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ZSM-5 Catalysts
S. Katiaguine and A. Maha y (Editors), Catalysis on the Energy SCl?ne '.~' 198-1 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
THE
PREPA~ATION
123
OF Pt/ZS'1-5 CATALYSTS
S. TAN, H.A. RANGWALA, J.A. SZYMURA, F.D. OTTO and S.E. WANKE Dept. of Chemical Engineerin~, University of Alberta, Edmonton, Canada T6G 2G6
AB5TP-ACT The aim of this work was to prepare Pt/ZS~-5 catalysts which are active for hydrogen-deuterium exchange between water and hydrogen. Such catalysts require high Pt loadings and relatively high Pt dispersions. Impregnation and ion exchange methods for the preparation of Pt/Z5M-5 containing about 5 wt% Pt are described. Impregnation of Z5M-5 with aqueous solutions of hexachloroplatinic acid resulted in catalysts with low (-0.1) Pt dispersions. Ion exchange with aqueous tetramine platinum (II) chloride yielded a catalysts with 5.6 wt% Pt and a dispersion of 0.4. This catalyst was active for the isotopic exchange between water and hydrogen. INTRODUCTI ON Zeolites have been used as supports for noble metals for over 20 years [lJ, and numerous zeolite supported metal catalysts are in use today [2J. The discovery of Z5M-5 zeolites a decade ago [3J has added a new dimension to shapeselective catalysis [4J which was first demonstrated by Weisz and ~rilette in 1960 [5J. ZSM-5 zeolites not only have a unique pore structure, but they are also hydrophobic [6J. This low affinity for water makes ZSH-5 a potentially usefu 1 support for meta1 cata 1ys ts whi ch a rl~ to be used in reaction mi xtures containing water. Host conventional supports (e.g. A1 203, 5i02 and the common zeolites) are very hydrophilic, and use of such supports at low temperatures in reaction mixtures containing water usually results in rapid catalyst deactivation due to waterlogging. One application of supported Pt catalysts which requires hydrophobic catalyst pellets is the isotopic hydrogen-deuterium exchange between hydrogen gas and liquid water. Hydrophobic catalysts for this exchange reaction have been and are being developed [7-9J. Although the current catalysts have high activities, further improvements may be possible. Pt supported on ZStl-5 is a possible alternative to the Pt on carbon used by Butler et al , [7J in their Pt-carbonTeflon-ceramic catalyst. However, Pt/Z5t1-5 catalysts with high Pt loadings and high Pt dispersions are required in order to be competitive with existing catalysts. In this paper we describe our attempts at obtaining such Pt/ZS~-5 catalysts.
124
EXPERIMENTAL METHODS Preparation of ZSM-5 ZSM-5 had to be prepared in the 1aboratory because it coul d not be purchased. The procedure we used was similar to that described in the literature [3,10]. An aqueous sodium aluminate solution, prepared by dissolving Al in aqueous NaOH (atomic Na/Al ratio about 1.2), was added to a silica gel in aqueous tetrapropyl ammonium hydroxide solution (Si/Al = 14). During the addition a white precipitate forms. The suspension was placed into a thick-walled tube, the tube was sealed and placed into an oven. The oven was heated to 1500C, and the tube was left in the oven at 1500C for 7 days. During these 7 days the tube was shaken twice a day to homogenize the suspension. After the 7 day treatment at 1500C the tube was cooled to OoC and then opened. The liquid was drained from the solid, and the solid was washed thoroughly with distilled water. The solid was left to dry at room temperature for 24 h and then calcined in air at 5400C for 2 h. The final solid product was slightly brownish in color. The crystallinity of the solid was determined by X-ray diffraction (see below). Preparation of Pt/ZSM-5 Impregnation and ion exchange methods for the preparation of Pt/ZSM-5 were investigated. The details of the preparation