Youn-Geun Kim | California Institute of Technology (original) (raw)
Papers by Youn-Geun Kim
Langmuir, Oct 3, 2006
The interaction of hydroquinone (H2Q) with well-defined Pd(111) surfaces at preselected potential... more The interaction of hydroquinone (H2Q) with well-defined Pd(111) surfaces at preselected potentials in dilute H2SO4 has been studied by molecule-resolved electrochemical scanning tunneling microscopy (EC-STM). H2Q spontaneously undergoes oxidative chemisorption to benzoquinone (Q), which adopts a slightly tilted parallel orientation. Evidently, the surface coordination is through the quinone pi-electron system. At potentials within the double-layer region, a close-packed well-ordered Pd(111)-(3 x 3)-Q adlattice was formed. A potential excursion to 0.7 V, a potential at which the solution-phase Q/H2Q redox reaction takes place, introduced disorder into the organic adlayer; this positive-potential-induced order-to-disorder phase transition is reversible because the ordered (3 x 3)-Q adlattice was regenerated when the potential reverted to 0.4 V. When the potential was poised at 0.2 V, a potential at which hydrogen evolution was initiated, an appreciable fraction of Q was (hydrogenatively) desorbed; the remnant Q molecules were agglomerated in small islands that retained the (3 x 3) symmetry of the full adlayer. Two possible structural models of the Pd(111)-(3 x 3)-Q adlattice are described.
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Journal of The Electrochemical Society, 2009
This paper describes the formation of Cu nanofilms using atomic layer deposition (ALD) via surfac... more This paper describes the formation of Cu nanofilms using atomic layer deposition (ALD) via surface-limited redox replacement, also referred to as monolayer-restricted galvanic displacement. An automated flow-cell electrodeposition system was employed to make Cu ...
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Langmuir, Jan 20, 2012
The growth of stoichiometric CuInSe(2) (CIS) on Au substrates using electrochemical atomic layer ... more The growth of stoichiometric CuInSe(2) (CIS) on Au substrates using electrochemical atomic layer deposition (E-ALD) is reported here. Parameters for a ternary E-ALD cycle were investigated and included potentials, step sequence, solution compositions and timing. CIS was also grown by combining cycles for two binary compounds, InSe and Cu(2)Se, using a superlattice sequence. The formation, composition, and crystal structure of each are discussed. Stoichiometric CIS samples were formed using the superlattice sequence by performing 25 periods, each consisting of 3 cycles of InSe and 1 cycle of Cu(2)Se. The deposits were grown using 0.14, -0.7, and -0.65 V for Cu, In, and Se precursor solutions, respectively. XRD patterns displayed peaks consistent with the chalcopyrite phase of CIS, for the as-deposited samples, with the (112) reflection as the most prominent. AFM images of deposits suggested conformal deposition, when compared with corresponding image of the Au on glass substrate.
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Bulletin of The Korean Chemical Society, 1991
The preparation and characterization of semiconductive electrodes of MgO doped were investigated.... more The preparation and characterization of semiconductive electrodes of MgO doped were investigated. Pellets of MgO doped were sintered at high temperatures between 1300C and 1400C and quenched rapidly in distilled water. The surfaces were analyzed by X-ray diffraction and scanning Auger electron spectroscopy. The surfaces of pellets contained both corundum structure () and spinel structure (). Electrodes made of this material gave comparable anodic and cathodic photocurrents under illumination. The cathodic and anodic photocurrent on these photoelectrodes were verified high at 5-10 wt. percent that is critical doping amounts.
