2505
J . Phys. Chem. 1993, 97, 2505-2506
Energetics for the Desorption of Hydroxyl Radicals from a Platinum Surface Charles E. Mooney, Louis C. Anderson, and Jack H. Lunsford' Department of Chemistry, Texas A&M University, College Station, Texas 77843 Received: December 29, 1992
When H20 and 0 2 , together with either H2 or D2, were passed over a heated Pt wire, the variation in E, with respect to the 02/(H2 or D2) ratio was the same. This result is taken as evidence that the Pt-OH bond strength is affected by the coverage of oxygen on the surface, rather than by the effect of temperature on the hydrogen atom coverage.
Introduction The desorption of OH' radicals from metal surfaces during the oxidation of H2 or CH4 has been the subject of several recent studies because of the importance of this radical chain carrier in catalytic combustion One interesting result is the strong dependence of the apparent activation energy on the 0 2 / H 2ratio. This was first observed by Fujimoto et al.,4 who found that over polycrystalline Pt, E, increased from 27.4 kcal/mol at 0 2 / H 2= 100 to 56.2 kcal/mol at 02/H2 = 0.038. A similar phenomenon exists during the reaction of H2 with 0 2 over polycrystalline Pd, only in this case a t O2/H2 > 4 and at T N 1215 K a discontinuity in E, was observed, which is attributed to a two-dimensional PdO phase." The value of E, generally is associated with the Pt-OH bond strength, although it is not clear whether the variation in E, is a result of a change in bond strength with oxygen coverage or whether a more subtle dynamic effect is responsible for the range of Ea'sthat have been observed. Gland et al.I2 showed that the heat of desorption of atomic oxygen increased from 40 kcal/mol at high fractional coverage to 110 kcal/mol at low fractional coverage; thus oxygen coverage can affect the Pt-O bond strength. Williams et a1.I0 have argued that the presence of 0, may have an effect on the Pt-OH bond, but that variations in E, may largely be attributed to competitive surface reactions. The formation of OH' may be described by the following set of reactions:
-+ + + -
H2,
2HS
(1)
02,
20s
(2)
H, 0, OH, H, OH,
H20, 0,
OHs
(3)
H20,
(4)
OH, 20H,
According to Williams et al., at lower temperatures thecoverage of H, is greater, and in the oxygen-rich regime more OH, species would be produced via reaction 3. Therefore the E , would be smaller. By contrast, in the oxygen-poor regime, fewer OH, species would survive reaction 4 and E , would be larger. They conclude that there is a unique activation energy (Le. bond energy) of 47 kcallmol for breaking the Pt-OH bond. Still a third explanation has been given by Lin and co-workers,4-I3 who proposed that strongly and weakly adsorbed OH, is present on the surface. The change in E, is a result of changes in the equilibrium concentrations of the strongly adsorbed OH, as a function of temperature, at different O2/H2 ratios. We believe that the results presented in this paper provide a test of the dynamic model set forth by Williams et a1.I0 In order to adequately understand the role of OH' radicals in catalytic
combustion, it is important to determine whether there is a unique Pt-OH bond strength, or whether the oxygen coverage has a major effect on this bond strength.
Experimental Section Hydroxyl radicals were detected using a laser-induced fluorescence spectrometer, similar to that described previously? Laser excitation of the OH' radical (A2Z,'v = 1 X2n, v = 0) near 282 nm was provided by the frequency-doubled output of a tunable dye laser (bandwidth, -0.3 cm-I; pulse width, - 5 ns; energy, -0.5 mJ/pulse in the UV). The dye laser was pumped by the second harmonic of a Q-switched Nd:YAG laser at 10 Hz. Fluorescence emission (A22,v' = 1 X 2 n , v" = 1, near 3 15 nm) was collected 90' from the laser beam into a spectrometer with a UV-sensitive photomultiplier tube (PMT). The Q1(4) band was used for excitation because the amplitude of this band relative to the total amplitudes of the bands changed less than 10% over the temperature region examined. In the Pyrex reactor H2 (Matheson, UHP) or D2 (Scientific Gas Products, 99.8% pure) and 0 2 (Matheson, extra dry), along with He and H20, flowed over a coil of 0.5-mm-diameter Pt wire (Johnson Matthey), which was suspended about 5 mm above the path of the laser beam. The H2O was introduced by bubbling the other gases through water. Fused-silica windows were attached to the reactor to permit entry of the laser beam and detection of the emitted photons. A thermocouple junction was spot welded on the Pt wire. The total pressure in the cell was less than 100 mTorr in order to minimize gas-phase reactions.
