552
The Journal of Physical Chemistry, Vol. 83, No. 4, 1979
1
1.0 r 0.8
40.4
P
TIME 1 picosecond Figure 1. Observed (0)and calculated (-) formation curves of the lowest triplet states of [IrCl,phen,]CI. Observed absorbances are accumulated values of 4,5, and 8 data for 95 % v/v DMF-H,O, 45 %
v/v DMF-H,O, and water, respectively.
TABLE I: Formation Rates ( h f )of Lowest Triplet States of [ IrCl,phen, ] C1 solvent
k f , s-'
95% V/V DMF-H,O 45% V / V DMF-H,O H,O
4.2 X 10" 2.9 3.6
pulse, and the other beam was focussed on the sample cell (0.2 cm light path), on which the exciting pulse is also focussed. After passing through a monochromator (Shimadzu 25-cm polychromator), both monitoring pulse trains were detected by a vidicon coupled to an optical multichannel analyzer (PAR, OMA 1205A/l205D), and were then displayed on an oscilloscope (Tektronix Model 556) together with the computer-analyzed (YHP 9825A) data. Triplet-triplet absorption of [IrCl,phen2]C1 was monitored at 480 nm. More than 15 measurements were carried out for each solvent. The absorbances measured under uniform conditions were selected and accumulated; four, five, and eight data for 95% v/v DMF-H20, 45% v/v DMF-H20, and water, respectively. Figure 1 shows the accumulated absorbances. To determine the formation rate, convolution was carried out;7B8the half-value of the ruby pulse is 20 ps. The calculated formation curves with the best fitted time constants are shown in Figure 1. At room temperature, the phosphorescence lifetime decreases as the solvent polarity increases, but even in the aqueous solution the lifetime is 13 ns, and the absorption does not decay in this time region. As is shown in Table I, the formation of T1was observed in a time range measurable by our apparatus. This result shows that intersystem crossing of this iridium complex is weak, compared with that reported for Cr(II1) comp l e x e ~ .From ~ the assignments of ground-state absorptions, the lowest excited singlet state (S,) of this iridium complex was assigned to a dn* state, and the lowest 3dn* and 3 ~ n - * states lie just below S1 within 1000 wavenumber. Then, the important intersystem crossing is of the , d ~ * - ~ d nor* l d ~ * - ~ ~ type. n - * On the other hand, in Cr(II1) complexes the lowest spin-allowed and spin-forbidden states are both 0022-3654/79/2083-0552$01 .OO/O
Communications to the Editor dd excited states. The spin-orbit interaction between metal d orbitals is very large, so dd-dd intersystem crossing is much stronger than dn-*-(d.rr*, na*)intersystem crossing. By changing the solvent, variation in the formation rates is found. Although experimental error inhibits a detailed discussion, the tendency for the change was reproducible in two other experiments. As was reported by authors12 TI is a 3dn* state in 95% v/v DMF-H20, and as the solvent polarity increases, 3d7r*shifts to higher energy and mixes with 37r7r*. S1 is located between 3dn-*and 3 ~ n in * 95% v/v DMF-HzO and also shifts to higher energy with a polarity increase. On the other hand, the l d ~ * - ~ n - ~ * interaction can be neglected, compared with the ' d ~ * - ~ d 7 ~ * interaction; the former is a two-center intergral while the latter is a one-center intergral. Therefore, the decrease in the kf value from 95 to 45% v/v DMF-H20 is caused by the decrease of a 3dn* component of T1,with mixing of the 3nn* state. In water, the solvent effect is not so simple, because Ida* shifts and becomes isoenergetic to or higher than the Tz state. Therefore, kf does not only depend on the dn-* component of T, state, but also on the d r * component of the T2 state.
