Homogeneous Catalysts Research Articles

A Review of Catalyst Integration in Hydrothermal Gasification

Industrial scale-up of hydrothermal supercritical water gasification process requires catalytic integration to reduce the high operational temperatures and pressures to enhance controlled chemical reaction pathways, product yields, and overall process economics. There is greater literature disparity in consensus on what is the best catalyst and reactor design for hydrothermal gasification. This arises from the limited research on catalysis in continuous flow hydrothermal systems and rudimentary lab-scale experimentation on simple biomasses. This review summarizes the literature status of catalytic hydrothermal processing, especially for continuous gasification and in situ catalyst handling. The rationale for using low and high temperatures during catalytic hydrothermal processing is highlighted. The role of homogeneous and heterogeneous catalysts in hydrothermal gasification is presented. In addition, the rationale behind certain designs and component selection for catalytic investigations in continuous hydrothermal conversion is highlighted. Furthermore, the effect of different classes of catalysts on the reactor and reactions are elaborated. Overall, design and infrastructural challenges such as plugging, corrosion, agglomeration of the catalysts, catalyst metal leaching, and practical assessment of catalyst integration towards enhancement of process economics still present open questions. Therefore, strategies for catalytic configuration in continuous hydrothermal process must be evaluated on a system-by-system basis depending on the feedstock and experimental goals.

Synthesis and Characterization of Transition Metal Complexes Supported by Phosphorus Ligands Obtained Using Hydrophosphination of Cyclic Internal Alkenes.

The design and study of rich, bulky phosphorus ligands is a key area of research for homogeneous catalysis. Here, we describe an original strategy using a hydrophosphination reaction to produce phosphines of interest for coordination chemistry and homogenous catalysis. In particular, the phosphine obtained by reacting diphenylphosphine with acenaphthylene (ligand 2) gives a ligand that adopts an unusual spatial geometry. The coordination chemistry of the ligand has been investigated with Au(I), Ag(I), Cu(I), and Pd(II), for which a complete characterization could be made, particularly in X-ray diffraction studies. The reactivity of some of these complexes has been demonstrated, particularly in Pd-catalyzed cross-coupling reactions and Au-catalyzed hydroaminations and in the hydration of alkynes.

Multi-Catalytic Metal-Based Homogeneous-Heterogeneous Systems in Organic Chemistry.

The combination of metal-based homogeneous and heterogeneous catalysts in the same reaction media is a powerful, yet relatively unexplored approach in organic chemistry. This strategy can address important limitations associated with purely homogeneous or heterogeneous catalysis such as the incompatibility of different catalytic species in solution, or the limited tunability of solid catalysts, respectively. Moreover, the facile reusability of the solid catalyst, contributes to increase the overall sustainability of the process. As a result, this semi-heterogeneous multi-catalytic approach has unlocked significant advances in organic chemistry, improving existing reactions and even enabling the discovery of novel transformations, exemplified by the formal alkane metathesis. This concept article aims to showcase the benefits of this strategy through the exploration of diverse relevant examples from the literature, hoping to spur research on new metal-based homogeneous-heterogeneous catalyst combinations that will result in reactivity challenging to achieve by conventional homogeneous or heterogeneous catalysis alone.

Synthesis, Structural Analysis, and Polymerization Activity of Noncyclopentadienyl Constrained Geometry Complex [Me2Si(Ph2C)(tBuN)]ZrCl2(thf)

ABSTRACTConstrained geometry complexes (CGCs) are homogeneous olefin polymerization catalysts that have been improved because of their considerably high activity and their copolymerization abilities for ethylene with α‐olefins. The ancillary ligands in CGCs comprise bridged 6π‐electron moieties (e.g., cyclopentadienyl (Cp) and fluorenyl) and amide donors. In this study, we investigated the effect of changing the 6π‐electron moieties to a diphenylmethyl moiety (Ph2C) on the polymerization activity of [Me2Si(Ph2C)(tBuN)]ZrCl2(thf) (thf = tetrahydrofuran). We synthesized [Me2Si(Ph2C)(tBuN)]ZrCl2(thf) and characterized it via 1H nuclear magnetic resonance analysis and single‐crystal X‐ray crystallography. Ethylene polymerization at 2 MPa using a catalyst of the [Me2Si(Ph2C)(tBuN)]ZrCl2(thf) and modified methylaluminoxane (MMAO) as a cocatalyst yielded a linear polyethylene (PE) with a high melting point (124°C–134°C); moreover, the polymerization activity of the catalyst was found to be up to 445 g (PE) mmol (Zr)−1 h−1. The polymerization of styrene using the zirconium complex and MAOs did not proceed, but the polymerization of 1,3‐butadiene produced a small amount of polybutadiene with ~60% cis‐1,4‐structure. These results indicate that non‐Cp‐type CGCs, which can be defined as metal complexes bearing an ancillary ligand in which non‐Cp moieties (benzyl moiety in this study) and an amido group are bridged by an organic linker, can become a new family of homogeneous olefin polymerization catalysts.

