The formation of a lithium-iridium complex hydride toward ammonia synthesis.

Noble metal elements are focal catalytic candidates for many chemical processes, but have received little attention in the field of nitrogen fixation except ruthenium and osmium. Iridium (Ir), as a representative, has been shown to be catalytically inactive for ammonia synthesis because of its weak nitrogen adsorption and severe competitive adsorption of H over N that strongly inhibits the activation of N2 molecules. Here we show that, upon compositing with lithium hydride (LiH), iridium can catalyze ammonia formation at much enhanced reaction rates. The catalytic performance of the LiH-Ir composite can be further improved by dispersion on a MgO support with a high specific surface area. At 400 °C and 10 bar, the MgO-supported LiH-Ir (LiH-Ir/MgO) catalyst shows a ca. 100-fold increase in activity compared to the bulk LiH-Ir composite and the MgO-supported Ir metal catalyst (Ir/MgO). The formation of a lithium-iridium complex hydride phase was identified and characterized, and this phase may be responsible for the activation and hydrogenation of N2 to NH3.

Barium hydride activates Ni for ammonia synthesis catalysis.

Nickel (Ni) metal has long been considered to be far less active for catalytic ammonia synthesis as compared to iron, cobalt, and ruthenium. Herein, we show that Ni metal synergized with barium hydride (BaH2) can catalyse ammonia synthesis with an activity comparable to that of an active Cs-Ru/MgO catalyst typically below 300 °C. Kinetic analyses show that the addition of BaH2 makes the apparent activation energy for the Ni catalyst decrease dramatically from 150 kJ mol-1 to 87 kJ mol-1. This result together with N2-TPR experiments suggests a strong synergistic effect between Ni and BaH2 for promoting N2 activation and hydrogenation to NH3. It is suggested that an intermediate [N-H] species is generated upon N2 fixation and then is hydrogenated to NH3 with the regeneration of hydride species, forming a catalytic cycle.

Enabling Semihydrogenation of Alkynes to Alkenes by Using a Calcium Palladium Complex Hydride.

Selective hydrogenation of alkynes to alkenes requires a catalytic site with suitable electronic properties for modulating the adsorption and conversion of alkyne, alkene as well as dihydrogen. Here, we report a complex palladium hydride, CaPdH2, featured by electron-rich [PdH2]δ- sites that are surrounded by Ca cations that interacts with C2H2 and C2H4 via σ-bonding to Pd and unusual cation-π interaction with Ca, resulting in a much weaker chemisorption than those of Pd metal catalysts. Concomitantly, the dissociation of H2 and hydrogenation of C2Hx (x = 2-4) species experience significant energy barriers over CaPdH2, which is fundamentally different from those reported Pd-based catalysts. Such a unique catalytic environment enables CaPdH2, the very first complex transition-metal hydride catalyst, to afford a high alkene selectivity for the semihydrogenation of alkynes.

In situ formed Co from a Co-Mg-O solid solution synergizing with LiH for efficient ammonia synthesis.

A cobalt magnesium oxide solid solution (Co-Mg-O) supported LiH catalyst has been synthesized, in which LiH functions both as a strong reductant for the in situ formation of Co metal nanoparticles and a key active component for ammonia synthesis catalysis. Dispersion of the Co-LiH composite on the Co-Mg-O support results in a significantly higher ammonia synthesis rate under mild reaction conditions (19 mmol g-1 h-1 at 300 °C, 10 bar).