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Stabilization of platinum on silica promoted with lanthanum oxide and zirconium oxide
~
AP PA LE IY D C AT L SS I A: GENERAL
ELSEVIER
Applied Catalysis A: General 124 (1995) 339-344
Stabilization of platinum on silica promoted with lanthanum oxide and zirconium oxide M i c h e l D e e b a *, R o b e r t J. Farrauto, Yiu K. Lui Engelhard Corporation 101 Wood Ave., lselin, NJ 08830-0770, USA
Received 26 August 1994; revised 29 November 1994; accepted 29 November 1994
Abstract A broad range of ZrO2 loadings on silica were found to be effective in stabilizing SiO2 against sintering. The combination of La203 and ZrO2 on SiO2 was most effective in stabilizing platinum against sintering in an oxidizing environment. This catalyst is a promising candidate for improved sulfuric acid catalyst since platinum and SiO2 have both been stabilized. Keywords: Lanthanum oxide; Platinum; Silica; Sintering; Stabilization; Zirconium oxide
1. Introduction
There are a number of applications where it would be preferred to use SiO2 as a carrier for precious metals instead of the more commonly used A1203. The strong Lewis acidity of A1203 [ 1] often causes undesirable side reactions such as cracking, isomerization, and aromatization in the platinum catalyzed dehydrogenation of straight chain hydrocarbons to linear monoalkenes in the production of detergents [ 2,3]. Currently, alkali is added to neutralize the acidic sites [4]. Silica might also have advantages over A1203 supports in the palladium catalyzed hydrogenation of dialkenes in pyrolysis gasoline to minimize polymerization and coking [ 5 ]. The high activity of A1203 towards reactions with precious metals leads to inactive catalytic compounds such as RhzO3/A1203 in auto exhaust abatement [6]. Similarly, reactions of SO3 with A1203 can lead to deactivation of the Pt/A1203 catalyst for sulfuric acid production and emissions from mobile and stationary sources [7,8]. The relative inertness of SiO2 would, therefore, extend the life of catalysts by minimizing metal/support or gas phase/support reactions. * Corresponding author. Tel. ( + 1-908) 2056581, fax. ( + 1-908) 2055300. 0926-860X/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI O 9 2 6 - 8 6 0 X ( 94 ) 0 0 2 7 6 - 2
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M. Deeba et a l . / Applied Catalysis A: General 124 (1995) 339-344
Two major problems have plagued SiO 2 in this respect; ( 1 ) inadequate retention of high surface area and, (2) insufficient stabilization of the precious metals against sintering [9]. This paper reports the findings of a study to render SiO2 more stable against sintering and to modify its surface to enhance stabilization of platinum against sintering. Retention of silica surface area by incorporating minor oxide components such as zirconia has been investigated by other researchers [ 10-12]. Using rare earth oxides to stabilize precious metal dispersions have been widely used in auto exhaust catalysts technology. Cerium oxide is considered an 'oxygen storage' component, and is commonly used with platinum to improve high-temperature performance. Cerium oxide has also been reported to stabilize PdO against reduction to palladium at high temperatures [9]. Lanthanum oxide on alumina was studied by Drozdev et al. [ 13,14] and showed improved platinum dispersion and stability. Bell and coworkers [ 15,16] reported that La203 as a support showed strong metal-support interactions (SMSI) with palladium and rhodium resulting in better catalytic activity for carbon monoxide hydrogenation. Zirconium oxide was reported to have little or no SMSI [ 17]. The Pt/La203/ZrO2/SiO2 reported here utilizes the combined features of ZrO2 stabilized SiO2 with strong SMSI from both La203 and ZrO2 to maintain platinum dispersion and stability at high temperatures.
2. Experimental
2.1. Catalystpreparation ZrO2/SiOe support Silica (Syloid 74) obtained from Davison was impregnated with zirconyl nitrate (from Alfa) solution using the incipient wetness technique. After drying at 100°C, the support was calcined in air at 700°C for 1 h. The final zirconia content was varied between 5 and 25%. Most of the results reported here were obtained on the catalyst containing 25% ZrO2-SiO2 except when specified.
