Studies of the optical properties and catalytic capabilities of noble metal nanoparticles (NPs), such as gold (Au) and silver (Ag), have formed the basis for the very recent fast expansion of the field of green photocatalysis: photocatalysis utilizing visible and ultraviolet light, a major part of the solar spectrum. The reason for this growth is the recognition that the localised surface plasmon resonance (LSPR) effect of Au NPs and Ag NPs can couple the light flux to the conduction electrons of metal NPs, and the excited electrons and enhanced electric fields in close proximity to the NPs can contribute to converting the solar energy to chemical energy by photon-driven photocatalytic reactions. Previously the LSPR effect of noble metal NPs was utilized almost exclusively to improve the performance of semiconductor photocatalysts (for example, TiO₂ and Ag halides), but recently, a conceptual breakthrough was made: studies on light driven reactions catalysed by NPs of Au or Ag on photocatalytically inactive supports (insulating solids with a very wide band gap) have demonstrated that these materials are a class of efficient photocatalysts working by mechanisms distinct from those of semiconducting photocatalysts. There are several reasons for the significant photocatalytic activity of Au and Ag NPs. (1) The conduction electrons of the particles gain the irradiation energy, resulting in high energy electrons at the NP surface which is desirable for activating molecules on the particles for chemical reactions. (2) In such a photocatalysis system, both light harvesting and the catalysing reaction take place on the nanoparticle, and so charge transfer between the NPs and support is not a prerequisite. (3) The density of the conduction electrons at the NP surface is much higher than that at the surface of any semiconductor, and these electrons can drive the reactions on the catalysts. (4) The metal NPs have much better affinity than semiconductors to many reactants, especially organic molecules. Recent progress in photocatalysis using Au and Ag NPs on insulator supports is reviewed. We focus on the mechanism differences between insulator and semiconductor-supported Au and Ag NPs when applied in photocatalytic processes, and the influence of important factors, light intensity and wavelength, in particular estimations of light irradiation contribution, by calculating the apparent activation energies of photo reactions and thermal reactions.

Direct photocatalytic CO₂reduction from primary sources, such as flue gas and air, into fuels, is highly desired, but the thermodynamically favored O₂reduction almost completely impedes this process. Herein, we report on the efficacy of a composite photocatalyst prepared by hyper-crosslinking porphyrin-based polymers on hollow TiO₂surface and subsequent coordinating with Pd(II). Such composite exhibits high resistance against O₂inhibition, leading to 12% conversion yield of CO₂from air after 2-h UV-visible light irradiation. In contrast, the CO₂reduction over Pd/TiO₂without the polymer is severely inhibited by the presence of O₂( ≥ 0.2 %). This study presents a feasible strategy, building Pd(II) sites into CO₂-adsorptive polymers on hollow TiO₂surface, for realizing CO₂reduction with H₂O in an aerobic environment by the high CO₂/O₂adsorption selectivity of polymers and efficient charge separation for CO₂reduction and H₂O oxidation on Pd(II) sites and hollow TiO₂, respectively.