Thermal Dehydrogenation Research Articles

ChemInform Abstract: Na3[BN2] and Na2K[BN2]: A Known and a Novel Alkali Metal Dinitridoborate Obtained via Mild Thermal Dehydrogenation.

AbstractNa3[BN2] (I) and Na2K[BN2] (II) are obtained from the reaction of the respective binary mixtures NaNH2:NaBH4 and NaNH2:KBH4 in a molar ratio of 2:1 via thermal dehydrogenation (873 K and 683 K, resp., under Ar).

The effects of the nanometric interstitial compounds TiC, ZrC and TiN on the mechanical and thermal dehydrogenation and rehydrogenation of the nanocomposite lithium alanate (LiAlH4) hydride

Profuse mechanical dehydrogenation occurs during controlled high energy ball milling of LiAlH4 containing 5 wt.% of the nanometric interstitial compounds such as n-TiC, n-TiN and n-ZrC which involves a gradual decomposition of LiAlH4 to the mixture of Li3AlH6 and Al (Stage I) followed by a further decomposition of Li3AlH6 to the mixture of Al and LiH (Stage II). XRD reveals that the interstitial compounds remain stable in the hydride matrix during entire ball milling duration. The effectiveness of the nanometric interstitial compound additives for mechanical dehydrogenation increases on the order of n-TiN > n-TiC > n-ZrC. X-ray diffraction (XRD) reveals that there is no measurable change in a unit cell volume of LiAlH4 after ball milling which indicates that an accelerated mechanical dehydrogenation of LiAlH4 containing the nanometric interstitial compounds is unrelated to the lattice expansion as we have already reported for the nanometric metal Fe (n-Fe). In addition, the observed strong catalytic activity of the nanometric interstitial compounds for mechanical dehydrogenation is not related to their valence electron concentration (VEC) number. However, the n-TiN additive, which is the most effective one for mechanical dehydrogenation, has the smallest average particle size of 20 nm and the largest Specific Surface Area (SSA > 80 m2/g). For thermal dehydrogenation in Stage I the average apparent activation energy, EA, for the interstitial compound additives is within the range of 87–96 kJ/mol whereas, for comparison, the nanometric metallic additives, n-Fe and n-Ni, exhibit drastically smaller apparent activation energy on the order of 55–70 kJ/mol. The average apparent activation energy for thermal dehydrogenation in Stage II is in the range of 63–80 kJ/mol in the order of EA(n-ZrC) < EA(n-Ti = n-TiC) and is lower than that for the nanometric metal additives n-Ni and n-Fe. In summary, the nanometric interstitial compounds do not substantially affect the apparent activation energy of Stage I but are able to reduce the apparent activation energy of thermal dehydrogenation in Stage II. XRD reveals that the interstitial compounds remain stable in the hydride matrix up to the dehydrogenation temperature of at least 165 °C. Ball milled LiAlH4 containing 5 wt.% n-TiC, n-TiN and n-ZrC is able to slowly discharge large quantities of H2 up to 5–6 wt.% during storage at 40 °C. Unfortunately, the results of rehydrogenation at 165 °C under 95 bar for 5 h indicate that LiAlH4 containing the nanometric interstitial compounds exhibits no rehydrogenation.

Na3[BN2] and Na2K[BN2]: A Known and a Novel Alkali Metal Dinitridoborate Obtained via Mild Thermal Dehydrogenation

