Catalysts 2019, 9, 1013; doi:10.3390/catal9121013 www.mdpi.com/journal/catalysts
Hydrogen Production from Steam Reforming of Acetic
Acid as a Model Compound of the Aqueous Fraction
of Microalgae HTL Using Co-M/SBA-15
(M: Cu, Ag, Ce, Cr) Catalysts
Pedro J. Megía, Alicia Carrero *, José A. Calles and Arturo J. Vizcaíno *
Chemical and Environmental Engineering Group, Rey Juan Carlos University, c/Tulipán s/n,
28933 Móstoles, Spain; firstname.lastname@example.org (P.J.M.); email@example.com (J.A.C.)
* Correspondence: firstname.lastname@example.org (A.C.); email@example.com (A.J.V.)
Tel.: +34-91-488-8088 (A.C.); +34-91-488-8096 (A.J.V.)
Received: 30 October 2019; Accepted: 28 November 2019; Published: 2 December 2019
Abstract: Hydrogen production derived from thermochemical processing of biomass is becoming
an interesting alternative to conventional routes using fossil fuels. In this sense, steam reforming of
the aqueous fraction of microalgae hydrothermal liquefaction (HTL) is a promising option for
renewable hydrogen production. Since the HTL aqueous fraction is a complex mixture, acetic acid
has been chosen as model compound. This work studies the modification of Co/SBA-15 catalyst
incorporating a second metal leading to Co-M/SBA-15 (M: Cu, Ag, Ce and Cr). All catalysts were
characterized by N
physisorption, ICP-AES, XRD, TEM, H
-TPD and Raman spectroscopy.
The characterization results evidenced that Cu and Ag incorporation decreased the cobalt oxides
reduction temperatures, while Cr addition led to smaller Co
crystallites better dispersed on the
support. Catalytic tests done at 600 °C, showed that Co-Cr/SBA-15 sample gave hydrogen selectivity
values above 70 mol % with a significant reduction in coke deposition.
Keywords: microalgae; acetic acid; steam reforming; hydrogen; cobalt; mesostructured materials
An increase in global pollution has resulted in a search for alternative energy resources that can
be substituted in place of widely used fossil fuels . It is known that energy provided from hydrogen
does not result in pollutant emissions when it is used in fuel cell applications [2–4]. In addition,
hydrogen is extensively used in chemical and petroleum industries [5,6]. Nowadays, a
hydrogen-based energy system must use renewable energy sources to be sustainable. In this sense,
hydrogen production processes such as biomass gasification, and steam reforming (SR) of pyrolysis
bio-oil have been widely described in the literature [7–10]. However, the use of microalgae
hydrothermal liquefaction integrated with the steam reforming of the aqueous fraction is less known.
Microalgae HTL requires temperatures between 250–350 °C and high pressures that can maintain the
water coming from the microalgae crops in liquid state (40–250 bar). This process provides a great
advantage when compared to the traditional biomass pyrolysis process, as it does not require a
previous stage for biomass drying associated with high energy consumption [11–13]. Microalgae HTL
products are a complex mixture of different compounds where carboxylic acids, ketones, phenols,
aldehydes, fatty acids and nitrogen compounds  can be easily found along with a high water
content. For this reason, they are not suitable for use as a fuel. However, this worthless aqueous
fraction can be revalorized by hydrogen production through catalytic steam reforming [15,16] but the
Catalysts 2019, 9, 1013 2 of 20
complex composition mentioned above usually forces the use of model compounds [17–20]. Among
them, acetic acid is a major component, which can account even for the 56% of the water-soluble
products . The overall equation of the acetic acid steam reforming is:
C2H4O2 + 2 H2O → 2 CO2+ 4 H2 (1)
Nowadays, SR catalysts are a critical point of study where activity, hydrogen selectivity and
deactivation are the main concerns of the scientific community. Many papers can be found using
different active phases such as Ni, Co, Pt or Ru, with Ni being the most studied . Hu et al. 
studied the performance of different transition metals supported over Al2O3 in acetic acid steam
reforming. Their study led to the conclusion that Ni and Co were more active than the other metals
tested (Fe and Cu). They attributed this behavior to the ability for cracking not only C-C bonds, but
also C-H bonds. However, Co-based catalysts have been less reported despite the fact that they also
provide high activity at moderate temperatures and also increases hydrogen yield [23,24].
Catalysts support selection is also an important point. For example, when Co was supported on
Al2O3 or TiO2 high metal dispersion was reported but cobalt aluminates or titanates were formed
avoiding the reduction of some Co species . On the other hand, the interaction of Co with silica
has been studied leading to the conclusion that this support does not affect to its reducibility but
instead promote the sintering of cobalt particles in the calcination and reduction steps [26,27]. Apart
from that, there are other advanced supports such as SBA-15, which is a mesostructured material
with high surface area that may allow higher metal dispersion when compared with the amorphous
silica. Furthermore, SBA-15 presents an uniform distribution of mesopores that hinders the formation
of Co agglomerates preventing also catalysts deactivation due to metal sintering .
Co-based catalysts have shown deactivation through sintering and surface cobalt oxidation .
