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Coke deactivation of Ni and Co catalysts in ethanolsteam reforming at mild temperatures in afluidized bed reactor

Jorge Vicente a, Carolina Montero a, Javier Ere~na a,*, Miren J. Azkoiti b,Javier Bilbao a, Ana G. Gayubo a

a Departamento de Ingenierıa Quımica, Universidad del Paıs Vasco UPV/EHU, Apartado 644, 48080 Bilbao, Spainb Departamento de Ingenierıa Quımica y Medio Ambiente, Universidad del Paıs Vasco UPV/EHU, Paseo Rafael

Moreno “Pitxitxi” 3, 48013 Bilbao, Spain

a r t i c l e i n f o

Article history:

Received 13 March 2014

Received in revised form

10 June 2014

Accepted 16 June 2014

Available online 11 July 2014

Keywords:

Ethanol steam reforming

Hydrogen

Ni catalyst

Co catalyst

Coke deactivation

* Corresponding author. Tel.: þ34 94 6015363E-mail address: javier.erena@ehu.es (J. Er

http://dx.doi.org/10.1016/j.ijhydene.2014.06.00360-3199/Copyright © 2014, Hydrogen Energ

a b s t r a c t

The deactivation by coke deposition of Ni and Co catalysts in the steam reforming of

ethanol has been studied in a fluidized bed reactor under the following conditions: 500 and

700 �C; steam/ethanol molar ratio, 6; space time, 0.14 gcatalyst h/gethanol, partial pressure of

ethanol in the feed, 0.11 bar, and time on stream up to 20 h. The decrease in activity de-

pends mainly on the nature of the coke deposited on the catalysts, as well as on the

physicalechemical properties (BET surface area, pore volume, metal surface area) of the

catalysts. At 500 �C (suitable temperature for enhancing the WGS reaction, decreasing

energy requirements and avoiding Ni sintering), the main cause of deactivation is the

encapsulating coke fraction (monoatomic and polymeric carbon) that blocks metallic sites,

whereas the fibrous coke fraction (filamentous carbon) coats catalyst particles and in-

creases their size with time on stream with a low effect on deactivation, especially for

catalysts with high surface area. The catalyst with 10 wt% Ni supported on SiO2 strikes a

suitable balance between reforming activity and stability, given that both the capability of

Ni for dehydrogenation and CeC breakage and the porous structure of SiO2 support

enhance the formation of filamentous coke with low deactivation. This catalyst is suitable

for use at 500 �C in a fluidized bed, in which the collision among particles causes the

removal of the external filamentous coke, thus minimizing the pore blockage of the SiO2.

At 700 �C, the coke content in the catalyst is low, with the coke being of filamentous nature

and with a highly graphitic structure.

Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

Introduction

The increasing demand of energy has driven the development

of processes for upgrading alternative sources to oil (coal,

; fax: þ34 94 6013500.e~na).93y Publications, LLC. Publ

natural gas, biomass) for the production of fuels and petro-

chemical raw materials. In this scenario, H2 plays a crucial

role as fuel (internal and external combustion engines and

fuel cells) and reactant (production of NH3 and hydro-

reforming reactions). The evolution towards sustainability

ished by Elsevier Ltd. All rights reserved.

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 9 ( 2 0 1 4 ) 1 2 5 8 6e1 2 5 9 6 12587

means a transition scenario between the periods of H2 pro-

duction from natural gas (by steam reforming) and water

(electrolysis), whose technology is not expected to be viable in

a short term for large-scale H2 production [1]. In this transi-

tion, the emerging raw material is the lignocellulosic biomass

(different types separated from the food chain), given that its

renewable nature means availability and zero net CO2 gener-

ation and the technology for its upgrading is considerably

developed [2].

The routes for producing H2 from lignocellulosic biomass

may be grouped into a) direct (high temperature pyrolysis,

catalytic pyrolysis, gasification and biological processes) and;

b) indirect, with intermediate steps for producing oxygenates

for their subsequent reforming [3]. Among the biomass-

derived oxygenates, bioethanol (i.e. aqueous ethanol pro-

duced by sugar fermentation), which is non-toxic and easy to

store, handle and transport in a safe way, is the one with the

best perspectives for large-scale production of H2 due to the

high worldwide production of bioethanol by sugar fermenta-

tion and, specially, to the improvements in its production

from lignocellulosic biomass, with this production being

enhanced by the rapid technological development of enzy-

matic hydrolysis [4].

Steam reforming is the most interesting process for

obtaining H2 from ethanol, as it gives way to the highest H2

yield at temperatures lower than those required for the steam

reforming of other fuels. Furthermore, although it is an

endothermal process, it avoids the energy-demanding pro-

cesses for water separation, which are required when bio-

ethanol is valorized as fuel or following other routes [5,6]. The

steam reforming of ethanol (SRE) occurs according to the

following stoichiometry:

CH3CH2OH þ 3H2O / 6H2 þ 2CO2 (1)

From the results of the effect of temperature on products

distribution [5,7e10] and of sensitive spectroscopic methods,

such as diffuse reflectance infrared Fourier spectroscopy

[11e14], reaction schemes have been proposed in which the

main route is the dissociative adsorption of ethanol as ethoxy

species, which further dehydrogenate to acetaldehyde.