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Journal of the American Chemical Society, Mar 7, 2016
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ECS transactions, Jan 6, 2017
The electrochemical reduction of CO_2 on copper electrodes is known to generate a wide variety of... more The electrochemical reduction of CO_2 on copper electrodes is known to generate a wide variety of products that include low-molecular-weight hydrocarbons and oxygenates. The product distribution can be regulated to yield a single liquid fuel by the control, at the atomic level, of the structure of the electrode surface. The reaction of interest was the selective CO-to-C_2H_5OH reduction at low potential in alkaline solution. The seriatim or sequential use of electrochemical scanning tunneling microscopy and differential electrochemical mass spectrometry allowed the identification of a particular surface structure responsible for a specific product selectivity. Monolayer-limited Cu ↔ Cu_2O oxidation-reduction cycles (ORC) in 0.1 M KOH transformed the reconstructed Cu(pc)-[Cu(100)] surface to an ordered stepped surface, Cu(S)-[3(100)×(111)], or Cu(511) that led to the exclusive production of ethanol. Despite the potential cycles, Cu(111) and (110) surfaces did not produce ethanol. The Cu(111) surface retained its pristine arrangement after a potential hold at -0.9 V and subsequent ORC. Under similar potentiostatic conditions, the Cu(110) surface became Cu(110)-[Cu(100)]; ORC of the reconstructed surface ultimately formed Cu(110)-[Cu(111)].
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Meeting abstracts, 2016
This work describes the employment of differential electrochemical mass spectrometry (DEMS) to in... more This work describes the employment of differential electrochemical mass spectrometry (DEMS) to investigate selectively pre-adsorbed reactants and (postulated) intermediates as a supplementary experimental approach in the study of the reaction mechanism of the Cu-catalyzed electrochemical reduction of CO2. The results show the following empirical inferences: (i) CO is one of the first products of CO2 reduction, as well as the first intermediate in more advanced reactions that include formation of hydrocarbons and oxygenates; this is in conformity with the (almost) unanimously held view. (ii) Formaldehyde, HCHO, is not a precursor for C=C double-bond formation. (iii) HCHO is an intermediate for the production of methane and ethanol. (iv) Methane and ethanol can be generated from adsorbed CO via two ways: one requires a theoretically postulated surface species, CO protonated on the C atom, and the other involves adsorbed HCHO, constituted after the rate-limiting CO-protonation step. (v) The generation of CH4 and CH3CH2OH from CO has a much higher activation barrier than from HCHO; not unexpected since the formaldehyde intermediate is formed after the slow Cu–OCH formation and, consequently, is not highly activated. This work also presents a study based on DEMS that tested the theoretical prediction suggesting the viability of a bimetallic near-surface alloy (NSA) made up of Au and W as a CO2-reduction electrocatalyst selective towards the formation of methanol as a product, away from methane, ethylene or ethanol typically produced using Cu as the catalyst. At an overlayer NSA that consisted of n monolayers (ML) of Au on a polycrystalline W electrode, W(pc)-n[(1×1)-Au], no methane, methanol, ethylene or ethanol were detected when the coverage of Au was at submonolayer (n = 0.5) or multilayer (n ≥ 2) coverages. However, when the overlayer contained only 1 ML of Au, methanol was generated exclusively. The anticipated CH3OH-product-selectivity of the W(pc)-(1×1)-Au NSA has thus been (qualitatively) confirmed. The CH3OH-selective activity was 52 µA cm-2 for a Faradaic efficiency of 0.50%; the bulk of the current was expended towards H2 evolution and, since the topmost layer was Au, most likely in the production of CO and formates that cannot be studied by DEMS.
