Solid State Chemistry in Catalysis - American Chemical Society

8 Characterization of Vanadium Oxide Catalysts in Relation to Activities and Selectivities for Oxidation and Ammoxidation of Alkylpyridines

Downloaded by EAST CAROLINA UNIV on October 26, 2014 | http://pubs.acs.org Publication Date: June 13, 1985 | doi: 10.1021/bk-1985-0279.ch008

ARNE ANDERSSON and S. LARS T. ANDERSSON Department of Chemical Technology, Chemical Center, Lund Institute of Technology, P.O. Box 124, S-221 00 Lund, Sweden ESCA, XRD, IR, SEM and ESR were used to characterize the composition and structure of V-Ti-O catalysts, i n both precursors and activated forms. Precursors consist at low V/Ti r a t i o s of non-stoichiometric r u t i l e containing Ti and V and with VO on the surface. With increasing V/Ti ratios V i s dissolved up to 6 atom % and V clusters are formed i n the r u t i l e . Excess vanadium forms non-stoichiometric V O crystals on the surface of the 25 µm r u t i l e p a r t i c l e s , which at higher concentrations are completely embedded. Cata l y s t s activated by reduction additionally contain non-stoichiometric V O and V O i n amounts increas ing with decreasing V/Ti r a t i o s . Isolated V ions i n the vanadium oxides also increase i n concentration. The c a t a l y t i c performance of these catalysts i n oxidation and ammoxidation of some alkylpyridines i s discussed. 3+

4+

2+

4+

2

6

13

2

5

4

4+

The V-Ti-0 system has been extensively studied i n connection with c a t a l y t i c oxidation and ammoxidation reactions of aromatic hydrocar bons. Two p r i n c i p a l l y d i f f e r e n t types of catalysts can be d i s tinguished. One type of catalyst i s prepared by impregnation, p r e c i p i t a t i o n or mixing of the vanadium and titanium phases followed by calcination i n a i r below the melting point of Vo°s (1-4). The simultaneous reduction of ν 0 and transformation of AIO^ (anatase) into r u t i l e when heating Below the V 0^ melting point has been demonstrated to be due to topotactic reactions (5). The formation of lower vanadium oxides can be of importance, because i t has been found that reduced phases determine the a c t i v i t y and s e l e c t i v i t y of catalysts (6,7). Another type of V-Ti-0 catalyst i s prepared by mixing V ^ and TiO (anatase) phases, followed by heating the mixture above the melting point of V 0 (8,9). Clauws and Vennik (10) have found a defect, associated with oxygen vacancies, by studying the o p t i c a l absorption of V O. c r y s t a l s . The same defect was found i n TiO^-promoted V 0 crystals (11), but the intensity was greatly enhanced. ?

2

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2

2

5

0097-6156/85/0279-0121$06.25/0 © 1985 American Chemical Society

In Solid State Chemistry in Catalysis; Grasselli, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

122

S O L I D STATE C H E M I S T R Y IN CATALYSIS

T h i s shows the importance o f the i n c o r p o r a t i o n o f T i i n creating oxygen v a c a n c i e s . The c a t a l y s t s d e a l t w i t h i n t h i s p r e s e n t a t i o n b e l o n g t o the second type o f c a t a l y s t s d e s c r i b e d above. They have been c h a r a c t e r i z e d by means o f XRD, ESCA, ESR, SEM and IR methods. I t has been d e s c r i b e d i n p a t e n t s t h a t ammoxidation c a t a l y s t s can be a c t i v a t e d by t r e a t m e n t w i t h ammonia a n d / o r hydrogen (12) o r w i t h c a r b o n monoxide ( 1 3 ) . T h e r e f o r e , b o t h p r e c u r s o r s and reduced c a t a l y s t s w i l l be c o n s i d e r e d i n t h i s p r e s e n t a t i o n . I t w i l l be shown t h a t the performance of the c a t a l y s t s a r e r e l a t e d t o t h e i r c h a r a c t e r i s t i c s . The adsorbed s t a t e o f r e a c t a n t s w i l l a l s o be d i s c u s s e d .


