This journal is c The Royal Society of Chemistry 2011 Catal. Sci. Technol., 2011, 1, 1013–1023 1013

Cite this: Catal. Sci. Technol., 2011, 1, 1013–1023

Support effects on the structure and performance of ruthenium catalysts

for the Fischer–Tropsch synthesisw

Juan Marıa Gonzalez Carballo,aElisabetta Finocchio,

bSergio Garcıa,

a

Sergio Rojas,*aManuel Ojeda,

aGuido Busca

band Jose Luis Garcıa Fierro

a

Received 15th April 2011, Accepted 7th June 2011

DOI: 10.1039/c1cy00136a

The influence of support and metal precursor on Ru-based catalysts has been studied in the

Fischer–Tropsch synthesis (FTS) combining flow reactor and quasi in situ infrared spectroscopy

experiments. A series of supported ruthenium catalysts (3 wt.%) have been prepared using two

different TiO2 (P25, 20% rutile and 80% anatase; Hombifine, 100% anatase) and SiO2�Al2O3

(28% Al2O3) as supports and RuCl3�nH2O as metal precursor. The catalysts were labeled as

RuTi0.8, RuTi1 and RuSA respectively. Another catalyst (RuTi0.8N) has been synthesized with

TiO2�P25 and Ru(NO)(NO3)3. After thermal treatments in air at 723 K and hydrogen at 443 K,

ruthenium metal particles are agglomerated when pure anatase TiO2 and SiO2�Al2O3 are used as

supports, leading to low active catalysts. In contrast, and despite the lower specific surface area of

TiO2�P25 as compared to that of the other supports, well dispersed Ru particles are stabilized

on titania P25. Remarkably, electronic microscopy studies demonstrate that Ru is deposited

exclusively on the rutile phase of TiO2�P25. The catalytic performance shown by all these

catalysts in FTS reactions follows the order: RuTi0.8 4 RuTi0.8N 4 RuSA c RuTi1. The same

trend is observed during quasi in situ FTS experiments conducted in an infrared (IR) spectroscopy

cell. The FTIR spectra of TiO2�P25 supported samples show that both samples behave similarly

under the FTS reaction. This work shows that the structure of the support, rather than its specific

surface area or the Ru precursor, is the parameter that determines the dispersion of Ru particles,

hence their catalytic performance.

1. Introduction

Crude oil reserves are the main source of current transportation

fuels, but environmental and political concerns require the

exploration of other sources.1 The Fischer–Tropsch Synthesis

(FTS) is a feasible and interesting process to produce sulfur,

nitrogen and aromatics-free liquid fuels from syngas (H2/CO

mixtures). The renewed interest in the FTS is related to the fact

that this syngas can be obtained from renewable biomass.

Iron- and cobalt-based catalysts are the archetypal and most

studied systems for the FTS; however, ruthenium is known

to be the most active metal for this reaction. Indeed, higher

CO/H2 conversion levels, hydrocarbon productivities, and chain

growth probabilities can be achieved when using Ru-based

catalysts.1–3 This, in turn, results in a decreased cost of FTS

fuels production, a specific requirement necessary for a wider

implementation of the biomass-to-liquids (BTL) process.1

Furthermore, Ru catalysts can operate in the presence of high

concentrations of water and other oxygenate-containing atmo-

spheres, which is an important requisite to successfully convert

biomass-derived syngas into hydrocarbons.4,5

Previous studies have reported contradicting results concerning

the influence of the support and metal particle size (dispersion)

on the activity shown by supported ruthenium catalysts in the

CO hydrogenation reaction. For instance, King6 reported a

significant role of the support and the metal loading in the

performance of ruthenium catalysts in FTS. In contrast, Vannice

and Garten7 observed moderate modification of turnover frequen-

cies and molecular weight product distribution with alumina-

and silica-supported Ru catalysts, although a significant effect

on titania was found,8 which was attributed to the development of

strong metal support interactions.9 Tauster10 suggested that the

increased activity (about one order of magnitude) with Ni/TiO2

a Grupo de Energıa y Quımica Sostenibles (EQS),Instituto de Catalisis y Petroleoquımica, CSIC, C/ Marie Curie 2,Cantoblanco, 28049 Madrid, Spain. E-mail: srojas@icp.csic.es;Fax: +34 91 585 4760

bDipartimento di Ingegneria Chimica e di Processo,Universita di Genova, Fiera del Mare, Pad. D, I-16/129 Genova,Italy

w Electronic supplementary information (ESI) available: Fig. S1, digitaldiffraction patterns of the areas shown in Fig. 3 in the manuscript usedto identify the rutile and anatase phases in P25; Fig. S2, HR-TEMimages of RuTi0.8 illustrating the deposition of Ru on the rutile phaseof P25; Fig. S3, FTIR spectra of CO adsorbed on the reduced catalystsat room temperature; Fig. S4, FT-IR spectra of surface and gas phasespecies formed with RuTi0.8N and RuSA catalysts after contact withCO + H2 at 523 K. See DOI: 10.1039/c1cy00136a

CatalysisScience & Technology

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1014 Catal. Sci. Technol., 2011, 1, 1013–1023 This journal is c The Royal Society of Chemistry 2011

and Pt/TiO2 catalysts was straightforward related to the

formation of new active sites in the metal–TiOx contact

perimeter.

Krishna and Bell11 investigated Ru/TiO2 catalysts and

found that anatase-supported Ru clusters were more active

than when deposited on rutile. The different performance was

attributed, however, to the effect of chlorine impurities and

different metal dispersion levels. Iglesia et al.12 showed that the

catalytic performance of supported Ru in FTS is moderately

affected by the support (silica, alumina, titania) or metal

dispersion. Perez-Zurita et al.13 studied the production of

higher alcohols from syngas with Ru clusters deposited on

different reducible supports, concluding that the catalytic

performance depends on their reducibility. They found that

the most active catalyst was obtained when Ru was supported

on a non-reducible support, such as Al2O3. However, as the

reducibility of the support increased, the selectivity to higher

alcohols also increased.

