ArticlePublisher preview available

Dicyclohexylmethane as a Liquid Organic Hydrogen Carrier: A Model Study on the Dehydrogenation Mechanism over Pd(111)

Authors:
To read the full-text of this research, you can request a copy directly from the authors.

Abstract and Figures

We have studied the dehydrogenation of the liquid organic hydrogen carrier (LOHC) dicyclohexylmethane (DCHM) to diphenylmethane (DPM) and its side reactions on a Pd(111) single crystal surface. The adsorption and thermal evolution of both DPM and DCHM was measured in situ in ultrahigh vacuum (UHV) using synchrotron radiation-based high-resolution X-ray photoelectron spectroscopy (HR-XPS). We found that after deposition at 170 K, the hydrogen-lean DPM undergoes C-H bond scission at the methylene bridge at 200 K and, starting at 360 K, complete dehydrogenation of the phenyl rings occurs. Above 600 K, atomic carbon incorporates into the Pd bulk. For the hydrogen-rich DCHM, the first stable dehydrogenation intermediate, a double π-allylic species, forms already at 190 K. Until 340 K, further dehydrogenation of the phenyl rings and of the methylene bridge occurs, yielding the same intermediate that is formed upon heating of DPM to this temperature, that is, DPM dehydrogenated at the methylene bridge. The onset for the complete dehydrogenation of this intermediate occurs at a much higher temperature than after adsorption of DPM. This behavior is mainly attributed to coadsorbed hydrogen from DCHM dehydrogenation. The results are discussed in comparison to our previous study of DPM and DCHM on Pt(111) revealing strong material dependencies. Graphical Abstract
This content is subject to copyright. Terms and conditions apply.
Dicyclohexylmethane as a Liquid Organic Hydrogen Carrier:
A Model Study on the Dehydrogenation Mechanism over Pd(111)
M. Amende
1
C. Gleichweit
1
T. Xu
1
O. Ho
¨fert
1
M. Koch
2
P. Wasserscheid
2,3,4
H.-P. Steinru
¨ck
1,3
Christian Papp
1
Jo
¨rg Libuda
1,3
Received: 31 January 2016 / Accepted: 3 February 2016 / Published online: 16 February 2016
Springer Science+Business Media New York 2016
Abstract We have studied the dehydrogenation of the liq-
uid organic hydrogen carrier (LOHC) dicyclohexylmethane
(DCHM) to diphenylmethane (DPM) and its side reactions on
a Pd(111) single crystal surface. The adsorption and thermal
evolution of both DPM and DCHM was measured in situ in
ultrahigh vacuum (UHV) using synchrotron radiation-based
high-resolution X-rayphotoelectron spectroscopy (HR-XPS).
We found that after deposition at 170 K, the hydrogen-lean
DPM undergoes C-H bond scission at the methylene bridge at
200 K and, starting at 360 K, complete dehydrogenation of
the phenyl rings occurs. Above 600 K, atomic carbon incor-
porates into the Pd bulk. For the hydrogen-rich DCHM, the
first stable dehydrogenation intermediate, a double p-allylic
species, forms already at 190 K. Until 340 K, further dehy-
drogenation of the phenyl rings and of the methylene bridge
occurs, yielding the same intermediate that is formed upon
heating of DPM to this temperature, that is, DPM dehydro-
genated at the methylene bridge. The onset for the complete
dehydrogenation of this intermediate occurs at a much higher
temperature than after adsorption of DPM. This behavior is
mainly attributed to coadsorbed hydrogen from DCHM
dehydrogenation. The results are discussed in comparison to
our previous study of DPM and DCHM on Pt(111) revealing
strong material dependencies.
