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Biologically inspired denitrification catalyst

By Tetsuo Satoh |

The development of denitrification catalysts that can reduce nitrate and nitrite to N2 is critical for sustaining the nitrogen cycle. However, regulating the catalytic selectivity has proven to be a challenge, due to the difficulty of controlling complex multi-electron/proton reactions. Now, Ryuhei Nakamura and coworkers at Riken (Wako City, Japan; have developed an artificial catalyst that imitates the denitrification enzyme of microorganisms, and succeeded in converting nitrite ions efficiently into harmless N2.

The researchers focused on the microorganisms that perform multi-step reactions under mild conditions using four enzymes that contain metals, such as Fe, Cu and Mo as the active center of the enzyme. They found that the catalyst composed of the enzyme containing Mo as the active center of the enzyme in the shape of a pterin-like structure, coordinated with the oxygen and sulfur, efficiently detoxifies the nitrite ions into N2 under mild conditions and without using a large-scale drainage treatment installation. Using a hydrothermal synthesis method, they fabricated the catalyst with the pterin-like structure and confirmed that this catalyst contained a MoS4 structure similar to the active site of the enzyme.

They also showed that utilizing sequential proton–electron transfer (SPET) pathways is a viable strategy to enhance the selectivity of electrochemical reactions. The selectivity of an oxo-molybdenum sulfide electro-catalyst toward nitrite reduction to dinitrogen exhibited a volcano-type pH dependency with a maximum at pH 5. The pH-dependent formation of the intermediate species (distorted Mo(V) oxo species), identified using operando electron paramagnetic resonance (EPR) and Raman spectroscopy, was in accord with a mathematical prediction that the pKa of the reaction intermediates determines the pH-dependence of the SPET-derived product. By utilizing this acute pH dependence, they achieved a Faradaic efficiency of 13.5% for nitrite reduction to N2, which is the highest value reported to date under neutral conditions.

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