Thermoelectric materials with high surface area for the application of thermoelectric promotion of catalysis

Abstract

Thermoelectric materials have been used for generating electricity from a temperature difference and noiseless cooling from electricity for several decades, but their application in the field of catalysis is still in its infancy. In fact, only a few years ago my supervisor and his colleagues used thermoelectric materials as a catalyst support and promoter for the first time in the world, and discovered that a Seebeck voltage can increase the catalytic activity by several tens to hundreds of times, and called this effect thermoelectric promotion of catalysis (TEPOC), or thermoelectrocatalysis. This effect has since been confirmed by other people in the world. This work aims to investigate other fundamental aspects of the application of thermoelectric materials for catalysis promotion, e.g., how to combine high surface area porous structure, how to achieve highest possible temperature difference in a given reaction chamber, and whether it is possible to use other thermoelectric materials which can work at high temperatures, for the thermoelectric promotion of catalysis. Numerical modelling has been used to simulate the transient thermal behaviors of thermoelectric materials in a custom-made reaction chamber during a typical chemical reaction run, and the modelling results were used to guide the sample design. Firstly, thicker sample was found to generate higher Seebeck voltage. Secondly, stacked samples were found to have the similar promotional effect as if it is one single sample. The second point also enables the sample to be consisted of a dense bulk and porous surface sample, so that both the high Seebeck voltage and high surface area for the sample could be achieved for a stacked sample. Sol-gel technique and the addition of polymer particles to thermoelectric powders have been used to obtained porous high surface area thermoelectric materials. Their microstructural and thermal transport properties have been characterized and their effect for the application of TEPOC on the reverse water-gas shift (RWGS) reactions has been investigated. Generally speaking, the porous sample has a much higher (>10 times) surface area than their non- porous counterparts, but possesses much lower Seebeck coefficient. This leads to a modest improvement of catalysis promotion as compared to the non- porous ones. The previously observed TEPOC dynamic rate equation, i.e., the linearity between the promotional ratio and the Seebeck voltage, was proved to be effective for porous thermoelectric materials. A new Figure of merit for thermoelectric materials for the application of TEPOC, has been proposed as S/κ, here S is the Seebeck coefficient and κ the thermal conductivity of the thermoelectric material. This Figure of merit is different from the usual dimensionless Figure of merit ZT which is best for the indication of energy conversion efficiency. A comprehensive literature survey has been carried out and it was found out that among all the non-single crystal thermoelectric materials, the previously used BiCuSeO have the highest S/κ, and another oxide Ca₃Co₄O₉ systems which are stable at very high temperatures also have a high S/κ. Following this, thermoelectric Ca₂.₆Tb₀.₄Co₄O₉ samples were prepared and applied in CO₂ hydrogenation reactions. The results reveal a strong enhancement in Seebeck coefficient by doping Tb into Ca₃Co₄O₉, while Ca₂.₆Tb₀.₄Co4O₉ samples showed excellent stability at the range of 373K-1073K. A highest CO₂ conversion of 64% was obtained from Ca₂.₆Tb₀.₄Co₄O₉ samples at 700K and inlet gas ratio H₂:CO₂=3:1. In summary, the high catalytic activity of porous BiCuSeO samples demonstrates the great potential of combining high porosity with the thermoelectric materials in the application of TEPOC. In the meantime, the use of Ca₂.₆Tb₀.₄Co₄O₉ enables higher temperature range in the field of TEPOC application.