Under appropriate conditions, significant microwave-specific enhancement of the reaction rate of an organic chemical reaction can be observed. Specifically, the unimolecular Claisen rearrangement of allyl p-nitrophenyl ether (ApNE) dissolved in naphthalene was studied under microwave heating and conventional convective (thermal) heating. Under constant microwave power, reaching a temperature of 185°C, a 4-fold rate enhancement was observed in the microwave over that using convective heating; this means that the microwave reaction was proceeding at an effective temperature of 202°C. Conversely, under constant temperature microwave conditions (200°C), a negligible (∼1.5-fold) microwavespecific rate enhancement was observed. The largest microwave-specific rate enhancement was observed when a series of 300 W pulses, programmed for 145− 175°C and 85−155°C cycles, where 2-and 9-fold rate enhancements, over what would be predicted by conventional thermal heating, was observed, respectively. The postulated origins of the microwave-specific effect are purely thermal and arise from selective heating of ApNE, a microwaveabsorbing reactant in a nonabsorbing solvent. Under these conditions, excess heat is accumulated in the domains around the ApNE solute so that it experiences a higher effective temperature than the measured temperature of the bulk medium, resulting in an accelerated unimolecular rearrangement.
Self-supported mesoporous Pt nanospheres
are prepared by the templated
method for the electrocatalytic oxygen reduction reaction (ORR). Single-crystal-like
MCM-48 nanospheres, synthesized via a new and simple route in a dilute
and alkaline solution of benzylcetyldimethylammonium chloride, are
applied as the hard template. A facile process of molten salt impregnation
followed by hydrogen reduction is developed to replicate both the
mesostructure and morphology of the hard template and to form mesoporous
Pt nanospheres. The high-surface-area mesoporous Pt nanospheres are
thermally stable in air up to 300 °C without showing significant
metal sintering. Electrocatalytic studies reveal that mesoporous Pt
nanospheres have a large fraction of {110} facets and exhibit high
activity for the ORR. The excellent electrocatalytic performance may
be mainly attributed to the facet-dependent ORR activities as well
as the presence of 3D interconnected mesopores to facilitate mass
transport.
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