Pereira et al.  proposed the preparation of bimetallic catalysts to stabilize Co/SiO2 catalyst to
safeguard the Co particles in a reduced state during the reforming. Combining diverse metals in the
same carrier has been reported as an effective way to improve the catalyst performance by facilitating
the metal reducibility . As reducibility promoters noble metals, transition metals or CeO2 among
others can be used. Wang et al.  reported that Cu addition to Ni/attapulgite catalyst decreased the
temperature for the reduction of nickel species. In line with this, Eschemann et al.  proved the
efficiency of silver as a reduction promoter in Co/TiO2 catalyst since Co-Ag bonds improve the
reducibility of cobalt oxides [32,33]. Besides, Harun et al.  achieved better Ni0 dispersion over
Al2O3 surface when Ag was included in the catalyst formulation. Similarly, it was described that
CeO2, presents a synergistic effect with cobalt oxides since more oxygen vacancies are formed leading
to higher reducibility . In addition to promoting the cobalt reducibility to avoid possible
crystallites oxidation, it is necessary to obtain a small crystallite size in order to increase activity and
reduce the coke formation according to its growth mechanism . Accordingly, Cerdá-Moreno et
al.  found that lower Co particle size for ethanol steam reforming led to better catalytic activity.
Recently, we have found that Ni-Cr/SBA-15 showed better catalytic behavior than Ni/SBA-15 in the
steam reforming of pyrolysis bio-oil aqueous fraction by decreasing Ni0 particles size .
Furthermore, Casanovas et al. [39,40] reported that the incorporation of Cr to Co/ZnO samples results
in better catalytic performance when these catalysts were tested in ethanol steam reforming.
So far, we have not been able to find any references using the promoters described above in
Co/SBA-15 catalysts to be tested in acetic acid steam reforming. Therefore, the main goal of this study
is the preparation of novel cobalt catalysts incorporating a second metal leading to Co-M/SBA-15 (M:
Cu, Ag, Ce and Cr) to achieve high hydrogen production rate through acetic acid steam reforming as
model compound of microalgae HTL aqueous fraction.
2. Results and Discussion
2.1. Catalysts Characterization
Nitrogen physisorption profiles displayed in Figure 1 show type IV isotherms with a H1-type
hysteresis loop according to the IUPAC classification, indicating the preservation of the initial
Catalysts 2019, 9, 1013 3 of 20
mesoestructure of SBA-15 used as the support of these samples. Textural properties calculated from
these analyses are summarized in Table 1 along with other physicochemical properties.
Figure 1. N2 physisorption isotherms of calcined (a) Co/SBA-15; (b) Co-Cu/SBA-15; (c) Co-Ag/SBA-
15; (d) Co-Ce/SBA-15; (e) Co-Cr/SBA-15 catalysts at 77K.
Table 1. Physicochemical properties of Co-M/SBA-15 (M: Cu, Ag, Ce, Cr) catalysts.
Catalyst Coa (wt.%) Ma (wt.%) SBET (m2·g−1) Dpore b (nm) Vpore c (cm3·g−1) DCo0 d
SBA-15 - - 550 ± 3 7.5 ± 0.1 0.97 ± 0.02 - -
Co/SBA-15 6.4 ± 0.1 - 503 ± 4 7.2 ± 0.1 0.83 ± 0.01 9.5 ± 0.5 7.5 ± 0.2
Co-Cu/SBA-15 6.5 ± 0.1 2.0 ± 0.1 476 ± 4 7.2 ± 0.1 0.79 ± 0.03 9.7 ± 0.3 6.3 ± 0.1
Co-Ag/SBA-15 6.4 ± 0.1 1.6 ± 0.1 419 ± 4 6.9 ± 0.1 0.71 ± 0.01 12.3 ± 0.4 3.9 ± 0.6
Co-Ce/SBA-15 6.6 ± 0.1 1.7 ± 0.1 494 ± 1 7.4 ± 0.1 0.84 ± 0.01 9.6 ± 0.2 6.5 ± 0.1
Co-Cr/SBA-15 6.8 ± 0.1 1.8 ± 0.1 469 ± 1 7.1 ± 0.1 0.81 ± 0.02 7.2 ± 0.1 9.9 ± 0.3
a Determined by ICP-AES (M: Cu, Ag, Ce or Cr) in reduced samples, b BJH desorption average pore
diameter, c Measured at P/P0 = 0.97, d Determined from XRD of reduced catalysts by Scherrer equation
from the (111) diffraction plane of Co0,e Determined from H2-TPD results using formula from Li et al.
 assuming H/Co = 1.
The metals loading is close to the nominal value used during the catalysts preparation. Metal
addition to bare SBA-15 leads to a decrease in BET surface area with Co-Ag/SBA-15 being the sample
with the smallest pore size, pore volume and surface area. This phenomena has been described
previously  and was ascribed to Ag structures growing in the mesopores of SBA-15. Similar
textural properties were found in Co-(Cu, Ce or Cr)/SBA-15 samples.