Infrared spectroscopy (IR) is a powerful technique that may

provide valuable information about the chemical nature,

structure, and chemical bond strengths of adsorbed species

during catalytic reactions.14 This technique has been previously

used to determine different adsorbed species and mechanistic

aspects in FTS with Fe15–17 and Co18,19 catalysts. In the case

of Ru catalysts, Ekerdt and Bell20 used IR spectroscopy to

explore CO/H2 reactions with Ru/SiO2 catalysts and observed

that neither CO pressure nor H2/CO ratio affected the position

and intensity of the bands of CO adsorbed species. However,

the intensity of this band decreased upon increasing the

reaction temperature because of lower CO coverage. These

authors assigned the IR bands at 2950 cm�1 to C–H stretching

vibrations in methyl groups, whereas symmetric and asymmetric

C–H vibrations in methylene groups were detected at 2910 and

2845 cm�1, respectively. They claimed that these species were

not derived from reaction products adsorbed from the gas

phase, but from reaction intermediates. Other authors have

observed that, under FTS conditions, a minimum temperature

or concentration of methylene groups on Ru/TiO2 is required

to form methane. Moreover, these methylene moieties may be

transformed into inactive hydrocarbon species upon thermal

treatment or after significant chain growth.21 McQuire and

Rochester22 detected different types of surface hydrocarbon on

Ru/SiO2 catalysts, including ethoxy groups on silica originated

via reaction of ethane with silanol groups, and methylene

chains bonded to Ru atoms. They proposed that these hydro-

carbons act as reaction intermediates in methane formation steps.

In order to clarify whether Ru–support interactions and

metal precursor play a significant role on the structure and

catalytic behavior of Ru clusters in FTS, IR measurements

during CO adsorption and CO/H2 reactions at different

temperatures were conducted. These investigations have

been combined with catalytic studies with a laboratory-scale

fixed bed reactor. Thus, a series of Ru-supported catalysts

were prepared using different supports: TiO2 (anatase), TiO2

(anatase and rutile, P25), and SiO2�Al2O3. With the aim of

studying the effect of the Ru precursor, two samples over

TiO2�P25 were synthesized using RuCl3 and Ru(NO)(NO3)3.

We have observed that after treating the catalytic precursors

in air at 723 K, Ru clusters deposited on pure anatase or

SiO2�Al2O3 do agglomerate, leading to low active FTS catalysts.

In contrast, the rutile/anatase mixture (TiO2) anchors the

ruthenium species more strongly and prevents, at least partially,

ruthenium sintering. Consequently, more active catalysts are

formed in this latter case. In addition, we have observed that

Ru clusters size depends on the identity of the metal precursor,

being smaller when Ru(NO)(NO3)3 is used. The IR spectra of

adsorbed CO evidence a higher concentration of step-like

defects as the mean Ru particles size decreases, which is a

key factor to obtain active FTS catalysts.

2. Experimental

2.1. Preparation of catalysts

The supported Ru catalysts (3 wt.%) were prepared by the

incipient wetness impregnation technique. TiO2 (Degussa P25,

20% rutile, 80% anatase), TiO2 (Hombifine, 100% anatase)

and SiO2�Al2O3 (Davison Chemical, 28% Al2O3) were used as

supports. RuCl3�nH2O (40.49%, Johnson Matthey) and

Ru(NO)(NO3)3 (31.30%, Alfa Aesar) were used as ruthenium

precursors.

Experimentally, an aqueous solution of the desired amount of

the ruthenium salt precursor was added to the support. The solid

was then dried at room temperature overnight and thermally

treated in air (calcination) at 723 K (10 K min�1) for 3 h. This

temperature was selected from the thermogravimetric analysis

recorded in air (not shown). The solids thus obtained were

denoted as RuM, where M is Ti0.8 (TiO2 80% anatase), Ti1

(TiO2 100% anatase) or SA (SiO2�AlO3). The catalyst prepared

from nitrosyl nitrate of ruthenium was designated as RuTi0.8N.

2.2. Characterization techniques

Nitrogen adsorption–desorption isotherms were recorded at

77 K with a Micromeritics ASAP 2000 apparatus. The samples

were degassed at 413 K for 16 h prior to the determination of

the adsorption–desorption isotherm.

Temperature programmed reduction (TPR) experiments were

carried out in a Micromeritics TPR/TPD 2900 equipment. Experi-

mentally, 30 mg of sample (0.25–0.30 mm pellet size) were loaded

into a U-shaped quartz reactor and then thermally treated under

flowing He at 393 K for 30 min. The TPR profiles were recorded

by heating the sample from room temperature to 1173 K at a rate

of 10 Kmin�1 under a 10 vol.%H2/Ar flow. H2 consumption was

monitored with a thermal conductivity detector (TCD).

H2 chemisorption experiments were performed in a conven-

tional glass vacuum apparatus. About 50 mg of the catalytic

precursor were placed in a quartz cell, degassed at 523 K for

30 min, and then reduced in H2 (133 mbar) for 1 h and evacuated

at 543 K for 20 min. The chemisorption temperature was

conducted at 373 K23 and the equilibration time was 30 min.

After recording the first isotherm, the cell was outgassed for

15 min and a second isotherm was subsequently measured.

The amount of irreversibly adsorbed hydrogen was determined

by subtraction of these two isotherms. Ru dispersion (D) was

calculated as follows assuming a Ru/H2 adsorption stoichiometry

equals to 2:

D ¼ME

MT¼ 2HT

MTð1Þ

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where ME and MT are, respectively, the number of exposed

and total Ru atoms normalized per gram of sample, and HT is

the number of irreversible chemisorbed H2 molecules per

gram. Average Ru clusters size (d) can be calculated from

dispersion data according to:

d ¼ 6M

rsNADð2Þ

where M is the ruthenium atomic weight (101.07 g mol�1), r is

the ruthenium density (12.3 g cm�3), s is the atomic surface

area of Ru (0.0817 nm2 at.�1), and NA is Avogadro’s number

(6.022 � 1023 at. mol�1).24

X-Ray photoelectron spectra (XPS) were acquired with a

VG ESCALAB 200 R spectrometer in the pulse-count mode at

a pass energy of 50 eV using a Mg Ka (hu = 1253.6 eV) X-ray

source. Kinetic energies of photoelectrons were measured

using a hemispherical electron analyzer working in the constant

pass energy mode. The background pressure in the analysis

chamber was kept below 3 � 10�8 mbar during data acquisition.

The powder samples were pressed into stainless steel holders

and then mounted on a support rod placed in the pretreatment

chamber. The XPS data were signal averaged for at least

200 scans and were taken in increments of 0.1 eV with dwell

times of 50 ms. Since the binding energy of the C 1s core level

(usually taken as reference) overlaps with the binding energy

of the Ru 3d core level, binding energies were calibrated

relative to the main lines of the supports;25 Ti 2p peak at

458.6 eV in the case of TiO2 samples and Si 2p peak at 103.4 eV

in the case of SiO2�Al2O3 sample to correct the contact

potential differences between the sample and the spectrometer.