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This work describes results from an attempt to employ differential electrochem. mass spectrometry... more This work describes results from an attempt to employ differential electrochem. mass spectrometry (DEMS) of selectively pre-adsorbed reactants and (postulated) intermediates as a supplementary exptl. approach in the study of the reaction mechanism of the Cu-catalyzed electrochem. redn. of CO2. The results prompt the following empirical inferences: (i) CO is the first product of CO2 redn., as well as the first intermediate in more advanced reactions that include formation of pure and oxygenated hydrocarbons; this is in conformity with the (almost) unanimously held view. (ii) HCHO is not a precursor for C=C double-bond formation. (iii) HCHO is an intermediate for the prodn. of methane and ethanol. (iv) The generation of CH4 and CH3CH2OH from adsorbed CO occurs via two pathways: one requires a theor. postulated surface species, CO protonated on the C atom, and the other involves adsorbed HCHO, constituted after the rate-limiting protonation step. (v) The generation of CH4 and CH3CH2OH from CO has a much higher activation barrier than conversions from HCHO; not unexpected since the reactions transpire after the slow Cu-OCH+ formation and, consequently, are not highly activated. This work also presents results from an exptl. study based on DEMS that tested the theor. prediction that suggested the viability of a bimetallic near-surface alloy (NSA) electrode made up of Au and W as a CO2-redn. electrocatalyst selective towards the formation of CH3OH as a product, away from methane, ethylene or ethanol. At an overlayer NSA that consisted of n monolayers (ML) of Au on a polycryst. W electrode, W(pc)-n[(1×1)-Au], no methane, ethylene or ethanol were detected, when the coverage of Au was at submonolayer (n = 0.5) or multilayer (n ≥ 2) coverage. However, when the overlayer contained only 1 ML of Au, methanol was generated exclusively. The anticipated CH3OH-product- selectivity of the W(pc)-(1×1)-Au NSA has thus been (qual.) confirmed. The CH3OH-selective activity was 52 μA cm-2 for a Faradaic efficiency of 0.50 %; the bulk of the current was expended towards H2 evolution and, since the topmost layer was Au, most likely in the prodn. of CO and formates that are undetectable by DEMS.
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Current Topics in Catalysis, May 30, 2017
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ECS transactions, Jan 4, 2017
This work describes the employment of differential electrochemical mass spectrometry (DEMS) as a ... more This work describes the employment of differential electrochemical mass spectrometry (DEMS) as a supplementary experimental approach to theory in the study of the reaction mechanism of the Cu-catalyzed electrochemical reduction of CO_2 by investigating the reduction of reactants and (postulated) intermediates. The empirical inferences: (i) CO is one of the first products of CO_2 reduction, as well as the first intermediate in the formation of more reduced products. (ii) Formaldehyde is not a precursor for C=C bond formation but is an intermediate for the production of methane and ethanol. (iii) Both methane and ethanol can be generated from CO_2 through the protonation of CO and through the HCHO intermediate. (iv) The generation of CH_4 and CH_3CH_2OH from CO and CO_2 has a much higher activation barrier than from HCHO; not unexpected since the formaldehyde intermediate is formed after the (computationally determined) rate-limiting CO-protonation step. In this work, DEMS was also used to test the theoretical prediction suggesting the viability of a bimetallic near-surface alloy (NSA) consisting of Au and W as a CO_2-reduction electrocatalyst selective towards the formation of methanol as a product, as opposed to methane, ethylene or ethanol. At an overlayer NSA that consisted of n monolayers (ML) of Au on a polycrystalline W electrode, W(pc)-n[(1×1)-Au], no methane, methanol, ethylene or ethanol was detected when the coverage of Au was at submonolayer (n = 0.5) or multilayer (n ≥ 2) coverages. However, when the NSA contained only 1 ML of Au, methanol was generated exclusively.
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Electrocatalysis, Sep 5, 2012
ABSTRACT Earlier studies on the chemisorption of hydroquinone (H2Q) on well-defined Pd(111) surfa... more ABSTRACT 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°.
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Journal of the American Chemical Society, Jul 8, 2015
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Langmuir, Dec 9, 2014
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Journal of Electroanalytical Chemistry, Mar 1, 2014
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Electrocatalysis, Jan 27, 2015
ABSTRACT IntroductionThe effective abatement of atmospheric carbon through its conversion via ele... more ABSTRACT IntroductionThe 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 CO2-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 [1–7]. However, much better catalyst performance is needed. The overall energy efficiency of copper is less than 40 % [1–4], 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 th ...
