Langmuir 1991, 7, 1436-1440
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Adsorption of Chloroplatinic Acid and Chloroiridic Acid on Composite Oxides S. Subrammiant and J. A. Schwarz' Department of Chemical Engineering and Materials Science, Syracuse University, Syracuse, New York 13244 Received October 1, 1990. I n Final Form: December 19, 1990 The objective of the present study is to investigate the adsorption of chloroplatinicacid and chloroiridic acid on alumina, titania, and composite oxides formed from alumina and titania. The adsorption results were analyzed by using the Langmuir equation. The adsorption kinetics and adsorption equilibrium amounts indicate that the metal ions have a higher affinity for alumina compared to titania; the difference is more significant in the case of Ir. It was also observed that the presence of titania inhibits aluminum dissolution when the TiOzAl2Oa supports are brought in contact with acidic impregnationsolutions. The adsorption results suggest that the metal ion can be selectivelypartitioned to alumina present in a &Os-Ti02 composite oxide. This selective adsorption process was explored by using temperature-programmed reduction. Our results suggest that the metal weight loading determines the selectivity of this partitioning.
Introduction Supported platinum and iridium catalysts are used in systems of industrial interest such as, automobile catalytic converters and reformers.lP2 A critical step in the preparation of supported catalysts is the impregnation of the active ingredient onto the support. Scientific interest in the impregnation step developed after itwas demonstrated that certain acids and salts contributed to the distribution of the active ingredient within a porous catalyst pelletS3p4 The investigation of the adsorption process during the wet impregnation of H2PtCLJA1203 became the subject of interest in subsequent studies.b** The impregnation step for several other precursor/support systems has been studieda7-l2 But no information is reported on the adsorption of HzIrCh. Also, the adsorption of catalytic precursors on more complex supports such as composite oxides has not been studied to date. Composite oxides form a class of supports for the potential design of supported metal catalysts with unique properties.13-18 The strength, number, and type of acid
* To whom correspondence should be addressed. 'Present address: Ford Motor Co., P.O. Box 2053, 2162 SRL, Dearborn, MI 48121. (1) Kummer, J. T. h o g . Energy Combust. Sci. 1980,6, 177. (2).Sinfelt, J. H. Bimetallic Catalysts: Discoveries, Concepts, and Apphcationu; John Wiley: New York, 1983. (3) Maatman, R. W.; Prater, C. D. Ind. Eng. Chem. 1957,49, 253. (4) Maatman, R. W. Ind. Eng. Chem. 1969,51,913. ( 5 )Santacasaria, E.; Canna, S.;Adami, I. Ind. Eng. Chem. R o d . Res. Dev. 1977, 16, 41. (6) Santacesaria, E.; Galli, C.; Canna, React. Kinet. Catal. Lett.
F.
1977,6, 301. (7) Chen, H. C.;Anderson,R.B. Ind. Eng. Chem. R o d . Res. Deu. 1973, 12, 122. (8) Chen, H. C.; Anderson, R. B. J. Catal. 1976,43,200. (9) Cervello, J.; Garcia dela Banda, J. F.;Hermana, E.; Jimenez,J. F. Chem. Eng. Technol. 1967,48,520. (10) Cervello, J.; Chou, T. S.; Summers, J. C.; Potter, N. M. In
Preparation of Catalysts I. Studies in Surface Science and Catalysis; Delmon, B., Grange, P., Jacobs, P., Poncelet, G., Ed.; Elsevier: Amstardam, 1976; Vol. 2, p 251. (11) Hegedus, L. L.; Chou, T. S.; Summers, J. C.; Potter, N. M. In Preparation of Catalysts II. Studies in Surface Science and Catalysis; Delmon, B., Grange, P., Jacobs, P., Poncelet, G., E&.; Elsevier: Amsterdam, 1979; Vol. 3, p 171. (12) Komiyama, M.; Merrill, R. P.; Harnsberger, H. F. J. Catal. 1980,
63, 35. (13) Jin, T.; Hattori, H.; Tanabe, K. Bull. Chem. Soe. Jpn. 1982,55, 2279.
sites in a composite oxide can be controlled by varying ta composition.13-ls Dumesic and co-workers have repon !d on the acidic DroDerties of a series of model binary oxic ?S formed by mou&ing a metal oxide at low concentrations onto the surface of a second metal oxide. Since the physical properties of the newly formed adsorbent are different from either of the pure components, we would expect that their adsorption properties during impregnation would also be different from that of the pure phases. In particular, the partitioning of the supported metal between the two phases of a composite oxide would be controlled by the adsorption kinetics on each phase. The objective of the present study is to investigate the adsorption of chloroplatinic and chloroiridic acid on pure and composite oxides formed from alumina and titania. Titania is of the strong metal-support interaction (SMSI) type and thus provides a unique anchor for catalytic However, pure titania is a low surface area support and alumina-titania composite oxides provide a convenient method for increasing the surface area of the supported titania phase.23 The rate of adsorption of the hexachlorinate Pt and Ir ions (MCh2-, where M = Pt or Ir) on AlzOs, TiO2, and TiOz/Al203composite oxides was measured and the results were analyzed by using the Langmuir equation.s Temperature-programmed reduction (TPRd) was used to assess the distribution of the metal species in the composite oxide supported precursors. The transient and equilibrium adsorption data and TPRd results suggest that, a t relatively low metal weight loadings, the selective adsorption of the hexachlorinate ion on alumina in a TiO2A1203 composite oxide is achieved. (14) Tanabe, K.; Itoh, M.; Morahige, K.; Hattori, H. In Preparation
B., Jacobs, P. A., Poncelet, G.,Ede.;Elsevier: Amsterdam, 1976; p 65. (15) Shibata, K.; Koyoura, K.; Kitagawa, J.; Samiyoahi, T.; Tanabe, K. Bull. Chem. SOC.Jpn. 1973,46, 2986. (16) Connell, G.; Dumesic, J. A. J. Catal. 1986, 101, 103. (17) Connell, G.; Dumesic, J. A. J . Catal. 1986, 102, 216. (18) Connell, G.; Dumesic, J. A. J . Catal. 1987, 105, 285. (19) Tauster, S. J.; Fung, S. C. J. Catal. 1978, 55, 29. (20) Tauster, S. J.; Fung,S.C.;Garten, R. L.J. Am. Chem. Soc. 1978, of Catalysts; Delmon,
105, 170.
(21) Vannice, M. A.; Garten, R. L. J. Catal. 1979,56, 236. (22) Vannice, M. A,; Twu, C. C.; Moon, S. H. J. Catal. 1983, 79, 70. (23) McVicker, G. B.; Ziemiak, J. J. J. Catal. 1985,95,473.
0743-7463/91/2407-1436$02.50/0 0 1991 American Chemical Society
Langmuir, Vol. 7, No. 7, 1991 1437
Adsorption of Acids on Composite Oxides
Theoretical Section The adsorption of a charged species onto a charged site may be described by a reaction of the form A + [active site] = A,, (1) Santacesaria et al.s have used the Langmuir equation to study the adsorption of chloroplatinic acid on alumina. In the present study, the adsorption of chloroplatinic and chloroiridic acid on A1203, TiO2, and TiOz/AlzOs was analyzed by using the Langmuir equation. According to the Langmuir equation, a t short adsorption times (t
