Abstract
Electrocatalytic conversion of nitrogen oxides to value-added chemicals is a promising strategy for mitigating the human-caused unbalance of the global nitrogen-cycle, but controlling product selectivity remains a great challenge. Here we show iron–nitrogen-doped carbon as an efficient and durable electrocatalyst for selective nitric oxide reduction into hydroxylamine. Using in operando spectroscopic techniques, the catalytic site is identified as isolated ferrous moieties, at which the rate for hydroxylamine production increases in a super-Nernstian way upon pH decrease. Computational multiscale modelling attributes the origin of unconventional pH dependence to the redox active (non-innocent) property of NO. This makes the rate-limiting NO adsorbate state more sensitive to surface charge which varies with the pH-dependent overpotential. Guided by these fundamental insights, we achieve a Faradaic efficiency of 71% and an unprecedented production rate of 215 μmol cm−2 h−1 at a short-circuit mode in a flow-type fuel cell without significant catalytic deactivation over 50 h operation.
Highlights
Electrocatalytic conversion of nitrogen oxides to value-added chemicals is a promising strategy for mitigating the human-caused unbalance of the global nitrogen-cycle, but controlling product selectivity remains a great challenge
57Fe Mössbauer spectroscopy and Fe K-edge extended X-ray absorption fine structure (EXAFS) reveal only two quadrupole doublets assigned to FeNx sites and Fe–N(O) interaction in FeNx sites, respectively, without any detectable spectroscopic signal from Fe clusters
In order to identify the gaseous products formed during NO reduction reaction (NORR), online differential electrochemical mass spectrometry (DEMS) coupled with a scanning flow cell (SFC; Supplementary Fig. 7) was introduced
Summary
Electrocatalytic conversion of nitrogen oxides to value-added chemicals is a promising strategy for mitigating the human-caused unbalance of the global nitrogen-cycle, but controlling product selectivity remains a great challenge. In the series of nitrogen reduction steps starting from nitrate, the catalytic reduction of nitric oxide (NO) is a key step to allow for the further reduction of nitrogen, determining the nature of the further reduced nitrogen products (e.g., N2O, NO, NH2OH, and NH3)[8,9] Noble metal electrocatalysts such as Pt and Pd typically produce N2O/N2 (low overpotential region) and NH2OH/NH3 (high overpotential region) from the NO reduction reaction (NORR)[7,9,10,11]. Two different NORR pathways were identified: pH-dependent (NH2OH formation) and pH-independent (N2O formation) pathways, the selectivity of which is affected by electrolyte pH, NO concentration, and electrode potential By controlling these experimental parameters, highly selective NO-to-NH2OH conversion was achieved with a rotating disk electrode (RDE) setup[21,22,23]. We achieved effective and durable NH2OH production on the single-atom Fe catalyst in a prototypical H2–NO fuel cell reactor
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