The effect of platinum on the SnO<Subscript>2</Subscript> sorption properties

Chemisorption of SO₂ and O₂ at Pt-modified SnO₂ is studied by using the vacuum static method, with simultaneous recording of electrical conductivity, over the 22 to 300°C temperature range. The SnO₂ surface modification results in the increasing of SO₂ adsorption and weakening of the gas-surface bonding. The chemisorption enhances the samples’ electrical conductivity. The surface pretreatment with oxygen leads to the decreasing of the successive SO₂ chemisorption.     

Adsorption of hydrogen on palladium film nanostructures

The mechanism and kinetics of hydrogen adsorption on Pd/Ni nanostructures on an Al₂O₃ surface were studied. The dependence of electrical conductivity on the degree of hydrogen adsorption and the temperature effect on the rate of adsorption were determined.     

Reversible adsorption of spherical particles from binary mixtures: long-time behaviour

Formation of monolayers of spherical particles in processes with reversible adsorption from mixtures of large and small particles was simulated in computer experiments. Computer program was based on an algorithm that took into account random sequential adsorption, desorption and lateral diffusion of adsorbed particles (RSA–DLD model). Computer experiments were performed for systems with rate constants of particle adsorption at least 10³ times higher than rate constants of desorption. In processes with very fast adsorption and slow desorption, formation of monolayer can be divided into two stages. During the first stage, the total surface coverage (the coverage with particles of both types) increases very fast and becomes very close to that at equilibrium. During the second stage, the total coverage changes very slowly and the system approaches equilibrium mainly by the replacement of large particles with the small ones. A simple kinetic model for evolution of the monolayer composition during the second stage has been proposed. Kinetic equations related to this model allow the determination of large particles’ desorption rate constants on the basis of changes in the surface concentrations of adsorbed large and small microspheres. The validity of the model has been tested comparing large particles’ desorption rate constants values that had been used for simulations with values of the corresponding rate constants determined using analytical equations, with a view to analysing the simulation results. To cite this article: S. Slomkowski et al., C. R. Chimie 6 (2003).     

Electroreduction of carbon dioxide: Part III. Adsorption and reduction of CO2 on platinum

The main properties of reductional adsorption of CO₂ on the platinum metals are studied. Chemisorbed particles are found to be produced only on platinum and rhodium. Electroreduction of CO₂ on these metals proceeds as a result of the interaction of CO₂ molecules activated in the course of adsorption on the metal surface with chemisorbed hydrogen. As a result, strongly chemisorbed particles are obtained on the surface which are the products of more profound reduction of CO₂ than to formic acid. The further reduction of these chemisorbed particles, accompanied by their desorption into the solution, is very slow due to very strong coupling of sorbed particles with the surface and to very fast backward adsorption of the reduction products. Neither reductional chemisorption of CO₂ nor interactions of CO₂ with adsorbed hydrogen were observed for iridium, palladium, osmium or ruthenium.     

Kinetics of Oleate Adsorption at a Nigerian Hematite-Water Interface

The rate of adsorption of oleate soap onto a Nigerian hematite in an aqueous medium was determined from 29 to 60°C using the differential analysis method. The activation energy and frequency factor were determined at 57.1 kJ mol⁻¹ K⁻¹ and 4.0 × 10³ liter mol⁻¹ min⁻¹, respectively, indicating that the chemical processes are the slow, rate-determining step and that the reaction proceeds relatively fast. The adsorption isotherm was the Langmuir type: chemisorption was considered the dominant mode of adsorption. The desorption isotherm indicated a minor contribution of physical adsorption to the overall adsorption process.     

On Reaction of Sulfur Dioxide with Aqueous Solutions of Carbamide

A mechanism of SO₂ chemisorption by aqueous solutions of carbamide was suggested. The effective rate constants of accumulation of hydrogen ions in the reaction of sulfur dioxide with aqueous solutions of carbamide in the range 278-313 K were determined.     

