Nature Of Photocatalysis Research Articles

Triazine-Carbazole-Based Covalent Organic Frameworks as Efficient Heterogeneous Photocatalysts for the Oxidation of N-aryltetrahydroisoquinolines.

Covalent organic frameworks (COFs) have attracted growing interests as new material platform for a range of applications. In this study, a triazine-carbazole-based covalent organic framework (COF-TCZ) was designed as highly porous material with conjugated donor-acceptor networks, and feasibly synthesized by the Schiff condensation of 4,4',4''-(1,3,5-triazine-2,4,6-triyl)tr ianiline (TAPB) and 9-(4-formylphenyl)-9H-carbazole-3,6-dicarbaldehyde (CZTA) under the solvothermal condition. Considering the effect of linkage, the imine-linked COF-TCZ was further oxidized to obtain an amide-linked covalent organic framework (COF-TCZ-O). The as-synthesized COFs show high crystallinity, good thermal and chemical stability, and excellent photoactive properties. Two π-conjugated triazine-carbazole-based COFs with tunable linkages are beneficial for light-harvesting capacity and charge separation efficiency, which are empolyed as photocatalysts for the oxidation reaction of N-aryltetrahydroisoquinoline. The COFs catalyst systems exhibit the outstanding photocatalytic performance with high conversion, photostability and recyclability. Photoelectrochemical tests were employed to examine the behavior of photogenerated charge carriers in photo-illumination system. The control experiments provide further insights into the nature of photocatalysis. In addition, the current research also provided a valuable approach for developing photofunctional COFs to meet challenge in achieving the great potential of COFs materials in organic conversion.

New Insights into the Fundamental Principle of Semiconductor Photocatalysis.

Although photocatalysis has been studied for many years as an attractive way to resolve energy and environmental problems, its principle still remains unclear. Some confusions and misunderstandings exist in photocatalytic studies. This research aims to elaborate some new thoughts on the fundamental principle of semiconductor photocatalysis. Starting from the basic laws of thermodynamics, we first defined the thermodynamic potential of photocatalysis. A concept, the Gibbs potential landscape, was thus then proposed to describe the kinetics of photocatalysis. Photocatalysis is therefore defined as a light-driven chemical reaction that still needs heat activation, in that light and heat play their different roles and interact with each other. Photocatalysis should feature an activation energy functioning with both light and heat. The roles of light and heat are correlative and mutually inhibit at both levels of thermodynamics and kinetics, so it is impossible for an intrinsic light–heat synergism to happen. Two criteria were further proposed to determine an intrinsic light–heat synergism in photocatalysis. Experiments were also carried out to calculate the thermodynamic potential and can agree well with the theory. Experimental results proved that there is no intrinsic light–heat synergism, in accordance with our theoretical prediction. This research clarified some misunderstandings and gained some new insights into the nature of photocatalysis; this is important for the discipline of semiconductor photocatalysis.

Turnovers and photocatalysis: A mathematical description

In previous articles we dwelled on the usage of relative photonic efficiencies [N. Serpone, G. Suave, R. Koch, H. Tahiri, P. Pichat, P. Piccinini, E. Pelizzetti, H. Hidaka, J. Photochem. Photobiol. A: Chem. 94 (1996) 191; N. Serpone, J. Photochem. Photobiol. A: Chem. 104 (1997) 1] and quantum yields Φ [N. Serpone, R. Terziaw, D. Lawless, P. Kennepohl, G. Suave, J. Photochem. Photobiol. A: Chem. 73 (1993) 11]. Recently, we also provided an experimental protocol to measure Φ in heterogeneous media [N. Serpone, A. Salinaro, Pure Appl. Chem. 71 (1999) 303] to infer which of several photocatalyzed processes might be the more significant and efficient process. In this article we revisit photocatalysis and discuss how to describe mathematically (photo)catalytic activity and how to compare (photo)catalytic activities of various materials. Specifically, we address the usage and provide a kinetic description of the three turnover quantities: turnover number (TON), turnover rate (TOR) and turnover frequency (TOF) as they bear on the (photo)catalytic activity of a given material in heterogeneous solid/liquid or solid/gas (photo)catalysis. We argue that these turnover quantities are conceptually distinct. TON and TOR require knowledge of the number of active sites on the (photo)catalyst’s surface, contrary to the requirement to determine TOF. Most significant, these turnovers also depend on the nature of the active state of the catalyst, and hence on how the active centers are described. This goes back to the differences in the nature of photocatalysis and photoinduced catalysis.