Salt-induced formation of hollow and mesoporous CoOx/SiO2 spheres and their catalytic behavior in toluene oxidation

Abstract

Hollow and mesoporous CoOx/SiO2 spheres (denoted as CoOx/hSiO(2) and CoOx/mSiO(2)) were synthesized via salt-assisted ultrasonic spray pyrolysis. Precursor solutions containing sodium silicate solution, a mineral acid (hydrochloric acid or nitric acid) and a cobalt salt were ultrasonically aerosolized and pyrolyzed. Results showed that CoOx/SiO2 spheres with hollow or mesoporous structure can be fabricated by using the NaCl and NaNO3 salts as in situ formed templates. Significantly, this approach avoids the need for post-calcination for template elimination, instead permitting aqueous removal with water. The influence of the sodium salts on the characteristics of CoOx/SiO2 spheres was investigated by means of XRD, nitrogen physisorption, SEM/TEM, ICP-MS, UV-Vis, XPS and H-2-TPR. On the basis of the experimental results, a possible mechanism on the formation of hollow and mesoporous spheres was proposed. The CoOx/SiO2 spheres were tested as catalysts for toluene oxidation. The mesoporous CoOx/mSiO(2) was found to exhibit superior activity to hollow CoOx/hSiO(2), probably attributed to a combination of several factors, including the predominant existence of a Co3O4 active phase, high surface Co3+ content, and easy reducibility of Co3+ at low temperature.