Langmuir 1998, 14, 245-247
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Surface Chemistry of Colloidal Platinum: Vibrational Coupling in the Infrared Spectrum of CO Adsorbed on Colloidal Platinum in Liquid Dispersion Dominique de Caro and John S. Bradley* Max-Planck-Institut fu¨ r Kohlenforschung, Kaiser-Wilhelm-Platz 1, D 45470 Mu¨ lheim an der Ruhr, Germany Received July 7, 1997. In Final Form: October 28, 1997X Infrared spectra are reported of 12CO and 12CO/13CO mixtures adsorbed on colloidal platinum with a mean particle size of 15 Å, stabilized in liquid dispersion by polyvinylpyrrolidone. A shift to higher frequency with increasing coverage of 12CO is observed in the high-frequency (on-top) CO band, and for 12CO/13CO mixtures at maximum coverage a shift of frequencies of the two components of the on-top CO stretching mode is observed with changing 12CO/13CO ratio. These results are due to the presence of vibrational coupling between adsorbed CO molecules on the colloidal platinum particles. The correspondence between these observations and similar results on supported platinum particles and platinum single-crystal faces suggests that, even in the presence of solvent and dissolved polymer, the colloidal metal particles can react with CO to give adsorbed overlayers with significant similarity to those resulting from CO adsorption on clean metal surfaces.
The chemistry of transition metal colloids in the particle size range between 1 and 10 nm is of current interest.1 The nature of the surfaces of colloidal metal particles is important in the context of their liquid phase catalytic application, and it is preferable that the surfaces of colloidal metal catalysts be investigated in the liquid phase, that is to say in the same environment in which their catalytic properties are evaluated. Accordingly we have for some time been investigating the surface composition and structure of metals and alloys in liquid dispersions.2-7 The stabilization of liquid dispersions of colloidal particles against agglomeration implies the presence at the surface of a stabilizing agent, in addition to the solvent, and so the surface of the colloidal metal particle in liquid suspension is expected to be far from ‘clean’. Analogies between liquid dispersions of metal colloid particles and nominally clean metal surfaces are thus problematic. The question is to what degree the presence of extraneous material affects the chemistry of the colloidal metal surface. We report here some infrared spectroscopic results which imply that carbon monoxide can adsorb from solution on platinum particles in liquid colloidal suspension in a manner similar to that observed for platinum surfaces under rigorously controlled high vacuum conditions and that the presence of adsorbed solvent or stabilizing polymer does not prevent the assembly of vibrationally interacting domains of adsorbed CO. The use of adsorbed CO in vibrational spectroscopic X Abstract published in Advance ACS Abstracts, December 1, 1997.
(1) Bradley, J. S. In Clusters and Colloids; Schmid, G., Ed.; VCH: Weinheim, 1994; pp 459. (2) Bradley, J. S.; Millar, J. M.; Hill, E. W.; Melchior, M. J. Chem. Soc., Chem. Commun. 1990, 705. (3) Bradley, J. S.; Millar, J. M.; Hill, E. W.; Behal, S. J. Catal. 1991, 129, 530. (4) Bradley, J. S.; Millar, J. M.; Hill, E. W.; Behal, S.; Chaudret, B.; Duteil, A. Faraday Discuss. Chem. Soc. 1991, 92, 255. (5) Bradley, J. S.; Millar, J. M.; Hill, E. W. J. Am. Chem. Soc. 1991, 113, 4016. (6) Bradley, J. S.; Hill, E. W.; Chaudret, B.; Duteil, A. Langmuir 1995, 11, 693. (7) Bradley, J. S.; Via, G. H.; Bonneviot, L.; Hill, E. W. Chem. Mater. 1996, 8, 1895.
