As Cast and Rapidly SolidifiedTi-V Alloys for Selective Hydrogen Absorption
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Steam reforming of hydrocarbons is a dominant technology in the commercial production of hydrogen. Hydrogen Sorption Enhanced Reforming (HSER) is a novel concept recently proposed by Statoil, with the aim of increasing the efficiency of conventional steam reforming. This goal can be achieved by using hydride forming alloys as hydrogen absorbers from the mixtures of hydrogen with active gases such as CH4, CO, CO2 and H2O.Thermodynamic analysis has shown that HSER increases fuel conversion and reduces operation temperatures. The optimal working conditions of the HSER are determined by the hydrogen absorption-desorption characteristics of the selected metal hydride system. In the present work, titanium-vanadium alloys have been chosen and studied as potential hydrogen absorbers in the HSER. Binary Ti-V alloys containing 10-30 at. % V have been characterized with respect to their hydrogen storage capacities, thermal stability of the formed hydrides and the kinetics of hydrogen exchange. Thermodynamics of the metal-hydrogen interaction has been tuned by partially replacing vanadium with chromium. Rapid Solidification (RS) was applied to optimize the microstructure of the alloys. Furthermore, to improve tolerance of the alloys to active gases, their surfaces were modified by depositing nanoparticles Pd, Pd/Pt, and Ni. Hydrogen absorption behaviours were characterized in pure hydrogen gas as well as in the mixtures of hydrogen with active gases containing substantial amounts of CO, in order to mimic the conditions of steam reforming. The experiments were performed both at static conditions and in a gaseous flow. At ambient temperatures, Ti-V alloys showed excellent hydrogen storage capacities, close to3.9 wt. % H, and fast kinetics of hydrogen absorption, allowing the synthesis of saturated hydrides in less than 60 seconds. At high operating temperatures, 450-550 ºC, the Ti-V alloys(10-20 at.%) have slightly decreased yet significant maximum absorption capacities close to3.5 wt. % H. Although the kinetics of hydrogen absorption at high temperatures decreases, it remains reasonably fast, with most of the hydrogen absorption completed in less than 5minutes. Hydrogen absorption-desorption studies performed in a gaseous flow showed that the hydrogen storage capacity of the Ti-V alloys is mostly temperature-controlled, with 1-3 wt.% hydrogen absorbed or released within several minutes. The process of hydrogen absorption is much less affected by the hydrogen partial pressure. Lower interaction temperatures result in higher hydrogen absorption capacities. Cycling resulted in a decreased reversibility of hydrogen storage which accelerated in presence of CO because of degradation. Deposition of Ni and mixed Pd/Pt nanoparticles on the surface of the alloys caused an improvement of the hydrogenation kinetics in presence of substantial amounts of CO in the gas stream. Ni and mixed Pd/Pt showed a better performance as compared to Pd alone. During cycling at ideal conditions, i.e. hydrogenation in pure H2 and desorption in vacuum,Ti0.9V0.1 showed a completely reversible behaviour. In contrast, a decrease in reversible capacities was observed when hydrogenation and dehydrogenation were done in a gas flow ofa mixture of H2 and Ar. This decrease is associated with the formation of stable Ti-V hydrides during the hydrogen desorption. The presence of CO accelerates the deactivation of the alloy and partly hinders hydrogen absorption. Further to the phase-structural changes, a partial oxidation and carbon deposition on the alloy surface take place after cycling in gas mixtures containing CO, as confirmed by the data of electron probe micro-analyser (EPMA) and Auger electron spectroscopy (AES) analysis. Rapid Solidification process applied to the alloys resulted in a decreased thermal stability of the corresponding Ti-V hydrides. This is due to Ti and V separation and redistribution in the rapidly quenched BCC alloy. The kinetics of hydrogen desorption was faster due to the refined microstructure of the RS alloy. In summary, the present work shows that Ti-V hydrides could potentially be applied as absorbents in the HSER process. Their hydrogen sorption properties are determined by thechemical composition of the parent alloys, their phase-structural composition, process parameters, including hydrogen pressure, temperature, and the content of active gases in the gas mixtures. The decrease in storage capacities during cycling is associated with the process parameters, inherent properties of the Ti-V alloys and their corresponding hydrides and presence of the metal catalysts.