Partial Hydrogenation of 1,3-Butadiene on Hydrogen-Precovered Pd

Oct 9, 2008 - H–D Exchange Mechanism of Butene on a D-Absorbed Pd–Au Alloy Surface. Shohei OguraSatoshi OhnoKozo MukaiJun YoshinobuKatsuyuki ...
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J. Phys. Chem. C 2008, 112, 17219–17224

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Partial Hydrogenation of 1,3-Butadiene on Hydrogen-Precovered Pd(110) in the Balance of π-Bonded C4 Hydrocarbon Reactions Satoshi Katano,†,| Hiroyuki S. Kato,† Maki Kawai,†,‡,* and Kazunari Domen§ Surface Chemistry Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan, Department of AdVanced Materials Science, UniVersity of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8651, Japan, and Department of Chemical System Engineering, UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan ReceiVed: May 13, 2008; ReVised Manuscript ReceiVed: August 08, 2008

The hydrogenation and dehydrogenation of C4 hydrocarbon molecules (1,3-butadiene, 1-butene, trans-2butene, cis-2-butene, and n-butane) on the hydrogen-precovered Pd(110) surface have been investigated by high-resolution electron energy loss spectroscopy (HREELS) and temperature-programmed desorption (TPD). 1,3-Butadiene was found to be adsorbed molecularly on the surface below 350 K. Further heating of the surface resulted in decomposition, forming hydrocarbons at 350 K and finally the graphite layer at 550 K. The butene isomers and n-butane adsorbed on the surface were, however, relatively unstable compared with 1,3-butadiene when heated. Some of the adsorbed butenes were desorbed, and the species that remained on the surface were dehydrogenated to 1,3-butadiene between 150 and 250 K. n-Butane on the surface showed similar reaction behavior except for the lower dehydrogenation and desorption temperature. Our findings indicate that the dehydrogenations of π-bonded C4 hydrocarbons on the Pd surface show significantly different pathways compared with those of the σ-bonded C4 hydrocarbon on Pt and Ru surfaces. Here, we discuss the selective partial hydrogenation of 1,3-butadiene on hydrogen-precovered Pd(110) in terms of the reactivity of the butenes and butanes. 1. Introduction The palladium metal catalyst plays an important role because it provides a high selectivity in the partial hydrogenation, such as that of 1,3-butadiene to butene.1,2 It was demonstrated that selective partial hydrogenation proceeds owing to the difference in the adsorption state between 1,3-butadiene and butene.3 Note that this selectivity also takes place in the ultrahigh vacuum (UHV) environment.4 Therefore, the characterization of the C4 hydrocarbon species adsorbed on the Pd surface is of considerable interest for the elucidation of such unique and complex reactions. Previous studies revealed that the bonding structure on metal surfaces, i.e., π bonding on Pd and σ bonding on Pt, strongly affects the selectivity in dehydrogenation.5,6 The nature of bonding between metal and unsaturated hydrocarbon is described by the hybridization between d orbitals of metals and π orbitals of molecules.In case of π bonding, the CdC bond can bind to metal center by donating π electron to an empty d orbital and by donating d electrons to empty π orbitals. The π bonding occurs mainly in late transition metals and is often accompanied with the retention of the double-bond character.7 As for the ethylene (C2H4) dehydrogenation, π bonded ethylene on Pd is dehydrogenated to ethynyl while the σ-bonded ethylene on Pt is dehydrogenated to ethylidyne.7 A similar selectivity is also expected in the case of the dehydrogenation of C4 hydrocarbon on metal surfaces.Although the adsorptions of 1,3* To whom correspondence should be addressed. E-mail: [email protected]. Phone: +81-48-467-9405. Fax: +81-48-462-4663. † RIKEN. ‡ Department of Advanced Materials Science, University of Tokyo. § Department of Chemical System Engineering, University of Tokyo. | Present address: Research Institute of Electrical Communication, Tohoku University.

