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

Hollow and mesoporous CoO_x/SiO₂ spheres (denoted as CoO_x/hSiO₂ and CoO_x/mSiO₂) 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 CoO_x/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 CoO_x/SiO₂ spheres was investigated by means of XRD, nitrogen physisorption, SEM/TEM, ICP-MS, UV-Vis, XPS and H₂-TPR. On the basis of the experimental results, a possible mechanism on the formation of hollow and mesoporous spheres was proposed. The CoO_x/SiO₂ spheres were tested as catalysts for toluene oxidation. The mesoporous CoO_x/mSiO₂ was found to exhibit superior activity to hollow CoO_x/hSiO₂, 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.