Highly active and poison-tolerant nickel catalysts for tar reforming synthesized through controlled hydrothermal synthesis

Article abstract of DOI:10.1016/j.apcata.2020.117779

Nickel catalysts suffer from the loss of tar reforming activity due to poisoning by syngas impurities. The strategy of increasing activity of catalysts through selection of suitable organic precipitants for hydrothermal synthesis was investigated. Catalysts were synthesized from nickel and aluminum nitrates using urea, N,N’-dimethylformamide, N-methylurea, 1,1-dimethylurea and N,N’-dimethylurea as precipitants and screened for naphthalene reforming. The catalyst prepared with urea maintained the highest reforming activity for 10 h in the presence of 50 ppmv H₂S (80 % naphthalene conversion, 800 °C). This was attributed to lower chemisorption of sulfur on Ni sites, due to the larger percentage of Ni⁰ and Ni aluminates which are less susceptible towards sulfidation than NiO. The precipitant also influenced the water gas shift activity which was higher in case of N,N’-dimethylformamide and urea probably due to Ni aluminate formation. Thus, a type of precipitant plays an essential role in tailoring of catalysts for syngas upgrading.

Full text of DOI:10.1016/j.apcata.2020.117779

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Highly active and poison-tolerant nickel catalysts for tar reforming
synthesized through controlled hydrothermal synthesis
Dara Khairunnisa Binte Mohamed (Conceptualization) (Data
curation) (Formal analysis) (Investigation) (Methodology) (Writing -
original draft) (Writing - review and editing), Andrei Veksha
(Conceptualization) (Data curation) (Formal analysis) (Methodology)
(Supervision) (Writing - original draft) (Writing - review and editing),
Teik-Thye Lim (Conceptualization) (Supervision) (Writing - review
and editing), Grzegorz Lisak (Conceptualization) (Resources)
(Supervision) (Writing - review and editing)
PII:
S0926-860X(20)30372-0
DOI:
https://doi.org/10.1016/j.apcata.2020.117779
APCATA 117779
Reference:
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Applied Catalysis A, General
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Revised Date:
Accepted Date:
27 May 2020
23 July 2020
12 August 2020
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Highly active and poison-tolerant nickel catalysts for tar reforming synthesized through
controlled hydrothermal synthesis
Dara Khairunnisa Binte Mohameda,b,c, Andrei Vekshac,*, Teik-Thye Lima,b,c, Grzegorz Lisaka,b,c,
*
a School of Civil and Environmental Engineering, Nanyang Technological University, Singapore
639798, Singapore.
b Interdisciplinary Graduate School, Nanyang Technological University, Singapore 639798,
Singapore.
c Residues and Resource Reclamation Centre, Nanyang Environment and Water Research Institute,
Nanyang Technological University, 1 Cleantech Loop, CleanTech One, Singapore 637141,
Singapore.
*Corresponding authors. Tel.: +65-6592-1756; fax: +65-6792-7319. Email: aveksha@ntu.edu.sg (A.
Veksha) and g.lisak@ntu.edu.sg (G. Lisak).
Graphical abstract
Highlights
 Hydrothermal synthesis of Ni catalysts using selected organic precipitants.
 Catalytic tar reforming in the presence of impurities for syngas purification.
 Catalysts exhibited high activity and resistance towards poisoning by impurities.
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 Precipitants influenced distribution of Ni, improving tolerance to H2S poisoning.
 Highly stable nickel aluminates contributed to high water-gas shift activities.
Abstract
Nickel catalysts suffer from the loss of tar reforming activity due to poisoning by syngas impurities.
The strategy of increasing activity of catalysts through selection of suitable organic precipitants for
hydrothermal synthesis was investigated. Catalysts were synthesized from nickel and aluminum
nitrates using urea, N,N’-dimethylformamide, N-methylurea, 1,1-dimethylurea and N,N’-
dimethylurea as precipitants and screened for naphthalene reforming. The catalyst prepared with urea
maintained the highest reforming activity for 10 h in the presence of 50 ppmv H2S (80% naphthalene
conversion, 800 °C). This was attributed to lower chemisorption of sulfur on Ni sites, due to the larger
percentage of Ni0 and Ni aluminates which are less susceptible towards sulfidation than NiO. The
precipitant also influenced the water gas shift activity which was higher in case of N,N’-
dimethylformamide and urea probably due to Ni aluminate formation. Thus, a type of precipitant
plays an essential role in tailoring of catalysts for syngas upgrading.
Keywords: catalytic reforming; hydrothermal synthesis; nanoflakes; nanospindles; naphthalene;
nickel catalyst.
1. Introduction
Municipal solid waste (MSW) gasification is considered a viable alternative to waste
incineration, as it produces syngas which can be further applied as a fuel for fuels cells and gas
engines, or serve as a feedstock for chemical synthesis [1]. Raw syngas contains significant amounts
of tar compounds, i.e., aromatic hydrocarbons formed from the thermal decomposition of complex
organic compounds as well as other reactions related to the decomposition of feedstock [2]. The high
content of tar in raw syngas poses an obstacle for syngas utilization. Tar compounds exist in the
vapour form at gasification temperatures but condense upon cooling of the syngas causing clogging
2

and corrosion of equipment. Tar removal is therefore an important aspect of syngas upgrading
technology [3, 4].
A promising method of tar removal is catalytic tar reforming, which is carried out at
temperatures close to the syngas temperature at the outlet of gasifier, and is advantageous as heat
losses are minimized and liquid waste streams are not produced [5, 6]. Typically, nickel has been
preferred as a reforming catalyst as it is highly active, low cost compared to noble metals, and
commercially available [7]. Despite these advantages, Ni catalysts are susceptible to deactivation due
to coke formation on the catalyst surface, Ni sintering as well as poisoning by syngas impurities [8].
It has been demonstrated that syngas impurities play essential role in the loss of catalytic activity of
Ni based catalysts [9-15]. Studies by the Richardson group reported that chloride species from
chlorocarbons had a detrimental effect on methane reforming and water-gas shift (WGS) reactions
[10, 11]. HCl concentrations up to 2000 ppmv present in the gas during naphthalene steam reforming
were shown to poison the WGS activity of Ni catalysts [12]. Sulfur species, such as H2S and COS,
presents a major problem for Ni reforming activity, and even a small amount of these species could
poison Ni catalysts [13, 16-18]. The chemisorption of sulfur species by a Ni catalyst leads to a
complexation between Ni and S, causing a reduction in available Ni active sites for hydrocarbon
reforming [19]:
Nix + xH2S →NiSx + xH2
(1)
The NiS complex has a high mobility over the catalyst surface due to the low melting point,
which increases the sintering ability causing further deactivation of the Ni catalyst [20]. Additionally,
studies have shown sulfur poisoning is likely to increase carbon deposition on a catalyst surface [16,
21-24]. According to Dou et al, when H2S and HCl present together in syngas, a more pronounced
poisoning of the WGS activity of the Ni catalyst was observed [9]. Considering that tar compounds
co-exist with H2S and HCl in syngas, development of tar reforming catalysts is essential to maintain
high and durable activity in the presence of both impurities.
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Several strategies have been explored to enhance the activity of tar reforming catalysts in the
presence of syngas impurities. They include using acidic supports such as Al2O3 or basic supports
such as CaO, doping of other metals to the Ni catalyst to form alloys, support modification as well as
control of catalyst morphology [19, 25]. Previous studies on the development of sulfur tolerant
catalysts have focused on using noble and rare earth metal dopants as promoters, such as La, Rh, Pd,
and Ce [26-28]. Although the perovskite-based LaNi0.5Mn0.5O3 catalyst was found to have higher tar
reforming activity than a LaNiO3 catalyst in the presence of 50 ppm H2S, the catalyst without Ni,
LaMnO3, was even more resistant to the sulfur poisoning with much higher activities [26]. Bimetallic
Rh-Ni/Ce-Al2O3 catalyst was shown to be more resistant to deactivation during a biogas tri-reforming
process in the presence of 25 ppm H2S, and was able to undergo regeneration and reverse the adverse
effects of H2S poisoning. [27]. High resistance to H2S was also achieved by doping Ni catalysts with
WO3 which showed high activity at low reforming temperatures even in the presence of H2S [29].
Although catalyst promoters have shown great promise in the development of poison-tolerant
catalysts, synthesis using conventional metals such as Ni and alumina are preferred due to the low
cost and their availability [30].
Catalysts on nanostructured supports also show superior catalytic activity than conventional
catalysts, due to a better active Ni metal dispersion on the support [25]. Various preparation methods
have been utilized in the synthesis of nanostructured catalysts [31]. One common synthesis method
is a hydrothermal synthesis, in which the catalyst precursors are dissolved in a solvent followed by
controlled precipitation under heat and pressure [32]. Reaction conditions, such as temperature and
time, as well as the types and amounts of precursors were shown to influence the physical properties
of the catalysts. [32]. A commonly used precipitant is urea, which slowly hydrolyses in water at
elevated temperature and pressure allowing a more controlled increase of pH and precipitation of
precursors [33, 34]. Other types of precipitants that have been used include sodium hydroxide [35],
potassium hydroxide [36], hydrazine hydrate [37] and tetra-alkyl ammonium hydroxides [38],
amongst others. Compared to urea which slowly increased the pH of the solution upon hydrolysis
4

