(Invited) Tuning of Catalytic Activity By Thermoelectric Effect

Abstract

Thermoelectric (TE) materials convert temperature difference directly into electrical voltage via the Seebeck effect, S= -V/ΔT where V is the voltage between the two ends of the TE material and ΔT the temperature difference, S is the Seebeck coefficient. It has been used for energy harvesting from waste heat, and for silent cooling via its converse phenomenon the Peltier effect. Here we report its novel application as a catalyst promoter. Catalyst promoters can generally be divided into structural and electronic promoters. Structural promoters enhance and stabilize the dispersion of the catalyst on the support. Electronic promoters induce changes of electronic state of the catalyst near the Fermi level. These promoters are added or fixed during the catalyst preparation, therefore can subject to degradation during catalytic process. We report here the realization of in situ, controllable, reversible, and significant modification of catalytic activity, through the generation of a Seebeck voltage. A chamber reactor which combine TE effect with catalytic chemical reaction was assembled.1, 2 The chamber was placed directly on a hot-plate, and its top cap was water cooled. A disc TE BiCuSeO (BCSO) sample of 20 mm in diameter and about 2 mm in thickness was then placed into a specific glass ceramic holder and attached to the cold surface of the cap for catalytic reaction investigation under the usual TE conditions. As the back side of the disc was in contact with the water cooled stainless steel cap, its temperature was never higher than 373 K, so a large temperature gradient therefore a large Seebeck voltage across the sample thickness was created when the front surface of the disc reached a high temperature Th. Under the reduced TE (RTE) condition, the back surface of the disc was not in contact with the cap, therefore the temperature gradient across the disc thickness and also the Seebeck voltage were much smaller. Figure 1a shows the ethylene oxidation reaction rates at different front surface temperatures Th for a sample of Pt thin film of 80 nm supported on BCSO with a large TE effect (red marks, Pt(80)/BCSO TE), and under a reduced TE effect (blue marks, Pt(80)/BCSO RTE) conditions respectively. For comparison purpose, the ethylene reaction rate on a typical catalyst for this reaction Pt thin film on Y2O3-stabilized ZrO2 (YSZ, green marks, Pt(80)/YSZ) is also depicted in Fig. 1a. For Pt(80)/BCSO TE, when the front surface Th was at 666 K, the backside temperature Tc was 349 K, so a temperature difference of 317 K existed across the BCSO thickness, which generated a Seebeck voltage -71 mV. For the same sample under the reduced TE effect (Pt(80)/BCSO RTE), at 553 K, the measured Seebeck voltage was -3.1 mV, and the reaction rate was much lower than that for Pt(80)/YSZ at a similar temperature 563 K. This was probably because the surface area of the Pt(80)/BCSO was much smaller than for Pt(80)/YSZ. However, with the increasing temperature, the reaction rates for Pt(80)/BCSO under TE and RTE conditions increased much faster than for non-TE YSZ supported catalyst. Further detailed investigation demonstrated that when the total reaction rate was lower than a certain value, a linear relationship existed between the logarithm of the reaction rate and the Seebeck voltage.1, 2 We called this thermoelectric promotion of catalysis (TEPOC). To confirm the above observed TEPOC can also be observed in other reactions, the same samples were used as catalysts for carbon dioxide hydrogenation. The inlet gases were CO2 and H2, and the reaction products observed were CO and CH4, with the vast majority (>90%) being CO. Similar as was observed for ethylene oxidation, for the same sample under the same inlet gas composition, the CO2 conversion rates r under TE conditions were much higher (between 10 to 30 times) than under RTE conditions. Again, a good linear relationship between Ln(r) and -eV/kbTh was observed (Figure 1b). The effects of the Seebeck voltage on the catalytic activity under different reaction conditions, such as fuel-lean and fuel-rich conditions for the ethylene oxidation, have also been investigated. The underlying mechanisms based on metal-semiconductor contact theory and work function change, as well as the comparison with the Electrochemical Promotion of Catalysis, will be discussed.