Carbon Alloys part 9 pptx

264 a h h Chapter 16 9 -&-*-e Fig. 5. Selective etching reaction of surface C atoms by exposing to 0, gas (1.3 x 10“ Pa) at RT in the same areavisualized by STM; (a) 0 L (before exposure, I,: 2.0 nA, V,: 10 mV); (b) 2.5 L (I,: 3.0 nA, V,: 20 mV), (c) 5.0 L (I,: 3.0 nA, V,: 20 mV), (d) 10.0 L (Z,: 2.0 nA, V,: 20 mV). 7.3 x 7.3 nm2. The STM tip was retracted during exposure to oxygen gas. Each image was measured under UHV without oxygen ambient. (e) A magnified image of the square region in (c) with lines of the (1 x 1) lattice. 3.6 x 3.6 nm2. (f) Model structure of (e). Empty circles and shaded circles represent Mo and C atoms, respectively. Figure 5d indicates that 100 carbon atoms were etched by -2.3 x lo3 molecules of oxygen impinging on the area. Etching of the surface carbon atoms by oxygen at room temperature is unexpected because both carbon and oxygen atoms are strong adsorbates on molybdenum metal surfaces. The carbon and oxygen atoms could not be removed from the molybdenum surfaces below 1200 K, when adsorbed as a single element [24,25]. If carbon and oxygen atoms co-exist on molybdenum metals, they desorb as carbon monoxide at temperatures of -1000 K [24,25]. An air-exposed Mo,C catalyst desorbs carbon monoxide and carbon dioxide at temperatures above 800 K [26]. Thus, stable adsorbed oxygen atoms cannot etch carbon atoms on a Mo,C(0001) surface. On the other hand, a dissociative adsorption of 0, on Al( 11 1) [27] or Pt( 11 1) [28] produces ‘hot’ oxygen atoms with translational energies parallel to the surface. Thermo- induced or photo-induced ‘hot’ oxygen atoms on Pt(ll1) react with adsorbed carbon monoxide to form carbon dioxide at temperatures below 150 K [29]. Such energetic- ally excited oxygen atoms formed on the surface should be responsible for the etching reactions reported above. The STM tip does not contribute to the etching reaction. The tip was retracted from the tunneling region during exposure to oxygen gas. Each frame of Fig. 5 is a sequential STM image measured under ultra-high-vacuum (UHV) conditions. The etched area was unchanged in the sequential images. Atomic-scale Structure and Reactivity of Metal Carbide Surfaces 265 The etching reaction does not occur homogeneously but starts from specific points and expands the (1 X 1) areas. Energetic oxygen atoms migrate and react with the carbon atoms at the edges of the ordered domains. The reaction rates (collision efficiency) are lower for carbon atoms at the edges of the (&x&)R3O0-honeycomb domains and at the c(2 x4)-zigzag row domains. Thus, preferential etching is probably due to different adsorption energies of carbon atoms in the two structures, i.e. the adsorption energy of carbon atoms in the (Ax &)R3O0-honeycomb domain is higher than that of carbon atoms in the c(2x4)-zigzag row domains. Certain atomic protrusions were found in the (Ax &)R30°-honeycomb region after exposure to oxygen. Consecutive STM imaging showed that they occupied three-fold hollow sites at the center of the triangles formed by three carbon atoms, and movement (hopping) between equivalent sites, frame by frame, was observed, assigned to oxygen atoms. On the other hand, such oxygen atoms were not found on the (1x1) and the c(2x4)-zigzag row regions, suggesting high mobility of oxygen atoms in those regions rendering them invisible to STM. This contrasts with immobile oxygen atoms on molybdenum metal surfaces. The high mobility of oxygen atoms explains the high etching rate of carbon atoms on the surface. A mild oxygen treatment of Mo,C or WC catalysts improves their catalytic activities [1,13,30,31], probably because of the formation of oxycarbide phases. Other oxide phases formed by more extensive oxidation inhibit catalysis. 