under heating, other precipitants were strongly basic leading to immediate precipitation of the catalyst
precursors and severe agglomeration of particles, and as such, do not have a large impact on the
control of the catalyst morphology. Ni-based catalysts used in hydrocarbon reforming that were
synthesized using the urea-assisted hydrothermal method were found to have high surface areas and
strong Ni-support interaction, which contributed to the high activities of the catalysts [39, 40]. Other
than urea, the use of other amide-based organic precipitants with slow hydrolysis have not been
extensively studied, and their effect on the catalyst activity and stability for tar reforming in the
presence of syngas impurities are unknown.
Given the success of urea as a precipitant for the development of nanostructured catalysts, it
is of great interest to explore the use of other types of amide-based organic precipitants with slow
hydrolysis and investigate their effect on the physical and chemical properties of the catalyst, as well
as on the catalyst activity. The objective of this study was to examine the effect of various precipitants
on the catalytic activity and stability of the resulted nanostructured catalysts during tar reforming.
Hydrothermal synthesis was employed to obtain the catalysts with controlled morphology and high
Ni dispersion over alumina support. The catalysts were used in the reforming of naphthalene over a
model syngas with H2S and HCl, which are common syngas impurities. The effects of various
precipitants on the nanostructure formation, Ni dispersion, Ni-support thermomechanical stability,
resistance of catalysts to poisoning as well as Ni activity during tar reforming and water gas shift
reactions are reported.
2. Experimental
2.1. Catalyst synthesis
Five catalysts were synthesized with different precipitants using the hydrothermal synthesis
method. Catalysts were denoted as Ni (x), where x represents the precipitant, i.e. ur = urea
(NH2CONH2, Sigma-Aldrich), dmf = N,N-dimethylformamide (HCON(CH3)2, Sigma-Aldrich), mur
= N-methylurea (CH3NHCONH2, Sigma-Aldrich), dmur = 1,1-dimethylurea ((CH3)2NCONH2,
5

Sigma-Aldrich), and mmur = N,N’-dimethylurea ((CH3NH)2CO, Sigma-Aldrich). The catalysts were
prepared by dissolving nickel(II) nitrate hexahydrate (Ni(NO3)2·6H2O, Sigma-Aldrich), aluminium
nitrate nonahydrate (Al(NO3)3·9H2O, Sigma Aldrich) at a Ni/Al molar ratio of 0.12 and the required
amount of catalyst precipitant in deionized water with a total solution volume of 120 mL. The total
mole ratio of catalyst precursors (Ni(NO3)2·6H2O and Al(NO3)3·9H2O) and precipitant, i.e. (Ni+Al)
: precipitant, was 1:2 for ur, 1:3 for mur, mmur and dmur, and 1:15 for dmf. The mole ratios between
(Ni+Al) : precipitant was chosen to ensure sufficient Ni and Al precipitation. The solution was
transferred to a hydrothermal autoclave reactor with a 200 mL capacity and placed in an oven at 180
°C for 18 h. After the reaction and cooling, the formed precipitate was collected through filtration
and washed for several times with water, dried in an oven at 100 °C overnight, and calcined for 5 h
at 400 °C with a 10 °C min-1 ramping rate. After calcination, catalysts were crushed and sieved, and
particles with sizes 100-315 μm were collected for testing.
A commercial catalyst with 6-hole monoliths (Xian Sunward Aeromat Co., China) was
crushed and sieved, and 100 to 315 m particle sizes were collected and used as a reference.
2.2. Catalyst characterization
Catalyst morphology was characterised with field emission scanning electron microscopy
(JEOL JSM-7600F FESEM). Transmission electron microscopy (TEM) images of the catalysts were
obtained using JEOL JEM-1400Plus Electron Microscope at 120 kV. Elemental mapping of the
synthesized catalysts was conducted with Energy-Dispersive X-ray spectroscopy (EDX) analysis
using an Oxford Instruments EDX detector coupled with TEM. High-resolution TEM was performed
using TEM JEOL 2010 HR at 200 kV. Particle sizes of Ni particles in the spent catalysts were
calculated from 150-200 Ni particles observed in TEM images using ImageJ software and assuming
the Ni particles were spherical. ICP-OES was used to measure nickel content in catalysts (Perkin
Elmer Optima 8300). BET surface areas, pore volumes and pore sizes of freshly prepared and spent
catalysts were calculated from the N2 adsorption and desorption isotherms measured at -196 °C with
6

a Quantachrome Autosorb-1 Analyzer. Crystalline structure of the prepared and spent catalysts were
obtained through X-Ray diffraction (XRD) analysis using a PANalytical XPert Pro XRD instrument
with a radiation source of Cu-Kα. Samples were measured in a 10-80° 2휃 range with a 0.035° step
size and 0.3 s per step. X-ray photoelectron spectroscopy (XPS) analysis to determine Ni, S and Cl
composition of spent catalysts was measured with a Kratos Axis Supra spectrophotometer with a dual
anode monochromatic Kα excitation source. XPS peaks were processed using a CASA XPS software,
and the binding energies for the elements analysed were calibrated using an adventitious carbon C 1s
binding energy (284.8 eV). Temperature programmed reduction (TPR) was conducted by increasing
the temperature to 900 °C at 10 °C min-1 in a gas mixture of 5% H2/N2 (30 mL min-1 flow rate).
Elemental analyser for CHNS (Elementar Vario EL Cube) was used to measure carbon content in the
fresh and spent catalysts.
2.3. Reforming of naphthalene
The prepared catalysts were applied for naphthalene reforming using a model syngas of H2,
steam, CO and CO2 content that represented as close as possible real syngas composition, with the
addition of H2S and HCl to represent syngas impurities. The experimental setup for catalyst testing
is shown in Fig. 1. Naphthalene, one of the major tar species and highly stable during reforming, was
used as the representative tar compound [41]. An evaporator was used to vaporise naphthalene, which
was then introduced into the system by flowing N2 gas. Steam and HCl were introduced by feeding
an aqueous solution of HCl with a syringe pump. Hydrogen gas was supplied during the experiments
for catalyst reduction, as nickel is typically in the oxidized form in commercially available and lab-
made reforming catalysts, and to provide a reducing environment throughout the reforming process.
Steam and hydrogen concentrations introduced were 24 vol.% and 9 vol.%, which are within their
reported ranges for the syngas produced from wet biomass and MSW [42, 43].
For each experimental run, a fixed-bed reactor with a frit (quartz, 50-90 μm openings) was
loaded with 0.5 mL catalyst. The reactor was then heated to 800 °C (ramping rate = 15 °C min-1) in
a 20 vol. % H2 – 80 vol. % N2 gas mixture with a 50 mL-STP min-1 gas flow, and the catalyst was
7

