Combined Theoretical and Experimental Investigation of Hydrogen Production Catalysis

• Main Author: Virca, Carolyn Noelle
• Format: Others
• Published: PDXScholar 2018
• Subjects: Chemistry
• Online Access: https://pdxscholar.library.pdx.edu/open_access_etds/4384; https://pdxscholar.library.pdx.edu/cgi/viewcontent.cgi?article=5429&context=open_access_etds

The widespread use of solar energy has been limited in part by the issue of effective storage. Water splitting is a means of converting solar energy into chemical fuel, in the form of hydrogen gas. It can be performed through both photochemical and electrochemical processes, via the two half reactions of water oxidation and proton reduction. Electrochemically, the catalyst receives electrons from an external circuit, which can be coupled to a renewable source such as a solar cell. Photochemically, a light-absorbing molecule provides an excited electron to a catalyst, which either generates oxygen or hydrogen gas. The work described herein studies both photochemical and electrochemical proton reduction using two different systems made of inexpensive transition metal and organic components. Nickel pyridine 2-thiolate (Ni(PyS)3-) (PyS=pyridinthiolate) has been demonstrated to have good stability and activity as a proton reduction catalyst. It is applied here as an electrocatalyst, due to the wealth of mechanistic information that can be obtained through electrochemical experiments. In our study of Ni(PyS)3-, a previously proposed catalytic pathway was first supported through Density Functional Theory (DFT) computations. Thermodynamic properties of the molecular nickel compound were investigated through analysis of free energy changes along various reaction coordinates, spin states, localization of charge and geometry of the intermediates and transition states. An experimental and theoretical investigation of the effects of ligand modification on hydrogen production and the catalytic mechanisms was then undertaken. Six derivatives of Ni(PyS)3- were synthesized through uniform ligand modification to all three PyS- ligands using a series of electron rich or poor substituents. The physical properties of interest were investigated experimentally through electrochemical methods and UV-vis absorbance spectroscopy. Specifically, the desired properties for a hydrogen production catalyst are high proton affinity, quantified through the pKa, and low overpotential, quantified through E0. Each compound was also studied in depth using computational modeling of the various possible catalytic pathways. By combining the results of computational study with experimental results, mechanistic insight could be gained. Electron poor catalysts maintained the normal mechanism undergone by the unmodified Ni(PyS)3- catalyst, but these compounds also yielded low electrochemical hydrogen production rates. The highest rate of hydrogen production was noted for the most electron rich catalyst, which was found to proceed through a unique mechanism. Ultimately, its unique mechanism was attributed to favorable changes to its physical properties. The same joint theoretical and experimental methodology has been used to study the effect of non-uniform ligand modification. Four heteroleptic compounds were selected for study, two containing electron poor ligands and two containing electron rich ligands in varied ratios. It was hypothesized that an appropriate ligand combination could target the desired properties necessary to increase hydrogen production rates. Catalysts were designed keeping in mind that more electron rich ligands were found to correlate with higher proton affinity, while electron poor ligands were found to promote lower overpotentials. By making heteroleptic compounds, these two physical properties are targeted independently from one another. What is found is that not only do the electronics of each ligand influence physical properties, but the placement of each ligand matters as well. Due to the unique structural features of heteroleptic compounds, the effectiveness of hydrogen production varies greatly. Thorough computational and experiential analysis provides insight into the underlying factors that govern these variations. Finally, photochemical hydrogen production was performed using polyvinylpyrrolidone (PVP)-coated carbon quantum dots (CQDs) as a photosensitizer, and nickel nanoparticles (NiNPs) as a catalyst. Total hydrogen production by CQD/NiNP composites as a function of the amount of PVP coating was investigated as well as various mechanistic and photophysical properties. Hydrogen production was studied using a custom-made photoreactor and a quadrupole mass analyzer. Both fluorescence quantum yields and hydrogen production quantum yields were determined using a phosphoremeter with an integrating sphere. The mechanism of hydrogen production was probed using fluorescence spectroscopy as well. Finally, an investigation into whether or not CQDs are capable of performing upconversion, as has been previously noted in the literature, was undertaken. The fluorescence quantum yield of the CQDs was found to increase along with increased addition of PVP coating. It was also noted that composites with more PVP had decreased rates of hydrogen production, but it was sustained over a longer period of time. Duel hydrogen production mechanisms were also found to be possible.