UV-vis Spectroelectrochemistry Research Articles

Probing catalyst behaviour with UV–Vis spectroelectrochemistry

Probing catalyst behaviour with UV–Vis spectroelectrochemistry

Catalyst Deactivation on Transition Metal Oxides during Ammonia Electrooxidation

Electrochemical ammonia oxidation reaction (AOR) offers a sustainable approach for waste ammonia remediation, energy conversion, and valuable product production.[1] Despite the promise of transition metal oxides as effective catalysts, the AOR mechanism on their surface remains elusive, along with factors leading to deactivation, which often involve surface poisoning species formation and site corrosion. Moreover, the AOR potential overlaps with the oxygen evolution reaction (OER), leading to the formation of O2 as a byproduct and an overall decline in energy efficiency. Therefore, it is critical to understand the molecular behavior of ammonia and other species on the oxide surface under reaction conditions.In this work, we systematically investigate the AOR mechanism on NiOOH and CuOOH, emphasizing the influence of OER and dissolved O2, which has been previously largely overlooked. Utilizing three in situ techniques: differential electrochemical mass spectrometry (DEMS) for detecting gaseous intermediates and products, Raman spectroelectrochemistry for monitoring surface transformations, and UV-vis spectroelectrochemistry for observing species within the electrical double layer, we reveal that the formation of NiOOH/CuOOH, the AOR catalytic activity and selectivity, and the deactivation pathway are all significantly modulated by OER and the presence of O2.It is established that AOR on NiOOH is potential-dependent, with N2 production being initiated at potentials coinciding with OER commencement and the generation of NOx species being favored at elevated potentials (Figure 1 left). Moreover, OER competes with the AOR to N2 pathway but promotes the AOR to NOx (N2O, NO, and NO2), which is further elucidated through the protective role of O2 to remove *N poisoning species, mitigating surface deactivation and decreasing charge transfer resistance (Figure 1 right). On CuOOH, however, the O2 can promote NO formation and Cu dissolution from the CuOOH layer, leading to complete deactivation at high potential. Our findings offer an in situ insight into the AOR mechanisms on metal oxides with co-existing OER and provide a critical direction for designing electrocatalysts for environmental applications, particularly for systems where long-term electrolysis is desired. Figure 1. Left: DEMS results during linear sweep voltammetry of NiOOH electrode in Ar/O2-saturated electrolytes (1.0 M KOH + 100 mM NH3). Right: Electrolysis results of NiOOH electrode in Ar/O2 saturated electrolytes (1.0 M KOH + 100 mM NH3) at various potentials. References MacFarlane, D. R.; Cherepanov, P. V.; Choi, J.; Suryanto, B. H. R.; Hodgetts, R. Y.; Bakker, J. M.; Ferrero Vallana, F. M.; Simonov, A. N. Joule, 4 (6), 1186-1205 (2020). Figure 1

Operando Spectroelectrochemistry Unravels the Mechanism of CO2 Electrocatalytic Reduction by an Fe Porphyrin.

Iron porphyrins are molecular catalysts recognized for their ability to electrochemically and photochemically reduce carbon dioxide (CO2). The main reduction product is carbon monoxide (CO). CO holds significant industrial importance as it serves as a precursor for various valuable chemical products containing either a single carbon atom (C1), like methanol or methane, or multiple carbon atoms (Cn), such as ethanol or ethylene. Despite the long-established efficiency of these catalysts, optimizing their catalytic activity and stability and comprehending the intricate reaction mechanisms remain a significant challenge. This article presents a comprehensive investigation of the mechanistic aspects of the selective electroreduction of CO2 to CO using an iron porphyrin substituted with four trimethylammonium groups in the para position [(pTMA)FeIII-Cl]4+. By employing infrared and UV/Visible spectroelectrochemistry, changes in the electronic structure and coordination environment of the iron center can be observed in real-time as the electrochemical potential is adjusted, offering new insights into the reaction mechanisms. Catalytic species were identified, and evidence of a secondary reaction pathway was uncovered, potentially prompting a re-evaluation of the nature of the catalytically active species.