butadiene and butene on σ-bonding surfaces such as Pt and Ru have been extensively studied, the characterization of those adsorbedspeciesontheπ-bondingsurfaceisnotyetsufficient.4-6,8-10 In this paper, the reactions (dehydrogenation, hydrogenation, desorption, and decomposition) of linear C4 hydrocarbon molecules (1,3-butadiene (CH2dCHsCHdCH2), 1-butene cisand trans-2-butene (CH2dCHsCH2sCH3), (CH3sCHdCHsCH3), n-butane (CH3sCH2sCH2sCH3)) on the hydrogen-precovered Pd(110) have been investigated using highresolution electron energy loss spectroscopy (HREELS) and temperature-programmed desorption (TPD) techniques. We focus on the identification of the adsorbed species derived from the adsorption of C4 hydrocarbon molecules on H/Pd(110) at several temperatures. The adsorbed species were characterized on the basis of the vibrational peaks in the HREEL spectra. The desorbed species were observed using TPD. Consequently, the determined reaction paths of the C4 hydrocarbons upon heating were clearly different from those of the σ-bonding metal surfaces, such as Pt and Ru.8,11 Here, we discuss the reactions of 1,3-butadiene and butene isomers on H/Pd(110), in relation to the selectivity in the partial hydrogenation of 1,3-butadiene. 2. Experimental Methods All the experiments were carried out in a UHV chamber at the base pressure of below 1 × 10-10 Torr. The Pd(110) surface was cleaned by repeating the Argon ion sputtering, annealing to 1100 K, and O2 treatment at 850 K. The surface cleanness was confirmed by the HREEL spectrum of the bare Pd(110) surface and low-energy electron diffraction (LEED). Gaseous molecules were introduced into the UHV chamber using a pulse gas doser. A set of a double-path monochromator and a single-path energy analyzer (Specs GmbH: DELTA 0.5) was used in the

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Figure 1. HREEL spectra of 1,3-butadiene adsorbed on the H(1.0 ML)/ Pd(110) surface at 100 K followed by heating the following temperatures: (a) 120, (b) 330, (c) 390, (d) 440, and (e) 550 K. Whole range spectrum was obtained by the specular mode and, additionally, 18° off-specular spectrum is depicted at the region of 300-420 meV. All the spectra were measured at 90 K.

HREELS experiment. The electron beam was input to the surface at the angle of 60°, and the HREEL spectra were obtained at the scatter angle of 60° from the surface normal in the scattering plane along [11j0] on Pd(110). The typical resolution of the HREEL spectra was 3.4 meV (27 cm-1). The TPD measurements were carried out with the heating rate of 6 K/s. The desorptions of 1,3-butadiene, butene, and butane were monitored with the parent species, m/e ) 54, 56, and 58, respectively, using a quadrupole mass spectrometer. All of the results shown in this paper were obtained with a Pd(110) surface covered with 1.0 monolayer (ML) of atomic hydrogen (1 ML ) 9.35 × 1014 cm-2) to relate the present study to the 1,3-butadiene hydrogenation. This surface was prepared by the exposing the sample to hydrogen gas at 100 K before introducing the hydrocarbon molecules. We confirmed that the coadsorption of hydrogen did not significantly affect the HREEL spectra of the C4 species on the clean Pd(110). This is because the absorption of hydrogen into the palladium subsurface takes place by the coadsorption of hydrocarbon molecules.12 While the absorbed hydrogen was detected in the TPD spectra, the subsurface hydrogen peak was not observed on the HREEL spectra because of the shielding effect of the metal surface.4 3. Results and Discussion 3.1. Reactions of 1,3-Butadiene on H/Pd(110). Figure 1 shows the HREEL spectra of 1,3-butadiene adsorbed on the H/Pd(110) surface at 100 K, which was subsequently heated to the indicated temperatures (120-550 K) followed immediately by quenching to 90 K. For the spectrum at 120 K (Figure 1a), strong peaks observed at 61, 107, and 113 meV have been assigned to the CH2 twisting, C-H bending, and CH2 wagging modes of adsorbed 1,3-butadiene, respectively.10 In addition, our previous study by near-edge X-ray absorption fine structure