then reduced at 800 °C for 30 min. To perform the naphthalene reforming, a gas mixture with 0.055
vol. % naphthalene, 24 vol. % H2O, 9 vol. % H2, 10 vol%. CO, 9 vol. % CO2, 0 or 50 ppmv H2S, 300
or 2000 ppmv HCl and N2 (balance) was flowed over the catalyst at 800 °C. The gas hourly space
velocity was kept at 24000 h-1 with 200 mL-STP min-1 total gas flow rate. Naphthalene and gas
samples were taken at hourly intervals for 10 h. Gas samples were collected into Tedlar bags for 5
min and analysed using a calibrated gas chromatograph (Agilent 7890B, USA) with two thermal
conductivity detectors and a flame ionization detector. Naphthalene samples (in approximately 200
mL of gas flow) were collected using an air pump (Gilian GilAir Plus) and measured by solid phase
adsorption (SPA) using Bond Elut NH2 (500 mg, 3 mL, Agilent). Naphthalene was eluted from SPA
tubes into a vial with 2 mL of dichloromethane and further diluted with a ratio of eluate to
dichloromethane of 1:10. The solution was analysed with a calibrated gas chromatograph-mass
spectrometer (Agilent 7890B GC and a 5977A MSD). Naphthalene samples at the outlet of the reactor
were collected before and after the reforming experiments. Naphthalene conversion was calculated
by using the following equation:
– C
Naphthalene conversion (%) = C
naphthalene, out × 100%
(2)
naphthalene, in
C
naphthalene, in
where C is the concentration of naphthalene derived from the GC-MS analysis in g Nm-3.
The results of all experiments are presented as averages of triplicated runs and standard
deviations (±). Statistical significance was determined using an ANOVA method with a 95%
confidence interval.
3. Results and discussion
3.1. Properties of catalysts
Chemical structures of the precipitants used in this study are shown in Fig. 2. During
hydrothermal synthesis, the organic precipitants selected for the synthesis of catalysts undergo slow
hydrolysis generating various reaction products. Specifically, the hydrolysis of urea generates
ammonia and carbon dioxide (equation 3) [33]. The hydrolysis of other methylated urea derivatives
8

generates carbon dioxide as well as methylamine and dimethylamine, depending on the amine
functional group (equations 4-6). In contrast, the hydrolysis of N,N’-dimethylformamide produces
dimethylamine and formic acid (equation 7). Hydrolysis of all precipitants increases the pH of
solution causing the precipitation of catalyst precursors.
(
)
Urea (ur):
CO NH2 2 + H2O → 2 NH3 + CO2
(3)
(4)
(5)
N-methylurea (mur):
CH3NHCONH2 + H2O → NH3 + NH2CH3 + CO2
N,N’-dimethylurea (mmur): CH3NHCONHCH3 + H2O → 2 NH2CH3 + CO2
1,1-dimethylurea (dmur):
N,N’-dimethylformamide (dmf): (CH3)2NHCOH + H2O → (CH3)2NH2 + HCOOH
(CH3)2NHCONH2 + H2O → NH3 + (CH3)2NH2 + CO2 (6)
(7)
The formation of different reaction products during hydrothermal synthesis had a drastic
effect on the shape and nanostructure of resulting catalysts. Fig. 3 shows the FESEM, TEM and
HRTEM images of prepared catalysts. Fig. 3a demonstrates the morphology of Ni(ur) prepared with
urea as a precipitant. According to FESEM and TEM images, the usage of urea resulted in the
formation of flat, mostly rectangular nanoflake structures with varying dimensions, ranging from 50
to 150 nm in length and 10 to 35 nm in width. Using methylated urea derivatives, i.e. N-methylurea
and N,N’-dimethylurea (Fig. 3b and Fig. 3c, respectively), catalysts with similar morphology to
Ni(ur) were produced. The nanoflake sizes of Ni(mur) were between 35-160 nm (length) and 12-30
nm (width), which are close to the sizes of Ni(ur). However, the sizes of Ni(mmur) nanoflakes were
larger, ranging from 75 to 200 nm and 30 to 115 nm in length and width, respectively. The HRTEM
images show the lattice spacing of the catalysts. The lattice spacings for Ni(ur), Ni(mur) and
Ni(mmur) were 0.44 nm, 0.46 nm and 0.46 nm respectively, corresponding to the (111) plane of
Al2O3, which suggests the preferential growth of crystallites along the (111) plane for these catalysts.
The differences in the lattice spacing values is due to the low crystallinity of the support which
prevents an accurate measurement of the spacings, of which the (111) plane corresponds to a lattice
spacing of 0.45 nm [44]. In contrast, the FESEM and TEM images of Ni(dmf) catalyst (Fig. 3d) show
the presence of nanospindles with the length of 11-45 nm. According to the HRTEM image, the
9

nanospindle shape was formed through the onion-like layering of alumina support containing well
dispersed nickel species. Ni(dmur) (Fig. 3e) consisted of a mixture of nanoflakes and nanospindles,
resembling the two nanostructures found in Ni(ur) and Ni(dmf), respectively. The nanoflake particle
sizes of Ni(dmur) were in the range of 16-46 nm for length and 2-18 nm for width. From the HRTEM
images of Ni(dmur) and Ni(dmf), the lattice spacings were 0.46 nm and 0.45 nm respectively,
corresponding to (111) plane, however, the difference in the lattice spacing values indicates
inaccurate measurement due to the low crystallinity of the support. According to these images, the
formation of nanoflakes was favored by the precipitants that produce ammonia and methylamine
during hydrolysis (i.e. urea, N-methylurea and N,N’-methylurea), while the formation of
nanospindles was favored by N,N’-dimethylformamide which hydrolyzes to dimethylamine. Since
1,1-dimethylurea (dmur) decomposes into both ammonia and dimethylamine upon hydrolysis,
Ni(dmur) contained both nanoflake and nanospindle structures. The differences in the morphologies
of the synthesized catalysts were due to the hydrolysis products of the precipitants formed during the
hydrothermal synthesis. The mixture of different compounds in the reaction solution (e.g. NH3 and
CO2 from the hydrolysis of urea, CH3NH2 and CO2 from the hydrolysis of N,N’-dimethylurea)
affected the precipitation of Ni and Al precursors differently according to the different precipitant
used [45]. The obtained morphology of catalysts was different from those reported previously [46,
47]. Controlled hydrothermal synthesis using hydrazine monohydrate was reported to yield square-
shaped nanoflake 훾-Al2O3 [46]. The hydrothermal synthesis of Ni-Al mixed oxide that was derived
from a Ni-Al double layer hydroxide using urea as a precipitant resulted in flat circular nanosheets
[47]. The differences in morphology obtained in this study could be due to the different synthesis
conditions.
Using the hydrothermal synthesis method allowed to obtain well dispersed Ni species over
the support as suggested by Fig. 4. According to the TEM-EDX images, Ni was uniformly distributed
in the Al oxide matrix of all catalysts. The isotherms of N2 adsorption-desorption is shown in Fig. 5a
suggesting that the synthesized catalysts were mesoporous. The N2 uptake occurred mainly at P/Po >
10

0.1, with hysteresis loops between the adsorption and desorption branches of the isotherms. The
properties of developed catalysts are shown in Table 1. The Ni content in the catalysts was 9-10 wt.%,
whereas the commercial catalyst contained 16 wt.% Ni. Commercial catalyst had the highest Ni load
per bed volume, due to the higher Ni content and larger bulk density. Among the synthesized
catalysts, the Ni load per bed was the highest for Ni(ur), i.e. 0.03 g per 0.5 mL of catalyst, due to the
largest bulk density (i.e. 0.62 g mL-1). BET surface areas and total pore volumes of the developed
catalysts were much larger than those of commercial catalyst (i.e. 183-243 m2 g-1 and 0.24-0.70 mL
g-1 for synthesized catalysts, respectively, compared to 20 m2 g-1 and 0.01 mL g-1 of the commercial
catalyst). Catalysts containing nanospindle structures had larger BET surface areas and total pore
volumes than catalysts containing nanoflake structures. These results suggest that the selected type
of precipitant can significantly influence the morphology and porosity of the catalyst during synthesis.
Porosity plays an important role in hosting Ni nanoparticles and increasing surface active sites due to
the deposition of Ni nanoparticles inside the porous cavities of the alumina support as well as
providing the reacting species access to the active sites, thus influencing the catalytic activity [48].
Fig. 5b shows the XRD patterns of the prepared catalysts. The XRD patterns had broad peaks
of both NiO and Al2O3 which indicated the formation of nanocrystallites of NiO, while Al2O3 in the
catalysts had nanosized and/or non-crystalline structures. Ni aluminate peaks were not easily
observed in the XRD pattern for all catalysts, due to the overlapping of peaks with those of 훾-Al2O3,
as well as the non-crystallinity and/ or high dispersion of Ni aluminates in the catalyst. Conversely,
the XRD spectrum of commercial catalyst consisted of sharp peaks of both NiO and Al2O3, suggesting
higher crystallinity of the Ni and Al compounds and lower dispersion of Ni. According to the XRD
patterns, alumina support in the synthesized catalysts was in the form of 훾-Al2O3, while in commercial
catalyst it was in the form of 훼-Al2O3 [44]. 훾-Al2O3 has a highly amorphous and porous structure,
which would explain the highly porous nature of the synthesized catalysts and higher Ni dispersion
[49]. Sharper NiO peaks could be observed in Ni(ur) indicating higher crystallinity of NiO compared
to the other synthesized catalysts.
11

