Green hydrogen plays a crucial role in decarbonization and the future of low-carbon society. Still, its transport/distribution and cost of production, mainly realized by electrolysis, are major hurdles. Liquid H₂ carriers reduce transport/distribution costs but add further expenses for their production. To address this challenge, we proposed a novel strategy for electrocatalytic production of a liquid organic hydrogen carrier with anodic valorization of the process. This review summarizes the state of the art and outlooks in this new concept. The electrocatalytic process is briefly introduced, and the main components are discussed. Subsequently, the electrocatalytic production of liquid organic hydrogen carriers and anodic oxidation from components to processes, together with the paired processes and reactors, are analyzed, highlighting challenges and prospects.

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To mitigate global warming and achieve a sustainable society, innovative technologies for efficient CO2 utilization are required. Integrated CO2 capture and reduction (CCR) using dual-function materials (DFMs) is favorable owing to its potentially low energy consumption, capital investment, and processing costs. Although numerous studies have focused on catalytic science, continuous and steady-state CCR operations have not been sufficiently addressed from an engineering perspective. In this study, a circulating fluidized bed (CFB) system is investigated for continuous CCR to syngas (CO + H2). In the CFB system, transition-metal-free DFM (Na/Al2O3) particles are circulated between two bubbling fluidized-bed reactors. The DFM captures CO2 in one reactor (CO2 capture reactor) and reduces the captured CO2 to CO by the reaction with H2 in the other reactor (H2 reactor). The effluent gas concentrations from both reactors reach steady state and are maintained for over 8 h. For the product gas from the H2 reactor, the CO2 conversion and CO selectivity exceed 80 % and 99 %, respectively. However, the H2 conversion is <20 %, indicating a potential challenge for the CFB system for integrated CCR. Furthermore, this study confirms that the H2/CO ratio for syngas can be controlled by adjusting the experimental conditions (particularly, the H2 flow rate). Consequently, the CFB system can be modified to facilitate the interaction between H2 gas and the DFM particles.@en

Multiphasic reaction of bicarbonate hydrogenation to form formate using homogeneous Ru PNP pincer catalyst in a continuous flow tubular reactor is reported. The reaction system consists of three phases. Catalyst is dissolved in toluene while potassium bicarbonate is dissolved in water. The significance of efficient mixing among the organic phase, aqueous phase and gaseous hydrogen to improve hydrogenation reaction by using different inert packing materials is studied by operando visualization and also quantitatively discussed. The bicarbonate conversion of up to 67% is achieved after optimization of important reaction and reactor parameters. The designed reactor setup comprised of effective recycling system that recycles the catalyst with >99% activity.

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The influence of nanostructures and interaction of Sn and Ir in oxygen evolution catalysts in a polymer electrolyte membrane electrolyzer were investigated. For this aim, two synthesis methods, namely, the one-step solution combustion method and the precipitation-deposition method with sodium borohydride reduction, were evaluated to prepare distinct nanostructures. Sn addition to Ir-based oxygen evolution reaction catalysts has been reported to yield materials with higher activity; however, in our case, this was observed only for Sn/Ir catalysts prepared by the precipitation-deposition method. The nanolayer of Sn/SnO₂ deposited over metallic Ir particles was identified to enhance the interfacial contacts, resulting in synergistic interactions. By deconvolution of the polarization curves into constituting contributions, the performance improvement was attributed to the higher exchange current density of the Sn/Ir powder as a consequence of a higher number of surface reaction sites created by the Sn-Ir interactions.