High-resolution spectral envelopes were obtained by curve

fitting using the XPS peak software.

A JEM-2100F 200 kV transmission electron microscope

(JEOL Ltd.) equipped with an Oxford INCAx-Sight EDS

detector (Oxford Instruments Ltd.) was used. High angle

annular dark field-scanning transmission electron microscopy

(HAADF-STEM) images were obtained upon operating the

microscope in scanning mode with an electron probe size of

1 nm and the signal was recorded with an annular dark field

detector with an inner collection angle of 59 mrad (Z-contrast)

and a maximum point resolution of 0.1 nm.

2.3. Quasi in situ infrared spectroscopy studies

Fourier Transform Infrared (FTIR) spectra were recorded

with a Nicolet Nexus instrument, using conventional IR cells

connected to a gas handling system, under static conditions.

Pressed disks of pure catalysts and support powders (B20 mg)

were thermally treated within the IR cell at 673 K under

vacuum (10�4 mbar). The catalysts were reduced in situ with

pure H2 (800 mbar) at 673 K. The reducing process was

conducted in two cycles of 30 min each followed by degassing

at the same temperature. CO adsorption (8 mbar) experiments

were performed at the temperature of liquid nitrogen, and

spectra were recorded in the range 133–298 K while outgassing.

In another set of experiments, CO was adsorbed (13 mbar)

at room temperature and then evacuated at increasing

temperatures. In the so-called quasi in situ FTS experiments,

after reducing the sample, a mixture of H2/CO= 6 (365 mbar

total pressure) was admitted into the IR cell. Spectra of the

catalyst and the gas phase were recorded at 298, 473,

523, and 573 K.

2.4. Catalytic experiments

Catalysts were tested in the CO hydrogenation reaction using

a fixed-bed microreactor (300 mm long and 9 mm i.d.). The

reactor temperature was measured with a K-type thermocouple

directly placed in the catalytic bed. Flow rates were controlled

with TOHO TTM 005 mass flow controllers. The catalytic bed

consisted of 0.1 g of the catalyst precursor diluted with 2.0 g of

quartz (0.25–0.30 mm pellet size) to avoid hot spots. The

catalysts were subjected to a thermal treatment in situ under

H2 at 443 K (10 K min�1). The temperature was afterwards

raised to 523 K (10 K min�1) under an N2 atmosphere and

immediately, the atmosphere was switched to H2/CO/N2

(62/31/7, GHSV= 1250 h�1) flow and the pressure was increased

to 4.04 MPa. Analysis of reactant gases and products was

performed on line with a gas chromatograph (Varian CP-3800)

equipped with a cryogenic unit which allowed the gas

chromatograph oven to be cooled down to 203 K. A Hayesep Q

(2 m � 0.32 cm) column connected to a TCD detector was

used to identify and quantify inorganic gases (H2, N2, CO and

CO2). Hydrocarbon compounds were identified and quantified

with an Rtx-1 column (60 m � 0.25 mm i.d. � 0.25 mm)

connected to a flame ionization detector (FID). N2 was used as

internal standard for chromatographic analyses.

3. Results and discussion

3.1. Characterization

The supported Ru samples display N2 adsorption–desorption

isotherms classified as type IV (not shown), typical of mesoporous

materials.26 A moderate decrease of the specific surface area of

Ti1 and SA supports is observed after Ru incorporation and

subsequent calcination (Table 1). However, neither the impreg-

nation with the Ru solution nor the calcination process affects the

surface area of the titania P25 support (Ti0.8 samples, Table 1).

The TPR-H2 profiles shown by the different supports and

catalytic precursors are depicted in Fig. 1. Each sample

presents a characteristic hydrogen consumption profile with

maximum reduction rates appearing at different temperatures.

The broad peaks observed in all cases might indicate the

presence of ruthenium species with different oxidation states.27

Furthermore, the profile of the hydrogen consumption process

depends also on the Ru precursor (RuTi0.8N vs. RuTi0.8),

suggesting that its identity may influence significantly the final

Ru state.

Table 1 Summary of textural properties (BET surface areas) andRu dispersion and particle size obtained from H2 chemisorptionmeasurements

Sample

BET surface area/m2 g�1 H2 chemisorption

Support Catalyst Dispersion (%) Particle size/nm

RuTi0.8 45 44 11 8.7RuTi0.8N 45 44 14 7.0RuTi1 120 101 — —RuSA 340 317 1 450

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Binding energy values and relative abundances of the

different surface species after treatment in H2 at 443 K are

shown in Table 2. The Ru 3d region exhibits Ru 3d5/2 peaks at

279.3–280.4 eV and 281.2–282.1 eV, attributed, respectively,

to Ru0 and oxidized ruthenium species. The actual oxida-

tion state of the latter species is not well defined, being

widely admitted that several Ru species, such as RuOx, RuClx,

RuO2�xH2O or RuOxHy, may coexist.28–32 The Ru/M atomic

ratio values are also shown in Table 2. The three TiO2-based

samples (RuTi1, RuTi0.8 and RuTi0.8N) display similar

Ru/M values (0.07–0.08), while that for the RuSA sample is

significantly higher (1.24).

The XPS analyses reveal the presence of chlorine species

on all titania-based samples. Remarkably, chlorine species are

also detected in RuTi0.8N, even though this solid was, in

principle, prepared with a Cl-free Ru precursor. Chen and

Goodwin24 have also reported the presence of residual Cl

species on commercial Ru(NO)(NO3)3 prepared from RuCl3.

Furthermore, sulfur species were detected on the surface of

RuTi1. These species may cause catalyst poisoning by avoiding

the adsorption of the reactants and/or leading to the formation

of catalytically inactive Ru–S species.33

Fig. 2 shows HAADF-STEM images of all H2-reduced

samples. Clearly, Ru species are deposited as large aggregates

on Ti1 (RuTi1, Fig. 2a) and particularly, in the case of RuSA

(Fig. 2b), where large Ru islands (450 nm) are deposited on

some silica–alumina particles. HAADF-STEM images of the

H2-reduced Ti0.8-based samples (Fig. 2c and d for RuTi0.8

and RuTi0.8N, respectively) show that TiO2 particles range

from 10 to 60 nm. Since atomic numbers of Ru and Ti are very

different (44 and 22, respectively), HAADF-STEM images

allow identifying the Ru clusters as the bright spots on the

border of the TiO2 particles.

