G. Cocco

Bauxite 'red mud' in the ceramic industry. Part 1: thermal behaviour

Samples of red mud, by-products of alumina production from bauxite, are studied in the 120–1400°C interval. An extensive characterization was performed by thermal and X-ray diffraction analyses. The identification of gaseous species released upon heating was carried out by coupling the thermal analizer with a gas-chromatographic/mass spectrometer. Density evolution was also determined as a function of the heat treatment. Results indicate primary H2O release from aluminium hydroxides, followed by carbonate decomposition with CO2 evolution below 900°C. Alkaline oxides, mainly CaO and Na2O, lead to the formation of Ca3Al2O6 and NaAlSiO4 between 900 and 1100°C. At the highest temperatures, reduction of Fe3+ to Fe2+, involving O2 release, promotes the formation of Fe2TiO4, with the disappearance of the rutile-TiO2 phase. The various solid state reactions, ascertained at different stages of the heating process, and possible mass balances are discussed with reference to the state diagrams of principal red mud components.

Influence of metal dispersion on selectivity and kinetics of phenylacetylene hydrogenation catalyzed by supported palladium

Abstract Several Pd metal dispersions on vitreous supports and on charcoal have been prepared and used in the half-hydrogenation of phenylacetylene. The metal particle size distributions have been determined both by small angle X-ray scattering (SAXS) and wide angle X-ray scattering (WAXS). The degree of dispersion was dependent on the nature of the support; in the most favorable cases the Pd load was dispersed in metallic particles smaller than 30 A in diameter. These catalysts hydrogenate phenylacetylene under mild conditions in heptane suspension. The kinetic results indicate that both half-hydrogenation rate and selectivity were largely affected by the degree of dispersion of Pd: a considerable reduction of catalytic effectiveness and selectivity has been observed as the averaged particle diameter was increased. Results have been interpreted and discussed in terms of hydrogen availability on the reacting catalytic center and of the metallic phase features.

The invariant laws of the amorphization processes by mechanical alloying: I. Experimental findings

Abstract Several Ti-, Zr-, Hf- and Nb-based alloys were synthesized using differing milling regimes. Metastable phases, either crystalline or amorphous, develop from the parent elements according to a general sigmoid-shaped behaviour ruled by an interface-controlled kinetic mechanism. The extent of the alloying reactions was related to the operating variables, experimentally determined in the course of the process. Although the transformation rates depended on the milling intensity, that is on the impact energy times the impact frequency, it was found that the reaction yield, defined by the ratio of the transformed fraction to the specific energy dose, is an invariant quantity characteristic of each system. The specific energy dose defines the mechanical work done on the system per mass unit of the reactants. A rationale for the observed behaviours was provided by the energy needed to reach a given level of the reactant dispersion. Ruling the total extent of the grain boundary area, and the overall kinetics of the alloying process, the work expended in the microstructure refinement was found to be another invariant property of the treated mixtures. The reaction yield is the reference parameter to compare milling trials on an absolute basis, so providing an opportunity towards a quantitative understanding of the mechanical alloying processes.

Mechanical alloying of the Al–Ti system

Abstract We have investigated the possibility of an amorphization reaction by mechanical alloying for two compositions of the Al–Ti binary system. While the Al-rich composition Al75Ti25 appears to give, after milling for 21 h, an Al(Ti) highly cubic phase, the Ti-rich composition Al25Ti75 does amorphize using various milling conditions. The progress of the amorphization as a function of time of milling was monitored by X-ray diffraction. At first, Al atoms diffuse into the host lattice of hexagonal Ti; subsequently, the milling accumulates a critical density of disorder that causes the Ti(Al) crystalline phase to collapse into an amorphous phase. The formation of amorphous alloys is discussed on the basis of thermodynamic models. The Miedema model is compared with a calculation of phase diagrams approach which has been modified to account for dependence of thermodynamic properties of the liquid upon temperature. T 0 curves are presented, showing a glass-forming range in agreement with experiments.

Hexagonal close packed nickel powder: Synthesis, structural characterization and thermal behavior

Abstract Hexagonal close packed nickel powder has been prepared upon reduction of Ni(II) with a K/B liquid alloy. This structural habit is stable at low temperature, undergoing a transition to a face centered cubic phase above 380°C.

