Analysis of H2 Release from Organic Polycyclics over Pd Catalysts

Article pubs.acs.org/JPCC

Analysis of H2 Release from Organic Polycyclics over Pd Catalysts Using DFT Farnaz Sotoodeh and Kevin J. Smith* Department of Chemical & Biological Engineering, University of British Columbia, 2360 East Mall, Vancouver, British Columbia, Canada V6T 1Z3 S Supporting Information *

ABSTRACT: Density functional theory (DFT) was used to study H2 release from dodecahydro-N-ethylcarbazole over Pd. Dehydrogenation of the molecule starts from the five-membered ring by weakening the axial C− H bonds adjacent to the N atom. In the adsorption mode identified for dodecahydro-N-ethylcarbazole, the five-membered ring was located on a hollow site and the axial H atoms adjacent to N were positioned atop Pd atoms with an adsorption energy of −95 kJ/mol. The two axial C−H bonds of the five-membered ring showed a large increase in bond distance upon adsorption, from 1.10 to 1.16 Å, that was responsible for the weakening of these bonds. The adsorption mode for dodecahydro-N-ethylcarbazole explained the product distribution obtained from the dehydrogenation reaction with octahydro-N-ethylcarbazole and tetrahydro-N-ethylcarbazole identified as primary and secondary products, respectively. DFT was also used to investigate the structure sensitivity of dodecahydro-N-ethylcarbazole dehydrogenation observed on Pd catalysts. The effect of the ethyl group and N atom on the rate of dodecahydro-N-ethylcarbazole dehydrogenation was also investigated by comparing the adsorption energies of dodecahydro-N-ethylcarbazole with two other H2 storage candidates, dodecahydrocarbazole and dodecahydrofluorene. The latter compounds had much slower H2 recovery rates, a consequence of the strong adsorption of these compounds relative to dodecahydro-N-ethylcarbazole. geometry,13−18 with multiple Pd atoms involved in the adsorption. A 4-fold hollow flat adsorption mode for benzene was also reported on Cu(100) in work by Lesnard et al.19 Recent experimental work by the present authors20,21 showed that dehydrogenation of dodecahydro-N-ethylcarbazole was structure sensitive over Pd. A series of SiO2-supported Pd catalysts with particle sizes varying from ∼3 to 15 nm were examined, and the activity was found to be optimal when a catalyst with a particle diameter of ∼9 nm was used. A sequential reaction pathway (Figure 1) was suggested based on the experimental product distribution. In the present work, results from density function theory (DFT) calculations of the dehydrogenation of dodecahydro-Nethylcarbazole over Pd are reported. Results are compared with

1. INTRODUCTION Hydrogen is an alternative energy carrier that is nontoxic and environmentally benign. However, despite several benefits, technology challenges associated with hydrogen storage that result from its low density have limited the widespread incorporation of hydrogen into the transportation sector. Onboard vehicular hydrogen storage, including compressed or cryogenic liquid hydrogen, requires high energy inputs and provides low gravimetric storage capacity. On the other hand, off-board regenerable materials have the potential to meet the storage capacity demands of automotive applications. Organic heteroaromatic compounds, such as carbazole and N-ethylcarbazole, have gravimetric storage capacities of more than 5.5 wt %,1−3 and their hydrogenated forms can release hydrogen at temperatures below 473 K.3−7 Hence, these liquids are potential candidates for hydrogen storage. However, in previous studies of the dehydrogenation of dodecahydro-Nethylcarbazole4−6 and similar compounds,7,8 the reaction rates were found to be slow and in most cases no selectivity to the completely dehydrogenated product was observed. Several literature studies have reported on the structure sensitivity of hydrogenation/dehydrogenation reactions of cyclo-organics.7,9−12 Experimental and theoretical studies of benzene adsorption on Pd(111) revealed that, depending on the surface coverage, benzene is adsorbed parallel to the surface (through interaction of π electrons) or in a tilted © 2012 American Chemical Society

Figure 1. Dodecahydro-N-ethylcarbazole dehydrogenation reaction network: (a) dodecahydro-N-ethylcarbazole, (b) octahydro-N-ethylcarbazole, (c) tetrahydro-N-ethylcarbazole, and (d) N-ethylcarbazole. Received: July 24, 2012 Revised: November 17, 2012 Published: November 27, 2012 194

dx.doi.org/10.1021/jp307325s | J. Phys. Chem. C 2013, 117, 194−204

The Journal of Physical Chemistry C

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metals, however, have been limited by the availability of computational resources required to create large surfaces with adequate surface sites. Furthermore, molecules of relatively large size such as dodecahydro-N-ethylcarbazole increase the number of possible adsorption modes and consequently computational costs. In this work, a thermodynamically stable29,30 Pd(111) surface, with closed-pack structure, was chosen to study the adsorption and dehydrogenation mechanism of dodecahydro-N-ethylcarbazole. Eight adsorption modes were considered for dodecahydro-N-ethylcarbazole (isomer 1) with respect to each axial C−H bond (normal to the xy plane) positioned above a Pd atom (referred to as atop adsorption mode). The hollow adsorption mode with respect to the five-membered ring and the atop mode with respect to the two axial C−H bonds, as shown in Figure 2a and 2b, was

experimental data to provide a better understanding of the reaction mechanism and the required surfaces that enhance the reaction rate.