Katano et al. (NEXAFS) measurements has revealed that 1,3-butadiene is π bonded on Pd(110) because a strong π resonance peak at 285 eV was observed.10 These findings suggest that 1,3-butadiene is adsorbed as intact on H/Pd(110) at 120 K. The HREEL spectrum at 330 K (Figure 1b) is almost identical to that at 120 K, indicating the intact adsorption when the surface is annealed. However, careful analysis of the CdC stretching peaks has shown that the chemisorption state of 1,3-butadiene slightly changes upon reaching the hydrogenation temperature (approximately 200 K), which has been discussed in the previous report.4 In brief, the broad peak at approximately 180-190 meV at 120 K (Figure 1a) consists of the two CdC stretching peaks at 183 and 188 meV associated with the strongly and weakly chemisorbed 1,3-butadiene. Upon heating to 330 K, the higher energy peak disappeared (Figure 1b), indicating that the weakly chemisorbed 1,3-butadiene proceeds to the hydrogenation and the strongly chemisorbed 1,3-butadiene remains on the surface. As the surface was heated to over 350 K, the HREEL spectrum significantly changed (Figure 1c). The change in the spectrum at 430 K is attributed to the decomposition of 1,3butadiene. This is consistent with the previous TPD study; the D2 desorption peak observed at 398 K is produced from the adsorbed deuteride 1,3-butadiene (C4D6).4 Structural analysis using scanning tunneling microscopy (STM) revealed that the decomposition products are formless protrusions of 1-2 nm diameter (see Figure S1 of Supporting Information). These formless particles are attributed to the hydrocarbon cluster (CxHy), in which the particles have various stoichiometries. Further increase in the substrate temperature to 550 K results in the disappearance of the C-H stretching peaks at approximately 350-390 meV (Figure 1d). This is consistent with the TPD results on deuteride 1,3-butadiene; the D2 desorption peak at 496 K associates with the recombined hydrogen from the hydrocarbon clusters.4 At this temperature, the Pd(110) surface was probably covered with graphite layer. The characteristic peaks observed at 65, 97, 121, and 196 meV in Figure 1d are close to the energy of the phonon resonances of the graphite layer observed on Ni(111).13,14 3.2. Reactions of Butenes and Butane on H/Pd(110). Parts a-c of Figure 2 show the HREEL spectra of butene isomers (a) 1-butene, (b) cis-2-butene, and (c) trans-2-butene on 1.0 MLH/Pd(110) at 120 K. The assignments of peaks in each spectrum are summarized in Table 1. The adsorption of butenes provides mutually different spectra while some vibrational peaks were commonly observed: CH3 deformation mode at 176-178 meV and CH3 stretching mode at 356-357 meV (parts a-c of Figure 2). These characteristic peaks are assigned to those of the sp3hybridized hydrocarbon and were absent in the case of 1,3butadiene. We conclude that butene isomers are molecularly adsorbed at 120 K. From the results of the NEXAFS studies, butene isomers are adsorbed with their CdC bond through a π character on Pd(110).15 STM observations also support the molecular adsorption of butene isomers.15,16 Figure 2d shows the HREEL spectrum of n-butane on H/Pd(110) at 120 K. The assignments of each peak are shown in Table 2. Peaks of the CH3 deformation mode at 178 meV and the CH3 stretching mode at 357 meV were similarly observed, indicating the n-butane in the sp3-hybridized state. Additionally, a characteristic peak observed at 88 meV is readily assigned to the -CH2- rocking mode. The observed peaks in Figure 2d suggest that n-butane is also adsorbed as intact on the Pd surface. Furthermore, the spectrum shows the unique feature in the C-H stretching region. A broad peak centered at 330 meV appeared beside the lower energy of the C-H

π-Bonded C4 Hydrocarbon Reactions

J. Phys. Chem. C, Vol. 112, No. 44, 2008 17221 TABLE 1: Assignment of Observed Vibrations of Butene Isomers (C4H8) on H/Pd(110)a

a The vibration energies are expressed in meV (1 meV ) 8.06548 cm-1). ∼ indicates inability to be resolved in the spectrum. b Reference 32. c Reference 33.