&Christian Papp
christian.papp@fau.de
&Jo
¨rg Libuda
joerg.libuda@fau.de
M. Amende
max.amende@fau.de
C. Gleichweit
christoph.gleichweit@fau.de
T. Xu
tao.xu@fau.de
O. Ho
¨fert
oliver.hoefert@fau.de
M. Koch
marcus.koch@fau.de
P. Wasserscheid
peter.wasserscheid@crt.cbi.uni-erlangen.de
H.-P. Steinru
¨ck
hans-peter.steinrueck@fau.de
1
Lehrstuhl fu
¨r Physikalische Chemie II, Friedrich-Alexander-
Universita
¨t Erlangen-Nu
¨rnberg, Egerlandstr. 3,
91058 Erlangen, Germany
2
Lehrstuhl fu
¨r Chemische Reaktionstechnik, Friedrich-
Alexander-Universita
¨t Erlangen-Nu
¨rnberg, Egerlandstr. 3,
91058 Erlangen, Germany
3
Erlangen Catalysis Resource Center, Friedrich-Alexander-
Universita
¨t Erlangen-Nu
¨rnberg, Egerlandstr. 3,
91058 Erlangen, Germany
4
Forschungszentrum Ju
¨lich, Helmholtz-Institut Erlangen-
Nu
¨rnberg fu
¨r Erneuerbare Energien (IEK 11), Na
¨gelsbachstr.
49b, 91052 Erlangen, Germany
123
Catal Lett (2016) 146:851–860
DOI 10.1007/s10562-016-1711-z
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
... This is especially important in the context of hydrogen storage with the liquid organic hydrogen carriers (LOHC), where the dehydrogenation reactions still occur at temperature not too far from 300°C. [4,5] For example, the thermally stable salts where successfully used for the dehydrogenation of indoline to indole with homogeneous Ir catalysts immobilized in the mixture of [Ph 4 P][NTf 2 ] and Cs[NTf 2 ]. [6] Nevertheless, the sufficient thermal stability of ionic liquids is required for processes involving LOHC. The thermal stability of ionic liquids is often defined as the onset temperature (T onset ) of the beginning of the measurable mass loss during the thermogravimetric heating of an IL sample. ...
Article
Full-text available
The two ionic compounds [Ph4P][NTf2] and Cs[NTf2] were qualified to be suitable liquid materials for different high temperature applications. Development and optimization of these application techniques require knowledge of the thermodynamic properties of vaporization. Vapor pressures and vaporization enthalpies have been measured by using quartz‐crystal microbalance. Solubility parameters and miscibility of ionic liquids in practically relevant solvents were assessed. The volatility of ionic liquids is not negligible at elevated temperatures. It is related to vapor pressures. We report the precise absolute vapor pressures and enthalpies of vaporization, of [Ph4P][NTf2] and Cs[NTf2], which were measured using the quartz‐crystal microbalance (QCM) method. Volatilities of both compounds is significantly lower compared to commonly used [Cnmim][NTf2] These values are useful for catalytic applications of ionic liquid as well as for qualitative assessment solubilities of ionic compounds in common ionic liquids of practical importance.
... It was concluded that no intact DPM was formed at any time neither on Pt(111) nor on Pd (111). 198,199 2. Theoretical Background ...
Thesis
The work presented in this thesis can be divided in two parts. The first part covers the physico-chemical investigation of chosen molten salts, all based on the bis(trifluoromethylsulfonyl)imide anion, one of the most common anions in ionic liquids (ILs). The Cs[NTf2] and [PPh4][NTf2] molten salts were of special interest. With melting points of 125 and 134 °C, respectively, they cannot be defined as ionic liquids, but they are highly thermally stable. The high thermal stability makes these molten salts very interesting as solvents for reactions at higher temperatures (between 150 and 350 °C) where common ionic liquids already decompose. Also, binary mixtures of these salts were investigated and a eutectic mixture containing 32 mol% [PPh4][NTf2] with a melting point of 98 °C was found. The density and viscosity of the mixtures lie between the ones of the pure salts. The molten salts were further investigated with vibrational spectroscopy which was extended by DFT calculations. The [NTf2]- anions of both salts were found to be in cis conformation where both CF3 groups are on the same side of the S-N-S plane. With high temperature Raman spectroscopy and mass spectrometry investigations all volatile decomposition products after prolonged heating of the salts could be identified. In order to use the molten salts as solvents for homogeneous catalysis the solvation of transition metal bis(trifluoromethylsulfonyl)imide compounds with the general formula M(NTf2)2 was studied. Raman spectroscopy, powder X-ray diffraction and mass spectrometry lead to the conclusion Co(NTf2)2 in [PPh4][NTf2] is octahedrally coordinated and the anion [Co(NTf2)3]- is formed. The mixtures of metal compounds and molten salts were investigated regarding their melting points, viscosity and thermal stability. The second part of the thesis deals with the question if the molten salts can serve as solvents for homogeneous catalysis. Thus, the M(NTf2)2 compounds (with M = Mn, Co, Ni, Cu, Zn) were used as catalyst dissolved in the IL [PMeBu3][NTf2] for the Friedel-Crafts acylation of toluene with benzoylchloride. The yields of 4-methylbenzophenone were moderate and the catalyst was found to be not stable under reaction conditions. In the context of renewable energy the storage of excess energy in the form of hydrogen has recently received a great deal of interest. One very advantageous storage method for H2 is by covalently binding it to liquid organic hydrogen carriers (LOHC). In this thesis hydrogenation and dehydrogenation were tested with homogeneous Ir catalysts immobilized in a molten salt to realize a liquid-liquid biphasic reaction with the substrates residing in a second phase of extracting agent. The dehydrogenation of indoline to indole was optimized and the extracting agent diphenyl ether was superior to dibutyl ether due to the different solubilites of the substrates. Also, the molten salts with aromatic cations showed slightly higher activity. The most active catalyst was the commercially available Crabtree catalyst [Ir(cod)(Py)(PCy3)][PF6]. Also, Co(NTf2)2(PPh3)2 was found to be active in the dehydrogenation of indoline. The biphasic approach was extended to the homogeneous Ir catalyzed hydrogenation of indole, which was found to be rather slow. However, a one-pot pressure swing reversible hydrogenation-dehydrogenation of two LOHC pairs, namely indole/indoline and quinaldine/tetrahydroquinaldine was possible. This is the first example of an ultra-low temperature hydrogen battery.
Article
Liquid organic hydrogen carriers (LOHCs) relying on eutectic diphenylmethane-biphenyl mixtures feature advantageous characteristics such as low melting points and large hydrogen storage capacities. For contributing to a reliable database of process-relevant thermophysical properties, the present study investigates the viscosity, surface tension, and density of the LOHC-system based on diphenylmethane, biphenyl, and benzophenone between (278 and 573) K. General agreement between the viscosity and surface tension results from surface light scattering and the data from capillary viscometry and pendant-drop tensiometry is found. Larger surface tension differences beyond 10% for systems containing benzophenone seem to originate from surface orientation effects. For the eutectic diphenylmethane-biphenyl mixture including its hydrogenated dicyclohexylmethane-bicyclohexyl analog, the densities, surface tensions, and viscosities are not significantly different from those of the corresponding pure compounds. By gradually replacing diphenylmethane by its oxidized form benzophenone in mixtures with biphenyl, an increase in density, surface tension, and especially viscosity is observed.
Article
For the efficient design of hydrogenation and dehydrogenation processes, a comprehensive database for the viscosity, surface tension, and density of mixtures of the diphenylmethane-based liquid organic hydrogen carrier system and the pure intermediate cyclohexylphenylmethane measured by complementary optical and conventional methods and calculated by molecular dynamics simulations at process-relevant temperatures up to 623 K is presented. The simulations employ self-developed force fields including a new one for cyclohexylphenylmethane and reveal surface enrichment and orientation effects influencing the surface tension. Relatively simple correlation and prediction approaches yield accurate representations as function of temperature and degree of hydrogenation (DoH) of the mixtures with average absolute relative deviations (AARD) of 0.07% for the density and 2.9% for the surface tension. Application of the extended hard-sphere theory considering the presented accurate density data allows capturing the highly nonlinear DoH-dependent behavior of the dynamic viscosity with an AARD of 2.9%.
Article
Efficient hydrogen release from liquid organic hydrogen carriers (LOHCs) requires a high level of control over the catalytic properties of supported noble metal nanoparticles. Here, the formation of carbon-containing phases...
Article
Hydrogen is widely considered an ideal energy source from the viewpoint of sustainability. However, as hydrogen is a gas under ambient conditions and needs to be handled with care, the development of safe and efficient hydrogen storage methods is indispensable for realizing advanced hydrogen technologies. For this reason, hydrogen storage systems that use organic compounds as a medium have garnered attention. In this review, catalytic systems for reversible dehydrogenation reactions and hydrogenation reactions using organic hydrogen carriers, those are promising to the application to hydrogen storage, are surveyed. Additionally, catalytic dehydrogenation of ammonia-borane and related compounds, those are also promising materials for hydrogen storage, are briefly introduced.
