Electrocatalysis represents a specific category of catalytic processes wherein an electric current induces a chemical reaction. This phenomenon typically occurs at the surface of an electrode, where a catalyst is present, thereby enhancing the reaction kinetics and diminishing the thermodynamic energy barrier that must be overcome for the reaction to proceed.
Electrocatalysts alter activation energies required for electrochemical transformations, allowing for a broader and more selective reaction scope through innovative reaction pathways.
This white paper presents examples from recent peer-reviewed research that show how the in-depth information and precise control provided by METTLER TOLEDO technologies aid in supporting green and sustainable chemistry across four thematic topics:
Electrochemistry and electrocatalysts are important in organic chemistry for several interconnected reasons. Transition metal complexes used in electrocatalytic synthesis facilitate the generation of novel pathways, mechanisms, and intermediates, enabling chemical transformations that are often challenging or not feasible with conventional synthetic strategies. By fine-tuning electrical potentials, researchers can achieve high selectivity while minimizing the formation of side products.
Electrocatalysis has proven effective in key chemical transformations, such as functionalizing C–H bonds, cross-coupling reactions, and oxidation and reduction processes. Additionally, using electrons to drive chemical transformations under mild conditions instead of relying on hazardous, toxic, or highly energetic chemical reagents aligns with the principles of green chemistry, reducing pollution and promoting sustainable production.
Electrocatalysis also plays a significant role in the global pursuit of clean energy production. Sustainable energy technologies like fuel cells rely on electrocatalysts for oxygen reduction, oxygen evolution, and hydrogen evolution reactions. The development of advanced electrocatalysts using earth-abundant metals such as iron and nickel, as well as carbon-based catalysts such as graphene, is progressing rapidly, further enhancing the potential of electrocatalysis in energy applications.
Electrocatalysis can be homogeneous or heterogeneous. Homogeneous electrocatalysis uses electrochemical redox moderators, which can be specific organic chemicals or transition metal complexes, to enhance the efficiency of electron transfer. In contrast, heterogeneous electrocatalysis occurs when the electrode surface itself acts as the catalyst, facilitating electron transfer to the reactants. The main distinction between heterogeneous and homogeneous electrocatalysis resides in the phase relationship of the catalyst with respect to the reactants and the electrolyte.
Spectroscopic techniques like Raman and FTIR are utilized in electrocatalysis to examine dynamic processes and clarify the mechanisms of electrochemical reactions, enhancing catalyst design and performance.
ReactIR™ provides real-time reaction trend information for reactants, reagents, intermediates, products, and by-products as the reaction proceeds. This generates precise information about kinetics, mechanism, pathways, catalytic cycles, and the influence of reaction variables on performance. Fiber-optic-connected immersion probes and inline flow cells offer the flexibility to monitor solution-phase chemistry directly in a reaction vessel or continuous flow system.
In-situ/operando FTIR spectroscopy tracks and analyzes electrocatalytic chemistry in real time within the reaction vessel, electrolytic cell, or via inline monitoring of an electrosynthesis flow cell. This technique enables researchers to observe how various factors—such as fine-tuning electrical potential, reagent concentration, and electrolyte composition—influence the synthesis by analyzing the changes in the infrared spectra of the reaction solution. FTIR spectroscopy allows for monitoring the changes in concentration of starting materials and products and key intermediates formed by the electrocatalytic process. The information gained often offers unparalleled insight into reaction kinetics and mechanism and a complete understanding of electron transfer, catalyst states, catalytic cycles, catalyst structure-activity relationships, and catalyst degradation.
The compact, high-performance ReactRaman™ spectrometer is optimized for in-situ applications through fiber optic connected sampling interfaces, including immersion probes, inline flow cells, and non-contact analysis probes. Comprehensive, intuitive iC Raman™ software provides high-quality data and reaction information through real-time reaction trend analysis.
In-situ/operando Raman spectroscopy is an effective technique for investigating electrocatalytic reactions, offering many similar advantages to FTIR for tracking and characterizing electrocatalytic reaction species in real time. Raman spectroscopy is particularly well-suited to analyzing syntheses performed in aqueous media. Raman spectroscopy also excels in investigating surface structures and the electrolyte-electrode interface and identifying molecular species bound to the catalyst surface, which can enhance understanding of the underlying mechanism in electrocatalysis.
Wang, Y., Yu, J., Wang, Y., Chen, Z., Dong, L., Cai, R., Hong, M., Long, X., & Yang, S. (2020). In situ templating synthesis of mesoporous Ni–Fe electrocatalyst for oxygen evolution reaction. RSC Advances, 10(39), 23321–23330. https://doi.org/10.1039/d0ra03111a
In support of electrocatalyst development, ReactRaman provides detailed information on bonding within and at the surface of the mesoporous Ni–Fe electrocatalyst.