Likewise, other reactions also occur in parallel, which

decrease H2 yield and cause the presence of by-products in the

reaction medium. These are: ethanol decomposition to CO,

CH4 and H2; ethanol dehydration to ethylene; acetaldehyde

decomposition to CO and CH4. These by-products can give

way to a decrease in H2 yield through reactions like metha-

nation of CO and CO2, or they can increase H2 yield according

to the WGS reaction and CH4 reforming. Likewise, undesired

reactions take place that contribute to catalyst deactivation by

coke formation, such as the following [5,7,8,14]:

Ethylene oligomerization: nC2H4 / oligomers / coke (2)

Dehydrogenation of methane: CH4 / C þ 2H2 (3)

Boudouard reaction: 2CO ⇔ C þ CO2 (4)

Catalysts of transition metals and of noble metals have

been used for obtaining high yields of H2 and minimizing the

yields of by-products. The most studied transition metal is Ni,

which has been used on different supports (Al2O3, Al2O3eLa,

SiO2, MgO, CeO2, CeO2eZr, CeO2ePr, TiO2, ZrO2) [15e28]. Co

catalysts on different supports (Al2O3, SiO2, MgO, ZnO, TiO2,

CeO2) [23,29e34] have also been widely studied. Noble metal

catalysts also perform well, particularly those of Rh due to its

activity for CeC bond breakage [35], although their high cost

does not justify their practical applications [36e39]. Other

authors have studied bimetallic catalysts prepared using

transition metals [40] and combining a noble metal with a

transition one [41,42].

In addition to high activity and H2 selectivity, a highly

efficient catalyst for SRE should be stable and, also, the oper-

ating conditions should be suitable for minimizing the causes

of catalyst deactivation. Operation at low temperature

(<600 �C) is interesting in order to i) avoid the sintering of

metallic sites, ii) lower the energy requirements and iii) favour

the thermodynamic equilibrium of water-gas shift (WGS) re-

action, which is important for increasing H2 yield and

lowering the CO concentration in the product stream. Never-

theless, under these conditions, Ni and Co catalysts are very

active for coke formation [43]. This formation depends on the

operating conditions and the properties of the catalyst, and an

increase in the steam/ethanol ratio in the feed contributes to

attenuating this problem [14]. Likewise, the properties of the

support also play an important role in the nature and growth

of the coke [44e46]. Thus, the capability of the support for

dehydration should be hindered in order to avoid the forma-

tion of ethylene, which is considered a precursor in the for-

mation of the encapsulating coke that deactivates the

catalysts (Eq. (2)) [20].

Comparisons are difficult to establish between the stability

of the different catalysts studied in literature and the infor-

mation on the deactivation mechanisms, given that different

authors have used different operating conditions (tempera-

ture, steam/ethanol molar ratio, space velocity). From the re-

sults, it is generally accepted that the main cause of activity

loss is blockage of metallic sites by encapsulating coke,

whereas the coke with filamentous structure and with a

noticeable porosity, does not hinder ethanol adsorption and

the formation of ethoxy intermediates. Consequently, when

coke is constituted by carbon structured filaments, the active

sites are not blocked and deactivation is only noticeable when

the growth of the filaments is very high and the access of

ethanol and water to the pores of the catalyst is blocked

[47e50].

The aim of this paper is to contribute to the knowledge of

deactivation by coke deposition on Ni and Co catalysts on

different supports (Al2O3 (alone or promoted with La), and

SiO2). Special attention has been paid to catalyst character-

ization in order to determine the relationship between the

nature and properties of the coke and the properties of the

catalysts and reaction temperature.

The kinetic runs have been carried out in a fluidized bed

reactor, whosemain advantage is the isothermicity of the bed,

which is important in this highly endothermic process.

Moreover, in the fluidized bed reactor the deactivation is not

affected by the longitudinal profile of reaction medium

component concentration along the reactor due to the mixing

regime of catalyst particles. It should be noted that particle

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 9 ( 2 0 1 4 ) 1 2 5 8 6e1 2 5 9 612588

motion in the fluidized reactor favours the breakage of parti-

cles enlarged with fibrous coke, which allows operating for

long periods under conditions of high coke depositionwithout

flow pattern deviation problems derived from bed plugging.

This behaviour of the fluidized bed is an advantage with

respect to the fixed bed for the scaling up, because the coke

formed could be separated continuously, thus keeping a

constant pressure drop in the reactor.

Experimental

Catalysts preparation and characterization

Ni catalysts have been prepared with 10 wt% nominal content

of Ni on SiO2, a-Al2O3 and La2O3eaAl2O3 supports. The catalyst

supported on SiO2 (called 10Ni/Si) has been prepared by

incipient wet impregnation at 70 �C under vacuum using an

aqueous Ni(NO3)2.6H2O solution (Carlo Erba Reagents, 99%) and

SiO2 (silica gel 100 ofMerck). The solid is dried at 110 �C for 24 h

and subsequently calcined in air at 550 �C (following a heating

ramp of 5 �C/min) for 4 h in order to ensure the complete

decomposition of the metallic precursor [51]. Ni/a-Al2O3 and

Ni/La2O3eaAl2O3 catalysts (denoted 10Ni/Al and 10Ni/LaAl,

respectively) have also been prepared by incipient wet

impregnation at 70 �C under vacuum [17]. La2O3eaAl2O3 sup-

port (with La2O3 nominal content of 10wt%) has been obtained

by impregnating the a-Al2O3 (supplied by Derivados del Fluor

S.A., Spain), with a 150e300 mm particle size under the condi-

tions indicated in the literature [52], using an aqueous

La(NO3)3$6H2O solution (Alfa Aesar, 99%). The solids obtained

are dried at 110 �C for 24 h and calcined at 650 �C (following a

heating ramp of 5 �C/min) for 3 h.

