Interaction of D2O with a Thick BaO Film: Formation of and Phase

Aug 10, 2009 - The interaction of D2O with a thick BaO film (g20 monolayer equivalent (MLE)) on ultrathin Al2O3/NiAl(110) was investigated with temper...
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J. Phys. Chem. C 2009, 113, 15692–15697

Interaction of D2O with a Thick BaO Film: Formation of and Phase Transitions in Barium Hydroxides Cheol-Woo Yi† and Ja´nos Szanyi* Institute for Interfacial Catalysis, Pacific Northwest National Laboratory, P.O. Box 999, MSIN: K8-87, Richland, Washington 99352 ReceiVed: April 24, 2009; ReVised Manuscript ReceiVed: June 16, 2009

The interaction of D2O with a thick BaO film (g20 monolayer equivalent (MLE)) on ultrathin Al2O3/NiAl(110) was investigated with temperature programmed desorption (TPD) and infrared reflection absorption spectroscopy (IRAS). Upon D2O exposure of a thick BaO film, amorphous barium hydroxide formed at room temperature that readily converted to crystalline Ba(OD)2 phases during annealing in ultrahigh vacuum (UHV). The formation of crystalline hydroxide phases depends on the initial D2O exposure at 300 K. Following low D2O exposure at room temperature that results in the formation of amorphous barium hydroxide with no hydrating water, only the R-Ba(OD)2 phase was observed after 400 K annealing. The sample that was exposed to D2O extensively (i.e., hydrated amorphous barium hydroxide formed) showed a series of phase transformations as the sample was annealed to increasingly higher temperatures: amorphous-Ba(OD)2 · xD2O (x > 1) f β-Ba(OD)2 · D2O f β-Ba(OD)2 f R-Ba(OD)2. The results of TPD experiments completely agreed with this phase transformation scheme: hydrating water molecules desorbed first at 425 K, allowing the formation of the β-Ba(OD)2 · D2O phase. Desorption of water from β-Ba(OD)2 · D2O at around 475 K leads to the formation of β-Ba(OD)2 and its subsequent conversion to R-Ba(OD)2. All of the barium hydroxides thermally decomposed at T < 550 K. When the BaO film was exposed to D2O at 425 K, crystalline β-Ba(OD)2 formed initially, which led to the formation of a small amount of R-Ba(OD)2 as well at low D2O exposures. At high D2O exposures, the dominant phase was β-Ba(OD)2 · xD2O, and no R-phase was seen. 1. Introduction The interaction of water with a solid surface is important in several areas such as corrosion, electrochemistry, as well as heterogeneous catalysis.1 For the field of heterogeneous catalysis, water may affect catalytic reactions as a poisoning agent and/ or a promoter.2 Water is also employed as one of the most useful probe molecules for surface defect site characterization because of its dissociation on specific sites in ultrahigh vacuum (UHV) conditions.3,4 As the concern about environmental contamination by emission from lean-burn (i.e., oxygen-rich) automobile engines, especially nitrogen oxide species (NOx), increases, the chemistry of supported BaO systems,5,6 the active component of promising NOx storage/reduction (NSR) catalysts, has been studied extensively. Gas phase species such as carbon dioxide (CO2) and water (H2O), which are present in substantial quantities in automotive exhausts, may significantly influence the nature and reactivity of the storage site, even though they may not directly be involved in the storage/reduction process. While most of the investigations of BaO-containing systems have focused on the interaction of NOx with BaO, recent studies7-10 have reported that the existence of water changes the morphology of the catalysts and affects the reaction mechanism between NO2 and BaO. Several anhydrous and hydrated barium hydroxide phases have been identified and characterized using vibrational spectroscopy, X-ray diffraction (XRD), and thermoanalytical measurements.11,12 In fact, two crystalline phases of anhydrous Ba(OH)2, R-phase at high temperature and β-phase at low * Corresponding author. E-mail: [email protected]. † Present address: Department of Chemistry and Institute of Basic Science, Sungshin Women’s University, Seoul 136-742, Korea (ROK).

