J. Phys. Chem. C 2007, 111, 10455-10460
10455
Hydrodebromination of Bromobenzene over Pt(111) Adam F. Lee,* Zhipeng Chang, Simon F. J. Hackett, Andrew D. Newman, and Karen Wilson Department of Chemistry, UniVersity of York, York YO10 5DD, United Kingdom ReceiVed: January 19, 2007; In Final Form: May 4, 2007
The surface chemistry of benzene and bromobenzene over Pt(111) has been studied by temperature-programmed XPS/MS and NEXAFS. Time-resolved XPS shows that benzene adopts a single chemically distinguishable environment during low-temperature adsorption within the monolayer, with a saturation coverage at θC6H6 ) 0.2 ML. Around 20% of a benzene monolayer desorbs molecularly, while the remainder dehydrogenates to surface carbon. Bromobenzene likewise adsorbs molecularly at 90 K, giving rise to two C 1s environments at 284.4 and 285.3 eV corresponding to the C-H and C-Br functions, respectively. The saturation C6H5Br monolayer coverage is 0.11 ML. NEXAFS reveals that bromobenzene adopts a tilted geometry, with the ring plane at 60 ( 5° to the surface. Bromobenzene multilayers desorb at ∼180 K, with higher temperatures promoting competitive molecular desorption versus C-Br scission within the monolayer. Approximately 30% of a saturated bromobenzene monolayer either desorbs reversibly or as reactively formed hydrocarbons. Debromination yields a stable (phenyl) surface intermediate and atomic bromine at 300 K. Further heating results in desorption of reactively formed H2, C6H6, and HBr; however, there was no evidence for either biphenyl or Br2 formation. Pt(111) is an efficient surface for low-temperature bromobenzene hydrodebromination to benzene and HBr.
Introduction Volatile organic compounds (VOCs) are major pollutants of both atmospheric and aqueous environments, and there is much recent concern over man-made contributions to these from halohydrocarbons. Water pollution resulting from the dissolution of toxic VOCs is of particular concern in industrialized areas. Waste electronic equipment and brominated flame retardants contain polybrominated and chlorinated biphenyls (hazardous to both aquatic environment and human health) and are a major source of such contamination. These polymers are traditionally destroyed by thermal incineration, which actually liberates VOCs into the environment; indeed, over 70% of the Br used within printed circuit boards is released as harmful bromobenzene by these noncatalytic technologies. The U.S. Environmental Protection Agency and the European Union have therefore taken the lead in mandating strict emission limits for the incineration of waste halogenated organics.1 There is, consequently, a growing pressure to develop catalytic routes2,3 for the destruction of bromobenzene evolved during incineration, such as hydrodebromination.4,5 Despite the toxicity and possible bioaccumulation of polybrominated diphenyl ethers in ecosystems, there are very few reports on aryl bromide catalytic dehalogenation in the scientific literature and none addressing the chemistry of bromoarenes over platinum surfaces. The surface chemistry of aryl halides has generally been investigated as a means of seeding surfaces with phenyl derivatives;6-11 however, studies of aryl halide adsorption over Pt surfaces are scarce, with only a few chloro12,13 and iodo14 derivatives reported. Iodobenzene exhibits lowtemperature C-I cleavage on Pt(111) to form phenyl and iodine atoms (which eventually desorb as molecular I2), in competition with limited C6H5I desorption. No biphenyl is formed from * Corresponding author. Tel: +44 (0)1904 434470; fax: +44 (0)1904 432516; e-mail:
[email protected].
subsequent homocoupling chemistry (in contrast to iodobenzene on Ag(111),7 Cu(111),15 and Au(111)16 surfaces, which facilite C-C coupling and biphenyl desorption), with benzene the major reactively formed desorption product. Surprisingly, the only previous surface science measurement on bromobenzene is a LEED/TDS/XPS study over stepped NiO(100)17 surfaces; hence, aryl bromides are ripe candidates for further investigation. Building on our earlier work demonstrating that Pt(111) single-crystal surfaces are effective for low-temperature dechlorination of 1,1,1-trichloroethane,18,19 we now report on the hydrodebromination of C6H5Br over the same surface, with a view to establishing the threshold temperature for C-Br cleavage, stability of surface intermediates, and their subsequent reaction pathways. Experimental Procedures XPS measurements were carried out at the SuperESCA beamline of the ELETTRA synchrotron radiation source using a Pt(111) single-crystal sample prepared by standard procedures and maintained under ultrahigh vacuum (∼1 × 10-10 Torr). Quoted exposures are given in Langmuirs (1 L ) 1 × 10-6 Torr s-1) and are uncorrected for ion-gauge sensitivity. The crystal was held at 90 K during dosing. Benzene and bromobenzene (both Aldrich 99%) were first purified by repeated freezepump-thaw cycles prior to background dosing through unheated stainless steel lines and valves to avoid thermal decomposition. C 1s and Br 3p XP spectra were acquired at a photon energy of 400 eV and energy referenced to the Fermi level. The limiting spectral resolution was ∼150 meV. Individual spectra were acquired approximately every 30 s during Fast XP measurements and were Shirley background-subtracted over the entire elemental region. Temperature-programmed XP spectra were acquired by application of a linear heating ramp (∼0.4 K s-1) to the exposed sample. A common line shape derived from graphitic
10.1021/jp070488r CCC: $37.00 © 2007 American Chemical Society Published on Web 06/20/2007
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Figure 1. C 1s XP spectra following C6H6 uptake over Pt(111) at 90 K. Inset shows integrated surface coverages of multilayer and monolayer benzene states.
