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Gas phase atomic hydrogen reacting with molecular and atomic oxygen to form water on the Pt(111) surface
Surface Science 419 (1999) 104–113
Gas phase atomic hydrogen reacting with molecular and atomic oxygen to form water on the Pt(111) surface Adam T. Capitano *, Aaron M. Gabelnick, John L. Gland University of Michigan, Department of Chemistry, Ann Arbor, MI 48109-1055, USA Received 2 April 1998; accepted for publication 11 September 1998
Abstract Gas phase atomic hydrogen is more reactive than coadsorbed hydrogen during reaction with both adsorbed atomic and molecular oxygen on the Pt(111) surface. The increased reactivity of gas phase atomic hydrogen is demonstrated by complete conversion of all adsorbed molecular oxygen to water below 180 K. In contrast, coadsorbed hydrogen induces less water formation under similar conditions. Preferential hydroxyl formation is observed for low gas phase atomic hydrogen exposures as evidenced by the predominance of water formed by hydroxyl recombination at 200 K. Under these conditions the sharp reduction in the intensity of the molecular oxygen desorption peak at 150 K together with an increase in the atomic oxygen desorption peak at 800 K provide direct evidence for gas phase atomic hydrogen-induced O–O bond cleavage. Adsorbed atomic oxygen is also completely converted to water at 110 K even for low exposures of gas phase atomic hydrogen. © 1999 Published by Elsevier Science B.V. All rights reserved. Keywords: Gas phase atomic hydrogen; Hydrogen ; Hydrogen radical; Hydroxyl; Molecular oxygen; Oxygen; Pt(111); Water
1. Introduction Platinum is a well-known catalyst for the reaction between hydrogen and oxygen. This reaction has been studied extensively since the time of Faraday because of its fundamental and applied importance [1–5]. Hydrogen oxidation continues to have current applications in fuel cell technology. To date, most studies conducted on platinum surfaces have focused on the reaction of atomic oxygen with coadsorbed hydrogen. Water formation has been characterized on close-packed Pt(111) surface using a variety of spectroscopies including: temperature-programmed reaction studies ( TPRS) [6–8], ultraviolet photoemission * Corresponding author. Fax: +1 734 7648776; e-mail: [email protected]
spectroscopy [6 ], X-ray photoemission spectroscopy ( XPS) [9], SSIMS [8], electron energy loss spectroscopy [10], and second harmonic generation [11]. The reaction of molecular oxygen and coadsorbed hydrogen was studied on the Pt(111) surface using TPRS and XPS, showing limited water formation at 200 K, with molecular oxygen desorbing at 150 K [9]. TPRS studies of the reaction of atomic oxygen and coadsorbed hydrogen identified three distinct temperature regions where water desorbed from the Pt(111) surface [4,8]. The first water desorption peak at 175 K is attributed to the molecular desorption of water. The second and third peaks at 220 and 300 K are reaction-limited peaks involving a combination of surface hydroxyl intermediates. These intermediates were examined using SSIMS and HREELS, and are believed to be
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water–hydroxyl complexes, or simply adsorbed surface hydroxyls [8,10]. The reaction of molecular oxygen and coadsorbed hydrogen has received less attention. Coadsorbed hydrogen does not react directly with molecular oxygen, but water formation occurs upon O dissociation at 150 K. Gas phase atomic 2 hydrogen has been shown to be effective in the activation of CNC, C–I, and C–C bonds [12–17]. Therefore, gas phase atomic hydrogen should be ideal for the study of the direct interaction between hydrogen and molecular oxygen. Saturation of the surface with molecular oxygen results in a higher oxygen content compared with the atomic oxygen saturated surface, providing the opportunity of synthesizing high concentrations of surface hydroxyl. Adsorbed hydroxyl intermediates are thought to play an important role in combustion and other catalytically important chemical reactions [18–20]. The reaction of gas phase atomic hydrogen with adsorbed atomic oxygen has recently received increasing attention. Investigations on the Ni(100) and Ru(100) surfaces have shown that adsorbed atomic oxygen is readily hydrogenated to water in a two-step process involving a hydroxyl intermediate creating through an Eley–Rideal mechanism [21–23]. In both cases, low exposures of gas phase atomic hydrogen were sufficient to completely induce water formation in adsorbed atomic oxygen. In this paper, we show that gas phase atomic hydrogen reacts directly with molecular oxygen on the Pt(111) surface to form high concentrations of water below 120 K.
2. Experimental All experiments were performed in an ultrahigh vacuum chamber equipped with turbomolecular, ion, and TSP pumps which combined to give a base pressure of 5×10−10 Torr. The system was equipped with a quadrupole mass spectrometer for TPRS, Auger electron spectroscopy to verify surface cleanness, and an atomic hydrogen source for the production of gas phase atomic hydrogen. A Pt(111) crystal oriented within 0.5° of the low index plane was mounted on a ceramic support
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with two 0.5 mm tantalum wires which allowed heating to 1050 K and liquid nitrogen cooling to 110 K. The sample was attached to an L-shaped manipulator that allows three-coordinate displacements and 360° rotation. This allowed exact positioning in front of all instruments and gas dosers. Temperature was measured with a 0.01 mm chromel–alumel (type K ) thermocouple spot welded to the back of the crystal. The crystal was cleaned by initial Ar+ ion sputtering followed by annealing to 1000 K. During experimental runs, the sample was cleaned using cycles of oxidation by heating in O at 2 8×10−8 Torr at 700 K for 5 min followed by annealing to 1000 K for 100 s. Auger electron spectroscopy verified that the surface was clean. A Hunt scientific control system allowed the simultaneous detection of up to eight masses with independent control of mass spectrometer electrometer amplifier sensitivities. For all experiments, a linear heating rate of 5 K s−1 was used. Reactive gases were inlet through a directional dosing system controlled by a leak valve. Hydrogen (Matheson 99.9999%) and oxygen (Matheson 99.9999%) were used without further purification. Gas phase atomic hydrogen was generated by passing molecular hydrogen over a 2100 K tungsten filament positioned 10 cm from the sample. We refer to the mixture of hydrogen atoms and molecules as gas phase atomic hydrogen for simplicity. Details of this method of hydrogen atom creation are discussed elsewhere [24]. In each experiment, reactive gases were adsorbed on the surface on 120 K. All exposures are expressed in terms of Langmuirs (1 L=1×10−6 Torr s) based on ionization gauge pressure readings and have not been corrected for large directional dosing fluxes and ion gauge sensitivity factors. Atomic oxygen coverages were obtained by dosing a saturation coverage of oxygen followed by annealing at 200 K. This method has been shown to produce an oxygen atom coverage of 0.25 ML [25].
