Chemisorption

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

Base catalysis for the synthesis of α,β-unsaturated ketones from the vapor-phase aldol condensation of acetone

Abstract: The vapor-phase aldol condensation of acetone was studied over MgO promoted with 0.7–1.0 wt.-% of alkali (Li, Na, K and Cs) or alkaline earth (Ca, Sr and Ba) metal ions. The basic properties of the samples were characterized by chemisorption of carbon dioxide. The basicity of MgO increased on addition of the promoter following the basicity order of the promoter oxide: the stronger the electron donor properties of the promoter, the greater the generation of surface basic sites. Major reaction products were mesityl oxide (MO), isomesityl oxide (IMO) and isophorone (IP). The selectivity to (MO + IMO + IP) over unpromoted MgO was practically 100%, thereby showing that magnesium oxide is suitable for selectively obtaining α,β-unsaturated ketones. The reaction was totally inhibited by co-feeding acetic acid along with acetone whereas the co-injection of pyridine did not affect the acetone conversion. This indicated that the self-condensation of acetone over MgO-based catalysts is catalyzed by basic sites. The promoter addition increased the activity of the MgO catalyst and a good correlation was obtained between catalyst activity and the concentration of basic sites. Such a proportionality between activity and surface basicity was an additional evidence that the rate-determining step in the aldol condensation mechanism is controlled by the surface base property. All the catalysts exhibited similar IP/(IMO+MO) selectivity ratio, except the Li/MgO sample which produced substantially larger amounts of isophorone. Because the tricondensation of acetone to give isophorone requires strong basic sites, the higher selectivity toward isophorone was indicative of the presence of stronger surface basic sites in the Li/MgO sample. Results from carbon dioxide chemisorption confirmed that Li/MgO exhibited the strongest basic properties. The generation of high-strength basic sites was explained by assuming that the addition of lithium causes a structural promotion of the MgO sample by replacing the Mg 2+ ions by Li + in the MgO lattice. The replacement would result in strained Mg O bonds and formation of [Li + O − ] species, which causes the generation of stronger basic sites.

Characterization of O2–CeO2 Interactions Using In Situ Raman Spectroscopy and First‐Principle Calculations

Abstract: Interactions between O(2) and CeO(2) are examined experimentally using in situ Raman spectroscopy and theoretically using density-functional slab-model calculations. Two distinct oxygen bands appear at 825 and 1131 cm(-1), corresponding to peroxo- and superoxo-like species, respectively, when partially reduced CeO(2) is exposed to 10 % O(2). Periodic density-functional theory (DFT) calculations aid the interpretation of spectroscopic observations and provide energetic and geometric information for the dioxygen species adsorbed on CeO(2). The O(2) adsorption energies on unreduced CeO(2) surfaces are endothermic (0.91