kJ Mol H2 Research Articles

Impact of hydronium ions on the Pd-catalyzed furfural hydrogenation

In aqueous mediums, the chemical environment for catalytic reactions is not only comprised of water molecules but also of corresponding ionized species, i.e., hydronium ions, which can impact the mechanism and kinetics of a reaction. Here we show that in aqueous-phase hydrogenation of furfural on Pd/C, increasing the hydronium ion activities by five orders of magnitude (from pH 7 to pH 1.6) leads to an increase of less than one order of magnitude in the reaction rate. Instead of a proton-coupled electron transfer pathway, our results show that a Langmuir-Hinshelwood mechanism describes the rate-limiting hydrogen addition step, where hydrogen atom adsorbed on Pd is transferred to the carbonyl C atom of the reactant. As such, the strength of hydrogen binding on Pd, which decreases with increasing hydronium ion concentration (i.e., 2 kJ molH2−1 per unit pH), is a decisive factor in hydrogenation kinetics (rate constant +270%). In comparison, furfural adsorption on Pd is pH-independent, maintaining a tilted geometry that favors hydrogen attack at the carbonyl group over the furan ring.

Boosting low-temperature de/re-hydrogenation performances of MgH2 with Pd-Ni bimetallic nanoparticles supported by mesoporous carbon

Bimetallic Pd-Ni nano-particles supported by a mesoporous carbon material CMK-3 (denoted as Pd30Ni70/CMK-3) were synthesized through solution impregnation and hydrogen reduction methods. Among those hierarchical Ni-Pd nano-particles, majorly large ones (>10 nm) are dispersed over the surface of CMK-3, while a litter small ones (<10 nm) are embedded into the pores. It significantly improves the de/re-hydrogenation performances of MgH2 at low temperature. The onset desorption temperature of MgH2-Pd30Ni70/CMK-3 is lowered by 150 K from that of pristine MgH2 (above 593 K). About 6 wt% hydrogen could be released during its decomposition below 561 K. Noticeably, MgH2-Pd30Ni70/CMK-3 is capable of releasing 1.3 wt% H2 even at 373 K. 4 wt% hydrogen can be absorbed at 343 K under a hydrogen pressure of 3 MPa within 18000 s. Activation energy values of both hydrogen decomposition (65.9 kJ mol−1) and absorption (78.9 kJ mol−1) for MgH2-Pd30Ni70/CMK-3 are greatly improved from those of as-milled MgH2. Thermal stability of the composite system is remarkably destabilized by 4.3 kJ mol H2−1 from pristine MgH2 according to pressure-composition isotherm curves and van't Hoff plots. The enhanced performances can be ascribed to the synergistic effects of both destabilization and catalysis from nano-dispersed Pd and Ni particles, respectively.

Hydrogen storage performances and reaction mechanism of non-stoichiometric compound Li1.3Na1.7AlH6 doped with Ti3C2

Our previous work found that Li1.3Na1.7AlH6 possesses the lowest dehydrogenation onset temperature and the most favorable dehydrogenation thermal stability among Li-Na-Al-H compounds (Fan et al., 2017). In this work, additional Ti3C2 was introduced into Li1.3Na1.7AlH6 by ball milling to further enhance its dehydrogenation performances. According to the volumetric release curves, the initial dehydrogenation temperature of the sample Li1.3Na1.7AlH6 + 5 wt.%Ti3C2 was lowered by 35 K from that of the pristine Li1.3Na1.7AlH6. The dehydrogenation enthalpy values of Li1.3Na1.7AlH6 + 5 wt.%Ti3C2 (46.9 KJ mol H2−1) was also slightly lower than that of pristine Li1.3Na1.7AlH6 (49.7 KJ mol H2−1). It means that the doped Ti3C2 was actively involved in the reaction with Li1.3Na1.7AlH6 during the dehydrogenation process. The investigation by X-ray photoelectron spectroscopy (XPS) demonstrated that the reaction between Ti element and Li1.3Na1.7AlH6 was definitely carried out in which the zero valence of Ti was transformed into Ti3+.

