Status and outlook of solid electrolyte membrane reactors for energy, chemical, and environmental applications

Solid electrolyte membrane reactors (SEMRs) can be operated at high temperatures with distinct reaction kinetics, or at lower temperatures (300-500 °C) for industrially relevant energy applications (such as solid oxide fuel/electrolysis cells, direct carbon fuel cells, and metal-air batteries), chemical (such as alkane dehydrogenation, C-C coupling, and NH3 synthesis), environmental (De-NO x , CO2 utilization, and separation), as well as their combined (one-step coupled CO2/H2O co-electrolysis and methanation reaction, power and chemical cogeneration) applications. SEMRs can efficiently integrate electrical, chemical, and thermal energy sectors, thereby circumventing thermodynamic constraints and production separation issues. They offer a promising way to achieve carbon neutrality and improve chemical manufacturing processes. This review thoroughly examines SEMRs utilizing various ionic conductors, namely O2-, H+, and hybrid types, with operations in different reactor/cell architectures (such as panel, tubular, single chamber, and porous electrolytes). The reactors operate in various modes including pumping, extraction, reversible, or electrical promoting modes, providing multiple functionalities. The discussion extends to examining critical materials for solid-state cells and catalysts essential for specific technologically important reactions, focusing on electrochemical performance, conversion efficiency, and selectivity. The review also serves as a first attempt to address the potential of process-intensified SEMRs through the integration of photo/solar, thermoelectric, and plasma energy and explores the unique phenomenon of electrochemical promotion of catalysis (EPOC) in membrane reactors. The ultimate goal is to offer insight into ongoing critical scientific and technical challenges like durability and operational cost hindering the widespread industrial implementation of SEMRs while exploring the opportunities in this rapidly growing research domain. Although still in an early stage with limited demonstrations and applications, advances in materials, catalysis science, solid-state ionics, and reactor design, as well as process intensification and/or system integration will fill the gaps in current high temperature operation of SEMRs and industrially relevant applications like sustainable clean chemical production, efficient energy conversion/storage, as well as environmental enhancement.

Advances in plasma-driven solution electrochemistry

Energetic species produced by gas-phase plasmas that impinge on a liquid surface can initiate physicochemical processes at the gas/liquid interface and in the liquid phase. The interaction of these energetic species with the liquid phase can initiate chemical reaction pathways referred to as plasma-driven solution electrochemistry (PDSE). There are several processing opportunities and challenges presented by PDSE. These include the potential use of PDSE to activate chemical pathways that are difficult to activate with other approaches as well as the use of renewable electricity to generate plasmas that could make these liquid-phase chemical conversion processes more sustainable and environmentally friendly. In this review, we focus on PDSE as an approach for controlled and selective chemical conversion including the synthesis of nanoparticles and polymers with desired but currently uncontrollable or unattainable properties as the next step in the use of PDSE. The underpinning redox chemistry and transport processes of PDSE are reviewed as many PDSE-driven processes are transport-limited due to the many short-lived highly reactive species involved.

An electricity-powered future for mixed plastic waste chemical recycling

In contemporary times, global plastic waste production has doubled in comparison to two decades ago, with only 9% effectively recycled. The polymer industry is undergoing a transition to address the disparity between plastic production and end-of-life waste management. Chemical recycling offers a solution by converting plastic waste into its constituent building blocks, or monomers, which can be utilized in the production of new, high-quality plastics. This concise review provides an overview of conventional chemical recycling technologies employing heated reactors, before delving into ongoing efforts towards electrifying the chemical recycling process. A conceptual framework for a fully electrified value chain aimed at achieving plastics circularity is outlined and analyzed. Additionally, attention is given to the challenges posed by industry inertia towards adopting electrified technologies, as well as performance issues stemming from the intermittent nature of renewable energy sources and the availability of long-duration renewable electricity storage options.

Electrification-Enhanced Low-Temperature NOx Storage-Reduction on Pt and K Co-Supported Antimony-Doped Tin Oxides

NOx storage-reduction (NSR), a promising approach for removing NOx pollutants from diesel vehicles, remains elusive to cope with the increasingly lower exhaust temperatures (especially below 250 °C). Here, we develop a conceptual electrified NSR strategy, where electricity with a low input power (0.5-4 W) is applied to conductive Pt and K co-supported antimony-doped tin oxides (Pt-K/ATO), with C3H6 as a reductant. The ignition temperature for 10% NOx conversion is nearly 100 °C lower than that of the traditional thermal counterpart. Furthermore, reducing the power in the fuel-lean period relative to that in the fuel-rich period increases the maximum energy efficiency by 23%. Electrically driven release of lattice oxygen is revealed to play vital roles in multiple steps in NSR, including NO adsorption, desorption, and reduction, for improved NSR activity. This work provides an electrification strategy for developing high-activity NSR catalysis utilizing electricity onboard hybrid vehicles.