AbstractNa3[BN2] and Na2K[BN2] were obtained as white polycrystalline powders from the reaction of the respective binary mixtures NaNH2:NaBH4 and NaNH2:KBH4 in molar ratio 2:1 at 873 K and 683 K, respectively, in an argon stream. According to the results of thermal analysis measurements, both compounds are thermally stable only up to 954 K (Na3[BN2]) and 712 K (Na2K[BN2]), respectively, decomposing under evolution of alkali metal and nitrogen to yield hexagonal BN as final residue, which was identified from powder patterns. The crystal structure of Na3[BN2] {β‐Li3[BN2] type; P21/c (No. 14); Z = 4} was confirmed and the unit cell parameters redetermined: a = 5.724(1) Å, b = 7.944(1) Å, c = 7.893(1) Å, β = 111.31(1)°. According to X‐ray powder data, Na2K[BN2] crystallizes isotypic to Na2KCuO2 in the tetragonal space group I4/mmm (No. 139) with a = 4.2359(1) Å, c = 10.3014(2) Å and Z = 2. The crystal structure of Na2K[BN2] is composed of linear [N–B–N]3– anions centering elongated M14 rhombic dodecahedra, which are formed by 8 sodium and 6 potassium atoms. The [BN2]@Na8/4K6/6 polyhedra are stacked along [001] and condensed via common tetragonal faces to generate a space‐filling 3D arrangement. The B–N bond lengths for the strictly linear [N–B–N]3– units are 1.357(4) Å. Vibrational spectra of the title compounds were measured and analyzed based on D∞h symmetry of the relevant [N–B–N]3– groups taking into account the site symmetry effects for Na3[BN2]. Both the wavenumbers, as well as the calculated valence force constants f(B–N) = 7.29 N·cm–1 (Na3[BN2]) and 7.33 N·cm–1 (Na2K[BN2]), respectively, are in good agreement with those of the known alkali and alkaline earth dinitridoborates.

Role of Co3O4 in improving the hydrogen storage properties of a LiBH4–2LiNH2 composite

Adding a small amount of Co3O4 significantly reduces the operating temperatures of dehydrogenation and improves the hydrogen storage reversibility of the LiBH4–2LiNH2 system. The LiBH4–2LiNH2–0.05/3Co3O4 composite desorbs ∼9.9 wt% hydrogen by a four-step reaction with a 96 °C reduction in the midpoint temperature with respect to the pristine sample. The first and third steps of the dehydrogenation of the 0.05/3Co3O4-added sample are endothermic in nature, which is different from the pristine sample. Upon thermal dehydrogenation, the Co3O4 additive undergoes a series of chemical transformations and finally converts to the metallic Co, which is responsible for the improved thermodynamics and kinetics of the Co3O4-added sample. More importantly, 1.7 wt% of hydrogen is recharged into the 0.05/3Co3O4-added system under 110 bar hydrogen at 220 °C, which is superior to the pristine system.

N-Butane dehydrogenation over Pt/Mg(In)(Al)O

The thermal dehydrogenation of butane to butene and hydrogen was investigated over Pt nanoparticles supported on calcined hydrotalcite containing indium, Mg(In)(Al)O. Prior work has shown that upon reduction in H2 at temperatures above 673K, bimetallic Pt-In particles of are formed, as evidenced by XANES and EXAFS. The performance of Pt/Mg(In)(Al)O for butane dehydrogenation was found to be highly dependent on the bulk In/Pt ratio. The optimal ratio was found to be between 0.33 and 0.88, yielding >95% selectivity to butenes. Hydrogen co-fed with butane was shown to suppress coke formation and catalyst deactivation, a ratio of H2/C4H10=2.5 providing the best catalytic performance. Regeneration of catalysts after removal of accumulated carbon and reduction in H2 restored the original catalyst activity and selectivity. Butane dehydrogenation above 803K resulted in higher formation of butadiene, a known precursor to coke. No evidence for butane cracking was found to occur on Pt/Mg(In)(Al)O due to moderately basic nature of the support. The present study shows that Pt/Mg(In)(Al)O exhibits superior performance for butane dehydrogenation compared to supported Pt catalysts promoted with Sn, Ge, Pb, and In prepared by successive incipient wetness impregnation of the Pt and promoter precursors.

Remarkable decrease in dehydrogenation temperature of Li–B–N–H hydrogen storage system with CoO additive

Cobalt monoxide (CoO) was introduced into the Li–B–N–H system as a catalyst precursor, and the hydrogen desorption behavior of the LiBH4–2LiNH2–xCoO (x = 0–0.20) composites was investigated. It was observed that the majority of hydrogen desorption from the CoO-added sample occurred simultaneously with the melting of α-Li4BN3H10. Moreover, the 0.05CoO-added sample exhibited optimized dehydrogenation properties, desorbing 9.9 wt% hydrogen completely with an onset temperature of 100 °C and exhibiting a decrease of more than 120 °C in the onset dehydrogenation temperature with respect to that of the additive-free sample. The activation energy of hydrogen desorption for the 0.05CoO-added sample was reduced by 30%. XAFS measurements showed that the CoO additive was first reduced chemically to metallic Co during the initial stage of thermal dehydrogenation, and the newly produced metallic Co acted as the catalytic active species in favor of the creation of B–N bonding. More importantly, approximately 1.1 wt% of hydrogen could be recharged into the fully dehydrogenated 0.05CoO-added sample at 350 °C and a hydrogen pressure of 110 atm, which represents much better performance than that exhibited by the pristine sample.