3. Results As reported in previous work [4,8] and replicated in Fig. 1a, the reaction of atomic oxygen
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Fig. 1. TPRS showing water formation resulting from the reaction of atomic oxygen (0.25 L) with 10 L of either coadsorbed hydrogen or gas phase atomic hydrogen. (a) Reaction with coadsorbed hydrogen results in water formation as seen from the three peaks at 175, 200 and 290 K. (b) Reaction with gas phase atomic hydrogen produces only desorption-limited water at 175 K.
with 10 L coadsorbed hydrogen results in three water formation peaks at 175 K, 200 K, and 290 K. The first desorption peak at 175 K is water desorption rate limited [9]. The second and third peaks are reaction-limited peaks involving reactions of surface hydroxyl intermediates. The broad peak near 800 K in the oxygen TPRS trace is the result of the recombination of unreacted atomic oxygen [25]. For the hydrogen TPRS trace, a broad peak at 350 K resulting from the recombination of unreacted hydrogen is observed. Since both unreacted oxygen and hydrogen are still present on the surface, a 10 L exposure of hydrogen is insufficient to derive the water formation reaction to completion during a TPD cycle. With higher coverages of surface hydrogen, all of the oxygen can be
converted to water, as indicated by the presence of only the desorption-limited water peak at 175 K and the absence of oxygen desorption at higher temperatures (data not shown). Comparison of the water yield from complete reaction of a saturated oxygen monolayer and hydrogen with the water yield from a 10 L hydrogen exposure ( Fig. 1a) indicates a reaction yield of approximately 15%, in agreement with observations by Ho et al. [26 ]. The reaction of atomic oxygen with 10 L gas phase atomic hydrogen is shown in Fig. 1b. In this case, the water desorption spectrum has a single, intense feature at 175 K corresponding to desorption-limited water. Comparison of this peak with the integrated area of the water desorption peaks
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found in Fig. 1a shows that over 8 times the water is formed from a similar hydrogen exposure. In the oxygen TPRS trace, no unreacted oxygen is detected, indicating complete reaction. The hydrogen TPRS has a feature at 170 K corresponding to a water fragment, and a higher temperature peak at 300 K from the desorption of excess surface hydrogen. Since the two high temperature water peaks from OH reaction are not present and no excess oxygen desorption is observed, the reaction between gas phase atomic hydrogen and atomic oxygen must be completed below 175 K. Unlike the case with coadsorbed hydrogen, the water formation reaction goes to completion even at low gas phase atomic hydrogen exposures. Molecular oxygen and coadsorbed surface hydrogen react to form water at 200 K. Fig. 2a shows the water, oxygen, and hydrogen TPRS traces from the coadsorption of 40 L H on a surf molecular oxygen presaturated surface. The water TPR spectra has two features at 200 and 290 K. These data are consistent with previous work by Fisher et al. in which water formation by hydroxyl recombination was observed [9]. The oxygen TPRS trace shows two peaks at 150 and 800 K. These correspond to the well-known oxygen molecular (150 K ) and atomic recombination (800 K ) desorption pathways [25]. Molecular desorption dominates indicating that only a small fraction of the oxygen dissociates. In the hydrogen TPR spectra, no desorption features are observed except for the small peaks from water fragmentation near 200 K. Fig. 2b shows the reaction of molecular oxygen and 40 L of gas phase atomic hydrogen. Most striking is the complete reduction in intensity of the molecular oxygen desorption peak that is normally found at 150 K. Concurrently, the formation of atomic oxygen is observed as evidenced by the O recombination peak at 800 K. These data indi2 cate that the gas phase atomic hydrogen must be interacting with molecular oxygen on the surface at 120 K. The hydrogen TPR spectra has peaks corresponding to water fragmentation as well as the molecular hydrogen desorption peak at 300 K. With two water desorption peaks at 175 and 200 K and a small shoulder near 290 K, the water (m/e=18) TPRS trace in Fig. 2b is similar to
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TPRS studies of atomic oxygen and coadsorbed hydrogen ( Fig. 1a). However, the ratio of the three water desorption peaks is very different from previously reported results [6–8]. A comparison of the 175 K relative to the 200 K peak in Fig. 2b and Fig. 1a clearly shows that the 175 K peak is dominant in the case of gas phase atomic hydrogen. The dominance of the 175 K desorptionlimited water peak suggests that gas phase atomic hydrogen directly induces water formation. Under similar conditions, this feature is not observed in the reaction of molecular oxygen and coadsorbed hydrogen ( Fig. 2a). Increasing gas phase atomic hydrogen exposure to adsorbed molecular oxygen causes a dramatic increase in the 175 K desorption-limited water peak. In Fig. 3, water (m/e=18) TPRS traces are shown for exposures of 20, 30, 40, and 210 L. With a 20 L gas phase atomic hydrogen dose, only the 200 K reaction channel is observed. Thus hydroxyl formation appears to play an important role in the reaction between gas phase atomic hydrogen and adsorbed molecular oxygen. With increasing gas phase atomic hydrogen exposure, the intensity of the 200 K peak decreases and the intensity of the desorption-limited water peak at 175 K increases. Following high exposures, all the molecular oxygen reacts to form water on the surface below 175 K. When 210 L of gas phase atomic hydrogen is exposed to the oxygen-covered Pt(111) surface, only the 175 K desorption-limited water peak is observed. Therefore, the water formation temperature increases with decreasing exposure to gas phase atomic hydrogen from the desorption-limited peak at 175 K to the reactionlimited peak at 200 K. Thus for small gas phase atomic hydrogen exposures when excess oxygen is present, hydroxyl intermediates dominate the surface and water formation is reaction limited at 200 K. The variations of the oxygen and hydrogen TPR spectra with increasing gas phase atomic hydrogen exposure were also monitored. Fig. 4 shows the hydrogen (m/e=2) and oxygen (m/e=32) TPRS traces resulting from the exposure of a molecular oxygen-saturated Pt(111) surface to 20, 30, 40 and 210 L of gas phase atomic hydrogen. For low exposures of gas phase atomic hydrogen, no excess
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Fig. 2. TPRS of the reaction of molecular oxygen with either 40 L of coadsorbed or gas phase atomic hydrogen. (a) Coadsorbed hydrogen leads to water formation by hydroxyl recombination. Excess oxygen desorbs molecularly (150 K ) and by recombination (800 K ). (b) Gas phase atomic hydrogen induces both desorption-limited water as well as hydroxyl recombination water. No molecular oxygen (150 K ) is observed.
