Thermal Chemistry of Water Adsorbed on Clean and Oxygen

Aug 18, 2007 - ... Christian Kramberger, Thomas Gemming, Alicja Bachmatiuk, Ryszard J. Kalenczuk, Bernd Rellinghaus, Bernd Büchner, and Thomas Pichle...
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J. Phys. Chem. C 2007, 111, 13570-13578

Thermal Chemistry of Water Adsorbed on Clean and Oxygen-Predosed V(100) Single-Crystal Surfaces Min Shen and Francisco Zaera* Department of Chemistry, UniVersity of California, RiVerside, California 92521 ReceiVed: June 1, 2007; In Final Form: June 29, 2007

The adsorption of water on V(100) single-crystal surfaces was characterized by temperature programmed desorption and X-ray photoelectron spectroscopy. On clean surfaces, the adsorption of water was determined to be dissociative, and dissociation to be favored at low coverages. With the aid of deuterium and 18O isotope labeling, it was found that coadsorbed surface oxygen blocks this dissociation but also opens a new reaction channel that relies on hydrogen bonding to enhance the generation of hydroxide surface species. The new reaction channel is seen throughout all stages of oxygen uptake, even after the formation of a thin surface oxide film. Three water desorption states were identified in the oxygen-treated substrates, corresponding to recombination of hydroxide surface groups, decomposition of hydrogen-bonded stabilized water, and molecular desorption, respectively.

1. Introduction The interaction of water with solid surfaces has received significant attention over the past decade.1-3 In heterogeneous catalysis in particular, water is often a reactant or a product and in many instances induces the formation of hydroxide and other surface species.2,4 Specifically, vanadium and vanadium oxides are known to catalyze many important industrial processes such as oxydehydrogenation5,6 and partial oxidation of alkanes7,8 that involve water, hydroxide groups, or both, and that are affected by the presence of those species in terms of catalytic activity and selectivity.9-11 Thus, a better understanding of the surface reactivity of water and hydroxide groups and, in particular, of how that chemistry is affected by the coexistence of surface oxygen atoms may help in the design of better catalysts. With this incentive in mind, a few water adsorption studies have been carried out on vanadium carbide12 and vanadium oxide13-16 surfaces. Nevertheless, little is known still about the chemistry of water on vanadium metal surfaces or about its interaction with coadsorbed oxygen. Studies on vanadium single crystals can shed some light on this chemistry, in particular, because those afford great control on the nature of the surface species involved.17,18 Our recent studies on this system have indicated that V(100) single-crystal surfaces can be treated with oxygen to produce a number of chemisorbed oxygen layers as well as thin oxide films.19 Here, we expand on the reactivity of those surfaces upon water exposures. To guide our studies on the chemistry of water on vanadium metal surfaces, we call upon the extensive past surface-science work available on similar systems.1,2 In particular, it has been argued that the ability of metals to dissociate adsorbed water may be correlated with the heat of formation of the corresponding metal oxides.2 It is therefore reasonable to expect that vanadium surfaces may be quite active toward water dissociation. Moreover, coadsorbed oxygen atoms can act as electron donors and bind to the hydrogen in surface water, hence facilitating its decomposition further. Only in a few cases, on Ru(0001)1,20 and Ni(111),1,2,21 this has proven not to be true. * Corresponding author. E-mail: [email protected].

On the other hand, the activation role of surface oxygen has often been shown to be restricted to low coverages; high oxygen coverages usually inhibit water dissociation due to site-blocking.1,2 On surface oxide layers grown using large oxygen exposures, water usually sticks only weakly, and desorbs without dissociation. Here, we present the first report on water adsorption and thermal conversion on V(100) single-crystal surfaces. On the clean surface, water dissociation was found to take place below 270 K and to be favored by low water coverages. The coadsorption of oxygen appears to inhibit this self-decomposition, perhaps by site blocking, but to also open another reaction channel involving hydrogen bonding to generate hydroxide groups. The latter chemistry dominates and persists throughout all oxygen pre-exposures, either before or after the formation of surface oxides. Three different adsorption states are induced by the presence of surface oxygen, and fill in sequentially throughout the water uptake. A detailed discussion of this behavior is presented in the following sections. 2. Experimental Section All of the experiments were performed in an ultrahigh vacuum chamber (UHV) turbo-pumped to a base pressure of around 2 × 10-10 Torr. A detailed description of this equipment can be found in previous publications.22,23 Briefly, the chamber is equipped with instrumentation for temperature programmed desorption (TPD), X-ray photoelectron (XPS), ion scattering (ISS), secondary ion mass (SIMS), and Auger electron (AES) spectroscopies. The TPD experiments are carried out by using a computer-interfaced Extrel quadrupole mass spectrometer capable of following up to 15 masses in each individual experiment. The solid sample is positioned within 1 mm of the front aperture of the spectrometer in order to selectively detect the species evolving from its front surface. A linear heating rate of 10 K/s was used in all of the TPD experiments reported here, as set by a homemade temperature controller. The TPD signals are reported in arbitrary units, but intensity scales are provided by the bars shown in each graph to allow for relative comparisons. The integrated TPD intensities were also calibrated

10.1021/jp074274l CCC: $37.00 © 2007 American Chemical Society Published on Web 08/18/2007

