Hydrophilic platinum‐polyoxotungstate colloidal particles formed by a self‐assembly of Cs4[{Pt(OH)2}2·SiW12O40] were successfully obtained by calcining a cesium salt of α‐Keggin‐type diplatinum(II)‐coordinated silicotungstate, Cs4[α‐SiW11O39{cis‐PtII(NH3)2}2]·11H2O (Cs‐Si‐Pt), in air at 300 °C. During the thermal treatment, the structure of the mono‐lacunary Keggin‐type silicotungstate ligand {SiW11O39} transformed to that of {SiW12O40}, and the diplatinum sites in Cs‐Si‐Pt transformed to the platinum hydroxides with the elimination of the ammonia molecules coordinated to the platinum sites. All four cesium ions in Cs4[{Pt(OH)2}2·SiW12O40] could be ion‐exchanged by four tetramethylammonium ions, and hydrophilic [(CH3)4N]4[{Pt(OH)2}2·SiW12O40] colloidal particles were also formed in aqueous solution. The calcined sample acted as a photocatalyst for hydrogen evolution from aqueous triethanolamine (TEOA) under visible light irradiation (λ = ≥440 nm) in the presence of Eosin Y, K5[α‐SiW11{Al(OH2)}O39]·7H2O, and titanium dioxide. Although relatively rapid deactivation was observed, the turnover frequency was 3588 h–1 after 20 min, and the turnover number exceeded 4600 after 3 h of light irradiation.
The tetramethylammonium salts of diplatinum(II) complexes composed of monolacunary Keggin-type silico and germanotungstates, [(CH 3 ) of TMA-Si-Pt and TMA-Ge-Pt showed signals assigned to the two sets of NH 3 ligands coordinated to the platinum sites. These three platinum compounds showed hydrogen evolution from aqueous triethanolamine solution under visible light irradiation (λ ≥ 400 nm) in the presence of eosin Y, α-Keggin mono-aluminum-substituted silicotungstate, and titanium dioxide. The catalytic activities were influenced by the central atoms, and TMA-P-Pt showed the highest activities among the three platinum compounds.
Two tungstates containing platinum nanoparticles (Pt Npts) were obtained by air-calcining α-Keggin-type diplatinum(II)-coordinated polyoxotungstates, Cs3[α-PW11O39{cis-Pt(NH3)2}2]⋅8H2O (Cs-P-Pt) and Cs4[α-SiW11O39{cis-Pt(NH3)2}2]⋅11H2O (Cs-Si-Pt), at 700–900 °C for 5 h. The polyoxotungstate Cs-P-Pt was transformed to a mixture of Pt Npts and Cs3PW12O40 upon calcination, while the Cs-Si-Pt structures were transformed to Pt Npts and Cs4W11O35. The Pt Npts generated by air-calcining Cs-P-Pt at 700 °C for 5 h were uniform with an average particle size of 3.6 ± 1.1 nm, which was much smaller than that of the Pt Npts obtained by calcining Cs-Si-Pt (19.9 ± 9.9 nm) under identical conditions. This demonstrated the significant inhibitory effect of Cs-P-Pt on aggregation during high-temperature air-calcination at a high platinum content (10.6 wt.%) and in the absence of a support. During calcination at 700–900 °C, Cs-P-Pt exhibited higher activities than Cs-Si-Pt with respect to hydrogen evolution from aqueous triethanolamine solutions under visible light irradiation in the presence of Eosin Y, α-Keggin-type mono-aluminum-substituted polyoxotungstate, and titanium dioxide. When Cs-P-Pt was calcined at 800 °C for 100 h, no decrease in activity was observed in comparison with that upon calcination for 5 h.
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