surface hydrogen remains on the surface and the only peaks in the TPR trace results from water fragmentation. Above 40 L exposures, molecular hydrogen desorption at 300 K begins to appear. At high exposures, the hydrogen concentration increases to fill the hydrogen b states at 280 K 2 [27]. As shown in Fig. 4b, low exposures of gas phase atomic hydrogen are insufficient to completely remove all molecular oxygen. For a 20 L hydrogen radical exposure, small molecular (150 K ) and large atomic (800 K ) oxygen desorption peaks are observed. The much greater intensity of the atomic peak demonstrates that even at this low exposures most of molecular oxygen has been converted to atomic oxygen. With further
increases in exposure, the molecular peak is completely consumed, and the intensity of the atomic oxygen peak is also reduced. Following a 210 L exposure of gas phase atomic hydrogen, no surface oxygen remains on the surface.
4. Discussion Gas phase atomic hydrogen has increased reactivity compared to surface hydrogen for reaction with molecular oxygen. As seen in Fig. 2a, when molecular oxygen is coadsorbed with surface hydrogen, a substantial amount of molecular oxygen desorbs and a small amount of water
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Fig. 3. Increasing gas phase atomic hydrogen exposure first causes water formation through the hydroxyl recombination pathway. As exposure increases, desorption-limited water becomes increasingly dominant.
formation is observed. Water formation does not occur from direct reaction of molecular oxygen with coadsorbed hydrogen. Instead, atomic oxygen is first formed by the dissociation of molecular oxygen at 150 K [25]. Upon formation, atomic oxygen reacts with surface hydrogen to form water through hydroxyl intermediates. In contrast, gas phase atomic hydrogen clearly induces O–O bond activation in molecular oxygen in the Pt(111) surface. Analysis of the molecular oxygen desorption trace (150 K ) in the TPRS spectra in Fig. 4b shows a substantial reduction in intensity compared with what was achieved with a similar exposure of coadsorbed hydrogen in Fig. 2a. Although gas phase atomic hydrogen is known to displace adsorbates from many surfaces, the cause of the reduction of the molecular oxygen
desorption peak can not be limited to displacement alone since an increase in the concentration of surface oxygen is concurrently observed [28]. A significant amount of the adsorbed molecular oxygen undergoes bond scission to form adsorbed atomic oxygen during reaction with gas phase atomic hydrogen. Comparison of the water TPORS in Fig. 2a and b provides further evidence from the increased reactivity of gas phase atomic hydrogen with molecular oxygen. The two panels in Fig. 2 compare water yields for identical apparent hydrogen exposures to a molecular oxygen-saturated surface. Panel 2a indicates that a small amount of water is formed following a 40 L hydrogen exposure with the dissociating filament off. Panel 2b indicates that a much larger amount of water is formed
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Fig. 4. TPRS showing the oxygen and hydrogen traces resulting from increasing gas phase atomic hydrogen exposure. (a) Excess surface hydrogen increases with increasing exposure. (b) At low gas phase atomic hydrogen exposures, some molecular oxygen remains (150 K ) and a substantial amount of atomic oxygen is formed. As exposure increases, both of these decrease until no surface oxygen remains.
following a 40 L exposure with the dissociating filament on (note the scaling factor in 2b). Thus for identical hydrogen exposures, the yield of water formation is much greater with gas phase atomic hydrogen. Enhanced reactivities for gas phase atomic hydrogen are further indicated by the large desorption-limited water peak at 175 K observed following atomic hydrogen exposure. In contrast, with coadsorbed hydrogen, a small reactionlimited water formation peak is observed at 200 K caused by hydroxyl recombination. The reaction of gas phase atomic hydrogen with adsorbed atomic oxygen is also very facile. Low exposures of hydrogen atoms drive this reaction to completion, resulting in only desorption-limited
water desorption at 175 K. In contrast, surface hydrogen coadsorbed with atomic oxygen leads to water formation at 175 and 200 K. Water formation at 200 K has been shown to result from the reaction of adsorbed hydroxyl [28]. Since hydroxyl formation has been shown to be the rate-limiting step for water formation, any reaction that favors OH formation will facilitate water production [29]. Hydroxyl formation was shown to be the intermediate in the reaction with gas phase atomic hydrogen and atomic oxygen on the Ru(100) and Ni(100) surfaces [21–23]. Based on the data presented here and by analogy with previous studies, a similar mechanism is proposed for this reaction on the Pt(111) surface.
