174
J. Phys. Chem. 1984,88, 174-175
other possibly the bridged radical but we cannot distinguish these two possibilities. We cannot be certain as to whether the simple Au/HCN iminyl is the cis or trans form but tentatively assign it to the cis form since it is clearly the most stable isomer and is likely to have the same structure as E which is the most stable of the two silver isomers. The g factors for MCH=N are all below the free-spin value (Ag -0.002) and are similar in magnitude to the values for the isoelectronic vinyls MCH=CH.3 The negative g shift suggests spin-orbit interaction with a low-lying unoccupied molecular orbital and is probably a consequence of the large spin-orbit coupling constants for metal atoms. The assignment of A and B and D and E to cis and trans isomers of the iminyl IV and V requires that the SOMO at the nitrogen has some s character to give a *A' ground state with C, symmetry. This is unexpected and unusual but we see no other alternative since the H13CN experiments eliminate imidoyls. The following factors may be operative: (i) The repulsive overlap between the electrons in the filled d orbitals of the metal with the electron in a nitrogen 2pz orbital may be sufficient to favor a state where the nitrogen lone pair and the electron are located in an spx ( x > 1) orbital which is directed away from the C=N bond.
-
H\
.C=N
0
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Such repulsive interactions will not occur in CH2N and hence the free electron orbital is 2p, in this case. (ii) This effect is supplemented by the increased separation and hence reduced repulsion which occurs as the angle between the lone pair and unpaired electron orbitals increases from ~ / 2 this ; may compensate the energy required for hybridization. If our assignments are correct this is an important case of hybridization encouraged by repulsive interactions rather than by bond formation at the hybridized center. Ending on a cautionary note it should be emphasized that although most of the experimental evidence supports two isomeric forms of group 1B organometallic iminyls the absence of a resolve hyperfine interaction with nitrogen for CuCH=N and AuCH=N and the observation of a small nitrogen coupling constant for AgCH=N makes unambiguous identification of cis and trans isomers difficult.20,21 Acknowledgment. We thank Drs. K. F. Preston, J. R. Morton, and J. S . Tse for many helpful discussions and the referees for some pertinent criticisms of an earlier version of this manuscript. J.A.H. and B.M. thank NATO for a Research Grant. Registry No. A, 88230-19-7; C, 88230-18-6; D, 88230-20-0; F, 88230-21-1; CU, 7440-50-8; Ag, 7440-22-4; Au, 7440-57-5; HCN, 7490-8.
(20) Fessenden, R. W. J. Phys. Chem. 1967, 71, 74-83. (21) We thank a referee for bringing this point to our attention.
Hydrogen Isotope Scrambling in Spillover on Pt/TiO, D. D. Beck and J. M. White* Department of Chemistry, University of Texas, Austin, Texas 78712 (Received: August 19, 1983)
Thermal desorption into vacuum, after sequential dosing of H2 and D2 on Pt/Ti02, shows incomplete isotope mixing of the desorbing products and provides evidence for the spillover of H and/or D from the Pt onto the oxide.
Introduction Hydrogen atom migration (spillover) from very active (for Hz dissociation) metal centers to sites located on relatively inactive oxide supports has been a subject of considerable interest and attention for many years in heterogeneous catalysis. Among the systems where such hydrogen spillover has been observed are M/Ti02 catalysts. In NMR/TPD studies of Rh/Ti02, two kinds of irreversibly bound hydrogen were observed; one of these is assigned to metal-bound hydrogen, and the other to surface hydroxyl groups.' ESR experiments have indicated the reversible formation of Ti3+upon exposure of M/Ti02 to H2at temperatures below 773 K.293 In these processes the presence of metal centers active for H2dissociation is necessary. Such centers also catalyze the reduction of Ti02. In this letter, we report preliminary results which show that H-D isotope scrambling is incomplete in molecular hydrogen desorption following sequential dosing of H2 and Dz on Pt/Ti02 under a variety of conditions. This incomplete scrambling is taken (1) T. M. Apple and C. Dybowski, Surf.Sci., 121, 243 (1982). ( 2 ) T. Huizinga and R. Prins, J. Phys. Chem., 85, 2156 (1981). (3) J. C. Conesa and J. Soria, J. Phys. Chem., 86, 1392 (1982).
as a strong indication of spillover from platinum onto T i 0 2 and the importance of diffusion kinetics in both the formation and removal of this spillover hydrogen.
