Estudi de la reductibilitat de catalitzadors basats en cobalt i el seu comportament en la reacció de desplaçament de gas d'aigua de CO

Projecte de recerca elaborat a partir d’una estada a la universitat d'Udine, Itàlia, entre setembre i desembre del 2006.S'han caracteritzat mitjançant la reducció a temperatura programada i tests catalítics catalitzadors en pols basats en cobalt i supostats en òxid de zinc i monòlits ceràmics funcionaliltzats també amb cobalt i òxid de zinc. L'addició de promotors (manganès, crom i ferro ) als catalitzadors en pols, preparats per impregnació i precipitació, no afecta significativament ni la temperatura a la qual té lloc la reducció ni al percentatge global de reducció. En els cicles de reducció-oxidació sí que s'observen diferències entre el primer perfil de reducció i els següents, especialment en el cas de la mostra que té ferro com a promotor, on les diferències s'accentuen en cicles successius (fins al quart). S'ha evaluat l'activitat d'aquests catalitzadors en la reacció de desplaçament de gas d'aigua, obtenint uns resultats satisfactoris. Finalment s'han realitzat reduccions a temperatura programada i tests catalítics en la reacció de desplaçament de gas d'aigua amb monòlits funcionalitzats amb cobalt i òxid de zinc (en cap d'ells s'ha introduït promotors). El nivell de conversió assolit és menor que en el cas de catalitzadors en pols, fet que s'associa a la geometria d'aquests sistemes catalítics, però la relació CH4/CO2 és més favorable que en els catalitzadors en pols, el que els converteix en sistemes molt selectius.

Hemilabile phosphine ligands in molybdenum and tungsten octahedral environments

The synthesis of three bidentate, hemilabile phosphine ligands, newly synthesized in the research group (TPOdiphos, DPPrPOdiphos and SODPdiphos), has been up-scaled and optimized. The ligand substitution reaction on Mo(CO)6 and W(CO)6 has been studied and the corresponding complexes fac-[MTPOdiphos(CO)3], fac-[MDPPrPOdiphos(CO)3], and fac-[MSODPdiphos(CO)3], (M= Mo, W) have been isolated in good yields and characterized by NMR, IR and HR MS. In the case of fac- [MoTPOdiphos(CO)3] the XRD crystal structure was resolved. The complexes were found to be octahedral, neutral molecules, with the metal in the zero oxidation state and the ligand adopting a facial P,P,O-coordination. The hard ligand atom (oxygen) is expected to exhibit special features the future applications of these novel ligands.

Synapses on motoneuron dendrites in the brachial section of the frog spinal cord: a computer-aided electron microscopic study of cobalt-filled cells.

Cobalt-labelled motoneuron dendrites of the frog spinal cord at the level of the second spinal nerve were photographed in the electron microscope from long series of ultrathin sections. Three-dimensional computer reconstructions of 120 dendrite segments were analysed. The samples were taken from two locations: proximal to cell body and distal, as defined in a transverse plane of the spinal cord. The dendrites showed highly irregular outlines with many 1-2 microns-long 'thorns' (on average 8.5 thorns per 100 microns 2 of dendritic area). Taken together, the reconstructed dendrite segments from the proximal sites had a total length of about 250 microns; those from the distal locations, 180 microns. On all segments together there were 699 synapses. Nine percent of the synapses were on thorns, and many more close to their base on the dendritic shaft. The synapses were classified in four groups. One third of the synapses were asymmetric with spherical vesicles; one half were symmetric with spherical vesicles; and one tenth were symmetric with flattened vesicles. A fourth, small class of asymmetric synapses had dense-core vesicles. The area of the active zones was large for the asymmetric synapses (median value 0.20 microns 2), and small for the symmetric ones (median value 0.10 microns 2), and the difference was significant. On average, the areas of the active zones of the synapses on thin dendrites were larger than those of synapses on large calibre dendrites. About every 4 microns 2 of dendritic area received one contact. There was a significant difference between the areas of the active zones of the synapses at the two locations. Moreover, the number per unit dendritic length was correlated with dendrite calibre. On average, the active zones covered more than 4% of the dendritic area; this value for thin dendrites was about twice as large as that of large calibre dendrites. We suggest that the larger active zones and the larger synaptic coverage of the thin dendrites compensate for the longer electrotonic distance of these synapses from the soma.

Divergent effect of cobalt and beryllium salts on the fate of peripheral blood monocytes and T lymphocytes.