Thermal Chemistry of Water Adsorbed on V(100) using established procedures24 and reported in monolayers (ML, in units of adsorbed molecules per vanadium surface atom). XPS spectra are acquired by using a magnesium-anode X-ray source and a hemispherical electron energy analyzer. A constant pass energy of 50 eV is used for all of the measurements, and the energy scale is referenced to a binding energy value for Cu 2p3/2 of 932.7 eV.25 Data analysis was performed with the aid of the XPSPEAK fit software, by first subtracting a Shirley background and then fitting appropriate mixed Lorentizian-Gaussian curves, as described before.19 The V(100) single crystal used as the solid sample (99.99% purity, Goodfellow Cambridge Limited, ∼12 mm diameter × ∼2 mm thickness) was cut and polished by using standard procedures, and spot-welded to two tantalum rods attached to an on-axis manipulator capable of rotation and translation in all three dimensions. The single-crystal could be cooled down to liquid nitrogen temperatures and resistively heated to above 1300 K. The temperature of the sample was measured by using a chromel-alumel thermocouple spot-welded to the back of the crystal and controlled with homemade electronics. The surface of the V(100) single crystal was subjected to numerous cycles of sputtering and annealing until no sulfur, carbon, or phosphor could be detected within the sensitivity of our instruments, as described before.19,26 A small amount of oxygen was unavoidable on the annealed surface, but could be kept to reasonably low levels. Previous experiments have indicated that this oxygen does not affect the surface reactivity in a significant way.19 The surface of the crystal was cleaned and annealed before each single TPD and XPS experiment. The 16O2 (99.99%) and 18O2 (95% isotope purity) gases were purchased from Matheson and Cambridge Isotope Laboratories, Inc., respectively, and used as supplied. The distilled water and D2O (99.5 atom % D, Aldrich) were used after purification by several freeze-pump-thaw cycles. Particular difficulties were encountered in the experiments with deuterated water because of isotope scrambling with regular water in the walls of the gas handling system and vacuum chamber. This lowered the isotope purity of the D2O on the surface to values around 50%, in particular because of the extensive use of regular water in the same experiments. Nevertheless, the final purity was consistent across all experiments in a given set and did not affect the qualitative trends observed. All gas dosings were performed by backfilling of the chamber via leak valves. The purities of the reactants were checked frequently in situ by using mass spectrometry. Gas doses were reported in langmuirs (1 L ) 10-6 Torr‚s), uncorrected for differences in ion gauge sensitivities.

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Figure 1. Main frame: temperature programmed desorption (TPD) spectra for molecular H2O on clean V(100) surfaces as a function of exposure at 110 K. A broad peak around 310 K is populated first, but a second sharp peak at 170 K grows after 1.0 L because of condensation on the second layer. Inset: TPD spectra for 2.0 L of water adsorbed on V(100), clean and after 1.0 and 50.0 L predoses of O2 at 320 K. The 310 K water peak broadens and shifts to higher temperatures in the 1.0 L O2 predose case and is totally inhibited after a 50.0 L O2 predose.

3. Results

Figure 2. TPD spectra for hydrogen (a, left panel) and water (b, right panel) from clean V(100) surfaces as a function of water dose at 180 K. Hydrogen desorption takes place at ∼270 K and is maximized at a water dose of 0.35 L. The peak for water desorption at ∼310 K grows steadily until reaching saturation after a water exposure of 0.5 L.

3.1. TPD Results on Clean V(100). The thermal chemistry of water on clean V(100) surfaces was first surveyed by temperature programmed desorption (TPD). The main frame of Figure 1 displays the TPD spectra obtained after dosing various amounts of water on the clean V(100) at 110 K, each starting from a fresh surface after sputtering and annealing. One broad water desorption peak is seen around 310 K for all exposures, either from recombination of hydroxide groups or from monolayer water desorption (see below). This feature appears to be populated even at low doses, and saturates around 0.5 L; further exposure of the surface to water leads to the growth of an additional sharp peak around 170 K ascribed to multilayer condensation.1,2 Oxygen pretreatments of the surface modify this behavior somewhat, as illustrated by the data in the inset of Figure 1. The coadsorbed oxygen does not change

the shape and position of the low-temperature water peak but leads to the broadening and upward shifting of the hightemperature desorption peak (see the trace for the 1.0 L O2 predose), until totally inhibiting this desorption after O2 preexposures of 50.0 L. In addition, the hydrogen desorption around 270 K observed on the clean surface is almost totally suppressed by the predosed oxygen (data not shown). Figure 2 shows the TPD spectra for the evolution of H2 (left panel) and H2O (right panel) obtained after dosing various amounts of water on clean V(100) at 180 K. That temperature was chosen to eliminate any water condensation, and to potentially enhance any dissociative adsorption. It was also determined that under these conditions adsorption from background gases is not a problem, a fact reflected by the absence of any signals in the blank TPD spectra for 0.0 L H2O shown

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Figure 3. TPD yields for hydrogen and water desorption from clean V(100) as a function of water dose, calculated from the data in Figure 2 and reported in ML. The total water uptake calculated both by adding all TPD yields and by using O 1s XPS data are also shown. Hydrogen production is maximized at 0.35 L H2O doses, and saturation is reached by 0.5 L water exposures.