Pt/Zr02/Si02 Platinum was impregnated into the Z r O 2 / S i O 2 support by using an aqueous solution of platinum ammonium salt or hexachloroplatinic acid. The concentration of the platinum on the finished catalyst was 2%. After impregnation, the catalysts were dried at 100°C and calcined in air at temperatures ranging between 500 and 700°C.
LaeO3-containing catalysts Lanthanum oxide was impregnated into the ZrO2/SiO 2 support by one of two procedures: (1) co-impregnating La(NO3)3 and the platinum salt followed by drying and calcination and (2) by separate impregnation of La(NO3)3 followed
M. Deeba et al. / Applied Catalysis A: General 124 (1995) 339-344
341
by calcination, followed by platinum addition. The platinum content in the finished catalysts was 2% by weight and the Pt:La203 molar ratio was 1 : 1. Pt/SiO 2 Platinum o n SiO 2 was used as the control and was prepared by direct impregnation of the platinum ammonium salt into SiO2 followed by drying and calcination at 500°C. The platinum content in the finished catalyst was 2%. 2.2. Catalyst characterization Surface areas of the finished catalysts were determined by the BET method using nitrogen. Platinum metal dispersion was determined by pulse carbon monoxide chemisorption at room temperature. The catalysts were first calcined at 500°C for 1 h followed by reduction in hydrogen for 1 h at 400°C. 2.3.
SO 2
oxidation test
The effect of platinum dispersion was tested in the S O 2 oxidation model reaction. A mixture of 2% SO2 and 98% air was passed through 0.2 g of catalyst in a tubular micro-reactor at 100 000 h - 1 GHSV. The percentage of SO2 converted was calculated by measuring the SO2 breakthrough with an on-line gas chromatography after trapping the SO3 in an ice bath.
3. Results and discussion 3.1. Stabilization of silica surface area The BET surface area for silica after exposure to air at the temperatures and times indicated in Fig. 1 clearly shows the absence of stability with time at 800°C in air. When a fresh sample is calcined to 900°C for only two hours, the surface area decreases from over 300 to 110 m2/g. Additional sintering would be observed at longer times. The addition of 5 wt.-% ZrO2 stabilizes the SiO2 against sintering. Fig. 1 shows no decrease in surface area after various times at 800°C. The initial area is slightly lower than that of the SiO2 only (278 vs. 318 m2/g) but the added stability is more desirable to insure catalytic durability. After 900°C for 2 h no sintering ( 278 fresh vs. 273 m2/g) is observed. Larger amounts of ZrO2, up to 25% by weight, also stabilize the SiO2 against sintering, with a minor penalty in a lower initial surface area. This material also retains its surface area after 2 h at 900°C. The lower initial surface area observed for the ZrO2/SiO2 support is due to loss in microporosity as determined by mercury porosimetry. The ZrO2 alone shows an area too low to be of value for most catalytic applications.
M. Deeba et al. /Applied Catalysis A: General 124 (1995) 339-344
342
300
E
200
.<:
¢/)
100
I.uJ =0
© 20
O 40 Calcination Time (Hours)
60
BO
Fig. 1. Thermal stability of SiO2 and ZrO2/SiO 2 in air at 800°C: SiO2 ( × ), 5%ZrO2/SiO 2 ( + ), 25% ZrO2/SiO 2 ( * ), and ZrO: ( O ) .
The precise chemical interactions between ZrO2 and SiO2 leading to stabilization are not clear, although an interesting observation of the ZrO2 structure was noted. The X-ray diffraction (XRD) pattern of commercial zirconia before and after air calcination at 800°C is similar to monoclinic ZrO2 or baddeleyite. However, the zirconia produced on the silica surface (25% ZrO2/SiO2) showed an XRD pattern matching tetragonal zirconia (/3-zirconia). This zirconia phase is usually formed by calcining zirconia at temperatures up to 1000°C or higher. No XRD patterns were detectable for the 5 or 10% ZrO2/SiO2 supports indicating the presence of highly dispersed zirconia with crystallites smaller than 40 ~,.
