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• Developed a molecular dynamics model to study the nano-scale transport of gaseous pollutants in titania-based catalysts. • Investigated the diffusivity under the impacts of different molecules, temperatures, pore widths and hydroxyl sites. • Studied the influences of competitive diffusion of mixed pollutant molecules on the diffusivity. Mass transfer can significantly affect the SCR process which is designed for NOx removal. However, it is still challenging to characterize the transport of gaseous species involved in the process, especially at nano-scale. This work probes into the application of non-equilibrium molecular dynamics (NEMD) simulations to study the mass transfer of multi-pollutants over a titania-based catalyst. A dual control-volume (DCV) model was proposed to simulate transport of typical gaseous molecules (e.g. NO, NH 3 and SO 2). The impacts of temperature, pore width, hydroxyl sites and competitive diffusion on diffusivity of objective molecules were studied in details. The results showed that temperature and surface sites could affect NH 3 more significantly than NO and SO 2 , yet the influence of surface sites was strongly size-dependent. The reduction in NH 3 diffusivity caused by the presence of surface sites decreased from 32.37 % to 2.97 % when the pore width grows from 25 Å to 75 Å. The competitive transport between NH 3 and SO 2 has also mitigated the impacts of surface sites on both molecules. [ABSTRACT FROM AUTHOR]