in the bottom traces. An asymmetric hydrogen peak is seen within the 250-280 K temperature region for all water exposures. This hydrogen peak first grows around 270 K after doses of up to 0.35 L and then decreases in size until reaching a constant yield at about 0.5 L as it shifts to lower (260 K) temperatures. Water molecular desorption takes place mainly around 310 K in a feature that develops steadily with increasing water doses, but a small water desorption tail is also observable in the high-temperature side of the TPD traces. Exposures of the surface to water above 0.5 L do not enhance the production of either hydrogen or water but do lead to the growth of a small H2O peak around 230 K, most likely because of adsorption on a second layer. The desorption yields of hydrogen and water derived from the TPD spectra in Figure 2 are reported as a function of initial dose in Figure 3. The water TPD yields were estimated by using the O 1s XPS peak areas for the water adsorbed at low temperatures and referred to those after O2 adsorption (which were calibrated in previous studies).19,27 The hydrogen yields were then calculated by using the relative response of the mass spectrometer to H2 and H2O, and also by using a mass balance argument. These results indicate that water decomposition reaches a maximum at an exposure of 0.35 L, at which point up to ∼55% undergoes total decomposition to hydrogen and surface oxygen. The extent of that reaction then decreases until reaching an approximately constant yield of ∼30% for exposures above 0.5 L, the dose needed for monolayer saturation. The total water uptake calculated by adding both H2O and H2 desorption yields together was found to agree well with that estimated by integration of the corresponding O 1s XPS peak areas. The decomposition of water on clean V(100) surfaces was further probed by monitoring the isotope scrambling between coadsorbed H2O and D2O. Figure 4 shows the 18 (H2O, left panel), 19 (HDO, center panel), and 20 (D2O, right panel) amu desorption traces acquired after first dosing H2O and then D2O (both at 180 K) on the V(100) surfaces while keeping the total water exposures constant at a value of 0.5 L. Although the original isotope purity of the D2O used was >99%, its actual isotope purity as it adsorbed on the surface was determined to be only around 50% because of the inevitable scrambling with

Shen and Zaera regular water that takes place in the gas lines, chamber walls, and mass spectrometer. This was unavoidable, as dosing of regular water was carried out concurrently in these experiments, but was carefully monitored and kept constant during a giving set of experiments to ensure the validity of the qualitative trends observed. The experiments in Figure 4 were repeated numerous times and in different sequences to corroborate the reported observations. Also, the signals for 18 amu shown in Figure 4 were corrected for contributions from D2O and HDO desorption. Clearly, the desorption yields of H2O (18 amu) and D2O (20 amu) follow opposite trends as a function of D2O exposure, with HDO (19 amu) production maximizing at intermediate H2O + D2O mixtures. This is better seen in the inset of Figure 4, which shows the calculated desorption yields for all three isotopomers. Note in particular that in going from the lowest (0.1 L D2O, 0.4 L H2O) to the highest (0.5 L D2O, 0.0 L H2O) D2O exposures, the HDO yield increases almost twofold, clearly pointing to the occurrence of substantial isotope scrambling. Also worth noticing is the maximum in HDO yield obtained with intermediate H2O + D2O mixtures. That maximum is seen at nominally high D2O doses only because the actual D2O content of the mixed layer is lower (due to the H2O impurity in the D2O); it seems that maximum isotope scrambling is achieved with roughly equimolar H2O + D2O surface mixtures. Also, an increase is seen in the total water desorption yield (calculated by adding the desorption yields of three isotopomers together) with increasing D2O content in the mixture, most likely because of a lower dissociation coefficient of D2O than that of H2O,28,29 or perhaps because of an isotope effect in the recombination of hydroxide groups.29 In fact, less D2 desorption is observed from thermal activation of D2O than H2 production from adsorbed H2O under similar conditions, indicating that D2O is indeed less reactive. Finally, all the observations discussed above appear not to be affected by the order of dosing; similar results were obtained when D2O was dosed first (data not shown). 3.2. Oxygen Coadsorption. The possible participation of surface oxygen atoms in the thermal chemistry of water adsorbed on V(100) was also probed by using isotope labeling. Figure 5 shows the TPD spectra for H2 (left panel), H216O (center panel), and H218O (right panel) obtained after 0.35 L doses of H216O on V(100) surfaces pre-exposed with various amounts of 18O2 at 320 K. Previous work has indicated that oxygen doses below 1.0 L lead to the deposition of chemisorbed oxygen, whereas exposures in excess of 1.0 L result in the formation of a thin surface oxide layer of a stoichiometry somewhere between V2O3 and V2O4.19,30 Here, it is seen that the H2 desorption peak seen around 260 K on the clean surface is suppressed significantly by 18O2 predoses as low as 0.25 L. In fact, that decrease in peak intensity is also accompanied by a downward shift to lower temperatures. Then, for 18O2 predoses above 0.5 L, this hydrogen desorption becomes almost negligible, and another desorption regime appears around 500 K. Finally, both hydrogen desorption states disappear after 18O2 predoses above 1.0 L. In terms of H216O desorption (Figure 5, center panel), its peak is initially centered around 310 K but broadens and develops a shoulder extending up to 500 K with the addition of surface oxygen. For 18O2 predoses above 1.0 L, the main H216O peak decreases in size and shifts upward, to around 380 K, while the high-temperature shoulder develops into a new peak around 525 K, and for 18O2 predoses above 3.0 L, these two desorption features are gradually inhibited until becoming negligible after 6.0 L predoses; by then, only one desorption peak at 230 K due to multilayer condensation is detected. H218O desorption