The TPR profiles of the developed catalysts are shown in Fig. 5c. From the TPR data the
reducibility of NiO can be deduced and the strength of the interaction between Ni species and the
Al2O3 surface can be confirmed. The TPR profiles of all catalysts contained varying reduction peaks,
which suggests that Ni dispersion and interaction with the support varied among the catalysts [50,
51]. Ni(ur) and Ni(mur) had low temperature reduction peaks at 410 °C and 330 °C respectively,
which correspond to the reduction of NiO weakly bound to the support [51]. Reduction peaks between
500-760 °C in the moderate temperature range correspond to NiO strongly bound to Al2O3 [50, 51].
Ni(mur), Ni(dmf) and Ni(dmur) had small reduction peak at 530 °C, while Ni(ur) had a reduction
peak at 580 °C. Ni(dmur) also had a peak at 690 °C, while Ni(mmur) had a maximum intensity peak
at 730 °C. The reduction peaks above 800 °C were also observed. The reduction peaks of Ni(mur)
and Ni(dmur) were at 845 °C, while the peaks of Ni(ur) and Ni(dmf) were at 920 °C and 945 °C
respectively, which could be attributed to nickel aluminates reduction [52-54].
The synthesized catalysts were characterized by Ni 2p XPS (Fig. 6). Apart from the binding
energy (BE) of ~862 eV due to the shake-up satellite peak, the deconvolution of main Ni peak showed
the presence of two Ni2+ states with BE of 855.6-855.7 and 856.6-856.7 eV, depending on the catalyst.
The BE of unsupported Ni2+ in Ni-based catalysts is ~854 eV and increases with the strength of NiO–
Al2O3 interactions to ~ 856 eV for NiO strongly bound to Al2O3 [55]. The Ni2+ peak at ~ 856-857 eV
can be ascribed to nickel aluminate [53, 56]. According to the XPS spectra, all the synthesized
catalysts contained strongly bound NiO and nickel aluminate on the surface. Table 2 provides the
distribution of Ni2+ species calculated from the corresponding peak areas. Among the catalysts,
Ni(dmur) contained higher amounts of nickel aluminate, suggesting NiO and alumina on the catalyst
surface underwent higher solid state reaction. The Al 2p and O 1s spectra for the synthesized catalysts
(Fig. S1) show peaks at ~74 eV and ~531 eV respectively, which indicated the presence of aluminium
and nickel oxides, as well as that of Ni aluminates [57-59]. The TPR data (Fig. 5c) for Ni(ur)
suggested the presence of weakly interacting NiO-Al2O3, which is not observed in surface of the
catalyst by XPS analysis. This could be due to the low XPS sampling depth, which is less than 10 nm
12

[60, 61] and unable to detect Ni beneath the top surface layer. It can be seen from the characterization
data that the properties of synthesized catalysts varied significantly with the type of precipitant. This
could be attributed to the differences in decomposition pathways of precipitants and reaction
products. In this respect, tailoring of catalyst properties using appropriate precipitants can provide a
valuable approach towards designing catalysts with high catalytic activity.
3.2. Effect of precipitants on naphthalene reforming
To investigate the influence of precipitants on catalytic activity, the catalysts were employed
in the reforming of naphthalene with the presence of syngas impurities such as H2S and HCl. Fig. 7a
shows the effect of impurities on the reforming efficiency of naphthalene over Ni(ur). Three
conditions were tested: 2000 ppmv HCl + 50 ppmv H2S, 300 ppmv HCl + 50 ppmv H2S and 2000
ppmv HCl + 0 ppmv H2S. From the analysis of the sampled gas, the products of the reforming for all
catalysts were H2, CO2 and CO whereas C1-C5 hydrocarbons were not detected. It can be seen that at
0 ppmv H2S and 2000 ppmv HCl, nearly complete conversion of naphthalene was maintained for 10
h. According to the thermodynamic calculations that were performed using an HSC Chemistry 9
software, the equilibrium naphthalene conversion was expected to be 100% in the presence of HCl,
which is consistent with the experimental results. When both 50 ppmv H2S and 2000 ppmv HCl were
present, the activity decreased to 80±7% due to poisoning. There was no statistically significant
increase in the reforming activity when HCl concentration was decreased from 2000 to 300 ppmv in
the presence of H2S (88±3% naphthalene conversion), suggesting that the loss of catalytic activity
was primarily due to the presence of H2S, whereas HCl had little effect on the reforming activity.
Considering the observed impact of low concentration of H2S on reforming activity, H2S is a more
potent catalyst poison compared to HCl. These results are consistent with the observations made
during naphthalene steam reforming over various commercial and synthetic Ni-based catalysts under
experimental conditions of 10 vol.% H2, 26 vol.% H2O and 14 vol.% N2 [9, 12]. The newly obtained
data suggested that the effects of HCl and H2S on catalytic activity persist with the addition of CO
and CO2 to the gas stream. The detrimental effect of H2S on reforming activity is typically attributed
13

to the reaction of Ni and S leading to 1) the formation of inactive Ni-S complex, and 2) increased Ni
sintering/decreased surface area due to higher Ni mobility caused by lower melting point of Ni-S
species .
Fig. 7b shows the naphthalene reforming activity of catalysts synthesized using various
precipitants in the presence of 50 ppmv H2S and 2000 ppmv HCl. Although the Ni content in Ni(ur)
used in the catalyst bed was 4 times lower than that in the commercial catalyst, the reforming activity
of Ni(ur) was better than that of the latter, while Ni(dmf) and Ni(dmur) activities were comparable to
the commercial catalyst even with lower Ni contents, suggesting superior activity. This could be
attributed to higher Ni dispersion in the prepared catalysts, which is essential for the high reforming
activity. The reforming activity of all tested catalysts decreased during 10 h tests due to poisoning.
However, the loss of catalytic activity varied depending on the catalyst as suggested by the conversion
efficiencies calculated within the last two hours of testing. While the reforming activity of Ni(ur),
Ni(dmf), Ni(mur) and Ni(mmur) continued to decrease throughout the 10 h tests, the naphthalene
conversion of Ni(dmur) stabilized to ~67% after 2 h until the end of the experiment. This could be
attributed to the high pore volume of Ni(dmur) which maintained access for reacting species to be
exposed to the active Ni. Ni(ur) catalyst had the highest reforming activity (naphthalene conversion
of 80%) followed by Ni(dmf) (naphthalene conversion of 71%) and the three other synthesized
catalysts. There were no statistically significant differences between the catalytic activities of
Ni(dmur), Ni(mur) and Ni(mmur). These reforming data cannot be attributed to the differences in Ni
contents per bed volume shown in Table 1. Among the synthesized catalysts, the content of Ni was
the lowest in Ni(dmf) while the activity of catalyst was one of the highest. To investigate whether the
morphology and porosity influenced the reforming activity, the BET surface areas and pore volume
data of the spent catalysts was measured and the results are shown in Table S1. After reforming, the
surface areas of the catalysts decreased, After reforming, the surface areas and pore volumes of the
catalysts decreased, with Ni(ur) having a surface area of 110 m2 g-1 and total pore volume of 0.269
mL g-1, while surface areas of Ni(dmf) and Ni(dmur) decreased more drastically (115 and 116 m2 g-1
14