2. COMPUTATIONAL METHODS DFT calculations were performed to determine the adsorption energies and energy barriers of different hydrogen removal steps using DMol322,23 implemented in Material Studio 4.4.0.0. (Accelrys Inc.). The geometry of the reactant, products, and surface was optimized using the double-numeric quality basis set (DNP) with the PBE (Perdew, Burke, and Ernzerhof) gradient-corrected functional for description of the exchange and correlation effects.24,25 A Fermi smearing of 2 × 10−3 Ha (1 Ha = 27.21 eV), a real-space cutoff of 4.0 Å, and a MEDIUMquality mesh size were used for numerical integration. Geometries of different stereoisomers of the reactant molecules were optimized in order to identify the most probable isomers. Adsorption modes of the reactants and intermediates on a Pd(111) surface were determined using a 4 × 4 unit cell consisting of four Pd layers (27 Pd atoms per layer and 108 atoms in total) and a 30 Å vacuum gap. The top layer of the slab was allowed to relax. Density functional semicore pseudopotentials (DSPP) were used to describe the core− electron treatment.26 The tolerances of energy, gradient, and displacement convergence were 2 × 10−5 Ha, 4 × 10−3 Ha/Å, and 5 × 10−3 Å, respectively. Adsorption energies were determined as E a d so r p t i o n = E a d so r b e d mo l e c u l e / s u r f a c e − Emolecule in vacuum − Esurface, in which Eadsorbed molecule/surface is the energy of the adsorption system, Emolecule in vacuum the energy of the adsorbed molecule in vacuum, and Esurface the energy of the surface. Further details as well as the chemical structure of dodecahydro-N-ethylcarbazole with the labels assigned to each H and C atom are provided in Figure S-1, Supporting Information.

Figure 2. Optimized geometry of dodecahydro-N-ethylcarbazole isomer 1 adsorption on Pd(111) surface: (a) front view, (b) side view with N−Pd distance of 3.84 Å and the five-membered ring at an angle of 10° to the surface, (c) top view of the hollow/atop adsorption mode (hollow with respect to the five-membered ring; atop with respect to the axial C−H bonds). Dots indicate the position of the two stretched axial C−H bonds.

3. RESULTS AND DISCUSSION 3.1. Dodecahydro-N-ethylcarbazole Adsorption on Pd(111) Surface. Molecular Structure. The molecular structure of different stereoisomers of dodecahydro-N-ethylcarbazole was optimized, and the obtained energies suggested three stable isomers for dodecahydro-N-ethylcarbazole (Table S-1, Supporting Information). For isomer 1 all four H atoms in the five-membered ring face down in the z direction. With this configuration, the ethyl group and the two six-membered rings tilt up from the xy plane. In isomer 2 H4aa and in isomer 3 H4aa and H9a face up in the z direction (refer to Figure S-1, Supporting Information, for atom labeling). The more negative energies of isomers 1 and 2 (with an energy difference of ∼3 kJ/mol between 1 and 2) implies higher stability of these two isomers in vacuum compared to isomer 3. Production of three isomers of dodecahydro-N-ethylcarbazole was confirmed experimentally by GC/MS analysis of liquid samples obtained from hydrogenation of N-ethylcarbazole,5 with one dominant peak and two small ones. Eblagon et al.27 also reported three peaks for dodecahydro-N-ethylcarbazole in the hydrogenation reaction of N-ethylcarbazole with the chemical structure of the compounds identified using 1D and 2D nuclear magnetic resonance (NMR). Adsorption on Pd(111). Benzene adsorption has been studied extensively as a model system for adsorption of aromatics on transition metal surfaces.28,29 Ab initio studies of the adsorption of other aromatic molecules on transition

identified as the most stable adsorption mode. Among the four C−H bonds in the five-membered ring, atop adsorptions of dodecahydro-N-ethylcarbazole through the H9a and H8aa atoms were found to be favored and had very similar adsorption energies of −95.0 and −94.6 kJ/mol, respectively. The fivemembered ring was adsorbed on the hollow site of the Pd(111) surface, and the two axial C−H bonds next to the N heteroatom were at atop position (Figure 2c). Detailed structural properties of the adsorbed dodecahydro-N-ethylcarbazole molecule are summarized in Table 1. The C−H bonds were weakened and characterized by a significant increase in the axial C9a−H9a and C8a−H8aa bond distance from 1.11 to about 1.16 Å, as shown in Table 1. The axial C−H bond length in cyclohexane increased from 1.11 to about 1.13 Å upon adsorption on a Pt(111) surface,28 which likely explains the higher adsorption energy of −95 kJ/mol for dodecahydroN-ethylcarbazole on Pd compared to −37 kJ/mol for cyclohexane on Pt. The H−Pd bond distances of 2.05 and 1.99 Å shown in Figure 2 are comparable to the H−Pt bond length of 1.6 Å for atop adsorbed hydrogen reported by Saeys et al.31 Considering the larger molecular size of dodecahydroN-ethylcarbazole relative to cyclohexane, a higher adsorption energy for the former is expected. A comparative first-principle study by Morin et al.32 also reported the adsorption energy 195



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Table 1. Structural Parameters for Dodecahydro-N-ethylcarbazole Adsorption on Pd(111) C−H bond (Å) atop H before optimizationa

gas phase

adsorbed

adsorbed

H9a

−95.0

H8aa

−94.6

2

H9a

−93.0

3

H3a

−42.4

C9a−H9a: 1.11 C8a−H8aa: 1.11 C9a−H9a: 1.11 C8a−H8aa: 1.11 C9a−H9a: 1.12 C8a−H8aa: 1.11 C3−H3a: 1.10 C6−H6a: 1.10 C8a−H8aa: 1.11 C4a−H4aa: 1.11

1.16 1.15 1.15 1.16 1.15 1.15 1.11 1.12 1.20 1.12

1.99 2.05 2.08 2.01 2.22 2.16 2.35 2.19 1.85 2.31

1

H8aab a

H−Pd (Å)

Eads (kJ/mol)

isomer no.

