Preferential oxidation of CO in a H2-rich stream over multi-walled carbon nanotubes confined Ru catalysts

ww.sciencedirect.com

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 6 6 6 5e1 6 6 7 6

Available online at w

journal homepage: www.elsevier .com/locate/he

Preferential oxidation of CO in a H2-rich streamover multi-walled carbon nanotubes confined Rucatalysts

Yuxian Gao, Kangmin Xie, Shiyang Mi, Ning Liu, Wendong Wang*,Weixin Huang*

CAS Key Laboratory of Materials for Energy Conversion and Department of Chemical Physics, University of Science

and Technology of China, Hefei 230026, China

a r t i c l e i n f o

Article history:

Received 1 November 2012

Received in revised form

22 March 2013

Accepted 13 April 2013

Available online 7 May 2013

Keywords:

Preferential CO oxidation

Ru catalysts

Carbon nanotubes

Confinement effects

* Corresponding authors. Tel.: þ86 551 63603E-mail addresses: [email protected] (

0360-3199/$ e see front matter Copyright ªhttp://dx.doi.org/10.1016/j.ijhydene.2013.04.0

a b s t r a c t

Multi-walled carbon nanotubes (MWNTs) confined Ru catalysts were prepared by a

modified procedure using ultrasonication-aided capillarity action to deposit Ru nano-

particles onto MWNTs inner surface. The structure properties of MWNTs supports and Ru

catalysts were extensively characterized by XRD, TGA, H2-TPR, XPS, TEM, FTIR and Raman

spectra. The catalytic performance in the preferential oxidation of CO in a H2-rich stream

was examined in detail with respect to the influences of Ru loading, MWNTs diameter,

various pretreatment conditions, and the presence of CO2 and H2O in the feed stream. In

contrast with Ru catalysts supported on MWNTs external surface and other carbon ma-

terials, the superior activity was observed for the MWNTs-confined Ru catalyst, which was

discussed intensively in terms of the confinement effect of carbon nanotubes. The opti-

mized catalyst of 5 wt.% Ru confined in MWNTs with diameter of 8e15 nm can achieve the

complete CO conversion in the wider temperature range and the favorable stability at 80 �C

under the simulated reformatted gas mixture, which proves a promising catalyst for

preferential CO oxidation in H2-rich stream.

Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction monoxide [2]. This remaining CO can easily poison the Pt-

The hydrogen production is one of the crucial technologies for

the future hydrogen energy economy [1e3], when the polymer

electrolytemembrane fuel cell (PEMFC) is considered as one of

the most promising fuel cell technologies due to its high en-

ergy efficiency and environment-friendly characteristics [4].

However, the fuel processing of hydrocarbons as the most

common hydrogen production method in commercial use,

mainly consisting of the reforming and water gas shift re-

actions, may also produce appreciable amounts of carbon

683; fax: þ86 551 6360043W. Wang), huangwx@ust2013, Hydrogen Energy P70

based anode of PEMFC and thus should be removed to a

trace level. Among the approaches to remove CO in a H2-rich

stream, the preferential oxidation (PROX) of CO is one of the

primarily developing techniques by catalytic CO trans-

formation in addition to the alternatives by physically cryo-

genic separation, pressure swing adsorption and selective

diffusion of hydrogen [2,5].

Among the reported catalysts systems active for the PROX

reaction, the supported noble metals (Ru, Pt and Rh) and gold

catalysts commonly exhibit appreciable activities at lower

7.c.edu.cn (W. Huang).ublications, LLC. Published by Elsevier Ltd. All rights reserved.

mailto:[email protected]

mailto:[email protected]

http://crossmark.crossref.org/dialog/?doi=10.1016/j.ijhydene.2013.04.070&domain=pdf

www.sciencedirect.com/science/journal/03603199

www.elsevier.com/locate/he

http://dx.doi.org/10.1016/j.ijhydene.2013.04.070



i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 6 6 6 5e1 6 6 7 616666

temperature (below 373 K) than themetal oxides counterparts

such as CuOeCeO2 catalysts as indicated in the recently

comprehensive review [5]. Under the practical conditions, this

lower temperature for PROX is quite preferred not only

because it is energy-economical and compatible with the

typical operating temperature of PEMFC, but also it may avoid

the possible reverse water gas shift reaction and thus hin-

dering complete CO removal. In particular, the Ru-based cat-

alysts have some advantages over other supported noble

metals catalysts in providing an extended temperature range

to achieve the acceptable CO removal since both oxidation

and methanation of CO can be accomplished at once [6e9]. It

has also been reported that the supported Ru catalysts can

performed better than the supported Pt, Rh, Pd and Au sup-

ported catalysts in some cases [5,10,11]. The Ru/Al2O3 cata-

lysts may have been considered as one of the most promising

based on the real operation data [5,12e14].

Carbon materials, with various forms and properties, have

been recognized to play an increasingly important role in

heterogeneous catalysis processes as an adsorbent, catalyst

support, or even catalyst on its own [15,16]. Carbon nanotubes

(CNTs) have proved particularly attractive and competitive in

catalytic processes due to the combination of their unique

morphology and electronic, adsorption, mechanical and

thermal properties [17,18]. Recently, growing research inter-

est has been triggered in creating novel composites through

the selective deposition of metal or oxide nanoparticles

within the well-defined channels of carbon nanotubes and

exploring their advantageous confinement effects on the

catalytic properties [19,20]. The CNTs-supported Ru-based

catalysts have demonstrated considerable modification to the

catalytic activity of varied reactions, such as FischereTropsch

synthesis [21], hydrogenation of cellobiose [22] and NH3

Synthesis [23].

Recently, the enhanced performance for CO-PROX has

been preliminarily reported over Ru/CNTs catalysts [24].

However, the confinement effects relating to the structure

properties of carbon nanotubes and Ru catalysts have not

been identified in detail. In this study, the catalytic perfor-

mance in CO-PROX over MWNTs confined Ru catalysts was

investigated to explore the confinement effect. The influences

of Ru loading, MWNTs diameter, various pretreatment con-

ditions, and the presence of CO2 and H2O in the feed stream

were probed to survey the interactions between MWNTs with

the confined Ru nanoparticles and with the reactants of CO-

PROX reaction.