Methods A c t i v i t y measurements. The measurements were performed a t atmos p h e r i c p r e s s u r e i n a g l a s s r e a c t o r . A thermocouple was p o s i t i o n e d i n the c e n t e r o f the r e a c t o r . I n the ammoxidation o f 3 - p i c o l i n e , the i n l e t r e a c t i o n m i x t u r e was a d m i t t e d a t a r a t e o f 32 l i t e r s / h r and c o n t a i n e d 232-254 moles o f a i r , 13-14 moles o f ammonia, and 56-62 moles o f water v a p o r f o r each mole o f 3 - p i c o l i n e . The r e a c t i o n was u s u a l l y performed i n the temperature i n t e r v a l 300-400 C . I n the o x i d a t i o n o f MEP ( 2 - m e t h y l - 5 - e t h y l p y r i d i n e ) the m o l a r r a t i o s o f O^/MEP and steam/MEP^ were 75 and 175 r e s p e c t i v e l y , and the space v e l o c i t y was 7000 h . I n ammoxidation s t u d i e s p r e r e d u c e d c a t a l y s t s were used and the measurements e x t r a p o l a t e d i n time t o g i v e d a t a a t the s t a r t o f the r e a c t i o n . I n o x i d a t i o n s t u d i e s unreduced c a t a l y s t s were used and the d a t a were o b t a i n e d at the steady s t a t e . XRD. X-Ray d i f f r a c t i o n a n a l y s e s were c a r r i e d out on c a t a l y s t s by a P h i l i p s X - r a y d i f f r a c t i o n i n s t r u m e n t u s i n g a PW 1310/01/01 g e n e r a t o r and Cu Κα r a d i a t i o n . IR. The i n f r a r e d s p e c t r a were r e c o r d e d on a P e r k i n - E l m e r 580B s p e c t r o p h o t o m e t e r connected t o a d a t a s t a t i o n from the same manufac t u r e r . The KBr d i s c method was u s e d . The s p e c t r a were s t o r e d on d i s k s and t r a n s f e r r e d t o a T e k t r o n i x 4051 computer f o r e v a l u a t i o n . ESCA. ESCA measurements were performed on an ΑΕΙ ES 200B e l e c t r o n s p e c t r o m e t e r equipped w i t h an A l - a n o d e (1486.6 e V ) . The f u l l w i d t h a t h a l f maximum (FWHM) o f the Au 4 f y l i n e was 1.8 e V . Sample c h a r g i n g was c o r r e c t e d f o r w i t h the 0 I s l i n e at 529.6 e V , w h i c h has been shown t o be a s u i t a b l e method i n t h i s system ( 1 4 ) . F o r the q u a n t i t a t i v e a n a l y s i s c a l i b r a t e d s e n s i t i v i t y f a c t o r s , o b t a i n e d from pure o x i d e s , were u s e d . These were 0 Is 1, T i 2p = 1.37 and V 2 p = 2.17. 2

s

3 / 2

3 / 2

ESR. A V a r i a n E - 3 s p e c t r o m e t e r was used f o r the ESR s t u d i e s . I n the q u a n t i t a t i v e measurements a c a l i b r a t e d V 0 / T i 0 sample was r u n between each c a t a l y s t . The e r r o r i n t h e s e r e l a t i v e measurements was l e s s t h a n 10 %. F o r the c a l i b r a t e d \/ 2 P P concen t r a t i o n was determined t o w i t h i n 3Ό % a c c u r a c y by c a l i b r a t i o n a g a i n s t a CuS0^ 5H 0 s i n g l e c r y s t a l . T h i s measurement, and measure ments f o r some or the s a m p l e s , were performed on a V a r i a n E9 equipped w i t h a d u a l c a v i t y . The g - v a l u e s were measured w i t h i n ±O.002. 2

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8.

ANDERSSON AND ANDERSSON

Characterization of Vanadium Oxide Catalysts 123

SEM. Scanning e l e c t r o n m i c r o s c o p i c i n v e s t i g a t i o n s w i t h a J e o l JSM-U3 o r an I S I - 1 0 0 A i n s t r u m e n t .

were

performed

Catalyst preparation. The c a t a l y s t s were prepared by h e a t i n g VÔ^ and T i O ^ (anatase) powders i n a q u a r t z c r u c i b l e i n a h i g h tempera t u r e oven f o r 3 h r s at 1 1 5 0 - 1 2 5 0 ° C . The fused c a t a l y s t s were d i v i d e d i n t o s m a l l p a r t i c l e s , and the O.71-1.41 mm f r a c t i o n was used i n the a c t i v i t y measurements. A c t i v a t e d c a t a l y s t s were prepared bv r e d u c t i o n o f the p r e c u r s o r s i n 1 atm o f hydrogen f o r 1 h r at 450 C . Q

