Renewable hydrogen production by a mild-temperature steam reforming of the model compound acetic acid derived from bio-oil

Article abstract of DOI:10.1016/j.molcata.2011.12.006

In this study, steam reforming of acetic acid over Ni/La₂O ₃ catalyst for hydrogen generation was investigated. The experiments were carried out by varying the reaction conditions such as Ni content, reaction temperature, liquid hourly space velocity (LHSV) and steam-to-carbon ratios (S/C), to evaluate the influences of these parameters on the steam reforming reactions for getting the best catalytic performances. The results showed that the Ni/La₂O₃ catalyst with the Ni loading of 20 wt.% presented the superior activity and selectivity. The reaction temperature, LHSV and S/C strongly influenced the reforming reactions. At T = 623 K, LHSV = 5.1 h^-1 and S/C = 7.5:1, the catalyst exhibited the best performances, acetic acid was converted completely, and selectivity to hydrogen exceeded 93%. Moreover, the catalyst presented rather stable performances for the 100 h time-on-stream without any deactivation. Besides, the reaction pathways were also discussed, methyl species was the important intermediate in the reforming reactions, which was the precursors for the formation of the gaseous products, while hydroxyl species was another important intermediate that was important for the formation of hydrogen and elimination of the by-products.

Full text of DOI:10.1016/j.molcata.2011.12.006

Journal of Molecular Catalysis A: Chemical 355 (2012) 123–133
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Renewable hydrogen production by a mild-temperature steam reforming of the
model compound acetic acid derived from bio-oil
Zhikun Lia,b, Xun Hua,c, Lijun Zhangc, Gongxuan Lua,∗
a State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China
b The Graduate School of Chinese Academy of Sciences, Beijing 100039, PR China
c Curtin University of Technology, Perth, WA 6845, Australia
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 4 October 2011
Received in revised form 5 December 2011
Accepted 6 December 2011
Available online 15 December 2011
In this study, steam reforming of acetic acid over Ni/La2O3 catalyst for hydrogen generation was investi-
gated. The experiments were carried out by varying the reaction conditions such as Ni content, reaction
temperature, liquid hourly space velocity (LHSV) and steam-to-carbon ratios (S/C), to evaluate the influ-
ences of these parameters on the steam reforming reactions for getting the best catalytic performances.
The results showed that the Ni/La2O3 catalyst with the Ni loading of 20 wt.% presented the superior
activity and selectivity. The reaction temperature, LHSV and S/C strongly influenced the reforming reac-
tions. At T = 623 K, LHSV = 5.1 h−1 and S/C = 7.5:1, the catalyst exhibited the best performances, acetic acid
was converted completely, and selectivity to hydrogen exceeded 93%. Moreover, the catalyst presented
rather stable performances for the 100 h time-on-stream without any deactivation. Besides, the reaction
pathways were also discussed, methyl species was the important intermediate in the reforming reac-
tions, which was the precursors for the formation of the gaseous products, while hydroxyl species was
another important intermediate that was important for the formation of hydrogen and elimination of
the by-products.
Keywords:
Acetic acid
Hydrogen
Steam reforming
Ni/La2O3
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
its subsequent steam reforming, for bio-oil has the advantages of
high energy density and ease of handling, and most importantly, it
Hydrogen, as the cleanest fuel and energy carrier, is not only
widely used for chemical and petrochemical industry, but also is
the important fuel for the high efficiency fuel cells [1]. Presently,
hydrogen is produced commercially, mostly from natural gas, liq-
uefied petroleum gas, and naphtha, by catalytic steam reforming
or partial oxidation of heavy oil fractions [2,3]. However, now it is
necessary to produce hydrogen from an alternative energy source,
owing to the concerns about the depletion of fossil fuel reserves and
the pollution caused by continuously increasingly energy demands.
Biomass is not only a resource of renewable and environmentally
preferable energy, but also can achieve the zero emission of car-
bon dioxide in the hydrogen production cycle, for the fact that
the carbon dioxide released into the atmosphere is offset by the
uptake of carbon dioxide during biomass growth. Thus, biomass
can be a sustainable and renewable source for the production of
chemicals including hydrogen [4,5]. The hydrogen from biomass
can be obtained using several methods, including pyrolysis, gasi-
fication and steam reforming [6]. Of special interest are methods
in which biomass is converted to intermediate liquid bio-oil and
is the fact that it can be used directly for the on-demand hydrogen
production for fuel cells. However, bio-oil is a complex mixture
of oxygenate compounds, and design of an effective catalyst for
bio-oil steam reforming require to investigate a model oxygenate
firstly to obtain the fundamental knowledge of steam reforming
the complicated mixture of the organic molecules. Acetic acid is
one of the major components in bio-oil [7,8], therefore, study of
steam reforming of acetic acid can provide valuable methods and
data for hydrogen production from bio-oil. In addition, acetic acid
itself is renewable, which can be easily obtained from biomass
by fermentation. And acetic acid, unlike methanol and ethanol, is
noninflammable, hence, it is a safe hydrogen carrier. Moreover,
acetic acid also can be easily converted to hydrogen with high
selectivity at relatively low reaction temperature over effective cat-
alysts; hence, it also may be one of the suitable fuels for the proton
exchange membrane fuel cells (PEMFC).
Up to now, although much work has been carried out on steam
reforming of methanol and ethanol, only a few papers appeared
in the open literature dealt with the conversion of acetic acid to
hydrogen [8–16]. And the catalysts investigated are limited to ICI
46-1 [8,9], UC G-90C [8–10], Ni–Al [11,12], Pt/ZrO2 [13–15], Pt, Rh
and Pd based catalysts [16]. Usually, the temperature conducted
in the reforming reactions over these catalysts was higher than
∗
Corresponding author. Tel.: +86 931 4968178; fax: +86 931 4968178.
E-mail address: gxlu@lzb.ac.cn (G. Lu).
1381-1169/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2011.12.006

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Z. Li et al. / Journal of Molecular Catalysis A: Chemical 355 (2012) 123–133
Fig. 1. Schematic diagram of the reactor system for steam reforming of acetic acid.