carbon was adopted for all C 1s components, based on a Gaussian/Lorentzian (70:30) mix with a common fwhm ) 0.58 eV. A similar line shape gave good fits and was employed for all Br components with a common fwhm ) 2.2 eV. Fitting was performed using CasaXPS version 2.3.5 using the minimum number of peaks required to minimize the R-factor. Coverages are defined in terms of monolayers (adsorbates/surface Pt atom) with 1 ML ) 1.5 × 1015 atoms cm-2. Absolute carbon coverages were determined by calibration with CO, with saturation benzene/bromobenzene coverages subsequently calculated as a sixth of the total carbon coverage. Temperatureprogrammed reaction spectra were acquired in a separate ultrahigh vacuum system using a VG 300 amu quadrupole mass spectrometer with a heating rate of ∼12 K s-1; spectra are uncorrected for ionization cross-section. The sample was grounded but not biased; however, a -110 V bias was applied to the mass spectrometer ionizer cage to prevent electron escape and surface damage. Temperatures were measured from a thermocouple spot-welded to the edge of the Pt(111) sample. Results and Discussion Benzene Adsorption on Pt(111). The low-temperature adsorption of benzene over a clean Pt(111) surface at 90 K was followed by C 1s time-resolved XPS. Figure 1 shows the C 1s XP uptake, which reveals a single C 1s environment at 284.6 eV for exposures below 1 L. The intensity of this feature rose linearly with coverage before saturating at ∼1.3 L, associated with molecular benzene chemisorption into a single adsorption state up to completion of the monolayer. Previous temperatureprogrammed desorption measurements revealed multiple, coverage-dependent, molecular desorption states20 from benzene monolayers. There is much debate over the origin of these states, with both lateral interactions21 and occupancy of multiple adsorption sites invoked.22 The presence of a single, well-defined carbon surface species within the monolayer favors the former’s influence on benzene desorption kinetics, rather than the coexistence of two distinct adsorption sites as postulated from DFT calculations.22 Higher benzene exposures led to the evolution of a new state at 285.1 eV, which grew continuously with exposure and could not be saturated in this experiment, indicative of multilayer formation. Figure 1 inset shows the corresponding coverage-dependent surface C coverage for components attributed to the monolayer and multilayer states. The saturated monolayer equates to a total carbon coverage of 1.2 ML, close to previous estimates for θC6H6 of 0.18 ML.23
Figure 2. C 1s temperature-programmed XP spectra of reacting saturated C6H6 adlayer on Pt(111). Inset shows total carbon coverage vs surface temperature.
Figure 3. C 1s XP spectra following C6H5Br uptake over Pt(111) at 90 K.
Thermal Chemistry of Benzene on Pt(111). Figure 2 shows the corresponding Fast XP TPRS spectra from a high benzene exposure dosed at 90 K, and the inset shows the corresponding temperature-dependent total surface C coverage. Multilayer benzene, centered around 285.1 eV, initially dominates the spectra at low temperature. The intensity of this feature drops significantly upon heating up to 180 K, accompanied by a slight shift to lower binding energy, these changes occurring precisely in the region where benzene multilayers desorb. As the temperature is raised further, the carbon coverage continues to fall by 0.21 ML carbon over the range where benzene desorbs from within the monolayer (300-500 K). This equates to a loss of 0.04 ML (roughly 20%) C6H6 from a saturated monolayer, somewhat lower than the 30% benzene loss observed by Campbell and co-workers,21 the remainder dehydrogenating as previously reported. Above 400 K, the overall C 1s envelope exhibits a notable shift to lower binding energy (∼284.1 eV), co-incident with benzene dehydrogenation and subsequent decomposition to CHx and surface carbon. Bromobenzene Adsorption on Pt(111). Having established the thermal chemistry and fingerprint XPS of benzene over Pt(111), the low-temperature adsorption of the brominated analogue was investigated. Figure 3 shows the C 1s XPS spectra acquired during bromobenzene uptake at 90 K. At low exposures, two well-resolved peaks are visible at 284.4 and
Hydrodebromination of Bromobenzene over Pt(111)
Figure 4. Fitted C 1s XP spectra for 1 and 4 L C6H5Br exposures on Pt(111) at 90 K showing monolayer and multilayer contributions. Inset shows integrated surface coverages of each bromobenzene state.
