Competitive Adsorption Process Research Articles

Cyclic voltammetric and <italic>in situ</italic> FTIR spectroscopic studies of redox of nitric oxide and carbon monoxide coadlayers on Pt electrode

The coadsorption behavior of nitric oxide (NO) and carbon monoxide (CO) on the polycrystalline Pt electrode and the influence of CO on the oxidation-reduction reactions of coadsorbed NO were studied by using cyclic voltammetric (CV) and <italic>in situ</italic> FTIR spectroscopy in acid media. The results demonstrated that the NO/CO adlayers can be adsorbed on the Pt electrode stably at 0.20 V (vs. SCE). CO existed with linear adsorption state (CO_L), and NO were coadsorded with both bridged adsorption state (NO_B) and linear adsorption state (NO_L). Due to the coadsorption of CO_L, the reduction peak potentials of NO were negatively shifted about 0.024 V, and NO_B which should have been not accessible to be oxidized were oxidized at 0.93 V. <italic>in situ</italic> FTIRS further indicated that NO can replace CO which is pre-adsorbed on the Pt electrode, and it is a competitive adsorption process between NO and CO. Both NO and CO can be converted into environment- friendly species, which are NO₃^- and CO₂ respectively. The quantity of NO₃^- depends on the adsorption quantity of CO on the Pt electrode.

Operando X-ray absorption and infrared fuel cell spectroscopy

A polymer electrolyte fuel cell enables operando X-ray absorption and infrared spectroscopy of the membrane electrode assembly catalytic layer with flowing fuel and air streams at controlled temperature. Time-dependent X-ray absorption near edge structure spectra of the Pt and Ni edge of Pt based catalysts of an air-breathing cathode show that catalyst restructuring, after a potential step, has time constants from minutes to hours. The infrared Stark tuning plots of CO adsorbed on Pt at 100, 200, 300 and 400 mV vs. hydrogen reference electrode were obtained. The Stark tuning plots of CO adsorbed at 400 mV exhibit a precipitous drop in frequency coincident with the adsorption potential. The turn-down potential decreases relative to the adsorption potential and is approximately constant after 300 mV. These Stark tuning characteristics are attributed to potential dependent adsorption site selection by CO and competitive adsorption processes.

Chemistry of Wet Treatment of GaAs(111)B and GaAs(111)A in Hydrazine-Sulfide Solutions

Chemical treatment by hydrazine-sulfide solutions, known to produce surface nitridation of GaAs(100), was applied to GaAs(111)A and B surfaces. The chemistries of these treatments for the Ga-terminated A surface and the As-terminated B one were investigated using synchrotron radiation photoemission and Auger electron spectroscopies. For the B surface, such treatment was found to produce an effective surface nitridation, via substitution of surface arsenic with nitrogen atoms from hydrazine molecules. The process automatically stops after formation of one monolayer. In contrast, the A surface is covered by sulfur bonded to underlying gallium. This extreme dependence on surface polarity is explained by competitive adsorption processes of and anions and of hydrazine molecules, on Ga-adsorption sites, which have distinct configurations on the A and B surfaces.

Competitive adsorption of binary mixture of Leptospirillum ferriphilum and Acidithiobacillus caldus onto pyrite

Leptospirillum ferriphilum and Acidithiobacillus caldus are two important acidophilic microorganisms involved in iron and sulfur oxidation during bioleaching. Cell adsorption to mineral surfaces is important for the direct leaching or contact leaching of minerals. In this study, we report the competitive adsorption of binary mixtures of L. ferriphilum LF-104 and A. caldus MTH-04 onto pyrite surfaces. The Langmuir adsorption parameter (CAm) indicated that these two bacteria underwent competitive adsorption to pyrite. Real-time quantitive PCR was used to quantify the relative amounts of L. ferriphilum and A. caldus adsorbed onto the surfaces of pyrite following exposure to a mixture of these two organisms. The adsorption of L. ferriphilum was not affected by A. caldus. However, adsorption of A. caldus was greatly affected by the presence of L. ferriphilum. Zeta-potential measurements and FT-IR spectroscopic studies showed that L. ferriphilum had a higher electrostatic attraction towards pyrite when compared to A. caldus. Based on the above results, we propose a competitive adsorption model to explain the mechanism by which L. ferriphilum and A. caldus compete in their adsorption to pyrite, although L. ferriphilum dominated in the competitive adsorption process. This work provides a better understanding of the adsorption behavior of mixed microbial populations onto mineral surfaces.

