Integrating Experiment and Theory in Electrochemical Surface Science: Studies on the Molecular Adsorption on Noble-Metal Electrode Surfaces by Density Functional Theory, Electron Spectroscopy, and Electrochemistry

Computational techniques based on density functional theory (DFT) and experimental methods based on electrochemistry (EC), electrochemical scanning tunneling microscopy (EC-STM), and high-resolution electron energy loss spectroscopy (HREELS) were employed to study the adsorption of (i) sulfuric acid on Pd(111), (ii) benzene on Pd(111), (iii) hydroquinone/benzoquinone on Pd(111), (iv) hydroquinone sulfonate/benzoquinone sulfonate on Pd(111), (v) 2,3-dimethylhydroquinone/2,3-dimethylbenzoquinone on Pd(111) and polycrystalline Pd, (vi) hydrogen on 1-6 monolayers (ML) of Pd deposited on a Pt(111) substrate, and (vii) a thiolated iron hydrogenase model complex on polycrystalline Au. In situ EC-STM and DFT investigations of sulfuric acid on a Pd(111) surface indicated that two layers of water molecules and hydronium ions are assembled, non-co-planar with one another, between the rows of surface-coordinated sulfate anions; the layer that is slightly elevated is composed of hydronium counter cations. The STM images of benzene chemisorbed on a Pd(111) electrode surface were simulated and the results suggested that, when the potential of the Pd electrode is held at 0.3 V, benzene is chemisorbed on a 3-fold site; while at 0.55 V, the molecule is adsorbed on a position between a 3-fold and a 2-fold site. Computational and experimental results implied that at low concentrations, hydroquinone sulfonate undergoes oxidative chemisorption forming benzoquinone sulfonate (BQS) on the Pd(111) surface, BQS adopts a flat orientation in which the quinone ring is centered over a 2-fold site, and the C–H and C–S bonds are no longer co-planar with the quinone ring and are slightly tilted, directed away from the surface. At very dilute concentrations, when hydroquinone (H_(2)Q) undergoes oxidative chemisorption producing benzoquinone oriented flat, albeit with a slight tilt, on the Pd(111) surface, the flat-adsorbed quinone ring is centered on a bridge site where the C_(2) axis is rotated 30degree from the [110] direction of the metal substrate, the p-oxygen atoms are located above two-fold sites, and the ring is slightly puckered with the C–H bonds tilted away from the surface at approximately 20degree. When 2,3-dimethylH_(2)Q is chemisorbed on the Pd surface, at low concentrations, 2,3-dimethylH_(2)Q is oxidatively chemisorbed producing 2,3-dimethyl-1,4-benzoquinone oriented flat on the surface, the flat-adsorbed rings are centered above 2-fold sites wherein the C=O bonds are pointing 30degree from the [110] direction of the substrate, the para-oxygen atoms are located above bridge sites, the peripheral bonds are tilted away from the surface at ca. 20degree, and at higher concentrations, oxidative chemisorption occurs through activation of the ring’s C–H bonds yielding edge-oriented 2,3-dimethylH_(2)Q. Electrochemistry and DFT results also implied that at 1-2 ML of Pd on Pt(111), hydrogen is only adsorbed on a hollow site while at 3 ML of Pd or more, atomic hydrogen may be chemisorbed on the 3-fold site or absorbed in the octahedral hole underneath the hollow site. Using Au electrodes, an unbound iron hydrogenase analogue complex studied was found to slightly catalyze the H_(2) evolution process. However, when the complex was immobilized unto the Au surface, the electrocatalytic activity was greatly improved

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

Reprint of "Selective conversion of CO into ethanol on Cu(511) surface reconstructed from Cu(pc): Operando studies by electrochemical scanning tunneling microscopy, mass spectrometry, quartz crystal nanobalance, and infrared spectroscopy"

A polycrystalline copper, surface-terminated by a well-defined (511)-oriented facet, was electrochemically generated by a series of step-wise surface reconstruction and iterations of mild oxidative-reductive processes in 0.1 M KOH. The electrochemical reduction of CO on the resultant stepped surface was investigated by four surface-sensitive operando methodologies: electrochemical scanning tunneling microscopy (STM), electrochemical quartz crystal nanobalance (EQCN), differential electrochemical mass spectrometry (DEMS), and polarization-modulation infrared spectroscopy (PMIRS). The stepped surface catalyzed the facile conversion of CO into ethanol, the exclusive alcohol product at a low overpotential of −1.06 V (SHE) or − 0.3 V (RHE). The chemisorption of CO was found to be a necessary prelude to ethanol production; i.e. the surface coverages, rather than solution concentrations, of CO and its surface-bound intermediates primarily dictate the reaction rates (current densities). Contrary to the expected predominance of undercoordinated step-site reactivity over the coordination chemistry of vicinal surfaces, vibrational spectroscopic evidence reveals the involvement of terrace-bound CO adsorbates during the multi-atomic transformations associated with the production of ethanol