-
-
Results and Discussion The relative production rate of OH' radicals, first with just H2O and 0 2 as reagents and then after the addition of increasing amounts of H2 or D2, is shown in Figure 1. Initially the OH' radicals are produced via reaction 6, but as H2 is added, reaction 3 contributes. At greater H2 pressures, however, reaction 4 becomes more important, and the formation of OH' radicals reaches a maximum. Several groups have reported such a maximum, although the H2 pressure at which it occurs depends on the conditions of the experiment. When Dl was added instead of H2, the rate of OH' production predictably decreased, as OD' radicals fluoresce at a different wavelength. Reactions 3 and 4 and, to a lesser extent, reaction 6 are responsible for this decrease.I4 Surface deuterium removes 0,,and also there is the formation of D2O and HDO. Although the functional effects of H2 and D2 are quite different (Figure l ) , the variation of E, with respect to the 02/(H2 or D2) ratio is the same within experimental error (Figure 2). These results strongly suggest that it is the oxygen coverage which dictates E, for OH' desorption. The range of E, values are in good agreement with those reported by Fujimoto et a1.,4 although
0022-365419312097-2505%04.00/0 0 1993 American Chemical Society
Letters
2506 The Journal of Physical Chemistry, Vol. 97, No. 1 1 , I993
At lower temperatures the coverage of D, would be greater, and in the oxygen-rich regime fewer OH, species would be produced via reaction 3. Therefore the E, would be larger, not smaller. In the oxygen-poor region fewer OH, species would be produced because of the enhanced D, coverage, and E, would again be larger. The presence of less D20 at lower temperatures would increase the production of OH, via reaction 6, but at higher deuterium pressures the consequences of this reaction would be small. The net effect would be that the variation in E, would be less with a change in the Oz/D2 ratio. This conclusion is contrary to the results of Figure 2.
A
g 40
w s !!
A 20
Conclusion
0
0
5
10
15
20
25
P(H2 or D2), mtorr Figure 1. Laser-induced fluorescence of OH' radicals produced over a pt wire a t 900 OC in the presence of H20,02, and increasing amounts of either H2 ( 0 )or D2 (A): P(H2O) = 1.5 mTorr; P ( 0 2 ) = 6.3 mTorr. 60
Acknowledgment. The authors gratefully acknowledge financial support of this research by the US.Department of Energy, Division of Basic Energy Sciences.
55
=E
50
1 45 Y
35 w;40
The identical variation of E, for hydroxyl radical desorption can best be understood in terms of the effect that 0, has on the Pt-OH bond energy. That is, increasing the 0, coverage brings about a decrease in the Pt-OH bond strength of up to 25 kcal/ mol. Energy level diagrams that describe the H2,02, Pt system should account for the variation of the Pt-O and Pt-OH bond strengths with oxygen coverage.
1
y
References and Notes ( I ) Talley, L. D.; Tevault, D. E.; Lin, M. C. Chem. Phys. Lett. 1979,66, 584.
(2) Tevault, D. E.; Talley, L. D.; Lin, M. C. J . Chem. Phys. 1980, 72, 3314. . ..
(3) Talley, L. D.; Sanders, W. A.; Bogan, D. J.; Lin, M. C. J . Chem. Phys. 1981, 75, 3107. (4) Fuiimoto. G.T.; Selwvn. G. W.; Keiser. J. T.; Lin. M.C. J . Phys. ( 5 ) Ljungstriim, S.; Hall, J.; Kasemo, B.; Rosen, A,; Wahnstrom. T. J . Caral. 1987, 107, 648.
(6) Wahnstriim, T.; Fridell, E.; Ljungstrom, S.;Hellsing, B.; Kasemo, Figure 2. Apparent activation energy of OH' radical formation over a Pt wire a s a function of the P(02)/P(H2) ( 0 )and P(Oz)/P(D2) (A) ratios: P(H2O) = 1.5 mTorr; P(02) = 6.3 mTorr.
the largest E, (56 kcal/mol) was obtained at a significantly lower 0 2 / H 2ratio (0.04) in the previous study. The addition of HzO in our study may have reduced the surface concentration of Os, thus requiring additional gas-phase 0 2 to achieve the results of Fujimoto et al.4 It is instructive to go through the analysis of Williams et a1.I0 to determine whether E, could be related to the coverage of D,.
B.; Rosen, A. Surf. Sci. Lett. 1989, 223, L905. (7) Pfefferle. L. D.; Griffin. T. A.: Winter, M.; Croslev. D.: Dyer. M. J. Combust. Flames 1989, 76, 325. (8) Marks, C. M.; Schmidt, L. D. Chem. Phys. 1991, 178, 358. (9) Mooney, C . E.; Anderson, L. C.; Lunsford, J. H. J . Phys. Chem.
1991, 95, 6070.
(IO) Williams, W. R.; Marks,C. M.;Schmidt, L. D. J . Phys. Chem. 1992, 96, 5922. (1 1) Anderson, L. C.; Mooney, C. E.;Lunsford, J. H. Chem. Phys. Lett.
1992, 196, 445.
(12) Gland, J. L.; Sexton, J. L.; Fisher, G . 8.Surf. Sci. 1980, 95, 587. (13) Hsu, D. S. Y.;Hoffbauer, M. A,; Lin, M. C. SurJ Sci. 1987, 184, 25. (14) As pointed out by a reviewer, the exchange reaction D, + OH, OD, + H, may also contribute to the decrease in production of OH. radicals.
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