Direct synthesis of nanocrystalline titanium dioxide/carbon composite and its catalytic effect on NaAlH4 for hydrogen storage

Nanocrystalline titanium dioxide/carbon composite (TiO₂/C) was synthesized through a direct solution-phase carburization using tetrabutyl titanate (Ti(OBu)₄) and resol as precursors. The prepared TiO₂/C composite was mainly in the anatase structure with an average particle size under 20 nm, which was then introduced in NaAlH₄ as a catalyst through ball milling. The desorption curves show that both nanocrystalline TiO₂/C and TiO₂ can obviously improve the kinetics of NaAlH₄, while NaAlH₄ with 3 mol% TiO₂/C exhibits better cycling stability than NaAlH₄ with 3 mol%TiO₂. The hydrogen storage capacity of NaAlH₄ with TiO₂/C remains stable after 5th cycle, and about 94% of initial hydrogen is released, while the capacity of NaAlH₄ with TiO₂ decreases continuously during cycling, and only 88% of initial hydrogen is released after 10th cycle. Furthermore, NaAlH₄ with 3 mol%TiO₂/C exhibits good reversibility at relatively low hydrogen pressures, and it can reload 4.16 and 1.63wt% hydrogen at 50 and 30 bar hydrogen pressures, respectively.     

A facile two-step synthesis and dehydrogenation properties of NaAlH4 catalyzed with Co–B

Co–B doped NaAlH₄ is successfully synthesized by two-step synthesis process. The first activation step is milling NaH/Al powder and Co–B mixtures under Ar atmosphere. The second step is milling in a lower hydrogen pressure atmosphere. XRD patterns and FTIR spectrum demonstrate that NaAlH₄ is completely formed after 15 h milling in Ar atmosphere following by 40 h milling in 2 MPa H₂ atmosphere. PCT curves of as-prepared NaAlH₄ show that it can release hydrogen at a low temperature of 90 °C. The activation energy value calculated by Arrhenius equation is only 67.95 kJ mol⁻¹. Moreover, the formation mechanism of NaAlH₄ is also discussed.     

Understanding the effect of TiO2, VCl3, and HfCl4 on hydrogen desorption/absorption of NaAlH4

The main objective of this work was to investigate the different effects of transition metals (TiO₂, VCl₃, HfCl₄) on the hydrogen desorption/absorption of NaAlH₄. The HfCl₄ doped NaAlH₄ showed the lowest temperature of the first desorption at 85 °C, while the one doped with VCl₃ or TiO₂ desorbed at 135 °C and 155 °C, respectively. Interestingly, the temperature of desorption in subsequent cycles of the NaAlH₄ doped with TiO₂ reduced to 140 °C. On the contrary, in the case of NaAlH₄ doped with HfCl₄ or VCl₃, the temperature of desorption increased to 150 °C and 175 °C, respectively. This may be because Ti can disperse in NaAlH₄ better than Hf and V; therefore, this affected segregation of the sample after the desorption. The maximum hydrogen absorption capacity can be restored up to 3.5 wt% by doping with TiO₂, while the amount of restored hydrogen was lower for HfCl₄ and VCl₃ doped samples. XRD analysis demonstrated that no Ti-compound was observed for the TiO₂ doped samples. In contrast, there was evidence of Al–V alloy in the VCl₃ doped sample and Al–Hf alloy in the HfCl₄ doped sample after subsequent desorption/absorption. As a result, the V- or Hf-doped NaAlH₄ showed the lower ability to reabsorb hydrogen and required higher temperature in the subsequent desorptions.     

Carbon nanostructures as catalyst for improving the hydrogen storage behavior of sodium aluminum hydride

The present paper reports the catalytic effect of carbon nanomaterials, particularly carbon nanotubes (CNTs) and graphitic nanofibres (GNFs) with two different structure morphology, namely planar GNFs (PGNFs) and helical GNFs (HGNFs) as the catalyst for improving the dehydrogenation and rehydrogenation behavior of sodium aluminum hydride (NaAlH₄). It has been observed that HGNFs posses superior catalytic activity than other carbon nanoforms in improving the desorption kinetics and decreasing the desorption temperature of NaAlH₄. Temperature programmed desorption (TPD) reveals that HGNFs admixed NaAlH₄ undergo hydrogen desorption at a much lower temperature than PGNFs and CNTs (SWCNTs and MWCNTs) admixed NaAlH₄. Thus for the heating rate of 2 °C/min, the peak desorption temperature corresponds to initial step decomposition of NaAlH₄ admixed with 2 wt.% HGNFs and 2 wt.% PGNFs has been lowered to 143.6 °C and 152.6 °C, respectively (for pristine NaAlH₄, it is ∼170 °C). In addition to the enhancement in desorption kinetics, the HGNFs admixed NaAlH₄ undergoes fast rehydrogenation at the moderate condition. Microstructural investigation reveals that the HGNFs were present on the surface of NaAlH₄ grains, whereas CNTs were tunneled into the grains of NaAlH₄ suggesting a distinct catalytic behavior of different carbon nanovariants.     