Figure 2. HREEL spectra of (a) 1-butene, (b) cis-2-butene, (c) trans2-butene, and (d) n-butane adsorbed on the H(1.0 ML)/Pd(110) surface at 100 K, followed by heating to 120 K. (a′-d′) HREEL spectra obtained after subsequently annealing parts a-d to the surface at 250 K, respectively. Whole range spectrum was obtained by the specular mode, and additionally, the 18° off-specular spectrum is depicted at the region of 300-420 meV. All the spectra were measured at 90 K.

stretching peak, which is particularly enhanced on the specular spectrum. This broad peak is associated with the C-H stretching mode interacted with the metal surface, the so-called softening mode.17 This feature is unique to the butane adsorption, in contrast with the adsorption of 1,3-butadiene and butenes that are adsorbed with π bonding. The HREEL spectra shown in parts a′-d′ of Figure 2 were obtained after annealing the surfaces shown in parts a-d of Figure 2 at 250 K, respectively. All the spectra exhibit strong peaks at 61, 107, and 113 meV. The appearance of these peaks is characteristic of the 1,3-butadiene spectra (parts a and b of Figure 1), which are assigned to the CH2 twisting, CH2 wagging, and CH bending modes, respectively.10 The lack of CH3 stretching modes at 356-357 meV is also a critical sign of their conformation change. Thus, we conclude that all the butene isomers and n-butane dehydrogenated to 1,3-butadiene during the annealing. The temperature-dependent HREEL spectra of trans-2-butene on H/Pd(110) are shown in Figure 3. Upon heating, characteristic peaks at 60, 108, and 115 meV increased in intensity with increasing temperature, while the intensity of each trans-2butene peak decreased. The CdC stretching region (160-200 meV) shown in Figure 3a is expanded in Figure 3b. At 120 K,

TABLE 2: Assignment of Observed Vibrations of n-Butane (C4H10) on H/Pd(110)a

a The vibration energies are expressed in meV (1 meV ) 8.06548 cm-1). ∼ indicates inability to be resolved in the spectrum. b Reference 34.

three peaks at 168, 176, and 183 meV are assigned to CH3 s-deformation, CH3 d-deformation, and CdC stretching modes of trans-2-butene on Pd(110), respectively. When the substrate was heated, these peaks gradually decreased at 150 K and finally disappeared at 250 K instead of the appearance of the 1,3butadiene peaks at 174 (CH2 scissoring) and 181 meV (CdC stretching). The position of the CdC stretching peak (181 meV) at 250 K indicates that the 1,3-butadiene produced from the

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Figure 3. (a) HREEL spectra taken after the adsorption of trans-2butene on the H(1.0 ML)/Pd(110) surface at 100 K and annealing at the indicated temperatures. All the spectra were measured at 90 K by the specular mode. (b) Expansion of part a in the CdC stretching region (160-200 meV).

Figure 4. (a) HREEL spectra taken after the adsorption of n-butane on H(1.0 ML)/Pd(110) surface at 100 K and annealing at the indicated temperatures. All the spectra were measured at 90 K by the specular mode.

dehydrogenation of trans-2-butene is in the strongly chemisorbed state (Figure 1b); this is consistent with the 1,3-butadiene species that remained after heating to 250 K. Other butene isomers show similar spectrum changes when annealed above 250 K. On the other hand, the temperature-dependent HREEL spectra of adsorbed n-butane show two-step dehydrogenation processes in Figure 4. Intense peaks of n-butane observed at 88, 117, and 178 meV decreased at 150 K and completely disappeared at 170 K, while several peaks newly appeared simultaneously. As referred from Table 1, new peaks at 105, 128, and 183 meV are assigned to the CsC stretching (or dCH2 wagging), CH3 rocking, and CdC stretching modes of adsorbed butenes, respectively. It is noteworthy that n-butane can be dehydroge-

Katano et al.