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The increased reactivity of gas phase atomic hydrogen reacting with both molecular and atomic hydrogen is supported by considering the thermochemistry of this system. When specific thermochemical data were not available, we have estimated their values [30]. The energy of adsorbed hydrogen is −9 kcal mol−1 relative to the gas phase [31]. Similarly, the energy for adsorbed atomic oxygen is −27.7 kcal mol−1 [32]. Through the comparison of experimental data with BOC-MP calculations, Sellers et al. determined the adsorption energy of adsorbed hydroxyl relative to the gas phase to be −60 cal mol−1 [33]. Taken with the formation energy of 9.5 kcal mol−1 for gas phase hydroxyl, this gives a surface formation energy of −50.5 kcal mol−1 for OH adsorbed on Pt(111) [34]. From these values, the DH for the rxn reaction O +H OH was calculated to be ad ad ad −13.8 kcal mol−1. This agrees with previous experimental results [1–10]. Using the thermodynamic values just described and the fact that gas phase atomic hydrogen has a potential energy of 52.1 kcal mol−1, DH for the reaction rxn O +H OH was estimated to be ad atomic ad −74.9 kcal mol−1 [35]. This implies that the reaction of gas phase atomic hydrogen with adsorbed atomic oxygen is more thermodynamically favorable than the reaction with surface hydrogen. Furthermore, the 52.1 kcal mol−1 potential energy of gas phase atomic hydrogen is high compared with the activation barrier to hydroxyl formation of 8.1 kcal mol−1 [36 ]. This also accounts for the
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enhanced ability of gas phase atomic hydrogen to induce water formation. For the reaction of gas phase atomic hydrogen and molecular oxygen, the reaction energetics are more favorable for gas phase atomic hydrogen than for coadsorbed hydrogen. With an adsorption energy of 8.8 kcal mol−1, molecular oxygen has a much weaker bond to the platinum surface than atomic oxygen does [25]. Overcoming an activation barrier of 7.8 kcal mol−1 is required to dissociate the oxygen atoms [37]. At 120 K, coadsorbed hydrogen does not have enough energy to surmount the activation barrier; therefore, no reaction occurs until oxygen dissociation occurs. The high potential energy of 52.1 kcal mol−1 for gas phase atomic hydrogen is sufficient to cleave the O–O bond. For the reaction O +H OH 2ads atomic ads +O , the DH is estimated to be ads rxn −86 kcal mol−1. Previous studies have shown that gas phase atomic hydrogen induces bond breaking through hydrogen addition in several species including C–I in alkyl halides and C–C in cyclopropane [12–17]. Cyclopropane has a C–C bond strength of 54.4 kcal mol−1 [38]. Since adsorbed molecular oxygen has a relatively weak bond energy of 20 kcal mol−1, it is likely that hydrogen radicals can add to adsorbed molecular oxygen in a similar manner to form an adsorbed hydroxyl and an atomic oxygen atom. Varying the amount of gas phase atomic hydrogen exposed to the surface provides addi-
Fig. 5. Proposed mechanism for reaction of gas phase atomic hydrogen and molecular oxygen on the Pt(111) surface. For low gas phase atomic hydrogen exposures, the O–O bond is cleaved and adsorbed hydroxyl is formed. Upon heating, the surface hydroxyls can combine to form water. As exposure increases, gas phase atomic hydrogen begins to add directly to the hydroxyls to form water.
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tional support for hydroxyl formation. As seen in Fig. 3, low exposures of gas phase atomic hydrogen result primarily in hydroxyl recombination pathways leading to water formation. As exposure increases, greater amounts of molecular water are formed until only desorption-limited water is present. Therefore, addition of gas phase atomic hydrogen to adsorbed molecular oxygen to form hydroxyl and an atomic oxygen appears to be the dominant initiation step. Further addition of gas phase atomic hydrogen to hydroxyl results in sequential hydrogen addition leading to water formation. Previous studies have shown that gas phase atomic hydrogen often interacts with adsorbates through an Eley–Rideal mechanism [39]. In most of these systems gas phase atomic hydrogen adds to the adsorbed species [12–17]. Therefore, we propose the following mechanism for the reaction of gas phase atomic hydrogen with molecular oxygen (as illustrated in Fig. 5). First, a hydrogen radical impinges on the adsorbed molecular oxygen and forms a surface hydroxyl and an oxygen atom. As discussed above, the reaction of atomic oxygen to form water and hydroxyl is very facile. Any atomic oxygen formed will readily undergo hydrogenation to form hydroxyl. As noted above, surface hydroxyl can also react with gas phase atomic hydrogen to form water. Therefore, after gas phase atomic hydrogen exposure, the surface contains atomic oxygen, free adsorbed hydrogen, hydroxyl, and water. Upon heating, the water formed desorbs at 175 K. Further heating leads to water formation through hydroxyl recombination and hydrogen addition.
5. Conclusions Gas phase atomic hydrogen can be used to make water efficiently from adsorbed molecular oxygen on the Pt(111) surface. Low exposures of hydrogen radicals add directly to the O–O of adsorbed molecular oxygen to form hydroxyl and surface oxygen. With increasing hydrogen atom exposure, both the hydroxyl and oxygen species can be reduced to form water. Since gas phase atomic hydrogen reacts directly with O , water formation 2
occurs to a much greater extent than what can be achieved with surface hydrogen.
Acknowledgements This work was supported by D.O.E. grant number DE-FG02-91ER1490, and is greatly appreciated. We would also like to thank Lauren Meyers for her assistance with this and many other manuscripts.