Thesis
The presented work addresses the need for dedicated reactor concepts for highly sophisticated reactions – particularly multiphase systems suffering from bad heat transport or hydrodynamic issues. Within the scope of process intensification, structured reactors have developed into interesting candidates to encounter these challenges. For heterogeneous catalysis certain steps and aspects must be considered when it comes to design, manufacturing and coating of these structures. Among various types of structures, the periodic open cellular structures (POCS) are suitable for defining dedicated structure morphologies. Therefore, the specific surface area and porosity of a diamond-based cell were derived as a function of cell size and strut thickness, respectively. In contrast to typical packed beds, each geometric parameter can be adjusted independently of one another. Due to freedom of component geometry and highly precise fabricating dimensions, the selective electron beam melting process (SEBM) was used for producing the supporting metal structures. In order to coat the metal surface with catalyst material, electrophoretic deposition was applied in order to form a defined layer thickness of boehmite particles. The addition of nitric acid and aluminium isopropoxide showed a significant influence on suspension stability and zeta potential. Low suspension conductivity and high zeta potential resulted in high deposition rates, but also bad adhesion between metal surface and particles. Good adhesion was achieved at lower Zetapotential with accordingly low deposition rates. An overall trade-off was accomplished at a nitric acid concentration of 25 mM in a suspension containing ethanol, 5 wt. % boehmite and 0,5 wt. % aluminium isopropoxide. Coating procedures starting with low electric field led to compact primer layers improving the overall grade of adhesion. Among the coating experiments, electric resistance was found to be an appropriate tool for monitoring layer thickness. With respect to upscaling the coating process to technical scale, easy preparation and handling of electrode configuration are crucial. Applying typical placement of external counter electrode could be correlated with depletion of particle deposition in the direction of structure center. Distributing the counter electrode as cylindrical struts into the structure’s free projection area led to well distributed layer thickness, but demands complicated insertion and preparation steps. For this challenge, additively manufactured InterPOCS offer a superior electrode configuration. Due to the interpenetrating structure configuration, there is no need for additional electrode preparing and insertion steps. As one structure is shifted by a half size in z-direction relatively to the second structure, both structures can be positioned without any contact allowing an overall homogeneous electric field between both structures. Thus, in the first step, one structure acts as the counter electrode in order to coat the second structure. In the next step, both structures’ polarities are switched, so that the second structure becomes the counter electrode to coat the first structure. Optimal deposition rates and layer qualities at an electric field of 20 V mm-1 were achieved at one polarity switch including a ramp of 2,5 V min-1 after switch step. The radial and axial distribution of boehmite layer thickness within the structure showed adequate homogeneity at a mean layer thickness of 60 µm. Besides the contactless structure positioning within InterPOCS, the grade of manufacturing precision allowed the possibility of moving one structure relatively to the second structure in z-direction. This allows an overall change of cell arrangement without changing porosity or specific surface area. Single phase pressure drop measurements using air as the model gas were performed, in order to establish a two-channel-model, that correlates the structures’ offset with the normalized change of pressure drop. In complete regular structure arrangement only one kind of channel was detected, whereas increasing the structure’ s offset led to two categories of channel size. Accordingly, increasing the offset led to drastic reduction of pressure drop within a range up to 50 % to 60 % compared to the minimal offset. Investigations addressing the gas holdup in two phase systems showed an inversely proportional correlation between offset value and gas hold up. Finally, the insights of structure morphology, electrophoretic coating procedure and hydrodynamic investigations of InterPOCS were condensed and applied in the dehydrogenation of liquid perhydro-dibenzyltoluene as a strongly endothermic reaction with strong hydrogen gas release. As a benchmark system, a typical packed bed was used as catalyst system in order to evaluate the thermal properties in the reactor. Significant axial and radial temperature gradients between 10 K and 75 K indicates poor heat transport properties. The lack of solid interconnecting matrix and the strong influence of flow regime on convective heat transport were suggested to be the most probable reasons for this phenomenon. Applying a catalytically coated InterPOCS system could reduce the overall temperature gradients below 10 K at remarkable productivity. In-operando structure modifications were performed during the dehydrogenation procedure. Increasing the offset led to enhanced radial heat transport without changing wall temperature or feed flowrate. In this way, an increase of hydrogen productivity up to 20 % were achieved within two minutes.
