J. Phys. Chem. 1987, 91, 2026-2028
2026
are seen at ca. 1591 and 1175 cm-I in the case of 14/15NH3and ND3 experiments, respectively. Despite these observations it is not possible to conclude that water as well as nitrous oxide is a primary product of the reaction between N H 2 and NO in a matrix. The weakness of the bands suggests that the formation of water is at best inefficient and could in fact arise from secondary chemistry as described in NzO + D2
h = 184.9 nm
Nz + D20
Indeed this reaction has been observed previously in our laboratory.I8 Two explanations may be proposed to account for the present results: (i) N2 and H 2 0are not primary products of the N H 2 + N O reaction in a matrix; there is a strong possibility that the water observed was from a secondary process. (18) Whyte, L. J.; Sodeau, J. R., unpublished data.
(ii) The 1,3 hydrogen migration from H 2 N N 0 to H N N O H , which involves the formation of a high-energy transition state, and upon which the production of H 2 0 as a final product is dependent, may not be possible in a rigid matrix environment. This explanation is unlikely however as N 2 0 is again the observed product in a matrix as fluid as Ne.
Conclusions The results described above conclusively prove for the first time that N 2 0 is a direct product of the reaction between N H 2 and NO. This is in contrast to earlier gas-phase work where the channel leading to N 2 0 and H2 was reported not to take place, with N 2 0 remaining undetected or cited as the product of bimolecular H N O reaction. The gas-phase-favored process yielding N 2 and H 2 0appears to be at best inefficient in a low-temperature matrix. Acknowledgment. We thank SERC for the award of a maintenance grant to J.N.C.
Observation of Combination Modes in Transmission I R Spectra of CO on Supported Platinum John L. Robbins* and Elise Marucchi-Soos Corporate Research Science Laboratories, Exxon Research and Engineering Company, Annandale, New Jersey 08801 (Received: November 17, 1986; In Final Form: February 23, 1987)
Transmission IR spectra of CO on Ti02-supported Pt show a weak high-frequency IR band at 2485 cm-' in addition to the previously observed terminal and bridging CO IR bands at 2083 and 1850 cm-I, respectively. The 2485-cm-' band is assigned to a combination band with C-O stretching (2083 cm-l) and Pt-CO stretching (402 cm-I) character. IR experiments with I3C- and '*O-labeled CO confirm this assignment. The use of combination modes to identify low-frequency fundamental vibrations which are masked by strong support IR absorption is proposed as a general method.
Transmission infrared spectroscopy has been used to characterize CO adsorbed on oxide-supported transition-metal particles for over 30 years.] Typically such studies have been limited to analysis of the strong C O stretching fundamental normally found between 1700 and 2200 cm-' for adsorbed CO. Direct observation of low-frequency fundamentals, such as the metal-C stretching or the metal-C-O bending modes expected between 300 and 600 cm-I, is precluded by intense absorbance of the oxide supports (e.g. SiOz, Alz03, TiOz) below 1000 cm-I. A photoacoustic cell with in-situ pretreatment capabilities has recently been described which can be used to study adsorbates on air-sensitive supported metals.2 The photoacoustic methods overcome some of the difficulties associated with observing adsorbate vibrations in the 1100-200-~m-~range where many metal oxide supports are opaque. However, low-frequency C O modes on supported catalysts have not yet been identified by these methods. Here we show that such low-frequency modes can be observed as combination absorptions with the fundamental CO stretching band. These combination bands occur in the 26002400-cm-I spectral region where the oxide supports are quite transparent. Although the bands are relatively weak, they can be observed with a conventional dispersive I R spectrometer. Figure 1 shows the transmission infrared spectrum of CO on a 35-mg wafer of 5 wt % Pt/Ti02 inside a stainless steel in-situ (1) Sheppard, N.; Nguyen, T. T. Advances in Infrared and Raman Spectroscopy, Vol. 5 , Clarke, R. J., Hester, R. E., Eds.; Heyden: London, 1978. (2) McGovern, S. J.; Royce, B. S. H.; Benziger, J. B. Appl. Surf.Sci. 1984, 18,401-413