R e s u l t s and D i s c u s s i o n


Catalyst Precursor XRD. X-Ray d i f f r a c t i o n p a t t e r n s o f the p r e c u r s o r s were composed of the p a t t e r n s o f V 0 and T i O ( r u t i l e ) , except f o r the O.5 and 1.0 mole % VÔ^ c a t a l y s t s f o r w h i c h o n l y T i O ^ l i n e s were observed ( 1 5 ) . P r e c u r s o r s w i t h more than 50 mole % V ^ O . a l s o c o n t a i n e d v e r y s m a l l amounts o f T i O ( a n a t a s e ) . Scanning o f the l i n e s i n the b a c k - r e f l e c t i o n r e g i o n , ΖΘ = 115-142 d e g r e e s , showed t h a t t h e r e was a s m a l l s h i f t o f the T i O ^ l i n e s . The l a t t i c e c o n s t a n t s o f the r u t i l e phase o f T i O ^ were c a l c u l a t e d . Cohen's l e a s t - s q u a r e s method o f e l i m i n a t i n g e r r o r s was used ( 1 6 ) . The r e s u l t s are g i v e n i n T a b l e I . The u n i t c e l l dimensions o f the c a t a l y s t r u t i l e phase has changed m a i n l y i n the a d i r e c t i o n . The l e n g t h o f the u n i t c e l l i n the c^ d i r e c t i o n was p r a c t i c a l l y the same as t h a t o f pure r u t i l e . Bond and coworkers have o b t a i n e d the same r e s u l t ( 4 ) ^ The c o n t r a c t i o n o f the u n i t c e l l can be due to i n c o r p o r a t i o n o f V i n the T i O ^ phase. No changes o f the l a t t i c e parameters o f the vanadium p e n t o x i d e phase c o u l d be d e t e c t e d , a l t h o u g h i t has been r e p o r t e d t h a t T i can be d i s s o l v e d i n 2°5 g h t be due to d e t e c t a b i l i t y p r o b l e m s . V 0 does not have any l i n e s i n the b a c k - r e f l e c t i o n r e g i o n , where s h i f t s are most e a s i l y s e e n . 2

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(11.).

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Table I .

T

h

i

s

m i

L a t t i c e constants of T i 0

2

(rutile)

Phase

C e l l dimensions c (Â) a (A)

T i 0 , pure VO , pure Ti6 , catalyst

4.594 4.530 4.583

2

2

2.959 2.869 2.958

Ref. (17) (18) t h i s work

ESCA. I n the ESCA measurements on the fused V ^ / T i O ^ c a t a l y s t s the 0 I s , V 2p«y« and T i 2p~ core l i n e s were o b s e r v e d . The b i n d i n g e n e r g i e s ( B . E . ) indicate'Ibhe presence o f VÔ- and T i O ^ f o r a l l samples and the v a l u e s were 5 2 9 . 6 , 516.6 and 4 5 7 . 9 e V , r e s p e c t i v e l y (See T a b l e I I ) . F o r the powder m i x t u r e s (un-fused) the v a l u e s were 5 2 9 . 6 , 516.6 and 458.5 e V , r e s p e c t i v e l y . Thus t h e r e i s a d i f f e r e n c e i n the T i 2p . B . E . w h i c h can be e x p l a i n e d by the f o r m a t i o n of r u t i l e d u r i n g h e a t i n g o f the samples i n a d d i t i o n t o the d o p i n g of the r u t i l e phase w i t h V .


124

S O L I D STATE C H E M I S T R Y IN CATALYSIS

Table I I .

