Langmuir 1998, 14, 7217-7221
7217
Water Dissociation and KOH Formation on Potassium-Covered MgO/Ru(001) H. H. Huang, X. Jiang, H. L. Siew, W. S. Chin, and G. Q. Xu* Department of Chemistry, National University of Singapore, 10 Kent Ridge, Singapore 119260 Received May 27, 1998. In Final Form: September 29, 1998 The sequential adsorption and reaction of H2O with K on MgO thin films have been studied using TDS and XPS. Upon H2O adsorption on K/MgO at 100 K, two hydrated species of K+(H2O)n- and K+(H2O)n(OH)are formed, resulting in corresponding H2O-TDS peaks at 200 and 270 K, respectively. The thermal conversion from hydrated K to hydrated KOH leads to H2 desorption at 190-220 K. The formation of KOH is evidenced by the O 1s bonding energies at 530.6-530.9 eV. Both additional K deposition and thermal activation favor H2O dissociation. Annealing the coadsorbed surface to 500-600 K results in desorption and decomposition of KOH. The coincident desorption of K2O, H2O, and K at θK > 0.3 ML strongly suggests the decomposition pathway 2KOH f K2O + H2O, which further produces K desorbing into the gaseous phase and oxygen being retained on the surface. The KH species formed during H2O dissociation is also evidenced by the coincident desorption of H2 and K at 470-490 K for θK g 1 ML.
Introduction Magnesium oxide has received extensive attention due to its fundamental importance and technological applications.1-8 However, the effects of surface modification of MgO by adsorbed molecules are much less understood, in particular coadsorption of H2O and alkali systems,4-6 compared to such studies on metal surfaces.9-14 On many metal surfaces, H2O chemisorbs molecularly in the absence of alkali.9,10,12-14 The dissociation of H2O on alkali-covered metal surfaces has been extensively studied. A critical alkali coverage is found to be necessary for H2O dissociation in most cases.11,13,15-17 For K + H2O/ Ag(111) system, the formation of solvated electrons, K+(H2O)n-, is evidenced, resulting in H2 and solvated OHupon annealing.10 In an EELS study on K + H2O/Ru(001),11 Thiel et al. concluded that KOH decomposes via KOH f K + O + H at low K coverages. This mechanism is supported by their observation of K-TDS and oxygen retention on the surface. It was noted that H2-TDS was not detected in their study. In addition, Chakarov et al. * Corresponding author. (1) Peng, X. D.; Barteau, M. A. Surf. Sci. 1989, 224, 327. (2) Peng, X. D.; Barteau, M. A. Langmuir 1989, 5, 1051. (3) Coluccia, S. A.; Tench, J.; Segall, R. L. J. Chem. Soc., Faraday Trans. 1979, 75, 1769. (4) Karolewski, M. A.; Cavell, R. G. Surf. Sci. 1992, 271, 128. (5) Onishi, H.; Egawa, C.; Aruga, T.; Iwasawa, Y. Surf. Sci. 1987, 191, 479. (6) Huang, H. H.; Jiang, X.; Zou, Z.; Chin, W. S.; Xu, G. Q.; Dai, W. L.; Fan, K. N.; Deng, J. F. Surf. Sci., accepted. (7) Huang, H. H.; Jiang, X.; Zou, Z.; Xu, G. Q. Surf. Sci. 1997, 376, 245. (8) Huang, H. H.; Jiang, X.; Zou, Z.; Xu, G. Q.; Dai, W. L.; Fan, K. N.; Deng, J. F. Surf. Sci. 1998, 398, 203. (9) Thiel, P. A.; Madey, T. E. Surf. Sci. Rep. 1987, 7, 211. (10) Blass, P. M.; Zhou, X. L.; White, J. M. J. Phys. Chem. 1990, 94, 3054. (11) Thiel, P. A.; Hrbek, J.; dePada, R. A.; Hoffmann, F. M. Chem. Phys. Lett. 1984, 108, 25. (12) Kiskinova, M.; Pirug, G.; Bonzel, H. P. Surf. Sci. 1985, 150, 319. (13) Bornemann, T.; Steinru¨ck, H. P.; Huber, W.; Eberle, K.; Glanz, M.; Menzel, D. Surf. Sci. 1991, 254, 105. (14) Klaua, M.; Madey, T. E. Surf. Sci. 1984, 136, L42. (15) Semancik, S.; Doering, D. L.; Madey, T. E. Surf. Sci. 1986, 176, 165. (16) Bonzel, H. P.; Pirug, G.; Winkler, A. Chem. Phys. Lett. 1985, 116, 133. (17) Bonzel, H. P.; Pirug, G.; Mu¨ller, J. E. Phys. Rev. Lett. 1987, 58, 2138.
