Search results for "physisorption"
showing 10 items of 53 documents
Confinement in micropores and enthalpies of physisorption
1994
Abstract The use of quasi-equilibrium volumetry associated with isothermal microcalorimetry at 77 K allows direct measurement of the interactions on adsorption of simple probe molecules within well crystallised aluminophosphate and zeolite molecular sieves. Four particular cases are presented which show: the effect of varying micropore diameter, the effect of adsorbing molecules with a favourable geometric compatibility with a given micropore system, the effect of varying the field potential within a given micropore system and finally, the effect of variable selective blocking of the micropore system.
Physisorption Instabilities during Dynamic Langmuir Wetting
1994
Continuous dynamic Langmuir wetting of lipid monolayers can create regular stripe patterns on the solid substrate surface. Monolayers doped with minor amounts of fluorescence dye may be deposited in stripes of alternating high and low dye concentration. In case of pure monolayers stripes are observed with AFM. The striations run normal to the dipping direction and have widths of typically several micrometres and distances varying from 1 to ≈ 100 μm. The stripe widths, distances, and fluorescence contrast depend on deposition speed, surface pressure, substrate surface preparation, and dye concentration. The patterns probably result from physisorption (meniscus) instabilities caused by a feed…
Adsorption of single Ag and Cu atoms on regular and defective MgO(001) substrates: an ab initio study
2004
Abstract The DFT slab calculations were performed for Ag and Cu atoms adsorbed on both regular and defective MgO(0 0 1) substrates. Both metal atoms and surface O vacancies ( F s centers) were distributed uniformly with a concentration of one Ag, Cu or F s per 2×2 surface supercell. Surface O 2− ions are energetically more preferable for metal-atom adsorption on a regular substrate as compared to Mg 2+ ions. The nature of the interaction between Ag or Cu adatoms and a defectless MgO substrate is physisorption (despite the difference in the adsorption energies: 0.62 vs. 0.39 eV per Cu and Ag adatom, respectively). Above the F s centers, metal atoms are bounded much stronger when compared wit…
Ab initio modelling of silver adhesion on the corundum (0001) surface
2003
The Ag/a-Al2O3(0001) interface was simulated using ab initio slab calculations. We have studied silver adhesion on both Al- and Oterminated corundum substrates. The latter case may be considered as silver adhesion on a defective Al-terminated corundum surface with external aluminium vacancies. The dependence of the adhesion energy on the interfacial distance has been analyzed for the two favorable Ag adsorption positions and for two metal coverages (a 1/3 monolayer of the Ag(111) crystallographic plane and a full Ag(111) monolayer, 1 ML). The two different terminations (Al- and O-) give rise to qualitatively different results. In the former case, the small adhesion energies per Ag atom are …
Investigation of the porosity of silica and alumina with chemically bonded polystyrene
1996
The effect of modification of porous silica and alumina with a copolymer of styrene and vinylsilane on the porosity of oxides have been investigated by means of low temperature nitrogen sorption. Only small changes of the specific surface area and of the specific pore volume were observed on modification of oxides with non-cross-linked copolymer. The specific surface area significantly increased after the cross-linking of the deposited polymeric layer while the specific pore volume remains almost unaffected. A broad hysteresis loop appears in the nitrogen sorption isotherm for the alumina modified with the cross-linked polymer. The porosity of oxides modified by chemisorption method differe…
Factors Controlling the Energy of Nitrogen Monolayer Coverage onHigh Surface Area Catalyst Oxide Carriers
2011
Factors affecting the strength of nitrogen physisorption at monolayer coverage on different catalytic oxide carriers (e.g., ZnO, MgO, Al2O3, ZrO2, TiO2, and SiO2) have been addressed. Isotherm elaboration by the two-parameter BET equation provides C-constant values (80–200) inversely related to the polarizing power (PP) of the oxide adsorbent irrespective of the surface area exposure. The energy of monolayer formation depends on the extent of charge-delocalization characterizing the surface cation-oxygen bond, which determines the acid–base character of the oxide and strength of van der Waals interactions with adsorbate molecules. Density functional theory (DFT) calculations on MgO and TiO2…
Influence of the covalent grafting of organic radicals to graphene on its magnetoresistance
2013
Graphene was obtained by direct exfoliation of graphite in o-dichlorobenzene (oDCB) or benzylamine, and further functionalized with 4,4′-[(1,3-dioxo-1,3-propanediyl)bis(oxy)]bis[2,2,6,6-tetramethyl-1-piperidinyloxy] (1-TEMPO) organic radicals by using the Bingel–Hirsch cyclopropanation reaction. Here, the use of different solvents permits variation of the density of radicals anchored to the carbon layers. Covalent grafting is unambiguously demonstrated by TGA, μ-Raman, XPS and EPR measurements, which also rule out spurious physisorption. Our transport measurements indicate that the conduction mechanism varies as a function of the density of radicals grafted to the carbon layers. Moreover, t…
Growth of sub-nanometric palladium clusters on boron nitride nanotubes: a DFT study.
2015
A QM/MM investigation is reported dealing with the nucleation and growth of small palladium clusters, up to Pd8, on the outer surface of a suitable model of boron nitride nanotubes (BNNTs). It is shown that BNNTs could have a template effect on the cluster growth, which is due to the interplay between Pd–N and Pd–Pd interactions as well as due to the matching of the B3N3 ring and the Pd(111) face arrangement. The values for the cluster adsorption energies reveal a relatively strong physisorption, which suggests that under particular conditions the BNNTs could be used as supports for the preparation of shape-controlled metal clusters.
Cluster calculations for H2dissociation on Cu and Ni
1988
Self-consistent cluster calculations have been carried out for hydrogen dissociation on Cu and Ni clusters using local-density theory and the LCAO-DVM expansion. We find physisorption, chemisorption and dissociation minima in the resulting two-dimensional potential energy surfaces, and for the Ni cluster, also an indication of the associative molecular chemisorption state. For Cu we find a considerable barrier at the seam separating the molecular chemisorption and dissociative minima. The analysis of one-electron levels supports the picture of Harris and Andersson that the s to d conversion present on Ni surfaces does not similarly lower the barrier on Cu surface.
Au/CeO2-SBA-15 catalysts for CO oxidation: Effect of ceria loading on physic-chemical properties and catalytic performances
2012
In this work gold catalysts supported over SBA-15 with different CeO 2 loadings (5-30 wt%) were prepared, characterized by N 2 physisorption analyses, SAXS, XRD, STEM and XPS techniques and their catalytic performances were evaluated in the CO oxidation, chosen as reaction test. Over a selected catalyst, Au/CeO 2(20 wt%)-SBA-15, the effect of CO 2 and of the mixture (CO 2 + H 2O) on the CO conversion to CO 2 was also evaluated. Characterizations by SAXS, XRD, STEM and XPS were carried out on selected spent catalysts after CO oxidation. The results were discussed in terms of relationship between morphological, structural, electronic and catalytic properties as a function of the ceria loading…