Direct Amination of Benzene with Ammonia by Flow Plasma Chemistry

Amine derivatives, including aniline and allylic amines, can be formed in a single-step process from benzene and an ammonia plasma in a microreactor. Different process parameters such as temperature, residence time, and plasma power were evaluated to improve the reaction yield and its selectivity toward aminated products and avoid hydrogenated or oligomerized products. In parallel, simulation studies of the process have been carried out to propose a global mechanism and gain a better understanding of the influence of the different process parameters. The exploration of diverse related alkenes showed that the double bonds, conjugation, and aromatization influenced the amination mechanism. Benzene was the best reactant for amination based on the lifetime of radical intermediates. Under optimized conditions, benzene was aminated in the absence of catalyst with a yield of 3.8 % and a selectivity of 49 % in various amino compounds.

Advanced (photo)electrocatalytic approaches to substitute the use of fossil fuels in chemical production

Electrification of the chemical industry for carbon-neutral production requires innovative (photo)electrocatalysis. This study highlights the contribution and discusses recent research projects in this area, which are relevant case examples to explore new directions but characterised by a little background research effort. It is organised into two main sections, where selected examples of innovative directions for electrocatalysis and photoelectrocatalysis are presented. The areas discussed include (i) new approaches to green energy or H2 vectors, (ii) the production of fertilisers directly from the air, (iii) the decoupling of the anodic and cathodic reactions in electrocatalytic or photoelectrocatalytic devices, (iv) the possibilities given by tandem/paired reactions in electrocatalytic devices, including the possibility to form the same product on both cathodic and anodic sides to "double" the efficiency, and (v) exploiting electrocatalytic cells to produce green H2 from biomass. The examples offer hits to expand current areas in electrocatalysis to accelerate the transformation to fossil-free chemical production.

Electrified Hydrogen Production from Methane for PEM Fuel Cells Feeding: A Review

The greatest challenge of our times is to identify low cost and environmentally friendly alternative energy sources to fossil fuels. From this point of view, the decarbonization of industrial chemical processes is fundamental and the use of hydrogen as an energy vector, usable by fuel cells, is strategic. It is possible to tackle the decarbonization of industrial chemical processes with the electrification of systems. The purpose of this review is to provide an overview of the latest research on the electrification of endothermic industrial chemical processes aimed at the production of H2 from methane and its use for energy production through proton exchange membrane fuel cells (PEMFC). In particular, two main electrification methods are examined, microwave heating (MW) and resistive heating (Joule), aimed at transferring heat directly on the surface of the catalyst. For cases, the catalyst formulation and reactor configuration were analyzed and compared. The key aspects of the use of H2 through PEM were also analyzed, highlighting the most used catalysts and their performance. With the information contained in this review, we want to give scientists and researchers the opportunity to compare, both in terms of reactor and energy efficiency, the different solutions proposed for the electrification of chemical processes available in the recent literature. In particular, through this review it is possible to identify the solutions that allow a possible scale-up of the electrified chemical process, imagining a distributed production of hydrogen and its consequent use with PEMs. As for PEMs, in the review it is possible to find interesting alternative solutions to platinum with the PGM (Platinum Group Metal) free-based catalysts, proposing the use of Fe or Co for PEM application.

Catalysis for e-Chemistry: Need and Gaps for a Future De-Fossilized Chemical Production, with Focus on the Role of Complex (Direct) Syntheses by Electrocatalysis

The prospects, needs and limits in current approaches in catalysis to accelerate the transition to e-chemistry, where this term indicates a fossil fuel-free chemical production, are discussed. It is suggested that e-chemistry is a necessary element of the transformation to meet the targets of net zero emissions by year 2050 and that this conversion from the current petrochemistry is feasible. However, the acceleration of the development of catalytic technologies based on the use of renewable energy sources (indicated as reactive catalysis) is necessary, evidencing that these are part of a system of changes and thus should be assessed from this perspective. However, it is perceived that the current studies in the area are not properly addressing the needs to develop the catalytic technologies required for e-chemistry, presenting a series of relevant aspects and directions in which research should be focused to develop the framework system transformation necessary to implement e-chemistry.

