Atomistic Mechanisms for Catalytic Transformations of NO to NH₃, N₂O, and N₂ by Pd

The industrial pollutant NO is a potential threat to the environment and to human health. Thus, selective catalytic reduction of NO into harmless N_2, NH_3, and/or N_2O gas is of great interest. Among many catalysts, metal Pd has been demonstrated to be most efficient for selectivity of reducing NO to N_2. However, the reduction mechanism of NO on Pd, especially the route of N-N bond formation, remains unclear, impeding the development of new, improved catalysts. We report here the elementary reaction steps in the reaction pathway of reducing NO to NH_3, N_2O, and N_2, based on density functional theory (DFT)-based quantum mechanics calculations. We show that the formation of N_2O proceeds through an Eley-Rideal (E-R) reaction pathway that couples one adsorbed NO^* with one non-adsorbed NO from the solvent or gas phase. This reaction requires high NO^* surface coverage, leading first to the formation of the trans-(NO)_2^* intermediate with a low N-N coupling barrier (0.58 eV). Notably, trans-(NO)_2^* will continue to react with NO in the solvent to form N_2O, that has not been reported. With the consumption of NO and the formation of N_2O^* in the solvent, the Langmuir-Hinshelwood (L-H) mechanism will dominate at this time, and N_2O^* will be reduced by hydrogenation at a low chemical barrier (0.42 eV) to form N_2. In contrast, NH_3 is completely formed by the L-H reaction, which has a higher chemical barrier (0.87 eV). Our predicted E-R reaction has not previously been reported, but it explains some existing experimental observations. In addition, we examine how catalyst activity might be improved by doping a single metal atom (M) at the NO^* adsorption site to form M/Pd and show its influence on the barrier for forming the N-N bond to provide control over the product distribution.