Photocatalytic reactions involving titanium dioxide (TiO2) are governed by complex kinetic and mechanistic pathways that determine reaction efficiency and selectivity. Understanding these processes requires analysis of surface reaction kinetics, identification of intermediate species, and elucidation of the roles of reactive oxygen species. The Langmuir-Hinshelwood (L-H) kinetic model is widely applied to describe the surface-mediated reactions occurring on TiO2, where adsorption and surface reactions are critical.
The L-H model assumes that photocatalytic degradation occurs when both the organic pollutant and hydroxyl radicals (•OH) are adsorbed on the TiO2 surface. The rate of reaction (r) is expressed as:
r = (k K C) / (1 + K C)
where k is the intrinsic rate constant, K is the adsorption equilibrium constant, and C is the concentration of the reactant. At low concentrations, the equation simplifies to a pseudo-first-order reaction, while at high concentrations, it approaches zero-order kinetics due to surface saturation. Deviations from L-H kinetics may occur if diffusion limitations or competing side reactions influence the process.
The rate-determining step in TiO2 photocatalysis often involves charge carrier dynamics. Upon UV irradiation, electron-hole pairs (e−/h+) are generated in TiO2. The holes (h+) oxidize surface hydroxyl groups or adsorbed water to produce •OH radicals, while electrons reduce adsorbed oxygen to form superoxide radicals (O2•−). The recombination of e−/h+ pairs competes with these redox processes, reducing photocatalytic efficiency. Studies indicate that the trapping of charge carriers by surface defects or dopants can suppress recombination, enhancing reaction rates.
Reactive oxygen species (ROS) play pivotal roles in photocatalytic degradation. Hydroxyl radicals (•OH) are highly oxidizing, capable of non-selectively degrading organic pollutants via hydrogen abstraction or electrophilic addition. Superoxide radicals (O2•−) contribute indirectly by forming hydrogen peroxide (H2O2), which further decomposes into •OH. The relative contribution of each ROS depends on reaction conditions such as pH, oxygen availability, and TiO2 crystal phase. For instance, anatase TiO2 exhibits higher •OH production than rutile due to its more favorable conduction band position.
Intermediate identification is crucial for mechanistic studies. Electron spin resonance (ESR) spectroscopy detects paramagnetic species such as •OH and O2•− using spin-trapping agents like 5,5-dimethyl-1-pyrroline-N-oxide (DMPO). ESR studies confirm the presence of DMPO-OH and DMPO-OOH adducts, providing direct evidence of ROS generation. Fourier-transform infrared spectroscopy (FTIR) monitors surface-bound intermediates by identifying characteristic vibrational modes. For example, carboxylate species (COO−) and hydroxylated fragments often appear during pollutant degradation, indicating progressive oxidation steps.
Additional techniques include mass spectrometry (MS) for tracking gaseous intermediates and photoluminescence spectroscopy for probing charge carrier dynamics. Transient absorption spectroscopy reveals short-lived reactive species, while electrochemical methods quantify interfacial charge transfer rates. Combining these methods allows reconstruction of reaction pathways, such as the sequential oxidation of methanol to formaldehyde, formic acid, and finally CO2.
The influence of surface properties on reaction kinetics cannot be understated. TiO2 facets exhibit varying reactivities; the (001) facet of anatase shows higher activity for •OH generation than (101) due to its higher surface energy. Surface hydroxylation also affects adsorption and ROS production, with hydrated surfaces favoring •OH formation. Defects such as oxygen vacancies alter electronic structure, creating trapping sites that modify charge transfer kinetics.
Quantitative studies reveal that under optimized conditions, the apparent rate constants for pollutant degradation range from 0.01 to 0.1 min−1, depending on substrate complexity and TiO2 properties. The activation energy for photocatalytic oxidation typically falls between 10–40 kJ/mol, suggesting surface reactions rather than diffusion control the process.
In summary, TiO2 photocatalysis follows L-H kinetics under ideal conditions, with ROS-mediated oxidation driving pollutant degradation. Mechanistic studies employing ESR, FTIR, and complementary techniques elucidate intermediate formation and charge carrier dynamics. Surface properties and ROS generation efficiency dictate overall reaction rates, providing a framework for optimizing photocatalytic performance. Future advancements in in-situ spectroscopic methods will further refine kinetic models and mechanistic understanding.