Potassium addition to Fe/MgO, Ni/MgO and Ni/SiO2: Reducibility, dispersity and stabilization of the promoter by the support

The influence of potassium addition on the morphological properties of Fe/MgO and Ni/MgO has been investigated: KNO₃ addition to precursors results in a decrease of the reducibility and in a smaller dispersity of the metallic phase. A loss of potassium is observed at reduction temperatures in excess of 773 K. Silica-supported Ni behaves differently: silica inhibits the potassium volatilization and nickel reducibility is enhanced. This illustrates how promoter effects can be influenced by the nature of support.

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Fe/MgO Ni/MgO: KNO₃ . , 773 K. , : . .

     

Influence of potassium nitrate addition to silica-supported Pd catalysts on chemisorption properties and on ethane hydrogenolysis

KNO₃ addition to Pd/SiO₂ results in a partial decoration of metal particles which inhibits ethane adsorption. Hydrogen chemisorption is less affected. The rate of ethane hydrogenolysis decreases and the activation energy increases.     

Study of the interactions between carbon monoxide and high specific surface area tin dioxideThermogravimetric analysis and FTIR spectroscopy

The interactions of CO with a high specific surface area tin dioxide was investigated by FTIR spectroscopy and thermogravimetric analysis. FTIR study of CO interactions have shown that CO can adsorb on cus (coordinatively unsaturated sites) Sn⁴⁺ cation sites (band at 2201 cm^-1). In addition, CO reacts with surface oxygen atoms. This leads to the partial reduction of SnO₂ surface and to the formation of ionised oxygen vacancies together with the release of free electrons, which are responsible for the loss of transmission. Formed CO₂ can chemisorb on specific surface sites: on basic sites to form carbonates species and on acidic sites (Sn⁴⁺-CO₂ species) which is in competition with the formation of Sn⁴⁺-CO species. TG experiment have shown that the reduction of SnO₂ by CO at 400°C occurs in two steps. First, the reduction of SnO₂ surface, which is a quick phenomenon. This has allowed to evaluate that more than 12% of reducible surface oxygens can react with CO, essentially because of the presence of a large amount of surface hydroxyl groups. The second step of the reduction of SnO₂ would be the progressive reduction of SnO₂ bulk by the slow diffusion of oxygen atoms from the bulk to the surface. This revised version was published online in July 2006 with corrections to the Cover Date.     

Influence of sodium salt addition on the morphological and catalytic properties (ethane hydrogenolysis) of aluminasupported cobalt

NaNO₃ addition to Co/Al₂O₃ results in an increase in the reducibility, in a smaller dispersity of the metal and in a decrease in the ethane hydrogenolysis rate due to a decorative effect.

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NaNO₃ Co/Al₂O₃ ,

     

CeO2-supported nickel catalysts. Evidence of a strong metal-support ineraction

An increase of the reduction temperature of a Ni/CeO₂ solid results in a continuous decrease of the catalytic activity toward C₂H₆ hydrogenolysis and of the quantity of adsorbed H₂. O₂ treatments with subsequent reduction at low temperature restore a part of the initial properties. This behavior, typical of a strong metal support interaction, is similar to that of Ni/TiO₂, Ni/SiO₂ and Ni/ZrO₂ solids and shows that the SMSI phenomenon is general.

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Ni/CeO₂ C₂H₆ H₂. . , (), Ni/TiO₂, Ni/SiO₂ Ni/ZrO₂ , .

     

Valence state and coordination number of molybdenum supported on SiO2 or MgO during the oxidation of propylene

The nature of the species occurring during the oxidation of propylene on molybdenum oxides supported on silica or magnesia has been investigated by diffuse reflectance spectroscopy. The reaction was performed in the uv cell itself. The oxidation occurs on partially reduced solids and the selectivity for acrolein is related to tetrahedral Mo⁵⁺.

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, , . . . Mo⁺⁵.