respectively), the total pore volumes remained high (0.394 and 0.461 mL g-1 respectively). While
porosity plays an important role in the reforming activity of the catalyst [62], it was found that high
reforming activities could also be achieved with catalysts having lower surface area and porosity, as
suggested by Veksha et al [12]. In this case, the differences in reforming activity of the catalysts were
not solely due to the different surface areas and pore volumes. Rather, the use of different precipitants
influenced Ni dispersion, as shown in the TPR results (Fig. 5c), with varied reduction peaks of NiO
indicating strong Ni-support interaction and thus higher activities.
Coke formation is known to influence catalyst deactivation [63]. To determine whether carbon
deposition occurred on the Ni particles during reforming, TEM images of the spent catalysts were
obtained and shown in Fig. 8. In the TEM images, Ni species are represented by the distinct dark
spherical particles scattered across the support matrix. From observation of the Ni particles in the
TEM images of the catalysts, filamentous or encapsulating carbon that can deactivate the catalyst
were not formed [63-65]. CHNS analysis was performed to determine the carbon, hydrogen, nitrogen
and sulfur content in the fresh and spent catalysts. From the analysis, an increase in the carbon
concentration of the spent catalysts compared to the fresh catalyst could indicate carbon deposition
on the catalysts. The carbon contents in the fresh and spent catalysts were below detection limits,
confirming that carbon deposition on the catalysts did not occur. This could be due to the high steam
content in gas (24 vol%) taking part in carbon gasification.
In order to investigate the role of syngas impurities on the loss of activity, the spent catalysts
were characterized using XPS. The wide survey XPS spectra suggested the presence of both Cl and
S species chemisorbed on the spent catalysts (Fig. S2). As it was discussed above, there was HCl did
not affect the reforming activity significantly and the poisoning effect could be primarily attributed
to S chemisorption on Ni surfaces [66, 67]. Fig. 9 shows the relationship between the naphthalene
conversion of catalysts and S/Ni ratios calculated from the XPS survey spectra. The S/Ni ratios
correspond to the surface coverage of Ni atoms with chemisorbed S species. Naphthalene conversion
15

was found to decrease with increasing S/Ni ratio, suggesting that the higher S chemisorption could
reduce the tar reforming activity of catalysts.
Using an HSC Chemistry 9 software, thermodynamic calculations were carried out to
investigate the reasons for different extents of Ni coverage by S among the synthesized catalysts. It
was assumed that Ni can be present in three states on the catalyst surface, namely Ni, NiO and
NiAl2O4. During reforming, metallic Ni is formed due to reduction by H2, while Ni2+ compounds can
exist on the catalyst surface due to the oxidation by H2O, which is more favourable in the presence
of alumina support facilitating NiO-support interactions [9]. The possible reactions between the Ni
compounds and H2S are as follows:
Ni + H2S → NiS + H2
∆G°= -30.3 kJ mol-1 (8)
∆G°= -76.7 kJ mol-1 (9)
∆G°= -46.0 kJ mol-1 (10)
NiO + H2S → NiS + H2O
NiAl2O4 + H2S → NiS + γ-Al2O3 + H2O
It can be seen from the negative standard Gibbs energies (ΔG°) at 800 °C that all three
reactions can occur spontaneously at the reforming temperature. However, the reaction between NiO
and H2S (equation 9) is the most thermodynamically favourable due to the lowest ∆G° followed by
NiAl2O4 and Ni (equations 10 and 8, respectively). These results suggest that, during the reforming,
sulfidation of oxidized Ni compounds is more favorable compared to metallic Ni. Furthermore, the
differences in the ∆G° values for NiO and NiAl2O4 imply that the Ni poisoning by H2S could be
influenced by the strength of NiO-Al2O3 interactions (i.e. the sulfidation of NiO weakly bound to the
support is more favorable). Previously, Blanchard et al. found that the loss of the NiAl2O4 phase of
an NiAl2O4/Al2O3-YSZ catalyst during methane dry reforming resulted in higher deactivation rates
from H2S poisoning, and that reforming the spinel NiAl2O4 phase could successfully regenerate the
catalyst [68]. This shows that H2S poisoning could be more detrimental to catalyst activity depending
on the different forms of Ni in the catalyst.
The XPS characterization of the spent catalysts was carried out to identify Ni species present
on the surface of the catalyst. The distribution of Ni species determined by deconvolution of the core
16

Ni 2p peaks is shown in Fig. 10. All catalysts contained the peaks at (1) 852.7-852.8 eV indicative of
Ni0 (Peak 1) (2) 855.7-855.9 eV indicative of strongly bound NiO (Peak 2), and (3) 856.7-856.9 eV
indicative of nickel aluminate (Peak 3). The quantification of identified species in accordance with
Peak 1, 2 and 3 intensities are shown in Table 3. Based on the thermodynamic data, catalysts with
high amounts of surface Ni0 (Peak 1) and nickel aluminates (Peak 3) are less likely to react with H2S.
The total contents of Ni0 and nickel aluminates were the highest in Ni(ur) (65.5%), followed by
Ni(dmf) (63.3%), Ni(dmur) (59.2%), Ni(mur) (57.8%) and Ni(mmur) (56.9%), which explains the
decrease in S chemisorption on catalysts in the same order. These data suggest that the tolerance to
sulfur poisoning and high catalytic stability of catalysts during reforming could be greatly influenced
by the state of Ni species. The contents of Ni0, NiO and nickel aluminates are determined by the type
of precipitant used for the catalyst synthesis.
Table 4 compares the reforming activity of the selected synthesized catalyst, i.e. Ni(ur), with
catalysts that were tested in the presence of H2S in other studies. Besides the higher activity compared
to the commercial catalyst, which was included into this work, Ni(ur) showed higher activity
compared to the reported Ni, Ni-Fe and Ni-Ce catalysts supported on Al2O3 [9, 17, 69, 70], Rh-Ni
catalyst supported on Ce-Al2O3 [27] and LaNi0.5Mn0.5O3 [26]. This demonstrates the high potential
of the proposed synthesis method towards the fabrication of poison-tolerant catalysts for tar
reforming.
3.3 Effect of precipitants on water-gas shift
Under steam reforming conditions, water-gas shift (WGS) reaction (equation 11) from the
reaction of excess steam with CO to form CO2 will occur. Similarly, the addition of steam to a dry
reforming process will allow this reaction to take place.
CO + H2O ↔ CO2 + H2
(11)
In order to examine the extent of WGS reaction during the reforming of naphthalene with 2000
ppmv HCl and 50 ppmv H2S, the CO/CO2 ratios were calculated for the outlet gas and are shown in
Fig. 11. A low CO/CO2 ratio indicates higher WGS reaction as CO is consumed and CO2 produced.
17

The dashed line corresponds to the CO/CO2 ratio at thermodynamic equilibrium. The CO/CO2 ratios
of all catalysts were close to the equilibrium value at the start of the reaction and subsequently
increased rapidly within 1 h before stabilization. The increase in CO/CO2 ratios was less prominent
between 1 and 10 h of testing. Higher CO/CO2 ratios suggest the loss of WGS activity by catalysts.
The WGS activities of the synthesized catalysts were higher than the commercial catalyst. Ni(dmf)
had the highest WGS activity followed by Ni(ur), Ni(dmur), Ni(mur) and, finally, Ni(mmur). This
trend was different compared to that for the reforming activities, suggesting that the type of
precipitant had different influence on the WGS activities.
One reason for the deactivation of WGS activity could be the sintering of Ni particles as a
result of chemisorption by HCl [12]. The Ni sintering is irreversible and occurs due to either Ni
migration and coalescence or Ostwald ripening [71, 72]. The XRD spectra of spent catalysts (Fig.
S3) showed the appearance of sharp peaks of metallic Ni indicating that Ni crystallite sizes increased
during reforming confirming the sintering, while more intense 훾-Al2O3 peaks were observed, due to
the increased crystallinity from the annealing of the support particles. The Ni particle size
distributions of the synthesized and commercial catalysts after 10 h of reforming with 2000 ppmv
HCl and 50 ppmv H2S calculated from the TEM images of 150-200 particles are shown in Fig. 12.
According to the smaller sizes of Ni particles, the synthesized catalysts were more resistant to
sintering compared to the commercial catalyst. In the synthesized catalysts, Ni particles had sizes
between 10 and 100 nm, while in the commercial catalyst, Ni particles with sizes only above 50 nm
were observed. The smaller sizes of Ni particles could explain higher WGS activities of the
synthesized catalysts compared to the commercial catalyst. These data reflect the excellent capability
of the synthesized catalysts towards sintering resistance in the presence of syngas impurities. The
sintering resistance is attributed to the strong interactions between Ni2+ and the alumina support
preventing agglomeration of Ni during reforming [73, 74]. XPS data suggest that all the spent
catalysts contained Ni2+ species with high binding energies, attributed to either strong NiO-support
interactions or NiAl2O4 formation (Peaks 2 and 3 in Table 3, respectively). It was suggested
18

