Abdenour Achour

Lignocellulosic biomass, especially lignin and extractives, has the potential to substitute fossil precursors in the chemical industry. The conversion of lignin has been intensively researched, but challenges remain as high temperatures and increased pressure are commonly used, which is not energy efficient. Lignin and wood extractives bear chromophoric groups that can absorb light in the ultraviolet (UV) region, which enables photochemical reactions. Ultraviolet light emitting diodes (UV-LEDs) are an emerging technology; they are cheap, versatile, and energy-efficient compared to existing mercury lamps. UV-LEDs were used in this study as a proof of concept for the valorisation of a lignocellulosic residual stream from the pulp and paper industry, sawdust. In a process at ambient temperature and pressure and without the use of a catalyst, we have shown that lignin and extractives can be valorised using light from UV-LEDs. Simplified lignin model compounds were used to pinpoint chemical reactions during irradiation, and to ease the analysis of the sawdust samples. The rate of conversion upon irradiation of the model compounds was found to be 0.7–2.3 g L⁻¹ h⁻¹, depending on the concentration of the starting compounds.

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 T_h. 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 T_h 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 T_h was at 666 K, the backside temperature T_c 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/k_bT_h 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. References: ¹A. Achour, J. Liu, P. Peng, C. Shaw, Z. Huang, In situ Tuning of catalytic activity by thermoelectric materials for ethylene oxidation, ACS catalysis, 8, 10164-10172 (2018). ²A. Achour, K. Chen, M. J. Reece, Z. Huang, Tuning of catalytic activity by thermoelectric materials for carbon dioxide hydrogenation, Advanced Energy Materials, 8, 1701430 (2018). Figure 1

An innovative use of a thermoelectric material (BiCuSeO) as a support and promoter of catalysis for CO₂ hydrogenation is reported here. It is proposed that the capability of thermoelectric materials to shift the Fermi level and work function of a catalyst lead to an exponential increase of catalytic activity for catalyst particles deposited on its surface. Experimental results show that the CO₂ conversion and CO selectivity are increased significantly by a thermoelectric Seebeck voltage. This suggests that the thermoelectric effect can not only increase the reaction rate but also change chemical equilibrium, which leads to the change of thermodynamic equilibrium for the conversion of CO₂ in its hydrogenation reactions. It is also shown that this thermoelectric promotion of catalysis enables BiCuSeO oxide itself to have a high catalytic activity for CO₂ hydrogenation. The generic nature of the mechanism suggests the possibility that many catalytic chemical reactions can be tuned in situ to achieve much higher reaction rates, or at lower temperatures, or have better desired selectivity through changing the backside temperature of the thermoelectric support.