High-throughput assessment of catalyst stability during autothermal reforming of model biogas

Article abstract of DOI:10.1039/c5cy00702j

The long-term stability of Ni-based catalysts for autothermal reforming of model biogas has been assessed in a six parallel-flow reactor set-up implemented to test simultaneously six different catalysts at 700 °C under a feed consisting of 42% H₂O, 14% CH₄, 9% CO₂, and 7% O₂ in argon. The reproducibility of catalyst performance measured in the 6 parallel reactors was ascertained using a commercial Ni-based catalyst. A screening of 12 catalyst formulations identified 5-0.05 wt% Ni-Rh/MgAl₂O₄ as a robust catalyst for autothermal reforming of model biogas over 200 h time-on-stream.

Full text of DOI:10.1039/c5cy00702j

View Article Online
View Journal
Catalysis
Sc ience &
Technology
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: M. Luneau, Y.
Schuurman, F. C. Meunier, C. Mirodatos and N. Guilhaume, Catal. Sci. Technol., 2015, DOI:
1
0.1039/C5CY00702J.
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the Ethical guidelines still
apply. In no event shall the Royal Society of Chemistry be held
responsible for any errors or omissions in this Accepted Manuscript
or any consequences arising from the use of any information it
contains.
www.rsc.org/catalysis

Page 1 of 8
Catalysis Science & Technology
View Article Online
DOI: 10.1039/C5CY00702J
Catalysis Science & Technology
RSCPublishing
ARTICLE
High-throughput assessment of catalyst stability
during autothermal reforming of model biogas
Cite this: DOI: 10.1039/x0xx00000x
M. Luneau, Y. Schuurman, F.C. Meunier , C. Mirodatos, N. Guilhaume*
The longꢀterm stability of Niꢀbased catalysts for autothermal reforming of model biogas has been assessed
in a six parallelꢀflow reactor implemented to test simultaneously six different catalysts at 700°C under a
Received
Accepted
feed consisting of 42% H
2 4 2 2
O, 14% CH , 9% CO , 7% O in argon. The reproducibility of catalyst
DOI:
performances measured in the 6 parallel reactors was ascertained using a commercial Niꢀbased catalyst. A
www.rsc.org/
screening of 12 catalyst formulations identified 5ꢀ0.05 wt.% NiꢀRh/MgAl
autothermal reforming of model biogas over 200 h timeꢀonꢀstream.
2 4
O as a robust catalyst for
Introduction
Waterꢀgas shift (WGS):
CO + H O ↔ CO + H
2
∆H = ꢀ 41.2 kJ/mol (Eq. 3)
2
2
Biogas is a complex gas mixture produced from the anaerobic
digestion of biomass, which contains methane and carbon Partial oxidation is a slightly exothermic reaction, actually the
dioxide as main components [1]. Biogas can be combusted result of two consecutive reactions: first the highly exothermic
directly, though exhibiting a lower heating value than natural combustion of methane at the beginning of the catalyst bed,
gas. Biogas can also be reformed into hydrogen, which is a base followed by steam and CO2 reforming of the remaining
feedstock for the chemical industry and can also be used as a methane and the WGS reaction.
fuel in fuelꢀcells to produce electricity [2].
Since biogas contains essentially methane and carbon dioxide, CH + ½O ↔ CO + 2H
2
Catalytic Partial Oxidation (CPO):
∆H = ꢀ36 kJ/mol
(Eq. 4)
4
2
the dry reforming (DR) of methane (Eq. 1) is an obvious
pathway to produce syngas:
Autothermal reforming (ATR) is seen as a practical option to
minimize energy supply to the reactor. ATR consist of adding
oxygen (or air), which results in combustion of a part of the
CH + CO ↔ 2CO + 2H
2
∆H = +247 kJ/mol (Eq. 1)
4
2
Yet, this reaction is strongly endothermic and must be methane into carbon dioxide and water followed by reforming
conducted at high temperatures (800ꢀ900°C), whereas the and WGS reactions.
CH /CO ratio in biogas (generally closer to 1.5 than to 1) is
4
2
not suitable for complete methane conversion. Methane can The general equation for ATR is:
also be converted into syngas by steam reforming (SR, Eq. 2):
CH + ½
x
O +
y
CO + (1ꢀxꢀy)H O ↔ (y+1)CO + (3ꢀxꢀy)H
2
4
2
2
2
CH + H O ↔ CO + 3H ∆H = +206.2 kJ/mol (Eq. 2)
(Eq. 5)
4
2
2
in which the O /CH ratio has to be adjusted to reach the
2
4
SR is also an endothermic reaction but yields more hydrogen thermal balance between exothermic and endothermic
than DR. SR limits carbon deposition and steam is generally reactions. According to thermodynamic calculations (HSC
introduced in excess in order to shift the waterꢀgas shift chemistry software) formation of CO and H by steam and dry
2
equilibrium (Eq. 3) towards more H and less CO production at reforming is favored at temperature above ca. 630°C.
2
the reactor outlet.