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IR cell of our own d e ~ i g n . To ~ obtain this spectrum the calcined (773 K; 0,; 2 h) but unreduced wafer was treated in flowing hydrogen for 2 h at 523 K, evacuated for 30 min at 573 K to 5 X 10" Torr, and then cooled to 310 K under dynamic vacuum. A base-line spectrum was recorded under vacuum and the wafer was then equilibrated with 20 Torr CO for 10 min. Spectra were then recorded in 20 Torr of CO and under vacuum after evacuation of gas-phase C O for 10 min. The spectrum shown represents the difference between the spectra measured under dynamic vacuum before and after equilibration of the sample with CO. In accord with the many previous IR studies of C O adsorbed on supported Pt,4-8 the intense band at 2080 cm-' and the weaker ones at 18809 and 1850 cm-' are respectively assigned to C O stretching modes for CO in terminal and twofold bridging sites (3) Material preparation: Degussa P-25 TiOZwas calcined in oxygen for 2 h at 1020 K to afford 40 m2/g TiOl which was a 50/50 mixture of crystalline anatase and rutile phases. Pt was introduced by incipient wetness methods using H2PtC16in acetone. The dry powder was calcined in oxygen for 2 h at 773 K. Infrared spectra were recorded on a Perkin-Elmer Model 684 spectrophotometer interfaced to a PE Model 3600 data station. The spectrometer frequency was calibrated with gas-phase CO and COz. Spectral resolution was 2 to 4 cm-'. No smoothing functions were employed, but spectra were averaged nine times to enhance signal/noise. (4) Primet, M.; Basset, J. M.; Mathieu, V.; Prettre, M. J . Cutul. 1973, 28, 368-375. ( 5 ) Vannice, M. A.; Twu, C. C.; Moon, S . H. J. Caral. 1983, 79, 70-80. (6) Peri, J. B. J . Caral. 1978, 52, 144-156. (7) Tanaka, K.; White, J. M. J. Calal. 1983, 79, 81-94. (8) Hammaker, R. M.; Francis, S. A,; Eischens; R. P. Spectrochim. Acta 1965, 21, 1295-1309. (9) This band, not apparent in the Figure, is observed as a weak peak under 10 Torr of CO.
0 1987 American Chemical Society
Letters
The Journal of Physical Chemistry, Vol. 91, No. 8, 1987 2027
TABLE Io
4Jm
adsorbate 12C160 13~160
12Cl80
obsd 2485 2433 2426
calcdb
obsd
2430 2425
2083 2036 2036
u(T-CO) calcdb 2036 2033
obsd
u(B-CO) calcdb
1844 1800 1794
v(HF) - v(T-CO)
1803 1799
obsd
calcdc
calcdd
402 397 390
395 388
389 397
OAll measurements made in 10 Torr of CO to ensure comparable CO coverage. All frequencies reported in cm-'. The l3CI6Ois 99.9 atom % I3C and 12 atom % lsO; the '2C180is 98 atom % leg;the I2CI6Ois 99 atom % lzC. bCalculatedby using eq 1 and data measured in '2C'60. cCalculated by using eq 2 and data measured in '2C160.dCalculated by using eq 3 and data measured in l2CI6O.
According to Braterman, the frequency of a C-0 stretching plus M-CO stretching or M-C-0 bending combination band for a metal carbonyl can be approximated to within a few cmI bl?l the arithmetic sum of the corresponding fundamental frequencies. Table I shows the difference v(HF) - u(T-CO) yields a value near 400 cm-' which is in the range where the Pt-CO stretching or Pt-C-0 bending vibrations are expected. The effect of I3C or lSO substitution on the Pt-CO stretching frequency can be estimated by using the frequency measured in I2Cl6Oand eq 21°
8C
t!