Acknowledgment. We express sincere gratitude to Professor S. Nagakura for his kind support and Mr. Ohtani for his help in computer analysis. References and Notes (1) R. J. Watts, G. A. Crosby, and J. L. Sansregret, Inorg. Chem., 11, 1474 (1972). (2) Y. Ohashi and T. Kobayashi, 26th IUPAC Congress Abstract, Session I1 and 111, 1977, p 633. (3) A. D. Kirk, P. E. Hoggard, G . 8. Porter, M. G. Rockley, and M. W. Windsor, Chem. Phys. Lett., 37, 199 (1976). (4) J. A. Broomhead and W. Grumley, Inofg. Chem., 10, 2002 (1971). (5) T. Kobayashi and S.Nagakura, Chem. Phys. Lett., 43, 429 (1976). (6) Y. Shichida, T. Kobayashi, H. Ohtani, T. Yoshizawa, and S.Nagakura, Photochem. Photobiol., in press. (7) R. W. Anderson, Jr., R. M. Hochstrasser,H. Lutz, and G. W. Scott, Chem. Phys. Lett., 28, 153 (1974). (8) R. W. Anderson, Jr., R. M. Hochstrasser, H. Lutz, and G. W. Scott, J . Chem. Phys., 61, 2500 (1974). The Institute of Physical and Chemical Research Wako, Saitama, Japan
Y. Ohashi" T. Kobayashl
Received March 28, 1978; Revised Manuscript Received November 20, 1978
Infrared Spectroscopic Study of the Adsorption of Isocyanic Acid
Sir: While isocyanate is formed with ease on suppoi ed noble metals in the NO + CO catalytic reaction,l in the absence of metals it is not detected on any of the support Nevertheless, there is strong evidence that, once formed on the metal, isocyanate migrates to the supports which possibly determine its r e a c t i ~ i t y . ~As - ~it has been assumed that isocyanate surface species may play a role in the undesirable formation of NH3 and HCN during the catalytic treatment of automobile exhaust g a ~ e s , l ,investigations ~-~ of the behavior of isocyanate over supporting materials are clearly required. As it is not possible to produce surface isocyanate on these materials via the NO + CO reaction, we tried to circumvent this problem by using isocyanic acid, HNCO, directly as adsorbing species. HNCO was prepared by the reaction of saturated aqueous KNCO solution with 95% H3P04at 25 "C.* The oxides used (A1,0, Degussa P 110 C1, SiOz Aerosil 200, 0 1979 American Chemical Society
The Journal of Physical Chemistry, Vol. 83, No. 4, 1979 553
Communications to the Editor
TABLE I: P o s i t i o n of t h e Bands d u e to Surface Isocyanate Species
23!3 cm-'
samplea
frequencies, cm-'
sampleb
frequencies, cm"
2318 2272 2223-2241 2210
SiO, A1,0, MgO TiO,
2313 2272 2223 2212
-__.
Pt/SiO, Pt/AI,O, PtIMgO Pt/TiO,
]IO%
-
in t h e NO t CO r e a c t i o n a t I s o c y a n a t e was f o r m e d d u r i n g t h e a d s o r p t i o n of HNCO a t 25 C. a I s o c y a n a t e was f o r m e d
400 C.
-2272 cm-l 2313 cm-1 -2223~6' 2195 cm-l
2313 cm-' -2212cm-1 2600 2oa0crri1 Figure 'I. Infrared spectra of adsorbed HNCO. Adsorption and subsequent evacuation of HNCO were made at 25 "C: (A) AI,O,, (e) MgO, (C) Ti02, ([I)SiO,, (E) SiO, (adsorption temperature of HNCO here was 200 "C),
(F)Pt/Si02.
