Masatoki Ito and Wataru Suetaka
1190 (4) M. Spoliti, G. Marini, S. Nanzionte Cesaro, and G. DeMaria, J. Mol. Struct., 19, 563 (1973). (5) P. S. Skell and J. E. Glrard, J. Am. Chem. Soc., 94, 5518 (1972). (6)"Handbook of Chemistry and Physics", 46th ed, Chemical Rubber Publishing Company, Cleveland, Ohio, 1965 p D96. (7) S.E. Novick, P. Davies, S. J. Harris, and W. Klemperer, J. Chem. Phys.,
59,2273 (1973). (8)C. E. Moore, Natl. Bur. Stand. US. Circ., No. 467 (1949).
(9)B. Meyer, "Low Temperature Spectroscopy", American Elsevier. New York, N.Y., 1971. (IO) W. Weltner, Jr., Adv. High Temp. Chem., 2, 85 (1970). (1 1) S.W. Benson and G. N. Stokes, J. Am. Chem. SOC., 89, 2525 (1967). (12)R. D. Present, "Kinetic Theory of Gases", McGraw-Hill, New York, N.Y.. 1958. (13)R. J. Rogers, H. L. Mitchell, Y. Fujirvara, and G. M. Whitesides, J. Org. Chem., 39,857 (1974).
Molecular Orientation of Chemisorbed Formates and Nitric Monoxides on Evaporated Metal Surfaces by Infrared Reflectance Spectroscopy Masatoki Ito* and Wataru Suetaka Laboratory of lnterface Science of Metals, Faculty of Engineering, Tohoku University, Aramaki, Sendai, 980, Japan (Received September 18, 1974: Revised Manuscript Received December 9, 1974) Publication costs assisted by Tohoku University
The adsorption of formic acid on evaporated copper and aluminum surfaces was studied using an infrared reflection method. The molecular orientation of the chemisorbed species on metal surfaces was successfully observed by this technique. When formic acid vapor was introduced over copper or aluminum surfaces, formate ions were observed. The reflection spectra of formate ions on copper showed the 1360-cm-l band of the symmetric OCO stretching vibration but not the 1600-cm-' antisymmetric OCO band; whereas the reverse was true of the transmission spectra of smooth copper films deposited on the polished KBr plates. These results lead to the conclusion that the chemisorbed formate molecules are oriented almost vertically on the smooth copper surface. Nitric monoxide and isoamyl nitrite adsorbed on evaporated copper, nickel, and iron surfaces were also investigated and discussed.
Introduction minum surface, but nitric monoxide, isoamyl nitrite (CH&H(CH3)CH&H2ONO) adsorbed on copper, iron, Infrared spectroscopic measurements of chemisorbed and nickel surfaces will be mentioned briefly in order to molecules on metal surfaces have been widely studied by elucidate the chemisorption to the metal surfaces. infrared transmission through thin evaporated fi1ms.l However, recent investigations on chemisorption on the Experimental Section metal surface using multiple reflection technique have indicated the feasibility of using the reflection m e t h ~ d . ~ B The whole apparatus is shown schematically in Figure 1. The advantage of this method can be seen in its application The infrared radiation was focused onto the sample mirror to bulk metal surfaces, and furthermore the great advan(S.M.) a t an incidence angle of 85'. The reflected radiation was collected by a spherical mirror (M4), and entered into tage of this method is its ability to determine orientations the spectrometer entrance slit of Jasco IR-G grating specof adsorbed species in relation to the plane of the metal trometer. The stainless-steel high vacuum cell equipped surface. Since Francis and Ellison first applied this method with the infrared transmitting KBr windows (64 X 12 mm) to the investigation of monolayers on a metal mirror4 and is also shown in this figure. The cell was pumped using an Greenler developed its theoretical and practical bases,5 this oil diffusion vacuum system. The ultimate pressure of 2.0 valuable technique has been considered as one of the powx Torr was measured with an ionization gauge. As erful tools for chemisorption study. substrates we used smooth quartz glass plates on which In all reflection spectra so far reported, however, the obgold films were previously deposited in another vacuum served chemisorbed species on bulk metal surfaces were evaporator. Their dimensions were 30 X 50 mm. After confined to carbon monoxide. Although chemisorbed forevaporating the metal to form the substrate, background mates have been investigated by many workers with transmission method,6,7 no direct evidence indicating the molecspectra were taken and stored in a spectrum computer JEC-5 (Japan