Hydrogen Spillover. Facts and Fiction

Chemoselective hydrogenation of unsaturated aldehydes and ketones over heterogeneous catalysts is a demanding task. The achieved selectivity levels depend both on the electronic and geometric structures of reactants and metal surfaces. Recent development in catalyst preparation, selection of new solvents, catalyst and reactor structures have been reviewed in this work. Additionally, catalyst characterization technique, especially alloy formation and in situ characterizations have been discussed in detail. The deep understanding of the catalyst structure is necessary for development of tailor-made catalysts for chemoselective hydrogenations. The state of art in both kinetic and molecular modelling used in chemoselective hydrogenations was presented. Some special features, like conversion–selectivity relationship and the performance in gas and liquid phase hydrogenations were discussed.

Chemoselective hydrogenation of carbonyl compounds over heterogeneous catalysts

The success or otherwise of catalyst preparation or modification depends on the availability of suitable characterization techniques to determine the condition of the catalyst. There are many techniques available for this purpose including x-ray powder diffraction, electron microscopy, photoelectron spectroscopy, and infrared spectroscopy. However, none of these techniques has proved to be wholly reliable or generally applicable to the characterization of catalysts under working conditions.

Temperature Programmed Reduction

High-surface area CuO-CeO2 catalysts prepared by a surfactant-templated method for low-temperature CO oxidation

Temperature-programmed isotopic exchange of 18O2 with 16O of several oxides was carried out in the 200−900 °C temperature range. The oxides can be ranked according to their maximal rates of exchange obtained at the following temperatures:  CeO2, 410 °C ≫ CeO2-Al2O3, 480 °C ≈ MgO, 490 °C > ZrO2, 530 °C ≫ γ-Al2O3, 620 °C ≫ SiO2, 850 °C. Except on CeO2 and on CeO2-Al2O3 a simple exchange yielding initially 18O16O can be observed. With ceria containing oxides, the reaction occurs in part via a multiple exchange mechanism yielding initially 16O2 which is indicative of the presence of binuclear species (O2, O2-, or O22-) at the ceria surface. Chlorine-free rhodium catalysts supported on these oxides were prepared with metal dispersions between 32 and 89%. The presence of rhodium accelerates considerably the oxygen exchange with the support:  the maximal rates of the exchange can be observed at much lower temperatures, by about 200−300 °C with respect to the bare oxides. This is attributed to a spillover of oxyg...

Mobility of Surface Species on Oxides. 1. Isotopic Exchange of 18O2 with 16O of SiO2, Al2O3, ZrO2, MgO, CeO2, and CeO2-Al2O3. Activation by Noble Metals. Correlation with Oxide Basicity†

M. Andre

Papers

Adsorption of atomic hydrogen on alumina by hydrogen spillover