectron l i n e s , and the (Nb/Al) vs. (Nb/Al) curve i s shown i n Figure 5. The S r e a k i n t h e curve corresponds t o ~ 1 9 % N b 0 / A l 0 and suggests t h a t the transition from a two-dimensional phase t o threedimensional particles, monolayer coverage, occurs a t this point. T h i s c o n c l u s i o n i s supported by XRD measurements which only detect crystalline Nb 0 p a r t i c l e s above 1 9 % N b 0 / A l 0 3 , and C0 chemisorpt ion measurements (see Figure 6) which i n d i c a t e t h a t the b a s i c alumina hydroxyls have been removed by the niobium oxide o v e r l a y e r [ 1 4 , 1 5 ] . The s l i g h t increase i n the C 0 chemisorpt ion above 1 9 % Nb 0 /Al 0 i s due t o C0 chemisorption on the c r y s t a l l i n e Nb 0 p a r t i c l e s . The Raman s p e c t r a i n Figure 7 reveal t h a t the 6 3 0 cm band of t h e s u r f a c e niobium oxide phase begins t o s h i f t towards the 6 9 0 cm" band o f c r y s t a l l i n e N b 0 above 1 9 % N b 0 / A l 0 3 due t o the presence of c r y s t a l l i n e Nb 0s particles. Thus, XPS, XRD, C 0 chemisorption, and Raman all demonstrate t h a t a monolayer o f s u r f a c e niobium oxide on alumina , ~ 1 8 0 m /g, corresponds to ~ 1 9 % g
surface
u r f a c
2
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bujk
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In Novel Materials in Heterogeneous Catalysis; Baker, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
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NOVEL MATERIALS IN HETEROGENEOUS CATALYSIS
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20.0
0.0
20.0
40.0
60.0
[Nb/AI]
bulk
80.0
100.0
(x 100)
Figure 5: Raman s h i f t s of ND2O5/AI2O3 (700°C) as a f u n c t i o n of niobium oxide coverage. 4.0
0
10
20 Nb 0 2
5
30
40
Loading (wt%)
Figure 6: XPS i n t e n s i t y r a t i o s of (Nb/Al) f u n c t i o n of (Nb/Al)
as a C e
In Novel Materials in Heterogeneous Catalysis; Baker, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
Niobium
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21. JEHNG AND WACHS
τ
1200
1000
1
Oxalate
1
800
241
1
ι
600
1
1
1
400
Γ
200
Raman Shift (cnr*)
Figure 7: C0 uptake of ND2O5/AI2O3 (500°C) as a f u n c t i o n of niobium oxide coverage. 2
ACKNOWLEDGMENTS
The support o f Niobium Products Company research p r o j e c t i s g r a t e f u l l y acknowledged.
f o r this
REFERENCES 1. Niobium Products Company Inc., Catalytic Applications of Niobium 2. J . M. Jehng, F. D. Hardcastle, and I. E . Wachs, Solid State Ionics, 32/33, 904(1989) 3. L . L . Murrell, D. C. Grenoble, C. J. Kim, and N. C. Dispenziere, Jr., J. Catal. 107, 463, (1987) 4. K. Y. Ng, X. Zhou, and E . Gulari, J. Phys. Chem. 89, 2477, (1985) 5. R. Y. Saleh, I. E . Wachs, S. S. Chan, and C. C. Chersich, J. Catal. 98, 102, (1986) 6. F. J. F a r r e l l , V. A. Maroni, and T. G. Spiro, Inorganic Chemistry 8(12), 2638, (1969) 7. R.S. Tobias, Can. J. Chem. 43, 1222, (1965) 8. C. Rocchiccioli-Deltcheff, R. Thouvenot, and M. Dabbabi, Spectrochimica Acta 33A, 143, (1977) 9. W. P. G r i f f i t h , and T. D. Wickins, J. Chem. Soc. (A), 590, (1967) 10. J . E . Guerchais, and B. Spinner, Bul1. Soc. Chim. France, 1122, (1965)
In Novel Materials in Heterogeneous Catalysis; Baker, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
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11. Ε. M. Zhurenkov, and N. Pobezhimovskaya, Radiokhimiya 12(1), 105, (1970) 12. C. Djordjevic, H. Gorican, and S. L. Tan, J. LessCommon Metals II, 342, (1966) 13. Ζ. X. Liu,Z. D. Lin, H. J . Fan, and F. H. Li, Appl. Phys. A 45, 159, (1988) 14. W. S. Milliam, Κ. I. Segawa, D. Smrz, and W. K. Hall, Polyhedron 5, 169, (1986) 15. C. L. O'Young, C. H. Yang, S. J . DeCanio, M. S. Patel, and D. A . Storm, J. Catal. 113, 307, (1988) Downloaded by NANYANG TECH UNIV LIB on November 2, 2014 | http://pubs.acs.org Publication Date: September 21, 1990 | doi: 10.1021/bk-1990-0437.ch021
RECEIVED May 9, 1990
In Novel Materials in Heterogeneous Catalysis; Baker, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
