Adsorption and Dehydrogenation of Decaborane on the Pt (111) Surface

Jul 7, 2009 - The adsorption and decomposition of decaborane (B10H14) on the Pt(111) surface was studied with reflection absorption infrared ...
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J. Phys. Chem. C 2009, 113, 13847–13854

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Adsorption and Dehydrogenation of Decaborane on the Pt(111) Surface Aashani Tillekaratne and Michael Trenary* Department of Chemistry, UniVersity of Illinois at Chicago, 845 West Taylor Street, Chicago, Illinois 60607-7061 ReceiVed: April 20, 2009; ReVised Manuscript ReceiVed: June 16, 2009

The adsorption and decomposition of decaborane (B10H14) on the Pt(111) surface was studied with reflection absorption infrared spectroscopy (RAIRS), temperature programmed desorption (TPD), and X-ray photoelectron spectroscopy (XPS). The molecule has a nido structure with 10 terminal B-H bonds and 4 bridging B-H-B bonds. Comparison of the experimental RAIR spectra with spectra calculated using density functional theory indicates that the molecule adsorbs at 85 K without dissociation and with an orientation in which the dipole moment is parallel to the surface. The TPD experiments show that the dissociation leads to H2 desorption up to ∼420 K but that no boron-containing species desorb. Dissociation occurs in stages via stable surface intermediates, as indicated by changes observed with RAIRS in the terminal B-H stretch region at ∼2600 cm-1. Boron, which is produced as a result of complete dissociation, is lost from the surface as indicated by XPS, most likely through dissolution into the bulk of the platinum crystal. Γvib ) 20A1 + 14A2 + 17B1 + 15B2

Introduction Complex hydrides are of interest for hydrogen-storage applications as they often contain more hydrogen per unit volume than found in highly compressed hydrogen gas or even in liquid hydrogen.1 To achieve the highest hydrogen weight percentage possible in the storage system, hydrides of the lightest elements are of most interest. Like carbon, boron forms a rich variety of compounds with hydrogen, and since boron is even lighter than carbon, boron hydrides (boranes) are of particular interest. Furthermore, since these are stable compounds that only decompose at elevated temperatures, there is interest in using transition metal catalysts to promote the low-temperature release of hydrogen from boranes. This provides the motivation for the present study of the adsorption and decomposition of a typical borane of high hydrogen content, decaborane (B10H14), on the surface of a catalytically active metal, platinum, which is wellknown to catalyze hydrogenation and dehydrogenation reactions of hydrocarbons. The structures, bonding, and reactions of boranes have all been widely studied.2-4 Although the interaction of boranes with metals to form metalloboranes has also been extensively studied,5-12 there is relatively little information on their reactions on metal surfaces.4,13-20 To probe the structure of the adsorbed molecule and to characterize its reactive chemistry under well-defined conditions, the experiments reported here were conducted on a clean, well-ordered Pt(111) single crystal using a variety of surface-sensitive techniques. In particular, we rely on the vibrational spectrum as measured with reflection absorption infrared spectroscopy (RAIRS) to establish that the molecule adsorbs molecularly at low temperature and decomposes via stable surfaces intermediates as the temperature is raised. Decaborane has a nido structure (Figure 1) with 4 bridging hydrogen atoms and 10 terminal hydrogen atoms, where the boron cage can be described as an 11-sided deltahedron lacking one B atom.5 The four bridging hydrogen atoms partake in the characteristic three-center-two-electron bonding found in boranes. The B10H14 molecule has C2V symmetry and its 66 normal modes can be assigned to the following irreducible representations: * To whom correspondence should be addressed. E-mail: mtrenary@ uic.edu.

Since the A2 modes are infrared inactive, only 52 normal modes are symmetry allowed. Keller and Johnston21 first reported rough assignments for the infrared spectrum of B10H14 in 1952, and Bellamy et al. have reviewed infrared spectra of compounds with boron bonds to oxygen, chlorine, methyl, phenyl, as well as terminal, bridge, and BH2 hydrogen atoms.22 Infrared spectra of many compounds of boron have also been discussed by Nakamoto.23 Although these are not surface studies, their assignments can be used as a guide in identifying the major vibrations of B10H14 adsorbed on the Pt(111) surface. Experimental Section The experiments were performed in two different ultra-high vacuum (UHV) chambers using two different Pt(111) single crystals. The X-ray photoelectron spectra were obtained in a chamber (chamber 1) with a base pressure of ∼4 × 10-11 Torr. The system has been described in detail elsewhere.24 In brief,

Figure 1. Structure of nido-decaborane (14), B10H14. Boron atoms are shown in blue. The molecule has 10 terminal hydrogen atoms (green) and 4 bridging hydrogen atoms (red). The molecule has C2V symmetry and can be considered as a fragment of an icosahedron in which 10 of the 12 vertices are occupied by boron atoms.

