Carbon and nitrogen fixation strategies are regarded as alternative routes to produce valuable chemicals used as energy carriers and fertilizers that are traditionally obtained from unsustainable and energy-intensive coal gasification (CO and CH 4 ), Fischer−Tropsch (C 2 H 4 ), and Haber−Bosch (NH 3 ) processes. Recently, the electrocatalytic CO 2 reduction reaction (CO 2 RR) and N 2 reduction reaction (NRR) have received tremendous attention, with the merits of being both efficient strategies to store renewable electricity while providing alternative preparation routes to fossil-fuel-driven reactions. To date, the development of the CO 2 RR and NRR processes is primarily hindered by the competitive hydrogen evolution reaction (HER); however, the corresponding strategies for inhibiting this undesired side reaction are still quite limited. Considering such complex reactions involve three gas−liquid−solid phases and successive proton-coupled electron transfers, it appears meaningful to review the current strategies for improving product selectivity in light of their respective reaction mechanisms, kinetics, and thermodynamics. By examining the developments and understanding in catalyst design, electrolyte engineering, and three-phase interface modulation, we discuss three key strategies for improving product selectivity for the CO 2 RR and NRR: (i) targeting molecularly defined active sites, (ii) increasing the local reactant concentration at the active sites, and (iii) stabilizing and confining product intermediates.
The recycling of tantalum (Ta) is becoming increasingly important due to the criticality of its supply from a conflict mineral. It is used extensively in modern electronics, such as in capacitors, and so electronic waste is a potentially valuable secondary source of this metal. However, the recycling of Ta is difficult, not least because of the challenges of its leaching and subsequent separation from other metals. In this work, we show that Ta(V) halides, such as TaCl5 and TaF5, which can potentially be accessed from Ta metal upon acid halide leaching, can be recovered by solvent extraction using a simple primary amide reagent. The need for high halide concentrations in the aqueous phase implies the formation of the hexahalide salts [TaX6]− (X = F, Cl) and that an anion-swing mechanism operates. While extraction of the fluorides is poor (up to 45%), excellent extraction under chloride conditions is found (>99%) and presents an alternative route to Ta recycling.
Lignin is an abundant natural polymer obtained from lignocellulosic biomass and rich in aromatic substructures. When efficiently depolymerized, it has great potential in the production of value-added chemicals. Fast pyrolysis is a promising depolymerization method, but current studies focus mainly on small quantities of lignin. In this Review, to determine the potential for upscaling, systems used in the most relevant unit operations of fast pyrolysis of lignin are evaluated. Fluidizedbed reactors have the most potential. It would be beneficial to combine them with the following: slug injectors for feeding, hot particle filters, cyclones, and fractional condensation for product separation and recovery. Moreover, upgrading lignin pyrolysis oil would allow the necessary quality parameters for particular applications to be reached.
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