3‐Arylcoumarin Scaffolds From 3‐Chlorocoumarin and Arylboronic Acids via Site‐Selective CCl Bond Activation With Palladium Complexes of N∩O Chelating Hydrazones

ABSTRACTUsing palladium hydrazone complex as a homogeneous catalyst for SMC, we synthesized a library of 15 variant 3‐arylcoumarins. In this context, we report a set of four coordinated palladium(II) hydrazone complexes 1 and 2 having the general formula [Pd(L)(Cl)(PPh3)] by reacting the precursor complex [PdCl2(PPh3)2] with the hydrazone ligands HL1 or HL2. Single crystal XRD and conventional spectroscopic techniques were employed to characterize both complexes. From the catalytic application point of view, the new complexes 1 and 2 were tested as homogeneous catalysts towards the conversion of 3‐chlorocoumarins into 3‐arylcoumarins by activating the challenging CCl functionality. A minimal catalyst loading of complex 2 (0.01 mol%), afforded a series of 3‐arylcoumarins with a maximum of 92% yield and reusability up to five cycles. The present methodology is simple and easy to carry out at room temperature utilizing a green solvent (ethanol) in an open flask with a spectrum of aryl boronic acids.

Covalent Organic Framework Catalyzed Amide Synthesis Directly from Alcohol Under Red Light Excitation.

The dehydrogenative coupling of alcohols and amines to form amide bonds is typically catalysed by homogeneous transition metal catalysts at high temperatures ranging from 130-140 °C. In our pursuit of an efficient and recyclable photocatalyst capable of conducting this transformation at room temperature, we report herein a COF-mediated dehydrogenative synthesis. The TTT-DHTD COF was strategically designed to incorporate a high density of functional units, specifically dithiophenedione, to trap photogenerated electrons and effectively facilitate hydrogen atom abstraction reactions. The photoactive TTT-DHTD COF, synthesized using solvothermal methods showed high crystallinity and moderate surface area, providing an ideal platform for heterogeneous amide synthesis. Light absorption by the COF across the entire visible range, narrow band gap, and valence band position make it well-suited for the efficient generation of excitons necessary for targeted dehydrogenation. Utilizing red light irradiation and employing extremely low loading of the COF, we have successfully prepared a wide range of amides, including challenging secondary amides, in good to excellent yields. The substrates' functional group tolerance, very mild reaction conditions, and the catalyst's significant recyclability represent substantial advancements over prior methodologies.

Covalent Organic Framework Catalyzed Amide Synthesis Directly from Alcohol Under Red Light Excitation

AbstractThe dehydrogenative coupling of alcohols and amines to form amide bonds is typically catalysed by homogeneous transition metal catalysts at high temperatures ranging from 130–140 °C. In our pursuit of an efficient and recyclable photocatalyst capable of conducting this transformation at room temperature, we report herein a COF‐mediated dehydrogenative synthesis. The TTT‐DHTD COF was strategically designed to incorporate a high density of functional units, specifically dithiophenedione, to trap photogenerated electrons and effectively facilitate hydrogen atom abstraction reactions. The photoactive TTT‐DHTD COF, synthesized using solvothermal methods showed high crystallinity and moderate surface area, providing an ideal platform for heterogeneous amide synthesis. Light absorption by the COF across the entire visible range, narrow band gap, and valence band position make it well‐suited for the efficient generation of excitons necessary for targeted dehydrogenation. Utilizing red light irradiation and employing extremely low loading of the COF, we have successfully prepared a wide range of amides, including challenging secondary amides, in good to excellent yields. The substrates’ functional group tolerance, very mild reaction conditions, and the catalyst's significant recyclability represent substantial advancements over prior methodologies.

