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We investigated the influence of the microstructure, texture and the relative amount of anatase and rutile phases on the photoelectrocatalytic H 2 evolution over mesoporous TiO 2 (m-TiO 2 ). This study was carried out by means of ex-situ powder X-ray diffraction (XRPD), N 2 physisorption, transmission electron microscopy (TEM), and photoelectrolysis measurements. The synthesis of nanocrystalline TiO 2 powders with different anatase/rutile weight ratios was successfully approached by evaporation-induced self assembly (EISA) method simply varying the volume amount of TiCl 4 and Ti(OBu) 4 (acid–base pair) as metal precursors. In particular, m-TiO 2 with anatase/rutile weight ratios similar to commercial TiO 2 (P25) was obtained. The microstructure, texture and photocatalytic properties toward hydrogen evolution reaction (HER) of the as-synthesized TiO 2 were compared with commercial P25. Interestingly, the m-TiO 2 with a high surface area of 196 m 2 /g, uniform pore size and 87/13 anatase/rutile weight ratio, showed a significant increase in the photoactivity with respect to the powders richer in either anatase or rutile phases. Furthermore, it was claimed that the crystallinity and specific surface area play a key role on the photocatalytic activity of the synthesized m-TiO 2 samples.

We focus on the chemical transformations involved in the 2NaNH2 + MgH2 system subjected to thermal and mechanical inputs. Transformations occurring on the powder mixture during thermochemical and mechanochemical processes are described by ex-situ X-ray powder diffraction (XRPD), FT-IR spectroscopy, differential scanning calorimetry and manometric measurements. In the thermally activated samples, the reaction take place through the fast formation of Mg(NH2)2 and NaH via metathesis reaction between NaNH2 and MgH2 at 125 °C. FT-IR analysis confirms the presence of unreacted NaNH2 and a new Na-amide phase that could be ascribable to tetramide, Na2Mg(NH2)4. At higher temperature, the formation of new imide-amide phase is detected, stables up to 300 °C. On the other hand when the initial mixture is subjected to mechanochemical processing for longer milling time (50 h), only Mg(NH2)2 and NaH are produced. The hydrogen desorption reaction of the as-milled Mg(NH2)2–NaH mixture starts at 100 °C together with the formation of the NaMg(NH2)(NH) imide-amide phase, equally to the initial mixture.

In situ two-dimensional synchrotron X-ray powder diffraction investigation combined with Rietveld method data analysis were performed in order to yield a complete and quantitative phases structure evolution of the polycrystalline mixture 2LiNH 2 + KBH 4 + LiH during H 2 desorption. While a first-principles, purely thermodynamics approach of the system predicted a single dehydrogenation step reaction at relatively low temperatures, it is assessed experimentally that the reaction occurs in two steps with first the formation of Li 2 NH at ca. 230 °C due to the reaction between LiNH 2 and LiH plus hydrogen and ammonia evolution, followed by an additional reaction of the resulting phases with KBH 4 at 360 °C, which releases hydrogen and leads to the formation of the monoclinic and tetragonal Li 3 BN 2 polymorphs. Besides pointing out possible limits of a purely thermodynamics approach inevitably relying exact knowledge of experimental quantities, it is concluded that before assuming it viable for on-board vehicle use, additional stoichiometries may be worth of investigation in order to assess any existence of lower hydrogen desorption temperature of such system.

The present investigation focuses on a new synthesis route of NaBH4 starting from the 2NaH + MgB2 system subjected to mechanochemical activation under reactive hydrogen atmosphere. The milling process was carried out under two different hydrogen pressures (1 and 120 bar) with two different rotation speeds (300 and 550 rpm). The reaction products were characterized by ex-situ solid state magic angle spinning (MAS) nuclear magnetic resonance (NMR), ex-situ X-ray powder diffraction (XRPD) and Infrared Spectroscopy (IR). From the results of these analyses, it can be concluded that milling in all the considered conditions led to the formation of NaBH4 (cubic-Fm-3m). In particular, a reaction yield of 5 and 14 wt% is obtained after 20 h of milling at 120 bar of H2 for the tests performed at 300 rpm and 550 rpm, respectively. The presence of MgH2 is also detected among the final products on the as milled powders. The influence of the milling conditions and the evaluation of the parameters related the mechanochemical process are here discussed.

Hot extrusion processing was employed to produce 2Mg–Fe bulk mixtures for hydrogen storage. 2Mg–Fe powder mixtures were prepared by high-energy ball milling. These mixtures were cold pressed into cylindrical pre-forms, which were then processed by hot extrusion (at 300 °C) to produce bulks. In this work, we analyzed the influence of the extrusion ratio (3/1, 5/1 and 7/1) on the sorption properties of the bulks. The nanometric grain size remained unaltered after all hot extrusion conditions. More porous bulks were produced at an extrusion ratio of 3/1. In the first cycle of hydrogenation, the sample processed at 3/1 absorbed more hydrogen (4 wt% of H) than the precursor powders (3 wt% of H). The results showed that the desorption temperature of bulks were very similar to that of 2Mg–Fe powders, which is good considering the lower surface area of bulks, and that samples with Fe in excess presented lower desorption temperatures.