Figure 2 shows the XRD patterns of the calcined samples. Peaks corresponding to cubic Co3O4
appear in all samples (JCDPS 01-071-4921). Attending to Co-Cu/SBA-15 sample, a small peak at 38.3°
can be observed due to the formation of monoclinic CuO (JCDPS 01-089-2531). In case of
Co-Ce/SBA-15, two small peaks over 28.5° and 47.5° can be seen due to the presence of cubic CeO2
(JCDPS 01-089-8436). Ag and Cr oxides were not detected by XRD due to the overlap of the main
diffraction peaks of cubic Ag2O (JCPDS 00-012-0793), rhombohedral Cr2O3 (JCPDS 00-002-1362) and
cubic CoCr2O4 spinel (JCPDS 00-022-1084), with the Co3O4 pattern. The higher Co content compared
absorbed volume (a.u.)
Relative pressure (P/P
0.2 0.4 0.6 0.8
Catalysts 2019, 9, 1013 4 of 20
to Ag and Cr also contributes to the non-detection of Ag and Cr oxides by XRD as were observed in
previous works [43,44]. XRD patterns corresponding to Co-(Cu, Ag or Ce)/SBA-15 present narrower
Co3O4 peaks and slightly larger Co3O4 crystallites were obtained comparing when compared to
Figure 2. XRD of calcined (a) Co/SBA-15; (b) Co-Cu/SBA-15; (c) Co-Ag/SBA-15; (d) Co-Ce/SBA-15; (e)
Co-Cr/SBA-15 catalysts. Co3O4 crystallites sizes calculated from the (311) diffraction plane using
Scherrer equation are displayed on the right.
Figure 3 shows the TEM micrographs of calcined samples. Irregular metal oxides particles can
be observed, some of them formed in the channels of SBA-15, while other particles were formed over
the external surface as previously reported . The presence of Co and promoters (Cu, Ag, Ce or
Cr) were evaluated in the corresponding sample by EDX indicating an intimate contact between Co
oxide and promoters. Co-Ag/SBA-15 catalyst has large metallic nanostructures through the SBA-15
channels and Ag2O particles can be also observed over the support . The incorporation of high
Ag loadings (> 1wt. %) affects support structure and distribution of Ag2O particles over the catalyst
because the probability of Ag-Ag bond formation increases [32,33]. On the other side, it is noticeable
how Co-Cr/SBA-15 sample clearly shows the highest dispersion over the support with very small
metal oxide particles, which is in agreement with the lower metal diameter calculated from XRD
30 40 50 60 70 80
Catalysts 2019, 9, 1013 5 of 20
Figure 3. TEM micrographs of calcined samples (a): Co/SBA-15; (b): Co-Cu/SBA-15; (c): Co-Ag/SBA-
15; (d): Co-Ce/SBA-15; (e): Co-Cr/SBA-15.
Figure 4 displays the H
-TPR profiles of the calcined catalysts. In the case of Co/SBA-15 sample,
the reduction profile shows two main reduction stages. The first one with maxima found at 248–267
°C and a shoulder around 332 °C. These peaks are attributed to the reduction of Co
to CoO and
subsequently to Co
. The reduction stage at high temperature, with a maximum placed at 494 °C, can
be attributed to the presence of Co-oxide species with stronger interaction with the support . Cu
addition led to a clear decrease of the reduction temperature as observed in Co-Cu/SBA-15 profile.
Catalysts 2019, 9, 1013 6 of 20
Figure 4. H2-TPR profiles for (a) Co/SBA-15; (b) Co-Cu/SBA-15; (c) Co-Ag/SBA-15; (d) Co-Ce/SBA-15;
(e) Co-Cr/SBA-15 samples. Red. (%) data displayed on the right correspond to reducibility.
The reduction zone is located at temperatures between 140-260 °C with two maxima at 150 and
194 °C. Whereas the lower temperature peak is ascribed to the simultaneous reduction of CuO and
Co3O4 to Cu0 and CoO respectively, the other one is related to the reduction of CoO to Co0 . This
effect of Cu in lowering reduction temperature of metal oxides was observed in previous works for
Ni-based catalysts . Co-Ag/SBA-15 catalyst showed two clearly different reduction areas, also at
low temperature. While the zone over 267 °C is related to the Co oxides next to Ag, the other one
around 166 °C is attributed to the reduction of segregated Ag2O particles to Ag0 . On the other
hand, Co-Ce/SBA-15 sample showed a reduction profile similar to Co/SBA-15 with the peak at 494
°C shifted to higher reduction temperature due to an emerging peak assigned to superficial cerium
oxide . Finally, in the reduction profile of Co-Cr/SBA-15 had a new peak around 182 °C, probably
due to the reduction of Cr-oxides to Cr3+ which can be affected by the presence of Co3O4  although
it could not be detected by XRD. The peak attributed to Co3O4 reduction at 271 °C remained unaltered
whereas the peak of CoO reduction shifts to higher temperatures due to the presence of Cr species
 or to the confinement of Co oxides into SBA-15 channels because of their smaller size. Based on
the literature, the most likely option is the formation of a cobalt chromate mixed oxide , although
none could be detected by XRD due to the overlap of the main diffraction lines of CoCr2O4 with those
of Co3O4. The XRD patterns of the samples after reduction at 700 °C under pure H2 flow are displayed
in Figure 5. No peaks ascribed to Co
3O4 pattern can be detected whereas cubic Co0 (JCDPS
00-001-1259) peaks corresponding to (111), (200) and (220) planes showing the reflection at 2θ = 44.4°,
51.3° and 75.4° can be observed in all samples after the reduction process.