10.1021/jp903624g CCC: $40.75  2009 American Chemical Society Published on Web 07/07/2009

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the UHV chamber is equipped with low-energy electron diffraction (LEED), an X-ray photoelectron spectrometer (XPS), a quadrupole mass spectrometer (QMS) for temperature programmed desorption (TPD), and a Fourier transform infrared (FTIR) spectrometer for reflection absorption infrared spectroscopy (RAIRS). The XPS system consists of a VG CLAM2 hemispherical analyzer and a dual-anode X-ray source. Mg KR radiation was used, and the spectrometer was calibrated with the Pt 4f7/2 peak at a binding energy of 71.2 eV. All RAIRS and TPD experiments were performed in a second chamber (chamber 2) with a base pressure of ∼8 × 10-11 Torr. The system has been described in detail elsewhere.25 In brief, this UHV chamber is equipped for LEED, Auger electron spectroscopy (AES), and TPD experiments with a QMS. The chamber is coupled to a commercial FTIR spectrometer, a Bruker IFS 66 v/S. The IR beam enters and exits the UHV chamber through differentially pumped O-ring sealed KBr windows and passes through a polarizer before reaching the infrared detector. To achieve maximum sensitivity, an InSb detector was used with a tungsten source for the spectra obtained only in the region above 2000 cm-1, while an MCT (HgCdTe) detector was used with a SiC source for the entire spectral range from 800 to 4000 cm-1. A resolution of 4 cm-1 was used. In cases where the crystal was annealed above 85 K, it was cooled back down to 85 K before acquiring the spectra. All background spectra were also acquired at 85 K. For the TPD results, signal from the QMS was recorded for each mass while heating the crystal at a linear rate of 2 K/s. The Pt(111) surfaces were cleaned and judged free of impurities by a standard procedure described earlier.26 Briefly, the crystal was heated in ∼3 × 10-7 Torr of O2 at ∼825 K for 1 h, after Ar+ ion bombardment. Before exposing to decaborane, the crystal was flashed to ∼1200 K and cooled to 85 K. Decaborane is a stable, white, crystalline solid with a melting point of 98.8 °C and a boiling point of 213 °C. Because it has a vapor pressure of about 100 mTorr at room temperature, it can be dosed onto surfaces under UHV conditions using standard gas-handling methods. Although it has been reported27 to be quite stable in air at room temperature with respect to both oxidation and hydrolysis compared to other boranes, a recent report28 states that it has a limited shelf life after which it polymerizes to give higher order BxHy molecules. This will turn the crystalline solid into a viscous substance, although the rate of the process can be decreased by storing decaborane in a refrigerator. Decaborane is toxic with a distinctly unpleasant odor, and appropriate precautions need to be taken when handling it.29 Decaborane is also known to react with hot water/ vapor releasing other toxic boranes like diborane and pentaborane.28 It dissolves latex rubber, so nitrile gloves should be worn when handling it. It can be removed from tools and glassware easily, as it dissolves readily in alcohols.29 The decaborane was purchased from Alfa Aesar with a quoted purity of >99%. It was further purified by transferring into a glass bulb in a glovebag with a continuous flow of dry nitrogen and subjecting it to several freeze-pump-thaw cycles using a dry ice/acetone bath. The decaborane was then condensed into another glass bulb cooled by liquid nitrogen. Finally, it was shielded from light to avoid any light-induced decomposition. Prior to each exposure, the gas above the decaborane solid was pumped down for several minutes in order to remove the gasphase impurities that slowly desorbed from the solid over time. For the RAIRS experiments, the Pt(111) surface was exposed to a few langmuirs (L, 1 L ) 1 × 10-6 Torr s) of decaborane at 85 K. Then, the surface was annealed to the designated temperatures, held for 30 s, and cooled down to 85 K before

Tillekaratne and Trenary

Figure 2. RAIR spectra of B10H14 on Pt(111) at 85 K as a function of exposure. The region from 800-2000 cm-1 is magnified by a factor of 2.

