Langmuir 1985,1, 526-528
526
Letters Kinetics of Hydroxyl Recombination on Clean and Oxygen-Covered Ag( 110) Scott W. Jorgensen, A. G. Sault, and R. J. Madix* Department of Chemical Engineering, Stanford University, Stanford, California 94305 Received March 25, 1985 The kinetics of hydroxyl disproportionation on the Ag(l10) surface was studied by temperature-programmed desorption. The activation energy of the reaction was determined to be 22.2 f 0.3 kcal/mol. The reaction was second order with a preexponential factor of 0.7 cm2/s, in good agreement with the preexponential expected for simple second-order recombination reactions of atomic species. It was observed that the presence of oxygen causes a stabilization of the OH groups on the surface; 0.3 monolayer of oxygen led to a stabilization of 0.05 monolayer of OH by approximately 2.5 kcal/mol due to the collective lateral interaction. The recombinative desorption of simple adsorbates has been extensively studied.' Adsorbed atoms such as On, N,, Ha, etc. recombine to form molecules which desorb immediately. These reactions are characterized by frequency factors of the order to loo cm2/s. We report here on the kinetics of the hydroxyl recombinative desorption. In contrast to simple adsorbates, hydroxyl groups disproportionate to yield H20 and 0,. The kinetics of this type of reaction has received little attention in the literature. The mechanism of the reaction of water with predosed oxygen on Ag(ll0) has been studied using a variety of technique^.^*^ If an oxygen-pretreated Ag(110) surface is dosed with water at 150 K and heated, three water desorption peaks are observed below room temperature. These peaks are due to desorption of an ice layer (170 K), a hydrogen bonded "second layer" (200 K), and a nonhydrogen-bonded first layer (240 K). At 320 K hydroxyl recombination occurs, resulting in further water evolution and leaving adsorbed oxygen on the surface.2 The experiments were carried out in a stainless steel ultrahigh vacuum chamber which has been described p r e v i ~ u s l y .Multiply ~~~ distilled water was further purified by pumping on the condensed solid to remove dissolved gas. No impurities could be identified in the mass spectrometer scan when the purified water was vaporized into the chamber. The operating base pressure was below 9 X torr; helium was the major component of the background. Surface cleanliness was verified using AES. Though a small amount of carbon on silver is difficult to detect with AES, it was removed by dosing oxygen onto the crystal and flashing the crystal to 700 K to form COz until no C02 could be detected. When an oxygen pretreated surface was exposed to water at or below room temperature to form hydroxyls directly by the reaction of water with 02 and the surface (1) Madix, R. J.; Benziger, J. Annu. Rev. Phys. Chem. 29, 1978, 285. (2) Stuve, E. J.; Madix, R. J. Surf. Sei.1981, I l l , 11. (3) Pockrand, I. Surf. Sci. 1982, 122, L569. (4) Benziger, J. B.; Madix, R. J. Surf. Sci. 1980, 94, 119. (5) Outka, D. A.; Friend, C. M.; Jorgensen, S.; Madix, R. J. J. Am. Chem. SOC.1983, 105, 3468.
0743-7463/85/2401-0526$01.50/0
Table I. Change in Desorption Energy with Excess Oxygen" eo, ML Tp,K AE,, kcal/mol 0.01 367 0.0 0.05 370 0.6 0.22 380 1.7 0.24 381 1.8 0.31 387 2.6 "The data of Figure 3 are tabulated by peak temperature and change in activation energy based on the assumption of constant preexponential factor.
was subsequently flashed to 400 K, the hydroxyl recombination proceeded to completion. As expected from the reversible reaction H 2 0 + 0, s 20H,, subsequent to recombination the surface retained the same amount of oxygen that was present prior to the water exposure. By dosing oxygen on the crystal once, being careful thereafter not to flash off any of the atomic oxygen, the hydroxyl coverage could be quite exactly reproduced several times in succession by saturating the surface with water and flashing off the molecular states by heating above 250 K. The crystal temperature was kept below 410 K to avoid possible loss of 0, into the crystal. In order to examine the kinetics of hydroxyl recombination, approximately 0.15 monolayer of oxygen was adsorbed initially onto the Ag(ll0) surface at 300 K. The crystal was then exposed to 5 L of H 2 0 at 210 K, giving rise to saturation in both of the water states that desorb above 200 K. Eight different flash desorptions were performed at different heating rates between 2 and 40 K/s using this one oxygen dose (Figure 1); thus the surface hydroxyl concentration was constant at 0.30 f 0.02 monolayer. From these results the activation energy was calculated to be 22.2 f 0.3 kcal/mo16 (Figure 2A). The OH coverage was also changed by variation of the initial 0, in order to determine the reaction order. The reaction was established to be second order using methods discussed previously (Figure 2B) .6 The second-order preexponential factor was calculated to be 0.7 cm2 s-l. This (6) Falconer, J. L.; Madix, R. J. J. Catal. 1978, 54, 414.
0 1985 American Chemical Society
Letters
Langmuir, Vol. 1, No. 4, 1985 527 H20/0H
eo;
OH Recombination Activation Energy
Ag(ll0)
A
0.3 ml
36.3 31.4 25.8 19 2 14.8
0
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1
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.
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300 350 Temperature
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.