temperature conditions, have been reported in the literature,13,14 the transition between these two phases is irreversible so that R-phase which is formed at high temperature remains unchanged at room temperature upon cooling under dry conditions. However, a reversible R- to β-phase transition was observed in the presence of water vapor.13 Recently, Nix and co-workers reported the formation of barium hydroxide upon exposure of a BaO/Cu(111) model system to water under ambient temperature conditions by X-ray photoelectron spectroscopy (XPS), temperature programmed desorption (TPD), and infrared reflection absorption spectroscopy (IRAS),15 and they assigned the IRAS feature to the R-barium hydroxide phase. The assignment of the TPD and IRAS results to the formation of R-barium hydroxide at 300 K was, however, not straightforward. In the present study, the interaction of water (in a wide exposure range) with a thick BaO film and its phase transformations were investigated with TPD and IRAS. 2. Experimental Section The experimental setup and data acquisition procedures employed in the current study have been discussed in detail elsewhere.4,16-19 All of the experiments were performed in an UHV surface analysis chamber equipped with various conventional surface spectroscopic techniques connected to an elevated pressure reactor cell with CaF2 windows for IRAS and pressure dependent experiments. The base pressure was less than 2 × 10-10 Torr in both chambers. The NiAl(110) single crystal used in this experiment (Princeton Scientific Corp., diameter ) 10 mm and thickness ) 2 mm) was spot-welded onto a U-shaped Ta wire (0.030′′ diameter). Temperature was measured by a C-type thermocouple spot-welded to the backside of the single

10.1021/jp903798z CCC: $40.75  2009 American Chemical Society Published on Web 08/10/2009

Interaction of D2O with a Thick BaO Film crystal. The NiAl(110) crystal was cleaned by repeated cycles of Ar ion sputtering and annealing at 1200 K in UHV conditions, and the cleanliness of the surface was verified with Auger electron spectroscopy (AES) and XPS. After a clean NiAl(110) substrate was prepared, an ultrathin alumina film was grown epitaxially by the oxidation of a NiAl(110) single crystal as described previously.4,16,17,20-22 Specifically, the oxide film was formed by 1200 L of O2 exposure of the clean NiAl(110) at 540 K and successive annealing at 1070 K in UHV. The alumina film thickness was estimated to be ∼2 monolayer equivalent (MLE),4,16,20,21,23-25 and the quality of the alumina films was checked with AES, XPS, low energy ion scattering spectroscopy (LEISS), and low energy electron diffraction (LEED). Then, Ba deposition onto a water multilayer as a reactive layer was carried out by the reactive layer-assisted deposition (RLAD) method using a resistively heated Ba doser (SAES Getters Inc.). As we have discussed in our previous publications,4,16-19 most of the variations reported in the literature for BaO-containing model systems have been caused by the influence of the substrate on which the base-metal oxide (i.e., BaO) is supported. This effect was clearly shown in the study of NO2 adsorption on a BaO (5-6 ML)/Cu(111) system by Tsami et al.15 In order to minimize the interaction between BaO and the underlying substrate (Al2O3 and/or NiAl),4,16-19 here a thick BaO film was prepared by RLAD of Ba onto multilayer D2O at 90 K. (The BaO film thickness was estimated from the Auger analysis of the deposited film as a function of deposition time and from the attenuation of the substrate XPS features (Al 2p and Ni 3p XPS intensities).18,24 The exact thickness of the BaO in this study is not known, and it is estimated to be between 20 and 25 ML. Our goal was to prepare a thick enough BaO film that exhibited chemical properties independent of the number of adsorption/ reaction/desorption cycles. LEISS measurements on the thick BaO film both prior to and after adsorption/desorption measurements revealed the presence of only Ba and O on the surface of the sample. High purity D2O (Aldrich, 99.99%) used in the experiments was further purified by several freeze-pump-thaw cycles. TPD experiments were conducted in the UHV chamber after dosing D2O onto the BaO film through a pinhole doser (the exposure of D2O during 15 s corresponds to ∼1 MLE). After D2O adsorption, the sample was moved in front of a quadrupole mass spectrometer (QMS) housed in a negatively biased metal shield (-70 V) to prevent the interaction between the adsorbates and the electrons from the ionization chamber of QMS. Infrared spectra were collected at 4 cm-1 resolution, using a grazing angle of approximately 85° with respect to the surface normal. All of the IRAS data collected were referenced to a background spectrum acquired from the clean sample prior to D2O adsorption. 3. Results and Discussion 3.1. TPD. Figure 1 shows a series of TPD spectra obtained from a thick BaO film supported on the alumina layer on NiAl(110) as a function of D2O exposure at 300 K sample temperature. At low D2O exposures, the TPD spectra follow a common leading edge and show a gradual shift of the maximum desorption rate from 470 to 510 K with increasing exposure. It suggests a zeroth-order desorption kinetics, in which the desorption rate is determined by the barrier to Ba-hydroxide decomposition. As the thickness of the Ba-hydroxide layer increases, the desorption rate maximum shifts to higher temperature, with no change in the onset temperature. However, upon a certain D2O exposure, the TPD profile suddenly changes

