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

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

Influence of the Surface Roughness of Platinum Electrodes on the Calibration of the Electrochemical Quartz-Crystal Nanobalance

The electrochemical quartz-crystal nanobalance (EQCN) is an in situ technique that measures mass changes (Δ<i>m</i>) associated with interfacial phenomena. Analysis of Δ<i>m</i> sheds light on the mass balance (in addition to the charge and energy balances) and provides new insight into the nature of electrochemical processes. The EQCN measures changes in frequency (Δ<i>f</i>) of a quartz-crystal resonator, which are converted into Δ<i>m</i> using the Sauerbrey equation containing the characteristic constant (<i>C</i>_f). The value of <i>C</i>_f is determined by physical parameters of the crystal and refers to an atomically smooth surface. However, real resonators are not smooth and electrodes have their intrinsic roughness. Thus, the conversion of Δ<i>f</i> to Δ<i>m</i> should be done using an experimentally determined characteristic constant (<i>C</i>_f,exp) for a given value of the surface roughness factor (<i>R</i>). Here, we calibrate the system using Ag electrodeposition on Pt electrodes of gradually increasing <i>R</i>; the latter is adjusted through Pt electrodeposition. The surface morphology of the Pt substrates prior to and after Ag electrodeposition is examined using atomic force microscopy. The values of <i>C</i>_f,exp are determined by analyzing the slopes of charge density versus Δ<i>f</i> plots for the Ag electrodeposition. They are different than <i>C</i>_f and increase logarithmically with <i>R</i>. The <i>C</i>_f and <i>C</i>_f,exp values are used in a comparative analysis of the mass changes (δΔ<i>m</i>) for complete cyclic voltammetry profiles covering the 0.05–1.40 V range. This reveals that the employment of <i>C</i>_f instead of <i>C</i>_f,exp provides inaccurate values of δΔ<i>m</i>, and the magnitude of the discrepancy increases with <i>R</i>

Interfacial structure of atomically flat polycrystalline Pt electrodes and modified Sauerbrey equation

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Identification and Analysis of Electrochemical Instrumentation Limitations through the Study of Platinum Surface Oxide Formation and Reduction

Anodic polarization of Pt electrodes in aqueous H₂SO₄ leads to the formation of a surface oxide (PtO). Herein, the surface oxide growth is accomplished using three different approaches: (i) chronoamperometry (CA); (ii) chronocoulometry (CC); and (iii) a combination of cyclic voltammetry (CV) and CA. The PtO reduction is accomplished potentiodynamically using voltammetry. The oxide growth takes place at defined polarization potentials (<i>E</i>_p), polarization times (<i>t</i>_p), and temperatures (<i>T</i>). The oxide charge density (<i>q</i>_ox) is determined for both the formation (<i>q</i>_ox,form) and reduction (<i>q</i>_ox,red) processes. The oxide reduction CV profiles are integrated to determine the charge density values for oxide reduction (<i>q</i>_ox,red,CV) which are compared with the <i>q</i>_ox,form,CA and <i>q</i>_ox,form,CC values. The values of <i>q</i>_ox,form,CC are greater than those of <i>q</i>_ox,form,CA, but both potentiotatic methods (CA and CC) produce <i>q</i>_ox,form values that are consistently lower than those of <i>q</i>_ox,red,CV. In the case of oxide formation with combined CV and CA, the values of <i>q</i>_{ox,form,CV+CA} are found to be lower than the values of <i>q</i>_ox,red,CV, although the difference is small. Electrochemical quartz crystal nanobalance (EQCN) is used to monitor the mass variation at the electrode surface during the oxide formation and reduction process at <i>E</i>_p = 1.20 V with various <i>t</i>_p values. Equal mass changes during oxide formation and reduction are detected by the EQCN. The nature of the differences in <i>q</i>_ox,form and <i>q</i>_ox,red encountered with the different experimental methods are discussed in terms of instrumental limitations