Depending on the dispersion degree and electronic states, alkali metals such as potassium can function as either promoters or inhibitors in catalytic reactions, but the underlying reasons for that are still obscure. Herein, we fabricated two kinds of single-site potassium catalysts (K-1/HMO and K-1/HWO) by using hollandite-type manganese oxide (HMO) and hexagonal tungsten oxide (HWO) as supports. K-1/HMO with the partially charged single K ions efficiently enhances the ability to activate both surface lattice oxygen and molecular oxygen, thereby leading to a higher catalytic activity for HCHO oxidation. In contrast, K-1/HWO is less active than HWO due to the electron-lean K ions. This work provides a feasible explanation for the functions played by alkali metals in catalytic oxidation, and thus assists the design of efficient catalysts promoted by alkali metals.