Indicated H atom was initially positioned atop a Pd surface atom. bComparison to the work by Eblagon et al.27

involved in dehydrogenation of dodecahydro-N-ethylcarbazole is isomer 1. Hirshfeld charges were calculated and compared for dodecahydro-N-ethylcarbazole in the gas phase as well as the adsorbed molecule on Pd (111) as summarized in Table S-2, Supporting Information. The electron densities at the two axial hydrogen atoms closest to the surface increase quite significantly upon adsorption. The negative charge increase for H9a and H8aa is about 0.06 au, comparable to the 0.04 au increase reported for the axial hydrogen atoms of cyclohexane upon adsorption on Pt(111).28 The corresponding Pd atoms, Pd9a and Pd8aa in Table S-2, Supporting Information, showed a negative charge decrease of the same amount, 0.06 au, indicative of a charge transfer from the surface to the H atoms. A charge increase of ∼0.03 au was calculated for the other two H atoms in the five-membered ring, H4aa and H4bb, upon adsorption, with a charge decrease of ∼0.05 au for the corresponding two closest Pd atoms, Pd4aa and Pd4bb. The negative charge on the N heteroatom in the molecule however decreased by ∼0.06 au, indicative of interaction of the N heteroatom to the closest Pd atom on the surface. The total negative charge on dodecahydro-N-ethylcarbazole increased by 0.22 au upon adsorption, representing a net charge density transfer from the surface to the adsorbed molecule with the charge mostly located at the H9a and H8aa atoms. The density of states (DOS) of the adsorbed dodecahydroN-ethylcarbazole was compared to that in vacuum (Figure S-2, Supporting Information). Comparison of Figure S-2(a), Supporting Information, with Figure S-2(b), Supporting Information, indicates stabilization of energies for dodecahydro-N-ethylcarbazole upon adsorption on Pd(111) through H9a or H8aa. The peaks corresponding to the energy value between 0.2 and 0.3 Ha for dodecahydro-N-ethylcarbazole in vacuum, shown in Figure S-2(a), Supporting Information, are broadened and shifted to negative values. Electronic states belonging to the adsorbed configuration of dodecahydro-N-ethylcarbazole in Figure S-2(b), Supporting Information, show contributions from s and p orbitals. The states above −0.35 Ha mainly have p characteristics, while the features below −0.45 Ha mainly come from s orbitals. From Figure S-2(a) and S-2(b), Supporting Information, it is clear that the density of states, especially those from s and p orbitals near and above the Fermi level, are modified due to the interaction between the molecule and the surface. Figure S-2(c), Supporting Information, compares the DOS of the adsorbed system with those of the Pd d states (Figure S-2(d), Supporting Information) upon adsorption of dodecahydro-N-ethylcarbazole. The d states of the Pd clean

trend for benzene, naphthalene, and anthracene over Pt(111) surface, with the adsorption energy increasing with molecular size from ∼−87 kJ/mol for benzene to −132 and −173 kJ/mol for naphthalene and anthracene, respectively. The stretch in the two axial C9a−H9a and C8a−H8aa bonds makes the molecule tilt from the surface as seen in Figure 2b, with the plane of the five-membered ring tilted about 10° away from the Pd(111) surface. Adsorption through H7a or H2b, with the H atom positioned above a Pd atom, resulted in similar adsorption geometry through H9a and H8aa after optimization (Figure 2c), with adsorption energies of −94.2 and −93.1 kJ/ mol, respectively. Geometry optimization of both systems led to the H7a and H2b atoms tilting away from the surface with a small C−H bond length increase from 1.10 to 1.11 Å and the H9a and H8aa atoms relocating to the atop position with a corresponding C−H bond length increase from 1.11 to ∼1.16 Å. This confirmed the stability of the adsorption mode through the axial H9a and H8aa and points to the structure-dependent adsorption of dodecahydro-N-ethylcarbazole on the Pd surface, where the molecule relocates so that it finds adequate and energetically proper adsorption sites. Positioning H4aa or H4bb on top of a Pd atom however resulted in weaker adsorption with similar adsorption energies of −88.8 and −86.7 kJ/mol, respectively. The obtained energies were found to be smaller compared to the optimum atop adsorption through H9a or H8aa. A small increase in the C4b− H4bb bond length from 1.10 to 1.11 Å was observed, with no change in the C4a−H4aa bond distance. The very small increase in the C−H bond length in this case implies that adsorption through H4aa or H4bb is not likely to occur. Adsorption through H3a or H6a resulted in the least favored adsorption mode among all those examined, with adsorption energies of −44.7 and −36.8 kJ/mol, respectively. In this case also, only a 0.01 Å bond stretch for C3−H3a and 0.02 Å for C6−H6a was calculated. An adsorption energy of −93.0 kJ/mol and similar adsorption geometry was obtained for dodecahydro-N-ethylcarbazole isomer 2 compared to that of isomer 1 adsorbed through H9a. The C9a−H9a and C8a−H8aa bond distances increased from their corresponding values in the gas phase to 1.15 Å, confirming the five-membered ring to be the favored site for the first C−H bond weakening of the molecule. Isomer 3 adsorbed weakly to the surface with an adsorption energy of −42.4 kJ/mol, and C3−H3a and C6−H6a bond distances changed by 0.01 and 0.02 Å, respectively. Comparing the adsorption energies of the three isomers of dodecahydro-Nethylcarbazole, it is very likely that the dominant isomer 196