Electronic Co.). The formic acid vapors were ular orientation of this species on bulk metal surfaces has effused in the cell by breaking the ampoule using a breaker. been obtained. The pressure of the vapor was about 1 Torr. Ten minutes The result to be reported here about formates on copper, later, the gas was evacuated to Torr, and the reflection a part of which was briefly communicated: is the first inspectra were measured. The spectrum was scanned at a frared reflection observation of oriented chemisorbed molrate of 0.8 cm-l sec-l. The differences between the backecules on metal surfaces. This paper is mainly concerned ground and the sample spectra were calculated a t 0.5-cm-' with the spectra of formates adsorbed on copper and aluThe Journal of Physical Chemistry, Voi. 79, No. 12, 1975
AdsorpHon of Formic Acld on Metal Swfaces
Flgure 1. Schematic view of the whole instrument and a drawing of the h evacuum cell: (a) KBr window: (b) shmer: (c) heater: (d) sample minu (S.M.): (e) thermocouple: (1) ampoule of gas source; (9) breaker: (h) tungsten filament:(i) viewing port (S)infrared source: (MI. M3, Ms)minors: (M2. Ma) spherical minors. frequency intervals over 1650-1250-cm-' region. Tenfold scale expansion was employed and multiscannings (eight times in ordinary cases) were adopted. The point-by-point digital difference data were plotted on a chart recorder. The signal (1-3% in transmittance) was sufficiently measurable in comparison with the noise level of 0.5%. The signal enhancement proeedure and repetitive scanning of the spectrum and computer averaging made it possible to obtain sufficiently measurable signals from a monomolecular layer. The transmission metal films, used in the transmission measurement, were evaporated on 30 X 30 mm KBr plates in such a way that both sides of the plates were coated with a film ea. 100 A thick. One plate was used in each measurement. The formic acid used in this work was obtained from Wako Chemical Co. and the isoamyl nitrite was from Nakarai Chemical Co. which was distilled three times. Pure metals of copper (99.999%). aluminum (99.99%), and nickel (99.998%) were used.
Result The transmission spectrum of the Cu(I1) formate tetrahydrate shows two strong absorption bands in the 16501250-cm-' region due to OCO stretching vibration^.^ The spectra of chemisorbed formate on copper show that the strongest absorption band at 1600 cm-l disappeared completely and the band a t 1360 cm-' occurred as shown in Figure 2e. On the other hand, in the spectra on aluminum both absorption bands appeared just like the transmission spectrum. The similar spectra were obtained for deuterated formate (dotted lines). These bands were relatively broad, having a half-width of about 80 cm-'. On deuteration, the band maxima shifted to lower frequencies (1590, 1355 em-'). These bands remained undiminished upon evacuation below lo-' Torr a t room temperature. The reflectivity decrease a t these bands is 1-3%. Figure 2 also shows the transmission spectra of the adsorbed formates on thin deposited copper film (100 A) on
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,100
,moo ,,oo
1.00
lo..nun8..
em-'
,300
Flgure 2. Transmissbn spectra of adsorbed HCOO- on a copper deposiled surface (a.b) and reflection spectra of adsorbed HCOO- and
DCOO- on copper or aluminum surlaces (c-f): (a) on wellground KBr plate: (b) on cleaved KBr plate: (c) DCOO- on aluminum: (d) HCOO- on aluminum: (e) DCOO- on copper: (1) HCOO- on copper.
Ib)
Replica electron micrographs of vacuum deposited metal surfaces: (a)on copper X10.000 (b) on aluminum X10.000. Flgure 3.
KBr plates. The solid line (Figure 2a) shows the spectrum of the well-ground specular surface and dotted line (Figure 2b) shows the spectrum from the cleaved surface which is assumed to be very rough. On the smooth surface only the 1 6 0 0 - ~ m -band ~ appeared and the band near 1350 cm-' disappeared, but the latter band appeared when the cleaved KBr plate was used as the substrate. As these differences in the spectrum may arise from surface roughness, some of the evaporated metal films used in this work were examined by electron microscopy. Figure 3a is the replica electron micrograph from the copper surface TbJaMlolPhyricelChsmirhy. Vol. 79. No. 12. 1975
Masatoki Ito and Wataru Suetaka
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z'
tP
E!
