Studies On Effect Of States Of Colloidal Platinum On Performance Of 12-tungstosilicate For Photocatalytic Hydrogen Generation And 9-tungstosilicate Properties

Colloidal platinum nanoparticles, the co-catalyst for hydrogen evolution, were prepared using 12-tungstosilicate as photocatalyst by two methods in which photolysis and reduction reaction took place at the same time and at different time, respectively. For the first method, 12-tungstosilicate was first photoreduced to heteropoly blue anion, and then the blue anion could reduce H₂PtCl₆ to colloidal platinum under no irradiation. For the second method, 12-tungstosilicate and H₂PtCl₆ were mixed directly under irradiation. H₂PtCl₆ was reduced directly to colloidal platinum under irradiation by photoreduced in situ to heteropoly blue anion.The Pt particle size and reactivity of 12-tungstosilicate were characterized by UV-vis and cyclic voltammetry. Preparation methods affect markedly the Pt particle size and reactivity of 12-tungstosilicate so that the photocatalytic activity and stability for hydrogen evolution are different. The number of the colloidal Pt particles plays a role in H₂ evolution. The possible mechanism was discussed. The results show that for the first method, the nucleation and crystal growth could be completed with in a few seconds due to a large amount of heteropoly blue anion ([SiW₁₂O₄₀]^5-) as reducing agent, which restrained the particles growth effectively and the Pt particles were kept in small size. But for the second method, Pt particles were formed in the process of the interaction between H₂PtCl₆ and a small amount of [SiW₁₂O₄₀]^5- produced in situ so that the induction period for nucleation and growth period were prolonged. Therefore, the Pt particles of larger size were formed to decrease the number of the particles and lower the activity of photocatalytic H₂ evolution. At the same time, we found a new compound prepared in the process of discussing the possible mechanism for the second method.The new compound from the second method was characterized by element analysis, UV-vis, IR, XRD, DTA and Cyclic voltammetry techniques. The element analysis shows that the composition of the new compound is H₁₀SiW₉O₃₄ 4H₂O.The properties of the H₁₀SiW₉O₃₄ 4H₂O were compared with H₄SiW₁₂O₄₀ and α-H₁₀SiW₉O₃₄ 4H₂O.