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Electrocatalysis, Jun 7, 2016
AbstractCu catalyzes the electrochemical reduction of CO2 or CO to an assortment of products, a b... more AbstractCu catalyzes the electrochemical reduction of CO2 or CO to an assortment of products, a behavior that is a detriment when only one reduced compound is desired. The present article provides an example in which, through the atomic-level control of the structure of the Cu electrode surface, the yield distribution is regulated to generate only one product. The reaction investigated was the preferential reduction of CO to C2H5OH on Cu at a low overpotential in alkaline solution. Experimental measurements combined electrochemical scanning tunneling microscopy (ECSTM) and differential electrochemical mass spectrometry (DEMS). An atomically ordered Cu(100) surface, prepared from either a single crystal or by Cu(pc)-to-Cu(100) reconstruction, did not produce ethanol. When the surfaces were subjected to monolayer-limited Cu↔Cu2O cycles, only the reconstructed surface underwent an additional structural transformation that spawned the selective production of ethanol at a potential 645 mV lower than that which generates multiple products. Quasi-operando ECSTM indicated transformation to an ordered stepped surface, Cu(S) − [3(100) × (111)], or Cu(511). The non-selective, multiple-product Cu-catalyzed reduction of CO had thus been regulated to yield only one liquid fuel by an atomic-level structural modification of the electrode surface. Graphical AbstractTOC GRAPHIC
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Langmuir, Jan 19, 2007
This article concerns the growth of an atomic layer of Pb on the Au(111)( radical3 x radical3)R30... more This article concerns the growth of an atomic layer of Pb on the Au(111)( radical3 x radical3)R30 degrees -I structure. The importance of this study lies in the use of Pb underpotential deposition (UPD) as a sacrificial layer in surface-limited redox replacement (SLRR). SLRR reactions are being applied in the formation of metal nanofilms via electrochemical atomic layer deposition (ALD). Pb UPD is a surface-limited reaction, and if it is placed in a solution of ions of a more noble metal, redox replacement can occur, but limited by the amount of Pb present. Pb UPD is a candidate for use as a sacrificial layer for replacement by any more noble element. It has been used by this group for both Cu and Pt nanofilm formation using electrochemical ALD. The I atom layer was intended to facilitate electrochemical annealing during nanofilm growth. Two distinctly different Pb atomic layer structures are reported, studied using in situ scanning tunneling microscopy (STM) with an electrochemical flow cell and ultrahigh vacuum surface analysis combined directly with electrochemical reactions (UHV-EC). Starting with the initial Au(111)( radical3 x radical3)R30 degrees -I, 1/3 monolayer of I on the Au(111) surface, Pb deposition began at approximately 0.1 V. The first Pb UPD structure was observed just below -0.2 V and displayed a (2 x radical3)-rect unit cell, for a structure composed of 1/4 monolayer each of Pb and I. The I atoms fit in Pb 4-fold sites, on the Au(111) surface. The structure was present in domains rotated by 120 degrees. Deposition to -0.4 V resulted in complete loss of the I atoms and formation of a Pb monolayer on the Au(111), which produced a Moiré pattern, due to the Pb and Au lattice mismatch. These structures represent two well-defined starting points for the growth of nanofilms of other more noble elements. It is apparent from these studies that the adsorption of I- on Pb is weak, and it will rinse away. If Pb is used as a sacrificial metal in an electrochemical ALD cycle and adsorbed I atoms are employed for electrochemical annealing, I atoms will need to be applied each cycle.
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Journal of Crystal Growth, Apr 1, 2010
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Journal of Electroanalytical Chemistry, Sep 1, 2008
The effect of Cl− on the structure of Cu nanofilms deposited with electrochemical ALD, using surf... more The effect of Cl− on the structure of Cu nanofilms deposited with electrochemical ALD, using surface limited redox replacement (SLRR), is described. These investigations involved ultrahigh vacuum analytical methodologies coupled directly with electrochemical studies ( ...
