Potential Induced Protonation of the Second Coordination Sphere in a Hangman-type Iron Porphyrin Complex Promotes HER: Insights via in Situ Raman Spectroelectrochemistry.

The iron-based porphyrin complex containing a bispyridine-based hanging unit termed Py2XPFe was previously used as an effective catalyst for the reduction of protons to molecular hydrogen in solution. Here, the molecular compound was immobilized on a modified gold electrode surface and investigated by spectroelectrochemical methods under catalytic conditions. Immobilization of the Py2XPFe was facilitated using a pyridine-based amine linker molecule grafted to the gold electrode by electrochemical amine oxidation. The linker molecule denoted in this report as Pyr-1 allows for effective coordination of the iron porphyrin compound to the modified gold surface through axial coordination of the pyridine component to the Fe center. Resonance Raman spectroelectrochemistry was performed on the immobilized catalyst in pH 7 buffer at increasing cathodic potentials. This facilitates the electrochemical hydrogen evolution reaction (HER) while concurrently allowing for the observation of the v4, v3, and v2 porphyrin marker bands, which are sensitive to oxidation and spin state changes at the metal center. The observed changes in these bands at decreasing potential indicate that the immobilized Py2XPFe exists in the formal high-spin FeIII state before being reduced to the low-spin FeII state resulting from axial interaction with the linker moiety. This FeII state likely acts as the precatalyst for the HER reaction. Surfaced enhanced Raman spectroelectrochemistry was also conducted on the system as the gold electrode provides a sufficient surface enhancement effect so as to observe the bonding nature of the pyridine substituents within the second coordination sphere. As the potential is lowered cathodically, the pyridine ring breathing modes at 999 cm-1 are shown to increase in intensity due to protonation, which reach an intensity saturated limit whereat HER is conducted. This suggests that in pH 7 buffer, the increase in cathodic potentials facilitates protonation of the pyridine-based second coordination sphere. The extent to which protonation occurs can be viewed as a function of decreasing potential due to an increase in proton flux at the immobilized catalyst which, at the required onset potential for catalysis, aids in the reduction of protons to molecular hydrogen.

Understanding active sites in molecular (photo)electrocatalysis through complementary vibrational spectroelectrochemistry.

Synthetic molecular (photo)electrocatalysts have been intensively studied due to their capability to drive key energy conversion reactions. In order to advance their potential through rational development, an in-depth mechanistic understanding of the catalytic reactions is required. In this article, we highlight in situ vibrational spectro-electrochemistry, specifically, confocal Raman and infrared absorption spectroscopy, as a highly capable method for obtaining profound insights into the structure and reactivity of electrode-immobilised molecular catalytic systems. Commonly employed experimental configurations for carrying out in situ studies and conditions for operating in the surface-enhanced mode are presented. This is followed by selected research examples to showcase the different aspects and features of molecular (photo)electrocatalysis that can be visualised by vibrational spectroelectrochemistry. Presented target systems include porphyrin-based systems, polypyridyl-based complexes as well as phthalocyanine-based two dimensional conjugated metal-organic frameworks, and photoactive conjugated polymers. The article concludes with a critical assessment of current limitations of the techniques and gives a brief outlook on anticipated future developments.

Electrochemically Triggered Dynamics within a Hybrid Metal-Organic Electrocatalyst.

A wide array of systems, ranging from enzymes to synthetic catalysts, exert adaptive motifs to maximize their functionality. In a related manner, select metal-organic frameworks (MOFs) and similar systems exhibit structural modulations under stimuli such as the infiltration of guest species. Probing their responsive behavior in situ is a challenging but important step toward understanding their function and subsequently building functional systems. In this report, we investigate the dynamic behavior of an electrocatalytic Mn-porphyrin-containing MOF system (Mn-MOF). We discover, using a combination of electrochemistry and in situ probes of UV-vis absorption, resonance Raman, and infrared spectroscopy, a restructuration of this system via a reversible cleavage of the porphyrin carboxylate ligands under an applied voltage. We further show, by combining experimental data and DFT calculations, as a proof of concept, the capacity to utilize the Mn-MOF for electrochemical CO2 fixation and to spectroscopically capture the reaction intermediates in its catalytic cycle. The findings of this work and the methodology developed open opportunities in the application of MOFs as dynamic, enzyme-inspired electrocatalytic systems.

Disparity of Cytochrome Utilization in Anodic and Cathodic Extracellular Electron Transfer Pathways of Geobacter sulfurreducens Biofilms.