3.2. Stabilization of platinum against sintering Inaccessibility of the catalytic component can occur both by sintering of the metal or by occlusion due to a collapsing of the porous network of the carrier. Occlusion has been minimized by the addition of ZrO2 so it is only necessary to address the problem of metal sintering. Platinum at the 2% level was deposited on a series of zirconia-stabilized silicas varying in the amount of ZrO2. After calcination at 500, 600 and 700°C in air for 1 h, samples were characterized by carbon monoxide chemisorption for retention of platinum dispersion. Carbon monoxide chemisorption values fell to almost zero for the platinum on 5% ZrO2/SiO 2 system from an initial value of 0.1 cm3/g while the 10 and 25 % ZrO:/SiO2 systems produced initial values close to 0.5 cm3/g and sintered to about 0.1 cm3/g after 1 h at 600°C. The addition of an equal molar amount of La203 to the platinum deposited on ZrO2-stabilized SiO2 generated a catalyst with a high initial carbon monoxide chemisorption value corresponding to about 50% dispersion with no negative effects on the SiO2 stabilization. Fig. 2 compares the carbon monoxide chemisorption values for a series of catalysts all prepared with 2% Pt. The results clearly
M. Deeba et al. /Applied Catalysis A: General 124 (1995) 339-344 1.4,
~
-
.
.
.
.
343
.
0.8
g ~
0.6
0 0 o 500
550
600 650 Calcination Temperature (oC)
700
Fig. 2. Carbon monoxide chemisorption on various supported platinum catalysts as a function of calcination temperature for I h. Pt/SiO2 ( × ). Pt/25% ZrO2/SiO2 ( + ), Pt/La~O3/ZrO2/SiO2 ( * ).
show that the presence of La203 produces catalysts with higher initial metal areas and aids in the maintenance of that area up to 650°C. At 650°C differences between platinum on La203/Zr02/SiO2, Pt/SiO2, and Pt/Zr02/Si02 became smaller; however, the superiority of the La203 containing system is evident especially at temperatures below 600°C. The precise mechanism for stabilization is unknown, however, one may speculate that a surface complex of platinum with La203 exists, the stability strength of which is greater than that with other oxides reported. Similar arguments have been put forth by researchers studying Pt/CeO2 interactions [ 18 ]. The Pt/La203/ZrO2 / SiO2 reported here utilizes the combined features of ZrO2 stabilized SiO2 with strong SMSI from both La203 and ZrO2 to maintain platinum dispersion and stability at high temperatures. The oxidation of SO2 to SO3 was used as a model reaction to establish the superiority of La203 as a stabilizer for platinum against sintering. This reaction represents an important chemical process. The only active catalysts at temperatures of 300-350°C are supported platinum catalysts. In contrast V205 catalysts are not active until temperatures greater than 380°C. Silica was used for this application because it is an inert support, which unlike high surface area alumina, resists sulfation. However, upon mild aging, the Pt/SiO2 catalyst showed severe deactivation due mainly to platinum sintering and loss in platinum dispersion. Moreover, high-temperature aging of the Pt/SiO 2 resulted also in loss of the silica surface area and thus encapsulation of the platinum within the silica pores. Addition of ZrO> as found here, stabilized the SiO2 against loss in surface area and helped in stabilizing the platinum. Platinum was further stabilized by adding rare-earth metal oxides, specifically La203. A SiO2 support containing 25% ZrO2 was impregnated with La(NO3)3, calcined then impregnated with a solution of platinum to give 2% Pt and a La203 to Pt molar ratio of 1:1. This catalyst along with a 2%Pt/SiO2 catalyst were aged at 650°C for
344
M. D e e b a et al./ Applied Catalysis A: General 124 (1995) 339-344
20 h. A mixture of 2% SO: in air was passed through the catalyst bed. The La203 stabilized catalyst gave 52% conversion at 375°C compared to 21% at 550°C for the 2% Pt/SiO2. The significant improvement in the SO2 conversion for the La203 stabilized catalyst is consistent with the maintenance of higher platinum dispersion.
