Cathodic regeneration of a clean and ordered Cu(100)-(1×1) surface from an air-oxidized and disordered electrode: An operando STM study

In work related to the electrocatalysis of the CO_2 reduction reactions, we recently reported in This Journal the structure and composition of a Cu(100) electrode surface, pre-dosed at low levels of O_(2(g)) to simulate a Cu electrocatalyst unprotected from air, before and after immersion in alkaline electrolyte at fairly negative potentials to ascertain if an oxide-to-metal reduction reaction can be effected; experimental measurements were based upon ex situ techniques, low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES). It was found that the mildly oxided surface remained ordered and could be cathodically reduced back to a well-ordered oxide-free Cu(100); the quality of the LEED pattern and AES spectrum was less than ideal, however, due to small amounts of base electrolyte remnant in the emersed layer. In this Short Communication, we present results from operando electrochemical scanning tunneling microscopy (EC-STM) that not only confirm the earlier observations but, more importantly, depict more accurately the actual electrocatalysis conditions. An as-received commercially oriented Cu(100) disk that had not been protected from air was observed to consist of narrow terraces encrusted with highly disordered oxides. Cyclic voltammetry and coulometry showed that the oxidized surface consisted of five monolayers of CuO and quarter of a monolayer of Cu_2O. Upon complete cathodic reduction of the interfacial oxides, the surface was found to have reverted to a single-crystalline Cu(100)-(1×1) structure. It may thus be inferred that, under the conditions of electrochemical CO_2 reduction, the Cu catalyst would exist as a zerovalent metal

C–H activation and metalation at electrode surfaces: 2,3-dimethyl-1,4-dihydroxybenzene on Pd(pc) and Pd(111) studied by TLE, HREELS and DFT

Previous studies, based on thin-layer electrochemistry (TLE), in situ scanning tunneling microscopy (EC-STM), high-resolution electron energy loss spectroscopy (HREELS) and density functional theory (DFT) computations, on the chemical adsorption of hydroquinone from aqueous solutions onto atomically smooth Pd (and Pt) electrode surfaces indicated two modes of attachment that depended upon the solution concentration. At low activities, the diphenol was oxidatively chemisorbed as benzoquinone in a flat orientation, suggestive of a Pd(2,3,5,6-η-C_6H_4O_2) surface complex; at higher concentrations, vertical chemisorption was effected via two C–H bond activations (or metalations) at the 2 and 3 ring positions, evocative of an o-phenylene organopalladium compound. We have extended the work to 2,3-dimethyl-1,4-dihydroxybenzene on Pd(pc) and Pd(111) electrodes to probe the effect of two methyl substituents on only one side of the diphenol ring. Surface coverage and adsorbed-molecule cross section data from TLE and HREELS measurements revealed non-random concentration-dependent adsorbate orientations similar to the oxidative chemisorption of hydroquinone: flat at low concentrations and edgewise at elevated concentrations. The DFT results suggested that, for the flat structure, surface coordination is via the two double bonds of the quinone ring as in [Pd(2,3,5,6-η)-2,3-dimethyl-p-quinone]. For the edge-vertical orientation, a structure analogous to an o-phenylene compound is generated in which C–H bonds at the 5 and 6 ring positions are activated and then metalated. DFT-simulated HREELS spectra helped identify the observed peaks that distinguish the surface-coordinated quinone from the surface-metalated diphenol

Spatially Sparse Precoding in Wideband Hybrid Terahertz Massive MIMO Systems

In terahertz (THz) massive multiple-input multiple-output (MIMO) systems, the combination of huge bandwidth and massive antennas results in severe beam split, thus making the conventional phase-shifter based hybrid precoding architecture ineffective. With the incorporation of true-time-delay (TTD) lines in the hardware implementation of the analog precoders, delay-phase precoding (DPP) emerges as a promising architecture to effectively overcome beam split. However, existing DPP approaches suffer from poor performance, high complexity, and weak robustness in practical THz channels. In this paper, we propose a novel DPP approach in wideband THz massive MIMO systems. First, the optimization problem is converted into a compressive sensing (CS) form, which can be solved by the extended spatially sparse precoding (SSP) algorithm. To compensate for beam split, frequency-dependent measurement matrices are introduced, which can be approximately realized by feasible phase and delay codebooks. Then, several efficient atom selection techniques are developed to further reduce the complexity of extended SSP. In simulation, the proposed DPP approach achieves superior performance, complexity, and robustness by using it alone or in combination with existing DPP approaches

Tracking the prelude of the electroreduction of carbon monoxide via its interaction with Cu(100): Studies by operando scanning tunneling microscopy and infrared spectroscopy

The first isolable intermediate in the electrochemical reduction of carbon dioxide is carbon monoxide. This species, or its hydrated form, formic acid, is also the primary end product from all but a handful of metallic electrodes; with the latter, hydrogen gas is generated, but it emanates from the reduction of water and not from CO₂. Only one electrode material, zerovalent copper, can spawn, in greater-than-trace quantities, a variety of species that are more highly reduced than CO. Hence, if the aim is to pursue a reaction trail of the reduction of CO₂ to products other than CO, it would be both logical and expedient to track the electrocatalytic reaction of CO itself. Heterogeneous electrocatalysis is a surface phenomenon; it transpires only when the reactant, CO in this case, chemisorbs on, or chemically interacts with, the Cu electrode surface. There is no electrocatalytic reaction if there is no CO adsorption. In ultrahigh vacuum, no CO resides on the Cu(100) surface at temperatures higher than 200 K. However, under electrochemical conditions, CO is chemisorbed on Cu at ambient temperatures at a given potential. We thus paired, in seriatim fashion, scanning tunneling microscopy (STM) and polarization-modulation IR reflection-absorption spectroscopy (PMIRS) to document the influence of applied potential on the coverage, the molecular orientation, and the adlattice structure of CO adsorbed on Cu(100) in alkaline solutions; the results are described in this paper

