SiO 2 supported Ag-Au bimetallic catalysts were prepared by sol adsorption method with 10/90, 20/80, 33/67, and 50/50 Ag/Au molar ratios. Reduction of HAuCl 4 in Ag sol resulted in alloyed AgAu colloid particles and that structure remained after calcination and reduction treatment. The alloy structure of the catalysts was confirmed by UV-visible spectroscopy and high resolution transmission electron microscopy. The Au-Ag bimetallic effect and its dependence on the Ag/Au molar ratio was studied in glucose oxidation where synergistic activity increase was observed compared to the Au/SiO 2 reference sample in case of the bimetallic samples with less than Ag/Au=50/50 molar ratio. The Ag/SiO 2 was inactive at the same conditions. The Ag/Au surface atomic ratios -calculated by X-ray photoelectron spectroscopy (XPS) -were slightly higher than in the bulk -determined by prompt gamma activation analysis (PGAA). The higher activity of the bimetallic samples is suggested to be caused by the improved O 2 activating ability provided by Ag sites. The further increase of Ag loading above the optimal concentration may dilute or cover the Au to such an extent that the number of gold ensembles necessary for glucose activation decreases deteriorating the activity.

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Given the rising socioeconomic issues of fossil fuels, efficient artificial photosynthesis would be an important milestone toward a sustainable world. A key step of photosynthesis is the catalytic photooxidation of water by photosystem II, which has a mean lifetime of 30 min under full sunlight. Since the efficiency of photosystem II is controlled by redox-active tyrosine–histidine pairs that regulate the light-induced flow of charges, research has recently focused on the utilization of redox-active ligands in artificial systems. Here we review the molecular catalysis of water oxidation with emphasis on redox cooperation modes between ligands and metal centers. Molecular systems involving redox-active ligands could achieve up to 100% efficiency with respect to oxygen production, overpotential of 200–300 mV and turnover frequency above 100 s−1, which is comparable to the natural process. Nonetheless, molecular catalysts are often prone to degradation of the organic ligand. The oxidative activation of ligands can contribute to the water oxidation reactivity of a metal–ligand complex, or lead to controlled catalyst film formation. We discuss the design of functional analogs to the tyrosine–histidine pair that for the most part rely on abundant elements and exploit redox-active molecular moieties to assist the catalytic centers. We highlight analogies with the cooperation between the natural oxygen-evolving complex and the redox-active tyrosine–histidine pairs found in photosystem II.

Pincer ligands occupy three coplanar sites at metal centers and often support both stability and reactivity. The five-coordinate [FeIIICl2(tia-BAI)] complex (tia-BAI− = 1,3-bis(2’-thiazolylimino)isoindolinate(−)) was considered as a potential pre-catalyst for water oxidation providing the active form via the exchange of chloride ligands to water molecules. The tia-BAI− pincer ligand renders water-insolubility to the Fe–(tia-BAI) assembly, but it tolerates the presence of water in acetone and produces electrocatalytic current in cyclic voltammetry associated with molecular water oxidation catalysis. Upon addition of water to [FeIIICl2(tia-BAI)] in acetone the changes in the Fe3+/2+ redox transition and the UV-visible spectra could be associated with solvent-dependent equilibria between the aqua and chloride complex forms. Immobilization of the complex from methanol on indium-tin-oxide (ITO) electrode by means of drop-casting resulted in water oxidation catalysis in borate buffer. The O2 detected by gas chromatography upon electrolysis at pH 8.3 indicates >80% Faraday efficiency by a TON > 193. The investigation of the complex/ITO assembly by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS) before and after electrolysis, and re-dissolution tests suggest that an immobilized molecular catalyst is responsible for catalysis and de-activation occurs by depletion of the metal.

A molecular pre-catalyst complex, [CuII(indH)(OClO3)(NCCH3)](ClO4).CH3CN (1.CH3CN) with the 3N pincer ligand 1,3-bis(2'-pyridyl)iminoisoindoline (indH) was immobilized on indium tin oxide (ITO) transparent conducting substrate to generate O2 electrocatalytically for over 20...

SiO 2 -supported Au nanoparticles derived from sol were promoted with 0.04À7.4 wt % CeO 2 using two methods. The addition of Ce precursor was done directly to the Au sols before sol immobilization step (method A) or to the suspension of parent Au/SiO 2 (method B). Both preparation routes resulted in CeO 2 decoration of 1À3 nm over Au nanoparticles, which induced high CO oxidation activity. However, above 0.6 wt % CeO 2 content, the activity did not change significantly, but it greatly exceeded that of pure Au/CeO 2 used for reference. High-resolution transmission microscopy (HRTEM) showed that up to this concentration ceria patches are attached onto gold surface, and the further increase in Ce-loading caused CeO 2 spread over the support surface as well. Strong interaction of Ce species with stabilizer ligands located around Au is suggested as the reason for CeO 2 localization on gold.

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