Ag-Cu2O Supported Biomass-Derived rGO for Catalyzing Suzuki-Miyaura Cross-Coupling.

The search for cost-effective, efficient, and ecofriendly heterogeneous catalysts for the Suzuki-Miyaura reaction is crucial due to challenges with expensive, toxic homogeneous catalysts. This study centrally aims at crafting a pioneering green catalyst by adorning reduced graphene oxide (rGO), sourced from basil seeds (Ocimum basilicum L.), with an Ag-Cu2O composite structure. Comprehensive characterization of the Ag-Cu2O/rGO nanocomposite was conducted through FTIR, SEM, hHR-TEM, EDS, XPS, XRD, TGA, and N2 adsorption/desorption analyses. Results showed that nanosized Ag-Cu2O particles were partially integrated into rGO sheets derived from basil seeds, acting as active species for oxidative addition with aryl halides in the SMR. The catalytic efficacy of this robust nanocatalyst was assessed in Suzuki-Miyaura cross-coupling reactions, targeting the synthesis of biaryls employing various aryl halides and aryl boronic acids. The findings underscore that the Ag-Cu2O/rGO nanocatalyst manifests rapid reaction kinetics (15 min) alongside commendable yields (99%). The Ag-Cu2O/rGO demonstrates impressive recyclability, maintaining catalytic efficiency over four cycles. Utilizing it as a green substrate for metal loading highlights its potential, offering well-defined coordination sites. This approach facilitates stable heterogeneous catalyst fabrication, crucial for significant bond formations. Notable features include broad applicability, exceptional functional tolerance, scalability, and practicality. Moreover, it holds promise for automating safe processes and enabling efficient late-stage functionalization of complex molecules with moderate to high efficiency, presenting promising prospects for various applications in chemical synthesis.

Intermolecular O-O Bond Formation between High-Valent Ru-oxo Species.

Despite extensive research on water oxidation catalysts over the past few decades, the relationship between high-valent metal-oxo intermediates and the O-O bond formation pathway has not been well clarified. Our previous study showed that the high spin density on O in RuV=O is pivotal for the interaction of two metal-oxyl radical (I2M) pathways. In this study, we found that introducing an axially coordinating ligand, which is favorable for bimolecular coupling, into the Ru-pda catalyst can rearrange its geometry. The shifts in geometric orientation altered its O-O bond formation pathway from water nucleophilic attack (WNA) to I2M, resulting in a 70-fold increase in water oxidation activity. This implies that the I2M pathway is concurrently influenced by the spin density on oxo and the geometry organization of the catalysts. The observed mechanistic switch and theoretical studies provide insights into controlling reaction pathways for homogeneous water oxidation catalysis.

Bioinspired octanuclear copper cubane complex as an efficient molecular electrocatalyst for water oxidation

Water oxidation catalysts (WOCs) with excellent activity and stability are highly desired. Numerous efforts have been dedicated to develop WOCs mimicking the cubic structure and activity of Mn4CaO5 cluster in oxygen-evolving center (OEC) of photosynthesis system II (PSII). Herein, a octanuclear Cu cluster [Cu8(dpy{OH}O)8(OAc)4]4+ (1, dpy{OH}O− = the monoanion of the hydrated, gem-diol form of di-2-pyridyl ketone) that contains two Cu4O4 cubic fragments with bioinspired structure mimicking the Mn4CaO5 cluster of OEC of natural PSII is developed as efficient homogeneous catalyst for electrochemical water oxidation under near neutral condition for the first time. It exhibits outstanding water oxidation catalytic activity via accelerating oxygen evolution with onset overpotential of 730 mV and appreciable turnover frequency of 20.1 s−1. Collective experiments together confirmed the homogeneity and stability of cluster 1 under long-term water oxidation catalytic cycle. Kinetic study reveals the water nucleophilic attack (WNA) catalytic mechanism occurs on the coordination unsaturated Cu center of the cluster. This work provides the inspiration of the development of robust bioinspired catalyst for electrochemical water oxidation.