Selective conversion of CO into ethanol on Cu(511) surface reconstructed from Cu(pc): Operando studies by electrochemical scanning tunneling microscopy, mass spectrometry, quartz crystal nanobalance, and infrared spectroscopy

A polycrystalline copper, surface-terminated by a well-defined (511)-oriented facet, was electrochemically generated by a series of step-wise surface reconstruction and iterations of mild oxidative-reductive processes in 0.1 M KOH. The electrochemical reduction of CO on the resultant stepped surface was investigated by four surface-sensitive operando methodologies: electrochemical scanning tunneling microscopy (STM), electrochemical quartz crystal nanobalance (EQCN), differential electrochemical mass spectrometry (DEMS), and polarization-modulation infrared spectroscopy (PMIRS). The stepped surface catalyzed the facile conversion of CO into ethanol, the exclusive alcohol product at a low overpotential of −1.06 V (SHE) or − 0.3 V (RHE). The chemisorption of CO was found to be a necessary prelude to ethanol production; i.e. the surface coverages, rather than solution concentrations, of CO and its surface-bound intermediates primarily dictate the reaction rates (current densities). Contrary to the expected predominance of undercoordinated step-site reactivity over the coordination chemistry of vicinal surfaces, vibrational spectroscopic evidence reveals the involvement of terrace-bound CO adsorbates during the multi-atomic transformations associated with the production of ethanol

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

Surface reconstruction of pure-Cu single-crystal electrodes under Co-reduction potentials in alkaline solutions: A study by seriatim ECSTM-DEMS

Quasi-operando electrochemical scanning tunneling microscopy (ECSTM) recently showed that a polycrystalline Cu electrode kept in 0.1 M KOH at − 0.9 V (SHE), a potential very close to that for electrochemical CO reduction, underwent a two-step surface reconstruction, initially to Cu(111), or Cu(pc)-[Cu(111)], and terminally to Cu(100), or Cu(pc)-[Cu(100)]. When subjected to monolayer-limited Cu_((s)) ↔ Cu_2O_((s)) oxidation-reduction cycles (ORC), the Cu(pc)-[Cu(100)] surface was further transformed to Cu(pc)-[Cu(511)] that produced C_2H_5OH exclusively, as detected by differential electrochemical mass spectrometry, at an overvoltage lower by 645 mV relative to that for the formation of hydrocarbons. In this paper, results are presented from studies with the native monocrystalline surfaces Cu(111), Cu(100) and Cu(110). Whereas the intermediate Cu(pc)-[Cu(111)] layer was eventually converted to Cu(pc)-[Cu(100)], the surface of a pristine Cu(111) single crystal itself showed no such conversion. The surface of an original Cu(100) electrode likewise proved impervious to potential perturbations. In contrast, the outer plane of a Cu(110) crystal underwent three transformations: first to disordered Cu(110)-d[Cu(110)], then to disordered Cu(110)-d[Cu(111)], and finally to an ordered Cu(110)-[Cu(100)] plane. After multiple ORC, the converted [Cu(100)] lattice atop the Cu(110) crystal did not generate ethanol, in contrast to the [Cu(100)] phase above the Cu(pc) bulk. Quasi-operando ECSTM captured the disparity: Post-ORC, Cu(110)-[Cu(100)] was converted, not to Cu(110)-[Cu(511)], but to an ordered but catalytically inactive Cu(110)-[Cu(111)]; hence, no C2H5OH production upon reduction of CO, as would have been the case for a stepped Cu(511) surface

Potential-Dependent Adsorption of CO and Its Low-Overpotential Reduction to CH_3CH_2OH on Cu(511) Surface Reconstructed from Cu(pc): Operando Studies by Seriatim STM-EQCN-DEMS

