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Performances of platinum metal catalysts supported on titania coated silica prepared from metal organics
Applied Surface Science 33/34 (1988) 269-276 North-Holland, Amsterdam
269
PERFORMANCES OF PLATINUM METAL CATALYSTS SUPPORTED ON TITANIA COATED SILICA PREPARED FROM METAL ORGANICS Kaori M A T S U O a n d K a t s u y u k i N A K A N O Department of Chemical Engineering, Fukuoka University, Fukuoka 814-01, Japan Received 23 August 1987; accepted for publication 31 October 1987
Titania-silica beads prepared by vapor-phase hydrolysis of titan alkoxide adsorbed on silica beads were observed to be active in promoting the perfect oxidation of acetone, but titania-silica beads prepared by pyrolysis of titan alkoxide and by adsorption of titania colloid onto silica beads showed no such activity. Further, a different performance in a photo-induced catalysis was observed among the platinum metal catalysts supported on these beads and on titania beads. In these reactions, the titania-silica interface seems to play an important role in relation to the excited electronic state of the catalytic surface. Next, the metal-support interaction, the so-called SMS1 effect, was examined in these catalysts and on other oxides (ZrO2, WO3, MOO3,Nb2Os, etc.) by using electronic and vibrational spectra. Based on the results, the ease of entry to the SMSI state was found to be correlated to the energy gap value of the metal oxides used. Further, the rates of some oxidation and hydrogenation reactions were found to be controlled by changing the degree of SMSI.
1. Introduction In recent years, considerable a t t e n t i o n has been given to m e t a l - s u p p o r t interactions in the p r e p a r a t i o n of supported metal catalysts, because the interface between a metal a n d a s e m i c o n d u c t o r oxide often produces a new function, b e y o n d our usual consideration, for catalysis [1]. T o prepare semic o n d u c t o r oxides, new approaches have been investigated using metal organics, such as metal alkoxides [2]. These situations stimulate the efforts to search for catalysts which are highly selective, active a n d durable. In this paper, three different methods were tried for coating silica surfaces with titania thin films utilizing (1) vapor-phase hydrolysis, (2) pyrolysis of titan alkoxides adsorbed on silica beads, a n d (3) a d s o r p t i o n of a t i t a n i a colloid (produced b y hydrolysis of titan alkoxides) o n t o silica beads. T h e surface characterization of these beads was m a d e b y using in situ I R - D R S a n d UV-Vis, etc. The p l a t i n u m metal catalysts supported o n these b e a d s were prepared a n d tested for p h o t o - i n d u c e d a n d perfect o x i d a t i o n reactions. As is well-known, p l a t i n u m metal supported o n s e m i c o n d u c t o r oxides, shows an interesting p h e n o m e n o n in catalysis, which is related to the strong m e t a l - s u p 0 1 6 9 - 4 3 3 2 / 8 8 / $ 0 3 . 5 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics P u b l i s h i n g Division)
270
K. Matsuo, K. Nakano / Performances of Pt metal catalysts supported on TiO: SiO: -
port interaction, the so-called SMSI effect [3]. This effect was examined for these catalysts and for the catalysts supported on other metal oxides (ZrO 2, WO), MOO3, Nb205 and anatase) by using electronic and vibrational spectra. Further, the rates of some surface reactions were correlated to the degree of SMSI in supported platinum metal catalysts.
2. Experimental
2.1. Methods of preparation for titania coated silica A flow diagram of the preparation methods of titania coated silica beads is shown in fig. 1.
(a) (TiO:/ SiO:.)n beads" The silica beads, Cariact 15 (3 m m diameter) supplied by F u j i - D a v i s o n Chemical Ltd., were added into the solution of titan isopropoxide (TIP) and isopropanol (IPA). After the adsorption of TIP on the silica beads was completed at 305 K, the beads were separated from the solution and kept overnight in a moist atmosphere, where the TIP on the sample was hydrolyzed by the H 2 0 vapor. The samples were dried in vacuo, and then the ( T i 0 2 / S i O 2 )~l beads were produced.
TIP (IPA)
]
TIP IPA
! Si02 Beads__! I Adsorption +
_
_
I (TIP/Si02) Beads Pyrolysis i n ~ an Ar s t r e a m ~ /
i
H~O
a/,/// [
TiO~ P a r t i c l e s
i [
TiO~
Collo{d i *
~Hydroiysis ~n
-
H20 vapor
4
1 ! Adsorption
C
~io~-sio~) I PdCiz or H2PtCIs-6H20
Impregnation -Z L Pd-orPt-(Ti02/Si02)p
! ?d-orPt-(Ti02/Si@2
B~ds I
I
PdCI2 or [ H2PtCI~-BH=Q i I I~pregnation
Pd-orPt-(YiO~-SiO2)
Fig. 1. Flow diagram of the preparation methods of titania coated silica beads and platinum catalysts supported on these beads.