5 Conclusions and Future Prospects STM is a powerful tool to image atomic arrangements of metal carbide surfaces. Each surface carbon atom of C-terminated Mo,C(OOOl) surfaces can be identified as a shallow sombrero protrusion using a low tunneling resistance less than 1 MR. Imag- ing of surface carbon atoms by STM elucidates chemical reactions on metal carbide surfaces. The etching by oxygen of surface carbon atoms of C-terminated Mo,C(0001) surfaces was observed. This etching reaction selectively occurs on the c(2 x 4)-zigzag row structure even at room temperature, leading to exposure of the underlying (1 x 1) molybdenum layer. This reaction may be an important intermediate in the under- standing of improvements to catalytic activity of Mo,C or WC catalysts by mild oxidation treatments. STM images indicate the local electronic density of states near to the Fermi level of a surface and relate to applied bias voltages between the tip and the surface. Local mapping of the electronic states of metal carbide surfaces is essential to the under- standing of their physical properties and chemical reactivities. STM images indicate that molybdenum atoms in the (&x~)R3O0-honeycomb structure and the ~(2x4)- zigzag row structure have continuous electronic states near to the Fermi level and that the (1 x 1) molybdenum atoms without carbon atoms have localized electronic states. Combinations of high resolution STM mapping with scanning tunneling spectroscopy (STS) should clarify the causes of high activities of Mo,C compared with noble metal catalysts. 266 Chapter 16 References 1. R.B. Levy and M. Boudart, Platinum-like behavior of tungsten carbide in surface catalysis. Science, 181: 547-549,1973. 2. S.T. Oyama, Introduction to the chemistry of transition metal carbides and nitrides. In: S.T. Oyama (Ed.), The Chemistry of Transition Metal Carbides and Nitrides, pp. 1-27. Blackie Academic and Professional, Glasgow, 1996. 3. S. Ramanathan and S.T. Oyama, New catalysts for hydroprocessing: transition metal car- bides and nitrides. J. Phys. Chem., 99 16365-16372,1995. 4. L.I. Johansson, Electronic and structural properties of transition metal carbide and nitride surfaces. Surf. Sci. Rep., 21: 177-250,1995. 5. A.T. Santhanam, Application of transition metal carbides and nitrides in industrial tools. In: S.T. Oyama (Ed.), The Chemistry of Transition Metal Carbides and Nitrides, pp. 28-52. Blackie Academic and Professional, Glasgow, 1996. 6. J.G. Chen, Carbide and nitride overlayers on early transition metal surfaces: preparation, characterization and reactivities. Chem. Rev., 96: 1477-1498,1996. 7. J K. Zuo, R.J. Warmack, D.M. Zehner and J.F. Wendelken, Periodic faceting on TaC( 110): Observations using high-resolution low-energy electron diffraction and scan- ning tunneling microscopy. Phys. Rev. B, 47: 10743,1993. 8. J K. Zuo, D.M. Zehner, J.F. Wendelken, R.J. Warmack and H N. Yang, TaC(ll0): a pe- riodic facet reconstruction studied by LEED and STM. Surf. Sci., 301: 233, 1994. 9. R.M. Tsong, M. Schmid, C. Nagl, P. Varga, R.F. Davis and I.S.T. Tsong, Scanning tunnel- ing microscopy studies of niobium carbide (100) and (110) surfaces. Surf. Sci., 366: 85-92, 1996. 10. M. Hammer, C. Tornevik, J. Rundgren, Y. Gauthier, S.A. Flodstrom, K.L. Hakansson, L.I. Johansson and J. Haglund, Surface atomic structure of reconstructed VC,,,,( 111) studied with scanning tunneling microscopy. Phys. Rev. B, 45: 6118,1992. 11. E. Pathe and V. Sadagopan, The structure of dimolybdenum carbide by neutron diffraction technique. Acta Crystallogr., 16 202-205,1963. 12. R L. Lo, K. Fukui, S. Otani, S.T. Oyama and Y. Iwasawa, C-termicated reconstruction and C-chain structure on Mo2C(0001) surface studied by LEED and STM. Jpn. J. Appl. Phys., 38: 3813-3815,1999. 