Competitive Desorption Research Articles

Competitive Adsorption-Desorption Kinetics of Phenoxyacetic Acids and a Chlorophenol in Volcanic Soil

Competitive adsorption and desorption kinetic experiments with 2,4-dichlorophenoxyacetic acid (2,4-D), 2-methyl-4-chlorophenoxyacetic acid (MCPA) and para-chloro-ortho-cresol in a volcanic soil clearly demonstrated that the rate of diffusion in mixtures of these chemicals are different compared to single solute values and therefore the assumption of single diffusion coefficient values may not be valid in many realistic situations. In the present article, the experimental data obtained from the batch reactor were compared with numerical calculations of a homogeneous surface diffusion model, developed for bicomponent systems. The model used the single component Freundlich parameters and the competition coefficients from bicomponent sorption experiments. The results showed that the adsorption surface diffusion coefficients were smaller by 12, 4 and 6 % for 2,4-D, MCPA and PCOC, respectively, while for desorption they were smaller by 4 – 8%, which is attributed to competition between the solutes. In general, good agreement between the numerical and experimental results was obtained for all cases considered.

Stochastic simulations of temperature programmed desorption kinetics

In principle, temperature programmed desorption (TPD) can yield a wealth of information on the rates of surface reactions. In practice, however, it is often difficult or impossible to extract such information from the experimental data. We describe an accurate stochastic modelling technique that facilitates the determination of mechanisms and rate constants from TPD measurements. The model for a particular reaction or process is cast in terms of a detailed mechanism, which can be set up in a variety of ways depending on the primary characteristics of the system under study. Starting from experimental rate constants obtained under limiting conditions, a series of simulations are used to estimate and refine unknown rate constants. Three types of examples are given to illustrate the methodology used. First, simulations of desorption of Xe from Pt(111) are used to show how a very simple system can be modelled. By comparing experimental data and calculations performed with several different mechanisms, we explore the kinetic consequences of various modes of desorption from defect sites. Next, we investigate the competitive desorption and dissociation of CO on W(110). In this case the model successfully reproduces experimental TPD and surface coverage data, allowing experimental rate constants to be optimized. Finally, we show that stochastic simulations can be used to study systems with strong repulsive lateral interactions using a model which explicitly tracks changes in the environment of a desorbing species with temperature and coverage. Previous Monte Carlo simulations of desorption from a square lattice are reproduced using this approach. It is also applied to desorption of CO from Ru(001), yielding simulated TPD spectra in excellent agreement with experiment and allowing the magnitude of repulsive interactions to be estimated. In both the square and hexagonal lattice systems TPD data are simulated with coverage-independent pre-exponential factors: all features of the spectra are attributable to variations in populations of surface species alone. The stochastic method is shown to be valuable as an integral part of an experimental study, allowing detailed models to be proposed and refined, and yielding microscopic data which are amenable to theoretical analysis.

Adsorption, desorption, and decomposition of HCl and HBr on Ge(100): Competitive pairing and near-first-order desorption kinetics

We have investigated the surface chemistry of coadsorbed hydrogen and halogen atoms on Ge(100), produced by dissociative chemisorption of HCl and HBr, by temperature-programmed desorption. The initial sticking probability S0 for HCl decreases from 0.6 at a substrate temperature of 270 K to 0.05 at 400 K, indicative of a precursor state to adsorption. For HBr S0 is constant at 0.7 over the same temperature range. A fraction f of adsorbed hydrogen atoms desorb associatively as H2 near 570 K, while the remaining (1−f) H atoms recombine with adsorbed halogen atoms and desorb as the hydrogen halide (HX) near 580–590 K. The activation energies for desorption of H2, HCl, and HBr are all approximately 40 kcal/mol. For both HCl and HBr f is 0.7 at low initial coverage and decreases slightly to 0.6 at saturation. The fraction f of adsorbed halogen atoms left on the surface following the competitive desorption of H2 and HX desorb as the dihalides GeCl2 and GeBr2 near 675 and 710 K, respectively. Desorption of H2, HCl, and HBr occurs with near-first-order kinetics, similar to the behavior of hydrogen adsorbed alone, which we attribute to preferential pairing induced by the π bond on unoccupied Ge dimers. We introduce and solve a generalized doubly occupied dimer model incorporating competitive pairing of H+H, H+X, and X+X on Ge dimers to explain the near-first-order kinetics. The model quantitatively accounts for both the desorption kinetics and the relative yields of H2 and HX with pairing energies of ≊3 kcal/mol. Implications of the present results for surface thermochemistry, chemical vapor deposition, and atomic layer epitaxy of Ge and Si(100)2×1 surfaces are discussed.