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Journal of Electroanalytical Chemistry, Sep 1, 2003
The chemisorption of benzene at well-defined Pd(111) electrode surfaces in 0.1 M HClO4 has been s... more The chemisorption of benzene at well-defined Pd(111) electrode surfaces in 0.1 M HClO4 has been studied by a combination of electrochemistry, molecular resolution scanning tunneling microscopy, and high-resolution electron energy loss spectroscopy. At potentials within the double-layer region, a well-ordered Pd(111)-c(2√3×3)-rect-C6H6 adlattice was formed in which the adsorbed benzene molecules are slightly tilted with respect to the metal surface. When
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Langmuir, Oct 3, 2006
The interaction of hydroquinone (H2Q) with well-defined Pd(111) surfaces at preselected potential... more The interaction of hydroquinone (H2Q) with well-defined Pd(111) surfaces at preselected potentials in dilute H2SO4 has been studied by molecule-resolved electrochemical scanning tunneling microscopy (EC-STM). H2Q spontaneously undergoes oxidative chemisorption to benzoquinone (Q), which adopts a slightly tilted parallel orientation. Evidently, the surface coordination is through the quinone pi-electron system. At potentials within the double-layer region, a close-packed well-ordered Pd(111)-(3 x 3)-Q adlattice was formed. A potential excursion to 0.7 V, a potential at which the solution-phase Q/H2Q redox reaction takes place, introduced disorder into the organic adlayer; this positive-potential-induced order-to-disorder phase transition is reversible because the ordered (3 x 3)-Q adlattice was regenerated when the potential reverted to 0.4 V. When the potential was poised at 0.2 V, a potential at which hydrogen evolution was initiated, an appreciable fraction of Q was (hydrogenatively) desorbed; the remnant Q molecules were agglomerated in small islands that retained the (3 x 3) symmetry of the full adlayer. Two possible structural models of the Pd(111)-(3 x 3)-Q adlattice are described.
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Journal of The Electrochemical Society, 2009
This paper describes the formation of Cu nanofilms using atomic layer deposition (ALD) via surfac... more This paper describes the formation of Cu nanofilms using atomic layer deposition (ALD) via surface-limited redox replacement, also referred to as monolayer-restricted galvanic displacement. An automated flow-cell electrodeposition system was employed to make Cu ...
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Langmuir, Jan 20, 2012
The growth of stoichiometric CuInSe(2) (CIS) on Au substrates using electrochemical atomic layer ... more The growth of stoichiometric CuInSe(2) (CIS) on Au substrates using electrochemical atomic layer deposition (E-ALD) is reported here. Parameters for a ternary E-ALD cycle were investigated and included potentials, step sequence, solution compositions and timing. CIS was also grown by combining cycles for two binary compounds, InSe and Cu(2)Se, using a superlattice sequence. The formation, composition, and crystal structure of each are discussed. Stoichiometric CIS samples were formed using the superlattice sequence by performing 25 periods, each consisting of 3 cycles of InSe and 1 cycle of Cu(2)Se. The deposits were grown using 0.14, -0.7, and -0.65 V for Cu, In, and Se precursor solutions, respectively. XRD patterns displayed peaks consistent with the chalcopyrite phase of CIS, for the as-deposited samples, with the (112) reflection as the most prominent. AFM images of deposits suggested conformal deposition, when compared with corresponding image of the Au on glass substrate.
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Bulletin of The Korean Chemical Society, 1991
The preparation and characterization of semiconductive electrodes of MgO doped were investigated.... more The preparation and characterization of semiconductive electrodes of MgO doped were investigated. Pellets of MgO doped were sintered at high temperatures between 1300C and 1400C and quenched rapidly in distilled water. The surfaces were analyzed by X-ray diffraction and scanning Auger electron spectroscopy. The surfaces of pellets contained both corundum structure () and spinel structure (). Electrodes made of this material gave comparable anodic and cathodic photocurrents under illumination. The cathodic and anodic photocurrent on these photoelectrodes were verified high at 5-10 wt. percent that is critical doping amounts.