873 K, and there were always appreciable amounts of organic and
gaseous by-products generation, which resulted in the low selec-
tivity to hydrogen. Therefore, an issue of major importance is then
to develop a catalyst exhibiting high activity and stability with high
hydrogen yields at relatively lower reaction temperature.
technique (TPR), X-ray photoelectron spectroscopy (XPS) and BET
method, respectively. The reaction pathways of steam reforming of
acetic acid were also discussed. The results showed that under the
selected experimental conditions, acetic acid could be converted
completely at the temperature as low as 623 K, with the selectiv-
ity toward hydrogen exceeded 93%, to our knowledge, such results
have not been reported at such low reaction temperature.
In our previous work [17,18], we have reported Fe–Co and Ni–Co
catalysts for acetic acid reforming, which showed good activity and
selectivity to hydrogen, complete conversion of acetic acid could be
achieved at 673 K. While in this paper, a novel supported catalyst
Ni/La2O3 for steam reforming of acetic acid was presented, which
was chosen based on the following consideration. Among transi-
tion metals, Ni was reported as the efficient material for the C–C
bond cleavage [19] and showed good activity for hydrogenation
[20], dehydrogenation [21] reactions, more importantly, it has been
widely investigated for methanol decomposition [22] and ethanol
reforming [23,24] reactions because of its high activity and low cost.
Therefore, in this paper Ni was selected as the active metal for acetic
acid reforming. Besides, the nature of the support also plays a key
role in determining the catalytic performance since it affects dis-
persion and stability of the metal as well as it may participate in the
reaction, hence choice of the support is crucial. Lanthanum oxide
was recognized as a good support for metals that catalyze reactions
such as methanol decomposition [25], ammonia oxidation [26], and
methane dry reforming [27]. Moreover, it has also shown that lan-
thanum oxides can substantially modify the chemical behavior of
highly dispersed metal catalysts [28], and it contained medium and
strong basic sites [29], which could favor water adsorption and OH
surface mobility and then aid the steam reforming [30,31]. Thus,
La2O3 was selected as the support. In addition, Ni/La2O3 catalyst
has been previously investigated for the steam reforming of ethanol
and exhibited excellent activity and selectivity [32–34]. Based on
the reasons mentioned above, the Ni/La2O3 catalyst was chosen
and investigated in detail for the steam reforming acetic acid in
this paper.
2. Experimental
2.1. Catalyst preparation
Ni/La2O3 catalysts were prepared by the method of incipient
wetness impregnation using nickel nitrate as a metal precursor. The
amount of Ni loadings on the catalyst was controlled at 0–26 wt.%
by varying the concentration of Ni(NO3)2 solution. Before impreg-
nation, La2O3 support was stabilized in air at 873 K for 3 h, surface
area of the treated sample was 10.67 m2 g−1. After impregnation,
the catalysts precursors were dried at room temperature for 24 h,
subsequently dried at 383 K for another 24 h and finally calcined at
773 K for 4 h. The catalyst power was tabletted and then crushed
into particles with 0.20–0.56 mm size for use.
2.2. Catalytic activity measurements
The experimental reactor system, consisting of a flow controller
unit, the reactor unit and the analysis unit, was constructed
as showed in Fig. 1. Catalytic performances tests were carried
out in a fixed bed continuous flow quartz reactor (i.d. 8 mm)
at atmospheric pressure and temperature from 523 to 873 K.
Typically, 0.8 g of catalyst was used in each run and diluted with
equal amount of quartz. Prior to each experiment, the calcined
catalyst was reduced in situ at T = 673 K, for 3 h, under a 50% H2
in N2 stream (flow rate 60 mL/min). The feed was then switched
to N2 and the reactor was equilibrated at the desired reaction
temperature, the pre-mixed acetic acid solution with a certain
steam to carbon ratio was then fed to the reactor by a syring at
a varied flow rate, which depended on the different runs of the
experiment. N2 was used as a carrier gas and internal standard for
The study was carried out by varying the operating reaction
conditions, such as Ni loadings, reaction temperature, steam to
carbon ratios, space velocity and so on, with the aim of getting
the maximum catalytic activity and hydrogen yields. The cat-
alyst was characterized by temperature-programmed reduction

Z. Li et al. / Journal of Molecular Catalysis A: Chemical 355 (2012) 123–133
125
gas analysis. The separation and quantification of the gas phase
effluents were attained on two on-line chromatograms equipped
with thermal-conductivity detectors (TCD). Hydrocarbons as well
as oxygenated products were separated and analyzed by a flame
ionization detector (FID). At the end of the catalytic tests, the flow
of acetic acid solution was stopped and the catalysts were cooled
under a N2 stream and stored for further characterizations. Steam
reforming activity was defined in terms of conversions of acetic
acid and selectivities to the products. The conversions of acetic acid
were denoted as C in the figures and tables. While the selectivities
to the products (hydrogen, carbon dioxide, methane, carbon
monoxide and so on) were denoted as Sproduct, which were calcu-
lated according to the corresponding equations: SH2 (%) = 100 ×
(moles of H2 production)/(moles of acetic acid consumed × 4);
the calculated method of other products’ selectivities were
similar to that of H2. Moreover, liquid hourly space velocity
(LHSV) here was defined as (volumetric flow rate of feed solution
(cm3 h−1))/(catalyst bed volume (cm3)). Another parameter used
to indicate the proportion of acetic acid and steam was steam to
carbon ratios (S/C), which was defined by the formula: S/C = (moles
of steam in the feed)/(moles of carbon in the feed).
2.3. Catalyst characterization
Fig. 2. H2 TPR for the series of Ni/La2O3 catalysts.
The H2 temperature-programmed reduction technique (TPR)
experiments over the series calcined catalysts were carried out in a
conventional flow system built in our laboratory. The curves were
obtained by passing a 5% H2 in Ar stream (flow rate: 40 mL/min)
through the catalysts (10 mg). The temperature was increased from
298 to 1023 K at a linearly programmed rate of 20 K/min. The efflu-
ent gas was passed through a 13× molecular sieve to remove the
steam from the exit stream and the amount of H2 consumed was
determined by a chromatogram.
The X-ray photoelectron spectroscopy (XPS) measurements
were carried out at a room temperature on a VG ESCALAB 210 spec-
trometer with Mg K␣ radiation (hv = 1253.6 eV). The binding energy
was calibrated by the C1s binding energy of 285.0 eV.
