The formation of carbon filaments upon decomposition of hydrocarbons catalysed by iron subgroup metals and their alloys

Abstract

The CP-conserving Minimal Supersymmetric Standard Model (MSSM) contains three neutral Higgs bosons [1 -5], the lighter and heavier CP-even scalars h and H, respectively, and the CP-odd pseudoscalar A. While the MSSM Higgs sector is CP-conserving at Born level even in the presence of CP-phases, loop effects mediated dominantly by third generation squarks can generate significant CP-violating scalar-pseudoscalar transitions, leading to mixing of the neutral Higgs states into the mass eigenstates H 1 , H 2 , H 3 , with no definite CP parities [6, 8 -14].It is well known that mixing of states with equal conserved quantum numbers is strongly enhanced when these states are nearly degenerate, i.e., when their mass difference is of the order of their widths [9,10]. This degeneracy occurs naturally in the Higgs decoupling limit of the MSSM, where the lightest Higgs boson has Standard Model-like couplings and decouples from the significantly heavier Higgs bosons [15]. In the decoupling limit, a resonance enhanced mixing of the states H and A can occur, which may result in nearly maximal CP-violating effects [8 -10]. The general formalism for mass mixing in extended Higgs sectors with explicit CP violation is well developed [11 -14], and sophisticated computer codes are available for numerical calculations [16 -18]. Detailed investigations of the fundamental properties of the Higgs bosons, both phenomenological and experimental, will be crucial for the understanding of the mechanism of electroweak symmetry breaking.In previous studies of the and CP-violating Higgs sector [28 -35], it was shown that the CP-properties and couplings of the heavy neutral Higgs bosons can be ideally tested in µ + µ − collisions. Such a muon collider is a superb machine for measuring the neutral Higgs masses, widths, and couplings with high precision, since the Higgs bosons are resonantly produced in the s-channel [36 -38]. The well controllable beam energy allows the study of the center-of-mass energy dependence of observables around the Higgs resonances. In particular, the beam polarization plays an essential role for analyzing the CP nature of the Higgs sector itself. Not only backgrounds can be reduced, but the CP-even and CP-odd contributions of the interfering Higgs resonances to the observables can be ideally studied, if the beam polarizations are properly adjusted [23 -33].Besides the initial beam polarization, the final fermion polarizations are essential to probe the Higgs interference. The secondary decays of the final fermions enable their spin analysis, and additional final state polarization observables allow for a complete determination of the CP-properties of the Higgs bosons [25 -28]. For final state SM fermions ff , with f = τ, b, t, such polarization observables have been classified according to their CP and CPT 1 transformation properties [28]. For the production of neutralinos [26] and charginos [27] with longitudinally polarized beams, it has been shown that asymmetries in the energy distributions of their decay products ar...

Coprecipitated iron-containing catalysts (Fe-Al2O3, Fe-Co-Al2O3, Fe-Ni-Al2O3) for methane decomposition at moderate temperatures

“…Despite the fact that the role of iron carbide in decomposition of hydrocarbons and CO disproportionation is widely discussed in the literature [14,19,33,[35][36][37][38], there is no clear answer to this question. The main problem is that it is impossible to carry out a comprehensive physicochemical study of working catalysts under in situ conditions.…”

“…Ni-Cu-Al 2 O 3 catalyst may serve as an example. It has been shown to have improved carbon capacity during methane decomposition and give CFC with different microstructural properties in comparison with Ni-Al 2 O 3 [12,19,21,22].…”

“…Traditional catalysts for methane decomposition are iron group metals (Fe, Ni, Co) [4][5][6][7][12][13][14][15][16][17][18][19][20]. Methane decomposition over iron-containing catalysts usually takes place at 680-1200 • C. The amount of carbon formed on such catalysts is typically low [4,[8][9][10][11]14].…”

Investigation of growth dynamics of carbon nanotubes

“…In the last decades, it was reported that metal nanoparticles can undergo partial carburization, i.e., the chemical transformation of metal into metal carbide, and subsequent reverse decomposition during the synthesis of carbon filaments and nanotubes by the catalytic thermal decomposition of hydrocarbons. In 1970–1980s, the growth of carbon filaments on metallic iron catalyst was actively studied by Buyanov and Chesnokov, and a carbide cycle mechanism of the growth was proposed [70–75]. According to this mechanism, a metastable carbide-like intermediate compound was formed in the subsurface layer of a catalytic particle as a result of the decomposition of hydrocarbon.…”

“…The decomposition of intermediate carbide led to the supersaturation of metal by carbon. Indeed, the degradation of iron carbide Fe 3 C results in the mixture of carbon and iron with a carbon content of 6–7 wt %, whereas the saturated solid solution of carbon in iron contains not less than 0.025 wt % carbon [75]. Thus, a large carbon concentration gradient was created in the bulk of the catalytic particle.…”