Modification of a three-way catalyst washcoat by aging: A study along the longitudinal axis

This work reports the research carried out by studying aliquots extracted at different axial coordinates from Three Way Catalyst (TWC) monoliths aged under real traffic conditions. Our study focused on the catalytic properties and on several chemical and physical effects caused in the Front and Rear monolith washcoat surfaces by vehicle aging after 60,000 km. Regarding the catalytic properties, all the used aliquots showed poorer activity than their corresponding fresh counterparts. The strongest deactivation was detected for NO and hydrocarbon conversion. CO conversion was less affected and the Rear monolith was as deactivated as the Front one. The characterisation techniques (TXRF, N₂ adsorption-desorption isotherms, XRD and H₂-TPR) detected - (i) the deposition of P, Zn and Pb; (ii) the formation of CePO₄ on account of the Ce from the washcoat; (iii) thermal sintering; (iv) inhibition of the reducibility of Ce oxides - as the main effects brought about by vehicle aging conditions. The deactivation observed at the beginning of the Front monolith was the result of a combination of the former effects. When moving downstream to higher axial coordinates, Pb accumulation and the loss of specific area appeared to be the only probable sources of deactivation.     

Adsorption of anionic surfactants on positively charged polystyrene particles II

In the case of cationic polystyrene latex, the adsorption of anionic surfactants involves a strong electrostatic interaction between both the particle and the surfactant, which may affect the conformation of the surfactant molecules adsorbed onto the latex-particle surface. The adsorption isotherms showed that adsorption takes place according to two different mechanisms. First, the initial adsorption of the anionic surfactant molecules on cationic polystyrene surface would be due to the attractive electrostatic interaction between both ionic groups, laying the alkyl-chains of surfactant molecules flat on the surface as a consequence of the hydrophobic interaction between these chains and the polystyrene particle surface, which is predominantly hydrophobic. Second, at higher surface coverage the adsorbed surfactant molecules may move into a partly vertical orientation with some head groups facing the solution. According to this second mechanism the hydrophobic interactions of hydrocarbon chains play an important role in the adsorption of surfactant molecules at high surface coverage. This would account for the very high negative mobilities obtained at surfactant concentration higher than 5×10^–7 M. Under high surface-coverage conditions, some electrophoretic mobility measurements were performed at different ionic strength. The appearance of a maximum in the mobility-ionic strength curves seems to depend upon alkyl-chain length. Also the effects of temperature and pH on mobilities of anionic surfactant-cationic latex particles have been studied. The mobility of the particles covered by alkyl-sulphonate surfactants varied with the pH in a similar manner as it does with negatively charged sulphated latex particles, which indicates that the surfactant now controls the surface charge and the hydrophobic-hydrophilic character of the surface.Dedicated to the memory of Dr. Safwan Al-Khouri IbrahimPresented at the Euchem Workshop on Adsorption of Surfactants and Macromolecules from Solution, Åbo (Turku), Finland, June 1989     

TXRF analysis of aged three way catalysts

A methodology for the quantitative analysis of the elements present in car exhaust catalysts by TXRF is described. Some of the most relevant catalytic components (Zr, Rh, Pd) and contaminant elements (P, Pb, Zn, Pt, Ca and Ni) on car Three Way Catalysts were quantified during vehicle aging. The study was conducted along the main axis of a Ford Focus 1.6i catalytic cartridge, aiming to obtain the axial profile of the elements retained or eliminated after more than 59,000 km. The analysis of a fresh Three Way Catalysts (0 km) was also conducted for comparative purposes. The existence of a strong differences for elemental concentration in the interphase between the two blocks of the catalytic cartridge has been quantitatively measured for first time. In addition, the behaviour of the contaminant absorption capacity of both catalytic blocks has been obtained. Finally, the loss of the contaminant elements Pb and Zn to the environment has been proven.     

Host-guest interactions at self-assembled monolayers of cyclodextrins on gold

We have developed synthesis routes for the introduction of short and long dialkylsulfides onto the primary side of alpha-, beta-, and gamma-cyclodextrins. Monolayers of these cyclodextrin adsorbates were characterized by electrochemistry, wettability studies, X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (TOF-SIMS), and atomic force microscopy (AFM). The differences in thickness and polarity of the outerface of the monolayers were measured by electro-chemistry and wettability studies. On average about 70% of the sulfide moieties were used for binding to the gold, as measured by XPS. Tof-SIMS measurements showed that the cyclodextrin adsorbates adsorb without any bond breakage. AFM measurements revealed for beta-cyclodextrin monolayers a quasi-hexagonal lattice with a lattice constant of 20.6 A, which matches the geometrical size of the adsorbate. The alpha-cyclodextrin and gamma-cyclodextrin monolayers are less ordered. Interactions of the anionic guests 1-anilinonaphthalene-8-sulfonic acid (1,8-ANS) and 2-(p-toluidinyl)naphthalene-6-sulfonic acid (2,6-TNS) and the highly ordered monolayers of heptapodant beta-cyclodextrin adsorbates were studied by surface plasmon resonance (SPR) and electrochemical impedance spectroscopy. The SPR measurements clearly showed interactions between a beta-cyclodextrin monolayer and 1,8-ANS. Electrochemical impedance spectroscopy measurements gave high responses even at low guest concentrations (< or = 5 microM). The association constant for the binding of 1,8-ANS (K = 289,000 +/- 13,000M-1) is considerably higher than the corresponding value in solution. (Partial) methylation of the secondary side of the beta-cyclodextrin strongly decreases the binding.     