Thermal Chemistry of Water Adsorbed on V(100)

J. Phys. Chem. C, Vol. 111, No. 36, 2007 13573

Figure 4. TPD spectra for H2O (a, left panel), HDO (b, center panel), and D2O (c, right panel) desorption from V(100) surfaces sequentially dosed with H2O and D2O at 180 K. Data are shown for different H2O/D2O dose ratios while keeping the total H2O + D2O exposure fixed at 0.5 L. Inset: desorption yields calculated from these TPD traces. The HDO yield is maximized at intermediate H2O + D2O mixtures.

Figure 5. H2 (a, left panel), H216O (b, center panel), and H218O (c, right panel) TPD spectra acquired after adsorbing 0.35 L of H216O on V(100) surfaces predosed with various amounts of 18O2 at 320 K. Hydrogen production is effectively inhibited by coadsorbed oxygen, and both H216O and H218O traces exhibit a similar trend in terms of the evolution of their desorption temperatures. The high-temperature features seen in the 20 amu desorption traces after large 18O2 predoses are due to desorption of imbedded argon.

(Figure 5, right panel) is seen throughout all 18O2 predoses, with trends similar to those observed for the H216O desorption. Curiously, though, an additional sharp and big feature is seen around 600 K in the 20 amu desorption traces for 18O2 predoses above 2.5 L. This desorption was found to originate from embedded argon introduced by the ion sputtering treatment used for cleaning of the sample (data not shown), a surprising result because all experiments were carried out with surfaces annealed to temperatures of about 1200 K. It is noteworthy here that this argon desorption is assisted by the oxygen that incorporates into the thin vanadium oxide layer obtained after high O2 exposures; it is not seen for lower O2 doses or when oxygen is adsorbed at lower temperatures. The desorption yields for H2, H216O, and H218O derived from the data in Figure 5, after subtraction of the contributions from condensed water and argon, are plotted as a function of 18O2 predose in Figure 6. The total water uptake was also calculated by adding all of these desorption yields together. A key result

highlighted in this figure is the clear decrease seen in the H2 yield, from 0.17 ML on clean V(100) to less than 0.02 ML after 1.0 L 18O2 predoses. The desorption yield of H216O increases slightly up to 18O2 doses of ∼0.35 L but drops afterward, with a pause between 2.0 and 3.0 L 18O2 predoses. The yield for H218O first exhibits a monotonic increase up to 3.0 L 18O2 predoses but then falls, and finally reaches a constant value of approximately 0.01 ML after 6.0 L predoses. The total water uptake decreases rapidly within two O2 predosing regimes, from 0.2 to 2.0 L and from 3.0 to 6.0 L. For reference, the XPS and ISS data corresponding to the surfaces obtained after the different O2 predoses indicate that while the oxygen uptake increases steadily as a function of O2 exposure over the whole dose range reported here, the surface composition of the topmost layer levels off after O2 exposures above 2.0 L.19 This indicates that 2.0 L of O2 is the threshold where deeper oxidation and the buildup of a thicker surface oxide are initiated on V(100) surfaces.

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Figure 6. H2, H216O, and H218O TPD yields calculated by integration of the TPD traces reported in Figure 5. The total water uptake was also calculated by adding these TPD yields together. Water decomposition is initially inhibited at the expense of molecular desorption, but both hydrogen and water yields decrease dramatically after 0.3 L 18O2 pre-exposures because of a corresponding drop in water uptake.

Figure 7 shows the desorption traces for hydrogen (left panel), H216O (center panel), and H218O (right panel) obtained after dosing different amounts of H216O on V(100) surfaces predosed with 0.7 L of 18O2 at 320 K (which produces a layer of chemisorbed oxygen). While a hydrogen desorption peak is seen around 500 K after low water doses, an additional broad signal is detected at lower temperatures after larger water exposures (although that may be due to cracking from water desorption). More complex behavior is seen in the desorptions of H216O and H218O. Equal amounts of desorption are seen initially around 480 K (R state) for both isotopomers, and additional features grow around 340 K (β state) after 0.06 L H216O exposures, with higher yields for H216O desorption. H216O exposures above 0.15 L lead to the development of a third desorption state around 310 K (γ state) in the H216O traces but only to a steady growth in the β state in the H218O desorption data. Similar trends were seen on surfaces predosed with 3.0 L of 18O2, that is, on a surface