methods are described below. Method 1: Impregnations with aqueous solutions of HzPtC1 6 Calcined ZSM-5 was wetted with distilled water (1.0 mL H20/g of ZSM-5), and an aqueous solution of H2PtC1 6, containing sufficient Pt to result in a 5 wt% Pt on ZSM-5 catalyst, was added to the paste. The total amount of water used for the wetting and impregnation was 2.8 mL/g of ZSM-5. The slurry was left at room temperature for 24 h; it was stirred intermittently during this time. The slurry was then slowly evaporated to dryness over a period of 8 h by heating on a hot plate. The resulting solid was crushed and dried in air at 1100C for an additional 24 h. The dried catalyst was either reduced (Cat. 1) or calcined before reduction (Cat. 2). Details of the reduction and calcination treatments are given in Table 1. Neutron activation analyses of the reduced catalysts yielded a Pt content of 4.96 wt% Pt. Method 2: Impregnation with aqueous sol utions of HzPtC1 6 - NH 3 This method was similar to Method 1, except that concentrated ammonia solution (30 mL/g ZSM-5) was added to the impregnation solution. The evaporation to dryness at 800C required 40 h compared to 8 h for Method 1. After evaporation to dryness the solids were dried in air at 1200C for 24 h. Additional treatments of catalysts prepared by Method 2 (Cat. 3 and 4) are given in Table 1. Method 3: Ion exchange with aqueous sol utions of Pt(NH 3)4C1 2 - NH 3 A solution of Pt(NH 3)4C1 2 was prepared by dissolving Pt(NH 3)4C1 2.H20 in an
125
TABLE 1 Treatments of dried catalysts prior to adsorption measurements Catalyst
Preparation Method
2
4
2 2
5
3
3
Treatment after drying at 110 or 120°C Reduced in flowing hydrogen at 150°C for 16 h, 250°C for 2 h and 500°C for 1 h. Calcined in flowing oxygen at 300°C for 2 h followed by reduction described for Catalyst 1. Same treatment as for Catalyst 2. Calcined in air at 300°C for 24 h and in oxygen at 300°C for 2 h, followed by reduction in hydrogen at 300°C for 2 h. Calcined in air at 300°C for 2 h followed by reduction in hydrogen at 300°C for 2 h.
aqueous ammonia solution (3 wt% NH 40H). The Pt content of the solution, as determined by atomic absorption (AA) was 0.0137 g Pt/mL of solution. A 50 mL portion of this solution was added to a wetted 4.0 g sample of ZSM-5. The mixture was heated to 900C and kept at 900C for 3 days. Distilled water was added periodically to maintain about 50 mL of liquiJ in the mixture. The mixture was then cooled to room temperature and filtered. The solids were washed three times with 20 mL portions of distilled water. The washed solids were dried in air at 120°C for 24 h, calcined in air at 300°C for 2 hand reduced in hydrogen at 300 0C. The filtrate and water from the washings were combined and evaporated to 100 mL. The Pt content of this 100 mL filtrate/ washings solution, determined by AA, was 0.0045 g Pt/mL. This yields a Pt loading of 5.6 wt% for the Pt/ZSM-5 prepared by Method 3 (Catalyst 5). Catalyst Characterization X-ray diffraction (XRD) and chemisorption were used to characterize the ZSM-5 and Pt/ZSM-5 catalysts. A Philips X-ray diffractometer, operated in the stepscan mode and equipped with a curved graphite monochromator and a Cu tube, was used to obtain XRD patterns. Steps of 0.2 0 of 26 with counting intervals of 40 seconds per step were used for all the XRD studies. The proportional X-ray detector was interfaced with a Hewlett-Packard 1000 computer, and the detector output was stored on disc and tape for subsequent plotting and subtraction of XRD patterns. Hydrogen and oxygen chemisorption and titration measurements were carried out at room temperature using a previously described dynamic pulse adsorption apparatus [11]. Nitrogen was used as the carrier gas during H2 uptake measurements, and helium was used during 02 uptake measurements.