Acid resistant sulphonated poly(vinylidene fluoride-co-hexafluoropropylene)/graphene oxide composite cation exchange for water splitting by iodine-sulfur bunsen process for hydrogen production

The acid/oxidative-resistant aliphatic-polymer-based cation-exchange membrane (CEM) is urgently required for water splitting by iodine-sulfur electrochemical Bunsen process (I-S cycle). Under operating environment of I-S cycle, commercially available CEM (perfluorosulfonic acid polymers such as Nafion) exhibits excessive mass transfer via electro-osmosis and its conductivity deteriorates due to membrane dehydration, while aromatic hydrocarbon based polymers showed short lifetimes due to the oxidative degradation. Herein, we achieved partial sulfonation of poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-co-HFP) to introduce hydrophilic character in the polymer matrix, and prepared sulphonated PVDF-co-HFP (SPVDF)/sulphonated GO (SGO) composite acid resistant CEMs with different composition for water splitting via I-S process for hydrogen production. These CEMs were extensively characterized for their morphological characteristics, ion-exchange capacity (IEC), water uptake, conductivity, and stabilities (mechanical, chemical, and thermal) in comparison with state-of-art Nafion117 membrane. About 3-fold high bound water content of suitable assessed SPVDF/SGO40 CEM (bound water content: 3.17%) in compare with Nafion 117 (bound water content: 1.04%) avoids the membrane dehydration and thus any deterioration in membrane conductivity.Iodine−sulfur (I−S) Bunsen process was successfully performed by membrane electrolysis in a two-compartment electrolytic cell (direct contact-mode) using SPVDF/SGO composite CEMs of different compositions. Theoretical and experimental values of acid produced in the anolyte and catholyte, showed good agreement. Further, relatively high current efficiency (close to 100%) for electrochemical Bunsen process using SPVDF/SGO40 CEM confirmed absence of any side reaction, while approximately 300 kJ mol-H2−1 energy was consumed to produce 1 mol of H2. In spite of low conductivity in compare with state-of-art Nafion 117 membrane, SPVDF/SGO40 composite CEM was assessed as suitable candidate for water splitting via I-S (Bunsen) process because of its low-cost nature, high bound water content and excellent stabilities in highly acidic environment attributed to the partial fluorinated segments in the membrane matrix.

H2 Adsorption on Cu(I)–SSZ-13

We report H2 adsorption capacities reaching 0.05 wt % at 303 K and at 1 atm H2 pressure on solid-state CuCl-exchanged [Al]-SSZ-13 and [B]-SSZ-13 zeolites. Differential heat of H2 adsorption is found in the range between 16 and 48 kJ mol H2–1 on Cu(I)-SSZ-13 at 323 K and isosteric heat of adsorption is found between 20 and 55 kJ mol H2–1 on Cu(I)-[B]-SSZ-13 at temperatures between 293 and 323 K. Strong interactions between H2 and the copper cations in Cu(I)-SSZ-13 are evidenced using Rietveld refinements of neutron powder diffraction patterns revealing Cu–deuterium (D2) distances of 2.3(2) and 2.41(1) A. A temperature-dependent Cu(I) migration—away from the six-membered ring (6MR) and eight-membered ring (8MR)—is related to the high adsorption capacities of the samples at 303 K. At 10 K, access to Cu(I) at 6MR is sterically hindered by framework oxygen atoms (Cu–Oframework distance of 2.196(5) A), rationalizing the low H2 adsorption capacities of Cu(I)-SSZ-13 samples as compared to Na-SSZ-13 at 77 K.