K. Matsuo, K. Nakano / Performancesof Pt metal catalysts supported on TiO2- SiO,
271
(b) ( T i O 2 / SiOe)p beads The TIP vapor (vaporized at 423 K) carried by an Ar gas stream was introduced into a tubular reactor packed with the silica beads, and the T I P coated silica beads were obtained. After the adsorption was completed, The temperature of the reactor was kept at 513-593 K to pyrolyze the T I P adsorbed on the silica beads in the Ar stream. The beads obtained were called (TiO2/SiO 2) p beads. (c) ( T i O : - S i 0 2 ) beads Prior to the preparation, we obtained the titania colloid by the use of the so-called sol-gel reaction of T I P under various reaction conditions [4]. The initial relative composition of TIP, IPA and water had a large effect on the nature of the products. The colloid obtained using the initial composition of the reactant, T I P / I P A / H 2 0 = 1 / 5 / 4 in molar ratio, was utilized. The colloid system obtained using lower and higher H z O / T I P ratios did not adsorb on the silica beads [5]. The silica beads were mixed with the titania colloid and the colloid was adsorbed on the surface of the silica beads to a thickness of about 5 t~m. We called the samples, after drying in vacuo, (TiO2-SiO2) beads. The beads were used in the catalytic reactions as freshly prepared. In addition, platinum metal was deposited onto these beads and the other semiconductor oxides. The method of metal deposition was adsorption from a solution of metal salt, as presented elsewhere [6]. 2.2. Methods of characterization and activity tests
The beads or particles obtained in section 2.1 were characterized in various ways, e.g. in situ I R - D R S and UV-Vis, etc. An activity test was done by the use of a fixed-bed reactor for the perfect oxidation of acetone on these beads or platinum metal catalysts supported on these beads [7]. A photo-induced reaction was observed by the use of an I R spectrometer (IR-810, made by Japan Spectroscopic Co. Ltd.) equipped with a DRS cell (DR-81). The catalyst samples, on which alkoxide was adsorbed, were ground in an agate mortar, and were set on the cell to be exposed to the irradiation of 365 nm UV rays. The thermal properties of the catalysts were measured by an IR spectrometer equipped with a temperature-controlled DRS cell (DR-31). UV-Vis spectra were obtained in the form of relative reflectance intensity for the sample compared to a standard alumina disk. 3. Results and discussion 3.1. Characteristics and catalytic activities of titania-silica beads and platinum metal supported on these beads
The energy gap and the absorbance properties in the UV-Vis spectra of the titania coated silica beads were found to be controlled by the preparation
272
K. Matsuo, K. Nakano / Performances of Pt metal catalysts supported on TiO, - Si02
100
2
,,i ,,
0 rCI 1~
....
TiOo colloid adsorption ~ " on SiO2 /I , ~ PyrOlysis of TIPISiO 2 /" ," I .~-Hydrolysis of TIP/SiO 2 / /I / // ~-,---Anatase
7
/ 50
(D O[:
/~/j
,"
360
460
s6o
Wave length [nm] Fig. 2. UV spectra of the titania coated silica beads prepared by the different methods. methods or the reaction conditions as shown in fig. 2. These differences in the properties may affect the catalytic activities of the beads. The catalytic activities of these beads were tested by the reaction of the perfect oxidation of acetone. The (TiO2/SiO 2)H beads were observed to be active for this reaction as shown in fig. 3, but the others, the (TiOJSiO2)l, and the (TiO2-SiO 2) beads, showed no activity. The activities of the platinum metal catalysts supported on these beads were compared with each other for the reaction mentioned above. Higher activity was observed on the Pd-(TiO2/SiO2)H beads compared with the Pd-(TiO2-SiO2) beads in the low acetone partial pressure region. This is due to the effect of active oxygen on the surface of the (TiO2/SiO2) u beads in addition to the active oxygen produced from the platinum surface, but there is no active oxygen on the surface of the (TiO2-SiO 2) beads as deduced from the fact mentioned above.