Article
The unsatisfactory performance of dehydrogenation catalysts has been the bottleneck for liquid organic hydrogen carrier (LOHC) development. After systematic experiments, the Au/Pd core/shell catalysts were screened from a series of Pd-M (M=Au, Ag, Ru, Rh) combinations for dehydrogenation of dodecahydro-N-ethylcarbazole (12H-NEC) through a one-pot wet chemical synthesis. The ratio of Pd to Au is also within the scope of the experiment and it was found that the catalytic activity was following the order of Au1Pd1.3 > Au1Pd2 > Au1Pd1 > Ru1Pd1.3 > Au1Pd0.7 > Rh1Pd1.3 > Ag1Pd1.3 supported on rGO for the dehydrogenation process. Au1Pd1.3/rGO greatly improves the efficiency of the dehydrogenation reaction, specifically, while maintaining selectivity and conversion rate of 100%, the reaction time was shortened by 43 % compared to the monometallic Pd/rGO catalyst with the highest activity we prepared before, and compared to the best performing bimetallic catalyst in literature, the optimal reaction time in this work was reduced by 71 % when the hydrogen storage requirements of US DoE (Department of Energy) are met. A cycle performance experiment was performed to verify its excellent catalytic stability. Further catalyst characterization also proves that it has good morphology and stability. Kinetics calculation was carried out to obtain fundamental reaction parameters.
Article
Full-text available
Liquid organic hydrogen carriers (LOHCs) have great potential as a hydrogen storage medium needed for a future sustainable energy system. Dehydrogenation of LOHCs requires a catalyst, such as supported Pd nanoparticles. Under reaction conditions, hydrogen and carbon may diffuse into the bulk of supported Pd catalyst particles and affect their activity and selectivity. The detailed understanding of this process is critical for the use of LOHCs in future hydrogen storage technologies. In this work, we studied these processes in-situ on a Pd model catalyst using high-energy grazing incidence X-ray diffraction. Pd nanoparticles were evaporated in ultra-high vacuum on a polished α-Al2O3(0001) substrate. The particles, with an initial average size of ~ 3.4 nm, were investigated at elevated temperature during their interaction with H2 and methylcyclohexane (MCH) representing a model LOHC. The interaction with H2 was studied in-situ at partial pressures up to 1 bar and temperatures between 300 and 500 K. At 300 K, the Pd nanoparticles (NPs) show a transition from α-PdH to β-PdH as a function of the H2 pressure. The transition occurs gradually, which is attributed to the heterogeneity of the NP system. The hydrogen uptake in β-PdHx at 300 K and 1 bar is estimated to be XH ~ 0.37 ± 0.03 indicating that the miscibility gap is narrowed for the nanoparticular system. With increasing temperature, XH decreases until no β-PdH phase is formed anymore at 500 K. At the same temperature, we studied the interaction of the Pd/sapphire model catalyst with MCH, both in the presence and in the absence of H2. In the absence of H2, carbon is formed and diffuses into the bulk yielding PdCx with a C concentration of around x ~ 0.05 ± 0.01. In the presence of H2 in the gas phase, bulk carbon formation in the Pd/sapphire model catalyst is completely suppressed. These results show that Pd nanoparticles act as an adequate catalyst for the dehydrogenation of MCH. Graphical Abstract
Article
Exhaustive hydrogenation of polyaromatic compounds with different extents of condensation (benzene, biphenyl, terphenyl) is studied. The reactions of catalytic dehydrogenation of the polycyclic naphthenic hydrocarbons (cyclohexane, bicyclohexyl, perhydroterphenyl) produced by hydrogenation of the aromatic substrates on a 3%Pt/Sibunit catalyst are explored in a flow setup in the temperature range 260–340 °С at the liquid hourly space velocity of 1 h− 1. The directions of dehydrogenation of each of the studied substrates are studied in detail. Correlation dependences showing the effect of the structure of the molecules under study on their conversion in the dehydrogenation process are revealed. The influence of isomerization of steric and structural isomers on the kinetics of the entire process is established.