126
RESULTS AND DISCUSSION Structure of ZSM-5 XRD was used to determine whether our procedure for preparation of ZSM-5 resul ted in a materi al wi th the sarre crystal structure as ZSM-5 obtained from Mobil. (Mobil Research and Development Corp. kindly supplied us with a 10 g sample of ZSM-5). Fig. 1 shows XRD patterns for our ZSM-5 preparatio.n and for the ZSM-5 obtained from Mobil. Also shown in.Fig. 1 is a XRD pattern for silicalite; silicalite is an essentially aluminum-free zeolite with a structure similar to that of ZSM-5 [12,13]. The results shown in Fig. 1 show that our preparation method did result in ZSM-5. Characterization of Pt/ZSM-5 Chemisorption and XRD were used to obtain information about the state of Pt in the Pt/ZSM-5 catalysts. The results of hydrogen adsorption and hydrogen oxygen titration measurements for the catalysts described in Table 1 are summarized in Table 2. After the reduction step (see Table 1) and prior to the hydrogen adsorption, catalysts were degassed in ultrapure nitrogen at 5000C for 1 to 2 h. Catalysts were treated in flowing oxygen at room temperature for 10 min prior to hydrogen titration measurements. A similar treatment in flowing hydrogen was done prior to oxygen titration measurements. The results in Table 2 show that the catalysts prepared by impregnation (Cat. 1 to 4) had very low dispersions, i.e. Pt dispersions were less than 0.14 if it
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Fig. 1. X-ray diffraction patterns for lSM-5 and silicalite. 250 counts/second for clarity.
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127
TABLE 2 Chemisorption and titration results for Pt/ZSM-5 Catalyst
Platinum Loadi ng (wt% Pt) 5.0
4
5.0 5.0 5.0
5
5.6
2 3
Adsorption and Titration Uptakes a HA HT OT 0.055 0.066 b 0.131 0.042 0.054
o.c.z> 0.39
0.143 0.175 0.409 0.101 0.139 0.180 1.11
0.059 0.181
0.57
HA
HT
OT
2.6 2.7 3.1 2.4 2.6 2.5 2.9
1.1 1.4
:
1.4
aHA, HT and OT are hydrogen adsorption, hydrogen titration and oxygen titration uptakes in H or 0 atoms per Pt atom. bUptakes measured after reduced catalyst (hydrogen covered) was heated to 750 0C and evacuated at 7500C for 2 h. is assumed that the hydrogen adsorption stoichiometry is one H atom per surface Pt atom. Pt dispersion is defined as the ratio of Pt surface atoms to total Pt atoms. The average ratio of hydrogen titration to hydrogen adsorption uptakes was 2.7 (see HA:HT values in Table 2). This value is somewhat lower than the value of 3.0 proposed by Benson and Boudart [14J. The OT:HA ratios are also lower than the value of 1.5 expected by the stoichiometry proposed by Benson and Boudart. The hydrophobic nature of the ZSM-5 is a possible cause for the low titration uptakes, i.e. some of the water formed during the titrations stays on the Pt surface and interferes with the subsequent adsorption of hydrogen or oxygen. Calcination of dried catalysts at 300 0C prior to reduction resulted in small increases in Pt dispersions (cf. Cat. 1 and 2, and Cat. 3 and 5; Table 2). However, calcination in oxygen at 500 to 5500C did not result in further improvements in Pt dispersions. High temperature treatments (750oC) of reduced catalysts resulted in marginal increases in adsorption uptakes (see Cat. 1 and 4; Table 2). However, none of the treatments of Pt/ZSM-5 prepared by impregnation (Methods 1 and 2) yielded catalysts which would be attractive for hydrogen deuterium exchange. Catalysts with higher Pt dispersions,i .e. better utilization of Pt, are requi red. The ion exchange method of preparation (Method 3) yielded a catalyst (Cat. 5) with a dispersion of 0.4 and a Pt loading of 5.6 wt%. Although a dispersion of 0.4 is not a very high dispersion, it represents a significant improvement in dispersion compared to the catalysts prepared by impregnation. XRD studies were done in order to determine whether a significant amount of Pt in Cat. 5 was