Extracellular electron transfer (EET) in microorganisms is prevalent in nature and has been utilized in functional bioelectrochemical systems. EET of Geobacter sulfurreducens has been extensively studied and has been revealed to be facilitated through c-type cytochromes, which mediate charge between the electrode and G. sulfurreducens in anodic mode. However, the EET pathway of cathodic conversion of fumarate to succinate is still under debate. Here, we apply a variety of analytical methods, including electrochemistry, UV-vis absorption and resonance Raman spectroscopy, quartz crystal microbalance with dissipation, and electron microscopy, to understand the involvement of cytochromes and other possible electron-mediating species in the switching between anodic and cathodic reaction modes. By switching the applied bias for a G. sulfurreducens biofilm coupled to investigating the quantity and function of cytochromes, as well as the emergence of Fe-containing particles on the cell membrane, we provide evidence of a diminished role of cytochromes in cathodic EET. This work sheds light on the mechanisms of G. sulfurreducens biofilm growth and suggests the possible existence of a nonheme, iron-involving EET process in cathodic mode.

Host-Guest Chemistry Meets Electrocatalysis: Cucurbit[6]uril on a Au Surface as a Hybrid System in CO2 Reduction.

The rational control of forming and stabilizing reaction intermediates to guide specific reaction pathways remains to be a major challenge in electrocatalysis. In this work, we report a surface active-site engineering approach for modulating electrocatalytic CO2 reduction using the macrocycle cucurbit[6]uril (CB[6]). A pristine gold surface functionalized with CB[6] nanocavities was studied as a hybrid organic-inorganic model system that utilizes host-guest chemistry to influence the heterogeneous electrocatalytic reaction. The combination of surface-enhanced infrared absorption (SEIRA) spectroscopy and electrocatalytic experiments in conjunction with theoretical calculations supports capture and reduction of CO2 inside the hydrophobic cavity of CB[6] on the gold surface in aqueous KHCO3 at negative potentials. SEIRA spectroscopic experiments show that the decoration of gold with the supramolecular host CB[6] leads to an increased local CO2 concentration close to the metal interface. Electrocatalytic CO2 reduction on a CB[6]-coated gold electrode indicates differences in the specific interactions between CO2 reduction intermediates within and outside the CB[6] molecular cavity, illustrated by a decrease in current density from CO generation, but almost invariant H2 production compared to unfunctionalized gold. The presented methodology and mechanistic insight can guide future design of molecularly engineered catalytic environments through interfacial host-guest chemistry.

Influence of Mesityl and Thiophene Peripheral Substituents on Surface Attachment, Redox Chemistry, and ORR Activity of Molecular Iron Porphyrin Catalysts on Electrodes.

Two iron porphyrin complexes with either mesityl (FeTMP) or thiophene (FeT3ThP) peripheral substituents were attached to basal pyrolytic graphite and Ag electrodes via different immobilization methods. By combining cyclic voltammetry and in-operando surface-enhanced Raman spectroscopy along with MD simulations and DFT calculations, their respective surface attachment, redox chemistry and activity toward electrocatalytic oxygen reduction was investigated. For both porphyrin complexes, it could be shown that catalytic activity is restricted to the first (few) molecular layer(s), although electrodes covered with thiophene-substituted complexes showed a better capability to consume the oxygen at a given overpotential even in thicker films. The spectroscopic data and simulations suggest that both porphyrin complexes attach to a Ag electrode surface in a way that maximum planarity and minimum distance between the catalytic iron site and the electrode is achieved. However, due to the distinctive design of the FeT3ThP complex, the thiophene rings are capable of occupying a conformation, via rotation around the bonding axis to the porphyrin, in which all four sulfur atoms can coordinate to the Ag surface. This effect creates a dense and planar surface coverage of the porphyrin on the electrode facilitating a fast (multi) electron transfer via several covalent Ag-S bonds. In contrast, bulky mesityl groups as peripheral substituents, which have been initially introduced to prevent aggregation and improve catalytic behavior in solution, exert a negative effect on the overall electrocatalytic performance in the immobilized state as a less dense coverage and less stable interactions with the surface are formed. Our results underline the importance of rationally designed heterogenized molecular catalysts to achieve optimal turnover, which not only strictly applies to the here discussed oxygen reduction reaction but eventually holds also true for other energy conversion reactions such as carbon dioxide reduction.

Probing CO2 Conversion Chemistry on Nanostructured Surfaces with Operando Vibrational Spectroscopy.