methods of surface analysis has played an important role in the elucidation of the surface chemistry of transition metal crystallites in the form of supported catalysts, often by analogy with surface spectroscopic studies on single crystals.8 Detailed studies have been reported in which the vibrational spectra of CO on terrace and step (defect) sites on stepped platinum single-crystal surfaces have been compared with spectra for CO on small supported metal particles,9-13 which would, as a consequence of their size and morphology, have a high incidence of edge and vertex (defect) sites. Colloidal metals are particularly amenable to IR spectroscopic study,1 since even at relatively high concentrations the suspensions do not scatter IR radiation, and the resulting spectra, which contain relatively high absorbance bands, can be handled with precision in a manner analogous to that for molecular solution spectra. CO adsorbed on colloidal metals has been investigated by several groups. Hydrosols of platinum, palladium,14-17 and rhodium18 and organosols of palladium,2,4,19,20 platinum,3,20-22 ruthenium,20 nickel,23 and palladium(8) Sheppard, N.; Nguyen, T. T. In Advances in Infrared and Raman Spectroscopy; Clarke, R. J., Hester, R. E., Eds.; Heyden and Son: London, 1978; p 106. (9) Hayden, B. E.; Kretzschmar, K.; Bradshaw, A. M.; Greenler, R. G. Surf. Sci. 1985, 149, 394. (10) Greenler, R. G.; Leibsle, F. M.; Sorbello, R. S. Phys. Rev. B 1985, 32, 8431. (11) Greenler, R. G.; Burch, K. D.; Kretzschmar, K.; Klauser, B.; Bradshaw, A. M.; Hayden, B. E. Surf. Sci. 1985, 152/3, 338. (12) Fox, S. G.; Browne, V. M.; Hollins, P. J. Electron Spectrosc. Relat. Phenom. 1990, 54/5, 749. (13) Brandt, R. K.; Hughes, M. R.; Bourget, L. P.; Truszkowska, K.; Greenler, R. G. Surf. Sci. 1993, 286, 15. (14) Mucalo, M. R.; Cooney, R. P. J. Chem. Soc., Chem. Commun. 1989. (15) Mucalo, M. R.; Cooney, R. P. Can. J. Chem. 1991, 69, 1649. (16) Mucalo, M. R.; Cooney, R. P. J. Chem. Soc., Faraday Trans. 1991, 87, 1221. (17) Mucalo, M. R.; Cooney, R. P. J. Colloid Interface Sci. 1992, 150, 486. (18) Mucalo, M. R.; Cooney, R. P. Chem. Mater. 1991, 3, 1081. (19) Bradley, J. S.; Hill, E. W.; Behal, S.; Klein, C.; Chaudret, B.; Duteil, A. Chem. Mater. 1992, 4, 1234. (20) Duteil, A.; Que´au, R.; Chaudret, B.; Mazel, R.; Roucau, C.; Bradley, J. S. Chem. Mater. 1993, 5, 341. (21) Lewis, L. N.; Lewis, N. J. Am. Chem. Soc. 1986, 108, 7228.
S0743-7463(97)00751-8 CCC: $15.00 © 1998 American Chemical Society Published on Web 01/20/1998
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copper alloys6,24,25 have been reported. The observation of IR absorptions due to adsorbed CO demonstrates that CO can occupy at least some of the surface of the colloidal metal particles even in the presence of high concentrations of organic molecules (i.e. neat liquid), but it is perhaps surprising that the spectra of CO adsorbed on colloidal metals in liquid dispersion, where the surface might be occluded or otherwise perturbed by solvent and stabilizer, often resemble those observed for CO adsorbed on nominally much cleaner supported metals under controlled atmosphere conditions, and this raises the prospect that more detailed studies might be equally as revealing for colloidal metal surfaces as they have been for metal crystallites. We report here the IR spectra of 12CO at partial coverage and of a range of 12CO/13CO mixtures, adsorbed on a 15 Å platinum organosol. These experiments were designed as analogs to those reported for CO on both supported and single-crystal metal surfaces with the object of obtaining partial coverage spectra9-13,26,27 which reveal vibrational coupling effects and cast light on the structure of the adsorbed CO overlayer and thus the underlying metal surface. Colloidal platinum was prepared by the reaction of solutions of Pt2(dba)320 (dba ) dibenzylideneacetone) in dichloromethane with hydrogen in the presence of poly(vinylpyrrolidone) (10 000 MW). This reaction yields a platinum sol with mean particle size ca. 15 Å, as determined by transmission electron microscopy. As is the case for all colloidal metal preparations, it cannot be asserted a priori that the particles are monocrystalline nor that their surfaces represent simple truncated lattice structures and certainly not that their surface chemistry will resemble that of clean single-crystal surfaces. It was not clear from electron micrographs whether the particles were well formed with well structured (faceted) surfaces, and in the absence of evidence to the contrary a range of surface morphologies might be expected. If the particles had an ideal cuboctahedral morphology and a close-packed structure, this particle size would correspond to clusters of ca. 200 atoms, with ca. 150 surface atoms. Spectra were obtained using a CaF2 window IR flow cell and an external MCT detector coupled to the external optical port of a Nicolet Magna 550 FTIR spectrometer, as previously reported.23 IR spectra of adsorbed 12CO at slowly increasing coverage were measured at 8 cm-1 resolution during the slow addition of 12CO (saturated with solvent vapor) to 20 mL of the sol in dichloromethane (ca. 5 mg‚mL-1 of metal) in a glass reactor initially under an argon atmosphere. Spectra of adsorbed 12CO + 13CO at slowly increasing 12CO/13CO ratios were measured during the slow addition of 12CO (saturated with solvent vapor) to the sol in dichloromethane (ca. 5 mg‚mL-1 of metal) which had been previously saturated with 13CO to constant absorbance. During acquisition of spectra the colloid suspension was circulated from the reactor through the flow cell at a rate of ca. 300 mL‚min-1 and the equilibration of the system was followed to completion. We chose conditions under which maximum coverage of the metal particles with CO or complete exchange of 12CO for 13CO was complete in less than 30 min, and spectra (22) Mucalo, M. R.; Cooney, R. P. J. Chem. Soc., Faraday Trans. 1991, 87, 3779. (23) de Caro, D.; Bradley, J. S. Langmuir 1997, 13, 3067. (24) Bradley, J. S.; Hill, E. W.; Klein, C.; Chaudret, B.; Duteil, A. Chem. Mater. 1993, 5, 254. (25) Bradley, J. S.; Via, G. H.; Bonneviot, L.; Hill, E. W. Chem. Mater. 1996, 8, 1895. (26) Eischens, R. P.; Pliskin, W. P. Adv. Catal. 1958, 10, 1. (27) Crossley, A.; King, D. A. Surf. Sci. 1977, 68, 528.
Letters
Figure 1. Infrared spectra of CO adsorbed on 15 Å Pt/PVP in dichloromethane (ca. 5 mg of Pt‚mL-1). Path length, 0.5 mm; resolution, 8 cm-1; total elapsed time, 1 h.
Figure 2. 13CO/12CO adsorbed on 15 Å Pt/PVP in dichloromethane (ca. 5 mg‚mL-1). Spectra were recorded during replacement of 13CO with 12CO in sols originally saturated with 13CO. Path length, 0.5 mm; resolution, 8 cm-1; total elapsed time, 30 min.
were averaged for time intervals of 1.17 s, the initial background absorbances of the sol before CO addition were subtracted, and the resulting spectra of adsorbed (and dissolved) CO were displayed in a manner which depended on the rate of change of the spectrum. Absorbance spectra obtained for CO (at natural isotopic abundances) on the 15 Å Pt sol are shown in Figure 1. At the lowest coverage recorded, 2 s after the beginning of CO addition, a weak band at 2023 cm-1 is observed, which progressively shifts to 2046 cm-1 at maximum coverage. This is assigned to vibrational transitions of adsorbed CO in terminal (‘ontop’) sites. A weak band near 1860 cm-1 (at maximum coverage) is also seen in the region normally associated with bridging sites. This is similar to spectra previously reported for colloidal platinum.16,20,21 (A weak band of variable low intensity at 2090 cm-1 is observed due to a small number of oxidized Pt sites). Because of the low intensity of the bridged CO band (