previously that while reforming activity to the larger extent depends on the presence of Ni0 species,
the WGS activity is facilitated by the presence of Ni2+ species formed at the Ni-alumina interface.
Hence, the strength of the Ni-support interaction is important in providing stable oxygenated Ni2+ for
WGS reaction [75, 76]. The interactions are higher in the case of smaller Ni particles due to the larger
contact area with the support.
The TPR profiles (Fig. 5c) of the catalysts show that both Ni(ur) and Ni(dmf) had reduction peaks
higher than the reforming temperature of 800 °C, suggesting highly stable nickel aluminates were
present. According to the reduction peaks, the stability of nickel aluminates was higher in Ni(dmf)
compared to Ni(ur) (reduction temperatures 945 and 920 °C, respectively). This could be a reason
for the higher WGS shift activity of the catalyst. The obtained data demonstrate the significant role
that a precipitant play in the synthesis of catalysts with high reforming and WGS activities in the
presence of poisons.
4. Conclusions
This study demonstrated that highly active and durable Ni-alumina catalysts can be synthesized
from hydrothermal synthesis using selected organic precipitants. The precipitants used for catalyst
synthesis were urea, N,N’-dimethylformamide, N-methylurea, 1,1-dimethylurea and N,N’-
dimethylurea. The catalysts were employed in the reforming of naphthalene in the presence of HCl
and H2S as syngas impurities. The hydrolysis products of the precipitants influenced the morphology,
Ni dispersion, porosity, Ni-alumina interaction and distribution of different Ni0, NiO and NiAl2O4
species. It was found that the catalyst prepared using urea as a precipitant, Ni(ur), had the highest
reforming activity. with 80% naphthalene conversion, followed by the catalyst prepared using N,N’-
dimethylformamide, Ni(dmf), with 71% conversion of and the catalysts prepared using the remaining
precipitants. The higher reforming activity was due to the lower chemisorption of S species on Ni
and increased resistance towards H2S poisoning. XPS data and thermodynamic calculations
suggested that this was caused by the larger percentage of Ni0 and Ni aluminates which are less
susceptible towards sulfidation than NiO. Both N,N’-dimethylformamide and urea also influenced
19

the high WGS activities for Ni(dmf) and Ni(ur). This is attributed to the high amount of Ni aluminates
that were stable at high temperatures, which served as active sites for WGS reactions to take place.
The choice of precipitant used during synthesis plays an important role in producing catalysts that
can maintain high reforming and WGS activities, thus opening new insights for synthesizing catalysts
that are highly active, stable, and tolerant towards poisoning for syngas upgrading.
Author contributions
Dara Khairunnisa Binte Mohamed: Conceptualization; Data curation; Formal analysis;
Investigation; Methodology; Writing - original draft; Writing - review & editing.
Andrei Veksha: Conceptualization; Data curation; Formal analysis; Methodology; Supervision;
Writing - original draft; Writing - review & editing.
Teik-Thye Lim: Conceptualization; Supervision; Writing - review & editing.
Grzegorz Lisak: Conceptualization; Resources; Supervision; Writing - review & editing.
Declaration of interests
The authors declare that they have no known competing financial interests or personal
relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors would like to thank the Interdisciplinary Graduate Programme (IGP) and Nanyang
Technological University (NTU) for the PhD scholarship. The authors would also like to thank the
Nanyang Environment & Water Research Institute (NEWRI) staff for their assistance in instrument
operations and analysis.
References
[1] W.X. Peng, L.S. Wang, M. Mirzaee, H. Ahmadi, M.J. Esfahani, S. Fremaux, Hydrogen and syngas
production by catalytic biomass gasification, Energy Conversion and Management, 135 (2017) 270-
273.
[2] B. Zhou, A. Dichiara, Y. Zhang, Q. Zhang, J. Zhou, Tar formation and evolution during biomass
gasification: An experimental and theoretical study, Fuel, 234 (2018) 944-953.
[3] Y. Shen, K. Yoshikawa, Recent progresses in catalytic tar elimination during biomass gasification
or pyrolysis—A review, Renewable and Sustainable Energy Reviews, 21 (2013) 371-392.
[4] K.-Y. Chiang, C.-H. Lu, M.-H. Lin, K.-L. Chien, Reducing tar yield in gasification of paper-reject
sludge by using a hot-gas cleaning system, Energy, 50 (2013) 47-53.
[5] J. Han, H. Kim, The reduction and control technology of tar during biomass gasification/pyrolysis:
An overview, Renewable and Sustainable Energy Reviews, 12 (2008) 397-416.
20

[6] L. Liu, Z. Zhang, S. Das, S. Kawi, Reforming of tar from biomass gasification in a hybrid
catalysis-plasma system: A review, Applied Catalysis B: Environmental, 250 (2019) 250-272.
[7] B. Zhoufeng, D. Sonali, W.M. Hui, H. Plaifa, K. Sibudjing, A Review on Bimetallic Nickel-Based
Catalysts for CO2 Reforming of Methane, ChemPhysChem, 18 (2017) 3117-3134.
[8] D.M. Argyle, H.C. Bartholomew, Heterogeneous Catalyst Deactivation and Regeneration: A
Review, Catalysts, 5 (2015).
[9] X. Dou, A. Veksha, W.P. Chan, W.-D. Oh, Y.N. Liang, F. Teoh, D.K.B. Mohamed, A. Giannis,
G. Lisak, T.-T. Lim, Poisoning effects of H2S and HCl on the naphthalene steam reforming and
water-gas shift activities of Ni and Fe catalysts, Fuel, 241 (2019) 1008-1018.
[10] J.D. Ortego, J.T. Richardson, M.V. Twigg, Catalytic steam reforming of chlorocarbons: methyl
chloride, Applied Catalysis B: Environmental, 12 (1997) 339-355.
[11] N. Couté, J.T. Richardson, Steam reforming of chlorocarbons: chlorinated aromatics, Applied
Catalysis B: Environmental, 26 (2000) 217-226.
[12] A. Veksha, A. Giannis, W.-D. Oh, V.W.C. Chang, G. Lisak, T.-T. Lim, Catalytic activities and
resistance to HCl poisoning of Ni-based catalysts during steam reforming of naphthalene, Applied
Catalysis A: General, 557 (2018) 25-38.
[13] I.F. Elbaba, P.T. Williams, Deactivation of Nickel Catalysts by Sulfur and Carbon for the
Pyrolysis–Catalytic Gasification/Reforming of Waste Tires for Hydrogen Production, Energy &
Fuels, 28 (2014) 2104-2113.
[14] R. Chein, Z.-W. Yang, H2S effect on dry reforming of biogas for syngas production,
International Journal of Energy Research, 43 (2019) 3330-3345.
[15] H. Madi, A. Lanzini, D. Papurello, S. Diethelm, C. Ludwig, M. Santarelli, J. Van herle, Solid
oxide fuel cell anode degradation by the effect of hydrogen chloride in stack and single cell
environments, Journal of Power Sources, 326 (2016) 349-356.
[16] U. Izquierdo, I. García-García, M.Á. Gutierrez, R.J. Arraibi, L.V. Barrio, F.J. Cambra, L.P.
Arias, Catalyst Deactivation and Regeneration Processes in Biogas Tri-Reforming Process. The
Effect of Hydrogen Sulfide Addition, Catalysts, 8 (2018).
[17] A.D. Hernandez, N. Kaisalo, P. Simell, M. Scarsella, Effect of H2S and thiophene on the steam
reforming activity of nickel and rhodium catalysts in a simulated coke oven gas stream, Applied
Catalysis B: Environmental, 258 (2019) 117977.
[18] D. Laprune, D. Farrusseng, Y. Schuurman, F.C. Meunier, J.A.Z. Pieterse, A.M. Steele, S.
Thorpe, Effects of H2S and phenanthrene on the activity of Ni and Rh-based catalysts for the
reforming of a simulated biomass-derived producer gas, Applied Catalysis B: Environmental, 221
(2018) 206-214.
[19] T.Y. Yeo, J. Ashok, S. Kawi, Recent developments in sulphur-resilient catalytic systems for
syngas production, Renewable and Sustainable Energy Reviews, 100 (2019) 52-70.
[20] J.C. Rodriguez, E. Romeo, J.L.G. Fierro, J. Santamaría, A. Monzón, Deactivation by coking and
poisoning of spinel-type Ni catalysts, Catal. Today, 37 (1997) 255-265.
[21] X. Chen, J. Jiang, F. Yan, K. Li, S. Tian, Y. Gao, H. Zhou, Dry Reforming of Model Biogas on
a Ni/SiO2 Catalyst: Overall Performance and Mechanisms of Sulfur Poisoning and Regeneration,
ACS Sustainable Chemistry & Engineering, 5 (2017) 10248-10257.
[22] M.D. Krcha, K.M. Dooley, M.J. Janik, Alkane reforming on partially sulfided CeO2 (111)
surfaces, Journal of Catalysis, 330 (2015) 167-176.
[23] C. Gillan, M. Fowles, S. French, S.D. Jackson, Ethane Steam Reforming over a
Platinum/Alumina Catalyst: Effect of Sulfur Poisoning, Industrial & Engineering Chemistry
Research, 52 (2013) 13350-13356.
[24] S. Cimino, L. Lisi, G. Mancino, Effect of phosphorous addition to Rh-supported catalysts for the
dry reforming of methane, International Journal of Hydrogen Energy, 42 (2017) 23587-23598.
[25] F.L. Chan, A. Tanksale, Review of recent developments in Ni-based catalysts for biomass
gasification, Renewable and Sustainable Energy Reviews, 38 (2014) 428-438.
21