Across the Board: Gabriele Centi on Decoupling Electrocatalytic Reactions to Electrify Chemical Production

In this series of articles, the board members of ChemSusChem discuss recent research articles that they consider of exceptional quality and importance for sustainability. This entry features Prof. G. Centi, who discusses the decoupling of the electrocatalytic reactions to realize spatiotemporal separation of the anodic and cathodic processes using redox mediators. This solution allows to potentially overcome the limitations due to intermittency of renewable energy production, besides a series of other advantages such as an improved energy efficiency.

Electrified methane steam reforming on a washcoated SiSiC foam for low‐carbon hydrogen production

In view of largely available renewable electricity as a green future resource, here we report the electrification of a Rh/Al₂O₃ washcoated SiSiC foam for methane steam reforming (MSR). We show that, thanks to the suitable bulk resistivity of the SiSiC foam, its direct Joule heating up to relevant temperatures is feasible; the interconnected geometry greatly reduces heat and mass transfer limitations, which results in a highly active and energy efficient system for low-carbon H₂ production. The foam-based electrified MSR (eMSR) system showed almost full methane conversion above 700°C and methane conversions approaching equilibrium were obtained in a range of conditions. Energy efficiency as high as 61% and specific power consumption as low as 2.0 kWh/ N m H 2 3 were measured at 650°C, at gas hourly space velocity (GHSV) of 150,000 cm³/h/g_cat. When driven by renewable electricity, the proposed reactor configuration promises a high potential to address the decarbonization challenge in the near-term future.

Recent Advances in the Development of Highly Conductive Structured Supports for the Intensification of Non-adiabatic Gas-Solid Catalytic Processes: The Methane Steam Reforming Case Study

Structured catalysts are strong candidates for the intensification of non-adiabatic gas-solid catalytic processes thanks to their superior heat and mass transfer properties combined with low pressure drops. In the past two decades, different types of substrates have been proposed, including honeycomb monoliths, open-cell foams and, more recently, periodic open cellular structures produced by additive manufacturing methods. Among others, thermally conductive metallic cellular substrates have been extensively tested in heat-transfer limited exo- or endo-thermic processes in tubular reactors, demonstrating significant potential for process intensification. The catalytic activation of these geometries is critical: on one hand, these structures can be washcoated with a thin layer of catalytic active phase, but the resulting catalyst inventory is limited. More recently, an alternative approach has been proposed, which relies on packing the cavities of the metallic matrix with catalyst pellets. In this paper, an up-to-date overview of the aforementioned topics will be provided. After a brief introduction concerning the concept of structured catalysts based on highly conductive supports, specific attention will be devoted to the most recent advances in their manufacturing and in their catalytic activation. Finally, the application to the methane steam reforming process will be presented as a relevant case study of process intensification. The results from a comparison of three different reactor layouts (i.e. conventional packed bed, washcoated copper foams and packed copper foams) will highlight the benefits for the overall reformer performance resulting from the adoption of highly conductive structured internals.

Direct Conversion of Methane to C2 Hydrocarbons in Solid-State Membrane Reactors at High Temperatures

Direct conversion of methane to C₂ compounds by oxidative and nonoxidative coupling reactions has been intensively studied in the past four decades; however, because these reactions have intrinsic severe thermodynamic constraints, they have not become viable industrially. Recently, with the increasing availability of inexpensive "green electrons" coming from renewable sources, electrochemical technologies are gaining momentum for reactions that have been challenging for more conventional catalysis. Using solid-state membranes to control the reacting species and separate products in a single step is a crucial advantage. Devices using ionic or mixed ionic-electronic conductors can be explored for methane coupling reactions with great potential to increase selectivity. Although these technologies are still in the early scaling stages, they offer a sustainable path for the utilization of methane and benefit from the advances in both solid oxide fuel cells and electrolyzers. This review identifies promising developments for solid-state methane conversion reactors by assessing multifunctional layers with microstructural control; combining solid electrolytes (proton and oxygen ion conductors) with active and selective electrodes/catalysts; applying more efficient reactor designs; understanding the reaction/degradation mechanisms; defining standards for performance evaluation; and carrying techno-economic analysis.

Mechanistic and multiscale aspects of thermo-catalytic CO2 conversion to C1 products

