Alnald Javier

The Evolution of the Polycrystalline Copper Surface, First to Cu(111) and Then to Cu(100), at a Fixed CO2RR Potential: A Study by Operando EC-STM

A study based on operando electrochemical scanning tunneling microscopy (EC-STM) has shown that a polycrystalline Cu electrode held at a fixed negative potential, -0.9 V (vs SHE), in the vicinity of CO2 reduction reactions (CO2RR) in 0.1 M KOH, undergoes stepwise surface reconstruction, first to Cu(111) within 30 min, and then to Cu(100) after another 30 min; no further surface transformations occurred after establishment of the Cu(100) surface. The results may help explain the Cu(100)-like behavior of Cu(pc) in terms of CO2RR product selectivity. They likewise suggest that products exclusive to Cu(100) single-crystal electrodes may be generated through the use of readily available inexpensive polycrystalline Cu electrodes. The study highlights the dynamic nature of heterogeneous electrocatalyst surfaces and also underscores the importance of operando interrogations when structure-composition-reactivity correlations are intended.

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.

Overlayer Au-on-W Near-Surface Alloy for the Selective Electrochemical Reduction of CO2 to Methanol: Empirical (DEMS) Corroboration of a Computational (DFT) Prediction

It is now widely known from extensive studies [1–3] over the past few decades on the heterogeneous electrochemical reduction of carbon dioxide in aqueous solutions that, across the vast landscape of CO_2-reduction electrocatalysts, copper stands alone as the single metal that can deliver a remarkable variety of products; unpredictably, however, the product distribution does not include methanol [1–5]. The overall energy conversion efficiency of Cu, defined [6] as the ratio of the free energy of the products generated and that consumed in the electrochemical reduction, is only 30 to 40 %, and the overpotential of Cu at benchmark current densities remains unacceptably large, ca. −1.4 V [1, 6]. The diversity of the product distribution also becomes a major hurdle if only one product is coveted. The desire for catalysts that can perform better than Cu, especially in the generation of methanol, a liquid transportation fuel, and feedstock for direct fuel cells, is thus understandable.

The Structure of Benzoquinone Chemisorbed on Pd(111): Simulation of EC-STM Images and HREELS Spectra by Density Functional Theory

Earlier studies on the chemisorption of hydroquinone (H2Q) on well-defined Pd(111) surfaces based on electrochemistry, high-resolution electron energy loss spectroscopy, and in situ scanning tunneling microscopy revealed that H2Q undergoes oxidative chemisorption to generate an adlayer of benzoquinone oriented flat, albeit with a slight tilt. Certain structural details, however, such as the actual adsorbate structure and the surface coordination site could not be unambiguously confirmed solely from the experimental measurements. Density functional theory was thus employed not only to calculate the total adsorption energies of the likely configurations but also to simulate their respective vibrational spectra. The results suggest that: (1) the flat-adsorbed quinone ring is centered on a bridge site in which the C2 axis that points along the para-oxygen atoms is rotated 30° from the [110] direction of the Pd(111) substrate; (2) the p-oxygen atoms are located above twofold sites; and (3) quinonoid ring is slightly puckered with the C–H bonds tilted away from the surface, at an angle of approximately 20°.

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.

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

Simulation of scanning tunneling microscope image of benzene chemisorbed on a Pd(111) electrode surface by density functional theory

A computational method based on density functional theory was used to simulate the scanning tunneling microscopy (STM) images of benzene chemisorbed on a Pd(111) electrode in order to confirm the adsorption site of the aromatic molecule on the metal surface held at a certain applied potential. The simulated STM images on various adsorption sites were obtained and compared with the experimental electrochemical STM images. The simulation results indicate that when the potential of the Pd electrode is held at 0.3 V, benzene is chemisorbed on a threefold hollow site; at 0.55 V, the molecule is adsorbed on a position between a threefold and a twofold bridge site. These findings corroborate previously published experimental elec - trochemical STM results.