Band Gap Engineering of Spin-Coated <i>α</i>-Fe<sub>2</sub>O<sub>3</sub> for Solar Water Splitting Utilizing a Facile Zinc Doping Route

Iron oxide (hematite, α-Fe2O3) is an earth-abundant material having a favorable band gap for solar energy harvesting by producing Hydrogen (H2) by photoelectrochemical (PEC) water splitting. Although Fe2O3 has a valence band maximum (VBM) more positive than the water oxidation potential in the normalized hydrogen energy scale (NHE), it can not be used as a photocathode because of the more positive conduction band minima (CBM) than the reduction potential of the electrons required for the hydrogen evolution reaction (HER). This paper reports the band gap engineering of Fe2O3 by doping it with Zinc (Zn) through a facile way to modify its CBM and VBM to enable it to work as an ideal and efficient photocatalyst. One-pot synthesis of α-Fe2O3 doped with Zn was achieved by spin-coating a precursor solution prepared with FeCl2 and Zinc acetate (Zn(CH3COO)2.2H2O) on fluorine-doped tin oxide (FTO) or glass substrates. The films were then annealed in air at around 450 °C. The surface morphology and the crystal structure of the Fe2O3 were studied using SEM (Scanning Electron Microscope) and XRD (X-ray diffractometer). The UV-VIS spectrophotometry and spectroelectrochemistry (SEC) determined the band energies of α-Fe2O3 and Zn-doped α-Fe2O3. The band gap and CBM of pristine α-Fe2O3 were found to be -2.09 eV and -5.31 eV vs. EVAC, whereas the band gap and CBM of Zn-doped α-Fe2O3 were -1.96 eV and -4.3 eV vs. EVAC respectively. The outcome of the photoelectrochemical study shows that α-Fe2O3 doped with Zn can have a reduced band gap with more negative CBM than the H+/H2 reduction potential that efficiently ensures both oxygen and hydrogen evolution reaction.

Spectroelectrochemical studies of TDMQ20: A potential drug against Alzheimer’s disease

Alzheimer's Disease (AD), reported for the first time in 1906, is a common disease that remains incurable to this day. In the past, a family of treatments using Cu(II) chelators failed during clinical trials, evidencing the importance of pre-clinical studies. In this work, we performed electrochemical characterisation of TDMQ20, a new potential drug against AD, using electrochemistry and spectroelectrochemistry. On the basis of voltammetry, we determined that TDMQ20 undergoes a two-step irreversible oxidation process and a one-step irreversible reduction process. Both oxidation and reduction reactions are pH-sensitive. Bidimensional UV-Vis spectroelectrochemistry (UV-Vis-SEC) allowed us to confirm that oxidation of TDMQ20 can occur both on the aliphatic chain and on the aromatic ring. The results expand the knowledge of the TDMQ20 redox activity in the human body which is important from the point of view of the toxicity of the proposed therapy.

Correlative in situ analysis of the role of oxygen on ammonia electrooxidation selectivity on NiOOH surfaces

Correlative in situ analysis of the role of oxygen on ammonia electrooxidation selectivity on NiOOH surfaces

Electrochemical, spectroelectrochemical, and electrochromic characterization of silicon(IV) phthalocyanine with axially ligated electropolymerizable 3,4-ethylenedioxythiophene moieties

Electrochemical, spectroelectrochemical, and electrochromic characterization of silicon(IV) phthalocyanine with axially ligated electropolymerizable 3,4-ethylenedioxythiophene moieties

Operando spectroscopic characterization of formamidinium lead iodide perovskite quantum dots for tracking electrochemical reactions

Multidimensional ABX3 hybrid perovskites three-dimensionally confined dot-shaped structure demonstrate versatile potential to photoelectrochemical cells for water splitting, hydrogen generation, solar cells, and light-emitting diodes. To apply perovskite quantum dots (PQDs) to solar-driven chemistry and optoelectronic devices, understanding the photoinduced charge carrier dynamics of PQDs under electrochemical conditions or applied bias are important. In this study, the detailed transformation mechanism of formamidinium lead iodide perovskite quantum dots under electrochemical conditions was studied by tracking the products of the reaction through cyclic voltammetry, X-ray photoemission spectroscopy, in-situ UV-visible spectroelectrochemistry, etc. Through comprehensive characterizations, the mechanism of irreversible oxidative transformation of perovskite quantum dots was presented. This study provides deeper insight into the electrochemical behavior of PQDs for successful solar-driven chemistry and optoelectronic device applications.

Electron-Triggered Imine Coupling: Synthesis and Characterization of Three Redox States (0,-1,-2) of a Ni(N2 S2 ) Complex.