Efficient removal of chromate and arsenate from individual and mixed system by malachite nanoparticles

Malachite nanoparticles of 100–150nm have been efficiently and for the first time used as an adsorbent for the removal of toxic arsenate and chromate. We report a high adsorption capacity for chromate and arsenate on malachite nanoparticle from both individual and mixed solution in pH ∼4–5. However, the adsorption efficiency decreases with the increase of solution pH. Batch studies revealed that initial pH, temperature, malachite nanoparticles dose and initial concentration of chromate and arsenate were important parameters for the adsorption process. Thermodynamic analysis showed that adsorption of chromate and arsenate on malachite nanoparticles is endothermic and spontaneous. The adsorption of these anions has also been investigated quantitatively with the help of adsorption kinetics, isotherm, and selectivity coefficient (K) analysis. The adsorption data for both chromate and arsenate were fitted well in Langmuir isotherm and preferentially followed the second order kinetics. The binding affinity of chromate is found to be slightly higher than arsenate in a competitive adsorption process which leads to the comparatively higher adsorption of chromate on malachite nanoparticles surface.

Selectivity of Cobalt-Based Non-Platinum Oxygen Reduction Catalysts in the Presence of Methanol and Formic Acid

The reaction selectivity of non-Pt based fuel cell oxygen reduction catalysts is studied in mixed methanol/O2 and formic acid/O2 environments. Technologically relevant fuel cell reaction processes are elucidated by combining experiment and theory. Structure-to-property correlations are established utilizing a mutivariant analysis technique (MVA) for catalyst surface species as determined by XPS and electrochemical performance parameters obtained in mixed reactant systems for a family of non-Pt fuel cell catalysts. Computations at the density-functional-theory (DFT) level support our experimental findings. We used first-principle simulations to explore the interactions of cobalt functionalized graphene with H2O2, O2, CO, and N2. We find that N2 and O2 adsorb as molecular species. In contrast H2O2 decomposes to form a strongly bound Co(OH)2 complex. O2 is intermediate, the oxygen−oxygen bond lengthens by ∼10% as compared to the gas phase and its binding energy (per atom) is less strong as compared to H2O2. Two distinct reactant adsorption processes are reported to occur in the respective methanol/O2 and formic acid/O2 systems. A noncompetitive reactant surface adsorption process is reported to occur in the methanol/O2 system whereas a competitive type adsorption process occurs in the formic acid/O2 environment.

Localised corrosion of heat-treated alloys Part II – Predicting grain boundary microchemistry and its effect on repassivation potential

A methodology has been developed for predicting the effect of thermal aging on the repassivation potential of austenitic alloys. The methodology combines two models, a grain boundary microchemistry model for calculating the chromium and molybdenum depletion profiles in the vicinity of grain boundaries and an electrochemical model that relates the repassivation potential to the microchemistry and environmental conditions, including temperature and solution chemistry. The grain boundary microchemistry model incorporates a thermodynamic paraequilibrium treatment of the formation of carbides and a kinetic treatment of the diffusion of Cr and Mo. With this model, experimental Cr and Mo depletion profiles can be reproduced for sensitised alloys 600, 825 and 316L. The repassivation potential model accounts for the effects of solution chemistry and temperature by considering competitive dissolution, adsorption and oxide formation processes at the interface between the metal and the occluded site solution. Using a previously developed relationship between the repassivation potential and bulk alloy composition, a procedure has been established for calculating the observable repassivation potential of thermally aged alloys by integrating the local Erp values that correspond to the alloy microchemistry in the depletion zone. The predicted repassivation potentials agree with experimental data for thermally aged alloys 600 and 825 in chloride solutions. Additionally, the model has been applied to estimate the repassivation potential of welded samples in which segregation of alloying elements is observed.

Confocal Microscopy and Competitive Adsorption: A New Look At Polymer-Enhanced Lung Surfactant Adsorption

Lung surfactant (LS) is a mixture of lipids and proteins that lines the air-liquid interface of the alveolar walls and modulates the surface tension in the lungs. It therefore greatly reduces the mechanical work of breathing as well as prevents alveolar collapse upon expiration. Blood serum leaking into the alveoli as a result of trauma can lead to LS inhibition, which is one characteristic of acute respiratory distress syndrome (ARDS). The competitive adsorption of serum proteins, such as albumin, to the air-liquid interface of the alveoli blocks LS from forming a functional monolayer during ARDS. The addition of hydrophilic polymers, such as polyethylene glycol and chitosan, to the liquid subphase has been shown to enhance interfacial LS adsorption in vitro. Optimal amounts of polymer allow LS to form a functional monolayer in the presence of albumin, thus reversing inhibition. Albumin must be displaced from the air-liquid interface in order for a functional monolayer of LS to form. Imaging of the competitive adsorption process with confocal microscopy has allowed us to better understand the mechanisms behind forming an interfacial LS monolayer under inhibitory conditions. We can simultaneously track LS, polymer, and albumin, as well as separately visualize phenomena occurring at the interface from those occurring in the bulk. As a result of these capabilities, we have studied how various parameters affect the transport of LS to the interface and the displacement of albumin in order to form a functional surfactant monolayer.