This is because the contrast in the HAADF-STEM technique

is a function of BZ2. Therefore, the brighter spots of the

image correspond directly to areas of higher mean atomic

number, provided that the thickness of these areas is identical.34,35

The Ru particles in RuTi0.8 and RuTi0.8N samples (arrowed

in Fig. 2c and d) can be described as epitaxially grown particles

of 1–2 nm thickness forming islands over the TiO2 surface.

These islands cover areas from 1 � 1 nm to 20 � 20 nm.

Substantial differences between the morphology of the Ru

particles on RuTi0.8 and RuTi0.8N samples are not found.

A positive effect of anionic oxygen vacancies on rutile for

avoiding the agglomeration of metallic particles has been

previously reported.36 The appearance of O2� ions is related

to the formation of Ti3+ species on the surface of rutile

Fig. 1 TPR-H2 profiles of the supports (dotted line) and catalytic

precursors (solid line).

Table 2 Summary of XPS results with supported Ru samples

Sample Region B.E./eV Species M/supporta

RuTi0.8 Ru 3d5/2 279.3 (85) Ru0 0.066281.2 (15) Rud+ 0.012

Ti 2p3/2 458.6 TiO2

Cl 2p3/2 198.0 0.030RuTi0.8N Ru 3d5/2 280.1 (63) Ru0 0.041

281.9 (37) Rud+ 0.024Ti 2p3/2 458.6 TiO2

Cl 2p3/2 198.3 0.014RuTi1 Ru 3d5/2 280.4 (70) Ru0 0.054

282.0 (30) Rud+ 0.024Ti 2p3/2 458.6 TiO2

Cl 2p3/2 199.7 0.014S 2p3/2 168.5 SO4

2� 0.013RuSA Ru 3d5/2 279.8 (88) Ru0 1.090

282.1 (12) Rud+ 0.150Si 2p3/2 103.4 SiO2

Al 2p3/2 74.9 Al2O3

Cl 2p3/2 —

a Ru or Cl or S surface atoms/Ti or (Si + Al) surface atoms from the

corresponding support. Values in parenthesis show the relative concen-

tration of Ru species.Fig. 2 HAADF-STEM images of RuTi1 (a), RuSA (b), RuTi0.8

(c) and RuTi0.8N (d) samples previously treated in H2 at 443 K.

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particles during anatase–rutile transformation, being the rutile

particles enriched in these Ti3+ ions and oxygen vacancies.36–38

It is likely that the anionic oxygen vacancies on rutile particles

may act as anchor sites for cationic species of ruthenium. It is

also well documented that in water solution, RuCl3 species

evolve with time to formRuClxd+ cationic species.39–41 Similarly,

Ru(NO)3+ cationic species can be stabilized from the nitrosyl

precursor of Ru.41 On anatase (Ti1), the Ru-precursor species

are poorly dispersed, and consequently, large Ru particles are

formed after the thermal treatment step in H2. In contrast, the

same species are highly dispersed on the rutile surface because

of the interaction with O2� ions, thus forming stable metallic

ruthenium particles.

Remarkably, a heterogeneous distribution of the Ru particles

on Ti0.8 is found. As a matter of fact, Ru is exclusively

deposited on the rutile phase of the TiO2�P25 substrate. This

feature is clearly illustrated in Fig. 3. The squared area

corresponds to a TiO2 particle in which the rutile and anatase

phases are identified (R and A, respectively). It can be seen

that the Ru particles are deposited exclusively on the rutile

component of titania. The assignment of the TiO2 structure to

either rutile or anatase phases is corroborated by indexing the

digital diffraction patterns (DDP) obtained after applying the

Fast Fourier Transform (FFT) to the high resolution images

of the particles, which is equivalent to indexing the electron

diffraction pattern42 (Fig. S1 in ESIw) or simply by the

identification of the d-spacing of the {110} planes of rutile,

d = 3.247 A, or {101} planes of anatase, d = 3.520 A43

(Fig. S1 in ESIw). The structure of more than 20 individual

Ru/TiO2 particles was analyzed, in all of them the only TiO2

phase observed was the rutile one.

On the other hand, the migration process of TiO2 crystals

over Ru particles has been studied. The strong metal–support

interaction effect for group 8–10 metals on TiO2 is well

documented.10 Such an interaction has been studied thoroughly

by Bernal44 and explained in terms of structural changes

and decorating microcrystals observed by high resolution-

transmission electronic microscopy (HR-TEM) for Rh and

Pt supported on CeO2. The interplanar spacings and angles

observed in the digital diffraction pattern (DDP) of RuTi0.8N

(Fig. S2 in ESIw) correspond to a particle with the rutile

structure oriented along the [121] zone axis. The coherence

between the crystal structure of the epitaxial Ru islands and

the structure of TiO2 impedes the identification of the Ru particles

by HR-TEM. Therefore, information about metal–support

interactions was obtained by an energy dispersive X-ray

spectroscopy (EDS) line scan.45

Fig. 4 shows the EDS profile recorded across one individual

TiO2 particle with Ru particles on its surface. The profiles

clearly show that Ru particles are deposited on the periphery

of the TiO2 particles with no evidence of coverage of those Ru

particles by TiO2 layers. Some small Ru islands are also

identified. The absence of TiO2 on the outmost layer of the

Ru particles indicates that no decoration process has occurred

under our experimental conditions, i.e. hydrogen flow at 443 K.

3.2. FTIR characterization of the catalysts

Fig. 5a shows the FTIR spectra of RuSA after outgassing, as

well as after reduction in hydrogen at 673 K and subsequent

outgassing at the same temperature. The spectrum of the

silica–alumina support (SA) recorded after outgassing at

673 K is also shown as a reference. The IR spectrum of the

SA support is consistent with the literature.46 The IR spectrum

of the outgassed RuSA sample shows an increased absorption

baseline, with a significantly reduced absorbance of the band

near 3745 cm�1 (detailed in the inset) associated to free surface

silanol groups. The broad absorptions in the 2100–1500 cm�1

region, due to overtones of bulk vibrations, are also reduced.