Catalytic reforming of methane has been widely studied and
carried out using different catalysts. Few studies have focused
on biogas. The most commonly used catalysts are nickelꢀ and
noble metalꢀbased catalysts (Ru, Rh, Pd, Ir, Pt) [3][4]. Nickelꢀ
based catalysts are often used because those combine low cost
and acceptable catalytic activity. The main issues with nickel
are the deactivation caused by deposition of carbon and/or
This journal is © The Royal Society of Chemistry 2013
J. Name., 2013, 00, 1-3 | 1

Catalysis Science & Technology
Page 2 of 8
View Article Online
ARTICLE
10.1039/C5CY00702J
CatalysisDOScI:ience & Technology
sintering of metal particles [5][6]. Noble metals, notably Pt and nitrates (SigmaꢀAldrich and Fluka) with a Al:Mg molar ratio of
ꢀ
1
Ir, are more active than Ni [7]. Noble metals present better 2:1 (concentrations of 0.66 and 0.33 mol L respectively) was
performances since they are less prone to oxidation, coking and added drop by drop into an aqueous solution of ammonium
sintering [8], but are more expensive and hence not the carbonate (SigmaꢀAldrich) under vigorous stirring. The
preferred option. Furthermore, the nature of the catalyst ammonium carbonate solution was prepared in order to contain
supports and metalꢀsupport interaction may play a significant large excess of carbonates, ensuring precipitation of all the
role in the catalytic activity. Alumina is widely used as a metallic cations. The precipitate was aged for 3 h at room
support. However, the nickel spinel phase (NiAl O ) can be temperature. After centrifugation and washing with deꢀionized
2
4
formed at high temperature during the reforming reaction. water, the powder was dried overnight at 120°C. The dry
NiAl O is inactive and its formation leads to a faster deposit was then calcined at 800°C for 5 h in air.
2
4
deactivation [9][10]. Therefore, supports have been designed in The same procedure was applied to synthesize ZnAl O spinel,
2
4
order to prevent coking and oxidation. Incorporation of basic using nitrates of Al and Zn (SigmaꢀAldrich) with a Al:Zn molar
alkaliꢀearth metals such as Mg and Ca into Al O ꢀbased ratio of 2:1.
2
3
catalysts has been carried out in order to improve both stability LaAlO was prepared by a modified solꢀgel method (Pechini).
3
and activity [11][12][13]. Other supports exhibiting oxygen Ethylenediaminetetraacetic acid (EDTA, SigmaꢀAldrich) was
mobility properties were studied in order to promote the added to an aqueous solution containing the metal nitrates
ꢀ
1
gasification of deposited carbon [14–16].
(concentration of 0.5 mol L , SigmaꢀAldrich) with a La:Al
Biogas composition varies depending on the origin of the molar ratio of 1:1. EDTA is used as a chelating agent. A few
biomass. Although mainly composed of methane and carbon milliliters of ammonia solution were added to obtain a clear
ꢀ
1
dioxide, biogas typically contains also sulphur compounds [17]. solution. Finally, a solution of citric acid (2.5 mol L , n(citric
H S presence is a major problem due to its irreversible acid) = n(Al) + n(La)) was added to the solution containing the
2
poisoning effect on metals [18]. Biogas can also contain chelated metals. The temperature of the solution was slowly
volatile organic compounds such as chlorinated compounds. heated to 100°C. Stirring was continued until evaporation of
Methyl chloride was shown to impact on the H /CO ratio by water lead to a transparent gel. The gel was then dried and
2
increasing the surface acidity of the alumina support [19]. calcined following four different steps (120°C – 3h ; 300°C –
Model biogas composed of clean methane and carbon dioxide 3h ; 400°C – 3h ; 800°C – 5h).
(
60:40 vol:vol) was used in our study.
The same procedure was applied to prepare LaNiO3 with
Many catalysts have been tested for methane or biogas nitrates of lanthanum and nickel with a La:Ni molar ratio of
reforming with different combinations of active metals on 1:1.
various supports [20]. However, these tests were performed CeO ꢀAl O was prepared by coꢀprecipation. An aqueous
2
2
3
under different conditions of temperature and pressure with solution containing Ce and Al nitrates (Fluka) with a Ce:Al
ꢀ
1
different feed ratios, flow rates and reactor designs, making a molar ratio of 1:2 (concentrations of 0.17 and 0.34 mol L ) was
fair comparison difficult. gradually added to an aqueous solution of ammonium carbonate
Highꢀthroughput catalyst testing has become a widely accepted under fast stirring. The carbonate solution was prepared in
tool in catalyst development. However, it is mainly used in the order to contain large excess of carbonates, assuring
screening and discovery of new catalysts formulations [21], precipitation of all metallic cations. The precipitate was aged
[
[
22], less commonly to characterize specific catalyst properties for 3 h. After centrifugation and several washing cycles with
23] or longꢀterm performances. On the other hand, this deꢀionized water, the precipitated powder was dried overnight
technology is hardly reported for the study of longꢀterm catalyst at 120°C, then calcined at 800°C for 5 h.
stability. This is rather surprising as an obvious gain in time can The same method was used to prepare ZrO ꢀAl O with nitrates
2
2
3
be obtained for these kind of tests, compared to testing each of zirconium and aluminium (Aldrich and Fluka) with a Zr:Al
sample separately.
molar ratio of 1:2.
In the present study, longꢀterm catalyst stability was assessed
using a six parallelꢀflow reactor, specially implemented to test Ni was deposited on the different supports (spinels, perovskites,
simultaneously six catalysts under essentially identical flow mixed oxides) by depositionꢀprecipitation using nickel nitrate
rate, temperature and feed composition. Four types of supports Ni(NO ) .6H O (Fluka) and urea (SigmaꢀAldrich). Stirring was
3
2
2
were investigated: spinels, mixed oxides, perovskites and continued for 4 h while the temperature was increased to 100°C
hydrotalciteꢀtype precursors. The supports were impregnated in order to remove excess urea. After centrifugation and several
with the same nickel amount and tested under similar washing with deꢀionized water, the samples were dried at
conditions.