Shen and Zaera covered with a thin VOx oxide layer, except that all of the desorption states were observed at higher temperatures (data not shown). The reaction between coadsorbed oxygen and water was further probed by isotope labeling experiments utilizing both D216O and 18O2. Figure 8 shows the TPD spectra acquired after adsorbing 0.4 L of D216O on V(100) surfaces predosed with 0.7 (left panel), 3.0 (center panel), and 10.0 L (right panel) of 18O at 320 K. In accordance with the results reported in Figure 2 5, hydrogen desorption is only seen in the case of the 0.7 L 18O predose. Notice also that, among the three possible 2 molecular hydrogen isotopomers, D2 desorbs at the highest temperature, perhaps because of a kinetic isotope effect. On the other hand, the generation of D218O (22 amu) and HD18O (21 amu) are prevalent in all three cases, even if it happens at higher temperatures from the surfaces predosed with 3.0 and 10.0 L of 18O2. Also, as reported above, extensive isotope scrambling is observed in these systems. The 20 amu desorption peak around 600 K is again seen only after the buildup of a surface oxide (the third-from-top traces in panels b and c), and its identification is seen as arising from the desorption of subsurface argon indicated by the fact that no such high-temperature feature is seen in the corresponding HD18O and D218O traces. Finally, the water uptake appears to decline as a function of 18O2 predose, a trend particularly evident by the contrast between the results obtained after 3.0 versus 10.0 L 18O2 predoses. 3.3. XPS Results. Figure 9 displays the O 1s XPS spectra acquired as a function of annealing temperature for 2.0 L of H2O adsorbed on clean (left panel) and oxygen-predosed (1.0 L O2, right panel) V(100) surfaces. Special care was taken to minimize the effect of X-ray radiation on the surface adsorbates during these experiments by acquiring the data at low temperatures and in short times; the spectra obtained for the thick condensed layers of ice matched those reported in previous studies. Also to note here is the fact that even in the experiments on the clean vanadium the spectra for the clean surface show the presence of some oxygen (left panel, top trace). This has been determined previously to correspond to approximately 0.3 ML but to be mostly due to subsurface oxygen and to not affect the surface chemistry in a significant way.19 In the case of the clean surface, heating to 180 K leads to the disappearance of the O 1s XPS peak at 534.7 eV, which

Figure 7. H2, (a, left panel), H216O (b, center panel), and H218O (c, right panel) TPD spectra acquired after dosing various amounts of H216O on V(100) surfaces predosed with 0.7 L of 18O2 at 320 K. A small amount of hydrogen desorption is seen at 500 K, but that peak becomes quickly saturated while three desorption states develop for both H216O and H218O at 480, 340, and 310 K. Significant 16O-18O isotope exchange is observed in the water detected in the two high-temperature states.

Thermal Chemistry of Water Adsorbed on V(100)

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Figure 8. TPD spectra acquired after adsorbing 0.4 L of D2O on V(100) surfaces predosed with 0.7 (a, left panel), 3.0 (b, center panel), and 10.0 L (c, right panel) 18O2 at 320 K. The formation of HD18O and D218O seen in all cases indicates a prevalent interaction between the coadsorbed oxygen and water regardless of the extent of surface oxidation.

Figure 9. O 1s X-ray photoelectron spectroscopy (XPS) data from water adsorbed on clean (a, left panel) and oxygen-predosed surfaces (b, right panel) as a function of annealing temperatures. On both surfaces, heating to 180 K leads to significant attenuation of the O 1s XPS intensity because of the removal of any condensed water. Heating the clean surface to 400 K induces the almost total elimination of the O 1s XPS signal initially seen at 533.7 eV, whereas some signal could still be detected at ∼533.2 eV on the oxygen-covered surfaces. We assign that peak to surface hydroxide species.

corresponds to molecular desorption of water; this is in agreement with the TPD results. Above that temperature, two well-resolved O 1s XPS peaks still remain at 531.2 and 533.7 eV, but heating to 300 K leads to the attenuation of the 533.7 eV feature and to an observable growth and downward shift of the O 1s XPS peak at 531.2 eV. The thermal evolution of the O 1s XPS signal at 533.7 eV is highly suggestive of its correspondence to surface hydroxide groups, and indicates a coverage of ∼0.3 ML of those species on the surface below 300 K.1,2,13,31,32 By 400 K, only one single peak is seen, at a lower binding energy (530.7 eV) and with a larger peak area than that obtained for the starting surface (compare the top vs bottom traces in the left panel of Figure 9). This points to some decomposition of water and to the corresponding deposition of oxygen adatoms on the surface. In contrast, in the case of the oxygen-covered surfaces, the O 1s XPS data for adsorbed water

Figure 10. NH3 (a, left panel) and H2O (b, right panel) TPD traces obtained after adsorbing 0.15 L of NH3 on V(100) surfaces pretreated with various amounts of oxygen. The NH3 desorption peaks shift to higher temperatures with increasing O2 predoses, and water production exhibits similar trends but is always detected at slightly higher temperatures.

at 110 K exhibits only two O 1s XPS peaks at 530.4 and 533.2 eV (Figure 9, right panel). Heating that surface to 180 K removes any water condensed in the multilayer but still leaves a remnant small O 1s XPS peak at 533.2 eV that persists after heating to 400 K. We again assign this feature to surface hydroxide species, in this case at a lower concentration because of the inhibition in water decomposition exerted by the coadsorbed oxygen. Finally, heating to above 500 K restitutes the starting surface (top trace). 3.4. Thermal Chemistry of Adsorbed Ammonia. Finally, we report some preliminary results on the thermal chemistry of ammonia adsorbed on clean and oxygen-treated V(100) surfaces. Ammonia has often been shown to react with surface oxygen in similar fashion as water.33-38 Figure 10 shows the ammonia (left) and water (right) TPD data acquired after 0.15 L doses of NH3 on V(100) surfaces pre-exposed to various amounts of O2 at 320 K. The ammonia desorption traces were corrected to account for the contribution of the desorbing water. On the clean surface, one ammonia desorption peak is initially seen around

