Comparison of catalytic performance of supported ruthenium and rhodium forhydrogenation of 9-ethylcarbazole for hydrogen storage applications

Katarzyna Morawa Eblagon, Kin Tam and Shik Chi Edman Tsang*

Received 27th April 2012, Accepted 29th May 2012

DOI: 10.1039/c2ee22066k

The stepwise hydrogenation of 9-ethylcarbazole to 9-ethyl-perhydro

hydrogenated intermediate(s) was studied over a number of support

stan

ion

l an

but

e in

ha

moderate activity gave a higher selectivity to the fully hydrogenated product under comparable

conditions. It was also found that the presence of a hydrophilic support such as alumina or rutile can

storage and supply to polymer

on board of a vehicle has

most of the LOH systems are

us they are compatible with

hich could drastically reduce

ng to hydrogen as an energy

drogen from these carriers is

ree and can be used directly to

l, most LOH systems with

Dynamic Article LinksC

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. View Article Onlineintrinsically high hydrogen capacities4 are safe and cost effective

for the future implementation of hydrogen economy.5

The use of 9-ethylcarbazole as a reversible hydrogen storage

material at moderate conditions was proposed by Air Products6,7

with other researchers independently proposing the use of

a general class of N-heterocycles.7,8 Initial theoretical and

experimental studies indicated that the partial substitution of

carbon atoms by nitrogen favours low temperature release of

hydrogen from organic liquids.5,7,8 Thus, 9-ethylcarbazole is one

promising material for hydrogen storage utilizing the LOH

approach. The reversible catalytic hydrogenation of 9-ethyl-

carbazole, studied in the present work, is shown in Fig. 1.

The primary proposal was to execute the catalytic dehydro-

genation reaction of 9-ethyl-perhydro-carbazole to supply

hydrogen in a PEMFC vehicle.6,7 As a result, the dehydrogena-

tion reaction and mechanism were extensively studied.913 On the

other hand, there is only limited information on the hydroge-

nation of 9-ethylcarbazole.1416 This is an important reaction

from the point of view of material regeneration at hydrogen

filling stations, although it is still likely that regeneration will be

carried out at a central facility or in a factory. Therefore, an in-

depth understanding of catalytic systems on reaction selectivity

and yield is of importance for the future implementation of the

LOH strategy in the application for mobile hydrogen storage.

We have previously reported a comparative study of the liquid

phase hydrogenation of 9-ethylcarbazole over a series of ruthe-

nium based catalysts dispersed on different carriers14 and the

studies for different unsupported metals for the same reaction

were also iniated.15 Similarly, the catalytic performances over

a commercial 5 wt% Ru/Al2O3 (ref. 10 and 13) and RANEYnickel catalyst16 were extensively studied by other researchers in

this area. In this study, the particular interest is paid to the nature

of metal and support on the overall catalytic performance for this

reaction.

Norskov and Hammer17 suggested that the adsorption prop-

erties of many substrates depend on the surface electronic

structure of the transition metal catalyst, which in turn is

determined by its chemical composition and morphology. A

good correlation between the position of the d-band centre of

Fig. 1 Reversible catalytic hydrogenation of 9-ethylcarbazole.a particular metal, adsorption strength of atoms/molecules17,18

and catalytic activity19,20 was established. In addition, the elec-

tronic properties of metals can also be influenced by the presence

and type of support used. The support can alter the electron

density of the metal leading to a change in its catalytic proper-

ties.21,22 Moreover, the support can sometimes increase the

catalytic activity of the metal by providing additional active sites

for substrate adsorption, as discussed by Chou and Vannice.23

In this paper, a detailed comparison of the catalytic perfor-

mances of ruthenium and rhodium over common support

8622 | Energy Environ. Sci., 2012, 5, 86218630materials alongside commercial supported palladium was carried

out in the liquid phase hydrogenation of 9-ethylcarbazole. The

focus on the two metals was motivated by their high activities

reported in the hydrogenation of pyrroles.24 Ruthenium was also

previously found by our group to be the most active catalyst in

the hydrogenation of 9-ethylcarbazole.15,25 Additionally,

rhodium was also found to be active in hydrogenation of

aromatic compounds.26 Thus, in the present work, the kinetics of

the stepwise hydrogenation of 9-ethylcarbazole based on

fundamental reaction pathways were compared over Ru and Rh

supported catalysts. The influences of the type of active metal

and support on the activity and selectivity of the catalysts in this

reaction will also be discussed.