4. Conclusions It has been found that a broad range of ZrO2 loadings on SiO2 are very effective in stabilizing SiO2 against sintering. The presence of ZrOa also imparts some stabilization against platinum sintering. However, the combination of La203, and ZrO2 on SiOz is most effective in stabilizing platinum against sintering in oxidizing environments.
References [ 1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [ 11 ] [ 12] [ 13] [ 14] [ 15] [ 16] [ 17] [ 18]
H. Kntzinger and P. Ratnasamy, Cat. Rev.- Sci. Eng., 17 ( 1) (1978) 31. K. Shuan, US Patent 3 920 615 (1975). F. Wilhelm, US Patent 3 998 900 (1976). W.D. Mross, Cat. Rev.-Eng. Sci., 25 (4) (1983) 591. D. Kronig, Hydrocarbon Processing, 49 (3) (1970) 121. C. Wong and R.W. McCabe, J. Catal., 119 (1989) 47. G, Joy and E.H. Homeiri, US Patent 4 849 399 (1989). R.M. Heck and R.J. Farrauto, Catalytic Air Pollution Control: Commercial Technology, Van Nostrand Reinhold, Ch. 5, 1995, p. 63. R. Lamber, N. Jaeger and G. Schulz-Ekloff, J. Catal., 23 (1990) 285. J.B. Miller, S.E. Rankin and E.I. Ko, J. Catal., 148 (1994) 673. S. Soled and G.B. McVicker, Catal. Today, 14 (1992) 189. T. Kennelly and R. J. Farrauto, US Patent 5 216 875 (1993). V.A. Drozdov, P.G. Tsyrulnikova, A.A. Davydov and E.M. Moroz, Kinet. Katal., 27 (1986) 162. V.A. Drozdov, P.G. Tsyrulnikova, A.N. Pestryakov, A.A. Davydov and V.V. Popovskii, Kinet. Katal., 29 (1989) 484. R,P. Underwood and A.T. Bell, J. Catal., 109 (1988) 61. J.S. Rieck and A.T. Bell, J. Catal., 103 (1987) 46. E.C. Akubuiro and X.E. Verykios, Appl. Catal., 14 (1985) 215. J.C. Summers and S.A. Ausen, J. Catal., 58 (1979) 131.
AP PA LE IY D C AT L SS I A: GENERAL
ELSEVIER
Applied Catalysis A: General 124 (1995) 339-344
Stabilization of platinum on silica promoted with lanthanum oxide and zirconium oxide M i c h e l D e e b a *, R o b e r t J. Farrauto, Yiu K. Lui Engelhard Corporation 101 Wood Ave., lselin, NJ 08830-0770, USA
Received 26 August 1994; revised 29 November 1994; accepted 29 November 1994
Abstract A broad range of ZrO2 loadings on silica were found to be effective in stabilizing SiO2 against sintering. The combination of La203 and ZrO2 on SiO2 was most effective in stabilizing platinum against sintering in an oxidizing environment. This catalyst is a promising candidate for improved sulfuric acid catalyst since platinum and SiO2 have both been stabilized. Keywords: Lanthanum oxide; Platinum; Silica; Sintering; Stabilization; Zirconium oxide
1. Introduction
There are a number of applications where it would be preferred to use SiO2 as a carrier for precious metals instead of the more commonly used A1203. The strong Lewis acidity of A1203 [ 1] often causes undesirable side reactions such as cracking, isomerization, and aromatization in the platinum catalyzed dehydrogenation of straight chain hydrocarbons to linear monoalkenes in the production of detergents [ 2,3]. Currently, alkali is added to neutralize the acidic sites [4]. Silica might also have advantages over A1203 supports in the palladium catalyzed hydrogenation of dialkenes in pyrolysis gasoline to minimize polymerization and coking [ 5 ]. The high activity of A1203 towards reactions with precious metals leads to inactive catalytic compounds such as RhzO3/A1203 in auto exhaust abatement [6]. Similarly, reactions of SO3 with A1203 can lead to deactivation of the Pt/A1203 catalyst for sulfuric acid production and emissions from mobile and stationary sources [7,8]. The relative inertness of SiO2 would, therefore, extend the life of catalysts by minimizing metal/support or gas phase/support reactions. * Corresponding author. Tel. ( + 1-908) 2056581, fax. ( + 1-908) 2055300. 0926-860X/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI O 9 2 6 - 8 6 0 X ( 94 ) 0 0 2 7 6 - 2
