Warm hydrogen direct adsorptive separation and purification with highly CO/H2S-tolerant rare earth alloys

Abstract

Direct H2 separation using hydrogen adsorption materials has been gaining research attention for several decades. Rare-earth-metal-based alloys have proved to be feasible hydrogen adsorbents; however, their applications are restricted because of the presence of poisonous impurities, especially CO. Herein, we demonstrate the utilization of fluorinated LaNi5-based materials in elevated temperature pressure swing adsorption (ET-PSA) method for H2 separation is feasible. The adsorbent, fluorinated LaNi4.3Al0.7, maintained satisfactory working capacity at 453 K in poisonous mixed gas, which contained up to 30% CO or 0.5% H2S. During the breakthrough tests performed on a fixed bed reactor packed with, clean separations of H2/CO2+CO+N2+CH4 were observed when the feed gas component was N2/CH4/CO2/CO/H2 (45/10/10/5/30), indicating hydrogen was efficiently separated from the mixed gas in only one step, thereby simplifying the H2 separation process and the device. The mechanism of poisoning of LaNi5 by CO, and the effect of the working temperature on the CO resistance of LaNi5, were investigated using the density functional theory and ab initio molecular dynamics simulation, whose outcomes showed that the presence of CO significantly reduces the adsorption energy of H atoms. Nevertheless, at elevated temperatures (over 453 K), CO molecules was easier to escape from LaNi5 surface and less likely to completely cover the adsorbent surface, resulting in more possibility of H adsorbing on LaNi5 surface.