Influence of titanium and nickel dopants on the dehydrogenation properties of Mg(AlH4)2: Electronic structure mechanisms

The structures and dehydrogenation properties of pure and Ti/Ni-doped Mg(AlH₄)₂ were investigated using the first-principles calculations. The dopants mainly affect the geometric and electronic structures of their vicinal AlH₄ units. Ti and Ni dopants improve the dehydrogenation of Mg(AlH₄)₂ in different mechanisms. In the Ti-doped case, Ti prefers to occupy the 13-hedral interstice (Ti_iA) and substitute for the Al atom (Ti_Al), to form a high-coordination structure TiH_n (n = 6, 7). The Ti 3d electrons hybridize markedly with the H 1s electrons in Ti_Al and with the Al 3p electrons in Ti_iA, which weakens the Al–H bond of adjacent AlH₄ units and facilitates the hydrogen dissociation. A TiAl₃H₁₃ intermediate in Ti_iA is inferred as the precursor of Mg(AlH₄)₂ dehydrogenation. In contrast, Ni tends to occupy the octahedral interstice to form the NiH₄ tetrahedron. The tight bind of the Ni with its surrounding H atoms inhibits their dissociation though the nearby Al–H bond also becomes weak. Therefore, Ti is the better dopant candidate than Ni for improving the dehydrogenation properties of Mg(AlH₄)₂ because of its abundant activated hydrogen atoms and low hydrogen removal energy.     

Bonding states of hydrogen for supported Ti clusters on pristine and defective graphene

To determine whether graphene-supported Ti clusters can synergistically store hydrogen through Kubas and spillover effect, we systematically investigate the growth pattern of Ti_n (n = 1–10) clusters on pristine and defective graphene and analyze multiple types of bonding states of hydrogen in detail. For pristine graphene, the most stable Ti_n (n = 1–10) clusters are the quasi-planar structure except for supported Ti₄ and Ti₅. The Ti dissociation energies and the binding energy of Ti_n clusters gradually increase with increasing n, which indicates that larger Ti_n clusters tend to form. Efficient spillover will occur on single-site Ti Catalysts at low hydrogen concentration due to the lower hydrogen spillover energy barrier (3.05 eV), while the energy barrier of hydrogen migration from Tin (n = 2–7) clusters to graphene on the cluster is 5.34–6.82 eV. The Ti: H ratio is a maximum of 1:8 for the single-site Ti catalyst, while decreases with the Tin cluster increases. Therefore, the pristine graphene-supported Ti nanoclusters are more suitable as substrates for hydrogen adsorbent rather than spillover. The introduced defects make Ti_n clusters have three-dimensional conical configurations from n = 4. Ti₃ and Ti₆ are the most stable clusters. Moreover, the migration energy barriers of H atoms on them decrease from 6.54 eV to 6.82 eV–4.32 eV and 3.42 eV, respectively. Our results explain recent experimental phenomena [Appl Phys Lett 2015; 106: 083,901. ACS Energy Lett 2022; 7 (7): 2297–2303] in depth at the molecular level.     

X-ray photoelectron spectroscopy investigation of magnetron sputtered Mg–Ti–H thin films

Thin film samples of Mg₈₀Ti₂₀ (Mg–Ti) and Mg, both with and without H, were investigated in a series of X-ray photoelectron spectroscopy (XPS) measurements. The samples were covered with a thin protective layer of Pd, which was removed by Ar⁺ sputtering prior to data acquisition. This sputtering was found to reduce both oxides and hydrides. A distinct, previously unknown peak was revealed in the Mg KLL spectrum of the Mg–Ti–H samples, located between the metallic and the MgO component. This peak was attributed to trapping of H in very stable interstitial sites at the interface between Ti nano-clusters and the Mg matrix, based on earlier density functional theory calculations and supported by so-called Bader analysis. The latter was performed in order to study the theoretical charge distribution between Mg, Ti and H, establishing a link between the position of the previously unknown peak and the effect of H on the valence state of Mg. The composition of the samples was studied both by energy dispersive spectroscopy using transmission electron microscopy and by quantitative XPS analysis. Final state Auger parameters (AP) were obtained for metallic Mg, MgO and MgH₂, as well as Mg affected by trapped H. No difference between the AP values from the metallic components was found between the Mg and the Mg–Ti samples. The AP values for MgO and MgH₂ were consistent with previous reports in literature; several eV lower than the metallic value. Mg in the vicinity of trapped hydrogen, on the other hand, showed a more metallic character, with its corresponding AP value less than 1 eV below the AP for pure Mg.     

