On the calculation of potential energy profile diagrams for processes in electrolytic metal deposition

The calculation of potential energy and free energy profile diagrams for successive and alternative steps in electrolytic metal deposition is described with reference to two extreme models of the entity resulting from the initial transfer of the metal particle from the solution to the surface of the metal.

Neutralization of the transferred ions to form adsorbed metal atoms is distinguished from ion-transfer processes in which the transferred entity maintains ionic character, with the appropriate number of stoichiometric electrons entering the metal lattice for each ion transferred.

The elementary processes considered are: transfer of ions from the solution to different types of surface sites upon the metal; surface diffusion of adsorbed ions; successive, dehydration of the adsorbed ions in lattice building.

The free energies of the transition states in successive steps in consecutive ion-transfer, surface-diffusion and lattice-building reactions are compared, and the probable rate-determining process in the over-all metal deposition reaction is deduced in the cases of Cu²⁺, Ni²⁺ and Ag⁺ ion deposition on to the respective metals. Uncertainties in the calculation are examined.

The heat of activation (ΔH^0≠) for transfer of ions from the solution to the metal surface depends upon the site to which transfer occurs, that to a planar site being significantly less than that to other sites (e.g., edges, kinks, etc.) Transfer to form completely non-polar neutral adatoms has prohibitively high values of ΔH^0≠

Direct deposition Of Cu²⁺ on to surface sites would be associated with a prohibitively high heat of activation. The path Cu²⁺ + e_M → Cu⁺ followed by Cu⁺ + e_M → (Cu_adion⁺+ e_M) is associated with heats of activation significantly lower than that for direct Cu²⁺ deposition in a single two-electron step. The free energy diagrams are consistent with the existence of a rate-determining reduction mechanism found experimentally. Near the Cu/Cu²⁺ reversible potential the free energy barrier for adion surface diffusion can become the highest. This is consistent with the experimental behaviour under these conditions. With Ag⁺ ion deposition the ion-transfer step has the highest free energy barrier at high negative overpotentials, whilst near the reversible potential the barrier for surface diffusion can become the highest. The kinetic behaviour found experimentally with silver supports the theoretical conclusions.

The low exchange current density for Ni²⁺ ion deposition is probably associated with the instability of the simple Ni⁺ ion in aqueous solutions.     

温度和氧含量对NH4Br改性稻壳焦汞氧化吸附特性的影响

采用10 g·L^-1 NH₄Br溶液对原始稻壳焦进行浸渍改性制备了脱汞吸附剂。利用比表面积及孔隙度分析仪、扫描电子显微镜/X射线能谱分析仪对改性前后稻壳焦吸附剂物理化学性质进行表征。在固定床汞吸附实验台上对吸附剂床层出口元素汞(Hg⁰)和二价汞(Hg²⁺)进行同步检测,研究了不同温度下N₂气氛和N₂+6.4 %(体积)O₂气氛中改性稻壳焦的汞氧化吸附特性。50℃下N₂气氛、N₂+6.4%(体积)O₂气氛以及150℃下N₂气氛中固定床出口均未明显检测到Hg²⁺,50℃和150℃时脱汞率均可达90%。温度为150℃时,在N₂+6.4%(体积)O₂气氛中吸附剂的总汞(Hg^T)脱除效率为98.2%,其中82.2%的Hg⁰被吸附脱除,其余则被O₂非均相氧化为Hg²⁺而氧化脱除。通过对程序升温管式炉出口Hg^T进行检测研究了吸附剂表面吸附态汞的脱附特性。汞脱附峰值温度300℃表明吸附态汞可能以Hg-Br化合物的形式赋存于改性稻壳焦表面,220~350℃的脱附温度说明吸附态汞有较高的热稳定性。100.14%~118.62%的汞平衡率验证了实验结果的准确性。     

Preparation and characterization of MgO powders having Ca or Ba as surface dopants