B i n d i n g e n e r g i e s and h a l f w i d t h s (eV) o f the V 2 p ^ Ti 2p« core l i n e s

and

2

/ 9

Sample

V

Ti0 ,

anatase

Ti0 , ζ V 0 2 5

rutile

o

o

o


2

5

2

catalyst o

5

i

2

P /2 3

516.6 (1.6) 516.6 (1.6) 516.1 (2.8) 516.2 (3.2)

Same a f t e r H S0,+NH„ treatment 2 4 3 V 0 / T i 0 ( 7 0 / 3 0 ) , a f t e r 1100°C i n vacuum f o r 15 h r . 2

T

'3/2

458.5 (1.7) 458.2 (1.9)

c

V 0 /Ti0 ,

2

2

457.9 (1.7) 458.1 (1.9) 457.9 (2.2)

I n an attempt t o study t h i s e f f e c t a V 0 « / T i 0 (70/30) sample w h i c h appears t o be a s o l i d s o l u t i o n from XKD d a t a ( o n l y r u t i l e l i n e s appear i n s p e c t r a ) was measured. The sanje^ T i 2p^# B . E . as f o r the c a t a l y s t s was o b t a i n e d . To measure the V i n the r u t i l e phase some samples were t r e a t e d w i t h s u l p h u r i c a c i d f o l l o w e d by ammonia t o d i s s o l v e the vanadium o x i d e p h a s e . The ESCA a n a l y s i s o f the 10 mole % V 0 c a t a l y s t t r e a t e d i n t h i s manner showed the presence o f 6 atom % V w i t h a B . E . o f 516.1 e V , o b t a i n e d a f t e r s u b t r a c t i o n o f the 0 l s ( K a ^ (i) ^ * f s p e c t r a . No vanadium o x i d e s a r e d e t e c t a b l e by XRD on t n i s sample. T h u s , a p p r o x i m a t e l y 6 atom % V seems t o be d i s s o l v e d i n the r u t i l e phase o f the c a t a l y s t s . The c o m p o s i t i o n V Ti 0 has been suggested i n the l i t e r a t u r e ( 4 ) . That i t i s p r o b a b l y * p r e s e n t as V i s i n d i c a t e d by the lower V 2 p * B . E . and i n correspondence w i t h the V 0 / T i 0 sample. I t i s i n t e r e s t i n g t o n o t e t h a t sample c h a r g i n g phenomena o c c u r f o r samples w i t h low vanadium l o a d i n g . F o r powder m i x t u r e s t h e s e a r e observed a t l e s s t h a n 50 atom % V and f o r fused samples a t l e s s t h a n 6 atom % V . Pure V 0 g i v e s almost no c h a r g i n g whereas pure T i O g i v e s a c h a r g i n g o f %-i V . W i t h enough 2 ^ 5 ^ a t there i s contact between o ° 5 P l t h e r e s h o u l d be a low c h a r g i n g e f f e c t . A t low V 0 ^ l o a d i n g s , however, t h e r e i s a c o n s i d e r a b l y worsened c o n t a c t between the V 0 ^ p a r t i c l e s throughout the b u l k o f the sample, and as f o r pure T i 0 a l a r g e c h a r g i n g e f f e c t a r i s e s . What i s v e r y i n t e r e s t i n g i s t h a t f o r the fused samples t h i s o c c u r s a t a much lower vanadium c o n t e n t . T h i s i n d i c a t e s the e x c e l l e n t coverage o f the V O. on the TiO~ p a r t i c l e s . I t i s f u r t h e r noteworthy t h a t the 50 atom % V powder m i x t u r e and the 6 atom % V fused sample c o n t a i n a p p r o x i m a t e l y the same atom % V as a n a l y s e d by ESCA, k e e p i n g i n mind the h i g h surface s e n s i t i v i t y . I n F i g u r e 1 the V c o n t e n t o f the v a r i o u s V - 0 / T i O - samples as r e v e a l e d by ESCA i s p l o t t e d a g a i n s t the n o m i n a l v - c o n t e n t . P o i n t s for ο ς / 2 l y l s o i n c l u d e d and i t i s e v i d e n t t h a t t h e s e tîénave s i m i l a r l y . The l i n e f o r the powder m i x t u r e s f a l l s c l o s e to the t h e o r e t i c a l l i n e (dashed l i n e ) . An almost p e r f e c t match c o u l d be o b t a i n e d by changing the s e n s i t i v i t y f a c t o r s . However, s i n c e t h i s d e v i a t i o n might be due to p a r t i c l e s i z e e f f e c t s , s e n s i t i v i t y f a c t o r s from pure o x i d e s were used i n s t e a d . F o r the fused samples a much ?

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V

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8.