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Fig. 2. Subtracted XRD patterns showing Pt 111 and 200 1ines: a = Cat. 1; b = Cat. 2; c = Cat. 5. Patterns are off-set by 75 counts/second for clarity. present in Pt crystallites greater than 3.0 nm in size. With the scan rates used in this work, only Pt crystallites larger than about 3.0 nm are detected by XRD. A Pt dispersion of 0.4 corresponds to an average Pt particle size of about 2.5 nm. XRD patterns, showing the Pt 111 and 200 lines, are presented in Fig. 2. These XRD patterns were obtained by point-by-point subtraction of the ZSM-5 pattern (Line A, Fig. 1) from the Pt/ZSM-5 patterns. The average Pt crystallite sizes, based on the width at half height of the 111 line [15J, are 25 to 30 nm for Cat. 1 and 2, and about 20 nm for Cat. 5. The average size of 20 nm obtained by XRD for Cat. 5 is much larger than the average size of 2.5 nm calculated from hydrogen chemisorption. The average size based on XRD is much larger than the size based on chemisorption because only a fraction of the Pt in Cat. 5 was detected by XRD. The amount of Pt detected by XRD is proportional to the area under the diffraction peaks. Although Cat. 5 contained more Pt than Cat. 1 or 2, the area under the Pt 111 line for Cat. 5 is less than half as large as the corresponding areas under the 111 lines for Cat. 1 or 2. This means that a significant fraction of the Pt in Cat. 5, probably more than 50%, was not detected by XRD. This undetected Pt is present in Pt crystallites smaller than 3.0 nm. XRD scans using smaller step sizes and longer counting times per step than those employed in this work are required in order to obtain more quantitative information on the fraction of Pt detected. However, the results obtained clearly show that the Pt in Cat. 5 existed in crystallites with a bimodal size distribution. A significant fraction of the Pt was present in small crystallites while the rest of the Pt was present in relatively large (10 to 20 nm) crystallites.
129
Preliminary hydrogen - deuterium exchange studies between water vapor and hydrogen showed that Cat. 5 was an active catalyst for this reaction. Cat. 5 had an activity per gram of Pt which was twice as large as that of Pt/A1 20 but 3 only half as large as the activity of Pt/C. However, the activity of Pt/ZSM-5 did not decrease with time of use, i .e . it did not become waterlogged. Further improvements in the preparation techniques of Pt/ZSM-5, which result in catalysts with higher Pt loadings and Pt dispersions above 0.8, should make Pt/ZSM-5 comparable and possibly superior to Pt/C for the isotopic exchange between water and hydrogen. ACKNOWLE DSME NT The isotopic exchange studies with Cat. 5 where carried out by J.H. Rolston et al. at Atomic Energy of Canada in Chalk River. We gratefully acknowledge this contribution. REFE:zENCES 1 J.A. Rabo, P.E. Pickert and R.L. Mays, Ind. Eng. Chem., 53 (1361) 733-736. 2 J. Scott (Ed.), Zeolite Technology and Applications, Recent Advances (Chemical Technology, Review No. 170), ~Ioyes Data Corp., Park Ridge, N.J., 1980. 3 R.J. Argauer and G.R. Landolt, US Patent 3,702,886, Nov. 14, 1972. 4 R.M. gessau, J. Catal., 77 (1982) 304-306. 5 P.B. Weisz and v.J. Frilette, J. Phys. Chem., 64 (1960) 382. 6 H. Nakamoto and H. Takahashi, Zeolites, 2 (1982) 67-68. 7 J.P. Butler, J.H. Rolston and W.H. Stevenson, in H.K. Rae (Ed.) Separation of Hydrogen Isotopes, ACS Symposium Series 68, American Chemical Society, Washington, D.C., 1978, pp. 91-109. 8 J.T. Enright and T.T. Chuang, Can. J. Chem. Eng., 56 (1978) 246-250. 9 I. Iida, J. Kato and K. Tamura, Z. physik.Chem. (N.F.), 107 (1977) 219-230. 10 E.G. Derouane, J.B. Nagy, P. Dejaifve, J.H.C. van Hooff, B.P. Spekman, J.C. Vedrine and C. Naccache, J. Catal., 53 (1978) 40-55. 11 S.E. Wanke, B.K. Lotochinski and H.C. Sidwell, Can. J. Chem. Eng., 59 (1981) 357-361. 12 R.W. Grose and Li1. Flanigen, US Patent 4,061,724, Dec. 6, 1977. 13 E.M. Flanigen, J.M. Bennett, R.W. Grose, J.P. Cohen, R.L. Patton, R.~1. Kirchner and J.V. Smith, Nature, 271 (1978) 512-516. 14 J.E. Benson and t1. Boudart, J. Catal., 4 (1965) 704-710. 15 B.D. Cullity, Elements of X-Ray Diffraction, Addison-Wesley, Don r·1il1s, On t .; 1967, p, 99.
