Langmuir 1986,2, 302-304
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Activation Energies for Thermal Desorption of Hydroxyl Radicals from Single-Crystal Platinum(11 1) and Polycrystalline Platinum Foil Surfaces David S. Y. Hsu,* Mark A. Hoffbauer,? and M. C. Lin* Chemistry Division, Code 6105, Naval Research Laboratory, Washington, D.C. 20375-5000 Received September 30, 1985 Apparent desorption energies were obtained for OH radicals formed in the associative reaction of chemisorbed 0 and H atoms on single-crystal Pt(ll1) and polycrystalline Pt foil surfaces by using the laser-induced fluorescence (LIF) technique coupled to an UHV surface science apparatus. As the reactant O/H gas mixture ratio is decreased from 10 to 0.1, the OH desorption energies increase from 27 to 36 kcal/mol for the Pt(ll1) crystal and from 32 to 42 kcal/mol for the polycrystalline Pt foil. We have recently measured the desorption energies of OH radicals formed in the catalytic oxidation of hydrogen on clean, well-characterized, single-crystal Pt(ll1) and polycrystalline platinum foil using laser-induced fluorescence (LIF) to detect OH radicals in the gas phase following desorption from the surfaces. For both the single crystal and the foil experiments, the OH desorption energies were found to decrease as the O/H reactant mixture ratio was increased. This confirmed the general trend observed in an earlier fast flow experiment in this laboratory on the thermal desorption of OH from an uncharacterized polycrystalline Pt wire catalyst.l This result is significant in view of the distinct experimental differences between the present work and the fast flow experiment. In the present experiment, the Pt(ll1) single crystal was well-ordered (as verified by LEED) and both the crystal and foil surfaces were cleaned by argon ion sputtering and periodically monitored for impurities by Auger electron spectroscopy. Measurements were taken with the reactant gas pressure in the torr range in order to ensure collisionless conditions for the desorbed OH radicals in the LIF detection zone. For the fast flow experiments, the polycrystalline wire surface was never thoroughly characterized and was only cleaned by heating in oxygen at high temperatures. Reactant gas pressures in the several millitorr range were used which precluded measurements of the directly desorbed OH radicals prior to any gas-phase collisional relaxation processes. In spite of these differences, we have found near-quantitative agreement in the measured apparent activation energies for the foil data in the UHV study and the wire data in the flow study. The reaction chamber used in the present experiments employed a conventional UHV surface science apparatus modified by the addition of laser and photomultiplier ports and coupled to an excimer pumped dye laser system for LIF detection of the desorbed OH radicals. The apparatus included an Auger electron spectrometer, low-energy diffraction (LEED) optics, an argon ion sputtering gun, a residual gas analyzer, and turbomolecular, ion, and sublimation pumps. The incorporation of a 1000 L/s turbomolecular pump was required to handle relatively large reactant gas throughput during the experiments. A vacuum in the torr range could be readily achieved after bake out. The 8 mm diameter X 1 mm thick Pt(ll1) crystal was cut from a stock ingot (Cornell Materials Research Center, research associate 1982-1984. Present address: Chemistry Division, CHM-2, Los Alamos National Laboratory, Los Alamos, NM 87545. t NRC/NRL
0743-7463/86/2402-0302$01.50/0
>99.995%) and was polished to a metallographic finish at NRL. Laue X-ray back-reflection analysis yielded a (111) surface orientation that was 3/40 from the perpendicular in the (100) direction. If it could be assumed that steps are of single atom height, the crystal would have terraces of (111)orientation that are about 75 atoms wide, separated by monoatomic high steps of (100) orientation. The 10 mm diameter X 0.5 mm thick platinum foil was cut from a platinum foil sheet (Alpha Products, >99.9995%). Surface impurities on the crystal and the foil (in particular C, Si, and Ca) were carefully monitored before, during, and after the experimental runs. Special care was taken to remove a small calcium impurity by repeated heating in oxygen followed by argon ion sputtering at 1200 K until Auger spectra taken between 300 and 1200 K showed no calcium or oxygen (from CaO). During the experiment either the crystal or foil was positioned with its face parallel to and about 1 mm away from the laser beam. The reaction was carried out by using an effusive source of premixed O2 and H2 of a certain O/H concentration ratio which was flowed over the heated surface. The OH radicals were formed by the recombination reaction of adsorbed 0 and H atoms, produced initially by dissociative chemisorption of O2 and H2, respecti~ely.