120°C over night and calcined at 550°C for 4 h in air.
5
wt.% Niꢀ0.05wt.% Rh/MgAl O and 5 wt.% Ni ꢀ0.05 wt.%
2 4
Experimental
Rh/LaAlO were also prepared by wet impregnation of
3
MgAl O or LaAlO with a solution containing nickel nitrates
2
4
3
Catalyst preparation
Ni(NO ) .6H O (Fluka), rhodium nitrates Rh(NO ) .2H O (Alfa
3 2 2 3 3 2
Magnesium spinel (MgAl O ) was prepared by coꢀ Aesar). Stirring was continued for 6 h. After evaporation of
2
4
precipitation. An aqueous solution containing Mg and Al
2
| J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 2012

Page 3 of 8
Journal Name
Catalysis Science & Technology
View Article Online
ARTICLE
DOI: 10.1039/C5CY00702J
water, the slurry was dried at 120°C overnight and calcined at +/ꢀ 4 mL/min for a flow of 80 mL/min per reactor. A 6ꢀport
5
50°C for 4 h in air.
automatic selection valve placed at the reactor exit allowed
selecting the effluent of one specific reactor to be sent to a mass
Hydrotalciteꢀtype precursors were prepared by coꢀprecipitation. spectrometer for gaseous product analysis. The selection valve
An aqueous solution containing nitrates of Ni, Mg and Al was automatically switched every five minutes in order to
(
Fluka and SigmaꢀAldrich) was added dropwise to a saturated measure sequentially the performances of each reactor using an
solution containing NaHCO and NaOH. The precipitate was online mass spectrometer. Pressure transducers placed upstream
3
aged for 1 h. After centrifugation and washing with deꢀionized and downstream the reactors also measured the pressure drop in
water, the deposit was dried over night at 120°C. The dry each reactor being analysed, to check for a possible increase in
deposit was then calcined at 800°C for 5 h in air.
the pressure due to extensive coke deposition.
A commercial Niꢀbased catalyst (HiFUEL R110, Alfa Aesar,
Ref. 45465) was used as reference catalyst.
Catalyst characterization
Powder Xꢀray diffraction patterns (XRD) were recorded
using a Bruker D5005 diffractometer with a CuK radiation at λ
=
1.5418 Å.
Elementary analysis of the fresh catalysts was performed by
ICPꢀAES using a Jobin Yvon Activa Spectrometer D ICPꢀOES
to determine the Ni loading.
Specific surface areas (SSA) were measured by nitrogen
adsorption on catalysts at ꢀ196°C on a BELSORPꢀMini (Belꢀ
Japan).
ꢀ
Thermal analyses (DTAꢀTGA) were performed in air with a
SETARAM analyser equipped with a mass spectrometer for
online gas analysis. The samples were placed in a Pt crucible
and heated at 10◦C/min under a flow of synthetic air (50
mL/min).
Figure 1: Setup for reactors inlet and outlet
Catalytic activity and stability measurements
Catalytic tests
A proprietary automated sixꢀparallel reactor technology was
implemented (Figure 1) to perform highꢀthroughput longꢀterm
testing of the catalysts. The liquid water flow rate was
controlled by a HPLC pump (Shimadzu LC 20ꢀAD) and
vaporized in a customꢀmade evaporator at 200°C, before
mixing with the gaseous reactants whose flow rate was
controlled by massꢀflow controllers. All gas lines and valves
were heated at 160°C to prevent steam condensation. Six
tubular quartz reactors (length 180 mm, 4 mm ID) were placed
in a sixꢀposition tubular furnace. The temperature in each
reactor was measured under inert gas flow and at 700°C
differences of +/ꢀ 8°C between the 6 reactors was found.
Taking an activation energy of 110 kJ/mol for the steam
reforming of methane [24], this will lead to a +/ꢀ 10% variation
in the reaction rate at 700°C. After mixing the reactants, the
main gas stream was split into six flows, each being directed
into a reactor. Special attention was given to distribute the
flows equally, since six different catalyst beds create each
different pressure drops and can cause unequal flow
distribution. To solve this issue, six identical stainless steel
capillaries with small internal diameters (1/16” OD, 0.01" ID,
Tests were performed at 700°C and at a pressure of 1.6 bar.
Prior to testing, the catalyst samples were pelletized, crushed
and sieved to obtain a particle size fraction between 100ꢀ200
microns. Catalyst beds were composed of 20 mg of catalyst and
diluted with quartz powder (100ꢀ200 microns), to obtain a
similar bed length of 8 mm whatever the catalyst density. The
catalysts were reduced in situ with a H argon mixture (4:1,
2
vol:vol) for 3 hours, at 700°C. The reaction mixture consisted
of 42% steam, 14% CH , 9% CO , 7% O in a balance of argon.
4
2
2
This composition represents a model bioꢀgas containing 60 %
CH and 40 % CO , which is reacted with a mixture of O and
4
2
2
H O under the conditions O /CH = 0.5 and H O/CH =3. If
2
2
4
2
4
oxygen were supplied as air, the balance gas would be nitrogen.