13. M.J. Ledoux, C. Pham-Huu, H. Dunlop and J. Guille, n-Hexane isomerization on high spe- cific surface Mo,C activated by an oxidative treatment. Proc. 10th Int. Cong. Catal., pp. 14. J. Ahn, H. Kawanowa and R. Souda, STM study of oxygen-adsorbed TiC(111) surface. Surf. Sci., 429: 33&344,1999. 15. R L. Lo, K. Fukui, S. Otani and Y. Iwasawa, High resolution images of Mo,C(OOOl)-( & x&)R30° structure by scanning tunneling microscopy. Surf. Sci., 440: L857-L862,1999. 16. K. Fukui, R L. Lo, S. Otani and Y. Iwasawa, Novel selective etching reaction of carbon at- oms on molybdenum carbide by oxygen at room temperature visualized by scanning tunnel- ing microscopy. Chem. Phys. Lett., 325: 275-280,2000. 17. R. Young, J. Ward and F. Scire, Metal-vacuum-metal tunneling, field emission, and the transition region. Phys. Rev. Lett., 27: 922-924, 1971. 18. R. Young, J. Ward and F. Scire, The Topografiner: An instrument for measuring surface microtopography. Rev. Sci. Instrum., 43: 999-1011,1972. 19. G. Binnig, H. Rohrer, Ch. Gerber and E. Weibel, Tunneling through a controllablevacuum gap. Appl. Phys. Lett., 40: 178-180, 1982. 20. G. Binnig, H. Rohrer, Ch. Gerber and E. Weibel, Surface studies by scanning tunneling mi- 955-967,1993. Atomic-scale Structure and Reactivity of Metal Carbide &$aces 267 croscopy. Phys. Rev. Lett., 49: 57-61, 1982. J. Cryst. Growth, 154: 202-204,1995. Iwasawa, Characterization of a-Mo,C(OOOl). Surf. Sci., 426: 187-198,1999. 21. S. Otani and Y. Ishizawa, Preparation of Mo2C single crystals by the floating zone method. 22. T.P. St. Clair, S.T. Oyama, D.F. Cox, S. Otani, Y. Ishizawa, R L. Lo, K. Fukui and Y. 23. NIST Surface Structure Database, ver. 3.0. 24. E.I. KO and R.J. Madix, Adlayer effects on adsorption/desorption kinetics: N2, H,, C,H,, and CO on Mo(lOO)-C. Surf. Sci., 100 L449-U53,1980. 25. K. Fukui, T. Aruga and Y. Iwasawa, Chemisorption of CO and H2 on clean and oxy- gen-modified Mo(112). Surf. Sci., 281: 241-252,1993. 26. G.S. Ranhotra, A.T. Bell and J.A. Reimer, Catalysis over molybdenum carbides and ni- trides. J. Catal., 108: 2439,1987. 27. H. Brune, J. Wintterlin, R.J. Behm and G. Ertl, Surface migration of hot adatoms in thc course of dissociative chemisorption of oxygen on Al(111). Phys. Rev. Lett., 68 624-626, 1992. 28. J. Wintterlin, R. Schuster and G. Ertl, Existence of a “hot” atom mechanism for the dissoci- ation of O2 on Pt(ll1). Phys. Rev. Lett., 77: 123-126,1996. 29. W.D. Mieher and W. Ho, Photochemistry of oriented molecules coadsorbed on solid sur- faces: the formation of CO,+O from photodissociation of O2 coadsorbed with CO on Pt(ll1). J. Chem. Phys., 91: 2755-2756,1989. 30. E. Iglesia, J.E. Baumgartner, F.H. Ribeiro and M. Boudart, Bifunctional reactions of al- kanes on Tungsten carbide modified by chemisorbed oxygen. J. Catal., 131: 523-544,1991. 31. A. Muller, V. Keller, R. Ducros and G. Maire, Catalytic activity and XPS surface determi- nation of tungsten carbide for hydrocarbon reforming. Influence of the oxygen. Catal. Lett., 35: 65-74,1995. 269 Chapter 17 Infra-Red Spectra, Electron Paramagnetic Resonance, and Proton Magnetic Thermal Analysis Osamu Ito", Tadaaki Ikoma" and Richard Sakurovsb "Institute of Multidisciplinary Research for Advanced Materiak, Tohoku University, Katahira, Sendai 980-8577, Japan; "CSIRO, Division of Energy Technology, Noah Ryde 1670, Australia. Abstract: This chapter describes the characterization of carbon alloys using infra-red spectroscopy (IR spectra), electron paramagnetic resonance (EPR) and proton magnetic resonance thermal analysis (PMRTA). The broad absorption bands of IR spectra observed using diffuse reflectance methods provide information about the ring size of aromatic molecules within a sample and the extent to which these are ordered. The sharp C-H stretching peaks are quantitatively compared with peaks using solid-state NMR. Two magnetic resonance methods employed pulse techniques where relative hydrogen contents are evaluated as ratios to carbon contents. PMRTA, which measures the temperature dependence of signal intensities and relaxation times of proton magnetic resonance, provides information about molecular motion in heat-treated carbons and in coals. These spectroscopic techniques give information about the composition of carbon precursors prepared at temperatures below 500400°C. Keywords: IR, UV, Vis, Near-IR, NMR, EPR, PMRTA. 