J. Phys. Chem. 1983,87,3181-3183
and so rhCN 1/11.. At lower pressures, when the rate is fallen off, the steady distribution above threshold is depleted, and can be attained in a shorter time than for the proper equilibrium distribution, hence rinc< 1/11.. What is not obvious is why pint should tend to a constant value as the second-order region is reached, nor why the limiting value should be the mean decay time constant for the manifold of reactive states. From a more practical viewpoint, it would seem that the incubation time, measured in a shock-wave heating process from a low starting temperature, could provide a rigorous method of classifying reactions into strong-collision and weak-collision types. Strong-collision behavior would be identified with a value of qncI ~,1, but, as true strong-
3181
collision behavior can never be observed since all relaxations must contain some sequential character, rincN rrel, as seems to be the case in the cyclopropane isomerization experiment, would appear to be the more likely occurrence. On the other hand, it is well established6p8J+12that in a ~ the 1 , indications are' weak-collision system, rinc> ~ ~ and that the magnitude of the disparity between these two quantities will increase with increasing departure from the strong-collision behavior of eq 2 or 3.
Acknowledgment. This work was supported by the Natural Sciences and Engineering Research Council of Canada; it is also a pleasure to thank Raj Vatsya for his helpful criticisms.
Multiple Adsorbates on Copper Surfaces in Formic Acid Vapor Observed by Polarization Modulation Infrared Spectrocopy Toshlmasa Wadayama, Klyoshl Monma, and Wataru Suetaka Laboratory of Interface Science of Metals, Faculty of Engineering, Tohoku University, Sendai 980, Japan (Received: April 28, 1983)
IR spectra of formic acid adsorbed on copper surfaces were observed in the presence of ita vapor at a relatively high pressure. The spectra show that formic acid dissociatively adsorbs on the metal at room temperatures to form bridged and monodentate formates. A new band, which appeared only in the presence of gas-phase formic acid, was observed at 1160 cm-' (1200 cm-I in deuterated species) in addition to the bands due to the formates. The weakly adsorbed species giving rise to the new band is discussed. The vibration spectra of adsorbed species on metal surfaces may provide information crucial for elucidating the mechanism of catalytic reactions on the metal. High-resolution electron energy loss spectroscopy is a powerful tool for obtaining vibration spectra of adsorbed species on solid surfaces in ultrahigh vacuum, but cannot be used for the observation of metal surfaces in gaseous medium. The polarization modulation (PM) technique, which has been successfully applied for improving the sensitivity of infrared absorption and emission spectroscopies,1*2is feasible for obtaining infrared absorption spectra of species on metal surfaces in a gaseous or liquid medium without hindrance of the absorption of the med i ~ m . ~ ~ ~ In catalytic reactions, weakly adsorbed species are generally present on the catalyst in addition to strongly adsorbed ones. Since not only the latter but also the former may play important roles in the reaction, in situ observation of both the species probably yields information valuable for shedding light on the reaction mechanism. However, infrared spectra of adsorbed species have generally been obtained from solid surfaces after evacuation of the gas-phase species so as to remove the absorption of infrared light by the gas. As a consequence, only the spectra of strongly adsorbed species have been obtained at room temperatures, because weakly adsorbed species (1) H. Pfnllr, D. Menzel, F. M. Hoffmann, A. Ortega, and A. M. Bredshew, Surf. Sci., 93,431-62(1980). (2) K. Wagatauma, K. Monma, and W. Suetaka, Appl. Surf. Sci., 7, 281-5 -- - - (1981). - - - - ,. (3) D. S. Dunn, M. W. Severson, W. G. Golden, and J. Overend, J. Catal., 65,271-80 (1980). (4)J. W. Russel, J. Overend, K. Scanlon, M. Severeon, and A. Bewick, J. Phys. Chem., 86, 3066-8 (1982).
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0022-3654/83/2087-3181$01.5QIO
were desorbed in the course of the evacuation. We applied the above-mentioned modulation technique for the in situ observation of adsorption of formic acid on evaporated copper films in the presence of the acid vapor, and were successful in detecting a weakly adsorbed species as well as strongly adsorbed formates. In the present Letter, the results obtained will briefly be reported. Figure 1shows the schematic diagram of the PM spectrometer used in the present work. The plane-polarized infrared beam was incident onto the sample at a high incident angle (ca. 80'). The polarizer was rotating at a constant speed of 1650 rpm to give a modulation frequency of 55 Hz. A thin KBr plate (1 mm in thickness) was inserted into the light path at an angle near the Brewster angle so as to compensate for the difference in reflectivity of the sample and mirrors for the p and s components of the infrared light. The resolution of the monochromator* filter used is about 15 cm-' over the region of measurement and is rather low. However, the filter does not change the intensity of the infrared light upon rotation of the plane of polarization in contrast to grating monochromators. A stainless-steel vacuum cell, which was equipped with NaCl infrared windows, was pumped out with an oil diffusion pump. Pure copper (purity 99.999%) was deposited onto a smooth glass plate in the cell at a background pressure of 1 X lo4 torr (1.33 X Pa). The temperature of the substrate glass was monitored with an alumel-chromel thermocouple embedded in the plate. Formic acid (purity 95%) and dideuterioformicacid (DCOOD, purity 99.9% ), which were used without further purification, were introduced into the cell after a degassing procedure in a glass vessel. Figure 2 shows the spectra of formic and deuterioformic acids adsorbed on copper films in the range of 1250-1800 0 1983 American Chemical Society
Letters
The Journal of Physical Ch8miSffy, Vol. 87,No. 17, 1983
3182
also observed that the monodentate formate (11) was an-
LS
0
HI
1I
0 I
I
M
M
I Figure 1. Schematic diagram of the PM spectrometer: LS, light swce;K, KBr plate; P, rotating w l r m polarizer;D,HgCdle detector; VC, vacuum cell; LIA, lock-in-amplifier: F, filter; S, sample.
