John Y. Gui

Surface chemistry of mercaptopyridines at Ag(111) electrodes studied by EELS, LEED, Auger spectroscopy and electrochemistry

Surface electrochemical studies of 2-pyridinethiol (2PyT, 2-mercaptopyridine) and 4-pyridinethiol (4PyT, 4-mercaptopyridine) at Ag(111) single-crystal electrode surfaces in aqueous fluoride solutions are reported. LEED patterns observed for 2PyT adsorbed at Ag(111) from a 1 mM aqueous solution containing 2 mM HF (pH 3) at potentials between −0.54 V and −0.64 V (vs. Ag/AgCl reference) reveal that an ordered layer is formed having a rectangular unit mesh, Ag(111) (2×0.53)R30°-2PyT, containing one molecule of 2PyT. Adsorption at potentials more positive than −0.54 V results in LEED patterns which are relatively diffuse and have not yet been identified. A weak but observable Ag(111) (30.53× 30.53)R30° LEED pattern is obtained for adsorption of 4PyT at −0.40 V from a 1 mM aqueous solution (pH 3). Molecular packing densities (nmolcm2) measured by means of Auger spectroscopy indicate that 2PyT and 4PyT molecules are adsorbed with the pyridine ring perpendicular to the Ag(111) surface. The packing density of these adsorbates is virtually independent of adsorbate concentration from 0.01 mM to near saturation (about 0.2 M) at −0.60 V. Packing density of 2PyT increases slightly but abruptly when the electrode potential is increased above about −0.4 V. The Ag(111) (2×0.53)R30°-2PyT structure contains about 0.57 nmolcm2, slightly less than the saturation packing density, 0.67 nmolcm2, observed at more positive electrode potentials. EELS spectra of adsorbed 2PyT indicate that attachment to the Ag(111) surface at potentials less than −0.2 V occurs primarily through the sulfur atom, with dissociation of the sulfhydryl hydrogen atom. At potentials more positive than 0.0 V, a coupling reaction between an adsorbed 2PyT and a dissolved 2PyT occurs at the Ag(111) surface. The occurrence of this coupling reaction is more apparent for 4PyT at the Ag(111) surface. Amplitudes of the EELS bands due to CH bending and CS stretching depend upon the electrode potential at which adsorption is carried out. EELS spectra of 2PyT adsorbed at Ag(111) at negative potentials (E < −0.4 V) closely resemble the IR spectrum of the unadsorbed compound; the same is true for 4PyT. Apart from potential-dependent adsorption/desorption and coupling reactions, adsorbed 4PyT and 2PyT are relatively inert towards electrochemical oxidation and reduction. The 2PyT and 4PyT adsorbed layers are stable in UHV.

Studies of adsorbed unsaturated alcohols at well-defined Pt (111) electrode surfaces by cyclic voltammetry assisted by vibrational spectroscopy (EELS)

Abstract Surface vibrational states (electron energy-loss spectroscopy, EELS), molecular packing density (Auger spectroscopy) and adsorbed layer electrochemical reactivity (cyclic voltammetry) were investigated for the following unsaturated alcohols adsorbed from neat liquids and aqueous solutions at Pt (111): benzyl alcohol (BZA); 4-pyridylcarbinol (4PDC); 3-pyridylcarbinol (3PDC); allyl alcohol (AAL); propargyl alcohol (PGA); cis-2-butene-1,4-diol (CBED); and 2-butyne-1,4-diol (BYD). Chemisorption of the unsaturated centers of these alcohols is the principal mode of surface attachment (phenyl, pyridyl, CC or CC). Electrochemical oxidation of adsorbed BZA, AAL. PGA, CBED and BYD in aqueous fluoride electrolyte proceeds efficiently to CO2, while adsorbed 4PDC and 3PDC are relatively inert. The adsorbed layer of each of these alcohols is stable in vacuum, such that surface spectroscopic methods in vacuum are directly applicable to the liquid/solid interfacial chemistry and electrochemistry of these compounds. 4PDC and 3PDC are adsorbed in a tilted vertical orientation with an angle near 70° between the pyridine ring and the Pt surface. The C-C bond-order for PGA and BYD is reduced dramatically to near that of a double bond as a result of the adsorption process. The aromatic or alkene moieties of BZA, 4PDC, 3PDC, AAL and CBED are retained and only small shifts in their vibrational frequencies are found. Ordered layers of these adsorbed alcohols were not found by LEED, although uniform molecular orientation is suggested by the packing density data and voltammetric behavior.

