IXFHARED SPECTRUM OF CARBON MONOXIDE CHEMISORBED ON NICKELFILMS
studies using vacuuin-evaporation techniques. In contrast to our results, no bands were observed below 1980 c m - l for vacuuni-prepared Rh films. In the case of Pd and I’t, thc results are in somewhat better agreenient, but, again, there are differences noted in bands observed below 2000 c ~ i i . - ~ . The probleni of filii1 sintering represents the major difficulty in obtaining the adsorptioridesorption data that are necessary for a detailed characterization of
1195
surface species. A possible solution to this problem would involve cooling the substrate to a point where sintering could largely be eliminated. In spite of this difficulty, the technique does offer the distinct advantage of permitting one to obtain spectra of chemisorbed CO on clean, unsupported nietal samples. A detailed study of CO on nickel films, presented in the following paper,12will indicate the kind of results possible in a favorable case.
Infrared Spectrum of Carbon Monoxide Chemisorbed on Evaporated Nickel Films
by C. W. Garland, R. C. Lord, and P. F. Troiano Department of Chemistry and the Spectroscopy Laboratory, ~vassachusettsInstitute of Technology, Cambridge, Massachusetts (Received October 1 , 1964)
The infrared spectrum has been investigated for CO cheniisorbed on nickel films which were evaporated in the presence of CO gas. For these unsupported samples, two limiting types of spectra were observed, depending on the CO gas pressure. When nickel is evaporated in 2 nim. of CO, the film is a dispersed, patchy deposit of 65-A. particles. Films evaporated in 12 mni. of CO consist of 200-A. particles connected in an open, chain-like network. The corresponding spectra are discussed in terms of these structural differences.
Introduction and Method Infrared spectra of GO chemisorbed on nickel have been reported previously for both supported and evaporated films. The earliest study was that of Eischens, Pliskin, and Francis1 using a nonporous silica support. The saniple contained about 8% Ni by weight and a t full coverage showed infrared absorption bands a t 2074, 2041, 1926, and approximately 1870 c111.-l. A more detailed study is that by Yates and Garland2 on aluniina-supported Si. In their work both adsorption-desorption studies arid variations in the metal content of the samples were used in the characterization of the surface species. At full coverage on samples containing 10% S i by weight, bands were observed at 2082, 20.57, 2035, 1963, and 1915 c1n-l. Sainples con-
taining 25% Xi by weight showed a decrease in the relative intensity of the bands above 2000 cni.-I when compared with the 10% samples. The band at 2082 cni. -I showed the most pronounced loss in intensity. Studies of CO chemisorbed on evaporated Ni films have been much less detailed than the studies on supported films. Reflectance spectra of CO on vacuumevaporated Ni films have been recorded by Pickering and Eckstroi~i.~In their work a doublet a t 2050 and 2060 c m - l is reported, as well as a weak band a t (1) 60, (2) (3)
R. P. Eischens, S. A. Francis, and W. A. Pliskin. J . Phys. Chem., 194 (1956). J. T. Yates and C. W. Garland, ihid., 65, 617 (1961). H. L. Pickering and H. C. Eckstrom, ibid., 63, 512 (1959).