340
M. Deeba et a l . / Applied Catalysis A: General 124 (1995) 339-344
Two major problems have plagued SiO 2 in this respect; ( 1 ) inadequate retention of high surface area and, (2) insufficient stabilization of the precious metals against sintering [9]. This paper reports the findings of a study to render SiO2 more stable against sintering and to modify its surface to enhance stabilization of platinum against sintering. Retention of silica surface area by incorporating minor oxide components such as zirconia has been investigated by other researchers [ 10-12]. Using rare earth oxides to stabilize precious metal dispersions have been widely used in auto exhaust catalysts technology. Cerium oxide is considered an 'oxygen storage' component, and is commonly used with platinum to improve high-temperature performance. Cerium oxide has also been reported to stabilize PdO against reduction to palladium at high temperatures [9]. Lanthanum oxide on alumina was studied by Drozdev et al. [ 13,14] and showed improved platinum dispersion and stability. Bell and coworkers [ 15,16] reported that La203 as a support showed strong metal-support interactions (SMSI) with palladium and rhodium resulting in better catalytic activity for carbon monoxide hydrogenation. Zirconium oxide was reported to have little or no SMSI [ 17]. The Pt/La203/ZrO2/SiO2 reported here utilizes the combined features of ZrO2 stabilized SiO2 with strong SMSI from both La203 and ZrO2 to maintain platinum dispersion and stability at high temperatures.
2. Experimental
2.1. Catalystpreparation ZrO2/SiOe support Silica (Syloid 74) obtained from Davison was impregnated with zirconyl nitrate (from Alfa) solution using the incipient wetness technique. After drying at 100°C, the support was calcined in air at 700°C for 1 h. The final zirconia content was varied between 5 and 25%. Most of the results reported here were obtained on the catalyst containing 25% ZrO2-SiO2 except when specified.
Pt/Zr02/Si02 Platinum was impregnated into the Z r O 2 / S i O 2 support by using an aqueous solution of platinum ammonium salt or hexachloroplatinic acid. The concentration of the platinum on the finished catalyst was 2%. After impregnation, the catalysts were dried at 100°C and calcined in air at temperatures ranging between 500 and 700°C.
LaeO3-containing catalysts Lanthanum oxide was impregnated into the ZrO2/SiO 2 support by one of two procedures: (1) co-impregnating La(NO3)3 and the platinum salt followed by drying and calcination and (2) by separate impregnation of La(NO3)3 followed
M. Deeba et al. / Applied Catalysis A: General 124 (1995) 339-344
341
by calcination, followed by platinum addition. The platinum content in the finished catalysts was 2% by weight and the Pt:La203 molar ratio was 1 : 1. Pt/SiO 2 Platinum o n SiO 2 was used as the control and was prepared by direct impregnation of the platinum ammonium salt into SiO2 followed by drying and calcination at 500°C. The platinum content in the finished catalyst was 2%. 2.2. Catalyst characterization Surface areas of the finished catalysts were determined by the BET method using nitrogen. Platinum metal dispersion was determined by pulse carbon monoxide chemisorption at room temperature. The catalysts were first calcined at 500°C for 1 h followed by reduction in hydrogen for 1 h at 400°C. 2.3.
SO 2
oxidation test
The effect of platinum dispersion was tested in the S O 2 oxidation model reaction. A mixture of 2% SO2 and 98% air was passed through 0.2 g of catalyst in a tubular micro-reactor at 100 000 h - 1 GHSV. The percentage of SO2 converted was calculated by measuring the SO2 breakthrough with an on-line gas chromatography after trapping the SO3 in an ice bath.
