Photophysical Processes Related to Photoadsorption and Photocatalysis on Wide Band Gap Solids: A Review

Abstract

Abstract. During the last two decades, various pathways describing photoexcitation of small molecules' surface reactions at the wide band gap metal oxides and halides (E g > 3 eV) have been recognized. Photogeneration of excitons and free charge carriers may occur in bands of: i) fundamental absorption; ii) extrinsic and intrinsic defect absorption, including those related to surface states; and iii) in UV-induced color centers. Considerable red shifts relative to the fundamental absorption threshold of wide band gap solids have been observed for the spectral limits of surface photoreactions induced in extrinsic absorption bands. This allows thinking about the wide band gap solids as a potential competitors for the relatively narrow band gap photocatalysts. This review discusses the concept of surface photoadsorption (photocatalytic) center while differentiating active and inactive states of the center. Electronically excited defect, surface self-trapped or bound exciton, and the surface defect with trapped photo carrier are considered as the active states of photoadsorption (photocatalytic) centers of different types. The decay pathway of active state determines the lifetime of a photocatalytic center, and in this connection the Langmuir-Hinshelwood kinetic approach is discussed.

Validation of a two-dimensional modeling of an externally irradiated slurry photoreactor

“…In all of these studies the knowledge of the radiant field, i.e. the specific radiation intensity at each point of the reaction space and in all the possible directions of irradiation, is essential because the photon absorption rate by the catalyst is the main phenomenon for initiating the photoreaction [4]. The radiation intensity values inside an irradiated scattering medium can be evaluated by solving the radiative transfer equation (RTE), which expresses the radiation intensity balance at a fixed wavelength.…”

Kinetic and equilibrium investigation of Cu(II) removal by Co(II)-doped iron oxide nanoparticle-immobilized in PVA–alginate recyclable adsorbent under dark and photo condition

“…The photogenerated electron on the conduction bands would increase the surface negativity of the semiconductor catalyst [45]. In order to balance the charges, the electron on the semiconductor would initiate the electrostatic attraction to draw all the positive cations in the solution to it surface, which cause simultaneous adsorption of Cu(II) and H + formed after the degradation of organic scavengers [45,52]. According to Rajeshwar et al [53], after the adsorption process, whether the sorbent would be chosen to be reduced or not was very much dependent on its standard reduction potential.…”

“…The simple mechanism proposed is shown in Eqs. 6-8 [45]: where S is the inactive active site; S ⁄ is the activated active site promoted by hv; k p is the rate of photo-excitation constant; k d is the decay constant and k a is the photo adsorption constant. The proposed first order kinetic model is described in Eq.…”