Operando scanning tunneling microscopy first revealed that application of a CO_2-reduction potential to a Cu(pc) electrode in 0.1 M KOH resulted in the reconstruction of the selvedge to an x-layer stack of well-ordered Cu(100) terraces, Cu(pc)-x[Cu(100)]. Subsequent Cu↔Cu_2O oxidation-reduction cycles between −0.90 V and 0.10 V SHE converted the reconstructed region to a stepped Cu(S)-[3(100) × (111)], or Cu(511), surface. Differential electrochemical mass spectrometry showed that reduction of CO produced only CH_3CH_2OH at the lowest overpotential. Later application of STM and surface infrared spectroscopy uncovered a potential, above which no CO adsorption occurs. In this study, electrochemical quartz crystal nanobalance was combined with STM and DEMS as a prelude to the acquisition of CO coverages as continuous functions of concentration and potential; in heterogeneous catalysis, surface coverage are important since the reaction rate are functions of those quantities. Also equally critical is the knowledge of the packing arrangement at the onset of the reaction because, if “CO dimers” were indeed the precursors to C_(2+) products, reduction can only be initiated when the adlayer consists of closely packed CO; otherwise, dimerization will not transpire if the molecules were far apart. The results indicate that the catalysis lags the adsorption, and starts only when CO adsorption is saturated

Surface reconstruction of pure-Cu single-crystal electrodes under Co-reduction potentials in alkaline solutions: A study by seriatim ECSTM-DEMS

Quasi-operando electrochemical scanning tunneling microscopy (ECSTM) recently showed that a polycrystalline Cu electrode kept in 0.1 M KOH at − 0.9 V (SHE), a potential very close to that for electrochemical CO reduction, underwent a two-step surface reconstruction, initially to Cu(111), or Cu(pc)-[Cu(111)], and terminally to Cu(100), or Cu(pc)-[Cu(100)]. When subjected to monolayer-limited Cu_((s)) ↔ Cu_2O_((s)) oxidation-reduction cycles (ORC), the Cu(pc)-[Cu(100)] surface was further transformed to Cu(pc)-[Cu(511)] that produced C_2H_5OH exclusively, as detected by differential electrochemical mass spectrometry, at an overvoltage lower by 645 mV relative to that for the formation of hydrocarbons. In this paper, results are presented from studies with the native monocrystalline surfaces Cu(111), Cu(100) and Cu(110). Whereas the intermediate Cu(pc)-[Cu(111)] layer was eventually converted to Cu(pc)-[Cu(100)], the surface of a pristine Cu(111) single crystal itself showed no such conversion. The surface of an original Cu(100) electrode likewise proved impervious to potential perturbations. In contrast, the outer plane of a Cu(110) crystal underwent three transformations: first to disordered Cu(110)-d[Cu(110)], then to disordered Cu(110)-d[Cu(111)], and finally to an ordered Cu(110)-[Cu(100)] plane. After multiple ORC, the converted [Cu(100)] lattice atop the Cu(110) crystal did not generate ethanol, in contrast to the [Cu(100)] phase above the Cu(pc) bulk. Quasi-operando ECSTM captured the disparity: Post-ORC, Cu(110)-[Cu(100)] was converted, not to Cu(110)-[Cu(511)], but to an ordered but catalytically inactive Cu(110)-[Cu(111)]; hence, no C2H5OH production upon reduction of CO, as would have been the case for a stepped Cu(511) surface

Nickel−Gallium-Catalyzed Electrochemical Reduction of CO_2 to Highly Reduced Products at Low Overpotentials

We report the electrocatalytic reduction of CO_2 to the highly reduced C_2 products, ethylene and ethane, as well as to the fully reduced C_1 product, methane, on three different phases of nickel–gallium (NiGa, Ni_3Ga, and Ni_5Ga_3) films prepared by drop-casting. In aqueous bicarbonate electrolytes at neutral pH, the onset potential for methane, ethylene, and ethane production on all three phases was found to be −0.48 V versus the reversible hydrogen electrode (RHE), among the lowest onset potentials reported to date for the production of C_2 products from CO_2. Similar product distributions and onset potentials were observed for all three nickel–gallium stoichiometries tested. The onset potential for the reduction of CO_2 to C_2 products at low current densities catalyzed by nickel–gallium was >250 mV more positive than that of polycrystalline copper, and approximately equal to that of single crystals of copper, which have some of the lowest overpotentials to date for the reduction of CO_2 to C_2 products and methane. The nickel–gallium films also reduced CO to ethylene, ethane, and methane, consistent with a CO_2 reduction mechanism that first involves the reduction of CO2 to CO. Isotopic labeling experiments with ^(13)CO_2 confirmed that the detected products were produced exclusively by the reduction of CO_2