THE
PREPA~ATION
123
OF Pt/ZS'1-5 CATALYSTS
S. TAN, H.A. RANGWALA, J.A. SZYMURA, F.D. OTTO and S.E. WANKE Dept. of Chemical Engineerin~, University of Alberta, Edmonton, Canada T6G 2G6
AB5TP-ACT The aim of this work was to prepare Pt/ZS~-5 catalysts which are active for hydrogen-deuterium exchange between water and hydrogen. Such catalysts require high Pt loadings and relatively high Pt dispersions. Impregnation and ion exchange methods for the preparation of Pt/Z5M-5 containing about 5 wt% Pt are described. Impregnation of Z5M-5 with aqueous solutions of hexachloroplatinic acid resulted in catalysts with low (-0.1) Pt dispersions. Ion exchange with aqueous tetramine platinum (II) chloride yielded a catalysts with 5.6 wt% Pt and a dispersion of 0.4. This catalyst was active for the isotopic exchange between water and hydrogen. INTRODUCTI ON Zeolites have been used as supports for noble metals for over 20 years [lJ, and numerous zeolite supported metal catalysts are in use today [2J. The discovery of Z5M-5 zeolites a decade ago [3J has added a new dimension to shapeselective catalysis [4J which was first demonstrated by Weisz and ~rilette in 1960 [5J. ZSM-5 zeolites not only have a unique pore structure, but they are also hydrophobic [6J. This low affinity for water makes ZSH-5 a potentially usefu 1 support for meta1 cata 1ys ts whi ch a rl~ to be used in reaction mi xtures containing water. Host conventional supports (e.g. A1 203, 5i02 and the common zeolites) are very hydrophilic, and use of such supports at low temperatures in reaction mixtures containing water usually results in rapid catalyst deactivation due to waterlogging. One application of supported Pt catalysts which requires hydrophobic catalyst pellets is the isotopic hydrogen-deuterium exchange between hydrogen gas and liquid water. Hydrophobic catalysts for this exchange reaction have been and are being developed [7-9J. Although the current catalysts have high activities, further improvements may be possible. Pt supported on ZStl-5 is a possible alternative to the Pt on carbon used by Butler et al , [7J in their Pt-carbonTeflon-ceramic catalyst. However, Pt/Z5t1-5 catalysts with high Pt loadings and high Pt dispersions are required in order to be competitive with existing catalysts. In this paper we describe our attempts at obtaining such Pt/ZS~-5 catalysts.
124
EXPERIMENTAL METHODS Preparation of ZSM-5 ZSM-5 had to be prepared in the 1aboratory because it coul d not be purchased. The procedure we used was similar to that described in the literature [3,10]. An aqueous sodium aluminate solution, prepared by dissolving Al in aqueous NaOH (atomic Na/Al ratio about 1.2), was added to a silica gel in aqueous tetrapropyl ammonium hydroxide solution (Si/Al = 14). During the addition a white precipitate forms. The suspension was placed into a thick-walled tube, the tube was sealed and placed into an oven. The oven was heated to 1500C, and the tube was left in the oven at 1500C for 7 days. During these 7 days the tube was shaken twice a day to homogenize the suspension. After the 7 day treatment at 1500C the tube was cooled to OoC and then opened. The liquid was drained from the solid, and the solid was washed thoroughly with distilled water. The solid was left to dry at room temperature for 24 h and then calcined in air at 5400C for 2 h. The final solid product was slightly brownish in color. The crystallinity of the solid was determined by X-ray diffraction (see below). Preparation of Pt/ZSM-5 Impregnation and ion exchange methods for the preparation of Pt/ZSM-5 were investigated. The details of the preparation methods are described below. Method 1: Impregnations with aqueous solutions of HzPtC1 6 Calcined ZSM-5 