548
Ind. Eng. Chem. Prod. Res. Dev. 1983, 22, 548-552
Pretzer, W. R.: Kobylinskl. T. P. Ann. N.Y. Aced. Sci. 1980, 333, 58. Pretzer, W. R.; Kobylinski. T. P.; Bozik, J. E. (to Gulf): U.S. Patent 4 133966, Jan 9, 1979. Pretzer, W. R.: Kobyiinski, T. P.; Bozlk, J. E. (to Gulf): U S . Patent 4 239 924, Dec 16, 1980a. Pretzer, W. R.: Kobyllnski. T. P.: Bozik. J. E. (to Gulf): U.S. Patent 4 239 925, Dec 16, 1980b. Pretzer, W. R.; Kobyllnski, T. P.; Bozik, J. E. (to Gulf): European Patent Appl. 13464 A l , July 23, 1980c. Rackett, H. G. J. Chem. Eng. Data 1970, 15, 514. Slaugh, L. H. (to Sheii): Dutch Patent Appl. 76/06138, June 8, 1976. Slinkard, W. E.; Baylls, A. B. (to Ceianese): U S . Patent 4 168391, Sept 18, 1979. Slocum, D. W. I n "Catalysis in Organic Synthesis": Jones, W. H., Ed.: Academic Press: New York, 1980: pp 245-276. Sugi, Y.; Bando, K.4.: Takaml, Y. Chem. Lett. 1981, 63. Taylor, P. D. (to Celanese): U S . Patent 4 111 837, Sept 5, 1978.
Van Boven, M.: Aiemdaroglu, N. H.: Penninger, J. M. L. Ind. Eng. Chem. Prod. Res. Dev. 1975, 14, 259. Walker, W. E. (to Union Carbide): U.S. Patent 4277634, July 7, 1981. Whyman, R. J. Organomet. Chem. 1974, 81, 97. Ziesecke, K. H. Brennst. Chem. 1952, 33, 385.
Received for review January 21, 1982 Revised manuscript received May 6, 1983 Accepted June 10, 1983 The authors wish to thank SOLVAY S.A. for financial support. PBF was granted a loan from the I.R.S.I.A. fund. Part of this paper was presented at the "Intemational Symposium on Catalytic Reactions of one Carbon Molecules" held in Bruges, Belgium, June 1982.
Effect of Pretreatment on Dispersion and Structure of Silica- and Alumina-Supported Pt Catalysts Janos Sarkanyt and Richard D. Gonzalez" Department of Chemistry, University of Rhode Island, Kingston, Rhode Island 0288 1
The dispersion and structure of silica- and alumina-supported Pt catalysts have been studied as a function of pretreatment. Initial pretreatment in He resulted in Pt/AI,03 catalysts having dispersions which were considerably larger than those obtained when H2 was used. This is explained by considering the enhanced mobility of the Pt surface complex in the presence of the He carrier gas. The choice of pretreatment was found to be less important for R/A1203catalysts with lower Pt loadings. Pt dispersions for the Pt/Si02 catalysts did not depend on the choice of pretreatment. When Pt/Si02 or F't/A1203catalysts were diluted with either pure alumina or silica prior to pretreatment, extensive interparticlediffusion of Pt occurred. The Interparticle transfer of Pt from silica to alumina during pretreatment for a series of Pt/Si02:A1,03 mixtures was studied by both selective chemisorption and infrared spectroscopy. The extent to which CO was bridge-bonded to Pt on a series of Pt/AI,03 catalysts was found to depend on crystallite size only when the catalysts were pretreated in H., This is explained in terms of possible preferential crystallographic orientations. Surface water and the extent to which the catalysts are dried prior to pretreatment play a prominent role in the surface diffusion of the resulting surface complexes.
Introduction The start of the reduction process is a particularly sensitive stage in the preparation of supported metal catalysts. In particular, the dispersion of supported metal catalysts prepared by the incipient wetness technique appears to be particularly sensitive to variations in the initial pretreatment. Because of the enormous industrial importance associated with supported Pt catalysts in catalytic reforming, these preparative variables have been the subject of considerable study and a coherent picture regarding the reduction process is beginning to emerge. The preparative variables which have received the most attention are: (1) the choice of Pt salts to be used in connection with a particular support (Dorling et al., 1971; Brunell et al., 1976); (2) the acidity of the support (Anderson, 1975);(3) the extent t~which the catalyst has been dried prior to reduction (Dorling et al., 1971; Dorling and Moss, 1967);(4) the decomposition of the surface complex (Shchukarev et al., 1956; Dorling et al., 1971; Lieske et al., 1983; Lietz, et al., 1983); (5) the interaction between the surface complex and the support during the initial pretreatment (Sarkany and Gonzalez, 1982a; Lieske et al., On leave from the Department of Organic Chemistry, Jozsef Attila University, Szeged, Hungary.