The special features of protein adsorption isotherms on silica adsorbents

The adsorption isotherms of hemoglobin, peroxidase, and β-galactosidase on silochrome and mesoporous and biporous silicas were comparatively studied. Adsorption developed in two stages, including fast “reversible” protein adsorption (equilibrium was reached in t ≤ 1–2 h) and a “slow stage” of irreversible binding in t ≫ 24 h (multipoint adsorption). The corresponding equilibrium constants were determined. The mechanism of unlimited linear association of peroxidase in the adsorption layer on the surface of silochrome was established.     

二维TiC 层表面H2的化学吸附与物理吸附

第一性原理计算研究发现由于二维TiC 单原子层具有高的比表面积与大量的暴露在表面的Ti 原子,其是一种非常有潜力的储氢材料. 计算结果显示H₂可以在二维TiC 单原子层表面进行物理吸附与化学吸附. 其中化学吸附能为每个氢分子0.36 eV,物理吸附能是每个氢分子0.09 eV. 覆盖度为1和1/4层（ML）时,H₂分子在二维TiC 单原子层表面的离解势垒分别为1.12 和0.33 eV. 因此,除了物理吸附与化学吸附,TiC 表面还存在H单原子吸附. 最大的H₂储存率可以达到7.69%（质量分数）. 其中,离解的H原子、化学吸附的H₂、物理吸附的H₂的储存率分别为1.54%、3.07%、3.07%. 符合Kubas吸附特征的储存率为3.07%. 化学吸附能随覆盖度的变化非常小,这有利于H₂分子的吸附与释放.     

Method for measuring the self-assembly of alkanethiols on gold at femtomolar concentrations

We describe a cantilever-based method for measuring the self-assembly of alkanethiols on a gold surface in a flow system that permits easy step changes in concentration and acquire a continuous in situ measure of the resulting chemisorption through the change in resonance frequency. A gold-coated (2.2 mm2), piezoelectric-excited, millimeter-sized cantilever (PEMC) sensor was exposed to 1-hexadecanethiol (HDT) in ethanol at concentrations ranging from 1 fM to 1 mM, sequentially and separately. A high-order flexural mode at approximately 850 kHz was monitored during the self-assembly. The resonance frequency decreases as a result of increased mass as chemisorption occurs on the surface. We show for the first time that the chemisorption of HDT at 1 fM is readily measurable and gave a response of 220 +/- 13 Hz (n = 4). At higher concentrations (10 and 100 fM; 1, 10, and 100 pM; 1, 10, and 100 nM; 1 microM; and 1 mM), the responses were proportionately, but nonlinearly, higher. At high concentrations (1 mM), the responses to C4, C8, C11, C16, and C18 alkanethiols were linearly proportional and were complete in approximately 25 min. We report for the first time that, once the Au surface is equilibrated at 1 pM, further chemisorption at a lower HDT concentration does not take place, even though over 99% of surface adsorption sites are available. At 1 fM, the overall chemisorption rate did not increase with a 2-fold increase in the HDT flow rate, suggesting that chemisorption at 1 fM is not transport-limited. The measured overall chemisorption rate constant at 1 fM was more rapid than 0.1 min-1.     

Influence of alloying platinum for the hydrogenation of p-chloronitrobenzene over PtM/Al2O3 catalysts with MSn,Pb, Ge,Al, Zn

