Chemisorption Of Probe Molecules Research Articles

Influence of thermal treatments on phase composition and acidity of mesoporous tungsten oxide

We report on the synthesis of mesoporous tungsten oxides via hard templating method using silicotungstic acid as W precursor and having surface areas and pore diameters up to 68m2/g and 12.8nm, respectively. The relation between thermal treatments (calcination temperature of W-KIT-6 composites and reductive treatment of tungsten oxides), crystalline structure and acidity has been systematically investigated. The acidity of mesoporous tungsten oxides was investigated using common pyridine adsorption methods and chemisorption of probe molecules (CO, H2). Combination of XRD and FTIR techniques showed that monoclinic and orthorhombic phases of crystalline wall structure coexist in the calcination temperatures 550–800°C. Increasing the calcination temperature and decreasing the W/Si mass ratio of composites favors the transformation of orthorhombic phase to more stable monoclinic. Moreover, the acidity as well as ability to chemisorbs CO decreases when the calcination temperature increases. The concentration of acid sites also depends on the temperature of reductive treatment of tungsten oxides: CO uptakes increase twice at the 250°C and decrease by four times when the temperature increases to 450°C.

Physisorption vs. chemisorption of probe molecules on boron nitride nanomaterials: the effect of surface curvature

The adsorption of H2O, NH3 and HCOOH as polar probe molecules and C6H6 and CH4 as non-polar ones on a series of zig-zag (n,0) single-walled boron nitride nanotubes (BNNTs) and on a boron-nitride mono-layer (BNML) has been studied by means of B3LYP-D* periodic calculations. Computed electrostatic potential maps for the pristine BN nanomaterials indicate that the smaller the radius, the larger the polar character. Polar molecules are found to be strongly chemisorbed on small radius BNNTs by means of dative interactions between electron donor atoms of the molecules and B atoms of the BNNTs, H-bonding, as well as dispersive forces. Remarkably, for HCOOH interacting with the (4,0) BNNT, this dative interaction is accompanied by a proton transfer to the nanotube. The corresponding computed adsorption energies decrease sharply with increasing tube radius, gradually approaching the values for physisorption on the BNML. Adsorption of non-polar molecules, mainly dictated by π-stacking (C6H6) and CH-π (CH4) dispersion interactions, is found to be energetically more favorable when physisorbed on large radius BNNTs, the most stable adducts being formed on the BNML.

Determination of Metal Dispersion and Surface Acidity of a Pd/Al2O3 Catalyst by Gas Chromatography

The present work is focused on the application of gas chromatography to the characterization of the catalyst activity of Pd/Al2O3 by chemisorption of probe molecules, considering both the metal phase and the acid sites on the catalyst surface. Gas chromatography has been used for this purpose, using pulses of hydrogen and oxygen to quantify active metal surface area, and pyridine and quinoline to determine the percentage of overall Lewis acid sites. Chromatographic results have been compared with those obtained by a dynamic pulse adsorption technique in a commercial apparatus.

ChemInform Abstract: Nature of Adsorption Sites on Sulfided Mo Catalysts and Their Selectivity in Chemisorption of Probe Molecules.

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Nature of Adsorption Sites on Sulfided Mo Catalysts and Their Selectivity in Chemisorption of Probe Molecules

Oxygen chemisorption has been studied on sulfided Mo catalysts supported on Si02 with differing dispersions as a function of pretreatment. It was found that presulfided samples purged in He at 350 C chemisorb about half as much 02 as samples reduced first in H2 at 350 C, followed by He purge at the same temperature. From this study in combination with other studies of chemisorption of probe molecules such as H2S, CO, 02, and NO on sulfided Mo catalysts, a two-site chemisorption model is proposed. The adsorption sites are intrinsically related to the geometric and electronic character of the MoS2 two-dimensional layer structure. Frontier orbital concepts and Hoffmann's theory of bonding on surfaces are employed to postulate the selectivity in chemisorption of probe molecules on the two adsorption sites and to correlate quantitatively the chemisorption amount of various probe molecules. The model suggests that Mo-S sites with unsaturated coordination located in S-Mo-S layers are adsorption sites for probe molecules HzS, CO, 02, and NO, while S sites located in S-S layers only adsorb 02 and NO.

Preparations and characterization of lanthana catalysts: study of their activity in CO/H2 reactions

Two original methods of preparation of lanthana with hexagonal structure as well as the stabilization of its cubic structure by addition of yttria were investigated. The physical properties and the catalytic behaviour of these samples are compared with classical preparations. The influence of the preparation techniques and of the pretreatment on the specific area, morphology and chemisorption of probe molecules were tested, the latter using mainly Fourier transform infrared spectroscopy. Catalytic tests, operated at high CO-H2 pressure and high temperature, led to the classical isosynthesis product distribution. The prepared lanthana samples are active in the hydrogenation of carbonate to formate species which can be involved in the formation of formaldehyde in the first step of the isosynthesis reaction. The evolution of the oxide catalysts to hydrates and carbonates during the reaction was studied.