GaAs(111)AandBsurfaces in hydrazine sulfide solutions: Extreme polarity dependence of surface adsorption processes

Chemical bonds formed by hydrazine-sulfide treatment of GaAs(111) were studied by synchrotron photoemission spectroscopy. At the B surface, the top arsenic atoms are replaced by nitrogen atoms, while GaAs(111)A is covered by sulfur, also bonded to underlying gallium, despite the sulfide molar concentration being 103 times smaller than that of the hydrazine. This extreme dependence on surface polarity is explained by competitive adsorption processes of HS- and OH- anions and of hydrazine molecules, on Ga- adsorption sites, which have distinct configurations on the A and B surfaces.

Competitive Adsorption of Toluene and n-Alkanes at Binary Solution/Silica Interfaces

The competitive adsorption of toluene and n-alkanes at binary solution/silica interfaces was studied at room temperature using IR-visible sum frequency generation vibrational spectroscopy. The surface coverage of toluene for toluene−pentane, toluene−heptane, and toluene−tetradecane mixtures was measured over the complete mole fraction range from 0 to 1. The competitive adsorption process was reversible, and the toluene coverage only depended on the bulk mole fraction, not on the history of the system. The estimated molar adsorption free energy of toluene is 3.4 ± 0.3, 1.8 ± 0.3, and 0.84 ± 0.3 kJ/mol higher than pentane, heptane, and tetradecane, respectively. Overall, toluene competes favorably on silica, and the molar adsorption free energy of alkanes increases as the chain length increases. It is consistent with the observed SFG spectra, indicating that the alkanes lie flat on the silica surface.

Fabrication and anti‐corrosion property of superhydrophobic hybrid film on copper surface and its formation mechanism

AbstractCompact and uniform superhydrophobic films were prepared on copper substrates using one‐step solution‐immersion process, and the appropriate preparation conditions were selected for mixed solutions. SEM shows that the hybrid film of 1‐dodecanethiol and tetradecanoic acid on copper substrate is more compact with the contact angle of 160°. The electrochemical impedance spectroscopy and polarization test demonstrate that the anti‐corrosion property of surface‐modified copper substrate is greatly improved, especially for the hybrid film. Moreover, the competitive adsorption process and adsorptive geometry of hybrid film were well explained based on the results of quantum chemistry calculations, SEM, and energy dispersive X‐ray analysis. Copyright © 2009 John Wiley &amp; Sons, Ltd.

Competitive Adsorption: A Physical Model for Lung Surfactant Inactivation

Charged, surface-active serum proteins can severely reduce or eliminate the adsorption of lung surfactant from the subphase to the alveolar air-liquid interface via a kinetically controlled competitive adsorption process. The decreased surfactant concentration at the interface leads to higher surface tensions during the compression of the interface during breathing. The correspondence between the factors governing colloid stability and competitive adsorption is validated via a new method of measuring surfactant and serum protein adsorption rates to the air-water interface, using quantitative Brewster angle microscopy (BAM). Competitive adsorption from a 10 mg/mL albumin subphase prevents the adsorption of lung surfactant from even high subphase concentrations due to the fast diffusion of the water-soluble proteins to the interface. The formation of an albumin film causes an electrostatic and steric barrier to subsequent surfactant adsorption, which can destroy the necessary properties of functional lung surfactant: low surface tension during compression and rapid respreading after film collapse. Surfactant inactivation is at least partially due to decreased surfactant adsorption; such decreased adsorption due to the presence of serum proteins may play a role in the development and severity of acute respiratory distress syndrome.

Adsorption behavior of acetonitrile on platinum and gold electrodes of various structures in solution of 0.5 M H 2SO 4

The variation of electrode nature and surface structure (the use of stepped single crystal faces with controlled width of (1 1 1) terraces and monoatomic steps of (1 0 0) or (1 1 0) orientation) allows to determine peculiarities of co-adsorption of acetonitrile molecules, hydrogen adatoms and (bi)sulfate anions. It has been shown that first of all acetonitrile blocks adsorption sites at the steps. Anion adsorption at terraces of stepped platinum surfaces in 0.5 M H 2SO 4 solution with additions of acetonitrile depends on terrace width and the step orientation. This demonstrates the important role of structural factors in competitive adsorption processes. The decrease in adsorption of hydrogen and anions on narrow terraces is substantially due to the influence of acetonitrile molecules placed at the steps or nearby sites. At E < 1.0 V, electrochemical conversion of acetonitrile has not been detected at single crystal Pt surfaces. However, acetonitrile oxidation might proceed on polycrystalline platinum followed by product desorption. On Au(1 1 1) surface acetonitrile adsorption is considerably weaker than that on platinum electrodes.