2. Experimental

2.1. Materials

All the materials were used as received, without further purifi-

cation. Sodium borohydride (99%), ruthenium(III) chloride

hydrate (3842%), tris(acetylacetonato)ruthenium(III) (Ru-70,

24.59%), rhodium(III) acetylacetonate (97%), oleic acid, dioctyl

ether (99%), oleylamine (70%) and TiO2 (rutile) were purchased

from Sigma-Aldrich. Commercial catalysts, namely: 5 wt% Ru

on activated carbon (AC), 5 wt% Rh on AC, 5 wt% Ru on Al2O3and 5 wt% Rh on Al2O3 were also purchased from Sigma

Aldrich. Activated carbon (activated, Ash 4%max), was received

from Johnson Matthey. The HPLC grade solvents (acetone,

cyclohexane, hexane and ethanol) were supplied by Fisher-

Scientific. Hydrogen gas (99.9%) was provided by Air-products.

2.2. Catalyst preparation

Supported noble metal catalysts were either purchased from

Sigma Aldrich (designated as COM) or were prepared using

mainly two methods in the present work. The as-prepared

catalysts were used without further pretreatments or activation.

The single step method was a mild chemical reduction (desig-

nated as CR) of the metal precursor impregnated on the

support. Briefly, an appropriate amount of ruthenium(III)

chloride hydrate salt (0.3 g) was dissolved in 10 mL of distilled

water. This solution was then added dropwise to the aqueous

slurry of the support (typically 2.9 g of support in 15 mL of

distilled water). After 48 hours of stirring, the mixture was

reduced by addition of a solution of sodium borohydride (0.4 g

in 10 mL of distilled water) at 90 C. Subsequently, the catalystwas washed five times with distilled water and acetone and

centrifuged at 8000 rpm for 10 minutes each time. Then it was

left to dry overnight at room temperature under ambient

atmosphere.

The two-step method was based on a modified polyol process

(designated as P), developed by Sun et al.27 followed by

a subsequent impregnation of the metal colloid on the support.

In this method, the sol of metal nanoparticles was produced

following the polyol process. Mainly, an appropriate amount of

tris(acetylacetonato)ruthenium(III)/rhodium(III) acetylacetonate

(0.16 g/0.11 g) was dissolved in 10 mL of dioctyl ether (99%

Aldrich). Then the appropriate amount of oleic acid and

oleylamine (65 ml/68 ml) for ruthenium, and (42 ml/45 ml) for

rhodium was added to the solution of the precursor, followedThis journal is The Royal Society of Chemistry 2012

Two main reaction intermediates, namely: 9-ethyl-tetrahy-

drocarbazole (Pl 4[H] addition of 4 hydrogen atoms to the

substrate) and 9-ethyl-octahydrocarbazole (Pl 8[H] addition of

8 hydrogen atoms) were identified as the main key intermediates

using GC-MS. The fully saturated product 9-ethyl-perhy-

drocarbazole (Pl 12[H] addition of 12 hydrogen atoms) was

separated into three different liquid fractions of isomers, namely

Fig. 3 Reaction model of direct and stepwise hydrogenation of 9-eth-

ylcarbazole to intermediate(s) and products over noble metal catalysts; Pl

0[H]: 9-ethylcarbazole, Pl 4[H]: 9-ethyl-tetrahydrocarbazole, Pl 8[H]: 9-

ethyl-octahydrocarbazole, Pl 12[H]: 9-ethyl-perhydrocarbazole. The

stereoisomers of 9-ethyl-perhydrocarbazole (A, B and C) are shown in

Fig. 2 and the numbering of the rate constants matches with those used

for modelling of the kinetics.

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. View Article Onlineby 0.14 g/0.9 g for ruthenium/rhodium of 1,2-hexadecanediol as

a reducing agent. The mixture was stirred continuously under

nitrogen atmosphere for 30 minutes in a refluxing set-up

without heating. Then it was refluxed at 240 C for 40 minutesunder nitrogen to form a black sol. After cooling to room

temperature, the catalysts were washed five times with a 50%/

50% mixture of ethanol and hexane and centrifuged at 5000 rpm

for 10 minutes each time. After washing was completed, the

catalysts were left to dry at ambient conditions for 12 h.

Subsequently, the produced metal catalyst was wet-impregnated

onto the support. The solution containing 0.015 g of dried

nanoparticles in 5 mL of ethanol was added dropwise to 0.285 g

of support dispersed in 10 mL of ethanol. Finally, the supported

nanocatalyst was dried at ambient conditions for 12 hours

before testing. The dry powder was used for catalysis without

any further pre-treatment.

2.3. Catalyst characterisation

The specific surface area of the catalyst was measured following

a standard N2-BET method.

The particle size and the dispersion were evaluated from the

HRTEM JEOL 2010 with an accelerating voltage of 300 kV at

a nominal magnification of 590k. Catalysts samples were

dispersed ultrasonically in ethanol . A droplet of the suspension

was then placed on a 200-mesh copper grid coated with formvar

carbon and left to dry at 80 C overnight. Determination of themean particle size distribution was based on the measurements of

the minimum of 100 particles from different areas of the sample

using the Scandium software from Olympus Soft Imaging

Solutions.

