Nearly half of the world population is fed by the 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 % of world annual energy supply. This is of the grand scientific challenge and practical need to develop efficient catalysts enabling NH3 synthesis under mild condition.
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 metal (TM) itself 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 determines the rate of NH3 synthesis over TM catalyst, and therefore, although tremendous research endeavors have been devoted, the industrial catalyst applied now-a-days is essentially the same as the original one developed by Mittasch in 1909.
Dalian Institute of Chemical Physics (DICP) 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 condition can be achieved at 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 negative charge that ensures LiH a strong reducing agent breaking the TM-N bonding, 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 NH3 formation rate is broken.
Figure Mechanistic proposal for the relayed two-active center catalysis of the TM-LiH system (a) and the catalytic performances of 3d TM-LiH composite catalysts (b and c).(Image by Guo Jianping and CHEN Ping)
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) orders of magnitude higher than the corresponding neat or supported TM(N) (See Fig. b and c), respectively.
Of equivalent importance is the superior low-temperature activities that have been achieved. The composites abovementioned perform extraordinarily well at lower temperatures, i.e., below 600 K (See Figure b). In particular, the Fe-LiH and Co-LiH shows constant activities of ca. 69 and 56 μmol g-1 h-1 at 423 K, respectively. Also worthy of highlighting 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 2-3 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 the 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.
The DICP group has been engaged in hydrogen storage over alkali hydrides, amides and imides for 14 years (Nature 2002). An accidental finding in year 2009 stimulated them to investigate the interaction of transition metals with those alkali compounds. The continuous research efforts over the years led them step-by-step from hydrogen storage to NH3 decomposition and now to NH3 synthesis.
This work is financially supported by National Science Funds for Distinguished Young Scholars (51225206), the Collaborative Innovation Center of Chemistry for Energy Materials (2011-iChEM) and Dalian Institute of Chemical Physics (DICP DMTO201504).(Text by CHEN Ping/Image by GUO Jianping and CHEN Ping)
Dr. LU Xinyi
Dalian Institute of Chemical Physics, Chinese Academy of Sciences,
457 Zhongshan Road, Dalian, 116023, China,