Electrochemical hydrogenation of aromatic compounds chemisorbed at polycrystalline and single-crystal Pd surfaces

The chemisorption and electrochemical hydrogenation of hydroquinone (H2Q) at polycrystalline (pc) Pd, well-ordered Pd(100), and Pd-modified Au(hkl) electrodes were studied using a combination of ultra-high vacuum (UHV) surface spectroscopy, electrochemistry (EC), and electrochemical mass spectrometry (EC-MS). H2Q was found to form a slightly tilted flat-oriented quinone (Q) adlayer, when adsorbed from low concentrations; when chemisorbed from high concentrations, an edgewise-oriented H2Q adlayer was indicated. The hydrogenation of the chemisorbed layer is initiated at potentials before the onset of the hydrogen evolution region. As expected, the kinetics increases as the applied potential is increased, but the hydrogenation pathway appears to be independent of the potential. Hydrogenation in the absence of absorbed hydrogen (sub-surface) was studied at ultra-thin Pd films on Au single-crystal substrates. Hydrogenation and/or potential induced desorption were established, although non-volatile and/or hydrophobic products were detected. In comparison, negative excursions with benzene-coated electrodes resulted in nothing more than potential-induced desorption of the starting material. Negative-potential electro-desorption was more facile at terraces than at steps. Vibrational spectroscopic measurements suggested that hydrogenation occurs one molecule at a time to the fullest extent that resulted in desorption of product; that is, partially hydrogenated species do not exist on the surface

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

Corrosión atmosférica tropical de Hierro Galvanizado en una atmósfera urbana leve, en el Valle de San José de Costa Rica

Costa Rica imports most of the metallic materials it uses. In construction, Galvanized Steel (GS) is one of the most used elements in urban areas, where atmospheric corrosion is the main problem of its environmental deterioration. The area of greatest population and economic activity in Costa Rica is the San José Valley, which has a tropical monsoon climate with low pollution, defined under ISO 9223 as light urban. The present study of the atmospheric corrosion of the GS, proposes a high correlation for simple linear models, with climatic parameters as main components and SO2 as secondary component. Seasonality and sampling sites are partially significant at the beginning of the oxidation process, but this effect is damped over time. The average corrosion rate after 2 years is in the order of 0.4 μm y-1, which represents a low level (C2 according to ISO 9223). Complex annual corrosion models, such as those indicated by ISO 9223, overestimate the real corrosion value.Costa Rica es un importador de la mayoría de los materiales metálicos que utiliza. En la construcción el Hierro Galvanizado (HG) es uno de los elementos de mayor uso en áreas urbanas, donde la corrosión atmosférica es el principal problema de su deterioro ambiental. El área de mayor población y actividad económica en Costa Rica es el Valle de San José, el cual tiene un clima tropical monzónico de baja contaminación, definido bajo la norma ISO 9223(2012) como urbano leve. El presente estudio de la corrosión atmosférica del HG, plantea una alta correlación para modelos lineales simples, con parámetros climáticos como principales componentes y SO2 como componente secundario. La estacionalidad y los sitios de muestreo son parcialmente significativos al inicio del proceso de oxidación, pero dicho efecto se amortigua con el tiempo. La velocidad de corrosión media luego de 2 años es del orden de 0.4 μm año-1, lo que representa un nivel bajo (C2 según la norma ISO 9223). Los modelos complejos de corrosión anual como los indicado por la norma ISO 9223, sobrestiman el valor de corrosión real.Universidad de Costa Rica/[804-B9-264]/UCR/Costa RicaCentro Nacional de Alta Tecnología/[CeNAT-VI-269-2017]/CeNAT/Costa RicaUniversidad Nacional/[UNA- SIA: 0600-17]/UNA/Costa RicaInstituto Tecnológico de Costa Rica/[ITCR-VIE 1490-021]/ITCR/Costa RicaMinisterio de Ciencia, Tecnología y Telecomunicaciones/[]/MICITT/Costa RicaUCR::Vicerrectoría de Investigación::Unidades de Investigación::Ciencias Básicas::Centro de Investigación en Electroquímica y Energía Química (CELEQ

