Chemical
Kinetic Method for Active-Site Quantification
in Fe-N‑C Catalysts and Correlation with Molecular Probe and
Spectroscopic Site-Counting Methods
posted on 2023-11-20, 19:40authored byJason S. Bates, Jesse J. Martinez, Melissa N. Hall, Abdulhadi A. Al-Omari, Eamonn Murphy, Yachao Zeng, Fang Luo, Mathias Primbs, Davide Menga, Nicolas Bibent, Moulay Tahar Sougrati, Friedrich E. Wagner, Plamen Atanassov, Gang Wu, Peter Strasser, Tim-Patrick Fellinger, Frédéric Jaouen, Thatcher W. Root, Shannon S. Stahl
Mononuclear
Fe ions ligated by nitrogen (FeNx) dispersed
on nitrogen-doped carbon (Fe-N-C) serve as active
centers for electrocatalytic O2 reduction and thermocatalytic
aerobic oxidations. Despite their promise as replacements for precious
metals in a variety of practical applications, such as fuel cells,
the discovery of new Fe-N-C catalysts has relied primarily on empirical
approaches. In this context, the development of quantitative structure–reactivity
relationships and benchmarking of catalysts prepared by different
synthetic routes and by different laboratories would be facilitated
by the broader adoption of methods to quantify atomically dispersed
FeNx active centers. In this study, we
develop a kinetic probe reaction method that uses the aerobic oxidation
of a model hydroquinone substrate to quantify the density of FeNx centers in Fe-N-C catalysts. The kinetic
method is compared with low-temperature Mössbauer spectroscopy,
CO pulse chemisorption, and electrochemical reductive stripping of
NO derived from NO2– on a suite of Fe-N-C
catalysts prepared by diverse routes and featuring either the exclusive
presence of Fe as FeNx sites or the coexistence
of aggregated Fe species in addition to FeNx. The FeNx site densities derived
from the kinetic method correlate well with those obtained from CO
pulse chemisorption and Mössbauer spectroscopy. The broad survey
of Fe-N-C materials also reveals the presence of outliers and challenges
associated with each site quantification approach. The kinetic method
developed here does not require pretreatments that may alter active-site
distributions or specialized equipment beyond reaction vessels and
standard analytical instrumentation.