Modulating Intermediate Reactivity for Nitrogen Oxide Selective Catalytic Reduction through Dual Scaling Laws: A Synergetic <i>In Situ</i> Diffuse Reflectance Infrared Fourier Transform Spectroscopy and Density Functional Theory Study

Nitrogen oxide (NO_<i>x</i>) abatement offers a viable way to mitigate air pollution, yet it continues to pose significant challenges in heterogeneous catalysis, calling for atomistic insights into how transition metal catalysts modulate reactivity. In this study, we integrate <i>in situ</i> diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) with density functional theory (DFT) calculations to uncover the mechanistic principles underlying NO_<i>x</i> selective catalytic reduction (SCR) on manganese-based catalysts. Our experimental-theoretical framework captures a sequence of NO_<i>x</i> intermediates with strong agreement between computed and measured IR signatures, enabling detailed mapping of charge redistribution and orbital interactions at Mn active sites. We discovered distinct reactivity scaling laws for intermediates with Mn³⁺ and Mn⁴⁺: a Sabatier-type volcano trend for Mn³⁺-bound intermediates and a linear relationship for Mn⁴⁺-containing systems. Moreover, we identified a unifying electronic descriptor, defined as the Fermi-level corrected energy of Mn 3<i>d</i> orbitals with a maximal overlap to NO_<i>x</i> O 2<i>p</i> orbitals, that quantitatively connects these dual regimes. This descriptor is consistent with ligand field theory and further captures the dynamic interplay of NO_<i>x</i> adsorbates, demonstrating that the symmetry and alignment of transition-metal 3<i>d</i> orbitals govern both charge distribution and chemical reactivity. Since multivalence is a ubiquitous feature of transition metal catalysts, the dual scaling laws discovered in this work could provide a generally valid framework. These findings not only establish a mechanistic foundation for understanding valence-dependent SCR reaction pathways but also suggest a broader framework with predictive power for catalyst design based on electronic structure engineering.