which neglects the minor. effects arising from coupling of the metal420 and the C-O stretching modes. To estimate the effects of isotopic substitution on a Pt-C-0 bending mode, eq 31° was
+
FB[phrPtC-2 porco-2
+ pC(rPtC-I+ rCo-1)2]- 5.8915 X 10-52 =
2400
2200
2000
1900
1800
Frequency (cm-') Figure 1. Infrared spectrum of CO on Pt/Ti02 following reduction and outgassing as described in the text. A base line recorded prior to admission of CO has been subtracted. Weak features near 2350 cm-' are due to imperfect compensation for atmospheric C 0 2 .
on the supported Pt particles. Expansion of the ordinate axis reveals an additional weak feature at 2483 cm-I which has not been noted in previous studies of C O on supported Pt. Isotope labeling experiments show the 2483-cm-I band is due to chemisorbed CO. Table I summarizes vibrational data obtained for l2CI6O, 13C160,and 12C1s0chemisorbed onto the same Pt/ T i 0 2 wafer following consecutive reductions and outgassings as above. The high-frequency (HF), terminal Pt-CO (T), and bridging Pt-CO (B) bands all shift to lower frequencies when CO labeled with heavier C or 0 isotopes is chemisorbed onto the reduced Pt/TiOz. Nearly identical spectral data are obtained when chemisorbed I2CI6Ois exchanged with excess l3CI6Oor 12C'80 at room temperature. Values of the HF, T, and B C O frequencies measured in 12C180or 13C160are in good agreement with those and the reduced expected from the frequencies measured in 1%160 mass relationships:
which should apply for fundamental CO stretching modes (Table I).Io Here F~~ refers to the reduced mass of the C O indicated. However, it is unlikely that the HF band represents the fundamental CO stretch for CO chemisorbed on a small number of Pt sites. There is no precedent for molecular or surface C O complexes with such high C O stretching frequencies.' Weak bands are sometimes observed in this region for molecular CO derivatives, but those bands are ascribed to combination modes with C O stretching and M C stretching or MCO bending charcter.1° We similarly find that the HF band positions in labeled C O can be accounted for if the bands represent a combination mode involving fundamental C-O and M-CO stretching vibrations. (10) Braterman, P. S. Metal Carbonyl Spectra; Academic: London, 1975; pp 1-33.
o
(3)
solved for the bending force constant, FB, by using the 402-cm-I frequency measured in l2CI6Oand assumed values for the Pt-C distance (rRC= 1.80 A) and the C-0 distance (rco = 1.18 A)." This value of FBwas then used along with the appropriate reduced masses for l3CI6Oor 12C'80to predict the positions of the bending mode in the labeled systems. In calculating the bending and stretching frequencies we have assumed infinite mass for the Pt substrate. The last columns of the table list frequencies for the Pt-C-0 bending and stretching modes calculated from these formalisms. The experimental values determined with I3Cl6Oand 12C180show good agreement with those expected for Pt-CO stretching modes. The significance of this fit can be questioned when the effects of experimental uncertainty on band location are considered. By means of spectral expansion, the HF and T C O peak positions can be located with a precision better than f2.5 and 1.O cm-l, respectively. The reproducibility error is somewhat larger. When a sample is cycled back and forth from saturation in l2Cl60 to, for example, I3Cl6O,the HF peak can vary by f 4 cm-I and the T C O peak by f1.5 cm-'. The observed values for v(HF) - v(T-CO) quoted in Table I thus have an uncertainty of f5.5 cm-'. Errors also propagate when eq 2 and 3 are used to calculate values for the Pt-C-0 low-frequency bending and stretching modes. The calculated values shown in the last columns of the table thus have uncertainties of f 6 cm-'. Because of these error limits a definitive assignment of a Pt-C stretching vs. PtC-O bending mode is not possible on the basis of these data alone. However, two trends lead us to prefer assigning the ca. 400-cm-I vibration to a Pt-C stretching mode. First, it is expected that the frequency of an M-CO stretching mode will decrease in the order YI~c'6o> Vl3c160 > u 1 2 ~ 1 sThis ~. is because eq 2 is governed by the mass of C O which is greater when " 0 is substituted than when I3C is. For an M-C-0 bending mode the order V l 2 c l 6 0 > V I ~ I> B U13cI60 ~ is expected. This is because in eq 3 the coefficient for wc is greater than the coefficient for po. These trends have been used to discriminate the low-frequency stretching and bending modes in molecular carbonyls.1° In an experiment where our sample was cycled four times through the sequence l2CI6O to 13C160to 12C'80to l2CI60,the difference v(HF) - v(T-CO) was consistently 5 to 10 cm-I greater for l3CI6Othan for I2ClSO. That ~~~~
~
~
(11) Chini, P. Inorg. Chim. Acta Rev. 1968, 2, 31-65.