I
v)
1900 Wavenurnber
I800 c rn-'
1700
1900 1800
Figure 4. Reflection spectra of adsorbed isoamyl nitrite (a) on copper, (b) on nickel, and (c) on iron. Reflection spectrum of adsorbed nitric monoxide (d) on iron. evaporated a t room temperature, whereas Figure 3b is from the aluminum surface. The 2-3% increase in reflectance was also observed just after the evaporation of copper, corresponding to sintering. These observed absorption bands were decreased in intensity by raising the substrate temperature to near E O 0 . Figure 4 shows the reflectance spectra of chemisorbed nitric monoxide on evaporated iron surface and chemisorbed isoamyl nitrite on evaporated copper, nickel, and iron surfaces. The spectrum of nitric monoxide on iron shows a weak band near 1825 cm-l (Figure 4d). The reflectance spectra of chemisorbed isoamyl nitrite show absorption bands at 1850 (on copper, Figure 4a) and 1750 and 1820 cm-l (on nickel and iron, Figure 4b,c). These spectra are in fair agreement with those from transmission studies on small particles of iron and iron oxide dispersed on silica, which has been already reported by Poling and Eischens.lo
Discussion Chemisorption of Formic Acid. The interaction between the oscillating molecular dipole in the adsorbed molecules and the electric vector of the incident radiation is highly related to the incident angle and the states of polarization of the beam as shown in Figure 5 . The amplitude ( A ) of the standing wave a t the metal surface increases with incident angle 0 and reaches its maximum a t a large incident angle, which changes depending upon the optical constants of metal. The parallel component (p) of the incident radiation can produce a t large incidence angles the strong resultant standing wave field as a result of interference between the incident and the reflected wave as shown in Figure 5b. The perpendicular component (s) cannot make the sizeable standing electric field because the phase change of the incident and reflected wave is about 180° and nearly complete cancellation occurs (Figure 5a). Accordingly the electric field a t the metal surface is predominantly perpendicular to the mirror surface, therefore, it can interact with the vibrational mode for which the effective component of the dipole moment change is perpendicular to the surface. The formate ion has strong infrared absorption bands a t 1360 and 1590 cm-l, and these bands are assigned to the symmetric OCO stretching vibration, in which the dipole moment change is parallel to the C2 axis of the ion, and the antisymmetric vibration, in which the change is perpendicular, respectively (Figure 6a,b). Therefore it can be said that the dipole moment change parallel to the C2 axis of the ion, namely, the symmetric OCO stretching vibration, does interact with the standing wave field and results in a The Journal of Physical Chemistry, Vol. 79,No. 12, 1975
(a)
(b)
Figure 5. The explanation for high-sensitivity reflection spectroscopy: (p) parallel component of radiation; (s)perpendicular component of radiation; ( A ) amplitude of standing wave field; (8) incident angle.
(C)
(d)
Flgure 6. Adsorptive form of formates on metal: (a) antisymmetric stretching vibrational mode; (b) symmetric stretching vibrational mode. Schematic view of adsorptive form: (c) on smooth copper surface; (d) on porous aluminum surface.
strong band in the reflection spectra, provided the Cz axes of the adsorbed species are normal to the surface. The antisymmetrical mode of vibration, on the contrary, cannot interact with the infrared radiation and the corresponding band does not appear in the reflection spectra. I t has been said that the evaporated copper surface is completely sintered at room temperature and its whole surface area corresponds to its geographical areaall On the other hand, the evaporated aluminum is porous with a very rough surface. The electron micrographs (Figure 3) of these metals clearly show the change in surface roughness. It can easily be seen from the photographs that the copper surface is very smooth whereas the aluminum surface is very rough. I t has been already suggested in a transmission study12 that the formate molecules adsorb on the metal surface by oxygen atoms and the C2 axis of the molecule becomes perpendicular to the surfaces. The missing 1 5 9 0 - ~ r n -OCO ~ antisymmetric stretching vibration band in the reflection spectra can be explained by the fact that the dipole moment change of the antisymmetric vibration becomes parallel to the mirror surface and it cannot interact with the electric field perpendicular to the surface. Similarly, the absence of the 1360-cm-l OCO symmetric stretching band in the transmission spectra (Figure 2a) indicates that the dipole moment change in the vibration becomes perpendicular to the surface and it cannot interact with the electric field parallel to the surface.