recording the spectra. All RAIR spectra have been baseline corrected. After being exposed to large amounts of decaborane, several cycles of Ar+ bombardment and O2 cleaning were performed in order to ensure a “boron-free” surface. Computational Method The density functional theory (DFT) calculations were performed with the GAUSSIAN 03 program package using the B3LYP (three-parameter hybrid Becke exchange and LeeYang-Parr correlation) functional. The triple-ζ plus polarization function 6-311G(d,p) basis set consisting of a polarization “d” function on B and a polarization “p” function on H was used. The structure of decaborane was optimized first and then vibrational frequencies and intensities were calculated for the optimized structure. Results Figure 2 shows RAIR spectra of B10H14 on Pt(111) as a function of increasing exposure at 85 K. The intensity is scaled by a factor of 2 in the 800-2000 cm-1 region. The most intense vibrations for all exposures are the B-H stretches of the terminal B-H bonds. At the lowest exposure of 0.5 L, two major peaks are evident in the B-H stretch region at 2592 and 2607 cm-1. As the B10H14 exposure is increased, these two peaks grow both in intensity and width. At an exposure of 1.5 L, a new B-H stretch peak is visible at 2567 cm-1, which grows as the exposure is increased to 2.0 L. The spectra at 5.0, 8.0, and 10.0 L are very similar in the B-H stretch region with a broad peak at 2594 cm-1 with a shoulder at 2607 cm-1. The 10.0 L spectrum corresponds to a multilayer of B10H14 on the Pt(111) surface, as indicated by the TPD results described below. The fact that the B-H stretch peak positions of a multilayer of B10H14 are almost identical to those of submonolayer coverages indicates that the adsorption of B10H14 on Pt(111) at 85 K is largely molecular.

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Figure 3. RAIR spectra of 0.5 L of B10H14, corresponding to a submonolayer coverage, on Pt(111) as a function of annealing temperature.

Figure 4. RAIR spectra in the B-H stretch region of 0.5 L of B10H14 as a function of annealing temperature.

The spectra in the range from 800 to 2000 cm-1 in Figure 2 show a few relatively weak but important features. Consistent with Keller et al.,21 the peaks at 1471, 1524, 1585, 1900, and 1942 cm-1 are associated with vibrations of the B-H-B bridging hydrogen atoms. The signal-to-noise level of the spectrum at the lower cutoff region of the MCT detector is very low, and the peaks are not clearly identified. However, peaks at ∼800-1100 cm-1 are generally associated with bending vibrations of terminal B-H bonds and the vibrations of the B10 cage. Figure 3 shows RAIR spectra as a function of annealing temperature following a 0.5 L exposure of B10H14, while Figure 4 shows the corresponding results from a separate experiment in the B-H stretch region only. As is clear from Figure 4, after annealing to only 200 K, there are significant changes in the adsorbed molecule with the most intense peaks at 2607 and 2592 cm-1 in the 85 K spectrum replaced by peaks at 2561, 2555, and 2536 cm-1, with a weaker peak remaining at 2602 cm-1. The changes in the B-H stretch region indicates a substantial rearrangement of the decaborane molecule or even dissociation after annealing to 200 K to form one or more surface intermediates that retain terminal B-H bonds. This is in contrast to the results of a previous study of C2B10H12 (ortho-carborane), which has a closo-structure, where the molecule was stable up to 300 K on Pt(111).30 Further transformation is evident from Figure 4 after annealing to 300 K, where the most intense peak is seen at 2569 cm-1, with two other peaks seen at 2575 and 2555 cm-1. Comparison with results in Figure 3, which include an annealing temperature of 250 K, reveals that there are large changes between the 200 and 250 K anneals, but relatively little difference in peak positions between 250 and 300 K, although

there is some difference in the relative intensities of the B-H stretch peaks. As both Figures 3 and 4 show, there is little change between the 300 and 350 K anneals. However, the peak that was at 2555 cm-1 in the 300 K spectrum in Figure 4, is absent in the 350 K spectrum. After annealing to 400 K, peaks are still present at 2569 and 2577 cm-1, although their intensities have decreased by 80% and by 450 K no peaks are observed in the B-H stretch region. As shown in Figure 3, several peaks in the lower wavenumber region (∼1400 cm-1) remain even after annealing the surface to 500 K. In other spectra not shown here these peaks are seen for annealing temperatures as high as 800 K. There is a very sharp feature that develops at ∼1429 cm-1 after annealing to 250 K, reaches its maximum intensity and mimimum width at 350 K, and then becomes broader and weaker as the surface is annealed further. This peak is accompanied by other sharp peaks at ∼1368, 1388, and 1475 cm-1 as the surface is annealed from 250 to 400 K. These peaks become broader but remain in the spectra up to very high temperatures. The 350 K anneal also produces the maximum in peak height of the B-H stretch at 2569 cm-1, which is due to the peak becoming much sharper since the integrated area of the B-H stretch peaks monotonically decrease with increasing annealing temperature. Evidently, the same ordering process leads to the peaks at both 1429 and 2569 cm-1 reaching their minimum widths at 350 K. Also apparent in the 300 and 350 K spectra are peaks at 3697 and 1043 cm-1, values that are likely due to O-H stretch and O-H bend vibrations of an OH group,31,32 raising the possibility of oxygen contamination. This is considered in more detail in the Discussion section.