Homogeneous Catalysis of Oxalic Acid Oxidation Reaction by Ozone in Strongly Acidic Solutions

ABSTRACT The possibility of catalysis by VO2+, Cr3+, Cr2O7 2–, Mn2+, MnO4 –, Fe3+, Co2+, Ni2+, Cu2+, Ce3+, BrO3 –, or IO3 – ions of oxalic acid oxidation reaction with ozone in strongly acidic aqueous solutions at pH 0.3–1.7 has been studied. The final product of the oxidation is carbon dioxide. The kinetic regularities of the reaction have been determined from the rates of ozone consumption and carbon dioxide release. Without a catalyst, oxalic acid does not interact with ozone. It has been found that Mn2+, MnO4 – and Co2+ ions are effective catalysts, while the catalytic effect of VO2+, Cr3+, Cr2O7 2–, Fe3+, Ni2+, Cu2+, Ce3+, BrO3 –, or IO3 – is weak or does not manifest itself at all. The effects of acidity and concentrations of the catalyst in solution and ozone in the gas phase on the kinetics of the process have been studied. Kinetic schemes of catalytic reactions and leading intermediates are proposed. The data obtained will help in choosing the optimal conditions for the process of oxidative mineralization of oxalic acid under the action of ozone in strongly acidic media.

Harnessing a Pd4 Water-Soluble Molecular Capsule as a Size-Selective Catalyst for Targeted Oxidation of Alkyl Aromatics.

Molecular hosts with functional cavities can emulate enzymatic behavior through selective encapsulation of substrates, resulting in high chemo-, regio-, and stereoselective product formation. It is still challenging to synthesize enzyme-mimicking hosts that exhibit a narrow substrate scope that relies upon the recognition of substrates based on the molecular size. Herein, we introduce a Pd4 self-assembled water-soluble molecular capsule [M 4 L 2] (MC) that was formed through the self-assembly of a ligand L (4',4‴'-(1,4-phenylene)bis(1',4'-dihydro-[4,2':6',4″-terpyridine]-3',5'-dicarbonitrile)) with the acceptor cis-[(en)Pd(NO3)2] [en = ethane-1,2-diamine] (M). The molecular capsule MC showed size-selective recognition towards xylene isomers. The redox property of MC was explored for efficient and selective oxidation of one of the alkyl groups of m-xylene and p-xylene to their corresponding toluic acids using molecular O2 as an oxidant upon photoirradiation. Employing host-guest chemistry, we demonstrate the homogeneous catalysis of alkyl aromatics to the corresponding monocarboxylic acids in water under mild conditions. Despite homogeneous catalysis, the products were separated from the reaction mixtures by simple filtration/extraction, and the catalyst was reused. The larger analogues of the alkyl aromatics failed to bind within the MC's hydrophobic cavity, resulting in a lower/negligible reaction outcome. The present study represents a facile approach for selective photo-oxidation of xylene isomers to their corresponding toluic acids in an aqueous medium under mild conditions.

Revolutionizing Electrochemical kinetics: Homogeneous Redox Catalysis unveiled through an innovative mathematical framework using the Homotopy Perturbation Method

Revolutionizing Electrochemical kinetics: Homogeneous Redox Catalysis unveiled through an innovative mathematical framework using the Homotopy Perturbation Method

A Radical Revolution in the 21st Century

AbstractThis Special Collection on “Radical Chemistry in Homogeneous Catalysis and Organic Synthesis” organized in collaboration with the European Journal of Organic Chemistry and ChemCatChem showcases the maturity of field. Overall, this Special Collection encompasses 11 reviews and 27 research articles, covering a diverse range of topics. It presents the latest advancements in the controlled generation of radical species using transition metal catalysis, photoredox catalysis, or electrochemistry. It highlights the importance of moving towards more sustainable chemical processes, with particular focus on the development of novel organo(photo)catalysts, or efficient catalyst‐free reactions. It presents new transformations to access complex scaffolds as well as new building blocks and reagents, further simplifying single‐electron disconnection logic. In addition, it also highlights the key advancements in addressing some of the current challenges in radical chemistry, such the development of enantioselective reactions or how to scale‐up radical processes to meet the needs of the fine chemical industry.

