Selective catalytic reduction (SCR) of NO (x) with NH3 has been widely adopted to reduce NO (x) emissions. Although MnO (x) -based catalysts exhibit higher NO (x) conversion, the underlying reaction mechanism is still unclear. Since the SCR is a gas-solid catalytic reaction, the adsorption of related gas species on the catalyst surface plays a key role. In this study, the adsorption of NO, NH3, and O-2 on beta-MnO2(110) surface was investigated by density functional theory calculations, showing their individual adsorption properties. Two different gas molecules can also simultaneously adsorb on the same adsorption site. When NO and O-2 co-adsorb on the surface, NO is oxidized by O-2 to form bridge nitrates and nitrites. This work provides a foundation for studying the mechanism of the SCR of NO (x) with NH3.