Littérature scientifique sur le sujet « Coadsorption complexes »

Heavy metals and metalloids are common cooccurrence in contaminated soils, making their behaviors more complex than their individual presences. Adsorption to soil minerals and organic components determines the solubility and mobility of heavy metals. However, little information is available regarding coadsorbing metals (e.g., Cd) and metalloids (e.g., Sb) to soil components, and whether there is a universal coadsorption rule needs to be illuminated. This study investigated the coadsorption behaviors of Cd(II) and Sb(V) to goethite, kaolinite, and bacteria (Bacillus cereus) at both acidic (pH 4.5) and alkaline pH (pH 8.5). Equilibrium adsorption experiments, coupled with scanning electron microscopy- (SEM-) energy-dispersive X-ray spectrum (EDS) and X-ray photoelectron spectroscopy (XPS), were applied to determine the batch adsorption phenomena and possible mechanisms. Batch results showed that Cd(II) adsorption was greater at pH 8.5 whereas Sb(V) adsorption was greater at pH 4.5. The presence of Cd or Sb promoted each other’s adsorption to goethite, kaolinite, and bacteria, but slight differences were that Sb(V) preferred to enhance Cd(II) adsorption at acidic pH, whereas Cd(II) was more able to increase Sb(V) adsorption at alkaline pH. SEM-EDS analyses further showed that the distribution of Cd and Sb was colocalized. The surface FeOH, AlOH, and COOH groups participated in the binding of Cd(II) and Sb(V), probably through the formation of inner-sphere complexes. Two possible ternary complexes, i.e., sorbent-Cd2+-Sb(OH)6– and sorbent-Sb(OH)6–-Cd2+, were possibly formed. Both the charge effect and the formation of ternary complexes were responsible for the collaborative coadsorbing of Cd-Sb. The universal synergistic rule obtained suggests that current models for predicting Cd(II) or Sb(V) sequestration based on single systems may underestimate their solid-to-liquid distribution ratio in a coexistence situation. The results obtained have important implications for understanding the chemical behavior of Sb and Cd in contaminated soils.

The coadsorption of oxygen and sodium on Ni(111) was studied by high resolution electron energy loss spectroscopy. Measurements were performed for a definite sodium precoverage, as a function of oxygen exposure and temperature. For a coverage of two layers of sodium, the loss spectra show the vibrational features related to Na– O 2 and O–O bonds of superoxide NaO 2 molecules. The annealing up to 450 K causes the decomposition of these complexes and the reappearance of the stretching frequencies of isolated Na–O and O–Ni species. At 550 K, only the oxidation of the Ni substrate occurs. The spectra are correlated to work function measurements, and the behavior of the work function versus oxygen exposure, for high alkali coverages, is ascribed to a reionization of the metallic Na layer, rather than to a penetration of oxygen atoms underneath the Na layer.