The specific surface area of the catalyst was measured by BET
method on a Micromeritics ASAP-2010 apparatus at a liquid nitro-
gen temperature with N2 as the absorbent at 77 K.
One point clarified here was that the XPS and BET characteriza-
tions were all conducted over the Ni/La2O3 catalyst with the Ni
loadings of 20 wt.%, for it exhibited the superior activity among
the series catalysts with different Ni contents, which would be
discussed below in detail (see Section 3.3).
the reduction temperature of which depended on the degrees of
its interaction with the support. The low-temperature peak might
be assigned to well-dispersed nickel oxide on the support while
the two high-temperature peaks suggested that the reducibility of
some of the nickel ions decreased by strong interactions with the
support. While for the catalyst with the Ni loadings higher than
20 wt.%, there were two types of Ni on La2O3 surface. The profiles
showed one shoulder at ca. 683 K and a principle peak at 733 K and a
tail peak at about 910 K. The shoulder and the principle peak should
be attributed to the reductions of the small and large particles of
NiO, respectively. The appearance of the shoulder was probably
caused by the slow hydrogen diffusion, suggesting an increase of
the mean sizes of NiO particles with the increase of Ni loadings.
The long tail peak in higher temperature region was corresponding
to the second types Ni that had strong interactions with the sup-
port. Besides, we also conducted the H2 TPR characterization over
the pure NiO oxides that come from the calcination of Ni(NO3)2, the
profiles showed one single peak at ca. 786 K. While the main reduc-
tion peak of NiO on La2O3 surface shifted to the lower temperature
regions by almost 70 K, indicating the promotion effect of La2O3. It
had also been reported that La2O3 could promote the dispersion of
nickel and further improve its reduction behaviors [35–37].
3. Results and discussion
The XPS measurements were also conducted to get the catalyst
surface information over the calcined, reduced and spent Ni/La2O3
catalysts, respectively. However, it could be seen from Fig. 3 that the
binding energy of Ni 2p3/2 in the reduced and spent catalysts were
overlapped by that of La 3d5/2, hence the binding energy was hard
to be calculated. Similar results also were reported by Sun et al. [33].
While the binding energy of Ni 2p3/2 at 854.87 eV in the calcined
catalyst indicated that Ni species existed as NiO state, but there
were positive shift of the binding energy when compared with the
standard values, which might be influenced by the formation of
LaNiO3 species in the calcined process. For the Ni species in the
reduced and spent catalysts, we speculated that it mainly existed
as metal Ni on catalyst surface, according to the TPR results and the
clear change of the spectroscopy of Ni 2p shown in Fig. 3. Evidently,
in the steam reforming process metal Ni was the active species. As
for La species, the binding energy of La 3d5/2 in the calcined catalyst
was about 834.91 eV (Fig. 4), indicating that La species existed as La
(III) species, however, in the reduced and spent catalyst the binding
3.1. Catalysts characterization
H2-TPR is a powerful tool to study the reduction behavior of the
oxides phase, from which it is possible to obtain useful information
about the degree of the interactions between the species. Results of
H2-TPR analysis profiles for the different metal loadings of Ni/La2O3
catalysts were presented in Fig. 2. It could be found that no hydro-
gen consumption was observed for La2O3, obviously, the support
could not be reduced under the reduction conditions. While for
the Ni-based catalysts, the amounts of hydrogen uptakes increased
with the increase of Ni loadings and the uptakes are approximately
in accordance with the catalyst compositions. However, although
the catalysts exhibited comparable TPR profiles, the profiles still
could be roughly divided into two types. For the catalysts with the
Ni loading lower than 20 wt.%, the TPR profiles showed mainly one
principle peak at ca. 700 K and two tail peaks at 930 and 1020 K,
respectively. Evidently, there were three types Ni on La2O3 surface,

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Z. Li et al. / Journal of Molecular Catalysis A: Chemical 355 (2012) 123–133
Fig. 5. The homogeneous reactions (in the absence catalyst), S/C ratio 7.5:1;
LHSV = 5.1 h−1; P = 1 atm.
Fig. 3. XPS spectra of Ni 2p for the calcined, reduced and spent Ni/La2O3 catalysts
(20 wt.%). (a) The calcined catalyst; (b) the reduced catalyst; (c) the spent catalyst.
come from the further decomposition of acetic acid. Besides, in the
whole temperature range investigated there was almost no H2 gen-
eration, suggesting that the steam reforming acetic acid [Eq. (4)]
could not take place in the absence of catalyst.
energy of it shifted to about 835.30 eV, probably owing to the strong
interactions between Ni and La species.
The results of BET measurements showed that the specific area
of the Ni/La2O3 catalyst was relatively little, 11.27 m2 g−1, the total
volume of pores and average pore diameter were 0.065 cm3 g−1 and
17.86 nm, respectively.
CH3COOH + 2H2O → 4H2 + 2CO2
(4)
3.3. Effects of Ni loadings
3.2. Homogeneous (non-catalytic) reaction
It is well established that catalyst composition has significant
influences on catalyst behaviors, such as the activity and selectiv-
ity. Therefore, the Ni loadings were varied in the range of 0–26 wt.%
to identify the optimal catalyst compositions for achieving the best
catalytic performances. The series catalysts were evaluated from
523 to 873 K and results were shown in Fig. 6 and Table 1. It could
be observed that the catalyst La2O3 without Ni loading showed very
poor activity for acetic acid reforming. Acetic acid conversion could
not attain 80% even at 873 K and only very small amount of H2 was
formed over the catalyst, in contrast, pronounced amount of ace-
tone, ketene and CH4 were produced, indicating that steam could
hardly participate in the reactions, only the ketonization, dehydra-
tion and decomposition of acetic acid itself occurred. Although the
La2O3 showed negligible activity toward the steam reforming, it
showed some activity for the conversion of acetic acid via the dehy-
dration and the ketonization reactions when compared with that
of the blank test (the homogeneous reaction). While for the Ni-
based catalysts, they all showed good activity for converting acetic
acid, obviously, Ni was the active species that catalyzed the steam
In order to gain an idea of the homogeneous reactions of the
reactants before them reaching the catalyst bed, the steam reform-
ing of acetic acid was primarily investigated in the absence of
catalyst. Results were shown in Fig. 5. It was observed that in
the temperature range of 523–623 K the homogeneous reaction
occurred just to a negligible extent. Only a trace amount of ace-
tone detected in the effluent products indicated the bimolecular
ketonization reaction of acetic acid [Eq. (1)] took place initially,
whereas at 673 K, the acetic acid dehydration and decomposition
proceeded [Eqs. (2) and (3)], resulting in the clear increase of ketene
and methane amounts.
