No abstract

Topological nodal line (DNL) semimetals, a closed loop of the inverted bands in its bulk phases, result in the almost flat drumhead-like non-trivial surface states (DNSSs) with an unusually high electronic density near the Fermi level. High catalytic active sites generally associated with high electronic densities around the Fermi level, high carrier mobility and a close-to-zero free energy of the adsorbed state of hydrogen (∆G H* ≈0) are prerequisite to design alternative of precious platinum for catalyzing electrochemical hydrogen production from water. By combining these two aspects, it is natural to consider if the DNLs are a good candidate for the hydrogen evolution reaction (HER) or not because its DNSSs provide a robust platform to activate chemical reactions. Here, through first-principles calculations we reported a new DNL TiSi-type family, exhibiting a closed Dirac nodal line due to the linear band crossings in k y =0 plane. The hydrogen adsorbed state on the surface yields ∆G H* to be almost zero and the topological charge carries participate in HER. The results highlight a new routine to design topological quantum catalyst utilizing the topological DNL-induced surface bands as active sites, rather than edge sites-, vacancy-, dopant-, strain-, or heterostructure-created active sites. Topologically protected surface states in gold-covered Bi 2 Se 3 topological insulators were theoretically suggested to serve as an effective electron path in the case of the CO oxidation [49]. Most recently, TWs (NbP, TaP, NbAs and TaAs) have been considered as excellent candidates of catalysts for hydrogen evolution reaction (HER) [50]. This key concept of TWs as catalysts was pioneered by topological surface states which provide an alternative way to create active sites, rather than by traditionally increasing the active edge sites or vacancies [51][52][53][54][55][56][57][58][59][60]. The possible bottleneck of TWs as electrocatalyst is their lower carrier density around Fermi level (Fig. 1a), because electrostatic screening strength in TWs is much weaker than that in the normal metals (e.g., Pt). DNL semimetal

La 0.67 Ca 0.33 Mn 1Ϫx Cu x O 3 (xϭ0 and 0.15͒ epitaxial thin films were grown on the ͑100͒ LaAlO 3 substrates, and the temperature dependence of their resistivity was measured in magnetic fields up to 12 T by a four-probe technique. We found that the competition between the ferromagnetic metallic ͑FM͒ and paramagnetic insulating ͑PI͒ phases plays an important role in the observed colossal magnetoresistance ͑CMR͒ effect. Based on a scenario that the doped manganites approximately consist of phase-separated FM and PI regions, a simple phenomenological model was proposed to describe the CMR effect. Using this model, we calculated the resistivity as functions of temperature and magnetic field. The model not only qualitatively accounts for some main features related to the CMR effect, but also quantitatively agrees with the experimental observations.

Rational use of abundant natural gas is gaining importance as petroleum oil reserves are diminishing. Methane, the main component of natural gas, can be converted to liquid fuels, hydrogen, and other value-added chemicals through a syngas intermediate, a mixture of CO and H 2 . Currently, syngas is produced by reacting methane with steam at high temperatures and pressures. This process is very energy-and capitalintensive, as the reaction is highly endothermic. An alternative process to produce syngas is the partial oxidation of methane (POM) with pure oxygen in the presence of a catalyst. [1,2] The exothermic nature of POM makes the process attractive in terms of energy consumption. The other advantage of POM over the steam-reforming process is that the H 2 /CO ratio of~2 of the as-produced syngas is highly suitable for subsequent conversion to environmentally friendly liquid fuels through a Fischer-Tropsch process. The main difficulty with POM lies in the consumption of large quantities of expensive pure oxygen that is produced by the cryogenic separation of air. A recent development in syngas production technology is the use of oxygen-permeable dense ceramic membranes [3,4] integrating the oxygen separation and POM processes in a single space.[5] The formidable problem for this approach is that the membrane must be chemically and mechanically stable at elevated temperatures in a large oxygen gradient with one side of the membrane exposed to oxidizing atmosphere (air) and the other side to the reducing atmosphere (the mixture of hydrogen and carbon monoxide). Herein we propose a two-stage membrane reactor, as depicted in Figure 1 a, which may reduce the requirement on the stability of the membrane materials. In this reactor, part of the methane is converted into CO 2 and H 2 O by reaction with oxygen permeated through the membrane from the air, and the resultant mixture is transferred to a catalyst bed where the remaining methane is reformed to syngas.A ceramic composite of Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3Àd (97.5 mol %) and Co 3 O 4 (2.5 mol %) was used to construct a membrane reactor. The major phase of the composite was intended for separating oxygen from air [6] and the minor phase at the surface for catalyzing the reaction of methane with permeated oxygen; [7] in terms of mechanics, small cobalt oxide particles embedded in the bulk may also reinforce the major phase. The dense tubular membrane of the required phase composition was prepared by extrusion followed by sintering at 1100 8C for 10 h. A g-Al 2 O 3 -supported catalyst was prepared with a nickel loading of 12.5 wt % and sieved to 40~60 mesh.[8] The reactor consisted of a membrane of length 2.14 cm, inner diameter 0.76 cm (membrane surface area 5.10 cm 2 ), and wall thickness 0.13 cm, and a catalyst bed containing 0.2 g Ni/g-Al 2 O 3 ; the membrane tube and the catalyst bed were separated by a distance of 2.5 cm (see Figure 1 b). In order to improve the flow pattern in the reactor, an alumina cylinder was placed inside the reactor (not shown in Figure...