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Journal of the American Chemical Society, Mar 7, 2016
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ECS transactions, Jan 6, 2017
The electrochemical reduction of CO_2 on copper electrodes is known to generate a wide variety of... more The electrochemical reduction of CO_2 on copper electrodes is known to generate a wide variety of products that include low-molecular-weight hydrocarbons and oxygenates. The product distribution can be regulated to yield a single liquid fuel by the control, at the atomic level, of the structure of the electrode surface. The reaction of interest was the selective CO-to-C_2H_5OH reduction at low potential in alkaline solution. The seriatim or sequential use of electrochemical scanning tunneling microscopy and differential electrochemical mass spectrometry allowed the identification of a particular surface structure responsible for a specific product selectivity. Monolayer-limited Cu ↔ Cu_2O oxidation-reduction cycles (ORC) in 0.1 M KOH transformed the reconstructed Cu(pc)-[Cu(100)] surface to an ordered stepped surface, Cu(S)-[3(100)×(111)], or Cu(511) that led to the exclusive production of ethanol. Despite the potential cycles, Cu(111) and (110) surfaces did not produce ethanol. The Cu(111) surface retained its pristine arrangement after a potential hold at -0.9 V and subsequent ORC. Under similar potentiostatic conditions, the Cu(110) surface became Cu(110)-[Cu(100)]; ORC of the reconstructed surface ultimately formed Cu(110)-[Cu(111)].
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Meeting abstracts, 2016
This work describes the employment of differential electrochemical mass spectrometry (DEMS) to in... more This work describes the employment of differential electrochemical mass spectrometry (DEMS) to investigate selectively pre-adsorbed reactants and (postulated) intermediates as a supplementary experimental approach in the study of the reaction mechanism of the Cu-catalyzed electrochemical reduction of CO2. The results show the following empirical inferences: (i) CO is one of the first products of CO2 reduction, as well as the first intermediate in more advanced reactions that include formation of hydrocarbons and oxygenates; this is in conformity with the (almost) unanimously held view. (ii) Formaldehyde, HCHO, is not a precursor for C=C double-bond formation. (iii) HCHO is an intermediate for the production of methane and ethanol. (iv) Methane and ethanol can be generated from adsorbed CO via two ways: one requires a theoretically postulated surface species, CO protonated on the C atom, and the other involves adsorbed HCHO, constituted after the rate-limiting CO-protonation step. (v) The generation of CH4 and CH3CH2OH from CO has a much higher activation barrier than from HCHO; not unexpected since the formaldehyde intermediate is formed after the slow Cu–OCH formation and, consequently, is not highly activated. This work also presents a study based on DEMS that tested the theoretical prediction suggesting the viability of a bimetallic near-surface alloy (NSA) made up of Au and W as a CO2-reduction electrocatalyst selective towards the formation of methanol as a product, away from methane, ethylene or ethanol typically produced using Cu as the catalyst. At an overlayer NSA that consisted of n monolayers (ML) of Au on a polycrystalline W electrode, W(pc)-n[(1×1)-Au], no methane, methanol, ethylene or ethanol were detected when the coverage of Au was at submonolayer (n = 0.5) or multilayer (n ≥ 2) coverages. However, when the overlayer contained only 1 ML of Au, methanol was generated exclusively. The anticipated CH3OH-product-selectivity of the W(pc)-(1×1)-Au NSA has thus been (qualitatively) confirmed. The CH3OH-selective activity was 52 µA cm-2 for a Faradaic efficiency of 0.50%; the bulk of the current was expended towards H2 evolution and, since the topmost layer was Au, most likely in the production of CO and formates that cannot be studied by DEMS.