The effect of mesoporous Co3O4, NiCo2O4 and NiO on the hydrogen sorption performance of MgH2 was investigated. These oxides were synthesized by multi-step nanocasting and introduced during the high-energy ball milling of MgH2 powder to act as catalysts. Hydrogen desorption on the as-milled powders was assessed upon heating the samples from room temperature to 400 °C. In all cases, the onset temperature for desorption was lowered by taking advantage of the introduced additives. The NiO-doped sample displayed the best response, the desorption rate being 7 times faster than in pure MgH2. Complementary kinetic studies on this particular sample revealed that the sorption activation energies were much lower (50 kJ/mol for absorption and 335 kJ/mol for desorption) than the corresponding ones for undoped MgH2 (57 kJ/mol for absorption and 345 kJ/mol for desorption), thus proving the catalytic activity of the mesoporous NiO oxide. Significantly, the X-ray powder diffraction (XRPD) patterns taken on the NiO-doped sample after discharging/charging cycles revealed that Mg could fully hydrogenate at the end of the charging process, while Mg metal was still detected in the undoped (pure) sample. Favored conditions for dissociative chemisorption of hydrogen could be ascribed to the formation of metallic Ni arising from complete or partial reduction of NiO, as observed in the XRPD patterns.

Ternary Mg–Nb2O5 – graphitic C mixtures (molar ratio % = 97.5:0.5:2.0) were prepared by high-energy ball milling (BM) under Ar for different times (from 0.25 h to 4 h) and thoroughly characterized by manometric, calorimetric, X-ray powder diffraction, and scanning electron microscopy analyses. The aims of the work were: - to assess the effect of the simultaneous presence of the two dopants on the reactivity and the sorption properties of the Mg–MgH2 system; - to study the influence of the milling time on the performance of the mixtures. Neither milling nor the high temperature/high pressure treatments led to reactions among the components of the mixtures, and Mg was the only hydrogen active phase. After 4 activation cycles at 623 K and 35 bar/1 bar charging/discharging pressure, the mixture milled for 1 h was the best performing one: it reversibly charged up to 6.8 wt% H2 with absorption/desorption rates 64/4.5 times higher than those of a pure Mg sample BM for the same time and activation energies 3.6/2 times lower. The desorption temperature and the dehydrogenation enthalpy of the ternary mixture were respectively 40 K and 4 kJ/mol H2 lower than those of pure MgH2.

The sorption properties of NaBH 4 /MH 2 (M = Mg, Ti) powder systems prepared by high-energy ball milling have been thoroughly investigated. Concerning the systems containing MgH 2 , the 2:1 and 1:2 molar compositions have been studied and both lead to a multi-step desorption pathway, where the formation of MgB 2 confirms the destabilization of NaBH 4 induced by the presence of MgH 2 . A noticeable kinetic enhancement is achieved for the MgH 2 -rich system (composition 1:2) if compared with the NaBH 4 -rich system (composition 2:1). Even though full re-absorption is obtained for neither of the two compositions, fast kinetics is achieved. During absorption, the unsuspected formation of the perovskite-type hydride NaMgH 3 is detected and it is showed that this ternary phase contributes to reduce the gravimetric capacity of the systems. Conversely, in the 2NaBH 4 /TiH 2 system, there is no formation of the intermetallic compound TiB 2 . Furthermore, a decrease in the sorption kinetics is found in comparison with the systems based on MgH 2 .

The study focuses on the hydrogenation of carbon monoxide over Co–Fe-based catalysts. The catalytic reactions were performed under both isothermal and mechanical activation conditions and their rates estimated by monitoring the gradual consumption of carbon monoxide. Aimed at carrying out a sound comparison, the rate of mechanically activated processes was referred to individual collisions by normalizing its value to the amount of powder involved in individual collisions and to the time interval of ongoing deformation. This allowed to point out that the amount of carbon monoxide converted per unit of catalyst mass and unit of time under mechanical activation conditions is at least four orders of magnitude larger than the one reacted under thermal activation conditions. This result is tentatively connected with the generation of local excited states at the catalyst surface.

Binary Mg–Ni mixtures and ternary Mg–Ni–C (graphite) samples with fixed proportions of metals (Mg 85%–Ni 15% by weight) and amount of C increasing in increments of 5 wt % from 5 wt % to 15 wt % were prepared by high energy ball milling (BM) in Ar for tBM = 2 h. The purpose of the study was to evaluate the effect of C addition on the reactivity, the sorption activation and the storage performance of the Mg–Ni system. Increasing the amount of C had the effect of decreasing (from 10 to 3) the number of cycles needed for activation (performed at 623 K and 40 bar/0.9 bar charging/discharging H2 pressure). After full activation, the 5 wt % C-containing sample exhibited the best absorption kinetics performance: the average rate to charge up to 5 wt % H2 was about 3 times higher than that observed for the undoped sample. Unfortunately, increasing the amount of C had a negative impact on the desorption behaviour, causing an increase in the dehydrogenation activation energy and a decrease in the discharging rates. Within the present study, C reacted neither with H2 nor with the H2 active phases (the two discharged phases Mg and Mg2Ni and the related hydrides) and consequently did not lead to variation in the sorption enthalpies of the Mg–Ni system. But, its presence did cause a small increase (4 K at 0.9 bar H2) in the minimum desorption temperatures of the hydrides and a consequent minor decrease (0.2 bar) in the equilibrium pressures. The best sorption properties were obtained for the 5 wt % C-sample, that on the whole worked better than the binary mixture.