Experimental Section The experiments were carried out in a two-chamber ultrahigh vacuum system. One chamber allows for dosing the substrate at pressures up to several torr and at temperatures between 140 and 600 K. The second chamber, which contains a quadrupole mass spectrometer, is used for temperature programmed desorption (TPD) into ultrahigh vacuum. The sample is moved between the two chambers on a sliding rod assembly. The sample mount allows for cooling to 140 K and resistive heating to 1200 K. The sample of interest is attached to a 0.008-in. diameter W wire which serves as the heating element. A chromel-alumel thermocouple is spot-welded to the center of this wire to indicate the temperature. Pt/Ti02 samples were prepared by impregnation of Degussa P25 titania powders with chloroplatinic acid using standard procedures to give 0.6 wt % Pt. A slurry of the dried form of this material was made and applied and air dried to the center 0.5 cm of the W heater. Weighing after the experiments were completed gave a sample weight of 0.5 mg. The samples were outgassed in the dosing chamber at 800 K for 1 h, and then cooled
0022-3654/84/2088-0174$01.50/00 1984 American Chemical Society
I
IW z
a a
3 0
I
I
I
A 0
I
----
/----
.' _ _ _ _ -------------.- _--= -/'
-
b
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+-
D2
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I
Results Figure 1 shows two sets of TPD s p t r a obtained after sequential dosing of H2and D2 In the first experiment, Figure la, the sample temperature was raised to 500 K and exposure to hydrogen was carried out for 100 s at 0.077 torr. The sample was then quickly evacuated and cooled to 300 K, a process taking about 2 min. It was then exposed to 0.077 torr of D2 for 100 s a t 300 K. Finally, the sample was quickly evacuated and cooled to 140 K to complete the adsorption sequence. After transferral to the TPD chamber, the sample was heated at a rate of 4 K s-' and ion signals for masses 2, 3, and 4 were followed as a function of time. The results, replotted as a function of temperature, are shown in Figure la. Interestingly, the Hz and D2 desorption peaks are separated by about 250 K with the Hz appearing at the higher temperatures. It is also of considerable interest that very little H D is desorbed. Figure l b shows desorption spectra for a second sequential dosing experiment in which Hzand D2 were adsorbed for the same time, pressure, and order as Figure la, but the dosing temperature was constant at 300 K. The separation between the peaks for H2 and D, is reduced to about 100 K. Moreover, there is a significant amount of H D appearing in the desorbed products. Reversing the order of adsorption reverses the desorption sequence.
Similar sequential dosing experiments done in the absence of platinum show only a very tiny amount of hydrogen desorption which peaks above 700 K. On Pt foil, in the absence of TiO,, sequential dosing experiments result in complete isotope mixing and no temperature separation of the peaks for Hz, HD, and DZ. These results, taken together, indicate spillover and the importance of the diffusion of hydrogen atoms away from active metal centers into the surrounding oxide regions. A simple qualitative model, which will be discussed more fully and quantitatively e l ~ e w h e r eis , ~presented here. We assume that dissociative adsorption of hydrogen occurs only at the Pt centers. Saturation of these centers occurs at very low exposures and there is subsequent, relatively slow, radial diffusion away from them into the surrounding oxide. The mean diffusion length increases nonlinearly with both temperature and exposure time. Within the framework of this model we can understand qualitatively the similarities and differences in Figure 1, a and b. In Figure l a , hydrogen is exposed at relatively high temper-
is now a slower radial diffusion rate because of the heavier isotope and because of the lower adsorption temperature. After these exposures we expect adsorbed hydrogen and deuterium to be spatially separated with hydrogen appearing at larger average distances from the platinum particles. Assuming that desorption occurs by migration back to the platinum particles, we then expect to be able to preserve the isotope separation as observed. The same arguments obtain for Figure 1b with the exception that the spatial separation between the isotopes is less pronounced because the dosing temperatures were identical. Similar experiments at other constant dose temperatures and experiments involving a single isotope but with different TPD sequences can be analyzed to show that there are at least two forms of activated spillover which are characterized by different activation energies for diffusion. These results will be reported in detail el~ewhere.~ Acknowledgment. Supported in part by the Office of Naval Research and the Robert A. Welch Foundation. Registry No. Pt, 7440-06-4;TiO,, 13463-67-7; H,,1333-74-0 (4) D. D. Beck, A. Bawagan, and J. M. White, to be submitted for pub-
lication.
Infrared Spectroscopic Study of Platinized Titania Photocatalysts Shinri Sato* and Keiji Kunimatsu Research Institute for Catalysis, Hokkaido University, Sapporo 060, Japan (Received: August 24, 1983; In Final Form: November 8, 1983)
The frequency shift of the IR absorption band of adsorbed CO was used as a probe to monitor the oxidation state of Pt in platinized TiOZ(anatase) photocatalysts. Photoelectrochemical (PEC) deposits of Pt on TiO, powder were found to be weakly oxidized due to exposure to air after its preparation. Oxidized Pt on TiO, was partially reduced to the zero valence state during the photocatalytic water-gas shift reaction, CO + H20(g) H, + C02. This result may be interpreted in terms of a gas-phase PEC mechanism on Pt/Ti02 photocatalysts.
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Metallized, powdered semiconductors have recently attracted considerable attention as a new type of photocatalyst available 0022-3654/84/2088-0175$01.50/0
for radiation-to-chemical energy conversion. A typical example of such a photocatalyst is platinized titania (Pt/TiO,) on which 0 1984 American Chemical Society