Figure 5. (a) TPD spectra of butene obtained from 1,3-butadiene adsorbed on the H(1.0 ML)/Pd(110) surface. TPD spectra of butene obtained from butene isomers (b) 1-butene, (c) cis-2-butene, and (d) trans-2-butene adsorbed on H(1.0 ML)/Pd(110). (e) TPD spectra of butane obtained from n-butane adsorbed on H(1.0 ML)/Pd(110). All the spectra were obtained at the heating rate of 6 K/s.

nated to butenes at 150-170 K, which is the temperature lower than that in the case of butene decompositions (150-250 K). Further heating of the surface led to the butene decomposition and finally the appearance of characteristic peaks at 60, 110, and 116 meV at 230 K. These are readily assigned to the vibrational peaks of adsorbed 1,3-butadiene as we discussed above. The spectral changes observed here demonstrate that successive transformation upon annealing occurs from n-butane to butenes and to 1,3-butadiene accompanied by the elimination of two hydrogen atoms at each dehydrogenation step. We found that the dehydrogenations of butenes and butane occur in competition with desorption during annealing. Figure 5 shows the TPD spectra from 1,3-butadiene, butene isomers, and butane adsorbed on H/Pd(110). For the butene-adsorbed surfaces (parts b-d of Figure 5), the peaks observed at 108 and 173 K are assigned to the desorption of butene from the multilayer and the first layer, respectively. No significant difference in the desorption temperature and amount were confirmed between the butene isomers.The n-butane surface shows behavior similar to that of butenes: the molecular desorption from the multilayer at 99 K and that from the first layer at 167 K, which shifted to lower temperature compared with the butene desorption. Note that the desorption yield of butane (Figure 5e) was about five times larger than that of butene from the butene-adsorbing surface (parts b-d of Figure 5). The loss peak intensity in butane HREEL spectrum observed after annealing up to 250 K was weak compared with those of the butene spectra, which implied that the amount of species that remained after heating was small. This evidence results from a weak interaction between n-butane and Pd. 3.3. Partial Hydroganation of 1,3-Butadiene on H/Pd(110). In our studies of the C4 hydrocarbon adsorbed on the atomichydrogen-precovered Pd(110) surface, we obtained the following results: (1) 1,3-Butadiene is adsorbed molecularly on Pd(110) below 350 K, followed by the decomposition to hydrocarbon

π-Bonded C4 Hydrocarbon Reactions

Figure 6. Summarized reaction scheme of C4 hydrocarbon molecules on H(1.0 ML)/Pd(110). (a) 1,3-Butadiene (C4H6), (b) butenes (C4H8), and (c) n-butane (C4H10). The desorption and hydrogenation temperatures indicated in the scheme are derived from the peak temperature of each TPD spectrum. The dehydrogenation temperatures indicated in the scheme represent the region of temperature being changed on the HREEL spectrum during heating.

