This page is designed for scientists who have some prior knowledge of chemistry, catalysis and spectroscopy, and is a quick summary of the research I have carried out into microwave catalysis. A simpler treatment is available here, and a much more detailed view is available in my thesis.

The image above is time resolved difference IR spectra obtained using a transmission IR microreactor cell of my own design. It shows how the oxidation of carbon monoxide over Europlatinum-1 (a standard Pt/Al₂O₃ catalyst) changes when heated using microwave radiation.

My PhD thesis covers the production of two novel microreactor cells which allow an assessment of catalytic activity under conventional and microwave heating. Comparing the IR spectra obtained, along with monitoring the exhaust gases using MS under both reaction regimes allows investigation into the possible 'microwave effect' --- the non-thermal enhancement in rate which has been reported for some reactions.

The basic reactor form holds the sample within a parallel plate applicator --- two conducting plates separated by a distance not more that one tenth of their width. These contain calcium fluoride windows to allow IR spectroscopy of the sample, and these windows are covered with gauze to approximate a flat conductor (needed for a homogeneous microwave field), and to prevent microwave leakage. An applicator plate from my transmission cell is pictured in Fig. 1 and also has a hole to house a cartridge heater.

The sample is held between two of these plates in a ceramic sample holder which is transparent to microwave radiation. The sample holder for the transmission cell is Fig. 2, and takes a pressed disc sample of 13mm diameter.

The attached pipes are 1/16 and 1/8 inch diameter and allow a flow of gas over the catalyst disc when the cell is assembled. Gas flow in is controlled by a number of mass flow controllers, and exhaust gases are monitored with a portable mass spectrometer. The assembled cell is shown below, with the two applicator plates clamped around the sample holder.

The plates are insulated from each other by a mica sheet under the clamping plate. Microwave power is supplied through the copper coaxial cable with SMA connector, which appears on the right side. The green cable is attached to a small, shielded thermocouple embedded in the ground applicator plate. The whole assembly is placed inside a standard FTIR (I used a Biorad FTS-6000), and spectra can then be recorded with the catalyst under a controlled atmosphere, and heated using microwaves, the cartridge heaters, or both.

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Historically microwaves inside the laboratory were used for rotational spectroscopy. The difference between rotational energy levels is very small, and similar to that of microwave photons. These energy levels are only observed in the gas phase, as rapid quenching through collisions occurs in condensed phases. It should be noted at this point that the energy required to break a chemical bond is orders of magnitude greater than the energy of a microwave photon, and any increase in activity under microwave heating can therefore not be due to a direct photochemical reaction. However, the interaction of microwaves with matter is complex, especially in non-uniform systems. Uneven or selective heating may occur, and complex temperature differentials may be established. In this situation, the rate measured in a reacting system will not correspond to the measured temperature. Indeed most of the controversy in the field is due to the difficulties of accurate temperature measurement.

In liquid systems, superheating can occur due to the absence of nucleation sites, and rate enhancements can generally be attributed to this. For solids or gases however, there is no equivalent theory which has established itself in the same way. Furthermore, it is possible that more than one effect is causing enhancements for different materials. There is evidence for the establishment of small intense temperature gradients in 'hot spots', but due to the difficulty of temperature measurement across small regions, direct observation is difficult.

The work I undertook to test my cells appears to show that the reaction I studied (CO oxidation over Europlatinum-1) is unaffected by the source of heat. The reaction rate is the same at a given temperature, whether heated conventionally or with a microwave. This agrees with previous results in the literature for catalytic CO oxidation under microwave heating. It is likely that the application of in-situ IR to other systems which show variation between the two heating methods could provide some enlightenment as to the method of this, and I hope that what I have written is helpful to whoever chooses to pursue this interesting topic further.

Feedback and questions to dr.silverwood@googlemail.com are welcome.