2CH3COOH → CH3COCH3 + CO2 + H2O
CH3COOH → CH2CO + H2O
CH3COOH → CH4 + CO2
(1)
(2)
(3)
Furthermore, as temperature increase up to about 823 K, small
quantities of CO were detected in the effluent gas, which might
Fig. 4. XPS spectra of La 3d for the calcined, reduced and spent Ni/La2O3 catalysts
(20 wt.%). (a) The calcined catalyst; (b) the reduced catalyst; (c) the spent catalyst.
Fig. 6. Acetic acid conversions as a function of temperature for the series of catalysts
with different Ni contents. S/C ratio 7.5:1; LHSV = 5.1 h−1; P = 1 atm.

Z. Li et al. / Journal of Molecular Catalysis A: Chemical 355 (2012) 123–133
127
Table 1
Influence of the Ni loadings on acetic acid conversions and distribution of the products. T = 623 K; S/C ratio 7.5:1; LHSV = 5.1 h−1; P = 1 atm.
Catalysts
C/%
30.1
90.7
91.8
SH2 /%
SCH4 /%
SCO/%
SCO2 /%
SCH
SCH
CO
2
COCH
3
3
La2O3
5.6
87.4
88.8
90.7
91.3
93.8
93.2
92.4
6.9
6.2
5.1
4.4
3.6
3.4
2.1
2.6
–
46.1
81.3
83.7
85.5
92.2
94.8
94.6
92.5
10.4
2.4
1.7
1.8
0.87
0.38
0.24
0.12
6.1
1.8
1.7
1.3
0.78
0.26
0.16
–
5% Ni/La2O3
9% Ni/La2O3
13% Ni/La2O3
16% Ni/La2O3
20% Ni/La2O3
23% Ni/La2O3
26% Ni/La2O3
1.4
1.9
1.1
0.8
0.3
1.8
2.9
100
100
100
100
100
reforming reaction. However, the catalytic behaviors were differ-
ent for the catalysts with different Ni loadings. It could be seen that
increasing Ni loadings from 5 to 20 wt.% resulted in a shift of the
acetic acid conversion curve toward lower temperatures, besides,
the selectivity to H2 also increased with the increase of Ni loadings,
owing to the clear decrease of the formation of CH4 and the organic
by-products, as shown in Table 1. However, further increase the Ni
loadings would result in a small but appreciable decrease in cat-
alytic activities, which was more clear at 573 K, where acetic acid
conversions increased with the increase of Ni contents and went
through the maximum values at 20 wt.% of Ni loadings. Low cat-
alyst loadings might not provide enough sites for the occurrence
of the steam reforming. It could be seen that for the catalysts with
the lower Ni loadings of 5 and 9 wt.%, acetic acid could not be con-
verted completely until the temperature increasing to 673 K, while
for the catalysts with the higher Ni loadings, the temperature at
which complete acetic acid conversion was achieved was lowered
by more than 50 K. However, it was not the case that the higher
Ni loadings, the better performances of the catalyst, because too
high Ni loadings would result in some extra Ni crystallites on cat-
alyst surface. The large NiO crystallites in size had no contribution
to improve the catalyst activity, vice versa; they would possibly
decrease the catalyst activity by forming larger size of Ni particles
in the reduction process and then caused the fast metal sintering
and coke deposition on catalyst [38]. Therefore, we speculated the
20 wt.% was the optimal metal loadings on La2O3 carrier for achiev-
ing the best catalytic performances. Therefore, a further research,
such as characterizations and influences of reaction temperature,
LHSV, S/C and time on stream on the reactions, over this catalyst
was conducted.
could not be cracked effectively because of the low reaction tem-
perature; consequently, the homogeneous reactions of acetic acid
itself proceeded. While as the temperature increasing to 623 K, the
desired steam reforming and shift reactions were predominant,
acetic acid was then converted completely, and the selectivity to
the by-product CO underwent a minimum, decreasing substantially
from about 14.1% at 523 K to about 0.24% at 623 K. Meanwhile, the
amount of undesired organic by-products acetone and ketene were
also presented in trace level. As a result, at 623 K, the selectivi-
ties to H2 and CO2 passed through the maximum values of 93.8
and 94.8%, respectively. Obviously, Steam reforming of acetic acid
could take place to a significant extent over Ni/La2O3 at temper-
ature above 623 K, as evidenced by the sharp increase of catalyst
activity and selectivity. However, the selectivity to H2 did not
increase further along with the temperature continuously increas-
ing to higher ranges, for the formation of CO recovered to increase
and the amounts increased rapidly with the increase of tempera-
ture. Furthermore, the selectivity to CH4 also started to increase.
Clearly, it was unfavorable for hydrogen production at higher tem-
perature because of the two main gaseous by-products, CH4 and
CO.
It is well-known that the production of CH4 and CO would
diminish hydrogen selectivity, 1 mol of CH4 generation will result
in 4 mol loss of H2, similarly, 1 mol CO will lose 1 mol of H2. Thus,
a further research on the pathways of their generations is neces-
sary. the decomposition of acetic acid, we speculated that another
possible route for the formation of methane was through methana-
tion of carbon oxides in the steam reforming process, since that Ni
had been widely investigated as the active species [39,40] for the
methanation of carbon oxides [Eqs. (6) and (7)] and the effluent
gas from acetic acid reforming was mainly H2 and CO2. While for
CO, the shift reaction itself is exothermic and therefore favored at
relatively low temperatures, at high temperatures the reverse shift
reaction [Eq. (8)] would be predominant [41], hence we speculated
that the reverse shift reaction also occurred in parallel with acetic
3.4. Effects of reaction temperature
Typical experimental results showing the catalytic behaviors
of Ni/La2O3 catalyst at different temperatures were presented in
Fig. 7, in which the conversions of acetic acid and selectivity to
each product were shown as a function of reaction temperature
that ranged from 523 to 873 K. It could be seen the catalyst showed
activity for the reforming reactions even at the initial lower reac-
tion temperature of 523 K, where acetic acid conversion attained
about 50%, along with a certain amount H2, CO2, CO, acetone, ketene
and CH4 as the products. Among them CO was the main gaseous
by-product, while acetone and ketene were the main liquid by-
products. The substantial amount of CO generation at 573 K might
be caused by that the CO adsorbed on catalyst surface could not be
removed effectively by the shift reaction [Eq. (5)], owing to the too
low reaction temperature.
