© Copyright 1997 American Chemical Society
JUNE 11, 1997 VOLUME 13, NUMBER 12
Letters Surface Spectroscopic Study of Carbon Monoxide Adsorption on Nanoscale Nickel Colloids Prepared from a Zerovalent Organometallic Precursor 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 December 6, 1996. In Final Form: March 18, 1997X
Solutions of bis(cyclooctadiene)nickel react in the presence of polyvinylpyrrolidone in dichloromethane to give stable colloidal suspensions of nickel with a mean particle size of 20 Å (10 wt % Ni/PVP) and 30 Å (15 wt % Ni/PVP). The PVP-stabilized metal particles in organic suspension adsorb CO in both bridged and linear geometries. Spectra obtained at partial coverages of CO show a coverage dependent frequency shift, which is interpreted in terms of vibrational coupling between adsorbed CO molecules, consistent with an ordered surface for the colloidal nickel particles.
Nanoscale metal particles are of great interest in the fields of catalysis and material science. In studies of the physical and chemical properties of metals in the form of nanoparticles, impurities resulting either from the preparative method used (such as metal borides from BH4reduction of metal salts1) or from surface oxidation of the highly reactive nanoparticles can have important effects. In addition to changes in surface chemistry and thus catalytic properties caused by surface or lattice impurities, for the ferromagnetic metals, Fe, Co, and Ni, also the magnetic properties will be sensitive to the presence of such impurities, and so the synthesis of nonagglomerated nanoparticles of controlled size and composition is an important challenge. The techniques of metal colloid chemistry have been increasingly applied to the preparation of metals in highly dispersed form.2 The use of solutions of molecular precursors as starting materials and mild chemical treatment * Author to whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, April 15, 1997. (1) Ravet, I.; Gourge, A.; Gabelica, Z.; Nagy, J. B. In Proceedings of the VIII International Congress on Catalysis; Verlag-Chemie: Berlin, 1984; pp 871-8. (2) Bradley, J. S. In Clusters and Colloids; Schmid, G., Ed.; VCH: Weinheim, 1994; p 459.
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for the generation of metal particles lends itself to careful control of synthesis parameters and the prospect of control of the composition, size, and morphology of the resulting particles. Several methods have been described in the literature for the synthesis of zerovalent nickel particles in colloidal form. Bo¨nnemann and co-workers have developed a general method for the preparation of surfactant-stabilized organic solutions of colloidal metals from groups 6-11 of the periodic table3,4 by reduction of the metal salts with hydridic reducing agents in the presence of surfactant stabilizers. Reetz and co-workers report the electrochemical synthesis of colloidal nickel stabilized with R4N+X-.5 A physical method for the preparation of highly dispersed nickel, this time in the form of an ultrafine powder, involves the condensation of nickel vapor into a cold hydrocarbon, resulting in the formation of ca. 2 nm particles contaminated with carbon.6,7 (3) Bo¨nnemann, H.; Brinkmann, R.; Ko¨ppler, R.; Neiteler, P.; Richter, J. Adv. Mater. 1992, 4, 804. (4) Bo¨nnemann, H.; Brijoux, W. In Active Metals; Fu¨rstner, A., Ed.; VCH: Weinheim, 1995; p 339. (5) Reetz, M. T.; Helbig, W. J. Am. Chem. Soc. 1994, 116, 7410. (6) Klabunde, K. J.; Tanaka, Y. J. Mol. Catal. 1983, 21, 57. (7) Klabunde, K. J.; Cardenas-Trevino, G. In Active Metals; Fu¨rstner, A., Ed.; VCH: Weinheim, 1995; p 237.