3. Results and discussion 3.1. Stabilization of silica surface area The BET surface area for silica after exposure to air at the temperatures and times indicated in Fig. 1 clearly shows the absence of stability with time at 800°C in air. When a fresh sample is calcined to 900°C for only two hours, the surface area decreases from over 300 to 110 m2/g. Additional sintering would be observed at longer times. The addition of 5 wt.-% ZrO2 stabilizes the SiO2 against sintering. Fig. 1 shows no decrease in surface area after various times at 800°C. The initial area is slightly lower than that of the SiO2 only (278 vs. 318 m2/g) but the added stability is more desirable to insure catalytic durability. After 900°C for 2 h no sintering ( 278 fresh vs. 273 m2/g) is observed. Larger amounts of ZrO2, up to 25% by weight, also stabilize the SiO2 against sintering, with a minor penalty in a lower initial surface area. This material also retains its surface area after 2 h at 900°C. The lower initial surface area observed for the ZrO2/SiO2 support is due to loss in microporosity as determined by mercury porosimetry. The ZrO2 alone shows an area too low to be of value for most catalytic applications.
M. Deeba et al. /Applied Catalysis A: General 124 (1995) 339-344
342
300
E
200
.<:
¢/)
100
I.uJ =0
© 20
O 40 Calcination Time (Hours)
60
BO
Fig. 1. Thermal stability of SiO2 and ZrO2/SiO 2 in air at 800°C: SiO2 ( × ), 5%ZrO2/SiO 2 ( + ), 25% ZrO2/SiO 2 ( * ), and ZrO: ( O ) .
The precise chemical interactions between ZrO2 and SiO2 leading to stabilization are not clear, although an interesting observation of the ZrO2 structure was noted. The X-ray diffraction (XRD) pattern of commercial zirconia before and after air calcination at 800°C is similar to monoclinic ZrO2 or baddeleyite. However, the zirconia produced on the silica surface (25% ZrO2/SiO2) showed an XRD pattern matching tetragonal zirconia (/3-zirconia). This zirconia phase is usually formed by calcining zirconia at temperatures up to 1000°C or higher. No XRD patterns were detectable for the 5 or 10% ZrO2/SiO2 supports indicating the presence of highly dispersed zirconia with crystallites smaller than 40 ~,.
3.2. Stabilization of platinum against sintering Inaccessibility of the catalytic component can occur both by sintering of the metal or by occlusion due to a collapsing of the porous network of the carrier. Occlusion has been minimized by the addition of ZrO2 so it is only necessary to address the problem of metal sintering. Platinum at the 2% level was deposited on a series of zirconia-stabilized silicas varying in the amount of ZrO2. After calcination at 500, 600 and 700°C in air for 1 h, samples were characterized by carbon monoxide chemisorption for retention of platinum dispersion. Carbon monoxide chemisorption values fell to almost zero for the platinum on 5% ZrO2/SiO 2 system from an initial value of 0.1 cm3/g while the 10 and 25 % ZrO:/SiO2 systems produced initial values close to 0.5 cm3/g and sintered to about 0.1 cm3/g after 1 h at 600°C. The addition of an equal molar amount of La203 to the platinum deposited on ZrO2-stabilized SiO2 generated a catalyst with a high initial carbon monoxide chemisorption value corresponding to about 50% dispersion with no negative effects on the SiO2 stabilization. Fig. 2 compares the carbon monoxide chemisorption values for a series of catalysts all prepared with 2% Pt. The results clearly
M. Deeba et al. /Applied Catalysis A: General 124 (1995) 339-344 1.4,
~
-
.
.
.
.
343
.
0.8
g ~
0.6
0 0 o 500
550
600 650 Calcination Temperature (oC)
700
Fig. 2. Carbon monoxide chemisorption on various supported platinum catalysts as a function of calcination temperature for I h. Pt/SiO2 ( × ). Pt/25% ZrO2/SiO2 ( + ), Pt/La~O3/ZrO2/SiO2 ( * ).