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Electrochemical ammonia (NH₃) synthesis from nitrate (NO₃⁻) offers a promising greener alternative to the fossil-fuel-based Haber-Bosch process to support the increasing demand for nitrogen fertilizers while removing environmental waste. Previous studies have mainly focused on designing catalysts to promote the direct conversion (NO₃⁻ → NH₃) while suppressing the two-step pathway (NO₃⁻ → NO₂⁻ → NH₃). We hypothesize that efficient nitrate reduction is possible on simple catalysts by instead promoting the two-step reaction and using chemical reactor principles in a membrane electrode assembly, despite NO₂⁻ intermediates. Here, we use an unmodified copper catalyst and control reactivity through current density, flow rate, and electrolyte recycling. Balancing the electrolyte flow rate with current density results in ideal residence times for NO₂⁻, allowing for 91% FE_NH3 in a 5 cm² electrolyzer with a NO₃⁻ to NH₃ partial current of 1.8 A. This work shows that traditional engineering principles can substantially boost the NO₃ reduction reaction, even for simple catalysts.

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Carbon dioxide (CO2) electrolysis on copper (Cu) catalysts has attracted interest due to its direct production of C2+ feedstocks. Using the knowledge that CO2 reduction on copper is primarily a tandem reaction of CO2 to CO and CO to C2+ products, we show that modulating CO concentrations within the liquid catalyst layer allows for a C2+ selectivity of >80% at 200 mA cm−2 under broad conversion conditions. The importance of CO pooling is demonstrated through residence time distribution curves, varying flow fields (serpentine/parallel/interdigitated), and flow rates. While serpentine flow fields require high conversions to limit CO selectivity and maximize C2+ selectivity, the longer CO residence times of parallel flow fields achieve similar selectivity over broad flow rates. Critically, we show that parts of the catalyst area predominantly reduce CO instead of CO2 as supported by CO reduction experiments, transport modelling, and achieving a CO2 utilization efficiency greater than the theoretical limit of 25% for C2+ products.@en

Carbon dioxide capture and reduction (CCR) process emerges as an efficient catalytic strategy for CO₂ capture and conversion to valuable chemicals. K-promoted Cu/Al₂O₃ catalysts exhibited promising CO₂ capture efficiency and highly selective conversion to syngas (CO + H₂). The dynamic nature of the Cu-K system at reaction conditions complicates the identification of the catalytically active phase and surface sites. The present work aims at more precise understanding of the roles of the potassium and copper and the contribution of the metal oxide support. While γ-Al₂O₃ guarantees high dispersion and destabilisation of the potassium phase, potassium and copper act synergistically to remove CO₂ from diluted streams and promote fast regeneration of the active phase for CO₂ capture releasing CO while passing H₂. A temperature of 350℃ is found necessary to activate H₂ dissociation and generate the active sites for CO₂ capture. The effects of synthesis parameters on the CCR activity are also described by combination of ex-situ characterisation of the materials and catalytic testing.

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When no hydrogen can reach the Pt catalyst in the anode for the hydrogen oxidation reaction (HOR) of an operating proton exchange membrane fuel cell (PEMFC), the anode potential increases and causes the cell potential to be reversed compared to normal operation conditions. During this reversal, the oxygen evolution reaction (OER) and carbon oxidation reaction (COR) will occur at the anode, where the COR has devastating consequences for the electrode. Introducing an OER catalyst limits the COR to occur, which makes a reversal tolerant anode (RTA). In this research, RTAs are differentiated by applying different ball milling times during catalyst layer processing, forming big and small OER (IrO_x/TiO_x) and HOR (Pt/C) catalyst particles. The two different particle sizes were electrochemically tested using a rotating disc electrode (RDE). Both catalyst sizes show a decrease in OER activity (mA cm⁻²) accompanied by loss of the ionomer in a self-developed accelerated stress test (AST). The small particle RTAs show higher OER activity as a result of increased surface area. However, during a chronopotentiometry measurement, which mimics a fuel cell reversal, the small particle coatings show a worse reversal tolerance. This phenomenon can be attributed to the increased difficulty in removing oxygen bubbles.