observed complicated thermal reactions of K + H2O on graphite(0001).18 The vibrational spectra of the adlayer indicate that KOH, KH, and KxOy are initially formed at 120-160 K, further converting to K2O, H2O, K, and CO2 upon thermal annealing. However, neither KOH- nor K2O-TDS was directly detected in those experiments mentioned above. On the other hand, well-ordered MgO single-crystal surfaces are unreactive toward adsorption and dissociation of H2O due to the high extent of bond saturation on surfaces.6,19 Recent studies of H2O adsorption on defect sites of MgO(001)20 and MgO(100)21 revealed that hydroxylation on low coordinated sites (n < 5) is energetically favored. Coadsorption of H2O with alkali also leads to the formation of hydroxyl groups on a perfect MgO(100) surface.4-6 In the present work, a MgO thin film grown on Ru(001) was used as the substrate for studies of interactions between K and H2O using XPS and TDS. In addition to the advantage of eliminating the possible charging problems occurring on MgO(100),6 the effects of defect sites in MgO thin films can also be explored. Detailed knowledge of alkali and H2O interaction on MgO is thus expected to provide valuable information for comparison with the corresponding results on metal and graphite surfaces. Experimental Section Two UHV systems with a base pressure of 1500 K and be cooled to 90 K by liquid nitrogen. The sample temperature was measured with a 0.003 in. C-type thermocouple (W-5%Re/ W-26%Re) spot-welded to the upper edge of the crystal. Ultrathin MgO films were synthesized under UHV conditions by evaporating Mg atoms onto a clean Ru(001) substrate, followed by oxidation in an oxygen background pressure of 1.0 × 10-7 Torr. The detailed preparation and characterization of MgO thin films have been reported elsewhere.22 The MgO thin film was cleaned by cycles of heating to 1100 K in an oxygen ambient of 1.0 × 10-8 Torr to remove any possible contaminants such as carbon.8 The thickness of the MgO films used in this work is ∼7.0 ML. Potassium was evaporated onto the surface at 150 K from a well-degassed SAES getter source. Its coverages were previously calibrated using TDS and XPS,23 whereby one monolayer of K was taken as the intersection point in a plot of K 2p/O 1s ratio against K-TDS peak intensity. Water was purified by cycles of freeze-pump-thaw prior to use and then introduced into the chamber through a stainless steel doser directed at the sample surface. The exposure of water from the doser is calibrated against the background dosage. After water adsorption at 100 K, the H2O/MgO/Ru(001) was annealed at 155 K for 1 min to desorb the physically condensed water molecules. The surface was then cooled to 100 K for TDS and XPS measurements. Annealing the adsorbed system at 155 K is a necessary step to resolve the O 1s of H2O interacting with K atoms from that of condensed water.
