Boosting Hydrogen Production via Selective Two-electron Mild Electrochemical Oxidation of Tetrahydroisoquinolines Completely to Dihydroisoquinolines

Different from the previous study that biomass derivatives replace water oxidation for enhancing hydrogen production, we found that mild oxidation was more conductive to cathodic hydrogen production. In this study, maximum Faradaic efficiency (>99 %) and lower energy consumption for hydrogen production was achieved by precisely controlling the two-electron mild electrochemical oxidation of tetrahydroisoquinolines (THIQs) to dihydroisoquinolines (DHIQs) in place of the four-electron deep oxidation to isoquinolines (IQs). Moreover, the high value-added DHIQs were prepared from THIQs with high selectivity (>99 %) at the low potential of 1.36 V. Operando electrochemical Raman and density functional theory proved that the high selectivity was attributed to the regulable active species of NiOOH induced by the interaction of Co and Fe for preferentially breaking C−H bond rather than N−H of THIQs. This novel method provides important insight into efficient biomass-assisted hydrogen production.     

Identification of Active Sites Formed on Cobalt Oxyhydroxide in Glucose Electrooxidation

Transition-metal-based oxyhydroxides are efficient catalysts in biomass electrooxidation towards fossil-fuel-free production of valuable chemicals. However, identification of active sites remains elusive. Herein, using cobalt oxyhydroxide (CoOOH) as the archetype and the electrocatalyzed glucose oxidation reaction (GOR) as the model reaction, we track dynamic transformation of the electronic and atomic structure of the catalyst using a suite of operando and ex situ techniques. We reveal that two types of reducible Co³⁺-oxo species are afforded for the GOR, including adsorbed hydroxyl on Co³⁺ ion (μ₁-OH−Co³⁺) and di-Co³⁺-bridged lattice oxygen (μ₂-O−Co³⁺). Moreover, theoretical calculations unveil that μ₁-OH−Co³⁺ is responsible for oxygenation, while μ₂-O−Co³⁺ mainly contributes to dehydrogenation, both as key oxidative steps in glucose-to-formate transformation. This work provides a framework for mechanistic understanding of the complex near-surface chemistry of metal oxyhydroxides in biomass electrorefining.     

Unveiling the CO Oxidation Mechanism over a Molecularly Defined Copper Single-Atom Catalyst Supported on a Metal–Organic Framework

Elucidating the reaction mechanism in heterogeneous catalysis is critically important for catalyst development, yet remains challenging because of the often unclear nature of the active sites. Using a molecularly defined copper single-atom catalyst supported by a UiO-66 metal–organic framework (Cu/UiO-66) allows a detailed mechanistic elucidation of the CO oxidation reaction. Based on a combination of in situ/operando spectroscopies, kinetic measurements including kinetic isotope effects, and density-functional-theory-based calculations, we identified the active site, reaction intermediates, and transition states of the dominant reaction cycle as well as the changes in oxidation/spin state during reaction. The reaction involves the continuous reactive dissociation of adsorbed O₂, by reaction of O_2,ad with CO_ad, leading to the formation of an O atom connecting the Cu center with a neighboring Zr⁴⁺ ion as the rate limiting step. This is removed in a second activated step.     

Breaking Local Charge Symmetry of Iron Single Atoms for Efficient Electrocatalytic Nitrate Reduction to Ammonia