Naphthalene Sulfonate Functionalized Dendrimers at the Solid−Liquid Interface: Influence of Core Type, Ionic Strength, and Competitive Ionic Adsorbates

The adsorption of naphthalene disulfonic acid surface-functionalized dendrimers (generation 4) on to colloidal alumina particles is reported, considering the role of dendrimer core type (ammonia vs benzylhydrylamine-polylysine) and electrolyte addition on the adsorption affinity and interfacial packing and competitive adsorption. Irrespective of the dendrimer core type, the maximum adsorbed amount increased with increasing ionic strength. The adsorption affinity of a benzylhydrylamine-cored SPL-7013 increased with increasing ionic strength, whereas a decrease was observed for the ammonia-cored SPL-2923. At high ionic strengths (>or=10(-1) M NaCl) dendrimers close pack at the interface as an array of equivalent hard spheres, whereas at lower ionic strengths both dendrimers occupy a lower area than theoretically predicted for either cubic or hexagonal close packing, based on double layer repulsion. The additional attraction between dendrimers is attributed to the intercalation of the neighboring dendrons. Adsorption of SPL-2923 is enhanced by the presence of Ca2+ ions and depressed by the presence of HCO3- and HPO4(2-) ions, whereas SPL-7013 adsorption is only depressed by the presence of HPO4(2-) ions, suggesting a dendrimer-specific competitive adsorption process. This work clearly demonstrates the role of dendrimer architecture on adsorption at an interface, a process of fundamental importance to a wide range of dendrimer applications.

A general model for the repassivation potential as a function of multiple aqueous species. 2. Effect of oxyanions on localized corrosion of Fe–Ni–Cr–Mo–W–N alloys

A recently developed model for predicting the repassivation potential has been applied to stainless steels and nickel-base alloys in aqueous environments containing chlorides and various inhibiting anions. The model accounts for the effects of solution chemistry and temperature on the repassivation of localized corrosion by considering competitive dissolution, adsorption, and oxide formation processes at the interface between the metal and the occluded site solution. An extensive database of repassivation potentials has been established for six alloys (UNS 31603, N06600, N06690, S31254, S32205, and UNS S41425) in contact with solutions that combine chlorides with hydroxides, molybdates, vanadates, sulfates, nitrates, and nitrites at various concentrations and temperatures. Also, repassivation potentials are reported for four alloys (UNS N08367, N08800, N06625, and N10276) in chloride solutions. The database has been used to establish the parameters of the model and verify its accuracy. The model quantitatively predicts the transition between concentrations at which localized corrosion is possible and those at which inhibition is expected. It is capable of predicting the repassivation potential over wide ranges of experimental conditions using parameters that can be generated from a limited number of experimental data. The parameters of the model have been generalized as a function of alloy composition, thus making it possible to predict the repassivation potential for alloys that have not been experimentally investigated.

Electrocatalytic reduction of nitrate on copper electrode in alkaline solution

This work is devoted to the study of the kinetics and reaction mechanism of nitrate reduction on a copper electrode in 0.1 M NaOH, which acts as the supporting electrolyte. The experimental methods include cyclic voltammetry (CV), cronoamperometry (CA), controlled-potential electrolysis (CPE), and coulometry. In CV, there are three potential regions where charge transfer reactions take place, reactions which are associated with NO 3 − and/or intermediates reduction. Two isopotential points observed in CV indicate the existence of some competitive adsorption processes at the electrode surface. The three charge transfer steps were also made evident in the CA, CPE and coulometry studies. The correlation of the experimental results with the literature data led to the conclusion that NO 3 − reduction on a copper electrode in 0.1 M NaOH has an intermediate (N 2O 2 2−) species, which reduces to N 2 at a potential of about −1.3 V and to NH 4OH at potential values lower than −1.4 V (both values are vs. SCE).

