Element table of TM-substituted polyoxotungstates for direct electrocatalytic reduction of nitric oxide to ammonia: a DFT guideline for experiments

Abstract

The electrochemical NO reduction (NOER) is one of the most promising routes for ammonia synthesis and for simultaneously removing the air pollutant, NO. However, the current electro-catalysts are mainly metal based, and the search for new and cost-efficient NOER catalysts is in the pipeline. Polyoxometalates, a class of metal–oxide clusters, exhibiting unique electrochemical redox behavior, have been widely applied to many electrocatalytic processes. In this work, density functional theory (DFT) calculations were adopted to investigate the NOER performance of a series of transition metal-substituted (TMPOMs, Fe^II ∼ Cu^II, Ru^II ∼ Ag^I, Os^II ∼ Au^I) heteropolytungstates. Firstly, we take the highly active SiW₁₁Fe^II catalyst reported in experiments as an example to investigate its detailed NOER mechanism, and thus provide some qualitative criteria for screening potential new NOER catalysts. The *NHO and *NHOH intermediates are confirmed based on their lower energies compared to *NOH and *NH₂O, respectively. The high activity and ammonia selectivity of SiW₁₁Fe^II are dominated by four factors: (i) a moderate NO adsorption ability, (ii) an acceptable N–O bond activation barrier (ΔG_b = 0.65 eV) at room temperature, (iii) low onset potential for the potential-limiting step (ca. −0.21 V), and (iv) effective suppression of the competitive HER and N₂O formation reaction. Inspired by this, Ni^II-, Co^II-, Cu^II-, Pd^II-, Pt^II-, and Ag^I-substituted heteropolytungstates are identified as candidate electrocatalysts for application in the NOER through a comprehensive comparison with the SiW₁₁Fe^II case. Particularly, SiW₁₁Ni^II was selected as the most optimal with an extremely low ΔG_b of only 0.07 eV for the *NO → *NHO step. It is suggested that the high electrocatalytic activity of SiW₁₁Ni^II originates from its high reduction ability to accept electrons in the electrocatalytic process, and the low basicity strength of O sites to promote the transfer of H. The present work provides an expansion of the POMs’ family in the field of NOER, and ultimately facilitates the designing of nitrogen-cycle catalysts for targeted environmental and synthesis applications.