Selective catalytic reduction of NOx with NH3 is one of the most effective ways to reduce NOx emissions. Manganese-based deNO(x) catalysts have attracted much attention owing to their unstable valence states. Nickel-foam-supported manganese-based catalysts show excellent low-temperature deNO(x) activities. However, the detailed deNO(x) mechanism is still unclear. In this paper, the adsorption mechanism of NH3, NO, and O-2 molecules over nickel-foam-supported manganese-based catalysts was studied by a density functional theory. The model of MnO2, Mn2O3, and Mn3O4 clusters loading on the Ni (111) surface was chosen. The results show that MnO2, Mn2O3, and Mn3O4 clusters can be stably adsorbed on the Ni (111) surface accompanied by charge redistributions, which provide a large number of stable adsorption sites for gas molecules. NH3 molecule can be adsorbed on the Ni-top and Mn sites as the coordinated NH3. For MnO2, Mn2O3, and Mn3O4 clusters, Mn3O4 clusters are the most favorable for NH3 adsorption, indicating that they can promote NH3-SCR reaction. NO exists in three adsorption states on the MnxOy/Ni (111) surface and forms different nitrite or nitrate species. Interestingly, NO molecule tends to adsorb on the hollow site of Ni (111) surface. O-2 molecule has various adsorption states on the MnxOy/Ni (111) surface. NO molecule is more likely to be adsorbed on the Mn site of the O-2-MnxOy/Ni (111) surface to form nitrosyl structure, and some of NO molecules can react with adsorbed oxygen to form the adsorbed NO2 spices, which can facilitate the SCR deNO(x) reaction. The catalytic activity of Mn3O4/Ni (111) surface is higher than those of MnO2/Ni (111) and Mn2O3/Ni (111) surfaces by comparing the work functions and adsorption properties of NH3, NO, and O-2 molecules.