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SINGLE SITE CATALYSTS – IDEAL MODELS
FOR UNDERSTANDING STRUCTURE-FUNCTION RELATIONSHIPS
Bell Alexis T.
Department of Chemical Engineering
University of California, Berkeley, USA
E-mail: bell@cchem.berkeley.edu
Most heterogeneous catalysts consist of nanoparticles of a metal, metal oxide, or
metal sulfide dispersed on a support. Such catalysts are difficult to characterize
precisely even with modern analytical tools because the particles have a range of
particle sizes and shapes. As a consequence, unambiguous definition of what is meant
by a catalytically active site is not possible. This difficulty is severely reduced when
the active sites are in the form of isolated metal cations or metal oxo groups, since
now virtually all sites are identical, i.e., the sites take on a “molecular” character.
For such materials it both experimental and theoretical methods can be used to a
clear perspective of the composition and structure of the active site and its catalytic
properties. This talk will examine several examples of such single-site catalysts.
The first illustration concerns Fe-porphyrin catalysts used for the epoxidation of
olefins by H 2 O 2 . Careful analysis of this reaction system has revealed the role of the
axial ligand in controlling the catalyst activity and the selectivity with respect to H 2 O 2 .
The second example examines the role of extra-framework Fe cations in
Fe-ZSM-5 on the decomposition of N 2 O and the effects of water vapor on this
process.
The third illustration examines with the properties of isolated Cu + cations
supported on meosporous silica and exchanged into zeolites Y and ZSM-5 and the
role of such cations in the oxidative carbonylation of methanol to dimethyl carbonate.
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