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surface appear at lower energy in the DOS spectrum with dodecahydro-N-ethylcarbazole adsorbed on the surface and indicate a charge transfer from occupied d orbitals of the Pd to the orbitals of the adsorbed molecule. 3.2. Dodecahydro-N-ethylcarbazole Dehydrogenation. Production of Decahydro-N-ethylcarbazole and Octahydro-N-ethylcarbazole. For the dodecahydro-N-ethylcarbazole adsorption mode shown in Figure 2, H9a and H8aa are in close proximity to the Pd surface at a distance of 1.99 and 2.05 Å from the nearest Pd atoms, respectively, and they can therefore react with the surface. As discussed earlier, from the C−H bond changes shown in Table 1, it is clear that a dehydrogenation pathway involving initial cleavage of either H9a or H8aa is most likely. Cleavage of the C−H bond at carbon positions 9a and 8a as the first reaction step was also confirmed through IR spectra reported by Sobota et al.33 for dehydrogenation of dodecahydro-N-ethylcarbazole. In order to gain insight into the relative reactivity of the four sequential hydrogen removal steps, the two reaction pathways proposed by Crawford et al.7 were considered for dehydrogenation of dodecahydro-N-ethylcarbazole to decahydro-N-ethylcarbazole and octahydro-N-ethylcarbazole over Pd(111), as shown in Figure 3. The molecular structure of different isomers of decahydro-N-ethylcarbazole and octahydro-N-ethylcarbazole were optimized (shown in Tables S-3 and S-4, Supporting Information, respectively). Decahydro-N-ethylcarbazole isomer 1, wherein one of the bonds in the five-membered ring is dehydrogenated, was the most stable one. In contrast to the hydrogenation reaction of N-ethylcarbazole, where decahydroN-ethylcarbazole was reported as one of the intermediates produced in small quantities,5 this compound was not observed during the dehydrogenation reaction.4,6 The most stable isomer of octahydro-N-ethylcarbazole was found to have two unsaturated bonds in the five-membered ring (Table S-4, Supporting Information). Unlike dodecahydro-Nethylcarbazole for which three different isomers were identified, only one peak for octahydro-N-ethylcarbazole was identified in the GC/MS analysis of the liquid samples obtained from dehydrogenation of dodecahydro-N-ethylcarbazole,20 implying that only one of the isomers is produced. The same structure for octahydro-N-ethylcarbazole was reported in the theoretical work of Eblagon et al.27 for hydrogenation of N-ethylcarbazole. Both pathways shown in Figure 3 start with dissociation of C9a−H9a (step 1) to form intermediate I, as C9a−H9a showed the largest bond stretch upon adsorption of dodecahydro-Nethylcarbazole (see Table 1 for structural properties). Dissociation of C4a−H4aa in pathway 1 and C8a−H8aa in pathway 2 led to production of two unique intermediates denoted as intermediate II, resulting in formation of a partial double bond between C9a and C4a in pathway 1 and a partial double bond between carbons in positions 9a and 8a and the N in pathway 2 (step 2). The produced intermediate II in pathway 1 corresponds to the stable structure of decahydro-Nethylcarbazole shown in Table S-3, Supporting Information. Formation of bonds between carbons in the five-membered ring and the N with partial double-bond nature was also identified by high-resolution electron energy-loss spectroscopy (HREELS) for pyrole adsorption on Pd(111)34 as well as dodecahydro-N-ethylcarbazole adsorption on Pd/Al2O3/NiAl(110).33 Intermediate III for each pathway is produced by the third dehydrogenation step (step 3), and both species subsequently lose a further H atom to produce octahydro-Nethylcarbazole (step 4). Octahydro-N-ethylcarbazole chemical

Figure 3. Two possible dehydrogenation pathways for production of octahydro-N-ethylcarbazole. Dehydrogenation begins with dissociation of H9a in both pathways (adapted from the work reported by Crawford et al.7).

structure resulting from the indicated reaction pathways shown in Figure 3 is consistent with the stable structure obtained for octahydro-N-ethylcarbazole reported in Table S-4, Supporting Information. Different H removal steps corresponding to pathway 1 are illustrated in Figure 4. The reaction energy barriers of each dehydrogenation step of the two pathways are compared in Figure 5. The calculated reaction barrier for dissociation of the C9a−H9a bond was 14.1 kJ/mol. The small energy barrier found for the first H removal step is consistent with the HR-XPS work of Sobota et al.,33 where facile initial dehydrogenation steps and more competitive later H removals were reported for dodecahydro-N-ethylcarbazole. For both pathways, the reactivity trend showed an increase in the energy barrier for dehydrogenation step 1 to 2. The energy barrier decreased in both pathways from dehydrogenation step 2 to 3. However, for dehydrogenation step 4, the reaction barrier along pathway 1 decreased significantly, whereas a large increase in the reaction barrier from step 3 to 4 was obtained in pathway 2. From the obtained reaction barriers shown in Figure 5, it follows that dehydrogenation pathway 1 of Figure 3 is more favorable. For the second dehydrogenation step, a larger bond distance increase was observed for C4a−H4aa compared to C8a− H8aa (Figure 4) and the activation barrier for H4aa removal in pathway 1 was ∼33 kJ/mol less than that of H8aa in pathway 2. The second H removal step in both pathways had the highest energy barrier. This step results in production of decahydro-Nethylcarbazole in pathway 1 after which a significant stretch of 0.05 Å in the C8a−H8aa bond length (Figure 4c) was observed and the H removal barriers decreased significantly for steps 3 and 4. This implies that decahydro-N-ethylcarbazole is produced but is also consumed very quickly and explains why no decahydro-N-ethylcarbazole was measured in the experimental work on dehydrogenation of dodecahydro-N-ethylcarbazole.20 Also, the cleavage barrier for C4b−H4bb leading to formation of a double bond between C8a and C4b and production of octahydro-N-ethylcarbazole in pathway 1 was 88 kJ/mol less than the corresponding value found for pathway 2. This is consistent with experimental observations for the 197