1983, Lietz et al., 1983); (6) calcination and reduction temperature (Jenkins, 1979); (7) the role played by anions added to the catalyst during pretreatment (Aboul-Gheit, 1979); (8) porosity of the support (Dorling et al., 1971); (9) the role played by H,O during the initial pretreatment (Dorling et al., 1971); and (10) the role played by chloride in the redispersion of Pt following high temperature treatment in O2 (Lieske et al., 1983; Leitz et al., 1983). In a previous paper (Sarkany and Gonzalez, 1982a),we reported on the rather extensive differences in migration of the surface complex formed as a result of the decomposition of H,PtCl, that occurred when the catalyst was pretreated in He rather than Hzprior to reduction. In particular, a new synthetic technique enabling the preparation of highly dispersed supported Pt catalysts having relatively high metal loadings was suggested. In this paper, we report further on the aspects of this surface migration.
Experimental Section The flow system which enables use of the reactor as either a pulse microreactor or a single-passreactor has been described in detail elsewhere (Miura and Gonzalez, 1982). In several experiments, an infrared cell also capable of operating either as a pulse microreactor or a single-pass differential reactor was used in place of the Pyrex microreactor. Details regarding the design of this infrared cell
0196-4321/83/1222-0548$01.50/00 1983 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 4, 1983 549 Table I. Effect of Pretreatment on Silica- and Alumina-Supported Pt Catalysts pretreatment gas uptake, pmol/g of cat. H, co
catalyst 6% Pt/Al,O,
6% Pt/A1203" 2% Pt/Al,O, 6% Pt/Al,O,:Al,O, 0.5% Pt/Al,O, 6% Pt/SiO,
(1:2)b
B
A
-
H/Pt
CO/H
35'1 26.5 30.5 15.3 16.7
0.11 0.12 0.12 0.24 o.23 0.49 0.47 0.46 0.39 0.44 0.81 0.82
0.57 0.53 0.52 0.60 0.62 0.77 0.73 0.74 0.67 0.68 0.74 0.79
70.0
0.26
0'87
17.4 18.3 18.7
19.8
36'2 35.1 25.1 23.9 23.8 19.8 22.5 10.5 10.6
43.6 38.7
40.2
19'3
Dried in vacuum desiccator for 2 weeks at 298 K.
gas uptake, wmol/g of cat. H, co
H/Pt
CO/H
88.1 86.7 90.1 81.4 79.7 28.0 28.0 29.8 40.7 40.3 11.4 11.6 43.3 42.4
0.57 0.56 0.57 0.53 0.52 0.55 0.55 0.58 0.79 0.79 0.88 0.90 0.28 0.27
0.71 0.72 0.70 0.72 0.74 0.70 0.70 0.72 0.71 0.72 0.69 0.65 0.89 0.88
124.6 127 117.8 39.4 39.4 43.2 58.1 15.7 15.1 76.9 74.6
All data calculated for 2% average metal content.
have been published (Sarkany and Gonzalez, 1982b; Miura and Gonzalez, 1982). However, an important feature of the infrared cell reador was that reactant gases were forced through the sample disk with little or no leakage around the edges of the sample. Materials. The gases used in this study were subjected to the following purification treatment: CO (research grade) was purified by passing it through a molecular sieve. H2 (research grade) was purified by passing it through a Deoxo unit to convert O2 impurities to H20, which was then removed by a molecular sieve maintained at 77 K by means of a liquid N2trap. O2(research grade) was purified by passing it through a dry ice-acetone bath to remove traces of H20. He (99.995%) was purified by passage through a Deoxo unit followed by a molecular sieve maintained at 77 K. The oxygen concentration measured at the catalyst was less than 0.1 ppm. The silica- or alumina-supported Pt samples used in this study were prepared by impregnation. Initially, the appropriate weight of H2PtC16.6H20(Strem Chem, Boston, MA) was dissolved in an amount of deionized water sufficient to ensure the complete wetting of the support. This solution was mixed with either Cab-O-Sil, Grade M-5, or Alon-C (Cabot Crop., Boston, MA) until a slurry having the consistency of a thin paste was formed. The slurry was then dried in a vacuum desiccator at room temperature for one or two days and stirred regularly during the drying process to retain uniformity. The dried catalyst was pressed into thin disks and ground into a fine powder before use. For use in the spectroscopic reactor, the dried catalyst was ground into a powder less than 45 pm in diameter and pressed into self-supporting disks 25 mm in diameter with an optical density of approximately 25 mg/cm2. Procedure. Fresh, siliea-supported Pt catalysts were treated according to the following pretreatment schedules: schedule A, heated in flowing H2 (20 mL min-l) as the temperature was increased from 298 to 523 K at a rate of 10 K min-l, reduced in flowing H2at 523 K for 1h followed by outgassing in flowing He for 30 min at 523 K, cooled in flowing He to 298 K; schedule B, heated in flowing He (20 mL min-') as the temperature was increased from 298 to 393 K at a rate of 10 K min-l, gas stream switched to H2 and temperature increased in flowing H2 from 393 to 523 K at a rate of 10 K min-', reduced in flowing H2 at 523 K for 1h followed by outgassing in flowing He for 30 min at 523 K, cooled in flowing He to 298 K. These different
I
' " ' ~ " " J " " " ' 1
p
300.