Hydrogenation of p-chloronitrobenzene (CNB) has been studied, in methanol suspension, at 303 K and atmospheric pressure, over a series of Pt/Al₂O₃ and PtM/Al₂O₃ catalysts (MSn, Pb, Ge, Al, Zn). The bimetallic catalysts were prepared by the so-called controlled surface reaction method by reacting organometallic compounds on the well-dispersed Pt/Al₂O₃ parent sample. The kinetics obeys a modified Langmuir-Hinshelwood model with competitive adsorption between CNB and hydrogen. The competition of adsorption no longer holds at high CNB concentration: on the Pt surface saturated by CNB, there are still Pt sites available for the chemisorption of the smaller hydrogen molecule. A rate law is then proposed which allows both adsorption and rate constants to be determined. Upon addition of the second metal M, the specific activity per Pt surface atom goes through a maximum value around a chemical composition M/Pt≌0.1–0.2. It is suggested that an electron-deficient species of the second metal promotes the reaction rate by activating the nitrogenoxygen bond; ionic tin species are better promoters for this purpose. At high CNB conversion (> 98%) the yield of the desired product, p-chloroaniline (CAN), increases from 82% on pure Pt/Al₂O₃ to 97.5% on PtSn/Al₂O₃ (Sn/Pt = 0.36). The improvement of selectivity to CAN is not due to a decrease in CAN dehalogenation rate, which is not affected by alloying. Actually, the promoting effect of the second metal decreases the relative strength of adsorption between CAN and CNB up to a factor of 10: CAN is then easily removed from the surface by CNB, thus preventing hydrodechlorination.     

Surface characterization and heats of adsorption of chromatographic alumina gel

The porous nature of chromatographic alumina gel has been investigated by adsorption/condensation processes and electron microscopy. Having 63% porosity, the gel is very porous. Total pore volume as determined by the fluid-displacement method is 0.497 cm³ g^–1. Its specific surface area, as determined by water vapor adsorption, is 225 m² g^–1. Micropore volume, as determined by utilizing Gurwitsch's rule, turns out to be 0.262 cm³ g^–1. The greater portion of the surface area and pore volume occurs in small and transitional pores, with average pore radii (hydraulic) less than 2.1 nm.Organic vapors, such as methyl ethyl ketone, acetone, methyl acetate, and methyl alcohol, were adsorbed on the gel between 0 and 36°C under vacuum, and the data were recorded on a Cahn-1000 electrobalance device. Isosteric heats of adsorption were calculated by applying the Clausius Clapeyron equation to the adsorption isosters at different surface coverages. Two types of adsorption processes, one with low activation energy and other with high activation energy can be distinguished. The increase in values ofq _st indicates that increasing temperature changes physical adsorption into chemisorption.     

Untersuchungen an Nickeloxid-Mischkatalysatoren. XI. Oberflächenchemische Eigenschaften von NiO/SiO2-Katalysatoren

Studies on Nickel Oxide Mixed Catalysts. XI. Surface Chemical Properties of NiO/SiO₂ Catalysts NiO/SiO₂ catalysts with different composition prepared by precipitation have been investigated which are characterized by a nickel layersilicate-like structure. The determination of the surface chemical properties was carried out by infrared spectroscopy (before and after pyridine adsorption), ¹H-NMR-measurements, chemisorption of ammonia, and titration with n-butylamine. It has been found three kinds of hydroxid groups which are assigned to Ni? OH and Si? OH groups with respect to investigations on definite nickel layersilicates. Furthermore, coordinatively unsaturated Ni^II surface sites were indicated. The number of the OH groups as well as the centers determined by chemisorption of bases increase with increasing NiO content. The obtained results allow the conclusion that the OH groups and the coordinatively unsaturated Ni^II surface sites are weakly acid.     

Thermodynamic parameters of adsorption of adamantane and its derivatives on graphitized thermal carbon black

The adsorption equilibrium constants for adamantane, 1-fluoro-, 1-chloro-, 1,3-difluoro-, 1,3-dichloro-, 1,3-dibromo-, and 1-hydroxyadamantane, and methyl 1-adamantyl ketone were determined by gas chromatography. The results were compared with molecular statistical calculations based on the known atomic-atomic potentials of the interaction of atoms of the sorbate molecule with the C atom of graphitized thermal carbon black (GTCB). The experimental adsorption heats exceed the calculated values by 3-10 kJ mol^-1. The reasons for this divergence are discussed. The changes in the adsorption entropy show that the molecules of the studied compounds form a layer of the ideal dimeric gas on the GTCB surface upon adsorption.     

