Langmuir 1994,10,4523-4529
4523
Interaction of Single and Multiple Layers of Copper Acetylacetonate Complexes with Silica Surfaces: An Infrared Diffuse Reflectance Study Mark B. Mitchell,*>? Vasumathi R. Chakravarthy,? and Mark G. White* Department of Chemistry, Clark Atlanta University, Atlanta, Georgia 30314-9987,and School of Chemical Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0100 Received November 1, 1993. I n Final Form: June 10, 1994@ The surface structure of copper acetylacetonate[Cu(acac)alon silica has been characterized using diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. Infrared spectroscopy has been used to determine the nature of the supported complex with regard to whether the complex is present on the surface as a single layer or as multiple layers. The copper acetylacetonate attaches to the silica surface from acetonitrile solution initially as a monolayer, with the subsequent formation of multiple layers as the impregnation solution concentration is increased. The integrated Kubelka-Munk intensities of the supported complexes vary linearly with the concentration of the complex on the support. In the 17001300 cm-l region, the infrared spectrum of the initial layer of the complex on silica is very similar to that of the complex observed in chloroform and bromoform, indicating that hydrogen bonding is the dominant mode of interaction with the surface. The spectrum of the complex in layers above the initial layer can be obtained by subtraction, and appears remarkably similar to that of crystalline Cu(acac)2. For species in the initial layer, the absorption intensities of vibrational modes with frequenciesof less than 1000 cm-' are vanishingly small, while the observed intensities for frequencies above 1200 cm-' are comparable to those in the second and third layers.
Introduction The use of organometallic complexes in general and metal carbonyls in particular for the preparation of supported catalysts has been the subject of numerous investigations and review papers over the past 20 years.' There has been a great deal of interest in the use of novel metal complex or organometallic precursors for the formulation of heterogeneous catalyst^.^^^ Much of the previous work has involved the use of transition metal carbonyl clusters as precursors. Rhs(C0)16,~-~ Fe3(C0)12,7-11O S ~ ( C O ) ~ ~ R ,U' ~~ -( ~C ~O ) Iand ~ , ~Cr(CO)6, ~ Mo(CO)~,and W(CO)621-26are all precursors which have been applied to silica and/or alumina, and much of the t Clark Atlanta University.
* Georgia Institute of Technology.
Abstract published inAdvance ACSAbstracts, August 1,1994. (1)See, for example, any of the following recent reviews: (a) Psaro, R.; Ugo, R. In Metal Clusters in Catalysis; Gates, B. C., Guczi, L., Knozinger, H., Eds.; Elsevier: Amsterdam, 1986;Studies in Surface Science and Catalysis, Vol. 29, p 427. (b) Hartley, F. R. Supported Metal Complexes; D. Reidel Publishing Co.: Boston, 1985. (c) Howe, R. F. In Tailored Metal Catalysts; Iwasawa, Y., Ed.; Reidel: Boston, 1986;p 141. (d) Yermakov, Yu. I.; Kuznetsov, B. N.; Zakharov, V. A. Catalysis by Supported Complexes; Elsevier: New York, 1981. (e) Brenner, A. In Metal Clusters; Moskovits, M., Ed.; Wiley: New York, 1986. (2)Yermakov, Yu. I.; Kuznetsov, B. N.; Zakharov, V. A. Catalysis by Supported Complexes; Elsevier: Amsterdam, 1981. (3)Uses of Metal Complexes in the Preparation of Catalysts, Proceedings of the Post Congress Symposium, Quebec, July 5, 6, 1988. (4)Bilhou, J. J.; Theolier, A.; Smith, A. K.; Basset, J. M. J . Mol. Catal. 