Thermal dehydrogenation of amorphous silicon: A time-evolution study

A model is proposed to describe the decrease of H content in hydrogenated amorphous silicon (a-Si:H), during annealing at a fixed temperature. H content has been measured in several a-Si:H samples (grown by plasma enhanced chemical vapor deposition) after being submitted to different annealing times at 400°C. Obtained data has been fitted to the proposed model and initial diffusion coefficients of 3.2×10−14cm2/s for intrinsic films and 4.2×10−14cm2/s for n-type films were obtained. Reversely, H content evolution can be predicted during a thermal treatment if diffusion coefficients are previously known.

Effect of boric acid on thermal dehydrogenation of ammonia borane: H2 yield and process characteristics

We have recently demonstrated that boric acid (H3BO3, BA) is a promising additive to decrease onset temperature as well as to enhance hydrogen release kinetics for thermolysis of ammonia borane (NH3BH3, AB). The observations suggest that tetrahydroxyborate ion released by heating BA serves as Lewis acid and catalyzes AB dehydrogenation. Using this approach, we obtained high H2 yield at 85°C, along with rapid kinetics. Various operating conditions were investigated, such as reactor temperature, AB wt %, and particle size of BA. Even in the presence of 10 wt % BA, high H2 yield (13 wt %) and trace amount of ammonia (10–20 ppm) were obtained at 80°C, proton exchange membrane (PEM) fuel cell operating temperature. To our knowledge, such H2 yield value is higher than from any other method using AB with additive or catalyst at PEM fuel cell operating temperatures. © 2013 American Institute of Chemical Engineers AIChE J, 59: 3359–3364, 2013

Combined DFT, Microkinetic, and Experimental Study of Ethanol Steam Reforming on Pt

Density functional theory (DFT) calculations for the thermal decomposition and oxidative dehydrogenation of ethanol, mechanistic aspects of water–gas shift reaction, and experimental kinetic data are integrated so as to develop and assess a comprehensive DFT-based microkinetic model of low temperature ethanol steam reforming on Pt catalysts. The DFT calculations show (1) that the C–C scission should occur late in the dehydrogenation sequence, (2) that the C–C scission barriers in highly dehydrogenated intermediates are comparable to early C–H abstraction barriers, and (3) that the oxidative dehydrogenation reactions should not be important under steam reforming conditions. The DFT-parametrized model shows good qualitative agreement with experiments, with reasonable deviations attributed to modeling only the metal chemistry (i.e., excluding support effects). Both the model and the experiments show that the overall mechanism is simply thermal decomposition of ethanol followed by incomplete water–gas shift. ...

Enhanced dehydrogenation and rehydrogenation properties of LiBH4 catalyzed by graphene

The investigation of thermally induced dehydrogenation of LiBH4 reveals that LiBH4 doped with the graphene catalysts shows superior dehydrogenation and rehydrogenation performance to that of Vulcan XC-72, carbon nanotube and BP2000 doped LiBH4. For doping with 20 wt.% graphene, thermal dehydrogenation of LiBH4 is found to start at ca. 230 °C and a total weight loss of 11.4 wt.% can be obtained below 700 °C. With increased loading of graphene within a LiBH4 sample, the onset dehydrogenation temperature and the two main desorption peaks from LiBH4 are found to decrease while the hydrogen release amount is found to increase. Moreover, variation of the equilibrium pressure obtained from isotherms measured at 350–450 °C indicate the dehydrogenation enthalpy is reduced from 74 kJ mol−1 H2 for pure LiBH4 to ca. 40 kJ mol−1 H2 for 20 wt.% graphene doped LiBH4. Importantly, the reversible dehydrogenation/rehydrogenation process was achieved under 3 MPa H2 at 400 °C for 10 h, with a capacity of ca. 4.0 wt.% in the tenth cycle. Especially, LiBH4 is reformed and new species, Li2B10H10, is detected after the rehydrogenation process.