Co catalysts have also been prepared by incipient wet

impregnation, using an aqueous Co(NO3)2.6H2O solution

(Panreac, 98%) and following themethodology described in the

literature [53]. The supports used are: SiO2, a-Al2O3 (the same

used in the preparation of Ni catalysts). After impregnation

the solids are dried at 110 �C for 24 h and calcined at 600 �C for

6 h, following a heating ramp of 5 �C/min. The catalysts have

been prepared with a nominal Co content of 10 wt% and have

been named 10Co/Si and 10Co/Al.

The metal content of the catalysts has been determined by

ICP-AES in an ARL 3410 with Ar as carrier and plasma torch.

The physical properties of fresh and deactivated catalysts

have been determined by N2 adsorption-desorption in a

Micromeritics ASAP 2010C and by Hg porosimetry in a Micro-

meritics Autopore 9220. The Micromeritics ASAP 2010C has also

been used for analysing the metal surface by H2 chemisorp-

tion. This device has also been used for hydrogen chemi-

sorption measurements for quantifying the metal dispersion

and the specific metal surface area (m2/gmetal) according to

the following procedure: after an initial evacuation step, the

metal phase was reduced for 6 h with H2 flow, following a

heating ramp to 700 �C or 850 �C. Then, impurities were

removed from the catalyst surface (evacuation with He) and

analyses were carried out at 35 �C. The irreversibly chem-

isorbed H2 (adsorbed on the metal surface) was determined

by the method of double isotherm, considering an irreversible

adsorption on the metal phase, a reversible adsorption on the

support and assuming an adsorption stoichiometry of

H:M ¼ 1:1 (M ¼ Ni or Co).

The temperature triggering the reduction for each catalyst

has been determined by temperature programmed reduction

(TPR, AutoChem 2920 from Micromeritics) by passing a 50 cm3/

min streamwith 10% volume of H2 in Ar. The X-ray diffraction

(XRD) analysis of the catalysts was performed on a Bruker D8

Advance diffractometer with a CuKa1 (l ¼ 0.154056 nm) radia-

tion. The reduced catalysts were transferred to the XRD

apparatus in a sealed container. The average particle size was

determined by applying the Scherrer formula to their corre-

sponding peaks.

The coke content deposited on the deactivated catalysts

has been measured directly by weight difference between the

fresh and deactivated catalyst. Additionally, for the operating

conditions of low coke deposition, the coke deposited was

determined by temperature programmed oxidation (TPO) in a

thermobalance (TA Instruments SDT 2960) connected online

with a mass spectrometer (Balzers Instruments Thermostar).

Temperature programmed hydrogenation (TPH) (in AutoChem

2920 from Micromeritics, by passing a 40 cm3/min stream with

10% volume of H2 in Ar), elemental analysis of coke deposits

(in a Leco CHN-932 analyzer, with ultra-microbalance Sartorius

M2P connected on line for data processing) and scanning

electron microscopy of the deactivated catalysts (JEOL/JSM-

7000f, equipped with analyzer EDX and operating under

30 KeV) have also been performed.

Catalyst particles grow through coke deposition, i.e., they

are progressively coatedwith the filamentous coke (formed on

the metallic sites) that is displaced through the pores towards

the outside of the catalyst particle. Nevertheless, although the

solid flow pattern is the one typical of fluidized beds (random),

the particles are not of uniform size because they undergo

attrition and the partial breakage of small fractions of the

external filamentous coke. Therefore, the proportion of the

two coke fractions (encapsulating and filamentous) in the

particle analysed depends on particle size. Accordingly, and in

order to study the coke deposited on the porous structure of

the catalyst (inside the particles), which is responsible for

deactivation, the analysis has been carried out mainly with

particles whose size is in the 150e300 mm range, which is that

corresponding to the fresh catalyst. The particles of deacti-

vated catalyst have been studied once they have been sepa-

rated into three fractions of different size by sieving.

Reaction conditions and equipment for reaction and productanalysis

The kinetic runs have been carried out in automated reaction

equipment (Microactivity Reference from PID Eng & Tech) pro-

vided with an isothermal fluidized-bed reactor (22 mm of in-

ternal diameter and total length of 460mm) connected on-line

to a gas chromatograph (Agilent Micro GC 3000) for product

analysis [54]. The hydrodynamic properties of the bed have

been improved by mixing the catalyst (particle size between

150 and 300 mm) with an inert solid (CSi, particle size between

60 and 90 mm) at a catalyst/inert ratio of 1:4. The Micro-GC is

provided with four modules for the analysis of: 1) permanent

gases; 2) oxygenates, light olefins (C2eC3) and water; 3) C2eC6

hydrocarbons; 4) C6eC12 hydrocarbons and oxygenate

0

0.2

0.4

0.6

0.8

1

0 4 8 12 16 20

Time on stream, h

X

10Ni/Si

10Ni/Al

10Ni/LaAl

10Co/Si

10Co/Al a

1YH2 10Ni/Si

10Ni/Al

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 9 ( 2 0 1 4 ) 1 2 5 8 6e1 2 5 9 6 12589

compounds. The compounds were quantified and identified

using calibration standards of known concentration. The

balance of atoms (C, H, O) is closed in all runs above 98%.

