Comparative Studies on Full Dehydrogenation of Dodecahydro-N-Ethylcarbazole on Pd(111) and Ni(111) Surfaces: Mechanism and Catalytic Enhancement.

Dodecahydro-N-ethylcarbazole (12H-NEC) is regarded as the most promising liquid organic hydrogen carrier for hydrogen storage and transportation. Understanding the mechanism of 12H-NEC dehydrogenation and developing cost-effective catalysts are significant. Pd is a high-performance catalyst for 12H-NEC but is not cost-effective, and Ni is just the opposite. How to understand the whole process of full dehydrogenation and improve the performance of Ni become two key questions. Herein, we systematically investigated the mechanism of the full dehydrogenation of 12H-NEC on Pd(111) and Ni(111) for the first time. By calculating all the barriers in the whole dehydrogenation process, we identified that 3H-NEC to 2H-NEC is the rate-determining step and Ni is catalytically less effective than Pd, which is attributed to its narrower d-band distribution and a 0.32 eV higher d-band center than that of Pd. To improve the performance of Ni, we further introduced dopants of Au, Ag, Cu, Pd, Pt, Ru, Rh, Zn, and Al. We found that Ag doping brings a downshift of the d-band center from -1.29 to -1.67 eV and reduces the barrier of 4H-NEC to NEC from 0.94 to 0.76 eV. This study provides new insights into the catalytic mechanism and performance-tuning strategy to help future experimental synthesis.

Zeolite Membrane-Based Low-Temperature Dehydrogenation of a Liquid Organic Hydrogen Carrier: A Key Step in the Development of a Hydrogen Economy.

Methylcyclohexane (MCH) dehydrogenation is an equilibrium-limited reaction that requires high temperatures (>300 °C) for complete conversion. However, high-temperature operation can degrade catalytic activity and produce unwanted side products. Thus, a hybrid zeolite membrane (Z) is prepared on the inner surface of a tubular support and used it as a wall in a membrane reactor (MR) configuration. Pt/C catalysts is packed diluted with quartz sand inside the Z-coated tube and applied the MR for MCH dehydrogenation at low temperatures (190-250 °C). Z showed a remarkable H2-permselectivity in the presence of both toluene and MCH, yielding separation factors over 350. The Z-based MR achieved higher MCH conversion (75.3% ± 0.8% at 220 °C) than the conventional packed-bed reactor (56.4% ± 0.3%) and the equilibrium state (53.2%), owing to the selective removal of H2 through Z. In summary, the hybrid zeolite MR enhances MCH dehydrogenation at low temperatures by overcoming thermodynamic limitations and improves the catalytic performance and product selectivity of the reaction.

Preparation of SBA-15 supported Ru nanocatalysts by electrostatic adsorption-ultrasonic in situ reduction method and its catalytic performance for hydrogen storage of N-ethylcarbazole.

N-ethylcarbazole (NEC) is an ideal liquid organic hydrogen storage carrier. The development of efficient hydrogen storage catalysts can promote the large-scale application of this process. In this paper, SBA-15 supported Ru nanocatalysts (Ru/S15-SU) were synthesized by strong electrostatic adsorption (SEA)-ultrasonic in situ reduction method (UR). Ru/S15-SU was characterized by N2 adsorption-desorption, TEM, H2 temperature program reduction, FT-IR, XRD, and XPS analysis measures. The results showed that ultrafine Ru NPs were evenly distributed on the surface of SBA-15, and ultrasonic in situ reduction not only reduced Ru3+ to Ru0, but also produced a coordination effect between Ru and O, enhancing the interaction between Ru NPs and the carrier. Ru/S15-SU exhibited excellent catalytic performance in the hydrogenation reaction of NEC, and the hydrogen storage efficiency reached 99.31% at 130°C and 6 MPa H2 pressure, which is superior to that of commercial 5wt%Ru/Al2O3. The excellent catalytic hydrogenation performance can be attributed to the selective anchoring of ruthenium ions on the surface of SBA-15 via electrostatic adsorption, preventing the aggregation of Ru NPs and enhancing the interaction between SBA-15 and Ru NPs by ultrasonic in situ reduction. Ru/S15-SU had a lower NEC hydrogenation apparent activated energy (Ea) of 68.45 kJ/mol than 5wt%Ru/Al2O3 catalyst. This method provides a new approach for the green preparation of nanocatalysts without using any chemical reducing agents.

Ultrasonic-assisted synthesis of highly stable RuPd bimetallic catalysts supported on MgAl-layered double hydroxide for N-ethylcarbazole hydrogenation.