2.4. Catalyst activity and selectivity

The hydrogenation of 9-ethylcarbazole was carried out in a Parr

stainless steel 300 mL batch reactor with continuous monitoring

and control of stirring speed, temperature, and pressure. In

a typical experiment, 1 gram of 9-ethylcarbazole was dissolved in

100 mL of cyclohexane followed by addition of 0.2 gram of

a selected catalyst. The reactor was then sealed, flushed with

hydrogen and heated to 130 C. When the desired temperaturewas reached, 70 bar of pure hydrogen was charged into the

reactor and the reaction time was taken as zero. After the

required reaction time, the autoclave was cooled using a water

bath. For study of the reaction kinetics, small aliquots of the

reaction mixture (usually less than 0.5 mL) were removed from

the reactor periodically and analysed. The composition of the

samples was analysed using a GC-MS instrument (Agilent 6890-

5975E GC-MS) equipped with a non-polar capillary column

(Agilent 19091s-433), an auto-sampler and an MSD-Triple Axis

Detector HED-EM. The experimentally obtained reproducibility

of the results showed a deviation of less than 5% on the values

obtained for both the conversion and selectivity in the repeated

series of experiments.

The catalytic activity and selectivity of the catalysts were

estimated based on the eqn (1) and (2) listed below.

Catalytic activity 9-ethylcarbazole convertedmolestotal metal contentgtimes (1)This journal is The Royal Society of Chemistry 2012Selectivity specific productmolessum of all the productsmoles 100 (2)

2.5. Modelling of the reaction kinetics

Fig. 2 Structures of the isomers of 9-ethyl-perhydrocarbazole obtained

using NMR techniques.25 Fraction B actually contained two stereo-

isomers indistinguishable by means of GC-MS. As a result, this mixture

was treated as a single product in the quantitative analysis of the reaction

kinetics. The numbering of the rate constants matches with the numbers

used in the modelling of the kinetics.Energy Environ. Sci., 2012, 5, 86218630 | 8623

Pl 12[H] A, B and C (structures shown in Fig. 2). Furthermore,

after the hydrogenation of the substrate was completed, the

isomers of 9-ethyl-perhydrocarbazole were found to be inter-

converted to each other at significant rates (see Fig. 2). More

detailed analysis of the reaction products using NMR tech-

niques was reported in our previous work.14,25 Based on the

analysis of the experimentally obtained concentration versus

time profiles, the reaction model was developed, which is shown

in Fig. 3.

In order to fit the experimental data into the proposed reaction

model, the appropriate differential equations were readily solved

to obtain the initial product concentrationtime profiles of each

species by using the 4th and 5th order RungeKutta formulas.28

For example, the rate equation describing the changes of

the concentration of 9-ethyl-tetrahydrocarbazole (Pl 4[H]) is

shown below.

dP4

dt k2P0 k5P4

where P0 is the concentration of 9-ethylcarbazole (Pl 0[H]) and

P4 is the concentration of 9-ethyl-tetrahydrocarbazole (Pl 4[H]).

Similar rate equationswere solved for eachof the reactions shown

in the reaction model in Fig. 3. The boundary conditions imposed

were: at time zero, concentration of the substrate (P0[H] 100(mol%)) and the concentration of other species was equal to 0.

The first rate constant k0 was determined by a regression

analysis of the substrate concentrationtime profile (derived

from the conversion) against a first order integrated rate law

where [P0]t0 represents the concentration of the substrate P0 attime zero 100 (mol%). d represents the correction factor for anylag time due to experimental error, which was also determined

from the regression analysis. The rate constant k0 represents the

overall first order decomposition rate constant of the starting

material, and it is equal to the summation of k1, k2, k3, k6 and k9.

The remaining rate constants were derived using a NelderMead

method (simplex method).29 The mismatch between experimental

and modelled values was calculated using a root mean square of

the difference (RMS).30 Thus, the unknown rate constants were

solved iteratively to fulfil the conditions described by the reaction

model and to optimise a set of k values that would result in

a minimum mismatch. The rate constant k0 was treated as

In Fig. 4, the obtained HRTEM images show distorted

(0.2

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[P0] [P0]t0exp(k0(t + d))

Table 1 Physical properties of the studied catalysts. COM arecommercial catalysts, CR are chemically reduced by the NaBH4 methodand P are catalysts synthesized using the polyol process

CatalystPreparationmethod TEM size (s.d.) [nm] BET [m2 g1]

5 wt% Ru on AC COM 3.0 (1.0) 8095 wt% Ru on TiO2 CR 2.1 (0.6) 1915 wt% Ru on TiO2 P 3.1 (0.6) 1805 wt% Ru on Al2O3 COM 9.1 (4.7) 835 wt% Rh on AC COM 3.9 (1.8) 4425 wt% Rh on Al2O3 COM 3.3 (0.4) 2725 wt% Rh on AC P 5 wt% Pd/AC COM 2.7 (0.6) 801

