Computational Modeling of Catalytic Pathways for Nitric Oxide Reduction: Evaluating the Efficiency of PtCu? and NiCu? Catalysts

Nitrogen oxides (NOₓ), particularly nitric oxide (NO), are major contributors to environmental issues such as acid rain deposition and ozone layer depletion. The NO reduction (NOR) reaction is therefore essential for mitigating these harmful effects; also, if NO can be fully reduced to ammonia (NH₃), it would provide a route to this critical chemical feedstock. In this study, computational modeling was used to analyze the performance of different catalysts for NOR, and the thermodynamics of different competing pathways were examined. Using density functional theory (DFT), the structures of all NOR to NH₃ reaction intermediates and their Gibbs free energy were calculated, allowing for identification of the most thermodynamically favorable NOR to NH₃ pathways.

The nanoparticle catalysts Cu₄ and Pt₄ were first examined, with Cu4 demonstrating more favorable thermodynamics for NOR. Next, PtCu₃ and NiCu₃ nanoparticles were examined for potential improvement, as bimetallic catalysts can show synergistic effects. PtCu₃ demonstrated improved catalytic activity compared to individual Cu₄ and Pt₄ structures, because it maintained a steady decrease in energy across steps, making the NOR process more favorable. In contrast, Cu₄ and Pt₄ displayed sharp energy spikes, or barriers, though Cu₄ performs better than Pt₄ because it had only one of these barriers (NO → N) and a fully downhill pathway (NO → NOH). Pt₄, however, has barriers in both key pathways, making the reaction less favorable. PtCu₃ avoids these entirely, resulting in smoother energy transitions and making it the most efficient catalyst for NOR.

The NiCu₃ catalyst also showed a favorable energy profile, as it had a significantly steeper slope in energy. This suggests it could potentially serve as an efficient catalyst for NOR to ammonia. Though it has much lower energy requirements for its pathways, PtCu₃ still performs better. In PtCu₃, all pathways starting from NO are downhill, which is ideal. In contrast, NiCu₃ has one downhill pathway(NO → N), but the other includes a barrier, making PtCu₃ more favorable overall for NOR.

This study examines the importance of selecting catalysts that not only reduce NO efficiently but also minimize the required activation energy, leading to more sustainable processes. The findings provide insights into catalyst design strategies that could help address the environmental challenges posed by nitrogen oxide emissions. By improving the catalytic performance of mixed metal systems like NiCu₃, there is potential for developing more effective solutions for NO removal in environmental protection efforts.