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Operando methodologies are widely used in heterogenous catalysis to understand unique state of catalyst materials emerging under specific reaction conditions and to establish catalyst structure-activity relationships. Recent studies highlight the importance of combining multiple operando techniques (multimodal approach) to gain complementary information as well as looking into chemical and material gradients and spatial variations on the reactor scale. In this work, we developed an operando UV–vis diffuse reflectance spectroscopy (DRS) setup compatible with a common fixed-bed tubular reactor. The design is based on optical calculations, validation experiments and signals considerations. A spatial resolution of 1 mm along the axial direction of the reactor was successfully demonstrated and combined with a time resolution of seconds with good signal to noise. CO oxidation over Pt/Al₂O₃ was performed as a proof of principle experiment demonstrating the capabilities of the new setup. The information gained by the space-resolved operando UV–vis DRS was combined with other space-resolved operando studies such as diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), gas sampling and temperature profiling. The study shows that the nature of active sites (Pt redox state) and thus the reaction mechanism alter with reaction temperature and also in space. Spatiotemporal UV–vis DRS is also demonstrated, showing the capability for transient studies with space-resolution.

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To introduce promotional H₂O effects for both CH₄ rate and C₂ selectivity, the OH radical formation, catalyzed through H₂O activation with O₂ surface species, was critical for modeling selective Mn-K₂WO₄/SiO₂ catalysts. Based on our reported experimental evidence, which demonstrates the formation of H₂O₂ through surface alkali peroxide intermediate, the elementary reactions that account for the OH-mediated pathway were added into the microkinetic model. The advanced model adeptly replicated the promotional H₂O effects on both OCM rate and selectivity. The data from a low-pressure microkinetic study were treated isothermally, and extended for near-industrially relevant pressures up to 901 kPa. Thermal visualization using an infrared camera found substantial temperature increases at undiluted high-pressure conditions which caused C₂ selectivity to drop significantly. When the furnace temperatures were decreased after ignition, side reactions after O₂ depletion (e.g., hydrocarbon reforming) were suppressed, obtaining 13.7 (11.8) % yields at 19.9 % CH₄ conversion with 68.6 (59.1) % selectivities for C_2-4 (C₂) at 901 kPa. The temperature was found to be the determining factor of C₂ yield which was perturbed by varying space velocity or CH₄/O₂ ratios. The optimum temperature for high-pressure conditions was predicted as 885 °C at 901 kPa. The study provides mechanistic and industrially relevant understandings for further OCM catalyst design and system application.

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Insight into mechanisms of heterogeneously catalyzed reactions holds importance in the development and optimization of new catalytic materials. Yet, the approaches often used in such investigations heavily rely on assumptions concerning the reactor and kinetics. Herein we report a new kind of kinetic investigation taking CO₂ hydrogenation reaction, specifically the reverse water–gas shift (RWGS) reaction over 3 wt% Pt/CeO₂, as an exemplifying case. The reported approach is based on spatially resolved steady-state isotopic transient kinetic analysis (SSITKA) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) identifying gaseous/surface species and their spatial variations along the reactor. This approach allows accurate evaluation of reaction mechanism by identifying correlations among the concentrations of gaseous/surface species and by quantitative description of their spatial variations by a kinetic model. Spatially resolved SSITKA-DRIFTS experiments show carbonate decomposition via a Pt-bound carbonyl to be the main route towards the production of carbon monoxide. Further kinetic modeling of the spatially resolved data confirms this mechanism proposal, and points to the production of water as the rate-limiting step.

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Surface intermediate species and oxygen vacancy-assisted mechanism over CeO2 catalyst in the direct dimethyl carbonate (DMC) synthesis from carbon dioxide and methanol are suggested by means of transient spectroscopic methodologies in conjunction with multivariate spectral analysis. How the two reactants, i.e. CO2 and methanol, interact with the CeO2 surface and how they form decisive surface intermediates leading to DMC are unraveled by DFT-based molecular dynamics simulation by precise statistical sampling of various configurations of surface states and intermediates. The atomistic simulations and uncovered stability of different intermediate states perfectly explain the unique DMC formation profile experimentally observed upon transient operations, strongly supporting the proposed oxygen vacancy-assisted reaction mechanism.@en