Synthesis of biochar-doped MgO catalyst for highly efficient conversion of glucose into formic acid at low temperatures

Formic acid (FA) is not only a significant platform chemical but also a promising candidate for hydrogen storage. Although homogeneous catalysts are commonly employed for the conversion of carbohydrate biomass into FA, which often come with substantial costs and inadequate reusability. It is noteworthy that there has been an increasing focus on the development of heterogeneous catalysts in this area, which aim to address these limitations and offer advantages such as easier separation and better reusability. Herein, an eco-friendly and efficient method for the catalytic conversion of glucose to FA by utilizing a biochar-MgO (BC-MgO) catalyst, which is prepared through pyrolysis of coconut husk biochar impregnated with magnesium chloride (MgCl2), is proposed. The FA yield at 74.6% is attained from glucose at 60 °C within only 3 h, utilizing a mere 0.1 g of catalyst and hydrogen peroxide (H2O2) amount of 100%. In conjunction with random forest (RF) algorithms and box-behnken experimental designs, further optimization resulted in an increased yield of 79.1%, with a selectivity of 82.7%, in only 3.8 h at 67 °C, employing 0.1 g of catalyst amount and 125% H2O2 amounts. Notably, the BC-MgO catalyst retains its catalytic efficiency over multiple cycles, only requiring a simple straightforward regeneration procedure. The mechanism underlying the glucose conversion to FA reaction system is demystified via kinetic modeling, time-gradient studies, and model compounds experiments. Life cycle assessment (LCA) indicates that when used for glucose conversion to FA, BC-MgO catalysts generate lower CO2 emissions than traditional homogeneous catalysts such as LiOH. Concurrently, an operation cost analysis indicates that operation cost of FA production from glucose is 4.1 $/kg with the BC-MgO catalyst, which is lower than LiOH catalyst (7.5 $/kg), evidencing the dual advantage of economic viability and an improved environmental impact inherent to this novel process.

20-Fold Increased Limiting Currents in Oxygen Reduction with Cu-tmpa by Replacing Flow-By with Flow-Through Electrodes.

Electrochemical oxygen reduction is a promising and sustainable alternative to the current industrial production method for hydrogen peroxide (H2O2), which is a green oxidant in many (emerging) applications in the chemical industry, water treatment, and fuel cells. Low solubility of O2 in water causes severe mass transfer limitations and loss of H2O2 selectivity at industrially relevant current densities, complicating the development of practical-scale electrochemical H2O2 synthesis systems. We tested a flow-by and flow-through configuration and suspension electrodes in an electrochemical flow cell to investigate the influence of electrode configuration and flow conditions on mass transfer and H2O2 production. We monitored the H2O2 production using Cu-tmpa (tmpa = tris(2-pyridylmethyl)amine) as a homogeneous copper-based catalyst in a pH-neutral phosphate buffer during 1 h of catalysis and estimated the limiting current density from CV scans. We achieve the highest H2O2 production and a 15-20 times higher geometrical limiting current density in the flow-through configuration compared to the flow-by configuration due to the increased surface area and foam structure that improved mass transfer. The activated carbon (AC) material in suspension electrodes, which have an even larger surface area, decomposes all produced H2O2 and proves unsuitable for H2O2 synthesis. Although the mass transfer limitations seem to be alleviated on the microscale in the flow-through system, the high O2 consumption and H2O2 production cause challenges in maintaining the initially reached current density and Faradaic efficiency (FE). The decreasing ratio between the concentrations of the O2 and H2O2 in the bulk electrolyte will likely pose a challenge when proceeding to larger systems with longer electrodes. Tuning the reactor design and operating conditions will be essential in maximizing the FE and current density.

Coke evolution study in a liquid atomization into gas-solid fluidized bed to convert fructose to value-added chemicals

Glucose and fructose are valuable compounds, which have the potential to displace fossil fuels as a feedstock for highly valued products. One of the promising reactions is the oxi-dehydration of fructose to produce Hydroxymethylfurfural, 2,5-diformylfuran, and furfural. Current processes to produce platform chemicals like Hydroxymethylfurfural, furfural, and 2,5-diformylfuran operate in liquid phase with homogeneous heterogeneous catalyst, but commercialization is limited by scale and environmental impact of the large volume solvent required. Here we developed a gas-phase process to convert fructose in a fluidized bed reactor over Mo − V − WO3/TiO2. However, in the catalytic oxi-dehydration of fructose, coke forms on active sites and decreases conversion but increases selectivity. Coke and reactor performance depends on fructose concentration and O2/fructose ratio. In the first 2 h, acidic amorphous coke accumulates (based on TPO, FTIR, and UV–vis). Feeding excess O2 maintains coke in an amorphous structure and liberates oxide sites. Whereas coke promotes, water block active site reducing yield. In this work, we optimized the operating parameters of fructose concentration (%) and O2/fructose ratio, while simultaneously mitigating the issues associated with coke formation and its detrimental effects.

