Search results for "Adsorption"
showing 10 items of 1326 documents
The Influence of Alkali Metal Ions in the Chemisorption of CO and CO2on Supported Palladium Catalysts: A Fourier Transform Infrared Spectroscopic Stu…
1996
Two series of palladium-based catalysts were compared on the basis of the adsorption of CO and CO2, monitored by Fourier transform infrared spectroscopy. The first series is represented by a silica-supported palladium catalyst and by some catalysts derived from it by addition of different amounts of sodium ion, 0 ≤ R ≤ 25.6, whereRis the atomic ratio Na/Pd. The second series consists of palladium catalysts supported on “model” and natural pumices. The model pumices, obtained by sol-gel techniques, are silico-aluminates containing variable amounts of sodium so that the corresponding Pd catalysts have anRvalue in the range 0 ≤ R ≤ 6.1. In the Pd/natural pumice catalysts, changes of the atomic…
Intrinsic Metal Size Effect on Adsorption of Organic Molecules on Platinum
2008
Di-σ adsorbed ethene, bridge(30) adsorbed benzene (with four di-σ-type and two π-type interactions), and η1 and η2 adsorbed acetone on nanosized platinum clusters consisting of 19 to 38 Pt atoms were studied theoretically by density functional theory (DFT) calculations with general gradient approximation (GGA) utilizing plane wave and local basis sets. The cluster results were compared to plane wave calculations employed with periodic boundary conditions. It was found that the geometries obtained with different methods are very similar but the adsorption energy depends prominently on the cluster size. Adsorption was strongest on the 22- and 26-atom clusters and weakest on the 35- and 38-ato…
Density Functional Theory Study on Propane and Propene Adsorption on Pt(111) and PtSn Alloy Surfaces
2011
Density functional theory calculations were performed to investigate the adsorption of propane, propene, and C and H atoms on Pt and PtSn surfaces employing the revised Perdew–Burke–Ernzerhof (RPBE) and vdW-DF functionals. Propane adsorption was found to be mediated by van der Waals interactions without significant site preference on any of the studied surfaces. The adsorption characteristics of propene are different: On the Pt(111) and Pt3Sn(111) surfaces, propene adsorption is covalent, and the molecule prefers a di-σ site to a π site. Alloying Pt(111) with Sn leads to weaker adsorption owing to geometric and relaxation effects, whereas electronic effects are found to be small. On the PtS…
Evaluation of the Stability of Pure Silica MCM-41 toward Water Vapor
1999
Water vapor adsorption/desorption isotherms at 298 K and XRD measurements and nitrogen isotherms at 77 K before and after exposure to water vapor were determined on pure silica MCM-41 samples; samples had different pore widths and were prepared by different synthesis methods, including hydrothermal and room-temperature procedures. It was found that prolonged exposure to water vapor provoked structural alterations in all of the MCM-41 materials studied, the most significant effects being a loss of pore shape uniformity and a large decrease in pore size and, in consequence, pore volume. Analysis of the results suggests that these alterations are due to expansion of the pore walls as well as, …
Nonlinear hairy layer theory of electrophoretic fingerprinting applied to consecutive layer by layer polyelectrolyte adsorption onto charged polystyr…
1997
The consecutive layer-by-layer adsorption of poly(allylamine hydrochloride) (PAH) and poly(styrenesulfonate) (PSS) on colloidal charged latex particles is investigated by measuring the electrophoretic mobility as a function of pH and ionic strength over a broad range (electrophoretic fingerprinting). Meaningful interpretation of the data required the development of a nonlinear approach to hairy particle electrophoresis including dissociation, adsorption, and association. Steric and electrostatic exclusion of mobile ions from the hairy layer has been considered. Also, the surface conductivity correction is extended to the case of charged hairy layer particles. We deposited up to three polyel…
Effects of pressure, temperature and atomic exchanges on phase separation dynamics in Au/Ni(111) surface alloy: Kinetic Monte Carlo study
2015
Abstract Instability of the Au/Ni(111) surface alloy is studied in different CO gas pressure, p , and temperature limits using kinetic Monte Carlo simulations. We analyze the reaction front dynamics and formation of Au clusters using the model which takes into account surface adatom pair and three-body interactions, CO adsorption and desorption, catalytic carbonyl formation reaction, Au and Ni adatom diffusion and their concerted exchange. Variation of interaction parameters allows us to identify three possible reaction front propagation limits with different pressure dependencies: (i) slow channel-like flow in agreement with experimental data [1] (step flow rate, R , increases with p ), (i…
Selective CO2 adsorption by a triazacyclononane-bridged microporous metal-organic framework.
2011
Metal-organic frameworks constructed by self-assembly of metal ions and organic linkers have recently been of great interest in the preparation of porous hybrid materials with a wide variety of functions. Despite much research in this area and the large choice of building blocks used to fine-tune pore size and structure, it remains a challenge to synthesise frameworks composed of polyamines to tailor the porosity and adsorption properties for CO(2). Herein, we describe a rigid and microporous three-dimensional metal-organic framework with the formula [Zn(2)(L)(H(2)O)]Cl (L=1,4,7-tris(4-carboxybenzyl)-1,4,7-triazacyclononane) synthesised in a one-pot solvothermal reaction between zinc ions a…
Adhesion and adhesion hysteresis of mica surfaces covered with bola-amphipiles in dry and humid air
1998
Abstract Bola-amphiphiles with spherical hydrophilic chinuclidinium head groups form laterally ordered monolayers on mica by adsorption from solution. To measure the surface adhesion, JKR-type compression/decompression cycles have been performed with the surface forces apparatus (SFA). In an inert atmosphere, the adhesion energy on increasing the contact area is much lower than predicted by wetting experiments. Furthermore, the pronounced adhesion hysteresis varies from sample to sample in a wide range, indicating a defect-rich surface. On increase of the relative humidity (rh), the reproducibility improves and the values for the adhesion energy measured on compression and decompression con…
Using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry for the characterization of functionalized carbon nanotubes
2013
RATIONALE Functionalization of carbon nanotubes (CNTs) generates complex systems that require the development of suitable characterization protocols. New techniques have been explored, and existing analytical and spectroscopic methods to characterize functionalized CNTs have been adapted. Presently, chemical characterization of functionalized CNTs (f-CNTs) remains a difficult task. METHODS Matrix-assisted laser desorption/ionization (MALDI) analysis is performed on f-MWCNT samples prepared via grafting or absorption of anti-oxidant (AO) molecules on both MWCNT-COOH and MWCNT-OH. Covalently functionalized MWCNTs were subjected to thermal degradation and/or hydrolysis reaction before analysis…
V-containing MCM-41 and MCM-48 catalysts for the selective oxidation of propane in gas phase
2001
Well-organised V-containing MCM-41 and -48 (0.3‐1 wt.% of V content) have been synthesised by one-pot synthesis or by grafting using VOSO4 or VOCl3 as V sources, respectively. The samples before and after the calcination step have been characterised by several physicochemical techniques (Ar and N2 adsorption, XRD, diffuse reflectance-UV‐VIS (DR-UV‐VIS) spectroscopy, temperature-programmed reduction). It was found that V species in the as-prepared catalysts were mainly as vanadyl ions (VO 2C ), while highly dispersed V 5C species with tetrahedral coordination were observed in the calcined materials. The catalytic behaviour of the calcined materials for the gas phase oxidation of propane has …