Article
Full-text available
Liquid Organic Hydrogen Carrier (LOHC) systems offer a very attractive way to store and transport hydrogen, a technical feature that is highly desirable to link unsteady energy production from renewables with the vision of a sustainable, CO2-free, hydrogen-based energy system. LOHCs can be charged and discharged with considerable amounts of hydrogen in cyclic, catalytic hydrogenation and dehydrogenation processes. As their physico-chemical properties are very similar to diesel, today’s infrastructure for liquid fuels can be used for their handling thus greatly facilitating the step-wise transition from today’s fossil system to a CO2 emission free energy supply for both, stationary and mobile applications. However, for a broader application of these liquids it is mandatory to study in addition to their technical performance also their potential impact on environment and human health. This paper presents the first account on the toxicological profile of some potential LOHC structures. Moreover, it documents the importance of an early integration of hazard assessment into technology development and reveals for the specific case of LOHC structures the need for additional research in order to overcome some challenges in the hazard assessment for these liquids.
Article
The potential for chemical H2 storage on liquid organic hydrogen carriers (LOHCs) has focused attention on the catalytic reactions needed to store and release H2 from the LOHCs. Herein we review our recent studies on the use of N‐ethylcarbazole and carbazole as LOHCs. Experimental data show that the hydrogenation reactions are relatively facile, although N‐ethylcarbazole hydrogenates 10×'s faster than carbazole on a 5 wt% Ru/Al2O3 catalyst at 150°C. Dehydrogenation of dodecahydro‐N‐ethylcarbazole is more difficult than hydrogenation and is structure sensitive on Pd catalysts. Maximum activity and 100% selectivity to the completely dehydrogenated product, N‐ethylcarbazole, was achieved over a 4 wt% Pd/SiO2 catalyst with dPd ∼ 9 nm. The dehydrogenation TOF of dodecahydrocarbazole and dodecahydrofluorene were much lower than dodecahydro‐N‐ethylcarbazole. DFT was used to identify the dehydrogenation mechanism and explain the experimental observations. Both theoretical and experimental results lead to the conclusion that dodecahydro‐N‐ethylcarbazole is a better H2storage candidate than dodecahydrocarbazole.
Article
A discussion is presented of the theories pertinent to catalysis; the origin of selectivity, selecting catalysts and, hermodynamics.
Article
We investigated the surface reaction of the liquid organic hydrogen carrier dicyclohexylmethane (DCHM) on Pt(111) in ultrahigh vacuum by high-resolution X-ray photoelectron spectroscopy, temperature-programmed desorption, near-edge X-ray absorption fine structure, and infrared reflection-absorption spectroscopy. Additionally, the hydrogen-lean molecule diphenylmethane and the relevant molecular fragments of DCHM, methylcyclohexane, and toluene were studied to elucidate the reaction steps of DCHM. We find dehydrogenation of DCHM in the range of 200-260 K, to form a double-sided π-allylic species coadsorbed with hydrogen. Subsequently, ∼30% of the molecules desorb, and for ∼70%, one of the π-allyls reacts to a phenyl group between 260 and 330 K, accompanied by associative hydrogen desorption. Above 360 K, the second π-allylic species is dehydrogenated to a phenyl ring. This is accompanied by C-H bond scission at the methylene group, which is an unwanted decomposition step in the hydrogen storage cycle, as it alters the original hydrogen carrier DCHM. Above 450 K, we find further decomposition steps which we assign to C-H abstraction at the phenyl rings.
Article
Liquid organic hydrides can act as energy storage from renewable sources in a cycle of hydrogenation and dehydrogenation. In this work, a feasibility study for heat storage by methylcyclohexane dehydrogenation as a system component in a Liquid Organic Reaction Cycle (LORC) is presented. The endothermic dehydrogenation of methylcyclohexane was investigated in a microstructured reactor, in order to supply reaction heat efficiently. The integration of a membrane into the microstructured reactor for in-situ pure hydrogen removal is intended in further studies.