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Gas phase atomic hydrogen reacting with molecular and atomic oxygen to form water on the Pt(111) surface Adam T. Capitano *, Aaron M. Gabelnick, John L. Gland University of Michigan, Department of Chemistry, Ann Arbor, MI 48109-1055, USA Received 2 April 1998; accepted for publication 11 September 1998
Abstract Gas phase atomic hydrogen is more reactive than coadsorbed hydrogen during reaction with both adsorbed atomic and molecular oxygen on the Pt(111) surface. The increased reactivity of gas phase atomic hydrogen is demonstrated by complete conversion of all adsorbed molecular oxygen to water below 180 K. In contrast, coadsorbed hydrogen induces less water formation under similar conditions. Preferential hydroxyl formation is observed for low gas phase atomic hydrogen exposures as evidenced by the predominance of water formed by hydroxyl recombination at 200 K. Under these conditions the sharp reduction in the intensity of the molecular oxygen desorption peak at 150 K together with an increase in the atomic oxygen desorption peak at 800 K provide direct evidence for gas phase atomic hydrogen-induced O–O bond cleavage. Adsorbed atomic oxygen is also completely converted to water at 110 K even for low exposures of gas phase atomic hydrogen. © 1999 Published by Elsevier Science B.V. All rights reserved. Keywords: Gas phase atomic hydrogen; Hydrogen ; Hydrogen radical; Hydroxyl; Molecular oxygen; Oxygen; Pt(111); Water
1. Introduction Platinum is a well-known catalyst for the reaction between hydrogen and oxygen. This reaction has been studied extensively since the time of Faraday because of its fundamental and applied importance [1–5]. Hydrogen oxidation continues to have current applications in fuel cell technology. To date, most studies conducted on platinum surfaces have focused on the reaction of atomic oxygen with coadsorbed hydrogen. Water formation has been characterized on close-packed Pt(111) surface using a variety of spectroscopies including: temperature-programmed reaction studies ( TPRS) [6–8], ultraviolet photoemission * Corresponding author. Fax: +1 734 7648776; e-mail: [email protected]
spectroscopy [6 ], X-ray photoemission spectroscopy ( XPS) [9], SSIMS [8], electron energy loss spectroscopy [10], and second harmonic generation [11]. The reaction of molecular oxygen and coadsorbed hydrogen was studied on the Pt(111) surface using TPRS and XPS, showing limited water formation at 200 K, with molecular oxygen desorbing at 150 K [9]. TPRS studies of the reaction of atomic oxygen and coadsorbed hydrogen identified three distinct temperature regions where water desorbed from the Pt(111) surface [4,8]. The first water desorption peak at 175 K is attributed to the molecular desorption of water. The second and third peaks at 220 and 300 K are reaction-limited peaks involving a combination of surface hydroxyl intermediates. These intermediates were examined using SSIMS and HREELS, and are believed to be
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water–hydroxyl complexes, or simply adsorbed surface hydroxyls [8,10]. The reaction of molecular oxygen and coadsorbed hydrogen has received less attention. Coadsorbed hydrogen does not react directly with molecular oxygen, but water formation occurs upon O dissociation at 150 K. Gas phase atomic 2 hydrogen has been shown to be effective in the activation of CNC, C–I, and C–C bonds [12–17]. Therefore, gas phase atomic hydrogen should be ideal for the study of the direct interaction between hydrogen and molecular oxygen. Saturation of the surface with molecular oxygen results in a higher oxygen content compared with the atomic oxygen saturated surface, providing the opportunity of synthesizing high concentrations of surface hydroxyl. Adsorbed hydroxyl intermediates are thought to play an important role in combustion and other catalytically important chemical reactions [18–20]. The reaction of gas phase atomic hydrogen with adsorbed atomic oxygen has recently received increasing attention. Investigations on the Ni(100) and Ru(100) surfaces have shown that adsorbed atomic oxygen is readily hydrogenated to water in a two-step process involving a hydroxyl intermediate creating through an Eley–Rideal mechanism [21–23]. In both cases, low exposures of gas phase atomic hydrogen were sufficient to completely induce water formation in adsorbed atomic oxygen. In this paper, we show that gas phase atomic hydrogen reacts directly with molecular oxygen on the Pt(111) surface to form high concentrations of water below 120 K.
2. Experimental All experiments were performed in an ultrahigh vacuum chamber equipped with turbomolecular, ion, and TSP pumps which combined to give a base pressure of 5×10−10 Torr. The system was equipped with a quadrupole mass spectrometer for TPRS, Auger electron spectroscopy to verify surface cleanness, and an atomic hydrogen source for the production of gas phase atomic hydrogen. A Pt(111) crystal oriented within 0.5° of the low index plane was mounted on a ceramic support
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with two 0.5 mm tantalum wires which allowed heating to 1050 K and liquid nitrogen cooling to 110 K. The sample was attached to an L-shaped manipulator that allows three-coordinate displacements and 360° rotation. This allowed exact positioning in front of all instruments and gas dosers. Temperature was measured with a 0.01 mm chromel–alumel (type K ) thermocouple spot welded to the back of the crystal. The crystal was cleaned by initial Ar+ ion sputtering followed by annealing to 1000 K. During experimental runs, the sample was cleaned using cycles of oxidation by heating in O at 2 8×10−8 Torr at 700 K for 5 min followed by annealing to 1000 K for 100 s. Auger electron spectroscopy verified that the surface was clean. A Hunt scientific control system allowed the simultaneous detection of up to eight masses with independent control of mass spectrometer electrometer amplifier sensitivities. For all experiments, a linear heating rate of 5 K s−1 was used. Reactive gases were inlet through a directional dosing system controlled by a leak valve. Hydrogen (Matheson 99.9999%) and oxygen (Matheson 99.9999%) were used without further purification. Gas phase atomic hydrogen was generated by passing molecular hydrogen over a 2100 K tungsten filament positioned 10 cm from the sample. We refer to the mixture of hydrogen atoms and molecules as gas phase atomic hydrogen for simplicity. Details of this method of hydrogen atom creation are discussed elsewhere [24]. In each experiment, reactive gases were adsorbed on the surface on 120 K. All exposures are expressed in terms of Langmuirs (1 L=1×10−6 Torr s) based on ionization gauge pressure readings and have not been corrected for large directional dosing fluxes and ion gauge sensitivity factors. Atomic oxygen coverages were obtained by dosing a saturation coverage of oxygen followed by annealing at 200 K. This method has been shown to produce an oxygen atom coverage of 0.25 ML [25].
