Hydrogen storage in liquid organic hydride: Selectivity of MCH dehydrogenation over monometallic and bimetallic Pt catalysts

Hydrogen storage for mobile and stationary applications is an expanding research topic. One of the more promising storage techniques relies on the reversibility, high selectivity, and high hydrogen density of liquid organic hydrides, in particular methylcyclohexane (MCH). Catalyst evaluation for MCH dehydrogenation to toluene is based on three catalytic parameters: activity, selectivity, and stability. Current catalysts, optimized for catalytic reforming, do not meet the targeted aromatic selectivity (+99%) for MCH dehydrogenation. Therefore, a range of Pt catalysts was prepared and compared with commercially available catalysts in a fixed-bed reactor under operating conditions suitable for mobile and stationary applications. The best overall performance was realized by a particular monometallic Pt catalyst. This catalyst showed superior activity, selectivity, and stability compared with other prepared and commercial catalysts. As an effort to further enhance the aromatic selectivity, this study identified the main side-reactions associated with MCH dehydrogenation, the effect of operating parameters on by-product yields, and the effect of catalyst deactivation on long-term selectivity.     

Liquid-film-type catalytic decalin dehydrogeno-aromatization for long-term storage and long-distance transportation of hydrogen

A new approach for mobile storage of hydrogen has been proposed with the use of a catalytic reaction pair of decalin dehydrogenation/naphthalene hydrogenation. With the complement of the industrialized naphthalene-hydrogenation catalysis, the other endothermic catalysis for decalin dehydrogenation was now performed at around 200°C with carbon-supported platinum-based catalysts. Under liquid-film conditions, hydrogen was evolved from decalin much more efficiently than the suspended ones due to the superheated states of dehydrogenation catalysts. It was confirmed that the catalytic conversions of decalin dehydrogeno-aromatization in the liquid-film states could surpass easily the equilibrium limit, because the conditions of suppressed reactant evaporation and reactive distillation were operative here. Exergy loss in the hydrogen storage system would be reduced tremendously by adopting this catalyst-assisted decalin/naphthalene pair as the medium of hydrogen carrier.     

Current situation and prospect of hydrogen storage technology with new organic liquid

This paper starts with the brief introduction to various methods of hydrogen storage, such as pressurized gaseous hydrogen storage, cryogenic liquefaction hydrogen storage, carbonaceous materials hydrogen storage, metal alloy hydrogen storage, complexation hydride hydrogen storage, glass microspheres hydrogen storage, liquid organic hydrogen storage, and so on. The corresponding principles of hydrogen storage were summarized with the analysis on advantages and disadvantages. Additionally, the characteristics of hydrogen storage with N-ethylcarbazole were profoundly discussed. The conditions and catalysts for hydrogenation and dehydrogenation (N-ethylcarbazole) were also analyzed at some length as well. According to the present situation of hydrogen storage with organic liquids, some ideas were put forward to get higher content and speed of absorbing and releasing hydrogen.     

A new hydrogen storage system based on efficient reversible catalytic hydrogenation/dehydrogenation of terphenyl

Noble metal catalysts on mesoporous SiO₂ and modified carbon supports were found to enhance the activities of terphenyl (TPh) hydrogenation and tercyclohexane (TCH) dehydrogenation without side reactions, such as cracking, hydrogenolysis, ring opening and/or coke formation. The noble metal catalysts could be used for a reversible hydrogen storage system. Five percent Pt/SiO₂ catalyst was highly active in TCH dehydrogenation without stirring, due to an easier diffusion of organic molecules to the small catalyst particles during dehydrogenation.     

Hydrogen delivery through liquid organic hydrides: Considerations for a potential technology

Carrying hydrogen in chemically bounded form as cycloalkanes and recovery of hydrogen via a subsequent dehydrogenation reaction is a potential option for hydrogen transport and delivery. We have earlier reported a novel method for transportation and delivery of hydrogen through liquid organic hydrides (LOH) such as cycloalkanes. The candidate cycloalkanes including cyclohexane, methylcyclohexane, decalin etc. contains 6 to 8 wt% hydrogen with volume basis capacity of hydrogen storage of 60–62 kg/m³. In view of several advantages of the system such as transportation by present infrastructure of lorries, no specific temperature pressure requirement and recyclable reactants/products, the LOH definitely pose for a potential technology for hydrogen delivery. A considerable development is reported in this field regarding various aspects of the catalytic dehydrogenation of the cycloalkanes for activity, selectivity and stability. We have earlier reported an account of development in chemical hydrides. This article reports a state-of-art in LOH as hydrogen carrier related to dehydrogenation catalysts, supports, reactors, kinetics, thermodynamic aspects, potential demand of technology in field, patent literature etc.     