Two novel derivatives of ammonia borane for hydrogen storage: synthesis, structure, and hydrogen desorption investigation

Two new derivatives of ammonia borane (AB), 1,2/1,3-di-aminopropane borane (1,2/1,3-TMDAB), were prepared through the coordination reaction between 1,2/1,3-di-aminopropane and BH3–THF, which were then characterized by HR-XRD, FT-IR, 13C and 11B NMR. The crystal structure of 1,3-TMDAB was obtained with a combined technique of HR-XRD data and DFT calculations. 1,3-TMDAB crystallizes in the space group of P212121 (no. 19) with an orthorhombic crystal system and lattice constants of a = 12.6439(8) A, b = 8.6289(3) A, c = 7.2322(2) A, and V = 789.05(3) A3. Both samples, with a theoretical hydrogen content of 9.8 wt%, were shown to release pure H2 during thermal dehydrogenation, presenting great advantages over AB which releases large amounts of impurities (such as NH3, B2H6 or borazine). Moreover, 1,2/1,3-TMDAB did not foam and showed a faster dehydrogenation rate compared with AB. Our newly synthesized 1,2/1,3-TMDAB may serve as a superior alternative to AB for hydrogen storage and enrich the research field of B–N–H hydrogen storage materials.

Effect of boric acid on thermal dehydrogenation of ammonia borane: Mechanistic studies

Ammonia borane (AB, NH3BH3) is a promising hydrogen storage material for use in proton exchange membrane (PEM) fuel cell applications. In this study, the effect of boric acid on AB dehydrogenation was investigated. Our study shows that boric acid is a promising additive to decrease onset temperature as well as to enhance hydrogen release kinetics for AB thermolysis. With heating, boric acid forms tetrahydroxyborate ion along with some water released from boric acid itself. It is believed that this ion serves as Lewis acid which catalyzes AB dehydrogenation. Using boric acid, we obtained high H2 yield (11.5 wt% overall H2 yield, 2.23 H2 equivalent) at 85 °C, PEM fuel cell operating temperatures, along with rapid kinetics. In addition, only trace amount of NH3 (20–30 ppm) was detected in the gaseous product. The spent AB solid product was found to be polyborazylene-like species. The results suggest that the addition of boric acid to AB is promising for hydrogen storage, and could be used in PEM fuel cell based vehicles.

Catalytic Thermal Decomposition of Ammonia–Borane by Well‐Dispersed Metal Nanoparticles on Mesoporous Substrates Prepared by Magnetron Sputtering

AbstractA method to prepare well‐dispersed metal nanoparticles on mesoporous substrates has been developed that uses a magnetron sputtering technique. Pd, Ni, and Pt nanoparticles are deposited on MCM‐48, a silica‐based mesoporous material, with good dispersion. The samples prepared by this method showed promising catalytic effects on the thermodynamic and kinetic properties of ammonia–borane (AB) thermal dehydrogenation. Catalyst‐doped AB samples started to release H2 at much lower temperatures and suppressed the emission of volatile byproducts effectively. Furthermore, foaming and volume expansion can be totally avoided. The excellent catalytic performance is due to the synergetic effect of nanoconfinement of MCM‐48 and the catalytic behavior of the metal particles.

Thermal dehydrogenation of amorphous silicon deposited on c‐Si: Effect of the substrate temperature during deposition

AbstractSamples of doped and undoped a‐Si:H were deposited at temperatures ranging from 100 ºC to 350 ºC and then submitted to different dehydrogenation temperatures (from 350 ºC to 550 ºC) and times (from 1 h to 4 h). a‐Si:H films were characterised after deposition through the measurements of specific material parameters such as: the optical gap, the conductivity at 25 ºC, the thermal activation energy of conductivity and its hydrogen content. Hydrogen content was measured after each thermal treatment. Substrate dopant contamination from phosphorus‐doped a‐Si thin films was evaluated by SIMS after complete dehydrogenation and a junction depth of 0.1 mm was obtained. Dehydrogenation results show a strong dependence of the hydrogen content of the as‐deposited film on the deposition temperature. Nevertheless, the dehydrogenation temperature seems to determine the final H content in a way almost independent from the initial content in the sample. H richer films dehydrogenate faster than films with lower hydrogen concentration (© 2012 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Immobilizing Highly Catalytically Active Pt Nanoparticles inside the Pores of Metal–Organic Framework: A Double Solvents Approach

