Surface Treatment for Conductive 12 CaO⋅7 Al₂O₃ Electride Powder by Rapid Thermal Annealing Processing and Its Application to Ammonia Synthesis

Abstract

The inorganic electride [Ca24Al28O64]4+(e−)4 (C12A7:e−) has unique properties, that is, chemical stability and a low work function comparable to that of metal K. However, its surface area is low (≈1 m2 g−1) because sintering occurs during high‐temperature annealing, which is needed to remove CaO layers formed by reaction with metal Ca that works as a reductant. We report a simple synthesis method for moderate‐surface‐area C12A7:e− (9–19 m2 g−1) by using a rapid thermal annealing (RTA) technique. The synthesis consists of 1) high‐temperature evacuation, 2) reaction with Ca metal, and 3) RTA. The influence of these synthesis conditions was first studied to achieve both high surface area and high electron concentration. Next, C12A7:e− samples were examined as Ru catalyst supports for NH3 synthesis. The activity of Ru‐loaded as‐prepared C12A7:e− was moderate but was increased fivefold by RTA processing prior to Ru loading as a result of surface structure reconstruction. The improvement in the Ru dispersion degree that results from the increased surface area trebled the overall activity (3550 μmol NH 3 gcat−1 h−1 at 340 °C) compared with that of conventional Ru/C12A7:e−.

Atmospheric‐pressure nonthermal plasma synthesis of ammonia over ruthenium catalysts

“…Furthermore, they reported that the effect of promoters follows the order of promoter IPs, with lower IPs resulting in greater effects as promoters (Cs > K> Na) . A similar mechanism was also confirmed for NH 3 synthesis over a Ru/electride catalyst (12CaO 7 Al 2 O 3 ) …”

The influence of pre-treatment gas mixture upon the ammonia synthesis activity of Co–Re catalysts

“…and 437 μmol g −1 h −1 for Re/MCM-41 [14] have been reported for conditions comparable to those applied in this study. In the case of Ru dispersed on electride, a rate of 3550 μmol g cat −1 h −1 has been reported at atmospheric pressure and 340°C [3]. Comparing the data in Table 1, a degree of influence of pretreatment gas can be seen.…”

“…This is the basis of the KBR Advanced Ammonia Process which is operated at lower pressure than in the Haber Bosch counterpart [2]. Investigations into Ru catalysed ammonia synthesis are on-going with very recent literature documenting very high activity for electride supported Ru catalysts [3]. Additional literature has reported that certain nitrides exhibit high activity, with Co 3 Mo 3 N based systems being particularly noteworthy [4][5][6], although high activity has also been documented for other systems such as Ni 2 Mo 3 N [4] which could offer advantages in terms of ease of nitridation [7].…”

Effect of pretreatment on ceria-supported cobalt catalyst for ammonia synthesis

“…So far, various transition metal catalysts such as Fe, Ru, Re and Co have been developed for this reaction. [3][4][5][6][7][8][9] Among them, Fe-based and Ru-based catalysts have been extensively used in ammonia plants all over the world. However, iron catalyst is kinetically sensitive to the changes in ammonia concentration, and the catalytic activity decreased greatly with increasing ammonia concentration of the reactant gas.…”