show that the presence of La203 produces catalysts with higher initial metal areas and aids in the maintenance of that area up to 650°C. At 650°C differences between platinum on La203/Zr02/SiO2, Pt/SiO2, and Pt/Zr02/Si02 became smaller; however, the superiority of the La203 containing system is evident especially at temperatures below 600°C. The precise mechanism for stabilization is unknown, however, one may speculate that a surface complex of platinum with La203 exists, the stability strength of which is greater than that with other oxides reported. Similar arguments have been put forth by researchers studying Pt/CeO2 interactions [ 18 ]. The Pt/La203/ZrO2 / SiO2 reported here utilizes the combined features of ZrO2 stabilized SiO2 with strong SMSI from both La203 and ZrO2 to maintain platinum dispersion and stability at high temperatures. The oxidation of SO2 to SO3 was used as a model reaction to establish the superiority of La203 as a stabilizer for platinum against sintering. This reaction represents an important chemical process. The only active catalysts at temperatures of 300-350°C are supported platinum catalysts. In contrast V205 catalysts are not active until temperatures greater than 380°C. Silica was used for this application because it is an inert support, which unlike high surface area alumina, resists sulfation. However, upon mild aging, the Pt/SiO2 catalyst showed severe deactivation due mainly to platinum sintering and loss in platinum dispersion. Moreover, high-temperature aging of the Pt/SiO 2 resulted also in loss of the silica surface area and thus encapsulation of the platinum within the silica pores. Addition of ZrO> as found here, stabilized the SiO2 against loss in surface area and helped in stabilizing the platinum. Platinum was further stabilized by adding rare-earth metal oxides, specifically La203. A SiO2 support containing 25% ZrO2 was impregnated with La(NO3)3, calcined then impregnated with a solution of platinum to give 2% Pt and a La203 to Pt molar ratio of 1:1. This catalyst along with a 2%Pt/SiO2 catalyst were aged at 650°C for
344
M. D e e b a et al./ Applied Catalysis A: General 124 (1995) 339-344
20 h. A mixture of 2% SO: in air was passed through the catalyst bed. The La203 stabilized catalyst gave 52% conversion at 375°C compared to 21% at 550°C for the 2% Pt/SiO2. The significant improvement in the SO2 conversion for the La203 stabilized catalyst is consistent with the maintenance of higher platinum dispersion.
4. Conclusions It has been found that a broad range of ZrO2 loadings on SiO2 are very effective in stabilizing SiO2 against sintering. The presence of ZrOa also imparts some stabilization against platinum sintering. However, the combination of La203, and ZrO2 on SiOz is most effective in stabilizing platinum against sintering in oxidizing environments.
References [ 1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [ 11 ] [ 12] [ 13] [ 14] [ 15] [ 16] [ 17] [ 18]
H. Kntzinger and P. Ratnasamy, Cat. Rev.- Sci. Eng., 17 ( 1) (1978) 31. K. Shuan, US Patent 3 920 615 (1975). F. Wilhelm, US Patent 3 998 900 (1976). W.D. Mross, Cat. Rev.-Eng. Sci., 25 (4) (1983) 591. D. Kronig, Hydrocarbon Processing, 49 (3) (1970) 121. C. Wong and R.W. McCabe, J. Catal., 119 (1989) 47. G, Joy and E.H. Homeiri, US Patent 4 849 399 (1989). R.M. Heck and R.J. Farrauto, Catalytic Air Pollution Control: Commercial Technology, Van Nostrand Reinhold, Ch. 5, 1995, p. 63. R. Lamber, N. Jaeger and G. Schulz-Ekloff, J. Catal., 23 (1990) 285. J.B. Miller, S.E. Rankin and E.I. Ko, J. Catal., 148 (1994) 673. S. Soled and G.B. McVicker, Catal. Today, 14 (1992) 189. T. Kennelly and R. J. Farrauto, US Patent 5 216 875 (1993). V.A. Drozdov, P.G. Tsyrulnikova, A.A. Davydov and E.M. Moroz, Kinet. Katal., 27 (1986) 162. V.A. Drozdov, P.G. Tsyrulnikova, A.N. Pestryakov, A.A. Davydov and V.V. Popovskii, Kinet. Katal., 29 (1989) 484. R,P. Underwood and A.T. Bell, J. Catal., 109 (1988) 61. J.S. Rieck and A.T. Bell, J. Catal., 103 (1987) 46. E.C. Akubuiro and X.E. Verykios, Appl. Catal., 14 (1985) 215. J.C. Summers and S.A. Ausen, J. Catal., 58 (1979) 131.