Catalysts Examined by Electron Spin Resonance-Examples from Hydrodesulfurization Catalysis
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CATALYSTS EXAMINED BY ELECTRON SPIN RESONANCE-EXAMPLES FROM HYDRODESULFURIZATION CATALYSIS
BERNARD G. SILBERNAGEL Corporate Research-Science Laboratories, Linden, New Jersey 07036
Exxon Research and Engineering Co.,
ABSTRACT The utility of electron spin resonance (ESR) for catalyst characterization is illustrated for the desulfurization catalysts used to remove sulfur, nitrogen, and organically bound metals from petroleum. These catalysts consist of active metals (Mo, W, Co, Ni) in mixed oxide and sulfide phases on high surface area alumina supports. Coordinated studies of unsupported sulfide and oxide model systems determine the chemical form and number of defect sites on the actual catalysts. Parallel catalysis studies correlate ESR defects and catalytic activity for many reactions.
INTRODUCTION Heterogeneous catalysis is widely used throughout the chemical and petroleum industries. Table I illustrates the broad range of current applications, from producing important chemical intermediates like styrene and ammonia, to cleaning and upgrading petroleum-based feedstocks, by desulfurization, cracking, and reforming processes, to producing synthetic fuels by such techniques as Fischer-Tropsch chemistry. The catalyst forms employed in
these processes represent almost every area of materials science: elemental metals and alloys, transition metal oxides, high surface area ceramics, and novel solid forms like zeolites and metal sulfides [1]. The future need for non-petroleum based energy sources will require significant improvements in our ability to do such catalytic chemistry. Increasingly sophisticated spectroscopic and surface characterization techniques are being employed in catalysis research. When used in conjunction with an active effort in solid state chemistry and in direct contact with experiments in catalytic chemistry, they can be extremely powerful probes of the chemistry of the catalyst and the nature of its interaction with the chemical species it transforms. A systematic application of these three tools: solid state chemistry, catalytic chemistry, and characterization techniques, provides a new dimension in the design of advanced catalyst systems. Electron spin resonance (ESR) is a particularly appropriate illustration of this interaction because it has been applied in one form or another to catalytic problems for the last two decades. The rationale for using ESR is apparent. The technique is sensitive to several of the most important constituents in catalytic systems: paramagnetic transition metal species with partially filled electron shells, and electronic defects in both inorganic and organic components of the catalyst systems. Such partially occupied electron states are good candidates for many of the electron transfer pro-
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