Photocatalytic Desulfurization of Thiophene with Chevrel Phase Ni₂Mo₆S₈ Synthesized by SHS

The Chevrel phase compounds Cu₄Mo₆S₈, Ni₂Mo₆S₈, and Fe₂Mo₆S₈, synthesized by self-propagating high temperature synthesis, were evaluated as photocatalysts for visible light photocatalytic desulfurization. Investigations began with reflectance measurements from which absorbance spectra were calculated using the Kubelka-Munk transformation. The absorbance data was then used to create Tauc plots to find the direct and indirect bandgaps of the Chevrel phase compounds. Bandgaps were found to be no more than the 1.74 eV for Ni₂Mo₆S₈, so it was selected for further study because this bandgap suggests it will use the sun's emission spectrum better than the other materials studied here. Photocatalytic desulfurization experiments studied the concentration of thiophene mixed into n-octane with and without exposure to Ni₂Mo₆S₈ with and without light exposure because of the relative difficulty of removing thiophenes from liquid fossil fuels by the industry standard hydrodesulfurization process. Ultraviolet-visible spectroscopy was used to analyze chemical changes in the thiophene-octane solution. Spectroscopic results demonstrate that the thiophene was effectively removed by exposure to Ni₂Mo₆S₈ and visible light together but not by exposure to Ni₂Mo₆S₈ or visible light alone. Multiple tests with the same Ni₂Mo₆S₈ sample demonstrate that the material is reusable as a catalyst for photocatalytic desulfurization. The proposed mechanism results in the release of SO species, which may be controlled and captured if separated from fossil fuels in bulk during industrial processing in contrast to their uncontrolled release as vehicle fuel exhaust. Controlled generation and collection of these species can allow for further processing into elemental sulfur in the same way that H₂S released by standard hydrodesulfurization is processed into elemental S.