CC BY
16XV International Symposium on Self-Propagating High-Temperature Synthesis
EXOTHERMIC HYDROGENATION KINETICS OF MG WITH CATALYTIC DISSOCIATION OF MOLECULAR HYDROGEN
M. Ohyanagi*", Y. Shimizu", M. Otowaki", and K. Shirai"
aDepartment of Materials Chemistry, Ryukoku University, Ohtsu, 520-2194 Japan *e-mail: ohyanagi@rins.ryukoku.ac.jp
DOI: 10.24411/9999-0014A-2019-10113
Most of materials synthesis with highly exothermic reaction of reactants is classified as combustion synthesis. The combination of melting of one reactant and dissolution of the other by the heat release from combustion wave front sustains the self-propagation of reaction. The apparent activation energy of combustion synthesis is calculated by the propagation velocity and maximum combustion temperature[1, 2]. However, in view point of the microscopic phenomenon for momentary time, the contact and diffusion of one reactant to the other before the melting in the combustion wave front initiate the exothermic reaction, leading subsequent both of the ignition and the combustion. When one of the reactant is gaseous, the reaction with the other solid reactant must initiate first by reaction-controlled step and then immediately occur with the diffusion-controlled step between gas and solid in the initial stage of combustion synthesis, often followed by the melting, because the most of gas-solid reaction becomes very fast at the surface of solid just after the ignition and the gas is consumed there so that the reaction rate is controlled by diffusion of gas in the boundary layer between gas and solid [3]. In case of the less exothermic combustion between gas and solid, the uniform heating of whole reactants often leads this kind of combustion regime, that is, the volume combustion. The criteria between the reaction-controlled step at the surface of solid and the diffusion-controlled step in the boundary layer between gas and solid is if the reaction rate of product or the consumption rate of reactant gas strongly depends on the temperature for the reaction. In the volume combustion synthesis, the apparent activation energy is not capable to calculate from the both of propagating velocity and the maximum combustion temperature. The activation energy for the volume combustion has to be calculated by the Arrhenius plot of reaction kinetic constant.
The exothermic reaction of Mg and hydrogen molecule takes place reversibly with reaction heat of 75.9 kJ/mol at 293 K [4]. The adiabatic temperature for the hydrogenation is 1255 K. The rate-determined step to form MgH2 from Mg + H2 is considered to be a dissociation-adsorption of molecular hydrogen, not diffusion of H into Mg. The dissociation-adsorption of molecular hydrogen also accelerates by the catalyst such as metal oxides [5]. The initial stage of hydrogenation of Mg using catalytic Nb2O5 and TiO was kinetically analyzed by the consumption of hydrogen gas from a decrease in hydrogen pressure in given volume using Sieverts method as shown in Fig. 1 with assumption of the rate-determining step of the dissociation-adsorption of molecular hydrogen as a first order reaction at the temperature below 313 K for Nb2O5 catalyst and 423 K for TiO catalyst. The lower activation energies for the catalysts compared to the value for Mg without catalyst, 72-97 kJ/mol [6] were calculated to be 45 and 31 kJ/mol in the lower temperature range, respectively. However, as shown in Fig. 2 the hydrogen pressure for the hydrogenation of Mg with Nb2O5 catalyst abruptly decreased in time at 303 K to 323 K, that is, the consumption rate of hydrogen intensively increased in the transient temperature range, just like ignition and combustion of Mg and molecular hydrogen. As the consumption rate of hydrogen is almost equal or slightly increase at the several initial temperatures, in other words, did not depend on the temperature so much, the rate constant was calculated based on a diffusion-controlled mechanism in the boundary layer (apparent first order reaction) for the combustion-type hydrogenation above the transient temperature which
ISHS 2019 Moscow, Russia
gives abrupt consumption of hydrogen gas. The apparent activation energies for the catalysts were calculated to be nearly equal to be zero above 313 K for Nb2O5 and 423 K for TiO, respectively.
Fig. 1. Illustration of pressure-composition-temperature measurement apparatus (Sieverts method); The pressure was recorded against time. Under the initial steady pressure, the time which the pressure started to decrease was defined as t = 0.
Time [see]
Fig. 2. A decrease of hydorge presure for the hydrogenation of Mg (dehydrogenated from MgH2 at 623 K for 2h in vacuo) with 10 wt % Nb2O5 catalyst at the given temperature.
1. C.-L. Yeh, C.-Y. Ke, Combustion synthesis of FeAl-based composites from thermitic and intermetallic reactions, Cryst., 2019, vol. 9, no. 3, p.127.
2. C. L. Yeh, W.H. Chen, Preparation of niobium borides NbB and NbB2 by self-propagating combustion synthesis, J. Alloys Compd., 2006, vol. 420, no. 1, p. 111.
3. G. Bruce, Clean coal engineering technology, 2ed, Chapter 4, 2017, 195 p.
4. J.M.W. Chase, et al., J. Phys. Chem. Ref. Data, 1985, vol. 14, no.1.
5. A. Borgschulte, J. H. Rector, B. Dam, R. Griessen, A. Zuttel, The role of niobium oxide as a surface catalyst for hydrogen absorption, J. Catal., 2005, vol. 235, no. 2, p. 353.
6. M. Pozzo, D. Alfe, Hydrogen dissociation and diffusion on transition metal (= Ti, Zr, V, Fe, Ru, Co, Rh, Ni, Pd, Cu, Ag)-doped Mg(0001) surfaces, Int. J. Hydrogen Energy, 2009, vol. 34, no. 4, p. 1922.