Nano-Pd/CeO2 catalysts for hydrogen storage by reversible benzene hydrogenation/dehydrogenation reactions

Nano-CeO₂ supports, which have different structure from different preparation methods, were used to prepare nano-Pd/CeO₂ catalysts. The hydrogen storage capacity of prepared nano-Pd/CeO₂ catalysts were studied via vapor phase benzene hydrogenation and cyclohexane dehydrogenation reactions. Results show that the prepared Pd/CeO₂ catalysts exhibit excellent benzene hydrogenation and cyclohexane dehydrogenation performances. The catalytic performance of the Pd/CeO₂ catalysts is related to the dispersion of metallic Pd, hydrogen adsorption-desorption ability and structure of Pd/CeO₂ catalysts and so on. And those properties are also directly affected by the morphology and mesoporous structure of the prepared nano-Pd/CeO₂ catalysts that can be regulated by CeO₂ support preparation methods. The synergistic effect between metal Pd, CeO₂ support and their structures can effectively promote benzene hydrogenation and cyclohexane dehydrogenation, thus promoting hydrogen storage capacity. The prepared Pd/CeO₂-HT catalyst, which has high specific surface area, developed pore structure and highly dispersed metal Pd species, exhibits superior catalytic performances. And, the Pd/CeO₂-HT catalyst exhibits superior catalytic hydrogen storage performances. The benzene conversion over it at 200 °C reaches 99.5%. Whereas the cyclohexane conversion at 450 °C is 65.3%, and the H₂ production capacity is 73.77 g/h.     

Progress of graphene and loaded transition metals on Mg-based hydrogen storage alloys

Mg-based hydrogen storage alloys have become a research hotspot in recent years owing to their high hydrogen storage capacity, good reversibility of hydrogen absorption/desorption, low cost, and abundant resources. However, its high thermodynamic stability and slow kinetics limit its application, so the modification of Mg-based hydrogen storage alloys has become the development direction of Mg-based alloys. Transition metals can be used as catalysts for the dehydrogenation of hydrogen storage alloys due to their excellent structural, electrical, and magnetic properties. Graphene, because of its unique sp² hybrid structure, excellent chemical stability, and a specific surface area of up to 2600 m²/g, can be used as a support for transition metal catalysts. In this paper, the internal mechanism of graphene as a catalyst for the catalysis of Mg-based hydrogen storage alloys was analyzed, and the hydrogen storage properties of graphene-catalyzed Mg-based hydrogen storage alloys were reviewed. The effects of graphene-supported different catalysts (transition metal, transition metal oxides, and transition metal compounds) on the hydrogen storage properties of Mg-based hydrogen storage alloys were also reviewed. The results showed that graphene played the roles of catalysis, co-catalysis, and inhibition of grain aggregation and growth in Mg-based hydrogen storage materials.     

Hydrogen storage by decalin dehydrogenation/naphthalene hydrogenation pair over platinum catalysts supported on activated carbon

Different platinum catalysts supported on activated carbon have been investigated for decalin dehydrogenation under several reaction conditions. Organic hydride as hydrogen storage media is an efficient source for fuel cells when the catalytic dehydrogenation reaction occurs rapidly under mild conditions. Among hydrides, the choice of decalin in this study is owed to the simplicity of analyses, as well as comparison with works reported in literature.     

Highly dispersed palladium nanoparticles anchored on polyethylenimine-modified carbon nanotubes for dehydrogenation of formic acid

Formic acid has been recognized as an excellent liquid hydrogen storage material. The development of catalysts with high performance for the dehydrogenation of formic acid is significant. Herein, we employed polyethylenimine-modified carbon nanotubes as a carrier for Pd nanoparticles to synthesize a novel catalyst by a wet chemical reduction method. It was found that the amino polymers on polyethylenimine-modified carbon nanotubes have great effects on reducing the size of Pd nanoparticles, changing the electronic state of Pd, and enhancing the hydrophilicity of catalyst. Therefore, the as-synthesized Pd/CNTs-A-PEI₁₈₀₀ catalyst showed superior activity for FA dehydrogenation in the absence of additives with an initial TOF value of 1506 h^?1 and a 100% selectivity of hydrogen.     