Occupational exposure to metals such as cobalt and beryllium represents a risk factor for respiratory health and can cause immune-mediated diseases. However, the way they act may be different. We show here that the two metals have a divergent effect on peripheral T lymphocytes and monocytes: BeSO(4) induces cell death in monocytes but not in T lymphocytes, which instead respond by producing Interferon gamma (IFN-γ); conversely, CoCl(2) induces apoptosis in T lymphocytes but not in monocytes. Interestingly, both metals induce p53 overexpression but with a dramatic different outcome. This is because the effect of p53 in CoCl(2)-treated monocytes is counteracted by the antiapoptotic activity of cytoplasmic p21(Cip1/WAF1), the activation of nuclear factor κB, and the inflammasome danger signaling pathway leading to the production of proinflammatory cytokines. However, CoCl(2)-treated monocytes do not fully differentiate into macrophage or dendritic cells, as inferred by the lack of expression of CD16 and CD83, respectively. Furthermore, the expression of HLA-class II molecules, as well as the capability of capturing and presenting the antigens, decreased with time. In conclusion, cobalt keeps monocytes in a partially activated, proinflammatory state that can contribute to some of the pathologies associated with the exposure to this metal.

Cobalt and molybdenum concentrated suspension for soybean seed treatment

The concentrated suspension (CS), the basis of Mo trioxide, allows high Mo concentrations and is therefore a technical advance for seed treatment, since it allows the recommendation of the Mo at lower dosage than with the liquid solution formulations (LS). The purpose of this research was to evaluate the efficiency and doses of fertilizer with Mo and Co in concentrated suspension in comparison with liquid solution as well as fertilizers associated with phytohormones, applied in seed treatments, and their effect on soybean yield. Two experiments were carried out in the growing seasons of 2004/2005 and 2005/2006 at the Universidade Federal de Uberlandia (UFU).The first was conducted in an experimental area on the Fazenda Capim Branco, with six treatments and four replications: (1) Mo and Co (CS) - 22 g ha-1 + 1.08 g ha-1; (2) Mo and Co (CS) - 22 g ha-1 + + 1.08 g ha-1 + phytohormone -200 mL ha-1; 3) Mo and Co (LS), 20.7 g ha-1 + 4.13 g ha-1; 4) Mo and Co (LS), 20.7 g ha-1 + 4.13 g ha-1 + phytohormone -200 mL ha-1; (5) + control phytohormone-200 mL ha-1; and (6) control (free of Mo and Co in the seed treatment). The phytohormone consisted of: auxin (11 mg L-1) and cytokynin (0.031 mg L-1). The soybean cultivar Monsoy 8004 was used and a fertilization of 400 kg ha-1 of 02-20-20 NPK fertilizer was applied at sowing. Based on the results of the first experiment, the second was conducted on the Fazenda Floresta do Lobo, in Uberlândia, MG, evaluated in a randomized block design with nine treatments and four replications. The treatments consisted of Mo and Co (g ha-1) doses applied to soybean seeds, as CS formulation (15, 25, 35, 45, 60 and 0.74; 1.23; 1.72; 2.21; 2.95) and LS- (15; 20; 25 and 3.18; 4.25; 5.31), respectively, and the control (free of Mo and Co in the seed treatment). The variety Monarch was used, fertilized with 300 kg ha-1 of NPK fertilizer (03-32-06) at sowing; and 78 kg ha-1 (K2O) in topdressing 30 days after soybean emergence. The Mo and Co doses in the seed treatment with LS and CS resulted in higher soybean yields than in the control, from 20 g ha-1 Mo and 4.25 g ha-1 Co in liquid solution and 35 g ha-1Mo and 1.72 g ha-1 Co in the concentrated suspension.

Entropy change of martensitic transformations in Cu-based shape-memory alloys

We have investigated the different contributions to the entropy change at the martensitic transition of different families of Cu-based shape-memory alloys. The total entropy change has been obtained through calorimetric measurements. By measuring the evolution of the magnetic susceptibility with temperature, the entropy change associated with conduction electrons has been evaluated. The contribution of the anharmonic vibrations of the lattice has also been estimated using various parameters associated with the anharmonic behavior of these alloys, collected from the literature. The results found in the present work have been compared to values published for the martensitic transition of group-IV metals. For Cu-based alloys, both electron and anharmonic contributions have been shown to be much smaller than the overall entropy change. This finding demonstrates that the harmonic vibrations of the lattice are the most relevant contribution to the stability of the bcc phase in Cu-based alloys.