2. Experimental

2.1. Catalysts preparation

Multi-walled carbon nanotubes with different diameter

(o.d.< 8, 8e15, 30e50 and>50 nm, respectively, manufacturer

data) were purchased from China Chengdu Organic Chem-

icals. All other chemicals were obtained from Sinopharm

Chemical Reagent and used as received without further

modification. Raw MWNTs were purified by refluxing in a

concentrated HNO3 (68 wt.%) solution at 140 �C with different

amount of acid solution and duration time. The mixture were

then filtered and washed with deionized water, followed by

drying in the oven at 110 �C for 10 h. The high purity ofMWNTs

was confirmed by inductively coupled plasma spectroscopy

(ICP). Transmission electron microscope (TEM) results indi-

cated that the obtained MWNTs exhibited open tips with

about 300e500 nm length.

The catalysts were prepared by following the reported

procedures [25e27] with some modifications. The preparation

of Ru catalysts confined inside MWNTs with open tips

(denoted by Ru/o-MWNTs-in) was conducted by a modified

approach using ultrasonication-aided capillarity action and

stirring to fill theMWNTs channels with the aceton solution of

RuCl3. The suspension was kept continuously stirring to

evaporate the solvent under ambient conditions, which was

then dried slowly during a carefully controlled process of

heating to 110 �C at a rate of 1 �C min�1. Meanwhile, two

different methods were employed to prepare the Ru catalysts

dispersed on the outer surface of MWNTs for comparison. Ru/

o-MWNTs-out catalyst was prepared by using the same open

MWNTs as for Ru/o-MWNTs-in, but with xylene as temporary

blocker of MWNTs channels before the impregnation of RuCl3water solution [23]. The other Ru/c-MWNTs-out catalyst was

prepared over MWNTswith closed ends, which were obtained

by refluxing the raw MWNTs in a diluted HNO3 (37 wt.%) so-

lution at 110 �C for 5 h, since this milder treatment could

effectively remove the impurity while keeping the caps intact.

Thus only the external surfaces were decorated with the Ru

nanoparticles for both Ru/o-MWNTs-out and Ru/c-MWNTs-

out catalysts, which were then subjected to the same drying

and heat process. Additionally, carbon nanoribbons and gra-

phene were produced according to the previously reported

methods [28,29], which were loaded with Ru to obtain Ru/

Carbon nanoribbon and Ru/Graphene catalysts for compari-

son by using the same process as for Ru/c-MWNTs-out. Before

characterization and reaction tests, catalysts were usually

prereduced at 400 �C in a flow of hydrogen for 2 h unless

otherwise stated. In some cases, catalysts or MWNTs supports

were subjected to various heating processes in a flow of argon

at given conditions.

2.2. Characterization methods

The actual Ru content was determined by ICP for the catalysts

with the nominal Ru loading between 1 and 7 wt.%. The phase

compositions of the catalysts were analyzed by powder X-ray

diffraction (XRD) with a Philips X’Pert Pro Super diffractom-

eter (Cu Ka ¼ 0.15406 nm). BET surface areas were measured

using a Micromeritics Tristar II 3020M system. Thermal

gravimetric analysis (TGA) were performed on a Shimadzu

DTG-60H system in a flow of air from 25 �C to 800 �C at a

heating rate of 10 �C min�1. Temperature programmed

reduction (TPR) was performedwith a conventional apparatus

equipped with a thermal conductivity detector at a heating

rate of 10 �C min�1 from room temperature to 550 �C in a flow

of 5% H2/Ar mixture. X-ray photoelectron spectroscopy (XPS)

measurements were carried out with an ESCALAB 250 spec-

trometer equipped with Al Ka radiation (1486.6 eV). Surface

atomic composition was estimated by calculating the integral

of each peak using the sensitivity factor provided by the

instrumental software. The spectra were analyzed by fitting



i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 6 6 6 5e1 6 6 7 6 16667

the experimental curve to mixed GaussianeLorentzian peaks.

TEM images were acquired on a JEOL-2010 transmission

electron microscope. Raman spectra were obtained on

LABRAM-HR Raman Microscope at an excitation wavelength

of 514 nm at ambient temperature. Fourier transformation

infrared (FTIR) spectra were recorded on a Nicolet 8700 FTIR

spectrometer in the transmission mode with a spectral reso-

lution of 4 cm�1 and 32 scans.

Fig. 1 e FTIR spectra of (a) raw MWNTs, (b) MWNTs

oxidized with diluted acid at 110 �C, (c) MWNTs oxidized

with concentrated acid at 140 �C, (d) sample in (c) heated at

800 �C in Ar.

2.3. Catalytic reaction testing

The catalytic performance of CO PROX reaction was tested

with a fixed-bed reactor at atmospheric pressure within the

reaction temperature region from room temperature to 200 �C.The comparison between different catalysts was made on the

basis of the same catalyst weight (30 mg) in all PROX reaction

except the stability test using 60 mg of catalyst. The catalyst

was loaded into a vertical tubular quartz reactor (4 mm i.d.)

and kept in the isothermal center part of the reactor between

two flocks of quartz wool. The reaction temperature was

controlled within 0.1 �C at the set point using a micropro-

cessor controller (Yudian AI-708P). The feed gas stream was

typically composed of 1.00% CO, 1.25% O2, 50% H2 and

balanced with N2 at a total flow rate of 100 ml min�1 regulated

with a set of mass flow controllers (Sevenstar D07-12A/ZM).

The weight hourly space velocity (WHSV) of total gaseous

reactant was as high as 200,000 ml h�1 g�1. In case of need,

certain amounts of CO2 and/or H2O were added to simulate

the real condition of the hydrogen-rich stream. The reactants

and products were analyzed by the gas chromatographs

(Shimadzu GC-14C and Fuli 9750) equipped with TCD and two

packed columns (Molecular sieve 5A and Carbon molecular

sieve) for the separation of CO, O2, H2, CH4 and CO2 respec-

tively. Concentrations of the monitored gas components were

calculated on the basis of the calibration of standard gas

mixture in the known quantities.

3. Results and discussion

3.1. Characterization of MWNTs supports

Nitric acid treatments are the most common technique

among those applied for the surface oxidation of carbon

nanotubes to introduce oxygen-containing functional groups

on MWNTs sidewalls and remove the impurities [17,30,31].