200 400 600 800
Catalysts 2019, 9, 1013 7 of 20
Figure 5. XRD of reduced (a) Co/SBA-15; (b) Co-Cu/SBA-15; (c) Co-Ag/SBA-15; (d) Co-Ce/SBA-15; (e)
Co-Cr/SBA-15 catalysts at 700 °C.
Cubic Ag0 (JCDPS 00-043-1038) diffraction peaks arose in the Co-Ag/SBA-15 sample at 2θ = 38.1°,
64.5° and 77°, ascribed to (111), (200) and (220) reflection planes, respectively. In this case, some Co-
oxides could remain in this sample explaining its low reducibility (see Figure 4) but they were not
detected because there is an overlapping between Ag0 and Co3O4 patterns at 38.1° and 64.5°. In Co-
Cr/SBA-15 catalyst a peak placed at 2θ = 63.7° was assigned to rhombohedral Cr2O3 (JCDPS 00-002-
1362) probably coming from the release of CoO from the spinel CoCr2O4. No diffraction peaks of
cubic Cu0 (JCPDS 00-001-1241) were distinguished in Co-Cu/SBA-15 sample due to the overlapping
between Cu0 and Co0 diffraction peaks. Co-Ce/SBA-15 reduced sample showed only the diffraction
peak of metallic Co. The absence of CeO2 diffraction peaks prompted us to think about the formation
of a non-stoichiometric CeO2−σ that cannot be detected by XRD .
Co0 crystallite sizes were calculated by the Scherrer equation from the diffraction plane (111). In
general, whereas Co-Cu/SBA-15 and Co-Ce/SBA-15 samples present a crystallite size similar to
Co/SBA-15, Co-Ag/SBA-15 had the largest crystallites (see Table 1) which differs from the literature
as silver loading in Co-Ag/SBA-15 is higher than in references [31,32]. In contrast, Co-Cr/SBA-15
presented the lowest Co crystallite size because making a parallelism with the paper of Amin et al.
 Cr-oxides can suppress the extension growth of Cu-oxides in that case, Co-oxides in our case.
H2-TPD analysis was carried out in order to measure the dispersion of the metallic phase over
the support. The results, summarized in Table 1, follow the opposite trend as Co0 crystallite sizes
calculated from the Scherrer equation. Co-Cr/SBA-15 sample reached the highest active phase
dispersion over the support. This effect can be clearly observed in Figure 6, where Co0 crystallite sizes
are displayed against dispersion and it is clear that the only promoter that improves the base
Co/SBA-15 catalyst is Cr. In addition, other authors have reported smaller crystallite size when Cr
was incorporated to the catalyst formulation suggesting the capacity of Cr2O3 to act as a textural
promoter preventing metallic sintering [55–57]. It should be noted that in a previous work we
reported the same behavior with Ni-Cr/SBA-15 sample [38,43], in line with the results obtained by
Xu et al. during the co-impregnation of Cr and Ni over char as support .
30 40 50 60 70 80
Catalysts 2019, 9, 1013 8 of 20
Figure 6. Comparison between Co0 crystallites size and Dispersion over the SBA-15 material used as
support for (a) Co/SBA-15; (b) Co-Cu/SBA-15; (c) Co-Ag/SBA-15; (d) Co-Ce/SBA-15; (e) Co-Cr/SBA-
2.2. Catalytic Tests
AASR (acetic acid steam reforming) reactions were carried out after the reduction of the
catalysts. All experiments were performed using an aqueous solution of acetic acid with a S/C molar
ratio = 2 and a WHSV = 30.1 h−1 at atmospheric pressure and 600 °C using N2 as carrier gas. Conversion
data are not shown because all catalysts reached complete conversion along 5 h of time-on-stream,
which implies high activity for all the samples in acetic acid conversion at these reaction conditions.
However, different product distributions were achieved indicating different activities in acetic acid
steam reforming reaction, ascribed to the role of a second metal in secondary reactions. In this sense,
hydrogen and carbon co-products distribution (dry basis) are displayed in Figure 7.
Figure 7. Products distribution in outlet gas stream produced in the acetic acid steam reforming over
Co-M/SBA-15 (M: Cu, Ag, Ce, Cr) catalysts at T = 600 °C, P = 1 atm, time-on-stream = 5 h.
The H2 content expected at equilibrium at the experimental conditions, predicted by means of
the software GasEQ, based on the method of free Gibbs energy minimization, is also shown.
Regarding products distribution, all catalysts reached high hydrogen concentration, above 53%. As
known, Co-based catalysts allow the breaking of C-C bonds (only methane is produced as hydrogen-
containing product) but also of C-H bonds . Moreover, an effective catalyst must also be active in
Co/SBA-15 Co-Cu/SBA-15 Co-Ag/SBA-15 Co-Ce/SBA-15 Co-Cr/SBA-15
Catalysts 2019, 9, 1013 9 of 20
WGS reaction in order to eliminate CO from the metal surface during steam reforming. Over Co,
methane reforming and WGS activity was presented and this clearly shown by products formation.