The tendency of large dopant cations towards surface segregation has here been utilised to prepare high surface area MgO powders carrying Ca²⁺ or Ba²⁺ as surface impurities in amounts equivalent to 0.2 to 5 monolayers of CaO or BaO. Computational approaches had pointed to possibilities for reconstruction of such impurity monolayers into “rumpled” arrangements as a means of minimizing their energies: the rumpled arrangement favoured for Ca²⁺/MgO would have alternate oxygen anions displaced outward from the original (100) plane, whereas that favoured for Ba²⁺/MgO would involve large outward displacement of alternate Ba²⁺ ions. Ions displaced in that way would necessarily feature lower coordination to counter-ions than species on non-rumpled surfaces, and hence could be expected to exhibit higher surface reactivity and/or catalytic activity. This paper presents results of experiments aimed at: (i) preparation of powdered MgO materials having monolayer amounts of CaO or BaO segregated upon their surfaces; and (ii) testing whether such monolayer-doped Ca²⁺/MgO and Ba²⁺/MgO materials exhibit physical and/or chemical properties indicative of the “rumpling” favoured by computations. Chemical comparisons made between surface reactivities of Ca²⁺/MgO and Ba²⁺/MgO versus the pure MgO support include: lability of surface oxide ions, as manifested in ease of heterophase oxygen isotope exchange at 623–873 K, and relative activities for oxidative coupling of methane at 923–958 K. Physical comparisons include: extent and spectral distribution of surface-sensitive luminescence; BET surface areas; X-ray diffraction and scanning electron microscopy.     

Untersuchung über die anionenadsorption mit hilfe markierter ionen

From previous work, the adsorption of anions is regarded as an essential factor for the different corrosion behaviour of metals in solutions containing different anions. Adsorption is measured by means of ³⁶Cl⁻, ⁸²Br⁻, ¹⁸ F⁻, ³⁶ClO₄⁻, ³⁵O₄²⁻, H³⁵S⁻ and ¹⁴CN⁻ on Pt, Ni and Fe in the form of sheets and evaporated films. Besides the determination of the adsorption after dipping into the solution, a method has been developed for the measurement of adsorption in contact with the solution and for the determination of its kinetics. The method can also be applied to O₂-free metal surfaces produced under vacuum. In this case, however, very rapid adsorption is observed, whereas normally saturation is reached only after many hours. It is concluded that, in general, exchange between oxygen on the metal and the anion takes place rather than simple adsorption.

The distribution of the anions adsorbed on the metal surface has been studied by autoradiography; adsorption takes place preferentially at the grain boundaries and increases when the crystal size decreases.

These results confirm the interpretation of passivation as a competition between various processes: metal dissolution, coverage by a passivating oxide film, and displacement of oxygen by anions.     

IR studies of CO and NO adsorbed on well characterized oxide single microcrystals

A systematic investigation of the surface morphology and of the vibrational properties of CO and NO adsorbed on simple oxides microcrystals (like MgO, NiO, NiO-MgO, CoO-MgO, ZnO, ZnO-CoO, -Cr₂O₃, -Al₂O₃, MgAl₂O₄ and other spinels, TiO₂, ZrO₂ and other oxides of a similar structure) with regular crystalline habit and exposing thermodynamically stable and neutral faces, is presented with the aim to elucidate the spectroscopic manifestations of CO and NO adsorbed on well defined crystallographic positions.

In particular the structure of CO and NO adsorbed on the cationic sites of extended faces of these model solids is presented and discussed with the aim of elucidating the nature of the Me^x+··· CO/NO bond (Me^x+ = non transition metal ion or transition metal ion). When non transition metal ions are involved, the molecule-cation interaction is predominantly electrostatic. This leads to an increase of the CO stretching frequency, which is roughly proportional to the polarizing field. On the contrary, when transition metal ions are involved, beside the predominant electrostatic interactions, a small contribution to the bond stability comes also from d-π overlap forces, which, although not very important from the energetic point of view, greatly influence the static and dynamic dipoles localized on the adsorbed molecules. Consequently, the strength of the dipole-dipole interactions occurring in the ordered adlayers of CO and NO adsorbed on transition and non transition metal oxide surfaces are resulted remarkably different.

On these well defined surfaces, the effects influencing the half-width (FWHM) of the CO and NO stretching peaks have also been considered. It has been calculated that the FWHM is a very sensitive parameter of the surface perfection. In a few cases (ZnO, -Cr₂O₃, etc.) FWHM values comprised in the 1.5–3.7 range have been obtained, which are indicative of a single-crystal quality of the exposed faces. These spectroscopic results were compared with those obtained with quantum calculations.