J. Phys. Chem. 1987, 91, 2028-2030
2028
trend is consistent with a Pt-C stretching mode assignment. This assignment is also anticipated on the basis of the surface dipole selection rule which states that only vibrations which belong to totally symmetric representations can be observed.12 In C,, symmetry, the C O and Pt-CO stretches transform as the totally symmetric Z representation while the Pt-C-0 bend transforms as a II representation. For combination modes we must consider the products of representations for the two fundamentals involved. The product 2x2 is again totally symmetric while the product Z X I I is not. Thus the Pt-C-O bending mode is dipole forbidden both as a fundamental and in combination with the C-O stretch. For comparison with our data, EELS studies of C O on a flat Pt( 111) and a stepped Pt[(611)X(11 l)] surface showed terminal and bridging C O stretches at 21 10 and 1870 cm-'.13 Loss features at 390 and 480 cm-' were respectively assigned to Pt-C stretches for bridging and terminal CO. The 390-cm-' loss showed coverage-dependent frequency shifts. A very weak loss at 2580 cm-' was assigned to a combination loss involving CO and Pt-CO stretching modes. The frequencies of the terminal Pt-CO and CO stretching modes we find for Pt/TiO, differ from those measured by Barb and Ibach on the flat and stepped Pt surfaces. Hydrogen chemisorption measurements on a sample of Pt/TiO, reduced a t 520 K indicate that the Pt particles are large, ca. 65 A, so bulklike Pt behavior is expected.16 The discrepancies may indicate that other crystal planes are preferentially exposed on the supported Pt particles. Blyholder and Sheets studied C O adsorbed onto Pt particles suspended in an oil film on salt wind o w ~ . ' Their ~ values for the CO stretching (2045 and 1815 cm-') and the Pt-C stretching (480 cm-I) modes were also different from those measured here. It may be that components of the hydrocarbon also chemically interact with the Pt particles. It is interesting to compare the effects of varying C O coverage on the frequencies of the T-CO and proposed Pt-CO bands. When the CO-saturated sample shown in Figure 1 is evacuated for 30 min at 523 K and then cooled to room temperature, we find the T-CO band intensity decreases by 20% and its position red shifts by 12 cm-l to 2071 cm-I. At the same time the H F band increases (12) Hoffmann, F. M. Surf. Sci. Rep. 1983, 3, 107-192. (13) BarB, A. M.; Ibach, H. H. J . Chem. Phys. 1979, 71, 4812-4816. (14) Blyholder, G.;Sheets, R. J . Phys. Chem. 1970, 74, 4335-4338.
