  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.