Preparations and de/re-hydrogenation properties of LixNa3-xAlH6 (x=0.9–1.3) non-stoichiometric compounds

Mixed alkali alanates LixNa3-xAlH6 have been successfully synthesized by means of grinding mixtures of Li3AlH6 and Na3AlH6 in specific molar ratios. Non-stoichiometric LixNa3-xAlH6 compounds with single perovskite-type structures (space group Fm-3m) can be formed only within the composition range of x = 0.9–1.3. Li1.3Na1.7AlH6 exhibits superior hydrogen storage properties over other LixNa3-xAlH6 compounds. Its onset dehydrogenation temperature (∼423 K) was lowered by more than 40 K from other samples in temperature programmed dehydrogenation (TPD) curves. Also, the dehydrogenation capacity of Li1.3Na1.7AlH6 (3.45 wt.%) is the highest among the compounds. The dehydrogenation enthalpy values of LixNa3-xAlH6 decreased as x increased from 0.9 to 1.3 according to the results by isothermal pressure-composition (PCI) curves and van't Hoff plots. It shows that the dehydrogenation Li1.3Na1.7AlH6 (49.7 kJ mol H2−1) was greatly destabilized from that of LiNa2AlH6 (68.1 kJ mol H2−1). Furthermore, the apparent activation energy of dehydrogenation for Li1.3Na1.7AlH6 (138.1 kJ mol−1) was remarkably lowered from that of LiNa2AlH6. This illustrates that Li1.3Na1.7AlH6 exhibits enhanced dehydrogenation kinetics from that of LiNa2AlH6.

The enhanced de/re-hydrogenation performances of LiNa2AlH6 combined with two-dimension lamellar Ti3C2

In the present work, two-dimension lamellar Ti3C2 was employed as additive of LiNa2AlH6. Volumetric dehydrogenation results informed that the onset dehydrogenation temperature of the ball milled LiNa2AlH6+5 wt% Ti3C2 sample was lowered by 68 K from that of pristine LiNa2AlH6. Isothermal pressure-composition (P-C-I) curves and van't Hoff plots demonstrated that the dehydrogenation enthalpy value (61 kJ mol H2−1) of LiNa2AlH6 + 5 wt% Ti3C2 was significantly reduced from the pristine LiNa2AlH6(68 kJ mol H2−1). By means of XPS analyses, it can be revealed that the destabilization of LiNa2AlH6 should be resulted from the involvement of Ti and the yield of unidentified Ti3+ species. Furthermore, it can be concluded that the lowering of dehydrogenation temperature was merely due to the thermal destabilization of LiNa2AlH6 by Ti3C2, since Ti3C2 hardly improved the dehydrogenation kinetics of LiNa2AlH6 and reduced its activation energy. However, hydrogenation kinetics was slightly enhanced from the released products of LiNa2AlH6+5 wt% Ti3C2.

2-Acrylamido-2-methyl-1-propanesulfonic Acid Grafted Poly(vinylidene fluoride-co-hexafluoropropylene)-Based Acid-/Oxidative-Resistant Cation Exchange for Membrane Electrolysis.

For developing acid-/oxidative-resistant aliphatic-polymer-based cation-exchange membrane (CEM), macromolecular modification of poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-co-HFP) was carried out by controlled chemical grafting of 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS). To introduce the unsaturation suitable for chemical grafting, dehydrofluorination of commercially available PVDF-co-HFP was achieved under alkaline medium. Sulfonated copolymer (SCP) was prepared by the free radical copolymerization of dehydofluorinated PVDF-co-HFP (DHPVDF-co-HFP) and AMPS in the presence of free radical initiator. Prepared SCP-based CEMs were analyzed for their morphological characteristics, ion-exchange capacity (IEC), water uptake, conductivity, and stabilities (mechanical, chemical, and thermal) in comparison with state-of-art Nafion117 membrane. High bound water content avoids the membrane dehydration, and most optimal (SCP-1.33) membrane exhibited about ∼2.5-fold high bound water content in comparison with that of Nafion117 membrane. Bunsen reaction of iodine-sulfur (I-S) was successfully performed by direct-contact-mode membrane electrolysis in a two-compartment electrolytic cell using different SCP membranes. High current efficiency (83-99%) confirmed absence of any side reaction and 328.05 kJ mol-H2(-1) energy was required for to produce 1 mol of H2 by electrolytic cell with SCP-1.33 membrane. In spite of low conductivity for reported SCP membrane in comparison with that of Nafion117 membrane, SCP-1.33 membrane was assessed as suitable candidate for electrolysis because of its low-cost nature and excellent stabilities in highly acidic environment may be due to partial fluorinated segments in the membrane structure.

