Effect of Ligand Modification on the Mechanism of Electrocatalytic Hydrogen Production by Ni(pyridinethiolate)3- Derivatives.

The effects of ligand modification on the catalytic mechanism of hydrogen production by Ni(PyS)3- derivatives, made with electron-withdrawing and -donating substitutions to the pyridinethiolate (PyS)- ligands, are studied experimentally and computationally using density functional theory. Thermodynamic data, spin density maps, and frontier molecular orbital diagrams were generated for reaction intermediates. Comparison of computed values for E0 and p Ka with experimental values supports the proposed mechanisms. The rate of electrochemical hydrogen production is correlated with the effect of ligand modification. Notably, the presence of an electron-donating substituent favors an alternative mechanism for hydrogen production. Computationally it was determined that the electron-donating substituent causes deviation from the original chemical-electrochemical-chemical-electrochemical (CECE) mechanism of Ni(PyS)3- to a CCEE mechanism, while the CECE mechanism is maintained for all catalysts substituted with electron-withdrawing groups.

Photocatalytic water reduction using a polymer coated carbon quantum dot sensitizer and a nickel nanoparticle catalyst.

Hydrogen gas is produced photocatalytically using 470 nm light, PVP-coated carbon quantum dots (CQDs) as the photosensitizer, and nickel nanoparticles (NiNPs) as the catalyst. The effect of the amount of polyvinylpyrrolidone (PVP) on the ability of the CQD/NiNP composites to catalyze proton reduction was studied. A maximum of 330 mmols H2/g CQD is produced using 68 µg ml-1 of CQDs and 6 µg ml-1 of NiNPs, with activity persisting for 4 h when 20 wt%-PVP-coated CQDs were used. The H2 production quantum yield under these conditions is 6%. It was found that composites having higher weight percent PVP had decreased rates of H2 production, but increased duration. Increasing the weight percent of PVP coating also increases the fluorescence quantum yield of CQDs. Fluorescence quenching titrations reveal that H2 production could occur by either a reductive or oxidative quenching mechanism. The nanomaterials, prepared using simple methods, are used as the photosensitizer and catalyst in the proton reduction system that operates using visible light.