Enhanced ammonia selectivity on electrochemical nitrate reduction: Cu–Ni metal–organic frameworks with tandem active sites for cascade catalysis

Abstract

A Cu–Ni metal–organic framework is proposed as a cascade catalyst to eliminate the nitrite product in nitrate reduction, providing an effective route for high-value green ammonia synthesis.

Cascade Electrocatalytic Reduction of Nitrate to Ammonia Using a Heterobimetallic Covalent Organic Framework Composed of Cu-Porphyrin and Co-Bipyridine

“…At the start of the catalytic cycle, the nitrate reactant could be adsorbed on both CuP and CoBpy sites; however, CoBpy units exhibit an energy barrier (1.04 eV) in the *NO 3 → *NO 3 H step. By contrast, there is nearly no energy barrier for the conversion of NO 3 – to NO 2 – over CuP units, indicating a preference for NO 2 – formation on CuP units . In the following conversion of NO 2 – to NH 3 , it is evident that CuP units show the highest free energy barrier at the *NO 2 H → *NO step.…”

“…By contrast, there is nearly no energy barrier for the conversion of NO 3 − to NO 2 − over CuP units, indicating a preference for NO 2 − formation on CuP units. 32 In the following conversion of NO 2 − to NH 3 , it is evident that CuP units show the highest free energy barrier at the *NO 2 H → *NO step. Fortunately, as shown in Table S3, the interaction between the NO 2 H intermediate and the CuP site is relatively weak, making it easy to desorb and be captured by the CoBpy sites.…”

“…The cooperation of multicatalytic sites could lead to greatly improved activity and selectivity of individual reactions, thus facilitating the overall chemical conversion. Upon integration of distinct catalytic sites, the thermodynamics of each reaction could be individually optimized. − For dynamics, the product (NO 2 – ) generated from the first reaction (NO 3 – to NO 2 – ) can be directly transferred to the catalytic sites of the second reaction (NO 2 – to NH 3 ). Meanwhile, the second reaction consumes NO 2 – and shifts the equilibrium of the first reaction forward. − As a result, the cascade catalysis provides sufficient reactant for the subsequent reaction and enhances the production of NH 3 while suppressing the accumulation of toxic NO 2 – .…”

Recent Progress and Perspectives on Transition Metal-Based Electrocatalysts for Efficient Nitrate Reduction

“…Furthermore, catalytic performance can be improved by the synergistic effect of tandem active sites in bimetallic MOFs. Yan et al 134 − and reduce it to NH 3 . This material has excellent stability, and the current density is only slightly reduced after 40 h of continuous operation.…”

“…Furthermore, catalytic performance can be improved by the synergistic effect of tandem active sites in bimetallic MOFs. Yan et al synthesized Ni 1.5 Cu 1.5 (HITP) 2 MOF materials with high NH 3 yield (130.93 μmol h –1 cm –2 ) and selectivity (72.45%), which were mainly caused by the synergistic effect between the ability of Cu active sites to adsorb NO 3 – and break N–O bonds and the ability of Ni active sites to adsorb the intermediate product NO 2 – and reduce it to NH 3 . This material has excellent stability, and the current density is only slightly reduced after 40 h of continuous operation.…”