Results and Discussion Figure 1 shows the thermal desorption of H2O and H2 upon 10.0 L of H2O sequential adsorption with K on MgO thin films at various K coverages. At zero coverage of K, molecular desorption of H2O is not detected in the temperature range studied (Figure 1a). However, a small H2 desorption peak appears at ∼175 K (Figure 1b), indicating the dissociative adsorption of H2O on the defects of the MgO thin film.23 This is in good agreement with the previous theoretical investigation on H2O/MgO(001)10 and the experimental study on H2O/MgO(100).21 The K-induced H2O adsorption and dissociation on MgO thin films are thus clearly illustrated in Figure 1a and b. At θK ) 0.1 ML, a broad H2O desorption peak is observed at 200-300 K. With increasing θK up to 1 ML, the peak at 270 K grows rapidly and dominates the spectrum. A shoulder at 200 K becomes resolvable at θK ) 0.3 ML. In addition, a much more stable water state desorbing at 560-580 K can be noticed at θK > 0.3 ML. However, further K deposition (θK > 1 ML) causes decreasing of the H2O desorption features at 270 and 200 K, respectively. Qualitatively similar H2O desorption features at low temperatures were previously reported in several K + H2O coadsorption systems. On Ag(111)10 and Pt(111),24 the H2O desorption at 260-270 K was assigned to water in hydrated KOH. On the other hand, adsorption of H2O on K-covered graphite(0001)18 resulted in two H2O desorption peaks at low temperatures, attributed to H2O molecules interacting with K and KOH. In the present work, we observed two H2O desorption features at 270 and 200 K, respectively. It is further noted that the H2O peak at 270 K in Figure 1a occurs after the corresponding H2 desorption at 190-220 K (see Figure 1b). This strongly suggests that the H2O desorption at 270 K is related to H2O molecules binding with KOH. The other H2O (22) Huang, H. H.; Jiang, X.; Siew, H. L.; Chin, W. S.; Xu, G. Q. Surf. Sci., submitted. (23) Huang, H. H.; Jiang, X.; Siew, H. L.; Chin, W. S.; Xu, G. Q. Surf. Sci., submitted. (24) Bonzel, H. P.; Pirug, G.; Winkler, A. Surf. Sci. 1986, 175, 287.
Figure 1. TDS of H2O (a) and H2 (b) after dosing 10.0 L of H2O onto K-covered MgO/Ru(001) at various θK.
desorption peak at a lower temperature of 200 K may, therefore, be attributed to H2O in hydrated K. The reduction of both H2O peaks with increasing θK at θK > 1 ML is consistent with the formation of a thicker KOH layer through H2O dissociation. The more stable water state desorbing at 560-580 K at θK > 0.3 ML (Figure 1a) can be attributed to KOH decomposition. This observation agrees well with previous studies on metal surfaces,10,24 in which KOH was reported to be stable up to 560-580 K. The amount of this H2O desorption was found to increase with increasing θK, attributable to the complete decomposition of KOH formed.10,24 In contrast, our results show that the H2O desorption peak at 560-580 K remains constant in intensity as θK increases. This observation suggests the partial decomposition of KOH formed on MgO thin films. H2-TDS spectra obtained after H2O adsorption on K-covered MgO thin films are shown in Figure 1b. The peak at ∼175 K initially observed from a clean MgO thin film is attributable to the H2 desorption from K-free defects in MgO. At θK ) 0.1 ML, an additional hydrogen species becomes detectable at a higher desorption temperature of 190 K with a shoulder at 260 K. This new peak grows dramatically and gradually shifts to the higher temperature side with increasing θK, eventually reaching 220 K at θK ) 3.3 ML. Combined with the related H2O desorption behavior (Figure 1a), the mechanism of H2O interaction with K on MgO films could be explained by the formation of solvated (25) Jortner, J.; Sharf, B. J. Chem. Phys. 1962, 37, 2506.
Potassium-Covered MgO/Ru(001)
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electrons, which is evidenced in frozen solutions25 and successfully applied to interpret the reaction of H2O with K on Ag(111).10 The interaction of H2O with K leads to the formation of hydrated K. The electron transfer from K to neighboring H2O molecules would produce negatively charged water clusters, (H2O)n-, which could react further to form hydrated OH- and H2:
K + nH2O f K+[(H2O)n-]
(1)
2K+[(H2O)n-] f H2(g) + 2K+[(H2O)n-1(OH)-] (2) This reaction mechanism is supported by our experimental observations. The H2O-TDS peaks at 200 and 270 K (Figure 1a) can be assigned to the desorption of H2O from the two hydrated species, that is, K+[(H2O)n-] and K+[(H2O)n-1(OH)-], respectively. The conversion from K+[(H2O)n-] to K+[(H2O)n-1(OH)-] gives H2 desorption at 190 K. Furthermore, it is interesting to note that a new H2 species desorbing at 470 K is detectable at θK > 0.6 ML (Figure 1b), which was not observed in previous studies.10,18,24 This feature grows gradually as θK increases. The fact that this H2 desorption occurs at a much lower temperature than H2O desorption at ∼580 K (Figure 1a) excludes the possibility of contribution from the decomposition of KOH. Using HREELS, Chakarov et al.18 reported that the KH species coexists with KOH in the K + H2O adlayer at 120-200 K. In our previous work on H/K/MgO(100),7 it was found that the thermal decomposition of KH hydride occurs at 420-450 K, with the K-TDS peak being ∼20 K higher than the corresponding H2 desorption. A similar desorption pattern is demonstrated in this study. The H2 desorption at 470 K (Figure 1b) is observed together with the corresponding K desorption at 490 K (see Figure 2a). Therefore, we attribute this high-temperature H2 desorption to the decomposition of KH. In fact, in addition to the surface reactions discussed above, an alternative water dissociation pathway could occur through step 3, where H(ad) reacts with K to produce KH:
K+[(H2O)n-] f H(ad) + K+[(H2O)n-1(OH)-]
(3)
H(ad) + K f KH
(4)
Figure 2 displays a set of corresponding K-, KOH-, and K2O-TDS spectra, which provide further evidence for H2O dissociation and KOH decomposition. In general, the coincident desorptions of three species at 550-600 K become sharper and shift gradually to the higher temperature side with increasing θK, indicating the formation of thicker KOH layers. These results together with the related H2O desorption at 560-580 K in Figure 1a clearly suggest that the decomposition of KOH goes through the reaction 2KOH f K2O + H2O. The desorption of KOH observed in Figure 2b is undoubtedly responsible for the constant peak intensity of H2O desorption at 560-580 K. The K-TDS peak at 490 K (Figure 2a) has been discussed together with the H2 desorption at 470 K (Figure 1b), attributable to the decomposition of KH formed upon water dissociation. The feature at 330 K observed at θK ) 3.3 ML is characteristic of metallic K desorption from MgO thin films,22 indicating the excess of K in the coadsorption system. However, the origin of the K peak at ∼580 K requires a careful interpretation. The concurrent desorption of K and K2O at θK > 0.3 ML implies the partial decomposition of K2O during thermal desorption. The
Figure 2. TDS of K (a), KOH (b), and K2O (c) after dosing 10.0 L of H2O onto K-covered MgO/Ru(001) at various θK.
absence of related O2 desorption might be due to its diffusion into bulk18 or retention on the surface.11 The existence of a low alkali coverage limit for water dissociation is one of the most challenging issues, which has been discussed previously for several alkali/metal systems.12,13,15,26 The critical coverage is very different for various adsorption systems: 0.05 ML for Li/Ru(001),15 0.25 ML for Na/Ru(001),26 0.06 ML for K/Pt(111),12 and 0.14 ML for K/Ni(111).13 The possible explanations for the coverage limit are derived from geometric arguments15 and related to the occupancy of the alkali valence s-orbitals.26 Partial occupancy does not, however, imply reactivity. It is possible that small changes in occupancy cause large differences in reactivity. The absence of a low-coverage limit for water dissociation was observed in H2O/K/Ag(111)10 and H2O/K/Ru(001),11 which was discussed in terms of a low work function. In the present work, the formation of KOH can occur even at a low θK of 0.1 ML (Figure 2b), at which the preadsorbed K is described as an ionic or highly polarized species.22 This result concludes that the low K coverage limit for H2O dissociation on MgO films must be 1 ML) results in a decrease in intensity of the O 1s peak at 533.5 eV with a concurrent increase of the lower O 1s peak at 530.9 eV. This can be qualitatively correlated with the decrease in H2O desorption at low temperatures (Figure 1a) and the enhanced KOH-TDS peak intensity (Figure 2b). The positive shift in the O 1s BE changing from 530.3 to 530.9 eV with
increasing θK clearly indicates the formation of thicker KOH layers. This result is in good agreement with published data.10,12,27 A thicker layer of KOH gives a higher O 1s BE due to the final state relaxation.10 The O 1s feature appearing at higher BEs of 533.5-534.0 eV can be attributed to water in the hydrated species, K+[(H2O)n-] and K+[(H2O)n-1(OH)-], coexisting with KOH at 100 K. This assignment is consistent with the H2OTDS results in Figure 1a and the fact that the O 1s BE of water is normally 4-5 eV higher than that of oxide.28 A negative shift of the O 1s BE from 534.0 to 533.5 eV with increasing θK might be caused by the conversion of hydrated K to hydrated KOH. Such a shifting trend was also observed in K + H2O adlayers on Ag(111) during the formation of KOH,10 although the absolute O 1s BEs are slightly different from ours. Our assignments of the O 1s peaks are further supported by the annealing experiments (dotted lines in Figure 3). Upon heating a K + H2O layer (θK ) 2 ML) up to 300 K, the O 1s peak at 533.5 eV disappears, consistent with the corresponding H2O-TDS (Figure 1a). At 300 K, all the adsorbed H2O molecules either desorb into the gaseous phase or react with K to form KOH. The O 1s feature at 530.9 eV associated with KOH is unaffected after annealing to 300 K. Further annealing the adlayer to 600 K causes the O 1s peak to shift from 530.9 eV back to 530.3 eV. This suggests the depletion of KOH from the surface due to desorption and decomposition in the temperature range 550-600 K (Figure 2). (27) Ito, T.; Kuramoto, M.; Yoshioka, M.; Tokuda, T. J. Phys. Chem. 1983, 87, 4411. (28) Fuggle, J.; Watson, L. M.; Fabian, D. J.; Affrossman, S. Surf. Sci. 1975, 49, 61.