Argon was substituted for nitrogen because N would forbid
2
analyzing CO by mass spectrometry (same molecular ion), and
Ar was also used as internal standard for products
quantification. The total flow rate of the reactant gases was 380
ꢀ
1
mL.min resulting in a Gas Hourly Space Velocity (GHSV) in
3
ꢀ3 ꢀ1
each reactor of 38,000 m m h and a Weigh Hourly Space
ꢀ
1
Velocity (WHSV) of 190 L(h g ) . Online analysis of the
cat
reactors exit stream was performed with a mass spectrometer
2
00 mm length) were placed upstream to the reactors. The
(
MS) monitoring m/z fragments 2 (H ), 4 (He), 15 (CH , this
2 4
pressure drop created by the capillaries was much more
important than that created by the catalyst bed. Measurement of
the individual flows through the reactor showed deviations of
fragment was chosen rather than the molecular ion because it is
nearly as intense (90%) as mass 16 in the methane
This journal is © The Royal Society of Chemistry 2012
J. Name., 2012, 00, 1-3 | 3

Catalysis Science & Technology
Page 4 of 8
View Article Online
ARTICLE
10.1039/C5CY00702J
CatalysisDOScI:ience & Technology
fragmentation spectrum, whereas O significantly contributes to coking due to cracking reactions taking place on the acid sites.
2
mass 16, and also CO, CO and H O to a lesser extent), 18 The basic character enhances steam activation as hydroxyl
2
2
(
H O), 28 (CO, corrected for the contribution of CO to this groups, which react with the coke and improve its gasification.
2 2
fragment), 32 (O ), 40 (Ar), 44 (CO ). Quantitative analysis Ceria combines a basic character with oxygen mobility
2
2
was performed by calibrating the MS response using different properties and has also been shown to improve the gasification
partial pressures of all relevant compounds diluted in argon. of coke in dry reforming reactions.
The Ar signal was used as internal standard and the other ion The MgꢀAlꢀNi hydrotalciteꢀtype catalyst (HT) has also a basic
current intensities were converted to molar flows knowing the character and a high surface area. Ni ions are initially
molar flow rate of Ar and the sensitivity coefficient of each m/z incorporated in a mixed hydroxyꢀcarbonate structure, then
fragment relative to Ar, which is constant. Accurate water extracted upon reduction as metallic Ni, which should lead to
quantification with a MS is difficult, but the liquid water flow the formation of well dispersed Ni particles. The LaNiO3
rate was precisely controlled by a calibrated HPLC pump, perovskite catalyst (LN) was intended to behave similarly
therefore the molar flow rate of steam at the reactor inlet was (synthesis of a mixed oxide and surface reduction leading to
known accurately.
well dispersed metallic Ni particles on LaNi1ꢀxO ), but its very
3
low SSA might forbid a good dispersion of Ni.
2
+
Results and Discussion
Since Ni tends to react with alumina at high temperature to
form the inactive spinel NiAl O4 (and any possible subꢀ
stoichiometric Ni1ꢀxAl O
2
Catalyst characterization
composition), MgAl O and
2 4
2
4ꢀx
The nickel loading and surface areas of the tested catalysts are
reported in Table 1. A similar amount of Ni, around 5 wt.%,
was generally deposited on the catalysts, with the exception of
sample NRL, which exhibited a lower loading (3.9 wt.%), and
sample LN (LaNiO3 perovskite) for which the bulk nickel
concentration was 26 wt.%. Four catalysts (NC, NS, NA, CC)
with higher Ni loadings (10ꢀ20%) and two catalysts doped with
ZnAl O spinels were used as supports because they combine a
2
4
high surface area, a basic character in the case of MgAl O ,
2
4
2+
2+
whereas Mg and Zn ions occupy the crystallographic
positions where Ni ions migrate in the alumina structure.
2+
Therefore, these supports should prevent, or at least limit, the
migration of nickel in the support.
a small amount of Rh (NRL and NRM) were also included in Six-parallel reactor set-up validation with a commercial
the screening. The catalysts exhibited substantial variations in catalyst
specific surface areas. It was generally high for spinel and
hydrotalciteꢀtype catalysts, with NM presenting the highest In order to obtain reliable information about the catalyst
ꢀ
1
value (158 m² g ). The perovskites had lower surface areas, performances and to assess the repeatability of the results, the
ꢀ
1
ranging from 2.7 to 15 m² g . Specific surface areas of mixed setꢀup was validated by testing the same mass of commercial
ꢀ
1
oxides catalysts were 57 and 119 m² g for NCA and NZA Niꢀbased catalyst in the 6 reactors (Fig. 2).
respectively.