3. Results As reported in previous work [4,8] and replicated in Fig. 1a, the reaction of atomic oxygen
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Fig. 1. TPRS showing water formation resulting from the reaction of atomic oxygen (0.25 L) with 10 L of either coadsorbed hydrogen or gas phase atomic hydrogen. (a) Reaction with coadsorbed hydrogen results in water formation as seen from the three peaks at 175, 200 and 290 K. (b) Reaction with gas phase atomic hydrogen produces only desorption-limited water at 175 K.
with 10 L coadsorbed hydrogen results in three water formation peaks at 175 K, 200 K, and 290 K. The first desorption peak at 175 K is water desorption rate limited [9]. The second and third peaks are reaction-limited peaks involving reactions of surface hydroxyl intermediates. The broad peak near 800 K in the oxygen TPRS trace is the result of the recombination of unreacted atomic oxygen [25]. For the hydrogen TPRS trace, a broad peak at 350 K resulting from the recombination of unreacted hydrogen is observed. Since both unreacted oxygen and hydrogen are still present on the surface, a 10 L exposure of hydrogen is insufficient to derive the water formation reaction to completion during a TPD cycle. With higher coverages of surface hydrogen, all of the oxygen can be
converted to water, as indicated by the presence of only the desorption-limited water peak at 175 K and the absence of oxygen desorption at higher temperatures (data not shown). Comparison of the water yield from complete reaction of a saturated oxygen monolayer and hydrogen with the water yield from a 10 L hydrogen exposure ( Fig. 1a) indicates a reaction yield of approximately 15%, in agreement with observations by Ho et al. [26 ]. The reaction of atomic oxygen with 10 L gas phase atomic hydrogen is shown in Fig. 1b. In this case, the water desorption spectrum has a single, intense feature at 175 K corresponding to desorption-limited water. Comparison of this peak with the integrated area of the water desorption peaks
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found in Fig. 1a shows that over 8 times the water is formed from a similar hydrogen exposure. In the oxygen TPRS trace, no unreacted oxygen is detected, indicating complete reaction. The hydrogen TPRS has a feature at 170 K corresponding to a water fragment, and a higher temperature peak at 300 K from the desorption of excess surface hydrogen. Since the two high temperature water peaks from OH reaction are not present and no excess oxygen desorption is observed, the reaction between gas phase atomic hydrogen and atomic oxygen must be completed below 175 K. Unlike the case with coadsorbed hydrogen, the water formation reaction goes to completion even at low gas phase atomic hydrogen exposures. Molecular oxygen and coadsorbed surface hydrogen react to form water at 200 K. Fig. 2a shows the water, oxygen, and hydrogen TPRS traces from the coadsorption of 40 L H on a surf molecular oxygen presaturated surface. The water TPR spectra has two features at 200 and 290 K. These data are consistent with previous work by Fisher et al. in which water formation by hydroxyl recombination was observed [9]. The oxygen TPRS trace shows two peaks at 150 and 800 K. These correspond to the well-known oxygen molecular (150 K ) and atomic recombination (800 K ) desorption pathways [25]. Molecular desorption dominates indicating that only a small fraction of the oxygen dissociates. In the hydrogen TPR spectra, no desorption features are observed except for the small peaks from water fragmentation near 200 K. Fig. 2b shows the reaction of molecular oxygen and 40 L of gas phase atomic hydrogen. Most striking is the complete reduction in intensity of the molecular oxygen desorption peak that is normally found at 150 K. Concurrently, the formation of atomic oxygen is observed as evidenced by the O recombination peak at 800 K. These data indi2 cate that the gas phase atomic hydrogen must be interacting with molecular oxygen on the surface at 120 K. The hydrogen TPR spectra has peaks corresponding to water fragmentation as well as the molecular hydrogen desorption peak at 300 K. With two water desorption peaks at 175 and 200 K and a small shoulder near 290 K, the water (m/e=18) TPRS trace in Fig. 2b is similar to
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TPRS studies of atomic oxygen and coadsorbed hydrogen ( Fig. 1a). However, the ratio of the three water desorption peaks is very different from previously reported results [6–8]. A comparison of the 175 K relative to the 200 K peak in Fig. 2b and Fig. 1a clearly shows that the 175 K peak is dominant in the case of gas phase atomic hydrogen. The dominance of the 175 K desorptionlimited water peak suggests that gas phase atomic hydrogen directly induces water formation. Under similar conditions, this feature is not observed in the reaction of molecular oxygen and coadsorbed hydrogen ( Fig. 2a). Increasing gas phase atomic hydrogen exposure to adsorbed molecular oxygen causes a dramatic increase in the 175 K desorption-limited water peak. In Fig. 3, water (m/e=18) TPRS traces are shown for exposures of 20, 30, 40, and 210 L. With a 20 L gas phase atomic hydrogen dose, only the 200 K reaction channel is observed. Thus hydroxyl formation appears to play an important role in the reaction between gas phase atomic hydrogen and adsorbed molecular oxygen. With increasing gas phase atomic hydrogen exposure, the intensity of the 200 K peak decreases and the intensity of the desorption-limited water peak at 175 K increases. Following high exposures, all the molecular oxygen reacts to form water on the surface below 175 K. When 210 L of gas phase atomic hydrogen is exposed to the oxygen-covered Pt(111) surface, only the 175 K desorption-limited water peak is observed. Therefore, the water formation temperature increases with decreasing exposure to gas phase atomic hydrogen from the desorption-limited peak at 175 K to the reactionlimited peak at 200 K. Thus for small gas phase atomic hydrogen exposures when excess oxygen is present, hydroxyl intermediates dominate the surface and water formation is reaction limited at 200 K. The variations of the oxygen and hydrogen TPR spectra with increasing gas phase atomic hydrogen exposure were also monitored. Fig. 4 shows the hydrogen (m/e=2) and oxygen (m/e=32) TPRS traces resulting from the exposure of a molecular oxygen-saturated Pt(111) surface to 20, 30, 40 and 210 L of gas phase atomic hydrogen. For low exposures of gas phase atomic hydrogen, no excess
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Fig. 2. TPRS of the reaction of molecular oxygen with either 40 L of coadsorbed or gas phase atomic hydrogen. (a) Coadsorbed hydrogen leads to water formation by hydroxyl recombination. Excess oxygen desorbs molecularly (150 K ) and by recombination (800 K ). (b) Gas phase atomic hydrogen induces both desorption-limited water as well as hydroxyl recombination water. No molecular oxygen (150 K ) is observed.