© 1997 American Chemical Society
3068 Langmuir, Vol. 13, No. 12, 1997
We have recently shown that metal particles in the nanoscale size range may be prepared in stable colloidal form by the mild chemical decomposition of low valent complexes of the metals in the presence of a dissolved stabilizer. Thus (cyclooctadiene)(cyclooctatetraene)ruthenium(0) reacts with hydrogen at 25 °C in solutions of polyvinylpyrrolidone (PVP) to give ca. 12 Å colloidal ruthenium.8,9 Similarly zerovalent complexes of palladium and platinum with dibenzylideneacetone9,10 react with hydrogen or carbon dioxide in polymer solutions to give highly dispersed Pd and Pt colloids. This method is promising for the preparation of metal particles uncontaminated by the byproducts, such as halide ions, from reductive synthesis from metal salts, or the formation of borides in borohydride reductions of metal salts. Surface adsorption of displaced organic ligands could, of course, still occur in the method we use, but this would not seem to be an added complication, since we are already dealing with organosols protected by organic polymers. We report here the preparation of colloidal nickel by the room temperature decomposition of bis(cyclooctadiene)nickel in dichloromethane solutions in the presence of PVP. In a typical preparation (10 wt % Ni/PVP) a yellow solution of Ni(COD)2 (230 mg) in CH2Cl2 (50 mL) was stirred in the presence of PVP (10 000 MW, 500 mg) under argon at room temperature. After 10 min the color of the mixture had changed to a dark brown-black with the decomposition of the starting complex. Stirring was continued for a further 15 h to ensure complete decomposition. Evaporation of the reaction mixture gave a black solid with solubility properties similar to those of the polymer. Transmission electron microscopic analysis showed that the product comprised colloidal nickel with particle sizes which depended on the metal loading. At 10 wt % Ni/PVP an approximate mean diameter of 20 Å was observed, and at 15 wt % the mean diameter was ca. 30 Å. (The particle images obtained were of insufficient clarity to show whether or not the particles were regular with well formed faces.) A similar dependence of particle size on metal/polymer ratio was reported in the analogous preparation of Ru/nitrocellulose colloids from Ru(COD)(COT).9 The nature of the surfaces of colloidal metal particles is important in the context of their catalytic application. In particular the stabilization of the particles against agglomeration implies the presence at the surface of a stabilizing agent, and the possibility of adsorption of byproducts from the preparation procedure cannot be ignored, as mentioned above. In addition, the potential oxidation of the zerovalent metal surface during routine nominally anaerobic manipulation cannot be ignored. The surface analysis of polymer-stabilized colloidal metals presents many problems, but comparative studies with surface vibrational spectroscopy of adsorbed CO on metal single crystals and supported crystallites have proved of use in forming conclusions about the surfaces of colloidal metals.8,10-12 In the present case we recorded the IR spectra of adsorbed 12CO under conditions of increasing coverage to make a preliminary assessment of the surface of the colloidal nickel particles. Spectra were obtained using an external IR flow cell (8) Bradley, J. S.; Millar, J. M.; Hill, E. W.; Behal, S.; Chaudret, B.; Duteil, A. Faraday Discuss. Chem. Soc. 1991, 92, 255. (9) Duteil, A.; Que´au, R.; Chaudret, B. M.; Roucau, C.; Bradley, J. S. Chem. Mater. 1993, 5, 341. (10) Bradley, J. S.; Hill, E. W.; Behal, S.; Klein, C.; Chaudret, B.; Duteil, A. Chem. Mater. 1992, 4, 1234. (11) Bradley, J. S.; Hill, E. W.; Klein, C.; Chaudret, B.; Duteil, A. Chem. Mater. 1993, 5, 254. (12) Bradley, J. S.; Hill, E. W.; Chaudret, B.; Duteil, A. Langmuir 1995, 11, 693.
Letters
Figure 1. Infrared spectra of 12CO adsorbed on 15 wt % Ni/ PVP (ca. 30 Å) in dichloromethane (ca. 5 mg‚mL-1 Ni), during slow addition of CO (1 bar). Resolution, 8 cm-1; path length, 0.5 mm. Spectra displayed are after 0.02, 0.2, 0.5, 1.0, 1.5, 2, 4, 6, 10, 15, 16, 20, and 24 min after beginning CO addition.
and a remote MCT detector coupled to the external optical port of a Nicolet Magna 550 FTIR spectrometer. IR spectra of adsorbed CO in colloidal suspensions in dichloromethane (ca. 5 mg‚mL-1 metal) were measured during the slow addition of a stream of 12CO (saturated with solvent) to 20 mL of the sol while it was circulating through the flow cell at a rate of ca. 300 mL‚min-1. The equilibration of the system was followed to completion, a process which is slowed to a rate convenient for acquisition of data by the slow addition of the CO coupled with poor gas-liquid diffusion achieved in the system, where agitation of the colloidal suspension in the presence of CO was achieved only by recirculation of the liquid through the flow cell. We chose conditions under which saturation of the metal particles with CO usually took less than 30 min. Spectra were averaged for time intervals of 1.17 s, the initial background absorbances of the sol were subtracted, and the spectra were selected for display in a manner which depended on the rate of change of the spectrum. IR spectra of adsorbed CO on a 10 wt % Ni/PVP sol are shown in Figure 1. The spectra obtained for the larger 15 wt % sol were indistinguishable from those observed for the 10 wt % sol. The spectra comprise three bands. The highest frequency band, at ca. 2140 cm-1 is due to dissolved CO, and its intensity increases over the course of the experiment. The reproducibility of spectral acquisition can be judged from the constancy of the frequency of this band. The band at ca. 2030 cm-1 in the spectrum after saturation is assigned to linear CO bound to the metal surface, and the lowest frequency band at 1915 cm-1 is assigned to doubly bridged CO. These assignments are made with reference to data on CO on both single-crystal Ni and supported Ni crystallites.13-20 We further interpret these spectra in terms of the formation of fully reduced, well ordered, relatively smooth metal particles, by comparison with published data on the analogous supported Ni systems.15,19,20 Three features are of interest in comparing these spectra with those reported for supported Ni particles. First, we observe no trace of a high-frequency band at 2195 cm-1, which has (13) Sheppard, N.; Nguyen, T. T. In Advances in Infrared and Raman Spectroscopy; Clark, R. J. H., Hester, R. E., Eds.; Heyden: London, 1978; Vol. 5, p 67. (14) Tracy, J. C. J. Chem. Phys. 1972, 56, 2736. (15) Primet, M.; Dalmon, J. A.; Martin, G. A. J. Catal. 1977, 46, 25. (16) Andersson, S. Solid State Commun. 1977, 21, 75. (17) Bertolini, J. C.; Dalmei-Imelik, G.; Rousseau, J. Surf. Sci. 1977, 68, 539. (18) Bertolini, J. C.; Tardy, G. Surf. Sci. 1981, 102, 131. (19) Blackmond, D. G.; Ko, E. I. J. Catal. 1985, 96, 210. (20) Blackmond, D. G.; Ko, E. I. J. Catal. 1985, 95, 343.