Table 2 Catalytic activity and selectivity quenched after 1 h of reaction

CatalystCatalytic activity 102 [mM ETC per g of metal per s]

5 wt% Ru/AC 27.85 wt% Ru on TiO2 (CR) 13.95 wt% Ru on Al2O3 (COM) 13.55 wt% Ru on TiO2 (P) 13.45 wt% Rh on AC (COM) 13.15 wt% Rh on Al2O3 (COM) 13.25 wt% Rh on AC (P) 8.35 wt% Pd/AC (COM) 4.78624 | Energy Environ. Sci., 2012, 5, 86218630spherical nanoparticles mostly uniformly distributed on the

supports. This suggests that the samples contain highly

g catalyst was used except in the case of 5 wt% Ru/AC (0.1 g))

Selectivity (%)

Pl 4[H] Pl 8[H] Pl 12[H]A Pl 12[H]B Pl 12[H]C

1 32 44 19 30 9 4 77 100.5 58 6 33 20.5 54 7 39 36 6 50 33 6

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. View Article Onlineunsaturated sites such as edges and corners on the sharp crystal

surfaces. From the image of 5 wt% Ru/Al2O3 (Fig. 4B), it can be

seen that the particles are agglomerated/sintered probably due

Fig. 4 Typical HRTEM images of the selected catalysts. A 5 wt% Ru/AC

Ru/TiO2 (CR).

This journal is The Royal Society of Chemistry 2012to the high temperature of reduction. In the HRTEM image of

5 wt% Ru/TiO2 (Fig. 4D), the metal fringes are clearly visible in

contrast to the support.

(COM), B 5 wt% Ru/Al2O3 (COM), C 5 wt% Rh/AC and D 5 wt%

Energy Environ. Sci., 2012, 5, 86218630 | 8625

3.2. Hydrogenation activity and selectivity

According to the Sabatier principle the catalytic activity is at

its maximum if the substrate/intermediate chemisorbs onto the

surface of the catalyst with an appropriate strength.31 If the

adsorption is too weak, the catalytic activity will be low and

the rate limiting step will be the adsorption of the substrate. On

the other hand, if the adsorption is too strong, the rate will be

limited by the regeneration of the surface sites of the catalyst. In

this case of the hydrogenation of 9-ethylcarbazole, the ability of

the surface d-band electrons of a particular metal to participate

in bonding with the substrate is likely to be involved in the rate

determining step. The shift of d band position across the periodic

table suggests that the metal situated more towards the right

hand side with a lower d band state as relative to the Fermi level

consequently has a weaker surface bonding with the substrate

and, as a result, has lower overall activity in the reaction.

It is interesting to note that the selectivity of the key inter-

mediate Pl 8[H] is proportional to the catalytic activity as

Fig. 5 Catalytic activity versus selectivity towards stable intermediate Pl

8[H] quenched after 1 h of the reaction.

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. View Article OnlineThe obtained catalytic activity and selectivity of the supported

metals in liquid phase hydrogenation of 9-ethylcarbazole in

a stirred tank reactor are summarised in Table 2.

The results were compared after the first hour of the reaction

due to the high substrate conversions obtained with longer

reaction time by some active catalysts. Based on the obtained

data, the influence of different factors such as type of the metal,

support and the synthesis method can be compared

systematically.

3.2.1. Influence of the different noble metals (Ru, Rh, Pd all

supported on AC). Comparing Ru, Rh and Pd commercial

catalysts dispersed on AC, Table 2 reveals that the activity per

gram of metal decreases in this reaction despite the large differ-

ences in the values of BET surface area (see Table 1) as the

metal d-band centre decreases (Ru 1.41, Rh 1.73,Pd 1.83).32 This observation is also consistent with ourprevious report of the poor activity of Pt/AC,25 which has the

lowest d band centre value (2.25 eV).32 The change of activity ofthe metals in this reaction can be described by the position of the

active metal in the periodic table, and decreases as elements shift

from the left hand to the right hand side in the second row of

transition metals.Fig. 6 Time dependent product distribution obtained with 0.1 g of

5 wt% Ru/AC (COM) catalyst during 4.5 hours of the reaction at 130 Cand 70 bar of hydrogen. The labels of different species are displayed.

8626 | Energy Environ. Sci., 2012, 5, 86218630shown in Fig. 5. The most active 5 wt% Ru/AC gave the highest

selectivity to Pl 8[H]. This suggests that the production of this

intermediate is favourable at higher substrate conversions in the

stepwise hydrogenation, but at the same time the readsorption

for further conversion of this intermediate on the same catalyst

surface is somewhat hindered on the surface of Ru metal. This

could be due to the competitive adsorption between substrates

(substrate, intermediates and products) on the same catalytic

sites. On the other hand, the difference of a surface structure of

Ru (hexagonal close packed structure) as compared to Rh, Pd

or Pt (all face centre cubic structures) may account for the

unusually high stability of the Pl 8[H] intermediate. In addition,

the difference in surface coverage of hydrogen species over

these different metals may also play a role in defining their

selectivity.