Q20 t/)
6, o
/ / ~
O%pm SV 400h-~
o U
0110
Oll2O
0~ 0
Oil40
Amount of titania adsorbed [ g/g-SiOz]
Fig. 3. Activity of (TiO2/SiO2) H beads for the perfect oxidation of acetone (CH~COCH~ + 4 02 3 CO2 + 3 H20).
K. Matsuo, K. Nakano / Performances of Pt metal catalysts supported on TiO 2- SiOe
(a)
J_
(b)
273
(c)
(5 u E
----.2L -----LL ---.4L_ 0.5
IB
r'l ,¢
320o
26o03200
1.0
---.,.._.__
26oo 3200
2600
Wave number {~crn-1 ] Fig. 4. Time dependencies of IR spectra for titania coated silica beads at 553 K: (a) (TiO 2 / S i O 2) H; (b) (TiO 2 / S i O 2) p; (c) (TiO 2 - S i O 2).
The strength of the interaction between the surface of these beads and the organic molecules was qualitatively examined by the use of temperature-controlled vibrational spectra measurements as shown in fig. 4. Alkoxide on the (TiO2/SiO2) n beads was easily desorbed when these beads were kept at 553 K in the DRS cell of the I R spectrometer. On the contrary, the signal due to the C H vibration (2975 c m - t ) continued to be observed on the (TiO2-SiOz) beads after 3 h heat treatment. From the figure, the strength of adsorption for organics on these beads was in the following order: ( T i O 2 - S i O z ) > (TiO2/SiO2) p > (TiO2/SiO2) HWhen the Pt-(TiO2/SiO2)H beads, on which alkoxide was adsorbed, were exposed to irradiation of 365 nm UV rays and the I R spectra were measured
l
i
4000
2000 1000 400 Wave number [ ¢ m -1 } Fig. 5. IR spectra for 0.67% P t - ( T i O 2 / S i O 2 ) H beads, on which alkoxide was adsorbed, after irradiation by UV rays.
274
K. Matsuo, k: Nakano / Performances of Pt metal catal}'sts supported on TiO: - SiO:
in situ, a sharp absorbance band was observed (fig. 5) at 2125 cm t which may be due to the on-top CO vibration. The intensity of this signal increased with increasing irradiation time, because the photo-induced reaction proceeded. This experiment was tried on a Pt TiO 2 and a Pt-SiO, catalyst instead of the P t - ( T i O J S i O 2 ) H or Pt-(TiO2/SiO2) p beads, but CO was not produced during 2 h of irradiation. This result shows that the interface between silica and titania plays an important role in the photo-induced catalysis, but the detailed mechanism is not clarified in the present situation.
3.2. Metal ,support interaction for platinum metal catalysts supported on titania-silica and the other semiconductor oxides As is generally known, platinum metal supported on semiconductor oxides shows an interesting phenomenon related to the strong m e t a l - s u p p o r t interaction (SMSI) [3]. When the titania supported platinum metal catalyst is strongly reduced to make the platinum lose its ability for dissociation of a hydrogen molecule, this phenomenon is called the " S M S I " effect. This effect was easily observed by comparing the in situ I R - D R S spectra of Pt- or Pd-TiO~ catalysts after the high temperature reduction to those without reduction. The typical results are shown in fig. 6. The intensity of the characteristic absorbance band by the oxide (920 cm i for Ti-O), which appeared in the normal state, decreased for the catalyst in the SMSI state. Another aspect ol the SMSI state is the disappearance of the O H vibrational band because of diminishing H 2 0 adsorption. This showed that oxygen desorbed irreversibly from the lattice matrix of Pd TiO 2 in the high temperature reduction, although oxygen can adsorb reversibly onto the Pd-TiO~ (x < 2.0) after low temperature reduction. After the desorption of oxygen in the high temperature reduction condition, hydride may be adsorbed in the metal-oxide lattice, which
Relative Activity . . . . . . . 1.0 . . . . . ~.7::.:~:~.~ . . . . . . . 0.91 7c ,.~- ~ _ -0.82 . I- . . . .
~__
-t_
#
_/
o
<
Treatment /~ (573,773) /'1 (773,773) ~ / / ( 773, - - ) -> ~ . ~ ' ~ . . ~ ; -"~,1
---076 ( 73 - )
1
Pd-Ti02( commercially available beads) 1 Treatment( Calcination Temp Reduction Temp) \\
\ "-...
4000
3000
2400 1600 1200 Wave number [" cml.~
800
400
Fig. 6. IR spectra and relative activity for the perfect oxidation of acetone, of 0.5% Pd TiO~ after the calcination and reduction treatment at various temperatures.
