Catalytic activity of rare earth and alkali metal promoted (Ce, La, Mg, K) Ni/Al2O3 nanocatalysts in reverse water gas shift reaction

Article abstract of DOI:10.1007/s11164-019-03905-1

Abstract: Nanocrystalline Ni/Al₂O₃ catalysts and promoted Ni-M/Al₂O₃ (M = Ce, La, Mg, K) catalysts were employed a in reverse water gas shift reaction. Among the prepared nickel catalysts 5% Ni/Al₂O₃ catalyst showed high CO₂ conversion and CO selectivity. To improve the CO₂ conversion and CO selectivity of 5% Ni/Al₂O₃, 1 or 2?wt% Ce, La, Mg or K were used. One percent La and 2% K showed highest CO₂ conversion and CO selectivity. The high activity and CO selectivity of promoted catalysts can be accredited to the increase of Ni dispersion (or smaller Ni particles), concentration of surface active sites and CO₂ adsorption by basic nature of promoters. The prepared samples were characterized by X-ray diffraction, inductively coupled plasma emission spectroscopy, N₂ adsorption–desorption (BET), temperature programmed reduction (TPR), scanning electron microscopy and transmission electron microscopy (TEM) techniques. The BET surface area of prepared γ-alumina support was 126.81?m²?g^?1, impregnation of the support with Ni and promotion of 5?wt% Ni with K and La reduced the specific surface area. The support and catalysts possessed mesoporous structure. The TPR-H₂ analysis revealed higher Ni dispersion and reducibility for promoted nickel catalysts; 5Ni–2K showed highest reducibility. TEM images of 5Ni and 5Ni–2K showed increase of Ni dispersion and decrease of Ni particle size for the promoted catalyst. After 50?h on stream at 600?°C, 5Ni–1La and 5Ni–2K performed great catalytic stability. Graphic abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s11164-019-03905-1

Research on Chemical Intermediates
https://doi.org/10.1007/s11164-019-03905-1
Catalytic activity of rare earth and alkali metal promoted
(Ce, La, Mg, K) Ni/Al2O3 nanocatalysts in reverse water gas
shift reaction
Atieh Ranjbar1 · Abdullah Irankhah2 · Seyed Foad Aghamiri1
Received: 10 January 2019 / Accepted: 14 June 2019
© Springer Nature B.V. 2019
Abstract
Nanocrystalline Ni/Al2O3 catalysts and promoted Ni-M/Al2O3 (M=Ce, La, Mg, K)
catalysts were employed a in reverse water gas shift reaction. Among the prepared
nickel catalysts 5% Ni/Al2O3 catalyst showed high CO2 conversion and CO selectiv-
ity. To improve the CO2 conversion and CO selectivity of 5% Ni/Al2O3, 1 or 2 wt%
Ce, La, Mg or K were used. One percent La and 2% K showed highest CO2 conver-
sion and CO selectivity. The high activity and CO selectivity of promoted catalysts
can be accredited to the increase of Ni dispersion (or smaller Ni particles), con-
centration of surface active sites and CO2 adsorption by basic nature of promoters.
The prepared samples were characterized by X-ray difraction, inductively coupled
plasma emission spectroscopy, N2 adsorption–desorption (BET), temperature pro-
grammed reduction (TPR), scanning electron microscopy and transmission electron
microscopy (TEM) techniques. The BET surface area of prepared γ-alumina support
was 126.81 m2 g−1, impregnation of the support with Ni and promotion of 5 wt%
Ni with K and La reduced the specifc surface area. The support and catalysts pos-
sessed mesoporous structure. The TPR-H2 analysis revealed higher Ni dispersion
and reducibility for promoted nickel catalysts; 5Ni–2K showed highest reducibility.
TEM images of 5Ni and 5Ni–2K showed increase of Ni dispersion and decrease of
Ni particle size for the promoted catalyst. After 50 h on stream at 600 °C, 5Ni–1La
and 5Ni–2K performed great catalytic stability.