Prior to the catalytic tests, the catalysts have been reduced

in situ in a hydrogen flow (10 vol.%) at the corresponding

reduction temperature, which is 500 �C for the catalysts sup-

ported on SiO2, and 700 �C for those supported on Al2O3 (alone

or promoted with La2O3). The kinetic runs have been carried

out under the following operating conditions: 500 and 700 �C;pressure, 1.2 bar, ethanol partial pressure in the feed (diluted

with He), 0.11 bar; steam/ethanol molar ratio, 6; space time,

0.14 gcatalyst h/gethanol; time on stream, up to 20 h. These con-

ditions have been selected based on a preliminary study [55]

for delimiting the suitable ranges of the operating conditions

required for a stable fluidized bed regime and also for

obtaining kinetic results with different levels of catalyst

deactivation. The cause of deactivation at 500 �C is exclusively

coke deposition, whereas at 700 �C the sintering of the

metallic sites contributes to deactivation.

0

0.2

0.4

0.6

0.8

0 4 8 12 16 20

Time on stream, h

10Ni/LaAl10Co/Si10Co/Al

b

Fig. 1 e Evolution with time on stream of ethanol

conversion (graph a) and hydrogen yield (graph b) for

different catalysts. Reaction conditions: 500 �C; steam/

ethanol molar ratio, 6; space time, 0.14 gcatalyst h/gethanol;

ethanol partial pressure in the feed, 0.11 bar.

Results

Catalysts' performance

Fig. 1 shows the evolution with time on stream of ethanol

conversion (graph a) and H2 yield (graph b) for the different

catalysts in the ethanol steam reforming at 500 �C, steam/

ethanol molar ratio of 6, space time of 0.14 gcatalyst h/gethanol,

and ethanol partial pressure of 0.11 bar. Ethanol conversion

has been defined as:

X ¼ FE;0 � FE

FE;0(5)

where FE,0 and FE are ethanol molar flow rate at the inlet and

outlet of the fluidized bed.

H2 yield has been calculated as the fraction of the

maximum that may be obtained according to stoichiometry

with the ethanol fed into the reactor:

YH2¼ FH2

6$FE;0(6)

It is observed that under these operating conditions,

ethanol conversion at zero time on stream is almost full with

all the catalysts (Fig. 1(a)). Furthermore, H2 yield is slightly

higher for Co catalysts (empty symbols) than for Ni catalysts

(solid symbols) (Fig. 1(b)), which is due to the latter favouring

methanation reactions at this temperature (500 �C) [56,57].

The rate of decrease in ethanol conversion with time on

stream is different for the different catalysts. For Co catalysts

(empty symbols) conversion decreases rapidly in 2 h (Fig. 1(a)),

and subsequently it remains almost constant for the catalysts

supported on a-Al2O3, whereas for the catalyst supported on

SiO2 the conversion continues decreasing slightly along the

following 18 h.

The initial deactivation rate for Ni catalysts is slower

than for Co catalysts, although it steadily decreases along

time on stream, especially for catalysts supported on a-

Al2O3 (alone or doped with La). It is remarkable the good

behaviour of the catalyst supported on SiO2 (10Ni/Si

catalyst), which undergo an increase in conversion for times

on stream higher than 10 h.

H2 yield decreases with time on stream due to catalyst

deactivation (Fig. 1(b)), although in a less pronounced way

than ethanol conversion, which indicates that the reforming

reaction is less affected by deactivation than other secondary

reactions in the kinetic scheme, which also contribute to

ethanol conversion. It is also observed that H2 yield goes

through a minimum between 4 and 8 h time on stream for

10Co/Si and 10Ni/Si catalysts.

It should be pointed out that the only cause of deactivation

of the catalysts studied at this temperature in Fig. 1 is coke

deposition, which is studied in detail in the following sections.

Furthermore, there is no sintering of metallic sites at 500 �C,given that X ray diffractometry recorded the same average

metal crystal size for fresh and deactivated catalysts (6 and

11 nm for 10Ni/LaAl and 10Ni/Si catalysts, respectively).

Furthermore, once deposited coke is removed by combustion,

the catalysts recover a similar value of metal surface area

(Table 3), determined by H2 chemisorption, which confirms

Fig. 2 e SEM images of 10Ni/Si catalyst (150e300 mm size)

used for 1 h (a) and for 20 h (b). Reaction conditions: 500 �C;steam/ethanol molar ratio, 6; space time, 0.14 gcatalyst h/

gethanol; ethanol partial pressure in the feed, 0.11 bar.

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 9 ( 2 0 1 4 ) 1 2 5 8 6e1 2 5 9 612590

that the catalysts studied at 500 �C do not undergo metal

sintering, and therefore the cause of deactivation is coke

deposition.