The types of these groups were identified from FTIR spectra as

shown in Fig. 1. The 1583 cm�1 peak assigned to aromatic ring

vibration [27,32e34] is present for all samples. In comparison

with raw MWNTs, those treated with nitric acid indicate the

peaks at 1210 and 1716 cm�1 associated with CeO stretching

vibrations in ethers and phenols [27,34] and C]O stretching

vibrations in carboxyls and carbonyls [27,32,35], respectively.

These oxygen-containing functional groups exhibit much

stronger adsorption peaks for MWNTs treated with concen-

trated HNO3 at 140 �C than that with diluted acid at 110 �C, andmostly removed due to thermal decomposition when

suffering from further heat treatment at 800 �C in a flow of

argon.

The presence of oxygen-containing functional groups was

confirmed and quantified by XPS analysis. Fig. 2 compares the

C 1s and O 1s core-level spectra for MWNTs oxidized with

concentrated and diluted acid. The curves fitting of C 1s

spectra suggests five discernible components related to the

existence of carbon in different chemical environments

[27,36,37], including CeC bonds of graphite (284.6 eV), CeO

bonds of phenol or ether groups (285.3 eV), C]O bonds of

carbonyls or aldehydes groups (286.9 eV), O]CeO bonds of

carboxyls or carboxylic anhydrides and esters groups

(289.1 eV) and pep* bonds of aromatic carbons characteristic

shake-up at 290.5 eV, respectively. Accordingly, two compo-

nents can be judged from O 1s spectra indicative of the pres-

ence of oxygen species in relation to C]O and CeO bonds

around 531.5 and 533.3 eV [37], respectively. The surface

chemical compositions of MWNTs under different treatment

conditions are summarized in Table 1. For the typical ratio of

acid solution amount per gram MWNTs at 50 ml g�1, MWNTs

treated with 68% HNO3 at 140 �C contain more surface oxygen

species than that with 37% HNO3 at 110 �C, which is in

agreement with the FTIR results.

Raman spectrawere also taken to investigate the defects in

MWNTs produced after varied treatments. As shown in Fig. 3,

all samples feature two characteristic vibration modes, one

peak of D-band around 1350 cm�1 caused from the disordered

structures in carbonmaterials and the other of G-band around

1590 cm�1 produced from the graphite carbon with high de-

gree of symmetry and order [27,38e40]. The integrated in-

tensity ratio of the D and G-band peaks (ID/IG) is also listed to

compare the relative degree of defects in the treated MWNTs.

A slightly smaller ID/IG value is obtained for the MWNTs

treated with diluted acid than the raw MWNTs, which sug-

gests that the oxidation with diluted acid may hardly produce

more defects since this treatment is mainly efficient at

removing the residual impurities and amorphous carbon. For

MWNTs oxidized with the concentrated acid, the ratio of ID/IGis apparently promoted because this harsh situation is



Fig. 2 e XPS spectra of C 1s for MWNTs oxidized with (a) concentrated and (b) diluted acid, and O 1s for MWNTs oxidized

with (c) concentrated and (d) diluted acid.


effective in producing more defect sites as well as functional

groups onMWNTs. It seems that there is a positive correlation

between the disordered structures from the ratio of ID/IG and

the oxygen-containing functional groups on MWNTs from the

XPS analysis. The increase in ID/IG ratio with the multiplied

acid amount and treatment duration indicates the more

defect sites created. When heated at 800 �C in a flow of argon,

the even higher ID/IG ratio implies new defects created by heat

treatment destroying a few carbon layers of MWNTs [27],

Table 1 e Surface chemical composition of MWNTs under diffe

Treatment condition [C] (at.%) [O] (at.%

Raw MWNTs 98.35 1.65

37% HNO3 (50 ml g�1) at 110 �C for 5 h 94.38 5.62

68% HNO3 (50 ml g�1) at 140 �C for 7 h 90.66 9.34

Calcined at 800 �C for 2 h in N2 97.07 2.93

though the oxygen-containing functional groups mostly

decompose during this treatment.

3.2. Characterization of Ru/MWNTs catalysts

Fig. 4 shows the TEM images of Ru/MWNTs catalysts to verify

the location of Ru particles. The image of catalyst 5%-Ru/o-

MWNTs-out (Fig. 4a) indicates that most of Ru particles are

deposited on the outer surfaces of MWNTs with high

rent treatment conditions by XPS analysis.

) Relative concentration (%)

CeC CeO C]O O]CeO pep*

66.9 17.2 6.7 4.0 5.2

62.5 18.7 7.9 5.3 5.6

60.1 19.3 8.3 7.1 5.2

66.3 14.6 7.8 4.8 6.4



Fig. 3 e Raman spectra of (a) raw MWNTs, (b) MWNTs

oxidized with diluted acid at 110 �C, (c) MWNTs oxidized

with concentrated acid at 140 �C, (d) sample in (c) heated at

800 �C in Ar.


dispersion. Themean Ru particle size, obtained on the basis of

measuring about 300 particles, is around 1.4 nm. For the

catalyst 5%-Ru/o-MWNTs-in (Fig. 4b), the almost identical Ru

particle size is obtained, but in contrast it features the uniform

Ru nanoparticles mainly confined inside the channels and

along the inner wall of MWNTs. When the catalyst was

calcined at 600 �C before reduction (Fig. 4c), the Ru particles is

irregularly enlarged with the mean particle size of about

2.6 nm. Similar situation arises for the MWNTs support

calcined at 800 �C prior to the introduction of Ru, and the

mean Ru particle size increased to 1.9 nm (Fig. 4d). It is also

noticed that themean particles sizes slightly increasewith the

Ru content in the case of Ru catalysts confined insideMWNTs,

as 1.3 and 1.9 nm observed for 2.5%-Ru/o-MWNTs-in and 7%-

Ru/o-MWNTs-in (Fig. 4e and f), respectively.

The XRD patterns of MWNTs and the supported Ru cata-

lysts are compared in Fig. 5. It is discerned that there are

mainly three diffraction peaks corresponding to the graphite

(002), (100) and (004) plane reflections of MWNTs (JCPDS 41-

1487). However, the XRD patterns of all the supported Ru

catalysts, almost identical to that of MWNTs support, are in

the absence of any diffraction peak related to either metallic

Ru or ruthenium oxides. This result is in line with the pres-

ence of small Ru nanoparticles with the mean particles size

less than 2 nm thatmay be beyond the detectable limitation of

XRD, as previously perceived in the case of Ru supported on

titania, ceria, alumina and carbon nanotubes support

[24,41,42].

