Understanding the degradation pathways of oxalic acid in different photocatalytic systems: Towards simultaneous photocatalytic hydrogen evolution

Abstract

The photocatalytic degradation of aqueous oxalic acid has been investigated employing different photocatalytic systems under constant pH conditions. A self-prepared TiO2 was utilized during the photocatalytic investigations in a bare and a platinized form. The synthesized pure anatase phase TiO2 had a 10 nm grain size and a BET surface area of ca. 121 m2 g−1 with relatively higher photocatalytic activity compared to the commercially available TiO2 photocatalyst UV100. Complete photocatalytic degradation of oxalic acid was observed within 60 min of illumination under aerobic condition with no by-product been detected. In the absence of molecular oxygen, a perceptible amount of formic acid was formed in the liquid phase, as determined quantitatively by means of ion chromatography. The formation of formic acid suggests that a photo-Kolbe reaction tacks place under oxygen-free conditions. The formation of formic acid was also noticed when platinized TiO2 (0.25 wt.%) was employed, together with an enhancement of the reactions photonic efficiency to the quadruple. A hydrogen evolution could only observe under oxygen-free condition with, again, a higher formation rate over the platinized material. However, upon complete photoreforming of oxalic acid the overall amount of the photocatalytically evolved hydrogen using Pt0.25%/TiO2 represented only 60% of the theoretical amount. Therefore, it is suggested that the source of H atoms could be the HC2O4− species rather than molecular oxalic acid. A detailed mechanism for the photocatalytic degradation of aqueous oxalic acid at different photocatalytic conditions is proposed and discussed.