The coadsorption of water and molecular oxygen on Ag(110): absence of OO bond activation by water

The coadsorption of molecular oxygen and water on Ag(110) have been examined using temperature-programmed reaction mass spectrometry. We find no evidence for a reaction between water and molecularly chemisorbed oxygen, in contrast to a recent report concerning water and oxygen on Ag(111). The data do suggest, however, a mechanism for the competitive desorption and dissociation of molecularly adsorbed oxygen on Ag(110). O 2 dissociation appears to displace some molecularly adsorbed oxygen into the gas phase. Furthermore, sites for O 2 dissociation are blocked by coadsorbed water, thus leading to altered kinetics for desorption of O 2(a).

Competitive Adsorption and Desorption of 2,4-D and PCOC in a Volcanic Soil

The competitive adsorption and desorption of 2,4-D and PCOC, two of the commonly used phenoxy herbicide chemicals, in a volcanic soil has been examined. A Freundlich-type isotherm incorporating competition between the solutes was used to analyze the adsorption and desorption data. The competition coefficients for 2,4-D and PCOC were linearly dependent on the initial concentration for adsorption and on the amount adsorbed for desorption. The adsorption capacities of 2,4-D and PCOC in a bicomponent system were reduced by 10% and 8%, respectively, in comparison to their single-component values. The desorption studies showed that both 2,4-D and PCOC were difficult to desorb in a bicomponent system.

The influence of temperature and surface composition upon the coordination of acetone to the Pd(111) surface

The adsorption and reaction of (CH 3) 2CO and (CD 3) 2CO were examined on both clean and oxygen-dosed Pd(111) surfaces using HREELS and TPD. On the clean surface acetone was initially adsorbed into a slightly disordered phase that rearranged into η 1-acetone (“end-on coordination”) and η 2-acetone (“side-on coordination”) species upon heating to 200 K. These two acetone adsorption configurations were identified by their vibrational spectra. Both HREELS and TPD demonstrated that acetone adsorbed in the η 1 configuration interacted weakly with the Pd(111) surface. These species either desorbed at 200 K without reacting or converted to η 2-acetone during annealing the adiayer. In contrast, the interaction between the palladium surface and η 2-acetone species was stronger than observed for η 1 species. The higher surface-ad-sorbate bond strengths of η 2-acetone resulted in competitive desorption and decomposition pathways for these species. On the oxygen-dosed Pd(111) surface, acetone adsorbed preferentially in the η 1 configuration due to electronic factors arising from the increased Lewis acidity of this surface. The sequence of reactions involved in acetone decomposition is discussed along with the factors influencing the coordination of acetone in the η 1 and η 2 configurations. These results are compared with previous studies of acetone adsorption on the Ru(001) and Pt(111) surfaces, and serve to illuminate the influence of the surf ace-adsorbate bond upon the selectivity of acetone reactions on surfaces of Group VIII metals.

Adsorption and dissociation of HCN on the Pt(111) and Pt(112) surfaces

The interaction of HCN with the Pt(111) and Pt(112) surfaces has been studied with temperature programmed desorption (TPD), scanning kinetic spectroscopy (SKS), LEED and work function measurements (Δφ) in the temperature range 87 to 1000 K. The only desorption products observed were H 2, C 2N 2 (cyanogen) and HCN. Labelling studies utilizing D 13CN + HCN revealed that HCN desorbs molecularly in three different low temperature states, while higher temperature β states were due to recombination of H(a) + (CN)(a). Different β-state desorption behavior was observed on Pt(112) which may be due to competitive desorption effects. Evidence suggests that the highest temperature molecular desorption state of HCN may act as a precursor to dissociation. The high step density Pt(112) surface was found to be more effective in promoting dissociation of HCN. The following sequence of LEED patterns was observed on Pt(111) with decreasing coverage of HCN beginning with monolayer coverage: (3×3) → c(4×2) → (2×2) → (1×1). Both HCN(a) and CN(a) were found to give an overall work function decrease on Pt(111) yielding values of −1.2 and −0.8 V, respectively at saturation. LEED measurements in conjunction with TPD and Δφ results suggest that HCN is bonded in a linear fashion at saturation with the nitrogen bonded to Pt. After dissociation, CN(a) is likely bonded with the molecular axis parallel to the Pt surface.