Metal imine-thiolate complexes, M(NS)2 are known to undergo imine C-C bond formation to give M(N2 S2 ) complexes (M=Co, Ni) containing a redox-active ligand. Although these transfor-mations are not typically quantitative, we demonstrate here that the one-electron reduction of a related Ni bis(imine-thiolate) complex affords the corresponding paramagnetic [Ni(N2 S2 )]- anion (2⋅- ) exclusively; subsequent oxidation with [Cp2 Fe]BF4 then affords a high yield of neutral 2 (Cp=η5 -cyclopentadienyl). Moreover, electrochemical studies indicate that a second one-electron reduction affords the diamagnetic dianion. Both anionic products were isolated and characterized by SC-XRD and their electronic structures were investigated by UV-vis spectro-electrochemistry, EPR and NMR spectroscopy, and DFT studies. These studies show that reduction proceeds primarily on the ligand, with (N2 S2 )4- containing both thiolate and ring-delocalized anions.

Spectroelectrochemical Detection of Acetaldehyde in Wines

The combination of electrochemical sensors and spectroscopy has been scarcely used due to the traditional instrumental limitations of spectroelectrochemical techniques. However, the development of new setups and commercial instruments enables the spectroelectrochemical detection of a variety of analytes [1]. In that way, UV-Vis spectroelectrochemistry joins the advantages of electrochemistry and UV-Vis spectroscopy, improving the analytical features that both techniques have separately.In this work, a new spectroelectrochemical two-enzyme sensor system has been developed for the detection of acetaldehyde in wine. Spectroelectrochemical detection is achieved by the analysis of the optical properties of the ferri/ferrocyanide redox couple involved in the enzymatic process: aldehyde dehydrogenase catalyzes the aldehyde oxidation using NAD+ as a cofactor and, simultaneously, diaphorase reoxidizes the NADH formed in the first enzymatic process due to the presence of ferricyanide. The analysis of the characteristic UV-Vis bands of ferricyanide at 310 and 420 nm allows the detection of acetaldehyde, since absorption bands are only related to the oxidation of this substrate and avoids the contribution of other interferents [2]. Two different wines have been evaluated in this work, obtaining excellent results that agree with the literature as well as with those obtained with a commercial fluorogenic kit.

Synthesis, characterization and electrochemistry of mono- and dirhodium(I) meso-diaryl dibenzoporphyrin(2.1.2.1) complexes

A series of mono- and dirhodium(I) meso-diaryl dibenzoporphyrin(2.1.2.1) complexes containing [Formula: see text]-tolyl, phenyl or pentafluorophenyl meso-substiuents were synthesized and characterized as to their electrochemical and spectroscopic properties in organic solvents and the data compared to that for same series of free base dibenzoporphyrins in their neutral and protonated forms. The redox behavior of the free base, mono- and di-metallic porphyrins was measured by cyclic voltammetry (CV) in methylene chloride or pyridine before and after the addition of acid in the form of TFA and the products of electron transfer are proposed on the basis of the CV data combined with results from thin-layer UV-visible spectroelectrochemistry. X-ray crystal structures of the dirhodium(I) complexes are also reported.

Completely o-Phenylene Bridged N4-Cyclophane: A Missing Link in the Phenylene Bridge N4-Cyclophane Family

Unprecedented types of metal-free complete o-phenylene bridged N4-cyclophanes (M1 and M2) have been synthesized via sequential palladium-catalyzed Buchwald-Hartwig N-arylation reactions. These cyclophanes may be considered as aromatic analogues of aliphatic group-spaced N4-macrocycles. These have been characterized fully using physicochemical characterization techniques and finally by single crystal X-ray structure determination. Their redox and spectral properties have been characterized by cyclic voltammetry, UV-vis spectro-electrochemistry, fluorescence spectral studies, and DFT calculations. These studies have shown rich redox, spectral, and photophysical properties that could make both M1 and M2 potential candidates for various applications.

Highly Robust and Antiaromatic Rhenium(I) Rosarin.

1ReH•Cl, a highly robust and antiaromatic rhenium(I) complex of triarylrosarin, is synthesized. The 1H NMR spectrum of 1ReH•Cl shows upfield-shifted pyrrole protons and highly downfield-shifted inner protons that confirm its antiaromatic nature, with density functional theory calculations strongly supporting this interpretation. Antiaromatic 1ReH•Cl absorbs from the UV to near-IR region of the optical spectrum; cyclic voltammetry, thin-layer UV-vis spectroelectrochemistry, and spin-density distributions clearly reveal that the rosarin backbone of 1ReH•Cl undergoes redox chemistry. The X-ray structure of 1ReH•Cl shows a fully coordinated and protonated inner cavity that effectively prevents proton-coupled electron transfer when treated with an acid. A remarkably negative NICS(0) value, clockwise anisotropy of the induced current density ring current, and the aromatic shielded inner cavity in the 2D ICSS(0) map reveal that the T1 state of 1ReH•Cl is aromatic based on Baird's rule.