13576 J. Phys. Chem. C, Vol. 111, No. 36, 2007 315 K, but O2 predosing attenuates this desorption and shifts it to high temperatures, until reaching 390 K for the case of a 0.5 L O2 predose. An additional peak shift is also seen after O2 predoses above 0.7 L, and the maximum shift in peak temperature is reached by 3.0 L O2 predoses, at which point the main ammonia desorption feature is centered around 520 K and is accompanied by some additional signal around 320 K. In terms of water desorption, a small amount can be seen even from the clean surface, perhaps because of background adsorption or oxygen impurities on the surface. Regardless, it is clear that oxygen predoses as low as 0.35 L effectively promote the generation of this water, albeit at higher temperatures (405 K). The production of water mirrors closely the desorption of ammonia, remaining almost the same up to 0.7 L O2 predoses and following a second upward temperature shift and growth in yield until 3.0 L O2 predoses. Notice, however, that the water desorption always occurs at slightly higher temperatures than those measured for ammonia. Isotope labeling experiments with ND3 and 18O2 unambiguously indicate that this water desorption state originates from a reaction between coadsorbed oxygen and ammonia (data not shown). 4. Discussion The TPD and XPS results reported here provide useful information about the reactivity of V(100) surfaces toward water. First, it is clear that some water decomposition takes place even on clean V(100) surfaces. Evidence for this behavior is provided not only by the TPD data in Figure 1 but also by the increase in O 1s XPS signal observed after annealing to 400 K (Figure 9) and by the extensive isotope scrambling observed between coadsorbed H2O and D2O (Figure 4). The detection of hydrogen desorption around 270 K (Figure 2) sets an upper limit for the temperature at which this reaction takes place. Water decomposition has been previously reported on VC(100),12 vanadium oxides,13-16 and other early transition metal surfaces such as Mo39 and W,40,41 where it has been argued that the relative strength of the metal-oxygen versus O-H bond defines the water adsorption mode, that is, dissociative versus molecular.1,2 Hence, the activation of water on V(100) suggests a high affinity of vanadium toward oxygen, as expected based on the stability of the different vanadium oxides.42 On the other hand, the extent of the decomposition appears to depend on the surface coverage of water. The observation that hydrogen desorption is maximized at intermediate water coverages suggests that water decomposition may require relatively large ensembles of vanadium surface atoms, and that it may be inhibited by site-blocking. Decomposition appears to occur in stepwise fashion and to be at least partly reversible. The stepwise nature of the process is indicated by the isolation of hydroxide surface groups, as indicated by the O 1s XPS data (Figure 9), whereas the reversibility of the surface reaction is proven by the detection of all possible isotopomers in TPD experiments with H2O + D2O mixtures (Figure 4). Our results also indicate that the presence of oxygen atoms on the surface significantly alters the chemistry of the adsorbed water. On the one hand, the presence of a fraction of a monolayer of oxygen inhibits the total dehydrogenation of water, as evidenced by the linear drop in hydrogen desorption yield as a function of oxygen predose shown in Figure 6. This occurs at the expense of an enhancement in water molecular desorption (since the total water uptake remains approximately constant for O2 pre-exposures of up to 0.3 L, see Figure 6), so it appears that the metal sites responsible for water dissociation are progressively passivated by adjacent oxygen atoms. On the other