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Figure 1. TPD: profiles for the 20 amu fragment after D2O adsorption onto a thick BaO/Al2O3/NiAl(110) system with different D2O exposures at 300 K.

and displays two clearly resolved desorption features with maximum desorption rates at ∼480 and ∼520 K. These spectra do not follow the common leading edge of previous traces, while the maximum desorption rate continuously shifts toward higher temperatures as the D2O exposure increases. With further D2O exposure, the integrated intensity of the TPD profile slowly increased, but even after 600 s exposure, it did not reach complete saturation. Note that the TPD spectra collected after the two highest D2O exposures show an additional small desorption feature with a maximum desorption rate at 423 K (shown in the inset of Figure 1) which can be assigned to the desorption of hydrating water molecules (see discussion later). Barium hydroxide is known to exist in amorphous and several crystalline forms, such as anhydrous β- and R-Ba(OH)2, as well as several crystalline or amorphous hydrates, Ba(OH)2 · xH2O (x ) 1, 3, and 8).11 Previous differential thermal analysis (DTA) and differential scanning calorimetry (DSC) analysis of barium hydroxide showed that β- to R-Ba(OH)2 phase transition temperature shifted to higher values as the number of water molecules in the hydrated barium hydroxide increased.11,12,26 Maneva-Petrova and Nikolova reported that the temperature of the last dehydration step shifted from 422 to 503 K as the number of hydrating water molecules increased (from the monoto octa-hydrated barium hydroxides).12 Furthermore, the β- to R-Ba(OH)2 phase transition temperature depended upon the hydration level of the starting hydrated hydroxide sample. For β-Ba(OH)2 obtained from the monohydrated hydroxide, this phase transition occurred at 485 K, while for the one prepared from the octa-hydrated form it took place at 533 K.12 The TPD results indicate that up to a certain D2O exposure the Bahydroxide formed in the reaction of water and BaO transforms into only one crystalline barium-hydroxide phase (we tentatively assign it to the R-Ba(OD)2) prior to decomposition. As the amount of water in the barium hydroxide phase reaches a level high enough to produce hydrates (evidenced by the appearance of the water desorption feature at around 423 K), initially β-Ba(OD)2 · D2O can form, which then releases its hydrating D2O at ∼470 K and transforms into β-Ba(OD)2. At even higher temperature, this phase transforms into anhydrous R-Ba(OD)2

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Figure 2. IRAS: a series of spectra obtained at low exposures of D2O onto a thick BaO/Al2O3/NiAl(110) system at 300 K (A), followed by annealing to the indicated temperatures (B).

and decomposes, with a maximum rate at around 520 K. In order to gain further understanding of these processes, we conducted IRAS experiments on D2O adsorption on the thick BaO layer, and the results are discussed in the following section. 3.2. IRAS. The interaction of D2O with the thick BaO layer was also investigated by IRAS in order to identify the hydroxide phases that can form under different experimental conditions, and to confirm the conclusions drawn from the TPD results. Figure 2A shows a series of IRAS spectra collected as a function of time at a constant D2O flux at 300 K in the 5-70 s time interval. In the corresponding D2O exposure range, the TPD profiles follow a common leading edge with an onset temperature of ∼425 K, and one single desorption feature develops with an increasing temperature of maximum desorption rate as the D2O exposure increases (Figure 1). At very low exposure of D2O (5-10 s D2O exposure time), a broad vibrational feature appears in the range of 2350-2650 cm-1, indicating the formation of amorphous Ba-hydroxide.15 As the D2O exposure

