We report on the size dependence of the melting temperature of silica-encapsulated gold nanoparticles. The melting point was determined using differential thermal analysis (DTA) coupled to thermal gravimetric analysis (TGA) techniques. The small gold particles, with sizes ranging from 1.5 to 20 nm, were synthesized using radiolytic and chemical reduction procedures and then coated with porous silica shells to isolate the particles from one another. The resulting silica-encapsulated gold particles show clear melting endotherms in the DTA scan with no accompanying weight loss of the material in the TGA examination. The silica shell acts as a nanocrucible for the melting gold with little effect on the melting temperature itself, even though the analytical procedure destroys the particles once they melt. Phenomenological thermodynamic predictions of the size dependence of the melting point of gold agree with the experimental observation. Implications of these observations to the self-diffusion coefficient of gold in the nanoparticles are discussed, especially as they relate to the spontaneous alloying of core-shell bimetallic particles.

Several cobalt porphyrins (CoP) have been reduced by radiation chemical, photochemical, and electrochemical methods, in aqueous and organic solvents. In aqueous solutions, the CoIP state is stable at high pH but is shorter lived in neutral and acidic solutions. Stable CoIP is also observed in organic solvents and is unreactive toward CO2. One-electron reduction of CoIP leads to formation of a species that is observed as a transient intermediate by pulse radiolysis in aqueous solutions and as a stable product following reduction by Na in tetrahydrofuran solutions. The spectrum of this species is not the characteristic spectrum of a metalloporphyrin π-radical anion and is ascribed to Co0P. This species binds and reduces CO2. Catalytic formation of CO and HCO2 - is confirmed by photochemical experiments in acetonitrile solutions containing triethylamine as a reductive quencher. Catalytic reduction of CO2 is also confirmed by cyclic voltammetry in acetonitrile and butyronitrile solutions and is shown to occur at the potential at which CoIP is reduced to Co0P. As compared with CoTPP, fluorinated derivatives are reduced, and catalyze CO2 reduction, at less negative potentials.

Iron and cobalt porphyrins (FeP and CoP) are utilized as electron-transfer mediators to effect photochemical reduction of CO 2 in homogeneous solutions. The species that activate and reduce CO 2 are the Fe 0 P and Co 0 P formed by reduction of the starting materials. Reduction of the metalloporphyrins (MP) is achieved by photolysis in dimethylformamide or acetonitrile solutions containing triethylamine (TEA) as a reductive quencher. The photoreduction is efficient for the M III P f M II P stage and probably occurs by an intramolecular electron transfer from an axially bound TEA. However, TEA does not bind to the reduced metal complexes, and the quantum efficiency is much lower for the subsequent reduction steps. Considerably higher quantum yields are obtained by adding p-terphenyl (TP) as a sensitizer. TP is very effectively photoreduced by TEA to form the radical anion, TP •-, which has a sufficiently negative reduction potential to reduce Co I P and Fe I P rapidly to their M 0 P state. The rate constants for these reactions, determined by pulse radiolysis, are found to be nearly diffusion-controlled. The quantum yield for the reduction of M II P to M I P and for reduction of CO 2 to CO are increased by more than an order of magnitude in the presence of TP. Side reactions involve hydrogenation of the porphyrin ring and production of H 2 . The hydrogenated porphyrins also catalyze reduction of CO 2 , but the photochemical production of CO eventually stops. This limit on catalytic activity is due to destruction of the porphyrin macrocycle and accumulation of CO. CO can bind strongly to Fe II P and to Fe I P but not to Fe 0 P, as demonstrated by electrochemical measurements and by optical spectra of the species produced by sodium reduction in tetrahydrofuran in the presence and absence of CO. Although binding of CO to Fe II P and Fe I P should not interfere with the formation of Fe 0 P, the active catalyst, the potential for reduction of Fe I P to Fe 0 P becomes more negative. However, CO probably binds to the hydrogenated products thereby inhibiting the catalytic process.

The role of cobalt and iron phthalocyanines in catalytic CO 2 reduction has been studied. Chemical, photochemical, and radiolytic reductions of the metal phthalocyanines (Pc) have been carried out in organic solvents, and reduction of their tetrasulfonated derivatives (TSPc) in aqueous solutions. Co II Pc and Fe II Pc are readily reduced to [Co I Pc]and [Fe I Pc] -, which do not react with CO 2 . Reduction of [Co I Pc]yields a product which is characterized as the radical anion, [Co I Pc •-] 2-, on the basis of its absorption spectra in the visible and IR regions. This species is stable under dry anaerobic conditions and reacts rapidly with CO 2 . Catalytic formation of CO and formate is confirmed by photochemical experiments in DMF and acetonitrile solutions containing triethylamine (TEA) as a reductive quencher. The photochemical yields are greatly enhanced by the addition of p-terphenyl (TP). The radical anion, TP •-, formed from the reductive quenching of the singlet excited state with TEA, reduces the phthalocyanines very rapidly. The rate constants for reduction of Co II Pc, [Co I Pc] -, and [Fe I Pc]by TP •-, determined by pulse radiolysis in DMF solutions, are nearly diffusioncontrolled. The mono-reduced species formed from [Co I Pc]is unstable under the pulse radiolysis conditions but is longer-lived under the flash photolysis conditions. The interaction of this species with CO 2 is either too weak or too slow to detect in the current experiments, where a competing reaction with protons predominates.