In order to compare the relative rates of competitive adsorp-

tions (conversions) for different species on catalyst surfaces, their

individual concentration versus time plots were obtained. The

results for hydrogenations over Ru/AC, Rh/AC and Pd/AC

commercial catalysts are shown in Fig. 68. A kinetic model for

the stepwise hydrogenations shown in Fig. 3 was used to derive

the fundamental rates for the elementary surface reactions.

The rate constants for each elementary step with 5 wt%

Ru/AC, 5 wt% Rh/AC and 5 wt% Pd/AC catalysts used for the

Fig. 7 Time dependent product distribution obtained with 0.2 g of

5 wt% Rh/AC (COM) catalyst during 5 hours of the reaction at 130 Cand 70 bar of hydrogen. The labels are the same as in Fig. 6.This journal is The Royal Society of Chemistry 2012

Table 3 The calculated rate constants in [h1] for 5 wt% Ru/AC (0.1 g catalyst), 5 wt% Rh/AC and 5 wt% Pd/AC

Catalyst k0 k1 k2 k3 k4 k5 k6 k7 k8 k9 k10 k11

5 wt% Ru/AC (COM) 3.4 0.6 1.6 1 0.001 3.7 0.22 0.001 1.058 0.002 0.13 0.0035 wt% Rh/AC (COM) 3.0 1.4 0.59 0.62 0.92 1.3 0.28 0.53 0.433 0.11 0.13 05 wt% Pd/AC (COM) 0.33 0.049 0.22 0 0.008 0.24 0.03 0.005 9.449 0.03 0.027 0.015

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in Table 3.

Careful inspection of the k values listed in Table 3 together

with corresponding concentrationtime profiles, shows that the

calculated k0 decreases in the order Ru > Rh > Pd, which is

the same trend as the comparison of activity after the first hour of

Fig. 8 Time dependent product distribution obtained with 0.2 g of

5 wt% Pd/AC (COM) catalyst during 5 hours of the reaction at 130 Cand 70 bar of hydrogen. The labels are the same as in Fig. 6.

Fig. 9 Time dependent product distribution obtained with 5 wt% Ru/

Al2O3 (COM) catalyst during 5 hours of the reaction at 130C and 70 bar

of hydrogen.

Table 4 The calculated values of rate constants for the 9-ethylcarbazole hyd

Catalyst k0 k1 k2 k3 k4

5 wt% Ru/AC (COM)a 3.4 0.60 1.6 1 0.0015 wt% Ru/TiO2 (CR) 13 0.008 0.57 7.5 1.35 wt% Ru/Al2O3 (COM) 12 0.93 0.65 9.2 1.15 wt% Ru/TiO2 (P) 6.3 2.7 0.1 4.9 0.3

a 0.1 g catalyst was used instead of 0.2 g.

This journal is The Royal Society of Chemistry 2012the reaction (Table 2). It is noted that the derived k0 is not

excessively large compared to other rates with the 5 wt% Ru/AC

catalyst. Hence with the high substrate conversion during the

first hour (Fig. 6), the rates (especially the k8) for Pl 8[H]

conversions would suggest that this intermediate is able to

Fig. 10 Time dependent product distribution obtained with 5 wt% Ru/

TiO2 (CR) catalyst during 4 hours of the reaction at 130C and 70 bar of

hydrogen. The labels are explained in Fig. 9.

Fig. 11 Time dependent product distribution obtained with 5 wt% Ru/

TiO2 (P) catalyst during 4 hours of the reaction at 130C and 70 bar of

hydrogen. The labels are explained in Fig. 9.

rogenation with supported ruthenium catalysts

k5 k6 k7 k8 k9 k10 k11

3.7 0.22 0.001 1.1 0.002 0.13 0.00314.4 2.0 0.47 0.035 3.5 0 0.1916 1.4 0.22 0.19 0 0.068 0.0334.0 0.3 0.003 0.16 0.36 0 0.021

Energy Environ. Sci., 2012, 5, 86218630 | 8627

Similarly, the main kinetic stable intermediate Pl 4[H] was found

when reaction was taking place over 5 wt% Pd/AC catalyst.

Comparison of the rate constants in Table 3 with corresponding

concentrationtime profiles (Fig. 68) revealed that over 5 wt%

Ru/AC and 5 wt% Pd/AC, the reaction takes place mostly by

consecutive addition of hydrogen atoms, whereas over 5 wt%Rh/

AC, the reaction appears to progress primarily via the direct

hydrogenation of the substrate, hence no significant accumula-

tion of any intermediate product(s) was observed.