K. Matsuo, K. Nakano
/
Performances of Pt metal catalysts supported on TiO: - SiO 2
275
100 //
t'
/// Nb205~
/
(1)~/:
.~ so
jA°a'ase 7
rr
l
," / / Zr02 / ~
o200
46o 56o Wave length Into}
36o
600
Fig. 7. UV spectra for several oxides and (1) (TiO 2/SiO2) H, (2) sol-gel titania (H20/TIP = ] 5)
and (3) the sol-gel titania sintered at 873 K. retards the dissociation of hydrogen. This phenomenon is often observed in the palladium-hydrogen system. We also found that the SMSI state is kept stable even in an oxidizing atmosphere at room temperature at least for a few years. As shown above, titania was easily reduced to enter the SMSI state, but it was difficult in the case of the titania-silica beads. The reason for this was examined by the use of electronic and vibrational spectra. For some semiconductor oxides, UV-Vis spectra were measured and are shown in fig. 7. Further, platinum metal was supported on these oxides and these samples were exposed in the hydrogen stream at various temperatures. The vibrational spectra were measured for these samples to check whether or not the samples entered the SMSI state. For ZrO 2 and the anatase (supplied by Rare Metallic Co. Ltd.) systems, the samples did not enter the SMSI state even at a reduction temperature above 773 K. For the platinum metal catalysts supported on Nb205 and the titania beads (supplied by Sakai Chemicals Ltd.), the SMSI state was achieved at a temperature around 773 K. Further, MoO 3 and O O 3 supported platinum catalysts entered easily into the SMSI state at 593 K. Thus, the ease with which the SMSI state could be entered was found to be correlated to the value of the energy gap of the metal oxides used as the supports, on which the reducibility of the oxides depends. We also measured UV-Vis spectra for the (TiO2/SiO2) H beads and sol-gel titania particles as shown in fig. 7. The Pt-(TiO2/SiO2) H beads did not enter the SMSI state even at 873 K; neither did P t - T i O 2, where TiO 2 particles were produced by the sol-gel reaction and Pt metal was deposited on the freshly prepared particles. On the contrary, TiO2, on which platinum metal was deposited after the freshly prepared particles were sintered at 873 K, was found to enter easily into the SMSI state when they were reduced at 773 K. These results are consistent with the fact that the ease of entry to the SMSI state depends on the reducibility of the oxides used as support.
276
K. Matsuo, K. Nakano / Performances of Pt metal catalysts supported on TiO: SiO:
The hardness and the electrical conductivity of the catalysts were observed to increase in the SMSI state compared with in the normal state. A change in the electrical conductivity must affect strongly the catalytic activity. The activities for the perfect oxidation of acetone and the IR spectra of the Pd-TiO2 catalysts in various reduction conditions were measured to correlate the activity with the intensity of the SMSI state (this is defined as the height ol the 920 cm 1 band based on the absorbance of 400 c m - I ) . The results are shown in fig. 6. It was found that the catalyst in the SMSI state was favorablc for enhancing the perfect oxidation of acetone. On the other hand, the reaction rate of benzene hydrogenation was found to be retarded on the platinum metal catalyst supported on titania in the SMS1 state as compared with in the normal state, which is due to the decrease in the dissociative adsorption for hydrogen [8]. From these results, the possibility was found for controlling the activity for some catalytic reactions by changing the degree of SMSI.
4. Conclusions Three different methods were tried for preparing titania coated silica beads using vapor-phase hydrolysis, pyrolysis of titan isopropoxide adsorbed on the silica beads, and also the adsorption of titania colloid onto the silica beads. These beads have different properties for the perfect oxidation of acetone. Platinum metal catalysts were prepared using these beads. The catalysts prepared were found to have unique properties in photo-induced catalysis and in the SMSI effect, compared with titania or silica supported platinum metal.
References [1] A. Morikawa, Catalyst (Shokubai) 29 (1987) 2: H. Arai and T. Ishihara, Catalyst (Shokubai) 29 (1987) 5. [2] J.D. Mackenzie, in: Science of Ceramic Chemical Processing, Eds. L . L Hench and D.R. Ulrich (Wiley, New York, 1986) p. 113. [3] S.J. Tauster, S.C. Fung and R.L. Garten, J. Am. Chem. Soc. 100 (1978) 170. [4] K. Matsuo and K. Nakano, Fukuoka Univ. Rev. Technol. Sci. 37 (t986) 333. [5] K. Matsuo and K. Nakano, Fukuoka Univ. Rev. Technoh Sci. 38 (1987) 227. [6] I. Furuoya, in: Shokubai Chosei Kagaku, Eds. A. OzakL T. Seiyama, K. Tanabe. Y. Ogmo, 5". Murata, T. Y a m a m o t o and S. Egashira (Kodansha, Tokyo, 1980) p. 49. [7] K. Matsuo and K. Nakano, in: Chem. Eng. Symp, Ser. 13 (The Society of Chemical Engineers, Japan, Tokyo, 1987) p. 43. [8] Y. Kabu, H. Okuda, K. Kusunoki and K. Nakano, Annual Reports, HVEM 1,uh., Kyuslm Univ. 8 (1984) 17.