was wetted with distilled water (1.0 mL H20/g of ZSM-5), and an aqueous solution of H2PtC1 6, containing sufficient Pt to result in a 5 wt% Pt on ZSM-5 catalyst, was added to the paste. The total amount of water used for the wetting and impregnation was 2.8 mL/g of ZSM-5. The slurry was left at room temperature for 24 h; it was stirred intermittently during this time. The slurry was then slowly evaporated to dryness over a period of 8 h by heating on a hot plate. The resulting solid was crushed and dried in air at 1100C for an additional 24 h. The dried catalyst was either reduced (Cat. 1) or calcined before reduction (Cat. 2). Details of the reduction and calcination treatments are given in Table 1. Neutron activation analyses of the reduced catalysts yielded a Pt content of 4.96 wt% Pt. Method 2: Impregnation with aqueous sol utions of HzPtC1 6 - NH 3 This method was similar to Method 1, except that concentrated ammonia solution (30 mL/g ZSM-5) was added to the impregnation solution. The evaporation to dryness at 800C required 40 h compared to 8 h for Method 1. After evaporation to dryness the solids were dried in air at 1200C for 24 h. Additional treatments of catalysts prepared by Method 2 (Cat. 3 and 4) are given in Table 1. Method 3: Ion exchange with aqueous sol utions of Pt(NH 3)4C1 2 - NH 3 A solution of Pt(NH 3)4C1 2 was prepared by dissolving Pt(NH 3)4C1 2.H20 in an
125
TABLE 1 Treatments of dried catalysts prior to adsorption measurements Catalyst
Preparation Method
2
4
2 2
5
3
3
Treatment after drying at 110 or 120°C Reduced in flowing hydrogen at 150°C for 16 h, 250°C for 2 h and 500°C for 1 h. Calcined in flowing oxygen at 300°C for 2 h followed by reduction described for Catalyst 1. Same treatment as for Catalyst 2. Calcined in air at 300°C for 24 h and in oxygen at 300°C for 2 h, followed by reduction in hydrogen at 300°C for 2 h. Calcined in air at 300°C for 2 h followed by reduction in hydrogen at 300°C for 2 h.
aqueous ammonia solution (3 wt% NH 40H). The Pt content of the solution, as determined by atomic absorption (AA) was 0.0137 g Pt/mL of solution. A 50 mL portion of this solution was added to a wetted 4.0 g sample of ZSM-5. The mixture was heated to 900C and kept at 900C for 3 days. Distilled water was added periodically to maintain about 50 mL of liquiJ in the mixture. The mixture was then cooled to room temperature and filtered. The solids were washed three times with 20 mL portions of distilled water. The washed solids were dried in air at 120°C for 24 h, calcined in air at 300°C for 2 hand reduced in hydrogen at 300 0C. The filtrate and water from the washings were combined and evaporated to 100 mL. The Pt content of this 100 mL filtrate/ washings solution, determined by AA, was 0.0045 g Pt/mL. This yields a Pt loading of 5.6 wt% for the Pt/ZSM-5 prepared by Method 3 (Catalyst 5). Catalyst Characterization X-ray diffraction (XRD) and chemisorption were used to characterize the ZSM-5 and Pt/ZSM-5 catalysts. A Philips X-ray diffractometer, operated in the stepscan mode and equipped with a curved graphite monochromator and a Cu tube, was used to obtain XRD patterns. Steps of 0.2 0 of 26 with counting intervals of 40 seconds per step were used for all the XRD studies. The proportional X-ray detector was interfaced with a Hewlett-Packard 1000 computer, and the detector output was stored on disc and tape for subsequent plotting and subtraction of XRD patterns. Hydrogen and oxygen chemisorption and titration measurements were carried out at room temperature using a previously described dynamic pulse adsorption apparatus [11]. Nitrogen was used as the carrier gas during H2 uptake measurements, and helium was used during 02 uptake measurements.