Highly dispersed and stable Schiff base nickel catalyst on multi-walled carbon nanotubes promote ethylene oligomerization

α-Olefins are essential chemical raw materials in the synthesis of emulsifiers, plasticizers, and other value-added chemicals, and ethylene oligomerization is the main method used for preparing α-olefins. However, catalysts employed in ethylene oligomerization have drawbacks such as poor selection of target products, catalyst particle aggregation, and deactivation. To prevent the aggregation of the catalyst and expose a large number of active sites, we propose an anchoring strategy to stabilize and uniformly disperse catalysts on multi-walled carbon nanotubes (MWCNTs). Hybrid catalysts (SCn@MWCNTs) fabricated using the strategy exhibited considerably higher catalytic performance compared with homogeneous Schiff base nickel catalysts (SCn) owing to the combined effects of MWCNTs and active nickel centers. Under optimal catalytic conditions (temperature: 25 °C; Al/Ni molar ratio: 500 (SCn); Al/Ni molar ratio: 700 (SCn@MWCNTs); pressure: 0.7 MPa), the activities of SC1 and SC1@MWCNTs were 6.56 × 104 and 8.25 × 104 g/(mol Ni·h), respectively. In particular, SC1@MWCNTs showed remarkable recyclability and catalytic stability.

Covalent organic framework membrane reactor for boosting catalytic performance

Membrane reactors are known for their efficiency and superior operability compared to traditional batch processes, but their limited diversity poses challenges in meeting various reaction requirements. Herein, we leverage the molecular tunability of covalent organic frameworks (COFs) to broaden their applicability in membrane reactors. Our COF membrane demonstrates an exceptional ability to achieve complete conversion in just 0.63 s at room temperature—a benchmark in efficiency for Knoevenagel condensation. This performance significantly surpasses that of the corresponding homogeneous catalyst and COF powder by factors of 176 and 375 in turnover frequency, respectively. The enhanced concentration of reactants and the rapid removal of generated water within the membrane greatly accelerate the reaction, reducing the apparent activation energy. Consequently, this membrane reactor enables reactions that are unattainable using both COF powders and homogeneous catalysts. Considering the versatility, our findings highlight the substantial promise of COF-based membrane reactors in organic transformations.

Understanding the Role of Entropy in Promoting Electrocatalytic CO2 Reduction

A global push towards increased sustainability requires technologies that enable the long-term storage of renewable electricity and the sustainable synthesis of chemicals. The electrochemical reduction of CO2 to fuels and chemicals represents a promising avenue to meet these goals. Yet, despite extensive research, many fundamental aspects of the interfacial interactions that enable this reaction remain unclear. One simple question that has not been addressed to date is the mechanism through which the applied potential controls the rate of CO2 reduction. To provide insight, we measured CO2 reduction activation energies on Ag electrodes in the presence of different cations. Building on our earlier work (J. Am. Chem. Soc. 2016, 138, 25, 7820–7823), where some of us demonstrated the important role of structural modifications to imidazolium cations in controlling the rate of CO2 reduction reactions, we expected that these structural modifications impact the reaction rate through changes to the activation energy. However, our findings revealed a much more fascinating picture of the impact of imidazolium cations on CO2 reduction rates.By measuring the activation energy in the presence of imidazolium cations with different structural features, we find that introducing 1-ethly-3-methylimidazolium (EMIM) results in an apparent activation energy of zero. Under these conditions, the applied potential modifies the reaction rate solely through changes to the pre-exponential factor of the Arrhenius equation.Based on literature on homogeneous catalysis, we suggest that our finding indicates that the rate of CO2 reduction in the presence of EMIM is entirely controlled by the formation entropy of the CO2 reduction transition state, with the potential likely influencing the rate by modifying the degree of ordering of imidazolium cations at the electrode surface. We further show that proton abstraction from EMIM forms a neutral compound, resulting in the elimination of the rate enhancement. Our results provide important new insight into the mechanism through which the applied potential controls the rate of electrocatalytic reactions and we expect them to be of broad importance to a better understanding of the connection between interface structure and electrocatalytic rates.