Effects of Alkaline Earth Metal Amides on Ru in Catalytic Ammonia Decomposition

Abstract

The effects of alkaline earth metal amides (Mg­(NH2)2, Ca­(NH2)2, and Ba­(NH2)2) on Ru in catalyzing NH3 decomposition were investigated. The catalytic activities rank in the order of Ru–Ba­(NH2)2 > Ru–Ca­(NH2)2 > Ru–Mg­(NH2)2, among which Ru–Ba­(NH2)2 and Ru–Ca­(NH2)2 catalysts have higher intrinsic activities (TOF) and lower apparent activation energies than those of Ru–Mg­(NH2)2 and Ru/MgO catalysts, indicating that Ca­(NH2)2 and Ba­(NH2)2 may have different roles from those of Mg­(NH2)2 and MgO. The TPR (temperature-programmed reaction) results show that Ca­(NH2)2 or Ba­(NH2)2 decomposes to N2 and H2 rather than NH3 in the presence of Ru. Ru may promote the NH x (x = 1, 2) coupling to H2 and N2 and change the decomposition pathways of Ca­(NH2)2 and Ba­(NH2)2. Kinetic analyses reveal that the Ru promoted NH x coupling to H2 and N2 may be the rate-determining step for catalytic ammonia decomposition. We suggest that the catalysis is very likely fulfilled via (1) Ru catalyzes the decomposition of amides to form H2, N2, and imides through an energy more favorable pathway and (2) imides react with NH3 to regenerate amides. The presence of Ca­(NH2)2 or Ba­(NH2)2 creates a NH x -rich environment, and Ru mediates the electron transfer from NH x to facilitate NH x coupling to N2 and H2.

Transition and Alkali Metal Complex Ternary Amides for Ammonia Synthesis and Decomposition

“…The decomposition of NH 3 over Rb 2 [Mn(NH 2 ) 4 ] and K 2 [Mn(NH 2 ) 4 ] starts at around 350 °C, which is close to the main appearance temperature of N 2 and H 2 in the Ar‐TPR spectra (Figure d). This ammonia decomposition temperature is similar to those of the noble metal catalysts for ammonia decomposition . At 500 °C, both amide systems show good NH 3 decomposition properties.…”

“…At this temperature, the NH 3 decomposition conversions of Rb 2 [Mn(NH 2 ) 4 ] and K 2 [Mn(NH 2 ) 4 ] reach 68 % and 48 %, respectively. These NH 3 decomposition conversions are much higher than that of MnN (15 %) and similar to that of Ru/MgO catalyst under the same reaction conditions. Similar phenomena were found in the previous results using composite catalysts composed of MnN and NaNH 2 /KNH 2 .…”

“…K 2 [Mn(NH 2 ) 4 ] was chosen based on its similarities with Rb 2 [Mn(NH 2 ) 4 ] (Figures S4 and S5). MnN is selected as a reference catalyst because it is stable under a certain concentration of ammonia, which is the precursor for NH 3 decomposition and the product of NH 3 synthesis . The decomposition of NH 3 over Rb 2 [Mn(NH 2 ) 4 ] and K 2 [Mn(NH 2 ) 4 ] starts at around 350 °C, which is close to the main appearance temperature of N 2 and H 2 in the Ar‐TPR spectra (Figure d).…”

Large Oblate Hemispheroidal Ruthenium Particles Supported on Calcium Amide as Efficient Catalysts for Ammonia Decomposition

“…These kinetic analyses suggest that ammonia decomposition over Ru/Ca(NH 2 ) 2 proceeds by the same reaction mechanism as with conventional Ru catalysts, that is, the Temkin–Pyzhev mechanism, in which the recombinative desorption of N adatoms from the metal surface dominates the ammonia decomposition rate. This result is significantly different from that of Yu et al., who reported that Ru/Ba(NH 2 ) 2 and Ru/Ca(NH 2 ) 2 have higher TOF and lower activation energy than Ru/MgO. Furthermore, they postulated a new reaction mechanism in which the decomposition of amides by the Ru catalyst to form N 2 and H 2 is the rate‐determining step of ammonia decomposition.…”

“…Furthermore, Guo et al . and Yu et al . reported ammonia decomposition by catalysts combining Ru particles and metal amides such as LiNH 2 –Ru/MgO and Ru/AE(NH 2 ) 2 (AE=Mg, Ca, Ba).…”