N-Ethylcarbazole (NEC), as a promising liquid organic hydrogen carrier (LOHC), can store and release hydrogen through a reversible catalytic hydrogenation and dehydrogenation reaction. In this paper, RuPd bimetallic nanocatalyst supported on MgAl-layered double hydroxide (RuPd/LDH) was prepared by ultrasonic-assisted reduction method, and its catalytic performance in NEC hydrogenation was also studied. Under the action of ultrasound, hydroxyl groups (-OH) on the surface of LDH support dissociated into highly reductive hydrogen radicals for the reduction of Ru3+ and Pd2+ to Ru0 and Pd0. For the 4Ru1Pd/LDH-(300-1) catalyst prepared under ultrasonic conditions of 25 kHz, 300 W, and 1 h, the average size of the metal nanoparticles was only 1.23 nm, which indicated that Ru, Pd, and RuPd NPs were highly dispersed on the support. The strong electronic effects between Ru and Pd improved its catalytic performance in NEC hydrogenation. With m(Ru+Pd)/m(NEC) = 0.2wt%, pressure of 6 MPa, and temperature of 120 °C, the selectivity of dodecahydro-N-ethylcarbazole (12H-NEC) was 98.07%, and the capacity and percentage of hydrogen storage were 5.75wt% and 99.3%, respectively. After the catalyst was recycled 8 times, the percentage of hydrogen storage still reached 98.9%, showing higher stability. The preparation method is simple and environmentally friendly, providing an idea for the preparation of ultrafine bimetallic catalysts with high catalytic activity and stability.

Preparation of highly dispersed Pd/SBA-15 catalysts for dodecahydro-N-ethylcarbazole dehydrogenation reaction by ion exchange-glow discharge.

Dehydrogenation reactions are critical in hydrogen storage based on a liquid organic hydrogen carrier (LOHC) system. Speeding up the dehydrogenation rate and lowering the reaction temperature are the main focuses of LOHC dehydrogenation catalysts. In this paper, Pd/SBA-15 catalysts (Pd-IP/S15) were prepared by NaOH treatment of surface hydroxyl groups on SBA-15, the ion exchange of Na+ with Pd(NH3)42+, and then reduction of Pd ions via glow discharge plasma. The dehydrogenation performance of dodecahydro-N-ethylcarbazole on the prepared catalysts is studied. The turnover frequency of Pd-IP/S15 is 13.94 min-1 at 170°C, which is 10.25 times that of commercial Pd/C. It is ensured via the ion exchange method that Pd(NH3)42+ could be precisely targeted at the Si-OH of SBA-15 to form Si-O-Pd(NH3)42+, which effectively prevents the aggregation and uncontrollable growth of Pd nanoparticles (NPs) during the in situ reduction by plasma. Pd NPs with high dispersion are obtained on SBA-15, which enhances the catalytic activity of Pd-IP/S15. The coordination of Pd NPs with O of Si-OH on SBA-15 enabled Pd-IP/S15 to exhibit excellent catalytic stability. After 7 dehydrogenation cycles at 180°C, the dehydrogenation efficiency remained above 97%.

Recent Advances in Catalysts and Membranes for MCH Dehydrogenation: A Mini Review.

Methylcyclohexane (MCH), one of the liquid organic hydrogen carriers (LOHCs), offers a convenient way to store, transport, and supply hydrogen. Some features of MCH such as its liquid state at ambient temperature and pressure, large hydrogen storage capacity, its well-known catalytic endothermic dehydrogenation reaction and ease at which its dehydrogenated counterpart (toluene) can be hydrogenated back to MCH and make it one of the serious contenders for the development of hydrogen storage and transportation system of the future. In addition to advances on catalysts for MCH dehydrogenation and inorganic membrane for selective and efficient separation of hydrogen, there are increasing research interests on catalytic membrane reactors (CMR) that combine a catalyst and hydrogen separation membrane together in a compact system for improved efficiency because of the shift of the equilibrium dehydrogenation reaction forwarded by the continuous removal of hydrogen from the reaction mixture. Development of efficient CMRs can serve as an important step toward commercially viable hydrogen production systems. The recently demonstrated commercial MCH-TOL based hydrogen storage plant, international transportation network and compact hydrogen producing plants by Chiyoda and some other companies serves as initial successful steps toward the development of full-fledged operation of manufacturing, transportation and storage of zero carbon emission hydrogen in the future. There have been initiatives by industries in the development of compact on-board dehydrogenation plants to fuel hydrogen-powered locomotives. This review mainly focuses on recent advances in different technical aspects of catalytic dehydrogenation of MCH and some significant achievements in the commercial development of MCH-TOL based hydrogen storage, transportation and supply systems, along with the challenges and future prospects.

Hydrogen production from the catalytic dehydrogenation of dodecahydro-N-ethylcarbazole: effect of Pd precursor on the catalytic performance of Pd/C catalysts.

