Results and Discussion

Colloidal dispersions of platinum and gold were prepared by γ -radiolysis of mixtures of Na₂PtCl₆ and NaAuCl₄, in nitrogen-saturated water at pH7 containing Carbowax 20M (1 × 10-³ mol/dm³) (1, 2). The total metal concentration was fixed at 5 × 10^-4 mol/dm³ and the mol fraction of each metal was varied from 0 to 1. The actual composition of each colloidal dispersion, after treatment with ion-exchange resins, was determined by atomic absorption and the average particle sizes were established by high resolution transmission electron microscopy. X-ray emission spectroscopy carried out in conjunction with scanning transmission electron microscopy confirmed that individual particles contained both platinum and gold but their relative intensities, compared to the atomic absorption results, suggested segregation with surface enrichment of gold.

The platinum-gold colloids were used to catalyse hydrogen-evolution from a well-established sacrificial photosystem using zinc tetrakis(N-methyl-4-pyridinium)porphyrin as photosensitiser, methyl viologen (MV²⁺) as electron relay, and dihydronicotinamide adenine dinucleotide (reduced form) as sacrificial electron donor in nitrogen-saturated water at pH4 (7). Upon illumination of this photosystem with visible light, the reducing radical MV⁺ is formed in high yield and it persists for many hours in the absence of oxygen. Photolysis in the presence of a colloid ([metal] =2 × 10^-4 mol/dm³) resulted in evolution of hydrogen. As the mol fraction of gold increases, there is an increase in the rate of evolution of hydrogen (R_H2) until an optimum value is reached at about 20 to 30 per cent gold after which R_H2 decreases. There is also a progressive increase in the total yield of hydrogen on exhaustive photolysis ([H₂]) as the mol fraction of gold increases. Overall, the optimum composition for the catalyst appears to be about Pt_0,7Au_0.3.

The increased yields of hydrogen obtained with increasing mol fraction of gold are attributed to inhibition of hydrogenation of the reactants by surface gold atoms. With mol fractions of gold exceeding 20 per cent, hydrogen is not consumed upon prolonged irradiation although this is a serious problem for colloids containing little or no gold. Many of the platinum atoms on the colloid surface are prevented from functioning as active catalytic hydrogen evolving sites because of specific adsorption. The adsorbate may be H atoms, MV^{2 +}, MV⁺ or surfactant used to protect the colloid against flocculation. Incorporating gold atoms into the colloid surface dilutes platinum-platinum co-ordination sites where specific adsorption occurs, allowing surface platinum atoms to operate as active hydrogen sites.

In hydrogen-saturated aqueous solution at pH4 containing colloidal platinum (2 × 10^-4 mol/dm³) hydrogenation of MV⁺ occurs very slowly. The reducing radical MV^+-, however, is hydrogenated readily under ambient conditions and it is apparent that this reaction is responsible for consumption of hydrogen during photolysis. The rate of hydrogenation of MV⁺ (R_Mv) as catalysed by the various colloids, was measured by electrochemical methods. It was found that R_MV decreased significantly with increasing mol fraction of gold. The inhibition of hydrogenation of MV⁺ on the bimetallic particles is a consequence of the decreased mobility of H-atoms (8) and the increased energy of adsorption of H₂ (9, 10).

This study has shown that the use of bimetallic platinum-gold colloids can improve the rates and yields of hydrogen evolution from photosystems in which one or more of the reactants is susceptible towards hydrogenation. This arises because gold atoms at the surface of the colloidal particle inhibit specific adsorption and hydrogenation of the reactants. This behaviour, which is well known for macroscopic catalysts (11, 12), shows the advantages that may be gained by using alloys in place of pure metals.