Co-effects of Tm2O3 and porous silica on reversible hydrogen storage in NaAlH4

The co-effects of lanthanide oxide Tm₂O₃ and porous silica on the hydrogen storage properties of sodium alanate are investigated. NaAlH₄-Tm₂O₃ (10 wt%) and NaAlH₄-Tm₂O₃ (10 wt%)-porous SiO₂ (10 wt%) are prepared by the ball milling method, and their hydrogen desorption/re-absorption capacities are compared. Dehydrogenation process was performed at 150 °C under vacuum and rehydrogenation was performed at 150 °C for 4 h under ∼9 MPa in highly pure hydrogen. The results show that Tm₂O₃ has a catalytic effect on the hydrogen desorption and re-absorption of NaAlH₄. The hydrogen desorption capacity of Tm₂O₃ single-doped NaAlH₄ is 4.6 wt%, higher than that of undoped NaAlH₄ (4.3 wt%). During the dehydrogenation process, NaAlH₄ is completely decomposed and no intermediate product Na₃AlH₆ is detected. The addition of porous silica improves the dehydrogenation performance of NaAlH₄. Tm₂O₃ and porous silica co-doped NaAlH₄ could release a maximum hydrogen amount of 4.7 wt%, higher than that of undoped NaAlH₄ and Tm₂O₃ single-doped NaAlH₄. Moreover, porous silica improves the reversibility of hydrogen storage in NaAlH₄.     

Improved hydrogen sorption performance of NbF5-catalysed NaAlH4

The effect of NbF₅ on the hydrogen sorption performance of NaAlH₄ has been investigated. It was found that the dehydrogenation/hydrogenation properties of NaAlH₄ were significantly enhanced by mechanically milling with 3 mol% NbF₅. Differential scanning calorimetry results indicate that the ball-milled NaAlH₄-0.03NbF₅ sample lowered the completion temperature for the first two steps dehydrogenation by 71 °C compared to the pristine NaAlH₄ sample. Isothermal hydrogen sorption measurements also revealed a significant enhancement in terms of the sorption rate and capacity, in particular, at reduced operation temperatures. The apparent activation energy for the first-step and the second-step dehydrogenation of the NaAlH₄-0.03NbF₅ sample is estimated to be 88.2 kJ/mol and 102.9 kJ/mol, respectively, by using Kissinger’s approach, which is much lower than for pristine NaAlH₄, indicating the reduced kinetic barrier. The rehydrogenation kinetics of NaAlH₄ was also improved with 3 mol% NbF₅ doping, absorbing ∼1.7 wt% hydrogen at 150 °C for 2 h under ∼5.5 MPa hydrogen pressure. In contrast, no hydrogen was absorbed by the pristine NaAlH₄ sample under the same conditions. The formation of Na₃AlH₆ was detected by X-ray diffraction on the rehydrogenated NaAlH₄-0.03NbF₅ sample. Furthermore, the structural changes in the NbF₅-doped NaAlH₄ sample after ball milling and the hydrogen sorption were carefully examined, and the active species and mechanism of catalysis in NbF₅-doped NaAlH₄ are discussed.     