Electrocatalytic Hydrogen Evolution of Bent Bis(dipyrrin) Ni(II) Complexes.

Planar Ni(II) porphyrinoid complexes have been widely used in electrochemical carbon dioxide reduction reaction and oxygen reduction reaction as well as hydrogen evolution reaction (HER). However, nonplanar Ni(II) tetra-pyrrolic complexes have not been thoroughly investigated thus far. In this study, three highly bent bis(dipyrrin) Ni(II) complexes have been synthesized to investigate their structure, electronic property, and electrocatalytic HER activities. Cyclic voltammetry and thin-layer UV-visible spectroelectrochemistry studies revealed four redox processes, yielding two reduced species as the final products. The ic/ip values of phenyl- and pentafluorophenyl-bearing bis(dipyrrin) Ni(II) complexes were >30 when trifluoroacetic acid was used as the proton source, and their Faradaic efficiencies for H2 generation were >93%. Density functional theory calculations of the HERs revealed low endothermic energies of bent bis(dipyrrin) Ni(II) complexes.

Electrocatalytic reduction of CO2 to CO by a series of organometallic Re(I)-tpy complexes.

A series of organometallic Re(I)(L)(CO)3Br complexes with 4'-substituted terpyridine ligands (L) has been synthesised as electrocatalysts for CO2 reduction. The complexes' spectroscopic characterisation and computationally optimised geometry demonstrate a facial geometry around Re(I) with three cis COs and the terpyridine ligand coordinating in a bidentate mode. The effect of substitution on the 4'-position of terpyridine (Re1-5) on CO2 electroreduction was investigated and compared with a known Lehn-type catalyst, Re(I)(bpy)(CO)3Br (Re7). All complexes catalyse CO evolution in homogeneous organic media at moderate overpotentials (0.75-0.95 V) with faradaic yields of 62-98%. The electrochemical catalytic activity was further evaluated in the presence of three Brønsted acids to demonstrate the influence of the pKa of the proton sources. The TDDFT and ultrafast transient absorption spectroscopy (TAS) studies showed combined charge transfer bands of ILCT and MLCT. Amongst the series, the Re-complex containing a ferrocenyl-substituted terpyridine ligand (Re5) shows an additional intra-ligand charge transfer band and was probed using UV-Vis spectroelectrochemistry.

Highly Chemoselective Catalytic Photooxidations by Using Solvent as a Sacrificial Electron Acceptor.

Catalyst recovery is an integral part of photoredox catalysis. It is often solved by adding another component-a sacrificial agent-whose role is to convert the catalyst back into its original oxidation state. However, an additive may cause a side reaction thus decreasing the selectivity and overall efficiency. Herein, we present a novel approach towards chemoselective photooxidation reactions based on suitable solvent-acetonitrile acting simultaneously as an electron acceptor for catalyst recovery, and on anaerobic conditions. This is allowed by the unique properties of the catalyst, 7,8-dimethoxy-3-methyl-5-phenyl-5-deazaflavinium chloride existing in both strongly oxidizing and reducing forms, whose strength is increased by excitation with visible light. Usefulness of this system is demonstrated in chemoselective dehydrogenations of 4-methoxy- and 4-chlorobenzyl alcohols to aldehydes without over-oxidation to benzoic acids achieving yields up to 70 %. 4-Substituted 1-phenylethanols were oxidized to ketones with yields 80-100 % and, moreover, with yields 31-98 % in the presence of benzylic methyl group, diphenylmethane or thioanisole which are readily oxidized in the presence of oxygen but these were untouched with our system. Mechanistic studies based on UV-Vis spectro-electrochemistry, EPR and time-resolved spectroscopy measurements showed that the process involving an electron release from an excited deazaflavin radical to acetonitrile under formation of solvated electron is crucial for the catalyst recovery.