In this paper, Pd/C catalysts are synthesized via Ar glow-discharge plasma reduction using activated carbon as the support and Pd(acac)2, Pd(NO3)2, K2PdCl4, and H2PdCl4 as the Pd precursors, and their catalytic performances are investigated by hydrogen production from dodecahydro-N-ethylcarbazole (H12-NEC). Pd/C-A, prepared from Pd(acac)2, which has the smallest palladium nanoparticles (1.7 nm), the highest dispersion (34%) and no residue of inorganic ions, exhibits the best catalytic activity with a hydrogen release of 5.28 wt.%, which is 2.2 times that of Pd/C-H. The order of the apparent activation energies of the prepared Pd/C catalysts, according to the kinetics of the H12-NEC dehydrogenation reaction, is as follows: Pd/C-A ≈ Pd/C-N < Pd/C-K < Pd/C-H. When Pd(acac)2 with a large ligand acts as a cation Pd precursor, the effect of coulombic attraction to Pd2+ during the plasma reduction process makes it difficult for Pd nanoparticles (NPs) to migrate, which leads to the formation of ultrafine Pd NPs.

Combustion in the future: The importance of chemistry.

Combustion involves chemical reactions that are often highly exothermic. Combustion systems utilize the energy of chemical compounds released during this reactive process for transportation, to generate electric power, or to provide heat for various applications. Chemistry and combustion are interlinked in several ways. The outcome of a combustion process in terms of its energy and material balance, regarding the delivery of useful work as well as the generation of harmful emissions, depends sensitively on the molecular nature of the respective fuel. The design of efficient, low-emission combustion processes in compliance with air quality and climate goals suggests a closer inspection of the molecular properties and reactions of conventional, bio-derived, and synthetic fuels. Information about flammability, reaction intensity, and potentially hazardous combustion by-products is important also for safety considerations. Moreover, some of the compounds that serve as fuels can assume important roles in chemical energy storage and conversion. Combustion processes can furthermore be used to synthesize materials with attractive properties. A systematic understanding of the combustion behavior thus demands chemical knowledge. Desirable information includes properties of the thermodynamic states before and after the combustion reactions and relevant details about the dynamic processes that occur during the reactive transformations from the fuel and oxidizer to the products under the given boundary conditions. Combustion systems can be described, tailored, and improved by taking chemical knowledge into account. Combining theory, experiment, model development, simulation, and a systematic analysis of uncertainties enables qualitative or even quantitative predictions for many combustion situations of practical relevance. This article can highlight only a few of the numerous investigations on chemical processes for combustion and combustion-related science and applications, with a main focus on gas-phase reaction systems. It attempts to provide a snapshot of recent progress and a guide to exciting opportunities that drive such research beyond fossil combustion.

Catalytic dehydrogenation of liquid organic hydrogen carrier dodecahydro-N-ethylcarbazole over palladium catalysts supported on different supports.

The system based on liquid organic hydrogen carrier (LOHC) is one of the technologies to solve the problem of hydrogen storage and transportation capacity in large-scale applications. In this paper, the catalytic dehydrogenation of LOHC dodecahydro-N-ethylcarbazole (H12-NEC) over supported Pd nanoparticles (NPs) catalyst on four kinds of different supports, such as Pd/C, Pd/Al2O3, Pd/TiO2, and Pd/SiO2, was studied. It was found from catalyst characterization and dehydrogenation reaction that the volcano-type dependence of the activity on the Pd particle size, catalytic activity improvement with large specific surface area, and high Pd reduction degree indicated that the structure, particle size, and reduction degree of Pd NPs and textural properties of supports had a synergistic effect on the catalytic performance. Among all the catalysts, Pd/C displayed outstanding catalytic performance with the H12-NEC conversion of 99.9% and hydrogen storage capacity of 5.69 wt% at 180 °C after 12 h. The particle size of Pd/C distributes in the range of 1.5-6.0 nm with an average size of 3.0 nm. The results of dehydrogenation reaction kinetics showed that the rate limiting step and rate constant for different catalysts were mainly related to the physicochemical properties and adsorption and activation abilities towards the reactants and intermediates. In terms of the stationarity of dehydrogenation process, Pd/Al2O3 was excellent, indicating that it was best for dehydrogenation of H12-NEC.

Model Catalytic Studies of Novel Liquid Organic Hydrogen Carriers: Indole, Indoline and Octahydroindole on Pt(111).

