Sheng-Pei Chen

Studies on the role of oxidation states of the platinum surface in electrocatalytic oxidation of small primary alcohols

Abstract The role of the oxidation state of a platinum polycrystalline surface in the electrocatalytic oxidation of C1 to C4 primary alcohols has been studied by using electrochemical techniques, in situ FTIR spectroscopy and X-ray photoelectron spectroscopy. The results revealed that the oxidation state of the Pt surface plays a key role in the oxidation of primary alcohols, and demonstrated that the oxidation of C1 to C4 primary alcohols on a Pt electrode is controlled by the formation of surface oxides on the Pt electrode at different potentials. It was found that the dependence of the reaction process on the oxidation states of the platinum surface yielded similar features in the cyclic voltammogram for oxidation of different primary alcohols at a Pt electrode. According to the effects in the oxidation of primary alcohols, the surface oxides of platinum may be classified as active and poison species. The Pt surface oxides of higher oxidation states (Pt(OH)3 and PtO2) formed at potentials above 1.0 V (SCE) were identified as poison species, while other lower oxidation states of Pt surface oxides such as PtOH, Pt(OH)2 and PtO may be identified as the possible active species for primary alcohol oxidation.

Electrochemical milling and faceting: Size reduction and catalytic activation of palladium nanoparticles

Improved performance through milling: A method for enhancing the catalytic activity of supported metal nanoparticles is reported. This method enhances the activity for the ethanol electro-oxidation of a supported palladium catalyst. The much higher catalytic performance is ascribed to the increased electrochemically active surface area as well as the generation of high-index facets at the milled nanoparticle surface.

Novel properties of dispersed Pt and Pd thin layers supported on GC for CO adsorption studied using in situ MS-FTIR reflection spectroscopy

Abstract The adsorption of CO on dispersed thin layers of platinum and palladium supported on glassy carbon (Pt/GC and Pd/GC) was studied using in sity multi-step FTIR spectroscopy (MS-FTIRS). Novel properties of the dispersed Pt and Pd thin layers with respect to CO adsorption have been observed for the first time. In comparison to the adsorption of CO on smooth Pt and Pd surfaces, the IR features of CO adsorbed on Pt/GC and Pd/GC electrodes become abnormal, consisting mainly in (1) the direction of the IR band being inverse, (2) the intensity of the IR band being enhanced significantly with an enhancement factor of 20 for linearly bonded CO on Pt/GC and 26 for bridge bonded CO on Pd/GC, and (3) the FWHM of the IR band being broadened by about 6–9 cm−1. The abnormal optic properties of the dispersed Pt and Pd thin layers with respect to CO adsorption were attributed to the particular structure of the thin layers, which is undergoing further investigation.

Chemical states of bismuth and sulfur adatoms on the polycrystalline Pt electrode surface towards HCOOH oxidation — combined studies of cyclic voltammetry, in situ FTIRS and XPS on the origin of electrocatalytic activity of adatoms

Abstract The modification of platinum electrode surfaces by bismuth and sulfur adatoms was studied using cyclic voltammetry, in situ FTIR spectroscopy and X-ray photoelectron spectroscopy (XPS). The apparent coverage of saturation adsorption of bismuth and sulfur on Pt electrode from solutions containing 10−3 M Bi3+ or S2− ions were measured at approximately 0.69 and 0.90, respectively. The in situ FTIR spectroscopic data demonstrated that both sulfur and bismuth adatoms can prevent, by a surface geometric arrangement, the formation of poison species which is identified as adsorbed CO species derived from the dissociative adsorption of HCOOH on Pt electrode. However, a big difference in electrocatalytic activity of Pt/Sad and Pt/Biad electrodes for HCOOH oxidation has been determined. HCOOH cannot be oxidized on a Pt/Sad electrode at the saturation adsorption of sulfur. Nevertheless, when the S adatoms have been partially removed by oxidation at potentials above 1.0 V (Pd/H), the oxidation of HCOOH on the Pt/Sad electrode can take place and yield a larger current than a Pt electrode does in the positive going potential sweep. It has been found that the Pt/Biad electrode at the saturation adsorption of bismuth maintains a high electrocatalytic activity towards HCOOH oxidation, which was determined both in the cyclic voltammetric studies and in the potential step experiments of a relatively long time window. The difference in electrocatalytic properties of bismuth and sulfur adatoms in HCOOH oxidation was attributed to the different chemical states of these adatoms on the Pt electrode surface. It has been revealed by combined studies of electrochemistry and X-ray photoelectron spectroscopy that the ions of S2− can discharge on Pt surface during adsorption forming sulfur adatom under conditions with or without electrochemical polarization. In addition, the adsorbed sulfur adatom is mainly in an atomic state, but charged partially with negative charge. The adsorbed sulfur (Sad) can be oxidized to sulfate species at potentials above 1.20 V (Pd/H). However, the Bi3+ ions in solution cannot be reduced on Pt surface during adsorption. It was determined that 67% adsorbed bismuth was reduced to its atomic state at 0.0 V (Pd/H) and 33% bismuth remained in an oxidized state even at this relatively low potential. A transition oxidized state of adsorbed Bi has been observed from the XPS spectrum recorded near 1.0 V (Pd/H), for which the higher binding energy of Biad appeared near 160 ( 4 f 7 2 ) and 165 eV ( 4 f 5 2 ). At potentials above 1.1 V (Pd/H), all adsorbed bismuth is in an oxidized state. The present study has placed emphasis on the importance of chemical states of electrode surface in electrocatalysis and thrown new insight to understand the origin of electrocatalytic effect of adatoms.