UV–Vis operando spectroelectrochemistry for (photo)electrocatalysis: Principles and guidelines

Electrocatalytic and photoelectrocatalytic technologies are key players in the transition to a circular economy. In order to rationally design the new generation of efficient (photo)electrocatalysts it is critical to establish which performance bottlenecks must be overcome during the complex catalytic transformations. Obtaining such detailed mechanistic and kinetic knowledge requires the use of operando spectroscopic techniques that are capable of probing the system under operating conditions. Of all the methods available UV–Vis spectroelectrochemistry stands out for its versatility and experimental simplicity which enables the systematic study of samples under a wide range of reaction conditions. From a mechanistic viewpoint, this technique allows us to quantify the accumulation of reactive species at the catalyst–electrolyte interface and, through a kinetic population model, opens the door to characterising the kinetics of the rate determining step of catalysis. Here, we review the different information that can be extracted from UV–Vis spectroelectrochemistry and how it can be used to understand catalytic mechanisms in solid (photo)electrodes.

Investigating Comproportionation in Multielectron Transfers via UV-Visible Spectroelectrochemistry: The Electroreduction of Anthraquinone-2-sulfonate in Aqueous Media.

UV-vis spectroelectrochemistry is assessed as a tool for the diagnosis and quantitative in situ investigation of the incidence of comproportionation in multielectron transfer processes. Thus, the sensitivity of the limiting current chronoabsorptometric signals related to the different redox states to the comproportionation kinetics is studied theoretically for different working modes (normal and parallel light beam arrangements) and mass transport regimes (from semi-infinite to thin layer diffusion). The theoretical results are applied to the spectroelectrochemical study of the two-electron reduction of the anthraquinone-2-sulfonate in alkaline aqueous solution, tuning the thermodynamic favorability of the comproportionation reaction through the electrolyte cation. The quantitative analysis of the experimental results reveals the occurrence of comproportionation in the three media examined, showing different kinetics depending on the cationic species in solution.

Direct Electrochemical CO2 Capture Using Substituted Anthraquinones in Homogeneous Solutions: A Joint Experimental and Theoretical Study.

Electrochemical capture of carbon dioxide (CO2) using organic quinones is a promising and intensively studied alternative to the industrially established scrubbing processes. While recent studies focused only on the influence of substituents having a simple mesomeric or nucleophilicity effect, we have systematically selected six anthraquinone (AQ) derivatives (X-AQ) with amino and hydroxy substituents in order to thoroughly study the influence thereof on the properties of electrochemical CO2 capture. Experimental data from cyclic voltammetry (CV) and UV–Vis spectroelectrochemistry of solutions in acetonitrile were analyzed and compared with innovative density functional tight binding computational results. Our experimental and theoretical results provide a coherent explanation of the influence of CO2 on the CV data in terms of weak and strong binding nomenclature of the dianions. In addition to this terminology, we have identified the dihydroxy substituted AQ as a new class of molecules forming rather unstable [X-AQ-(CO2)n]2– adducts. In contrast to the commonly used dianion consideration, the results presented herein reveal opposite trends in stability for the X-AQ-CO2•– radical species for the first time. To the best of our knowledge, this study presents theoretically calculated UV–Vis spectra for the various CO2-AQ reduction products for the first time, enabling a detailed decomposition of the spectroelectrochemical data. Thus, this work provides an extension of the existing classification with proof of the existence of X-AQ-CO2 species, which will be the basis of future studies focusing on improved materials for electrochemical CO2 capture.

Radical Complexes of Nickel(II)/Copper(II) and Redox Non-innocent MB-DIPY Ligands: Unusual Stability and Strong Near-Infrared Absorption at λmax ∼1300 nm.

The preparation of radicals with intense and redox-switchable absorption beyond 1000 nm is a long-standing challenge in the chemistry of functional dyes. Here we report the preparation of a series of unprecedented stable neutral nickel(II) and copper(II) complexes of "Manitoba dipyrromethenes" (MB-DIPYs) in which the organic chromophore is present in the radical-anion state. The new stable radicals have an intense absorption at λmax ∼1300 nm and can be either oxidized to regular [MII (MB-DIPY)]+ (M=Cu or Ni) or reduced to [MII (MB-DIPY)]- compounds. The radical nature of the stable [MII (MB-DIPY)] complexes was confirmed by EPR spectroscopy with additional insight into their electronic structure obtained by UV-Vis spectroscopy, electro- and spectroelectrochemistry, magnetic measurements, and X-ray crystallography. The electronic structures and spectroscopic properties of the radical-based chromophores were also probed by density functional theory (DFT) and time-dependent DFT (TDDFT) calculations. These nickel(II) and copper(II) complexes represent the first stable radical compounds with a MB-DIPY ligand.