CO + H2O → H2 + CO2
(5)
The production of acetone and ketene implied that besides
steam reforming acetic acid the dehydration and bimolecular
ketonization of acetic acid also involved as the parallel reactions,
which competed with the steam reforming reaction to hydrogen.
It might be caused by that the C–C bond of acetic acid molecule
Fig. 7. Effects of reaction temperature on the steam reforming reactions. Catalyst:
Ni/La2O3 (20 wt.%); S/C ratio 7.5:1; LHSV = 5.1 h−1; P = 1 atm.

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Z. Li et al. / Journal of Molecular Catalysis A: Chemical 355 (2012) 123–133
Fig. 8. Distribution of the products in the methanation reactions. The temperature
of the first reactor was fixed at 873 K. Catalyst: Ni/La2O3 (20 wt.%); GHSV = 2400 h−1
;
Fig. 9. The steam reforming of methane. Catalyst: Ni/La2O3 (20 wt.%); S/C ratio 5:1;
GHSV = 8400 h−1; P = 1 atm.
P = 1 atm.
higher temperature ranges it was mainly governed by the reverse
shift reaction.
acid steam reforming, especially in higher temperature ranges. To
prove the hypothesis, we conducted the methanation reactions
over Ni/La2O3 catalyst using two reactors in the temperature region
of 523–873 K. Acetic acid was firstly reformed with steam in the
first reactor to generate the effluent gas, which was then intro-
duced into the second reactor to figure out methanation activity of
the catalyst. The distribution of the products obtained from the sec-
ond reactor as a function of reaction temperature was summarized
in Fig. 8. It could be observed that in the lower temperature region
of 523–723 K, the selectivity to CH4 increased rapidly and reached
the maximum value of 53.1% at 723 K, leading to the substantial
decrease of H2 and CO2 selectivities. However, further increasing
temperature did not result in the further increase of CH4 selectiv-
ity, owing to the limitations of thermodynamic equilibrium of the
reaction. The curve of CH4 selectivity as a function of reaction tem-
perature in the acetic acid reforming process was shown in Fig. 8,
in which CH4 amounts increased slowly but linearly with the tem-
perature increase from 523 to 773 K and then decreased slightly
with the temperature continuously increase. It was approximately
in agreement with the results of the methanation experiments,
suggesting that in the reforming process the methanation reaction
might be one of the pathways for the CH4 generation, especially at
mild temperatures such as 673 K.
In addition to the methanation and reverse shift reactions in
the reforming reactions, we speculated the steam reforming of CH4
[Eqs. (9) and (10)] also might have some impact on the distribution
of the products at higher temperatures, since steam was excess
in the reactants and the Ni-based catalysts were effective for CH4
reforming [42].
CH4 + 2H2O → CO2 + 4H2
CH4 + H2O → CO + 3H2
(9)
(10)
Besides, the steam reforming CH4 also could explain the
decrease in the formation of CH4 and increase of CO amounts at
higher temperature. Therefore, In order to examine this possibility,
additional experiments of steam reforming CH4 were conducted
over Ni/La2O3 catalyst under the similar reaction conditions of
acetic acid reforming. Results were shown in Fig. 9. It could be seen
that at the temperature below 773 K, the catalyst showed negligi-
ble activity. While at the maximum reaction temperature of 873 K,
the catalyst still showed very poor activity to convert CH4. Only
a small amount of H2 and CO2 generated and no CO formation
were observed, indicating that the steam reforming of CH4 had an
insignificant influence on the distribution of the products in the
reforming process. To summarize, in higher temperature region,
the methanation reaction was the important pathways for CH4 gen-
eration, while the reverse shift reaction was responsible for the
appreciable amount of CO formation.
CO + 3H2 → CH4 + H2O
CO2 + 4H2 → CH4 + 2H2O
H2 + CO2 → H2O + CO
(6)
(7)
(8)
Besides, it could be observed an interesting phenomenon. The
production of CO exhibited a different characteristic from that of
CH4. It could be observed that the amounts of CO decreased ini-
tially but then increased quickly with temperature increase and
reached a remarkable level at higher temperature, as shown in
Fig. 8. Evidently, the methanation of CO took place at the lower
temperature of 523 K, the formation of CO then decreased and
underwent a minimum at 623 K, which was well in accordance with
the results shown in Fig. 7. Therefore, we concluded that the shift
reaction and the methanation of CO were the two main reasons
for the sharp decrease of CO formation at 623 K in the reforming
process. While along with the temperature continuously increase,
the rate of the reverse shift reaction accelerated and exceeded that
of CO methanation, as a consequence, CO amounts recovered to
increase and reached a remarkable level in higher temperature
ranges. Summarized the above results, we concluded that CO con-
centration was governed by the shift and methanation reactions in
the lower temperature ranges in the reforming process, while in
3.5. Effects of LHSV
The effects of LHSV on the catalytic performances of Ni/La2O3
catalyst were investigated at 573 and 623 K, respectively. Results
were illustrated in Figs. 10 and 11. It could be observed the dis-
tinct effects of LHSV on the reforming reactions under the different
reaction temperature. At lower temperature of 573 K, increasing
the LHSV resulted in the sharp decrease of acetic acid conversions,
together with a significant decrease in H2 and CO2 production.