The thermal behavior was investigated by Thermogravi-

metric analysis in order to evaluate the stability of Ru/o-

MWNTs-in catalysts. Fig. 6a shows the evolution of the puri-

fied MWNTs during calcination in air up to 800 �C, exhibitingan apparent weight loss due to the oxidation of carbon around

600 �C. When Ru is loaded, the MWNTs support can be more

and more readily gasified with increasing Ru content. For 5%-

Ru/o-MWNTs-in catalyst (Fig. 6d), the temperature where the

apparent decrease in mass occurs is even blow 400 �C. The

difference in the thermal stability of MWNTs may be attrib-

uted to the presence of Ru in the catalysts promoting the

gasification of carbon at the lower temperature [43], which

also suggests a strong interaction between the Ru nano-

particles and MWNTs support.

The H2-TPR profiles of 5%-Ru/o-MWNTs-in and 5%-Ru/o-

MWNTs-out catalysts are plotted in Fig. 7 to compare their

redox properties. The as-dried samples mainly exhibit two H2

consumption peaks (Fig. 7b and c) from 50 up to about 500 �Cas reported elsewhere [23,24]. One peak locating around 293 �Cwith shoulder at lower temperature may come from the

reduction of Ru(III) species from RuCl3 precursor to metallic

Ru, which is almost identical for both samples. The other

starting above 400 �C is associated with the reduction of

oxygen-containing functional groups on MWNTs surface as

evidenced from the reduction of bare oxidized MWNTs

(Fig. 7a). It seems that the reduction of the functional groups

on MWNTs surface is more facilitated for 5%-Ru/o-MWNTs-

out catalyst at lower temperature, which suggests that the

outside Ru particles may promote the reduction of functional

groups in comparison with Ru particles inside MWNTs. When

the reduced samples were reoxidized with 5% O2 in Ar at

250 �C and then subjected to the second running of TPR (Fig. 7d

and e), the reduction peak of oxygen-containing functional

groups on MWNTs is almost absent, but in contrast only a

symmetric peak corresponding to the reduction of ruthenium

oxides is present. It is noticed that the reduction of RuOx takes

place at 171 �C for 5%-Ru/o-MWNTs-in, whereas it occurs at a

higher temperature around 183 �C for 5%-Ru/o-MWNTs-out,

which demonstrates that the inside Ru species are easier to

reduce compared to the outside counterpart. This observation

is well consistent with the previous reports on the trend

toward facilitated reduction of the metal oxides confined in-

side CNTs channels [23,24,44].

3.3. Catalytic performance

3.3.1. Influence of Ru loading and MWNTs diameterFor the typical support MWNTs with diameter of 8e15 nm and

treated with concentrated acid at 140 �C, the catalytic per-

formances of Ru catalysts confined inside MWNTs for CO-

PROX reaction are shown in Fig. 8 to investigate the influ-

ence of Ru loading. All the catalysts are active for the CO-PROX

reaction and can achieve complete CO conversion under the

space velocity as high as 200,000ml h�1 g�1. The conversion of

CO (Fig. 8a) increases apparently with the content of Ru from 1

to 5%, which exhibits the decrease in the lowest temperature

for complete CO conversion from 150 to 80 �C. When further

increasing Ru content to 7%, the activity is almost identical to

that with 5% Ru loading before 130 �C, however the conversion

of CO begins descending more rapidly at higher reaction

temperature. This reduction in catalytic activity with further

increasing loading of Ru may arise from the slightly increased

mean sizes of Ru particles. The O2 selectivity toward CO2

(Fig. 8b) can attain nearly 100% at low CO conversion but

apparently decrease with the rapidly increasing CO conver-

sion. When complete CO conversion is reached, all the cata-

lysts exhibit the similar selectivity around 40% due to the full

O2 consumption with the excess H2 according the present

reactant composition, which is commonly observed for Ru-



Fig. 4 e TEM images for (a) 5%-Ru/o-MWNTs-out, (b) 5%-Ru/o-MWNTs-in, (c) 5%-Ru/o-MWNTs-in heated at 600 �C in Ar

before reduction, (d) 5%-Ru/o-MWNTs-in with MWNTs support heated at 800 �C in Ar, (e) 2.5%-Ru/o-MWNTs-in, (f) 7%-Ru/o-

MWNTs-in. (The particle size distribution of Ru is overlaid.)


based catalysts in PROX reaction [11,14,24]. It suggests that the

optimized loading of Ru for the catalysts confined inside

MWNTs is about 5 wt.%, as previous works on Ru-based cat-

alysts for selective removal of CO in a H2-rich stream [24,45],

which is adopted for the following investigations.

The influence of MWNTs diameter on the catalytic per-

formances for the CO-PROX reaction over 5%-Ru/o-MWNTs-

in catalysts is shown in Fig. 9. It can be observed that the

catalysts with MWNTs outer diameter present comparable

activity despite the great difference in the surface area of

MWNTs supports (about 500 vs. 40 m2 g�1). Similar perfor-

mances are also obtained for the catalysts with MWNTs of

8e15 and 30e50 nm o.d. (SSA about 200 vs. 60 m2 g�1), both

of which are more active than the former pairs by

exhibiting the decrease in the lowest temperature for

complete CO conversion from 90 to 80 �C. This result may

originate from the confinement effects in the presence of

different diameter MWNTs [19] as discussed later. Since the

catalyst with MWNTs of 8e15 nm displays the highest ac-

tivity, which is chosen as the typical support for the sub-

sequent studies.

3.3.2. Influence of different pretreatment conditionsFig. 10 indicates the influence of acid treatments of MWNTs

support on the catalytic performances for CO-PROX reaction

over 5%-Ru/o-MWNTs-in catalysts.WhenMWNTs treatedwith

68% HNO3 at 140 �C, the optimized activity is observed for the

catalyst with MWNTs obtained under the typical conditions of



Fig. 5 e XRD patterns of (a) MWNTs oxidized with

concentrated acid at 140 �C and Ru-based catalysts (b) 2.5%-

Ru/o-MWNTs-in, (c) 5%-Ru/o-MWNTs-in, (d) 7%-Ru/o-

MWNTs-in and (e) 5%-Ru/o-MWNTs-out.

