Donald A. Stern

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.

Characterization of single-crystal electrode surfaces as a function of potential and pH by Auger spectroscopy and LEED: Pt (111) in aqueous CaCl2 and HCl solutions

Experiments were performed in which a well-characterized Pt (111) surface was immersed into aqueous CaCl2/HCl solutions at controlled potential, after which the surface was removed from solution, evacuated, and characterized by LEED, Auger spectroscopy and related techniques. Potential-dependent and pH-dependent adsorption of halogen was observed, predominantly as Cl atoms. Stable chemisorption of Cl occurred only when the pH was less than about 4 with the electrode potential more positive than about 0.5 V (vs. Ag/AgCl reference). An order adsorbed layer, Pt (111)(3×3)−Cl, containing θCl = 0.25, was formed at potentials at which Cl was the predominant adsorbed species. Chemisorbed oxides or hydroxides were formed at the more positive potentials. Retention of Ca2+ by the surface was potential-dependent, with a minimum at about 0.3 V, and never exceeded θCa = 0.08 (Ca2+ ions per surface atom). Water was retained by the surface following evacuation, to the extent of 10–20 water molecules per Ca2+ ion, depending upon the pH.

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 reactivity of 2,2',5,5'-tetrahydroxybiphenyl and related compounds adsorbed at pt (111) surfaces: studies by eels, leed, auger spectroscopy and cyclic voltammetry

Abstract Adsorption and surface electrode reactions of a family of compounds containing the hydroquinone moiety, namely 2,2,5,5-tetrahydroxybiphenyl (THBP), 2,5-dihydroxy-4-methylbenzylmercaptan (DMBM) and hydroquinone (HQ), have been studied at well-defined Pt (111) surfaces by means of cyclic voltammetry assisted by electron energy-loss spectroscopy (EELS), Auger electron spectroscopy and low-energy electron diffraction (LEED). Packing densities of THBP measured by Auger spectroscopy indicated adsorption predominantly with the rings parallel to the surface, although reversible electroactivity of the layer increased with increasing THBP concentration, indicating a small fraction of verticallyoriented molecules. In contrast, DMBM is adsorbed entirely through its sulfur atom with its ring perpendicular to the surface; the pendant 2,5-diphenyl moiety in adsorbed DMBM was reversibly electroactive. Adsorbed HQ was not reversibly electroactive at any adsorbate concentration. Vibrational spectra of the adsorbed species were obtained by means of EELS, and were compared with the infrared spectra of the parent compounds in KBr. EELS and IR spectra were closely similar except where adsorption changed the nature of the molecule: based upon virtual absence of the characteristic EELS O-H stretching band (near 3500 cm −1 ), the phenolic hydroxyl hydrogens of HQ and the horizontallyoriented form of THBP were removed upon adsorption, as was the mercaptan sulfhydryl hydrogen of DMBM. The EELS bands of polar groups such as OH are not broadened to the same extent as in the IR spectra of the solid compounds, evidently due to lesser intermolecular hydrogen bonding among such groups at the surface. LEED observations indicated that the THBP layer was structurally diffuse, while HQ formed a Pt (111) (3×3)-HQ adlattice and DMBM formed a Pt (111) (23 0.5 × 2 0.5 )R30 °-DMBM adlattice at packing densities slightly below saturation. Adsorbate orientation and mode of surface bonding exert a systematic influence on the product distribution of electrocatalytic oxidation of these well-characterized adsorbed organic intermediates, based upon cyclic voltammetry and potential-step chronocoulometry experiments. Also, these results have revealed that evacuation does not alter the composition or electrochemical properties of the chemisorbed layer formed from solution.

Surface analysis of electrodes by ultra-high vacuum techniques: acetonitrile solvent chemisorption at Pt(111)

Abstract Electrochemical studies at a carefully characterized Pt(111) electrode surface are yielding useful insights into atomic, ionic and molecular electrochemistry at interfaces. Reviewed here are studies of the chemisorption of the common polar aprotic solvent acetonitrile (CH 3 CN) at a Pt(111) surface. Electrosorption of CH 3 CN under typical electrolytic conditions from CH 3 CN electrolyte, CH 3 CN aqueous electrolytic solutions and CH 3 CN vapor was investigated by high-resolution electron energy-loss spectroscopy (HREELS, vibrational spectroscopy), Auger electron spectroscopy (AES, elemental analysis), low-energy electron-diffraction (LEED, structure/symmetry probing) and linear potential scan cyclic voltammetry ( cv , exploration of electrochemical maturity). The results indicate that a chemisorbed layer is formed from CH 3 CN liquid, vapor and typical aqueous solutions. The chemisorbed layer consists of a mixture of species related to CH 3 CN and acetamide (CH 3 CONH 2 ), contains about θ = 0.15 molecules per surface Pt atom is stable in vacuum and in solution over a wide range of electrode potentials, is replaced only slowly by other strong adsorbates such as iodide and lacks long-range order in the absence of anions such as iodide. Related studies are exploring ionic absorption, organic molecular adsorption and electrodeposition of metallic monolayers. A brief overview and references are included.

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 Cue5fbC 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 Cue5fbC moiety is parallel to the Pt (111) surface. EELS spectra indicate that the Cue5fbC 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 Cue5fbC 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. Oue5f8H 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 Cue5fbC 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 Cue5fcC 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 Ruue5f8O and Oue5f8H 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 Ruue5f8O and Oue5f8H 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 Cue5f8H and Nue5f8H 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 Cue5fcN stretch with increasing potential. Packing densities were virtually independent of potential, except at extremely positive potentials where a separate Ague5f8SCN phase is formed or where the Pt/SCN system undergoes extensive oxidation, and the surface becomes disordered as indicated by LEED.