This observation would be in line with the exchange of Ru

species with the protons of surface silanol groups. It is also

possible that the overtones in the 2100–1500 cm�1 region have

a predominant surface character and may be relaxed due to

the presence of surface Ru species. Both these effects are

attenuated after the reduction treatment, suggesting that Ru

metal atoms coalesce into large particles, thus regenerating the

surface hydroxyl groups and decreasing the Ru surface coverage.Fig. 3 TEM images of the RuTi0.8N sample treated at 773 K in

H2. R: rutile; A: anatase.

Fig. 4 EDS line scans recorded for the RuNTi0.8N sample reduced at

443 K: (a) HAADF-STEM image, (b) HAADF-STEM image with the

EDS intensity profile of Ru–La1 (red) and Ti–Ka1 (blue) energy lines

overlapping, (c) Ru–La1 and (d) Ti–Ka1 profiles, (e) sum up spectra.

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The sample reduced in H2 shows a further increased absor-

bance baseline, possibly because of the continuous absorption

of Ru metal particles. This behavior is consistent with our

previous characterization data (STEM and H2 chemisorption)

showing large ruthenium clusters in RuSA, see also XPS

results.

The spectra of TiO2 and Ru–TiO2 samples are shown in

Fig. 5b for Ti0.8 and Fig. 5c for Ti1. Irrespective of the

support used, a pronounced increase of the absorbance is

observed in the spectra of the outgassed RuTi1 and RuTi0.8

samples. Upon thermal treatment in hydrogen at 673 K,

spectra with decreasing absorbance are recorded. Altogether,

these facts point to a strong electronic interaction between

Ru species and bulk TiO2. It is interesting to remark that

RuO2, which displays high conductivity, is stable in the

rutile structure, probably because both phases share the

same crystal structure (see discussion below). Furthermore,

RuxTi1�xO2 rutile-type solid solutions may be formed, which

also display high conductivity and light absorption properties.47

Our data suggest that after impregnation, a strong interaction

occurs between the bulk titania and the supported Ru oxide

phase, which is responsible for the increased absorption of the

IR radiation. After reduction, the Ru centers would coalesce

into metal particles, thus decreasing the electronic interaction

with titania.48 In fact, our microscopy study (see above) shows

no evidence of any decoration of the Ru particles by TiO2

layers after thermal treatment in hydrogen.

The region of the FTIR spectra corresponding to the

surface hydroxyl groups of the two titania powders (insets in

Fig. 5b and c) are akin to those reported in the literature

for similar systems.49,50 The addition of ruthenium causes the

disappearance of the sharp bands of the surface hydroxyl

groups, which are now apparent as a broad absorption in

the 3700–3500 cm�1 region. This provides evidence of the

perturbation of the surface by ruthenium oxide species, which

persists after reduction in hydrogen.

It is important to remark the presence of a sharp band at

1365 cm�1 in the spectrum of Ti1, which is characteristic of

surface sulfate species, a typical contaminant in some anatase

preparations.49 This band seems to evolve into another sharp

one centered at 1268 cm�1 after ruthenium addition, which we

tentatively assign to ruthenium sulfate species.

The catalysts were further characterized by FTIR after CO

adsorption at low temperature, which allows CO to inter-

act with both metallic Ru sites and support. Moreover, the

oxidation of the ruthenium particles by CO is impeded at low

temperatures.51 The spectra of CO adsorbed (COad) at low

temperature on reduced RuSA after different evacuation

temperatures are shown in Fig. 6. The high frequency bands

at 2228 and 2157 cm�1 (strong) are due to CO adsorbed on the

support and disappear almost completely after outgassing.

The most intense band due to the CO stretching mode of

Ru-carbonyls appears at 2035 cm�1 with a shoulder at 1995 cm�1

(Fig. 6, inset). Upon outgassing, the main band is shifted

to 2014 cm�1. Other broad and less intense bands appear at

1885 and 1845 cm�1. Similar bands have been reported after

CO adsorption on reduced Ru/alumina, Ru/ZSM5 zeolite and

Fig. 5 (a) FTIR spectra of silica–alumina (SA) and RuSA catalyst

disks after outgassing and reduction. Inset: OH stretching region.

(b) FTIR spectra of TiO2 (Ti0.8) and RuTi0.8 catalyst disks after

outgassing and reduction. Inset: OH stretching region. (c) FTIR

spectra of TiO2 (Ti1) and RuTi1 catalyst disks after outgassing and

reduction. Inset: OH stretching region.

Fig. 6 FTIR spectra of CO adsorbed on reduced RuSA at 133 K and

upon warming under outgassing up to 293 K. Inset: enlargement of the

metal-carbonyl region.

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Ru/silica catalysts. These bands are associated to weakly

adsorbed species, more likely bridging carbonyl species,58

which disappear after outgassing at room temperature. On

the other hand, the band centered at 1995 cm�1 is associated to

a second, more labile, on-top carbonyl species.

CO species adsorbed on Ru particles are more stable than

CO adsorbed on the support, as revealed by the evolution of

the COad bands with the outgassing temperature shown in

Fig. 6. This is a typical behavior of on-top carbonyls on

extended metal particles, due to vibrational coupling effects.59

Fig. 7a shows the spectra of CO adsorbed at low tempera-

ture on the reduced RuTi0.8 sample. The sharp strong band

observed at 2178 cm�1 is assigned to CO adsorbed on Ti4+

sites.49 Upon outgassing, the position of the bands shifts to

2193 cm�1, and it eventually appears at 2207 cm�1. Another

component at 2156 cm�1 is due to CO weakly interacting with

the support through H-bonds. The broader band at 2033 cm�1

reveals CO species linearly coordinated to metallic Ru particles.

This band shifts to a lower frequency (2012 cm�1; Dn= 21 cm�1)

with decreasing the CO coverage. The spectrum of CO

adsorbed on RuTi0.8N follows a similar pattern to that

recorded for RuTi0.8, as shown in Fig. 7b. Nonetheless,

certain differences are appreciable. In this sample, the features

of CO adsorbed on the strongest anatase Ti4+ sites at

2206 cm�1 are very weak, if any. The presence of bands

accounting for CO adsorbed on anatase is in good agreement

with the preferential deposition of Ru on the rutile phase. It

also suggests that the amount of anatase sites exposed on the

surface of RuTi0.8 is higher than on RuTi0.8N. Additionally,

the broad band assigned to on-top carbonyls adsorbed on Ru

metallic particles is recorded between 2010–1990 cm�1; a

significantly lower frequency compared to that on RuTi0.8.

