Thesis advisor: Dunwei . Wang / C₁ chemistry, referring to CO₂ reduction and CH₄ conversion, is very important not only from environmental consideration, but also from chemical perspective. However, high activation barrier and poor product selectivity control hinder the development of both reactions. New approach like photo-assisted method is promising to solve these challenges. In addition, understanding the reaction mechanism and designing new catalysts could provide further insights and improve the performance. Specifically, I successfully demonstrated selective photoelectrochemical (PEC) CO₂ reduction to CO by Si photocathode and metal complex catalysts, resulting in low applied bias. Then I further studied how proton donor would affect the electrode-catalyst interaction by comparing two Re complex with almost identical structure, which further results in different product selectivity. Further, I studied PEC CH₄ oxidation by TiO₂, where photoexcited species participate in the chemical reactions. Selective CH₄ oxidation to CO was demonstrated under room temperature and ambient pressure. Detailed mechanistic investigation by experimental and theoretical efforts revealed a synergistic effect by adjacent Ti³⁺ for selective CO production. I also focused on direct CH₄ conversion to oxygenate by thermocatalysis. Single atom and dinuclear Ir center were created on silicon oxide (SBA-15), which showed great activity towards oxygenate production. Systematic experimental investigations implied a synergistic effect by adjacent Ir atom for acetic acid formation. / Thesis (PhD) — Boston College, 2019. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.