While as regard the by-products, the case was complicated. A mild
decrease in CH4 selectivity was observed, however, along with
the increase of LHSV, the selectivity to other by-products such as
CO, acetone and ketene all increased remarkably, especially the
CO selectivity, which increased progressively from less than 5% at
3.3 h−1 to about 30% at 12.3 h−1. We speculated the low conver-
sions of acetic acid might be caused by the limited specific area
of the catalyst and the low steam reforming rates at the low reac-
tion temperature. While at higher space velocity the lower contact

Z. Li et al. / Journal of Molecular Catalysis A: Chemical 355 (2012) 123–133
129
Fig. 12. Effects of S/C on the conversions of acetic acid and distribution of the prod-
ucts at 573 K. Catalyst: Ni/La2O3 (20 wt.%); T = 573 K; LHSV = 5.1 h−1; P = 1 atm.
Fig. 10. Influence of LHSV on the conversions of acetic acid and selectivities to the
products at 573 K. Catalyst: Ni/La2O3 (20 wt.%); T = 573 K; S/C ratio 7.5:1; P = 1 atm.
3.6. Effects of S/C
S/C was one of the key parameters in the reforming reactions,
influences of which on the steam reforming reaction were also
investigated at 573 and 623 K, respectively. Results were given in
Figs. 12 and 13, where acetic acid conversions and distribution of
the products were plotted as functions of the S/C ratios that ranged
from 0 to 10:1. It could be seen that whether acetic acid conver-
sions or selectivities to H2 and CO2 all increased monotonously
with the increase of S/C ratios, conversely, the formation of the by-
products decreased at high S/C ratios. Clearly, excess steam could
favor the steam reforming reactions and depress the generation
of the by-products. Marquevich et al. also reported low S/C ratios
would result in low rate of the steam reforming and large amount
by-products generation, owing to the low partial pressure of steam
[8]. Moreover, it could be seen in the absence of steam, that is,
at the S/C ratio of 0, the acetic acid ketonization, dehydration and
decomposition reactions were dominant, resulting in almost no H2
formation. While in the presence of steam, these reactions were
depressed to a significant extent. Such as at the S/C ratio of 2.5:1,
the steam reforming of acetic acid proceeded, acetic acid conver-
sion and H2 yield increased substantially. However, the selectivity
to H2 was still relatively low because of the significant amount of
CO generation. It might be caused by that CO was more difficult to
be removed by the shift reaction at low steam ratios, which was
proved by the subsequent runs conducted at higher S/C ratios, CO
selectivity decreased linearly along with the increase of S/C ratios.
Obviously, at high steam ratios, not only the shift reaction was
time was unfavorable for the adsorption and activation of steam
and acetic acid, as a result, the steam reforming and shift reactions
could not work sufficiently and then the ketonization and dehy-
dration reactions proceeded, leading to the large amount of CO,
acetone and ketene generation. As for CH4, we considered a part of
it come from the methanation of carbon oxides. When higher LHSV
was imposed, the limited catalyst surface was mainly adsorbed by
the molecules of acetic acid and steam. Consequently, the H2 and
carbon oxides might have less chance to contact with catalyst active
sites to form CH4, thus, it was the results that the higher LHSV, the
lower selectivity to CH4. However, increasing temperature by 50 K
could result in a remarkable improvement of the catalytic activ-
ity and selectivity. As shown in Fig. 11. Acetic acid conversion kept
100% for the whole range of LHSV investigated. Although the selec-
tivities to H2 and CO2 still decreased at higher space velocity, they
did not drop so dramatically as that of 573 K. There still was a sig-
nificant amount of CO formed at higher space velocity, but the
amount was in trace level at lower space velocity, and selectivi-
ties to other by-products were all decreased with the increase of
LHSV. Apparently, at 623 K, the catalyst showed relatively better
tolerance space velocity, since the steam reforming rates acceler-
ated and the reactants could be activated enough to form H2 and
diminish the by-products generation.
Fig. 11. Influence of LHSV on the conversions of acetic acid and selectivities to the
products at 623 K. Catalyst: Ni/La2O3 (20 wt.%); T = 623 K; S/C ratio 7.5:1; P = 1 atm.
Fig. 13. Effects of S/C on the conversions of acetic acid and distribution of the prod-
ucts at 623 K. Catalyst: Ni/La2O3 (20 wt.%); T = 623 K; LHSV = 5.1 h−1; P = 1 atm.

130
Z. Li et al. / Journal of Molecular Catalysis A: Chemical 355 (2012) 123–133
promoted but also the reverse shift reaction was inhibited to some
extent. Higher temperature could result in higher steam reforming
rates and better activation of the reactants, and then better catalytic
performances were obtained, as the results shown in Fig. 13. At
623 K, the catalytic activity and selectivity were improved remark-
ably. At the same S/C ratios, higher acetic acid conversions and H2
yields were achieved. Moreover, the formation of the by-products
were all suppressed to a significant extent, especially for CO, at the
S/C ratios above 7.5:1, it was only be detected in trace amount.
At 573 K, there were pronounced amounts of CO and CH4 gen-
eration and the amounts increased substantially along with the
decrease of S/C ratios. Hence, it was worthy to discuss the influences
of S/C ratios on the by-products generation here at this low reaction
temperature. In Section 3.4, we had elucidated that in higher tem-
perature ranges part of CO and CH4 were the secondary products,
the methanation and reverse shift reactions were the important
pathways for their generations. However, at lower reaction tem-
perature such as 573 K, it seemed not to be the case, especially
for CO. It was well established in the literature that the reverse
shift reaction was favored at higher temperature, in lower temper-
ature region, the shift reaction was dominant [41]. Apparently, the
reverse shift reaction could not explain the significant amount of
CO formation at the temperature below 623 K. CO might be the pri-
mary products that come directly from the steam reforming acetic
acid. As for CH4, we thought another route that CH4 was formed
directly by the decomposition of acetic acid [Eq. (3)] might work at
lower S/C ratios, for the catalyst showed relatively lower activity
for the methanation reaction at temperature below 623 K and the
decomposition of acetic acid was favored at lower steam ratios.