Hydrogen absorption kinetics and structural features of NaAlH4 enhanced with transition-metal and Ti-based nanoparticles

The hydrogen cycled (H) planetary milled (PM) NaAlH₄ + xM (x < 0.1) system (M = 30 nm Ag, 80 nm Al, 2–3 nm C, 30 nm Cr, 25 nm Fe, 30 nm Ni, 25 nm Pd, 65 nm Ti) has been studied by high resolution synchrotron powder X-ray diffraction. Isothermal absorption kinetic isotherms have been measured over the first two H cycles. The PM NaAlH₄ + 0.1Ti system has also been studied by high resolution transmission electron microscopy (TEM). 80 nm Al and 2–3 nm C were inactive, and would not allow hydrogen (H) desorption from NaAlH₄. 30 nm Cr, 25 nm Fe, 30 nm Ni, and 25 nm Pd showed activity, but with weak kinetics of only ca. 1 wt.% H/hour. The NaAlH₄ + 0.1Ti system displays absorption kinetics of ca. 7 wt.% H/hour, comparable to TiCl₃ enhanced NaAlH₄ after five H cycles. After H cycling the PM NaAlH₄ + 0.1Ti system, we observe a body centred tetragonal (bct) χ-TiH₂ phase, which displays intense anisotropic peak broadening. The broadening is evident as a massive dislocation density of ca. 1.20 × 10¹⁷/m² in high resolution TEM images of the χ-TiH₂ phase. All originally added Ti can be accounted for in the bct χ-TiH₂ phase by quantitative phase analysis (QPA) after five H cycles. The PM NaH + Al + 0.02 (Ti-nano-alloy) system shows absorption kinetic rates in the order TiO₂ > TiN > TiC > Ti, with rapid hydrogenation kinetics of ca. 23 wt.% H/hour for TiO₂ enhanced NaAlH₄, equivalent to TiCl₃ enhanced NaAlH₄. The TiN and TiC are partially reduced by ca. 7 and 22% respectively, and the TiO₂ is completely reduced. The location of the reduced Ti cannot be discerned by X-ray diffraction at these minor proportions.     

Hydrogen penetration and diffusion on Mg17Al12 (110) surface: A density functional theory investigation

The adsorption, diffusion and penetration of H on the Mg₁₇Al₁₂ (110) surface are investigated systematically by means of the density functional theory calculations. Results indicate that H and H₂ prefer to adsorb on the Mg-Mg bridge sites of the Mg₁₇Al₁₂ (110) surface. The lowest barrier energy of molecular hydrogen dissociation on the (110) surface is ～0.87 eV. The penetration processes of atomic hydrogen incorporation into the Mg₁₇Al₁₂ (110) surface are discussed. It is obtained that the H penetrates from the Mg₁₇Al₁₂ (110) surface into the subsurface with the minimum barrier of ～0.63 eV, while the hydrogen atom spreads into the deeper Mg₁₇Al₁₂ (110) surface with lower barrier.     

Improved charge transfer and morphology on Ti-modified Cu/γ-Al2O3/Al catalyst enhance the activity for methanol steam reforming

The metal-oxide interaction has been considered as an effective factor for catalytic performance in methanol steam reforming. In this work, Ti modified Cu/γ-Al₂O₃/Al catalyst was prepared by anodization technology. It is found that the addition of Ti can largely increase the surface area of the carrier and thus improve the dispersion of copper. The co-existence of Ti⁴⁺ and Ti³⁺ makes the charge transfer between Cu and Ti easier, which improves the redox performance of copper. The DFT calculations reveal that Ti also enhance the adsorption capacity of water and methanol on the surface of the catalysts. Besides, Ti also reduce the acid density on the carrier, inhibit methanol dehydration reaction and thereby reduce the selectivity of the DME. The optimal catalyst CuTi_1.9/γ-Al₂O₃/Al achieves nearly 100% conversion at 275 °C, while the methanol conversion of Cu/γ-Al₂O₃/Al is 82%. And the H₂ output of CuTi_1.9/γ-Al₂O₃/Al reached 69.17 mol/(kg_cat·h) at 300 °C.     