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315
320
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100 OH Recombination Reaction Order
450
B
/
(K)
Figure 1. Temperature-programmeddesorption curves for water
formation via hydroxyl recombination. A constant hydroxyl concentrationof 0.3 ML was maintained by preservingthe oxygen left after recombination. The peak above 300 K is due to hydroxyl disproportionation. The change in peak size is due to the change in desorption rate as the heating rate, 8, was increased from 2.7 to 36.3 K s-l. The recombination event is observed on a rising background. value is in close agreement with the frequency factors of simple recombination. The activation energy is nearly 20 kcal/mol lower than the recombinativk desorption of oxygen atoms.' When the atomic oxygen was only partially reacted with water, an adlayer of hydroxyls in excess atomic oxygen resulted. When hydroxyl recombination took place in the presence of excess oxygen, the desorption of water occurred at a higher temperature. The increase in desorption temperature was proportional to the amount of excess oxygen, as shown in Figure 3 and Table I. These experiments were performed using a constant water dose and different oxygen predoses. The heating rate was kept constant at P = 19.5 K/s. In order to monitor the concentration of OH, the area of each water desorption peak was calculated. The excess oxygen varied from 0.02 ML to 0.30 ML. The excess oxygen caused a linear temperature shift of 20 K, equivalent to a change in activation energy of about 2.5 kcal/mol. With 0.3 ML of excess oxygen present the hydroxyl recombination at 6OH = 0.05 occurred nearly 15 K higher than on the clean surface at any OH coverage. There was no apparent change in peak shape; it is therefore likely that the kinetic order was unchanged. This temperature shift is due to stabilization of the OH adlayer either by hydrogen bonding to the excess 0 atoms or via a collective interaction. The work of Stuve, using EELS,2 and Pockrand, using SERS,3are inconclusive regarding hydrogen bonding between OH groups. If hydrogen bonding with oxygen atoms did occur and were responsible for this stabilization, each hydroxyl could only bond to one oxygen atom. This would lead to stabilization = 6oH and no further stabilization a t higher up to Bo oxygen coverages. A similar argument holds for intermolecular OH interactions. However, the stability of the (7) Bowker, M. Surf. Sei. 1980, 100, L472.
-Q
IO
B
2
I
I
2
5 10 Ln(ow)
20
50
Figure 2. (A) Determinationof Ea. The peak temperatures from the data in Figure 1me used to fiid the activation energy by using a plot of Ln (@/Tp2) vs. l/Tp,where j3 is the heating rate, Tp is the peak temperature, and the slope of the plot is equal to the activation energy divided by the gas constant. E, is found to be 22.2 h 0.3 kcal/mol. (B)Determination of reaction order. The reaction order is determined from a plot of Ln (rate) vs. Ln (eoH) at a constant heating rate of 19.5 Ks-'. The slope of the plot gives the reaction order based on the assumption that the coverage dependence is of the form (60,)". In this case the slope is 1.9 A 0.15.
hydroxyl groups increases linearly even at oxygen coverages 6 times the hydroxyl coverage. It therefore appears that there exists a collective lateral interaction between the OH groups and the atomic oxygen which stabilizes the hydroxyls. The possibility of OH island formation on the preadsorbed oxygen must also be considered. Oxygen atoms at 300 K are mobile and form ordered (nxl)structures. If the OH groups form islands, the effect of excess oxygen would be to stabilize the OH as discussed above, since the excess oxygen is distributed external to the islands, the OH coverage being held constant. If, on the other hand, OH groups are randomly distributed upon the preadsorbed oxygen, they may be farther apart at low coverage. Cor-
528 Langmuir, Vol. 1, No. 4, 1985
300
350
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Temperoture
450 (K)
Fmre 3. Hydroxyl recombination in the presence of 0,. A nearly constant OH coverage was maintained by careful H20dosing. The hydroxyl coverage was always less than 0.05 ML. Variations in the OH coverage are responsible for the variation in area of the peaks. At this coverage a 100% change in OH coverage would only cause a 5 K change in desorption temperature. A heating rate of 19.5 Ks-* was used.
respondingly, the migration distance for recombination is greater, and the peak temperature is expected to be higher in contrast to the observed behavior. Thus, although the magnitude of the stabilization by oxygen may be greater
Letters
than calculated from the peak shift, its origin appears to be collective lateral interactions. Evidence exists for 0-0 interactions on Ag(ll0). Oxygen adsorbed at 300 K is known to align in evenly spaced rows perpendicular to the troughs.8-10 The lower the oxygen coverage, the farther the rows are separated. Bowker has, in fact, successfully interpreted the apparent first-order recombination of adsorbed oxygen atoms to form O2from this surface in terms of a second-order reaction with strong attractive lateral interactions.' The range of hydroxyl stabilization due to 0, in this case must be greater than one lattice spacing. When 0.30 ML of excess oxygen is present with 0.05 ML of hydroxyl it is not possible, even if the hydroxyls are mobile, for each 0, to be a nearest neighbor to a hydroxyl. Some oxygen atoms must be at least two sites away, yet a smooth increase in the OH stabilization continues. Thus the range of the interaction is at least two lattice sites. We conclude that atomic oxygen stabilizes the hydroxyl group via attractive lateral interactions which can extend up to two lattice distances.
Acknowledgment. We gratefully acknowledge the support of this work through NSF Grant CPE-8320072. S.W.J. and A.G.S. also acknowledge predoctoral fellowships granted by the NSF.
(8) Engelhardt, H. A.; Menzel, D. Surf.Sci. 1976,57, 591. (9) Rovida, G. J . Phys. Chem. 1976,80, 150.
(10)Bowker,M.; Barteau, M. A.; Madix, R. J. Surf.Sci. 1980, 92, 528.