Shen and Zaera hand, a new reaction channel, the abstraction of hydrogen from water by surface oxygen, opens up with the addition of oxygen. This step leads to the generation of stable hydroxide groups on the surface; those disproportionate back to water and surface oxygen only at relatively high temperatures. The main supporting evidence for this conclusion comes from the observation of substantial amounts of H218O generated after coadsorbing H216O with 18O on V(100) surfaces, a phenomenon unambiguously pointing to extensive 16O-18O scrambling between the coadsorbed oxygen atoms and the water (Figure 5). The formation and stability of surface hydroxide group on oxygen-treated V(100) surfaces can also be seen directly in the O 1s XPS data provided in Figure 9. The additional isotope labeling experiments utilizing both D216O and 18O2 confirm the occurrence of extensive hydrogen transfer from water to surface oxygen, since both D218O and HD18O formation are detected throughout all of the oxygen precoverages tested (Figure 8). Finally, the production of water from surfaces with coadsorbed ammonia and oxygen is consistent with this hydrogen transfer model as well.34-36 We contend that the behavior reported here is indicative of extensive hydrogen bonding among all of the oxygen-containing adsorbed species. The thermal chemistry of water on V(100) surfaces discussed above was found to depend on the precoverage of oxygen (Figure 5). Indeed, several oxygen pre-exposures regimes may be sorted according to the types of water reaction observed: (1) From 0.1 to 0.5 L O2 predoses, where the water uptake leads to the deposition of chemisorbed oxygen adatoms. The interaction between surface oxygen and adsorbed water is evidenced by a significant broadening of the water desorption peak in the TPD and by a substantial hydrogen transfer from water to oxygen. The details of such interaction were found to be fairly complex and will be discussed below. (2) From 0.5 to 2.0 L O2 predoses, where the growth of surface vanadium oxide starts to take place. During this stage, the decrease in water uptake is accompanied by the appearance of a high-temperature (∼500 K) H2 TPD peak and by a shift in the water main desorption peak toward higher temperatures. This indicates a stronger bonding perhaps associated with surface ionic species. (3) From 2.0 to 3.0 L, where the oxygen uptake leads to a steady increase in oxygen coverage19 and to the generation of more ionic species. The water desorption profile remains almost unchanged, but the extent of the reaction between adsorbed water and surface oxygen increases substantially. (4) From 3.0 to 6.0 L O2 predoses, where further surface oxidation takes place but does not lead to a significant increase in oxygen uptake.19 Water chemisorption in this regime was found to decrease steadily and to become weaker. The origin of this behavior may not be simply explained by a decrease in surface vanadium sites, since our previous ISS results suggest an almost constant surface stoichiometry of the topmost layer in this regime19 and may be due to a surface reconstruction instead.1,43 (5) Above 6.0 L O2 predoses, where a stable thin oxide film is formed and no significant change in water chemistry is observed. The oxide film is fairly inert, the same as in other systems.1,2,34 It is worth noticing that the production of ammonia and water from adsorbed ammonia on the same surfaces (Figure 10) exhibit similar trends as those discussed above in terms of the peaktemperature shifts as a function of surface oxidation, even though, in contrast, considerable desorbing yields for both species are still seen even after the formation of the thin oxide film (Figure 10, top traces). This observation may imply that adsorbed ammonia in fact behaves somewhat differently than adsorbed water and always adopts a bonding geometry favoring

Thermal Chemistry of Water Adsorbed on V(100) interactions with coadsorbed oxygen regardless of any surface reconstruction of the surface induced by surface oxidation. On the basis of the evolution of several desorption features seen when exposing 18O-covered surfaces to increasing amount of H216O (Figure 7), it is inferred that the local structure of water adsorbed on oxygen-covered V(100) surfaces may be quite complex. In particular, three water desorption states were found to populate sequentially. Initially, equal amounts of H216O and H218O desorption are seen at 480 K, in what we call here the R state. This state is most likely associated with the disproportionation of hydroxide groups generated from a stoichiometric reaction between coadsorbed water and oxygen. Such hydroxide groups exhibit high thermal stability, in particular in the presence of coadsorbed oxygen, but decompose to some extent to produce the small amount of hydrogen that desorbs at these temperatures. The second water desorption feature, seen around 340 K and labeled β state, is also seen in both H216O and H218O traces but to a lesser extent in the latter. The origin of this state is not evident, but may be related to water molecules stabilized by adjacent oxygen or hydroxide groups via hydrogen bonding.1,44 The limited 16O-18O exchange observed in the water that desorbs in this state may imply low but detectable hydrogen mobility within such a (H2O)x-OH network or the formation of these complex only in the periphery of oxygen surface islands.45 The third desorption state at 310 K (γ state) is only seen after high water exposures, and grows steadily until saturation of the first layer. The absence of isotope exchange within this state suggests a weak interaction of this water with either the adsorbed oxygen or the hydroxide surface groups. It should be mentioned that although these water states are reported in Figure 7 for a surface precovered with a submonolayer of chemisorbed oxygen, they all can also be observed on the surface oxide thin film that grow after higher oxygen exposures (3.0 L, data not shown) and thus appear not to be affected by the extent of surface oxidation. Finally, we briefly discuss the meaning of the O 1s XPS spectra obtained for adsorbed water on both clean and oxygencovered V(100) surfaces, in particular, their implications in terms of the formation of surface hydroxide groups (Figure 9). O 1s XPS binding energies ranging from 531 to 534 eV have previously been assigned to either hydroxide groups or water.1,2,31,34-36,46 Here, in the case of clean V(100) surfaces, the O 1s XPS peak at 533.7 eV that dominates after multilayer desorption by 180 K can be assigned to either hydroxide groups, submonolayer coverages of adsorbed water, or a mixture of both. The extensive isotope scrambling seen in the TPD data in Figure 4 and the enhancement of this chemistry in the presence of coadsorbed oxygen strongly suggest the formation of hydroxide species, or at the very least hydrogen-bonded H2O-O surface complexes. Annealing those surfaces to 400 K leads to the desorption of the remaining water and leaves chemisorbed atomic oxygen behind on the surface (as indicated by the growth of the O 1s XPS peak at 530.7 eV). On oxygen-covered surfaces, both submonolayer water and hydroxide species exhibit one O 1s XPS feature at 533.2 eV, a value close to that for water multilayers on the same surfaces, perhaps implying a weaker interaction between the chemisorbed water and vanadium. Also, in contrast to the case of the clean surface, annealing oxygencovered surfaces to above 400 K still results in considerable O 1s XPS intensity from hydroxide groups, hydrogen-bonded water, or both. All of these observations are in good accordance with those from TPD experiments.