Fig. 7 e H2-TPR profiles of (a) MWNTs oxidized with

concentrated acid at 140 �C, (b) as-dried 5%-Ru/o-MWNTs-

in, (c) as-dried 5%-Ru/o-MWNTs-out, (d) reduced 5%-Ru/o-

MWNTs-in reoxidized with 5% O2 in Ar at 250 �C and (e)

reduced 5%-Ru/o-MWNTs-out reoxidized with 5% O2 in Ar

at 250 �C.


the ratio of acid solution amount per gramMWNTs at 50ml g�1

and treatmentduration for 7h.Thevariedacidsolutionamount

and theprolonged treatment durationmay lead to thedecrease

inactivity to someextent.Thedifference inactivitymight come

from different amount and distribution of functional groups as

well as the defects generated on MWNTs surface, which may

result in the different electronic interaction between Ru parti-

cles and MWNTs for MWNTs-confined Ru catalysts.

The influence of reduction temperature on the catalytic

activity for CO-PROX reaction over 5%-Ru/o-MWNTs-in cata-

lysts is shown in Fig. 11. The highest performance is observed

for the catalyst under reduction at 400 �C, which is adopted for

other tests in this study when no alternative is specified. The

lower reduction temperature at 300 �C result in an inferior

activity, an increase in the lowest temperature for complete

CO conversion from 80 to 90 �C, which may be due to the

Fig. 6 e Thermogravimetric analysis of (a) MWNTs

oxidized with concentrated acid at 140 �C, and (b) 1%-Ru/o-

MWNTs-in, (c) 2.5%-Ru/o-MWNTs-in and 5%-Ru/o-

MWNTs-in catalysts.

Fig. 8 e Temperature dependence of (a) CO conversion and

(b) O2 selectivity for CO-PROX over Ru/o-MWNTs-in

catalysts with different Ru loadings. (Reactants: 1% CO,

1.25% O2, 50% H2 and N2 balanced,

WHSV [ 200,000 ml hL1 gL1.)



Fig. 9 e Temperature dependence of CO conversion for CO-

PROX over 5%-Ru/o-MWNTs-in catalysts with different

MWNTs diameter. (Outer diameters indicated in brackets

of legend. Reactants: 1% CO, 1.25% O2, 50% H2 and N2

balanced, WHSV [ 200,000 ml hL1 gL1.)

Fig. 11 e Temperature dependence of CO conversion for

CO-PROX over 5%-Ru/o-MWNTs-in catalysts with different

reduction temperature. (Reactants: 1% CO, 1.25% O2, 50% H2

and N2 balanced, WHSV [ 200,000 ml hL1 gL1.)


chlorine residue in the catalyst as evidenced by TPR profile

(Fig. 7b). On the other hand, the higher reduction temperature

at 500 �C leads to the slightly decreasing activity above 120 �C,which may be related to the somewhat agglomerated Ru

particles as observed by TEM.

The influence of heating processes in a flow of Ar at

different temperature before reduction on the catalytic ac-

tivity for CO-PROX reaction over 5%-Ru/o-MWNTs-in catalysts

is also examined as shown in Fig. 12. In comparison with the

as-dried sample without calcination before reduction, the

catalysts heated at 400 and 500 �C exhibit less activity by

shifting the lowest temperature for complete CO conversion

from 80 to 90 �C. When the catalyst is heated at 600 �C, the CO

conversion can not reach a maximum of 100% and decreases

drastically at higher reaction temperature, which is correlated


CO-PROX over 5%-Ru/o-MWNTs-in catalysts with MWNTs

treated with different amount of acid solution and

treatment duration. (Reactants: 1% CO, 1.25% O2, 50% H2

and N2 balanced, WHSV [ 200,000 ml hL1 gL1.)

with the apparent agglomeration of Ru particles as observed

from TEM image (Fig. 4c). This result suggests the detrimental

effect of calcination before reduction on the activity for CO-

PROX reaction over Ru/MWNTs catalysts.

3.3.3. Confinement effectsFig. 13 compares the catalytic activity for CO-PROX reaction of

Ru catalysts confined in MWNTs and supported on several

different carbon materials. It is manifested that the optimal

performance observed for 5%-Ru/o-MWNTs-in catalyst fea-

tures the complete CO removal at the widest temperature

ranges from 80 to 150 �C. However, 5%-Ru/o-MWNTs-out

catalyst can only afford CO conversion of 14.7% at 80 �C and

the complete CO conversion at 100 �C. The comparable ac-

tivity is obtained as well for 5%-Ru/c-MWNTs-out catalyst,

prepared over MWNTs with closed ends, which gives CO


CO-PROX over 5%-Ru/o-MWNTs-in catalysts heated at

different temperature in Ar before reduction. (Reactants:

1% CO, 1.25% O2, 50% H2 and N2 balanced,

WHSV [ 200,000 ml hL1 gL1.)




CO-PROX over Ru catalysts with different carbon materials.

(Reactants: 1% CO, 1.25% O2, 50% H2 and N2 balanced,

WHSV [ 200,000 ml hL1 gL1.)


conversion of 13.3% at 80 �C and the complete CO removal at

the temperature ranges from 100 to 140 �C. This difference in

activity is also identified between Ru catalysts confined inside

MWNTs and the counterparts supported on outside of

MWNTs with varied diameters (shown as Supplementary

data). When the Ru nanoparticles are supported on other

carbon materials, such as carbon nanoribbon or graphene

derived from the same raw MWNTs, they show rather lower

catalytic performance under the identical CO-PROX reaction

conditions, as previously observed for Ru catalysts supported

on activated carbon and graphite [24]. These results suggest

the distinct confinement effects of Ru catalysts located inside

MWNTs on the removal of CO in a H2-rich stream.

The confinement effects of carbon nanotubes on catalysis

may be understood mainly in terms of the interactions be-

tween CNTs with the confined materials and between CNTs

with the reactants involved [19,20]. With respect to the pre-

sent MWNTs-confined Ru catalysts for CO-PROX reaction,

these beneficial effects may originate from the electronic

interaction of the confined Ru nanoparticles withMWNTs, the

space restriction of Ru particles in the nanosized channels of

MWNTs and the enrichment of reactants inside MWNTs.