Surface chemistry of five-membered heteroaromatics at Pt(III) electrodes studied by EELS, LEED, Auger spectroscopy and electrochemistry: furan, pyrrole and thiophene

Abstract Reported here are studies of the chemisorption, surface vibrational spectroscopy, elemental and molecular packing densities, and electrochemical reactivity of three related five-membered heterocycles at Pt(III): pyrrole (PRR), furan (FRN), and thiophene (TPE). Packing density and stoichiometry were investigated by the use of Auger spectroscopy. None of the adsorbed layers exhibited long-range order detectable by LEED. Vibrational spectra of the chemisorbed species were obtained by means of electron energy-loss spectroscopy (EELS), and were compared with infrared spectra of the unadsorbed compounds. Electrochemical oxidation of the adsorbed molecules was explored by the use of cyclic voltammetry (CV). The results reveal that PRR adsorbs at Pt(III) mainly in a horizontal orientation with the ring intact; electrochemical oxidation of adsorbed PRR proceeds completely to NO2 and CO2. However, FRN is converted by hydrolytic, ring-opening isomerization to adsorbed pi-bonded butenoic acid (BTA); electrochemical oxidation of adsorbed BTA proceeds to CO2. TPE forms a mixed layer consisting of S atoms and adsorbed TPE molecules S-bonded and near-vertically oriented; adsorbed TPE is oxidized to CO2 and SO42−.

Oriented adsorption at well-defined electrode surfaces studied by Auger, leed, and eels spectroscopy

Abstract Quantitation by use of Auger spectroscopy and cyclic voltammetry of molecular layers adsorbed at Pt(111) and Pt(100) surfaces from aqueous electrolytes is examined in this work for the following compounds: hydroquinone (HQ); phenol (PL); catechol (CT); 3,4-dihydroxyphenylacetic acid (DOPAC); L-3,4-dihydroxyphenylalanine (DOPA); l -tyrosine (TYR); l -phenylalanine (PHE); nicotinic acid (NA); 2,5-dihydroxy-4-methyl-benzyl mercaptan (DMBM); thiophenol (TP); benzylmercaptan (BM); 3-thiophene carboxylic acid; and 2,5,2′,5′-tetrahydroxybiphenyl (THBP). Two independent methods of measurement of packing density based upon Auger spectroscopy, and one based upon cyclic voltammetry are employed and the results compared. Voltammetric oxidation/reduction of adsorbed layers formed from these compounds at Pt surfaces in aqueous electrolyte is found to be essentially the same whether carried out before or after lengthy evacuation. Therefore, the results of surface spectroscopy in UHV are directly applicable to the liquid—solid chemistry and electrochemistry of these adsorbed compounds. Packing densities measured by means of two Auger spectroscopic methods were in good agreement with each other and with the voltammetric measurements.

Electrochemical oxidation of adsorbed terminal alkenes as a function of chain length at Pt (111) electrodes: Studies by cyclic voltammetry, EELS, and auger spectroscopy

Abstract Studies are reported of the surface packing density, vibrational spectroscopy and electrochemical oxidation of a series of straight-chain terminal alkenes adsorbed from the vapor at a Pt (111) electrode surface. Compounds studied were: ethene (ETE), propene (PPE), 1-butene (BTE), 1-pentene (PTE), 1-hexene (HXE), 1-octene (OCE) and 1-decene (DCE). Vibrational spectra of the adsorbed layers were obtained by use of electron energy-loss spectroscopy (EELS). Molecular packing densities (nmol/cm2) in the adsorbed layer were measured by use of Auger spectroscopy. Electrochemical oxidation of each adsorbed layer in aqueous inert electrolyte (KF+ HF) was investigated by means of linear potential scan voltammetry. Attachment to the surface is through the CC bond. Based upon molecular packing densities, the aliphatic chains are pendant; regardless of its chain length, each chemisorbed alkene molecule occupies an area similar to that of chemisorbed PPE. The packing densities of ETE and PPE indicate an average orientation in which the CC moiety is parallel to the Pt (111) surface. EELS spectra indicate that the CC double bond is preserved in the adsorbed state of each 1-alkene studied. Measurement of the average number of electrons, nox, required to desorb an adsorbed 1-alkene molecule electro-oxidatively reveals that the catalytic electrochemical oxidation process involves primarily the CC double bond and one adjacent saturated carbon atom.

