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

• Published in: Applied catalysis. B, Environmental Vol. 83; no. 3-4; pp. 277 - 285
• Main Authors: Yu, Zhiqiang, Chuang, Steven S.C.
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 23.09.2008; Elsevier
• Subjects: Catalysis; Chemistry; Exact sciences and technology; General and physical chemistry; Hydroxyl group; Infrared spectroscopy; Photocatalytic degradation; Photochemistry; Photogenerated electron; Physical chemistry of induced reactions (with radiations, particles and ultrasonics); Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry; Titanium dioxide; Hydroxyl group; Photocatalytic degradation; Infrared spectroscopy; Titanium dioxide; Photogenerated electron; Photoelectron; Binary compound; Correlation; Support; Scission; Degradation; Proton; Deposition; Oxygen; Photocatalysis; Transition element compounds; Blocking; Air; Methylene blue; Hydrophily; Infrared spectrometry; Heterogeneous catalysis; Titanium oxide; Hydroxyl; Electrons; Environmental protection
• ISSN: 0926-3373; 1873-3883
• DOI: 10.1016/j.apcatb.2008.01.040

Photocatalytic degradation pathway of methylene blue (MB) has been studied over TiO2, 0.5wt.% Pt/TiO2, and 3wt.% Pt/TiO2 at ambient conditions (30°C and 1atm 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 3672cm−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 0.5wt.% 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 3632cm−1 before reaction and –OH at 3672cm−1 during the reaction on TiO2 and 0.5wt.% Pt/TiO2 indicates that OH generation sites were not blocked by low Pt loading (i.e., 0.5wt.%). 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 3wt.% resulted in the total elimination of –OH and the significant decrease in the MB degradation rate, indicating that the low activity of 3wt.% Pt/TiO2 could be attributed to the blocking of OH generation sites by the high Pt loading.