The FTIR spectra recorded after room temperature CO

adsorption on the reduced Ru catalysts show similar bands

to those recorded after low temperature CO adsorption

(see Fig. S3 in the ESIw). The most intense band of COad

species on metallic Ru appears at similar frequencies than

those reported for the low temperature experiments. However,

two main differences are observed: (i) the spectra lack COad

bands on the support, and (ii) a further set of less intense

bands can be detected at the higher frequency side of the most

intense COad band. These bands account for CO adsorbed on

oxidized Ru species, which are formed during CO adsorption

at room temperature.14 Accordingly, these species are not

recorded for RuTi1, where the dispersion of ruthenium is

very low.

3.3. FTS studies by quasi in situ FTIR

The performance of the Ru catalysts in CO hydrogenation

(FTS) has been followed by quasi in situ FTIR experiments.

First, samples are reduced in situ under a H2 atmosphere at

673 K. Then, a CO/H2 mixture (365 mbar total pressure) is

admitted into the FTIR cell at the desired reaction temperature

for 5 min. Three reaction temperatures for the CO hydrogenation

reaction are studied, 473, 523 and 573 K; the FTIR spectra were

recorded at room temperature.

RuTi0.8 and RuTi0.8N samples display very similar FTIR

spectra during FTS at different temperatures (Fig. 8). The

most intense COad band appears at 2027 cm�1, which shifts to

lower frequency and its intensity decreases with the increasing

reaction temperature. The next most intense band appears in

the range 1930–1910 cm�1, and their intensity is found to

be independent of the reaction temperature. Additionally, two

weaker components are also observed at 2132 and 2068 cm�1

and assigned to CO adsorbed on oxidized Ru species (see above).

Again, the shape of the COad bands recorded on RuSA and

RuTi1 is similar, although different to those observed for

RuTi0.8 and RuTi0.8N. The main COad band in RuSA and

RuTi1 appears at 2020 cm�1 and 2007 cm�1, respectively; only

faint components at the lower and higher frequency to the

main COad band are recorded.

Fig. 7 FTIR spectra of CO adsorbed on reduced RuTi0.8 (a) and

RuTi0.8N (b) upon warming under outgassing up to 293 K.

Fig. 8 FTIR spectra of CO adsorbed on the four reduced catalysts

after contact with CO + H2 at different temperatures.

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Fig. 9 compares the spectra of the adsorbed species formed

during the CO hydrogenation reaction at 523 K with the most

active catalyst, RuTi0.8, and the less active catalyst, RuTi1.

For comparison, both spectra have been normalized to show

the same intensity of the COad band at ca. 2007 cm�1. It is

evident that on the RuTi0.8 catalyst, strong bands due to C–H

stretching of polymethylene chains at 2924 and 2853 cm�1 are

formed, consistent with its higher activity in the FTS (Table 3).

On the almost inactive RuTi1 catalyst, such bands are very

weak, and actually are only observed after spectrum magnifi-

cation. In both cases, other C–H stretching modes appear,

evidencing the presence of other CHx species. The analysis of

the IR spectrum of the gas phase after CO hydrogenation on

RuTi0.8 (see inset in Fig. 9) reveals a number of additional

bands due to the presence of methane, CO2, water, ethylene

(H2CQwagging mode at 949 cm�1), propene (H2CQwagging

mode at 912 cm�1) and butenes (H2CQ wagging mode 889

at cm�1). The appearance of these species confirms that even

under these mild conditions of pressure and temperature, this

catalyst shows a detectable FTS activity by FTIR and that this

technique is suitable to follow both the reaction products and

the evolution of Ru centers during FTS. The results obtained

with RuTi0.8N (Fig. 4 in ESIw) are similar to those obtained

with RuTi0.8, in good agreement with their comparable FTS

activity recorded in a fixed bed reactor under more realistic

FTS conditions.

3.4. Catalytic activity

All supported Ru catalysts display a high initial activity

(not shown), reaching the steady-state after 10–12 h on stream.

Table 3 summarizes the catalytic performance of Ru-based

samples in FTS at CO conversion of 36%. The CO conversion

rate follows the order: RuTi0.8ERuTi0.8N4RuSAcRuTi1.

The identity of the support is of extreme importance to

obtain active Ru-based catalysts. Those prepared with titania

P25 (RuTi0.8 and RuTi0.8N) show the best FTS performance

in terms of CO conversion rate and selectivity to long chain

hydrocarbons, as deduced from the chain growth probability

values (a) followed by RuSA. In contrast, RuTi1 is inactive

under typical FTS reaction conditions.

We note that RuTi0.8 shows a similar reaction rate compared

to RuTi0.8N, even though smaller Ru clusters are found in this

latter case. In contrast, reaction selectivity to the different

hydrocarbons and olefins/paraffins distribution is not affected

by the identity of the Ru precursor.

4. Discussion

H2 chemisorption and electron microscopy experiments show

that Ru dispersion, or particle size, varies with the support. In

principle, solids with high specific surface area should lead to

higher metallic dispersion when used as support; however, the

dispersion of the Ru particles derived from H2 chemisorption

analysis (see Table 1) records the highest value for the catalysts

prepared on TiO2�P25, the support showing the lower specific

surface area value in the series; 45 m2 g�1 vs. 120 m2 g�1 and

340 m2 g�1 for Ti1 and SiO2�Al2O3, respectively. The particle

size of Ru calculated from H2 chemisorption data is 8.7 nm

for RuTi0.8 and 7.0 nm for RuTi0.8N. HAADF-STEM

images show larger Ru clusters on RuTi1, and especially on

RuSA, consistent with H2 chemisorption data. These results

indicate that the effects of both the specific surface area of the

support and that of the Ru precursor on the particle size

is negligible as compared to the effect of the structure of

the support itself (see below). The XPS results are in good

agreement with the aforementioned tendencies, except for

RuSA, where the surface atomic abundance of Ru is higher

than for the other catalysts (Table 2). In principle, the high

Ru/support atomic ratio derived from the XPS analysis of

RuSA would suggest that this sample consists of highly

dispersed Ru particles. However, H2 chemisorption data point

otherwise, a very low dispersion of B1% (Table 1). The

microscopy images of RuSA show that this sample consists

of very large Ru particles deposited on some silica–alumina

particles. This explains the low Ru dispersion value found by

H2 chemisorption. Moreover, since Ru islands actually cover

the whole surface of the support particles, those would not be

detected by a surface sensitive technique such as XPS, hence

recording a surface enriched in Ru atoms.