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. View Article OnlineFig. 12 Time dependent product distribution obtained with 5 wt% Rh/

Al2O3 (COM) catalyst during 5.5 hours of reaction at 130C and 70 bar

of hydrogen.further convert to the fully hydrogenated products. It is thus

interesting to find out why the selectivity for Pl 8[H] reaches

>32% after the first hour of the reaction (Table 2). On the other

hand, from Table 3 it can be also seen that in the case of 5 wt%

Ru/AC, the rates of Pl 8[H] formation (k3, k5) are much higher

than the rates of its further conversion (k4, k7, k8). But, the rates

of Pl 8[H] formation over 5 wt% Pd/AC and 5 wt% Rh/AC are

much lower than the rates of its further conversion over these two

catalysts. Therefore, the unusual high stability of the Pl 8[H]

intermediate using 5 wt% Ru/AC could be attributed to its weak

interaction with the surface of this catalyst. This interaction is

further decreased by the distorted structure of this intermediate

resulting in the steric hindrance with the catalysts surface.

Fig. 13 Time dependent product distribution obtained with 5 wt% Rh/

AC (P) catalyst during 5.5 hours of reaction at 130 C and 70 bar ofhydrogen.

Table 5 The calculated rate constants for 5 wt% Rh/Al2O3 (COM), 5 wt% R

Catalyst k0 k1 k2 k3 k4

5 wt% Rh/Al2O3 (COM) 1.6 1.1 0.50 0 0.0045 wt% Rh/AC (COM) 3.0 1.4 0.59 0.62 0.925 wt% Rh/AC (P) 1.1 0.73 0.52 0 0.87

8628 | Energy Environ. Sci., 2012, 5, 86218630In addition, from Table 2, it can be seen that the composition

of the fully saturated product (9-ethyl-perhydrocarbazole) is

similar for the Ru/AC, Rh/AC and Pd/AC commercial catalysts

with the most thermodynamically stable Pl 12[H]A as the main

product. Thus, the formation of the isomers of Pl 12[H] does not

seem to be affected by the type of the metal used but rather by the

type of support.

3.2.2. Influence of the type of support on catalytic performance

of Ru based catalysts. Activated carbon is known to be an inert

material giving insignificant metalsupport interaction. In order

to evaluate the influence of support on catalytic performance,

ruthenium was deposited on Al2O3 (alumina) and TiO2 (rutile) in

order to compare with the results of ruthenium supported on

activated carbon. The concentrationtime profiles obtained over

these supported ruthenium catalysts are presented in Fig. 911.

The experimental data analysed with the aid of our kinetic model

(Fig. 3) generally showed a good fit. The calculated rate

constants for these supported ruthenium catalysts are presented

in Table 4.

Table 4 shows that 5 wt% Ru/AC gives the significantly lowest

k0 rate compared with other supports. We believe that this could

be an artefact due to the use of a smaller amount of catalyst (0.1 g

instead of 0.2 g) in this experiment. Although the calculated rates

account for the quantity of metal, the coverage of the species on

metal surfaces can be different. However, comparing the rate

constants of direct production of Pl 12[H] isomers from the

substrate (k1, k6 and k9) with the rate constants of the stepwise

hydrogenations (k2 and k3), the reaction pathway clearly adopts

the consecutive hydrogenation pathway over all these supported

ruthenium catalysts, regardless of the type of the support used. In

addition, the high selectivity to the Pl 8[H] intermediate is very

pronounced over all supports as the rates of Pl 8[H] formation

(k3, k5) are consistently higher than the rates of its further

conversion (k4, k7, k8).

One main difference when using different carriers is the

composition of the final product Pl 12[H]. In the case of 5 wt%

Ru/AC (COM), as mentioned before, the main product was the

isomer Pl 12[H]A (see Fig. 6). This isomer accounted for over

65% of the total sum of isomers of the 9-ethyl-perhydrocarbazole

only after 1 hour of the reaction (Table 2). But, in the case of

h/AC (COM) and 5 wt% Rh/AC (P)

k5 k6 k7 k8 k9 k10 k11

3.6 0 0.71 1.9 0 0.21 0.0041.3 0.28 0.53 0.43 0.11 0.126 01.0 0.09 0 0 0.43 0.046 3.7This journal is The Royal Society of Chemistry 2012

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. View Article Onlinebased catalysts. As discussed, the catalysts based on rhodium

displayed slightly lower activity than those based on ruthenium,

but they did not produce large quantities of intermediate(s),

resulting in higher selectivity towards the fully hydrogenated

products under comparable conditions. Therefore, it seems of

importance to identify the influence of the type of support on Rh

based catalysts.

The concentrationtime profiles are shown in Fig. 12 and 13.

The profile for 5 wt% Rh/AC was presented in Fig. 7 for

comparison. The rate constants derived from our model are

presented in Table 5.