1978,3,245. (5)Evans, J.; McNulty, G. S.J . Chem. Soc.,Dalton Trans. 1984,587. (6)Lawson, D.N.; Wilkinson, G. J . Chem. SOC.1966,1900. (7)Hugues, F.; Basset, J. M.; BenTaarit,Y.; Choplin,A.; Primet, M.; Rojas, D.; Smith, A. K. J . Am. Chem. SOC.1982,104,7020. (8)Iwasawa, Y.; Yamada, M.; Ogasawara, S.; Sato, Y.; Kuroda, H. Chem. Lett. 1983,621. (9)Hugues, F.; Smith, A. K; BenTarrit,Y.; Basset, J.M.; Commereuc, D.; Chauvin, Y.J . Chem. SOC.,Chem. Commun. 1980,68. (10)Yamamoto, S.;Lewis, R. M.; Hotta, H.; Kuroda, H. Inorg. Chem. 1989,28,3092. (11)Hanson, B. E.; Bergmeister, J. J. 111;Petty, J. T.; Connaway, M. C. Inorg. Chem. 1986,25,3089. (12)Choplin, A.; Besson, B.; D'Ornelas, L.; Sanchez-Delgado, R.; Basset, J . M.J . A m . Chem. SOC.1988,110, 2783. (13)Smith, A. K.; Besson, B.; Basset, J. M.; Psaro, R.; Fusi, A.; Ugo, R. J . Organomet. Chem. 1980,192,C31. @
characterization of these species has involved the use of infrared spectroscopy. Metal carbonyl clusters have been applied to magnesia2' and titania28as well. Aqueous impregnation technologies do not ensure uniform surface coverage in the preparation of metal oxide catalysts. The nonuniform surfaces show catalytic properties which depend upon the surface morphology. When a process demands high selectivity and activity, it becomes important to produce uniform surfaces. In this connection, we were motivated to explore catalyst preparation techniques which use stable metal chelate complexes as the precursors to a mixed metal oxide. Our early results with cationic complexes on silica (using the perchlorate salts) suggested that the cations formed in solution become attached firmly to the surface siloxide anions via ion exchange with surface s i l a n ~ l s ,with ~ ~ -the ~ ~ligands acting as spacers between the metal atoms. We have prepared (14)Psaro, R.; Ugo, R.; Zanderighi, G. M.; Besson, B.; Smith, A. K.; Basset, J. M. J . Organometal. Chem. 1981,213,215. (15)Basset, J. M.;Besson, B.; Choplin, A.; Hugues, F.; Leconte, M.; Rojas, D.; Smith,A. K.; Theolier,A.; Chauvin,Y.; Commereuc,D.; Psaro, R.; Ugo, R.; Zanderighi, G. M. InFundamentalResearch in Homogeneous Catalysis; Giongo, M., Graziani, M., Eds.; Plenum: New York, 1984; Vol. 4 p 19;see also references therein. (16)Barth, R.;Gates, B. C.; Zhao, Y.; Knozinger, H.; Hulse, J. J . Catal. 1983,82,147. (17)Venter, J. J.; Chen,A.; Vannice, M. A. J . Catal. 1989,117,170. (18)Venter, J. J.;Vannice, M. A. Carbon 1988,26,889. (19)Venter, J.J.;Vannice, M. A. J.Am. Chem. SOC.1987,109,6204. (20)Tholier, A.; Choplin,A.; D'Ornelas, L.; Basset, J. M.; Sourisseau, C.; Zanderighi, G. M.; Ugo, R.; Psaro, R. Polyhedron 1983,2,119. (21)Zecchina, A.; Platero, E. E.; Arean, C. 0.Inorg. Chem. 1988,27, 102. (22)Kazusaka, A.; Howe, R. F. J. Mol. Catal. 1980,9,183. (23)Kazusaka, A.;Howe, R. F. J . Mol. Catal. 1980,9,199. (24)Shirley, W. M.; Abdul-Manan, N.; Frye, J. S. Inorg. Chem. 1988, 27,3846. (25)Bunvell, R. L. Jr.; Brenner, A. J . Mol. Catal. 1976,I , 77. (26)Brenner, A.; Hucul, D. A.; Hardwick, S. J. Inorg. Chem. 1979, 18,1478. (27)Lamb, H. H.; Gates, B. C. J . Am. Chem. SOC.1986,108,81. (28)Deeba, M., Scott, J . P., Barth, R., and Gates, B. C. J . Catal. 1981,71,373. (29)Babb, K. H.; White, M. G. J . Catal. 1986,98,343. (30)Beckler, R.K.; White, M. G. J . Catal. 1986,102,252. (31)Beckler, R. K.; White, M. G. Langmuir 1987,3, 1074.
0743-746319412410-4523$04.50/0 0 1994 American Chemical Society
Mitchell et al.
