Dehydrogenation kinetics and catalysis of organic heteroaromatics for hydrogen storage

The complete recovery of the H₂ stored on dodecahydro-N-ethylcarbazole was achieved at 443 K and 101 kPa using Pd catalysts prepared by incipient wetness impregnation and calcination in He rather than air. Over a 4 wt% Pd/SiO₂ catalyst, the reaction proceeded to complete conversion within 22 min and complete H₂ recovery (5.8 wt%) within 1.6 h. The dehydrogenation rate of dodecahydro-N-ethylcarbazole and selectivity to the completely dehydrogenated product, N-ethylcarbazole, were dependent upon the Pd particle size. The dehydrogenation rate of dodecahydro-N-ethylcarbazole was compared to that of dodecahydrocarbazole and dodecahydrofluorene. The lower turn-over frequency (TOF) for dodecahydrocarbazole was attributed to a strong adsorption of the dehydrogenated products to Pd through the N atom, whereas the ethyl group in dodecahydro-N-ethylcarbazole prevented a strong N interaction with the surface. Density functional theory (DFT) results showed that dodecahydrocarbazole and dodecahydrofluorene were more strongly adsorbed on Pd than dodecahydro-N-ethylcarbazole leading to a significant decrease in their TOFs for H₂ recovery.     

Nitrogen-phosphorus co-functionalized reduced graphene oxide supported NiCoPd-CeOx nanoparticles as a highly efficient and stable catalyst for formic acid dehydrogenation

Formic acid (HCOOH, FA), a common liquid hydrogen storage material, has attracted tremendous research interest. However, the development of efficient, low-cost and high-stable heterogeneous catalyst for selective dehydrogenation of FA remains a major challenge. In this paper, a simple co-reduction method is proposed to synthesize nitrogen-phosphorus co-functionalized rGO (NPG) supported ultrafine NiCoPd-CeO_x nanoparticles (NPs) with a mean size of 1.2 nm. Remarkably, the as-prepared Ni_0.2Co_0.2Pd_0.6-CeO_x/NPG shows outstanding catalytic activity for FA dehydrogenation, affording a high TOF value of 6506.8 mol H₂ mol Pd^?1 h^?1 at 303 K and a low activation energy of 17.7 kJ mol^?1, which is better than most of the reported heterogeneous catalysts, and can be ascribed to the combined effect of well-dispersed ultrafine NiCoPd-CeO_x NPs, modified Pd electronic structure, and abundant active sites. The reaction mechanism of dehydrogenation of FA is also discussed. Furthermore, the optimized Ni_0.2Co_0.2Pd_0.6-CeO_x/NPG shows excellent stability over 10^th run with 100% conversion and 100% H₂ selectivity, which may provide more possibilities for practical application of FA system on fuel cells.     

Remarkable hydrogen properties of MgH2 via combination of an in-situ formed amorphous carbon

Carbon-based materials have been proposed as an ideal medium to reduce the reaction energy barriers and improve the (de)hydrogenation kinetics of magnesium-based hydrogen storage material (MgH₂) in term of their excellent dispersion. However, tedious preparation process and uneven distribution of carbon restrict the application. Therefore, in this paper, we cover MgH₂ by in-situ formed amorphous carbon via a facile approach of co-sintering Mg with fluorene followed by hydriding combustion and ball milling processes, named as MgH₂-carbonization product of fluorene (MgH₂-CPF). As a result, the MgH₂-CPF composite prepared at 823 K initially dehydrogenates at 557 K, 94 K lower than the as-milled MgH₂ (651 K). Meanwhile, the composite can release 5.67 wt% H₂ within 1000 s at 623 K. Even at a lower temperature of 423 K, the MgH₂-CPF composite still reabsorbs 5.62 wt% H₂ within 3600 s, while the as-milled Mg can hardly absorb hydrogen under a same condition. Furthermore, by addition of CPF, the apparent activation energy of the system is decreased from 161.2 kJ/mol to 87.2 kJ/mol. Our finding suggests that the carbon layer can keep the MgH₂ from aggregation, promote hydrogen transport and improve the efficiency of hydrogen absorption and desorption.     

Solvent-free synthesis of Rh/meso-Al2O3 via mechanochemistry for hydrolytic dehydrogenation of ammonia borane

An effective strategy synthesis of Rh/meso-Al₂O₃ catalysts was demonstrated by mechanochemistry for hydrolytic dehydrogenation of ammonia borane (AB). These catalysts are characterized systematically by N₂ adsorption-desorption isotherms, X-ray diffraction (XRD), X-ray photoelectron spectrometry (XPS), scanning electron microscope (SEM), and transmission electron microscope (TEM). The results show that the turnover frequency (TOF) and activation energy (E_a) are 246.8 mol_H2·mol_Rh^?1·min^?1 and 47.9 kJ mol^?1 for hydrolytic dehydrogenation of at 298 K catalyzed by Rh/Al₂O₃-CTAB-400, obviously higher than those previously reported catalysts. Furthermore, catalyst Rh/Al₂O₃-CTAB-400 can be recycled by simple centrifugal separation and the catalytic activity is still well maintained after five cycles. In addition, a plausible mechanism for hydrolytic dehydrogenation of AB has also been proposed. This mechanochemical synthesis method exhibits great application prospects for the preparation of heterogeneous catalysts.     