1
00%
Table 1: Ni loading and specific surface area of prepared catalysts
Ni
R1
R2
R3
R4
R5
R6
80%
60%
40%
20%
0%
BET
Surface
Area
catalysts
Code
loading
%)
(
(
m².gꢀ1)
158
118
99
5
wt.% Ni/MgAl
2
O
4
NM
NRM
NZ
NL
NRL
LN
NCA
NZA
HT
NC
NS
5.1
5.1
4.5
4.5
3.9
26
5
5
ꢀ0.05wt.%NiRh/MgAl
2
O
4
5
wt.% Ni/ZnAl
wt.% Ni/LaAlO
ꢀ0.05 wt.% NiRh/LaAlO
LaNiO
wt.% Ni/CeO
wt.% Ni/ZrO
2 4
O
5
3
15
0
10
20
30
40
5
3
7.8
2.7
57
119
116
10
40
123
16
Time (h)
3
5
5
2
ꢀAl
ꢀAl
2
O
3
5
4
3
2
1
0
1.4
2
2
O
3
5
H2
Hydrotalciteꢀtype precursor
NiO/CaAl12
0 wt.% Ni/SiO
0 wt.% Ni/Al
Commercial catalyst
4.3
14
20
20
10
1.2
1
O
19
2
2
0.8
0.6
0.4
0.2
0
2
2
O
3
NA
CC
CO
The selection of catalysts and support for the screening was
based on general trends found in the literature related to steam
and dry reforming of methane over Niꢀbased catalysts: basic
supports are preferred to acidic supports, which are prone to
0
10
20
30
40
0
10 20 30 40
Time (h)
Time (h)
4
| J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 2012

Page 5 of 8
Journal Name
Catalysis Science & Technology
View Article Online
ARTICLE
DOI: 10.1039/C5CY00702J
Figure 2: Autothermal reforming of model biogas (60%-40% ; CH4-CO
T=700°C with S/C =3, O/C=0.5 a) Methane conversion (%) b) H and CO
production c) H /CO ratio over time
2
) at
started to convert methane again. Before oxygen removal, the
O /CH ratio in the feed stream is 0.5, which means that 25% of
2
2
4
2
the methane is immediately burnt into CO at the reactor inlet
2
(
the stoichiometry for methane combustion is O /CH =2).
2 4
Methane was fully converted at t=0 in all reactors, but declined
steadily. The production of hydrogen and of carbon monoxide
were similar in each reactor at full conversion and decreased
with decreasing methane conversion. At t=0, the production of
H2 was of about 1 µmol/min and that of CO about 0.3
Oxygen is totally converted and Ni is likely to be oxidized in
the CH4 combustion zone, but the metallic Ni still present
downstream in the reactor ensures the reaction of the remaining
methane with steam, and possibly with CO . As soon as the
2
catalyst is sufficiently deactivated that some of the oxygen is
unconverted (this should theoretically happen when the
methane conversion falls below ≈25%, but it might also happen
before since NiO is not a very good catalyst for methane
combustion [25], the gaseous oxygen will rapidly oxidize the
metallic Ni and deactivate the catalyst. These catalysts will not
µmol/min. The H /CO ratio remained around four until the
2
sharp drop in the methane conversion. The deactivation profiles
were similar: a slow deactivation was observed followed by an
abrupt loss of the activity though scattered over a range of
about 5 h. As the methane conversion fell to zero, the hydrogen
and monoxide productions were simultaneously suppressed.
Figure 3 shows the composition (on dry basis, i.e. not taking the
steam into account) of reactor #6 exit stream. With a S/C ratio
of 3, the steam is in large excess and is only partly consumed
be reactivated when O is removed from the feed because Ni is
2
fully oxidized and unable to catalyse the steam reforming
reaction. In reactors 2 and 5 the catalyst was not completely
deactivated and still produced small amounts of H2 + CO,
which suggests that some metallic Ni was still present, and
upon oxygen removal the hydrogen produced by methane steam
reforming was able to reduce again the oxidized Ni. After ca.
during the reaction. H and CO molar fractions reach about
2
4
0% and 10% respectively at full methane conversion.
Complete conversion of methane and oxygen is observed
during the first hours. After 15 hours, the methane molar
fraction increases while the oxygen molar fraction remains
zero. When the methane molar fraction reaches 10%
4
7 hours, hydrogen was added to the feed. Methane
conversions ranging from ca. 65 to 70% was observed in all six
reactors. Oxidation of Ni appears to play an important part in
the deactivation process.
(
corresponding to 60% conversion), the oxygen conversion is
no longer complete. Methane and oxygen conversions suddenly
drop to zero and hydrogen and carbon monoxide productions
drop similarly. The catalyst is fully deactivated. Similar
behavior was observed on all reactors.
100%
a) O2
b) H2
8
6
4
2
0%
0%
0%
0%
removed
added
50%
40%
30%
20%
10%
R1
R2
R3
R4
R5
R6
CH4
CO2
CO
O2
H2
0
%
43
45
47
49
Time (h)
0%
Figure 4: Methane conversion (%) during autothermal reforming of model biogas
a) (60%-40% ; CH4-CO ) at T=700°C with S/C =3 and absence of oxygen b) (60%-
0% ; CH4-CO ) at T=700°C with S/C =3 and addition of hydrogen
0
10
20
Time (h)
30
40
2
4
2
Figure 3: Dry outlet gas concentrations during autothermal reforming of model
biogas (60%-40% ; CH4-CO ) at T=700°C with S/C =3, O/C=0.5
Catalyst screening for ATR of model biogas
2
The results of catalyst screening in terms of methane
The catalyst stability in the six reactors varied between 40 +/ꢀ 5 conversion are shown in Figure 5.
hours. This corresponds rather well to the variation of the The catalyst performances at 700°C versus timeꢀonꢀstream
methane steam reforming rate of +/ꢀ 10% which can be depended strongly on catalyst composition.
attributed to slight differences in the reactor temperatures, Comparing the behaviour of the ≈5 wt.% Ni catalysts loaded on
catalyst loadings and reactant flow rates. The six deactivation different supports, our screening shows that the support
profiles, however, were sufficiently similar to allow properties that were shown to be beneficial in methane steam
discriminating unambiguously between fastꢀdeactivating and reforming do not appear as relevant when autothermal
stable catalysts.
