Chemisorption

About: Chemisorption is a research topic. Over the lifetime, 16298 publications have been published within this topic receiving 554989 citations.

Evaluation of the acid-base surface properties of several oxides and supported metal catalysts by means of model reactions

Abstract: Acid-base properties of oxides (Al2O3, SiO2, ZrO2, CeO2, MgO, SiO2Al2O3 and CeO2Al2O3) were investigated by means of model reactions: 3,3-dimethylbut-1-ene isomerization (33DMB1), methylene cyclohexane isomerization (MECH), cyclohexanol conversion (CHOL) and CO2 chemisorption at room temperature. The effect of acid (Cl−, SO2−4) and basic (K+) promoters on certain oxides (Al2O3, ZrO2) was also studied. Surface acidity was evaluated by means of 33DMB1 or MECH isomerization and of CHOL dehydration into cyclohexene. CO2 chemisorption as well as the cyclohexanone to cyclohexene ratio in CHOL conversion were used to measure the surface basicity of the solids. Except for very few cases, all the tests gave coherent results which led to well-defined scales of acidity and of basicity. Contrary to what could be observed with the conventional isopropanol test, the CHOL test proves to be little sensitive to the redox sites of the oxides. The presence of metals can create significant perturbations in these acid-base tests, except in the case of 33DMB1 isomerization. The latter reaction is not catalyzed by Pt and Rh surfaces, which allows to measure (in the presence of these metals) the acid properties of the support and to investigate the changes in surface acidity, resulting from the metal impregnations (metal → support electronic effect, presence of anions,…).

Heterogeneous catalysis on the molecular scale

Abstract: The techniques of modern surface science permit the study of the structure, the composition, and the oxidation states in the topmost surface layer. A new instrument has been developed for the combined determination of the kinetic parameters of catalytic surface reactions at high pressures (atmospheres), and for surface characterization at low pressures (approx. 10/sup -9/ torr) by using small area (approx. 1 cm/sup 2/) model catalysts. Single-crystal catalyst studies reveal the structure sensitivity of most catalytic reactions. During hydrocarbon conversion, a carbonaceous deposit forms on the catalyst surface whose bonding and stoichiometry influences the surface reactions. A molecular model of the working metal catalysts for hydrocarbon reactions has been proposed. The oxidation states of surfaces atoms also markedly influence the rate and selectivity of catalyzed surface reactions. Using the molecular ingredients of hetergeneous catalysis that have been identified now makes it possible to build new, high-technology catalysts. 15 figures, 1 table.

Plasma Treatment Process for Palladium Chemisorption onto Polymers before Electroless Deposition

Abstract: Before electroless plating, polymer surfaces must be sensitized and/or activated by using either the conventional two-step or one-step process. The latter stage is a compulsory one to make such surfaces catalytic, e.g., for Ni-P deposition. These processes are performed here using O{sub 2}, NH{sub 3}, or N{sub 2} plasma pretreatments. Reaction mechanisms are proposed for each of the processes and for each type of surface considered (oxygenated or both oxygenated and nitrogenated by the plasma treatment). Direct palladium chemisorption onto nitrogenated groups is highlighted. This allows one to simplify the process making the surface catalytic via elimination of the use of SnCl{sub 2} and to extend the method to any polymer. An additional interest of the plasma treatments, besides their high efficiency in grafting chemical functions, is to perform this grafting at will on selected areas which results in selective metallization.