As a catalyst, single-atom platinum may provide an ideal structure for platinum minimization. The single-atom catalyst of platinum supported on titanium nitride nanoparticles were successfully prepared with an aid of chlorine ligands. Unlike platinum nanoparticles, the single-atom active sites predominantly produced hydrogen peroxide in the electrochemical oxygen reduction with the highest mass activity reported so far. The electrocatalytic oxidation of small organic molecules, such as formic acid and methanol, also exhibited unique selectivity on the single-atom platinum catalyst. A lack of platinum ensemble sites changed the reaction pathway for oxygen reduction reaction toward two electrons pathway and formic acid oxidation toward direct dehydrogenation, and also induced no activity for methanol oxidation. The single-atom platinum showed high mass activity and unique selectivity for the electrochemical reactions. Additionally, the role of the support may have a significant impact on the catalytic properties similar to that of the ligand molecules in homogeneous catalysts. The support effect was demonstrated by preparing the single-atom platinum catalyst on two different supports, titanium carbide and titanium nitride. The formation of single-atom Pt was confirmed by STEM, EXAFS, and in-situ IR spectroscopy. Pt1/TiC showed higher activity, selectivity, and stability for electrochemical H2O2 production than Pt1/TiN. Density functional theory calculation presented that oxygen species have strong affinity into Pt1/TiN possibly acting as surface poisoning species, and Pt1/TiC preserves oxygen-oxygen bond more with higher selectivity towards H2O2 production. The support in single-atom catalysts actively participates in the surface reaction, not just acting as anchoring sites for single-atoms. The Pt single-atom electrocatalyst was also prepared on Sb-doped SnO2 (Pt1/ATO), and it was used to fabricate direct formic acid fuel cell.