Surface vibrational spectroscopy. A comparison of the EELS spectra of organic adsorbates at Pt(111) with IR and Raman spectra of the unadsorbed organics

In this study EELS spectra obtained for the adsorbed species formed from aqueous electrolytes at Pt(111) electrode surfaces are compared with the IR and Raman spectra of the unadsorbed compounds in order to reveal the changes in vibrational spectra resulting from chemisorption of various important functional groups, and to explore the differences in vibrational absorptivities between EELS spectra of adsorbed species and IR and Raman spectra of the corresponding unadsorbed compounds. Of particular interest are the variations in EELS vibrational frequency, bandwidth and absorptivity due to bonding with the surface, intermolecular interactions of adsorbed molecules and changes in adsorbate molecular orientation. The influence of surface bonding on the EELS spectrum of a functional group was explored through studies of phenol (PL), phenol-d6 (PLD6), benzyl alcohol (BZOH), catechol (CT), benzoic acid (BA), 2-picolinic acid (PA), 2,6-pyridine dicarboxylic acid (26PDCA), and propenoic acid (PPEA). The aromatic ring of adsorbed PL, PLD6, BZA, CT, BA, PA and 26PDCA is oriented parallel to the Pt(111) surface. The resulting strong interactions affect the frequencies and relative intensities of the EELS bands: weak CH stretching modes; a large CC stretching band (1600–1650 cm−1), and weak CH bending (700–800 cm−1). The carboxylic acid moieties of BA and PA interact strongly with the Pt surface, while those of 26PDCA do so only when adsorbed at relatively positive electrode potentials. OH stretching and bending are absent from the EELS spectra of adsorbed PL, BZOH and CT, perhaps due to dissociation of the hydroxyl hydrogen during adsorption of the molecule. Adsorption of alkenes at Pt(111) from solution preserves the characteristic CC stretching band near 1650 cm−1; examples are: PPEA; 1-hexene (HXE); propenol (PPEOH); 4-pentenol (PTEOH); and cis-2-butene-1,4-diol (CBED); adsorption of ethene, propene and butene from vacuum at room temperature has been reported to result in loss of double-bond character. Adsorption of propynol (PPYOH) lowers the frequency of the CC triple bond to approximately that of a double bond. EELS spectra of adsorbed pyridine (PY), 2-methylpyridine (2MPY) and 2,6-dimethylpyridine (26DMPY) were examined in order to contrast the vibrational behavior of adsorbates having the aromatic ring perpendicular to the Pt surface (PY and 2MPY), giving rise to strong perturbation of the vibrational spectra or parallel (26DMPY), leading to limited perturbation of the vibrational spectra. Compounds for which the surface interaction is limited primarily to bonding of a single sulfur atom were studied in order to observe the vibrational behavior of a comparatively unperturbed pendant ring or chain: thiophenol (TP); benzylmercaptan (BM); cysteine (CYS); and thiophene (TPE). The expected lack of perturbation was found, making TP, BM and CYS particularly suitable reference compounds for surface vibrational studies of aromatic rings and amino acids. In contrast, L-phenylalanine (PHE) interacts with the Pt(111) surface through the phenyl ring as well as the amino acid functionality. In general, chemisorption and intra-layer intermolecular interaction both lower the surface vibrational frequencies (EELS) and incidentally affect peak amplitudes. Orientation affects peak amplitudes and incidentally affects frequencies insofar as a change in orientation is accompanied by a change in mode of surface bonding. Surface roughness leads to a scrambling of adsorbed states which affects bandwidths, chemical/electrochemical reactivities, and in turn the frequencies and amplitudes of EELS peaks. Specific findings and conclusions are presented for each compound and correlated.