The increasing environmental concerns due to anthropogenic CO2 emissions have called for an alternate sustainable source to fulfill rising chemical and energy demands and reduce environmental problems. The thermo-catalytic activation and conversion of abundantly available CO2, a thermodynamically stable and kinetically inert molecule, can significantly pave the way to sustainably produce chemicals and fuels and mitigate the additional CO2 load. This can be done through comprehensive knowledge and understanding of catalyst behavior, reaction kinetics, and reactor design. This review aims to catalog and summarize the advances in the experimental and theoretical approaches for CO2 activation and conversion to C1 products via heterogeneous catalytic routes. To this aim, we analyze the current literature works describing experimental analyses (e.g., catalyst characterization and kinetics measurement) as well as computational studies (e.g., microkinetic modeling and first-principles calculations). The catalytic reactions of CO2 activation and conversion reviewed in detail are: (i) reverse water-gas shift (RWGS), (ii) CO2 methanation, (iii) CO2 hydrogenation to methanol, and (iv) dry reforming of methane (DRM). This review is divided into six sections. The first section provides an overview of the energy and environmental problems of our society, in which promising strategies and possible pathways to utilize anthropogenic CO2 are highlighted. In the second section, the discussion follows with the description of materials and mechanisms of the available thermo-catalytic processes for CO2 utilization. In the third section, the process of catalyst deactivation by coking is presented, and possible solutions to the problem are recommended based on experimental and theoretical literature works. In the fourth section, kinetic models are reviewed. In the fifth section, reaction technologies associated with the conversion of CO2 are described, and, finally, in the sixth section, concluding remarks and future directions are provided.

Inertially enhanced mass transport using 3D-printed porous flow-through electrodes with periodic lattice structures

Electrochemical reactors utilizing flow-through electrodes (FTEs) provide an attractive path toward the efficient utilization of electrical energy, but their commercial viability and ultimate adoption hinge on attaining high currents to drive productivity and cost competitiveness. Conventional FTEs composed of random, porous media provide limited opportunity for architectural control and engineering of microscale transport. Alternatively, the design freedom engendered by additively manufacturing FTEs yields additional opportunities to further drive performance via flow engineering. Through experiment and validated continuum computation we analyze the mass transfer in three-dimensional (3D)-printed porous FTEs with periodic lattice structures and show that, in contrast to conventional electrodes, the mesoscopic length scales in 3D-printed electrodes lead to an increase in the mass correlation exponent as inertial flow effects dominate. The inertially enhanced mass transport yields mass transfer coefficients that exceed previously reported 3D-printed FTEs by 10 to 100 times, bringing 3D-printed FTE performance on par with conventional materials.

Catalyst Heating Characteristics in the Traveling-Wave Microwave Reactor

Traveling-Wave Microwave Reactor (TMR) presents a novel heterogeneous catalytic reactor concept based on a coaxial waveguide structure. In the current paper, both modeling and experimental studies of catalyst heating in the TMR are presented. The developed 3D multiphysics model was validated from the electromagnetic and heat transfer points of view. Extrudes of silicon carbide (SiC) were selected as catalyst supports and microwave absorbing media in a packed-bed configuration. The packed-bed temperature evolution was in good agreement with experimental data, with an average deviation of less than 10%. Both experimental and simulation results show that the homogeneous temperature distribution is possible in the TMR system. It is envisioned that the TMR concept may facilitate process scale-up while providing temperature homogeneity beyond the intrinsic restrictions of microwave cavity systems.

Cavitation intensifying bags improve ultrasonic advanced oxidation with Pd/Al2O3 catalyst

Advanced oxidation processes can potentially eliminate organic contaminants from industrial waste streams as well as persistent pharmaceutical components in drinking water. We explore for the first time the utilization of Cavitation Intensifying Bags (CIB) in combination with Pd/Al2O3 catalyst as possible advanced oxidation technology for wastewater streams, oxidizing terephthalic acid (TA) to 2-hydroxyterephthalic acid (HTA). The detailed characterization of this novel reaction system reveals that, during sonication, the presence of surface pits of the CIB improves the reproducibility and thus the control of the sonication process, when compared to oxidation in non-pitted bags. Detailed reaction kinetics shows that in the CIB reactor the reaction order to TA is zero, which is attributed to the large excess of TA in the system. The rate of HTA formation increased ten-fold from ~0.01 μM*min-1 during sonication in the CIB, to ~0.10 μM*min-1 for CIB in the presence of the Pd/Al2O3 catalyst. This enhancement was ascribed to a combination of improved mass transport, the creation of thermal gradients, and Pd/Al2O3 catalyst near the cavitating bubbles. Further analysis of the kinetics of HTA formation on Pd/Al2O3 indicated that initially the reaction underwent through an induction period of 20 min, where the HTA concentration was ~0.3 μM. After this, the reaction rate increased reaching HTA concentrations ~6 μM after 40 min. This behavior resembled that observed during oxidation of hydrocarbons on metal catalysts, where the slow rate formation of hydroperoxides on the metal surface is followed by rapid product formation upon reaching a critical concentration. Finally, a global analysis using the Intensification Factor (IF) reveals that CIB in combination with the Pd/Al2O3 catalyst is a desirable option for the oxidation of TA when considering increased oxidation rates and costs.