Wang et al. reported that acetic acid was dissociative adsorp-
tion to form COx, CHx and Hads species on catalyst surface in the
steam reforming process [10]. Low S/C ratios meant less propor-
tion of steam in the reactants, which would inevitably affect the
adsorption of steam and then result in less OHads species formed
by the dissociation of steam. Consequently, the CHx species would
has more chance to combine with Hads species, and then desorbed
to form the by-product CH4, or combined with only one OHads
species and then dehydrogenated to form COads species. However,
the COads species was difficult to be removed by the shift reaction
at lower S/C ratios due to the less OHads species formed on catalyst
surface, consequently, another by-product CO formed. Therefore,
we thought that low S/C would affect the adsorption of steam and
then the shift reaction, which, as a result, lead to the lower conver-
sions of acetic acid and higher selectivities to CO and CH4. Besides,
Marquevich et al. pointed out that the organic and steam competed
in metal sites in the reforming process, S/C would have remark-
able influence on catalytic activity and selectivity if the active sites
on catalyst surface were limited [43]. The characterization showed
that Ni/La2O3 catalyst had limited specific area, which might result
in the competing for active sites between acetic acid and steam,
and then influenced the distribution of the products, as the results
presented above.
Fig. 14. Stability test. Catalyst: Ni/La2O3 (20 wt.%). T = 623 K; S/C ratio: 7.5:1;
LHSV = 5.1 h−1; P = 1 atm.
by-product CO decreased with the reaction time and after 40 h
in the effluent gas, its presence in the effluent gas was in trace
amount, which was of significance for the use of acetic acid as the
promising fuel for PEMFC. Besides, there was negligible amount
of acetone generation at the initial reaction time, but it could not
be detected anymore after several hours of time-on-stream, thus,
we did not express it in Fig. 14. However, the amount of another
main by-product CH4 increased remarkably after 20 h but then sta-
bilized after 50 h. Consequently, the selectivities to H2 and CO2
dropped correspondingly, but the values still exceeded 90%, and
further exposure of the catalyst to the reactants mixture did not
result in the further decrease of the selectivities. The stable inter-
nal structure of Ni/La2O3 catalyst might be one of the reasons for its
good performances in the stability test. Liang et al. reported that the
rare earth oxide, La2O3, could prevent Ni metal from agglomerat-
ing by supporting and isolating the particles, and then promoted Ni
dispersion and restricted the increase of the particle size [44]. From
the XRD results, we also could observe that there was no remark-
able increase of Ni particle size in spent catalyst, which is very
advantageous for the steam reforming reactions. While Fleys et al.
also reported that nickel particles could be dispersed in the lan-
thanum oxide matrix and formed an overlayer that contained both
lanthanum and nickel which could explain the stable and active
performances of the catalyst [37].
There are some reports [45–47] that hydrocarbons are easily
polymerized and form coke on metal sites, which will make the cat-
alyst lose activity. However, of special interest is the uniqueness of
the Ni/La2O3 in terms of its long term stability and good resistibility
to carbon deposition, which also has been observed for reforming
of CH4 with CO2 [48]. Fatsikostas et al. have explained this phe-
nomenon by taking into account a mechanistic model that a thin
overlayer of lanthana is formed on top of the nickel particles during
their work of steam reforming of ethanol [49]. Under the reform-
ing reaction conditions, lanthanum oxides species, which decorated
the Ni particles, reacted with CO2 to form La2O2CO3 [Eq. (11)]. The
lanthanum oxycarbonate species then reacted with surface carbon,
at their periphery, thus cleaning the carbon deposits on Ni surface
[Eq. (12)].
3.7. Stability test
Taking into account that Ni/La2O3 catalyst showed the best
performances at 623 K, stability test was carried out at this tem-
perature to better insight into its catalytic behaviors. The test
performed for 100 h, results were shown in Fig. 14, in which acetic
acid conversions, and selectivities to the products were plotted as
functions of time. It could be seen that Ni/La2O3 catalyst exhib-
ited the stable performances for the 100 h investigated, showing no
deactivation signs. Conversions of acetic acid kept the practically
constant values of 100% for all the time-on-stream, while distri-
bution of the products varied with the prolonged reaction time.
It was noteworthy that the formation of the highly undesirable
La2O3 + CO2 → La2O2CO3
(11)
(12)
La2O2CO3 + C → La2O3 + 2CO
In this way, the surface carbon at the periphery of the oxy-
carbonate particles was removed, leading to the observed good
stability of the catalyst. There is also a body of other work on the
beneficial effect of lanthanide oxides in the stability of Ni-based cat-
alysts since these oxides are known promoters of carbon removal

Z. Li et al. / Journal of Molecular Catalysis A: Chemical 355 (2012) 123–133
131
Fig. 16. Acetic acid pulse experiments in the absence and presence of steam over
Ni/La2O3 (20 wt.%). Catalyst: Ni/La2O3 (20 wt.%). T = 623 K; LHSV = 5.1 h−1; P = 1 atm.
Fig. 15. Acetic acid pulse experiments in the absence and presence of steam over
La2O3. Catalyst: La2O3. T = 623 K; LHSV = 5.1 h−1; P = 1 atm.
from metallic surfaces [50,51]. Besides, the steam reforming reac-
tion was conducted at a high steam to carbon ratio, the excess steam
also was a very effective factor to resist carbon deposition, which
could gasify [Eq. (13)] and then eliminated the surface carbon on
catalyst.
COx species, which then reacted with steam to form H2. Although
La2O3 had poor activity of activating acetic acid, it was an impor-
tant medium for the activation of steam. It was reported that steam
could be adsorbed on La2O3 surface and then mobilized to metal
sites to participate in the steam reforming reaction [30]. Besides,
as discussed in Section 3.7, La2O3 was one of the important factors
for the elimination of the carbon deposits on catalyst surface.
C + H2O → CO + H2
(13)
The carbon balance calculated from carbon products was higher
than 97% throughout the stability test. Moreover, from the XPS
results of the spent catalyst, although we observed the carbon con-
centration increase on the surface of the catalyst, the increase was
very little and negligible, suggesting that the carbon deposition rate
was relatively low under the experimental conditions employed,
the catalyst showed good resistibility to carbon deposition.
3.9. Reaction pathways
Based on the results outlined above, we suggested a schematic
representation of reaction pathways involved in the formation
of H2, CO2, CO, CH4, acetone and ketene from acetic acid steam
reforming over Ni/La2O3 catalyst. As illustrated in Figs. 17 and 18.