126
RESULTS AND DISCUSSION Structure of ZSM-5 XRD was used to determine whether our procedure for preparation of ZSM-5 resul ted in a materi al wi th the sarre crystal structure as ZSM-5 obtained from Mobil. (Mobil Research and Development Corp. kindly supplied us with a 10 g sample of ZSM-5). Fig. 1 shows XRD patterns for our ZSM-5 preparatio.n and for the ZSM-5 obtained from Mobil. Also shown in.Fig. 1 is a XRD pattern for silicalite; silicalite is an essentially aluminum-free zeolite with a structure similar to that of ZSM-5 [12,13]. The results shown in Fig. 1 show that our preparation method did result in ZSM-5. Characterization of Pt/ZSM-5 Chemisorption and XRD were used to obtain information about the state of Pt in the Pt/ZSM-5 catalysts. The results of hydrogen adsorption and hydrogen oxygen titration measurements for the catalysts described in Table 1 are summarized in Table 2. After the reduction step (see Table 1) and prior to the hydrogen adsorption, catalysts were degassed in ultrapure nitrogen at 5000C for 1 to 2 h. Catalysts were treated in flowing oxygen at room temperature for 10 min prior to hydrogen titration measurements. A similar treatment in flowing hydrogen was done prior to oxygen titration measurements. The results in Table 2 show that the catalysts prepared by impregnation (Cat. 1 to 4) had very low dispersions, i.e. Pt dispersions were less than 0.14 if it
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Fig. 1. X-ray diffraction patterns for lSM-5 and silicalite. 250 counts/second for clarity.
Patterns off-set
127
TABLE 2 Chemisorption and titration results for Pt/ZSM-5 Catalyst
Platinum Loadi ng (wt% Pt) 5.0
4
5.0 5.0 5.0
5
5.6
2 3
Adsorption and Titration Uptakes a HA HT OT 0.055 0.066 b 0.131 0.042 0.054
o.c.z> 0.39
0.143 0.175 0.409 0.101 0.139 0.180 1.11
0.059 0.181
0.57
HA
HT
OT
2.6 2.7 3.1 2.4 2.6 2.5 2.9
1.1 1.4
:
1.4
aHA, HT and OT are hydrogen adsorption, hydrogen titration and oxygen titration uptakes in H or 0 atoms per Pt atom. bUptakes measured after reduced catalyst (hydrogen covered) was heated to 750 0C and evacuated at 7500C for 2 h. is assumed that the hydrogen adsorption stoichiometry is one H atom per surface Pt atom. Pt dispersion is defined as the ratio of Pt surface atoms to total Pt atoms. The average ratio of hydrogen titration to hydrogen adsorption uptakes was 2.7 (see HA:HT values in Table 2). This value is somewhat lower than the value of 3.0 proposed by Benson and Boudart [14J. The OT:HA ratios are also lower than the value of 1.5 expected by the stoichiometry proposed by Benson and Boudart. The hydrophobic nature of the ZSM-5 is a possible cause for the low titration uptakes, i.e. some of the water formed during the titrations stays on the Pt surface and interferes with the subsequent adsorption of hydrogen or oxygen. Calcination of dried catalysts at 300 0C prior to reduction resulted in small increases in Pt dispersions (cf. Cat. 1 and 2, and Cat. 3 and 5; Table 2). However, calcination in oxygen at 500 to 5500C did not result in further improvements in Pt dispersions. High temperature treatments (750oC) of reduced catalysts resulted in marginal increases in adsorption uptakes (see Cat. 1 and 4; Table 2). However, none of the treatments of Pt/ZSM-5 prepared by impregnation (Methods 1 and 2) yielded catalysts which would be attractive for hydrogen deuterium exchange. Catalysts with higher Pt dispersions,i .e. better utilization of Pt, are requi red. The ion exchange method of preparation (Method 3) yielded a catalyst (Cat. 5) with a dispersion of 0.4 and a Pt loading of 5.6 wt%. Although a dispersion of 0.4 is not a very high dispersion, it represents a significant improvement in dispersion compared to the catalysts prepared by impregnation. XRD studies were done in order to determine whether a significant amount of Pt in Cat. 5 was
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DEGREES 2
50 THET~
Fig. 2. Subtracted XRD patterns showing Pt 111 and 200 1ines: a = Cat. 1; b = Cat. 2; c = Cat. 5. Patterns are off-set by 75 counts/second for clarity. present in Pt crystallites greater than 3.0 nm in size. With the scan rates used in this work, only Pt crystallites larger than about 3.0 nm are detected by XRD. A Pt dispersion of 0.4 corresponds to an average Pt particle size of about 2.5 nm. XRD patterns, showing the Pt 111 and 200 lines, are presented in Fig. 2. These XRD patterns were obtained by point-by-point subtraction of the ZSM-5 pattern (Line A, Fig. 1) from the Pt/ZSM-5 patterns. The average Pt crystallite sizes, based on the width at half height of the 111 line [15J, are 25 to 30 nm for Cat. 1 and 2, and about 20 nm for Cat. 5. The average size of 20 nm obtained by XRD for Cat. 5 is much larger than the average size of 2.5 nm calculated from hydrogen chemisorption. The average size based on XRD is much larger than the size based on chemisorption because only a fraction of the Pt in Cat. 5 was detected by XRD. The amount of Pt detected by XRD is proportional to the area under the diffraction peaks. Although Cat. 5 contained more Pt than Cat. 1 or 2, the area under the Pt 111 line for Cat. 5 is less than half as large as the corresponding areas under the 111 lines for Cat. 1 or 2. This means that a significant fraction of the Pt in Cat. 5, probably more than 50%, was not detected by XRD. This undetected Pt is present in Pt crystallites smaller than 3.0 nm. XRD scans using smaller step sizes and longer counting times per step than those employed in this work are required in order to obtain more quantitative information on the fraction of Pt detected. However, the results obtained clearly show that the Pt in Cat. 5 existed in crystallites with a bimodal size distribution. A significant fraction of the Pt was present in small crystallites while the rest of the Pt was present in relatively large (10 to 20 nm) crystallites.