269
PERFORMANCES OF PLATINUM METAL CATALYSTS SUPPORTED ON TITANIA COATED SILICA PREPARED FROM METAL ORGANICS Kaori M A T S U O a n d K a t s u y u k i N A K A N O Department of Chemical Engineering, Fukuoka University, Fukuoka 814-01, Japan Received 23 August 1987; accepted for publication 31 October 1987
Titania-silica beads prepared by vapor-phase hydrolysis of titan alkoxide adsorbed on silica beads were observed to be active in promoting the perfect oxidation of acetone, but titania-silica beads prepared by pyrolysis of titan alkoxide and by adsorption of titania colloid onto silica beads showed no such activity. Further, a different performance in a photo-induced catalysis was observed among the platinum metal catalysts supported on these beads and on titania beads. In these reactions, the titania-silica interface seems to play an important role in relation to the excited electronic state of the catalytic surface. Next, the metal-support interaction, the so-called SMS1 effect, was examined in these catalysts and on other oxides (ZrO2, WO3, MOO3,Nb2Os, etc.) by using electronic and vibrational spectra. Based on the results, the ease of entry to the SMSI state was found to be correlated to the energy gap value of the metal oxides used. Further, the rates of some oxidation and hydrogenation reactions were found to be controlled by changing the degree of SMSI.
1. Introduction In recent years, considerable a t t e n t i o n has been given to m e t a l - s u p p o r t interactions in the p r e p a r a t i o n of supported metal catalysts, because the interface between a metal a n d a s e m i c o n d u c t o r oxide often produces a new function, b e y o n d our usual consideration, for catalysis [1]. T o prepare semic o n d u c t o r oxides, new approaches have been investigated using metal organics, such as metal alkoxides [2]. These situations stimulate the efforts to search for catalysts which are highly selective, active a n d durable. In this paper, three different methods were tried for coating silica surfaces with titania thin films utilizing (1) vapor-phase hydrolysis, (2) pyrolysis of titan alkoxides adsorbed on silica beads, a n d (3) a d s o r p t i o n of a t i t a n i a colloid (produced b y hydrolysis of titan alkoxides) o n t o silica beads. T h e surface characterization of these beads was m a d e b y using in situ I R - D R S a n d UV-Vis, etc. The p l a t i n u m metal catalysts supported o n these b e a d s were prepared a n d tested for p h o t o - i n d u c e d a n d perfect o x i d a t i o n reactions. As is well-known, p l a t i n u m metal supported o n s e m i c o n d u c t o r oxides, shows an interesting p h e n o m e n o n in catalysis, which is related to the strong m e t a l - s u p 0 1 6 9 - 4 3 3 2 / 8 8 / $ 0 3 . 5 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics P u b l i s h i n g Division)
270
K. Matsuo, K. Nakano / Performances of Pt metal catalysts supported on TiO: SiO: -
port interaction, the so-called SMSI effect [3]. This effect was examined for these catalysts and for the catalysts supported on other metal oxides (ZrO 2, WO), MOO3, Nb205 and anatase) by using electronic and vibrational spectra. Further, the rates of some surface reactions were correlated to the degree of SMSI in supported platinum metal catalysts.
2. Experimental
2.1. Methods of preparation for titania coated silica A flow diagram of the preparation methods of titania coated silica beads is shown in fig. 1.
(a) (TiO:/ SiO:.)n beads" The silica beads, Cariact 15 (3 m m diameter) supplied by F u j i - D a v i s o n Chemical Ltd., were added into the solution of titan isopropoxide (TIP) and isopropanol (IPA). After the adsorption of TIP on the silica beads was completed at 305 K, the beads were separated from the solution and kept overnight in a moist atmosphere, where the TIP on the sample was hydrolyzed by the H 2 0 vapor. The samples were dried in vacuo, and then the ( T i 0 2 / S i O 2 )~l beads were produced.
