Ammonia (NH₃) is vital for synthesizing fertilizers and has gained great attention as a carbon-free hydrogen carrier and a hydrogen-rich fuel. Electrochemical ammonia synthesis from nitrate in a water-fed polymer electrolyte membrane electrolyzer is an innovative approach to wastewater treatment. However, the major hurdles to practical implementation are competing hydrogen evolution reactions (HERs) and constrained catalytic efficiency. Herein, we demonstrate the use of polyvinylpyrrolidone (PVP)-modified ruthenium (Ru) nanoparticles as a strategy to drive the desired reaction of nitrate to ammonia. The particle size of Ru was controlled by PVP, enhancing the metal-utilization efficiency and the electrochemical active surface area. PVP modification was found to alter the electron density on Ru, suppressing the HER by increasing the energy barrier of hydrogen coupling to form H₂, while promoting absorbed hydrogen (H*) formation, facilitating the hydrogenation of intermediates to ammonia. Benefiting from the combined effects, PVP-10 wt % Ru/C achieved an ammonia production rate of 3800 μg·mg_Ru^–1·h^–1, compared to 590 μg·mg_Ru^–1·h^–1 for 40 wt % Ru/C at 2 V.

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.

Integrated CO₂ capture and reduction (CCR) using dual-function materials (DFMs) has emerged as a promising strategy for effective utilization of CO₂. A thorough understanding of the reaction mechanisms of CCR using the DFMs is important for enhancing their performances. In this study, Na/Al₂O₃ and Na/Cu/Al₂O₃ were compared to investigate the role of Na-based DFMs, particularly transition-metal-free DFMs, in facilitating CCR to CO. Fixed-bed experiments comprising of CO₂ capture and reduction phase were performed to examine their performances at various temperatures. The two DFMs exhibited similar behaviors during the CO₂ capture phase. In the subsequent reduction phase, the DFMs produced nearly equal amounts of CO, reaching 0.21 mmol/g, at temperatures exceeding 450 °C. By increasing the temperature to 500 °C, the CO production rates reached an identical level of 0.18 mmol/(min·g). Time-resolved in-situ spectroscopy confirmed the formation of carbonate species during the capture phase. Carbonates were further reduced to CO directly or via intermediate formate species in the reduction phase. The formation of formates was predominant on Na/Cu/Al₂O₃ at temperatures below 400 °C. However, at higher temperatures, the direct reductive decomposition of carbonates to form CO became the dominant pathway for both the DFMs. Elucidation of a more detailed mechanism of the direct reductive decomposition pathway is critical, particularly the role of Na sites during the reduction phase.

Single-atom catalysts, which consist of isolated metal sites immobilized on the support, have attracted significant attention in heterogeneous catalysis due to their high catalytic performance. The so-called strong electrostatic adsorption (SEA), in which a metal precursor is deposited onto an oxide support by electrostatic attraction, is widely employed to obtain single-atom catalysts in wet synthesis. In this work, we investigated the adsorption behavior of Re precursor, perrhenate, on anatase titania as an example of SEA studied by in situ attenuated total reflection infrared (ATR-IR) spectroscopy. The study confirms that the adsorption of perrhenate on titania is enhanced at a lower pH, which is consistent with a SEA model, and that the adsorption and desorption processes are reversible at pH 3.0. The ATR-IR spectroscopic kinetic analysis of the adsorption processes of perrhenate in nitric acid, assuming the Langmuir adsorption model, reveals that the kinetics of the perrhenate adsorption onto titania is influenced by the ionic strength. Furthermore, the adsorption mechanism of perrhenate changes depending on pH between 3.0 and 5.0. This study demonstrates that in situ ATR-IR spectroscopy is a powerful tool for the real-time monitoring and the kinetic study of SEA processes to design atomically engineered catalytic active sites.

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.

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.

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.

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.

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.

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.

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.