[26] C.P.B. Quitete, R.L. Manfro, M.M.V.M. Souza, Perovskite-based catalysts for tar removal by
steam reforming: Effect of the presence of hydrogen sulfide, International Journal of Hydrogen
Energy, 42 (2017) 9873-9880.
[27] U. Izquierdo, I. García-García, Á.M. Gutierrez, J.R. Arraibi, V.L. Barrio, J.F. Cambra, P.L.
Arias, Catalyst Deactivation and Regeneration Processes in Biogas Tri-Reforming Process. The
Effect of Hydrogen Sulfide Addition, Catalysts, 8 (2018) 12.
[28] S.L. Lakhapatri, M.A. Abraham, Analysis of catalyst deactivation during steam reforming of jet
fuel on Ni-(PdRh)/γ-Al2O3 catalyst, Applied Catalysis A: General, 405 (2011) 149-159.
[29] K. Sato, K. Fujimoto, Development of new nickel based catalyst for tar reforming with superior
resistance to sulfur poisoning and coking in biomass gasification, Catalysis Communications, 8
(2007) 1697-1701.
[30] C.A. Schwengber, F.A. da Silva, R.A. Schaffner, N.R.C. Fernandes-Machado, R.J. Ferracin,
V.R. Bach, H.J. Alves, Methane dry reforming using Ni/Al2O3 catalysts: Evaluation of the effects of
temperature, space velocity and reaction time, Journal of Environmental Chemical Engineering, 4
(2016) 3688-3695.
[31] Z. Zhang, L. Liu, B. Shen, C. Wu, Preparation, modification and development of Ni-based
catalysts for catalytic reforming of tar produced from biomass gasification, Renewable and
Sustainable Energy Reviews, 94 (2018) 1086-1109.
[32] S.H. Feng, G.H. Li, Chapter 4 - Hydrothermal and Solvothermal Syntheses, in: R. Xu, Y. Xu
(Eds.) Modern Inorganic Synthetic Chemistry (Second Edition), Elsevier, Amsterdam, 2017, pp. 73-
104.
[33] M. Adachi-Pagano, C. Forano, J.-P. Besse, Synthesis of Al-rich hydrotalcite-like compounds by
using the urea hydrolysis reaction—control of size and morphology, Journal of Materials Chemistry,
13 (2003) 1988-1993.
[34] L.H. Chagas, G.S.G. De Carvalho, W.R. Do Carmo, R.A.S. San Gil, S.S.X. Chiaro, A.A. Leitão,
R. Diniz, L.A. De Sena, C.A. Achete, MgCoAl and NiCoAl LDHs synthesized by the hydrothermal
urea hydrolysis method: Structural characterization and thermal decomposition, Materials Research
Bulletin, 64 (2015) 207-215.
[35] W. Wang, X. Huang, S. Wu, Y. Zhou, L. Wang, H. Shi, Y. Liang, B. Zou, Preparation of p–n
junction Cu2O/BiVO4 heterogeneous nanostructures with enhanced visible-light photocatalytic
activity, Applied Catalysis B: Environmental, 134-135 (2013) 293-301.
[36] S. Li, H. Zhu, Z. Qin, G. Wang, Y. Zhang, Z. Wu, Z. Li, G. Chen, W. Dong, Z. Wu, L. Zheng,
J. Zhang, T. Hu, J. Wang, Morphologic effects of nano CeO2–TiO2 on the performance of Au/CeO2–
TiO2 catalysts in low-temperature CO oxidation, Applied Catalysis B: Environmental, 144 (2014)
498-506.
[37] Z. Yang, Y. Huang, D. Ji, G. Xiong, H. Luo, Y. Wan, Hydrazine hydrate-induced hydrothermal
synthesis of MnFe2O4 nanoparticles dispersed on graphene as high-performance anode material for
lithium ion batteries, Ceramics International, 43 (2017) 10905-10912.
[38] J. Yang, S. Mei, J.M.F. Ferreira, Hydrothermal Synthesis of Nanosized Titania Powders:
Influence of Tetraalkyl Ammonium Hydroxides on Particle Characteristics, Journal of the American
Ceramic Society, 84 (2001) 1696-1702.
[39] Q. Zhang, T. Wu, P. Zhang, R. Qi, R. Huang, X. Song, L. Gao, Facile synthesis of hollow
hierarchical Ni/γ-Al2O3 nanocomposites for methane dry reforming catalysis, RSC Advances, 4
(2014) 51184-51193.
[40] Z. Bian, S. Kawi, Highly carbon-resistant Ni–Co/SiO2 catalysts derived from phyllosilicates for
dry reforming of methane, Journal of CO2 Utilization, 18 (2017) 345-352.
[41] R. Coll, J. Salvadó, X. Farriol, D. Montané, Steam reforming model compounds of biomass
gasification tars: conversion at different operating conditions and tendency towards coke formation,
Fuel Processing Technology, 74 (2001) 19-31.
[42] B. Abdullah, N.A. Abd Ghani, D.-V.N. Vo, Recent advances in dry reforming of methane over
Ni-based catalysts, Journal of Cleaner Production, 162 (2017) 170-185.
22