Modeling Phosphate Adsorption by Agricultural and Natural Soils

The nonideal competitive adsorption model (NICA) was developed from the Langmuir adsorption theory and the Freundlich empirical equation. It is well suited to describing ion adsorption in complex systems like soils, which have multiple ions, highly heterogeneous surfaces, and a variety of adsorption sites on the particle surfaces. It has been successfully used to model the binding of protons and metal ions to humic substances, but its application for the adsorption of anions to heterogeneous surfaces has not been documented. The purpose of this study was to adapt the NICA model to describe hydroxyl and phosphate adsorption. Results show that by considering two types of surface sites, the NICA model can provide an excellent fit (R2 &gt; 0.99) of the hydroxyl adsorption data obtained from −11 to −4 of log[OH], which corresponds to soil pH from 3 to 10. By using the parameters generated from hydroxyl adsorption, including adsorption maxima (Qmax,OH), binding strength (KOH), and nonideality (m), the NICA model gave a remarkable goodness of fit (R2 &gt; 0.98) for the phosphate adsorption data obtained at different pH values. The model sensitivity test showed that the Type 1 surface (SOH2+) has up to 100 times greater contribution to phosphate adsorption than the Type 2 surface (SOH). Thus, the model may be simplified into a three‐parameter model by only considering the Type 1 surface for phosphate adsorption in acidic soils. The adapted NICA model can thus describe phosphate adsorption combined with hydroxyl adsorption and the parameters (nPO4,1/nOH,1 &lt; 1, where nPO4,1 accounts for nonideality of PO43− and nOH,1 accounts for nonideality of OH−, both on a Type 1 surface) reveal the multidentate binding of phosphate. It provides a promising tool for analyzing competitive anion adsorption processes in soils.

X-ray spectromicroscopy study of competitive adsorption of protein and peptide onto polystyrene-poly(methyl methacrylate)

A synchrotron-based x-ray photoemission electron microscope (X-PEEM) was used to investigate the coadsorption of a mixture of human albumin serum and SUB-6, a synthetic antimicrobial peptide, to a phase-segregated polystyrene/poly(methyl methacrylate) (PMMA) substrate at varying concentrations and pH. The authors show that X-PEEM could detect the peptide adsorbed from solution at concentrations as low as 5.5 x 10(-9)M and could differentiate the four components via near-edge x-ray absorption fine structure spectromicroscopy. At neutral pH the SUB-6 peptide adsorbed preferentially to PMMA. At a pH of 11.8 where the charge on the peptide was neutralized, there was a more balanced adsorption of both species on the PMMA domains. The authors interpret these observations as indicative of the formation of an electrostatic complex between positive peptide and negative protein at pH of 7.0. This solution complex had an adsorption behavior that depended on the polarity of the substrate domains, and favored adsorption to the electronegative PMMA regions. At a pH of 11.8 the complex formation was suppressed and a more competitive adsorption process was observed.

Competitive protein adsorption to polymer surfaces from human serum

Surface modification by "soft" plasma polymerisation to obtain a hydrophilic and non-fouling polymer surface has been validated using radioactive labelling. Adsorption to unmodified and modified polymer surfaces, from both single protein and human serum solutions, has been investigated. By using different radioisotopes, albumin and Immunoglobulin G (IgG) adsorption has been monitored simultaneously during competitive adsorption processes, which to our knowledge has not been reported in the literature before. Results show that albumin and IgG adsorption is dependent on adsorption time and on the presence and concentration of other proteins in bulk solutions during adsorption. Generally, lower albumin and IgG adsorption was observed on the modified and more hydrophilic polymer surfaces, but otherwise the modified and unmodified polymer surfaces showed the same adsorption characteristics.

The influence of surface energy on competitive protein adsorption on oxidized NiTi surfaces

NiTi shape memory alloy surfaces, untreated, and oxidized by a new oxidation treatment (OT) in order to obtain a Ni-free surface, have been compared in terms of surface energy and protein adsorption behavior. The polar and dispersive components of the surface energy have been determined. A competitive adsorption process between fibronectin and albumin has been carried out by 125I-radiolabeling. Moreover, the adhesion strength between both proteins and NiTi surfaces has been evaluated by performing an elution test. The results show that the OT treatment enhances the hydrophilic character of NiTi surfaces by significantly increasing the polar component of their surface energy. Moreover, the OT treatment increases the amount of fibronectin and albumin adsorbed. It also increases the fibronectin affinity for NiTi surfaces. The elution test results could suggest a conformational change of fibronectin as a function of chemical composition of NiTi material and of surface treatment. Finally, a linear correlation between the amount of adsorbed albumin and the polar component of the surface energy of NiTi surfaces has been demonstrated. This work indicates that the OT treatment has an influence on the surface energy value of NiTi materials, which in turn influences the protein adsorption process.