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Figure 6. TPD of hydrogen (m/e ) 2) for different exposures of B10H14 to the Pt(111) surface. The bottom-most trace labeled b/g was for a control experiment without any B10H14 exposure and indicates the temperature for hydrogen desorption from the clean surface and the amount of hydrogen that adsorbs from the background during the time of a typical experiment. All exposures were at 85 K and the surface was cleaned after each TPD experiment. Figure 5. RAIR spectra of 10.0 L of B10H14 on Pt(111) as a function of annealing temperature.

The results of an annealing experiment following a 10 L B10H14 exposure at 85 K is shown in Figure 5. This exposure results in adsorption of a decaborane multilayer, as revealed by thermal desorption experiments. The anneal to 200 K, a temperature below the multilayer desorption temperature, leads to a sharpening of all peaks so that at least five separate B-H stretch peaks are resolved, but otherwise little change, suggesting that there is increased order in the multilayer but no chemical reaction or desorption. In contrast, the 300 K anneal leads to the loss of all of the sharp decaborane peaks, with only relatively weak B-H stretch peaks remaining at 2555 and 2598 cm-1. Following the 450 K anneal, no peaks attributable to decaborane or its decomposition products are evident in the spectra. The broad feature at ∼1000 cm-1 is likely an artifact due to miscancellation of a strong feature present in the single beam spectra. The small positive peak at ∼2100 cm-1 is due to CO that was present on the surface before decaborane exposure, which was displaced by the decaborane. There is no indication of the O-H stretch peak at 3697 cm-1 or the peaks in 1360-1480 cm-1 range that are seen in the spectra of Figure 3. TPD was used to determine the stages of hydrogen release from decaborane decomposition and to establish if any other reaction products desorb. Figure 6 shows the TPD results for H2 (m/e ) 2) as a function of increasing B10H14 exposure to the Pt(111) surface at 85 K. The surface was cleaned by several cycles of Ar+ ion sputtering followed by heating in O2 after each TPD experiment. All the spectra in Figure 6 have a peak at 136 K that is assumed to be an artifact, possibly due to desorption from the sample holder. The trace labeled b/g at the

bottom of the figure, which is scaled by a factor of 4 relative to the traces for 1.0 L and above, is a control experiment in which the m/e ) 2 channel was monitored without any decaborane exposure and indicates the small amount of hydrogen adsorption from the background. The H2 desorption peaks at 413-410 K for the 0.5, 1.0, and 1.5 L exposures are clearly at a higher temperature than the temperature at which hydrogen desorbs from the clean surface and are therefore attributed to reactionlimited desorption associated with the decomposition of a hydrogen-containing surface intermediate. In contrast, the peaks at 338-335 K for the higher exposures are desorption limited, with B-H bond dissociation occurring at a lower temperature. The H2 peak at ∼200 K is attributed to a cracking fragment associated with desorption of molecular B10H14 from a multilayer, as revealed by a desorption signal at m/e ) 124, corresponding to the parent ion, B10H14+, as shown in Figure 7. Decaborane desorbs at 224 K, which gives rise to the fragment peak at ∼222 K in the TPD results for H2. Also shown in Figure 7 are desorption results for elemental boron (m/e ) 11), which shows that the only boron-containing species that desorbs from the surfaces is associated with the decaborance multilayer desorption at 224 K. The absence of desorption of either boron or molecular B10H14 at exposures corresponding to submonolayer coverages indicates complete dissociation of molecules in direct contact with the platinum surface. The fact that the m/e ) 11 signal is stronger than the parent ion at m/e ) 124 is due to a combination of reduced sensitivity at higher masses, and the fact that the m/e ) 124 signal corresponds to the all 11B isotopomer of B10H14 whereas the isotopomers containing one and two 10B atoms are more abundant, even though the 11B/10B natural abundance ratio is 4:1. The large 11B fragment signal associated with decaborane desorption would presumably apply to the desorption of other

Adsorption and Dehydrogenation of Decaborane

Figure 7. TPD results for desorption of boron (m/e ) 11) and B10H14 (m/e ) 124) as a function of exposure. The 3.0 and 5.0 L spectra for m/e ) 124 were multiplied by a factor of 10 in order to compensate for the low sensitivity of the mass spectrometer in this region.