Potassium-Covered MgO/Ru(001)
Figure 4 shows the related K 2p XPS spectra after 10.0 L of H2O exposure to K covered surfaces. The K 2p3/2 BE shifts gradually from 293.6 eV at θK ) 0.25 ML to 294.3 eV up to 1 ML. This is in excellent agreement with the O 1s behavior shown in Figure 3, indicating the formation of KOH. A similar positive shift of the K 2p3/2 BE was previously observed for H2O/K/Ag(111),10 as K coverage becomes higher. For θK > 1 ML, the K 2p3/2 does not shift in BE but increases in its intensity, suggesting the thickening of KOH layers. To obtain further insight into the interaction of H2O with K on MgO thin films, the effect of thermal activation on the initial formation of KOH was also studied by preparing K + H2O adlayers at different surface temperatures. Figure 5 presents the O 1s XPS spectra after exposing a 1.3 ML K-precovered surface to 10.0 L of H2O at 100, 200, and 300 K, respectively. At a low temperature of 100 K, two O 1s peaks appearing at 533.6 and 530.9 eV are attributable to the contributions from both H2O in hydrated K and KOH, and the formation of KOH, respectively, as discussed above. When the adsorption temperature increases to 200 K, the higher O1s peak decreases considerably in intensity with its BE shifting to a lower value of 533.3 eV, attributable to the existence of hydrated KOH, since hydrated K would not be formed at 200 K (see Figure 1a). This O 1s feature at 533.3 eV is hardly observable when H2O adsorbs at 300 K. This indicates that molecular water adsorption does not occur at this temperature, consistent with our TDS results (Figure 1a). It is interesting to note that the O 1s peak related to KOH remains at 530.9 eV with increasing adsorption temperature. However, its intensity is found to increase at a higher temperature, which is clearly
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presented in the inset of Figure 5. On the basis of the above observation, we conclude that H2O dissociation on K/MgO is also a thermally activated process. Conclusions The coadsorption and reaction of H2O with K-covered MgO thin films have been studied using TDS and XPS. In addition to the formation of hydrated K+(H2O)n- and K+(H2O)n(OH)- species, the adsorbed H2O has a high reactivity toward the formation of KOH even at the low temperature 100 K, by the observation of the O 1s BEs at 530.6-530.9 eV. This dissociation process can be promoted by both additional K deposition and thermal activation. No evidence for a low K coverage limit for H2O dissociation is found at any of the θK values studied. The KOH species is stable up to 500-600 K. The decomposition pathway of KOH strongly depends on the precoverage of K. At a low θK of e 0.3 ML, KOH desorbs directly without decomposition, while its partial decomposition to K2O and H2O occurs above 0.3 ML. A further dissociation of K2O to K desorbing into the gaseous phase and oxygen being retained on/in MgO films is proposed with the evidence of K-TDS. In addition, the KH species formed during H2O dissociation is also observed by the coincident desorption of H2 and K at 470-490 K for θK g 1 ML. Acknowledgment. This work was supported by National University of Singapore under Grant No. RP910681. LA980616Q
