Voltammetric studies of silane-modified SnO2 surfaces in selected aqueous and non-aqueous media

Voltammetric studies in selected aqueous and non-aqueous media were conducted for a series of silane-modified SnO₂ electrodes which had been further modified with a variety of metal phthalocyanines and metal cations. It is shown that tetrasulfonated, transitionmetal phthalocyanines can be both adsorbed and covalently attached to silane-modified surfaces where the silane has a coordinating ligand functionality such as a primary or secondary amine. It is further shown that the presence of the silane on the SnO₂ surface causes the appearance of a reversible voltammetric process following exposure of the surface to solution cation species. This reversible voltammetric wave is hypothesized to be due to the reduction of a surface bound Sn(IV) species to Sn(II) in a strongly hydroxylated environment.     

Study of the dynamics of surface molecules by time-resolved sum-frequency generation spectroscopy

Sum-frequency generation (SFG) is a nonlinear laser-spectroscopy technique suitable for analysis of adsorbed molecules. The sub-monolayer sensitivity of SFG spectroscopy enables vibrational spectra to be obtained with high specificity for a variety of molecules on a range of surfaces, including metals, oxides, and semiconductors. The use of ultra-short laser pulses on time-scales of picoseconds also makes time-resolved measurements possible; this can reveal ultrafast transient changes in molecular arrangements. This article reviews recent time-resolved SFG spectroscopy studies revealing site-hopping of adsorbed CO on metal surfaces and the dynamics of energy relaxation at water/metal interfaces. Time-resolved sum frequency generation spectroscopy at surfaces with non-resonant laser pulse irradiation     

Adsorption of carbon monoxide and oxygen on transition metal surfaces: A comparative study of the results from electron spectroscopy and theoretical calculations

Results of investigations on the adsorption of CO andO ₂ on transition metal surfaces by employinguv and x-ray photoelectron spectroscopy and electron energy loss spectroscopy (eels) are presented. Results of molecular orbital calculations on adsorbed CO and O₂ are also discussed. Some of the interesting aspects discussed are, satellites in the O(ls) region due to adsorbed CO, vibrationaleels of adsorbed O₂ and dissociation energy profiles of adsorbed O₂ on clean surfaces as well as surfaces covered with potassium or presorbed atomic oxygen. Contribution No 245 from the Solid State and Structural Chemistry Unit.     

Adsorption of bisulfate on the Ru(0001) single crystal electrode surface

Adsorption of anions from sulfuric acid solutions has been studied on Ru(0001) single crystal and polycrystalline surfaces by electrochemical techniques and in-situ Fourier transform infrared spectroscopy. Infrared spectroscopy shows that bisulfate is the anion adsorbed on the Ru(0001) surface. The bisulfate adsorption is detected at the H₂ evolution potential and extends into the potential region where the Ru surface is oxidized. A method for extracting unipolar bands from bipolar bands has been presented. The tuning rate of adsorbed bisulfate in the double layer potential region of Ru(0001) was found to be significantly smaller than those observed for other platinum metals. This has been ascribed to a small change in bisulfate coverage on Ru(0001) in this potential range. Bisulfate vibration frequencies are higher on this surface than at any face-centered cubic metal with the (111) orientation. Oxidation of the Ru(0001) surface is limited to one electron per Ru atom, distinctly different from the high degree of oxidation seen in polycrystalline surfaces. For oxidized polycrystalline Ru, only solution phase sulfates and bisulfates are observed in the IR spectra.     

Immobilization of substituted metall phthalocyanines by nonwoven polypropylene

The specifics of immobilization of water-soluble phthalocyanines on a nonwoven polypropylene (PP), depending on the nature of substitutents and the metal, was studied. The spectral characteristics of the phthalocyanines adsorbed on PP were analyzed.     

Recent developments in the vibrational spectroscopies (infrared,Raman, electron energy loss etc.) as applied to the structural analysis of species chemisorbed on metal surfaces

An account is given of the methods of vibrational spectroscopy that are nowadays used to obtain structural information about species adsorbed on metal catalyst surfaces. The discussion deals separately with experiments involving a) oxide-supported metal catalysts in the finely divided form, and b) specific surfaces of metal single crystals. A discussion is also given of the advantages and limitations of each method, e.g. infrared transmission and Raman spectroscopy in the case of finely-divided metals, and electron energy loss spectroscopy and reflection-absorption infrared spectroscopy as used for metal single crystals.Infrared and Raman spectroscopic studies of ligands attached to multi-atom metal clusters are shown to lead to convincing identifications of species adsorbed on single crystal surfaces. The latter results in turn contribute to the understanding of the overlapping spectra from several different adsorbed species that commonly occur on finely-divided metal catalysts.     