Indole derivatives were recently proposed as potential liquid organic hydrogen carriers (LOHC) for storage of renewable energies. In this work, we have investigated the adsorption, dehydrogenation and degradation mechanisms in the indole/indoline/octahydroindole system on Pt(111). We have combined infrared reflection absorption spectroscopy (IRAS), X-ray photoelectron spectroscopy (XPS) and DFT calculations. Indole multilayers show a crystallization transition at 200 K, in which the molecules adopt a strongly tilted orientation, before the multilayer desorbs at 220 K. For indoline, a less pronounced restructuring transition occurs at 150 K and multilayer desorption is observed at 200 K. Octahydroindole multilayers desorb already at 185 K, without any indication for restructuring. Adsorbed monolayers of all three compounds are stable up to room temperature and undergo deprotonation at the NH bond above 300 K. For indoline, the reaction is followed by partial dehydrogenation at the 5-membered ring, leading to the formation of a flat-lying di-σ-indolide in the temperature range from 330-390 K. Noteworthy, the same surface intermediate is formed from indole. In contrast, the reaction of octahydroindole with Pt(111) leads to the formation of a different intermediate, which originates from partial dehydrogenation of the 6-membered ring. Above 390 K, all three compounds again form the same strongly dehydrogenated and partially decomposed surface species.

Vacuum Surface Science Meets Heterogeneous Catalysis: Dehydrogenation of a Liquid Organic Hydrogen Carrier in the Liquid State.

Ultrahigh vacuum (UHV) surface science techniques are used to study the heterogeneous catalytic dehydrogenation of a liquid organic hydrogen carrier in its liquid state close to the conditions of real catalysis. For this purpose, perhydrocarbazole (PH), otherwise volatile under UHV, is covalently linked as functional group to an imidazolium cation, forming a non-volatile ionic liquid (IL). The catalysed dehydrogenation of the PH unit as a function of temperature is investigated for a Pt foil covered by a macroscopically thick PH-IL film and for Pd particles suspended in the PH-IL film, and for PH-IL on Au as inert support. X-ray photoelectron spectroscopy and thermal desorption spectroscopy allows us to follow in situ the catalysed transition of perhydrocarbazole to carbazole at technical reaction temperatures. The data demonstrate the crucial role of the Pt and Pd catalysts in order to shift the dehydrogenation temperature below the critical temperature of thermal decomposition.

Model Catalytic Studies of Liquid Organic Hydrogen Carriers: Dehydrogenation and Decomposition Mechanisms of Dodecahydro-N-ethylcarbazole on Pt(111).

Liquid organic hydrogen carriers (LOHC) are compounds that enable chemical energy storage through reversible hydrogenation. They are considered a promising technology to decouple energy production and consumption by combining high-energy densities with easy handling. A prominent LOHC is N-ethylcarbazole (NEC), which is reversibly hydrogenated to dodecahydro-N-ethylcarbazole (H12-NEC). We studied the reaction of H12-NEC on Pt(111) under ultrahigh vacuum (UHV) conditions by applying infrared reflection-absorption spectroscopy, synchrotron radiation-based high resolution X-ray photoelectron spectroscopy, and temperature-programmed molecular beam methods. We show that molecular adsorption of H12-NEC on Pt(111) occurs at temperatures between 173 and 223 K, followed by initial C-H bond activation in direct proximity to the N atom. As the first stable dehydrogenation product, we identify octahydro-N-ethylcarbazole (H8-NEC). Dehydrogenation to H8-NEC occurs slowly between 223 and 273 K and much faster above 273 K. Stepwise dehydrogenation to NEC proceeds while heating to 380 K. An undesired side reaction, C-N bond scission, was observed above 390 K. H8-NEC and H8-carbazole are the dominant products desorbing from the surface. Desorption occurs at higher temperatures than H8-NEC formation. We show that desorption and dehydrogenation activity are directly linked to the number of adsorption sites being blocked by reaction intermediates.

Size and Structure Effects Controlling the Stability of the Liquid Organic Hydrogen Carrier Dodecahydro-N-ethylcarbazole during Dehydrogenation over Pt Model Catalysts.

Hydrogen can be stored conveniently using so-called liquid organic hydrogen carriers (LOHCs), for example, N-ethylcarbazole (NEC), which can be reversibly hydrogenated to dodecahydro-N-ethylcarbazole (H12-NEC). In this study, we focus on the dealkylation of H12-NEC, an undesired side reaction, which competes with dehydrogenation. The structural sensivity of dealkylation was studied by high-resolution X-ray photoelectron spectroscopy (HR-XPS) on Al2O3-supported Pt model catalysts and Pt(111) single crystals. We show that the morphology of the Pt deposit strongly influences LOHC degradation via C-N bond breakage. On smaller, defect-rich Pt particles, the onset of dealkylation is shifted by 90 K to lower temperatures as compared to large, well-shaped particles and well-ordered Pt(111). We attribute these effects to a reduced activation barrier for C-N bond breakage at low-coordinated Pt sites, which are abundant on small Pt aggregates but are rare on large particles and single crystal surfaces.