Synthesis of helical carbon nanofibres and its application in hydrogen desorption

In this communication, we report the synthesis of helical carbon nanofibres (HCNFs) by employing hydrogen storage intermetallic LaNi₅ as the catalyst precursor. It was observed that oxidative dissociation of LaNi₅ alloy (2LaNi₅ + 3/2O₂ → La₂O₃ + 10Ni) occurred during synthesis. The Ni particles obtained through this process instantly interacted with C₂H₂ and H₂ gases, and fragmented to nanoparticles of Ni (∼150 nm) with polygonal shape. These polygonal shapes of Ni nanoparticles were decisive for the growth of helical carbon nanofibres (HCNFs) at 650 °C. TEM, SAED and EDAX studies have shown that HCNFs have grown on Ni nanoparticles. Typical diameter and length of the HCNFs are ∼150 nm and 6-8 μm respectively. BET surface area of these typical HCNFs has been found to be 127 m²/g. It was found that at temperature 750 °C, spherical shapes of Ni nanoparticles were produced and decisive for the growth of planar carbon nanofibres (PCNFs). The diameter and length of the PCNFs are ∼200 nm and 6-8 μm respectively. In order to explore the application potential of the present as-synthesized CNFs, they were used as a catalyst for enhancing the hydrogen desorption kinetics of sodium aluminum hydride (NaAlH₄). We have found that the present as-synthesized HCNFs, with metallic impurities, indeed work as an effective catalyst. The pristine NaAlH₄ and 8 mol% as-synthesized HCNFs admixed NaAlH₄, at 160 °C-180 °C and for the duration of 5 h, liberate 0.8 wt% and 4.36 wt% of hydrogen, respectively. Thus there is an enhancement of ∼5 times in kinetics when as-synthesized HCNFs are used as the catalyst. To the best of our knowledge, the use of hydrogen storage alloy LaNi₅ as the catalyst precursor for the growth of HCNFs has not yet been done and thus represents a new feature relating to the growth of HCNFs. Furthermore, we have shown that the as-synthesized HCNFs work as an effective new catalyst for improving the dehydrogenation kinetics of the complex hydride, NaAlH₄.     

Improvement in desorption kinetics of NaAlH4 catalyzed with TiO2 nanopowder

NaAlH₄ has been homogeneously mixed with micron- and nano-sized TiO₂ powders by high-energy ball milling and their sorption properties have been investigated during hydrogen absorption/desorption cycles. NaAlH₄ with TiO₂ nanopowder exhibits as good desorption kinetics as NaAlH₄ with TiCl₃, whereas poor desorption kinetics is observed with micron-sized TiO₂ powder. NaAlH₄ with TiO₂ nanopowder also provides improved cyclic property compared to NaAlH₄ with TiCl₃ in terms of both desorption rate and hydrogen capacity. X-ray diffraction analysis shows that micron-sized TiO₂ remains stable with NaAlH₄ after milling, although thermodynamic calculation predicts that TiO₂ reacts with NaAlH₄.     

Catalytic effect of carbon nanostructures on the hydrogen storage properties of MgH2–NaAlH4 composite

The present investigation describes the hydrogen storage properties of 2:1 molar ratio of MgH₂–NaAlH₄ composite. De/rehydrogenation study reveals that MgH₂–NaAlH₄ composite offers beneficial hydrogen storage characteristics as compared to pristine NaAlH₄ and MgH₂. To investigate the effect of carbon nanostructures (CNS) on the de/rehydrogenation behavior of MgH₂–NaAlH₄ composite, we have employed 2 wt.% CNS namely, single wall carbon nanotubes (SWCNT) and graphene nano sheets (GNS). It is found that the hydrogen storage behavior of composite gets improved by the addition of 2 wt.% CNS. In particular, catalytic effect of GNS + SWCNT improves the hydrogen storage behavior and cyclability of the composite. De/rehydrogenation experiments performed up to six cycles show loss of 1.50 wt.% and 0.84 wt.% hydrogen capacity in MgH₂–NaAlH₄ catalyzed with 2 wt.% SWCNT and 2 wt.% GNS respectively. On the other hand, the loss of hydrogen capacity after six rehydrogenation cycles in GNS + SWCNT (1.5 + 0.5) wt.% catalyzed MgH₂–NaAlH₄ is diminished to 0.45 wt.%.     

