Photocatalytic methanol assisted production of hydrogen with simultaneous degradation of methyl orange

Platinized TiO₂ prepared by photodeposition was evaluated for activity in the simultaneous conversion of methyl orange (MO), and methanol assisted formation of hydrogen. Low concentrations of MO were found ineffective for generation of hydrogen in measurable quantities upon illumination of Pt/TiO₂ in water. On the other hand, methanol induced hydrogen formation was significant. Surprisingly, when methyl orange was added to the methanol/water solution, hydrogen formation was significantly suppressed. The origin of this detrimental effect of methyl orange lies in the strong and preferred adsorption of the dye on the Pt sites of the catalyst, resulting in hydrogenation of the azo bond and suppression of the catalyzed formation of hydrogen. The hydrogenation of the azo bond is corroborated by dis-colorization of the solution and the observation of a mass fragment in LC-MS analysis corresponding to a hydrogenated product of MO (m/z = 172). Similar to hydrogen formation, dye dis-colorization is stimulated by the presence of methanol, without the formation of new chemical compounds, confirming the role of methanol as a hole scavenger in the photocatalytic processes. Finally the presence of oxygen (in lean conditions) delays dye hydrogenation and hydrogen formation, which we discuss is due to oxygen adsorption and formation of superoxide anions over the Pt sites (leading to oxidation of methanol), which is preferred over N=N bond hydrogenation, and proton reduction.