TIP (IPA)
]
TIP IPA
! Si02 Beads__! I Adsorption +
_
_
I (TIP/Si02) Beads Pyrolysis i n ~ an Ar s t r e a m ~ /
i
H~O
a/,/// [
TiO~ P a r t i c l e s
i [
TiO~
Collo{d i *
~Hydroiysis ~n
-
H20 vapor
4
1 ! Adsorption
C
~io~-sio~) I PdCiz or H2PtCIs-6H20
Impregnation -Z L Pd-orPt-(Ti02/Si02)p
! ?d-orPt-(Ti02/Si@2
B~ds I
I
PdCI2 or [ H2PtCI~-BH=Q i I I~pregnation
Pd-orPt-(YiO~-SiO2)
Fig. 1. Flow diagram of the preparation methods of titania coated silica beads and platinum catalysts supported on these beads.
K. Matsuo, K. Nakano / Performancesof Pt metal catalysts supported on TiO2- SiO,
271
(b) ( T i O 2 / SiOe)p beads The TIP vapor (vaporized at 423 K) carried by an Ar gas stream was introduced into a tubular reactor packed with the silica beads, and the T I P coated silica beads were obtained. After the adsorption was completed, The temperature of the reactor was kept at 513-593 K to pyrolyze the T I P adsorbed on the silica beads in the Ar stream. The beads obtained were called (TiO2/SiO 2) p beads. (c) ( T i O : - S i 0 2 ) beads Prior to the preparation, we obtained the titania colloid by the use of the so-called sol-gel reaction of T I P under various reaction conditions [4]. The initial relative composition of TIP, IPA and water had a large effect on the nature of the products. The colloid obtained using the initial composition of the reactant, T I P / I P A / H 2 0 = 1 / 5 / 4 in molar ratio, was utilized. The colloid system obtained using lower and higher H z O / T I P ratios did not adsorb on the silica beads [5]. The silica beads were mixed with the titania colloid and the colloid was adsorbed on the surface of the silica beads to a thickness of about 5 t~m. We called the samples, after drying in vacuo, (TiO2-SiO2) beads. The beads were used in the catalytic reactions as freshly prepared. In addition, platinum metal was deposited onto these beads and the other semiconductor oxides. The method of metal deposition was adsorption from a solution of metal salt, as presented elsewhere [6]. 2.2. Methods of characterization and activity tests
The beads or particles obtained in section 2.1 were characterized in various ways, e.g. in situ I R - D R S and UV-Vis, etc. An activity test was done by the use of a fixed-bed reactor for the perfect oxidation of acetone on these beads or platinum metal catalysts supported on these beads [7]. A photo-induced reaction was observed by the use of an I R spectrometer (IR-810, made by Japan Spectroscopic Co. Ltd.) equipped with a DRS cell (DR-81). The catalyst samples, on which alkoxide was adsorbed, were ground in an agate mortar, and were set on the cell to be exposed to the irradiation of 365 nm UV rays. The thermal properties of the catalysts were measured by an IR spectrometer equipped with a temperature-controlled DRS cell (DR-31). UV-Vis spectra were obtained in the form of relative reflectance intensity for the sample compared to a standard alumina disk. 3. Results and discussion 3.1. Characteristics and catalytic activities of titania-silica beads and platinum metal supported on these beads
The energy gap and the absorbance properties in the UV-Vis spectra of the titania coated silica beads were found to be controlled by the preparation
272
K. Matsuo, K. Nakano / Performances of Pt metal catalysts supported on TiO, - Si02
100
2
,,i ,,
0 rCI 1~
....
TiOo colloid adsorption ~ " on SiO2 /I , ~ PyrOlysis of TIPISiO 2 /" ," I .~-Hydrolysis of TIP/SiO 2 / /I / // ~-,---Anatase
7
/ 50
(D O[:
/~/j
,"
360
460
s6o
Wave length [nm] Fig. 2. UV spectra of the titania coated silica beads prepared by the different methods. methods or the reaction conditions as shown in fig. 2. These differences in the properties may affect the catalytic activities of the beads. The catalytic activities of these beads were tested by the reaction of the perfect oxidation of acetone. The (TiO2/SiO 2)H beads were observed to be active for this reaction as shown in fig. 3, but the others, the (TiOJSiO2)l, and the (TiO2-SiO 2) beads, showed no activity. The activities of the platinum metal catalysts supported on these beads were compared with each other for the reaction mentioned above. Higher activity was observed on the Pd-(TiO2/SiO2)H beads compared with the Pd-(TiO2-SiO2) beads in the low acetone partial pressure region. This is due to the effect of active oxygen on the surface of the (TiO2/SiO2) u beads in addition to the active oxygen produced from the platinum surface, but there is no active oxygen on the surface of the (TiO2-SiO 2) beads as deduced from the fact mentioned above.