__.
I
0
50
.
,
.
I
/
100
I
I
.
l
l
,
l
150 Time(mn)
Figure 1. Catalyst pretreatment schedules A and B.
heating schedules are summarized in Figure 1. Prior to the H2 and CO chemisorption measurements, the supported Pt catalysts were treated as follows: they were heated in flowing O2 from 298 to 523 K at a rate of 10 K min-', treated in flowing O2 at 523 K for 30 min, followed by outgassing in He (20 mL min-') for 10 min, reduced in flowing H2 (20 mL min-l) at 523 K, followed by outgassing at the same temperature in flowing He for 30 min. The sample was then cooled to room temperature in flowing He. Because alumina-supported Pt catalysts are more difficult to reduce than Pt silica catalysts, the programmed heating schedules were continued until the final reduction temperature of 673 K was reached. Chemisorption measurements were performed using the dynamic pulse method. Details regarding the use of this technique have been outlined in a recent report (Sarkany and Gonzalez, 1982~).
Results and Discussion Effect of Pretreatment Schedule on Dispersion. The effect of using the two different pretreatment schedules in reducing a series of Pt/A1203and Pt/Si02 catalysts was investigated. The results of this study are summarized in Table I. In the case of the Pt/A120, catalysts, pretreatment B resulted in Pt metal dispersions (H/Pt X 100) which were considerably larger than those obtained using pretreatment A. However, as the Pt loading was decreased, differences in dispersion as a result of the different pretreatments were much less noticeable. Differences in dispersion as a result of the different pretreatment schedules were not noticeable for the Pt/Si02 catalysts provided that the catalysts were thoroughly dried prior to
550
Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 4, 1983
the initiation of the pretreatment schedule. Of particular interest was the dispersion obtained when a 2% Pt/Al2O3catalyst was prepared by mixing a 6% in a 1:2 ratio. When this Pt/A1203catalyst with pure A1203 physical mixture was treated according to schedule B, the dispersion increased from 0.55 for the undiluted 2% Pt/A1203catalyst to 0.79. When the dilution of the original 6% Pt/A1,03 was performed by adding A1203following reduction, no change in dispersion from the original 0.55 was observed. These results strongly suggest that extensive migration of the resulting surface complex occurred during pretreatment. When pretreatment A was used, little or no surface migration occurred for the 6% Pt/A1203catalyst diluted with pure A1203 before the start of the pretreatment. These results suggest that the start of the reduction occurs well below 400 K. When the initial pretreatment is performed in Hg,Pt reduction is well underway at temperatures only slightly above room temperature. After the Pt is reduced, it will not migrate to additional surface sites on the alumina support. When the catalyst is initially heated in He to 400 K (pretreatment B), the resulting surface complex can desorb from its primary adsorption sites on the alumina and readsorb on secondary sites prior to the start of the reduction process. At lower Pt loadings, the choice of pretreatment becomes less important due to the stronger adsorption of the Pt surface complex. The Effect of Incomplete Drying on Pt Dispersion. To assess the influence of incomplete drying prior to reduction, two 6% Pt/A120, catalysts which differed only in the extent to which they had been dried were studied. An increase in dispersion from 11 to 24% clearly showed that as the H 2 0content was decreased, incomplete drying was an important variable when the catalyst was heated in H2prior to reduction. In one case, a 6% Pt/Si02 sample pressed into a disk for use in the IR spectroscopic reactor was reduced according to pretreatment schedule A without first purging the reactor with He. The resulting Pt dispersion for this catalyst was only 6%. This low dispersion can be attributed to the formation of H 2 0 as the result of a reaction between air initially present in the IR cell and H2 from the carrier gas over the Pt catalyst. when the IR cell was flushed with flowing He at room temperature for a short period of time prior to the start of the reduction process, a dispersion of 26% Pt was obtained. When the 6% Pt/A1203catalysts were treated according to schedule B, the Pt dispersion appeared to be less sensitive to the extent to which the catalyst had been dried prior to the initiation of the pretreatment schedule. In order to understand the effect of the different pretreatment schedules on the dispersion of Pt/A1,0, and Pt/SiOz catalysts, it is useful to consider the genesis of supported Pt catalysts prepared by impregnation of the support with H2PtC&.6Hz0.It is well-known that H,PtC1, is strongly adsorbed on alumina but only slightly adsorbed on silica due to the absence of amphoteric OH- groups. Adsorption equilibrium constants are 1 X lo4 and