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Characterization of Vanadium Oxide Catalysts 125

h i g h e r atom % o f V i s o b t a i n e d by ESCA t h a n was e x p e c t e d . E v i d e n t l y the VÔ^ phase c o m p l e t e l y c o v e r s the r u t i l e p a r t i c l e s . T h i s was a l s o i n d i c a t e d by the c h a r g i n g phenomena d i s c u s s e d above. I n c o n c l u s i o n f r o m ^ t h e ESCA r e s u l t s , the fused V 0JTIO^ c a t a l y s t s c o n s i s t of a V doped T i O ( r u t i l e ) phase embedded i n the V 0 phase. +

?

2

5

ESR. I n F i g u r e 2 the ESR s p e c t r a a t R . T . f o r some f r e s h l y p r e p a r e d 2°5^ 2 n « Even a f t e r r e d u c t i o n i n H JL£ 450 C f o r 1 hour pure T i O ^ showed o n l y v e r y weak bands from T i which t j j e j e f o r e does not i n t e r f e r e w i t h the q u a n t i t a t i v e measurements o f V d i s c u s s e d b e l o w . F o r O.05 mole % 2 ° 5 ^ band showing some h y p e r f i n e s p l i t t i n g was o b s e r v e d . T h i s was i n t e r p r e t e d as VO ions on the r u t i l e s u r f a c e i n a c c o r d w i t h the assignments o f some slightly different spectra ( 1 9 , 2 0 ) . These seem t o broaden and d i s a ç j j i e a r on i n c r e a s i n g t h e °5 * b r o a d s i n g l e band due to V i n 2?5 P P « T h u s , almost no f i n e s t r u c t u r e i s seen f o r 10 mole % ^ ^ » ^ d i s c o m p l e t e l y absent f o r the samples 30-100 mole % ^ 2 ^ 5 ' The g - v a l u e f o r the broad resonance was 1.972. The l i n e w i d t h i n c r e a s e s from 100 t o 150 Gauss w i t h i n c r e a s i n g 9 ° 5 from 10 t o 100 %. The 10 % sample showed no s i g n a l a f t e r t h e remova^. of the vanadium o x i d e s . T h i s f a c t s t r o n g l y i n d i c a t e s t h a t t h e V i o n s i n the r u t i l e phase a r e not seen a t R . T . due t o s t r o n g s p i n l a t t i c e coupling>4 and thus do not c o n t r i b u t e t o the q u a n t i t a t i v e measurements o f V I n F i g u r e 3 the ESR s p e c t r a at 7Ί& a r e shown. The reduced T i 0 sample shows a s p e c t r a a s s i g n e d to T i ( 2 1 ) . The i n t e n s i t y o f the Ti s i g n a l i s c o m p l e t e l y n e g l i g i b l e compared t o the i n t e n s i t y o f the o t h e r samples a t 77K. The i n c r e a s e i n i n t e n s i t y w i t h the d e c r e a s e i n temperature ( r e l a t i v e i n t e n s i t y 7 7 K / R . T . ) i s v e r y l a r g e f o r a l l samples except f o r V 0 . Here a 3.6 f o l d i n c r e a s e i s o b s e r v e d , w h i c h corresponds t o tne normal change i n s p i n p o p u l a t i o n w i t h decreased t e m p e r a t u r e . E s t i m a t e s from i n s t r u m e n t g a i n s e t t i n g s p o i n t t o an a p p r o x i m a t e l y 10 -10 f o l d i n c r e a s e f o r the fused V 0 / T i O samples. The i n t e n s i t y at 77K f o r the pure V 0 sample i s n e g l i g i b l e - a few p e r c e n t a t the most - compared w i t h t h a t o f the T i 0 c o n t a i n i n g samples. The most s i g n i f i c a n t e f f e c t i s observed f o r the H-SO^ and NH^ t r e a t e d samples w h i c h show no s i g n a l at a l l at R . T . but one o f s i m i l a r magnitude as f o r a l l V 0 - / T i O « samples a t 77K. E v i d e n t l y the low temperature s i g n a l i s almost e n t i r e l y due t o the r u t i l e p h a s e . I t i s p o s s i b l e t h a t the temperature e f f e c t may J>£ due t o a s p i n - l a t t i c e r e l a x a t i o n e f f e c t whereby s u b s t i t u t i o n a l V would not be o b s e r v a b l e a t R . T . ( 2 2 ) . I t i s known t h a t V - V bonds a r e formed i n t h e ^ V 0 - T i 0 s y s t e m , a p p a r e n t l y by p a i r i n g o f randomly distributed V i o n s at low c o n c e n t r a t i o n s o f V ( 2 3 ) . These would undoubtedly g i v e temperature dependent s p i n - s p i n r e l a x a t i o n e f f e c t s . The c a l c u l a t i o n s o f s p i n c o n c e n t r a t i o n s from measurements a t 77K d o , however, g i v e a b s u r d l y h i g h v a l u e s . I t i s t h e r e f o r e suggested t h a t a l a r g e p a r t o f the i n c r e a s e i n s i g n a l i n t e n s i t y i s due t o a f e r r o m a g n e t i c phase t r a n s i t i o n , ^ t h i g h e r c o n c e n t r a t i o n s i t seems v e r y l i k e l y that c l u s t e r s of V o r V 0 i s l a n d s are formed i n the r u t i l e p h a s e . Anatase doped w i t h up t o 2 atom % V was suggested t o c o n t a i n a l a r g e p r o p o r t i o n o f V 0 i s l a n d s ( 2 4 ) . Pure V 0 does not g i v e any s i g n a l s i n the ESR s p e c t r a . I t i s w e l l known t h a t temperature dependent phase t r a n s i t i o n s do o c c u r i n V 0 and t h a t t h e s e a r e

