Site Sensitivity Resulting in Selective Hydroxylamine Electrosynthesis

Abstract The electroreduction of nitrogen oxides offers a promising route for green synthesis of hydroxylamine (NH2OH). Although Pd exhibited high selectivity toward NH2OH production experimentally, we found the dominant hollow sites of pristine Pd prefer ammonia production via the NOH*-pathway. This discrepancy was resolved by unveiling the critical roles of in situ hydriding of Pd under operando conditions. Using Grand Canonical Monte Carlo simulations driven by machine learning potential, we demonstrated the potential-dependent structural evolution from pristine Pd to palladium hydride (PdHx) under −0.11 to −0.51 V vs RHE. For the in situ-generated PdHx surfaces with some amorphous or disordering features, the dominant hollow sites of Pd are blocked by adsorbed H* species, thereby repelling NO* to adsorb obliquely on PdHx top sites. The adsorption configuration of NO* steers the reaction pathway from NOH* to HNO* formation, leading to NH2OH* production eventually. In addition, the PdHx top sites exhibit generally downshifted d-band centers, promoting NH2OH* desorption and preventing its over-reduction to ammonia. At last, a site sensitivity was proposed for hydroxylamine electrosynthesis based on the analysis on a variety of catalysts; the formation energies of HNO* vs NOH* and the adsorption energy of NH2OH* were identified as important features resulting in high hydroxylamine selectivity.