*
Abdullah Irankhah
irankhah@kashanu.ac.ir
1
2
Department of Chemical Engineering, Faculty of Engineering, University of Isfahan, Isfahan,
Iran
Hydrogen and Fuel Cell Research Laboratory, Department of Chemical Engineering, Faculty
of Engineering, University of Kashan, Kashan, Iran
Vol.:(0123456789)
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A. Ranjbar et al.
Graphic abstract
Keywords CO2 mitigation · RWGS · Nickel catalyst · Promoter · γ-Al2O3
Introduction
The negative efect of CO2 on climate change and global warming is well known.
Utilization of CO2 in catalytic reactions such as CO2 hydrogenation can decrease its
concentration in the atmosphere. Reverse water gas shift (RWGS) reaction at atmos-
pheric pressure recycles CO2 into CO (CO2 +H2 ↔CO+H2O), which is the feed-
stock for production of methanol (via carbon dioxide hydrogenation to form metha-
nol via a reverse-water-gas-shift reaction, or CAMERE, process) and other valuable
hydrocarbons. The catalytic reactions that directly convert CO2 into valuable prod-
ucts sufer from low CO2 conversion.
Several types of catalysts have been reported for RWGS reaction. Noble metals
Pt, Rh, Au, Pd and their bimetallic combinations such as Pt–K, Au–Mo, Rh–Fe were
used for the reaction [1–6]. These catalysts are active and CO-selective, but they
are expensive and can’t be used for large-scale CO production [7]. Cost-efective
non-noble metals such as Fe, Mn, Cu, Ni, Co have also been evaluated. Fe- and
Mn-based catalysts supported on Al2O3 and CeO2 showed high CO selectivity, but
low CO2 conversions [8, 9]. Cu based catalysts supported on Al2O3, CeO2 and SiO2
showed better CO2 conversions, but sufered from low thermal stability at tempera-
tures higher than 500 °C [10]. RWGS reaction is an endothermic reaction and higher
temperatures are preferred. Addition of promoters such as Fe, K, Ni to Cu-based
catalysts prevented Cu particles from agglomeration at higher temperatures and
induced better dispersion of Cu particles [11–13]. Ni based catalysts supported on
Al2O3, CeO2, SiO2, B-Mo2C, SBA-15 and Ce–Zr–O exhibited high CO2 conversion,
1 3

Catalytic activity of rare earth and alkali metal promoted…
but low CO selectivity especially at lower temperatures [14–20]. Low CO selectiv-
ity is a consequence of CO and CO2 methanation reactions occurring along with the
RWGS reaction.
CO2 + 4H2 ↔ CH4 + 2H2O ΔH = −252.9 kJ mol−1
(1)
CO + 3H2 ↔ CH4 + H2O ΔH = −165.0 kJ mol−1
(2)
Addition of a basic promoter can hinder the parallel methanation reactions
occurring along with the RWGS process [21]. Addition of K promoter to Ni/CeO2
catalysts decreased CH4 selectivity in medium-temperature shift reaction [22, 23].
A basic promoter partially blocks acidic sites available and aids the adsorption of
acidic CO2 on the created active sites [24]; consequently the formation of CH4 dur-
ing reaction is prevented. Ce, La, Ca, Mg and K or their oxides have been widely
added as promoters to Ni-based catalysts for reactions such as dry reforming of
methane or biogas dry reforming [25–29]. Furthermore, addition of a rare earth pro-
moter increased the Ni dispersity and stabilized catalytic activity of Ni/Al2O3 cata-
lysts at high temperatures in dry reforming of methane reaction [30, 31]. Higher Ni
dispersion and smaller Ni particles increase the CO selectivity in RWGS reaction
[15].
Until now addition of Ce, La, Mg and K as promoters to Ni/Al2O3 catalysts and
their efect on the CO selectivity in RWGS reaction has not been evaluated. Dif-
ferent basic promoters (rare earth metal and alkali) are chosen to fgure out which
is more efcient for RWGS process. Before promoting the catalysts, we have also
experimented Ni/Al2O3 catalysts with diferent nickel loadings to fnd out the proper
nickel loading. The catalysts were prepared by impregnation and co-impregnation
technique. The γ-alumina support used in this work was prepared by precipitation
method.