X-ray diffraction study of the amorphization process by mechanical alloying of the NiTi system☆

Abstract X-ray Fourier methods were used to follow fragmentation phenomena and structural defects during mechanical alloying processes in the NiTi system. Furthermore, a methodological X-ray diffraction approach was developed to study the kinetics of the process up to complete amorphization of the powders. Results show that the effective crystallite size and defect concentration of the unreacted powders achieve asymptotic values after mechanical alloying for about 5 h, whereas the amorphization process is accomplished after 30 h. The limiting value of the effective sizes, calculated after correction for the strain contribution, is around 30 nm, which is one order of magnitude larger than the values reported in these materials from a line-broadening analysis using the Scherrer equation. As for the defect concentration, elemental titanium and nickel powders milled separately show approximately the same strain content as the NiTi mixture treated under the same experimental conditions. However, the pre-induced disorder does not speed up the solid state reaction; in fact, when the single elements milled separately are reacted, the amorphization process is also completed in 30 h.

Thermal properties of mechanically alloyed Ni50Ti50 powders

Abstract Ni 50 Ti 50 amorphous powders were prepared by the novel technique of ball milling pure elements. Amorphization was complete after milling for about 35 h. X-ray diffraction was used to determine the amount of amorphous phase produced. Differential scanning calorimetry (DSC) was employed for thermal analysis. At various stages of alloying, two phenomena were detected. In the temperature range 450–650 K, the amorphization of part of the alloy occurs, giving a large exothermal signal. Peak shape analysis was used to estimate the coefficient of interdiffusion in the amorphous alloy. In the temperature range 680–780 K, crystallization occurs. The DSC peak changes its shape and area with milling time. Increases in the temperature of the maximum and of the heat crystallization were observed. The role of the specific heat difference between liquid and crystal phases for solid state amorphization is outlined.

Hydrogenation of nitrocompounds with supported palladium catalysts: Influence of metal dispersion and nitrocompound nature

Nitrobenzene, Et-NO2, and t-Bu-NO2 are hydrogenated to corresponding amines using Pd catalysts in n-octane suspension at 90 °C and at constant H2 pressure. Nitrobenzene reduction to aniline has been studied with several Pd catalysts having a different degree of metal dispersion determined by X-ray methods and chemisorption analysis. Results indicate that the process is a “structure sensitive reaction”; a peculiar lowering in catalytic activity as the degree of Pd dispersion increases is observed. This fact is discussed in terms of metallic surface oxidation due to the sorbed nitro-compound. Hydrogenation kinetic patterns change with the nature of the nitrocompound. Reduction of Et-NO2 and t-Bu-NO2 depends on substrate concentration, while nitrobenzene hydrogenation is independent of this parameter. The relevant kinetic experiments allow the formulation of a general reaction mechanism accounting for the different kinetic patterns observed on changing the substrate. The discussion illustrates the possibility that in nitrocompound reduction with metal catalysts the rate determining step may be hydrogenation of the metallic surface oxidized by the sorbed nitrocompound.

Bimodal metal particle size distributions of Pd and Pt on vitreous supports from SAXS data

Abstract A study of metal particle size distribution using small-angle X-ray scattering (SAXS) is reported for low-content Pd and Pt catalysts, prepared by reduction with LiH or H2 of M(C3H5)2 (M = Pd,Pt) treated with vitreous supports holding −OH groups on the surface. In the mathematical treatment of SAXS data, both analytical and numerical methods have been employed and compared. Two different sizes in the aggregation state of the metal have been ascertained in all the examined cases, one of which under 30 A. A comparison with transmission electron microscopy (TEM) results is also presented. The reliability of SAXS technique is discussed in terms of absolute scattering power.

Role of molecular interactions during the preparation of Pd and Pt/glass hydrogenation catalysts, in determining the physical state and chemical reactivity of the metal

Pd and Pt catalysts on vitreous materials, with measured physicochemical properties, have been prepared starting from the corresponding M(C3H5)2 precursors. The catalytic activity in the hydrogenation of 1-hexene was found to be strictly dependent on the metal particle size determined by SAXS techniques. The preparative procedure for the catalysts which involves prior chemical interaction of M(C3H5)2 with the −OH groups of the support is demonstrated to be the fundamental step in achieving a good metal dispersion (clusters < 30 A in diameter), the sorber M(C3H5)2 species bearing larger and practically unreactive metal particles. The different chemical reactivity between small and large metal particles, suggested by catalysis data, is tested by the reaction of dispersed Pt metal with CO and phosphines to give carbonyl-phosphino complexes; this is discussed and compared with the chemical reactivity of known organometallic clusters.