Active sites of silica-based quantum photocatalysts for non-oxidative reactions

Abstract

Various photocatalytic reactions have been developed in recent years. Among them, non-oxidative reactions by silica-based quantum photocatalysts are quite unique. Pure silica materials, especially mesoporous silica evacuated at high temperature, were found to promote olefin metathesis upon photoirradiation at room temperature. A kind of surface defect on silica was generated through dehydroxylation at high temperature before the photoreaction, which can be photoexcited to form the first surface intermediate. On the other hand, non-oxidative direct methane coupling was revealed to proceed on silica materials and silica-based quantum photocatalysts. Also in these cases, evacuation at high temperature was required for the activation of catalyst before the reaction. The active sites and feature of silica-based quantum photocatalysts for these non-oxidative reaction systems are summarized and discussed.

Chromium oxide loaded silica aerogels: Novel visible light photocatalytic materials for environmental remediation

“…Then depending of the availability of inter system crossing mechanisms, the excited electron could transfer to a triplet state yielding phosphorescence, which has been detected using a photoluminescence spectroscopic method. [6] We were able to observe similar emission fine structure which consist of intensity maxima corresponding to each vibrational energy level of the bond in the photoactive site, in our case (Cr -O). Note our solid state photoluminescence studies (Figure 8).…”

“…Thus, for our systems chromium was chosen as the doping agent for our more in-depth study of Cr-SiO 2 and mixed TiO 2 -SiO 2 samples. [5,6,7,8] Herein we report the observed UV and visible light activities of chromium ions loaded silica and titania based materials and possible mechanisms for the observed photocatalytic performances.…”

“…According to Yoshida and coworkers studies, when loded metal oxides are highly dispersed a photo driven excitation can occurs at the molecular orbital level at localized reaction sites [6] During the excitation process, an electron in the ground state of the M-O bond gets excited to an unoccupied singlet orbital. Then depending of the availability of inter system crossing mechanisms, the excited electron could transfer to a triplet state yielding phosphorescence, which has been detected using a photoluminescence spectroscopic method.…”

Semiconductor Photocatalysts for Non-oxidative Coupling, Dry Reforming and Steam Reforming of Methane

“…Therefore, they can promote even difficult reactions through the oxidative and reductive half reactions. On the other hand, highly dispersed photocatalysts are another type of the heterogeneous photocatalysts, which have been mentioned as single site photocatalysts, quantum photocatalysts or local excitation photocatalysts, with a unique structures and excitation states of the active sites [5,6]. In this type of photocatalysts, the active sites such as Ti cations are supported by insulating materials such as silica and zeolite, and the metal cation is coordinated by some oxygen anions on the surface of the support.…”

Photomethanation of Gaseous CO2 over Ru/Silicon Nanowire Catalysts with Visible and Near‐Infrared Photons

“…Since this initial study, numerous catalysts have been tested for photoactivated CO 2 3 , ZnO, and MgO and found that of these materials, only ZrO 2 and MgO exhibited photoactivity for the reduction of CO 2 to CO in a H 2 atmosphere. [ 20,21 ] Lo et al also demonstrated the photoreduction of CO 2 with H 2 over ZrO 2 in a circulating photocatalytic reactor. [ 22 ] More recently, CO 2 photoreduction to methanol in a hydrogen environment has been reported to occur over graphene oxide [ 23 ] and zinc-copper-gallium layered double hydroxide catalysts.…”