This study comparatively examines the efficiency and mechanism of peroxymonosulfate (PMS) activation by twenty metal and metalloid nanoparticles loaded on alumina. Among the tested metals, Co exhibited the highest capacity for PMS activation and accompanying oxidative degradation of trichlorophenol (TCP), a representative organic pollutant in water. Other transition metals such as Mn, Cu, Mo, Ni, and W exhibited moderate activity, while Ti, Zn, Fe, V, Cr, Al, and Si were mostly ineffective. In contrast, all of the tested noble metals (Ru, Rh, Pd, Ir, Pt, and Au) except Ag enabled rapid PMS activation and TCP degradation, outperforming Co at acidic pH. Transition metals with noticeable PMS activation capacity differed from noble metals in several aspects, such as the effect of radical quenching on 4-chlorophenol (4-CP) degradation, electron paramagnetic resonance spectral features, oxidative conversion of bromide into bromate, and oxidation intermediate distribution. They were also distinguishable with respect to the dependence of PMS degradation on the presence of an electron donor (i.e., TCP), the capacity to activate peroxydisulfate (PDS), and the electrochemical response upon addition of PMS and 4-CP when fabricated into electrodes. Based on these observations, we categorized surface-loaded metal nanoparticles into two groups with distinctive PMS activation mechanisms: (i) transition metals such as Co, Cu, and Mo that activate PMS to produce highly reactive sulfate radicals (SO4-); and (ii) noble metals such as Rh, Ir, and Au that mediated direct electron transfer from organic compound (electron donor) to persulfate (electron acceptor) without involving the formation of radical species.