Yi and Szanyi increases, the intensity of this broad IRAS feature monotonically increases, and the peak maximum after 70 s D2O exposure time is positioned at ∼2532 cm-1. Significant changes in the IRAS spectra were observed during annealing of the sample as shown in Figure 2B. In the temperature range of 300-375 K, IRA spectra remained unchanged. However, the spectrum collected after annealing the sample at 400 K shows the transformation of the broad, unresolved IRAS band into two distinct absorption features. A sharp vibrational feature appears at 2595 cm-1 with a broad shoulder at 2100-2400 cm-1. In order to compare the results of our IRAS measurements with those published in the open literature,11 we have also performed an experiment in which the thick BaO film was exposed to H2O at 300 K, and then annealed to increasingly higher temperature in vacuum. The sharp IRAS feature obtained at 2595 cm-1 upon room temperature D2O exposure and annealing to 400 K is shifted to 3530 cm-1 with H2O adsorption and annealing under the same conditions, in accord with the vibrational frequency shift caused by the substitution of D for H in the hydroxyl group. Concomitantly, the broad feature between 2100 and 2400 cm-1 observed after annealing the D2O-exposed sample to 400 K shifted to ∼3150 cm-1. It is noteworthy that a small, but clearly distinguishable IRAS feature was also observed at 3585 cm-1 when H2O was used, corresponding to the 2634 cm-1 lower intensity feature observed for D2O adsorption. These two features at 3350 cm-1 and ∼3150 cm-1 in H2O exposed/ annealed BaO (2595 and 2100 - 2400 cm-1 for D2O) can be assigned to the R-barium hydroxide phase, one of the anhydrous barium-hydroxide polymorphs. Lutz and co-workers11 have investigated hydrous and anhydrous barium hydroxides, and the transitions between the different hydroxide phases by Raman and IR spectroscopies. They reported two IR features characteristic of the R-Ba(OH)2 phase: a broad band centered at 3360 cm-1, and a sharp, but low intensity shoulder at 3584 cm-1 at 295 K. In a study on the phase transitions of Ba(OH)2, Cordfunke et al.13 showed the same absorption bands in the IR spectrum of R-Ba(OH)2 centered around 3400 cm-1 with a very low intensity but a sharp shoulder at ∼3600 cm-1. The results obtained in these two previous studies qualitatively agree with our findings, although the intensity ratios of these two IR bands of R-Ba(OH)2 are somewhat different, which may be attributed to the fundamentally different systems and experimental conditions applied in the previous and in our studies. In both of the references cited, bulk barium hydroxides were studied, while here we investigated the formation of barium hydroxides and their phase transformations starting with an ultrathin film of BaO. With further increase of sample temperature, the intensities of these two vibrational features simultaneously decrease and completely disappear as the annealing temperature reaches 500 K. These observations together with the results of TPD shown in Figure 1 allow us to conclude that at low D2O exposures (in this study tD2O exposure e 70 s) the amorphous Ba(OD)2 initially formed transforms to crystalline R-Ba(OD)2 at around 400 K. The thus-formed R-Ba(OD)2 subsequently decomposed at 475 K < T < 500 K. The D2O TPD profile of the 70 s exposed BaO film shows that the onset temperature of water desorption is ∼425 K and that the maximum desorption temperature is ∼500 K. The direct formation of R-Ba(OD)2 from the amorphous phase is surprising in the light of previous studies that showed the formation of the R-phase from the β-phase only. However, in those studies the starting Ba(OH)2 · xH2O (1 e x e 8) materials were always hydrated, and even the initially formed β-Ba(OH)2 contained one hydrating H2O molecule. The R-phase formed only from the anhydrous β-Ba(OH)2 that, in

