Olga S. Morozova

Mechanism and kinetics of mechanically induced transformation of titanium and titanium hydride: Effect of reaction medium on microstructure, morphology and hydrogen-uptake properties

The stimulating effect of graphite addition on hydrogen sorption/desorption properties of titanium activated under ball milling and titanium hydride activated or prepared under ball milling is studied using X-ray diffraction, scanning and transmission electron microscopy, and temperature-programmed reaction/desorption techniques. The formation of microporous carbon matrix containing randomly distributed titanium or titanium hydride fragments of several nm in size is found to be the major effect of graphite addition. This kind of morphology allows the hydrogen transport to titanium or from titanium hydride surfaces without hindrances and improves titanium-hydrogen interaction through modifying the titanium surface and subsurface layers by interstitial C atoms.

Co and Co2 hydrogenation under mechanochemical treatment

Abstract The effect of mechanochemical treatment on the hydrogenation of CO and CO2 was studied using a flow milling vial as a catalytic reactor. Traditional catalysts for the methane formation (Ni, Zr, Zr-Ni-containing samples, Zr hydride and Zr-Ni hydrides of various genesis), as well as inactive NiO and ZrO2, were examined. The following features of the mechanically driven CO hydrogenation were observed: (i) Zr and Zr-Ni hydrides demonstrated the highest activity in the formation of CH4; (ii) the hydrogen present in the reaction mixture suppressed the formation of CH4; (iii) Ni was found to be ineffective in the formation of CH4 but active in the reaction of CO disproportionation; (iv) on the oxide samples, a small amount of CH4 was formed owing to the mechanical activation. Deep structural transformations of the metal and hydride samples under milling in the reaction mixture were found to be responsible for the changes in their catalytic activity. Only zirconium hydride exhibited catalytic activity for CO2 hydrogenation. In this case, no phase transformation was observed.

Hydrogenation of Co and C during Mechanical Treatment of Zr and Ni Containing Systems

Processes of CH 4 and C 2 H 6 formation during mechanical treatment of NiZrH x , ZrH x , NiZr, Zr, Ni, and Zr+Ni in the presence of CO + H 2 or CO atmosphere as well as the mechanochemical reaction of hydrides with carbon (graphite) have been investigated using X-ray, chemisorption, thermodesorption, mass-spectroscopy and chromatography. It was found that I) the rate of methane formation decreases when Ni is included into the systems, 2) 100% hydrogen of hydrides may be converted into methane in mixtures of the hydrides with carbon, 3) in the presence of CO the major part of hydrides hydrogen is evolved in a molecular form while only <15% of H 2 is expended for the methane formation. The results are discussed in terms of mechanochemical reactions of Zr with gases.

Size distributions of nanoscopic holes in Ti/h-BN and Ti/B nanocomposites

Positron annihilation spectroscopy was employed for defect studies of “Ti”-based nanocomposites prepared by high-energy ball milling and consisting of Ti nanoparticles separated by hexagonal boron nitride (h-BN) or boron (B) additive. The size distribution of nanoscopic holes in nanocomposites was determined directly from measurement of ortho-positronium (Ps) lifetimes. Chemical environment of defects was characterized using coincidence Doppler broadening. It was found that size of nanoscopic holes is reduced with increasing milling time in H2/He atmosphere and also probability of Ps formation in holes decreases. At the same time the Ti content in the vicinity of holes increases. This can be explained by (i) increased intermixing of Ti particles with h-BN or B additive and by (ii) filling the nanoscopic holes with absorbed hydrogen. Analysis of obtained results showed that both these processes take place during milling of nanocomposites. In addition, it was found that the effect of filling the nanoscopic ...

Ti/C and Ti/B Nanocomposites: Comparison of Sorption-Desorption Properties

Hydrogen sorption-desorption properties of Ti/B and Ti/C nanocomposites prepared under ball-milling of the corresponding powders in H2 flow were studied using kinetic, microscopic and spectroscopic techniques. Amorphous boron was found to be more effective in spurring Ti – H2 interaction than carbon because of the following properties: (1) significant fragmentation of Ti powder by preventing agglomeration of the particles; (2) unhindered hydrogen access to the surface of Ti nanoparticles through the boron matrix; (3) appearance of new occupation sites available for H atoms, which are characterized by low H2 desorption temperature. The dynamics of the formation of these sites and the H2 distribution between different occupation sites in dependence on phase composition and morphology were studied for the Ti/B system.

Effect of additives on titanium-hydrogen interaction under ball milling of Ti powder probed by hard x-ray emission spectroscopy

Titanium is well known as a light-weight hydrogen storage material that is applied as a component of hydrogen storage composites together with Mg and other metals. Amorphous boron, boron nitride, and graphite were used as additives to improve Ti-H2 reactivity during ball-milling due to its anti-sticking and matrix-forming properties. The chemical state and local electronic structure of Ti atoms were studied by hard x-ray emission spectroscopy (XES). We have measured fluorescent Ti Kβ5 (4p→1s transition) x-ray emission spectra, which are very sensitive to the local surroundings of exciting atoms, and found additional features coinciding in energy with spectra of reference samples TiB2, TiN, and TiC. Based on these measurements, it is concluded that atoms of additives form chemical bonding with Ti due to the occupation of interstitials in the host Ti-lattice.

Boron Enhanced Synthesis of Ti-hydride Nanoparticles by Milling Ti/B in Hydrogen Flow

Morphological, structural and chemical evolution in Ti/B/H2 system is studied in detail as a function of mechanical treatment. Ti/B powder continuously changes both in composition and morphology during ball-milling in H2 flow: The powder composition varies from Ti/B to TiH2-x/B causing a change in mechanical properties. The role of boron additive also changes from preventing the Ti nanoparticles from sticking together in the early stages to a matrix material participating in Ti - B interface reactions in the intermediate and final stages of the process. Boron atoms participating in the formation of nanoscopic holes give rise to new H states in the hydride by changing the local atomic state of Ti atoms. The dynamics of the formation of these sites and the redistribution of hydrogen between dif- ferent types of occupation sites in dependence of phase composition and milling time of the powders are also studied.