Significantly improved dehydrogenation of LiAlH4 catalysed with TiO2 nanopowder

The effects of TiO₂ nanopowder addition on the dehydrogenation behaviour of LiAlH₄ have been studied. The 5 wt.% TiO₂-added LiAlH₄ sample showed a significant improvement in dehydrogenation rate compared to that of undoped LiAlH₄, with the dehydrogenation temperature reduced from 150 °C to 60 °C. Kinetic desorption results show that the added LiAlH₄ released about 5.2 wt% hydrogen within 30 min at 100 °C, while the as-received LiAlH₄ just released below 0.2 wt.% hydrogen within same time at 120 °C. From the Arrhenius plot of the hydrogen desorption kinetics, the apparent activation energy is 114 kJ/mol for pure LiAlH₄ and 49 kJ/mol for the 5 wt.% TiO₂ added LiAlH₄, indicating that TiO₂ nanopowder adding significantly decreased the activation energy for hydrogen desorption of LiAlH₄. X-ray diffraction and Fourier transform infrared analysis show that there is no phase change in the cell volume or on the Al-H bonds of the LiAlH₄ due to admixture of TiO₂ after milling. X-ray photoelectron spectroscopy results show no changes in the Ti 2p spectra for TiO₂ after milling and after dehydrogenation. The improved dehydrogenation behaviour of LiAlH₄ in the presence of TiO₂ is believed to be due to the high defect density introduced at the surfaces of the TiO₂ particles during the milling process.     

Improved hydrogen storage performance of MgH2–NaAlH4 composite by addition of TiF3

In a previous paper, it was demonstrated that a MgH₂–NaAlH₄ composite system had improved dehydrogenation performance compared with as-milled pure NaAlH₄ and pure MgH₂ alone. The purpose of the present study was to investigate the hydrogen storage properties of the MgH₂–NaAlH₄ composite in the presence of TiF₃. 10 wt.% TiF₃ was added to the MgH₂–NaAlH₄ mixture, and its catalytic effects were investigated. The reaction mechanism and the hydrogen storage properties were studied by X-ray diffraction, thermogravimetric analysis, differential scanning calorimetry (DSC), temperature-programmed-desorption and isothermal sorption measurements. The DSC results show that MgH₂–NaAlH₄ composite milled with 10 wt.% TiF₃ had lower dehydrogenation temperatures, by 100, 73, 30, and 25 °C, respectively, for each step in the four-step dehydrogenation process compared to the neat MgH₂–NaAlH₄ composite. Kinetic desorption results show that the MgH₂–NaAlH₄–TiF₃ composite released about 2.4 wt.% hydrogen within 10 min at 300 °C, while the neat MgH₂–NaAlH₄ sample only released less than 1.0 wt.% hydrogen under the same conditions. From the Kissinger plot, the apparent activation energy, E_A, for the decomposition of MgH₂, NaMgH₃, and NaH in the MgH₂–NaAlH₄–TiF₃ composite was reduced to 71, 104, and 124 kJ/mol, respectively, compared with 148, 142, and 138 kJ/mol in the neat MgH₂–NaAlH₄ composite. The high catalytic activity of TiF₃ is associated with in situ formation of a microcrystalline intermetallic Ti–Al phase from TiF₃ and NaAlH₄ during ball milling or the dehydrogenation process. Once formed, the Ti–Al phase acts as a real catalyst in the MgH₂–NaAlH₄–TiF₃ composite system.     

Synthesis and hydriding/dehydriding properties of nanosized sodium alanates prepared by reactive ball-milling

TiF₃-doped NaH/Al mixture was hydrogenated into Na₃AlH₆ and NaAlH₄ complex hydrides by reactive ball-milling at room temperature through the optimization of milling duration and hydrogen pressure. The analysis of the preparation of NaAlH₄ samples during reactive ball-milling process has been performed by XRD and TG/DSC. It has been found that Na₃AlH₆ was formed under 0.5 MPa hydrogen pressure and 30 h milling duration, while NaAlH₄ was formed under 0.8 MPa hydrogen pressure and 45 h milling duration. The process of preparing NaAlH₄ by ball-milling was found accomplished via two reaction steps, namely: (1) NaH + Al + H₂ → Na₃AlH₆ and (2) Na₃AlH₆ + Al + H₂ → NaAlH₄. As the hydrogen pressure and milling duration increase, the synthetic yield of NaAlH₄ and its corresponding dehydriding capacity are both increased. With increased hydrogen pressure (0.8-3 MPa) and milling duration (45-60 h), the cell volume of Na₃AlH₆ decreases while that of NaAlH₄ increases gradually. The abundance of Na₃AlH₆ phase decreases from 57.76 (x = 0.8, y = 45) to 8.69 wt.% (x = 3, y = 60), and the abundance of NaAlH₄ phase increases from 20.63 (x = 0.8, y = 45) to 86.50 wt.% (x = 3, y = 60). All the samples prepared in this way have fairly good activation behavior and fast hydriding/dehydriding reaction kinetics, which are capable of absorbing 4.26 wt.% hydrogen at 120 °C and desorbing 4.12 wt.% hydrogen at 150 °C, respectively. The improvement of hydriding/dehydriding properties is ascribed to the favorable microstructure and ultrafine particle features of nanosized NaAlH₄ formed during ball-milling at the optimum synthetic condition.     