Interaction of D2O with a Thick BaO Film turn, was produced by the elimination of the hydrating H2O molecule. In our case, however, at the low D2O exposure limit, all the water molecules impinging onto the BaO surface take part in the formation of barium hydroxide, excluding the possibility of the formation of hydrated species. In the absence of hydrates, the β-phase cannot form when the sample is heated instead it converts to R-Ba(OD)2 directly, which then decomposes as the temperature approaches 500 K. The temperature of the phase transition observed in this study (375-400 K) is slightly lower than those reported previously for the formation of the R-phase. Maneva-Pertrova et al.12 reported that the temperature of phase transition that led to the formation of R-Ba(OH)2 depended on the number of water molecules that were present in the starting amorphous Ba-hydroxide used to prepare the β-Ba(OH)2. The β to R phase transition for Ba(OH)2 · 8H2O occurred at ∼533 K, while it took place at 485 K for Ba(OH)2 · H2O, showing that the temperature of phase transition shifted to lower temperatures as the starting material contained fewer number of hydrated water molecules. In the sample we prepared at low D2O exposure, the phase transition from amorphous Ba(OD)2 to R-Ba(OD)2 occurred at even lower temperature (between 375 and 400 K) in the absence of any hydrating D2O molecules. The results of the TPD experiments obtained at long water exposure times indicate that fundamentally different processes take place when the sample is exposed to large amounts of water. To understand these phenomena, we investigated the changes that occurred in the IRA spectra following higher D2O exposures at 300 K and subsequent annealing until all the water desorbed. IRA spectra collected at a constant D2O flux at 300 K sample temperature as a function of exposure time (from 30 to 600 s) are shown in Figure 3A. The IRA spectra collected at long exposure times have the same characteristics as the ones recorded at short exposure times (compare with Figure 2A), i.e., one broad, featureless band centered at around 2545 cm-1. These results suggest that as the D2O exposure of the BaO film increases, it gradually converts to amorphous barium hydroxide. The intensity of this broad feature increases with the D2O exposure time, without any change in the shape of the band as a function of D2O uptake. However, significant changes take place in the IRA spectra as the sample is annealed to gradually higher temperatures. Even after the first annealing step (heating from 300 to 350 K), IRAS shows a small drop in the intensity of the broad feature centered at 2545 cm-1, and the concomitant appearance of a new, sharp band at around 2589 cm-1. The decrease in the intensity of the 2545 cm-1 feature accelerates when the sample is heated to 375 and then to 400 K, while the intensity of the 2589 cm-1 band does not change appreciably. In the TPD spectrum of a 600 s D2O-exposed BaO sample in the 300-400 K temperature range, no D2O desorption was observed. Therefore, the changes in the IRA spectra observed in this temperature range should be associated with the transition of the disordered amorphous Ba(OD)2 phase to an ordered one, most probably to the β-phase. In this case, due to the long D2O exposure time, the resulting β-Ba(OD)2 contains more than one hydrating D2O. The shape of the IR spectrum collected after 400 K annealing suggests the presence of mixed phases, i.e., both β- and amorphous Ba(OD)2 with hydrating D2O. A dramatic change in the IR spectrum occurs when the sample is annealed to 425 K. Two sharp features appear simultaneously at 2496 and 2645 cm-1, while the intensity of the sharp band at 2589 cm-1 reduced significantly. In this temperature range, as the inset in Figure 1 shows, a low intensity D2O desorption peak appears in the TPD spectra due, most probably, to the

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Figure 3. IRAS: spectra for high exposures of D2O onto a thick BaO/ Al2O3/NiAl(110) system at 300 K (A) and subsequent annealing to the indicated temperatures (B).

desorption of hydrating water molecules. These results are consistent with the elimination of hydrating water and the formation of crystalline β-Ba(OD)2 · D2O (monohydrate). The formation of this ordered phase by the elimination of excess water was reported to occur between 365 and 425 K,12 in good agreement with the findings of this study. In our control IRAS experiment performed with H2O, the corresponding peak positions were at 3274 and 3504 cm-1, and these peaks can be assigned to β-Ba(OH)2 · H2O. The vibrational features at 3274 and 3504 cm-1 are in good agreement with those of a previous infrared spectroscopic studies of Lutz et al. (3278 and 3496 cm-1)11 and Cordfunke et al. (3260 and 3490 cm-1)13 (although, they assigned these bands to anhydrous β-Ba(OH)2). In addition, the νOH/νOD ratio is 1.32, consistent with the value reported in ref 27 by measuring this ratio for the two isotopes in solid argon matrix. The peak position of the IRAS feature at 2589 cm-1 is very close to that we have assigned to R-Ba(OD)2 discussed above (Figure 2B). However, the appearance of this band at this low temperature, together with the absence of the very broadband between 2100 and 2400 cm-1 that is characteristic