Metal-oxide surfaces

Over the past two decades the amount of effort devoted to the study of metal oxides by surface scientists has increased significantly. The general characteristics of the electronic structure of metal-oxide surfaces are now fairly well understood, although transition-metal oxides have been more thoroughly studied than have non-transition-metal oxides. The geometric arrangement of atoms on the surfaces of a variety of metal oxides has also been determined. Extensive studies have been performed of the interaction of both molecules and metal atoms with metal oxides, where point defects are found to play a dominant role. However, our understanding of the surface properties of metal oxides is still much less compete than it is for metals and semiconductors, and there are several areas where more experimental and theoretical effort needs to be concentrated.     

Recent progress in surface scientific approaches to oxide catalysis

This paper attempts to review recent work on in-situ characterizations of metals and metal oxides supported on oxide surfaces relevant to oxide catalysis. The active well-defined surfaces can provide novel information on the key issues in catalytic research such as the structure, composition and distribution of active sites, ensembles and phases, behavior of adsorbed active species, electronic property participating in catalysis, etc., which are well characterized by recent in-situ spectroscopy and also by traditional spectroscopy.     

Synthesis and Aggregation of Cationic Zinc and Magnesium Phthalocyanines Containing 4-(3,5-Dimethyl-1<Emphasis Type="Italic">H</Emphasis>-pyrazol-1-yl)phenoxy Groups

Tetracationic metal phthalocyanines were synthesized by the quaternization of the C_s-regioisomers of tetrasubstituted zinc and magnesium phthalocyanines containing 3,5-dimethylpyrazole fragments. The individuality of the synthesized compounds was confirmed by HPLC?mass spectrometry. The aggregation of the cationic metal phthalocyanines in water and aqueous-organic media, as well as in the presence of Triton X- 100 was studied. The dependence of the dimerization constant on the polarity of mixed solvents was determined.     

Arg–Cys and Arg–cysteamine adsorbed on gold and the G-protein–adsorbate interaction

The dipeptide, Arg–Cys, and the related molecule, Arg–cysteamine, are adsorbed to gold surfaces and the monolayers are characterized. Chemical binding and electronic structure of the monolayers are obtained by X-ray photoelectron spectroscopy (XPS). Strong molecular binding of the adsorbates to gold surface through the sulfur atom is attained. Orientation of the adsorbates on gold is studied using infrared reflection absorption spectroscopy (IRAS). Arg–Cys is interpreted to be adsorbed on gold in a compact configuration. The Arg–cysteamine molecule is adsorbed on gold with the main molecular axis perpendicular to the surface. Interaction of G-protein with the adsorbates was studied using the surface plasmon resonance (SPR) technique. It is believed that arginine has a major role in G-protein recognition since the G-protein-coupled receptor (GPCR) ₂A has an arginine-rich region in the G-protein-binding part of the third intracellular loop.     

Paradoxical effect of the redox potential of adsorbed metallophthalocyanines on their activity for the oxidation of 2-mercaptoethanol. Inner versus outer sphere electrocatalysis

We have studied the effect of the redox potential of metallophthalocyanines (M-Pcs) adsorbed on graphite on their electrocatalytic activity for the oxidation of 2-mercaptoethanol (ME). This was achieved by: (i) changing the metal in the phthalocyanine (M-Pc where M=Cr, Mn, Fe, Co, Ni and Cu) and (ii) using cobalt phthalocyanines with electron-donor and electron-withdrawing substituents on the macrocyclic ligand. For phthalocyanines of different metals a plot of log k versus the redox potential of the catalyst gives a straight line of slope 0.10 V decade⁻¹ which is close to the value obtained (0.12 V decade⁻¹) from Tafel plots for all M-Pcs investigated. In contrast, when different substituted cobalt phthalocyanines are compared, a plot of log k versus redox potential gives a straight line of negative slope (−0.240 V decade⁻¹) and the rate decreases with driving force. Since ME electrooxidation in aqueous media most likely proceeds via an inner-sphere mechanism and the rate-determining step is the same for all cobalt phthalocyanines investigated, the decrease in rate constant with driving force may be due to a decrease in the electronic coupling between the cobalt center and the sulfur in ME. Preliminary PM3 semi-empirical theoretical calculations of the electronic coupling associated with the interaction of the metal in the phthalocyanine and the ME molecule support this explanation.     