Experimental
Support preparation
For preparation of Al2O3 appropriate amount of aluminum nitrate (Al(NO3)3·9H2O)
is dissolved in distilled water. Ammonia was added drop wise to the solution
at 30 °C by careful adjustment of pH to 7.5. After precipitation, the mixture was
refuxed at 60 °C for another 12 h under stirring. The precipitate was fltered and
washed with distilled water. The product was dried at 80 °C for 24 h and calcined at
700 °C for 4 h.
Preparation of Ni/Al2O3
Nickel catalysts were prepared by impregnation. Primarily, the prepared support was
dispersed in distilled water then Ni(NO3)2·6H2O was added to the solution to obtain
2, 5, 7 and 10 wt% Ni. The slurry was stirred for 4 h at room temperature. The
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A. Ranjbar et al.
mixtures were dried at 100 °C for 24 h and calcined at a rate of 3 °C min−1 at 600 °C
and kept for 4 h at the fnal temperature.
Preparation of Ni‑M/Al2O3
Promoted nickel catalysts were prepared by co-impregnation. After dispersing the
support in distilled water, Ni (NO3)2·6H2O and the rare earth metal or alkali pro-
moter (Ce(NO3)3·6H2O, LaNO3, Mg(NO3)2·6H2O, KNO3) were simultaneously
added to the solution to obtain 5 wt% Ni and 1 or 2 wt% M where M=Ce, La, Mg
or K. The slurry was stirred for 4 h at room temperature. The prepared samples were
dried and calcined at the same condition as Ni/Al2O3 catalysts.
Characterization
The XRD patterns were recorded on an X-ray difractometer (PANalytical X’Pert-
Pro) using a Cu-Kα monochromatized radiation source and a Ni flter in the range
2θ=10° to 80°. Analyses were compared with the Inorganic Crystal Structure Data-
base (ICSD) using powder difraction fle no. 75-1862 for Al2O3, no. 47-1049 for
NiO, 10-0339 for NiAl2O4 and 87-0712 for Ni. The surface areas (BET) were deter-
mined by nitrogen adsorption at −196 °C using an automated gas adsorption ana-
lyzer (Tristar 3000, Micromeritics). The total metal loading (wt%) of the calcined
catalysts were determined by inductively coupled plasma atomic emission spec-
troscopy (ICP-OES) using Varian Vista-Pro CCD Simultaneous. Temperature pro-
grammed reduction (TPR–H2) profles were obtained by using Micromeritics Auto-
Chem 2920 TPR apparatus, in temperature range from 30 to 1000 °C at a rate of
10 °C min−1 under fow rate of 20 ml min−1 of 5 vol% H2/Ar. The catalyst samples
were degassed under inert atmosphere of Ar at 200 °C prior to the TPR analysis.
Scanning electron microscopy (SEM) was performed with VEGA TESCAN, oper-
ated at 30 kV for the surface morphology. SEM analysis was followed by energy
dispersive spectrometry (EDS) analysis for the elemental composition. Morphologi-
cal and structural characteristics of the reduced and used catalysts were also exam-
ined using a transmission electron microscope (TEM Philips CM120) operated at
120 keV.
Catalyst testing
Catalytic activity experiments were carried out in a fxed-bed continuous-fow reac-
tor at atmospheric pressure; see Fig. 1. The reactor is made of an 8-mm-i.d. quartz
tube. The reactor was charged with 200 mg catalyst for each test. Prior to the reac-
tion, the catalyst was reduced in situ in fow of H2 (30 ml min−1) at 600 °C for 4 h
and cooled down afterwards to 400 °C in a fow of an inert gas (Ar). The reactant
gases consisting of H2:CO2 =1:1 and (50 ml min−1) were introduced into the reactor.
Inlet fow of reactant gases were monitored by mass fow controllers. The activity
measurement tests were carried out at temperatures ranging from 400 to 700 °C in
steps of 100 °C. The experiments were repeated three times at each temperature. The
1 3

Catalytic activity of rare earth and alkali metal promoted…
Fig. 1 Process fow diagram of experimental apparatus for reverse water gas shift reaction over Ni/Al2O3
catalysts and Ni-M/Al2O3 catalysts
accuracy of the tests were controlled by carbon balance (Cin −Cout <ε). The product
stream was analyzed by a gas chromatograph equipped with thermal conductivity
detector (TCD) and a Carbosieve column. The long-term stability was checked out
for 50 h at 600 °C.
