Two-dimensional functionalized <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mi>Mo</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:mrow><mml:mrow><mml:mi mathvariant="normal">N</mml:mi></mml:mrow></mml:mrow><mml:m

This study employs first-principles calculations to investigate functionalized ${\mathrm{Mo}}_{2}\mathrm{N}{T}_{2}$ (T = \O{}, $\mathrm{H}$, $\mathrm{O}$, $\mathrm{F}$) MXenes as dual-functional materials for toxic gas capture and sensing applications. The surface of the pristine ${\mathrm{Mo}}_{2}\mathrm{N}$ surface is unsaturated, possesses dangling bonds, and exhibits a strong capability to capture various environmental gases. Surface functionalization significantly modulates this behavior: ${\mathrm{Mo}}_{2}{\mathrm{NH}}_{2}$ demonstrates remarkable selectivity toward ${\mathrm{NO}}_{x}$ capture, while ${\mathrm{Mo}}_{2}{\mathrm{NO}}_{2}$ emerges as a superior $\mathrm{NO}$ sensor due to its optimal adsorption strength (\ensuremath{-}0.342 eV), substantial charge transfer (\ensuremath{-}0.197 e), and 14%--18% current reduction in nanodevice measurements. Notably, investigations of mixed-terminated ${\mathrm{Mo}}_{2}{\mathrm{NO}}_{x}(\mathrm{OH})y$ systems reveal that hydroxyl concentration dictates $\mathrm{NO}$ reaction pathways: low $\mathrm{OH}$ triggers proton abstraction, forming $\mathrm{O}$-$\mathrm{N}$-$\mathrm{H}$ structures, whereas high $\mathrm{OH}$ induces direct $\mathrm{O}\text{\ensuremath{-}}\mathrm{H}$ cleavage, generating ${\mathrm{H}}_{2}\mathrm{N}\text{\ensuremath{-}}\mathrm{OH}$. Although water passivation mitigates these reactions, irreversible chemical transformations persist, underscoring that precise control of surface chemistry, particularly terminal-group engineering, is essential for achieving the selective capture capabilities and reversible sensing performance required for next-generation MXene-based gas management platforms.