clusters over 350 K and finally forming graphite layer at 550 K. (2) The butene isomers (1-butene, cis-2-butene, and trans2-butene) and n-butane are adsorbed molecularly on Pd(110) at 120 K and are distinct from each other in the HREEL spectra. All of the butenes desorbed and dehydrogenated to 1,3-butadiene between 150 and 250 K, and n-butane also shows the same behavior at slightly lower temperature (150-170 K). As above, the C4 hydrocarbons on H/Pd(110) show a tendency to dehydrogenate themselves during the thermal processes. The summarized reaction schemes of 1,3-butadiene, butenes, and butane on the atomic-hydrogen-precovered Pd(110) surface are shown in Figure 6. It is generally accepted that these dehydrogenation paths strongly depend on the metals used in the catalyst.18 The difference in the dehydrogenation path for the metals can be attributed to the difference in the chemisorbed state of molecules. Our previous studies revealed that 1,3-butadiene and butene isomers are adsorbed on Pd(110) with the π character,4,10,15 while they are σ bonded on Pt, Ru, and Mo surfaces.8,11,19 The metal-molecule interaction is much weaker for the π-bonded system than for the σ-bonded system. In fact, upon 1,3-butadiene adsorption on Pt(111) and Ru(0001),8,11 1,4-di-σ-bonding species is formed at 210 K followed by the decomposition to a (CH)4 metallacycle at 260 K and to a CCHCHC metallacycle at 400 K. Compared with this thermal activity of σ-bonded 1,3butadiene, the π-bonded 1,3-butadiene remains intact on Pd(110) at a much higher temperature up to 350 K. The higher thermal stability of the π-bonded 1,3-butadiene until decomposition would allow itself to be hydrogenated by the thermally activated hydrogen adatoms. The bonding structure also affects the reaction selectivity in butene dehydrogenation. In the case of butene isomers on Pt(111), Ru(0001), and Mo(110), 1-butene is dehydrogenated to alkyldyne, and 2-butenes are dehydrogenated to butyne.20-22 This implies that the σ-bonded molecule is dehydrogenated preferentially from the R-hydrogen. In contrast, the butene isomers on Pd(110) are commonly dehydrogenated to 1,3butadiene, maintaining the favorable flat structures by the π-bonded interactions. If the hydrogenation of 1,3-butadiene occurs via the reversed reaction paths, the conversion from 1,3butadiene to butenes should also be easy for the π-bonded systems. Such π-bonding properties of unsaturated hydrocarbons cause the selective hydrogenation of 1,3-butadiene to butene on

J. Phys. Chem. C, Vol. 112, No. 44, 2008 17223 H/Pd(110). From the TPD measurement, 1,3-butadiene was hydrogenated to butene at approximately 200 K (Figure 6a). This temperature is slightly but meaningfully higher than those of butenes and butane reactions; some of the butene molecules are desorbed with a peak at 173 K, and the remaining butene molecules dehydrogenated at approximately 150-250 K (Figure 6b), while butane is desorbed at a slightly lower temperature with a peak at 167 K and dehydrogenated at approximately 150-170 K (Figure 6c). Thus far, by use of density functional theory (DFT) calculations, the elucidation of the selectivity observed for the 1,3butadiene hydrogenation on Pt(111) and Pd(111) has been attempted.23 The adsorption energy difference between 1,3butadiene and 1-butene was estimated to be 67 kJ mol-1 on Pt(111) and 79 kJ mol-1 on Pd(111). Then, they pointed out that, by using only the estimated energy difference, it is difficult to explain the difference in the selectivity of the facile butene desorption on the Pd surface. Here, we showed that butenes and butane molecules cannot be stabilized at the hydrogenation temperature (200 K as shown in Figure 5a) because of the desorption or the dehydrogenation. Consequently, the instability of butene and butane molecules on Pd(110) at the hydrogenation temperature must lead to the selective hydrogenation of 1,3butadiene only to butene and not to butane. The rate-determining step in the hydrogenation has been considered as the addition of the first hydrogen.24 Two desorption peaks of butene were obtained at 215 and 355 K when we performed the TPD experiment on the less hydrogen coadsorbed surface, i.e., 0.5 ML of hydrogen (Figure S2 in Supporting Information). The peak appeared at 215 K is assigned to the interaction with preadsorbed hydrogen atoms (similar to the 1.0 ML-H/Pd(110) spectrum shown in Figure 5a). On the other hand, the desorption peak appeared at 355 K is assigned to selfhydrogenation of 1,3-butadiene, which is close to the decomposition temperature of 1,3-butadiene. The hydrogenation temperature of approximately 200 K is close to the desorption temperature of bulk hydrogen25,26 that should be the reactive species for the hydrogenation.27-29 The theoretical study pointed out the high reactivity for subsurface hydrogen. Ledentu et al. have reported that the higher reactivity of subsurface hydrogen originates form the extra energy acquired by emerging from the subsurface site, which leads to the formation of “hot” reactant at surface.30 This mechanism was also supported by the angular distribution measurement of hydrogen desorption.31 Actually, TPD measurements using deuteride 1,3-butadiene (C4D6) clarified that the excess amount of hydrogen atoms desorbed from the subsurface site and those reacted with 1,3butadiene (Figure S3 in Supporting Information), indicating the hydrogenation of 1,3-butadiene by the subsurface hydrogen. 4. Conclusions We investigated the hydrogenation and dehydrogenation of C4 hydrocarbon molecules (1,3-butadiene, 1-butene, trans-2butene, cis-2-butene, and n-butane) on hydrogen-precovered Pd(110) by HREELS and TPD. It was found that the dehydrogenation products of π-bonded C4 hydrocarbons on Pd(110) are significantly different from those of the σ-bonded C4 hydrocarbon on Pt and Ru surfaces. During annealing, the 1,3butadiene molecule remains intact on Pd(110) below 350 K, followed by the decomposition to hydrocarbon clusters over 350 K. Finally, a graphite layer is formed at 550 K. On the other hand, the butene isomers and n-butane are adsorbed intact on Pd(110) at 120 K. Some of the butene isomers were desorbed, and the remaining adsorbed butenes dehydrogenated to 1,3-

