Energetics of CO₂ adsorption on mg-al layered double hydroxides and related mixed metal oxides

Energetics of CO₂ uptake by layered double hydroxides (LDH) of Mg and Al and their corresponding mixed metal oxides (MMO) of two different compositions (Mg/Al = 2:1 and 3:1) were investigated by gas adsorption calorimetry. The initial adsorption enthalpies for all the LDH and MMO are similar and in the range -90 to -120 kJ/mol, indicating strong chemisorption of CO₂. However, the differential adsorption enthalpy varies substantially with increasing coverage for different samples. LDH prepared by coprecipitation (both Mg/Al = 2 and 3) and their corresponding MMO exhibit similar CO₂ uptake, which are in the working sorption capacity range (0.6-1 mmol/g), but the one prepared by urea hydrolysis shows poor sorption capacity (0.02-0.35 mmol/g). An integral enthalpy of adsorption of -54 kJ/mol was observed for [Mg-Al-CO₃] LDH prepared by urea hydrolysis and -59 kJ/mol and -57 kJ/mol for the ones prepared by coprecipitation with composition 3:1 and 2:1, respectively. Attenuated total reflectance spectroscopy measurements of samples after CO₂ uptake aid in identifying the mode of binding of adsorbed carbonate, which gives information about strength of basic sites. The presence of monodentate carbonate in all samples suggests that strong basic sites are available in LDH and MMO, in agreement with the large exothermic initial adsorption enthalpies observed, indicating strong chemisorption.