V

T i 0

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v

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2 c l u s t e r s g i v e f e r r o m a g n e t i c s p e c i e s a t l o w e r t e m p e r a t u r e s . C o n c e r n i n g the c u r v e forms i t i s seen t h a t f o r the O.05 mole % V«0 sample t h e r e i s ,a h y p e r f i n e s p l i t t i n g i n d i c a t i v e o f a s u b s t i t u t i o n a l s o l u t i o n of V i n r u t i l e ( 2 2 , 2 6 ) . I t i s q u i t e c l e a r t h a t on i n c r e a s i n g the c o n t e n t from O.05 t o O.5 mole %, the ESR s p e c trum i s s t r o n g l y broadened, p r o b a b l y due t o l a r g e s p i n - s p i n r e l a x ation effects for V i n the r u t i l e p h a s e . Any t r a c e s o f h y p e r f i n e s p l i t t i n g have almost d i s a p p e a r e d a t 1 mole % 2 ° 5 * ^ P^f 10 t o 90 mole % showed i d e n t i c a l s i g n a l s , w h i c h was a l s o the case f o r the 10 % H ^ O ^ and NH^ t r e a t e d sample. The g - v a l u e f o r the broad resonance was 1.934 f o r the 30 mole % V«0 sample and i n c r e a s e d c o n t i n u o u s l y t o 1.957 f o r the 90 mole Τ vÔ^ sample w h i c h differs from the v a l u e s at R . T . S i m u l t a n e o u s l y , the l i n e w i d t h i n c r e a s e d from 180 to 210 Gauss. A

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SEM. The p r e c u r s o r s w i t h 50 t o 10 mole % V 0^ t r e a t e d Η SO^ and NH^ were s t u d i e d . F i g u r e s 4(a) and 4(b) show tne T i O ^ p a r t i c l e s i z e i n t h e s e s a m p l e s , w h i c h i s about 25ym i n b o t h c a s e s . F i g u r e 4 ( c ) shows s i n t e r e d T i O ^ . By comparing t h i s w i t h F i g u r e s 4(a) and 4 ( b ) , i t can be c o n c l u d e d t h a t t h é T i O ^ p a r t i c l e s become l a r g e r w h e n ^ i n t e r e d i n the p r e s e n c e o f V ^ O - . The r e a s o n f o r t h i s can be t h a t V is incorp o r a t e d i n t o the T i O ^ l a t t i c e , w h i c h l e a d s t o a d e c r e a s e o f the m e l t i n g p o i n t o f T i O . . T h i s d e c r e a s e i s then r e f l e c t e d i n a g r e a t e r l a t t i c e m o v a b i l i t y . T h e f a c t t h a t the T i 0 « p a r t i c l e s i z e i s the same^ i n p r e c u r s o r s w i t h b o t h 50 and 10 mole % *

Solid State Chemistry in Catalysis - American Chemical Society

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