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Figure 4. Schematic of pathway 1 for dehydrogenation of dodecahydro-N-ethylcarbazole over Pd(111) surface: (a) adsorption geometry of dodecahydro-N-ethylcarbazole showing the interaction of H9a and H8aa with the nearest Pd atoms at 1.99 and 2.05 Å, respectively; (b) dissociation of the C9a−H9a bond, bond distance of C4a−H4aa and C8a−H8aa increased to 1.13 and 1.11 Å, respectively; (c) removal of H4aa with the C8a−H8aa bond distance increased to 1.16 Å; (d) removal of H8aa with the C4b−H4bb bond distance increased to 1.18 Å; (e) octahydro-N-ethylcarbazole production, N−Pd = 2.89 Å and five-membered ring plane at an angle of 5° to the surface; (f) top view of octahydro-N-ethylcarbazole hollow di-σ adsorption structure.

Figure 6. Product distribution for dodecahydro-N-ethylcarbazole dehydrogenation at 443 K and 101 kPa over a 4 wt % Pd/SiO2; left y axis: (■) dodecahydro-N-ethylcarbazole, (▲) octahydro-N-ethylcarbazole, (●) tetrahydro-N-ethylcarbazole, (◊) N-ethylcarbazole; right y axis shows the evolved H2 profile (□). Details of the experimental procedure are reported elsewhere.20

Figure 5. Comparison of the reaction barriers for the pathways shown in Figure 3 (adapted from the work by Crawford et al.7).

Table 2. Gas-Phase (vacuum) and Adsorbed Octahydro-Nethylcarbazole Bond Distances

product distributions shown in Figure 6, where octahydro-Nethylcarbazole was found to be a primary product in the dehydrogenation of dodecahydro-N-ethylcarbazole. Production of Hexahydro-N-ethylcarbazole and Tetrahydro-N-ethylcarbazole. As shown earlier, octahydro-N-ethylcarbazole adsorbs to the surface in a di-σ mode through C9a and C4a atoms in the five-membered ring with σC−Pd bond distances of 2.25 and 2.29 Å, respectively (Figure 4e and 4f). An adsorption energy of −56.0 kJ/mol was obtained for octahydroN-ethylcarbazole. C−Pd bond distances are comparable to the σC−Pt distances of 2.19 and 2.16 Å obtained for di-σ adsorption of 1,3-cyclohexadiene on Pt(111)28 and σC−Pt distances of 2.16 Å for adsorbed benzene.29 The changes in the structural properties of octahydro-N-ethylcarbazole before and after adsorption are summarized in Table 2. As seen in Table 2, the bond distance between C9a and C4a increased from 1.39 Å in the gas phase to 1.49 Å upon adsorption on the Pd(111) surface. This increase in the C9aC4a bond distance supports the di-σ adsorption of the molecule through C9a and C4a. Saeys et al.28 reported a bond distance of 1.51 Å for the CC bond

bond C9a−Pda C4a−Pda C9a−C4a a

gas phase (Å)

adsorbed (Å)

1.39

2.25 2.29 1.49

Corresponding Pd atom shown in Figure 4e.

in the di-σ adsorption of 1,3-cyclohexadiene on Pt(111). In the optimized conformation shown in Figure 4, two aliphatic rings as well as two C atoms in the five-membered ring tilt away from the Pd(111) surface. Figure S-3, Supporting Information, compares the density of states for C9a and C4a in vacuum and upon adsorption to the surface obtained from periodic calculations. A significant shift and broadening of the energy levels of carbon p orbitals are observed for both C9a and C4a, supporting the strong interaction of carbon atoms with the Pd surface atoms. 198



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Figure 7. H removal steps from octahydro-N-ethylcarbazole on Pd(111): (a) adsorbed octahydro-N-ethylcarbazole showing the interaction between H1b and the nearest Pd atom at 2.53 Å, (b) removal of H1b and increased interaction between H2b and the nearest Pd atom at 2.24 Å, (c) increased interaction between H4a with the nearest Pd atom at 2.16 Å, (d) dissociation of H4a and increased interaction between H3a and surface at 2.5 Å, (e) production of tetrahydro-N-ethylcarbazole with a N−Pd distance of 3.28 Å and the five-membered ring plane at an angle of 25° to the surface, (f) aromatic bridge 1,2-di-σ/3,4-π adsorption of tetrahydro-N-ethylcarbazole.

membered rings is completely dehydrogenated. The higher stability of this isomer is expected as partial dehydrogenation of one of the six-membered rings results in loss of resonance of the sextet and hence increasing instability of the molecule.35,36 However, the suggested dehydrogenation pathway for octahydro-N-ethylcarbazole based on the reaction energy barriers reported in Table 3 does not lead to formation of hexahydro-Nethylcarbazole isomer 3. This explains the product distribution shown in Figure 6 where no hexahydro-N-ethylcarbazole was detected, likely because isomer 1 undergoes fast cleavage of C4a−H4a and C3−H3a producing tetrahydro-N-ethylcarbazole (Figure 7d and 7e). Table S-6, Supporting Information, summarizes the energies and structures of different isomers of tetrahydro-N-ethylcarbazole. An isomer with an unsaturated five-membered ring and one of the six-membered rings (shown as isomer 2 in Table S-6, Supporting Information) was found to be the most stable structure in vacuum. Similar structures for 1,2,3,4-tetrahydrocarbazole adsorption on Pd(111)7 and for tetrahydro-Nethylcarbazole obtained from hydrogenation of N-ethylcarbazole27 were suggested in previous studies. Tetrahydro-Nethylcarbazole adsorbs to the surface through C1−4, and the five-membered ring tilts away from the surface at an angle of 25° (Figure 7e) with a N−Pd distance equal to 3.28 Å. The adsorption energy of tetrahydro-N-ethylcarbazole on the Pd(111) surface was calculated as −117.5 kJ/mol, significantly smaller than the −143.8 kJ/mol value reported for adsorption of tetrahydrocarbazole on Pd(111).7 This is likely due to the effect of the ethyl group which decreases the interaction of the N heteroatom with the nearby Pd atoms and moves the molecule away from the surface. C1−Pd and C2−Pd distances are 2.24 and 2.25 Å, respectively, as shown in Table 4. These values are comparable to the σC−Pt distance of 2.16 Å for