1 Infra-Red (IR) Spectra I .I Dime Rejkctance Absorption (DR4) Spectroscopy. IR spectra of carbons from the KBr pellet transmittance method contain a broad background (from scattering ofthe monitoring light) that needs to be subtracted from the spectrum. With decreasing particle size these broad backgrounds are reduced and must be subtracted to obtain reliable IR spectra. An example is shown in Fig. la for a bituminous coal [l]. For IR spectra from the diffuse reflectance absorption (DRA) method, the absorption intensities, represented as a Kuberka-Munk function (f(R,) or F(R,)) show weak broad bands in the higherwave-number region (Fig. lb). By the DRA method, high rank coals, heat-treated organic materials and coal-tar pitches (CTP) also show broad absorptions attributable to the absorption tail of electronic transitions extending from the WNis regions [2]. Such broad bands are observed by 270 Chapter 17 Wavenumber (om-') Fig. 1. IR spectra of a bituminous coal (sample/KBr = 1/150): (a) transmittance KBr pellet method; (b) diffise reflectance absorption method [l]. DRA and increase with decreasing particle size of the sample [2]. Thus, it is not necessary to subtract these broad absorptions in the entire region measured by the DRA method. FT-IR spectra observed by DRA for various coals are shown in Fig. 2 [2]. The baseline of a low rank coal (Monvell coal: C = 67.4 wt%) is flat in the region of 4000-6000 cm-'. For bituminous coal (Oyubari coal: C = 85.4 wt%), small absorp- tions were observed at 4000-6000 cm-' with a decrease in the IR-band intensities of hydrogen bonds at 3000-3500 cm-' and carbonyl groups at 1700 cm-'. With a further increase in coal rank, Moura (C = 85.6 wt%) and Hongei (C = 93.7 wt%), the absorption tails extend to lower wave-numbers with a decrease in intensity of alkyl group band at 2940 cm-'. For active carbon, only the broad band was observed, the absorption edge extending to wave-numbers less than 1000 cm-', indicating a narrow band gap in the active carbon. Absorptions in the near-IR region are used as a measure of coal rank [2]. For a bituminous coal (Shin-Yubari: C = 86.9 wt%), the intensity of the broad band increases with increasing heat treatment temperature (HTT) to 400°C accompanied by a decrease in the intensities of the alkyl bands. At an HTT of 6OO0C, the steep rise of the near-IR absorption ceases when it then resembles the absorption spectrum of activated carbon as in Fig. 2. These changes in absorption spectra in the near-IR region are used to study effects of HTT on organic materials. On heat-treating decacyclene to 5OO0C, the broad band in the near-IR region appeared after 1 h increasing in intensity after 2 h, but decreased after 3 h. At an HTT IR Spectra, Electron Paramagnetic Resonance, and Proton Magnetic Thermal Analysis 271 0.60 1 0.30 - activated carbon 0.00 . I . 1 . 6000 6000 4000 3000 2000 1000 Wavenumber cm-' Fig. 2. IR/near-IR spectra of several coal samples [2] of 550°C, the absorption spectrum in the near-IR region is flat showing decreases in the intensities of the C-H bands. These changes in the near-IR region relate to absorption bands in the Vis regions (see below). 