With the rising emphasis on renewable energy research, the field of electrocatalytic CO2 conversion to fuels has grown tremendously in recent years. Advances in nanomaterial synthesis and characterization have enabled researchers to screen effects of elemental composition, size, and surface chemistry on catalyst performance. However, direct links from structure and active state to catalytic function are difficult to establish. To this end, operando spectroscopic techniques, those conducted simultaneously as catalysts operate, can provide key complementary information by investigating electrocatalysis under turnover conditions. In particular, Raman and infrared spectroscopy have the potential to reveal the identity of surface-bound intermediates, catalyst active state, and possible reaction sites to supplement the insights extracted from conventional electrochemistry. Such research aims to work in tandem synthetic and catalytic efforts to guide the development of next-generation CO2 electrocatalytic systems through rational design. In this Mini Review, we examine the latest developments in the operando probing of electrochemical CO2 reduction on nanostructured electrocatalysts and detail how this research accelerates the advancement of this field.

Advancing Techniques for Investigating the Enzyme-Electrode Interface.

Enzymes are the essential catalytic components of biology and adsorbing redox-active enzymes on electrode surfaces enables the direct probing of their function. Through standard electrochemical measurements, catalytic activity, reversibility and stability, potentials of redox-active cofactors, and interfacial electron transfer rates can be readily measured. Mechanistic investigations on the high electrocatalytic rates and selectivity of enzymes may yield inspiration for the design of synthetic molecular and heterogeneous electrocatalysts. Electrochemical investigations of enzymes also aid in our understanding of their activity within their biological environment and why they evolved in their present structure and function. However, the conventional array of electrochemical techniques (e.g., voltammetry and chronoamperometry) alone offers a limited picture of the enzyme-electrode interface. How many enzymes are loaded onto an electrode? In which orientation(s) are they bound? What fraction is active, and are single or multilayers formed? Does this static picture change over time, applied voltage, or chemical environment? How does charge transfer through various intraprotein cofactors contribute to the overall performance and catalytic bias? What is the distribution of individual enzyme activities within an ensemble of active protein films? These are central questions for the understanding of the enzyme-electrode interface, and a multidisciplinary approach is required to deliver insightful answers. Complementing standard electrochemical experiments with an orthogonal set of techniques has recently allowed to provide a more complete picture of enzyme-electrode systems. Within this framework, we first discuss a brief history of achievements and challenges in enzyme electrochemistry. We subsequently describe how the aforementioned challenges can be overcome by applying advanced electrochemical techniques, quartz-crystal microbalance measurements, and spectroscopic, namely, resonance Raman and infrared, analysis. For example, rotating ring disk electrochemistry permits the simultaneous determination of reaction kinetics and quantification of generated products. In addition, recording changes in frequency and dissipation in a quartz crystal microbalance allows to shed light into enzyme loading, relative orientation, clustering, and denaturation at the electrode surface. Resonance Raman spectroscopy yields information on ligation and redox state of enzyme cofactors, whereas infrared spectroscopy provides insights into active site states and the protein secondary and tertiary structure. The development of these emerging methods for the analysis of the enzyme-electrode interface is the primary focus of this Account. We also take a critical look at the remaining gaps in our understanding and challenges lying ahead toward attaining a complete mechanistic picture of the enzyme-electrode interface.

Artificial photosynthesis with metal and covalent organic frameworks (MOFs and COFs): challenges and prospects in fuel-forming electrocatalysis.

Mimicking photosynthesis in generating chemical fuels from sunlight is a promising strategy to alleviate society's demand for fossil fuels. However, this approach involves a number of challenges that must be overcome before this concept can emerge as a viable solution to society's energy demand. Particularly in artificial photosynthesis, the catalytic chemistry that converts energy in the form of electricity into carbon-based fuels and chemicals has yet to be developed. Here, we describe the foundational work and future prospects of an emerging and promising class of materials: metal- and covalent-organic frameworks (MOFs and COFs). Within this context, these porous and tuneable framework materials have achieved initial success in converting abundant feedstocks (H2 O and CO2 ) into chemicals and fuels. In this review, we first highlight key achievements in this direction. We then follow with a perspective on precisely how MOFs and COFs can perform in ways not possible with conventional molecular or heterogeneous catalysts. We conclude with a view on how spectroscopically probing MOF and COF catalysis can be used to elucidate reaction mechanisms and material dynamics throughout the course of reaction.

Aerobic Conditions Enhance the Photocatalytic Stability of CdS/CdOx Quantum Dots.