Among them, CO2 formation is highest and followed by CO, CH4, thus WGS is more pronounced
compared to other disproportionation and decomposition reactions. Cu, Ag and Ce addition to
Co/SBA-15 decreases the hydrogen content in the gas outlet stream in line with higher co-carbon
products percentages. In contrast, Co-Cr/SBA-15 reached the highest hydrogen concentration in the
product stream. This behavior is related to the small Co crystallite size (see Table 1) leading to higher
active sites surface area [59,60]. Therefore, Cr addition improved the catalytic performance by
preventing Co agglomeration. In fact, Casanovas et al.  have published similar behavior adding
Cr to Co/ZnO being more active and selective for ethanol steam reforming. On the other side,
Co-Ag/SBA-15 achieved the lowest hydrogen concentration and therefore carbon containing
products composition was higher, probably due to the pore blocking effect and the highest Co
crystallite size. Co-Cu/SBA-15 and Co-Ce/SBA-15 showed higher CO2/CO molar ratio compared to
the other samples (3/2) suggesting that the activity for WGS reaction was increased . If WGS
reaction is favored, an increase in the hydrogen production is expected but the hydrogen content
reached with these two catalysts was lower than with Co/SBA-15 (CO2/CO ratio = 1.7) thus, it is
possible to assume that the presence of a second metal hinders reactants access to Co active centers,
thereby avoiding their catalytic role breaking C-H bonds. Finally, Co-Cu/SBA-15, Co-Ag/SBA-15 and
Co-Ce/SBA-15 showed an increase of CH4 from 2% to almost 5% in comparison to the Co/SBA-15
sample. CH4 formation can be due to the decomposition of acetic acid or methanation .
Particularly, Co-Cu/SBA-15 and Co-Ce/SBA-15 produce more CH4 in line with the reduction of H2
and CO content which indicates that Cu and Ce promote the methanation reaction (3H2 + CO → CH4
+ H2O) . Instead, the increase of produced methane with Co-Ag/SBA-15 could be due to the
decomposition of acetic acid since the CO content was kept constant while both CO2 and CH4
concentrations increase, which would be in accordance with the stoichiometry of the reaction
CH3COOH → CH4 + CO2. However, other parallel and consecutive reactions varying the CO, CO2
and CH4 content can be taking place.
Regarding the evolution of H2 selectivity, calculated as the ratio between hydrogen produced
and 4 times the reacted acetic acid (stoichiometry), with reaction time showed in Figure 8 Co-Cu/SBA-
15 and Co-Ag/SBA-15 samples exhibited a decrease at 2 h but after that, it remains almost constant.
Regardless, the H2 selectivity of the rest of catalysts remains almost unaltered with time-on-stream.
Therefore, no deactivation was detected for Co/SBA-15, Co-Ce/SBA-15 and Co-Cr/SBA-15 samples.
In addition, it can be assessed that Co-Cr/SBA-15 sample also achieved the highest H2 selectivity close
to the thermodynamic value at the present reaction conditions. This result is promising compared to
those obtained by Ni-based catalysts widely referenced in literature for acetic acid steam reforming
reactions. In this sense, Thaicharoensutcharittham et al.  reported that Ni/Ce0.75Zr0.25O2 catalyst
with a Ni loading of 5 wt.% reached hydrogen selectivity of 33.54 mol % with a S/C = 1, and 64.39 mol
% using a S/C = 3. On the other hand, Wang et al.  achieved hydrogen selectivity between 54.5
and 70.9 mol % for reaction tests carried out at 550 °C and 650 °C respectively, with S/C = 3 using
Ni/Attapulgite catalysts. In another work, Nogueira et al. published the catalytic performance of
Ni catalysts supported on (MgO)-modified γ-Al2O3 reaching, a H2 selectivity of 67.5 mol % at higher
S/C ratio (S/C = 4). Additionally, our group tested at similar operation conditions (600 °C, GHSV:
11000 h−1) Ni-based catalysts in AASR with a S/C = 4 . In that work, we achieved up to 60 mol %
of hydrogen content for both Ni/SBA-15 and Ni-Cr/SBA-15, which implied H2 selectivities between
56.6–59.9 mol %. These values are lower than those achieved with Co-M/SBA-15 catalysts in the
present work, even though lower S/C ratio has been used that should lead to worse catalytic results.
Despite differences in reaction conditions, mainly S/C molar ratio, these H2 selectivity values are
lower than that achieved by Co-Cr/SBA-15 sample. Furthermore, we also observed the beneficial
effect of adding Cr to catalysts in our recently published works [38,43], where we reached using Cr
as promoter added to Ni/SBA-15 catalysts, better catalytic performance using different feedstock in
steam reforming reaction.
Catalysts 2019, 9, 1013 10 of 20
Figure 8. Hydrogen selectivity of gas stream produced in the acetic acid steam reforming over (a)
Co-Cr/SBA-15; (b) Co/SBA-15; (c) Co-Ce/SBA-15; (d) Co-Ag/SBA-15; (e) Co-Cu/SBA-15 catalysts at T
= 600 °C, P = 1 atm.