z P
~
1
0.5%
1300
1400
1500
WAVENUMBER
1600 ( cm-'
1700 1800
1
Flgwe 2. I R spectra of formic acid adsorbed on evaporated copper films (1 250-1 800 cm-') at room temperature unless otherwise stated; A, spectrum recorded before introduction of formic acM (1 X 10" torr); 6,spectrum in the presence of HCOOH (3 torr); C, spectrum after torr, 420 K); D, spectrum in the presence of evacuation (1 X WOOD (3 torr).
cm-'. Figure 2A is the background spectrum before the introduction of formic acid. The spectrum recorded at a room temperature in the presence of gas-phase formic acid (3 torr) is also shown in this figure (spectrum B). Absorption bands can be located at 1350 and 1640 cm-' in the spectrum. Both the bands persisted upon evacuation torr. of gas-phase formic acid to 1 X It is generally recognized that formic acid adsorbs dissociatively to form formate on oxygen-covered (or clean at low temperatures) copper surfaces. Since the vacuum used in the present work was poor, copper surfaces should be contaminated with oxygen and other residual gases. The formation of surface formate, therefore, is quite likely. When a sample, exhibiting these two bands, was annealed at 350 K for several minutes under vacuum, the band at 1640 cm-' decreased remarkably in intensity. At the same time, the intensity of the 1350-cm-' band increased appreciably. Sexton investigated formic acid adsorption on the Cu(100) surface with high-resolution electron energy loss spectroscopy.6 He observed a strong band at 1330 cm-I at 400 K in the above-mentioned region. Upon cooling to 100 K another strong band appeared at 1640 cm.-' He interpreted the appearance of the latter band in terms of the change in molecular orientation of the bidentate formate species. However, the band at 1640 cm-I has been reassigned to the monodentate formate based on the band separation between the two bands by Avery? He ( 5 ) B.A. Sexton, S u ~ f Sci., . 88,319-30 (1979).
I
M
I
nealed to the bridged species (I) on oxygen-covered platinum surface. The spectral change obtained at 350 K probably indicates that partial conversion of the monodentate species into the bridged form took place on the copper surface at that temperature. In our previous paper: the formate adsorbed on a very rough copper surface showed absorption bands at 1350 and 1600 cm.-' The former and the latter are assigned to the v,(OCO) and the v,(OCO) modes of the bridged species, respectively. When the copper surface was smooth, the band at 1600 cm-I disappeared completely in the reflection spectrum, because of the perpendicular orientation of the bridged species and the surface selection rule.7 The band at 1350 cm-' in Figure 2B is in agreement in wavenumber with the v,(OCO) mode of the bridged species. Although no band is located around 1600 cm-' in spectrum B where the v,(OCO) band of the bridged species may appear, the absence of an antisymmetric stretching band is in accord with the surface selection rule, because the surface used in the present work was smooth. It was noted that slight changes in the experimental condition gave rise to noticeable changes in the intensity ratio between the 1350cm-' band and that at 1640 cm;' suggesting the coexistence of two kinds of species. For these reasons, the band at 1350 cm-' can be assigned to the bridged formate on the copper surface. The band at 1640 cm-' may be attributed to the C=O stretching vibration of the monodentate species. The absorption arising from the v(C-0) of the monodentate species is probably overlapping with the v,(OCO) band of the bridged species. The sample, which gave spectrum B of Figure 2, was heated to 420 K under vacuum at 1 X torr and a spectrum was recorded at the same temperature. The spectrum (Figure 2C) is in agreement with the background spectrum. The bands at 1350 and 1640 cm-l disappeared completely, indicating the decomposition of the surface formates. The introduction of dideuterioformic acid onto a newly evaporated copper film resulted in spectrum D of Figure 2. Two strong bands can be seen in the spectrum at slightly lower frequencies than their counterparts from formic acid. This fact indicates that the 1640-cm-l band can not be ascribed to adsorbed water, supporting the above-mentioned assignment of the two bands. Similar shifts of the bands of the formates on a rough aluminum surface were observed on deuteration in our previous paper.7 The spectra obtained in the region of 1050-1230 cm-I are shown in Figure 3. The background spectrum A changed into spectrum B, exhibiting a new band at 1160 cm,-l upon introduction of formic acid at room temperature. The introduction of deuterioformic acid gave a new band at 1200 cm-' as can be seen in spectrum C of this figure. Spectra B and C were recorded in the presence of 3 torr of acid vapor, and the evacuation of gas-phase molecules resulted in the complete disappearance of these bands. This fact shows that these bands should be at(6)N.R.Avery, Appl. Surf. Sci., 14,149-56 (1982-83). (7)M.Ito and W.Su(taka, J . Phys. Chem., 79,1190-3 (1975).
