Decomposition of nitric oxide (NO) gas on a reactive transition-metal cluster of W2TcO6 has been examined and investigated via selective catalytic reduction by ammonia (NH3-SCR) using the M06-L density functional method. The transition-metal cluster of W2TcO6 can be employed to transform NO to N2 gas efficiently over an active site of tungsten (W). A reaction mechanism of NO conversion based on the NH3-SCR process has been elucidated by a potential energy surface along the reaction pathways. The reaction pathways of this NH3-SCR process begin with adsorption of NH3, adsorption of NO to the cluster, formation of nitrosamine (NH2NO) and NHNO/NHNOH intermediates, and rearrangement of NHNO/NHNOH to obtain N2 and H2O, respectively. Notably, a significant NH2NO as a key intermediate, namely, "nitrosamine", must be formed before further steps can take place in the generation of N2 from NO, followed by the involvement of the NHNO or NHNOH intermediate. From our calculated results, the NHNO intermediate via TS3a is found in pathway a, while NHNOH is found in pathway b via TS3b. Pathway b has a lower energy barrier of 35.1 kcal/mol than pathway a with an energy barrier of 41.8 kcal/mol, indicating that pathway b should be more energetically favorable. The step for NHNO intermediate rearrangement is a rate-determining step for the reaction occurring through pathway a, which is found to be more difficult in accordance with a difficult N-H bond cleavage to form the NNOH intermediate before N2 formation. The overall reaction is an exothermic process with thermodynamic and kinetic favors. Thus, this bimetallic W2TcO6 cluster could be used as a promising and active catalyst for NO decomposition via the NH3-SCR process to an eco-friendly gas, that is, N2.