Figure 7a illustrates the optimized adsorbed geometry of octahydro-N-ethylcarbazole with the interaction of H1b with the surface. Both C1−H1b and C2−H2b bond distances increased to 1.11 Å upon adsorption and therefore are likely to be the first bonds to dissociate. However, the energy barrier for C1−H1b cleavage was found to be 137.7 kJ/mol (Table 3), while this Table 3. Reaction Barriers Corresponding to the Transition States (TSs) of Figure 7 for Stepwise H Removal from Octahydro-N-ethylcarbazole transition state (TS)

removed H

barrier (kJ/mol)

TS1 TS2 TS3 TS4

H1b H2b H4a H3a

137.7 81.2 83.9 198.9

value was ∼259.7 kJ/mol for cleavage of C2−H2b, indicating that H removal starting from C1−H1b bond cleavage is more favorable. Upon cleavage of the C1−H1b bond (Figure 7b), C9a and C4a detach from the surface and C1 makes a σC−Pd bond with the nearest Pd atom. The second dehydrogenation step resulted in removal of H2b (Figure 7c) and formation of a σ bond between C2 and the nearest Pd on the surface with an energy barrier of 81.2 kJ/mol. Removal of H2b results in formation of one of the isomers of hexahydro-N-ethylcarbazole (isomer 1 in Table S-5, Supporting Information). Hexahydro-N-ethylcarbazole was detected as one of the intermediates of the hydrogenation reaction of Nethylcarbazole produced in small quantities,5 whereas it was not observed in the dehydrogenation reaction of dodecahydro-Nethylcarbazole as shown in Figure 6.20 The most stable isomer of hexahydro-N-ethylcarbazole was found to be isomer 3 (Table S-5, Supporting Information) wherein one of the six199



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ethylcarbazole (137.7 kJ/mol, see Table 3) produced from octahydro-N-ethylcarbazole indicates that H removal from the five-membered ring is favored over the six-membered ring. This is in good agreement with the obtained experimental apparent activation energy of 144.3 kJ/mol for production of tetrahydroN-ethylcarbazole compared to 67.1 kJ/mol obtained for production of octahydro-N-ethylcarbazole.4 It was pointed out by Cui et al.37 that incorporation of a N heteroatom in the five-membered ring of 1-methyloctahydroindole decreased the energy barrier for dehydrogenation of that ring but increased the dehydrogenation barrier of the sextet, resulting in the dehydrogenation reaction starting from the five-membered ring. This clearly supports the suggested reaction pathway in the present study for H removal from the five-membered ring of dodecahydro-N-ethylcarbazole to produce octahydro-N-ethylcarbazole followed by H removal from the six-membered ring and production of tetrahydro-N-ethylcarbazole, and these results are in agreement with the experimental product distribution reported previously.20 It is interesting to note that the product distribution observed for dehydrogenation of dodecahydro-N-ethylcarbazole was different from the reverse reaction, hydrogenation of N-ethylcarbazole. No hexahydro-N-ethylcarbazole was detected in the dehydrogenation reaction, whereas this compound was observed as one of the hydrogenation intermediates. The difference in the observed product distribution points to the fact that depending on the geometry of the reactant and

Table 4. Gas-Phase (vacuum) and Adsorbed Tetrahydro-Nethylcarbazole Bond Distances bond C1−Pda C2−Pda C3−Pda C4−Pda C1−C2 C3−C4 a

gas phase (Å)

adsorbed (Å)

1.39 1.39

2.24 2.25 2.28 2.29 1.46 1.43

Corresponding Pd atom shown in Figure 7.

adsorbed benzene.28 Longer bond distances of 2.28 and 2.29 Å were obtained for C3−Pd and C4−Pd, respectively, indicative of π bonds formed through C3 and C4 with the surface. This is supported by the larger increase of 0.07 Å in the C1−C2 bond distance compared to 0.04 Å for the weak C3−C4 π bond (Table 4). The suggested octahydro-N-ethylcarbazole dehydrogenation pathway resulted in production of the stable isomer tetrahydro-N-ethylcarbazole. Experimental observation confirmed the proposed dehydrogenation pathway in which tetrahydro-N-ethylcarbazole was observed as a secondary intermediate in the dehydrogenation reaction of dodecahydro-N-ethylcarbazole (Figure 6). Comparing the first step reaction barrier for production of octahydro-N-ethylcarbazole from dodecahydro-N-ethylcarbazole (14.1 kJ/mol in Figure 5) with that of tetrahydro-N-

Table 5. Dodecahydro-N-ethylcarbazole Adsorption on Pd(100) and Pd(110) Surfaces

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Figure 8. Optimized geometry of the adsorbed (a) dodecahydro-N-ethylcarbazole, (b) dodecahydrocarbazole, (c) dodecahydrofluorene, (d) tetrahydro-N-ethylcarbazole, (e) tetrahydrocarbazole, and (f) tetrahydrofluorene over Pd(111).

