Photoreactions occurring on metal-oxide surfaces are not all photocatalytic - Description of criteria and conditions for processes to be photocatalytic

Studies involving the TiO2-assisted photodegradation of organic substances report that the processes are photocatalytic in nature. Yet, no evidence exists confirming such assertions. Previously, we examined the usage of relative photonic efficiencies [N. Serpone, G. Sauve, 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 Phi empty set [N. Serpone, R. Terzian, D. Lawless, P. Kennepohl, G. Sauve, J. Photochem. Photobiol. A: Chem. 73 (1993) 11] to systematize discrepant claims about process efficiencies. An experimental protocol is now available [N. Serpone, A. Salinaro, Pure Appl. Chem. 71 (1999) 303] to measure true Phi empty set in heterogeneous media. Photoinduced reduction of O-2 and photooxidation of H-2 occurring on oxidized and reduced surfaces of ZrO2 were recently examined [A.V. Emeline, G.N. Kuzmin, L.L. Basov, N. Serpone, J. Photochem. Photobiol. A: Chem. 174 (2005) 214] to probe the spectral variations of the photoactivity and photo-selectivity of ZrO2 by determining Phi empty set for the two redox reactions at various wavelengths of photoexcitation (200 < lambda < 400 nm). Irradiation of ZrO2 in the intrinsic absorption region (lambda < 260 nm) led predominantly to photoreduction of O-2, whereas photooxidation of H-2 predominated on irradiation in the extrinsic spectral region (260 < lambda < 360 nm). A difficult task in heterogeneous catalysis and photocatalysis is determination of the actual number of active sites, an issue that has heretofore been elusive to assess the (photo)catalytic activity of a given material in heterogeneous solid-liquid and solid-gas (photo)catalysis. A kinetic description of the three turnover quantities, viz., turnover number (TON), turnover rate (TOR) and turnover frequency (TOF) has been described [N. Serpone, A. Salinaro, A.V. Emetine, V.K. Ryabchuk, J. Photochem. Photobiol. A: Chem. 130 (2000) 83], concluding that turnover quantities are conceptually distinct, with TON and TOR requiring knowledge of the number of active sites on the (photo)catalyst's surface. Apparently, turnovers depend on the nature of the active state of the catalyst and how it is described. The number of surface-active sites on the ZrO2 particle surface have been determined quantitatively (similar to 10(16) active centers) through thermoprogrammed desorption spectroscopy, affording an estimate of TONs for the photooxidation of H-2 (TON > 14.5) and photoreduction of O-2 (TON > 6.6) on ZrO2 and demonstrating for the first time that a photoreaction occurring on the surface of a metal oxide is truly photocatalytic [A.V Emeline, A. V. Panasuk, N. Sheremetyeva, N. Serpone, J. Phys. Chem. B 109 (2005) 2785]. Photocoloration of a metal oxide such as ZrO2 (process of photoinduced formation of Zr3+, F- and V-type color centers) during a surface photochemical reaction was also used to assess whether a reaction is photocatalytic. Our study on the influence of simple photoreactions involving the photoreduction of O-2, photooxidation of H-2, photooxidation of H-2 by adsorbed O-2, and photoinduced transformation of NH3 and CO2 on the photocoloration of ZrO2 concluded that photoreactions involving NH3 and CO2 arenon-photocatalytic processes, in contrast to the photooxidation of H-2 which is photocatalytic [A. V. Emeline, G.V. Kataeva, A.V. Panasuk, V.K. Ryabchuk, N.V. Sheremetyeva, N. Serpone, J. Phys. Chem. B 109 (2005) 5175]. In this article, we describe the criteria and conditions by which a photoreaction taking place on the surface of a solid can be said to be photocatalytic by considering both a chemical approach and a physical approach. (c) 2007 Elsevier B.V. All rights reserved.