Figure 8. XPS as a function of annealing temperature following a 5 L exposure of B10H14 to the Pt(111) surface at 90 K.

boranes and therefore the absence of a 11B signal except at the multilayer desorption temperature makes it unlikely that any other boron-containing species desorb from the surface. Figure 8 shows XPS spectra of the B 1s region for increasing annealing temperatures following a 5 L decaborane exposure at 90 K. The spectra were obtained with a pass energy of 20 eV and a step size of 0.4 eV, and each spectrum is the average of 30 scans. There is a major peak around 191 eV at 90 K, which shifts to ∼190 eV and broadens as the temperature is increased to 250 K. At 300 K, a shoulder is observed at 189.5

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Figure 9. Comparison of the experimental spectrum of 10.0 L of B10H14 on Pt(111) after annealing to 200 K, with a simulated spectrum based on DFT for randomly oriented molecules.

eV on the main peak at 190.6 eV. The peak remains at 191 eV up to an annealing temperature of 450 K with almost the same intensity. At the same time, the peak becomes sharper. The presence of the B 1s peak for annealing temperatures up to 450 K confirms that boron remains on the surface after all features disappear from the RAIR spectra and after all hydrogen has desorbed, as indicated by the TPD results of Figure 6. The surface was annealed to temperatures as high as 900 K in a separate experiment and a plot of B 1s peak areas as a function of annealing temperature (Supporting Information) shows a sharp drop for desorption of the multilayer, followed by a roughly constant value from 200 to 400 K, after which the intensity falls to zero by 900 K. The decrease in the B 1s peak is attributed to dissolution of boron into the bulk of the crystal, a property of boron on metal surfaces that has been noted in other studies.15,33 Figure 9 shows a comparison of the simulated spectrum based on a DFT calculation of an isolated molecule of B10H14 with the experimental spectrum for a multilayer of B10H14 on Pt(111) after annealing to 200 K following a 10 L exposure at 85 K. A fwhm of 5 cm-1 and a Lorentzian line shape was assumed for the simulated spectrum. The raw DFT frequencies were scaled by a factor of 0.9613, as recommended for the B3LYP hybrid functional and the 6-311G(d,p) basis set.34 A random orientation for the molecules was assumed in calculating the intensities for the simulated spectrum, which shows good agreement with both a previously reported experimental spectrum of molecular decaborane and with the multilayer spectrum of Figure 9, particularly in the terminal B-H stretch region. The main discrepancies are that the calculated peak at 1622 cm-1 is missing in the experimental spectrum, and the peaks at 1518 and 1468 cm-1 are weaker, broader, and considerably red-shifted relative to the calculated peaks at 1555 and 1534 cm-1. Also, the calculated two peaks due to the B-H-B stretches of the bridging hydrogen atoms at 1946 and 1928 cm-1 have the opposite intensity ratio compared to the corresponding experimental peaks at 1936 and 1896 cm-1.

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Discussion Each of the three experimental techniques used here, RAIRS, TPD, and XPS, provide complementary information about the adsorption and decomposition of decaborane on Pt(111). We first consider the RAIRS data that indicate molecular adsorption at 85 K and what can be learned about the orientation of the molecule from the spectra. Figure 3 shows that at the submonolayer coverages corresponding to a 0.5 L exposure, the terminal B-H stretches appear at 2590 and 2606 cm-1, essentially the same values as observed for the multilayer obtained after a 10.0 L exposure. Although it would be reasonable to simply assume that the multilayer consists of intact molecular B10H14, this conclusion is also supported by the strong similarity between the experimental multilayer spectrum in Figure 9 and the spectrum simulated as described below from the frequencies and intensities obtained from a DFT calculation of the isolated molecule. The assumption of molecular adsorption at 85 K appears valid regardless of coverage, whereas the behavior at higher temperature depends markedly on coverage. In the multilayer spectra of Figure 5, the lack of significant frequency shifts between spectra obtained immediately following adsorption at 85 K and spectra obtained after annealing to 200 K indicates that the molecule remains intact up to 200 K at this coverage. In contrast, the spectra of Figure 4 following a 0.5 L exposure show the B-H stretches strongly red-shifted upon annealing to 200 K. The observation that decaborane adsorbed at 85 K on Pt(111) is only weakly perturbed by its interaction with the surface implies that further information on the orientation of the molecule can be deduced from comparison of the experimental spectrum with spectra calculated for various assumed orientations. This is based on the surface dipole selection rule that states that only normal modes with dipole moments with nonzero components along the surface normal will be observed.35 Figure 10 shows calculated spectra for B10H14 with the molecule’s X, Y, or Z axis (as defined in Figure 1) oriented along the surface normal together with the experimental spectrum for an exposure of 0.5 L at 85 K. The 10 terminal B-H stretches of B10H14 belong to the following irreducible representations of the C2V point group:

ΓB-H ) 4A1 + A2 + 3B1 + 2B2 This implies that there are nine infrared-active B-H stretch fundamentals for the randomly oriented molecule since the A2 mode is inactive. If the molecule is oriented with its 2-fold rotation axis (the Z axis in Figure 1) along the surface normal, then only the A1 modes will be active. Similarly, if it is oriented with the X or Y axis along the surface normal, only the B1 or B2 modes, respectively, will be active. On the other hand, if the molecules are randomly adsorbed on the surface, then the experimental spectrum should represent a simple sum of the three spectra labeled X, Y, and Z in Figure 10. Vibrational spectra of boron hydrides are complicated by the presence of the 11B and 10B isotopes in a natural abundance ratio of 4:1. The simulated spectra of both Figures 9 and 10 represent a weighted sum of spectra calculated for the five most abundant isotopomers of B10H14; these and their weightings are 11B10H14 (0.11), 11B910B1H14 (0.27), 11B810B2H14 (0.30), 11B710B3H14 (0.20), and 11B610B4H14 (0.09). The probability of finding five or more 10B atoms in the molecule was too small to consider. The fact that there are several nonequivalent boron atom positions was also taken into account. For

Figure 10. Comparison in the B-H stretch region of the experimental RAIR spectrum for a 0.5 L exposure of B10H14 to the Pt(111) surface at 85 K with calculated spectra for an isolated B10H14 with either the molecules’s X, Y or Z axis oriented along the surface normal.

example, in the case of 11B810B2H14, all possible structures in which two 11B atoms are replaced by two 10B atoms were calculated and their spectra summed. Then all such spectra for the five isotopomers were summed with the correct statistical weighting factor applied for each of the three assumed orientations (X, Y, or Z), to obtain the simulated spectra shown in Figure 10. A Lorentzian line shape with a fwhm of 5 cm-1 for each peak was assumed and the calculated frequencies were scaled by a factor of 0.965 in order to best align the calculated and experimental B-H stretch positions. The standard scaling factor for the B3LYP functional and 6-311G(d,p) basis set in Gaussian is 0.9613. Irikura et al.36 have reported this factor to be 0.9669 ( 0.0205 for the B3LYP functional and the 6-311G(d,p) basis set and have noted that these scaling factors are accurate only to two significant figures. Therefore, the use of a scaling factor of 0.965 instead of 0.9613 is fully justified. Of the three simulated spectra in Figure 10, the spectrum labeled Y best matches the experimental spectrum. This is particularly true of the two peaks at 2605 and 2615 cm-1, while the calculated intensities of the two peaks at 2591 and 2584 cm-1 differ noticeably from those of the corresponding experimental peaks. This is likely due to the presence of some molecules with the other orientations. Nevertheless, the comparisons in Figure 10 do suggest a preferred orientation on the surface. This is likely due to the interaction of the dipole moment with its image induced in the metal surface.37,38 Decaborane has a large permanent dipole moment of 3.52 ( 0.02 D39 that by symmetry must be along the Z axis so that a preferred orientation with the molecule’s Y axis along the surface normal means that the dipole moment is then parallel to the surface. This orientation is the most energetically favorable for the dipole induced-dipole interaction. The reason for a preferred orientation with the Y