also point to the fact that the surface structure plays a role in the reaction of the molecule, as observed experimentally.20 Surface statistics, calculated using the methods proposed by Van Hardeveld and Hartog,38 were used to estimate the particle diameter for a Pd fcc cuboctahedron. Results are summarized in Table S-7, Supporting Information. As shown previously in Figure 2c, dodecahydro-N-ethylcarbazole needs at least 5 edge atoms to adsorb on the Pd(111) surface. According to Table S7, Supporting Information, for a surface with 5 edge atoms (i.e., m = 5), the corresponding particle size will be ∼3.3 nm. These results are consistent with the experimental observation published by the authors20 where a 0.5 wt % Pd/SiO2 with a particle diameter of ∼3.6 ± 0.1 nm was found to be active for dehydrogenation of dodecahydro-N-ethylcarbazole. The activity, however, increased to an optimum value by increasing the particle size to ∼7.3 ± 0.4 nm on a 4 wt % Pd/SiO2 catalyst. Comparing the experimental results with the surface statistics (Table S-7), the optimum catalyst Pd particle size corresponds to 10 edge atoms (m = 10). The larger size observed experimentally for maximum TOF compared to the theoretical calculation likely reflects the heterogeneous nature of the real catalyst Pd particles. 3.4. Effect of the Heteroatom on Reactivity. The effect of the N heteroatom on the dehydrogenation reaction was studied by comparing the adsorption energies and surface/ adsorbate charge transfer of dodecahydro-N-ethylcarbazole, dodecahydrocarbazole, and dodecahydrofluorene. Adsorption of dodecahydro-N-ethylcarbazole to Pd(111) was characterized by a significant stretch of two of the C−H bonds in the fivemembered ring, resulting in a 10° angle between the fivemembered ring plane and the surface (Figure 8a). Dodecahydrocarbazole and dodecahydrofluorene, however, showed a stronger adsorption with adsorption energies of −109.4 kJ/mo and −180.8 kJ/mol compared to −95.0 kJ/mol for dodecahydro-N-ethylcarbazole,21 which as a result caused a larger angle of 19° and 22° between the five-membered ring plane and the surface, respectively (Figure 8b and 8c). The shorter N−Pd distance obtained for dodecahydrocarbazole compared to dodecahydro-N-ethylcarbazole is indicative of a stronger interaction of the docecahydrocarbazole N heteroatom with the Pd(111) surface, whereas for dodecahydro-N-ethylcarbazole the ethyl group prevents a strong interaction between N and Pd. Adsorption of dodecahydro-

intermediates especially for larger molecules and their adsorption orientation on the surface, different reaction pathways may be favored. In dehydrogenation of dodecahydro-N-ethylcarbazole, since octahydro-N-ethylcarbazole is the first intermediate, dehydrogenation of this compound results in formation of isomer 1 of hexahydro-N-ethylcarbazole which has a lower stability than isomer 3. Also, the second and third H removal steps from octahydro-N-ethylcarbazole have a much lower energy barrier compared to the first H removal and therefore are favored to produce the more stable tetrahydro-Nethylcarbazole. In the hydrogenation reaction of N-ethylcarbazole, however, strong adsorption of the produced hexahydro-N-ethylcarbazole isomer 1 to the surface is likely the reason for observing this compound and its very slow conversion to octahydro-N-ethylcarbazole.4,5 3.3. Adsorption on Pd(110) and Pd(100). The adsorption geometry of dodecahydro-N-ethylcarbazole on Pd(111) has also been compared to that on Pd(110) and Pd(100). Table 5 shows that Pd(100) and Pd(110) have a less packed structure compared to Pd(111). Adsorption energies of dodecahydro-N-ethylcarbazole on Pd(100) and Pd(110) were calculated as −93.0 and −83.2 kJ/mol, respectively, smaller than the −95.0 kJ/mol obtained on Pd(111). Both surfaces showed similar adsorption modes for dodecahydro-N-ethylcarbazole with H9a and H8aa located at atop positions and a corresponding increase in the C−H bond length upon adsorption. The low adsorption energy obtained on Pd(110) compared to the other two surfaces is a consequence of the less packed structure of the Pd(110) surface. As shown in Table 5, the distance of ∼4 Å between the other two H atoms in the five-membered ring and the Pd atoms of the second layer on Pd(110) prevents the H atom from interacting with the surface. On the Pd(100), however, a very similar adsorption energy compared to the Pd(111) surface was obtained, due to the smaller distance of the Pd atoms (2.75 Å) compared to the Pd(110) (3.89 Å), making interaction of the H atoms in the five-membered ring possible. The preferred adsorption geometry maximized the interaction of H9a and H8aa with the Pd surface. The specific adsorption mode required by dodecahydro-N-ethylcarbazole is indicative of the structure sensitivity of the dehydrogenation reaction. The different adsorption energies shown in Table 5 201



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Table 6. Bond Length Changes for Dodecahydrofluorene and Dodecahydrocarbazole upon Adsorption on Pd(111) dodecahydrofluorene

dodecahydrocarbazole

C−H bond distance (Å)

C−H bond distance (Å)

adsorbing atom

gas phase

adsorbed

adsorbing atom

gas phase

adsorbed

C9−Pd (2.11 Å)

C9−H9: 1.09 C9a−H9a: 1.11 C8a−H8aa: 1.11 C4a−H4aa: 1.10 C4b−H4bb: 1.10

1.10 1.12 1.13 1.11 1.10

N−Pd (2.22 Å)

