Synthesis of Zn _x Co_3−x O₄ spinels at low temperature and atmospheric pressure

In order to synthesize cobalt-based spinel oxides, air was bubbled through aqueous hydroxides suspensions of cobalt and zinc with atomic ratios of Co:Zn = 100:0, 85:15, 67:33 (precisely 2:1), 50:50, 30:70, 15:85, and 0:100 at 70 °C and atmospheric pressure. When cobalt was absent in the suspensions, hexagonal ZnO nanocrystals with ca. 82.2 nm size were observed, whereas cubic Co₃O₄ ones with ca. 52.7 nm were seen when zinc was not present. Zinc-contained cobalt spinel oxides, i.e., Zn _x Co_3−x O₄ (x = 0–1), were obtained when both hydroxides were present, e.g., spherical nanoparticles with ca. 107.6 and 85.0 nm diameters were observed for Co:Zn = 50:50 and 67:33, respectively. The lattice constant, a, for the cubic spinel increased with the increase in zinc atomic %, suggesting the increase in zinc concentration in the spinel. The Zn _x Co_3−x O₄ synthesized was normal spinel with a cubic crystal structure, whereas it was also suggested that a very small portion of zinc ion was incorporated into the octahedrally coordinated sites (inversion of spinel). BET surface area of the synthesized catalyst increased with increasing the cobalt atomic % except for Co:Zn = 67:33, for which local minimal surface area was obtained. Oxygen storage capacity of the catalyst was the largest for Co:Zn = 85:15 at 150 and 200 °C, whereas it was for 50:50 at 250 °C. After reducing the synthesized catalysts with hydrogen, metallic cobalt was formed on zinc oxide. CO chemisorption number on the cobalt was the largest for Co:Zn = 67:33, for which the smallest metallic cobalt diameter was also obtained. On the other hand, catalytic CO₂ hydrogenation activity and methane selectivity were the highest for Co:Zn = 50:50, suggesting that zinc oxide as a basic support played an important role in the hydrogenation of acidic CO₂. It was also shown that the catalytic hydrogenation activity and the methane selectivity were higher for the catalysts prepared by the present liquid-phase approach than for those prepared by conventional coprecipitation and impregnation methods, which were ascribed to larger surface area and number of active sites for the former preparation technique than for the latter two.