Highly destabilized Mg-Ti-Ni-H system investigated by density functional theory and hydrogenography

Using hydrogenography, we recently mapped the thermodynamic properties of a large range of compositions in the quaternary Mg-Ti-Ni-H system. The enthalpy of hydride formation of Mg-Ni alloys is significantly altered upon Ti doping. For a small range of compositions, we find a hydrogenation enthalpy H = �40 kJ mol H2 �1 , which is the desired enthalpy for hydrogen storage at moderate temperature and pressure. This enthalpy value is surprising since it is significantly less negative than the H of the Mg-Ni and Mg-Ti hydrides. The nanostructure of the Mg-Ti-Ni-H films hinders a direct determination of the hydride phases involved by x-ray diffraction. Using density functional theory calculations for various hydrogenation reaction paths, we establish that the destabilization of the Mg-Ni-H system by Ti doping is due to the formation of Mg2Ni and Ti-Ni intermetallics in the as-deposited state, which transform into a metastable Ti-doped Mg2NiH4 phase upon hydrogenation. The Ti-doped Mg2NiH4 phase can be considered as a heavily doped semiconductor.

Substitution effect of elements in Zr-based alloys with Laves phase for nickel-hydride battery

By examining the electrochemical and thermodynamic properties of Zr1−xA′x(VyNizMnuB′v)2+α (0 ⩽ x ⩽ 0.1; 0.10 ⩽ y ⩽ 0.15; 0.41 ⩽ z ⩽ 0.62; 0.28 ⩽ u ⩽ 0.39; 0 ⩽ v ⩽ 0.05; 0 ⩽ α ⩽ 0.02; y + z + u + v = 1;A′ ≡ Ti, Nb; B′ ≡ Co, Cr, Cu, Fe) alloy systems with C15-type Laves phase structure, the results described below were obtained. The alloy electrodes were completely activated even by one or two charge-discharge cycles at a current density of 17 mA g−1. Some of the alloys had discharge capacities larger than 400 mA h g−1 at 17 mA g−1. They also had capacities larger than 380 mA h g−1 even at 220 mA g−1. Moreover, the high rate capability for discharge increased as the absolute value of the partial molar enthalpy ΔH0 for hydride formation decreased. The ΔH0 values of such alloys with excellent electrochemical properties were found to range from −36 to −42 kJ mol H2−1. On the other hand, the substitution effect of the elements in the alloys was qualitatively and quantitatively evaluated as a function of the atomic radii and electronegativities of the constituent elements. ΔH0, which shows the stability of the absorbed hydrogen, had a good correlation individually with the average electronegativity ξav and the average atomic radius rav. The correlation was further improved when these two factors were considered together. The cell volume of C15 type had a correlation with ΔH0 and a good correlation with both the average atomic radius and the difference ΔξB−A in electronegativity between the A and B atoms. From the above description it can be concluded that the ranges of rav and ξav of the alloys with excellent capacity and high rate efficiency are 1.516 ⩽ rav ⩽ 1.525 and 1.621 ⩽ ξav ⩽ 1.640 respectively.

Adsorption and catalytic properties of CoxFe3–xO4 spinels. Part 2.—Hydrogen chemisorption on precursors to ammonia synthesis catalysts

At temperatures in the range 323–423 K H2 adsorbs largely irreversibly onto the surfaces of CoxFe3–xO4(0 ⩽x⩽ 3) spinels to an extent (0.91–4.76 molecules nm–2) which increases with x and temperature. Although the enthalpy of adsorption (60–210 kJ mol H2) is greater than bulk spinel reduction at these temperatures the latter process is not thought to be significant. Adsorption isotherms obey the Langmuir model well. Crystal-field considerations suggest sites responsible for adsorbing H2 which are contrary to recent reports (J-P. Beaufils, Y. Barbaux and B. Saubat, J. Chem. Soc., Chem. Commun., 1982, 1212). Cobalt additions improve the extent and enthalpy of the fast adsorption of hydrogen on these spinel precursors of non-promoted ammonia-synthesis catalysts.