The electrochemical conversion of nitrate pollutants into value-added ammonia is a feasible way to achieve artificial nitrogen cycle. However, the development of electrocatalytic nitrate-to-ammonia reduction reaction (NO₃⁻RR) has been hampered by high overpotential and low Faradaic efficiency. Here we develop an iron single-atom catalyst coordinated with nitrogen and phosphorus on hollow carbon polyhedron (denoted as Fe−N/P−C) as a NO₃⁻RR electrocatalyst. Owing to the tuning effect of phosphorus atoms on breaking local charge symmetry of the single-Fe-atom catalyst, it facilitates the adsorption of nitrate ions and enrichment of some key reaction intermediates during the NO₃⁻RR process. The Fe−N/P−C catalyst exhibits 90.3 % ammonia Faradaic efficiency with a yield rate of 17980 μg h⁻¹ mg_cat⁻¹, greatly outperforming the reported Fe-based catalysts. Furthermore, operando SR-FTIR spectroscopy measurements reveal the reaction pathway based on key intermediates observed under different applied potentials and reaction durations. Density functional theory calculations demonstrate that the optimized free energy of NO₃⁻RR intermediates is ascribed to the asymmetric atomic interface configuration, which achieves the optimal electron density distribution. This work demonstrates the critical role of atomic-level precision modulation by heteroatom doping for the NO₃⁻RR, providing an effective strategy for improving the catalytic performance of single atom catalysts in different electrochemical reactions.     

一种双光束红外光谱及其在气固相多相催化反应实时原位表征中的应用

单光束红外光谱技术(原位傅立叶透射变换红外、原位漫反射红外和衰减全反射红外光谱技术)虽然已经用于气固相多相催化反应的原位表征中,但这些光谱在真实反应条件下会受到气体分子振动光谱和加热条件下产生的发射光谱的严重影响,不能实时获得催化剂表面的真实信息.另外,由于在真实的气固相多相催化反应过程中,催化剂本底的信息会随着反应时间的延长而发生变化,因此单光束红外光谱技术在扣除本底信息方面存在误差.为了实现在反应条件下,实时、原位表征催化剂表面的状态,我们报道了一种双光束红外光谱表征技术.该技术包括双光束红外光谱系统及双光束红外反应池.其特征在于:实时双光束原位红外光谱系统由两台完全相同的红外光谱仪和双光束红外反应池组成.双光束红外反应池由完全相同的样品池和参考池连接而成,样品池和参考池处于同一水平线上分别对应于样品光谱仪和参考光谱仪,由计算机同步控制两台红外光谱仪,排除实时状态下的气体分子振动光谱干扰和加热条件下产生的发射光谱干扰.该技术可以对真实反应条件下的气固相多相催化反应进行实时原位表征.通过应用程序的关联可以实时、同步采集样品光束和背景光束谱图来得到催化剂表面物种随反应时间变化的真实信息.该技术克服了单光束红外光谱在原位多相催化反应表征方面的缺陷,使表征结果变得更加精确可靠.该技术还可以在变化的气相组分条件下,获得不同温度下、实时的催化剂表面活性中心、活性相和中间物种的信息.采用上述双光束红外光谱仪对丁烯在纳米HZSM-5催化剂上芳构化反应过程进行了实时、原位观测,首次在实际反应条件下,观察到了异丁烯在纳米HZSM-5沸石的表面Br?nsted酸中心上经历吸附、活化、聚合、环化等反应步骤生成芳烃的过程.     

Direct Monitoring of Li2S2 Evolution and Its Influence on the Reversible Capacities of Lithium-Sulfur Batteries

The polysulfide (PS) dissolution and low conductivity of lithium sulfides (Li₂S) are generally considered the main reasons for limiting the reversible capacity of the lithium-sulfur (Li-S) system. However, as the inevitable intermediate between PSs and Li₂S, lithium disulfide (Li₂S₂) evolutions are always overlooked. Herein, Li₂S₂ evolutions are monitored from the operando measurements on the pouch cell level. Results indicate that Li₂S₂ undergoes slow electrochemical reduction and chemical disproportionation simultaneously during the discharging process, leading to further PS dissolution and Li₂S generation without capacity contribution. Compared with the fully oxidized Li₂S, Li₂S₂ still residues at the end of the charging state. Therefore, instead of the considered Li₂S and PSs, slow electrochemical conversions and side chemical reactions of Li₂S₂ are the determining factors in limiting the sulfur utilization, corresponding to the poor reversible capacity of Li-S batteries.     

