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, l > 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 GCMS.
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 mmol1 g1 min1 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 OHc and O2c
play key roles
in the oxidation of CHA. |