~-~ A small fraction of the OH radicals formed on the surface can desorb and were detected by LIF using an excimer pumped dye laser which was frequency-doubled to the 280-285-nm region suitable for OH (%+,v’ = 1 211,u = 0) excitation. In these experiments, the dye laser was fixed at 281.132 nm corresponding to the R1(5) (band head) transition in OH. LIF signals resulting from the OH (22+, u’ = 1 211, u r r= 1)transition near 309 nm were filtered and detected with a photomultiplier tube. The signals were processed by a boxcar integrator and digitized and stored in a microcomputer. For a given O/H ratio, the LIF intensity measurements were made as a function of temperature. The typical reaction chamber pressure was 5 x torr. Some test runs were also made at 9.5 X and 2.5 X torr. At a fixed temperature the signals in both the foil and single-crystal experiments were observed to be roughly proportional to the pressures used. Figure 1 shows typical Arrhenius plots of the OH LIF intensity data for the Pt(ll1) crystal for O/H ratios of 1/5,
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(1) Fujimoto, G. T.; Selwyn, G. S.; Keiser, J. T.; Lin, M. C. J.Phys. Chem. 1983,87, 1906. (2) Talley, L. D.; Tevault, D. E.; Lin, M. C. Chem. Phys. Lett. 1979,
66, 584. (3) Unstead, M. E.; Talley, L. D.; Tevault, D. E.; Lin, M. C. Opt. Eng. 1980, 10, 94. (4) Tevault, D. E.; Talley, L. D.; Lin, M. C. J. Chem. Phys. 1980, 72, 3314.
0 1986 American Chemical Society
Activation Energies for Hydroxyl Radical Desorption
Langmuir, Vol. 2, No. 3, 1986 303
P i Foil
0.65
_.0.86
0.75 1000/T
0.85 (K-'
0.95
1
Figure 1. Typical Arrhenius plote for OH desorption rate obtained from the OH LIF intensity data for the Pt(ll1) single crystal for O/Hreactant mixture ratios of (squares),1 (circlea), torr. The and 5 (triangles) at a total reactant preasure of 5 x solid lines are nonlinear least-squaresfits to the data (see text).
1, and 5. The magnitudes of the intensities of the three data sets cannot be directly compared, since they were measured under different conditions. The curvature in these plots at the higher temperature end is reproducible and could be due to the low concentration of adsorbed 0 and H atoms on the surface due to the increased rate for desorption of H2 and O2at the higher temperatures." In this temperature region (-1350 K), the total formation rate of OH on the surface could become so slow as to compete with the desorption rate of OH as the rate-limiting step. Kinetic modeling is under way to examine this effect by taking into account the change in sticking coefficients of H2 and O2 a t high temperatures. This conjecture is supported by the observation that the curvature is the greatest in test runs carried out at the lowest pressure of 2.5 X torr, and no such curvature was observed in our previous fast-flow experiments under higher pressure condition^.^?^ In the leasbsquarea analysis, data points in the curving regions of the plots were not included. For the three sets of data with O/H ratios of 1/5, 1, and 5 as shown in Figure 1, we obtain apparent OH desorption energies of 34.7 f 1.2, 29.7 f 2.3, and 26.1 f 1.9 kcal/mol, respectively. These should be taken as lower limits due to the effecta of curvature in the Arrhenius plots. Another possible explanation for the curvature is dynamic in origin, as suggested by trajectory calculations on thermal desorption of Xe and NO from Pt(ll1),loJ1which (5) Christmann, K.; Ertl, G.; P i p e t , T. Surf. Sei. 1976, 54, 365. (6) Norton, P. R.; Richards, P. J. Surf. Sci. 1974, 44, 129. (7) Gland, J. L.; Fisher, G. B.; Kollin, E. B. J. Cotal. 1982, 77, 263. (8)Gland, J. L.; Korchak, V. N. Surf. Sci. 1978, 75, 733. (9) Gland, J. L. Surf. Sei. 1980,93,487. (10) Grimmelmann, E. K.;. Tullv, - . J. C.; Helfand. E. J. Chem. Phvs. 1981, 74, 5300. (11) Muhlhausen, C. W.; Williams, L. R.; Tully, J. C. J . Chem. Phys. 1986,83,2594.