Finally the activity towards CO and NO of perfect, low index faces and of more defective situations (like those associated with edges, steps and corners) are compared, in order to have a better insight on the role of surface defectivity in catalytic reactions.     

REVERSE POTENTIAL PHENOMENA IN THE MEMBRANE-ELECTRODE (M-E) PROCESS

Conventional water treatment techniques typically suffer from severe limitations in selective removal and recovery of heavy metals from dilute solutions. For instance, treatment by pH control, chemical reduction, and chemical oxidation result in precipitation of heavy metaJs, which are subsequently landfilled. Ion exchange suffers from lack of selectivity in cation removal from multi-component mixtures, while electrochemical reduction has severe limitations in dilute solutions. In order to address the problem of selective heavy metal removal and recovery, development of the Membrane-Electrode (M-E) process was undertaken. The M-E process is a hybrid of electrochemical reduction and ion-exchange technologies and permits selective ion-exchange from dilute aqueous solutions.

High cation selectivities in the M-E process is due to controlled rate of ion-exchange. It has been discovered that cation exchange rates can be controlled by application of an electrical potential difference (pd), and that an inverse relationship exists between pd and the rate of ion-exchange. This behavior is termed as the “Reverse-Potential Phenomena”. Typically, the rate of ion-exchange in conventional ion-exchange processes is dependent upon the cation concentration in solution, and cation loading on the ion-exchange material; this rate cannot be controlled by external means.

Thus far, the M-E process has been effectively demonstrated for selective recovery of Pb²⁺ and Cu²⁺ ions from dilute aqueous binary and ternary cation solutions [,]. This paper focuses on the effect of Reverse-Potential phenomena on selectivity in the Cu²⁺ and Ni²⁺ binary solution mixture.     

On the nature of the adsorbed species and mechanism of adsorption of In(III) from iodide solutions at the dme

The reduction of In(III) in solutions containing 0.05, 0.1 and 0.25 M of NaI and with an ionic strength of 1.00 maintained with sodium perchlorate, has been studied by means of interfacial admittance measurements at different frequencies and dc potentials of the dropping mercury electrode.

Our results demonstrate that the process takes place with specific adsorption of electroactive species. The adsorption parameters and their dependence with potential have been determined. From the analysis of our results, together with those obtained in a previous study at higher concentrations of I⁻, it can be deduced than InI⁺₂ is the species preferentially adsorbed, this adsorption taking place via a bridge-like mechanism, through I⁻ ions previously adsorbed.     

Highly efficient and selective membrane transport of silver(I) using hexathia-18-crown-6 as a specific ion carrier

Hexathia-18-crown-6 (HT18C6) was used as a specific ion carrier for the transport of silver ion through a chloroform bulk liquid membrane. In the presence of thiosulfate ion as a suitable stripping agent in the receiving phase, the amount of silver transported across the liquid membrane after 60 min is 99.5±1.0%. The selectivity of Ag⁺ transport from the aqueous solutions containing other Mⁿ⁺ cations such as Tl⁺, Mg²⁺, Ca²⁺, Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Pb²⁺, Hg²⁺, and Cr³⁺ ions was investigated. Except with Hg²⁺ ion, non of the cations used interfere the silver transport, even at a Mⁿ⁺/Ag⁺ molar ratio of 500. The interfering effect of Hg²⁺ ion was successfully eliminated in the presence of EDTA at pH 5.     

SO2活化改性石油焦吸附剂的汞吸附特性

利用SO₂气体对石化工业副产物石油焦进行活化改性以制成富硫高活性脱汞吸附剂（SAPC）。在固定床实验装置上进行SAPC吸附脱除汞的实验研究,考察吸附温度、入口Hg⁰浓度、烟气成分以及热再生等因素对脱汞特性的影响规律,同时结合比表面积及孔隙度分析、元素分析和X射线光电子能谱（XPS）等表征手段深入分析SAPC的汞吸附机理。结果表明,SO₂活化改性石油焦的物理和化学特性得到极大改善,羰基、酯基以及非氧化态硫是Hg⁰的主要活性吸附位。吸附温度的升高会抑制对Hg⁰的吸附脱除,烟气中较高的Hg⁰浓度会降低汞脱除效率,但对其汞吸附速率有促进作用。SO₂对SAPC的脱汞性能影响较小,O₂易将Hg⁰氧化成为更容易与含氧、含硫官能团结合的氧化态汞,从而促进对Hg⁰的脱除。热再生时吸附态汞化合物受热分解的过程伴随着吸附剂表面化学活性位的损失,导致再生后汞吸附性能大幅下降。     

