Revealing Correlation of Valence State with Nanoporous Structure in Cobalt Catalyst Nanoparticles by in Situ Environmental TEM

TL;DR

This study uses in situ environmental TEM to simultaneously analyze the electronic structure and nanoporous morphology of cobalt catalysts, revealing how valence states correlate with structural changes during reduction under reaction conditions.

Contribution

It demonstrates the first in situ correlation of cobalt valence states with nanoporous structures in catalysts using environmental TEM and EELS, advancing understanding of catalyst behavior.

Findings

Cobalt valence states change from oxide to metallic during reduction.

Nanoporous structures coarsen as reduction proceeds.

In situ analysis aids in optimizing catalyst performance.

Abstract

Simultaneously probing the electronic structure and morphology of materials at the nanometer or atomic scale while a chemical reaction proceeds is significant for understanding the underlying reaction mechanisms and optimizing a materials design. This is especially important in the study of nanoparticle catalysts, yet such experiments have rarely been achieved. Utilizing an environmental transmission electron microscope (ETEM) equipped with a differentially pumped gas cell, we are able to conduct nanoscopic imaging and electron energy loss spectroscopy (EELS) in situ for cobalt catalysts under reaction conditions. Analysis revealed quantitative correlation of the cobalt valence states to the particles' nanoporous structures. The in situ experiments were performed on nanoporous cobalt particles coated with silica while a 15 mTorr hydrogen environment was maintained at various temperatures (300-600\degreeC). When the nanoporous particles were reduced, the valence state changed from cobalt oxide to metallic cobalt and concurrent structural coarsening was observed. In situ mapping of the valence state and the corresponding nanoporous structures allows quantitatively analysis necessary for understanding and improving the mass activity and lifetime of cobalt-based catalysts, i.e., for Fischer-Tropsch synthesis that converts carbon monoxide and hydrogen into fuels, and uncovering the catalyst optimization mechanisms.