N−H9: 1.02 C9a−H9a: 1.11 C8a−H8aa: 1.11 C4a−H4aa: 1.10 C4b−H4bb: 1.10

1.02 1.11 1.11 1.10 1.11

almost two times lower than the 0.22 au charge increase obtained for dodecahydro-N-ethylcarbazole. The difference can be explained by the large amount of electron donation through N (∼0.08 au) in dodecahydrocarbazole that compensates the net charge transfer from the Pd surface to the molecule. The smaller overall charge increase on the molecule is likely the reason for the small change in the C−H bond distances of the five-membered ring of the adsorbed dodecahydrocarbazole, and therefore, a slower dehydrogenation rate was observed for dodecahydrocarbazole compared to dodecahydro-N-ethylcarbazole.21 For dodecahydrofluorene, on the other hand, the charge on the replaced C atom (C9) increased by about 0.05 au (Table S-8, Supporting Information) with a significant charge decrease of ∼0.16 au on the attaching Pd atom (Pd9). The overall charge on the dodecahydrofluorene molecule was increased by about 0.31 au, indicative of a net flow of charge transfer from the surface to the molecule mostly concentrated on the σ bond between C9 and Pd9, and this is responsible for the strong adsorption of the molecule to the surface. The very small change in the C−H bond distances of the molecule (Table 6) explains the very slow H2 recovery from this molecule at 443 K and 101 kPa as shown previously,21 implying that the reaction requires a higher temperature to overcome the large activation barrier. Tetrahydro-N-ethylcarbazole adsorption occurred through four carbon atoms in the aromatic six-membered ring (Figure 8d), with a N−Pd distance of 3.28 Å, larger than the N−Pd bond distance of 3.05 Å obtained for tetrahydrocarbazole (Figure 8e) and the 2.89 Å bond distance for C9−Pd9 in tetrahydrofluorene (Figure 8f), indicative of a stronger interaction of the latter to the surface. The very slow dehydrogenation reaction rate observed for dodecahydrofluorene is very well supported by the strong adsorption of dodecahydrofluorene and tetrahydrofluorene to the catalyst surface.

carbazole through N and dodecahydrofluorene through C resulted in the molecule tilting away from the surface as shown in Figure 8, such that the C−H bonds were no longer in close proximity with the surface, and therefore, no significant change in the C−H bond distances of the five-membered ring was observed for dodecahydrocarbazole and dodecahydrofluorene upon adsorption (Table 6). The stronger interaction of the N heteroatom in dodecahydrocarbazole is likely responsible for the much slower observed dehydrogenation reaction rate compared to dodecahydro-N-ethylcarbazole.21 Figure 9 compares the energy pathway for removal of the first H from dodecahydrofluorene with that of dodecahydro-N-

Figure 9. Comparison of energy pathway for first H removal from dodecahydro-N-ethylcarbazole and dodecahydrofluorene. Zero energy associates with the Pd(111) surface and the free molecule in each case.

ethylcarbazole. The energy barrier for removal of H9a from dodecahydrofluorene was found to be ∼101 kJ/mol, significantly higher than the 14 kJ/mol barrier for dodecahydro-Nethylcarbazole. The high energy barrier for dodecahydrofluorene can be explained by the adsorption structure of the molecule on the surface, where the molecule tilts away from the surface by 22°, making interaction of the H atoms in the fivemembered ring with the closest Pd atoms less probable. Hirshfield charges for dodecahydrocarbazole and dodecahydrofluorene upon adsorption to the surface were obtained from periodic calculations in order to make a comparison with dodecahydro-N-ethylcarbazole. The large negative charge decrease of ∼0.13 au on the Pd9 atom attached to N (Table S-8, Supporting Information) in dodecahydrocarbazole is likely responsible for formation of a σN−Pd bond. Total charge on the dodecahydrocarbazole molecule increased by 0.095 au, which is

4. CONCLUSIONS DFT calculations show that dehydrogenation of dodecahydroN-ethylcarbazole was initiated by removal of H atoms from the five-membered ring to produce octahydro-N-ethylcarbazole, followed by stepwise H removal from one of the aliphatic rings to produce tetrahydro-N-ethylcarbazole. Results support observed experimental product distribution data for dodecahydro-N-ethylcarbazole dehydrogenation, where both hexahydroN-ethylcarbazole and decahydro-N-ethylcarbazole are absent from the products. Adsorption of dodecahydro-N-ethylcarbazole on the Pd surface required multiple sites, and the adsorption mode of dodecahydro-N-ethylcarbazole suggested that the molecule locates on the surface in a proper adsorption site, indicative of the structure dependency of the adsorption. The strong interaction of the N heteroatom in dodecahy202



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drocarbazole was found to be responsible for the slow dehydrogenation rate of dodecahydrocarbazole compared to that of dodecahydro-N-ethylcarbazole. The rate of dodecahydrofluorene dehydrogenation was much lower than dodecahydrocarbazole, due to its very strong adsorption. It can be concluded that the N heteroatom favors the dehydrogenation reaction of dodecahydrocarbazole compared to dodecahydrofluorene, although it was shown that, in dodecahydrocarbazole dehydrogenation, product inhibition decreased the reaction rate due to the strong interaction of N with the surface.

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ASSOCIATED CONTENT

S Supporting Information *

Additional detail of calculation procedures and results. This material is available free of charge via the Internet at http:// pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Funding for the present study from the Natural Sciences and Engineering Research Council of Canada is gratefully acknowledged.

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REFERENCES

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Analysis of H2 Release from Organic Polycyclics over Pd Catalysts

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