Influence of Co/Fe molar ratio on hydrotalcite catalysts prepared with or without microwave

Two series of mixed cobalt-iron oxides were synthesized by the conventional hydrotalcite route, without (CoxFe2-HT) or with microwave irradiations (CoxFe2-MW). Different molar ratios Co2+/Fe3+ were also investigated with 2 ≤ x ​≤ ​8. X-ray diffraction (XRD) analysis confirmed the formation of the desired hydrotalcite phase for all the materials before calcination whatever the molar ratio Co2+/Fe3+ is. After calcination of the latter materials at 500 ​°C, XRD evidenced the complete destruction of the hydrotalcite phase and, consequently, the formation of mixed oxides of cobalt and iron. After being characterized, the oxides materials were tested towards the total oxidation of the volatile organic compound (VOC) propene. Both the molar ratio and the preparation method have a significant effect on the catalytic properties. The light-off curves of CoxFe2-HT500 materials showed that the catalytic activity is improved for a molar ratio Co2+/Fe3+ > 2. Co6Fe2-HT500, with the ratio Co2+/Fe3+ equal to 3, showed the best catalytic performance. Furthermore, except for Co8Fe2-MW500, the materials that were subjected to microwave irradiations gave higher catalytic activities compared to those of the homologous ones prepared without such irradiations. Co6Fe2-MW500 was found to be the most catalytically active among all the materials of both series due to the higher specific surface area, the higher amount and easier reducibility of active species, higher proportion of mobile oxygen and of Co2+ on the surface. Furthermore, this catalyst did not show tendency to deactivation and coking even after 100 ​h under stream.