High-pressure ultrasonic study of vibrational anharmonicity in bcc Cu-Al-Be alloys

To quantify the vibrational anharmonicity of the long-wavelength acoustic modes of bcc Cu74.1Al23.1Be2.8 near its martensitic transition temperature Ms (261 K), the hydrostatic pressure derivatives (¿CIJ/¿P)P=0 of the elastic stiffness moduli have been measured. The Grüneisen parameters at 268 K (just above Ms), especially of longitudinal modes, which become smaller than those of the shear modes, are quite different from those at 295 K: the anharmonicity changes markedly in the vicinity of the transition. Similar trends are noted for Cu66.5Al12.7Zn20.8. Experimental data near Ms are used to estimate cubic invariants in the strain order parameters in a Landau formalism.

Influence of configurational atomic order on the relative stability of bcc and close-packed structures in Cu-based alloys

We present a study of the influence of atomic order on the relative stability of the bcc and the 18R martensitic structures in a Cu2.96Al0.92Be0.12 crystal. Calorimetric measurements have shown that disorder increases the stability of the 18R phase, contrary to what happens in Cu-Zn-Al alloys for which it is the bcc phase that is stabilized by disordering the system. This different behavior has been explained in terms of a model recently reported. We have also proved that the entropy change at the martensitic transition is independent of the state of atomic order of the crystal, as predicted theoretically. Our results suggest that differences in the vibrational spectrum of the crystal due to different states of atomic order must be equal in the bcc and in the close-packed phases.

Lattice-dynamical study of the premartensitic state of the Cu-Al-Be alloys

Neutron-scattering techniques have been used to study the premartensitic state of a family of Cu-Al-Be alloys, which transform from the bcc phase to an 18R martensitic structure. We find that the phonon modes of the TA2[110] branch have very low energies with anomalous temperature dependence. A slight anomaly at q=2/3 was observed; this anomaly, however, does not change significantly with temperature. No elastic peaks, related to the martensite structure, were found in the premartensitic state of these alloys. The results are compared with measurements, performed under the same instrumental conditions, on two Cu-Al-Ni and a Cu-Zn-Al martensitic alloy.

Elastic constants of bcc Cu-Al-Ni alloys

We have measured the adiabatic elastic constants of two Cu-Al-Ni martensitic alloys using ultrasonic methods and we have compared the results to recent neutron-scattering experiments. It is shown that the elastic behavior of Cu-Al-Ni alloys follows the same trends exhibited by other Cu-based alloys; in particular, the TA2 long-wavelength acoustic modes are softer than all other modes.

Monte Carlo study of the relation between vacancy diffusion and domain growth in two-dimensional binary alloys

Domain growth in a two-dimensional binary alloy is studied by means of Monte Carlo simulation of an ABV model. The dynamics consists of exchanges of particles with a small concentration of vacancies. The influence of changing the vacancy concentration and finite-size effects has been analyzed. Features of the vacancy diffusion during domain growth are also studied. The anomalous character of the diffusion due to its correlation with local order is responsible for the obtained fast-growth behavior.

Vibrational behaviour of bcc Cu-based shape memory alloys close to the martensitic transition

Experimental data from ultrasonic and inelastic neutron scattering measurements are analyzed for different families of Cu-based shape-memory alloys. It is shown that the transition occurs at a value, independent of composition and alloy family, of the ratio between the elastic constants associated with the two shears necessary to accomplish the lattice distortion from the bcc to the close-packed structure. The zone boundary frequency of the TA2[110] branch evaluated at the transition point (TM), weakly depends, for each family, on composition. A linear relationship between this frequency and the inverse of the elastic constant C', both quantities evaluated at TM, has been found, in agreement with the prediction of a Landau model proposed for martensitic transformations.

On the stability of the bcc phase in Cu-Al-Mn shape memory alloys

Measurements of the entropy change at the martensitic transition of two composition-related sets of Cu-Al-Mn shape-memory alloys are reported. It is found that most of the entropy change has a vibrational origin, and depends only on the particular close-packed structure of the low-temperature phase. Using data from the literature for other Cu-based alloys, this result is shown to be general. In addition, it is shown that the martensitic structure changes from 18R to 2H when the ratio of conduction electrons per atom reaches the same value as the eutectoid point in the equilibrium phase diagram. This finding indicates that the structure of the metastable low-temperature phase is reminiscent of the equilibrium structure.