Q20 t/)
6, o
/ / ~
O%pm SV 400h-~
o U
0110
Oll2O
0~ 0
Oil40
Amount of titania adsorbed [ g/g-SiOz]
Fig. 3. Activity of (TiO2/SiO2) H beads for the perfect oxidation of acetone (CH~COCH~ + 4 02 3 CO2 + 3 H20).
K. Matsuo, K. Nakano / Performances of Pt metal catalysts supported on TiO 2- SiOe
(a)
J_
(b)
273
(c)
(5 u E
----.2L -----LL ---.4L_ 0.5
IB
r'l ,¢
320o
26o03200
1.0
---.,.._.__
26oo 3200
2600
Wave number {~crn-1 ] Fig. 4. Time dependencies of IR spectra for titania coated silica beads at 553 K: (a) (TiO 2 / S i O 2) H; (b) (TiO 2 / S i O 2) p; (c) (TiO 2 - S i O 2).
The strength of the interaction between the surface of these beads and the organic molecules was qualitatively examined by the use of temperature-controlled vibrational spectra measurements as shown in fig. 4. Alkoxide on the (TiO2/SiO2) n beads was easily desorbed when these beads were kept at 553 K in the DRS cell of the I R spectrometer. On the contrary, the signal due to the C H vibration (2975 c m - t ) continued to be observed on the (TiO2-SiOz) beads after 3 h heat treatment. From the figure, the strength of adsorption for organics on these beads was in the following order: ( T i O 2 - S i O z ) > (TiO2/SiO2) p > (TiO2/SiO2) HWhen the Pt-(TiO2/SiO2)H beads, on which alkoxide was adsorbed, were exposed to irradiation of 365 nm UV rays and the I R spectra were measured
l
i
4000
2000 1000 400 Wave number [ ¢ m -1 } Fig. 5. IR spectra for 0.67% P t - ( T i O 2 / S i O 2 ) H beads, on which alkoxide was adsorbed, after irradiation by UV rays.
274
K. Matsuo, k: Nakano / Performances of Pt metal catal}'sts supported on TiO: - SiO:
in situ, a sharp absorbance band was observed (fig. 5) at 2125 cm t which may be due to the on-top CO vibration. The intensity of this signal increased with increasing irradiation time, because the photo-induced reaction proceeded. This experiment was tried on a Pt TiO 2 and a Pt-SiO, catalyst instead of the P t - ( T i O J S i O 2 ) H or Pt-(TiO2/SiO2) p beads, but CO was not produced during 2 h of irradiation. This result shows that the interface between silica and titania plays an important role in the photo-induced catalysis, but the detailed mechanism is not clarified in the present situation.
3.2. Metal ,support interaction for platinum metal catalysts supported on titania-silica and the other semiconductor oxides As is generally known, platinum metal supported on semiconductor oxides shows an interesting phenomenon related to the strong m e t a l - s u p p o r t interaction (SMSI) [3]. When the titania supported platinum metal catalyst is strongly reduced to make the platinum lose its ability for dissociation of a hydrogen molecule, this phenomenon is called the " S M S I " effect. This effect was easily observed by comparing the in situ I R - D R S spectra of Pt- or Pd-TiO~ catalysts after the high temperature reduction to those without reduction. The typical results are shown in fig. 6. The intensity of the characteristic absorbance band by the oxide (920 cm i for Ti-O), which appeared in the normal state, decreased for the catalyst in the SMSI state. Another aspect ol the SMSI state is the disappearance of the O H vibrational band because of diminishing H 2 0 adsorption. This showed that oxygen desorbed irreversibly from the lattice matrix of Pd TiO 2 in the high temperature reduction, although oxygen can adsorb reversibly onto the Pd-TiO~ (x < 2.0) after low temperature reduction. After the desorption of oxygen in the high temperature reduction condition, hydride may be adsorbed in the metal-oxide lattice, which
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K. Matsuo, K. Nakano
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Performances of Pt metal catalysts supported on TiO: - SiO 2
275