Ultrafine Pt nanoparticles were successfully immobilized inside the pores of a metal-organic framework, MIL-101, without aggregation of Pt nanoparticles on the external surfaces of framework by using a "double solvents" method. TEM and electron tomographic measurements clearly demonstrated the uniform three-dimensional distribution of the ultrafine Pt NPs throughout the interior cavities of MIL-101. The resulting Pt@MIL-101 composites represent the first highly active MOF-immobilized metal nanocatalysts for catalytic reactions in all three phases: liquid-phase ammonia borane hydrolysis, solid-phase ammonia borane thermal dehydrogenation, and gas-phase CO oxidation.

Synthesis and the Thermal and Catalytic Dehydrogenation Reactions of Amine-Thioboranes

A series of trimethylamine-thioborane adducts, Me(3)N·BH(2)SR (R = tBu [2a], nBu [2b], iPr [2c], Ph [2d], C(6)F(5) [2e]) have been prepared and characterized. Attempts to access secondary and primary amine adducts of thioboranes via amine-exchange reactions involving these species proved unsuccessful, with the thiolate moiety shown to be vulnerable to displacement by free amine. However, treatment of the arylthioboranes, [BH(2)-SPh](3) (9) and C(6)F(5)SBH(2)·SMe(2) (10) with Me(2)NH and iPr(2)NH successfully yielded the adducts Me(2)NH·BH(2)SR (R = Ph [11a], C(6)F(5) [12a]) and iPr(2)NH·BH(2)SR (R = Ph [11b], C(6)F(5) [12b]) in high yield. These adducts were also shown to be accessible via thermally induced hydrothiolation of the aminoboranes Me(2)N═BH(2), derived from the cyclic dimer [Me(2)N-BH(2)](2) (13), and iPr(2)N═BH(2) (14), respectively. Attempts to prepare the aliphatic thiolate substituted adducts R(2)NH·BH(2)SR' (R = Me, iPr; R' = tBu, nBu, iPr) via this method, however, proved unsuccessful, with the temperatures required to facilitate hydrothiolation also inducing thermal dehydrogenation of the amine-thioborane products to form aminothioboranes, R(2)N═BH(SR'). Thermal and catalytic dehydrogenation of the targeted amine-thioboranes, 11a/11b and 12a/12b were also investigated. Adducts 11b and 12b were cleanly dehydrogenated to yield iPr(2)N═BH(SPh) (22) and iPr(2)N═BH(SC(6)F(5)) (23), respectively, at 100 °C (18 h, toluene), with dehydrogenation also possible at 20 °C (42 h, toluene) with a 2 mol % loading of [Rh(μ-Cl)cod](2) in the case of the former species. Similar studies with adduct 11a evidenced a competitive elimination of H(2) and HSPh upon thermolysis, and other complex reactivity under catalytic conditions, whereas the fluorinated analogue 12a was found to be resistant to dehydrogenation.

Preparation of Graphene-Like Carbon Materials as Electrodes of Electric Double Layer Capacitors

Graphene-like carbon materials (GCMs) are prepared by the carbonization of liquid acrylonitrile oligomer (LANO) at 600-1400 °C for 4 h in an Ar atmosphere. LANO undergoes thermal oxidative dehydrogenation and preliminary cyclization by pre-process in air at low temperature (120 °C) and high temperature (220 °C) before the carbonization. Surface chemistry，crystalline and pore structures of the GCMs are characterized by scanning electron microscope (SEM), power X-ray diffraction (XRD) and N2 adsorption respectively. Comparison of the effect of various carbonization temperatures on the electrochemical performance of the electric double layer capacitors (EDLCs) is specifically investigated. The GCMs carbonized at 1300 °C for 4 h shows a fairly good electrochemical performance. The BET surface area of GCM1300 is 81.58 m2/g and the specific capacitance of the GCM1300 is 22.52 F/g in 1 mol/ L Et4NBF4/PC electrolytes.