2006/2007
The first part of this thesis work concerns the development of an ultra–high–vacuum experimental system, hosting a commercial Low Temperature Scanning Tunneling Microscope (LT-STM), that has been built for characterizing and manipulating single molecules and molecular complexes adsorbed on metal surfaces. The design from scratch, the commissioning and the performance tests of the various components of the system are presented. The preparation chamber that has been developed provides the fundamental surface preparation and analysis instruments, combining the STM analysis with other experimental techniques and allowing for easy setup of additional preparation/analysis instruments. The manipulator sample stage allows for a wide temperature range, needed for the preparation of a variety of surfaces to be investigated in the LT-STM. In the second part of this work, we present two examples of how molecules and molecular complexes can be studied and manipulated with the LT-STM technique. In the first example, within a collaboration established with the group of Dr. Leonhard Grill at the Freie Universität in Berlin, we have characterized an azobenzene derivative adsorbed on the Au(111) surface, a known molecular switch based on a trans–cis isomerization, which can be reversibly induced by the tip of the STM with controlled voltage pulses. We could show how the molecule–molecule interactions play a critical role in determining the switching abilities of the molecules between different self–assembled molecular islands, and how the molecule–substrate interaction can efficiently determine two different spatial periodicities of the switching molecules. In the second example, we have presented preliminary results regarding the characterization of the NH3-NO complex which forms on the Pt(111) surface, showing that by exploiting the manipulation and spectroscopic tools of the LT-STM, it will be possible to investigate at the atomic scale the properties of the hydrogen bond most likely involved the complex.
La prima parte di questa tesi descrive lo sviluppo di un sistema sperimentale da ultra–alto–vuoto, dotato di un microscopio a scansione a effetto tunnel a bassa temperatura (LT-STM) disponibile in commercio. Il sistema è stato concepito e costruito per caratterizzare e manipolare singole molecole e complessi di molecole adsorbiti su superfici metalliche. Viene descritta la realizzazione del sistema sperimentale, a partire dalla fase di progettazione fino alla messa in opera e ai vari test di funzionamento che sono stati effettuati. La camera di preparazione che è stata costruita è provvista della strumentazione di base per la preparazione e l’analisi dei campioni di misura, e permette di integrare la tecnica STM con altre tecniche sperimentali e di installare in modo semplice e veloce nuovi strumenti. Il manipolatore è dotato di un sistema di riscaldamento campioni che permette di raggiungere un largo intervallo di temperature, necessario per la preparazione di svariati tipi di campioni da poter studiare tramite l’LT-STM. Nella seconda parte della tesi, vengono presentati due esempi di come, grazie alle potenzialità dell’LT-STM, è possibile caratterizzare e manipolare molecole e complessi di molecole. Nel primo dei due esempi, nell’ambito di una collaborazione con il gruppo del Dr. Leonhard Grill presso la Freie Universität a Berlino, è stato studiato sulla superficie (111) dell’oro il comportamento di un derivato dell’azobenzene, un “interruttore molecolare” basato sul processo di isomerizzazione trans–cis che può essere indotto in modo reversibile tramite impulsi di voltaggio dalla punta del microscopio STM. Si è dimostrato come le interazioni inter–molecolari giocano un ruolo fondamentale nel determinare se, all’interno di isole molecolari con diversa struttura, le molecole possono isomerizzare o meno; inoltre è stato scoperto che l’interazione delle molecole con il substrato può determinare l’ordinamento delle molecole in grado di isomerizzare, in strutture di periodicità definita all’interno delle isole molecolari. Nel secondo esempio che viene descritto, vengono presentati i risultati preliminari della caratterizzazione del complesso NH3-NO che si forma sulla superficie (111) del platino, e viene indicato come sarà possibile caratterizzare la natura dei legami idrogeno, che probabilmente stabilizzano il complesso, tramite le tecniche spettroscopiche e di manipolazione messe a disposizione dall’LT-STM.
XX Ciclo
1980

Abstract Objective/Scope The CO2 emission is one of the main causes for the global warming and it may be controlled by sequestrating CO2 into the geological formation. The coalbed formation provides a dual advantage for CO2 sequestration as CO2 may be stored in coal forever with enhancing the coalbed methane recovery. Thus, the cost of CO2 sequestration may be offset completely or partially. The main objective of the paper was to comprehend the CO2-ECBM displacement using the three concepts viz. competitive adsorption, diffusion, and thermodynamic modelling of coal Methods, procedure, and process In this paper, the pure gas isotherm on coal for CH4, and CO2 was evaluated using manometric method. The binary gas isotherm or competitive adsorption was studied using IAST modelling. MATLAB code was developed for the solution of IAST model and Newton Raphson approach was followed. The IAST modelling was done by taking 50%/%50 mole fraction of CH4/CO2. By analyzing the binary gas isotherm, the optimum injection pressure was evaluated. On the same injection pressure, co adsorption isotherm was drawn at different mole fraction of CO2 in gas phase. Separation factor was calculated by taking ratio of CO2 and CH4 in the gas and adsorbed phase respectively. Furthermore, adsorption data was used for discussing the sorption kinetics in coal and diffusion coefficient was evaluated. Furthermore, the thermodynamic parameters were also calculated and integrated with above noted parameters for the methane displacement in CO2-ECBM process. Results, observations, and calculations The CO2-ECBM displacement is very much dependent on the competitive adsorption and diffusion process in coal. The surface potential and Henry constant are important parameters for defining the CO2-ECBM displacement. The coadsorption isotherm was drawn at the optimum injection pressure and it shows that methane displacement would be the optimum by taking 11 %/89% mole fraction of CO2 and CH4 for two temperatures i.e., 288 K, 308 K. It is identified through diffusion regime that diffusion coefficient for the binary gas isotherm is the average of the diffusion coefficients of pure CO2 and CH4. Novel/Additive information This is the first kind of study which provides the completely integrated approach for describing the methane displacement in CO2-ECBM process. This novel study promotes our understanding of the complex mechanisms of CO2-ECBM displacement process.