The curvature of CNTs surface results in an electric po-

tential difference by shifting the p electron density from the

concave inner to the convex outer graphene layers. When

metallic nanoparticles is deposited on CNTs, the modified

electronic structure of nanomaterials can influence the redox

properties of metal/metal oxide nanoparticles and thus the

catalytic activity of reactions involving charge transfer be-

tween reactants and catalysts [19,20]. Theoretical studies

indicated the lower electron density of Ru confined inside

CNTs than Ru located outside by the inside Ru donating more

electrons to the CNTs concave surface, while experimental

results congruously suggested weaker adsorption sites for CO

on Ru confined inside CNTs than Ru located outside [23]. The

H2-TPR results in Fig. 7 revealed the enhanced reducibility of

the oxidized 5%-Ru/o-MWNTs-in catalysts due to the stronger

interaction between ruthenium oxides and MWNTs inner

surface, which could imply the facilitated activation of oxygen

by the formation of intermediate RuOx species during reac-

tion. This electronic interaction and the resultantly promoted

redox property of the confined Ru nanoparticles with MWNTs

may give a reason for the superior activity of 5%-Ru/o-

MWNTs-in catalyst for CO-PROX reaction to Ru catalysts

supported on outside of MWNTs and other carbon materials

as observed in Fig. 13. The strength of electronic interaction

may vary with the size of CNTs, that is, the smaller the

diameter of CNTs, the stronger the intriguing curvature effect

[19,20]. The effect of MWNTs diameter (Fig. 9) demonstrated

that the Ru catalysts with MWNTs of 8e15 and 30e50 nm o.d.

are more active than that with larger MWNTs outer diameter

> 50 nm as expected. However, the activity decreased for the

catalyst with the rather smaller MWNTs outer diameter

<8 nm, which could be explained by the low filling efficiency

of Ru into MWNTs with small diameter because of the size

restriction on the capillarity and the wetting behavior [26,46].

The space restriction can prevent the aggregation of Ru

particles in the nanosized channels of MWNTs. TEM images

(Fig. 4c) shown that the calcination at 600 �C made the Ru

particles outside the channels ofMWNTs aggregate noticeably

as large as about 8 nm, in comparison to themean particle size

around 2.6 nm for Ru particles inside the channels of MWNTs.

This was unfavorable to the activity for CO-PROX reaction

over Ru/MWNTs catalysts (Fig. 12), since the aggregation of

nanoparticles often leads to the deactivation of catalysts.

Meanwhile, it was noted that the surface oxygen-containing

functional groups on MWNTs played the important roles in

regulating the size of supported nanoparticles and thus the

catalytic performance [21,22,47,48]. When the MWNTs sup-

port was pretreated by heating at 800 �C to decompose the

oxygen-containing functional groups, TEM images (Fig. 4d)

indicated the increase in mean Ru particle size to 1.9 nm,

corresponding to the decrease in the activity for CO-PROX

reaction (shown as Supplementary data). Additionally, the

influence of acid treatments (Fig. 10) shown that the opti-

mized activity was observed for the catalyst over MWNTs

treated with the medium amount of acid solution at 50 ml g�1

and duration for 7 h, suggesting that themedium strength and

amount of functional groups favored the activity for CO-PROX

reaction over MWNTs-confined Ru catalysts.

The interactions betweenMWNTswith the reactants inCO-

PROX reaction manly involved the enrichment of CO and O2

rather than H2 inside carbon nanotubes. The theoretical study

shown that CO could be preferentially enriched inside CNTs to

cause a notably higher CO/H2 ratio in comparison with that on

the outside of CNTs [49]. The simulated adsorption isothermof

O2 also revealed close similarity to that of CO in CNTs [50]. The

enrichment ofCOandO2practically enhanced the ratios ofCO/

H2 and O2/H2 in the channels of CNTs, which could provide an

effective approach to increase the probability of CO reacting

with O2 by confining the reaction inside CNTs. This effect may

also favor the advanced activity for CO-PROX reaction over 5%-

Ru/o-MWNTs-in catalyst (Fig. 13).

3.3.4. Influence of CO2 and H2OThe H2-rich stream produced by the fuel processing of hy-

drocarbons generally contains CO2 and H2O under realistic

conditions. Therefore, it is essential to expose the influence of




CO-PROX over 5%-Ru/o-MWNTs-in and 5%-Ru/o-MWNTs-

out catalysts with (A) CO2 or (B) H2O in the feed stream.

(Reactants: 1% CO, 1.25% O2, 50% H2, 15% CO2 or 10% H2O,

N2 balanced, WHSV [ 200,000 ml hL1 gL1.)

Fig. 15 e CO conversion for CO-PROX over 5%-Ru/o-

MWNTs-in with reaction time on stream. (Reaction

temperature: 80 �C. Reactants: 1% CO, 1.25% O2, 15% CO2,

10% H2O, 50% H2, N2 balanced,

WHSV [ 100,000 ml hL1 gL1.)


these components on the activity and stability of the MWNTs-

confined Ru catalysts for the CO-PROX reaction.

Fig. 14A presents the effect of adding 15% CO2 into the feed

stream on the activity of Ru catalysts inside and outside

MWNTs channels. It is noticed that the light-off behavior is

almost identical for the CO-PROX reaction in the presence and

absence of CO2 over 5%-Ru/o-MWNTs-in catalyst, although

the complete CO conversion is attained at 80e120 �C in the

CO2 containing stream, inferior to 80e150 �C in the CO2-free

stream. With respect to 5%-Ru/o-MWNTs-out catalyst, the

addition of CO2 in the reaction mixture not only shifts the

lowest temperature for complete CO conversion from 100 to

110 �C, but also leads to the decrease in the temperature

window of complete CO conversion from 100e140 �C to

110e130 �C. The result suggests that the presence of CO2 has

slight negative effect on the catalytic performance of both Ru

catalysts inside and outside MWNTs channels. However, the

Ru/o-MWNTs-in catalyst exhibits the favorable resistance to

CO2, especially at lower reaction temperature region below

120 �C, which is crucial for the PROX reactor compatible with a

PEMFC system. On the other hand, the presence of 10% H2O in

the feed stream (Fig. 14B) virtually has no effect on the activity

for CO-PROX reaction over both 5%-Ru/o-MWNTs-in and 5%-

Ru/o-MWNTs-out catalysts, except for slightly fluctuating in

the low CO conversion region.

The influence of co-adding 15% CO2 and 10% H2O in the

feed stream is examined along with the stability of CO-PROX

reaction over 5%-Ru/o-MWNTs-in catalyst. As presented in

Fig. 15, the complete CO conversion can be sustained for

longer than 90 h at 80 �C, a typical operating temperature of

PEMFC system [4]. Although the catalytic performance grad-

ually drops with the further prolonged reaction time possibly

due to the surface coverage of carbonate species and the

partial oxidation of Ru particles, the CO conversion still re-

mains beyond 97%, and the deactivated catalyst could be

readily regenerated by reduction in a hydrogen flow.