The CO2 conversion, CO and CH4 selectivity were evaluated as follows:
([
]
[
]
) [
∕ CO2 in × 100
out
]
CO2 conversion (%) = CO2 − CO2
(3)
(4)
(5)
in
([
]
[
]
)
Selectivity of CO (%) = [CO]out∕ CO2 − CO2
× 100
in
out
[
]
([
]
[
]
out
)
Selectivity of CH4 (%) = CH4 ∕ CO2 − CO2
× 100
out
in
Results and discussion
Catalytic characterization
Figure 2a presents the XRD patterns of prepared support and diferent nickel load-
ings on Al2O3. For the alumina support, formation of pure crystalline phase is evi-
dent from the peaks indexed as Al2O3. For nickel catalysts, no obvious peaks related
to NiO are observed. NiO refections at 37.3° and 43.3° overlap with the refections
of alumina at 37.7° and 43.3°, respectively. Addition of nickel to Al2O3 has reduced
its crystallinity. Characteristic lines related to nickel aluminate phase (NiAl2O4) at
37.96° and 43.53° are also overlapped with alumina and NiO refections. Figure 2b
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A. Ranjbar et al.
Fig. 2 XRD patterns of prepared support and catalysts; a Al2O3 and Ni/Al2O3 catalysts, b 5Ni-M/Al2O3
(M=Ce, Mg, La, K) catalysts, c reduced Ni/Al2O3 catalysts and d reduced 5Ni-M/Al2O3 (M=Ce, Mg,
La, K) catalysts
shows the XRD patterns of promoted nickel catalysts. Because of the low promoter
content no refections related to promoters were detected. As it can be seen from dif-
fractions of reduced nickel catalysts (Fig. 2c), for 5% Ni and higher loadings, peaks
related to Ni at 44.49° and 51.85° are observed. The difractograms of promoted
5Ni-M/Al2O3 catalysts are shown in Fig. 2d. For these catalysts, as well as the non-
reduced samples, no peaks related to promoters could be detected.
The structural properties of prepared samples are shown in Table 1. The average
crystallite size of prepared support is calculated from the half-width of difraction
peaks (β) using Scherrer’s formula. The prepared support showed nanocrystalline
structure with average crystallite size of 31.26 nm. Ni crystallite sizes are calculated
on (111) and (200) planes, also by using Scherrer’s formula. Increase of Ni crystal-
lite size is observed with increase of nickel loading. The prepared support showed
higher specifc surface area in comparison with the nickel catalysts and nickel pro-
moted catalysts. The average pore diameter increased with addition of nickel and
promoters to the support. Incorporation of Ni and promoters with the support
declined the structure of Al2O3.
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Catalytic activity of rare earth and alkali metal promoted…
Table 1 Structural properties of prepared samples
Sample
DXRD avg. crys-
Ni crystallite BET (m2 g−1
)
Total pore vol- Average pore
tallite sizea (nm) size (nm)a
ume (cm3 g−1
)
diameter (nm)
Al2O3 support
2Ni
31.3
–
–
–
126.8
–
0.2182
–
6.9
–
5Ni
7Ni
10Ni
5Ni–2K
–
–
–
–
–
111.1
–
–
103.1
105.2
0.2215
–
–
0.2131
0.2149
7.9
–
–
8.3
8.1
12.3
13.8
12.4
–
5Ni–1La
–
aCalculated from the XRD patterns
Table 2 The ICP analysis of
Sample
Ni (wt%)
La (wt%)
K (wt%)
5Ni–1La and 5Ni–2K
5Ni–2K
5Ni–1La
4.22
4.05
0
0.83
1.96
0
Fig. 3 a Pore size distributions and b N2 adsorption/desorption isotherms of Al2O3, 5Ni/Al2O3, 5Ni–2K/
Al2O3 and 5Ni–1La
The ICP results of 5Ni–1La and 5Ni–2K showed that the metal levels close to
desired levels were achieved for these catalysts (see Table 2).
