Jean Sanabria-Chinchilla

Electrocatalysis of hydrogen production by active site analogues of the iron hydrogenase enzyme: structure/function relationships

A series of binuclear FeIFeI complexes, (μ-SEt)2[Fe(CO)2L]2 (L = CO (1), PMe3 (1-P)), (μ-SRS)[Fe(CO)2L]2 (R = CH2CH2 (μ-edt): L = CO (2), PMe3 (2-P); R = CH2CH2CH2(μ-pdt): L = CO (3), PMe3 (3-P); and R = o-CH2C6H4CH2 (μ-o-xyldt): L = CO (4), PMe3 (4-P)), that serve as structural models for the active site of Fe-hydrogenase are shown to be electrocatalysts for H2 production in the presence of acetic acid in acetonitrile. The redox levels for H2 production were established by spectroelectrochemistry to be Fe0Fe0 for the all-CO complexes and FeIFe0 for the PMe3-substituted derivatives. As electrocatalysts, the PMe3 derivatives are more stable and more sensitive to acid concentration than the all-CO complexes. The electrocatalysis is initiated by electrochemical reduction of these diiron complexes, which subsequently, under weak acid conditions, undergo protonation of the reduced iron center to produce H2. An (η2-H2)FeII–Fe0/I intermediate is suggested and probable electrochemical mechanisms are discussed.

A DEMS Study of the Reduction of CO 2 , CO, and HCHO Pre-Adsorbed on Cu Electrodes: Empirical Inferences on the CO 2 RR Mechanism

The effective abatement of atmospheric carbon through its conversion via electrochemical reduction to pure and oxygenated hydrocarbon fuels relies on the ability to control product selectivity at viable current densities and faradaic efficiencies. One critical aspect is the choice of the electrode and, in the CO_2-reduction electrocatalyst landscape, copper sits as the only metal known to deliver a remarkable variety of reduction products other than carbon monoxide and formic acid. However, much better catalyst performance is needed. The overall energy efficiency of copper is less than 40 %, and its nominal overvoltage at benchmark current densities remains unacceptably large at ca. 1 V. The diversity of the product distribution also becomes a major inconvenience in the likelihood that only one product is desired; unless, of course, if the selectivity window for such product is already known. Several experimental parameters influence the product selectivity of the CO_2 reduction reactions (hereafter referred to as CO_2RR); the more obvious include the composition and the crystal structure of the catalyst surface, the applied potential, the solution pH, and the supporting electrolyte. The documentation, at the atomic level, of the mechanistic origins of the CO_2RR selectivity of copper demands a systematic combination of ex situ, in situ, and operando techniques to interrogate the electrode surface, pristine and modified, prior to, during, and after the reduction reaction; the task includes not only the analysis of reaction-product distributions but also the identification of surface intermediates that serve as the precursor states for each reaction pathway. We recently studied the nature of well-defined Cu(hkl) single-crystal surfaces that, similar to “real-world” catalysts, were handled in air. Such investigation is pertinent since Cu is a well-known scavenger of molecular oxygen; hence, CO_2RR electrocatalysis must first contend with the initial presence of multilayers of disordered copper oxides. It was found that the oxides are actually easily reduced electrochemically back to the metal; in addition, even if the oxided single-crystal surface is severely disordered, cathodic reduction completely regenerates the original ordered structure. Most recently, we discovered that a polycrystalline Cu electrode held at a fixed negative potential in the CO_2RR region in KOH, undergoes stepwise surface reconstruction, first to Cu(111) and then to Cu(100). The results help explain the Cu(100)-like behavior of Cu(pc) in terms of CO_2RR product selectivity. In the work described in this Letter, we have applied differential electrochemical mass spectrometry (DEMS) of pre-adsorbed reactants and intermediates as a complementary experimental approach in the study of the mechanistic pathways for the Cu-catalyzed CO_2 reduction reactions; the reactant was CO_2 and the intermediates were CO and HCHO. The reduction products monitored by mass spectrometry were H_2, CO (from CO_2), CH_4, H_2C=CH_2 and CH_3CH_2OH.

Immobilization-Enabled Proton Reduction Catalysis by a Di-iron Hydrogenase Mimic

We have long been interested in the influence of surface immobilization on the electrochemical integrity of redox-active moieties [1–5]. Our studies have shown that, if the electroactive group itself is directly chemisorbed on (coordinated to) the electrode surface, profound alterations result in both the thermodynamics and kinetics of the electron transfer processes; the oxidative chemisorption of the iodide anion (to zerovalent iodine atoms) or the hydroquinone molecule (to benzoquinone) are prototypical examples. The changes are more subtle and less dramatic if the electroactive site is only a pendant moiety tethered to the surface via an anchor group; mercapto hydroquinone bound exclusively via the –SH group is a well-known specimen. We recently extended our investigations to include enzyme-inspired molecular electrocatalysts in which the multinuclear reactive site may require a certain entatic state to carry out its catalytic function; the anticipation is that the motion-restricted surface-tethered species would suffer diminished catalytic activity. The results are described in this brief communication.