Deactivated catalysts characterization

Evolution with time on stream of the amount and structure ofthe cokeThe high coke deposition rate of Ni and Co catalysts and the

growth of coke towards the exterior of catalyst particles cause

a significant increase in particle size, which is a well known

fact in the literature [58]. Moreover, the use of a fluidized bed

reactor favours the breakage of the catalysts particles

enlarged with coke, which gives way to a heterogeneous

particle size distribution with different coke contents. As an

example of the results of this effect of coke deposition, the

values of particle size distribution of the solid in the bed

(constituted by used 10Ni/Si catalyst particles with coke) for

times on stream of 1, 3 and 20 h are set out in Table 1, together

with values of the corresponding total coke content (deter-

mined by weight difference between the fresh and the used

catalyst). It is observed that coke content increases almost

linearly with time on stream, and catalyst particle size also

increases. As observed, 21 wt% of catalyst particles have a size

higher than 630 mm after 20 h time on stream. The remaining

catalyst particles undergomore severe attrition, with external

filamentous coke being released.

The content and structure of the coke deposited as well as

the physical properties of the deactivated catalyst particles

vary with time on stream and particle size. Fig. 2 shows SEM

images of the particles with 150e300 mm size for 10Ni/Si

catalyst used for 1 h (Fig. 2(a)) and 20 h (Fig. 2(b)). These par-

ticles are the same size as the fresh catalyst, and therefore the

analyses correspond either to the coke deposited within the

porous structure or to a thin layer coating the particle. Fig. 3

corresponds to particles larger than 630 mm of the same

catalyst used for 20 h, and so they contain mainly filamentous

coke on the outside. Table 2 gathers the values of coke content

(determined by TPO analysis), C/H ratio (determined by

elemental analysis) and physical properties (BET surface area,

pore diameter and pore volume, determined by adsorp-

tionedesorption of N2) corresponding to fractions with

different size of 10Ni/Si catalysts used for 20 h.

The SEM images in Figs. 2 and 3 evidence the filamentous

structure of the coke deposited. For catalyst particles with

150e300 mm size and used for 1 h (Fig. 2(a)), Ni particles are

observed at the end of the carbon filaments, whose diameter

is similar to that of themetal particle, which is consistent with

Table 1 e Evolution with time on stream of coke contentand catalyst particles size distribution for 10Ni/Sicatalyst. Reaction conditions: 500 �C; steam/ethanolmolar ratio, 6; space time, 0.14 gcatalyst h/gethanol; ethanolpartial pressure in the feed 0.11 bar.

t, h Cc, wt% Fraction, wt%

150e300, mm 300e630, mm >630, mm

1 44 63 31 3

3 131 35 51 13

20 563 27 52 21

the mechanism proposed for filamentous coke formation [59].

For particles with the same size used for 20 h (Fig. 2(b)), a high

content of coke filaments is observed, with their sizes being

highly heterogeneous. In the fraction of particles with higher

size, >630 mm (Fig. 3), the content of carbon filaments is much

higher than on the smaller particles, and agglomerates are

formed, which makes it difficult to appreciate Ni metal

particles.

Given the fast growth of carbon filaments, coke content

increases as particle size increases (Table 2) and, moreover,

the C/H ratio of the coke (very high in all cases) also increases

with particle size, which evidences the steady development of

the carbon filaments towards more graphitic structures.

In all the fractions of the used 10Ni/Si catalyst with

different particle size, coke deposition causes a slight

decrease in BET surface area and pore diameter compared to

the values corresponding to fresh catalyst, whereas pore vol-

ume is more affected and decreases noticeably (Table 2).

These results are explained by the contribution of the surface

area of the filamentous coke, whereas the catalyst pores are

Fig. 3 e SEM image of 10Ni/Si catalyst (size > 630 mm) used

for 20 h. Reaction conditions: 500 �C; steam/ethanol molar

ratio, 6; space time, 0.14 gcatalyst h/gethanol; ethanol partial

pressure in the feed, 0.11 bar.

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 9 ( 2 0 1 4 ) 1 2 5 8 6e1 2 5 9 6 12591

partially blocked by the coke fractions (encapsulating and

filamentous) within the particle.

Comparison of coke deposition on different catalystsTable 3 sets out the results of coke deposited on the different

catalysts at 500 �C after 20 h time on stream. This table also

compares the physical properties (BET surface area, pore

volume and average pore size) of the fresh and deactivated

catalysts (fraction of particles with 150e300 mm size). As

observed, coke content is very high, specially that corre-

sponding to 10Ni/Si catalyst (563 wt%), and coke deposition is

significantly attenuated by the addition of La to the a-Al2O3

Table 2e Characteristics of used 10Ni/Si catalyst particlesof different size. Reaction conditions: 500 �C; steam/ethanol molar ratio, 6; space time, 0.14 gcatalysth/gethanol;ethanol partial pressure in the feed 0.11 bar; time onstream, 20 h.

dp, mm Cc, wt% C/H SBET, m2/g dpore, nm Vpore, cm

3/g

150e300

(fresh)

e e 290 9.5 0.92

150e300 175 8.8 256 7.1 0.37

300e630 483 9.9 269 7.3 0.40

>630 1478 10.0 280 7.2 0.43

Table 3 e Coke content and physical properties of fresh and deethanol molar ratio, 6; space time, 0.14 gcatalysth/gethanol; ethano

