  We present first-principles calculations in the framework of
density-functional theory and the pseudopotential approach, aiming to model the
intermediate stages of the reduction of NO in the presence of MoO$_3$(010). In
particular, we study the formation of dinitrosyl, which proves to be an
important intermediate stage in the catalytic reduction. We find that the
replacement of an oxygen of MoO$_3$ by NO is energetically favorable, and that
the system lowers further its energy by the formation of (NO)$_2$. Moreover,
the geometry and charge distribution for the adsorbed dinitrosyl indicates a
metal-oxide mediated coupling between the two nitrogen and the two oxygen
atoms. We discuss the mechanisms for the dinitrosyl formation and the role of
the oxide in the reaction.
