Several new heteropoly compounds catalysts with pronounced activities in NOx conversion via adsorption-decomposition process in a fixed-bed reactor were evaluated concerning the principal internal and external factors having marked effects on the process of adsorption. Results showed that substitution of H+ in HPW (H3PW12O40•6H2O) by NH4+ results in marked enhancement of the catalyst activity, however, substitution of coordination atom W in the [PW12O40]3- anion by one or more Mo atoms tends to lower the catalyst activity and such tendency deepens with the increase in the Mo atom numbers introduced. It is for the first time that the nano-effect was observed with nano (NH4)3PW12O40 catalyst, in which a 1.6% to 10.6% enhancement in NOx adsorption efficiency was obtained in comparison with the same catalyst in normal particle size. By IR analysis, NO was found to be adsorbed via interaction with the proton of HPW structure. With the aid of GC-MS analysis, the process effectiveness for NOx decomposition into N2 was confirmed.