Interface effects in NaAlH4–carbon nanocomposites for hydrogen storage

For practical solid-state hydrogen storage, reversibility under mild conditions is crucial. Complex metal hydrides such as NaAlH₄ and LiBH₄ have attractive hydrogen contents. However, hydrogen release and especially uptake after desorption are sluggish and require high temperatures and pressures. Kinetics can be greatly enhanced by nanostructuring, for instance by confining metal hydrides in a porous carbon scaffold. We present for a detailed study of the impact of the nature of the carbon–metal hydride interface on the hydrogen storage properties. Nanostructures were prepared by melt infiltration of either NaAlH₄ or LiBH₄ into a carbon scaffold, of which the surface had been modified, varying from H-terminated to oxidized (up to 4.4 O/nm²). It has been suggested that the chemical and electronic properties of the carbon/metal hydride interface can have a large influence on hydrogen storage properties. However, no significant impact on the first H₂ release temperatures was found. In contrast, the surface properties of the carbon played a major role in determining the reversible hydrogen storage capacity. Only a part of the oxygen-containing groups reacted with hydrides during melt infiltration, but further reaction during cycling led to significant losses, with reversible hydrogen storage capacity loss up to 40% for surface oxidized carbon. However, if the carbon surface had been hydrogen terminated, ∼6 wt% with respect to the NaAlH₄ weight was released in the second cycle, corresponding to 95% reversibility. This clearly shows that control over the nature and amount of surface groups offers a strategy to achieve fully reversible hydrogen storage in complex metal hydride-carbon nanocomposites.     

Enhanced desorption properties of LiBH4 incorporated into mesoporous TiO2

LiBH₄ nano-particles are incorporated into mesoporous TiO₂ scaffolds via a chemical impregnation method. And the enhanced desorption properties of the composite have been investigated. The LiBH₄/TiO₂ sample starts to release hydrogen at 220 °C and the maximal desorption peak occurs at about 330 °C, much lower compared to the bulk LiBH₄. Furthermore, the composite exhibits excellent dehydrogenation kinetics, with 11 wt% of hydrogen liberated from LiBH₄ at 300 °C within 3 h. X-ray diffraction and Fourier transform infrared spectroscopy are used to confirm the nanostructure of LiBH₄ in the TiO₂ scaffold. This work demonstrates that confinement within active porous scaffold host is a promising approach for enhancing hydrogen decomposition properties of light-metal complex hydrides.     

Direct synthesis of sodium alanate using mischmetal nanocatalyst

This study reports the synthesis of NaAlH₄ by ball milling of NaH and Al mixture along with 3 mol % Mischmetal (Mm) nanocatalyst under hydrogen atmosphere. It is observed that synthesis of the intermediate phase Na₃AlH₆ can be achieved by ball milling even under 1 atm hydrogen at room temperature. Ball milling of the NaH + Al with 3 mol % Mm with 3 atm hydrogen in excess of 40 h time did not lead to the formation of NaAlH₄ but charging of the milled material at 100 atm hydrogen pressure at 120 °C lead to formation of NaAlH₄ phase. Direct synthesis of NaAlH₄ was achieved by milling of NaH + Al with 3 mol % Mm under 100 atm hydrogen pressure. Direct synthesis is possible even without any catalyst by high pressure milling. However catalyst is required to improve the hydrogen sorption characteristics of the synthesized material. The as-prepared Mm catalyzed NaAlH₄ is also found to reversibly store hydrogen up to 4.2 wt% hydrogen. Catalytic activity is attributed to defects promoted by ball milling and catalysts.     

Transition metal-decorated activated carbon catalysts for dehydrogenation of NaAlH4

Dehydrogenation of NaAlH₄ can be greatly facilitated by activated carbon catalysts. The catalytic function can be further enhanced by decorating the carbon with Co, Ni, or Cu nanoparticles. The decomposition temperature was lowered by as much as 100 °C using a 3 wt.% Co or Ni-decorated activated carbon, comparable to a Ti-based catalyst, which were the most effective among the metals tested. The catalytic effect is likely due to a combination of hydrogen spillover effect, high contact area between carbon and the hydride, and confinement of the hydride as nano-sized domains in the pores of the carbon matrix. The catalysts were also effective in facilitating rehydrogenation of NaAlH₄ under moderate pressure (75.8 bar H₂) and low temperature (120 °C), when no rehydrogenation would occur without the catalyst. The fact that this new catalyst system is not specific to any hydride offers many potential applications.     

