LiH mediates low-temperature ammonia synthesis
Nearly half of the world's population is fed by industrial N2 fixation, i. e., the Harbor-Bosch process. Although exergonic in nature, NH3 synthesis from N2 and H2 catalyzed by the fused Fe has to be conducted at elevated temperatures and high pressures. It consumes over 1 percent of the world's annual energy supply. Developing efficient catalysts that enable NH3 synthesis under mild conditions is a grand scientific challenge and is of great practical need.
The ideal catalyst for NH3 synthesis should have strong activation to N2 (small activation energy Ea) but relatively weak binding to the activated N species (small EN), which is, unfortunately, unattainable by transition metals (TM) themselves because of the linear scaling relations between Ea and EN, i.e., a transition metal catalyst having strong activation to N2 will have strong binding to the activated N, and vice versa. Such relations determine the rate of NH3 synthesis over the TM catalyst, and therefore, although tremendous research efforts have been applied, the industrial catalyst used today is essentially the same as the original one developed by Mittasch in 1909.
The Dalian Institute of Chemical Physics (DICP) research group led by Prof. CHEN Ping demonstrates, for the first time, that the scaling relations on catalytic NH3 synthesis can be broken. Thus, NH3 synthesis under mild reaction conditions can be achieved at an unprecedentedly high rate over a new set of catalysts.
The key element leading to this change is the employment of ionic hydride LiH. Distinctly different from proton or atomic H applied in biochemical, organometallic, and heterogeneous NH3 formation, H in LiH bears a negative charge that ensures that LiH is a strong reducing agent breaking the TM-N bond, and an immediate H source abstracting N to Li to form LiNH2. LiNH2 can further split H2, heterolytically giving off NH3 and regenerating LiH. Through this mechanism (See Figure a), the activation of N2 and the subsequent hydrogenation of N are carried out separately over the two reactive centers, i.e., TM and LiH, respectively, so that the direct influence of TM on the NH3 formation rate is broken.
Such a "relayed" two-active center catalysis enables the 3d TM(N)-LiH composites (3d TM spread from V to Ni) universal and unprecedentedly high NH3 synthesis activities. The DICP researchers found that, at 573 K, their activities are at least four (Cr-, Mn- and Co-LiH), three (V-LiH), two (Ni-LiH), and one (Fe-LiH) order of magnitude higher than the corresponding neat or supported TM(N) (See Fig. b and c), respectively.
Of equivalent importance are the superior low-temperature activities that have been achieved. The composites mentioned above perform extraordinarily well at lower temperatures, i.e., below 600 K (See Figure b). In particular, Fe-LiH and Co-LiH show constant activities of ca. 69 and 56 μmol g-1 h-1 at 423 K, respectively. Also notable is that the Cr-, Mn-, Fe- and Co-LiH composite catalysts outperform the Cs-promoted Ru catalyst, one of the most active NH3 synthesis catalysts, by two to three times at 573 K and 12-20 times at 523 K.
The dissociative activation of N2 on transition metals has long been regarded as the rate-determining step in NH3 synthesis. For TM-LiH composites, however, the rate-determining step is found to be the hydrogenation of LiNHx species, showing remarkable changes in the energetics of catalysis.
NH3 synthesis is regarded as a bellwether reaction in heterogeneous catalysis. The strategy in the circumvention of the scaling relations, the employment of ionic hydride in hydrogenation, and the abnormal behavior of early transition metal nitrides etc., shown in this work should be highly relevant to other catalytic processes.
The DICP group has been engaged in hydrogen storage over alkali hydrides, amides and imides for 14 years (Nature 2002). An accidental finding in the year 2009 stimulated the researchers to investigate the interaction of transition metals with those alkali compounds. The continuous research efforts over the years led them from hydrogen storage to NH3 decomposition and now to NH3 synthesis.