Adsorption of Formic Acid on Metal Surfaces Figure 6c,d shows the orientation of the adsorbed molecules to the microscopic metal surface. On the copper surface, all of the formate ions oriented vertically to the smooth copper surface and on the aluminum surface the adsorbed formate ions appear in a relatively random orientation to the macroscopic metal surface due to the surface roughness. The above-mentioned transmission data on smooth and rough KBr plates confirm these features of orientation on the metal surfaces. Chemisorption of Nitric Monoxide and Isoamyl Nitrite. The infrared study of butyl nitrite and nitric monoxide on iron and iron oxide has been investigated by Poling and Eischens using a transmission method.1° Both nitric monoxide and butyl nitrite retard the corrosion of iron and steel probably by the same mechanism, because butyl nitrite dissociates during chemisorption and the fragment appears to be nitric monoxide. They reported that the nitric monoxide on a preoxidized iron surface produces a band at 1820 cm-l which is assigned to Fe2+:N=O+, and oxidative chemisorption on initially reduced iron produces a band at 1750 cm-l which is assigned to Fe-0-N=O. The reflectance spectrum of nitric monoxide on the iron surface shows a band at 1825 cm-l, which is in good accord with the transmission spectra of chemisorbed nitric monoxide on the nickel surface.13 Recently, Calahorra and Folman studied the adsorption of nitric monoxide on germanium.14 They ascribed two peaks a t 1853 and 1750 cm-l to the NO dimer formed on the surface and assumed the higher frequency is that of the symmetric mode, whereas the lower one is due to the asymmetric mode of the NO bond. In the present investigation, the reflectance spectra of isoamyl nitrite on evaporated nickel and iron showed two absorption bands at 1820 and 1750 cm-l, and showed a band at 1850 cm-l on copper. These results suggest that the nitric monoxide, formed as fragments of the isoamyl nitrite, adsorbs perpendicularly on the metal surfaces because the species adsorbed parallel to the smooth metal surface give rise to extremely weak absorptions as mentioned above.
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Our samples were probably contaminated by the residual gas or the diffusion pump oil under our vacuum system near 2 X Torr. The ultrahigh vacuum system is essential to investigate the accurate molecular orientation on atomically clean metal surfaces, which is now in progress in our laboratory. The reflection spectra of a monolayer or less of carbon monoxide adsorbed on clean metal surfaces have already been obtained from the single reflection technique.315The chemisorbed formates and nitric monoxides show strong absorption bands as well as carbon monoxide, and the measurable signals were also detected from these species with a single reflection apparatus. This infrared reflection absorption technique, if applied to the surface phenomena of the catalytic reaction, will become a very powerful technique to obtain information on chemisorbed species. Acknowledgment. Part of the cost of this research was met by the Scientific Research Grant of the Ministry of Education (No. 58706), to which the authors’ thanks are due.
References and Notes (1) For example, L. H. Little, “Infrared Spectra of Adsorbed Species”, Academic Press, London, 1966. (2) R. G. Greenler, J. Chem. fhys., 44, 310 (1966). (3) M. A. Chesters, J. Prltchard, and M. L. Slrns, Chem. Commun., 1454 (1970). (4) S.A. Francis and A. H. Ellison, J. Opt. SOC.Am., 49, 131 (1959). (5) H.0. Tompklns and R. G. Greenler, Surface Sci., 28 194 (1971). (6) K. Klshl, T. Ogawa, and K. Hlrota, J. Catab, 5, 464 (1966). (7) A. Ueno, T. Onlshl, and K. Tarnaru, Trans. Faraday Soc., 65, 756 (1969). (8) M. Ito and W. Suetaka, Chem. Left., 757 (1973). (9) Y. Kuroda and M. Kubo, Spectrochim. Acfa, Sect. A, 23, 2779 (1967). (10) G. W. Poling and R. P Elschens, J. Necfrochem. SOC.,113, No. 3, 218 (1966). (11) A. R. L. Moss and 8. H. Blott, Surface Sci., 17, 240 (1969). (12) K. Hlrota and Y. Nakal, Nihon Kagaku Zashi, 80, No. 7, 700 (1959). (13) G. Blyholder and M. C. Allen, J. fhys. Chem., 69, 3998 (1965). (14) 2. Calahorra and M. Folman, The Second International Conference on Solid Surface, Kyoto, March 1974, No. 29a C2 4.
The Journal of Physical Chemistry, Voi. 79, No. 12, 1976