Mg2−xTixNi (x = 0, 0.5) alloys prepared by mechanical alloying for electrochemical hydrogen storage: Experiments and first-principles calculations

Mg_2−xTi_xNi (x = 0, 0.5) electrode alloys have been prepared by mechanical alloying (MA) under argon atmosphere at room temperature using a planetary high-energy ball mill. The microstructures of synthesized alloys are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The effects of substitutional doping of Ti in Mg₂Ni phase have been investigated by first-principles density functional theory calculations. XRD analysis results indicate that Ti substitution for Mg in Mg₂Ni-type alloys results in the formation of TiNi (Pm-3m) and TiNi₃ intermetallics. With the increase of milling time, the TiNi phase captures Ni from Mg₂Ni to further form TiNi₃ phase and the MgO phase increases. The calculated results of enthalpy of formation indicate that the most preferable site of Ti substitution in Mg₂Ni lattice is Mg(6i) position and the stability of phase gradually decreases along the sequence TiNi₃ phase > TiNi phase > Mg₉Ti_3Mg(6i)Ni₆ Ti-doped phase > Mg₂Ni phase. SEM observations show that the average particle sizes of Mg₂Ni and Mg_1.5Ti_0.5Ni milled alloys decrease and increase, respectively with increasing the milling time. The TEM analysis results reveal that TiNi and Mg₂Ni coexist as nanocrystallites in the Mg_1.5Ti_0.5Ni alloy milled for 20 h. Electrochemical measurements indicate that the maximum discharge capacities of Mg₂Ni and Mg_1.5Ti_0.5Ni alloys rise and decline, respectively with the prolongation of milling time. The Mg_1.5Ti_0.5Ni alloy milled for 20 h shows the highest discharge capacity among all milled alloys. The capacity retaining rate of Mg_1.5Ti_0.5Ni milled alloys is better than that of Mg₂Ni milled alloys.     

Catalytic effect of Ti2C MXene on the dehydrogenation of MgH2

In the present work, the Ti₂C MXene is prepared by selective etching Al layer from Ti₂AlC. The catalytic effect of Ti₂C MXene on the dehydrogenation of MgH₂ is investigated. Compared with the pure MgH₂, the onset desorption temperature, apparent activation energy (E_a) and the overall enthalpy changes (△H) of dehydrogenation of MgH₂-5 wt%Ti₂C decreased by 37 °C, 36.5%, 11%, respectively. The catalytic mechanism of Ti₂C on the dehydrogenation of MgH₂ is investigated by means of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscope (TEM). It is suggested that the surface Ti atoms with multivalence serve as the intermediate for electrons shifting between H⁻ and Mg²⁺, which makes the dehydrogenation of MgH₂ easier. In addition, the good hydrogen adsorption ability and thermal conductivity of Ti₂C MXene could also contribute to the improvement of thermodynamics of MgH₂.     

Atom doping in α-Fe2O3 thin films to prevent hydrogen permeation

Prevention of hydrogen (H) penetration into passive films and steels plays a vital role in lowering hydrogen damage. This work reports effects of atom (Al, Cr, or Ni) doping on hydrogen adsorption on the α-Fe₂O₃ (001) thin films and permeation into the films based on density functional theory. We found that the H₂ molecule prefers to dissociate on the surface of pure α-Fe₂O₃ thin film with adsorption energy of −1.18 eV. Doping Al or Cr atoms in the subsurface of α-Fe₂O₃ (001) films can reduce the adsorption energy by 0.03 eV (Al) or 0.09 eV (Cr) for H surface adsorption. In contrast, Ni doping substantially enhances the H adsorption energy by 1.08 eV. As H permeates into the subsurface of the film, H occupies the octahedral interstitial site and forms chemical bond with an O atom. Comparing with H subsurface absorption in the pure film, the absorption energy decreases by 0.01–0.22 eV for the Al- and Cr-doped films, whereas increases by 0.82–0.96 eV for the Ni-doped film. These results suggest that doping Al or Cr prevents H adsorption on the surface or permeation into the passive film, which effectively reduces the possibility of hydrogen embrittlement of the underlying steel.     