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for the R-Ba(OD)2 strongly argue against the assignment of this band to the R-phase. As the sample temperature further increased from 425 to 475 K, the intensities of the IRAS peaks just developed after annealing to 425 K (2496 and 2645 cm-1) drastically reduced. The fact that these two IRAS features appear and disappear simultaneously strongly suggests that they originate from the same species. In this temperature range, TPD shows D2O desorption with maximum desorption rate at around 470 K. Both the TPD and IRAS results can be rationalized by the loss of hydrating water from β-Ba(OD)2 · D2O, i.e., the formation of the anhydrous β-Ba(OD)2 phase. Comparing the IR spectra recorded after annealing the sample at 450 and 475 K also reveals that besides the formation of the anhydrous, crystalline β-Ba(OD)2, a new phase also began to form as evidenced by the appearance of the broad vibrational feature between 2100 and 2400 cm-1 that we have attributed to crystalline R-Ba(OD)2. Further evidence for the formation of R-Ba(OD)2 is the slight increase in the intensity of the 2589 cm-1 peak as the broad band in the low frequency region appears. The intensity of this band reaches its maximum as the sample is heated to 500 K. Note that as the intensity of this band reaches its maximum, the band centered at 2496 cm-1 loses a significant portion of its intensity. This observation indicates that the vibrational feature at 2589 cm-1 is a composite band consisting of contributions from characteristic features of both anhydrous R- and β-Ba(OD)2. At even higher temperatures (>500 K), the intensites of all IRAS features decrease and completely disappear after annealing at 550 K, in agreement with the TPD results of Figure 1. On the basis of these results, we propose that the intense D2O TPD feature is assigned to the decomposition of anhydrous crystalline Ba(OD)2, probably both R- and β-phase. During the TPD run above 500 K, the anhydrous crystalline β-Ba(OD)2 gradually transforms to the R-phase; however, at these temperatures the R-phase starts to decompose, and the decomposition is complete at a sample temperature of 550 K. The TPD and IRAS results we have discussed so far substantiated the formation of amorphous barium hydroxides with and without hydrating water molecules, depending on the H2O exposure at 300 K, and the subsequent series of phase transformations, and eventual decomposition to BaO as the sample was heated from 300 to 550 K. The results of TPD experiments suggested that the formation of the β-Ba(OD)2 phase required the presence of excess water, i.e., hydrating water. Once D2O evolution is observed at around 425 K, the main desorption peak splits into a low and a high temperature part. The IRAS results of Figure 3B indicated that critical changes in the barium hydroxide phase occurred upon annealing to 425 K. In order to understand these processes that are taking place during D2O exposure and subsequent annealing of the thick BaO film, we conducted a D2O exposure experiment at 425 K sample temperature and collected IR spectra as a function of exposure time. Subsequently, the sample was annealed to 550 K, the temperature sufficiently high enough to remove all of the water molecules and hydroxyl groups from the sample (i.e., decompose the barium hydroxides). A series of IRA spectra obtained during D2O exposure of the BaO film at 425 K sample temperature as a function of exposure time at a constant water flux are displayed in Figure 4A. At low D2O exposures (15-300 s), the IR spectra show the development of an intense absorption feature centered at 2595 cm-1 and the development of a broad, low intensity feature in the 2100-2400 cm-1 range. The position of these IR features are consistent with the formation of both R- and β-Ba(OD)2 phases. The fact that the intensity of the broad

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Figure 4. IRAS: spectra for high exposures of D2O onto a thick BaO/ Al2O3/NiAl(110) system at 425 K and subsequent annealing to the indicated temperatures (B).