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This work describes results from an attempt to employ differential electrochem. mass spectrometry... more This work describes results from an attempt to employ differential electrochem. mass spectrometry (DEMS) of selectively pre-adsorbed reactants and (postulated) intermediates as a supplementary exptl. approach in the study of the reaction mechanism of the Cu-catalyzed electrochem. redn. of CO2. The results prompt the following empirical inferences: (i) CO is the first product of CO2 redn., as well as the first intermediate in more advanced reactions that include formation of pure and oxygenated hydrocarbons; this is in conformity with the (almost) unanimously held view. (ii) HCHO is not a precursor for C=C double-bond formation. (iii) HCHO is an intermediate for the prodn. of methane and ethanol. (iv) The generation of CH4 and CH3CH2OH from adsorbed CO occurs via two pathways: one requires a theor. postulated surface species, CO protonated on the C atom, and the other involves adsorbed HCHO, constituted after the rate-limiting protonation step. (v) The generation of CH4 and CH3CH2OH from CO has a much higher activation barrier than conversions from HCHO; not unexpected since the reactions transpire after the slow Cu-OCH+ formation and, consequently, are not highly activated. This work also presents results from an exptl. study based on DEMS that tested the theor. prediction that suggested the viability of a bimetallic near-surface alloy (NSA) electrode made up of Au and W as a CO2-redn. electrocatalyst selective towards the formation of CH3OH as a product, away from methane, ethylene or ethanol. At an overlayer NSA that consisted of n monolayers (ML) of Au on a polycryst. W electrode, W(pc)-n[(1×1)-Au], no methane, ethylene or ethanol were detected, when the coverage of Au was at submonolayer (n = 0.5) or multilayer (n ≥ 2) coverage. However, when the overlayer contained only 1 ML of Au, methanol was generated exclusively. The anticipated CH3OH-product- selectivity of the W(pc)-(1×1)-Au NSA has thus been (qual.) confirmed. The CH3OH-selective activity was 52 μA cm-2 for a Faradaic efficiency of 0.50 %; the bulk of the current was expended towards H2 evolution and, since the topmost layer was Au, most likely in the prodn. of CO and formates that are undetectable by DEMS.
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Current Topics in Catalysis, May 30, 2017
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ECS transactions, Jan 4, 2017
This work describes the employment of differential electrochemical mass spectrometry (DEMS) as a ... more This work describes the employment of differential electrochemical mass spectrometry (DEMS) as a supplementary experimental approach to theory in the study of the reaction mechanism of the Cu-catalyzed electrochemical reduction of CO_2 by investigating the reduction of reactants and (postulated) intermediates. The empirical inferences: (i) CO is one of the first products of CO_2 reduction, as well as the first intermediate in the formation of more reduced products. (ii) Formaldehyde is not a precursor for C=C bond formation but is an intermediate for the production of methane and ethanol. (iii) Both methane and ethanol can be generated from CO_2 through the protonation of CO and through the HCHO intermediate. (iv) The generation of CH_4 and CH_3CH_2OH from CO and CO_2 has a much higher activation barrier than from HCHO; not unexpected since the formaldehyde intermediate is formed after the (computationally determined) rate-limiting CO-protonation step. In this work, DEMS was also used to test the theoretical prediction suggesting the viability of a bimetallic near-surface alloy (NSA) consisting of Au and W as a CO_2-reduction electrocatalyst selective towards the formation of methanol as a product, as opposed to methane, ethylene or ethanol. At an overlayer NSA that consisted of n monolayers (ML) of Au on a polycrystalline W electrode, W(pc)-n[(1×1)-Au], no methane, methanol, ethylene or ethanol was detected when the coverage of Au was at submonolayer (n = 0.5) or multilayer (n ≥ 2) coverages. However, when the NSA contained only 1 ML of Au, methanol was generated exclusively.
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Electrocatalysis, Sep 5, 2012
ABSTRACT Earlier studies on the chemisorption of hydroquinone (H2Q) on well-defined Pd(111) surfa... more ABSTRACT 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°.
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Journal of the American Chemical Society, Jul 8, 2015
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Langmuir, Dec 9, 2014
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Journal of Electroanalytical Chemistry, Mar 1, 2014
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Electrocatalysis, Jan 27, 2015
ABSTRACT IntroductionThe effective abatement of atmospheric carbon through its conversion via ele... more ABSTRACT IntroductionThe 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 CO2-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 [1–7]. However, much better catalyst performance is needed. The overall energy efficiency of copper is less than 40 % [1–4], 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 th ...
