The multicenter and multi-step kinetic mechanism of oxygen production by water oxidation was revealed

Recently, the team of Li Can, academician and researcher Wang Xiuli, academician and researcher of Dalian Institute of Chemical Physics, Chinese Academy of Sciences, has made new progress in the research of photocatalytic kinetics. Using the self-developed reaction timescale transient absorption spectroscopy method, the team revealed the multi-center and multi-step continuous valence kinetic microprocess of water oxidation oxygen production (OER) reaction on a typical catalyst cobalt tetroxide (Co3O4), and demonstrated the kinetic characteristics of fast generation and slow conversion of reaction intermediates. The results were published in the Journal of the American Chemical Society.

Microscopic process of multi-center and multi-step continuous valence dynamics during OER reaction (Photo courtesy of Dalian)

As a key reaction to provide hydrogen protons and electrons, OER reaction plays a crucial role in natural photosynthesis and artificial photosynthesis, and exploring the microscopic mechanism of valence dynamics of its catalytic process is of great significance for rational design and optimization of OER catalyst.

However, due to the complexity of the multi-electron and multiproton transfer process of OER reaction collection, the study of the intermediate reaction mechanism is very difficult, and the time scale of the reaction is very broad. Although a lot of work has been done on charge transfer on ultrafast timescales, the statistical time of actual reaction kinetics is mostly in nanoseconds to nanoseconds, and studying the microscopic mechanism of water oxidation on nanosecond scales faces new challenges.

In this work, the team focused on the variable kinetics of water oxidation reactions, constructed a photosensitized water oxidation system with tripyridine ruthenium as a dye molecule, selected cobalt tetroxide as a model catalyst, and studied OER reaction kinetics by using self-developed slow timescale transient absorption spectroscopy technology.

It is found that the rapid oxidation of the surface tetracoordinated Co2+ site initiates the OER reaction, and then the surface tetracoordinated Co2+ and hexacoordinated Co3+ jointly act as active sites to catalyze the multi-step and multicenter OER reaction. Subsequently, the team further revealed the multi-step continuous valence dynamics mechanism of the OER process on Co3O4, and the multi-step dynamic transformation characteristics of Co2+→ Co3+→Co4+ are the dynamic characteristics of fast generation and slow transformation of all cobalt intermediate species. This characteristic makes the distribution of Co4+ intermediate species play a decisive role in OER activity, which is the essential reason for the slow overall OER dynamics.

This work reveals the multicenter and variable catalytic cycling mechanism on artificial OER catalysts, which provides an experimental basis and theoretical support for in-depth understanding of OER reaction mechanism and design of efficient artificial OER catalysts. (Source: Sun Danning, China Science News)

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