Coke formation during steam reforming has been reported as the main cause of SR catalyst
deactivation . It must be emphasized that catalyst deactivation is not only related to the amount
of coke, but also to the nature of the coke formed, the morphology and the location over the catalyst
structure . In this sense, XRD patterns of used catalysts after 5 h (TOS) are shown in Figure 9.
Figure 9. XRD patterns of used (a) Co/SBA-15; (b) Co-Cu/SBA-15; (c) Co-Ag/SBA-15; (d)
Co-Ce/SBA-15; (e) Co-Cr/SBA-15 catalysts. Co0 crystallites sizes calculated from the (111) diffraction
plane using Scherrer equation are displayed on the right.
Peaks corresponding to cubic Co0 (JCDPS 00 001 1259) at 2θ = 44.4°, 51.3° and 75.4° can be still
distinguished. In contrast to reduced samples (Figure 5), reflection peaks corresponding to graphitic
carbon (JCDPS 00-041-1487) at 2θ = 26.5°, 42.6°, 53.9° and 78.8° ascribed to (002), (100), (004) and (006)
reflection planes, respectively, appear as a consequence of the coke deposition along the acetic acid
steam reforming being more pronounced in Co-Ag/SBA-15 sample. Cobalt crystallites sizes of used
10 20 30 40 50 60 70 80
Catalysts 2019, 9, 1013 11 of 20
catalysts (calculated from Scherrer equation) are shown on the right side of Figure 9. Comparing
these results with those found in reduced samples (Table 1), it can be concluded that cobalt crystallites
sizes were very similar, which indicates no significant sintering throughout the reforming reaction.
TGA can be used for the identification of the type of coke formed during the reaction since more
ordered coke will need higher temperature to be oxidized . It is normally reported that
amorphous carbon is more reactive than graphitic in reactions with O2  because it oxidizes at low
temperatures whereas filamentous or graphitic carbon does at higher temperatures [69–71]. Figure
10 displays the derivative thermogravimetric (DTG) curves of the used catalysts along with the
amount of coke formed during the reaction in terms of mgcoke·gcat−1·h−1.
Figure 10. DTG curves of used (a) Co/SBA-15; (b) Co-Cu/SBA-15; (c) Co-Ag/SBA-15; (d)
Co-Ce/SBA-15; (e) Co-Cr/SBA-15 samples after 5 h time-on-stream.
There are significant differences in the total coke content, in the order Co-Ag/SBA-15 > Co-
Ce/SBA-15 > Co-Cu/SBA-15 > Co/SBA-15 > Co-Cr/SBA-15 which follows the reverse order of the
hydrogen content in the outlet stream during AASR (see Figure 6). In general, all DTG profiles show
a maximum around 500 °C and a shoulder around 550 °C, indicating the formation of some kind of
carbon nanofibers with different ordering degree [69,70]. Co-Ag/SBA-15 showed a maximum around
441 °C which can be related to the formation of some defective carbon deposits. Co-Ag/SBA-15
obtained the worst catalytic results (high CH4 concentration and the lowest H2 concentration), in line
with the highest carbon deposition. Besides, it is noteworthy that Co-Cr/SBA-15 reduced the coke
production two times compared to Co/SBA-15. It is known that Cr2O3 has been used as an oxide
catalyst with outstanding carbon deposition resistance properties [72,73]. In our case, the reduction
in carbon deposition can be also ascribed to the role of chromium avoiding the formation of large Co
crystallites as it could be observed by TEM and measured by the Scherrer equation, because smaller
Co crystallites will prevent the initiation of carbon nucleation leading to coke formation . On the
other hand, Cr2O3 has catalytic activity in the WGS reaction, lowering the CO concentration into the
gas phase surrounding the catalytic bed, thus favoring the formation of H2 and CO2 . In this sense,
the extent Boudouard reaction (2 CO (g) → CO2 (g) + C (s)), which is one of the main routes for coking,
will be reduced.
200 300 400 500 600 700 800 900
15.4 ± 1.5 mg
90.0 ± 8.9 mg
282.6 ± 28.3 mgcoke·gcat-1·h-1
51.6 ± 5.2 mgcoke·gcat-1·h-1
Deriv. Weight (a.u.)
34.8 ± 3.5 mgcoke·gcat-1·h-1
Catalysts 2019, 9, 1013 12 of 20
Used catalysts were also analyzed by TEM as shown in Figure 11. In all cases, carbon nanofibers
with different ordering degree can be observed. Besides, Co-Ag/SBA-15 micrograph shows some
zones of defective coke deposits, in concordance with DTG results.
Figure 11. TEM micrographs of used (a): Co/SBA-15; (b): Co-Cu/SBA-15; (c): Co-Ag/SBA-15; (d): Co
Ce/SBA 15; (e): Co-Cr/SBA-15 (5 h time-on-stream at 600 °C).
