Photo-Induced Electron Transfer Between a Reactant Molecule and Semiconductor Photocatalyst: In Situ Doping

Abstract

The possibility of the direct electron transition between the donor/acceptor level generated by adsorbed molecules and the conduction/valence band for photoilluminated semiconductor-type metal oxide is discussed. The effective wavelength is shifted to a longer wavelength by the formation of donor/acceptor level derived from adsorbed molecule (called here ''in situ doping''). This photo-activation mechanism by ''in situ doping'' gives us attractive ways for the removing the limit of band-gap energy, and the utilization of visible light.

Selective Catalytic Reduction of NO by NH3 over Photocatalysts (Photo‐SCR): Mechanistic Investigations and Developments

“…The action spectrum clearly showed the visible‐light response (Figure ), and the response to visible light was explained by a unique photo‐activation mechanism of NH 3 molecules: the direct electron transfer from the N 2p level of the adsorbed NH 3 to the conduction band of TiO 2 (in situ doping, Figure ). This mechanism was evidenced by the ESR spectroscopy and DFT calculations 24,25. The NO conversion was 52% at a GHSV of 4,000 h −1 , and N 2 O byproducts were not detected (i.e., N 2 selectivity: 100%).…”

Reaction Mechanism of Selective Photooxidation of Amines over Niobium Oxide: Visible-Light-Induced Electron Transfer between Adsorbed Amine and Nb2O5

“…The major difference in the photooxidation of amines and alcohols is whether dimerization occurs or not. We previously proposed a similar pathway (direct electron excitation from an adsorbed molecule to the conduction band of Nb 2 O 5 ) in photoactivation of ammonia molecule over TiO 2 . − In this case, a TiO 2 –NH 2 surface complex (dissociatively adsorbed ammonia on TiO 2 ) can absorb visible light (>390 nm) to generate an amide radical and an excited electron in the conduction band of TiO 2 . However, both interband excitation and direct electron transfer are involved, in contrast to the present study.…”

“…This direct excitation from a new donor level to the conduction band of the photocatalyst can be categorized as a ligand-to-metal charge transfer (LMCT) transition in a broad sense, which has been reported in TiO 2 -based systems, for example, sulfoxidation of alkanes, activation of ammonia, − oxidation of alcohols − and amines, and other adsorbate-induced photocatalyses. , According to these systems, it can be assumed that amine oxidation over Nb 2 O 5 occurs through similar chemistry to that of alcohol oxidation. However, the detailed reaction mechanism of amine oxidation is still unclear.…”

Partial Oxidation of Alcohols on Visible-Light-Responsive WO3 Photocatalysts Loaded with Palladium Oxide Cocatalyst

“…On the other hand, a considerable portion of the holes generated on TiO 2 photocatalysts react directly with benzene molecules, enhancing undesirable peroxidations, including cleavage of the aromatic ring . These findings motivated us to apply the WO 3 photocatalysts to other synthetic reactions initiated by • OH radicals, such as the oxidation of alcohols. − Partial oxidation of primary and secondary alcohols to the corresponding aldehydes and ketones, respectively, are important processes in the chemical industry, and therefore the development of efficient catalytic or photocatalytic systems has been extensively studied to achieve highly selective reactions. − In the present study, a visible-light-responsive WO 3 photocatalyst was applied to the partial oxidation of alcohols in water using molecular O 2 as the oxidant. The oxidation of 2-propanol was first examined in detail as a model reaction to understand the properties of WO 3 photocatalysts in comparison with those of TiO 2 , before the oxidation of other alcohols was also examined to evaluate the feasibility and scope of this procedure.…”