Adsorption and electrophysical properties of semiconductors of the InSb-CdS system

Adsorption and the electrophysical properties (changes in electrical conductivity) of thin film solid solutions and binary components of the new InSb-CdS system are studied by means of piezoquartz microweighing, IR spectroscopy and probe compensation in the temperature range of 273–353 K and a range of adsorbate gas (NH₃, NO₂) pressures of 0.2–9.8 Pa. The areas of physical and chemical adsorption accompanied by the charging of the (mostly positive) surface are determined. The mechanisms and regularities of adsorption as a function of external conditions and composition of the system are established, and certain similarities and patterns of changes in the adsorption and electroconductivity, and parallelisms of the “acidbase characteristic-composition,” “adsorption characteristic-composition,” and “electrophysical characteristics-composition” dependences are described. These prove to be useful in studying the adsorption mechanism and in identifying the most active components of the system relative to NH₃ and NO₂, proposed as materials for respective sensors/detectors.     

Kinetics of adsorption and desorption of sodium alkyl sulfates to and from nylon particles

The desorption rate of surfactant ions from nylon particles was investigated at the concentrations below the critical micelle concentration by applying the stopped-flow method. A mixing cell of stopped-flow spectrophotometer was modified with platinum electrodes for electric conductivity detection. The change in electric conductivity with time in the desorption process was monitored by a memory-recorder system. The surfactants used were sodium decyl, sodium dodecyl, sodium tetradecyl, and sodium hexadecyl sulfates. The desorption rate was independent of the surfactant concentration and the rate constants were obtained by applying the first-order reaction scheme. The adsorption rate constants were estimated from the experimental desorption rate constants and equilibrium constants assuming the second-order kinetics. The desorption rate constants were determined to be 1–6 sec^–1 and the adsorption rate constants to be 2–8×10⁴ mol^–1 dm³ sec^–1; the former decreased and the latter increased with increasing number of carbon atoms in alkyl chain of the surfactants.     

Reactivity of titanium dioxide with oxygen at room temperature and the related charge transfer

The measurements of electron work function were applied for in situ monitoring of the charge transfer during oxidation and reduction for well-defined titanium dioxide, TiO 2, at room temperature. The TiO 2 specimen was initially standardized at 1173 K in the gas phase of controlled oxygen activity, at p(O 2) = 10 Pa, and then cooled down in the same gas phase. The work function changes were monitored during oxidation at room temperature at p(O 2) = 75 kPa and subsequent reduction at p(O 2) = 10 Pa. It is shown that oxidation of TiO 2 at room temperature results in fast oxygen chemisorption, involving initially the formation of singly ionized molecular oxygen species, followed by the formation of singly ionized atomic oxygen species, and subsequent slow oxygen incorporation. Although all these processes lead to work function increase, the components of the work function changes related to the individual processes may be distinguished based on different kinetics. The obtained work function data indicate that oxidation results in rapid surface coverage with singly ionized molecular oxygen species, which are subsequently dissociated leading to the formation of singly ionized atomic species. The related chemisorption equilibria are established within 2 and 5 h, respectively. Oxygen incorporation leads to slow work function changes, which achieve a maximum within 100 h. The determined work function data were assessed by using a theoretical model that describes the electrical effects related to different mechanisms of TiO 2 oxidation. The work function data indicate that oxygen incorporation leads to structural changes of the outermost surface layer resulting, in consequence, in a change of the external work function component. Reimposition of the initial gas phase, p(O 2) = 10 Pa, leads to partial desorption of weakly adsorbed molecular species formed during oxidation.     