MgO DAB 6, TiOz Degussa P 25), were compressed into self-supporting disks (20-30 mg/cm2). All treatments and reactions were carried out in a vacuum infrared cell described e l ~ e w h e r e .A~ Specord ~~ 71 IR spectrometer was used to record the spectra. The resolution was better than &5 cm I. Prior to infrared spectroscopic measurements the disks were evacuated a t 400 "C for 60 min, then kept in 100 torr of O2 a t 400 "C for 30 min and evacuated again a t 400 c'C for 30 min. In evaluating the spectra we restrict ourself a t present only to the most important frequency range (2000-:?500 cm-l) for isocyanate groups. In Figure 1 we show the spectra of oxides after HNCO adsorption. On introduction of 10 torr of HNCO onto Si02a t 25 "C, a very strong band appeared a t 2278 cm-l. This band is partly due to i,he gaseous and partly due to weakly bonded HNCO. On evacuation of the gas phase at 25 "C, this band was completely eliminated. The residual very weak band was located at 2313 cm-l. With the rise of adsorption temperature the intensity of this band markedly increased. The adsorbed species responsible for this band proved to be very stable even a t 400 "C. The admission of HNCO onto alumina a t 25 "C caused the appearance of a new strong, broad band at 2272 cm-l. The band became somewhat sharper after degassing, but the location i3nd the intensity of the band remained practically the same. On admittance of HNCO onto MgO or TiOz, new bands a t 2225 cm-l on MgO, and a t 2230 cm-l on TiOz were produced in the frequency range of interest. After degassing the first one was shifted to 2223 cm-l, and the second one to 2212 cm-l. From a comparison of the frequencies observed at 2212-2313 cm with those of known metal isocyanates,*10 we assign the above stable bands to the surface M-NCO species. Table I shows a comparison of the positions of
the bands due to surface isocyanate formed in the NO C CO reaction on various supported Pt catalysts4 and in the present work on supports alone. The agreement of the band positions is very striking indeed. This coincidence provides additional evidence for the assumption that the isocyanate formed on these supported Pt catalysts resides mainly on the support.z4 The similar thermal behaviors and the stability order of surface isocyanate on these materials with or without Pt are also in harmony with this conclusion. As HNCO is only weakly adsorbed on silica a t 25 "C, in the case of P t / S i 0 2 we examined whether, after adsorption of HNCO on this sample, it is possible to detect surface species formed on Pt. Admission of HNCO onto 5% P t / S i 0 2 led to a dramatic change compared to the spectra obtained on pure silica. This was particularly the case after degassing the sample (Figure 1). A strong and stable band a t 2313 cm-l appeared even on room-temperature adsorption, and a new weaker band a t 2195 cm-' appeared. With the rise of temperature the intensity of the band at 2313 cm-l increased, whereas that at 2195 cm-l decreased, and finally disappeared above 200 "C. Control measurements showed that this band cannot be attributed to the chemisorption of any of the likely impurities (CO, HCN, COz, or NH,) of HNCO. Accordingly, the presence of Pt activated the HK'CO molecule, promoting its dissociative chemisorption on silica. It is very likely that in the first step HNCO adsorbed dissociatively on Pt 2 P t HNCO 6 Pt-H + Pt-NCO and then the NCO group migrated from the Pt to the sillica. The transitory nature observed for the weak band at 2195 cm-l can be tentatively assigned to isocyanate bonded to surface Pt atoms.
+
References and Notes ( 1 ) M. L. Unland, J. Phys. Chem., 77, 1952 (1973). (2) R. A. Dalla Betta and M. Shelef, J . Mol. Catal., 1, 431 (1976). (3) F. Soiymosi, J . Kiss, and J. SBrk5ny in "Proceedings of the 7th
(4) (5)
(6) (7) (8) (9)
International Vacuum Congress and 3rd International Conference on Solid Surfaces", R. Dobrozemsky, Ed., Vienna, 1977, p 819. F. Solymosi, L. Voigyesi, and J. SBrkBny, J . Catal., 54, 336 (1978). R. J. H. Voorhoeve, C. K. N. Patel, L. E. Trimble, R J. Kerl, and P. K. Gallagher, J . Catal., 45, 297 (1976). R. J . H. Voorhoeve and L. E. Trimble, J . Catal., 38, 80 (1975). F. Solymosi and J. RaskB, J . Catal., 49, 240 (1977). R. A. Ashby and R. L. Werner, J . Mol. Spectrosc., 18, 184 (19165). L. J . Beliamy, "Advances in Infrared Group Frequencies", Methuen and Co.. London. 1968. 58 -B. A. Morrow and I. A. Cody, J . Chem. Sac., Faraday Trans 1 , 71, 1021 (1975). - I
(IO)
Reaction Kinetical Research Group The University 670 I Szeged, Hungary Received March 30, 1978
F. Solymosi*
T. Blrnsilrgl