Catalyst Smetallic m2/gmetal Cc, wt% SBET

Fresh

10Ni/Si 14.9 563 290

10Ni/Al 28.1 134 69

10Ni/LaAl 25.4 15 43

10Co/Al 5.7 135 61

10Co/Si 4.7 67 278

support (15 wt% coke content). This result is consistent with

those in the literature, and is explained by the formation of

La2O2CO3, which is active for reacting with the coke and thus

cleaning the Ni crystals [18,52]. Furthermore, the basic

character of La2O3 could also contribute to this result, as it

may attenuate the low acidity of a-Al2O3 under operating

conditions (with a high water content at high temperature)

and the subsequent formation of ethylene (which is the

precursor of the encapsulating coke, Eq. (2)) by ethanol

dehydration.

Furthermore, the comparison of the physical properties of

the fresh and deactivated catalysts evidences the porous

structure of coke deposits, which are constituted by carbon

filaments. The SEM images in Fig. 4 for the different deacti-

vated catalysts confirm this filamentous structure of the coke,

which is formed by carbon filaments highly heterogeneous in

both size and shape.

The presence of carbon filaments has a higher effect on

the physical properties of the catalysts on supports of low

porosity (Al2O3), for which the BET surface area and pore

volume of the deactivated catalysts increase significantly

compared to the fresh catalysts. The decrease in pore

diameter may be due to a partial blockage of the support

porous structure, which is evident for the catalysts with

higher values of BET surface area and pore volume, such as

10Ni/Si and 10Co/Si, in which the development of carbon

filaments is more favoured. For these catalysts, the partial

pore blockage explains the decrease in the physical proper-

ties. These results are consistent with those by Karim et al.

[59] for Co/Zn catalysts and by Wu and Williams [21,22] for a

Ni/Si catalyst.

The increase in reaction temperature above 600 �C has the

favourable effect of attenuating coke deposition due to gasi-

fication, which is favoured by the increase in temperature

and water content, although Ni sintering also occurs. The

coke content deposited above 600 �C is lower than that

deposited at 500 �C and is a consequence of a balance be-

tween its formation and removal by gasification. Table 4 sets

out the low coke contents deposited on the catalysts deacti-

vated at 700 �C (determined from the TPO curves shown in

Section 3.3). The values of metal crystal size (determined by

applying the correlation by DebyeeScherrer to the peaks

corresponding to NiO and Co3O4 species in the XRD dif-

fractograms) are also reported, which undergo a noticeable

sintering at this temperature. Nevertheless, the reaction

indices remain constant with time on stream and are very

similar for all the catalysts studied (H2 yield higher than 80%)

because operation under the conditions in Table 4

activated catalysts. Reaction conditions: 500 �C; steam/l partial pressure in the feed 0.11 bar; time on stream, 20 h.

, m2/g Vpore, cm3/g dpore, nm

20 h Fresh 20 h Fresh 20 h

256 0.92 0.37 9.5 7.1

179 0.18 0.25 9.0 7.1

89 0.15 0.15 11.5 8.1

181 0.17 0.43 9.2 10.0

266 0.90 0.46 9.9 7.1

Fig. 4 e SEM images of the catalysts (size 150e300 mm) used for 20 h. Reaction conditions: 500 �C; steam/ethanol molar ratio,

6; space time, 0.14 gcatalyst h/gethanol; ethanol partial pressure in the feed, 0.11 bar.

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 9 ( 2 0 1 4 ) 1 2 5 8 6e1 2 5 9 612592

corresponds to thermodynamic equilibrium (excess of cata-

lyst), which does not allow monitoring the deactivation that

actually occurs. The BET surface area and pore volume

decrease compared to fresh catalyst, whereas pore diameter

increases slightly, with these changes being attributable to

coke deposition (porous and with fibrous nature) on the

porous structure of the catalysts, which explains that the

changes are more noticeable for the catalysts with a higher

coke content. It should be noted that the coke content

deposited at this temperature is very low or even negligible in

some cases, due to coke gasification.

Relationship between deactivation and coke content and natureThere is no clear relationship between the content of the

coke deposited on the catalysts (Table 3) and the corre-

sponding deactivation rate (Fig. 1), which evidences that

catalyst deactivation also depends on other factors, such as

Table 4 e Coke content, average size of metal crystals and phyconditions: 700 �C; steam/ethanol molar ratio, 6; space time, 00.11 bar; time on stream, 20 h.

Catalyst Cc, wt% dMO, nm SBET

Fresh 20 h Fresh

10Ni/Si 0.30 11 17 290

10Ni/Al 0.26 8 e 69

10Ni/LaAl 0 6 10 43

10Co/Si 0 11 19 278

the nature of the coke and the physicalechemical properties

of the catalysts. The limited deactivation observed for 10Ni/

Si catalyst even for a high coke content deposited on this

catalyst may be explained by the fibrous nature of this coke,

which does not block metallic sites. Moreover, pore blockage

is minimized by the accessibility of reactants to the porous

structure of the SiO2 support. The results of BET surface area

for the fresh catalysts supported on SiO2 are 290 m2/g (10Ni/

Si) and 278 m2/g (10Co/Si), which are much higher than

those corresponding to the catalysts prepared on other

supports.