reforming of methane is involved: NM, NZ and NCA exhibited
The role of oxygen in the catalyst deactivation is evidenced by full initial conversion but deactivated within 2 hours, while
the experiments shown in Figure 4. After 43 hours, oxygen was NZA deactivated almost instantly. HT was found fully inactive,
removed from the feed gas, and the catalyst in reactors 2 and 5 but the catalyst reduction temperature was probably too low to
This journal is © The Royal Society of Chemistry 2012
J. Name., 2012, 00, 1-3 | 5

Catalysis Science & Technology
Page 6 of 8
View Article Online
ARTICLE
10.1039/C5CY00702J
CatalysisDOScI:ience & Technology
reduce and extract Ni from the mixedꢀoxide structure, since the
Ni reduction in NiꢀMgꢀAl hydrotalcite catalysts has been shown
to take place between 750 and 900°C [26]. NL catalyst (5%
Ni/LaAlO3) deactivated more gradually and was fully
NM
NRM
NZ
1
.5
NL
deactivated after 16 h. Therefore LaAlO appears an interesting
1
3
NRL
LN
support, despite its low SSA.
NCA
NZA
HT
NM
1
00%
0.5
0
NRM
NZ
80%
60%
40%
20%
NS
NL
NA
NRL
LN
0
5
10
Time (h)
15
NC
NCA
NZA
HT
Figure 6: Hydrogen production (µmol/min) during autothermal reforming of
model biogas (60%-40% ; CH4-CO ) at T=700°C with S/C =3, O/C=0.5
2
NS
The hydrogen production profiles matched well the conversion
profiles of methane shown in Figure 5. In agreement with the
low methane conversion, the reaction on LN produced much
less hydrogen than the other catalysts.
The abrupt changes in the slopes of the deactivation profiles
exhibited by most catalysts suggested different deactivation
0%
NA
0
5
10
15
50
150
NC
Time (h)
Figure 5: Methane conversion (%) during autothermal reforming of model biogas
60%-40% ; CH4-CO ) at T=700°C with S/C =3, O/C=0.5
(
2
Increasing the Ni loading was expected to delay the catalyst mechanisms for each stage. For the slow initial deactivation:
deactivation, since more Ni is available in the reactor, provided nickel and/or support sintering eventually combined with coke
that it is dispersed enough and that the support exhibits the deposition may progressively deactivate the catalysts. Our
appropriate acidꢀbase properties. This was generally verified reaction conditions imply working with an excess of steam
since all 5 wt. Ni catalysts except NL deactivated before CC (H O/CH =3) that can promote the sintering of oxides and
2
4
(
10% Ni) and NC (14% Ni) catalysts, which in turn deactivated metals, known to be a rather slow process. However, whatever
before NA (20% Ni). A noticeable exception was NS (20% be its origin, such a slow initial deactivation cannot account for
Ni/SiO ), which deactivated so rapidly that no methane the subsequent sharp collapse in the catalytic activity. The latter
2
conversion was observed, evidencing that SiO is not a good might be related to the presence of oxygen in the feed stream,
2
support in this reaction. LN catalyst (26 wt.% Ni) revealed leading to nickel oxidation. As mentioned previously, ATR of
peculiar performances, with a moderate but constant methane methane combines the combustion of methane (to provide heat)
conversion (≈30%), which gradually dropped to zero after 20 h with steam/dry reforming of the remaining methane. Since
(
not shown), probably due to the negative combination of high methane combustion is much faster than reforming reactions
Ni loading and very low surface area.
and strongly exothermic ( H=ꢀ801 kJ/mol), it takes place at the
∆
Rh doping had a very strong influence on the stability of NM catalyst bed inlet and leads to the formation of hotspots and
since NRM still fully converted methane after 200 h of temperature gradients along the bed [23, 24]. As shown in the
reaction. In contrast, NRL deactivated before NL, which might case of methane partial oxidation [36], the combination of high
be attributed to the very low surface area of the catalyst after its temperature and presence of oxygen in the feed stream is likely
second impregnation by Rh. NiꢀRh bimetallic catalysts have to oxidize the nickel particles, which no longer participate in
been shown to exhibit enhanced stability for methane triꢀ the reforming reactions. The high temperature front will diffuse
reforming [27] and dryꢀreforming reactions [28], similarly to further down the bed, leaving less metallic Ni available for the
NiꢀPd [29–31] NiꢀPt [32] and NiꢀCo formulations [33][34].
reforming reactions. Once the combustion zone reaches the end
With the exception of LN, the deactivation profiles of all tested of the catalyst bed the methane conversion will drop to zero.
catalysts were rather similar to that of the commercial catalyst:
the methane conversion decreased slowly for a certain time, Postꢀreaction characterization of catalysts showed that XRD
then rapidly fell to zero. Hydrogen production versus time is patterns were not significantly modified compared to fresh
shown in Figure 6. The order of deactivation time NSꢀHTꢀNZA catalysts. Figure 7 presents the diffractograms of NM catalyst
<
NZ < NCA < NM < NRL < NC < NL < NA < NRM was before and after ATR reaction. Both diffractograms exhibited
respected.
the characteristic lines of the spinel phase MgAl O , whereas
2
4
no Ni or NiO lines were evidenced.