17224 J. Phys. Chem. C, Vol. 112, No. 44, 2008 butadiene above 150-250 K, and n-butane shows the same behavior at slightly lower temperature (150-170 K). The dehydrogenation path of C4 hydrocarbon on the Pd surface is due to the weak interaction with the metal surface compared with the σ-bonding surface. By relating the obtained results with the selective hydrogenation of 1,3-butadiene to butene, we concluded that the instability of the butene isomers and n-butane at the hydrogenation temperature (approximately 200 K) inhibit the further hydrogenation of the produced butene to butane. Supporting Information Available: STM images of 1,3butadiene adsorbed on the H(1.0 ML)/Pd(110) surface at 100 K followed by heating to 130 K (below the decomposition temperature of 1,3-butadiene) and 450 K (above the decomposition temperature of 1,3-butadiene). TPD spectrum of butene obtained from 1,3-butadiene adsorbed on the H(0.5 ML)/Pd(110) surface. TPD spectra of hydrogen (H2 and D2) obtained from deuteride 1,3-butadiene (C4D6) adsorbed on the H(0.5, 1.0, 1.5, and 2.5 ML)/Pd(110) surface. These materials are available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Arnold, H. , Dobert, F.; Gaube, J. In Handbook of Heterogeneous Catalysis; Ertl, G., Weitkamp, J. , Eds.; WILEY-VCH: Weinheim, 1997; Vol 5, p 2165. (2) Bertolini, J. C.; Massardier, J. In The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis; King, D. A., Woodruff, D. P., Eds.; Elsevier: Amsterdam, 1990; Vol. 3B, p 107. (3) Bond, G. C.; Wells, P. B. AdV. Catal. 1964, 15, 91. (4) Katano, S.; Kato, H. S.; Kawai, M.; Domen, K. J. Phys. Chem. B 2003, 107, 3671. (5) Bertolini, J. C.; Cassuto, A.; Jugnet, Y.; Massardier, J.; Tardy, B.; Tourillon, G. Surf. Sci. 1996, 349, 88. (6) Tourillon, G.; Cassuto, A.; Jugnet, Y.; Massardier, J.; Bertolini, J. C. J. Chem. Soc., Faraday Trans. 1996, 92, 4835. (7) Zaera, F. Chem. ReV. 1995, 95, 2651. (8) Weiss, M. J.; Hagedorn, C. J.; Weinberg, W. H. J. Vac. Sci. Technol. A 2000, 18, 1443. (9) Guo, X. C.; Madix, R. J. J. Catal. 1995, 155, 336.