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Electrocatalysis, Jun 7, 2016
AbstractCu catalyzes the electrochemical reduction of CO2 or CO to an assortment of products, a b... more AbstractCu catalyzes the electrochemical reduction of CO2 or CO to an assortment of products, a behavior that is a detriment when only one reduced compound is desired. The present article provides an example in which, through the atomic-level control of the structure of the Cu electrode surface, the yield distribution is regulated to generate only one product. The reaction investigated was the preferential reduction of CO to C2H5OH on Cu at a low overpotential in alkaline solution. Experimental measurements combined electrochemical scanning tunneling microscopy (ECSTM) and differential electrochemical mass spectrometry (DEMS). An atomically ordered Cu(100) surface, prepared from either a single crystal or by Cu(pc)-to-Cu(100) reconstruction, did not produce ethanol. When the surfaces were subjected to monolayer-limited Cu↔Cu2O cycles, only the reconstructed surface underwent an additional structural transformation that spawned the selective production of ethanol at a potential 645 mV lower than that which generates multiple products. Quasi-operando ECSTM indicated transformation to an ordered stepped surface, Cu(S) − [3(100) × (111)], or Cu(511). The non-selective, multiple-product Cu-catalyzed reduction of CO had thus been regulated to yield only one liquid fuel by an atomic-level structural modification of the electrode surface. Graphical AbstractTOC GRAPHIC
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Langmuir, Jan 19, 2007
This article concerns the growth of an atomic layer of Pb on the Au(111)( radical3 x radical3)R30... more This article concerns the growth of an atomic layer of Pb on the Au(111)( radical3 x radical3)R30 degrees -I structure. The importance of this study lies in the use of Pb underpotential deposition (UPD) as a sacrificial layer in surface-limited redox replacement (SLRR). SLRR reactions are being applied in the formation of metal nanofilms via electrochemical atomic layer deposition (ALD). Pb UPD is a surface-limited reaction, and if it is placed in a solution of ions of a more noble metal, redox replacement can occur, but limited by the amount of Pb present. Pb UPD is a candidate for use as a sacrificial layer for replacement by any more noble element. It has been used by this group for both Cu and Pt nanofilm formation using electrochemical ALD. The I atom layer was intended to facilitate electrochemical annealing during nanofilm growth. Two distinctly different Pb atomic layer structures are reported, studied using in situ scanning tunneling microscopy (STM) with an electrochemical flow cell and ultrahigh vacuum surface analysis combined directly with electrochemical reactions (UHV-EC). Starting with the initial Au(111)( radical3 x radical3)R30 degrees -I, 1/3 monolayer of I on the Au(111) surface, Pb deposition began at approximately 0.1 V. The first Pb UPD structure was observed just below -0.2 V and displayed a (2 x radical3)-rect unit cell, for a structure composed of 1/4 monolayer each of Pb and I. The I atoms fit in Pb 4-fold sites, on the Au(111) surface. The structure was present in domains rotated by 120 degrees. Deposition to -0.4 V resulted in complete loss of the I atoms and formation of a Pb monolayer on the Au(111), which produced a Moiré pattern, due to the Pb and Au lattice mismatch. These structures represent two well-defined starting points for the growth of nanofilms of other more noble elements. It is apparent from these studies that the adsorption of I- on Pb is weak, and it will rinse away. If Pb is used as a sacrificial metal in an electrochemical ALD cycle and adsorbed I atoms are employed for electrochemical annealing, I atoms will need to be applied each cycle.
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Journal of Crystal Growth, Apr 1, 2010
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Journal of Electroanalytical Chemistry, Sep 1, 2008
The effect of Cl− on the structure of Cu nanofilms deposited with electrochemical ALD, using surf... more The effect of Cl− on the structure of Cu nanofilms deposited with electrochemical ALD, using surface limited redox replacement (SLRR), is described. These investigations involved ultrahigh vacuum analytical methodologies coupled directly with electrochemical studies ( ...
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Journal of Electroanalytical Chemistry, Sep 1, 2003
The chemisorption of benzene at well-defined Pd(111) electrode surfaces in 0.1 M HClO4 has been s... more The chemisorption of benzene at well-defined Pd(111) electrode surfaces in 0.1 M HClO4 has been studied by a combination of electrochemistry, molecular resolution scanning tunneling microscopy, and high-resolution electron energy loss spectroscopy. At potentials within the double-layer region, a well-ordered Pd(111)-c(2√3×3)-rect-C6H6 adlattice was formed in which the adsorbed benzene molecules are slightly tilted with respect to the metal surface. When
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