band between 2100 and 2400 cm-1 did not increase proportionally with the increase of the 2595 cm-1 feature strongly suggests the formation of a mixed R- and β-barium hydroxide phase. The formation of these phases can be rationalized by the results presented above: upon low exposure of D2O, the sample temperature (425 K) is high enough to induce the formation of R-Ba(OD)2 and prevent the formation of hydrated species. Therefore, the Ba(OD)2 that forms at this temperature is the mixed crystalline R- and β -Ba(OD)2 with no hydrates. As the sample is exposed to water for 360 s, the appearance of two new features can be observed at 2520 and 2641 cm-1, concomitant with the intensity drop of the broadband in the 2100-2400 cm-1 region. The appearance of the 2520 and 2641 cm-1 IRAS features indicates the onset of the transformation to β-Ba(OD)2 · D2O, which is expected in the presence of excess water. Parallel to the formation of monohydrated β-Ba(OD)2 is the reconversion of the R-Ba(OD)2 phase to the β-phase, a reversible process in the presence of water.12 Increasing the water exposure time to 1200 s results in the initially fast, then

Interaction of D2O with a Thick BaO Film slow increases in the intensities of both the 2520 and 2641 cm-1 bands and the complete disappearance of the broad feature of R-Ba(OD)2 in the low frequency region. After 1200 s of D2O exposure, the IR cell was evacuated for an extended period of time at 425 K sample temperature, which resulted in no change in the intensities of the IRAS absorption features. This result indicates the high stability of the monohydrated β-Ba(OD)2 in vacuum at 425 K. The series of IR spectra collected in the subsequent annealing experiments (from 425 to 550 K) is shown in Figure 4B. Increasing the sample temperature from 425 to 450 K results in large decreases in the intensities of both the 2520 and 2641 cm-1 bands, while seemingly no change in the intensity of the 2596 cm-1 peak is observed. Also note that no IRAS feature is observed in the 2100-2400 cm-1 spectral region after annealing to 425 K. The IR spectrum collected following the 475 K annealing step shows dramatic changes in the sample. The 2520 and 2645 cm-1 bands completely disappear, while the broad band between 2100 and 2400 cm-1 develops with high intensity. Annealing to even higher temperatures (up to 550 K) brings about the gradual decreases and ultimate disappearance in the intensities of the two remaining IRAS bands (2587 and 2100-2400 cm-1). These results are consistent with those we discussed above: β-Ba(OD)2 · D2O formed during the extensive water exposure begins to lose water as the sample is heated to 450 K and forms the β-Ba(OD)2 phase. During the 475 K annealing step, all of the hydrating water is lost from the β-phase resulting in the formation of the mixed R/βBa(OD)2. Above 475 K, the β-hydroxide readily converts to R-hydroxide which then completely decomposes to BaO as the sample temperature reaches 550 K. Here again, the 2587 cm-1 IRAS feature has contributions from both anhydrous crystalline Ba(OD)2 phases (R- and β-phases). The results of a TPD experiment (not shown) obtained under conditions identical to those of the 600 s D2O exposed sample at 425 K in the IRAS experiment showed the same desorption features as we have discussed previously for the sample that was exposed to D2O at 300 K for 600 s, except the absence of the 425 K desorption feature. 4. Conclusions In this study, the reaction of water with BaO was investigated by TPD and IRAS at 300 and 425 K sample temperatures. Upon room temperature water exposure, BaO converted into amorphous Ba(OD)2 with or without hydrating D2O, depending on the water exposure. Anhydrous, amorphous barium hydroxide (formed at short water exposure times at 300 K) directly converted to crystalline R-Ba(OD)2 at 375 < Tanneal < 400 K, without going through the β-Ba(OD)2 phase. During annealing of an extensively water exposed sample (hydrating water was present in the amorphous Ba-hydroxide at 300 K), through a series of phase transformations, the thermodynamically most stable R-Ba(OD)2 phase formed. When the sample was exposed to water at 425 K, a mixture of crystalline R- and β-Ba(OD)2 formed initially (the amount of R-phase is much smaller than that of the β), which converted to β-Ba(OD)2 · D2O at long