Adsorption and Dehydrogenation of Decaborane rather than the X axis along the surface normal must involve more subtle aspects of the molecule-surface interaction. The data obtained here indicate that the dissociation of decaborane on Pt(111) releases only hydrogen into the gas phase, leaving boron on the surface. The TPD results indicate that the hydrogen desorption is completed by the relatively low temperature of 450 K. This is also the temperature by which B-H stretch peaks are no longer observed with RAIRS. The decomposition chemistry depends on decaborane coverage, as seen by a comparison of the RAIRS spectra as a function of annealing temperature for the 0.5 and 10 L B10H14 exposures in Figures 4 and 5. In the 0.5 L case, there is a sharp red-shift of the most intense B-H stretch peak with a 200 K anneal, whereas no red-shift is seen after annealing to 200 K for the 10 L exposure. In the TPD, only the lower exposures lead to reaction-limited H2 desorption, i.e., to desorption at temperatures higher than where H2 desorbs from the platinum surface. This implies that empty platinum sites are needed to form a species that has a higher stability than what can form on the more crowded surface. The RAIRS results shown in Figure 4 reveal that a significant molecular transformation occurs at temperatures as low as 200 K, whereas molecular decaborane desorption does not occur until 224 K, and then only for multilayers. Thus there is no decrease in decaborane coverage during the TPD temperature ramp before the molecule begins to decompose. This would explain in a general way how there can be hydrogen desorption peaks above 400 K for the lowest exposures but not for higher exposures. Although B-H stretch peaks are not observed at 450 K or above, the RAIRS results alone do not preclude the presence of one or more BxHy species if they possess B-H stretch modes with intensities below our detection limits. However, the absence of hydrogen desorption at temperatures higher than ∼420 K precludes that possibility. This is in contrast to the behavior of decaborane on Si(111) where Chen et al. observed the persistence of the terminal B-H stretch up to a temperature of 900 K.17 The lack of desorption of any boron-containing species (other than decaborane from multilayers) is consistent with the XPS results showing a B 1s peak that only disappears after heating to 800 K, at which point the boron diffuses into the bulk of the crystal. The release of hydrogen from decaborane on Pt(111) occurs at temperatures well below the usual minimum thermal decomposition temperature of gas phase decaborane of ∼450 K.40 This implies that platinum can act as an effective dehydrogenation catalyst for boranes, just as it does for hydrocarbons. The RAIRS spectrum is quite sensitive to the molecular transformations that take place as the decaborane covered surface is heated and indicates that one or more surface intermediates are formed that contain terminal B-H bonds. However, it is difficult to identify specific intermediates from the measured spectra. This is largely because only the B-H stretches of the intermediates are strong enough to clearly observe and because the terminal B-H stretches of a wide variety of boranes all have about the same frequencies. Brint et al.41 have noted a correlation between B-H stretch frequencies in neutral boranes and in borane anions and the state of hybridization at the boron atom, just as there is a corresponding correlation between C-H stretch frequencies and carbon atom hybridization in hydrocarbons. For the neutral molecules, they found typical B-H stretch values of 2368, 2497, and 2763 cm-1 for hybridizations at the boron atom of sp3, sp2, and sp, respectively. For closo borane anions of the form BnHn2- with n ) 6, 8, 9, 10, 11, and 12, they found that the B-H stretch frequencies fell in a narrow range of 2440-2540 cm-1. This

J. Phys. Chem. C, Vol. 113, No. 31, 2009 13853 indicates that the boron atoms in both decaborane and in its decomposition products on Pt(111) have a hybridization between sp2 and sp3 and that the intermediate is not likely to be a BnHn2anion. This is in contrast to our previous study of C2B10H12 on Pt(111), where an intermediate formed with a strong B-H stretch at ∼2500 cm-1, a value most consistent with a BnHn2species. Despite the difficulty in deducing specific surface intermediates from the value of the B-H stretch alone, a recent DFT calculation42 of the BH species on the Pt(111) surface predicted a B-H stretch frequency of 2565 cm-1, remarkably close to the value of 2569 cm-1 observed for the most intense peak observed in Figure 4 for the 300, 350, and 400 K anneals. If this peak were due to the 11BH molecule, then a weaker satellite peak due to 10BH should be observed about 10 cm-1 higher. Thus, it is plausible that the peak observed at 2577 cm-1 is due to 10BH. However, the predicted 11BH/10BH peak area ratio is 4:1, whereas after peak fitting we obtain a ratio of only 1.6:1. This is consistent with the peak-height ratio, which appears to be about 2:1 in Figure 4. A principal reason that the higher frequency peak might have enhanced intensity relative to the lower frequency peak in isotopic mixtures is that dipole coupling interactions are well-known to effectively transfer intensity to the higher frequency component. This has been thoroughly investigated for adsorbed mixtures of diatomic molecules such as 12CO/13CO and 14N2/15N2.43-45 For this reason, the observed intensity ratio does not necessarily preclude assigning the 2569/ 2577 cm-1 peaks to 11BH/10BH and we consider the formation of BH on the Pt(111) from decaborane decomposition plausible. Finally, we consider the origin of the RAIRS peaks from 1368 to 1475 cm-1 seen in Figure 3, which persist to high temperature. As hydrogen desorption is complete below 450 K, these peaks cannot be associated with any BxHy species. They also occur at higher wavenumbers than those associated with the vibrations of various forms of elemental boron. Hydrogen adsorbs at the 3-fold hollow sites on Pt(111) to give a dipole-active vibrational mode at 550 cm-1, as observed with high resolution electron energy loss spectroscopy,46 ruling out the possibility that these peaks are due to Pt-H vibrations. However, these peaks do match those that can occur for boron oxides. Belyansky et al. observed a broad and intense band centered at 1485 cm-1 in a RAIRS study of the room temperature oxidation of boron thin films grown on the Hf(0001) surface.47 A Raman spectroscopy study of B2O3 associated these peaks with vibrations of the BO3 network.48 The presence of oxygen is also indicated by the 3697 cm-1 peak seen in Figure 3 for the 300 and 350 K spectra, which is highly characteristic of an O-H stretch. Confirmation that these peaks are associated with B-O bonds is evident from spectra (Supporting Information) that were obtained by directly dosing the decaborane-covered surface with O2. Greenwood et al.49 have observed the hydroxyl compound, 6-hydroxy-nidodecaborane (6-(OH)-B10H13), by mass spectroscopy, as an impurity in B10H14, although in low abundance. The OH stretch of this compound occurs at 3570 cm-1.49,50 The source of oxygen is most likely water that is either present as a contaminant in the decaborane itself or that adsorbs from the background in the UHV chamber. The difficulty of removing any residual water from decaborane was noted in a study where it was used to deposit boron on a SiC surface.16 Nevertheless, there are several facts that argue against an oxygen-containing impurity in the decaborane itself. First, decaborane from several different sources was used in these experiments and in all cases we purchased the highest purity available and yet the same features were observed regardless of the source of the decaborane.