Characterization of Pd-based FCC CO/NOx control additives by in situ FTIR and extended X-ray absorption fine structure spectroscopies

Pdⁿ⁺/Ceⁿ⁺/Na⁺/γ-Al₂O₃-type materials used as FCC additives for CO/NO_x control were characterized by extended X-ray absorption fine structure (EXAFS) spectroscopy and in situ FTIR. The EXAFS data indicate that in freshly prepared samples palladium is present in the form of highly dispersed PdO species. Reduction with H₂ at 500 °C leads to the formation of small Pd clusters incorporating on average approximately six to eight metal atoms at a Pd−Pd bond distance of 2.76 Å. All components of these materials can interact with NO and promote the formation of nitrate/nitrite species, essentially “trapping” NO_x species on the catalyst surface. However, the Na⁺ species dominate the surface chemistry and readily form sodium nitrates with a characteristic IR band at 1370–1385 cm⁻¹. Finally, hydroxyls from the support are also actively participating in the formation of HNO_x type compounds with characteristic stretching vibrations in the 3500–3572 cm⁻¹ region.     

Spin localization for NO adsorption on surface O atoms of metal oxides

The adsorption of NO on the oxygen site of several metal oxide surfaces is discussed. It is shown that the strength of the interaction and the variation of the bond lengths are not always correlated to the electron transfer from NO to the surface atoms. In cases of irreducible metal oxides, NO₂²⁻ may be strongly adsorbed. The formation of NO₂⁻ on reducible metal oxide is difficult unless terminal oxygen is present on the surface. Then, the reduction of the surface by transferring the unpaired electron from the NO to the surface appears in DFT calculations (VASP code).     

NO–NH3 coadsorption on vanadia/titania catalysts: determination of the reduction degree of vanadium

Diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) has been used to study NH₃ and NO adsorption over a 15% w/w vanadia/titania catalyst. NH₃ is adsorbed as coordinate NH₃ and NH₄⁺ species over the oxidised catalyst, leading to the reduction of the vanadia surface. At 300°C, adsorbed nitrosyls species are detected, suggesting that the oxidation of gaseous or adsorbed ammonia species takes place over the V=O sites. Coadsorption experiments show that NO is able to reoxidise about the 57% of the reduced V=O groups, resulting in N₂, according to a NO+V_□→1/2N₂+V=O reaction. On the other hand, NO is only adsorbed over vanadia reduced surfaces. The measure of the area of the 2ν(V=O) bands results in an estimate of the oxidation state of vanadium. From this estimate it can be concluded that nitrosyls species are adsorbed on the catalyst surface for vanadium atoms having an oxidation state ranging from +4 to +3.1.     

Biosorption of mercury and lead by aqueous Streptomyces VITSVK9 sp. isolated from marine sediments from the bay of Bengal,India

Toxic heavy metals are increasingly accumulating in the environment worldwide and are considered to be life threatening contaminants. The biosorption of mercury and lead by marine actinomycetes isolated from marine sediment collected from the Bay of Bengal coast of Puducherry, India, was evaluated. The maximum tolerance concentration (MTC) of Streptomyces sp. was determined by a well diffusion method and a broth dilution method. The effects of the initial metal ion concentration, the pH and the biomass dosage on the biosorption of mercury and lead ions were investigated. The MTC of the isolate to metals was 200 mg·L^-1 for mercury and 1800 mg·L^-1 for lead. At neutral pH, the isolate had a maximum biosorption of metal ions of 200 mg·L^-1 and 150 mg·L^-1 for mercury and lead respectively. Fourier transform infrared (FTIR) absorption spectra showed the chemical interactions between the functional groups in the biomass such as hydroxyl (-OH), amine (-NH₂), carboxyl (-COOH) and the metal ions. The isolate was further characterized by molecular taxonomy and identified as a member of the genus Streptomyces. Based on the phenotypic and phylogenetic analysis, the strain was classified as a new species of the genus Streptomyces and designated as Streptomyces VITSVK9 sp. (HM137310). A blast search of the 16S rDNA sequence of the strain showed the most similarity (95%) with Streptomyces sp. A515 Ydz-FQ (EU384279). Based on the results, it can be concluded that this marine Streptomyces could be used as a biosorbent for the removal of heavy metal ions from aqueous environments.     

