EFFECT OF PLATINUM LOADING ON THE PHOTOCATALYTIC ACTIVITY OF CADMIUM(II) SULFIDE PARTICLES SUSPENDED IN AQUEOUS AMINO-ACID SOLUTIONS

Loading of small amount of Pt by colloid deposition or platinum(IV) ox ide (PtO2) by mechanical mixing onto cadmium(II) sulfide (CdS) particl es inhibited the photocatalytic racemization of L-lysine (Lys) in aque ous suspension at ambient temperature under deaerated conditions, but promoted deamino-N-cyclization of Lys into pipecolinic acid (PCA). Hig h performance liquid chromatography and gas chromatography-mass spectr ometry analyses revealed that oxidation of Lys to release ammonia occu rs in parallel with the racemization and PCA formation. The critical a mount of the deposits for apparently complete retardation of the Lys r acemization depended on whether Pt or PtO2 was used; a smaller amount of Pt was necessary than of PtO2, suggesting superior contact of Pt pa rticles with the CdS surface. Transmission electron microscopy analyse s supported this. Electron spin resonance measurements of these cataly sts revealed that photoexcited electrons move to the Pt site and recom bine with positive holes in the absence of an appropriate substrate fo r oxidation. These findings led to the conclusion that Pt (or PtO2) de posits act as the reduction site for photoexcited electron in CdS and prohibit a reduction of a surface-adsorbed Lys derivative that is prod uced through the oxidation with a positive hole.