Microwave gas-solid reactivity in industrially relevant systems

摘要

Gas-Solid reactivity was extensively studied throughout the early 20th century and the kinetics of these systems have become well established and well understood. Recently, microwave active materials (conductive or magnetic materials that absorb microwave irradiation) have been shown to produce increased reactivity in a significantly different way when compared to conventional heating. Any of these materials can be used to improve reactivity in industrially relevant gas-solid systems. Many of these rate enhancements can be measured by using reaction kinetics, and these kinetic rates can be compared to the previously studied, well established, thermal measurements. By understanding the difference between microwaves and conventional heating we may better predict which systems would be ideal candidates for increased reactivity. Specifically the reaction between steam and carbon has been measured extensively in the past and could be ideal to benefit from microwave irradiation. "C + H2O " ?? " H2 + CO " At 131 kJ/mol this endothermic reaction uses carbon as its microwave active material. This solid can be any form of carbon (activated carbon, graphite, coal etc.) and it selectively heats in a microwave reactor. This reaction was shown to have a large difference in apparent activation energies and kinetic rates when compared to the thermal rates and energies. By using an Arrhenius plot, the apparent microwave equilibrium constants were calculated at various wattages and shown to be lower when matched against comparable temperature ranges of the conventional thermal reactions. The enthalpy and entropy of the systems were then calculated to give an effective thermodynamic value to describe the energy differences. Not only was the reaction more efficient in the microwave, but the microwave composition of the product gases included less CO2, which would be produced from a water gas shift side reaction. These findings, of a system that produces less side products at lower temperatures, are evidence that microwave gas-solid reactions could provide unique chemistry that should be applied to more industrially relevant systems. Probing the mechanisms of these results was done by using a nitrous oxide and carbon system to observe the compositional difference in reactivity. "2C + 2NO2" ?? " N2 + 2CO2 " The interfacial polarization of the carbon is understood to be the method of heating in a microwave reactor. Electron hole pairs are created as the charges separate and become trapped at grain boundaries across the surface of the material. These electron hole pairs create an active site on the surface that helps facilitate reactivity and sometimes leads to different compositional makeup of product gases. Probing this mechanism was important to help describe which systems would be good candidates to study in further research endeavors.