Direct valorization of methane via oxidative coupling of methane (OCM) is an encouraging alternative to conventional oil-based processes for the production of light hydrocarbons (ethane and ethylene). Abundant, inexpensive simple oxides such as MgO and La₂O₃ possess the ability to selectively activate methane. However, during OCM, the selective conversion to ethane and the following dehydrogenation to ethylene are threatened by the thermodynamically favored partial and total oxidation reactions to form CO and CO₂, respectively. With the aid of spatially resolved operando analysis of temperature and gas concentration along the catalytic bed, we demonstrate the relevance of highly exothermic reaction paths developed in the gas phase, i.e., the homogeneous reaction, during OCM conditions at the front of the catalytic bed, largely determining the total C₂ yield obtained on those systems. With the new insights provided by the analysis of temperature and concentration gradients along the bed, we redefine the positive effect of promoters (Li, Sr), which enhance the influence of catalyst surfaces. The effect of promoters is recognized in the suppression of the exothermic oxidation paths leading to undesired CO_x, thus limiting the formation of hotspots and driving the reaction toward the desired C₂ products.

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Recently, carbon capture and reduction (CCR) technology has gained interest to directly convert CO₂ to value-added products without requiring purification of CO₂ and its subsequent transportation. CCR to methanol in one dual function material (DFM) poses mechanistic and kinetic challenges. To counteract this, a process combining Na/Al₂O₃ as a capture component and Cu/ZnO/Al₂O₃ (CZA) as methanol synthesis catalyst was developed to allow CCR to methanol. With a 5 vol% CO₂ flow for capture and subsequent H₂ stream combined with a temperature swing, a methanol selectivity of 26 % was achieved at 9 bar. Further investigation found that Na/Al₂O₃ significantly increased methanol yield, while a stacked configuration of Na/Al₂O₃ followed by CZA significantly outperformed a mixed configuration of the two catalysts. With further investigation of operation at higher pressure and surface mechanism, an effective CCR to methanol process using two affordable yet readily available catalysts can be realized.

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The direct synthesis of methanol via the hydrogenation of CO₂, if performed efficiently and selectively, is potentially a powerful technology for CO₂ mitigation. Here, we develop an active and selective Cu-Zn/SiO₂ catalyst for the hydrogenation of CO₂ by introducing copper and zinc onto dehydroxylated silica via surface organometallic chemistry and atomic layer deposition, respectively. At 230 °C and 25 bar, the optimized catalyst shows an intrinsic methanol formation rate of 4.3 g h^-1 g_Cu^-1 and selectivity to methanol of 83%, with a space-time yield of 0.073 g h^-1 g_cat^-1 at a contact time of 0.06 s g mL^-1. X-ray absorption spectroscopy at the Cu and Zn K-edges and X-ray photoelectron spectroscopy studies reveal that the CuZn alloy displays reactive metal support interactions; that is, it is stable under H₂ atmosphere and unstable under conditions of CO₂ hydrogenation, indicating that the dealloyed structure contains the sites promoting methanol synthesis. While solid-state nuclear magnetic resonance studies identify methoxy species as the main stable surface adsorbate, transient operando diffuse reflectance infrared Fourier transform spectroscopy indicates that μ-HCOO*(ZnO_x) species that form on the Cu-Zn/SiO₂ catalyst are hydrogenated to methanol faster than the μ-HCOO*(Cu) species that are found in the Zn-free Cu/SiO₂ catalyst, supporting the role of Zn in providing a higher activity in the Cu-Zn system.

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Direct electroreduction of nitric oxide offers a promising avenue to produce valuable chemicals, such as ammonia, which is an essential chemical to produce fertilizers. Direct ammonia synthesis from NO in a polymer electrolyte membrane (PEM) electrolyzer is advantageous for its continuous operation and excellent mass transport characteristics. However, at a high current density, the faradaic efficiency of NO electroreduction reaction is limited by the competing hydrogen evolution reaction (HER). Herein, we report a CO-mediated selective poisoning strategy to enhance the faradaic efficiency (FE) towards ammonia by suppressing the HER. In the presence of only NO at the cathode, Pt/C and Pd/C catalysts showed a lower FE towards NH₃ than to H₂ due to the dominating HER. Cu/C catalyst showed a 78 % FE towards NH₃ at 2.0 V due to the stronger binding affinity to NO* compared to H*. By co-feeding CO, the FE of Cu/C catalyst towards NH₃ was improved by 12 %. More strikingly, for Pd/C, the FE towards NH₃ was enhanced by 95 % with CO co-feeding, by effectively suppressing HER. This is attributed to the change of the favorable surface coverage resulting from the selective and competitive binding of CO* to H* binding sites, thereby improving NH₃ selectivity.