Finally, the Raman spectra of used catalysts in the range 1200–1700 cm
are presented in Figure
12. As it can be observed, two main bands appear in all cases, at 1330–1340 (D-band) and 1586–1591
(G-band). G-band is ascribed to the stretching mode of carbon sp
bonds of condensed graphitic
aromatic structures such as graphite layer , whereas D-band is related to the carbon atoms
vibration of disordered aromatic structures such as amorphous or defective filamentous carbon
[70,77–79]. The presence of both bands exhibits the heterogeneity of carbon species constituting the
coke formed during the AASR reaction. It has been reported that the intensity of the D band relative
to the G band can be used as a qualitative measure of the formation of different kinds of carbon with
different degree of graphitization or disorder in the carbon structure [78–80]. Smaller I
indicate higher crystallinity due to higher contribution of the graphitic carbon structures formed
[81,82] but it also implies more layers constituting the deposited carbon . In these sense, the
estimated values are summarized also in Figure 12. As can be seen, the I
ratio decreases in the
following order: Co-Cr/SBA-15 (I
= 0.80) > Co/SBA-15 (I
= 0.65) > Co-Ce/SBA-15 (I
= 0.61) >
= 0.53) > Co-Cu/SBA-15 (I
= 0.48). These results indicate that carbon
deposition over the Co-Cu/SBA-15 sample occurs in larger extent on the Co surface when compared
with the other samples, leading to the growth of well-ordered carbon, which may be responsible of
catalyst deactivation since it act as a shell covering the active Co sites layer by layer . It must be
highlighted that the H
selectivity represented in Figure 8, decreases in the same order as I
Therefore, the H
selectivity is directly related to the kind of carbon deposited on the catalyst.
Catalysts 2019, 9, 1013 13 of 20
Figure 12. Raman spectra of used (a) Co/SBA-15; (b) Co-Cu/SBA-15; (c) Co-Ag/SBA-15; (d)
Co-Ce/SBA-15; (e) Co-Cr/SBA-15 catalysts.
An AASR test done at long time-on-stream displayed in Figure 13 showed that Co-Cr/SBA-15
achieved good stability after 50 h time-on-stream. Conversion values were near 95% at the end of the
reaction, while almost constant hydrogen selectivity (~72 mol %) was obtained. These results
evidence that Co-Cr/SBA-15 sample is a promising option for acetic acid steam reforming, since
hydrogen selectivity remains close to the equilibrium value for a long period and, in addition, this
value is greater than those obtained with the Ni-based catalysts described in literature.
Figure 13. Acetic acid conversion (■) and hydrogen selectivity (●) during stability test of Co-Cr/SBA-
15 catalyst at T = 600 °C, P = 1 atm.
3. Experimental Section
3.1. Catalysts Synthesis
Mesostructured SBA-15 material, synthesized using the hydrothermal method described
elsewhere , was used as catalysts support. Pluronic 123 and TEOS were used as surfactant and
silica precursor (Aldrich, St. Louis, MO, USA) respectively.
Synthesis of Co-M/SBA-15 (M: Cu, Ag, Ce or Cr) catalysts was accomplished by the incipient
wetness impregnation method described in previous work . Metal loading was selected as 7 wt.%
1300 1400 1500 1600 1700
Raman shift (cm
10 20 30 40 50
Catalysts 2019, 9, 1013 14 of 20
of Co and 2 wt.% of promoter . In this way, mixed aqueous solutions of the corresponding nitrates
were used for the co-impregnation: Co(NO3)2·6H2O (Acros Organics, Morris Plains, NJ, USA) and
Cu(NO3)2·3H2O, Cr(NO3)3·9H2O, Ce(NO3)3·6H2O, AgNO3 (Aldrich, St. Louis, MO, USA).
Subsequently, the prepared samples were calcined under air at 550 °C.
3.2. Catalysts Characterization
N2 adsorption/desorption at 77 K on a TRISTAR 3000 sorptometer (Micromeritics, Norcross, GA,
USA) was used for the measurement of textural properties. Prior to the analysis samples were
outgassed under vacuum at 200 °C for 4 h. To determine the chemical composition of the catalysts,
ICP-AES technique was used. The equipment was a VISTA-PRO AX CCD-Simultaneous ICP-AES
spectrophotometer (Varian, Palo Alto, CA, USA). Samples were previously treated by acidic
digestion. XRD measurements were recorded using an X’pert PRO diffractometer (Philips,
Eindhoven, The Netherlands) using Cu Kα radiation. The Scherrer equation was used to estimate the
metal crystallites mean diameter. Reducibility of the samples was studied by TPR analyses. A
Micromeritics (Norcross, GA, USA) AUTOCHEM 2910 system was used. The experiment is carried
out flowing 35 N mL/min of gas (10% H2/Ar) through the sample and increasing temperature up to
980 °C with a 5 °C/min heating ramp. Samples were previously outgassed under Ar flow at 110 °C
for 30 min. Co dispersion of the catalysts was determined by hydrogen TPD in the same apparatus.
For that, the samples were first reduced under 35 N mL/min of gas (10% H2/Ar), then cooled to 50 °C,
and saturated with H2. After that, the physically absorbed H2 is removed by flushing Ar and finally
heated up to 700 °C at 5 °C/min in Ar flow (30 N mL/min). TEM micrographs were obtained on a 200
kV JEM 2100 microscope (JEOL, Tokyo, Japan), with a resolution of 0.25 nm at the National Centre
for Electron Microscopy (CNME, Complutense University of Madrid, Madrid, Spain). It also has the
possibility to achieve microanalysis results by energy dispersive X-ray spectroscopy (EDX). Samples
preparation involve their suspension in acetone and subsequently deposition on a carbon-coated
copper or nickel grid. Carbon deposited during catalytic tests was measured by thermogravimetric
analysis (TGA), TEM and Raman spectroscopy. TGA analysis were performed in airflow with a
heating rate of 5 °C/min up to 1000 °C on a SDT 2960 thermobalance (TA Instruments, New Castle,
DE, USA). Raman spectra were recorded using a NRS-5000/7000 series Raman spectrometer (JASCO,
Tokyo, Japan) at the IMDEA Energy Institute.