4. Conclusions

The MWNTs-confined Ru catalysts have been prepared by

depositing Ru nanoparticles inside MWNTs channels via

ultrasonication-aided capillarity action, which has been veri-

fied by versatile characterization techniques. The results of

FTIR, XPS and Raman spectra shown the evolution of surface

functional groups and defects in MWNTs produced under

varied treatments, which was considered to have effects on

the preparation of theMWNTs-confined Ru catalysts. TEM and

XRD results revealed the high dispersion of Ru nanoparticles

mainly inside MWNTs channels with the mean size of less

than 2 nm,which could induce the strong interaction between

Ru particles andMWNTs as indicated by themodified thermal

and redox properties from TGA and H2-TPR results. The

confinement of Ru particles inside MWNTs channels

enhanced the catalytic performance in CO-PROX reaction

when compared with those supported on MWNTs external

surface and other carbon materials. The optimal activity was




obtained for the catalyst with 5 wt.% Ru loading confined in

MWNTs with diameter of 8e15 nm, attaining the complete CO

conversion in the temperature range of 80e150 �C under the

space velocity as high as 200,000 ml h�1 g�1. The influence of

different pretreatment conditions suggested that the catalytic

activity was favored by the medium strength and amount of

functional groups on MWNTs and smaller Ru particles. The

confinement effects were considered to be beneficial for the

activity involving the interactions between MWNTs with the

confined Ru nanoparticles and with the reactants of CO-PROX

reaction, which could promoted the activation and enrich-

ment of CO and O2 in the MWNTs channels. The MWNTs-

confined Ru catalyst could also exhibited the effective resis-

tance to CO2 and water vapor, which afforded the favorable

stability and regeneration capability in the simulated refor-

matted gasmixture at a typical PEMFC operating temperature.

Acknowledgment

The authors are grateful for the financial supports from the

National Basic Research Program of China (Nos.

2010CB923300, 2013CB933104), Fundamental Research Funds

for the Central Universities (No. WK2060030010) and National

Natural Science Foundation of China (No. J1030412).

Appendix A. Supplementary data

Supplementary data related to this article can be found at

http://dx.doi.org/10.1016/j.ijhydene.2013.04.070.

r e f e r e n c e s

[1] Winter CJ. The hydrogen energy economy: an industrytycoon in sight? Int J Hydrogen Energy 2011;36:12653e4.

[2] Holladay JD, Hu J, King DL, Wang Y. An overview of hydrogenproduction technologies. Catal Today 2009;139:244e60.

[3] Farrauto RJ, Liu Y, Ruettinger W, Ilinich O, Shore L, Giroux T.Precious metal catalysts supported on ceramic and metalmonolithic structures for the hydrogen economy. Catal RevSci Eng 2007;49:141e96.

[4] Yuan XZ, Li H, Zhang SS, Martin J, Wang HJ. A review ofpolymer electrolyte membrane fuel cell durability testprotocols. J Power Sources 2011;196:9107e16.

[5] Park ED, Lee D, Lee HC. Recent progress in selective COremoval in a H2-rich stream. Catal Today 2009;139:280e90.

[6] Echigo M, Tabata T. A study of CO removal on an activatedRu catalyst for polymer electrolyte fuel cell applications.Appl Catal A 2003;251:157e66.

[7] Echigo M, Tabata T. Development of novel Ru catalyst ofpreferential CO oxidation for residential polymer electrolytefuel cell systems. Catal Today 2004;90:269e75.

[8] Xu GW, Zhang ZG. Preferential CO oxidation on Ru/Al2O3

catalyst: an investigation by considering the simultaneouslyinvolved methanation. J Power Sources 2006;157:64e77.

[9] Kim YH, Park ED, Lee HC, Lee D. Selective CO removal in a H2-rich stream over supported Ru catalysts for the polymerelectrolyte membrane fuel cell (PEMFC). Appl Catal A2009;366:363e9.

[10] Perkas N, Teo J, Shen SC, Wang Z, Highfield J, Zhong ZY, et al.Supported Ru catalysts prepared by two sonication-assistedmethods for preferential oxidation of CO in H2. Phys ChemChem Phys 2011;13:15690e8.

[11] Snytnikov PV, Sobyanin VA, Belyaev VD, Tsyrulnikov PG,Shitova NB, Shlyapin DA. Selective oxidation of carbonmonoxide in excess hydrogen over Pt-, Ru- and Pd-supportedcatalysts. Appl Catal A 2003;239:149e56.

[12] Echigo M, Shinke N, Takami S, Tabata T. Performance of anatural gas fuel processor for residential PEFC system using anovel CO preferential oxidation catalyst. J Power Sources2004;132:29e35.

[13] Echigo M, Shinke N, Takami S, Higashiguchi S, Hirai K,Tabata T. Development of residential PEFC cogenerationsystems: Ru catalyst for CO preferential oxidation inreformed gas. Catal Today 2003;84:209e15.

[14] Rosso I, Antonini M, Galletti C, Saracco G, Specchia V.Selective CO-oxidation over Ru-based catalysts in H2-rich gasfor fuel cell applications. Top Catal 2004;30e31:475e80.

[15] Rodriguez-Reinoso F. The role of carbon materials inheterogeneous catalysis. Carbon 1998;36:159e75.

[16] Auer E, Freund A, Pietsch J, Tacke T. Carbons as supports forindustrial precious metal catalysts. Appl Catal A1998;173:259e71.

[17] Serp P, Corrias M, Kalck P. Carbon nanotubes and nanofibersin catalysis. Appl Catal A 2003;253:337e58.

[18] Serp P, Figueiredo JL. Carbon materials for catalysis.Hoboken: John Wiley & Sons; 2009579.

[19] Pan XL, Bao XH. The effects of confinement inside carbonnanotubes on catalysis. Acc Chem Res 2011;44:553e62.

[20] Serp P, Castillejos E. Catalysis in carbon nanotubes.ChemCatChem 2010;2:41e7.