Local density functional study on adsorption of Cu and Fe on a Si (100) surface

Using a local density functional calculation, we investigate the adsorption energies, geometries and electronic structures of single Cu²⁺ and Fe²⁺ ions on a Si(100) surface. The adsorption energy results reveal that both ions are stably adsorbed in a hollow site; from this site, Cu²⁺ is located at 0.51 Å and Fe²⁺ is located at 0.41 Å from the surface, respectively. The adsorption energy of Cu²⁺ is about 1.5 times larger than that of Fe²⁺. This means that Cu²⁺ is strongly adsorbed on the silicon surface. From the analysis of spin density, we find that Cu²⁺strongly attracts the electrons of the silicon surface, because spin density is delocalized in the silicon backbone. As a result, the 3d orbital of Cu²⁺ becomes more similar to the closed-shell state compared to that of Fe ²⁺. This means that the core electronic state of Cu²⁺ is closer to the neutral atom, which is qualitatively confirmed by XPS measurements.     

Catalytic behavior of La–Sr–Ce–Fe–O mixed oxidic/perovskitic systems for the NO+CO and NO+CH4+O2 (lean-NOx) reactions

Mixed oxides of the general formula La_0.5Sr_xCe_yFeO_z were prepared by using the nitrate method and characterized by XRD and Mössbauer techniques. The crystal phases detected were perovskites LaFeO₃ and SrFeO_3−x and oxides -Fe₂O₃ and CeO₂ depending on x and y values. The low surface area ceramic materials have been tested for the NO+CO and NO+CH₄+O₂ (“lean-NO_x”) reactions in the temperature range 250–550°C. A noticeable enhancement in NO conversion was achieved by the substitution of La³⁺ cation at A-site with divalent Sr⁺² and tetravalent Ce⁺⁴ cations. Comparison of the activity of the present and other perovskite-type materials has pointed out that the ability of the La_0.5Sr_xCe_yFeO_z materials to reduce NO by CO or by CH₄ under “lean-NO_x” conditions is very satisfying. In particular, for the NO+CO reaction estimation of turnover frequencies (TOFs, s⁻¹) at 300°C (based on NO chemisorption) revealed values comparable to Rh/-Al₂O₃ catalyst. This is an important result considering the current tendency for replacing the very active but expensive Rh and Pt metals. It was found that there is a direct correlation between the percentage of crystal phases containing iron in La_0.5Sr_xCe_yFeO_z solids and their catalytic activity. O₂ TPD (temperature-programmed desorption) and NO TPD studies confirmed that the catalytic activity for both tested reactions is related to the defect positions in the lattice of the catalysts (e.g., oxygen vacancies, cationic defects). Additionally, a remarkable oscillatory behavior during O₂ TPD studies was observed for the La_0.5Sr_0.2Ce_0.3FeO_z and La_0.5Sr_0.5FeO_z solids.     

Adsorption on perfect and reduced surfaces of metal oxides

Two major chemical processes, acidobasic and redox, track the adsorption mechanism on metal oxides. Molecular and dissociative adsorption on stoichiometric surfaces can be understood as acid–base processes. Clean and anhydrous surfaces of metal oxides have two different active sites: cations and anions. Electron-rich molecules or fragments arising from a heterolytic bond cleavage (Lewis bases) react with Mⁿ⁺, while electron poor ones (Lewis acids) react with O²⁻. The MgO and TiO₂ surfaces clearly appear to be predominantly acidic and molecules that do not dissociate generally bind to the metal cation. The electronic structure, insulating character for the stoichiometric surface, is preserved upon adsorption.

When the initial system does not favor an energy gap (open-shell adsorbates, defective surfaces), the best adsorption mode is via a redox mechanism that restores the situation of an insulator and the highest oxidation states for all the atoms.