The dispersion of the Ru particles is improved on Ti0.8 irres-

pectively of the Ru precursor used to prepare the catalysts,

consistent with previous studies for other metals. Thus, Jongsomjit

et al.60 have recently reported that the highest degree of

metallic Co dispersion is achieved when TiO2 (anatase/rutile

ratio of 81/19) is used as a support. These authors claimed that

Fig. 9 FTIR spectra of the adsorbed species formed on the active

catalyst RuTi0.8 and on the inactive catalyst RuTi1 after contact with

CO + H2 at 523 K for 5 min. Inset: gas phase species detected in the

same conditions over the RuTi0.8 catalyst.

Table 3 Catalytic performance of Ru catalysts in FTS (CO conver-sion 36%, 523 K, 4.04 MPa, H2/CO = 2, WHSV for RuTi0.8 andRuTi0.8N = 120 h�1, WHSV for RuSA = 36 h�1)

CatalystCO conversion ratea/mmolCO h�1 molRu

�1

Selectivity (%)

abC1 C2–C5 C6+

RuTi0.8 246 14 29 57 0.75RuTi0.8N 245 11 29 60 0.76RuSA 70 25 50 25 0.68

a Conversion rate value at the steady-state. b Chain growth probability

calculated for the C5–C10 fraction.

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this effect is the result of the higher reducibility of the cobalt

species deposited on the rutile phase.

STEM images of RuTi0.8N clearly show (Fig. 3) that the

preferred anchor site for the Ru particles is the rutile phase of

Ti0.8. In line with our results, the preferential location of

IrO2 on the rutile phase of TiO2�P25 has been previously

reported.61,62 It should be recalled that both rutile and anatase

phases coexist in Ti0.8 and according to XRD calculations,

rutile accounts for ca. 20% of the crystalline particles in P25.42

The preferential deposition of Ru on the rutile phase can be

explained by taking into account that both RuO2 and rutile

crystallize in the same space group P42/mnm, whereas anatase

space group is I41/amd, see Scheme 1. That is, rutile can

accommodate the Ru oxide particles within its structure. This

also explains the epitaxial growth of the Ru particles on Ti0.8

as discussed above. This feature explains the predominant role

of the structure of the support in the dispersion of the Ru

particles as compared to other features such as specific surface

area of the support or the nature of the Ru precursor.

After thermal treatment in hydrogen, the Ir particles

referred above become re-distributed homogeneously on both

the rutile and anatase phase of TiO2�P25.62 This is not the case,however, for our Ru samples, where the preferential location

of Ru on the rutile crystals is maintained even after thermal

treatment in hydrogen (Fig. 3b). The high stability of the Ru

particles towards thermal treatment in H2 could be explained

by the formation of RuxTi1�xO2 rutile type solid solutions. As

a matter of fact, the high absorbance recorded for the degassed

spectrum of RuTi0.8 (Fig. 5b) could be indicative of the

presence of such species.

The FTIR spectra recorded after either low temperature or

room temperature CO adsorption show a single COad broad

band, as expected for Ru-based samples. As summarized

recently by Payne et al.,63 CO adsorption on Ru(0001) single

crystal surfaces gives rise essentially to a single IRAS and

EELS band at any coverage shifting from about 1980 cm�1 to

about 2060 cm�1 with the increasing CO loading from zero

coverage to saturation, due to static and dynamic coupling

effects. This band is due to on-top CO species forming the

(O3 � O3)R301 pattern evident in LEED experiments. A

similar situation was found on some stepped surfaces such

as Ru(109), where terminal carbonyls absorb at 2063 cm�1 at

the highest coverage, and at 1973 cm�1 at the lowest coverage,

where CO has been also found to dissociate producing Ru–C

and Ru–O species at the step sites and recombine near 530 K.58

On the Ru(11%20) face, terminal carbonyls are observed to shift

from 1937 to 2058 cm�1 by increasing coverage. Another COad

species characterized by unusually low stretching frequency

(1558 cm�1) together with an unusually high deformation

frequency (694 cm�1) has been identified as the precursor of

CO dissociation.64 This species is supposed to be tilted,

bonded to a kind of fourfold hollow site. On Ru(10%10),

terminal carbonyls near 2000 cm�1 were found together with

a second species absorbing at 1810 cm�1.65

The position of the COad band recorded at high CO coverage

for the Ru-supported catalysts in the present study ranges

between 2039 and 2007 cm�1, which is a significantly lower

value than that observed on Ru single crystal faces (typically

near 2060 cm�1). This shifting to lower frequencies suggests

that the surfaces of the supported Ru metal particles are more

electron rich than that of the extended Ru crystal faces, hence

being able to back-donate more electrons into the p* orbital ofCO, thus recording a lower nCO frequency. This behavior is

usually associated to the small crystal size of the Ru particles

in the supported catalyst and/or to the co-presence of adsorbed

hydrogen species in our experiments. It is not possible,

however, to establish a direct correlation between the COad

band position and the size of the Ru particles on the support.

This is because the presence of ions such as Cl� and/or sulfate

species (as detected by XPS) can affect the electron donating

ability of the Ru particles, masking thus pure size effects.

Concerning the support effects, the FTIR spectra shown in

Fig. 6 and 7 illustrate that COad species on the support are

detected only after low temperature CO adsorption. After

outgassing, these COad species disappear and only those

ascribed to COad on Ru remain. COad species on the support

were not detected in the high temperature experiments.

The FTIR spectra after FTS reaction at 473 K in the IR cell

(Fig. 8) show broad COad bands centered at 2007 cm�1

(RuTi1), 2020 cm�1 (RuSA), and 2027 cm�1 (RuTi0.8, and

RuTi0.8N). Noticeably, the position of these bands shifts to

lower frequency values with the increasing reaction tempera-

ture, except for RuTi1 and RuSA. Furthermore, the intensity

of this band also decreases with the reaction temperature with

the exception of RuTi1. On the other hand, the catalytic

experiments conducted in a fixed bed reactor show that both

RuTi0.8 and RuTi0.8N render the most active catalysts in

FTS, whereas RuTi1 is almost inactive for the synthesis

of hydrocarbons. While the FTIR spectra of COad on the

Ti0.8-based catalysts show certain differences (not shown)

as discussed above, identical spectra are recorded for both

samples during quasi in situ FTS experiments at every reaction

temperature. In addition to the main COad band at 2027 cm�1,

COad bands at 2132 and 2068 cm�1, along with a broad

absorptionB1900 cm�1 are found. The latter band is assigned

to nCO of bridging carbonyls, which are usually not found on

Ru single crystal surfaces. The COad bands in the high

frequency region are usually ascribed to COad on partially

Scheme 1 Cartoon type illustration on how RuO2 can accommodate

on the crystal structure of the rutile phase.