6
| J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 2012

Page 7 of 8
Journal Name
Catalysis Science & Technology
View Article Online
ARTICLE
DOI: 10.1039/C5CY00702J
MgAl O4
2
a)
b)
b)
a)
Figure 8: TEM images of deactivated commercial catalyst after ATR reaction
showing the presence of a) oxidized nickel b) carbon deposits.
Figure 7: X-ray diffractogram of NM catalyst a) after reduction at 700°C b) after
deactivation
Conclusions
Assessing the stability of catalysts in ATR of bioꢀgas requires
longꢀterm experiments, since catalysts might deactivate rapidly
after exhibiting apparently stable performances for several
hours. A sixꢀparallel flow reactor is a powerful tool to assess
the longꢀterm stability of six catalysts simultaneously in ATR
of biogas. A screening of catalysts was performed in a short
time demonstrating an order of deactivation time NSꢀHTꢀNZA
However, the small amount of catalyst in each reactor (20 mg,
mixed with quartz powder) provided only a lowꢀresolution
diffractogram, which did not allow a thorough comparison of
fresh and used catalyst diffraction patterns. TGAꢀDTAꢀTPO
analysis of the commercial catalyst after reaction revealed two
small mass losses with maxima at ca. 350°C (ꢀ0.12%,
exothermic) and 690°C (ꢀ0.11%, endothermic), associated with
CO production in subꢀppm amounts. The CO production at
<
NZ < NCA < NM < NRL < NC < NL < NA < NRM. A slow
2
2
initial deactivation process could be related to some carbon
deposition, but the main fast final deactivation process is likely
associated to nickel oxidation. Rh promotes Ni stability and
thus 5ꢀ0.05 wt.% NiꢀRh/MgAl O was identified as a robust
low temperature might be attributed to the combustion of very
reactive carbon, in direct contact with Ni. The high temperature
CO2 production is endothermic and thereby suggests the
decomposition of carbonates formed on the basic support.
Limited carbon deposition is consistent with the reaction
conditions in which the H O/CH ratio of 3 and the high CO
2
4
catalyst for autothermal reforming of model biogas.
2
4
2
partial pressure should prevent extensive coking (carbon can be
Acknowledgements
The research leading to these results has received funding from the
gasified by steam and by CO through the reverse Boudouard
2
reaction [37]). TEM examination of the deactivated commercial
catalyst samples also revealed the presence of a carbon film on
some Ni particles, although not on all of them (Figure 8), which
probably corresponds to the carbon burning at low temperature
in TGAꢀTPO experiments. Such carbon films are likely to
participate in the slow initial deactivation, by progressively
encapsulating the metal particles. There was no evidence of
carbon whisker formation, also known as a potential longꢀterm
deactivation process. High resolution and electron diffraction
also revealed the presence of oxidized nickel, which is clearly
an issue for longꢀterm stability in ATR.
European
Community's
Seventh
Framework
Programme
([FP7/2007ꢀ2013] under grant agreement n° 325383 (BioRobur).
The authors thank Bernadette Jouguet for performing timely TGAꢀ
DTA analyses.
References
1
A. Demirbas, Energ. Convers. Manage., 2008, 49, 2106–2116.
2
J. Xuan, M. K. H. Leung, D. Y. C. Leung and M. Ni, Renew. Sust.
Energ. Rev., 2009, 13, 1301–1313.
3
4
J. R. RostrupꢀNielsen and J.ꢀH. Bak Hansen, J. Catal., 1993, 144, 38–49.
Depending on its level of deactivation, the initially blackꢀ
coloured catalyst bed exhibited after reaction a ring of light
blueꢀgreen colour at its inlet, or was found fully blueꢀgreen for
deactivated catalysts, which suggests the formation of inactive
NiO or NiAl O . In this sense, the addition of small amounts of
P. FerreiraꢀAparicio, A. GuerreroꢀRuiz and I. RodrıguezꢀRamos, Appl.
́
Catal. A-Gen., 1998, 170, 177–187.
5
6
J. Sehested, Catal. Today, 2006, 111, 103ꢀ110.
S. Helveg, J. Sehested and J. R. RostrupꢀNielsen, Catal. Today, 2011,
2
4
1
78, 42–46.
Rh to Ni appears to effectively promote nickel reduction and
prevent bulk oxidation. Rh addition is also effective to inhibit
carbon deposition [38]. Therefore, a 0.05 wt.% Rh addition has
a positive effect on the catalyst resistance to deactivation under
ATR conditions.
7
8
J. Wei, E. Iglesia, J. Catal., 2004, 224, 370ꢀ383.
P. O. Sharma, M. A. Abraham and S. Chattopadhyay, Ind. Eng. Chem.
Res., 2007, 46, 9053–9060.
9
A. M. Gadalla and B. Bower, Chem. Eng. J., 1988, 43, 3049–3062.
This journal is © The Royal Society of Chemistry 2012
J. Name., 2012, 00, 1-3 | 7

Catalysis Science & Technology
Page 8 of 8
View Article Online
ARTICLE
10.1039/C5CY00702J
CatalysisDOScI:ience & Technology
1
1
1
1
0
1
2
3
K.ꢀH. Lin, H.ꢀF. Chang and A. C.ꢀC. Chang, Int. J. Hydrogen Energ., 34 X. You, X. Wang, Y. Ma, J. Liu, W. Liu, X. Xu, H. Peng, C. Li, W.