It is noted from Table 5 that regardless of the synthetic method

or type of support used, the catalysts give a high rate for direct

hydrogenation of 9-ethylcarbazole to Pl 12[H]A (k1). The main

product for all the supported Rh catalysts is the isomer Pl 12[H]

A, which is similar to the result with 5 wt% Ru/AC.

It is interesting to note that the use of more hydrophilic

alumina as the support (5 wt% Rh/Al2O3) resulted in the main

initial product being Pl 12[H]B (see Fig. 12), which is similar to

the case of Ru. However, during the progress of the reaction, the

isomer Pl 12[H]B was further converted to Pl 12[H]A by an

isomerisation step (k10). This result agrees very well with our

previous observation that the composition of the final products

(isomers of 9-ethyl-perhydrocarbazole) is influenced mainly by

the type of support used. It is clear that the use of the activated

carbon support tends to favour the thermodynamically most

stable isomer Pl 12[H]A, whereas over alumina and rutile

supports, the production of Pl 12[H]B is facilitated.

4. Discussions and conclusion

Fundamental understanding of the elementary steps of the

reversible hydrogenation of 9-ethylcarbazole over different cata-

lytic surfaces is important regarding the supply of hydrogen gas

for mobile PEMFC devices. From the point of view of spent fuel

recovery, the rate of hydrogenation for the carrier compound

should be as high as possible. On the other hand, the existence of

kinetically stable intermediates (for example, Pl 4[H] over Pd

systems or Pl 8[H]withRu)may create technical difficulties. Thus,

an ideal liquid organic hydride (LOH) storage material should be

able to interchange between loaded and unloaded forms during

hydrogenation (material regeneration) and dehydrogenation

(delivery of hydrogen gas). Any stable intermediates would mean

the reduction of total storage capacity and would definitely resultother supports, the main product was Pl 12[H]B instead. Thus, it

seems that the final isomer distribution can be significantly

influenced by the type of support used. According to the calcu-

lated rate constants in Table 4, the stepwise hydrogenation of Pl

8[H] to isomer Pl 12[H]A is via k4 and to isomer Pl 12[H]B is via

k8 route (Fig. 3). While it is known that Pl 12[H]A is the most

thermodynamically stable product, the surface of hydrophilic

supports such as TiO2 and Al2O3 must apparently influence the

position of the hydrogen addition resulting in the formation of

the symmetrical Pl 12[H]B product (much higher k4). Thus, the

role of the type of support can direct the stepwise hydrogenations

to form the kinetically favoured but thermodynamically less

stable Pl 12[H]B product.

3.2.3. Influence of the support on catalytic performance of RhThis journal is The Royal Society of Chemistry 2012in longer times for hydrogen delivery and regeneration. In addi-

tion, the produced undesirable isomers such as trans- isomers of

9-ethyl-perhydrocarbazole in the hydrogenation of 9-ethyl-

carbazole may not possess the stereochemistry required for the

dehydrogenation steps. It should be noted that it is more favoured

to remove two hydrogen atoms from amolecule in the cis form on

terrace sites of the metal catalysts.7

The catalytic hydrogenation of 9-ethylcarbazole on the noble

metal surfaces appears to involve a series of consecutive reactions

of addition of hydrogen to double bonds to produce the final

fully hydrogenated 9-ethyl-perhydrocarbazole product. These

elementary steps can take place purely on the metal surface as

a direct hydrogenation process depending on the adsorption

strengths of substrate/intermediate(s). Alternatively, the rela-

tively weak adsorption of some intermediate(s) onto the metal

surface with respect to their molecular solvation can result in the

formation of kinetically stable intermediates in solution. As

a result, only a combined concentrationtime profile analysis

with rate constants elucidation from a kinetic model may allow

systematic comparison of catalyst performances.

In the present work it was found that Ru is the most active

metal followed by Rh and Pd in liquid phase hydrogenation of

9-ethylcarbazole, regardless of the type of support used. Here, we

invoke the general d-band model which describes the interaction

between renormalized valence states of a substrate molecule and

the d-states of the transition metal catalyst. In atomistic sight; the

bond strength of the substrate molecule on the catalyst surface

would depend on the d-band position in relation to the Fermi

level.17 Due to the fact that antibonding states are always above

d-states, the position of the d-band centre with respect to the

Fermi level is a good factor describing the reactivity of the

transition metal surfaces.17 In principle, the higher the energy of

the d-band centre in relation to the Fermi level, the higher the

antibonding orbital and the stronger the bond.17,32 However, the

changes in substrate, surface feature and geometry of adsorption

can affect the adsorbate levels that couple to the metal and as

a result can modify the bond strength. In the light of the relative

positions of the d-band centres of the studied metals (Ru, Rh and

Pd), the catalytic activities appear to correlate well with the

position of the d-band. This clearly indicates that the rate

determining step of hydrogenation of 9-ethylcarbazole lies in the

ability of 9-ethylcarbazole adsorption onto the metal catalyst

surface, which is not significantly influenced by the presence of

support. It is thus clear that ruthenium is the most active metal in

this reaction, however it suffers from relatively poor selectivity

towards fully hydrogenated products, due to the production and

accumulation of large quantities of the Pl 8[H] intermediate. Our

results show that the surface interaction of Pl 8[H] with the

surface of the ruthenium catalyst seems to be weak. This is due to

the change in the compound structure from Pl 0[H] to Pl 8[H]

(containing a stable pyrrole moiety which hinders the re-

adsorption of this intermediate on terrace sites of the catalyst14)

influencing its conversion into the fully saturated product.