In situ FTIR spectra of pyridine adsorbed on SiO2–Al2O3, TiO2, ZrO2 and CeO2: general considerations for the identification of acid sites on surfaces of finely divided metal oxides

Exposure of strong Lewis (coordinatively unsaturated metal atoms) and Bronsted (proton donor OH-groups) acid sites on solid surfaces is a prime demand for potential adsorptive and catalytic applications. In situ FTIR spectroscopy of small adsorbed base molecules, often NH₃, pyridine, CH₃CN, NO or CO molecules, has been well established as a powerful surface analytical technique for characterization of nature, strength and concentration of acid sites. Pyridine (Py) has been preferred as an IR probe molecule of finely divided metal oxide surfaces at room (RT) and higher temperature regimes, since it is (i) more selective and stable than NH₃; (ii) much more strongly adsorbed than CO and CH₃CN; and (iii) relatively more sensitive to the strength of Lewis acid sites than NO. In the present work, in situ IR spectra of Py adsorbed at ≥RT on characterized alumina, silica, silica–alumina, titania, zirconia and ceria were measured, and compared with RT-spectra of liquid and gas phase Py obtained under identical spectroscopic conditions, in order to characterize spectral consequences of mutual Py–Py interactions in the adsorbed phase. It has been concluded that the availability of Lewis acid sites can be unequivocally monitored by formation of coordinated Py molecules giving rise to IR-absorption(s) due to the ν_8a mode of νCCN vibrations at 1630–1600 cm⁻¹, where the higher the frequency assumed, the stronger the acidity of the site. Formation of pyridinium surface species (PyH⁺) is identifiable by (i) an ν_8a-absorption at ≥1630 cm⁻¹; (ii) an ν_19b-absorption at 1550–1530 cm⁻¹; as well as (iii) νN⁺---H and δN⁺---H absorptions occurring, respectively, near 2450 and 1580 cm⁻¹, and, thus, the availability of Bronsted acid sites. Moreover, products and IR-characteristics of Py surface reactions at >RT have been identified, and used to imply nature of surface base sites (OH⁻and O²⁻) involved in formation of acid–base site pairs.     

Synthesis and spectral characteristics of cyclohexylmethoxy-substituted phthalocyanines of rare-earth elements

New symmetrical metal complexes of the rare-earth element phthalocyanines were synthesized by the reaction of 4,5-bis(cyclohexylmethoxy)phthalonitrile, obtained for the first time, with the rare-earth element salts, as well as starting from the corresponding free phthalocyanine. A correlation between method of the synthesis and the reaction product compositions was studied. Structures of the complexes obtained were confirmed by mass spectrometry, X-ray crystallography, and electronic absorption spectroscopy. All the metal complexes are well soluble in organic solvents. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2343–2349, December, 2007.     

An application of the coincidence Doppler spectroscopy for substances of chemical interest: phthalocyanine and acetylacetonate complexes

Coincidence Doppler spectroscopy, which is particularly powerful when one is concerned with high momentum components of positron annihilation gamma-rays, has been applied to two different kinds of organo-metalic ligands: metal phthalocyanines and metal acetylacetonates. The energy (momentum) profiles of the annihilation gamma-rays were the same for metal phthalocyanines indicating that positron and/or positronium are not interacting with the metal ions. However, the profiles for the metal acetylacetonates evidently showed a dependence on the kind of metal ions. Discussion is made on the features of positron interaction which are different for phthalocyanines and acetylacetonates.     

Interfacial electron transfer energetics studied by high spatial resolution tip-enhanced Raman spectroscopic imaging

Interfacial electron transfer (ET) in TiO?-based systems is important in artificial solar energy harvesting systems, catalysis, and in advanced oxidative waste water treatment. The fundamental importance of ET processes and impending applications make the study of interfacial ET a promising research area. Photoexcitation of dye molecules adsorbed on the surface of wide band gap semiconductors, such as TiO?, results in the injection of electrons from the dye molecules to the conduction band of the semiconductor or energetically accessible surface electronic states. Using Raman spectroscopy and ensemble-averaging approaches,t he chemical bonding and vibrational relaxation of the ET processes have been extensively studied. However, due to the complexity of the interfacial ET energetics and dynamics, significant questions remain on characterizing the source of the observed complexities. To address these important issues, we have applied advanced spectroscopic and imaging techniques such as confocal and tip-enhanced near-field Raman as well as photoluminescence spectroscopic and topographic imaging. Here we explore single surface states on TiO? as well as the interfacial electronic coupling of alizarin to TiO? single crystalline surfaces.     