in frequency by 11 cm-' to 2494 cm-' and the calculated position of the Pt-CO fundamental increases in frequency by 21 cm-' to 423 cm-I. Analogous coverage-dependent frequency behavior has been found for the T-CO fundamental on supported and unsupported Pt. On a Pt foil the T-CO band appears near 2065 cm-' at low CO coverage and increases in frequency to 2100 cm-' as C O coverage approaches ~aturati0n.I~This frequency shift is in large part ascribed to increasing intermolecular dipole cou~ 1 i n g . lAt ~ very low coverages, donation of Pt d-electron density to empty 2?r* antibonding molecular orbitals on C O (Le. ?r backbonding) is thought to play a major role in defining the C O stretching frequency. On Pt/Ti02 the T-CO frequency drops with decreasing C O coverage as expected while the Pt-CO stretching frequency follows the opposite trend. That a low-frequency fundamental such as the Pt-C stretch increases in frequency with decreasing coverage can be rationalized in terms of the backbonding model referred to earlier. As the CO coverage decreases, filled Pt d levels of 7r symmetry can donate more electron density to empty 2?r* orbitals on C O and the C-0 bond is weakened as reflected by a reduction in the C-O stretching frequency. At the same time this interaction strengthens the Pt-C bond and an increase in the metal-CO stretching frequency is anticipated. Alternatively, reduced intermolecular repulsion at the lower CO coverage can also be expected to increase the Pt-C stretching frequency. Barb and Ibach also found the Pt-CO stretch for bridging CO increases in frequency with decreasing CO coverage.13 The influence of the intermolecular dipole coupling, which may also play a role in defining the frequency of the metal-C fundamental, is difficult to anticipate theoretically at this time. Finally, we point out that observation of the high-frequency combination band is not limited to Ti02-supported Pt or the rather pristine conditions of low-pressure CO chemisorption. A similar high-frequency band is found for C O on a 5% Pt/A1203sample with intense C O stretching bands. We also observe these highfrequency bands in IR spectra of Pt on T i 0 2 and A1203during steady-state CO hydrogenation. These results will be reported later in a more detailed report. (15) Crossley, A.; King, D. A. Surf. Sci. 1977, 68, 528-538. (16) X-ray line broadening measurements on the Pt(200) reflection indicate a somewhat larger average Pt particle size, 90 A.
Femtosecond Laser- Induced Kerr Responses in Liquid CS, C. Kalpouzos, W. T. Lotshaw, D. McMorrow, and G. A. Kenney-Wallace* Lash Miller Laboratories, University of Toronto, Toronto, Canada MSS 1A1 (Received: December 30, 1986)
Femtosecond optical Kerr studies of liquid CS2show the electronic hyperpolarizabilitycomponent separable from two molecular relaxations with lifetimes of 160 fs and 1.61 ps. These nuclear contributions to the nonlinear polarizability are delayed with respect to the time origin indicating the need for an inertial aspect to the field-driven response in prevailing Kerr theories.
W e report new results on the femtosecond (fs) optical Kerr effect (OKE) of liquid CS2,identifying three novel observations linked to specific molecular relaxation responses in this liquid at 295 K. Similar behavior is observed for other Kerr liquids.' We present, for the first time, (1) the direct observation of an instantaneous component of the Kerr signal which follows the pulse autocorrelation and is identified as arising from the electronic part ~~
~
~
(1) Lotshaw, W. T.; McMorrow, D.; Kalpouzos C.; Kenney-Wallace, G. A. Chem. Phys. Lett., in press. Kenney-Wallace, G.A,; Dickson, T.; Gol o m b k , M. Faraday Discuss., in press.
0022-3654/87/2091-2028$01.50/0
of x&, the third-order nonlinear susceptibility; (2) evidence that the ultrafast relaxation component of ~ $ 2 ,is significantly faster than previously reported:-' with T , / ~= 160 f 15 fs, and separable from the above electronic component; and (3) a demonstration that the full temporal profile of the Kerr signal cannot be fitted nor predicted by using the generally applied theory.5 In agreement (2) Halbout, J. M.; Tang, C. L. Appl. Phys. Lett. 1982.40, 765. In Time Resolved Spectroscopy, Atkinson, G.,Ed.; Academic: New York, 1983; pp 13-82. (3) Green, B. I.; Farrow, R. C. J . Chem. Phys. 1982, 77, 4779. (4) Green, B. I.; Farrow, R. C. Chem. Phys. Lett. 1983, 98, 273.
0 1987 American Chemical Society