J. Phys. Chem. 1983,87,3183-3186
W V Z
s V
W
-1
LL
w
LL
1050 1100
1200
WAVENUMBER (
cm-')
Fbgure 3. I R spectra of formic acid adsorbed on evaporated copper films recorded at room temperatures (lower frequency region): A, spectrum recorded just after film evaporation (1 X lo-' torr); B, spectrum in the presence of HCOOH (3 torr); C, spectrum in the presence of DCOOD (3 torr).
tributed to weakly adsorbed species. Because the species predominant in the gas phase was formic acid, the new bands might be ascribed to physically adsorbed formic and dideuterioformic acid. The gas-phase formic acid monomer as well as its dimer show three absorption bands in and near this region, which are assigned to C-H out-of-plane deformation, C-0 stretching, and CO-H bending vibrations.* Since the (8)W. V. F. Brooks and C. M. Haas, J.Phys. Chem., 71,650-5 (1967).
3183
deuteration results in a small upward shift of the band as can be seen in Figure 3, the new band should be assigned to the C-0 stretching vibration. Formic acid dimer and monomer have a very strong C=O stretching band, but no band is located around 1750 cm;' where a strong C=O band should appear, as can be seen in parts B and D of Figure 2. A possible explanation for the missing band is molecular orientation of weakly adsorbed formic acid. If the C=O bond of formic acid is oriented parallel to the copper surface, the absorption band arising from the C = O stretching vibration will be very weak because of the surface selection rule. The orientational model of a formic acid molecule with its plane parallel to the metal surface, however, is ruled out, because the C-0 bond is oriented parallel to the metal surface in this model and cannot give rise to the strong v(C-0) band. The adsorption of the acid molecule with its plane perpendicular to the metal surface may give rise to an orientation of the C=O bond parallel and the C-O inclined steeply to the metal surface, resulting in feeble v(C=O) and strong v(C-0) bands. Although it is difficult at present to draw a definite conclusion about the weakly adsorbed species, it is clearly shown in the present work that PM infrared spectroscopy is feasible for the investigation of the mechanism of catalytic reaction on metal surfaces, because it can provide us information which cannot be obtained with EELS or conventional infrared spectroscopy. We hope that the PM technique will enable us to get IR spectra of metastable reaction intermediates, because the presence of gas-phase species at a relatively high pressure may result in an increase in the number of intermediates on the metal surface.
Acknowledgment. This work was supported by a Grant-in-Aid for Scientific Research (No. 56850169) from the Ministry of Education, Science and Culture of Japan.
CIDEP Enhanced ENDOR of Short-Lived Radicals R. 2. Sagdeev, Instltute of Chemical Kinetics and Combustion, Slberian Branch of the Academy of Science of the USSR, Novosibirsk, USSR
W. Mohl, and K. Mobius' Institute of Molecular mysics, Free University of Berlin, PI000 Beriin 33, Arnlmallee 14, West Germany (Received May 16, 1983)
During photolysis of 1,4-benzoquinone in ethylene glycol NMR transitions from the monoprotonated p benzosemiquinone radicals were detected via intensity changes of the CIDEP-polarizedESR signal. The CIDEP detection of rf-pumped NMR transitions offers a novel method for ENDOR spectroscopy of transient radical intermediates in chemical reactions.
Introduction In magnetic resonance spectroscopy of stable radicals, ENDOR has proved to be extremely useful.' Application of ENDOR to short-lived paramagnetic species, which occur in the course of chemical reactions, is hampered by sensitivity problems, since for short lifetimes the equilibrium concentration of radicals in the cavity is often too small even for ESR detection. Possible solutions for this sensitivity problem are offered by chemically induced (1)K.Mabius, M.Plato, and W. Lubitz, Phys. Rep., 87,171 (1982).
polarization effects which may result in strong deviations from Boltzmann spin level populations. Consequently, taking advantage of chemically induced nuclear polarization (CIDNP), the method of "CIDNP-detected NMR" was recently p r o p o ~ e dand ~ * ~experimentally realized in the time-resolved mode.& The main idea of this method is (2) R. Z. Sagdeev, Yu. A. Grishin, and A. Z. Gogolev, Zh. Strukt. Khim., 20, 1132 (1979). (3) R.Z.Sagdeev, Yu. N. M o l i , Yu. A. Grishin, A. V. Dushkin, K. M. Salickov, and A. Z. Gogolev, Proceedings of XXI Ampere Colloque, Bull. Magn. Res., 27 (1981).
0022-3654/83/2087-3183$01.50/00 1983 American Chemical Society