J. Phys. Chem. C, Vol. 111, No. 36, 2007 13577 5. Conclusions In summary, water adsorption on clean V(100) surfaces was determined to be partially dissociative, to an extent that is maximized at intermediate coverages. The coexistence of surface oxygen significantly affects the thermal chemistry of adsorbed water in at least two ways, namely, by inhibiting the selfdissociation of water, and by opening up another reaction channel involving a hydrogen abstraction mechanism and leading to the generation of hydroxide groups. This second reaction channel was found to persist throughout all of the oxygen coverages tested, even after the formation of surface oxide thin films. However, the uptake and bonding of water appear to depend on the precoverage of oxygen, and may be related to the changes in the surface structural and electronic properties associated with this uptake. Three water adsorption states populate sequentially during the uptake on oxygen precovered surface associated with hydroxide groups (R state), hydrogen-bonded water (β state), and weakly interacting molecular water (γ state), respectively. Acknowledgment. Financial support for this project was provided by the U.S. National Science Foundation. References and Notes (1) Henderson, M. A. Surf. Sci. Rep. 2002, 46, 1. (2) Thiel, P. A.; Madey, T. E. Surf. Sci. Rep. 1987, 7, 211. (3) Verdaguer, A.; Sacha, G. M.; Bluhm, H.; Salmeron, M. Chem. ReV. 2006, 106, 1478. (4) Okuhara, T. Chem. ReV. 2002, 102. (5) Mamedov, E. A.; Corberan, V. C. Appl. Catal. A 1995, 127, 1. (6) Blasco, T.; Nieto, J. M. L. Appl. Catal. A 1997, 157, 117. (7) Arena, F.; Parmaliana, A. Acc. Chem. Res. 2003, 36, 867. (8) Herman, R. G.; Sun, Q.; Shi, C. L.; Klier, K.; Wang, C. B.; Wachs, I. E.; Bhasin, M. M. Catal. Today 1997, 37, 1. (9) Argyle, M. D.; Chen, K. D.; Bell, A. T.; Iglesia, E. J. Phys. Chem. B 2002, 106, 5421. (10) Oyama, S. T.; Middlebrook, A. M.; Somorjai, G. A. J. Phys. Chem. B 1990, 94, 5029. (11) Chen, K. D.; Khodakov, A.; Yang, J.; Bell, A. T.; Iglesia, E. J. Catal. 1999, 186, 325. (12) Didziulis, S. V.; Frantz, P.; Perry, S. S.; El-bjeirami, O.; Imaduddin, S.; MerriII, P. B. J. Phys. Chem. B 1999, 103, 11129. (13) Toledano, D. S.; Metcalf, P.; Henrich, V. E. Surf. Sci. 2001, 472, 21. (14) Abu Huija, M.; Guimond, S.; Uhl, A.; Kuhlenbeck, H.; Freund, H.-J. Surf. Sci. 2006, 600, 1040. (15) Schoiswohl, J.; Tzvetkov, G.; Pfuner, F.; Ramsey, M. G.; Surnev, S.; Netzer, F. P. Phys. Chem. Chem. Phys. 2006, 8, 1614. (16) Kurtz, R. L.; Henrich, V. E. Phys. ReV. B 1983, 28, 6699. (17) Somorjai, G. A. Introduction to Surface Chemistry and Catalysis; Wiley: New York, 1994. (18) Zaera, F. Prog. Surf. Sci. 2001, 69, 1. (19) Shen, M.; Ma, Q.; Lee, I.; Zaera, F. J. Phys. Chem. C 2007, 111, 6033. (20) Doering, D. L.; Madey, T. E. Surf. Sci. 1982, 123, 305. (21) Nobl, C.; Benndorf, C. Surf. Sci. 1987, 182, 499. (22) Zaera, F. Surf. Sci. 1989, 219, 453. (23) Tjandra, S.; Zaera, F. Langmuir 1992, 8, 2090. (24) Wilson, J.; Guo, H.; Morales, R.; Podgornov, E.; Lee, I.; Zaera, F. Phys. Chem. Chem. Phys., 2007, 9, 3830. (25) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Maulder, J. F.; Muilenberg, G. E. Handbook of X-ray Photoelectron Spectroscopy; Perkin Elmer: Eden Prairie, MN, 1978. (26) Jensen, V.; Andersen, J. N.; Nielsen, H. B.; Adams, D. L. Surf. Sci. 1982, 116, 66. (27) Koller, R.; Bergemayer, W.; Kresse, G.; Hebenstreit, E. L. D.; Konvicka, C.; Schmid, M.; Podloucky, R.; Varga, P. Surf. Sci. 2001, 480, 11. (28) Andersson, K.; Nikitin, A.; Pettersson, L. G. M.; Nilsson, A.; Ogasawara, H. Phys. ReV. Lett. 2004, 93, 6101. (29) Denzler, D. N.; Wagner, S.; Wolf, M.; Ertl, G. Surf. Sci. 2003, 532-535, 113. (30) Szalkowski, F. J.; Somorjai, G. A. J. Chem. Phys. 1972, 56, 6097. (31) De Jesu´s, J. C.; Carrazza, J.; Pereira, P.; Zaera, F. Surf. Sci. 1998, 397, 34. (32) Kaya, S.; Sun, Y.-N.; Weissenrieder, J.; Stacchiola, D.; Shaikhutdinov, S.; Freund, H.-J. J. Phys. Chem. C 2007, 111, 5337.

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