ABSTRACT
Ag/γ-Al2O3 is widely used for catalyzing various reactions, and its performance depends on the valence state, morphology and dispersion of Ag species. However, detailed anchoring mechanism of Ag species on γ-Al2O3 remains largely unknown. Herein, we reveal that the terminal hydroxyls on γ-Al2O3 are responsible for anchoring Ag species. The abundant terminal hydroxyls existed on nanosized γ-Al2O3 can lead to single-atom silver dispersion, thereby resulting in markedly enhanced performance than the Ag cluster on microsized γ-Al2O3. Density-functional-theory calculations confirm that Ag atom is mainly anchored by the terminal hydroxyls on (100) surface, forming a staple-like local structure with each Ag atom bonded with two or three terminal hydroxyls. Our finding resolves the puzzle on why the single-atom silver dispersion can be spontaneously achieved only on nanosized γ-Al2O3, but not on microsized γ-Al2O3. The obtained insight into the Ag species dispersion will benefit future design of more efficient supported Ag catalysts.
ABSTRACT
Motivated by the discovery of new phases of carbon under cold high-pressure compression, we performed a global structure search of high-pressure phases of boron nitride (BN). Ten new bulk phases were identified, each energetically more stable than the graphite-like hexagonal BN (h-BN) under high pressures. All ten high-pressure phases could be viewed as involving a stacking of buckled h-BN layers. Some of these solid structures can be fabricated through the cold high-pressure compression of h-BN films. According to the buckling of the h-BN layers, the new BN phases could be classified into three groups. The atomic structures, relative stabilities, electronic structures, and mechanical properties were studied in detail. A strong dependence of the relative stability, band structure, and mechanical properties on the buckling of h-BN was observed. The computed electronic band structures suggested that most of the high-pressure BN phases were insulators with wide and indirect band gaps. The calculated elastic constants and hardness suggested that several of the BN structures were superhard materials with potential applications in materials science and engineering. The computed transition paths indicated that the direct transition from h-BN to four of the new sp3-hybridized BN structures, or specifically to w-BN or bct-BN, were likely to occur through cold compression. For the other five of the new BN structures, although deeper local minima existed in the transition path, their formation through cold compression of h-BN was still plausible due to the low transition barrier from the deeper local minima to the targeted structure.
ABSTRACT
Ferromagnetism is usually deemed incompatible with superconductivity. Consequently, the coexistence of superconductivity and ferromagnetism is usually observed only in elegantly designed multi-ingredient structures in which the two competing electronic states originate from separate structural components. Here we report the use of surface molecular adsorption to induce ferromagnetism in two-dimensional superconducting NbSe2, representing the freestanding case of the coexistence of superconductivity and ferromagnetism in one two-dimensional nanomaterial. Surface-structural modulation of the ultrathin superconducting NbSe2 by polar reductive hydrazine molecules triggers a slight elongation of the covalent Nb-Se bond, which weakens the covalent interaction and enhances the ionicity of the tetravalent Nb with unpaired electrons, yielding ferromagnetic ordering. The induced ferromagnetic momentum couples with conduction electrons generating unique correlated effects of intrinsic negative magnetoresistance and the Kondo effect. We anticipate that the surface molecular adsorption will be a powerful tool to regulate spin ordering in the two-dimensional paradigm.