Theoretical and experimental study of the titanium substituted Mg2-xTixNi alloys and Mg2-xTixNiH4 hydrides

The formation of the Ti substituted Mg₂Ni alloys, a promising hydrogen storage material for various applications is studied in detail. Mg_1.95Ti_0.05Ni alloy and ribbons are successfully prepared by vacuum arc melting and melt spinning methods. The phases, microstructures, and thermal behavior of the alloys and ribbons are characterized by XRD, SEM, TEM, DTA/TG. Sievert-type apparatus is used to study hydrogen sorption properties. Apart from the dominant Mg₂Ni phase, the formation of MgNi₂, Mg, and Ni₃Ti phases is seen in both Mg_1·95Ti_0·05Ni alloy and ribbons. During the initial three cycles, Mg_1·95Ti_0·05Ni ribbons showed 2 wt % hydrogen storage capacity. To explain the atomic-scale influence of Ti dopant in the studied alloys and hydrides, FP(L)APW + lo method based on Density Functional Theory (DFT) is applied to Mg_2-xTi_xNi (x = 0.25 and 0.5) alloys and Mg_2-xTi_xNiH₄ (x = 0.25 and 0.5) hydrides. An increase in the Ti dopant on the Mg site leads to the hydrides destabilization. Bader's charge density topology analysis provides insight into the charge transfer and bonding between the constituent atoms.     

Effects of vanadium,vanadium carbide,and vanadium oxide catalysts on hydrogenation of Mg17Al12 (110) surface: A first principles study

Effects of V and V-based compounds (VC, VO, VO₂) on hydrogenation performances of the Mg₁₇Al₁₂ (110) surfaces are studied by first principles calculations. Results indicate that V and V-based compounds are stably adsorbed on the (110) surfaces. Additionally, the V-doped Mg₁₇Al₁₂ (110) systems are simulated. Calculations indicate that compared with the pure (110) system, H₂ adsorption and dissociation on the Mg₁₇Al₁₂ (110) surfaces with adding V, VC, VO, and VO₂ are dramatically promoted. Particularly, H₂ are spontaneously dissociated on the vanadium oxide-adsorbed (110) surfaces. Analyses of density of states for the VO-adsorbed and VO₂-adsorbed systems exhibit stronger hybridizations between V d sates and H s states, leading to the improvement of hydrogenation for the Mg₁₇Al₁₂ (110) surface. The vanadium oxide displays a preferred catalytic effect in our investigations, and the relative catalytic mechanism is revealed.     

Hydrogen adsorption and dissociation on nickel-adsorbed and -substituted Mg17Al12 (100) surface: A density functional theory study

Adsorption and dissociation properties of hydrogen on Ni-adsorbed and -substituted Mg₁₇Al₁₂ (100) surface are investigated systematically by means of the density functional theory calculations. Results show that one Ni atom prefers to adsorb on MgMg bridge site of the surface with adsorption energy −4.90 eV. For substitution systems, the Mg₁₇Al₁₂ (100) surface doped with 3.94 wt% and 7.69 wt% of Ni are considered. It is obtained that Ni atoms tend to replace Mg atoms occupied at the subsurfaces. With the addition of Ni, the energy of atomic (molecular) hydrogen adsorption (dissociation) on the Mg₁₇Al₁₂ (100) surface are significantly improved. The dissociation of H₂ on the Ni-adsorbed surface is spontaneous. The mechanisms analyses based on the density functional theory are in line with the experimental results.     