129
Preliminary hydrogen - deuterium exchange studies between water vapor and hydrogen showed that Cat. 5 was an active catalyst for this reaction. Cat. 5 had an activity per gram of Pt which was twice as large as that of Pt/A1 20 but 3 only half as large as the activity of Pt/C. However, the activity of Pt/ZSM-5 did not decrease with time of use, i .e . it did not become waterlogged. Further improvements in the preparation techniques of Pt/ZSM-5, which result in catalysts with higher Pt loadings and Pt dispersions above 0.8, should make Pt/ZSM-5 comparable and possibly superior to Pt/C for the isotopic exchange between water and hydrogen. ACKNOWLE DSME NT The isotopic exchange studies with Cat. 5 where carried out by J.H. Rolston et al. at Atomic Energy of Canada in Chalk River. We gratefully acknowledge this contribution. REFE:zENCES 1 J.A. Rabo, P.E. Pickert and R.L. Mays, Ind. Eng. Chem., 53 (1361) 733-736. 2 J. Scott (Ed.), Zeolite Technology and Applications, Recent Advances (Chemical Technology, Review No. 170), ~Ioyes Data Corp., Park Ridge, N.J., 1980. 3 R.J. Argauer and G.R. Landolt, US Patent 3,702,886, Nov. 14, 1972. 4 R.M. gessau, J. Catal., 77 (1982) 304-306. 5 P.B. Weisz and v.J. Frilette, J. Phys. Chem., 64 (1960) 382. 6 H. Nakamoto and H. Takahashi, Zeolites, 2 (1982) 67-68. 7 J.P. Butler, J.H. Rolston and W.H. Stevenson, in H.K. Rae (Ed.) Separation of Hydrogen Isotopes, ACS Symposium Series 68, American Chemical Society, Washington, D.C., 1978, pp. 91-109. 8 J.T. Enright and T.T. Chuang, Can. J. Chem. Eng., 56 (1978) 246-250. 9 I. Iida, J. Kato and K. Tamura, Z. physik.Chem. (N.F.), 107 (1977) 219-230. 10 E.G. Derouane, J.B. Nagy, P. Dejaifve, J.H.C. van Hooff, B.P. Spekman, J.C. Vedrine and C. Naccache, J. Catal., 53 (1978) 40-55. 11 S.E. Wanke, B.K. Lotochinski and H.C. Sidwell, Can. J. Chem. Eng., 59 (1981) 357-361. 12 R.W. Grose and Li1. Flanigen, US Patent 4,061,724, Dec. 6, 1977. 13 E.M. Flanigen, J.M. Bennett, R.W. Grose, J.P. Cohen, R.L. Patton, R.~1. Kirchner and J.V. Smith, Nature, 271 (1978) 512-516. 14 J.E. Benson and t1. Boudart, J. Catal., 4 (1965) 704-710. 15 B.D. Cullity, Elements of X-Ray Diffraction, Addison-Wesley, Don r·1il1s, On t .; 1967, p, 99.