Summary Using temperature-programmed desorption (TPD), we have investigated the interaction of carbon dioxide with alkali-metal cation-exchanged faujasite type zeolites (LSX, X and Y). TPD in the temperature range between 300 and 500 K results in desorption profiles of different intensities depending on the kind of cation and the aluminium content of zeolites. For NaX the desorbed amount corresponds to about one percent of the saturation capacity at 298 K. In case of NaX and X type zeolites exchanged with Cs+ ions an additional desorption peak above 500 K could be observed. Taking into account desorption curves of different heating rates, desorption energy distribution functions were calculated by using an extended integral equation. Initial adsorbed CO2 could be assigned to carbonate species in different environments by DRIFT spectroscopy.
There are very few examples in nature for U(VI) compounds with carbonate ligands other than the well known tricarbonates. Especially examples of U(VI) dicarbonato compounds are nearly completely missing. Even in aqueous solutions, the dicarbonato complex was found as a species of minorimportance only. On the basis of structural data on the ligands H2O and carbonate as well as the available data on U(VI) coordination compounds, steric requirements of equatorial coordination are studied for aqueous solution species. A pentagonally coordinated monocarbonato species [UO2CO3(H2O)3] is found as the most likely coordination. For the dicarbonato species, hexagonally coordinated [UO2(CO3)2(H2O)2] with D2h symmetry is found as most probable structure. Possible causes of the instability of U(VI) dicarbonato species are discussed.
Electronspin resonance (ESR) studies of -irradiated LaNiO3 revealed the formation of chemisorbed superoxide ion (O
) and F centers (electrons trapped in anion vacancies). X-ray photoelectron spectroscopy (XPS) showed that the -irradiation of LaNiO3 in the presence of moisture leads to the reduction of the transition metal (Ni3+ to Ni2+) which in turn facilitates the formation of O
and surface carbonate species (CO
). A qualitative molecular orbital model has been proposed for the chemisorption of O
on the reduced transition metal centers (Ni2+). The hydrated electron generated by the radiolysis of moisture reduces the transition metal. Gamma-irradiated LaNiO3 shows enhanced catalytic activity for the decomposition of hydrogen peroxide (H2O2) and the increase in catalytic activity is attributed to the reduced metal content. The formation of chemisorbed oxygen decreases the electrical conductivity by trapping the charge carriers.
Bayer hydrotalcites prepared using the seawater neutralisation (SWN) process of Bayer liquors are characterised using X-ray
diffraction and thermal analysis techniques. The Bayer hydrotalcites are synthesised at four different temperatures (0, 25,
55, and 75 °C) to determine the effect of synthesis temperature on the thermal stability of the Bayer hydrotalcite structures
and the mineralogical phases that form. The interlayer distance increased with increasing synthesis temperature, up to 55 °C,
and then decreased by 0.14 Å for Bayer hydrotalcites prepared at 75 °C. The three mineralogical phases identified in this
investigation are; (1) Bayer hydrotalcite, (2), calcium carbonate species, and (3) hydromagnesite. The DTG curve can be separated
into four decomposition steps; (1) the removal of adsorbed water and free interlayer water in hydrotalcite (30–230 °C), (2)
the dehydroxylation of hydrotalcite and the decarbonation of hydrotalcite (250–400 °C), (3) the decarbonation of hydromagnesite
(400–550 °C), and (4) the decarbonation of aragonite (550–650 °C).
Catalysis of mixed oxide LaMnO3 was studied for the decomposition of hydrogen peroxide (H2O2). The catalyst was -irradiated in open petri dishes, vacuum, dry oxygen and moist oxygen. LaMnO3 irradiated in moist oxygen showed highest catalytic activity. X-ray photoelectron spectroscopic (XPS) studies were carried out to investigate the surface modifications occurred during -irradiaiton of LaMnO3. No significant change in the surface was noticed in LaMnO3 irradiated in vacuum and dry oxygen. However, LaMnO3 irradiated in moist oxygen and in open petri dishes showed the reduction of transition metal (MN3+ to Mn2+) which in turn leads to the formation of chemisorbed superoxide ions (O
) and surface carbonate species (CO
). The latter processes decreases the electrical conductivity by trapping the charge carriers. The hydrated electron generated by the radiolysis of moisture reduces the transition metal. A qualitative molecular orbital model has been proposed for the chemisorption of O
on the reduced transition metal centers (Mn2+).
Authors:N. Sergent, P. Gélin, L. Périer-Camby, H. Praliaud, and G. Thomas
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.
Authors:H. Nasser, Á. Rédey, Tatiana Yuzhakova, and J. Kovács
In order to explore the influence of CeO2 on the structure and surface characteristics of molybdena, an investigation was undertaken by using N2 adsorption (BET method), thermal analysis and in-situ diffuse reflectance infrared (DRIFT) techniques. In this work, the
Mo/CeO2 and Ce-Mo/Al2O3 samples were prepared by impregnation and co-precipitation methods with high Mo loadings. Combining the results one may notice
that the presence of ceria led to the increase of polymerized surface Mo species so as to forming Mo-O-Ce linkages besides
the formation of coupled O=Mo=O bonds indicative of polymeric MoO3.
From thermal analysis, it can be inferred that Mo/Al2O3 is the thermally most stable material in the temperature range used in the experiment (up to 900°C), whereas Ce-Mo/Al2O3 and Mo/CeO2 samples undergo morphological modifications above 700°C resulting in lattice defects, which motivate the mobility of Mo and
Ce ions and thus enhance the possibility of interaction between them. Additionally, their activity towards CO adsorption needs
reduced ceria and molybdena containing coordinatively unsaturated sites (CUS), oxygen vacancies and hydroxyl groups to form
various carbonate species.
Authors:C. Amairia, S. Fessi, A. Ghorbel, and A. Rîves
the full oxidation of the palladium particles [ 21 ], the poisoning effect of the formed water and carbon dioxide [ 22 ], or the effect of formed carbonatespecies [ 23 ]. Nevertheless, some authors have succeeded to prepare a Pd/Al 2 O 3 catalyst