Bioactivity of prenylated hydroxybenzoic acids from Piper garagaranum C. DC

A bio-assay guided fractionation of Piper garagaranum C. DC. led to the isolation of two prenylated hydroxybenzoic acids (1-2) with anti-inflammatory and cytotoxic activities. The anti-inflammatory action was determined in an LPS stimulated RAW 264.7 murine macrophage assay with IC50 values for inhibition of NO production of (18 ± 3) and (26 ± 5) μM, for 1 and 2, respectively. These compounds do not inhibit NO production by a competitive inhibition of the iNOS enzyme and show anti-inflammatory properties by lowering the expression of pro-inflammatory genes (TNF-α, IL-1β, CXCL2 and CCL2), as determined by qRT-PCR. Electrochemical measurements using cyclic voltammetry (CV) show that compound 1 exhibits anti-oxidant properties. This is the first phytochemical study of this plant, and we report a preliminary study of the biological activity of the isolated compounds.Universidad de Costa Rica/[809-C0-073]/UCR/Costa RicaUCR::Vicerrectoría de Investigación::Unidades de Investigación::Ciencias Básicas::Centro de Investigaciones en Productos Naturales (CIPRONA)UCR::Vicerrectoría de Investigación::Unidades de Investigación::Ciencias Básicas::Centro de Investigación en Biología Celular y Molecular (CIBCM)UCR::Vicerrectoría de Investigación::Unidades de Investigación::Ciencias Básicas::Centro de Investigación en Electroquímica y Energía Química (CELEQ)UCR::Vicerrectoría de Docencia::Ciencias Básicas::Facultad de Ciencias::Escuela de Químic

Combined effect of the exposure angle and face orientation on the atmospheric corrosion behavior of low carbon steel

Tropical atmospheric corrosion in Central America and The Caribbean has been studied preferentially in coastal areas, using samples placed at 30° angles and under ISO 9223 and ISO 8407 standards. Recent studies have shown that the variation of the exposure angle influences the grade of corrosion between both sides of the sample; the side facing up (skyward) and the side facing down (groundward). An evaluation of the annual corrosion at different angles with respect to the horizontal in the San Jose Valley in Costa Rica was carried out, to consider this. The results were analyzed by classical methodologies (gravimetric methods, thickness measurement, XRD and SEM), as well as by electrochemical methods (LPR, EIS and EFM). The values obtained show a decrease in corrosion with increasing angle, as well as differences in corrosion levels between faces. Electrochemical measurements were consistent with each other and with those obtained using classical measurements.This study was funded by the National Council of University Presidents (CONARE) of Costa Rica, as part of a collaborative project among the State Distance University (UNED-VINVES-6–10–50), the University of Costa Rica (UCR-VI-805-B8–650 and UCR-VI-804-B9–264), the National Center for High Technology (CeNAT-VI-269–2017), the National University (UNA- SIA: 0600–17), and the Costa Rica Institute of Technology (ITCR-VIE 1490–021).Peer reviewe

A DEMS Study of the Reduction of CO_2, CO, and HCHO Pre-Adsorbed on Cu Electrodes: Empirical Inferences on the CO_2RR 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

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

Electrochemical surface science twenty years later: Expeditions into the electrocatalysis of reactions at the core of artificial photosynthesis

Surface science research fixated on phenomena and processes that transpire at the electrode-electrolyte interface has been pursued in the past. A considerable proportion of the earlier work was on materials and reactions pertinent to the operation of small-molecule fuel cells. The experimental approach integrated a handful of surface-sensitive physical–analytical methods with traditional electrochemical techniques, all harbored in a single environment-controlled electrochemistry-surface science apparatus (EC-SSA); the catalyst samples were typically precious noble metals constituted of well-defined single-crystal surfaces. More recently, attention has been diverted from fuel-to-energy generation to its converse, (solar) energy-to-fuel transformation; e.g., instead of water synthesis (from hydrogen and oxygen) in fuel cells, water decomposition (to hydrogen and oxygen) in artificial photosynthesis. The rigorous surface-science protocols remain unchanged but the experimental capabilities have been expanded by the addition of several characterization techniques, either as EC-SSA components or as stand-alone instruments. The present manuscript describes results selected from on-going studies of earth-abundant electrocatalysts for the reactions that underpin artificial photosynthesis: nickel-molybdenum alloys for the hydrogen evolution reaction, calcium birnessite as a heterogeneous analogue for the oxygen-evolving complex in natural photosynthesis, and single-crystalline copper in relation to the carbon dioxide reduction reaction