Abstract
A mix-valenced cobalt oxide, CoOx, was prepared from cobalt nitrate aqueous solution through a precipitation with sodium hydroxide and an oxidation by hydrogen peroxide. Further, other pure cobalt oxide species were refined from the CoOx by temperature-programmed reduction (TPR) at 170, 230 and 300 °C (labeled as R-170, R-230 and R-300, respectively). They were characterized by X-ray (XRD), infrared (IR), thermogravimetry (TG) and TPR. The major composition of CoOx is CoO(OH), with a small amount of Co4+ species; R-170 is CoO(OH) with a hexagonal structure; R-230 is Co3O4 with a spinel structure and R-300 is CoO with a cubic structure. Their catalytic activities toward the CO oxidation were further studied in a continuous flow microreactor. The results indicated that the relative activity decreased significantly with the oxidation state of cobalt, i.e., CoO(+2)≳Co3O4(+8/3)≫CoO(OH)( +3)≳CoOx(>+3).
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