Katano et al. (10) Katano, S.; Ichihara, S.; Ogasawara, H.; Kato, H. S.; Komeda, T.; Kawai, M.; Domen, K. Surf. Sci. 2002, 502, 164. (11) Avery, N. R.; Sheppard, N. Proc. R. Soc. London 1986, 405, 27. (12) Ichihara, S.; Okuyama, H.; Kato, H.; Kawai, M.; Domen, K. Chem. Lett. 2000, 112. (13) Shikin, A. M.; Farias, D.; Adamchuk, V. K.; Rieder, K. H. Surf. Sci. 1999, 424, 155. (14) Shikin, A. M.; Prudnikova, G. V.; Adamchuk, V. K.; Moresco, F.; Rieder, K. H. Phys. ReV. B 2000, 62, 13202. (15) Katano, S.; Kim, Y.; Furukawa, M.; Ogasawara, H.; Komeda, T.; Kato, H. S.; Nilsson, A.; Kawai, M.; Domen, K. Jpn. J. Appl. Phys. 2002, 41, 4911. (16) Sainoo, Y.; Kim, Y.; Fukidome, H.; Komeda, T.; Kawai, M.; Shigekawa, H Jpn. J. Appl. Phys. 2002, 41, 4976. (17) Raval, R.; Pemble, M. E.; Chesters, M. A. Surf. Sci. 1989, 210, 187. (18) Somorjai, G. A. Introduction to Surface Chemistry and Catalysis; John Wiley & Sons Ltd: New York, 1994. (19) Bredael, G.; Tysoe, W. T.; Zaera, F. Langmuir 1989, 5, 899. (20) Chesters, M. A.; Horn, A. B.; Ilharco, L. M.; Ransley, I. A.; Sakakini, B. H.; Vickerman, J. C. Surf. Sci. 1991, 251, 291. (21) Avery, N. R.; Sheppard, N. Surf. Sci. 1986, 169, L367. (22) Eng, J.; Chen, J. G. Surf. Sci. 1998, 414, 374. (23) Valcarcel, A.; Clotet, A.; Ricart, J. M.; Delbecq, F.; Sautet, P. Surf. Sci. 2004, 549, 121. (24) Masel, R. I. Principles of Adsorption and Reaction on Solid Surface; Wiley Interscience: Weinheim, 1996. (25) Behm, R. J.; Penka, V.; Cattania, M. G.; Christmann, K.; Ertl, G. J. Chem. Phys. 1983, 78, 7486. (26) Christmann, K. Surf. Sci. Rep. 1988, 9, 1. (27) Ceyer, S. T. Acc. Chem. Res. 2001, 34, 737. (28) Johnson, A. D.; Daley, S. P.; Utz, A. L.; Ceyer, S. T. Science 1992, 257, 223. (29) Teschner, D.; Borsodi, J.; Wootsch, A.; Revay, Z.; Havecker, M.; Knop-Gericke, A.; Jackson, S. D.; Schlogl, R. Science 2008, 320, 86. (30) Ledentu, V.; Dong, W.; Sautet, P. J. Am. Chem. Soc. 2000, 122, 1796. (31) Wright, S.; Skelly, J. F.; Hodgson, A. Chem. Phys. Lett. 2002, 364, 522. (32) Gallinella, E.; Cadioli, B. Vib. Spectrosc. 1997, 13, 163. (33) Mckean, D. C.; Mackenzie, M. W.; Morrisson, A. R.; Lavalley, J. C.; Janin, A.; Fawcett, V.; Edwards, H. G. M. Spectrochim. Acta A 1985, 41, 435. (34) Shimanouchi, T. Tables of Molecular Vibrational Frequencies; NSRDS-NBS: 1972.

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