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been reported in other systems and attributed to incompletely reduced nickel,15 and we would expect a similar high-frequency absorption in the event that oxidized nickel sites were present. The absence of such a band confirms that the CO adsorbing surface is in the zero oxidation state. The absence of low coordination number metal atoms (defect sites) on otherwise regular faces of the colloidal particles is evidenced by a second comparison with supported Ni. In agreement with Blackmond and Ko we observe absorptions for bridged CO and for linear CO, but those authors observed a second high-frequency band near 2080 cm-1 assigned to polycarbonyl Ni(CO)x sites.19 For supported Ni particles with sizes comparable to those of the current study the band was of similar intensity to the linear CO absorption. The presence of this band and its variable intensity were attributed to surface inhomogeneity, resulting from the variable abundance of low coordinate metal atoms in defect sites on the supported crystallites. We observe no absorption band attributable to such defect sites and conclude that the colloidal metal surfaces are regular. Support for this conclusion is found in the fact that we observe no trace of Ni(CO)4 formation even after prolonged addition of CO at 25 °C. Ni(CO)4 formation might be more facile at low coordination number Ni atoms on irregular crystallites, and Blackmond and Ko reported that Ni(CO)4 formed readily when CO adsorbed on certain supported Ni catalysts.20 The formation of Ni(CO)4 at low levels would be easy to detect in solution under these conditions, where a sharp band at ca. 2055 cm-1 would be seen. Both of these observations support the suggestion that in the low-temperature colloidal preparation even small particles grow in a more uniform manner than classical supported nickel catalysts, resulting in relatively smooth surfaces.
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Poorly crystalline colloidal cobalt prepared from cobalt vapor has been shown to react readily with CO to form Co2(CO)8.21 The order we impute to the surface of the Ni colloidal particles is supported by a third aspect of the IR spectra in Figure 1. We observe a coverage dependent frequency shift for both the terminal and bridged CO bands. In the case of the 20 Å sample the linear CO band shifts from 2010 to 2033 cm-1 over the coverage range observed, and the bridged CO absorption shifts from 1865 to 1910 cm-1. The latter shift is difficult to measure precisely, since at the lowest coverages there may be occupancy of triply bridged sites at low frequencies. For the 30 Å particles the corresponding shifts were from 2000 to 2037 cm-1 and from 1865 to 1915 cm-1. These phenomena are the subject of further investigation, for these Ni sols and for analogous Pd and Pt sols, but here we briefly interpret this shift, which is probably due to vibrational coupling between CO molecules in ordered arrays on the metal particle surfaces,22-24 as further evidence for a relatively ordered structure for the surfaces of the colloidal nickel particles. Acknowledgment. A postdoctoral fellowship from the Max-Planck-Gesellschaft is gratefully acknowledged (D. de C.). A sample of Ni(COD)2, kindly provided by the group of Prof. G. Wilke, and helpful discussions with Prof. P. Jolly are also gratefully acknowledged. LA9620824 (21) Hill, E. W.; Bradley, J. S. Unpublished observations. (22) Eischens, R. P.; Pliskin, W. P. Adv. Catal. 1958, 10, 1. (23) Crossley, A.; King, D. A. Surf. Sci. 1977, 68, 528. (24) Moskovits, M.; Hulse, J. E. Surf. Sci. 1978, 78, 397.
