The effect of Pt on the photocatalytic degradation pathway of methylene blue over TiO2 under ambient conditions

Photocatalytic degradation pathway of methylene blue (MB) has been studied over TiO2, 0.5 wt.% Pt/TiO2, and 3 wt.% Pt/TiO2 at ambient conditions (30 °C and 1 atm of air) by infrared (IR) spectroscopy. The reaction was proposed to be initiated via the abstraction of H from MB molecule by hydroxyl radical (·OH), followed by –CH3 elimination and CAr–N scission. The correlation in IR intensity between the decrease in C–H bond in MB molecule and the increase in hydroxyl group (–OH) at 3672 cm−1 during the reaction (i) provides an indirect evidence to support the proposed ·OH-initiating pathway, (ii) suggests that the –OH sites could be related to OH generation sites, and (iii) offers new insights into the photoinduced hydrophilicity of the TiO2 surface. Subsequent breakup of the MB central ring via accepting protons and photogenerated electrons resulted in the formation of intermediates containing CO, COO−, and N–H groups. Deposition of 0.5 wt.% Pt to TiO2 enhanced (i) the scission of and C–H bonds and (ii) the formation of intermediates containing CO and COO− groups. The latter suggests that Pt provided the sites for oxygen absorption, accelerating the formation rate of oxygen-containing intermediates. The comparable IR intensity of –OH at 3632 cm−1 before reaction and –OH at 3672 cm−1 during the reaction on TiO2 and 0.5 wt.% Pt/TiO2 indicates that OH generation sites were not blocked by low Pt loading (i.e., 0.5 wt.%). The simultaneous presence of OH generation, photoelectron generation, and Pt sites could play a synergetic role in enhancing and C–H bond scission and formation of CO and COO− species. Increasing Pt loading to 3 wt.% resulted in the total elimination of –OH and the significant decrease in the MB degradation rate, indicating that the low activity of 3 wt.% Pt/TiO2 could be attributed to the blocking of OH generation sites by the high Pt loading.