ABSTRACT
Ceria and ceria-zirconia nanomaterials of different origin were studied in order to elucidate the role of their structural and textural characteristics in controlling the performance towards CO2 capture. Two commercial cerias and two home-prepared samples, CeO2 and CeO2-ZrO2 (75% CeO2) mixed oxide, were investigated. The samples were characterized by a number of analytical techniques including XRD, TEM, N2-adsorption, XPS, H2-TPR, Raman and FTIR spectroscopy. Static and dynamic CO2 adsorption experiments were applied to assess the CO2 capture performance. The type of surface species formed and their thermal stability were evaluated by in situ FTIR spectroscopy and CO2-TPD analysis. The two commercial ceria samples possessed similar structural and textural characteristics, formed the same types of carbonate-like surface species upon CO2 adsorption and, consequently, demonstrated almost identical CO2 capture performance under both static and dynamic conditions. The thermal stability of the adsorbed species increased in the order bidentate (B) carbonates, hydrogen carbonates (HC) and tridentate carbonates (T-III, T-II, T-I). Reduction of CeO2 increased the relative amount of the most strongly bonded T-I tridentate carbonates. Preadsorbed water led to hydroxylation and enhanced formation of hydrogen carbonates. Although the synthesized CeO2 sample had a higher surface area (by 30%) it showed a disadvantageous long mass transfer zone in the CO2-adsorption breakthrough curves. Because of its complex pore structure, this sample probably experiences severe intraparticle CO2 diffusion resistance. Having the same surface area as the synthesized CeO2, the mixed CeO2-ZrO2 oxide exhibited the highest CO2 capture capacity of 136 µmol g-1 under dynamic conditions. This was related to the highest concentration of CO2 adsorption sites (including defects) on this sample. The CeO2-ZrO2 system showed the lowest sensitivity to the presence of water vapor in the gas stream due to the lack of dissociative water adsorption on this material.
ABSTRACT
The storage of energy by means of reversible intercalation of bivalent magnesium ions represents, nowadays, the shortest route for doubling the energy density of conventional lithium ion batteries. Contrary to the intercalation of monovalent lithium ions, the intercalation of Mg2+ is a kinetically limited process. Herein we demonstrate a new approach for improving Mg2+ intercalation, which is based on dual intercalation of Li+ and Mg2+ ions with a synergic effect. The concept is proved on the basis of eco-compatible oxides such as magnesium manganate spinels, MgMn2O4, and lithium titanates Li2TiO3 and Li4Ti5O12 with a monoclinic and spinel structure. These two types of oxides are selected since they exhibit high and low potentials of ion intercalation due to the redox couples Mn2,3+/Mn3,4+ and Ti3+/Ti4+, respectively. Through a newly developed method of synthesis, we succeeded in the preparation of well-crystallized nanosized spinels with a specific cationic distribution. The intercalation properties of MgMn2O4, Li2TiO3 and Li4Ti5O12 are first examined in model cells versus the metallic Li anode. The Li+ and Mg2+ intercalation is directed by the kind of the used electrolyte: a lithium electrolyte consisting of 1 M LiPF6 solution in EC : DMC and a magnesium electrolyte consisting of 0.5 Mg(TFSI)2 solution in diglyme. The mechanism of Li+ and Mg2+ co-intercalation is assessed by ex situ X-ray diffraction and high-resolution transmission electron microscopy (HR-TEM). Finally, a new type of hybrid Li-Mg ion cell combining MgMn2O4 and Li4Ti5O12 oxides as electrodes is constructed. The cell configuration allows reaching an operating voltage of around 1.7 V by using an electrolyte containing 0.5 M LiTFSI in diglyme.
ABSTRACT
The reduction of a 1.3% Rh/TiO2 sample with carbon monoxide leads to the formation of uniform Rh nanoparticles with a mean diameter of dp ≈ 2.2 nm. Adsorption of CO on the reduced Rh/TiO2 produces linear and bridged carbonyls bound to metallic Rh(0) sites and only a few geminal dicarbonyls of Rh(I). The ν(CO) of linear Rh(0)-CO complexes is strongly coverage dependent: it is observed at 2078 cm(-1) at full coverage and at ca. 2025 cm(-1) at approximated zero coverage. At low coverage, this shift is mainly caused by a dipole-dipole interaction between the adsorbed CO molecules while at high coverage, the chemical shift also becomes important. Hydrogen hardly affects the CO adlayer at high CO coverages. However, on a partially CO-covered surface (θCO ≈ 0.5), the adsorption of H2 at increasing pressure leads to a gradual shift in the band of linear Rh(0)-CO from 2041 to 2062 cm(-1). Subsequent evacuation almost restores the original spectrum, demonstrating the reversibility of the hydrogen effect. Through the use of (12)CO + (13)CO isotopic mixtures, it is established that the addition of hydrogen to the CO-Rh/TiO2 system leads to an increase in the dynamic interaction between the adsorbed CO molecules. This evidences an increase in the density of the adsorbed CO molecules and indicates segregation of the CO and hydrogen adlayers. When CO is adsorbed on a hydrogen-precovered surface, the carbonyl band maximum is practically coverage independent and is observed at 2175-2173 cm(-1). These results are explained by a model according to which CO successively occupies different rhodium nanoparticles.
ABSTRACT
A series of manganese-ceria supported on alumina catalysts with various Mn/Ce ratios are investigated in both methanol decomposition to CO and hydrogen and SCR of NO(x) with CO. The study is aimed at the potential application of both reactions in integrated devices, where NO(x) is reduced with the products of the decomposed methanol. The samples are characterized by nitrogen physisorption, XRD, TEM, XPS, UV-Vis, and TPR. It was established that manganese-ceria supported on alumina catalysts are perspective in both methanol decomposition and NO reduction at temperatures above 723 K, which are typical for exhausted gases from the vehicles and some stationary stations. The best catalytic activity and selectivity to the desired products under these conditions was found for the samples with Mn/Mn+Ce ratio of 0.5 and 0.7. This superior catalytic performance is related to the formation of mixed valence Mn(3+)/Ce(4+) and Mn(4+)/Ce(3+) active sites.
ABSTRACT
The effect of Ce addition to alumina-supported copper, cobalt, and copper-cobalt oxides with low loadings on the catalysts efficiency in NO reduction with CO was studied. The attention was focused on varying the impregnation procedure in the ternary-supported catalysts in order to determine the best catalyst as well as the reasons for the enhanced catalytic activity. Ternary Co-Cu-Ce and binary Co-Ce, Cu-Ce, and Cu-Co-supported alumina were prepared and characterized by ICP, XRD, TEM, adsorption studies, XPS, H(2)-TPR, and catalytic investigations. The high activity of the ternary and the binary catalysts was determined by the favorable influence of the added cerium on the dispersion of the copper and cobalt active phases. The presence of ceria contributes to the formation of appropriate active phases, resulting in catalytic sites on the surface of the samples that promote the reduction of NO with CO.