[21] Kang JC, Zhang SL, Zhang QH, Wang Y. Rutheniumnanoparticles supported on carbon nanotubes as efficientcatalysts for selective conversion of synthesis gas to dieselfuel. Angew Chem Int Ed Engl 2009;48:2565e8.

[22] Deng WP, Liu M, Tan XS, Zhang QH, Wang Y. Conversion ofcellobiose into sorbitol in neutral water medium over carbonnanotube-supported ruthenium catalysts. J Catal2010;271:22e32.

[23] Guo SJ, Pan XL, Gao HL, Yang ZQ, Zhao JJ, Bao XH. Probing theelectronic effect of carbon nanotubes in catalysis: NH3

synthesis with Ru nanoparticles. Chem Eur J2010;16:5379e84.

[24] Li BD, Wang C, Yi GQ, Lin HQ, Yuan YZ. Enhancedperformance of Ru nanoparticles confined in carbonnanotubes for CO preferential oxidation in a H2-rich stream.Catal Today 2011;164:74e9.

[25] Pan XL, Fan ZL, Chen W, Ding YJ, Luo HY, Bao XH. Enhancedethanol production inside carbon-nanotube reactorscontaining catalytic particles. Nat Mater 2007;6:507e11.

[26] Wang CF, Guo SJ, Pan XL, ChenW, Bao XH. Tailored cutting ofcarbon nanotubes and controlled dispersion of metalnanoparticles inside their channels. J Mater Chem2008;18:5782e6.

[27] Wang L, Ge L, Rufford TE, Chen JL, Zhou W, Zhu ZH, et al. Acomparison study of catalytic oxidation and acid oxidationto prepare carbon nanotubes for filling with Runanoparticles. Carbon 2011;49:2022e32.

[28] Higginbotham AL, Kosynkin DV, Sinitskii A, Sun ZZ, Tour JM.Lower-defect graphene oxide nanoribbons from multiwalledcarbon nanotubes. ACS Nano 2010;4:2059e69.

[29] Gao W, Alemany LB, Ci LJ, Ajayan PM. New insights into thestructure and reduction of graphite oxide. Nat Chem2009;1:403e8.

[30] Datsyuk V, Kalyva M, Papagelis K, Parthenios J, Tasis D,Siokou A, et al. Chemical oxidation of multiwalled carbonnanotubes. Carbon 2008;46:833e40.





[31] Karousis N, Tagmatarchis N, Tasis D. Current progress on thechemical modification of carbon nanotubes. Chem Rev2010;110:5366e97.

[32] Ros TG, van Dillen AJ, Geus JW, Koningsberger DC. Surfaceoxidation of carbon nanofibres. Chem Eur J 2002;8:1151e62.

[33] Liu YQ, Gao L. A study of the electrical properties of carbonnanotube-NiFe2O4 composites: effect of the surfacetreatment of the carbon nanotubes. Carbon 2005;43:47e52.

[34] Chen J, Zhu ZH, Ma Q, Li L, Rudolph V, Lu GQ. Effects of pre-treatment in air microwave plasma on the structure of CNTsand the activity of Ru/CNTs catalysts for ammoniadecomposition. Catal Today 2009;148:97e102.

[35] Shaffer MSP, Fan X, Windle AH. Dispersion and packing ofcarbon nanotubes. Carbon 1998;36:1603e12.

[36] Wepasnick KA, Smith BA, Bitter JL, Fairbrother DH. Chemicaland structural characterization of carbon nanotube surfaces.Anal Bioanal Chem 2010;396:1003e14.

[37] Kundu S, Wang YM, Xia W, Muhler M. Thermal stability andreducibility of oxygen-containing functional groups onmultiwalled carbon nanotube surfaces: a quantitative high-resolution XPS and TPD/TPR study. J Phys Chem C2008;112:16869e78.

[38] Rao AM, Richter E, Bandow S, Chase B, Eklund PC,Williams KA, et al. Diameter-selective Raman scatteringfrom vibrational modes in carbon nanotubes. Science1997;275:187e91.

[39] Ferrari AC, Robertson J. Interpretation of Raman spectra ofdisordered and amorphous carbon. Phys Rev B2000;61:14095e107.

[40] Liu YL, Pan CX, Wang JB. Raman spectra of carbon nanotubesand nanofibers prepared by ethanol flames. J Mater Sci2004;39:1091e4.

[41] Abe T, Tanizawa M, Watanabe K, Taguchi A. CO2

methanation property of Ru nanoparticle-loaded TiO2

prepared by a polygonal barrel-sputtering method. EnergyEnviron Sci 2009;2:315e21.

[42] Okal J, Zawadzki M, Krajczyk L. Light alkane oxidation overRu supported on ZnAl2O4, CeO2 and Al2O3. Catal Today2011;176:173e6.

[43] Wang WD, Serp P, Kalck P, Faria JL. Photocatalyticdegradation of phenol on MWNT and titania compositecatalysts prepared by a modified sol-gel method. Appl Catal B2005;56:305e12.

[44] Chen W, Fan Z, Pan X, Bao X. Effect of confinement in carbonnanotubes on the activity of Fischer-Tropsch iron catalyst. JAm Chem Soc 2008;130:9414e9.

[45] Panagiotopoulou P, Kondarides DI, Verykios XE. Selectivemethanation of CO over supported Ru catalysts. Appl Catal B2009;88:470e8.

[46] Ugarte D, Chatelain A, de Heer WA. Nanocapillarityand chemistry in carbon nanotubes. Science1996;274:1897e9.

[47] Zheng WQ, Zhang J, Zhu B, Blume R, Zhang YL, Schlichte K,et al. Structure-function correlations for Ru/CNT in thecatalytic decomposition of ammonia. ChemSusChem2010;3:226e30.

[48] Li C, Shao ZF, Pang M, Williams CT, Liang CH. Carbonnanotubes supported Pt catalysts for phenylacetylenehydrogenation: effects of oxygen containing surface groupson Pt dispersion and catalytic performance. Catal Today2012;186:69e75.

[49] Guan J, Pan X, Liu X, Bao X. Syngas segregation induced byconfinement in carbon nanotubes: a combined first-principles and Monte Carlo study. J Phys Chem C2009;113:21687e92.

[50] Kowalczyk P. Molecular insight into the high selectivity ofdouble-walled carbon nanotubes. Phys Chem Chem Phys2012;14:2784e90.



Documents

Preferential oxidation of CO in a H2-rich stream over multi-walled carbon nanotubes confined Ru catalysts