Synthesis and Li electroactivity of Fe2P2O7 microspheres composed of self-assembled nanorods

Iron phosphate (Fe(II)2P2O7) microspheres composed of self-assembled nanorods were prepared by means of thermal dehydrogenation followed by thermochemical reduction of hydrothermally-synthesized FePO4·2H2O at 400°C under a H2 atmosphere. The phase and morphological evolutions of FePO4·2H2O and Fe2P2O7 products were characterized by X-ray diffraction. The Fe2P2O7 electrode as a new anode showed higher specific capacities (490mAhg−1 after 10 cycles at a rate of 100mAg−1) and better cyclic performances than FePO4·2H2O.

Highly Efficient Dehydrogenation of 5‐Benzyl‐3‐phenyl‐2‐thioxoimidazolidin‐4‐one: Microwave versus Flash Vacuum Pyrolysis Conditions

Abstract5‐Benzyl‐3‐phenyl‐2‐thioxoimidazolidin‐4‐one underwent thermal dehydrogenation to afford 5‐benzylidene‐2‐thioxoimidaxolidin‐4‐one under microwave and flash vacuum pyrolysis conditions. A high predominance of the Z‐isomer over the E‐isomer of the imidazolidinone product was achieved. By using DFT and NBO calculations, the mechanism of the dehydrogenation and the selectivity were also explored.

The effects of the micrometric and nanometric iron (Fe) additives on the mechanical and thermal dehydrogenation of lithium alanate (LiAlH4), its self-discharge at low temperatures and rehydrogenation

LiAlH4 containing 5 wt.% of nanometric Fe (n-Fe) shows a profound mechanical dehydrogenation by continuously desorbing hydrogen (H2) during high energy ball milling reaching ∼3.5 wt.% H2 after 5 h of milling. In contrast, no H2 desorption is observed during low energy milling of LiAlH4 containing n-Fe. Similarly, no H2 desorption occurs during high energy ball milling for LiAlH4 containing micrometric Fe (μ-Fe) and, for comparison, both the micrometric and nanometric Ni (μ-Ni and n-Ni) additive. X-ray diffraction studies show that ball milling results in a varying degree of the lattice expansion of LiAlH4 for both the Fe and Ni additives. A volumetric lattice expansion larger than 1% results in the profound destabilization of LiAlH4 accompanied by continuous H2 desorption during milling according to reaction: LiAlH4 (solid) → 1/3Li3AlH6 + 2/3Al + H2. It is hypothesized that the Fe ions are able to dissolve in the lattice of LiAlH4 by the action of mechanical energy, replacing the Al ions and forming a substitutional solid solution. The quantity of dissolved metal ions depends primarily on the total energy of milling per unit mass of powder generated within a prescribed milling time, the type of additive ion e.g. Fe vs. Ni and on the particle size (micrometric vs. nanometric) of metal additive. For thermal dehydrogenation the average apparent activation energy of Stage I (LiAlH4 (solid) → 1/3Li3AlH6 + 2/3Al + H2) is reduced from the range 76 to 96 kJ/mol for the μ-Fe additive to about 60 kJ/mol for the n-Fe additive. For Stage II dehydrogenation (1/3Li3AlH6 → LiH+1/3Al + 0.5H2) the average apparent activation energy is within the range 77–93 kJ/mol, regardless of the particle size of the Fe additive (μ-Fe vs. n-Fe). The n-Fe and n-Ni additives, the latter used for comparison, provide nearly identical enhancement of dehydrogenation rate during isothermal dehydrogenation at 100 °C. Ball milled (LiAlH4 + 5 wt.% n-Fe) slowly self-discharges up to ∼5 wt.% H2 during storage at room temperature (RT), 40 and 80 °C. Fully dehydrogenated (LiAlH4 + 5 wt.% n-Fe) has been partially rehydrogenated up to 0.5 wt.% H2 under 100 bar/160°C/24 h. However, the rehydrogenation parameters are not optimized yet.