surface hydrogen remains on the surface and the only peaks in the TPR trace results from water fragmentation. Above 40 L exposures, molecular hydrogen desorption at 300 K begins to appear. At high exposures, the hydrogen concentration increases to fill the hydrogen b states at 280 K 2 [27]. As shown in Fig. 4b, low exposures of gas phase atomic hydrogen are insufficient to completely remove all molecular oxygen. For a 20 L hydrogen radical exposure, small molecular (150 K ) and large atomic (800 K ) oxygen desorption peaks are observed. The much greater intensity of the atomic peak demonstrates that even at this low exposures most of molecular oxygen has been converted to atomic oxygen. With further
increases in exposure, the molecular peak is completely consumed, and the intensity of the atomic oxygen peak is also reduced. Following a 210 L exposure of gas phase atomic hydrogen, no surface oxygen remains on the surface.
4. Discussion Gas phase atomic hydrogen has increased reactivity compared to surface hydrogen for reaction with molecular oxygen. As seen in Fig. 2a, when molecular oxygen is coadsorbed with surface hydrogen, a substantial amount of molecular oxygen desorbs and a small amount of water
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Fig. 3. Increasing gas phase atomic hydrogen exposure first causes water formation through the hydroxyl recombination pathway. As exposure increases, desorption-limited water becomes increasingly dominant.
formation is observed. Water formation does not occur from direct reaction of molecular oxygen with coadsorbed hydrogen. Instead, atomic oxygen is first formed by the dissociation of molecular oxygen at 150 K [25]. Upon formation, atomic oxygen reacts with surface hydrogen to form water through hydroxyl intermediates. In contrast, gas phase atomic hydrogen clearly induces O–O bond activation in molecular oxygen in the Pt(111) surface. Analysis of the molecular oxygen desorption trace (150 K ) in the TPRS spectra in Fig. 4b shows a substantial reduction in intensity compared with what was achieved with a similar exposure of coadsorbed hydrogen in Fig. 2a. Although gas phase atomic hydrogen is known to displace adsorbates from many surfaces, the cause of the reduction of the molecular oxygen
desorption peak can not be limited to displacement alone since an increase in the concentration of surface oxygen is concurrently observed [28]. A significant amount of the adsorbed molecular oxygen undergoes bond scission to form adsorbed atomic oxygen during reaction with gas phase atomic hydrogen. Comparison of the water TPORS in Fig. 2a and b provides further evidence from the increased reactivity of gas phase atomic hydrogen with molecular oxygen. The two panels in Fig. 2 compare water yields for identical apparent hydrogen exposures to a molecular oxygen-saturated surface. Panel 2a indicates that a small amount of water is formed following a 40 L hydrogen exposure with the dissociating filament off. Panel 2b indicates that a much larger amount of water is formed
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Fig. 4. TPRS showing the oxygen and hydrogen traces resulting from increasing gas phase atomic hydrogen exposure. (a) Excess surface hydrogen increases with increasing exposure. (b) At low gas phase atomic hydrogen exposures, some molecular oxygen remains (150 K ) and a substantial amount of atomic oxygen is formed. As exposure increases, both of these decrease until no surface oxygen remains.
following a 40 L exposure with the dissociating filament on (note the scaling factor in 2b). Thus for identical hydrogen exposures, the yield of water formation is much greater with gas phase atomic hydrogen. Enhanced reactivities for gas phase atomic hydrogen are further indicated by the large desorption-limited water peak at 175 K observed following atomic hydrogen exposure. In contrast, with coadsorbed hydrogen, a small reactionlimited water formation peak is observed at 200 K caused by hydroxyl recombination. The reaction of gas phase atomic hydrogen with adsorbed atomic oxygen is also very facile. Low exposures of hydrogen atoms drive this reaction to completion, resulting in only desorption-limited
water desorption at 175 K. In contrast, surface hydrogen coadsorbed with atomic oxygen leads to water formation at 175 and 200 K. Water formation at 200 K has been shown to result from the reaction of adsorbed hydroxyl [28]. Since hydroxyl formation has been shown to be the rate-limiting step for water formation, any reaction that favors OH formation will facilitate water production [29]. Hydroxyl formation was shown to be the intermediate in the reaction with gas phase atomic hydrogen and atomic oxygen on the Ru(100) and Ni(100) surfaces [21–23]. Based on the data presented here and by analogy with previous studies, a similar mechanism is proposed for this reaction on the Pt(111) surface.