Similarly, the Pd based catalyst suffers from the accumulation of

Pl 4[H] and poorer activity as compared to Ru in the 9-ethyl-

carbazole hydrogenation reaction. Rh based catalysts with some

characteristic surface features appeared to provide moderate

binding strengths for both substrate and intermediates in this

reaction, hence resulting in a good overall activity and selectivity.Energy Environ. Sci., 2012, 5, 86218630 | 8629

Based on the obtained results, it is clear that the interaction of the

substrate and intermediates with a metal catalyst may be modi-

fied through the use of a support or by alloying the active metal

with another metal. The alloying of two metals shifts the position

of the d-band centre of the metal and, as a result, modifies the

strength of the interaction between catalyst and reaction

substrates/intermediates/products.17 Regarding the support

effect, it was found that the use of oxide support such as Al2O3 or

TiO2 can facilitate the production of kinetically favoured cis-

isomers of 9-ethyl-perhydrocarbazole (Pl 12[H]B). It is thought

that hydrophilic surfaces at the metalmetal oxide interface can

affect the adsorption of intermediate(s). However, the isomer-

isation of Pl 12[H]B would further convert this product into the

thermodynamically most stable isomer Pl 12[H]A.

In conclusion, the type and stability of the intermediates in the

hydrogenation of 9-ethylcarbazole is governed in majority by the

electronic structure of the active metal whereas the initial

Catal. Today, 2003, 82, 119.3 P. D. Tien, T. Satoh, M. Miura and M. Nomura, Fuel Process.

7 G. Pez, A. Scott, A. Cooper, H. Cheng, F. Wilhelm andA. Abdourazak, US Pat., 7351395, Air Products and Chemicals,Inc., USA, April 11, 2008.

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15 K. M. Eblagon, K. Tam, K. M. K. Yu and S. C. E. Tsang, J. Phys.Chem. C, 2012, 116, 7421.

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. View Article OnlineTechnol., 2008, 89, 415.4 R. B. Binwale, S. Rayalu, S. Devotta and M. Ichikawa, Int. J.Hydrogen Energy, 2008, 33, 360.

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supports of this project.

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Acknowledgements

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Comparison of catalytic performance of supported ruthenium and rhodium for hydrogenation of 9-ethylcarbazole for hydrogen storage applicationsComparison of catalytic performance of supported ruthenium and rhodium for hydrogenation of 9-ethylcarbazole for hydrogen storage applicationsComparison of catalytic performance of supported ruthenium and rhodium for hydrogenation of 9-ethylcarbazole for hydrogen storage applicationsComparison of catalytic performance of supported ruthenium and rhodium for hydrogenation of 9-ethylcarbazole for hydrogen storage applicationsComparison of catalytic performance of supported ruthenium and rhodium for hydrogenation of 9-ethylcarbazole for hydrogen storage applicationsComparison of catalytic performance of supported ruthenium and rhodium for hydrogenation of 9-ethylcarbazole for hydrogen storage applicationsComparison of catalytic performance of supported ruthenium and rhodium for hydrogenation of 9-ethylcarbazole for hydrogen storage applicationsComparison of catalytic performance of supported ruthenium and rhodium for hydrogenation of 9-ethylcarbazole for hydrogen storage applications

Comparison of catalytic performance of supported ruthenium and rhodium for hydrogenation of 9-ethylcarbazole for hydrogen storage applicationsComparison of catalytic performance of supported ruthenium and rhodium for hydrogenation of 9-ethylcarbazole for hydrogen storage applicationsComparison of catalytic performance of supported ruthenium and rhodium for hydrogenation of 9-ethylcarbazole for hydrogen storage applicationsComparison of catalytic performance of supported ruthenium and rhodium for hydrogenation of 9-ethylcarbazole for hydrogen storage applicationsComparison of catalytic performance of supported ruthenium and rhodium for hydrogenation of 9-ethylcarbazole for hydrogen storage applicationsComparison of catalytic performance of supported ruthenium and rhodium for hydrogenation of 9-ethylcarbazole for hydrogen storage applications

Comparison of catalytic performance of supported ruthenium and rhodium for hydrogenation of 9-ethylcarbazole for hydrogen storage applicationsComparison of catalytic performance of supported ruthenium and rhodium for hydrogenation of 9-ethylcarbazole for hydrogen storage applications