Synthesis, Characterization, and Reactivity of Ethynyl- and Propynyl-Terminated Si(111) Surfaces

Ethynyl- and propynyl-terminated Si(111) surfaces synthesized using a two-step halogenation/alkylation method have been characterized by transmission infrared spectroscopy (TIRS), high-resolution electron energy-loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), atomic-force microscopy (AFM), electrochemical scanning–tunneling microscopy (EC-STM) and measurements of surface recombination velocities (S). For the ethynyl-terminated Si(111) surface, TIRS revealed signals corresponding to ethynyl ≡C–H and C≡C stretching oriented perpendicular to the surface, HREELS revealed a Si–C stretching signal, and XPS data showed the presence of C bound to Si with a fractional monolayer (ML) coverage (Φ) of Φ_(Si–CCH) = 0.63 ± 0.08 ML. The ethynyl-terminated surfaces were also partially terminated by Si–OH groups (Φ_(Si–OH) = 0.35 ± 0.03 ML) with limited formation of Si^(3+) and Si^(4+) oxides. For the propynyl-terminated Si(111) surface, TIRS revealed the presence of a (C–H)CH_3 symmetric bending, or “umbrella,” peak oriented perpendicular to the surface, while HREELS revealed signals corresponding to Si–C and C≡C stretching, and XPS showed C bound to Si with Φ_(Si–CCCH_3) = 1.05 ± 0.06 ML. The LEED patterns were consistent with a (1 × 1) surface unit cell for both surfaces, but room-temperature EC-STM indicated that the surfaces did not exhibit long-range ordering. HCC–Si(111) and CH_3CC–Si(111) surfaces yielded S values of (3.5 ± 0.1) × 10^3 and (5 ± 1) × 10^2 cm s^(–1), respectively, after 581 h exposure to air. These observations are consistent with the covalent binding of ethynyl and propynyl groups, respectively, to the Si(111) surface

Domestication and divergence of Saccharomyces cerevisiae beer yeasts

Whereas domestication of livestock, pets, and crops is well documented, it is still unclear to what extent microbes associated with the production of food have also undergone human selection and where the plethora of industrial strains originates from. Here, we present the genomes and phenomes of 157 industrial Saccharomyces cerevisiae yeasts. Our analyses reveal that today's industrial yeasts can be divided into five sublineages that are genetically and phenotypically separated from wild strains and originate from only a few ancestors through complex patterns of domestication and local divergence. Large-scale phenotyping and genome analysis further show strong industry-specific selection for stress tolerance, sugar utilization, and flavor production, while the sexual cycle and other phenotypes related to survival in nature show decay, particularly in beer yeasts. Together, these results shed light on the origins, evolutionary history, and phenotypic diversity of industrial yeasts and provide a resource for further selection of superior strains

Molecular catalysis that transpires only when the complex is heterogenized: Studies of a hydrogenase complex surface-tethered on polycrystalline and (1 1 1)-faceted gold by EC, PM-FT-IRRAS, HREELS, XPS and STM

The proton-reduction catalytic activity of two di-iron hydrogenase complexes, [(μ-S_(2)C_(3)H_6)[Fe(CO)_3][Fe(CO)_(2)(PPh_3)] (1) and (μ-S_(2)C_(3)H_6)[Fe(CO)_3][Fe(CO)2(PPh2{(CH2)2SH})] (2), was investigated at polycrystalline and (1 1 1)-faceted Au electrodes in nonaqueous electrolyte. Compound (2) was irreversibly tethered to the surface through the single bondSH group; (1) was present only in the unadsorbed (dissolved) state. No enhancement of the proton reduction reaction was observed with the homogeneous complex. Pronounced catalysis was exhibited by the heterogenized (surface-attached) material. Neither increase nor decrease in activity was observed when unadsorbed complex (2) was added to the solution of the heterogenized catalyst. The conclusion from these observations, that no catalysis transpires unless the subject molecular complex is tethered to the electrode surface, is totally unexpected; it runs counter to conventional wisdom that an untethered homogeneous electrocatalyst, especially one that requires a particular entatic (partially rotated) configuration to complete its function, would invariably perform better than its surface-immobilized counterpart. The heterogenized complex, present at rather low coverages due to its sizable adsorbed-molecule cross section, was further investigated by polarization-modulation Fourier transform infrared reflection absorption spectroscopy (PM-FT-IRRAS), high-resolution electron-energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). The electrochemistry (EC) and STM results indicated that the catalytic activity of the immobilized complex is a function of its surface coverage but not of its spatial configuration; the catalytic sites are accessible regardless of the particular arrangement of the pendant active site with respect to the surface. The surface-immobilized complex suffered a non-negligible loss in catalytic activity after the ex situ experiments, perhaps due to (partial) decarbonylation