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Ammonia synthesis reaction under time interrupted conditions

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2019
Aslan, Mustafa Yasin
The objective of this thesis study is to demonstrate a solution for a sustainable ammonia production process based on the unsteady state operating conditions under milder temperatures and atmospheric pressure. In this framework, the elimination of the ammonia synthesis catalyst deactivation was investigated to improve the rates under milder operating conditions. The hydrogen adsorption /desorption characteristics over SiO2 and Vulcan supported Ru catalysts with different Ru metal loadings were investigated. It was shown that Ru metal dispersion decreased with increasing Ru metal loading. In addition, It was observed that Vulcan support Ru catalysts accommodated higher amounts of hydrogen compared to SiO2 supported Ru catalysts. It was demonstrated that dissociated hydrogen over Ru metal migrated from Ru metal surface to support surface and higher temperatures were needed to desorbed the spilled over hydrogen from the support surface. The inhibition effect of ammonia was conducted in the context of the study. Ammonia synthesis reaction experiments were performed with zeolite-Y, hydroxyapatite (HAp), and Vulcan supported Ru catalysts at 300 – 400 °C and atmospheric pressure. It was observed that ammonia synthesis catalyst was poisoned and deactivated by synthesized ammonia within 1 h., regardless of surface acidities of the supports. N2 pulses were used to diminish the poisoning effect of ammonia. It was demonstrated that under pulsed flow conditions, the inhibition effect of ammonia was eliminated. In the final part of the study, Co3Mo3N as a next generation ammonia synthesis catalyst was investigated. This catalyst operates through a different mechanism, by involving lattice nitrogen in the process. Ammonia synthesis reaction experiments were performed with different H2:N2 ratios between 0.05 and 3.0. It was observed that ammonia synthesis rate did not change between H2:N2 ratio of 3.0 and 0.5. Besides, ammonia synthesis rate decreased with decreasing H2:N2 ratio below 0.5. On the other hand, N2 pulses was also applied to the ammonia synthesis reaction over Co3Mo3N, but no improvement was obtained. As a result, it was demonstrated that ammonia synthesis reaction over Co3Mo3N catalysts can be carried out with similar rates using lower hydrogen amount (H2:N2=0.5:1) compared to stoichiometric H2:N2 ratio of 3:1 under steady flow conditions.