Gold loaded titanium dioxide–carbon nanotube composites as active photocatalysts for cyclohexane oxidation at ambient conditions

Abstract

Photocatalytic oxidation of neat cyclohexane (CHA) with H₂O₂ as an oxidant was carried out using gold modified versions of several types of materials, including titania nanotubes (Au/TNT), reduced graphene oxide (Au/RGO) as well as titania nanotubes–multiwalled carbon nanotubes composite (Au/TNT–MWCNT) under UV irradiation (125 W, λ > 296 nm). The synthesized nanoparticles were characterized using physical adsorption of nitrogen, X-ray diffraction, transmission electron microscopy and ultraviolet-visible diffuse reflectance spectroscopy, and the reaction products were analyzed by GC-MS. Both Au/RGO and Au/TNT–MWCNT catalysts promoted partial CHA oxidation with higher conversion (6–9.0%) and selectivity (60–75%) for cyclohexanone, exceeding Au/TNT, TNT–MWCNT and TNT catalysts (conv. 2.1–4%, sel. 32–55%). Au/TNT–MWCNT synthesized using hydrothermal deposition methods exhibited the highest catalytic activity. This was chiefly attributable to the high surface hydrophobicity of MWCNT that accelerated CHA adsorption, bonding of cyclohexanol and cyclohexanone to TNT as well as decomposition of H₂O₂ on gold nanoparticles. Increasing the surface area as well as decreasing the average particle size of Au⁰ to 15 nm of hexagonal shape contributed to the superior catalytic activity of Au/TNT–MWCNT, in achieving an average rate of 0.0035 mmol⁻¹ g⁻¹ min⁻¹ and conversion was 9.0% after 12 h of reaction. The latter catalyst exceeded industrially synthesized Co based catalysts (3.6%) operated at high temperatures. For confirming the autoxidation process, a radical scavenger offered a proof that the oxidation follows a radical-chain mechanism. The differences in surface morphology, light absorption and surface properties of Au/TNT when incorporated with MWCNT were well investigated. The photocatalytic oxidation mechanism elucidated using active scavengers suggested that OH˙ and O₂˙⁻ play key roles in the oxidation of CHA.