Enhancing selectivity and reducing cost for dehydrogenation of dodecahydro-N-ethylcarbazole by supporting platinum on titanium dioxide

N-ethylcarbazole/dodecahydro-N-ethylcarbazole (NECZ/12H-NECZ) was a promising system for hydrogen storage applications. 1.0 wt% Pt/TiO₂ was regarded as the optimal loading in Pt/TiO₂ catalyst applied in the 12H-NECZ dehydrogenation reaction. The hydrogen release amount, selectivity to NECZ and TOF of 12H-NECZ dehydrogenation are 5.75 wt %, 98% and 229.73 min⁻¹ at 453 K. Compared with the commercial 5.0 wt% Pd and Pt-based catalysts, it exhibited very high activity, selectivity and stability for 12H-NECZ dehydrogenation with low Pt loading. Combined with the XRD, XPS, HRTEM, TPR analysis, it was indicated that the enhanced catalytic performance was due to the SMSI (strong metal-supporting interaction) between Pt and TiO₂ support, which accelerated the rate-limiting step and enhanced the whole dehydrogenation reaction. This work may be beneficial for the commercial application of Pt/TiO₂ catalysts in the Liquid Organic Hydrogen Carrier (LOHC) system.     

Preparation of Pt supported on mesoporous Mg–Al oxide catalysts for efficient dehydrogenation of methylcyclohexane

Hydrogen energy, characterizing by high-energy density, non-pollution and renewability, is regarded as an ideal clean green energy, and the chemical hydrogen storage is an optimal strategy to realize its large-scale utilization. In this study, to enhance the hydrogen evolution rate in the dehydrogenation of methylcyclohexane (MCH), Pt supported on Mg–Al oxide catalysts were prepared and the effects of the co-precipitation reaction time during the preparation of Mg–Al hydrotalcite on their structural properties were studied in detail. The results showed that both the pore diameter and Pt dispersion were increased after prolonging the precipitation reaction time. During the dehydrogenation of MCH, these resultant catalysts presented high activity and good stability: hydrogen evolution rate reached up to 1892 mmol·g_Pt^?1 min^?1 at 623 K and the conversion was still held at 92% after 218 h. Of course, a slight decrease on the conversion during the dehydrogenation reaction was also observed, which was mainly attributed to the aggregation of Pt particles at high temperature.     

A experimental study on the dehydrogenation performance of dodecahydro-N-ethylcarbazole on M/TiO2 catalysts

Liquid organic hydrogen carrier (LOHC) is considered as a promising candidate for large-scale hydrogen storage. In this work, we found that Pt/TiO₂ catalysts exhibited better catalytic activity and selectivity compared to Pd/TiO₂ and commercial Pd/Al₂O₃ catalysts in the dehydrogenation of dodecahydro-N-ethylcarbazole (12H-NECZ) at 453 K. The catalytic activity of the noble metal catalysts followed the trend of Pt/TiO₂ > Pd/TiO₂ > Rh/TiO₂ > Au/TiO₂ > Ru/TiO₂. Compared with the commercial Pd/Al₂O₃, Pt/TiO₂ greatly improved the selectivity and conversion rate, the reaction time was also shortened. In addition, kinetics calculation was carried out to obtain fundamental reaction parameters. It was found that the third step of 4H-NECZ dehydrogenation to NECZ was the rate-limiting step of the entire dehydrogenation reaction for all catalysts.     

Mg-based nanocomposites with improved hydrogen storage performances

MgH₂ is one of the most attractive candidates for on-board H₂ storage. However, the practical application of MgH₂ has not been achieved due to its slow hydrogenation/dehydrogenation kinetics and high thermodynamic stability. Many strategies have been adopted to improve the hydrogen storage properties of Mg-based materials, including modifying microstructure by ball milling, alloying with other elements, doping with catalysts, and nanosizing. To further improve the hydrogen storage properties, the nanostructured Mg is combined with other materials to form nanocomposite. Herein, we review the recent development of the Mg-based nanocomposites produced by hydrogen plasma-metal reaction (HPMR), rapid solidification (RS) technique, and other approaches. These nanocomposites effectively enhance the sorption kinetics of Mg by facilitating hydrogen dissociation and diffusion, and prevent particle sintering and grain growth of Mg during hydrogenation/dehydrogenation process.     