Selective decoration of nitrogenated holey graphene (C2N) with titanium clusters for enhanced hydrogen storage application

For an envisioned hydrogen (H₂) economy, the design of new multifunctional two-dimensional (2D) materials have been a subject of intense research for the last several decades. Here, we report the thriving H₂ storage capacity of 2D nitrogenated holey graphene (C₂N), functionalized with Ti_n (n = 1–5) clusters. By using spin polarized density functional theory (DFT) calculations implemented with the van der Waals corrections, the most favourable adsorption site for the Ti_n clusters on C₂N has been revealed. With the monomer Ti, the functionalization was evenly covered on C₂N having 5% doping concentration (C₂N–Ti). For C₂N–Ti sheet, Ti binds to C₂N with a strong binding energy of ~6 eV per Ti which is robust enough to hinder any Ti–Ti clustering. Bader charge analysis reveals that the Ti_n clusters donate significant charges to C₂N sheet and become cationic to polarize the H₂ molecules, thus act as efficient anchoring agents to adhere multiple H₂ molecules. Each Ti in C₂N–Ti could adsorb a maximum of 10H₂ molecules, with the adsorption energies in the range of ?0.2 to ?0.4 eV per H₂ molecule, resulting into a high H₂ storage capacity of 6.8 wt%, which is promising for practical H₂ storage applications at room temperature. Furthermore, Ti_m (m = 2, 3, 4, 5) clusters have been selectively decorated over C₂N. However, with Ti_m functionalization H₂ storage capacities fall short of the desirable range due to large molecular weights of the systems. In addition, the H₂ desorption mechanism at different conditions of pressure and temperature were also studied by means of thermodynamic analysis that further reinforce the potential of C₂N–Ti as an efficient H₂ storage material.     

Enhanced low temperature hydrogen desorption properties and mechanism of Mg(BH4)2 composited with 2D MXene

Mg(BH₄)₂ occupies a large hydrogen storage capacity of 14.7 wt%, and has been widely recognized to be one of the potential candidates for hydrogen storage. In this work, 2D MXene Ti₃C₂ was introduced into Mg(BH₄)₂ by a facile ball-milling method in order to improve its dehydrogenation properties. After milling with Ti₃C₂, Mg(BH₄)₂–Ti₃C₂ composites exhibit a novel “layered cake” structure. Mg(BH₄)₂ with greatly reduced particle sizes are found to disperse uniformly on Ti₃C₂ layered structure. The initial dehydrogenation temperature of Mg(BH₄)₂ has been decreased to 124.6 °C with Ti₃C₂ additive and the hydrogen liberation process can be fully accomplished below 400 °C. Besides, more than 10.8 wt% H₂ is able to be liberated from Mg(BH₄)₂–40Ti₃C₂ composite at 330 °C within 15 min, while pristine Mg(BH₄)₂ merely releases 5.3 wt% hydrogen. Moreover, the improved dehydrogenation kinetics can be retained during the subsequent second and third cycles. Detailed investigations reveal that not only Ti₃C₂ keeps Mg(BH₄)₂ particles from aggregation during de/rehydrogenation, but also the metallic Ti formed in-situ serves as the active sites to catalyze the decomposition of Mg(BH₄)₂ by destabilizing the B–H covalent bonds. This synergistic effect of size reduction and catalysis actually contributes to the greatly advanced hydrogen storage characteristics of Mg(BH₄)₂.     

Structural characterization and electrochemical hydrogen storage properties of Ti2−xZrxNi (x = 0, 0.1, 0.2) alloys prepared by mechanical alloying

Nominal Ti₂Ni was synthesized under argon atmosphere at room temperature using a planetary high-energy ball mill. The effect of milling time and Zr substitution for Ti on the microstructure was characterized by XRD, SEM and TEM, and the discharge capacities of Ti_2−xZr_xNi (x = 0, 0.1, 0.2) were examined by electrochemical measurements at galvanostatic conditions. XRD analysis shows that amorphous phase of Ti₂Ni can be elaborated by 60 h of milling, whereas Zr substitution hinders amorphization process of the system. The products of ball milling nominal Ti_2−xZr_xNi (x = 0.1, 0.2) were austenitic (Ti, Zr)Ni and partly TiO, despite the fact that the operation was carried out under argon atmosphere. By comparing the SEM micrographs, it is found that the amorphous phase of Ti₂Ni was formed in the stage of cold-welding during milling, while with Zr substitution particles were flaky and finer, inhomogeneous in size distribution with massive agglomeration. TEM analysis was carried out and confirmed the observations via XRD. In the electrochemical tests, amorphous Ti₂Ni shows the best discharge capacity at 102 mAh/g at a current density of 40 mA/g. Without need of activation, it exhibits extraordinary cycling stability under room temperature. On the other hand, the effect of Zr substitution on the electrochemical property of Ti₂Ni is tricky, as superficially the discharge capacity drops drastically with Zr substitution, but with increase of Zr content (from x = 0.1 to x = 0.2), the discharge capacity increases generally, which credits to larger unit-cell-volume provided by ZrNi compared to TiNi. It is also found that the Ti–Ni system becomes significantly susceptible to oxidation when Zr is introduced to the initial powders as mechanical alloying is deployed as a synthesis method.     