285.3 eV, which evolve in a constant 5:1 ratio. This peak separation and intensity ratio is entirely consistent with the expected C-H versus C-Br chemical shifts24,25 and the number of equivalent carbons within the parent bromobenzene, indicating non-dissociative chemisorption within the monolayer. These molecular features continue to grow up to ∼5 L C6H5Br, with higher exposures resulting in a loss of resolution and spectral broadening to higher binding energy, suggesting the formation of an additional adsorption state. In this high-exposure regime, the total C 1s signal continued to rise monotonically with dosage; hence, we associate this shift and broadening to bromobenzene multilayers. Indeed, all spectra could be fitted successfully using two sets of doublets with a common peak splitting of 0.9 eV and 5:1 intensity ratios. Representative lowand high-coverage spectra are shown in Figure 4 along with their fitted components. These monolayer and multilayer contributions are shown in the inset to Figure 4 and yield a saturation monolayer coverage of 0.67 ML carbon, which equates to a C6H5Br coverage of 0.11 ML. This value is in good agreement with that of 0.1 ML reported for C6H5I on Pt(111)14 and significantly less than our estimate for benzene. A small contribution from photodissociated bromobenzene is also apparent to low binding energy in the multilayer spectra and is most likely attributed to phenyl moieties formed due to beam damage during extended irradiation of the sample over the course of an uptake series (approximately 30 min). Figure 5 shows the angle-dependent C K-edge NEXAFS for a saturated bromobenzene monolayer. To aid the assignment of transitions, a bromobenzene multilayer spectrum recorded at normal and grazing emission is shown for comparison. For the multilayer, a series of resonances is observed at 285, 286.3, 287, 288.7, 293.5, and 300 eV, many akin to those observed for a saturated benzene overlayer (Figure 5, inset) and attributed to transitions to vacant σ* and π* states of the aromatic ring. The benzene resonances at 285, 288.7, 293.4, and 301 eV are associated with electronic transitions from C 1s to π*(e2u), π*(b2g), σ*(e1u), and σ*(e2g+a2g) molecular orbitals26 in accordance with previous work by Weiss and co-workers.27 The bromobenzene features are assigned in light of the benzene NEXAFS, and those of chloro- and iodobenzene adlayers over Cu(111),28 to C 1s f π*, σ*(C-Br), σ*(C-H), and σ*(C-C) transitions involving both the aromatic ring and the heteroatom as indicated in Figure 5. The low-energy π*(e2u) resonance observed for benzene is now split into two π*(a2,b1) components at 285 and 287.7 eV due to
J. Phys. Chem. C, Vol. 111, No. 28, 2007 10457
Figure 5. Angle-dependent C K-edge NEXAFS spectra of saturated C6H5Br monolayer and multilayer on Pt(111) at 90 K. Inset shows a C K-edge NEXAFS spectra of saturated C6H6 monolayer on Pt(111) at 90 K.
the lower symmetry of the aromatic ring upon Br incorporation (D6h f C2V), compounded by a chemical shift of the C 1s core level of the carbon atom bound to bromine. An additional resonance observed for bromobenzene at 286.3 eV is attributed to the C 1s f σ*(C-Br) transition since it does not exhibit the same angular dependency as the π* resonances. To our knowledge, this is the first reported NEXAFS spectrum of C6H5Br, and our observations are in good accord with previous measurements for multilayer C6H5I and C6H5Cl adlayers on Cu(111). Indeed, our assigned C 1s f σ*(C-Br) resonance falls precisely in between those for the σ*(C-I) and σ*(C-Cl) resonances at 285 and 287.7 eV, respectively, possibly reflecting the relative carbon-halogen bond strengths (and thus aryl halide orbital energies) down the group. A strong interaction between chemisorbed C6H5Br and Pt(111) is immediately evident from examining the corresponding monolayer NEXAFS: the strong π*(a2,b1) resonance observed in the gas and condensed phases is almost completely attenuated within the monolayer, indicating that this LUMO is partially filled by charge transfer from the surface. The π*(b1) resonance is also significantly downshifted to 288 eV, consistent with strong rehybridization of the aromatic system. The angular-dependent intensities of the π*(b1) + σ*(C-H) and σ*(C-Br) transitions, which decrease and increase, respectively, as the incident photon angle rises from normal to grazing incidence, can be used to calculate the orientation of the bromobenzene ring with respect to the surface. Adopting the approach of Sto¨hr and Outka,29 the grazing/normal incidence intensity ratio I70/I0 for these resonances translates to a tilt angle of ∼60° relative to the surface. This compares favorably with observations for C6H5I and C6H5Cl on Cu(111), where an angle of 45° is reported.28 It is interesting to note that White and coworkers suggest that iodobenzene prefers to adopt a flat-lying configuration over Pt(111), although they note that their HREELS spectra are also equally consistent with a C6 plane tilted away from parallel.14 Thermal Chemistry of Bromobenzene on Pt(111). Figure 6 presents bromobenzene desorption mass spectra as a function of the initial halide exposure on Pt(111). At coverages