Surface combinatorial studies of IR properties of nanostructured Ru film electrodes using CO as probe molecule

Abstract An individually addressable array of Pt microelectrodes was designed and prepared. Ru film of different nanostructures was prepared electrochemically on platinum microelectrodes of the array under cyclic voltammetric conditions. The electrochemical behavior and surface structure of the Ru film were investigated, respectively, using cyclic voltammetry and scanning tunneling microscopy. In combining the individually addressable array with an in situ microscope FTIR reflection spectroscopy (MFTIRS), surface combinatorial studies of IR properties of different nanostructured Ru films have been conveniently carried out. In situ microscope FTIR spectral library of CO adsorbed on different nanostructured Ru films and at different electrode potentials was acquired rapidly. Particular IR properties of nanostructured Ru film were revealed. IR absorption of both bridge-bonded CO (CO B , around 1800 cm −1 ) and linearly bonded CO (CO L , near 2000 cm −1 ) was significantly enhanced, and an enhancement factor of IR absorption has been determined to be varied between 11.8 and 15.5 along with the variation of nanostructure of Ru film on the array. Following consecutive increase in thickness of Ru film and in size of Ru islands that form the film, CO B species yielded always a broad band appearing in anti-absorption direction, while CO L species produced an IR band that was transformed from a bipolar shape to monopolar shape of anti-absorption direction. Together with the shift of IR band center, the large increase in IR bandwidth and in Stark tuning rate, the particular IR properties were illustrated as a change from Fano-like asymmetric spectral characteristics to abnormal IR features, and attributed to the consecutive variation of nanostructure of Ru film on the individually addressable array.

In situ scanning FTIR microscopy and IR imaging of Pt electrode surface towards CO adsorption

In situ scanning FTIR microscopy was built up for the first time in the present work, which consists of an FTIR apparatus, an IR microscope, an X-Y mapping stage, and the specially designed electrochemical IR cell and computer software. It has been demonstrated that this new space-resolvdin situ IR technique can be used to study vibration properties of micro-area, and to perform IR imaging of electrode surface. The chemical image obtained using this technique for CO adsorption on Pt electrode illustrated, at a space-resolution of 10-2 cm, the inhomogeneity and the distribu-tion of reactivity of micro-area of electrode surface.

In situ time-resolved FTIRS study of adsorption and oxidation of ethylene glycol on Pt(100) electrode

Adsorption and oxidation of ethylene glycol (EG) on Pt(100) electrode were studied by in situ time-resolved FTIRS (TRFTIRS). The TRFTIR spectra recorded at 0.10 V illustrate that an IR band appears near 2050 cm−1 at t > 5 s, corresponding to linearly bonded CO formed in dissociative adsorption of EG. The TRFTIR results have confirmed also that CO species are distributed uniformly on Pt(100) surface. Another band appears near 2342 cm−1 at t > 70 s, associating with IR absorption of CO2 produced in the direct oxidation of EG. With the increase of electrode potential, the direct oxidation of EG becomes gradually the main reaction. When the potential is above 0.40 V, the oxidation of EG occurs mainly via the reactive intermediates, i.e. species containing -COOH determined by in situ TRFTIRS.