Adsorption Properties of Hydrogen and Carbon Dioxide in Prussian Blue Analogues M₃[Co(CN)₆]₂, M = Co, Zn

Abstract

H2 and CO2 adsorption were studied in dehydrated Prussian blue analogues M3[Co(CN)6]2 (M = Co, Zn) using volumetric isotherm measurements. Both materials adsorbed 1.2−1.3 wt % of H2 at 77 K and 760 Torr with isosteric heats of adsorption ranging from 5.9 to 6.8 kJ/mol. High-pressure H2 isotherms at 77 K showed that Co3[Co(CN)6]2 started to saturate well above 6 atm with a saturation coverage of ∼1.9 wt %. These materials adsorbed approximately 17.6−19.7 wt % of CO2 at 253 K and 760 Torr with isosteric heats of adsorption of ∼25−28 kJ/mol. The CO2 saturation coverages from high-pressure isotherms at 263 K and 15 atm were ∼27.4−29.0 wt %. The displacement of CO2 by H2 in these compounds was investigated with Fourier transform infrared spectroscopy (FTIR). The FTIR experiments showed that CO2 physisorption at cryogenic temperatures produced an infrared peak at 2335 cm-1. Co-adsorption experiments revealed that H2 was able to displace preadsorbed CO2 if the PH2/PCO2 ratio was well above 100. The infrared results from the co-adsorption experiments also showed that H2 and CO2 competed for adsorption in the same pores under these conditions.