The authors comment on the importance of developing electrocatalysts for use in oxygen evolution reactions (OER). These electrocatalysts must have specific key efficiency characteristics, such as active sites that are evenly distributed and readily available across the surface of the catalyst while being cost-effective and sustainable. To meet these goals, they researched and developed a method to prepare a mesoporous fumed silica (MFS) framework that dispersed Ni²⁺ and Fe³⁺ using a straightforward approach. This method uses commercially available MFS as a 3D support to attach the metal ions. By etching the MFS metal-impregnated structure with KOH, the Ni−Fe−O electrocatalyst formed has key characteristics for OER, such as good charge transfer ability, large electrochemical active surface area, and overall excellent stability.
A series of NiFe-MFS catalysts were synthesized with different mole ratios of metal-ion aqueous solutions. A series of techniques were used to develop a detailed understanding of the microstructure of these electrocatalysts. This included transmission electron microscopy to investigate the nanostructure, energy-dispersive X-ray to map the Ni, Fe, Si, and O element distribution, and X-ray diffraction to analyze the crystallinity of the samples. Morphology was studied by emission scanning electron microscopy; X-ray photoelectron spectroscopy analyzed the element binding energy, and elemental analysis was performed using an inductively coupled plasma-atomic emission spectrometer. Surface area and structure porosity were investigated using nitrogen gas adsorption-desorption measurements.
The bonding of the Ni/Fe metals to the fumed silica support was analyzed and verified with ReactRaman spectroscopy. For undoped MFS, bands at 345–450, 575, 750, 973, and 1070 cm⁻¹ are observed, arising from a series of Si−O−Si and Si−OH vibrational bonds. For samples that are impregnated with high iron content, bands at 332, 495, and 1163 cm⁻¹ are observed arising from O–Fe–O bending, Fe–O–Si bending, and the Fe–O–Si asymmetric stretch, respectively. These observations indicated that the iron was effectively incorporated into the silica lattice. In contrast, when nickel was impregnated into the fumed silica, the 973 cm⁻¹ surface Si−OH stretching band was significantly weakened, and no additional bands were observed. Evaluating the X-ray photoelectron and Raman measurements led to the conclusion that, whereas Fe³⁺ prefers to insert into the framework of the fumed silica and forms Fe–O–Si bonding, Ni²⁺ binds covalently with Si−OH groups on the fumed silica surface.
A series of electroanalysis studies were performed that showed the importance of the metal ion ratio to performance and that Ni and Fe bonding at an optimal content led to optimum OER efficiency and improved reaction kinetics. The 1Ni1Fe-MFS sample demonstrated the highest OER intrinsic activity, while 2Ni1Fe-MFS catalyst had the larger double-layer capacitance and electrochemically active surface area. A series of spectroscopic investigations were carried out to determine the changes in the catalyst after OER. They showed that in the presence of the KOH, the Si was etched, exposing Ni and Fe elements, which were the OER active centers. Further work showed that even after long-term electrical operation, 2Ni1Fe-MFS catalyst remains both highly efficient and stable during the OER process, in comparison testing with IrO₂ and RuO₂ electrodes.
Sauermann, N., Mei, R., & Ackermann, L. (2018). Electrochemical C−H amination by cobalt catalysis in a renewable solvent. Angewandte Chemie International Edition, 57(18), 5090–5094. https://doi.org/10.1002/anie.201802206
ReactIR provides kinetic and mechanistic information for proposed C−H amination electrocatalytic cycle.
The authors comment on the wide-ranging importance of substituted amines and the various methods to synthesize these key compounds. These include metal-catalyzed cross-coupling reactions using a palladium catalyst or direct amination of stable C−H bonds, which eliminates the need for using aryl halides. In the latter case, stoichiometric quantities of copper(II) or silver(I) salts are required, and these can lead to the formation of undesirable metal-containing by-products. In contrast, the work presented by this team uses electricity as a sustainable and inexpensive means to achieve C−H activated aminations. Their work includes using renewable solvents for C−H functionalization and cobalt-catalyzed, electrochemical C−H aminations with high selectivity. Significant effort went into optimizing the electrocatalytic aminations, including identifying cobalt salts as effective catalysts and biomass-derived, renewable solvent, g-valerolactone, used as the reaction solvent. Also, it was determined that the electrochemical amination required the presence of carboxylate, and potassium acetate was identified as the most effective additive.