Studies of Ru(001) electrodes in aqueous electrolytes containing silver ions and methane: LEED, HREELS, Auger spectroscopy and electrochemistry

Abstract Potent catalysis by Ru electrodes has been reported by various workers. Reported here are studies of chemisorption, surface vibrational spectroscope and electrochemical reactivity at Ru(001) single-crystal electrode surfaces. Electrochemical oxidation of methane on these Ru electrode surfaces in aqueous electrolytes was investigated. The influence of surface oxide and electrodeposited silver on methane oxidation were explored. Immersion of Ru(001) into pure water at open circuit forms a layer of adsorbed hydrous oxides with an ordered (2 × 2) structure as measured by Auger spectroscopy and low energy electron diffraction (LEED). Anodization of Ru(001) in 1 M HClO 4 produces a disordered Ru O/OH film consisting of several atomic layers. The high resolution energy electron loss spectrum of this O/OH layer exhibits RuO and OH stretching bands, and the layer is not removed by subsequent electrolysis at negative potentials. Various submonolayer and multiple-layer amounts of silver were electrodeposited on Ru(001). A continuous film is formed, based upon attenuation of the substrate Auger signal. The silver layer lacks long-range order, as judged by LEED. Under the present conditions, namely Ru(001) single-crystal surfaces with or without the O/OH and/or silver layers in aqueous electrolytes, the faradaic current due to oxidation of methane is generally less than 1 μA cm −2 . Experiments were performed with Ru(001) surfaces brought to an atomically clean ordered state by Ar + ion bombardment and annealing in UHV. Immersion of Ru(001) into water at open circuit formed a submonolayer of oxide/hydroxide with an ordered (2 × 2) structure and HREELS vibrational bands attributable to RuO and OH libration and traces of adsorbed CO. Cyclic voltammetry of Ru(001) in aqueous KF and HClO 4 electrolytes illustrated the substantial irreversibility of oxide formation—reduction and the near reversibility of hydrogen adsorption—desorption. Electrochemical oxidation of CH 4 in 10 mM KF electrolyte at pH 3 produced an increase in current density equal to 1 μA cm −2 at potentials between − 0.1 and 0.2 V vs. Ag/AgCl. Electrodeposition of Ag onto the bare Ru(001) surface did not alter the current density for CH 4 oxidation significantly. Electrochemical oxidation of the Ru(001) surface to form a (1 × 1) oxide/hydroxide layer decreased the activity of the surface for anodic oxidation of CH 4 in aqueous KF electrolyte. Electrodeposition of a fractional monolayer or multilayer of Ag onto the electrochemically pre-oxidized surface did not detectably influence the rate of CH 4 electrochemical oxidation. The clearly demonstrated catalytic activity of metal-doped Ru oxide electrodes [1–4] is all the more intriguing in the light of the present results which suggest that the observed catalytic behavior is not due primarily to the metallic Ru surface, layers of adsorbed oxide on Ru or electrodeposited metallic Ag.

Adsorption and electrochemistry of SCN−: Comparative studies at Ag(111) and Pt(111) electrodes by means of AES, CV, HREELS and LEED☆

Abstract Surface electrochemical studies are reported of SCN − at Ag(111) and Pt(111) electrode surfaces in aqueous solutions. Adsorbate packing density and stoichiometry were investigated by use of Auger electron spectroscopy (AES). Adsorbate molecular constitution and surface chemical bonding were characterized by means of high-resolution electron energy-loss spectroscopy (HREELS), which yielded vibrational spectra spanning the entire IR frequency range. Surface electrochemical behavior was explored by cyclic voltammetry (CV). Packing densities of SCN − at Pt and Ag are essentially the saturation values at all practical concentrations, and at all electrode potentials for which the metal surface is stable. The SCN − ion remains intact during adsorption at Pt and Ag surfaces unless the potential is strongly oxidizing. When adsorbed onto Pt(111) at pH 3, SCN − is protonated (SCNH), while at pH 10 it is not protonated (SCN − K + ). Protonation of Pt/SCN − leads to CH and NH vibrational modes in HREELS, suggesting that the adsorbed layer consists of at least two species (PtSCNH and PtNCHS). However, when adsorbed at Ag(111), SCN − is not protonated, even at pH 3. The C:S stoichiometric ratio (evaluated from AES) was essentially 1:1 under all conditions studied; the packing densities were about 0.5 SCN per surface Pt atom, or 0.25 SCN per surface Ag atom. The HREELS spectra suggest that the axis of the SCN moiety is approximately perpendicular to the Pt or Ag surface. Low energy electron diffraction (LEED) studies revealed a Pt(111)(1 × 2)-SCN structure (θ SCN -2~ 0.5), which was converted by heating (700°C) to Pt(111)(2 × 2)-S (θ s -2~ 0.25), and an Ag(111)(2 × 3√3, rectangular)-SCN structure (θ SCN -2 0.25). Adsorption of SCN − at Pt(111) or Ag(111) as a function of electrode potential revealed noticeable changes in the HREELS spectra, including a blue-shift of the CN stretch with increasing potential. Packing densities were virtually independent of potential, except at extremely positive potentials where a separate AgSCN phase is formed or where the Pt/SCN system undergoes extensive oxidation, and the surface becomes disordered as indicated by LEED.

