Thermochemical CO_2 splitting reaction with supported La_x_A(1-x)Fe_yB_(1-y)O_3 (A = Sr, Ce, B = Co, Mn; 0≤x, y≤1) perovskite oxides

摘要

An efficient redox material for two-step thermochemical CO_2 splitting reaction requires high chemical yield at relatively low reduction temperature. Herein, the oxides with perovskite structure of formula La_x_A(1-x)Fe_yB_(1-y)O_3 (A = Sr, Ce, B = Co, Mn; 0 ≤ x, y ≤ 1) start to release O_2 at 800 ℃ and the largest O_2 production is 11.8 ml/g_(perovskite) at 1300 ℃. However, for these unsupported La_x_A(1-x)Fe_yB_(1-y)O_3 materials, the CO production is low in spite of high reduction yield. ZrO_2, Al_2O_3 and SiO_2 are thus considered as supports to disperse La_x_A(1-x)Fe_yB_(1-y)O_3 materials and different supports induce great differences in the reaction activity. By A-site or B-site substitution of LaFeO_3, the O_2 releasing temperature has fallen from 1230 ℃ to 800-950 ℃ and the CO production is enhanced 2-3 times. LaFe_(0.7)Co_(0.3-)O_3 (25 wt%)/SiO_2 shows the highest reaction activity among these investigated materials with the O_2 production of 4.0 ml/g_(material) (16.0 ml/g_(perovskite) and CO production of 7.6 ml/g_(material) (30.4 ml/g_(perovskite) when it is reduced at 1300 ℃ and re-oxidized at 1100 ℃, and the activity is relatively stable even after 10 cycles of the reaction. By contrast, the CO production is 4.5 ml/g for CeO_2 when it is reduced at 1400 ℃. The estimated activation energy for the reduction step of LaFeo.7Co_(0.3)O_3 (25 wt%)/SiO_2 is around 89-149 KJ/ mol according to different models. The CO generation step of LaFe_(0.7)Co_(0.3)O_3 (25 wt%)/SiO_2 is mainly controlled by bulk diffusion (D1) at 1000 ℃ and then it turns to first order surface reaction (F1) at 1100 ℃.