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In spectroscopy and diffraction methods, the signatures of catalytically active sites are often submerged by the contribution of spectator species. In some cases, the signals may also superimpose with each other, hindering proper peak identification. Rationalizing a reaction pathway becomes very challenging, if not impossible, under these circumstances. Accordingly, the implementation of transient, dynamic methods such as modulation/modulated excitation spectroscopy (MES) can improve the analysis of these signals. In MES, the catalyst sample is subjected to periodic changes in the environment (e.g., reactant concentration) that stimulate the active species periodically while spectra or diffractograms are recorded with sufficient time resolution. Combined with phase-sensitive detection (PSD) analysis, this approach selectively enhances the signals of the responsive (and possibly active) species and at the same time attenuates the contribution from the spectator and static species. Overall, this results in increased analytical capabilities irrespective of the type of spectroscopy or diffraction technique that is used for the experiment. In this chapter, we introduce the basic concepts of MES, discuss the theory of PSD, and provide general guidelines that are useful for whoever encounters PSD data for the first time.

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The continuous electrochemical NO reduction to ammonia in a PEM cell was investigated in this work. We used a ruthenium-based catalyst at the cathode and an iridium oxide catalyst at the anode. The highest ammonia faradaic efficiency was observed at 1.9 V cell voltage. Adjusting the NO flow allowed to achieve 97% NO conversion and 93% ammonia faradaic efficiency for a 5.2% NO/He feed. The ammonia yield was 0.51 mmol cm^-2 h^-1, among the highest reported to date with the advantage of continuous operation. Experiments with a low NO concentration feed of 983 ppm showed 98% conversion at 0 V vs pseudo-RHE. Achieving this performance under such mild conditions indicates the great potential of the PEM cells for NO_x abatement applications and the production of valuable NH₃

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Oxidative dehydrogenation of ethane (ODHE) is an essential reaction in modern society to produce ethylene. The orthorhombic Mo₃VO_x catalyst (MoVO) was reported as one of the best catalysts for this reaction after a particular redox treatment to generate lattice oxygen defects. This study elucidates the location and nature of the defect sites as well as how molecular oxygen is activated toward the ODHE. The insights into the active site formation and guidelines for its design were gained through structural characterization, quantification of oxygen defects, and experimental and theoretical investigation of the reaction mechanisms. Peroxo formed at the defect sites of the bridging oxygen facing the heptagonal channel was found to drive the reaction efficiently.

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The electrochemical CO₂ reduction reaction (CO₂RR) is an attractive method to produce renewable fuel and chemical feedstock using clean energy sources. Formate production represents one of the most economical target products from CO₂RR but is primarily produced using post-transition metal catalysts that require comparatively high overpotentials. Here a composition of bimetallic Cu–Pd is formulated on 2D Ti₃C₂T_x (MXene) nanosheets that are lyophilized into a highly porous 3D aerogel, resulting in formate production much more efficient than post-transition metals. Using a membrane electrode assembly (MEA), formate selectivities >90% are achieved with a current density of 150 mA cm⁻² resulting in the highest ever reported overall energy efficiency of 47% (cell potentials of −2.8 V), over 5 h of operation. A comparable Cu-Pd aerogel achieves near-unity CO production without the MXene templating. This simple strategy represents an important step toward the experimental demonstration of 3D-MXenes-based electrocatalysts for CO₂RR application and opens a new platform for the fabrication of macroscale aerogel MXene-based electrocatalysts.

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