3.3. Catalytic Tests
Acetic acid steam reforming reactions were performed at 600 °C on a MICROACTIVITY-PRO
unit (PID Eng. & Tech. S.L., Alcobendas, Madrid, Spain) as described in previous works [7,38,85,87].
The reactor consists in a fixed-bed tubular reactor in stainless steel 316 (i.d. = 9.2 mm, L = 300 mm).
The reactor is located inside an electric oven of low thermal, where temperature in the catalytic bed
was measured by means of a K-thermocouple. All the components inside the hot box were
maintained at 200 °C to prevent condensation in the pipes and to preheat the reactants. A schematic
diagram is displayed in Figure 14.
Catalysts 2019, 9, 1013 15 of 20
Figure 14. Schematic diagram of the catalytic testing setup .
The reactions were carried out isothermally at atmospheric pressure. Before tests, all catalysts
were reduced under pure hydrogen (30 mL/min) up to 700 °C with a heating rate of 2 °C/min.
Temperature was maintained for 30 min. Reaction feed was a mixture of acetic acid and water using
a steam to carbon molar ratio of 2, using N2 as carrier and internal standard (GHSV = 11,000 h−1). The
composition of the outlet gas was measured online with an 490 Micro-GC (Agilent, Santa Clara, CA,
USA) equipped with a thermal conductivity detector (TCD), a PoraPlot U column (10 m) and a
Molecular Sieve 5A column (20 m) using He and Ar as carrier gas, respectively. Condensable vapors
were trapped in the condenser at 4 °C and analyzed in a Varian (Palo Alto, CA, USA) CP-3900
chromatograph equipped with a CP-WAX 52 CB (30 m × 0.25 mm, DF = 0.25) column and flame
ionization detector (FID).
The incorporation of a second metal like Cu, Ag, Ce or Cr into Co/SBA-15 sample catalyst
resulted in bimetallic catalysts with very different properties and catalytic behavior in acetic acid
steam reforming. Co-Ag/SBA-15 presented some pore blockage of the SBA-15 structure due to the
presence of isolated silver oxide particles. Cu and Ag addition to Co/SBA-15 led to a significant
decrease in the reduction temperature, as shown in H2-TPR profiles. Cu addition to Co/SBA-15 favors
Co oxide reducibility, while maintaining almost unaltered Co0 crystallites size. In contrast, Co-
Ag/SBA-15 showed also lower reduction temperatures but larger Co0 crystallites than Co/SBA-15.
However, Ce addition does not affect significantly neither reducibility nor Co0 crystallite size. Finally,
Cr addition to Co/SBA-15 strongly decreases Co crystallites size, induced by the presence of
chromium oxides, improving metal dispersion with a slight decrease in the reduction temperature.
Regarding acetic acid steam reforming, Co-Cu/SBA-15 and Co-Ag/SBA-15 gave lower hydrogen
selectivity than unmodified Co/SBA-15 catalyst. However, Cr addition improved the catalytic
behavior reaching the highest hydrogen selectivity next to the thermodynamic equilibrium. After the
steam reforming tests, cobalt crystallites sizes in the used catalysts were very similar to those in fresh
samples, indicating that coke deposition and not sintering is the cause of catalysts deactivation.
Catalysts 2019, 9, 1013 16 of 20
Besides, the amount of coke formed on Co-Cr/SBA-15 was much lower than on the rest of the catalysts
after 5 h of time on stream. Another difference resided in the nature of coke deposited because
disordered aromatic structures such as amorphous or defective filamentous carbon were formed in a
higher extent on Co-Cr/SBA-15 (ID/IG = 0.80) while the contribution of condensed graphitic aromatic
structures increased in Co-Cu/SBA-15 (ID/IG = 0.48). Thus, Cr addition to Co/SBA-15 resulted in the
best catalytic performance on acetic acid steam reforming, but Cr toxicity opens the way to the search
for other metals providing similar catalytic properties.
Author Contributions: Conceptualization, A.C. and J.A.C.; methodology, A.J.V.; validation, A.J.V.; formal
analysis, A.C.; investigation, P.J.M.; writing—original draft preparation, P.J.M.; writing—review and editing,
A.C., J.A.C. and A.J.V.; supervision, A.C., J.A.C. and A.J.V.; project administration, A.C. and J.A.C.; funding
acquisition, A.C. and J.A.C.
Funding: This research was funded by the Spanish Ministry of Economy and Competiveness (project ENE2017-
83696-R) and the Regional Government of Madrid (project S2018/EMT-4344).
Acknowledgments: The authors acknowledge the IMDEA Energy Institute and the Complutense University of
Madrid for the Raman and TEM analyses, respectively.
Conflicts of Interest: The authors declare no conflict of interest.
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