Adsorption and protein-induced metal release from chromium metal and stainless steel

A research effort is undertaken to understand the mechanism of metal release from, e.g., inhaled metal particles or metal implants in the presence of proteins. The effect of protein adsorption on the metal release process from oxidized chromium metal surfaces and stainless steel surfaces was therefore examined by quartz crystal microbalance with energy dissipation monitoring (QCM-D) and graphite furnace atomic absorption spectroscopy (GFAAS). Differently charged and sized proteins, relevant for the inhalation and dermal exposure route were chosen including human and bovine serum albumin (HSA, BSA), mucin (BSM), and lysozyme (LYS). The results show that all proteins have high affinities for chromium and stainless steel (AISI 316) when deposited from solutions at pH 4 and at pH 7.4 where the protein adsorbed amount was very similar. Adsorption of albumin and mucin was substantially higher at pH 4 compared to pH 7.4 with approximately monolayer coverage at pH 7.4, whereas lysozyme adsorbed in multilayers at both investigated pH. The protein-surface interaction was strong since proteins were irreversibly adsorbed with respect to rinsing. Due to the passive nature of chromium and stainless steel (AISI 316) surfaces, very low metal release concentrations from the QCM metal surfaces in the presence of proteins were obtained on the time scale of the adsorption experiment. Therefore, metal release studies from massive metal sheets in contact with protein solutions were carried out in parallel. The presence of proteins increased the extent of metals released for chromium metal and stainless steel grades of different microstructure and alloy content, all with passive chromium(III)-rich surface oxides, such as QCM (AISI 316), ferritic (AISI 430), austentic (AISI 304, 316L), and duplex (LDX 2205).     

Electron spin resonance of Langmuir-Blodgett films fabricated with metal derivatives of polyazamacrocycles

This review analyses the results obtained through the Electron Spin Resonance (ESR) spectroscopy on multilayer Langmuir-Blodgett films fabricated with metal derivatives (mainly Cu²⁺, VO²⁺ and low-spin CO²⁺) of polyazamacrocycles (porphyrins, tetraazaannulenes, phthalocyanines). These metal ions are particularly suitable for an ESR analysis because of their relatively long electron spin relaxation times that render possible their observation even at room temperature and in all co-ordination symmetry. This allows us to obtain valuable data on both mobility and orientation of the metal-macrocycle systems adsorbed onto a solid support. The preliminary characterisation of the spreading monolayer obtained from the systems considered, as it is typically done by surface pressure/area isotherms, optical (UV-Vis) and ESR spectroscopies, is also reviewed.     

Synthesis and solution properties of new metal-free and metallo-phthalocyanines containing four bis(indol-3-yl)methane groups

Metallo-phthalocyanines bearing four bis(indol-3-yl)methane groups were successfully prepared by reaction of the corresponding phthalonitriles with anhydrous metal salts [Zn(CH₃COO)₂, NiCl₂ and CoCl₂] in the presence of a catalytic amount of DBU in 2-(dimethylamino)ethanol. The metal-free phthalocyanine was obtained by treating a mixture of the phthalonitrile derivative in similar conditions but in the absence of a metal salt. All of these phthalocyanines are soluble in DMSO, DMF, and pyridine. The products were characterized by IR, NMR, and UV-vis spectroscopy, MALDI-TOF-MS, and thermogravimetric analysis. The aggregation properties of the phthalocyanines were investigated at different concentrations in DMSO. All the phthalocyanines showed monomeric behavior in solution.     

2,9,16,23-四联苯基金属酞菁的合成、溶解性及光学性质

合成了2,9,16,23-四联苯基金属(Fe,Co,Ni,Cu和Zn)酞菁。研究了合成产物的质谱、溶解性、紫外光谱以及荧光光谱．结果表明,取代基使金属酞菁的溶解性增加,不发生二聚,紫外光谱发生红移。     

H2 adsorption on 3d transition metal clusters: a combined infrared spectroscopy and density functional study

The adsorption of H2 on a series of gas-phase transition metal (scandium, vanadium, iron, cobalt, and nickel) clusters containing up to 20 metal atoms is studied using IR-multiple photon dissociation spectroscopy complemented with density functional theory based calculations. Comparison of the experimental and calculated spectra gives information on hydrogen-bonding geometries. The adsorption of H2 is found to be exclusively dissociative on Sc(n)O+, V(n)+, Fe(n)+, and Co(n)+, and both atomic and molecularly chemisorbed hydrogen is present in Ni(n)H(m)+ complexes. It is shown that hydrogen adsorption geometries depend on the elemental composition as well as on the cluster size and that the adsorption sites are different for clusters and extended surfaces. In contrast to what is observed for extended metal surfaces, where hydrogen has a preference for high coordination sites, hydrogen can be both 2- or 3-fold coordinated to cationic metal clusters.