Article
Hydrogenation reaction of N-ethylcarbazole was investigated over ruthenium catalyst supported by alumina powder. The catalytic conversion of 100% and selectivity of 98% towards N-perhydroethylcarbazole were achieved over 1 g catalyst. The effect of temperature, hydrogen pressure, stirring speed and the dosage of catalyst on the hydrogen capacity of N-ethylcarbazole in liquid phase were studied, respectively. The optimum reaction condition was 6.0 MPa, 413 K, 1.0 gRu/γ-Al2O3, 600rpm, and the corresponding hydrogen storage capacity reached 5.6 wt% approximately. The kinetics of hydrogenation was also discussed and the apparent activation energy was 27.01 kJ/mol.
Article
Bismuth Postdosing Thermal Desorption Spectroscopy (BPTDS) and the related D2-BPTDS are mass spectroscopic techniques for quantitative surface analysis based on surface derivatization (with bismuth and deuterium) prior to Temperature Programmed Desorption (TPD). These techniques are briefly described, and the rules for assigning peaks are presented. New applications of these techniques in elucidating the mechanism and kinetics of the decomposition of methylcyclohexane (MCH, c-C6H11-CH3) on Pt(111) are presented. Adsorbed MCH dehydrogenates at 230 K to form a π-allyl c-C6H8-CH3,a species and three hydrogen adatoms. The π-allyl species dehydrogenates at ∼335 K (∼270 K on defects) to an adsorbed benzyl species (c-C6H5-CH2,a) and H2,gas. This benzyl species dehydrogenates further at ∼480K (∼405 K at defects) to form a species of stoichiometry C7H3,a, although at defects it also produces some adsorbed benzene. This benzene dehydrogenates at ∼500 K. The activation energies and prefactors for all of these processes are determined by fitting coverage versus flash temperature curves.
Article
The potential for chemical H2 storage on liquid organic hydrogen carriers (LOHCs) has focused attention on the catalytic reactions needed to store and release H2 from the LOHCs. Herein we review our recent studies on the use of N-ethylcarbazole and carbazole as LOHCs. Experimental data show that the hydrogenation reactions are relatively facile, although N-ethylcarbazole hydrogenates 10×'s faster than carbazole on a 5 wt% Ru/Al2O3 catalyst at 150°C. Dehydrogenation of dodecahydro-N-ethylcarbazole is more difficult than hydrogenation and is structure sensitive on Pd catalysts. Maximum activity and 100% selectivity to the completely dehydrogenated product, N-ethylcarbazole, was achieved over a 4 wt% Pd/SiO2 catalyst with dPd ∼ 9 nm. The dehydrogenation TOF of dodecahydrocarbazole and dodecahydrofluorene were much lower than dodecahydro-N-ethylcarbazole. DFT was used to identify the dehydrogenation mechanism and explain the experimental observations. Both theoretical and experimental results lead to the conclusion that dodecahydro-N-ethylcarbazole is a better H2storage candidate than dodecahydrocarbazole.
Article
We review recent results towards a molecular understanding of the adsorption and dehydrogenation of carbazole-derived liquid organic hydrogen carriers on platinum and palladium single crystals and on Al2O3-supported Pt and Pd nanoparticles. By combining synchrotron-based high-resolution X-ray photoelectron spectroscopy, infrared reflection-absorption spectroscopy, advanced molecular beam methods and temperature-programmed desorption spectroscopy, detailed insights into the reaction mechanism are obtained. On Pt(111), dehydrogenation of perhydro-N-ethylcarbazole, H12-NEC, starts with activation of the hydrogen atoms at the pyrrole unit, yielding H8-NEC as the first stable reaction intermediate at ∼340 K, followed by further dehydrogenation to NEC at ∼380 K. Above 390 K, dealkylation starts, yielding carbazole as an undesired byproduct. On small supported Pt particles, the dealkylation sets in at lower temperatures, due to the higher reactivity of low-coordinated sites, while on larger particles with (111) facets a reactivity as on the flat surface is observed. Carbazole derivatives with ethyl, propyl and butyl chains show an overall very similar reactivity, both on Pt(111) and on Pt nanoparticles. When comparing the dealkylation behavior of H12-NEC on Pt(111) and Pt nanoparticles to that on Pd(111) and Pd nanoparticles, we find a higher reactivity for the Pd systems.
Article
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.