Tungsten(VI) oxide supported rhodium(0) nanoparticles; highly efficient catalyst for H2 production from dimethylamine borane

It reports the preparation and characterization of tungsten(VI) oxide supported rhodium(0) nanoparticles (Rh⁰/WO₃ NPs) being used as catalysts in releasing H₂ from dimethylamine borane (DMAB). The reducible nature of WO₃ plays a significant role in the catalytic efficiency of rhodium(0) nanoparticles in the dehydrogenation of DMAB. The Rh⁰/WO₃ NPs were in-situ generated from the reduction of Rh²⁺ ions on the surface of WO₃ during the catalytic dehydrogenation of dimethylamine borane in toluene and isolated from the reaction solution after the dehydrogenation to be characterized by using SEM, TEM, XPS, ATR-IR and XRD. The results reveal the formation of Rh⁰ NPs with a mean particle size of 1.92 ± 0.34 nm dispersed on the surface of tungsten(VI) oxide. Rh⁰/WO₃ NPs are found to be very active catalyst releasing 1.0 equiv. H₂ per mole of dimethylamine borane under ambient conditions. Among the various WO₃ supported Rh⁰ NPs with different metal loadings, the sample with 0.1% wt. Rh provide the record catalytic activity (TOF = 2816 h^?1) which is one of the highest value ever reported for rhodium-based catalysts in H₂ generation from DMAB at 60.0 ± 0.5 °C. Rh⁰/WO₃ NPs were also reusable catalyst in dehydrogenation of DMAB retaining 55% of their initial catalytic activity in the 3rd run of the dehydrogenation reaction. Control experiments were performed at various catalyst concentrations and temperatures to investigate the kinetics of dehydrogenation and to calculate the activation parameters for the reaction.     

Density functional theory study on dehydrogenation of methylcyclohexane on Ni–Pt(111)

Methylcyclohexane is a very promising liquid organic hydrogen carrier, but its dehydrogenation mechanism on Pt-based bimetallic catalysts is not yet clear. In order to understand the catalytic dehydrogenation of methylcyclohexane on Ni–Pt(111), DFT calculations were performed and the calculation results were compared with the corresponding values on Pt(111). It is shown that because the electronegativity of Ni atoms is less than that of Pt atoms, electrons transfer from Ni atoms to Pt atoms. Compared with Pt(111), the binding energy (the absolute value of the adsorption energy) of related species on Ni–Pt(111) surface was smaller, indicating that the binding strength between these species and the surface metal atoms on Ni–Pt(111) is weaker. In the stable adsorption configurations on Ni–Pt(111), almost all the metal atoms forming chemical bonds with the adsorbates were Pt atoms, indicating that Pt was the main active component. Although the actual catalytic reaction is more complicated, this study provided some insights into one of the important aspects.     

Hydrogen sorption properties of Li–N–F–H pellets in laboratory and small tank scales

We propose the improvement of de/rehydrogenation kinetics and reversibility of LiNH₂–LiH system via F substitution for H in LiNH₂. Slight content of LiF–TiH₂ composite is milled with LiNH₂–3LiH and the obtained mixture is compacted into the pellet. By compositing LiNH₂ with excess LiH and compaction, NH₃ emission poisoning fuel cell catalysts and degrading hydrogen capacity can be prevented. The formation of LiNH_(2-x)F_x achieved from F substitution in LiNH₂ significantly enhances kinetic properties, for example, the 1st dehydrogenation at 280 °C completes within 5 min with the capacity of 3.6 wt % H₂. Although LiNH_(2-x)F_x cannot be recovered, good kinetics and reversibility upon five de/absorption cycles of LiNH₂–LiH (up to 3.0 wt % H₂ within 20 min) are preserved due to catalytic effects of LiF. Phase compositions and hydrogen capacities in laboratory and tank scales are comparable. This suggests the maintained de/rehydrogenation performance of Li–N–F–H pellets even after upscaling.     

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.     

浅析循环气体压缩机停车事故的处理程序

循环气体压缩机的紧急停车,是烷基苯联合装置非计划停车中的一个比较典型的事故案例。通过对这个案例的分析和讨论,便于我们了解和掌握脱氢装置的操作要点。     

Influence of Pt nanoparticles modified by La and Ce oxides on catalytic dehydrocyclization of n-alkanes

Catalytic reforming accounts for a large share of the world’s gasoline production, it is the most important source of aromatics for the petrochemical industry. In addition, reforming of hydrocarbon on the dual-function catalysts has been found to form fundamentally different products in hydrogen diluents. Typical catalysts employed for this reforming process are Pt/Al₂O₃ and Pt-M/Al₂O₃, M being the promoter. These solids are characterized by both acid and metal functions which catalyze dehydrocyclization, dehydrogenation, isomerization and cracking processes. In this regard, information about cerium and lanthanum, as promoters, is hardly revealed. The present work aims to study the performance of Pt/Al₂O₃ catalysts modified by lanthanum or cerium during the conversion of cyclohexane, n-hexane and n-heptane. Catalytic activities of the prepared catalysts were tested using a micro catalytic pulse technique. Physicochemical characterization of the solid catalysts such as, surface area (S_BET), Fourier transform infrared (FTIR), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), hydrogen-temperature programed reduction (H₂-TPR), hydrogen-temperature-programed desorption (H₂-TPD), CO₂-TPD, NH₃-TPD, high resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) were depicted. Results indicated clearly that Pt/Al₂O₃ catalyst is selective toward dehydrogenation to benzene which could be explained as due to the decrease in the active acid sites and the comparative segregation of the alumina support especially at 3% load of CeO. The presence of La₂O₃ in the Pt/Al₂O₃ catalyst promotes aromatization of n-hexane and n-heptane, also the dehydrocyclization of n-hexane is more difficult than that of n-heptane. Thus, modification of the Pt/Al₂O₃ catalyst by La, resulted in a more active and selective reforming catalyst.