Nature and evolution of coke deposits

The temperature programmed hydrogenation (TPH) profiles

for the different used catalysts (particles with 150e300 mm

size) (Fig. 5) show a major peak at 500e550 �C corresponding

sical properties of fresh and deactivated catalysts. Reaction.14 gcatalyst h/gethanol; ethanol partial pressure in the feed

, m2/g Vpore, cm3/g dpore, nm

20 h Fresh 20 h Fresh 20 h

231 0.92 0.88 9.5 11.6

54 0.18 0.18 9.0 10.7

e 0.15 e 11.5 e

e 0.90 e 9.9 e

0 200 400 600 800 1000Temperature, ºC

TCD

sig

nal,

a.u.

10Ni/LaAl

10Ni/Si

10Co/Al

Fig. 5 e TPH profiles of the catalysts (size 150e300 mm) used

for 20 h. Reaction conditions: 500 �C; steam/ethanol molar

ratio, 6; space time, 0.14 gcatalyst h/gethanol; ethanol partial

pressure in the feed, 0.11 bar.

Fig. 6 e TPO profiles of the catalysts (size 150e300 mm) used

for 20 h. Graph a: Ni catalysts. Graph b: Co catalysts.

Reaction conditions: 500 �C; steam/ethanol molar ratio, 6;

space time, 0.14 gcatalyst h/gethanol; ethanol partial pressure

in the feed, 0.11 bar.

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 9 ( 2 0 1 4 ) 1 2 5 8 6e1 2 5 9 6 12593

to the hydrogenation of carbon filaments, and another minor

peak at 300e350 �C corresponding to other fraction of poly-

meric and non-structured coke [50]. The TPH profile of the

used 10Ni/LaAl catalyst has another peak at low temperature

(~220 �C) that may be assigned to monoatomic coke. Both

types of non-structured cokes (polymeric and monoatomic)

are adsorbed on metal sites and are efficient for their block-

ing, thereby hindering the adsorption of ethanol and water

[60]. Consequently, the loss of catalyst activity due to the

blockage of metallic sites is faster for the catalysts with lower

dispersion of metallic sites and with supports of lower

accessibility for reactants, which explains that Co and Ni

catalysts supported on Al2O3 have a more noticeable deacti-

vation than the Ni catalyst supported on SiO2 (10Ni/Si cata-

lyst) (Fig. 1), even though the latter has a higher coke content

(Table 3). Furthermore, the presence of monoatomic carbon

in the coke deposited on 10Ni/LaAl explains that its deacti-

vation is similar to that of 10Ni/Al catalyst (Fig. 1), which has

a higher coke content (Table 3), but with a more structured

nature.

Fig. 6 shows the temperature programmed oxidation (TPO)

profiles for Ni (Fig. 6(a)) and Co (Fig. 6(b)) deactivated catalysts.

According to the literature, the peaks at low temperature

(<450 �C) correspond to amorphous coke (monoatomic or

polymeric), whose combustion is activated by the metal on

which it is adsorbed. The oxidation of fibrous coke (not

adsorbed on the metallic sites) occurs above 450 �C, and the

peaks at higher temperature (550e600 �C) correspond to the

combustion of coke deposits with different graphitization

degrees, including highly developed carbon filaments

[18,38,50,61,62].

As observed in Fig. 6(a), Ni catalysts have a major peak at

550 �C corresponding to the combustion of filamentous coke.

The shift of this peak towards a slightly higher temperature

for 10Ni/Si catalyst is explained by its higher coke content,

which is more evolved towards a graphitic structure, as has

been previously mentioned in the analysis of SEM images.

Although this catalyst has a high total coke content (Table 3),

the low content of encapsulating coke (that burns below

450 �C) explains its lower deactivation rate (Fig. 1).

These results evidence the different role of the different

types of coke (encapsulating and filamentous) in the deacti-

vation, and also explain the fact that both ethanol conversion

and H2 yield pass through aminimumwith time on stream for

10Ni/Si catalyst (Fig. 1). This minimum is explained by the

evolution of the fibrous structure of the coke, which enhances

the vacancy of metallic sites occupied by encapsulating coke

for low values of time on stream. Moreover, an increase in

activity caused by the re-dispersion of Ni particles due to the

growth of carbon filaments should not be excluded. This result

is consistent with that obtained by Carre~no et al. [63] with a Ni

catalyst supported on carbon.

The TPO profiles for Co catalysts (Fig. 6(b)) are slightly

different to those corresponding to Ni catalysts, as evidenced

by the three clearly differentiated peaks at 410, 480 and 550 �C.The first one corresponds to encapsulating coke with different

polymerization degree, which is adsorbed on metal sites, and

the others correspond to filamentous coke with different

graphitization degree. The high encapsulating coke content

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 9 ( 2 0 1 4 ) 1 2 5 8 6e1 2 5 9 612594

explains the fast deactivation of Co catalysts for low values of

time on stream (Fig. 1).

The values of coke content of the used catalysts (deter-

mined from the TPO results in Fig. 6) and of C/H ratio of the

coke (determined by elemental analysis) are set out in Table 5.

