Ni- and Cu-co-Intercalated Layered Manganese Oxide for Highly Efficient Electro-Oxidation of Ammonia Selective to Nitrogen

Abstract

We fabricated a thin film of layered MnO2 whose interlayer space was occupied by hydrated Ni2+ and Cu2+ ions. The process consisted of electrodeposition of layered MnO2 intercalated with tetrabutylammonium cations (TBA+) by anodic oxidation of aqueous Mn2+ ions in the presence of TBA+, followed by ion exchange of the initially incorporated bulkier TBA+ with the denser transition metals in solution. The resulting layered MnO2 co-intercalated with Ni2+ and Cu2+ ions (NiCu/MnO2) catalyzed the ammonia oxidation reaction (AOR) in an alkaline electrolyte with a much lower overpotential than its Ni2+- and Cu2+-intercalated single-cation counterparts. Surprisingly, the NiCu/MnO2 electrode achieved a faradic efficiency as high as nearly 100% (97.4%) for nitrogen evolution at a constant potential of +0.6 V vs Hg/HgO. This can be ascribed to the occurrence of the AOR in the potential region where water is stable and dimerization of the partially dehydrogenated ammonia species is preferred, thereby forming an N–N bond, rather than to be further oxidized into NO x species.

Catalysts for electrochemical ammonia oxidation: Trend, challenge, and promise

“…In other words, the unique properties of Mn and Cu probably promote the formation of NH x + cation radical and suppress the side reaction of generating soluble metal-NH 3 complex for achieving a high yield of oxidizing NH 3 to N 2 . The addressed roles of Cu and Mn correspond to the previous study of NiCu/ MnO 2 /CP [92].…”

“…The synergistic effects of NiCu to improve eAOR activity and achieve high selectivity to NO 3 − and NO 2 − have been covered enough [88][89][90][91], but studies of NH 3 conversion into N 2 are rare. Recently, Nagita et al [92] In combination with Ni and Cu, another element such as cobalt (Co) has been explored to achieve high N 2 selectivity because most of NiCu studies derive oxygen-containing nitrogen species (NO 2 − and NO 3 − ) as major products except for NiCu/ MnO 2 [92]. Co can be one of the candidates to overcome the limited N 2 selectivity of NiCu bimetal since it strongly bonds to N but weakly bonds to hydrazine, which is a possible intermediate for N 2 formation [93].…”

Layered Manganese Dioxide Thin Films Intercalated with Ag+ Ions Reduceable In Situ for Oxygen Reduction Reaction

“…We have reported that MnO 2 grown electrochemically serves not only as a catalyst but also as a support for immobilizing various catalytic ions. 1 − 3 …”

“…16 We have introduced transition metals into the interlayers of layered MnO 2 and used them as catalysts for various reactions. [1][2][3]17 The transition metals exist as aqua-complexes isolated in the interlayers, unlike common heterogeneous catalysts such as transition-metal oxides and hydroxides. In this study, we intercalated Ag + ions into the interlayers of layered MnO 2 (birnessite) and reduced them to metallic Ag 0 species in situ by injecting electrons from an external circuit through the MnO 2 layers.…”

“…In addition, no polymer binders or conducting additives are required, which makes it easier to determine the relationship between electrochemical performance and physicochemical properties of the active materials. We have reported that MnO 2 grown electrochemically serves not only as a catalyst but also as a support for immobilizing various catalytic ions. − …”