I
I
0.75
0.85
1000/T
(K-'
A 0.95
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Figure 2. Typical Arrhenius plots for OH desorption rate data for the polycrystalline Pt foil for O/Hreactant mixture ratios of l / 6 (squares), 1 (circles), and 8 (triangles) at a total reactant pressure of 5 X lob torr. Each set was measured on a different day. Solid lines are nonlinear least-squares fits to the data.
exhibited similar leveling off in the Arrhenius plots at high temperatures due to the steep decrease in the sticking probability with increasing temperature. The similarity between NO and OH is particularly significant because the desorption energies for these two systems are about the same. Similar calculations for OH desorption could be very useful in elucidating the origin of possible dynamical effects. Interestingly, the curvature in the platinum foil data is much less, as could be seen from Figure 2. One likely reason is that the more abundant surface irregularities such as steps and kinks etc. on the foil enhance dissociative chemisorption58J2J3of O2and H2 as well as the reactivity of the foil surface.14 The adsorbed 0 and H atom concentrations on the foil could be expected to be higher than on the single crystal. The resulting higher formation rate of OH on the foil surface would maintain an adequate supply of OH on the surface, ensuring that the OH desorption process is the rate-limiting step. The observed energies also appear to be higher than those observed in the single-crystal experiments at the same O/H ratios. The observations of the curvature in the Arrheniw plots at the higher temperature end as well as the scalability of the LIF signal with reactant pressure suggest that the coverage of the reactants on the surfaces was low. The measured desorption energies would then reflect largely the effective interaction potential of the OH radicals with the Pt surfaces, without significant interference from other adsorbants. Figure 3 summarizes the apparent desorption energy results as a function of the O/H ratio for both the Pt foil and the P t ( l l 1 ) single crystal. The reactant mixture pressures used were either 5 X or 9.5 X 10" torr. A t (12) Hopster, H.; Ibach, H.; Comsa, G. J. Catol. 1977, 46, 37. (13) Poelsema, B.; Verheij, L. K.; Comsa, G. Surf. Sei. 1985,152/153, 496. (14) Lang, B.; Joyner, R. W.; Somorjai, G. A. Surf. Sei. 1972,30,454.
Hsu et al.
304 Langmuir, Vol. 2, No. 3, 1986
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1
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8
Pi wire
A
Pt(lll)-UHV
0 R foil
- UHV
. 0.0 1
1 .o
0.1
O/H
10
100
RATIO
Figure 3. Apparent OH desorption energies as a function of the O/H reactant mixture ratio. The triangles and circles are data obtained in this work for the Pt(ll1) single crystal and the polycrystalline Pt foil, respectively (see text). The filled squares are data obtained from an earlier flow experiment using polycrystalline Pt wire.'
a fixed O/H ratio, the desorption energies measured were independent of either of these pressures. In cases with more than one experimental run at one O/H ratio, the results shown represent the averages. In Figure 3, the single-crystal data points (triangles) represent results from 24 experimental runs and the foil data points (open circles) are from 51 experimental runs. As an experimental check on the validity of using a single OH rovibronic transition for these studies, we used results from a separate experiment,15in which rotational energy populations for a O/H = 1 mixture at different surface temperatures were measured by wavelength scanning the laser over the Rzl and R1 branches. The rotational populations (usually 18 points) at each temperature were summed to give one point on the Arrhenius plot. Rotational population data from five different temperatures for the Pt(ll1) single crystal and five different temperatures for the Pt foil were used. The resulting two Arrhenius plots yielded OH desorption energies of 27.3 f 2.4 and 30.4 f 2.2 kcal/mol for the single crystal and the foil, respectively. These points are shown by the filled triangle and circle, respectively, in Figure 3. Also shown as filled squares in Figure 3 are results from the previously mentioned fast flow experiment using polycrystalline Pt wire.l The gradual increase in the desorption energy for the process O(ads) + H(ads) ( = ) OH(ads)
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OH(g)
with decreasing O/H ratios was attributed to the removal of OH(ads), progressively from weaker to stronger surface sites by the dominant reaction H(ads)
+ OH(ads)
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H,O(ads)
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H20(g)
as the concentration of H(ads) increases.' Thus, at high O/H ratios, the relatively high OH(ads) concentrations would occupy a wide spectrum of weak and strong surface sites. Under these conditions, OH radicals from the weak surface sites would dominate the desorption process, ~
~~~
(15) Hoffbauer, M. A.; Hsu, D. S. Y.; Lin, M. C. J.Chem. Phys. 1986, 84, 532.
