Metal-exchanged zeolites have been widely used in industrial catalysis and separation, but fundamental understanding of their structure-property relationships has remained challenging, largely due to the lack of quantitative information concerning the atomic structures and reactionrelevant adsorption properties of the embedded metal active sites. We report on the use of lowtemperature chemisorption to titrate Cu-exchanged ZSM5. Quantitative descriptors of the atomic structures and adsorption properties of Cu-ZSM5 are established by combining atomistic simulation, DFT calculations, operando molecular spectroscopy, chemisorption and titration measurements. These descriptors are then applied to interpret the catalytic performance of Cu-ZSM5 for NO decomposition. Linear correlations are established to bridge the low-temperature adsorption analytics and high-temperature reaction kinetics, which are demonstrated to be generally applicable for understanding the structure-property relationships of metal exchanged zeolites and foregrounded for guiding the development of advanced catalytic materials.
Catalysts composed of platinum dispersed on zeolite supports
are
widely applied in industry, and coking and sintering of platinum during
operation under reactive conditions require their oxidative regeneration,
with the platinum cycling between clusters and cations. The intermediate
platinum species have remained only incompletely understood. Here,
we report an experimental and theoretical investigation of the structure,
bonding, and local environment of cationic platinum species in zeolite
ZSM-5, which are key intermediates in this cycling. Upon exposure
of platinum clusters to O2 at 700 °C, oxidative fragmentation
occurs, and Pt2+ ions are stabilized at six-membered rings
in the zeolite that contain paired aluminum sites. When exposed to
CO under mild conditions, these Pt2+ ions form highly uniform
platinum gem-dicarbonyls, which can be converted
in H2 to Ptδ+ monocarbonyls. This conversion,
which weakens the platinum–zeolite bonding, is a first step
toward platinum migration and aggregation into clusters. X-ray absorption
and infrared spectra provide evidence of the reductive and oxidative
transformations in various gas environments. The chemistry is general,
as shown by the observation of platinum gem-dicarbonyls
in several commercially used zeolites (ZSM-5, Beta, mordenite, and
Y).
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