J. Phys. Chem. C, Vol. 113, No. 35, 2009 15697 exposure times. Upon annealing, this sample converted back to β-Ba(OD)2 and at even higher temperature (450 < T < 475 K) to R-Ba(OD)2 before it decomposed to BaO and D2O as the sample temperature approached 550 K. Acknowledgment. We gratefully acknowledge the US Department of Energy (DOE), Office of Basic Energy Sciences, and Division of Chemical Sciences for the support of this work. The research described in this article was performed at the Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by the DOE Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory (PNNL). PNNL is operated for the US DOE by Battelle Memorial Institute under contract number DE-AC05-76RL01830. This work was also supported by the Sungshin Women’s University Research Grant of 2009. References and Notes (1) Carrasco, J.; Illas, F.; Lopez, N. Phys. ReV. Lett. 2008, 100, 016101. (2) Gronbeck, H.; Panas, I. Phys. ReV. B 2008, 77, 245419. (3) Henderson, M. A. Surf. Sci. Rep. 2002, 46, 5. (4) Yi, C. W.; Szanyi, J. J. Phys. Chem. C 2007, 111, 17597. (5) Matsumoto, S. CATTECH 2000, 4, 102. (6) Kwak, J. H.; Mei, D.; Yi, C.-W.; Kim, D. H.; Peden, C. H. F.; Allard, L. F.; Szanyi, J. o. J. Catal. 2009, 261, 17. (7) Kim, D.; Chin, Y.-H.; Kwak, J.; Szanyi, J.; Peden, C. Catal. Lett. 2005, 105, 259. (8) Kim, D. H.; Kwak, J. H.; Szanyi, J.; Burton, S. D.; Peden, C. H. F. Appl. Catal., B 2007, 72, 233. (9) Szanyi, J.; Kwak, J. H.; Kim, D. H.; Wang, X.; Chimentao, R.; Hanson, J.; Epling, W. S.; Peden, C. H. F. J. Phys. Chem. C 2007, 111, 4678. (10) Szanyi, J.; Kwak, J. H.; Kim, D. H.; Wang, X.; Hanson, J.; Chimentao, R. J.; Peden, C. H. F. Chem. Commun. 2007, 984. (11) Lutz, H. D.; Eckers, W.; Schneider, G.; Haeuseler, H. Spectrochim. Acta A 1981, 37, 561. (12) Maneva-Petrova, M.; Nikolova, D. Thermochim. Acta 1985, 92, 287. (13) Cordfunke, E. H. P.; Booij, A. S.; Konings, R. J. M.; vanderLaan, R. R.; SmitGreen, V. M.; vanVlaanderen, P. Thermochim. Acta 1996, 273, 1. (14) Friedrich, A.; Kunz, M.; Suard, E. Acta Crystallorg., Sect. A 2001, 57, 747. (15) Tsami, A.; Grillo, F.; Bowker, M.; Nix, R. M. Surf. Sci. 2006, 600, 3403. (16) Yi, C. W.; Kwak, J. H.; Peden, C. H. F.; Wang, C.; Szanyi, J. J. Phys. Chem. C 2007, 111, 14942. (17) Yi, C. W.; Kwak, J. H.; Szanyi, J. J. Phys. Chem. C 2007, 111, 15299. (18) Yi, C.-W.; Szanyi, J. J. Phys. Chem. C 2009, 113, 716. (19) Yi, C.-W.; Szanyi, J. J. Phys. Chem. C 2009, 113, 2134. (20) Franchy, R. Surf. Sci. Rep. 2000, 38, 195. (21) Kresse, G.; Schmid, M.; Napetschnig, E.; Shishkin, M.; Kohler, L.; Varga, P. Science 2005, 308, 1440. (22) Lay, T. T.; Yoshitake, M.; Song, W. Appl. Surf. Sci. 2005, 239, 451. (23) Stierle, A.; Renner, F.; Streitel, R.; Dosch, H.; Drube, W.; Cowie, B. C. Science 2004, 303, 1652. (24) Yi, C. W.; Luo, K.; Wei, T.; Goodman, D. W. J. Phys. Chem. B 2005, 109, 18535. (25) Yoshitake, M.; Lay, T. T.; Song, W. J. Surf. Sci. 2004, 564, 211. (26) Habashy, G. M.; Kolta, G. A. J. Inorg. Nucl. Chem. 1972, 34, 57. (27) Andrews, L.; Wang, X. Inorg. Chem. 2005, 44, 11.

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