In general, the samples with higher coke content have higher

C/H ratio, which evidences that coke filaments evolve towards

more graphitic structures.

In the SEM image of 10Ni/Si catalyst deactivated at 700 �C(Fig. 7) carbon filaments are not observed due to the low coke

content, because it undergoes gasification at this temperature.

In the TPO profiles in Fig. 8, which correspond to the coke

combustion of catalysts deactivated at 700 �C, a major peak is

observed, which shifts from 605 to 650 �C as coke content in-

creases, and corresponds to the oxidation of a highly graphitic

coke.

Fig. 7 e SEM image of 10Ni/Si catalyst (150e300 mm size)

used for 20 h. Reaction conditions: 700 �C; steam/ethanol

molar ratio, 6; space time, 0.14 gcatalyst h/gethanol; ethanol

partial pressure in the feed, 0.11 bar.

Conclusions

It has been proven that the coke nature and the physical

properties of the support are the determining factors on the

catalyst deactivation severity in the steam reforming of

ethanol at 500 �C. The cause of deactivation of Ni and Co

catalysts at 500 �C is coke deposition, which is constituted by

two fractions of different nature, from which that of mon-

oatomic and polymeric coke (encapsulating coke) adsorbed

on the metal (with TPO peaks below 450 �C) is the main

responsible for deactivation by blocking metallic sites. A high

metal dispersion hinders metallic site blockage, which ex-

plains the higher stability of Ni catalysts compared to Co

catalysts.

The fraction of coke with fibrous nature (with TPO peaks

above 500 �C), constituted by filamentous carbon, has a

lower effect on deactivation. Furthermore, the porous

structure of SiO2 (high BET surface area) favours the devel-

opment of the filaments and diffusion of reactants, which

explains that Ni catalyst supported on SiO2 (10 Ni/Si catalyst)

undergoes a moderate deactivation with a high coke

content.

The size of used catalyst particles increases with time on

stream, as they are progressively enlarged by coke filaments,

but the collisions between particles (favoured by the mixing

regime of the fluidized reactor) cause partial breakage of the

coke on their surface.

Consequently, the 10Ni/Si catalyst has an interesting

behaviour at 500 �C, as the synergy between the

Table 5 e Coke content and C/H atomic ratio of usedcatalyst particles with 150e300 mm size. Reactionconditions: 500 �C; steam/ethanol molar ratio, 6; spacetime, 0.14 gcatalyst h/gethanol; ethanol partial pressure inthe feed 0.11 bar; time on stream, 20 h.

Catalyst Cc, wt% C/H

10Ni/Si 201 8.76

10Ni/Al 80 6.14

10Ni/LaAl 15 4.26

10Co/Al 92 6.81

10Co/Si 50 6.49

dehydrogenating capability of Ni and the porous structure of

the support (SiO2) favours the rapid formation of mainly

fibrous coke, thus minimizing the blockage of Ni crystals.

Moreover, this catalyst is suitable for use in a fluidized bed

reactor, as the filamentous coke is released by the collisions

between particles, which contributes (together with the suit-

able porous structure of SiO2) to minimizing pore blockage

problems. The interest of operating at a mild temperature lies

in avoiding catalyst sintering, thereby reducing energy re-

quirements and enhancing the WGS reaction.

Coke deposition is drastically attenuated at 700 �C, tem-

perature at which coke gasification is very rapid, and the

remaining coke is highly condensed (high C/H ratio). Never-

theless, metal site sintering occurs at this temperature, with a

noticeable increase in metal crystal size.

Fig. 8 e TPO profiles of the catalysts (size 150e300 mm) used

for 20 h. Reaction conditions: 700 �C; steam/ethanol molar

ratio, 6; space time, 0.14 gcatalyst h/gethanol; ethanol partial

pressure in the feed, 0.11 bar.

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 9 ( 2 0 1 4 ) 1 2 5 8 6e1 2 5 9 6 12595

Acknowledgements

This work has been carried out with financial support from

the Ministry of Science and Technology of the Spanish Gov-

ernment (Projects CTQ2009-13428 and CTQ2012-35263), the

University of the Basque Country (UFI 11/39) and the Basque

Government (Project IT748-13). Carolina Montero is grateful

for the Ph.D. grant from the National Secretariat of Higher

Education, Science, Technology and Innovation of Ecuador-

SENESCYT (Contract 20110560).

Nomenclature

10Co/Si, 10Co/Al Co catalysts (with 10 wt% nominal content

of Co) supported on SiO2 and a-Al2O3,

respectively

10Ni/Si, 10Ni/Al, 10Ni/LaAl Ni catalysts (with 10 wt% nominal

content of Ni) supported on SiO2,

a-Al2O3 and La2O3eaAl2O3,

respectively

Cc coke content, wt%

dpore pore diameter, nm

dMO average size of the metal crystal, nm

FE, FE,0 molar flow rate of ethanol at the reactor outlet and in

the feed, respectively

FH2 molar flow rate of H2 at the reactor outlet

SBET BET surface area, m2/g

Smetallic metal surface area, m2/gmetal

SRE steam reforming of ethanol

Vpore pore volume, cm3/g

WGS water-gas shift reaction

X ethanol conversion

YH2 hydrogen yield

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