4524 Langmuir, Vol. 10,No. 12, 1994
a number of other catalysts by attaching neutral metal acetylacetonates to silica surfaces without the benefit of ion exchange or ligand exchange reaction^.^^-^^ We were able to develop monolayer films of the metal complexes even at high concentrations. This strong interaction of the metal acetylacetonates with ceramic surfaces has been observed on alumina and titania for complexes of iron or ~ a n a d y l .This ~ ~ technique of applying complexes to surfaces shows promise as a general method to prepare well-dispersed metal oxides on ceramic supports. In the investigation reported here, a neutral metal acetylacetone complex, formed by a copper ion and 2,4pentanedione, is used to form a surface-supported metal complex. The P-diketone typically loses one hydrogen atom when it forms a bidentate complex with a metal ion and takes on a charge of -1. Metals such as Cu2+form a neutral complex with two acetylacetonate (acac)ligands. The general form of such complexes is Mn+(acac),. A schematic diagram for bis-square-planar complexes involving two acac ligands is shown below: H C 3 \
H
/ CH3
\ /
H
/M\
Supported copper catalysts have potential for a variety of applications including NO, reduction and SO2 abatement. We have previously characterized silica-supported Cu(acac)zusing a variety of techniques including elemental analysis, infrared spectroscopy, and thermal gravimetric a n a l y ~ i s . ~In ~ ,those ~ ’ earlier investigations, the elemental analysis of the copper complex supported on the silica showed the same C/Cu stoichiometry (10/1) as the unsupported copper acetylacetonate, indicating that the Cu(acac)zcomplex did not react with the solvent or the surface to lose a ligand. Diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy of these same samples revealed that the infrared spectra of the supported metal complexes appeared to be very similar to that of the parent complex, with variations in relative intensity among bands which depended on the weight loading of the sample. One band in particular, the overtone of the C-H out-of-plane bend, appeared to disappear for metal loadings of less than -3.0 d w t % (which corresponds to 1.42 x lo1* molecules of Cu(acac)2/m2of silica surface). By analogy with spectroscopic investigations of benzene on silica, we concluded that this band shifted to higher frequency for those complexes in intimate contact with the silica surface, and we were able to use the “reappearance”ofthis overtone absorption as a n indication of the formation of multilayer structures on the surface. The investigation reported here is a detailed study of the spectroscopy of this unique substrate/adsorbate sys(32)Beckler, R. K.; White, M. G. J. Catal. 1988,109,25. (33)Beckler, R. K.; White, M. G. J. Catal. 1988,110, 364. (34)Beckler, R. K.; White, M. G. J. Catal. 1988,112,157. (35)Beckler, R. K. Ph.D. Thesis, Georgia Institute of Technology, Atlanta, GA, 1987. (36)Kenvin, J. C.; White, M. G.; Mitchell, M. B. Langmuir 1991,7, 1198. (37)Kenvin, J. C.; White, M. G. J. Catal. 1991,130, 447. (38)Kenvin, J . C. Ph.D. Thesis, Georpia - Institute of Technology, __ Atlanta, GA, 1991. (39)van Ommen, J.; Ross, J. R. H.; Gellings, P. J.App1. Catal. 1983, R -.
(40) Kenvin, J . C.; White, M. G. J. CataE. 1992,135,81
tem, with the goal of determining the applicability of the Kubelka-Munk relation for the DRIFT spectra of such systems. There is little doubt of the utility of DRIFT spectroscopy for qualitative investigations of supported species as has been shown by a number of researcher^.^'-^^,^^-^^ However, the nature of this sample is different from that typically studied using DRIFT spectroscopy. Due to the formation of surface layers, Cu(acacIz on silica provides a unique test of the linearity of the Kubelka-Munk relati0n,5~9~~ the relationship typically used in diffuse reflectance spectroscopy to relate the observed signal to concentration, when applied to the study of heterogeneous catalytic materials and catalytic precursors. In this study, we have improved on earlier results obtained with the Cu(acac)z complex,36and are able to demonstrate the linearity of the spectral response with the concentration of the complex on the support.
Experimental Section The Cu(acac)z was purchased from Aldrich Chemicals and used as received. The silica (Cab-0-Sil)was purchased from Eastman Kodak (Grade M-5) and used as received. The surface area of the nonporous Cab-0-Si1 was nominally 200 mVg. The acetonitrile used for impregnation was purchased from Fischer Scientific and dried over molecular sieves. Metal analysis for the copper on silica was carried out using atomic absorption by Applied Technical Services of Marietta, GA. Impregnation of the silica with the Cu(acac)zcomplexby batch impregnation has been described previously.36A suitable amount of Cu(acac)z dissolved in dry acetonitrile is placed in a polyethylene bottle, the Cab-0-Si1 is added, and the mixture is stirred for approximately 24 h. After that time, the impregnated Cab0-Si1 is filtered from the solution and dried. The copper impregnation has been shown t o yield supported copper complexes with high yield. The concentrations of Cu(acac)z on the silica surface were determined by convertingthe measured copper content into the corresponding amount of Cu(acac)z,assuming that the complex remains intact upon adsorption on the silica surface, an assumption which has already been shown to be rea~onable,3~ and dividing by the available surface area of the Cab-0-Sil. For the DRIFT studies, we used an optical accessory from Harrick Scientific Corp., DRA-2CO. A Nicolet 510 with a new data station and a mercury-cadmium-telluride (MCT)detector was the FT-IR spectrometer used in these experiments. All powdered samples studied were sieved through a standard testing sieve with a mesh size of