012, 37, 15696–15703. Zhou, P. Yuan, X. Chen, ChemCatChem, 2014, 6, 3377ꢀ3386.
2
G. Xu, K. Shi, Y. Gao, H. Xu and Y. Wei, J. Mol. Catal. A-Chem., 1999, 35 J. A. C. Dias and J. M. Assaf, Appl. Catal. A-Gen., 2008, 334, 243–250.
1
47, 47–54.
3
3
3
6
7
8
D. Dissanayake, M. P. Rosynek, K. C. C. Kharas and J. H. Lunsford, J.
Catal., 1991, 132,117–127.
A. A. Lemonidou and I. A. Vasalos, Appl. Catal. A-Gen., 2002, 228,
2
27–235.
H. Düdder, K. Kähler, B. Krause, K. Mette, S. Kühl, M. Behrens, V.
Scherer, M. Muhler, Catal. Sci. Technol., 2014, 4, 3317ꢀ3328.
C. Papadopoulou, H. Matralis and X. Verykios, “Utilization of Biogas as
a Renewable Carbon Source: Dry Reforming of Methane” in Catalysis
for Alternative Energy Generation, L. Guczi and A. Erdôhelyi, Eds.
Springer New York, 2012, 57–127. ISBN: 978ꢀ1ꢀ4614ꢀ0343ꢀ2.
M. Nurunnabi, K. Fujimoto, K. Suzuki, B. Li, S. Kado, K. Kunimori and
K. Tomishige, Catal. Commun., 2006, 7, 73–78.
1
1
4
5
Y. J. O. Asencios, J. D. A. Bellido and E. M. Assaf, Appl. Catal. A-Gen.,
2
011, 397, 138–144.
Y. J. O. Asencios, C. B. Rodella and E. M. Assaf, Appl. Catal. B-
Environ., 2013, 132–133, 1–12.
1
1
1
6
7
8
O. A. Bereketidou and M. A. Goula, Catal. Today, 2012, 195, 93–100.
S. Rasi, A. Veijanen and J. Rintala, Energy, 2007, 32, 1375–1380.
S. Appari, V. M. Janardhanan, R. Bauri, S. Jayanti and O. Deutschmann,
Appl. Catal. A-Gen., 2014, 471, 118–125.
1
2
2
2
2
2
9
0
1
2
3
4
M. P. Kohn, M. J. Castaldi and R. J. Farrauto, Appl. Catal. B-Environ.,
2
014, 144, 353–361.
H. J. Alves, C. Bley Junior, R. R. Niklevicz, E. P. Frigo, M. S. Frigo and
C. H. CoimbraꢀAraújo, Int. J. Hydrogen Energ., 2013, 38, 5215–5225.
J. Pérezꢀramirez, R. J. Berger, G. Mul, F. Kapteijn and J. A. Moulijn,
Catal. Today, 2000, 60, 93–109.
J. K. van der Waal, G. Klaus, M. Smit and C. M. Lok, Catal. Today,
2
011, 171, 07–210.
P. Laveille, G. Biausque, H. Zhu, J.ꢀM. Basset, V. Caps, Catal. Today,
2
013, 203, 3ꢀ9.
J. R. RostrupꢀNielsen, "Catalytic Steam Reforming" in Catalysis, J. R.
Anderson and M. Bouchard, Eds. SpringerꢀVerlag Berlin, 1984, 5, 1–
1
17. ISBN: 978ꢀ3ꢀ642ꢀ93249ꢀ6.
2
2
5
6
O. Dewaele, G. F. Froment, J. Catal., 1999, 184, 499ꢀ513.
K. Takehira, T. Shishido, P. Wang, T. Kosaka, K. Takaki, J. Catal.,
2
004, 221, 43ꢀ54.
2
2
7
8
U. Izquierdo, V.L. Barrio, J. Requies, J.F. Cambra, M.B. Guëmez, P.L.
Arias, Int. J. Hydrogen Energy, 2013, 38, 7623ꢀ7631.
M. Nowosielska, W. K. Jozwiak, J. Rynkowski, Catal. Lett., 2009, 128,
8
3ꢀ93.
2
3
9
0
J. Zhang, Y. Wang, R. Ma, D. Wu, Appl. Catal. A, 2003, 243, 251ꢀ259.
K. Yoshida, K. Okumura, T. Miyao, S. Naito, S. Ito, K. Kunimori and K.
Tomishige, Appl. Catal. A-Gen., 2008, 351, 217–225..
3
3
3
1
2
3
I.Z. Ismagilov, E.V. Matus, V.V. Kuznetsov, M.A. Kerzhentsev, S.A.
Yashnik, I.P. Prosvirin, N. Mota, R.M. Navarro, J.L.G. Fierro, Z.R.
Ismagilov, Int. J. Hydrogen Energy, 2014, 39, 20992ꢀ21006.
L. Li, L. Zhou, S. OuldꢀChikh, D. H. Anjum, M. B. Kanoun, J. Scaranto,
M. N. Hedhili, S. Khalid, P. V. Laveille, L. D'Souza, A. Clo, J.ꢀM.
Basset, ChemcatChem, 2015, 7, 819ꢀ829.
K. Takanabe, K. Nagaoka, K. Nariai, K. Aika, J. Catal., 2005, 232, 268ꢀ
2
75.
8
| J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 2012