Highly efficient dehydrogenation of hydrazine borane over CoIr/TiO2 catalyst

In recent years, hydrazine borane (HB) as an excellent hydrogen material has been extensively studied by researchers because of its substantial hydrogen content (15.4 wt%), favourable chemical stability, eco-friendliness and being easy to synthesize. With Higher activity, catalysts of HB dehydrogenation will undoubtedly be more desirable for practical applications. In this work, CoIr nanoparticles (NPs) are successfully immobilized on TiO₂ substrate, achieving outstanding catalytic performance and 100% conversion for HB dehydrogenation. Particularly, Co_0.6Ir_0.4/TiO₂ can complete the reaction for HB dehydrogenation at 323 K within 32 s (0.53 min), and shows a fairly high turnover frequency (TOF) value (5625 h^?1), which is higher than the values achieved by most Ni-based catalysts reported so far in the same condition. This superior catalytic performance can be attributed to uniform dispersion of metal NPs with small size and strong interaction among the CoIr NPs and the substrate. It is unquestionable that our work will help to promote the use of HB as a promising hydrogen storage material for fuel cells.     

Engineering PdCu and PdNi bimetallic catalysts with adjustable alloying degree for the dehydrogenation reaction of dodecahydro-N-ethylcarbazole

The NECZ/12H-NECZ (N-ethylcarbazole/dodecahydro-N-ethylcarbazole) system is regarded as the most potential liquid organic hydrogen carrier. However, the low activity, selectivity of NECZ and high cost of catalysts for the dehydrogenation reaction restrict its efficiency and commercial applications. In this work, a series of bimetallic Pd-M(M = Cu, Ni)/SiO₂ catalysts were prepared and employed to enhance catalytic activity and selectivity of NECZ for the 12H-NECZ dehydrogenation reaction. Pd₃Ni₁/SiO₂ exhibited high catalytic performance with 100% conversion, 91.1% selectivity of NECZ and 5.63 wt% hydrogen release amount at 453 K, 101.325 kPa for 8 h. The TOF (turnover frequency) of Pd₃Ni₁/SiO₂ is enhanced by 42.4% compared with Pd/SiO₂. Combined with the characterization analysis, it was found that adjusting the alloying degree or the alloy phase in the PdCu and PdNi bimetallic catalysts could significantly enhance the dehydrogenation activity and selectivity, which were dependent on the component of bimetallic catalysts. This work may provide theoretical guidance for designing the efficient and low-cost bimetallic catalysts for the dehydrogenation of 12H-NECZ, which could boost the commercial applications of liquid organic hydrogen carriers.     

Boosting the dehydrogenation efficiency of dodecahydro-N-ethylcarbazole by assembling Pt nanoparticles on the single-layer Ti3C2Tx MXene

As the candidates for large-scale hydrogen storage, liquid organic hydrogen carriers (LOHCs) exhibit evident advantages in hydrogen storage density and convenience of storage and transportation. Among them, NECZ (N-ethylcarbazole)/12H-NECZ (dodecahydro-N-ethylcarbazole) is considered as a typical system with the lower hydrogenation/dehydrogenation temperature. However, the low dehydrogenation efficiency restrict its commercial applications. In this work, the single-layer Ti₃C₂T_x MXene was employed as the support to load the Pt nanoparticles for the 12H-NECZ dehydrogenation reaction. The effect of transition metals, loading amounts and morphologies of catalysts were analyzed. It was found that the 3 wt% Pt/S–Ti₃C₂T_x catalyst exhibited the best catalytic performance with 100% conversion, 91.55% selectivity of NECZ and 5.62 wt% hydrogen release amount at 453 K, 101.325 kPa for 7 h. The product distributions and kinetics analysis suggested that the elementary reaction from 4H-NECZ to NECZ was the rate-limiting step. The selectivity of NECZ is sensitive to the dehydrogenation temperature. Combined with the XRD, SEM, HRTEM, XPS, BET and FT-IR results, it could be indicated that the special two-dimension structure of S–Ti₃C₂T_x and electronic effect between Pt and S–Ti₃C₂T_x enhanced the dehydrogenation efficiency of 12H-NECZ. The measurements of cyclic dehydrogenation indicated that the Pt/S–Ti₃C₂T_x catalyst exhibited good stability after 42 h. This work brought a new strategy for the design of efficient catalysts using two-dimensional materials in the applications of the liquid organic storage hydrogen technology.     