Volume 69, S u m b e r 4
April 1963
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2030 cm.-’. A similar reflectance study by Gardner and Petrucci4revealed only a single band a t 2060 cm. -1. Sardisco5 has recorded the transmission spectra of CO adsorbed on S i films prepared in a vacuum metallizer used to make specimens for electron microscopes. In this work a doublet at 2020 and 1980 cm.-l was reported. In view of the differences observed for silica- and alumina-supported samples and the conflicting data presented for. evaporated films, an extensive investigation has been carried out of the spectra of CO chemisorbed on high-area, unsupported nickel films. The experimental method is new and is described in detail in the preceding paper.6 The technique of film evaporation used for nickel is the same as that described for platinum, palladium, and rhodium. Nickel films were prepared by evaporating the metal (Baker Chemical, 99.9% pure) in the presence of CO a t pressures ranging from to 44 mm. Spectra were then recorded from 4000 to 300 cm. -1. I t was found, however, that films prepared at pressures below 1 mm. have poor infrared transmission properties and show only weak absorption spectra superimposed on interference fringes. These films are thus quite similar to those obtained by evaporation in argon.6 Films prepared a t higher pressures transmit between 40 and 60% at 2000 depending upon their mass, and show reasonably intense spectra of chemisorbed CO in the region from 2100 to 1600 em.-’. It should be noted that, for purposes of comparison, the values of the yo transmission of the metal films have arbitrarily been referred to 2000 cm.-I. These values were obtained by drawing in a linear background from 1500 to 2300 em.-’. These background lines were used to obtain the values of Io required to prepare the point-by-point plots of 1001/10 (shown in Figures 2 and 3). All the original spectra were recorded using fivefold expansion of the intensity scale on a Perkin-Elmer Model 521 spectrometer.
Experimental Results Preliminary Observations. With regard to the stability of the evaporated nickel films and the reproducibility of the spectra, it should be pointed out that the frequencies of the absorption band maxima have been observed to shift as a function of time in the spectra of many of the films prepared in this study. These shifts are most pronounced during the early stages of filii1 development, Le., iinniediately after evaporation. Spectra obtained from films evaporated in 2 min. of CO with initial transmissions above 60% at 2000 cm. are subject to the largest variations on standing. These changes involve not only a shift of the band The Journal of Physical Chemistry
C. W. GARLAND, R. C. LORD,AND P. F. TROIANO
maxima to lower frequencies but also marked changes in the relative intensities of the bands. Frequency shifts in the spectra are less pronounced in f i l m with lower initial transmissions, and the relative band intensities are unchanged on standing several hours. The band positions appear to reach nearly constant values after an “aging” period of about 30 min. The total shift in the band maxima amounts to something of the order of 5 cm. -I. In view of this, detailed study has been limited to the more stable f i l m having initial transmissions between 40 and 60% at 2000 cni. -I. Spectra presented below are typical of these “stabilized” films. Xi films evaporated in 12 mm. of CO appear to be relatively more stable than those evaporated in 2 mm. of CO, and they show no appreciable spectral changes on standing. They also differ from the films prepared at lower CO pressures in that the transmission throughout the infrared region increases more rapidly with increasing wave length. Sickel films prepared in 2 mm. and in 12 mm. of CO exhibit reproducibly different spectra, and both have been examined in detail. These samples will be termed 2-mm. films and 12-inm. films, respectively. Intermediate CO pressures give rise to spectra which show a gradual transition from spectra characteristic of the 2-mni. films to those of the 12-nim. films. Higher pressures merely serve to shift the typical 12-mni. spectra to lower frequencies by amounts of the order of 10 em. -I. Electron Microscope Studies. Electron micrographs of 2- and 12-mm. Ni films have been obtained in an effort to characterize the structure of these spectroscopically distinguishable metal deposits. Unfortunately, the method used to obtain the micrographs required that the films be exposed to air during the transfer to the microscope and that the films be evacuated to lo-* nim. prior to viewing.6 Since both exposure to air and pumping were found to cause pronounced transmission losses, marked changes in the structure of the deposit before and even during the viewing of the film were anticipated. In an effort to establish the effects of this “pretreatment,” films were exposed to air for periods ranging up to 4 days and were pumped on for periods up to 1 hr. prior to viewing. The results of these studies indicate that there are no major changes in the over-all film structure owing to this (4) R. A. Gardrier and
R. H . Petrucci. J . Am. Chem. Soc.. 8 2 , 5051
(1960).
(5) J. B. Sardisco, Perkin-Elmer Instrument S e w s , 15, No. 1, 13 (1963). (6) C. W.Garland, I