Hydrogen storage of dual-Ti-doped single-walled carbon nanotubes

The hydrogen adsorption capacity of dual-Ti-doped (7, 7) single-walled carbon nanotube (Ti-SWCNTs) has been studied by the first principles calculations. Ti atoms show different characters at different locations due to local doping environment and patterns. The dual-Ti-doped SWCNTs can stably adsorb up to six H₂ molecules through Kubas interaction at the Ti₂ active center. The intrinsic curvature and the different doping pattern of Ti-SWCNTs induce charge discrepancy between these two Ti atoms, and result in different hydrogen adsorption capacity. Particularly, eight H₂ molecules can be adsorbed on both sides of the dual-Ti decorated SWCNT with ideal adsorption energy of 0.198 eV/H₂, and the physisorption H₂ on the inside Ti atom has desirable adsorption energy of 0.107 eV/H₂, ideal for efficient reversible storage of hydrogen. The synergistic effect of Ti atoms with different doping patterns enhances the hydrogen adsorption capacity 4.5H₂s/Ti of the Ti-doped SWCNT (VIII), and this awaits experimental trial.     

The effect of functional groups (O,F, or OH) on reversible hydrogen storage properties of Ti2X (X=C or N) monolayer

The effect of functional groups (O, F, or OH) on the hydrogen storage properties of Ti₂X (X = C or N) monolayer was systematically investigated by first-principles calculations. The results show that the reversible hydrogen storage capacity of Ti₂X(OH)₂ monolayer is approximately 2.7 wt%, which is larger than that of Ti₂XO₂ and Ti₂XF₂ monolayers. The binding energy of the OH group at the F site is stronger than H atom. Thus, H₂ molecules will not be dissociated on Ti₂X(OH)₂ monolayer. At this time, the loss of 1.8 wt% hydrogen storage capacity is not produced in Ti₂X(OH)₂ monolayer. Furthermore, the PDOS, the population analysis, and the electron density difference explore that electron transfer appears between Ti and the second layer H₂ molecules on Ti₂X(OH)₂ monolayer, and a Dewar-Kubas interaction lies between second layer H₂ molecules and Ti₂X(OH)₂ monolayer. For Ti₂X(OH)₂ monolayer, the molecular dynamic simulation indicates that the H₂ molecules by Dewar-Kubas interaction sable adsorption at 300 K, and desorption at 400 K. Therefore, Ti₂X(OH)₂ is appropriate for reversible hydrogen sorbent storage materials under ambient conditions.     

Hydrogen storage properties of Ti1.4V0.6Ni + x Mg (x = 1–3, wt.%) alloys

We prepared Ti_1.4V_0.6Ni ribbons by arc-melting and subsequent melt-spinning techniques. Ti_1.4V_0.6Ni + x Mg (x = 1, 1.5, 2, 2.5 and 3, wt.%) composite alloys were obtained by the mechanical ball-milling method. The structures and hydrogen storage properties of alloys were investigated. Ti_1.4V_0.6Ni + x Mg composite alloys contained icosahedral quasicrystalline phase, Ti₂Ni-type phase, β-Ti solid-solution phase and metallic Mg. The electrochemical and gaseous hydrogen storage properties of alloys were improved with Mg addition. Ti_1.4V_0.6Ni + 2 Mg alloy showed maximum electrochemical discharge capacity of 282.5 mAh g⁻¹ as well as copacetic high-rate discharge ability of 82.3% at the discharge current density of 240 mA g⁻¹ compared with that of 30 mA g⁻¹, and the cycling life achieved above 200 mAh g⁻¹ after 50 consecutive cycles of charging and discharging. The hydrogen absorption/desorption properties of Ti_1.4V_0.6Ni + x Mg (x = 1, 2 and 3, wt.%) alloys were better than Ti_1.4V_0.6Ni. Ti_1.4V_0.6Ni + 3 Mg alloy also exhibited a favorable hydrogen absorption capacity of 1.53 wt.%. The improvement in the hydrogen storage characteristics caused by adding Mg may be ascribed to better hydrogen diffusion and anti-corrosion ability.     

