CO2 Capture by Na2TeO4 : Structure of Na2– xHxTeO4 and Kinetic Aspects
Résumé
β-Na2TeO4 is able to trap CO2 in a humid atmosphere due to a partial Na+/H+ exchange and the formation of NaHCO3. The RT powder X-ray diffraction pattern of the resulting Na2–xHxTeO4 shows broad and narrow hkl lines preventing the structural study. We show by the DIFFaX program that Na+/H+ exchange is topotactic since the structure, as in the mother form, consists of [TeO4]n2n– chains of TeO6 octahedra. We also show that the broadening of some hkl lines is due to stacking faults which result from the weakness of H–O···H bonds connecting the [TeO4]n2n– chains. Upon heating, a progressive structural organization takes place which has been followed by powder X-ray diffraction, Raman, and NMR spectroscopies. Around 300 °C, a well organized structure can be described from powder X-ray diffraction refinements in the monoclinic P21/n space group while ab initio computations allowed location of the hydrogen atoms with satisfactory H–O bonds. In addition, we present the CO2 sorption/desorption by Na2TeO4 and compare its performance to that of Na2SiO3. Finally, the existence of a Na2–xLixTeO4 solid solution (0 ≤ x ≤ 0.9) is evidenced, and we show that the presence of lithium in the structure leads to the disappearance of the structural transition observed for β-Na2TeO4 and to a progressive decrease of the CO2 capture ability.