Cerium hydride generated during ball milling and enhanced by graphene for tailoring hydrogen sorption properties of sodium alanate

In this study, NaAlH₄?based hydrogen storage materials with dopants were prepared by a two-steps in-situ ball milling method. The dopants adopted included Ce, few layer graphene (FLG), Ce + FLG, and CeH_2.51. The hydrogen storage materials were studied by non-isothermal and isothermal hydrogen desorption measurements, X-ray diffractions analysis, cycling sorption tests, and morphology analysis. The hydrogen storage performance of the as-prepared NaAlH₄ with Ce addition is much better than that with CeH_2.51 addition. This is due to that the impact of Ce occurs from the body to the surface of the materials. The addition of FLG further enhances the impact of Ce on the hydrogen storage performance of the materials. The hydrogen storage capacity, hydrogen sorption kinetics, and cycle performance of NaAlH₄ with Ce + FLG additions are all better than NaAlH₄ materials with the addition of either Ce or FLG alone. The NaAlH₄ with Ce and FLG addition starts to release hydrogen at 85 °C and achieves a capacity of 5.06 wt% after heated to 200 °C. The capacity maintains at 4.91 wt% (94.7% of the theoretical value) for up to 8 cycles. At 110 °C, the material can release isothermally a hydrogen capacity of 2.8 wt% within 2 h. The activation energies for the two hydrogen desorption steps of NaAlH₄ with Ce and FLG addition are estimated to be 106.99 and 125.91 kJ mol^?1 H₂, respectively. The related mechanisms were studied with first-principle and experimental methods.     

Enhanced hydrogen storage properties of TiN–LiAlH4 composite

The hydrogen storage properties of LiAlH₄ doped efficient TiN catalyst were systematically investigated. We observe that TiN catalyst enhances the dehydrogenation kinetics and decreases the dehydrogenation temperature of LiAlH₄. The dehydrogenation behaviors of 2%TiN–LiAlH₄ are investigated using temperature programmed desorption (TPD), differential scanning calorimetry (DSC) and fourier transform infrared spectroscopy (FTIR). Interestingly, the onset hydrogen desorption temperature of 2%TiN–LiAlH₄ sample gets lowered from 151.0 °C to 90.0 °C with a faster kinetics, and the dehydrogenation rate reached a maximum value at 137.2 °C. By adding a small amount of as-prepared TiN, approximately 7.1 wt% of hydrogen can be released from the LiAlH₄ at 130 °C. Interestingly, the result of the FTIR indicates that the 2%TiN–LiAlH₄ maybe restore hydrogen under 5.5 MPa hydrogen. Moreover, 2%TiN–LiAlH₄ displayed a substantially reduced activation energy for LiAlH₄ dehydrogenation.     

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.     

The decoration of TiO2/reduced graphene oxide by Pd and Pt nanoparticles for hydrogen gas sensing

Reduced graphene oxide (RGO) was used to improve the hydrogen sensing properties of Pd and Pt-decorated TiO₂ nanoparticles by facile production routes. The TiO₂ nanoparticles were synthesized by sol–gel method and coupled on GO sheets via a photoreduction process. The Pd or Pt nanoparticles were decorated on the TiO₂/RGO hybrid structures by chemical reduction. X-ray photoelectron spectroscopy demonstrated that GO reduction is done by the TiO₂ nanoparticles and Ti–C bonds are formed between the TiO₂ and the RGO sheets as well. Gas sensing was studied with different concentrations of hydrogen ranging from 100 to 10,000 ppm at various temperatures. High sensitivity (92%) and fast response time (less than 20 s) at 500 ppm of hydrogen were observed for the sample with low concentration of Pd (2 wt.%) decorated on the TiO₂/RGO sample at a relatively low temperature (180 °C). The RGO sheets, by playing scaffold role in these hybrid structures, provide new pathways for gas diffusion and preferential channels for electrical current. Based on the proposed mechanisms, Pd/TiO₂/RGO sample indicated better sensing performance compared to the Pt/TiO₂/RGO. Greater rate of spill-over effect and dissociation of hydrogen molecules on Pd are considered as possible causes of the enhanced sensitivity in Pd/TiO₂/RGO.