An ab initio study of dissociative adsorption of H2 on FeTi surfaces

Dissociative adsorption of H₂ on clean FeTi (001), (110) and (111) surfaces is investigated via ab initio pseudopotential-plane wave method. Adsorption energies of H atom and H₂ molecule on Fe and Ti terminated (001) and (111) and FeTi (110) surfaces are calculated on high symmetry adsorption sites. It is shown that, top site is the most stable site for horizontal H₂ molecule adsorption on (001) and (111) surfaces for both terminations. The most favorable site for H atom adsorption on these surfaces however, is the bridge site. In (110) surface, the 3-fold hollow site which is composed of a long Ti–Ti bridge and an Fe atom, (Ti–Ti)_L–Fe, and again a 3-fold hollow site this time composed of a short Ti–Ti bridge and an Fe atom, (Ti–Ti)_S–Fe, are the most stable sites for H₂ and H adsorption, respectively. With the analysis of the above favorable adsorption sites, probable dissociation paths for H₂ molecule over these surfaces are proposed. Activation energies of these dissociations are also determined with the use of the dynamics of the H₂ relaxation and climbing image nudged elastic band method. It is found that H₂ dissociation on (110) and Fe terminated (111) surfaces has no activation energy barrier. On other surfaces however, activation energies are calculated to be 0.178 and 0.190 eV per H₂ molecule for Fe and Ti terminated (001) surfaces respectively, and 1.164 eV for Ti terminated (111) surface.     

The synthesis of nanoscopic Ti based alloys and their effects on the MgH2 system compared with the MgH2 + 0.01Nb2O5 benchmark

The MgH₂ + 0.02Ti-additive system (additives = 35 nm Ti, 50 nm TiB₂, 40 nm TiC, <5 nm TiN, 10 × 40 nm TiO₂) has been studied by high-resolution synchrotron X-ray diffraction, after planetary milling and hydrogen (H) cycling. TiB₂ and TiN nanoparticles were synthesised mechanochemically whilst other additives were commercially available. The absorption kinetics and temperature programmed desorption (TPD) profiles have been determined, and compared to the benchmark system MgH₂ + 0.01Nb₂O₅ (20 nm). TiC and TiN retain their structures after milling and H cycling. The TiB₂ reflections appear compressed in d-spacing, suggesting Mg/Ti exchange has occurred in the TiB₂ structure. TiO₂ is reduced, commensurate with the formation of MgO, however, the Ti is not evident anywhere in the diffraction pattern. The 35 nm Ti initially forms an fcc Mg_47.5Ti_52.5 phase during milling, which then phase separates and hydrides to TiH₂ and MgH₂. At 300 °C, the MgH₂ + 0.02 (Ti, TiB₂, TiC, TiN, TiO₂) samples display equivalent absorption kinetics, which are slightly faster than the MgH₂ + 0.01Nb₂O₅ (20 nm) benchmark. All samples are contaminated with MgO from the use of a ZrO₂ vial, and display rapid absorption to ca. 90% of capacity within 20 s at 300 °C. TPD profiles of all samples show peak decreases compared to the pure MgH₂ milled sample, with many peak profiles displaying bi-modal splitting. TPD measurements on two separate instruments demonstrate that on a 30 min milling time scale, all samples are highly inhomogenous, and samplings from the exact same batch of milled MgH₂ + 0.02Ti-additive can display differences in TPD profiles of up to 30 °C in peak maxima. The most efficient Ti based additive cannot be discerned on this basis, and milling times ? 30 min are necessary to obtain homogenous samples, which may lead to artefactual benefits, such as reduction in diffusion distances by powder grinding or formation of dense microstructure. For the hydrogen cycled MgH₂ + 0.01Nb₂O₅ system, we observe a face centred cubic Mg/Nb exchanged Mg_0.165Nb_0.835O phase, which accounts for ca. 60% of the originally added Nb atoms.