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The increased reactivity of gas phase atomic hydrogen reacting with both molecular and atomic hydrogen is supported by considering the thermochemistry of this system. When specific thermochemical data were not available, we have estimated their values [30]. The energy of adsorbed hydrogen is −9 kcal mol−1 relative to the gas phase [31]. Similarly, the energy for adsorbed atomic oxygen is −27.7 kcal mol−1 [32]. Through the comparison of experimental data with BOC-MP calculations, Sellers et al. determined the adsorption energy of adsorbed hydroxyl relative to the gas phase to be −60 cal mol−1 [33]. Taken with the formation energy of 9.5 kcal mol−1 for gas phase hydroxyl, this gives a surface formation energy of −50.5 kcal mol−1 for OH adsorbed on Pt(111) [34]. From these values, the DH for the rxn reaction O +H OH was calculated to be ad ad ad −13.8 kcal mol−1. This agrees with previous experimental results [1–10]. Using the thermodynamic values just described and the fact that gas phase atomic hydrogen has a potential energy of 52.1 kcal mol−1, DH for the reaction rxn O +H OH was estimated to be ad atomic ad −74.9 kcal mol−1 [35]. This implies that the reaction of gas phase atomic hydrogen with adsorbed atomic oxygen is more thermodynamically favorable than the reaction with surface hydrogen. Furthermore, the 52.1 kcal mol−1 potential energy of gas phase atomic hydrogen is high compared with the activation barrier to hydroxyl formation of 8.1 kcal mol−1 [36 ]. This also accounts for the
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enhanced ability of gas phase atomic hydrogen to induce water formation. For the reaction of gas phase atomic hydrogen and molecular oxygen, the reaction energetics are more favorable for gas phase atomic hydrogen than for coadsorbed hydrogen. With an adsorption energy of 8.8 kcal mol−1, molecular oxygen has a much weaker bond to the platinum surface than atomic oxygen does [25]. Overcoming an activation barrier of 7.8 kcal mol−1 is required to dissociate the oxygen atoms [37]. At 120 K, coadsorbed hydrogen does not have enough energy to surmount the activation barrier; therefore, no reaction occurs until oxygen dissociation occurs. The high potential energy of 52.1 kcal mol−1 for gas phase atomic hydrogen is sufficient to cleave the O–O bond. For the reaction O +H OH 2ads atomic ads +O , the DH is estimated to be ads rxn −86 kcal mol−1. Previous studies have shown that gas phase atomic hydrogen induces bond breaking through hydrogen addition in several species including C–I in alkyl halides and C–C in cyclopropane [12–17]. Cyclopropane has a C–C bond strength of 54.4 kcal mol−1 [38]. Since adsorbed molecular oxygen has a relatively weak bond energy of 20 kcal mol−1, it is likely that hydrogen radicals can add to adsorbed molecular oxygen in a similar manner to form an adsorbed hydroxyl and an atomic oxygen atom. Varying the amount of gas phase atomic hydrogen exposed to the surface provides addi-
Fig. 5. Proposed mechanism for reaction of gas phase atomic hydrogen and molecular oxygen on the Pt(111) surface. For low gas phase atomic hydrogen exposures, the O–O bond is cleaved and adsorbed hydroxyl is formed. Upon heating, the surface hydroxyls can combine to form water. As exposure increases, gas phase atomic hydrogen begins to add directly to the hydroxyls to form water.
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tional support for hydroxyl formation. As seen in Fig. 3, low exposures of gas phase atomic hydrogen result primarily in hydroxyl recombination pathways leading to water formation. As exposure increases, greater amounts of molecular water are formed until only desorption-limited water is present. Therefore, addition of gas phase atomic hydrogen to adsorbed molecular oxygen to form hydroxyl and an atomic oxygen appears to be the dominant initiation step. Further addition of gas phase atomic hydrogen to hydroxyl results in sequential hydrogen addition leading to water formation. Previous studies have shown that gas phase atomic hydrogen often interacts with adsorbates through an Eley–Rideal mechanism [39]. In most of these systems gas phase atomic hydrogen adds to the adsorbed species [12–17]. Therefore, we propose the following mechanism for the reaction of gas phase atomic hydrogen with molecular oxygen (as illustrated in Fig. 5). First, a hydrogen radical impinges on the adsorbed molecular oxygen and forms a surface hydroxyl and an oxygen atom. As discussed above, the reaction of atomic oxygen to form water and hydroxyl is very facile. Any atomic oxygen formed will readily undergo hydrogenation to form hydroxyl. As noted above, surface hydroxyl can also react with gas phase atomic hydrogen to form water. Therefore, after gas phase atomic hydrogen exposure, the surface contains atomic oxygen, free adsorbed hydrogen, hydroxyl, and water. Upon heating, the water formed desorbs at 175 K. Further heating leads to water formation through hydroxyl recombination and hydrogen addition.
5. Conclusions Gas phase atomic hydrogen can be used to make water efficiently from adsorbed molecular oxygen on the Pt(111) surface. Low exposures of hydrogen radicals add directly to the O–O of adsorbed molecular oxygen to form hydroxyl and surface oxygen. With increasing hydrogen atom exposure, both the hydroxyl and oxygen species can be reduced to form water. Since gas phase atomic hydrogen reacts directly with O , water formation 2
occurs to a much greater extent than what can be achieved with surface hydrogen.
Acknowledgements This work was supported by D.O.E. grant number DE-FG02-91ER1490, and is greatly appreciated. We would also like to thank Lauren Meyers for her assistance with this and many other manuscripts.
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