Metal-oxide-supported Ru catalysts, typically in the form of Ru particles dispersed on oxides, have emerged as the most promising catalysts for the ammonia synthesis reaction. Generally, Ru particles are regarded as catalytically active components and are mainly responsible for the formation of ammonia during ammonia synthesis. Here we show that subnano Ru clusters on Sm2O3 (Ru/Sm2O3) favor the formation of surface hydride (Sm–H) species under reaction conditions, which can trigger the activation of Ru clusters in the ammonia synthesis reaction. The synergy of surface hydride species and Ru clusters boosts the activity relative to Ru clusters alone and affords ammonia yields as high as 90.1–100% of the thermodynamic equilibrium value at 400 °C. Mechanistic studies and theoretical calculations reveal that Sm–H species can not only work in synergy with Ru clusters to reduce the energy barrier for nitrogen dissociation, but also directly participate in the formation of ammonia on the Ru clusters. The discovery of the synergy role of surface hydride species can deepen the molecular-level understanding of the ammonia synthesis reaction process, which will eventually help to rationally design efficient oxide-supported metal catalysts in the heterogeneous catalysis process where hydrogen is involved.
Stable mesoporous black anatase TiO2 nanosheets (MBTNs) are synthesized via an earth-abundant low-cost biotemplate method combined with an ethanediamine encircling process, and subsequent high-temperature calcinations and surface hydrogenation. The MBTNs, which can be synthesized on a large scale with a low cost, are composed of Ti3+ in frameworks and surface disorders. In this case, the employment of ethanediamine encircling plays a vital role in fabricating the stable MBTNs, and not only favors high-temperature hydrogenation (600 °C), but also retains the mesoporous network as well as inhibiting the anatase-to-rutile phase transformation and grain growth. The resultant MBTNs possess a relatively high surface area of ∼74 m2 g−1, and large pore size and pore volume of ∼10 nm and 0.15 cm3 g−1, respectively. The MBTNs, with a narrow bandgap of ∼2.85 eV, can extend the photoresponse from the ultraviolet to visible light region, and exhibit an excellent solar-driven photocatalytic hydrogen evolution rate (∼165 μmol h−1 0.05 g−1), which is about twice as high as that of pristine mesoporous TiO2 nanosheets (∼82 μmol h−1 0.05 g−1). This efficient solar-driven photocatalytic hydrogen evolution is ascribed to the synergistic effects of the highly crystalline, ultrathin 2D mesoporous structure of Ti3+ in frameworks and surface disorders.
Catalytic hydrogenation of alkynes to alkenes is pivotal in both petrochemical and fine chemical industries. Its implementation relies on the use of transition metals, especially those precious Pd-based catalysts. In this work, we utilize a melt infiltration method to prepare magnesium oxide-supported potassium hydride (KH/MgO), which is illustrated as a transition-metal-free catalyst for selective hydrogenation of alkynes. H 2 -D 2 exchange experiment proves that dihydrogen activation and dissociation on KH/MgO is plausible at room temperature and ambient pressure. KH/MgO catalyzes hydrogenation of diphenylacetylene (DPA) already at 40 ℃, implying its high intrinsic activity at very mild conditions. Under an optimized condition of 80°C, 2 bar H 2 , and 40 h, 75 % conversion of DPA is achieved, affording 82 % selectivity to stilbenes. Mechanistic study suggests that surface hydride on KH/MgO plays an important role toward dihydrogen activation, and involved in the hydrogenation step to form stilbenes. This work shows the promise for using light metal hydrides consisting of earth-abundant elements as alternative hydrogenation catalysts.
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