Basic Sites on Alumina with Preadsorbed Ethanol and Ammonia-An IR Study

The adsorption of ethanol and ammonia changes the basic properties of alumina, and new basic sites are created. Ethanol reacts with surface Al-OH groups, forming ethoxy group Al-O-C2H5. The substitution of Al-OH by Al-O-C2H5 increases the negative charge of neighbouring oxygen atoms, and they became sufficiently basic to react with adsorbed CO2 forming carbonate species CO32-. These carbonates were found to be monodentate and bidentate species. Preadsorption of ammonia also increases the basicity of alumina, but the mechanism is different than for ethanol adsorption. Adsorbed ammonia interacts with surface Lewis acid sites being three-coordinated aluminium atoms. This interaction is accompanied by an electron transfer from ammonia molecules to surface sites, and increases the basicity of the neighbouring oxygens, which can react with the absorbed CO2. The carbonate species formed are polydentate ones.

Ethoxy Groups on ZrO2, CuO, CuO/ZrO2 Al2O3, Ga2O3, SiO2 and NiO: Formation and Reactivity

The reaction of ethanol with surface OH groups on ZrO₂, CuO/ZrO₂, CuO, Al₂O₃, Ga₂O₃, NiO, and SiO₂ was studied by IR spectroscopy. The basicity of oxides was followed by CO₂ adsorption, and their ability to oxidize was investigated by H₂-TPR. It has been found that ethanol reacts with surface OH groups forming ethoxy groups and water. Some oxides: ZrO₂, CuO/ZrO₂, Al₂O₃, and Ga₂O₃ contain several kinds of OH groups (terminal, bidentate, and tridentate) and terminal hydroxyls react with ethanol in the first order. Two kinds of ethoxyls are formed on these oxides: monodental and bidental ones. On the other hand, only one kind of ethoxy group is formed on CuO and NiO. The amount of ethoxy groups correlates with the basicity of oxides. The biggest amount of ethoxyls is produced on the most basic: ZrO₂, CuO/ZrO₂, and Al₂O₃, whereas the smallest amount of ethoxyls is produced on CuO, NiO, and Ga₂O₃, i.e., on oxides of lower basicity. SiO₂ does not form ethoxy groups. Above 370 K ethoxy groups on CuO/ZrO₂, CuO, and NiO are oxidized to acetate ions. The ability of oxides to oxidize ethoxyl groups increases in the order NiO < CuO < CuO/ZrO₂. The temperature of the peak in the H₂-TPR diagram decreases in the same order.

IR Studies of Ethoxy Groups on CeO(2)

The reaction of ethanol with a surface of CeO₂ was studied using IR spectroscopy. In some experiments, CeO₂ was pretreated in a vacuum at 820 K which caused a partial reduction. In other experiments, CeO₂ was reduced with hydrogen at 770 K. We also used CeO₂ oxidized by oxygen treatment at 670 K. At low coverages, ethoxy groups and new surface OH groups were formed and water was not produced. On the other hand, at higher loading surfaces, Ce-OH was consumed and ethoxy groups and water were formed. Three kinds of ethoxyls were found on CeO₂: monodentate, bidentate, and tridentate ones. They were characterized by various frequencies of symmetrical, asymmetrical, and combinational bands of C-C-O units. The reduction of CeO₂ increased the contribution of tridentate ethoxyls and the oxidation increased the contribution of monodentate ones. At higher temperatures, ethoxy groups were oxidized to acetate ions with the formation of new surface OH groups. Monodentate ethoxyls were the most reactive and tridentate ones were the least reactive during oxidation. The amounts of acetate species were the highest for the oxidized CeO₂.

Ethoxy Groups on ZrO2, CuO, and CuO/ZrO2 Studied by IR Spectroscopy

The formation, properties, decomposition and reactions of ethoxy groups on ZrO₂, CuO, and CuO/ZrO₂ were followed by IR spectroscopy. The reaction of ethanol with terminal Zr-OH groups leads to the formation of monodendate ethoxy groups (type I), whereas the reaction of ethanol with tribridged Zr-OH grups results in the formation of bidendate ethoxyls (type II). In both cases, water is produced. Ethoxy groups of type II were also formed on CuO. The type of the surface species detected after interaction of ethanol with CuO/ZrO₂ was the same as detected for both oxides (i.e., ZrO₂ and CuO) separately. This suggests that no new phase was formed in the mixed oxide system. At higher temperatures, ethoxy groups were oxidized forming acetate ions. Gaseous ethanol present in the cell was oxidized to acetaldehyde without the intermediacy of ethoxy groups.