Comprehensive Kinetic and Mechanistic Analysis of TiO₂ Photocatalytic Reactions According to the Direct–Indirect Model: (II) Experimental Validation

Abstract

As a continuation of the Direct–Indirect (D-I) model theoretical approach presented in Part I of this publication, concerning the photocatalytic oxidation of organic molecules in contact with TiO2 dispersions, a comparative photooxidation kinetic analysis of three model organic molecules, benzene (BZ) dissolved in acetonitrile (ACN), phenol (PhOH) dissolved in either water or acetonitrile, and formic acid (FA) dissolved in water, is presented to test the applicability of the D-I model under both equilibrium and nonequilibrium adsorption–desorption conditions. A previous analysis involving diffuse reflectance ultraviolet–visible (DRUVS) and Fourier transform infrared (FTIR) spectroscopy, combined with adsorption isotherm plots, shows that BZ chemisorption on the TiO2 surface is not allowed, physisorption being in this case the only possible adsorption mode. In line with D-I model predictions, BZ photooxidation is observed to take place via an adiabatic indirect transfer (IT) mechanism, with the participation of photogenerated terminal −Os •– radicals as oxidizing agents. In contrast, because of their strong chemisorption, FA species dissolved in water are found to be mainly photooxidized via inelastic direct transfer (DT) trapping of photogenerated valence-band free holes (h f +). Finally, when dissolved in water, PhOH chemisorption is not favored because of the strong electronic affinity of water molecules with the TiO2 surface, while chemisorption strength considerably increases when PhOH is dissolved in ACN, as far as the electronic interaction of solvent molecules with the TiO2 surface is negligible. Consequently, as predicted by the D-I model, PhOH dissolved in water is photooxidized via a combination of IT and DT mechanisms, the IT photooxidation rate (v ox IT) being about 1 order of magnitude higher than DT photooxidation rate (v ox DT). In contrast, when ACN is used as solvent, v ox IT remains practically unchanged, while v ox DT increases by about 2 orders of magnitude. These photooxidation results sustain the central D-I model hypothesis that the degree of substrate species interaction with the TiO2 surface is a decisive factor determining the kinetics of photocatalytic reactions. The effect of adsorption–desorption equilibrium rupture on the photooxidation kinetics of dissolved substrate species, predicted by the D-I model, is analyzed for the first time from experimental kinetic data concerning the photooxidation of PhOH dissolved in water under high enough illumination intensity (ρ ≈ 1017 cm–2 s–1).

Photocatalysis: Basic Principles, Diverse Forms of Implementations and Emerging Scientific Opportunities

“…It was discovered that different interactions of TiO 2 with formic acid, benzene, and phenol resulted in different oxidation mechanisms of these molecules. [105] For instance, formic acid strongly chemisorbed on TiO 2 in water (Figure 14a), resulting in direct photo oxidation by trapping photogenerated valence band holes, hence the direct transfer (DT) mechanism. In contrast, chemisorption of benzene on TiO 2 surface in acetonitrile was not favored.…”

Photocatalytic and photoelectrochemical oxidation mechanisms of methanol on TiO2 in aqueous solution

“…Following the early work by Fujishima and Honda [1] reporting the photolysis of water on TiO 2 electrodes employing an external bias potential, and the suggestion that surface defect states may play a crucial role in the decomposition of water into H 2 and O 2 , many research efforts have been devoted to the understanding of the fundamental processes in TiO 2 photocatalysis and photoelectrochemistry [2][3][4][5][6][7][8][9][10]. It is generally accepted that, when TiO 2 is exposed to light the energy of which exceeds its band gap energy, electrons are excited from the valence band into the conduction band.…”

The Relevance of ATR-FTIR Spectroscopy in Semiconductor Photocatalysis

“…It is worth to note that TiO 2 nanoparticles are much more extensively used as substrates as compared with other metal oxides. The adsorption of organic compounds bearing common functional groups such as acids [23,45,46], amino acids [47], phenolic compounds [11,48], and a few complex heteroaromatic compounds [49][50][51][52] has been studied in detail (cf. Table 3).…”

“…This is due to the fact that the reactivity and the pathways for product formation are determined by the structures of the formed surface species during the dark adsorption. Recently, Montoya et al [11] have investigated the interaction of the TiO 2 surface with three probe molecules, e.g., formic acid, benzene, and phenol employing ATR-FTIR spectroscopy. Based upon the analysis of the IR spectra ( Figure 10), assumptions have been made concerning the physisorption of benzene (no changes have been observed in the spectra with and without the TiO 2 layer), the strong chemisorption of formic acid, and also the role of the solvent (water or acetonitrile) for the adsorption mode of phenol.…”

“…Surface science plays a prominent role in mechanistic investigations concerning the photocatalytic process, providing a unique approach to understand bulk, surface, and interfacial phenomena occurring at the TiO 2 surface [6,8,9]. According to several proposed photocatalytic mechanisms [10,11], an important point for the conversion of the molecules on TiO 2 is the physical and electronic structure of the adsorbed state of the molecules. How a molecule binds onto the TiO 2 surface influences its electronic structure, as well as its redox properties.…”