[43] W.P. Chan, A. Veksha, J. Lei, W.-D. Oh, X. Dou, A. Giannis, G. Lisak, T.-T. Lim, A hot syngas
purification system integrated with downdraft gasification of municipal solid waste, Applied Energy,
237 (2019) 227-240.
[44] R.-S. Zhou, R.L. Snyder, Structures and transformation mechanisms of the [eta], [gamma] and
[theta] transition aluminas, Acta Crystallographica Section B, 47 (1991) 617-630.
[45] X. Li, G.M. Kale, Urea-based hydrothermal synthesis of ITO nanopowders, Journal of Physics:
Conference Series, 26 (2006) 319-322.
[46] X.Y. Chen, Z.J. Zhang, X.L. Li, S.W. Lee, Controlled hydrothermal synthesis of colloidal
boehmite (γ-AlOOH) nanorods and nanoflakes and their conversion into γ- Al2O3 nanocrystals, Solid
State Communications, 145 (2008) 368-373.
[47] X. Hu, P. Li, X. Zhang, B. Yu, C. Lv, N. Zeng, J. Luo, Z. Zhang, J. Song, Y. Liu, Ni-Based
Catalyst Derived from NiAl Layered Double Hydroxide for Vapor Phase Catalytic Exchange between
Hydrogen and Water, Nanomaterials, 9 (2019).
[48] Z. Shang, S. Li, L. Li, G. Liu, X. Liang, Highly active and stable alumina supported nickel
nanoparticle catalysts for dry reforming of methane, Applied Catalysis B: Environmental, 201 (2017)
302-309.
[49] L. Samain, A. Jaworski, M. Edén, D.M. Ladd, D.-K. Seo, F. Javier Garcia-Garcia, U.
Häussermann, Structural analysis of highly porous γ-Al2O3, Journal of Solid State Chemistry, 217
(2014) 1-8.
[50] S.V. Moghaddam, M. Rezaei, F. Meshkani, Surfactant-Free Sol–Gel Synthesis Method for the
Preparation of Mesoporous High Surface Area NiO–Al2O3 Nanopowder and Its Application in
Catalytic CO2 Methanation, Energy Technology, 8 (2020) 1900778.
[51] A. Lechkar, A. Barroso Bogeat, G. Blanco, J.M. Pintado, M. Soussi el Begrani, Methanation of
carbon dioxide over ceria-praseodymia promoted Ni-alumina catalysts. Influence of metal loading,
promoter composition and alumina modifier, Fuel, 234 (2018) 1401-1413.
[52] R.-j. Zhang, G.-f. Xia, M.-f. Li, Y. Wu, H. Nie, D.-d. Li, Effect of support on the performance
of Ni-based catalyst in methane dry reforming, Journal of Fuel Chemistry and Technology, 43 (2015)
1359-1365.
[53] C. Jiménez-González, Z. Boukha, B. de Rivas, J.R. González-Velasco, J.I. Gutiérrez-Ortiz, R.
López-Fonseca, Behavior of Coprecipitated NiAl2O4/Al2O3 Catalysts for Low-Temperature
Methane Steam Reforming, Energy & Fuels, 28 (2014) 7109-7121.
[54] A. Remiro, A. Arandia, L. Oar-Arteta, J. Bilbao, A.G. Gayubo, Regeneration of NiAl2O4 spinel
type catalysts used in the reforming of raw bio-oil, Applied Catalysis B: Environmental, 237 (2018)
353-365.
[55] A.P. Grosvenor, M.C. Biesinger, R.S.C. Smart, N.S. McIntyre, New interpretations of XPS
spectra of nickel metal and oxides, Surface Science, 600 (2006) 1771-1779.
[56] L. Zhang, X. Wang, C. Chen, X. Zou, W. Ding, X. Lu, Dry reforming of methane to syngas over
lanthanum-modified mesoporous nickel aluminate/γ-alumina nanocomposites by one-pot synthesis,
International Journal of Hydrogen Energy, 42 (2017) 11333-11345.
[57] F. Qin, J.W. Anderegg, C.J. Jenks, B. Gleeson, D.J. Sordelet, P.A. Thiel, X-ray photoelectron
spectroscopy studies of the early-stage oxidation behavior of (Pt, Ni)3Al(111) surfaces in air, Surface
Science, 602 (2008) 205-215.
[58] J.F. Moulder, Handbook of X-Ray Photoelectron Spectroscopy, Physical Electronics, (1995)
230-232.
[59] M.H. Amin, S. Putla, S. Bee Abd Hamid, S.K. Bhargava, Understanding the role of lanthanide
promoters on the structure–activity of nanosized Ni/γ-Al2O3 catalysts in carbon dioxide reforming
of methane, Applied Catalysis A: General, 492 (2015) 160-168.
[60] C.-F. Sun, B.J. Glaz, M. Okada, E. Baker III, X.-Y. Cheng, S.P. Karna, Y. Wang, Blocking
Oxidation Failures of Carbon Nanotubes through Selective Protection of Defects, Advanced
Materials, 28 (2016) 6672-6679.
23

[61] K.A. Wepasnick, B.A. Smith, J.L. Bitter, D. Howard Fairbrother, Chemical and structural
characterization of carbon nanotube surfaces, Analytical and Bioanalytical Chemistry, 396 (2010)
1003-1014.
[62] M.H. Amin, Relationship Between the Pore Structure of Mesoporous Silica Supports and the
Activity of Nickel Nanocatalysts in the CO2 Reforming of Methane, Catalysts, 10 (2020) 51.
[63] A. Ochoa, J. Bilbao, A.G. Gayubo, P. Castaño, Coke formation and deactivation during catalytic
reforming of biomass and waste pyrolysis products: A review, Renewable and Sustainable Energy
Reviews, 119 (2020) 109600.
[64] J. Vicente, C. Montero, J. Ereña, M.J. Azkoiti, J. Bilbao, A.G. Gayubo, Coke deactivation of Ni
and Co catalysts in ethanol steam reforming at mild temperatures in a fluidized bed reactor,
International Journal of Hydrogen Energy, 39 (2014) 12586-12596.
[65] C. Montero, A. Remiro, B. Valle, L. Oar-Arteta, J. Bilbao, A.G. Gayubo, Origin and Nature of
Coke in Ethanol Steam Reforming and Its Role in Deactivation of Ni/La2O3–αAl2O3 Catalyst,
Industrial & Engineering Chemistry Research, 58 (2019) 14736-14751.
[66] M. Riegraf, M.P. Hoerlein, R. Costa, G. Schiller, K.A. Friedrich, Sulfur Poisoning of
Electrochemical Reformate Conversion on Nickel/Gadolinium-Doped Ceria Electrodes, ACS
Catalysis, 7 (2017) 7760-7771.
[67] G. Garbarino, A. Romero Perez, E. Finocchio, G. Busca, A study of the deactivation of low
loading Ni/Al2O3 steam reforming catalyst by tetrahydrothiophene, Catalysis Communications, 38
(2013) 67-73.
[68] J. Blanchard, I. Achouri, N. Abatzoglou, H2S poisoning of NiAl2O4/Al2O3-YSZ catalyst during
methane dry reforming, The Canadian Journal of Chemical Engineering, 94 (2016) 650-654.
[69] S. Appari, V.M. Janardhanan, R. Bauri, S. Jayanti, Deactivation and regeneration of Ni catalyst
during steam reforming of model biogas: An experimental investigation, International Journal of
Hydrogen Energy, 39 (2014) 297-304.
[70] X. Yang, An experimental investigation on the deactivation and regeneration of a steam
reforming catalyst, Renewable Energy, 112 (2017) 17-24.
[71] T.W. Hansen, A.T. DeLaRiva, S.R. Challa, A.K. Datye, Sintering of Catalytic Nanoparticles:
Particle Migration or Ostwald Ripening?, Accounts of Chemical Research, 46 (2013) 1720-1730.
[72] A.T. DeLaRiva, T.W. Hansen, S.R. Challa, A.K. Datye, In situ Transmission Electron
Microscopy of catalyst sintering, Journal of Catalysis, 308 (2013) 291-305.
[73] J. Du, J. Gao, F. Gu, J. Zhuang, B. Lu, L. Jia, G. Xu, Q. Liu, F. Su, A strategy to regenerate
coked and sintered Ni/Al2O3 catalyst for methanation reaction, International Journal of Hydrogen
Energy, 43 (2018) 20661-20670.
[74] Y. Liu, J. Gao, Q. Liu, F. Gu, X. Lu, L. Jia, G. Xu, Z. Zhong, F. Su, Preparation of high-surface-
area Ni/α-Al2O3 catalysts for improved CO methanation, RSC Advances, 5 (2015) 7539-7546.
[75] D. Li, M. Koike, L. Wang, Y. Nakagawa, Y. Xu, K. Tomishige, Regenerability of Hydrotalcite-
Derived Nickel–Iron Alloy Nanoparticles for Syngas Production from Biomass Tar, ChemSusChem,
7 (2014) 510-522.
[76] L. Foppa, T. Margossian, S.M. Kim, C. Müller, C. Copéret, K. Larmier, A. Comas-Vives,
Contrasting the Role of Ni/Al2O3 Interfaces in Water–Gas Shift and Dry Reforming of Methane, J.
Am. Chem. Soc., 139 (2017) 17128-17139.
24

Fig. 1. Bench-scale catalytic steam reforming setup.
25

N,N’-dimethylformamide
urea
N-methylurea
1,1-dimethylurea
N,N’-dimethylurea
Fig. 2. Chemical structures of precipitants used for catalysts synthesis.
26

Fig. 3. FESEM, TEM and HRTEM images of (a) Ni(ur), (b) Ni(mur), (c) Ni(mmur), (d) Ni(dmf)
and Ni(dmur) catalysts.
27

200
(a)
(b)
(c)
(d)
100
100
100
100
(e)
Fig. 4. TEM-EDS images of (a) Ni(ur), (b) Ni(mur), (c) Ni(mmur), (d) Ni(dmf) and (e) Ni(dmur)
showing the dispersion of Ni over the alumina support.
28