Figure 3a reveals the pore size distribution for Al2O3 support, 5Ni, 5Ni–2K and
5Ni–1La. Pore sizes calculated by the BJH method are mainly about 2–10 nm, indicat-
ing mesoporous structure for all the prepared samples. The nitrogen adsorption/desorp-
tion isotherms are shown in Fig. 3b. According to the IUPAC classifcation the N2 iso-
therm is a type IV isotherm with a large type H3 hysteresis loop. This type of isotherm
is usually given by mesoporous structures where monolayer-multilayer adsorption is
followed by pore condensation (gas condenses in pores at pressure P less than P0) [32].
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A. Ranjbar et al.
Fig. 4 TPR profles of 5Ni/
Al2O3, 5Ni–1Ce/Al2O3, 5Ni–
1Mg/Al2O3, 5Ni–1La/Al2O3 and
5Ni–2K/Al2O3 (10% vol H2/Ar,
heating rate 10 °C min−1
)
Table 3 Total hydrogen
Catalyst
Total H2
consumption values of 5Ni/
Al2O3 and 5Ni-M/Al2O3
consumption
(μmol g−1
)
(M=Mg, Ce, La, K) catalysts
5Ni
871.2
969.5
937.7
977.7
1061.2
5Ni–1Mg
5Ni–1Ce
5Ni–1La
5Ni–2K
Figure 4 shows the TPR profles of 5% nickel and 5% nickel promoted cata-
lysts. The reduction is observed over a wide range of temperature from 300 to
950 °C. The peak β is related to NiO species, which have strong interaction with
the support. γ1 peak, which appears around 700 °C and is attributed to the pres-
ence of Ni2+ species in the octahedral coordination. γ2 peak appearing around
800 °C and higher is assigned to Ni2+ species in the tetrahedral environment that
corresponds to the spinel structure of NiAl2O4 [33, 34]. The peaks β, γ1 and γ2 are
only distinguished in 5Ni–2K, and for the rest of prepared catalysts these peaks
are merged together. In addition to the peaks for 5Ni–2K, 5Ni–1La and 5Ni–1Mg
a small peak labeled as α at left shoulder of these profles around 380 °C is
detected, which is associated with well dispersed NiO with weak interaction to
the support. Addition of promoters has increased the reduction peaks and shifted
the maximum temperature of these peaks to slightly lower temperatures. The
increasing reducibility of promoted Ni catalysts is confrmed by increase of H2
consumption. Table 3 gives the total H2 consumption of 5Ni and 5Ni-promoted
catalysts. Among the prepared catalysts 5Ni–2K showed the highest H2 consump-
tion. The appearance of NiO species with lower interaction with the Al2O3 sup-
port and shifting of maximum temperature of the peaks to lower temperatures,
after impregnation with alkali promoters has been reported by many research-
ers [25, 35–37]. Mazumder et al. showed that after impregnation of Ni/Al2O3
with La2O3, a small peak around 390 °C appeared. They illustrated that highly
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Catalytic activity of rare earth and alkali metal promoted…
dispersed NiO on the support surface was responsible for this low temperature
peak [29]. The same results have also been reported for CeO2 as promoter [38],
but here we haven’t seen much change in reduction of the Ce promoted catalyst,
except a small temperature shift.
Figure 5 shows SEM images of Al2O3 support, 5% Ni/Al2O3 and promoted Ni
catalysts. As can be seen, they don’t have any particular shape. The promoted cata-
lysts showed similar particle size distribution (an average between 60 and 90 nm)
except 5Ni–1Mg, which showed larger particles among the promoted catalysts.
Since we had low promoter loadings that could not be detected by XRD analysis, we
used energy dispersive analysis (EDS), as shown in Table 4. All available elements
in the prepared catalysts were detected by EDS analysis. The reported amounts were
close to desired loadings in catalysts.
Figure 6 shows TEM images of reduced 5Ni/Al2O3 and 5Ni–2K/Al2O3 catalysts.
