Redirecting Configuration of Atomically Dispersed Selenium Catalytic Sites for Efficient Hydrazine Oxidation
Authors: Pang Kanglei , Pang Kanglei 
Published: December 2024

Thorough understanding of reconstructed active sites evolving from their initial states is crucial for a variety of catalytic reactions, as it not only promotes an atomic-level comprehension of the catalytic mechanism but also guides the design of practically usable catalysts. In particular under electrochemical operation, the reconstruction phenomenon of surface sites during reactions. After reconstruction, the active site is no longer the original pristine structure but rather the newly formed surface site. Exploring surface reconstruction under Operando catalytic conditions can aid in a better understanding of the active sites and catalytic pathways, helping designing more efficient catalyst systems.
Atomically dispersed catalysts (ADCs), specifically the ones based on carbon, show tremendous potential in critical electrochemical reactions, including the hydrazine oxidation reaction (HzOR), which exhibits zero-carbon emissions and possesses a high energy density, presents considerable promise for the application of hydrazine fuel cells with improved power density. The dynamic reconstruction of atomically dispersed catalytic sites during electrocatalytic processes has emerged as a subject of intensive investigation. On one hand, knowledge of the reconstruction process is crucial, as it provides deep insights into the atomically dispersed active site and catalytic mechanism, laying a firm basis for material innovation. On the other hand, atomically dispersed catalytic sites have clear and uniform coordination structures, making them ideal quasi-model catalysts for revealing reconstruction mechanisms of active sites. However, discovering these reconstruction behaviors has focused primarily on metallic atomically dispersed catalytic sites so far, leaving non-metallic ones much unexplored, despite the fact that their performance in catalysis is comparable to or even superior to that of costly noble metals. An accurate vision of active sites during reactions is vital in developing non-metallic atomically dispersed catalysts; from a scientific perspective, it is essential to extract information about possible dynamic reconstruction processes in non-metallic atomically dispersed catalytic sites, similar to that of metallic ones.
Combining ex situ high resolution electron microscopy and in situ X-ray adsorption spectroscopy to monitor the structurally uniform and well-defined single atomic site of atomically dispersed catalyst as an ideal model system could provide valuable atomic insights to active sites and the corresponding catalytic reaction mechanism. In situ techniques, particularly operando near and extended X-ray absorption fine structure (EXAFS and XANES) spectroscopies, in which the probe parameters such as the edge intensity, “white line”, and its energy position are affected by spectator species adsorbed on the electrode surface and intermediates formed upon reaction, may deliver chemical fingerprints that capture evolution of active sites during reaction
In this study, we synthesized a carbonaceous, non-metallic atomically dispersed selenium (Se) catalyst and confirmed the atomically dispersed configuration of Se in a SeC4 configuration. The as-prepared Se ADCs was found active in the hydrazine oxidation reaction (HzOR) in alkaline media (-114 mV (vs. reversible hydrogen electrode) at a current density of 1 mA cm-2), even surpassing that of noble-metal platinum (Pt) catalysts. Using the Se ADCs as a quasi-model catalyst, in situ X-ray absorption spectroscopy and Fourier-Transform infrared spectroscopy were used to trace the dynamic reconstruction process of atomically dispersed Se catalytic sites. We found that pristine SeC4 will pre-adsorb an *OH ligand in alkaline aqueous solutions, and the electrochemical oxidation of the N2H4 molecule will subsequently occur on the opposite side of the OH-SeC4. Theoretical calculations suggest that the pre-adsorbed *OH group pulls electrons from the Se site, resulting in a more positively charged Se and a higher polarity of Se-C bonds, and consequently higher surface reactivity toward HzOR. Our findings provide valuable insights into the reconstruction mechanism of atomically dispersed semimetallic catalytic sites and promote their practical use in a wide range of energy systems.

Figure 1