Photocatalytic H2 production through water splitting represents an attractive route to generate a renewable fuel. These systems are typically limited to anaerobic conditions due to the inhibiting effects of O2 . Here, we report that sacrificial H2 evolution with CdS quantum dots does not necessarily suffer from O2 inhibition and can even be stabilised under aerobic conditions. The introduction of O2 prevents a key inactivation pathway of CdS (over-accumulation of metallic Cd and particle agglomeration) and thereby affords particles with higher stability. These findings represent a possibility to exploit the O2 reduction reaction to inhibit deactivation, rather than catalysis, offering a strategy to stabilise photocatalysts that suffer from similar degradation reactions.

Tuning Product Selectivity for Aqueous CO2 Reduction with a Mn(bipyridine)-pyrene Catalyst Immobilized on a Carbon Nanotube Electrode.

The development of high-performance electrocatalytic systems for the controlled reduction of CO2 to value-added chemicals is a key goal in emerging renewable energy technologies. The lack of selective and scalable catalysts in aqueous solution currently hampers the implementation of such a process. Here, the assembly of a [MnBr(2,2'-bipyridine)(CO)3] complex anchored to a carbon nanotube electrode via a pyrene unit is reported. Immobilization of the molecular catalyst allows electrocatalytic reduction of CO2 under fully aqueous conditions with a catalytic onset overpotential of η = 360 mV, and controlled potential electrolysis generated more than 1000 turnovers at η = 550 mV. The product selectivity can be tuned by alteration of the catalyst loading on the nanotube surface. CO was observed as the main product at high catalyst loadings, whereas formate was the dominant CO2 reduction product at low catalyst loadings. Using UV-vis and surface-sensitive IR spectroelectrochemical techniques, two different intermediates were identified as responsible for the change in selectivity of the heterogenized Mn catalyst. The formation of a dimeric Mn0 species at higher surface loading was shown to preferentially lead to CO formation, whereas at lower surface loading the electrochemical generation of a monomeric Mn-hydride is suggested to greatly enhance the production of formate. These results emphasize the advantages of integrating molecular catalysts onto electrode surfaces for enhancing catalytic activity while allowing excellent control and a deeper understanding of the catalytic mechanisms.

High Performance Reduction of H2O2 with an Electron Transport Decaheme Cytochrome on a Porous ITO Electrode.

The decaheme cytochrome MtrC from Shewanella oneidensis MR-1 immobilized on an ITO electrode displays unprecedented H2O2 reduction activity. Although MtrC showed lower peroxidase activity in solution compared to horseradish peroxidase, the ten heme cofactors enable excellent electronic communication and a superior activity on the electrode surface. A hierarchical ITO electrode enabled optimal immobilization of MtrC and a high current density of 1 mA cm-2 at 0.4 V vs SHE could be obtained at pH 6.5 (Eonset = 0.72 V). UV-visible and Resonance Raman spectroelectrochemical studies suggest the formation of a high valent iron-oxo species as the catalytic intermediate. Our findings demonstrate the potential of multiheme cytochromes to catalyze technologically relevant reactions and establish MtrC as a new benchmark in biotechnological H2O2 reduction with scope for applications in fuel cells and biosensors.

Redox-linked protein dynamics of cytochrome c probed by time-resolved surface enhanced infrared absorption spectroscopy.

Time-resolved surface enhanced infrared absorption (SEIRA) spectroscopy is employed to analyse the dynamics of the protein structural changes coupled to the electron transfer process of immobilised cytochrome c (Cyt-c). Upon electrostatic binding of Cyt-c to Au electrodes coated with self-assembled monolayers (SAMs) of carboxyl-terminated thiols, cyclic voltammetric measurements demonstrate a reversible redox process with a redox potential that is similar to that of Cyt-c in solution, and a non-exponential distance-dependence of the electron transfer rate as observed previously (D. H. Murgida and P. Hildebrandt, Chem. Soc. Rev. 2008, 37, 937). On the basis of characteristic redox-state-sensitive amide I bands, the protein structural changes triggered by the electron transfer are monitored by rapid scan and step scan SEIRA spectroscopy in combination with the potential jump technique. Whereas the temporal evolution of the conjugate bands at 1693 and 1673 cm(-1) displays the same rate constants as electron transfer, the time-dependent changes of the 1660-cm(-1) band are slower by about a factor of 2. The study demonstrates that time-resolved SEIRA spectroscopy provides further information about the dynamics and mechanism of interfacial processes of redox proteins, thereby complementing the results obtained from other surface-sensitive techniques. In comparison with previous surface enhanced resonance Raman spectroscopic findings, the present results are discussed in terms of the local electric field strengths at the Au/SAM/Cyt-c interface.