Electrocatalytic Reactions of Chemisorbed Aromatic Compounds: Studies by ES, DEMS, STM and EC

The interaction of organic molecules with, and their subsequent reactivities at, electrode surfaces are among the more critical aspects of modern electrochemical surface science. However, the study of these processes is an exceedingly difficult proposition. In the past, experimental probes were limited to conventional electrochemical techniques. But the information content of these methods is limited to the macroscopic properties of the electrodeelectrolyte interface. Consequently, results from surface studies based merely on ensemble thermodynamic and kinetic measurements can be rationalized only phenomenologically with little basis for interpretations at the molecular level. Over the past few decades, a slew of surface-physics techniques were developed for the study of interfacial processes, and present-day research in surface electrochemistry has taken advantage of such methods. Since it is clear that no single empirical technique can ever hope to unravel all the nuances of heterogeneous reactions, the use of multiple complementary techniques in surface science has become the standard approach. Unfortunately, the surface-characterization methods, while quite powerful, are also rather intricate and quite expensive to implement. As a consequence, less than a handful of (judiciously selected) surface-physics methods have actually been employed in electrochemical research laboratories.

Heterogenization of a Water-Insoluble Molecular Complex for Catalysis of the Proton-Reduction Reaction in Highly Acidic Aqueous Solutions

Our long-held interest in the resiliency of electrochemical functionalities upon surface immobilization has herded us from directly chemisorbed electroactive moieties, to anchor group-leashed redox-active couples and to surface-tethered enzyme-inspired molecular catalysts. The latter represent the most intricate because the electrocatalytic activities involve mixed-valence states and may require certain entatic (fractionally rotated) configurations. In this regard, we recently investigated the proton-reduction electrocatalysis by hydrogenase-inspired di-iron complexes at polycrystalline and (111)-faceted Au electrodes.

Addendum to Immobilization-Enabled Proton-Reduction Catalysis by a Di-iron Hydrogenase Mimic

It may perhaps be conjectured that the mercapto-free di-iron hydrogenase complex, (μ-pdt)[Fe(CO)3][Fe(CO)2(PPh3)], is inactive towards proton reduction because it is decomposed upon exposure to the Au electrode and that the decomposition products poison the catalysis. If decomposition and catalyst inhibition actually transpired, it would only be because the interaction between the surface and the molecular fragment is much stronger than the bond between that fragment and the constituents within the intact complex. The presence of the chemisorbed residue can thus be established by simple cyclic voltammetry of the Au electrode in 1.0 M H2SO4 after immersion in pure acetonitrile and after exposure to the hydrogenase mimic dissolved in acetonitrile. The electrode is rinsed in sulfuric acid solution prior to the voltammetric experiments. The results are shown in Fig. 1. The fact that the two sets of current-potential curves are superimposable provides clear evidence that the mercapto-free complex is neither decomposed nor chemisorbed when exposed to the Au electrode surface. Fig. 1 Cyclic voltammogram in 1.0 M H2SO4 of a smooth polycrystalline Au electrode exposed to pure acetonitrile (spittled line) and to acetonitrile that contained 0.5 mM of (μ-pdt)[Fe(CO)3][Fe(CO)2(PPh3)] (solid line). Potential sweep rate, 10 mV s

Chemisorption-Isotherm Measurements at Electrode Surfaces by Quantitative High-Resolution Electron Energy Loss Spectroscopy

The chemisorption isotherm of benzoquinone at a well-defined Pd(100) surface was obtained by quantitative high-resolution electron energy loss spectroscopy (HREELS). Extraction of surface-coverage information from HREELS required the normalization of integrated peak intensities to compensate for differences in the backscattered electron flux brought about by the organic adlayer. A common procedure rests on a match of the elastic-peak heights, but it fails for organic adsorbates since those introduce surface roughness that result in a higher stream of inelastically scattered electrons. A more accurate method is based on the equalization of the incident electron beam currents. This is attained only when the background intensities integrated over a peak-free spectral region are set equal to one another. The HREELS-generated isotherm was compared with that acquired by thin-layer electrochemical measurements; excellent agreement was observed.

The Interfacial Chemistry of Grignard Reagent Formation: Reactions of Clean Mg(0001) Surfaces

The Grignard reagent, RMgX, where R is a hydrocarbon group and X is a halogen, is one of the more important and versatile reagents for organic synthesis [1]. It is formed in a heterogeneous reaction between magnesium and an organic halide in an appropriate organic solvent [2, 3, 4, 5]

Electrocatalytic hydrogenation and oxidation of aromatic compounds studied by DEMS: Benzene and p-dihydroxybenzene at ultrathin Pd films electrodeposited on Au(hkl) surfaces.

{\rm{RX + Mg}} \to {\rm{RMgX + RR + Mg}}{{\rm{X}}_{\rm{2}}}{\rm{ + other by - products}}