Modification of NaAlH4 properties using catalysts for solid-state hydrogen storage: A review

Energy is an essential requirement in our daily lives. Currently, most of our energy demands are fulfilled by fossil fuels. After 20 years, non-renewable fossil fuels are estimated to plummet rapidly. The world will face energy shortage and will seek for a new environmental method of energy generation for transportation, economy and application. Hydrogen is a fascinating energy carrier that is considered as ‘hydrogen economy’ for the future. The key challenge in developing the hydrogen economy is the context of hydrogen storage. Storing hydrogen via the solid-state method has received special attention and consideration because of its safety and larger storage capacity. A light complex hydride, NaAlH₄, is considered as an attractive material for solid-state hydrogen storage owing to its high hydrogen capacity, bulk in availability and low cost. Sluggish sorption kinetics and poor reversibility have driven research into various catalysts to enhance its hydrogen storage properties. This review article examines the development of different catalysts and their effects on the hydrogen storage properties of NaAlH₄. The addition of catalyst offers synergistic catalytic effect on the dehydrogenation performance of NaAlH₄. Doping NaAlH₄ with catalyst promote promising results such as lower decomposition temperature, improved kinetics and reduced activation energy. Superior performance on the dehydrogenation performance of NaAlH₄ doping with the catalyst may be due to the nanosized catalyst particle and in situ formed active species that may serve as nucleation sites at the surface of the NaAlH₄ matrix and benefiting the kinetics properties of NaAlH₄.     

Effect of LDH composition on the catalytic activity of Ru/LDH for the hydrolytic dehydrogenation of ammonia borane

The hydrolytic dehydrogenation of ammonia borane (NH₃-BH₃, AB for short) in the presence of catalysts has been identified to be a safe and efficient way for hydrogen release. Understanding the dehydrogenation mechanism of AB is helpful and important to design efficient catalysts. So far, although the effects of various factors on dehydrogenation of AB have been studied, such as the noble metal particle size effect, crystal-phase effect and the support crystal plane effect, the effect of support composition on dehydrogenation of AB has rarely been reported yet. In this study, we choose composition-adjustable layered double hydroxide (MgAl-LDHs) as support for Ru nanoparticles, and use the as-prepared catalysts for comparing their catalytic activity towards the dehydrogenation of AB. The catalytic results demonstrate the catalytic activity of Ru/MgAl-LDHs is related to MgAl-LDHs composition, exhibiting a support-composition effect in the hydrolytic dehydrogenation of AB. Combining various characterizations, the different composition of MgAl-LDHs has an effect on the interaction between Ru nanoparticles and MgAl-LDHs, which directly affects the catalytic activity for the hydrolysis of AB. This study provides new important fundamental knowledge on the mechanism of AB hydrolysis over practical supported metal catalysts which can be used for a better catalyst design.     

Effect of crystallite size of Al on the reversible hydrogen storage of NaAlH4 and few aspects of catalysts and catalysis

NaAlH₄ has been catalyzed by MWCNT, Ce rich mischmetal (Mm), MmNi₅ and TiO₂ catalysts. The general aspect which relates every catalyst with the hydrogen storage capacity of NaAlH₄ system has been verified through XRD analysis. Interesting features like chemical reduction, grain size variation, hydrogenation/dehydrogenation and phase transformation of the catalytic species are noticed. In the case of reversible hydrogen uptake, an interesting relationship exists between the restored hydrogen capacity, crystallite size of Al (desorbed in the dehydrogenation reaction) and the applied hydrogen pressure. Thus, as far as the reversible hydrogen storage in NaAlH₄ is concerned, the mysterious role of catalyst seems to be a process of restricting the size of Al in a narrow range. The factors considered for analyzing this claim are discussed in detail.     

Balancing molecular level influences of intermolecular frustrated Lewis pairs (FLP) for successful design of FLP catalysts for hydrogen storage applications

The rise in energy demands and the deleterious environmental issues related to fossil fuels has led to a surge of interest in hydrogen as a “green” alternative. Hydrogen's extraordinary energy density makes it a potential energy and economic “power”-house. Significant research has been dedicated to materials-based hydrogen storage. One area, liquid organic hydrogen carriers (LOHC) is of substantial interest for the reversible transportation of hydrogen from production to end-use facilities. There are challenges associated with this technology including the dependency on precious metal-based catalysts. Recent work in frustrated Lewis pair (FLP) catalysis demonstrates promise for addressing these challenges. This review is focused on assessing recent literature on the utilization of intermolecular FLP main group catalysts for improved hydrogenation/dehydrogenation of various substrates including potential LOHC complexes. This review will present an overview of FLPs, highlight potential hydrogen storage applications, and propose areas where knowledge gaps exist that require further investigations.