For radical adsorption a first solution occurring on irreducible oxides is to couple the electrons and form two opposite ions adsorbed on the two surface sites, as for H₂/MgO, involving an acid–base mechanism. Another possibility occurring on reducible oxide is via an electron transfer to or from the oxide (redox mechanism). The electron transfer occurs from the substrate to the adsorbate for electronegative group (Cl adsorption on O) or the other way around for an electropositive one (NO adsorption on M). The reactivity at surfaces deviating from stoichiometry differs from that on the perfect ones. The difference does not only originate from the modification of the coordination number but also from the electron counting.     

Density functional complexation study of metal ions with poly(carboxylic acid) ligands. Part 1. Poly(acrylic acid) and poly(α-hydroxy acrylic acid)

We have studied metal ion complexation with poly(carboxylic acid) ligands using density functional methods and a continuum-solvation model (COSMO). Geometry optimisations have been carried out for metal complexes of poly(acrylic acid-co-maleic acid), poly(methyl vinyl ether-co-maleic acid), and poly(epoxy succinic acid) oligomers. The complexation energies for Mg²⁺, Ca²⁺, Mn²⁺, and Fe³⁺ have been calculated and they have been corrected with previously determined metal specific correction parameters. The most effective ligand for all the metal ions was found to be poly(epoxy succinic acid). With Ca²⁺, poly(epoxy succinic acid) was found to form 6-coordinated complex with three metal-coordinating carboxylate oxygen, two ether oxygens, and one hydroxyl oxygen atom. All the other metals favoured 5-coordinated complexation geometry with four metal-coordinating carboxyl oxygens and one ether oxygen atom.     

Voltammetric behavior of the C60-γ-cyclodextrin inclusion complex (1:2) on hmde in an aqueous solution

Electrochemical behavior of the water-soluble C₆₀-γ-CD (1:2) inclusion complex has been studied on the hanging mercury drop electrode. A one-electron reversible adsorptive electro-reduction and three irreversible adsorptive electro-reductions were detected by cyclic voltammetry. The amount of C₆₀-γ-CD adsorbed at saturation is 2.50 × 10⁻¹¹ mol cm⁻², the diffusion coefficient is 4.36 × 10⁻⁶cm²s⁻¹ and the standard rate constant of the surface reaction k_s are 0.745 s⁻¹, 0.612 s⁻¹ and 0.513s⁻¹, respectively.     

Electro-optical studies of submonolayers of lead formed on gold electrodes by faradaic adsorption in 1 M HClO4

Current-potential and specular reflectivity-potential curves of a Au electrode have been measured in 1 M CHlO₄ in the presence of 1·0 × 10⁻³ M Pb²⁺. Four peaks were found on a linear sweep voltammogram at potentials more positive than the Nernst reversible potential of Pb⁰/Pb²⁺ couple, which are attributed to the Faradaic adsorption of Pb²⁺.

Specular reflectivity-potential curves recorded simultaneously at varied wavelengths gave a clear indication of the adsorption which increased the reflectivity at shorter wavelengths while decreased at longer wavelengths.

From the analysis of the current-potential curve of four peaks in relation to the reflectivity change, the following results were obtained:

(1) The existence of four peaks on the voltammogram is the indication of four different adsorbed states.

(2) The Pb atoms in a submonolayer on Au electrode have rather similar optical properties to those of metallic Pb except for the adsorbed atoms formed at the most potential region.

(3) The adsorbed Pb formed at the most positive potential region have the shortest distance to the surface Au atom, and accordingly, have the strongest interaction with the surface.     

Pb2+识别响应型智能高分子功能材料的研究进展

发酵生物甲烷的菌群可以吸附Pb²⁺等重金属而在沼液和沼渣中富集;因此,在生物甲烷系统研究过程中,Pb²⁺等重金属离子的检测和处理是沼液和沼渣处理的关键问题之一。18-冠-6具有选择性络合Pb²⁺的能力,研究者们将18-冠-6的Pb²⁺识别特性与聚N-异丙基丙烯酰胺的相变行为特性相结合制备了一系列Pb²⁺识别响应型智能高分子功能材料。本文综述了基于18-冠-6和聚N-异丙基丙烯酰胺的离子识别响应型智能高分子材料在检测和去除Pb²⁺等方面的研究进展。