Understanding Electrochemical Reaction Mechanisms of Sulfur in All-Solid-State Batteries through Operando and Theoretical Studies **

Due to its outstanding safety and high energy density, all-solid-state lithium-sulfur batteries (ASLSBs) are considered as a potential future energy storage technology. The electrochemical reaction pathway in ASLSBs with inorganic solid-state electrolytes is different from Li-S batteries with liquid electrolytes, but the mechanism remains unclear. By combining operando Raman spectroscopy and ex situ X-ray absorption spectroscopy, we investigated the reaction mechanism of sulfur (S₈) in ASLSBs. Our results revealed that no Li₂S_8, Li₂S_6, and Li₂S₄ were formed, yet Li₂S₂ was detected. Furthermore, first-principles structural calculations were employed to disclose the formation energy of solid state Li₂S_n (1≤n≤8), in which Li₂S₂ was a metastable phase, consistent with experimental observations. Meanwhile, partial S₈ and Li₂S₂ remained at the full lithiation stage, suggesting incomplete reaction due to sluggish reaction kinetics in ASLSBs.     

Time-Resolved Product Observation for CO2 Electroreduction Using Synchronised Electrochemistry-Mass Spectrometry with Soft Ionisation (sEC-MS-SI)

The mechanistic understanding of electrochemical CO₂ reduction reaction (CO₂RR) requires a rapid and accurate characterisation of product distribution to unravel the activity and selectivity, which is yet hampered by the lack of advanced correlative approaches. Here, we present the time-resolved identification of CO₂RR products by using the synchronised electrochemistry-mass spectrometry (sEC-MS). Transients in product formation can be readily captured in relation to electrochemical conditions. Moreover, a soft ionisation (SI) strategy is developed in MS for the direct observation of CO, immune to the interference of CO₂ fragments. With the sEC-MS-SI, the kinetic information, such as Tafel slopes and onset potentials, for a myriad of CO₂RR products are revealed and we show the hysteresis seen for the evolution of some species may originate from the potential-driven changes in surface coverage of intermediates. This work provides a real-time picture of the dynamic formation of CO₂RR products.     

Multivariate curve resolution applied to in situ X-ray absorption spectroscopy data: An efficient tool for data processing and analysis

Large datasets containing many spectra commonly associated with in situ or operando experiments call for new data treatment strategies as conventional scan by scan data analysis methods have become a time-consuming bottleneck. Several convenient automated data processing procedures like least square fitting of reference spectra exist but are based on assumptions. Here we present the application of multivariate curve resolution (MCR) as a blind-source separation method to efficiently process a large data set of an in situ X-ray absorption spectroscopy experiment where the sample undergoes a periodic concentration perturbation. MCR was applied to data from a reversible reduction–oxidation reaction of a rhenium promoted cobalt Fischer–Tropsch synthesis catalyst. The MCR algorithm was capable of extracting in a highly automated manner the component spectra with a different kinetic evolution together with their respective concentration profiles without the use of reference spectra. The modulative nature of our experiments allows for averaging of a number of identical periods and hence an increase in the signal to noise ratio (S/N) which is efficiently exploited by MCR. The practical and added value of the approach in extracting information from large and complex datasets, typical for in situ and operando studies, is highlighted.     

Operando XAFS Imaging of Distribution of Pt Cathode Catalysts in PEFC MEA

Three‐dimensional imaging using X‐ray as a probe is state‐of‐the‐art for the characterization of heterogeneous materials. In addition to simple imaging of sample morphology, imaging of elemental distribution and chemical states provides advanced maps of key structural parameters of functional materials. The combination of X‐ray absorption fine structure (XAFS) spectroscopy and three‐dimensional imaging such as computed tomography (CT) can visualize the three‐dimensional distribution of target elements, their valence states, and local structures in a non‐destructive manner. In this personal account, our recent results on the three‐dimensional XAFS imaging for Pt cathode catalysts in the membrane electrode assembly (MEA) of polymer electrolyte fuel cell (PEFC) are introduced. The distribution and chemical states of Pt cathode catalysts in MEAs remarkably change under PEFC operating conditions, and the 3D XAFS imaging revealed essential events in PEFC MEAs.