Dynamics in physisorbed monolayers of 5-alkoxy-isophthalic acid derivatives at the liquid/solid interface investigated by scanning tunneling microscopy

Monolayers of isophthalic acid derivatives at the liquid/solid interface have been studied with scanning tunneling microscopy (STM). We have investigated the dynamics related to the phenomenon of solvent co-deposition, which was previously observed by our research group when using octan-1-ol or undecan-1-ol as solvents for 5-alkoxy-isophthalic acid derivatives. This solvent co-deposition has now been visualized in real-time (two frames per second) for the first time. Dynamics of individual molecules were investigated in mixtures of semi-fluorinated molecules with video-STM. The specific contrast arising from fluorine atoms in STM images allows us to use this functionality as a probe to analyze the data obtained for the mixtures under investigation. Upon imaging the same region of a monolayer for a period of time we observed that non-fluorinated molecules progressively substitute the fluorinated molecules. These findings illustrate the metastable equilibrium that exists at the liquid/solid interface, between the physisorbed molecules and the supernatant solution.     

STM imaging of a heptanuclear ruthenium(II) dendrimer, mono-add layer on graphite

Scanning tunneling microscopy (STM) and molecular mechanics calculations were used to investigate the long-range packing and the structure of an heptanuclear ruthenium (II) dendritic species, as a PF6- salt. STM imaging was carried out on a mono-add layer of the ruthenium dendrimer formed by physisorption from a 1,2,4-trichlorobenzene solution at the liquid-graphite interface. The packing of the molecules on the surface was visualised by the formation of ordered patterns and a distance of 27 +/- 2 A was measured between two adjacent lamellae. The comparison of this dimension with the molecular-modelling data indicates that the lamellae were formed by rows of dendrimer molecules in which the counterions (PF6-) were strongly associated with the Ru atoms. The images acquired with higher spatial resolution revealed the presence of repeating units within the lamellae. The comparison of the STM images with the modelling results allowed the attribution of the repeating units observed in the imaged pattern to the STM signature of single dendrimer molecules.     

On the adsorption of IgG onto polystyrene particles: electrophoretic mobility and critical coagulation concentration

An experimental investigation on the adsorption of immuno -globulin molecules on polystyrene microspheres is described. Three different IgG samples were adsorbed on latex particles. One was of polyclonal nature with a broad range of isoelectric points (6.1–8.7), whereas the other samples were of monoclonal nature, Mab 1 and Mab 2 with i.e.p. of (5.65±0.15) and (7.7±0.1), respectively. Adsorption isotherms at different ionic strengths and pH were performed. Most of the adsorption isotherms showed well-defined plateaus. Because of instability in solution of Mab 2 in the pH values of 7 and 8, no plateau values were found in the adsorption isotherms at both pH-values. Maximum protein adsorption was found around the i.e.p. of the protein. According to the findings, the IgG adsorption on polystyrene surface is strongly irreversible with respect to pH changes. The ionic-strength changes, however, exert a pronounced effect on the adsorption-desorption processes of IgG on negatively charged polystyrene surface. Also, electrophoresis experiments were performed to gain information on the electrostatic interaction between the IgG molecules and the PS latex. With increasing the adsorbed amount of IgG the absolute value of mobility decreases to reach a plateau value. The isoelectric pH of the IgG-PS complex is always smaller than the i.e.p. of the dissolved IgG, indicating that the PS surface charge must partly compensate the positive charge on the protein. Finally, the colloidal stability of the rabbit IgG/PS complex is always very low, whereas the Mab/PS complexes are very stable when the charge electrokinetically mobilized by these systems is very large.     

Structure alteration and immunological properties of 60Co-gamma-rays irradiated bothropstoxin-I

In this work, the authors investigated the immunological behavior of bothropstoxin-I (BTHX-1), before and after irradiation process, and also the influence of scavengers substances on protein alterations induced by free radical production. Structural modifications were investigated by SDS-PAGE in reducing or non-reducing conditions. In vitro cytotoxicity assay was performed to test average toxic activities of BTHX-I. BALB/c Isogenic mice were immunized with irradiated or non-irradiated (native) forms of BTHX-I and antibody titers and isotypes were determined by ELISA method. Expression of murine cytokines was analyzed by using expression data obtained by quantitative real-time PCR (qPCR) assays. The results indicate that irradiation of proteins leads to significant structural modifications, and also changes the cytokines profile during immunization process, regarding a suitable approach to new immunogenic production.