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Second, the decaborane was transferred in a glovebag from the container in which it was shipped to the container that was attached to the UHV chamber and was therefore never exposed to air. Third, we obtained NMR spectra of our decaborane sample and found no evidence for any impurity containing B-O bonds. Fourth, any volatile compounds including water should have been removed through the various freeze-pump-thaw cycles used and by pumping above the solid in our gas line prior to dosing. Fifth, neither the O-H stretch peak nor the peak at 1429 cm-1 was seen in Figures 2 or 5 where the exposures are much higher than used for Figure 3. Sixth, the XPS spectrum does not show any oxygen on the surface and more significantly, the B 1s peak does not show a peak due to a boron oxide, where the B 1s binding energy is generally at 193-195 eV. Unlike with RAIRS, XPS provides quantitative information and so the XPS results imply that the amount of oxygen is quite low. Thus the prominence of the RAIRS peaks at 1333, 1429, and 1479 cm-1 in Figure 3 is likely due to the very high intrinsic intensity of the B-O stretch vibrations, rather than due to any significant contamination. It is likely that the surface oxygen originates from water adsorption from the background and that this requires areas of the surface not covered by decaborane, which is why these peaks are present for the low B10H14 exposures, but not for the higher exposures. The water presumably reacts with the decaborane as the surface is annealed producing intense B-O stretch peaks due to a species present at only low coverages. We therefore conclude that although a surface species containing boron-oxygen bonds forms under the conditions of our experiment, the amount is not significant and therefore is largely irrelevant to our conclusions regarding the surface chemistry of B10H14 on Pt(111). Conclusions Decaborane adsorbs molecularly on the Pt(111) surface at 85 K with a preferential orientation in which the dipole lies parallel to the surface. The surface catalyzes the release of hydrogen from the adsorbed molecule at temperatures well below the gas-phase decomposition temperature of decaborane. The decomposition occurs by way of stable surface intermediates containing terminal B-H bonds, as indicated by changes with annealing temperature in the RAIR spectra of the B-H stretch peaks, which are observable up to 400 K. Identification of the specific intermediates is not possible as terminal B-H stretch values show little variation among various boranes. No desorption of any boron-containing compound, other than decaborane from the multilayer, is observed with TPD, implying that all of the boron initially deposited in the form of decaborane remains on the surface after all of the hydrogen has desorbed. This is confirmed by XPS, in which a B 1s signal is observed up to 800 K, after which it decreases to zero, presumably due to boron dissolution into the bulk of the crystal. Acknowledgment. This work is supported by the Department of Energy under Grant No. DE-FG02-05ER15726 and by the National Science Foundation under Grant No. CHE-0714562. We appreciate the many helpful discussions we had with Professor Todd Lee on the properties of decaborane. We thank Professor Michael J. Janik for making results available to us prior to publication. Supporting Information Available: Additional spectra. This material is available free of charge via the Internet at http:// pubs.acs.org.

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