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oxidized Ru species. Over an oxidized Ru(0001) surface, the C–O

stretching tends to increase with respect to the clean surface.

According to Jakob and Schiffer,66 the observed frequency for

the Ru(0001)–(O3 � O3)R301 CO pattern is 2030.8 cm�1,

shifting to 2050.7 cm�1 for the Ru(0001)–(2 � 2)–(CO–O) mode,

and to 2090.3 cm�1 for the Ru(0001)–(2 � 2)–(CO+ 2O) mode.

Instead, for the Ru(0001)–(2 � 2)–(2CO + O) mode, the

frequency of terminal carbonyls is observed at 2068.6 cm�1,

while a second species considered to be triply bridging CO on

fcc-type sites is observed at 1849.7 cm�1. Two groups reported

the adsorption and reaction of CO on the RuO2(110) single

crystal surface. Using HREELS, Wang et al.67 found that CO

adsorbs on-top on coordinatively unsaturated Ru species absorb-

ing at 2115 cm�1, while after reduction of the surface by CO

(forming CO2), new bands appeared at 1975 and 1895 cm�1,

associated to species adsorbed on centers similar to reduced

ruthenium. Using IRAS, Farkas et al.68 found on the stoichio-

metric RuO2(110) surface CO adsorbed on-top shifting from

2110 to 2123 cm�1 by increasing coverage, with an additional

weaker feature at 2001 cm�1 attributed to asymmetrically bridging

CO. On the mildly reduced RuO2(110) surface, the main band

shifts from 2016 to 2086 cm�1, with the weaker feature at

2001–1994 cm�1. A symmetrically bridging species is also observed

in some conditions at 1860–1880 cm�1. Both groups also showed

several complex effects associated to the different oxygen coverage

of the surface during oxidation or reduction treatments.69,70

Indeed, multiple bands in the region 2150–2080 cm�1 have been

frequently observed on supported Ru catalysts and attributed to

polycarbonyls on incompletely reduced ruthenium.51–57

Interestingly, the shape and intensity of the COad band

at B1900 cm�1 remains invariable during the FTS reaction.

In contrast, the bands at the higher frequency values disappear

with the increasing FTS temperature. This is because the

partially oxidized Ru species are reduced within the reducing

atmosphere of the FT reaction.

On the other hand, strong bands of polymethylene

chains are observed on the spectra of RuTi0.8 and RuTi0.8N.

Faint bands of those species are observed in the spectra

of RuTi1 or RuSA. This result is in good agreement with

the actual FTS performance of the catalysts in the fixed

bed reactor. In addition, a strong broad band is observed

at ca. 1620 cm�1 due to water molecules produced during

FTS. This broad band would overlap with those of CO

tilted species, proposed as intermediate species in the FT

process.

Early FTS literature suggested that larger Ru particles yield

more active catalysts. It is also proposed that the surface

carbon species formed by hydrogen-assisted CO dissociation

participate in the FT synthesis reaction and as precursor for

the species that causes catalyst deactivation.11,71,72 Recent

theoretical studies suggest, however, that the CO dissociation

ability is favored on couples of atoms on monoatomic steps

being the key requirement for Ru to develop a good FTS

catalyst,73,74 followed by hydrogenation and coupling of

CHx fragments75 or CO insertion.74 Experimental studies have

shown that water presents a positive effect on the reaction rate

and selectivity, in part due to its ability to remove surface

carbon atoms,4 and in part because Ru particles around 8 nm

perform better than smaller particles for FTS.76

This study shows that Ru particles are accommodated and

stabilized on the rutile phase of TiO2 showing a size between

7–8 nm. Both Ti0.8 supported catalysts show higher activity

for the FTS reaction than those supported on Ti1 and SA, due

to the smaller size of the Ru clusters. Also, the catalytic

performance of RuTi1 is affected negatively by the formation

of Ru-sulfate like species, which are known to be inactive for

the FT synthesis reaction. Quasi in situ FTS studies by FTIR

demonstrate that both RuTi0.8 and RuTi0.8N yield similar

Ru species within the reaction atmosphere, hence their

comparable performance in FTS.

5. Conclusions

The dispersion of Ru particles is dominated by the structure of

the support rather than by features such as metal precursor or

specific surface area of the support. Remarkably, Ru deposition

occurs preferentially on the rutile phase of TiO2�P25, renderingparticles of ca. 7–8 nm which are stable after successive

thermal treatments in air and hydrogen. This behavior has

been ascribed to the similar crystal phase of rutile and

Ru which favors the epitaxial growth of the Ru particles,

impeding their agglomeration. Furthermore, irrespectively of

the Ru precursor, when supported on TiO2�P25 Ru is an active

FTS catalyst. On the other hand, when supported on pure

anatase or SiO2�Al2O3, the catalytic performance drops

dramatically. The performance of the catalysts in FTS follows

the order: RuTi0.8 4 RuTi0.8N 4 RuSA c RuTi1. The

same trend was observed during the quasi in situ FTS experi-

ments recorded in an IR cell at 473–573 K and 667 mbar. The

formation of surface polymethylene chains is evident on the

Ti0.8 catalyst. Methane, ethylene and propylene were detected

in the spectra of the gas phase. The FTIR spectra of CO

adsorbed on the different Ru catalysts, characteristic of each

sample, are mostly determined by the Ru size and the presence

of other ions. However, after quasi in situ FTS, both RuTi0.8

and RuTi0.8N display similar spectra, in line with their

catalytic performance in FTS.

Acknowledgements

J. M. Gonzalez-Carballo acknowledges the Ministerio de

Educacion of Spain through the Formacion de Profesorado

Universitario program (FPU) for financial support. The authors

also acknowledge projects ENE2007-67533-C02-02/ALT from

Ministerio de Ciencia e Innovacion and Project S2009ENE-

1743 from Comunidad de Madrid. Programa de Actividades

de I + D entre Grupos de Investigacion en Tecnologıas for

funding this work.

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