leading to lower apparent desorption energies. It has also been observed that adsorbed oxygen can block strong binding sites as suggested by work on NO desorption from Pt(lll).16J7In the high O/H ratio regime in the present work, O(ads) could also block the stronger binding sites, making them less accessible to H and OH radicals. As the O/H ratio decreases by increasing H, the OH(ads) concentration decreases, as manifested by the progressively decreasing OH LIF signals. This is due to its removal through H 2 0 formation. An increasingly larger fraction of surface OH would occupy the strong surface sites, thus leading to higher desorption energies. In the fast flow experiment,' the apparent desorption energy was found to increase from 27 f 1kcal/mol at O/H = 100 to 50 f 3 kcal/mol at O/H = 0.03. The extrapolated value of 60 f 3 kcal/mol at O/H = 0 would represent the bond dissociation energy of the strongest Pt-OH bond. Results from the present experiments show the same trend in the desorption energy with O/H ratio with the foil data at some 5-10 kcal/mol higher than the single-crystal data in the O/H ratio range studied. In addition, there is better quantitative agreement between the foil results and those from the fast flow study using the polycrystalline Pt wire. This is not surprising, as the surface properties of the polycrystalline Pt wire can be expected to be more similar to those of the Pt foil than to those of the Pt(ll1) single crystal. The observation that the OH desorption energy from the polycrystalline Pt foil is higher than that from the Pt(ll1) single crystal is reasonable since it has been shown in other studies that surface irregularities and defects can increase the desorption energy due to stronger gas-surface attractive interactions at these sites.17-19 In conclusion, using single-crystal Pt(ll1) and polycrystalline Pt foil surfaces under clean, low-coverage conditions, we have found that the apparent activation energies for OH desorption from the surfaces increases from 27 to 36 k d / m o l and from 32 to 42 kcal/mol, respectively, as the O/H ratio was decreased from 10 to 0.1. This qualitative trend was similar to the one observed in an earlier fast flow study and can be attributed to the desorption of OH radicals bonded to increasingly stronger surface sites as the O/H ratio was decreased. The desorption energies from the Pt foil experiment are in near quantitative agreement with those from the flow study and are slightly higher than those from the Pt(ll1) singlecrystal experiment. These observations could be satisfactorily rationalized by considering the nature of the surfaces involved. If our assumption that the measured apparent desorption energy at O/H 0 corresponds to the OH desorption energy from stronger sites, the result summarized in Figure 3 for Pt(ll1) indicates &[OH/Pt(lll)] 45 kcal/mol at O/H 0. This value suggests that the reaction O(ads) H(ads) OH(ads) on Pt(ll1) is slightly exothermic rather than endothermic20 and the ready production of H 2 0 at 120 K from adsorbed hydrogen and oxygen atoms can be reasonably explained. observed by Fisher et al.7921-23 Registry No. OH,3352-57-6; Pt, 7440-06-4.
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(16) Serri, J. A.; Cardillo, M. J.; Becker, G. E. J. Chem. Phys. 1982, 77, 2175. (17) Campbell, C. T.; Ertl, G.; Segner, J. Surf. Sci. 1982, 115, 309. (18) Lin, T. H.; Somorjai, G. A. Surf. Sci. 1981, 107, 573. (19) Serri, J. A.; Tully, J. C.; Cardillo, M. J. J. Chem. Phys. 1983, 79, 1530. (20) E+ G. Catal.: Sci. Technol. 1983, 4, 209. (21) Fisher, G. B.; Gland, J. L. Surf. Sci. 1980,94,446. (22) Fisher, G. B.; Sexton, B. A. Phys. Reu. Lett. 1980, 44, 683. (23) Fisher, G. B.; Gland, J. L.; Schmieg, S. J. J. Vac. Sci. Technol. 1982, 20, 518.