Surface electrochemistry and molecular orientation: Studies of pyridyl hydroquinones adsorbed at Pt(111) by cyclic voltammetry, Auger electron spectroscopy and electron energy loss spectroscopy

Abstract Reported here are surface electrochemical studies of three newly synthesized compounds: (4-pyridyl) hydroquinone (4PHQ), (3-pyridyl) hydroquinone (3PHQ) and (2-pyridyl) hydroquinone (2PHQ). These compounds possess essentially the same electrochemical reactivity in the dissolved form, but exhibit different electron transfer behavior in the adsorbed state. Each chemisorbs at Pt(111) electrode surfaces from aqueous solutions to form a close-packed and highly oriented monolayer which is stable in vacuum and in solution. Molecular packing density (in nanomoles per square centimeter) was measured by means of Auger electron spectroscopy and coulometry. Surface molecular vibrations were determined by use of high resolution electron energy loss spectroscopy (EELS). Electrochemical reactivity was investigated by use of cyclic voltammetry. Long range surface structure was monitored by low energy electron diffraction. The results indicate that 3PHQ and 4PHQ are attached to the platinum surface exclusively through the nitrogen atom with the pyridine ring in a tilted vertical orientation. Such a surface molecular orientation keeps the hydroquinone moiety pendant and thus reversibly electroactive. Since none of the molecular conformations of adsorbed 4PHQ permits direct contact between the hydroquinone moiety and the Pt(111) surface, electron transfer between the hydroquinone moiety and the surface evidently proceeds by electron hopping and/or tunneling through the chemisorbed pyridine ring. In contrast with the behavior of 3PHQ and 4PHQ, the HQ moiety of 2PHQ is directly attached to the Pt(111) surface in addition to the PtN surface bond of the pyridine ring, as required by the 2PHQ molecular structure. Accordingly, adsorbed 2PHQ possesses virtually no reversible electroactivity and only one adsorbed state in the electrode potential range from −0.1 to 0.4 V, based upon EELS spectra.

Ordered Hg0.8Cd0.2Te (111)A and (111)B surfaces: Preparation by annealing in Hg vapor and characterization by low‐energy electron diffraction, Auger electron spectroscopy, and electron energy‐loss spectroscopy

Reported here are studies in which an oriented single‐crystal, Hg0.8Cd0.2Te (111), was etched with methanolic bromine solution, ion bombarded with Ar+, and annealed in Hg vapor. Both parallel faces of the crystal, the metal‐terminated (111)A surface and the Te‐terminated (111)B surface, were characterized after each of the above stages of preparation by Auger electron spectroscopy (AES), electron energy‐loss spectroscopy (EELS) and low‐energy electron diffraction (LEED). After polishing in air the (111)A and (111)B surfaces were found to be contaminated with C and S‐containing substances, and deficient in Cd; the LEED patterns were diffuse. EELS spectra showed a broad C–H stretching band near 2980 cm−1 and another broad band centered at 1300 cm−1 due to C–H bending and C–C stretching of surface hydrocarbons. Etching with bromine solution in air removed S‐containing surface contaminants, without significantly changing the LEED and EELS results. Argon ion‐bombardment removed the C contamination and increase...