The dissociation pattern of acetic acid and the pathways for the
formation of acetone and ketene was shown in Fig. 17. Here
the * represented the metal sites. There were mainly two ways
3.8. Roles of Ni and La2O3 in the steam reforming reaction
It was recognized that the steam reforming process involved the
important steps of the adsorption and activation of acetic acid. To
make sure where acetic acid was activated, we conducted the pulse
experiments over La2O3 and Ni/La2O3, respectively. The experi-
ments was carried out by introducing pure acetic acid into the
reactor for 10 min to make sure catalyst surface was adsorbed
sufficiently, then the pulse of acetic acid was stopped and steam
was introduced into the reactor for another 10 min to gasify the
adsorbed species. The products were detected on-line by the chro-
matograms. It could be seen from Fig. 15 that acetone and ketene
was the main by-products over La2O3 in the absence of steam,
which mainly came from the homogeneous reactions. When acetic
acid pulse was stopped and steam introduced, there was no H2 for-
mation, and the organic products and CO2 productions declined
very quickly along with time, indicating that acetic acid could not
be dissociably adsorbed on La2O3 surface to form the methyl, COx
and other species that were the important intermediate for the pro-
ceeding of the steam reforming process. In Section 3.3, we also had
shown that over La2O3 there was almost no hydrogen formation,
the support itself showed no steam reforming activity, probably
due to its poor capacity of activating acetic acid. While for Ni/La2O3
catalyst, the case was different. As shown in Fig. 16. The catalyst
showed remarkably high activity for converting acetic acid, and
there was some H2 generation, which mainly came from acetic acid
decomposition. More importantly, when steam was introduced
into the reactor, the yields of typical steam reforming products
such as H2, CO2, CO and CH4, increased substantially. It was clear
that metal Ni was the active sites for the reforming reaction. Acetic
acid was adsorbed and dissociated over it to form the methyl and
for the dissociation of acetic acid, cleavage the H–O and C O
bonds formed the acetate and acyl species, respectively [52]. Here
the acetate species would undergo further decompose to form
adsorbed methyl and CO2, similarly, the acyl species could decom-
pose to form methyl and CO*, but it also could integrate with
the methyl group to form acetone. It could be observed here that
whether acetic acid dissociated to acetate species or acyl species,
further decomposition of them all gave methyl species, which was
an important intermediate in the steam reforming process, it was
the precursors for the formation of gaseous products, as shown in
Fig. 18. The methyl group could combine with one *OH species
and then dehydrogenated to form H2 and CO*, the CO* was then
removed by the shift reaction. It also could further dehydrogenate
to form CH2* and then combined with two *OH species, which then
gave the products of H2 and CO2. In addition, the methyl also could
combine with *H to form the by-product CH4, or further decompose
to form C* species, which would accumulate and block the surface
sites and then decreased catalyst activity. Nevertheless, under the
experimental conditions employed, most of the C* formed could be
gasified by steam and CO2. Besides, the dissociation of steam and
the recombination of two H* species to form H2 also were impor-
tant steps involved in the steam reforming process. From Fig. 18,
we could find another intermediate that was very important for
the formation H2 in the steam reforming process was *OH species,
it could integrate with CHx (x = 0–3) to form the gaseous carbon
products and, most importantly, to form H2. Furthermore, *OH
was also an important species to suppress the by-products genera-
tion. It could not only prevent the further dissociation of methyl to
carbon deposits and the combination of methyl group with *H to

132
Z. Li et al. / Journal of Molecular Catalysis A: Chemical 355 (2012) 123–133
Fig. 17. The dissociation patterns of acetic acid on catalyst surface.
Fig. 18. Assumed reaction pathways involved in the steam reforming process.
CH4, but also could eliminate CO generation through the shift reac-
tion.
in the reforming process, it could form CH4, CO, CO2 and acetone
through the combination with H*, *OH or acyl species, respectively,
while *OH was another important intermediate that was impor-
tant for the formation of H2 and elimination of the by-products
generation.
4. Conclusion
At lower reaction temperature 573 K, space velocity and S/C had
remarkable influence on the conversion of acetic acid and distribu-
tion of the products, particularly at higher space velocity and lower
S/C, acetic acid conversion was very low and a significant amount
of by-products were produced. However, increasing the reaction
temperature and the S/C ratios could remarkably improve the cat-
alytic activity and selectivity, since lower reaction temperature and
S/C resulted in lower steam reforming rates. At a selected exper-
imental condition of T = 623 K, LHSV = 5.1 h−1 and S/C = 7.5:1, the
catalyst showed high activity, selectivity and, most importantly,
good long-term stability for hydrogen production. Therefore, we
thought Ni/La2O3 was a promising catalyst, which made the steam
reforming acetic acid become feasible and open an opportu-
nity to the use of a new renewable energy source for hydrogen
production.
To sum up, the present results demonstrated that the novel
Ni/La2O3 catalyst could produce hydrogen via acetic acid steam
reforming at relatively mild temperature effectively. At the Ni
loading of 20 wt.%, the catalyst exhibited the best catalytic
performances. The characterizations showed there were strong
interactions between Ni and La species. Ni was the active sites for
the proceeding of the reforming reactions. While La2O3 itself had no
contribution for the activation of acetic acid, but it could effectively
adsorb steam and react with CO2 to form La2O2CO3, which could
further react with the surface carbon deposits and then restore the
catalyst activity.
In the steam reforming reactions, acetone and ketene was the
main organic by-products at the temperature lower than 673 K,
they mainly come from bimolecular ketonization reaction and
acetic acid dehydration, respectively. While CH4 and CO was the
main gaseous by-products in the effluent gas, the decomposi-
tion of acetic acid and the methanation of carbon oxides were
the two main pathways for CH4 generation; as for the produc-
tion of CO, the amount was mainly governed by the shift and the
reverse shift reactions. According to the experimental results, we
speculated that methyl species was the important intermediate
Acknowledgements
We acknowledge the financial support of the 973 Project of
China (G2003CB214503). We are grateful to Qunyang Gao and Xiao-
jie Zhang for their help in the characterizations of the catalyst.

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133
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