Elsa M. Arce-Estrada

Characterization of CoCu mechanical alloys by linear sweep voltammetry

Abstract The mechanical alloying process has shown that it is possible to obtain supersaturated solid solutions at any composition in immiscible metallic systems by ball milling of elemental powder mixtures. The mechanically alloyed CoCu powders are chemically homogeneous supersaturated solid solutions and exhibit crystallite sizes of a nanometer order. In the present work, the mechanically alloyed CoCu powders have been characterized by linear sweep voltammetry in 0.5 M NaOH and 0.15 M Na2B4O7 aqueous solutions. The electrochemical behavior has been related to the characteristics of enhanced solubility found in CoCu mechanical alloys. In order to perform a systematic study of this system, the voltammograms of pure Cu, pure Co, CoCu original powder mixtures and CoCu alloys, obtained by melting and casting, were also analyzed for comparison. The results showed that the voltammograms of mechanically alloyed CoCu powders presented peaks of dissolution at potentials intermediate between those exhibited in pure Cu and Co samples. Moreover, a third dissolution peak occurred at more noble potentials in mechanical alloys, but not in as-cast alloys. This peak is associated with the formation of the solid solution, in mechanical alloys. This suggests that both elements are dissolved simultaneously, which confirms that CoCu mechanical alloys are true supersaturated solid solutions. Additionally, it was found that the MA CoCu alloys showed corrosion rates higher than those in as-cast alloys, which is attributed to the high value of stored energy presented in MA alloys as a result of the severe plastic deformation during ball milling.

Electrocatalytic properties of mechanically alloyed Co-20wt%Ni-10wt%Mo and Co-70wt%Ni-10wt%Mo alloy powders

Mechanically alloyed Co-20wt%Ni-10wt%Mo and Co-70wt%Ni-10wt%Mo (nominal compositions) alloy powders were produced by milling of pure elemental powders. Mechanically alloyed powders were characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. MA powder specimens were tested electrollitically in a 30% KOH aqueous solution at 298 K. X-ray diffraction analysis and transmission electron microscopy of milled powders showed the presence of two phases, an fcc solid solution and intermetallic compounds of Ni or Co with Mo. These phases showed a nanometric size. The linear sweep voltammograms confirmed also the presence of two phases in both mechanically alloyed alloy powders. The Co-20wt%Ni-10wt%Mo alloy powders showed the best electrocatalytic activity for hydrogen evolution reaction.

Influence of Alumina Crystal Size on the Hydrotreating Activity of Supported NiMo Catalysts Using Real Feedstock

Three different alumina substrates having different crystal sizes were used as supports for NiMoS x catalysts. The supports were analyzed by XRD, pyridine FTIR-TPD, and nitrogen physisorption in order to determine their crystal size, surface acidity, and textural properties, respectively. After the NiMo was loaded on the γ-Al 2 O 3 supports, the metallic particles were characterized during the preparation steps (annealing and sulfidation) using transmission electron microscopy (TEM). Hydrogen TPR studies of the NiMo catalysts were carried out to correlate their hydrogenating properties and their catalytic functionality, such as hydrodesulfurization (HDS), hydrodenitrogenation (HDN), and aromatic hydrogenation (HDA) using Mexican gas oil (LGO). Experiments were carried out in a pilot plant at temperatures of 613°, 633°, and 653°K at pressure of 54 Kg cm -2 . XRD measurements were performed on the alumina supports A, B, and C to determine their crystal size (3, 5, and 10 nm, respectively), using Scherers equation. Transmission electron microscopy was used to analyze the metallic NiMo species dispersion; the dimensions and distribution of non-sulfided and sulfided metallic crystallites were confirmed by TEM. The catalytic evaluation results showed that the rate constants of hydrodesulfurization (HDS), hydrodenitrogenation (HDN), and hydrodearomatizing (HDA) at 613-653°K decreased in the following order, A > B > C, corresponding to the increase of NiMoS crystal size associated with these catalysts.