1.2 Relutionship of IR spectra with UVlEslNear-IR Spectra On heat-treatment of aromatic hydrocarbons, polycondensation reactions occur resulting in formation of mesophase which is the precursor of graphitizable carbon fibers and carbons. Changes in the molecular structures and molecular arrangements during this process are monitored as electronic transitions in the Vis and near-IR region. 272 Chapter 17 I Fig. 3. Correlation of observed p-band wave-number with calculated energy gap (A&) between LUMO and HOMO in p-unit for several hydrocarbons [3]. The absorption spectra of solid carbon precursors are measured using the KBr-CsI pellet transmittance method. At an HTT of 5OO0C, decacyclene first forms mesophase spheres which coalesce into anisotropic flow textures. The absorption band of parent decacyclene is only slightly broader than when in solution, because of intermolecular interactions in a concentrated solution and the solid state. With heat-treatment, the absorption bands at wavelengths longer than 500 nm increase in intensity. Inter- mediates of the carbonization reactions of decacyclene are zethrene derivatives, with absorption bands beyond 550 nm. The relationship between the energy gap from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) and the absorption maximum at longest wavelength of the aromatic hydrocarbons is shown in Fig. 3. By comparing the observed absorption bands with Fig. 3, it is seen that a zethrene dimer is a main component in pyrolyzed decacyclene [3]. For coal-tar pitch, the diffusion reflectance method gives more reliable absorp- tion spectra in Vidnear-IR region compared with the transmittance method. At 500"C, coal-tar pitch forms mesophase spheres [4], a process described by the absorption spectra in Vis/near-IR/IR region as measured by the DRA (Fig. 4). Here, the absorption intensities of the Vis region increase with heat-treatment time (1 h). The absorption intensity in the 300-500 nm region reaches a maximum at -3 h and then decreases. The initial increase in absorption intensity is attributed to the aroma- tization of the coal-tar pitch, the later decrease in intensities being due to the broadening of absorption bands because of inter-aromatic interactions. The IR Spectra, Electron Paramagnetic Resonance, and Proton Magnetic Thermal Analysis 273 wavenumber x1~3cm-1 Fig. 4. Separation of the observed spectra into intensities due to lamella (L-part) and to isolated aromatic hydrocarbons (A-part). Inset: Shaded parts are plots of Eq. (1) [4]. absorption band of the lamellar aromatic hydrocarbons is described by the following equation, which has been applied to amorphous semiconductors [5]: where AE is the band gap energy and B to a proportional factor increasing with extents of ordering of the lamellar aromatic hydrocarbons. The straight line of the inset in Fig. 4 shows values as calculated from Eq. (1). The hatched area in Fig. 4 results from absorption by lamella (L-part), the remaining white area being due to absorption by single aromatic molecules (A-part). After heat treatment for 0.5 h, the L-part increases, but the A-part decreases, indicating increased interactions between aromatic molecules due to the improved alignment and ordering of the aromatic molecules. 1.3 Relationship with Solid-state NMR 13C-NMR spectra of solid carbon samples are measured using magic-angle NMR. However, the NMR signals for carbons and coals samples that are rich in quaternary carbon atoms surrounded by other carbon atoms exhibit ‘side-bands’. In the 13C-NMR signals from coronene, the main signal is at 125 ppm, but with side-bands in the wings of the 125 ppm signal (Fig. 5a) [l]. The positions and relative intensities of the side-bands change with the rotation speed of the sample probe. NMR spectra of coals observed under the same conditions are composed of signals and side-bands (Fig. 5b), the principal signal, 100-150 ppm, being assigned to aromatic carbon atoms. 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