selective photoelectrocatalytic oxidation of 5-(hydroxymethyl)-2-furaldehyde in water by Pt loaded nanotube structure of TiO2 electrodes

In this research, photoelectrocatalytic (PEC) synthesis of 2,5-furandicarbaldehyde (FDC) from 5-(hydroxymethyl)-2-furaldehyde (HMF) has been performed in water by using TiO2 films on Ti layer prepared by thermal annealing of Ti (Ti/TiO2) and TiO2 nanotube on Ti (Ti/TiO2NT) prepared by anodic oxidation as photoanodes. The latter ones showed better activity. In order to improve both activity and selectivity, Ti/TiO2NT was loaded with Pt by photoreduction method. 5-Formyl-2-furoic acid (FFA) was also produced by using Pt loaded Ti/TiO2NT photoanodes. The photoanodes were characterised by using XRD, SEM and XPS techniques. Allotropic phase of TiO2 photoanodes was mainly rutile, whereas trace amounts of anatase phase were found in some samples of TiO2 nanotube photoanodes. The distribution of TiO2 nanotubes on Ti are well organised. In addition, primary particle sizes of TiO2 were increased by increasing thermal treatment temperature; however primary particle sizes of TiO2 in Ti/TiO2NT are higher than that of other samples. XPS results showed that the loaded platiniums are in metallic form. Qualitative and quantitative analyses of the samples taken from reaction medium at fixed times were performed by HPLC analysis and the conversion and selectivity values were calculated. The photoanode preparation temperature, nanotube length, loaded Pt amount, bias voltage and medium pH were optimised for the reaction rate and product selectivity. The optimum thermal treatment temperature, applied voltage and pH are 500 °C, 0,5V and 5, respectively. The electrodes with longer nanotubes were more active than that of short ones, while selectivities showed opposite trend. Pt loading increased both activity and selectivity. Photocatalytic and electrocatalytic HMF oxidations were also performed for comparison and negligible activities were observed. Finally, efficient Pt loaded Ti/TiO2NT photoanodes were investigated for PEC selective and active FDC (selectivity up to 40%) and FFA (selectivity up to 19%) syntheses in environmental friendly conditions.