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and (3) the sol-gel titania sintered at 873 K. retards the dissociation of hydrogen. This phenomenon is often observed in the palladium-hydrogen system. We also found that the SMSI state is kept stable even in an oxidizing atmosphere at room temperature at least for a few years. As shown above, titania was easily reduced to enter the SMSI state, but it was difficult in the case of the titania-silica beads. The reason for this was examined by the use of electronic and vibrational spectra. For some semiconductor oxides, UV-Vis spectra were measured and are shown in fig. 7. Further, platinum metal was supported on these oxides and these samples were exposed in the hydrogen stream at various temperatures. The vibrational spectra were measured for these samples to check whether or not the samples entered the SMSI state. For ZrO 2 and the anatase (supplied by Rare Metallic Co. Ltd.) systems, the samples did not enter the SMSI state even at a reduction temperature above 773 K. For the platinum metal catalysts supported on Nb205 and the titania beads (supplied by Sakai Chemicals Ltd.), the SMSI state was achieved at a temperature around 773 K. Further, MoO 3 and O O 3 supported platinum catalysts entered easily into the SMSI state at 593 K. Thus, the ease with which the SMSI state could be entered was found to be correlated to the value of the energy gap of the metal oxides used as the supports, on which the reducibility of the oxides depends. We also measured UV-Vis spectra for the (TiO2/SiO2) H beads and sol-gel titania particles as shown in fig. 7. The Pt-(TiO2/SiO2) H beads did not enter the SMSI state even at 873 K; neither did P t - T i O 2, where TiO 2 particles were produced by the sol-gel reaction and Pt metal was deposited on the freshly prepared particles. On the contrary, TiO2, on which platinum metal was deposited after the freshly prepared particles were sintered at 873 K, was found to enter easily into the SMSI state when they were reduced at 773 K. These results are consistent with the fact that the ease of entry to the SMSI state depends on the reducibility of the oxides used as support.
276
K. Matsuo, K. Nakano / Performances of Pt metal catalysts supported on TiO: SiO:
The hardness and the electrical conductivity of the catalysts were observed to increase in the SMSI state compared with in the normal state. A change in the electrical conductivity must affect strongly the catalytic activity. The activities for the perfect oxidation of acetone and the IR spectra of the Pd-TiO2 catalysts in various reduction conditions were measured to correlate the activity with the intensity of the SMSI state (this is defined as the height ol the 920 cm 1 band based on the absorbance of 400 c m - I ) . The results are shown in fig. 6. It was found that the catalyst in the SMSI state was favorablc for enhancing the perfect oxidation of acetone. On the other hand, the reaction rate of benzene hydrogenation was found to be retarded on the platinum metal catalyst supported on titania in the SMS1 state as compared with in the normal state, which is due to the decrease in the dissociative adsorption for hydrogen [8]. From these results, the possibility was found for controlling the activity for some catalytic reactions by changing the degree of SMSI.
4. Conclusions Three different methods were tried for preparing titania coated silica beads using vapor-phase hydrolysis, pyrolysis of titan isopropoxide adsorbed on the silica beads, and also the adsorption of titania colloid onto the silica beads. These beads have different properties for the perfect oxidation of acetone. Platinum metal catalysts were prepared using these beads. The catalysts prepared were found to have unique properties in photo-induced catalysis and in the SMSI effect, compared with titania or silica supported platinum metal.
References [1] A. Morikawa, Catalyst (Shokubai) 29 (1987) 2: H. Arai and T. Ishihara, Catalyst (Shokubai) 29 (1987) 5. [2] J.D. Mackenzie, in: Science of Ceramic Chemical Processing, Eds. L . L Hench and D.R. Ulrich (Wiley, New York, 1986) p. 113. [3] S.J. Tauster, S.C. Fung and R.L. Garten, J. Am. Chem. Soc. 100 (1978) 170. [4] K. Matsuo and K. Nakano, Fukuoka Univ. Rev. Technol. Sci. 37 (t986) 333. [5] K. Matsuo and K. Nakano, Fukuoka Univ. Rev. Technoh Sci. 38 (1987) 227. [6] I. Furuoya, in: Shokubai Chosei Kagaku, Eds. A. OzakL T. Seiyama, K. Tanabe. Y. Ogmo, 5". Murata, T. Y a m a m o t o and S. Egashira (Kodansha, Tokyo, 1980) p. 49. [7] K. Matsuo and K. Nakano, in: Chem. Eng. Symp, Ser. 13 (The Society of Chemical Engineers, Japan, Tokyo, 1987) p. 43. [8] Y. Kabu, H. Okuda, K. Kusunoki and K. Nakano, Annual Reports, HVEM 1,uh., Kyuslm Univ. 8 (1984) 17.