Interactions between Human Plasma Proteins and Heparin-Poly(Methyl Methacrylate) Copolymer

A solid Heparin-PMMA copolymer has been synthetized by a radical polymerization of methyl methacrylate from oxidative reaction initiated by Ce4+ ions in the presence of heparin. Covalently linked heparin was 10% of copolymer weight. The antithrombin activity of the copolymer corresponded to 1% of grafted heparin. PMMA sequence of the copolymer played the leading role in fibrinogen, immunoglobulins, transferrin and albumin adsorption. These proteins adsorbed on the copolymer, showed different competitive desorption pattern in the presence of whole plasma: fibrinogen presented the highest degree of affinity for the copolymer. The heparin part of the copolymer was responsible for antithrombin III adsorption and for decrease of factor V activity. Active antithrombin III was eluted. An inactivation of factor V in plasma was observed using high concentrations of soluble heparin. This result suggested that copolymer heparin chains, even devoid of antithrombin activity were involved in this inactivation. With Heparin-PMMA copolymer, plasma clotting pro-enzymes behaved differently than on heparin-sepharose copolymer:disappearance of factor XI activity, decrease in prekallikrein activity and activation of factor IX were observed. PMMA sequences were responsible for factor IX activation.

Surface chemistry of the non-basal planes of cobalt: The structure, stability, and reactivity of Co(101̄2)-CO

The structural and chemisorptive properties of the stepped, non-unique, (101̄2) surface of cobalt have been investigated by standard LEED/Auger/Δφ/thermal desorption methods. The clean surface is well-ordered, unreconstructed, and reversibly undergoes the predicted structural changes on cycling through the phase transition. CO chemisorption is rapid and non-dissociative at 300 K, leading ultimately to a (3 × 1) structure with a COCO spacing of 3.8 Å. Heating of the adlayer can, depending on the conditions, lead to competitive desorption and dissociation reactions. The data suggest that the transition state to desorption is mobile whereas that for dissociation is localised. Dissociation is accompanied by diffusion of oxygen into the bulk and formation of a very well-ordered (2 × 3) carbon structure. This structure is interpreted in terms of epitaxial growth of the (001) plane of Co 3C. The carbide surface is still capable of chemisorbing a substantial amount of CO, but cannot dissociate it. Some other ordered phases of the CoCCO system are also observed, and an attempt is made to interpret them in a consistent way. The CO chemistry of the (101̄2) surface is very different from that of the basal plane.

Quantification of the EEG in Fourier analysis

Metal hydroxides (e.g. ferrihydrite) present in geomedia play significant roles in regulating the environmental mobilities of arsenate (As(V)) and inorganic phosphate (Pi) because of their high adsorption affinities for these oxyanions. In this study, results are presented of experiments aimed at determining individual and competitive adsorption/desorption kinetics of As(V) and Pi on ferrihydrite at pH 4 and 8. Selected samples were also subjected to As K-edge EXAFS study for understanding the changes with time in As(V) complexation on ferrihydrite in the presence/absence of Pi. Both oxyanions showed similar behavior in single ion adsorption experiments. However, when both oxyanions were loaded together, more As(V) was adsorbed than Pi. Furthermore, more pre-equilibrated Pi was desorbed by sequentially added As(V) than vice versa. Interactions of As(V) and Pi with ferrihydrite slowed down after the initial rapid adsorption/desorption. The experimentally determined adsorption/desorption kinetic data for As(V) and Pi showed good compliance with pseudo-second order, Elovich, and power-function equations. Both oxyanions competed for adsorption on ferrihydrite, and each of them showed a limited capacity to desorb the other. EXAFS analysis of selected samples indicated the presence of mononuclear (2E) and binuclear (2C) bidentate As(V) surface complexes. The Fe coordination numbers (CN) increased with increasing time and decreased with addition of Pi into the system. A higher proportion of Fe CN associated with 2E As(V) surface complexes decreased after the addition of Pi, compared to Fe CN associated with 2C As(V) surface complexes. The competitive desorption study indicates that the excessive input of Pi due to the overuse of fertilizers could mobilize As(V) from contaminated geomedia. Furthermore, insights into Pi-induced desorption of As(V) could also provide an opportunity for developing chemical treatment methods to intercept the mobilized As(V) by co-precipitation in apatite-like phases.