Oxygen and nitrogen tailoring carbon fiber aerogel with platinum electrocatalysis interfaced lithium/sulfur (Li/S) batteries

Sluggish kinetics of lithium/sulfur (Li/S) conversion chemistry and the ion channels formation in the cathode is still a bottleneck for developing future Li/S batteries with high-rate, long-cycling and high-energy. Here, a rational cathode structure design of an oxygen (O) and nitrogen (N) tailoring carbon fiber aerogel (OCNF) as a host material integrated with platinum (Pt) electrocatalysis interface is employed to regulate Li/S conversion chemistry and ion channel. The Pt nanoparticles were uniformly sprayed onto the S surface to construct the electrocatalysis interface (Pt/S/OCNF) for generating ion channels to promote the effective penetration of electrolyte into the cathode. This Pt/S/OCNF gives the cathode a high sulfur utilization of 77.5%, an excellent rate capacity of 813.2 mAh/g (2 C), and an outstanding long-cycling performance with a capacitance retention of 82.6% and a decay of 0.086% per cycle after 200 cycles at 0.5 C. Density functional theory (DFT) calculations reveal that the Pt electrocatalysis interface makes the cathode a high density of state (DOS) at Fermi level to facilitate the electrical conductivity, charge transfer kinetics and electrocatalysis to accelerate the lithium polysulfides (LiPSs) electrochemical conversion. Furthermore, the unique chemisorption structure and adsorption ability of Li₂S_n (n = 1, 2, 4, 6, 8) and S₈ on OCNF are attributed to the bridging effects of interfacial Pt and the bonding of N-Li. The Pt electrocatalysis interface combined with the unique 3D hierarchical porous structure and abundant functional active sites at OCNF guarantee strong adsorption confinement, fast Li/S electrocatalytic conversion and unblocked ion channels for electrolyte permeation in cathode.     

ATR-FTIR studies on the adsorption/desorption kinetics of dimethylarsinic acid on iron-(oxyhydr)oxides

Dimethylarsinic acid (DMA) is an organoarsenical compound that, along with monomethylarsonic acid, poses a health and an environmental risk, and a challenge to the energy industry. Little is known about the surface chemistry of DMA at the molecular level with materials relevant to geochemical environments and industrial sectors. We report herein the first in situ and surface-sensitive rapid kinetic studies on the adsorption and desorption of DMA to/from hematite and goethite at pH 7 and I = 0.01 M KCl using ATR-FTIR. Values for the apparent rates of adsorption and desorption were extracted from experimental data as a function of spectral components, flow rate of the aqueous phase, film thickness of hematite, and using chloride and hydrogen phosphate as desorbing agents. The adsorption kinetic data show fast and slow rates, consistent with the formation of more than one type of adsorbed DMA. Apparent adsorption and desorption rate constants were extracted from the dependency of the initial adsorption rates on [DMA(aq)]. Desorption rate constants were also extracted from desorption experiments using hydrogen phosphate and chloride solutions, and were found to be higher by 1-2 orders of magnitude than those using chloride. In light of the complex ligand exchange reaction mechanism of DMA desorption by phosphate species at pH 7, apparent desorption rate constants were found to depend on [hydrogen phosphate] with an order of 0.3. The impact of our studies on the environmental fate of DMA in geochemical environments, and the design of technologies to reduce arsenic content in fuels is discussed.     

FTIR investigation of 2-chlorophenol chemisorption on a silica surface from 200 to 500 degrees C

The time-dependent chemisorption of 2-chlorophenol on a fumed silica surface was studied in situ from 200 to 500 degrees C using a temperature-controlled dosing cell and FTIR absorption spectroscopy. 2-Chlorophenol was found to chemisorb at isolated and geminal surface hydroxyl sites. 2-Chlorophenol chemisorption and subsequent surface oxidation resulted in a mixture of chlorophenolate and partial oxidation products, such as formates and acetates. The rates of chemisorption were measured, and the activation energy of adsorption was found to be 15 +/- 4 kJ mol(-1) for a fast, initial reaction and 22 +/- 2 kJ mol(-1) for a slower reaction at higher surface coverage. This work was motivated by the observation that combustion-generated fly ash mediates the formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F) at temperatures between 250 and 450 degrees C. Although transition metals such as copper are known to catalyze or mediate this reaction, silica is the major component of fly ash and chemisorption at higher concentration surface sites of silica must have a significant impact on the surface-mediated PCDD/F formation on fly ash surfaces.