Ni particles in both samples exhibit nearly spherical shaped particles and fall in the
nanoscale range with uniform size distribution. The TEM results revealed smaller
particles and higher dispersion of Ni particles in promoted catalyst. The portion of
larger particles is higher in unpromoted catalyst.
Catalytic performance
The CO2 conversion, CO and CH4 selectivity over Ni/Al2O3 at diferent tempera-
tures are shown in Fig. 7a–c, respectively. The results showed that with increasing
the nickel content up to 5%, CO2 conversion increased. Increasing the nickel content
higher than 5% did not have much efect on catalyst activity and it just increased the
methanation. Two percent showed lowest catalytic activity and highest CO selectiv-
ity in comparison with other Ni loadings. High activity and CO selectivity of 5Ni
(in comparison with 7Ni and 10Ni) is due to higher dispersion or incorporation of
Ni species with the Al2O3 support. Ni species with low dispersity and large parti-
cles and an increase in H2 coverage on active sites, which is in favor of CH4 forma-
tion (CO2 +4H2 ↔CH4 +2H2O) [15, 17]. To evaluate the efect of promoter on CO
selectivity, we chose 5% nickel to proceed.
The CO2 conversion and CO selectivity of alkali and rare earth promoted 5% Ni/
Al2O3 catalysts are presented in Fig. 8a–d, respectively. Results showed that addi-
tion of rare earth metal or alkali promoters decreased the catalytic activity at 400 °C,
but at higher temperatures they exhibited almost the same activity and even higher.
The lower activity at temperatures below 500 °C is related to decrease of methana-
tion reactions occurring along with the RWGS reaction. In other words, there is no
decrease in RWGS catalytic activity and this decrease is attributed to decrease of
side reactions. Among the alkali and rare earth promoted catalysts 1% La and 2% K
showed higher CO2 conversion especially at temperatures higher than 500 °C.
CH4 formation was reduced during the reaction for all the promoted catalysts.
One percent Ce increased CO selectivity by about 4%, 1% Mg and 1% La by about
9% and 1% K by about 20% at 400 °C. After that the results of 1% promoters were
obtained, we continued our work with 2% La and 2% K on 5% Ni/Al2O3. Two per-
cent K decreased the catalytic activity and methanation more than 1% K, where 2%
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A. Ranjbar et al.
Fig. 5 SEM images of prepared support and catalysts; a Al2O3, b 5Ni/Al2O3, c 5Ni–1Ce/Al2O3, d 5Ni–
1Mg/Al2O3, e 5Ni–1La/Al2O3, f 5Ni–2La/Al2O3, g 5Ni–1K/Al2O3, h 5Ni–2K/Al2O3
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Catalytic activity of rare earth and alkali metal promoted…
Table 4 Elemental composition of promoted catalysts detected by EDS analysis
Element
5Ni–1Ce
(wt%)
5Ni–1Mg
(wt%)
5Ni–1La
(wt%)
5Ni–2K
(wt%)
(at.%)
(at.%)
(at.%)
(at.%)
O
55.12
39.87
4.59
0.43
–
–
–
68.83
29.52
1.60
0.08
–
–
–
58.28
34.94
5.10
–
1.68
–
71.52
25.42
1.70
–
1.36
–
57.52
37.45
3.98
–
71.07
27.44
1.34
–
54.60
39.68
3.93
–
68.46
29.49
1.23
–
Al
Ni
Ce
Mg
La
K
–
–
–
–
1.05
–
0.15
–
–
1.59
–
0.81
–
–
Fig. 6 TEM images of reduced catalysts; a, b 5Ni/Al2O3, c, d 5Ni–2K/Al2O3
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A. Ranjbar et al.
Fig. 7 a CO2 conversion, b CO selectivity and c CH4 selectivity of Ni/Al2O3 catalysts at diferent tem-
peratures, H2:CO2 =1:1 (GHSV=1.5×104 ml g−1 h−1
)
La in comparison with 1% La showed almost the same activity and lower CO selec-
tivity. In general, the CO selectivity of catalysts exhibited the following order: 2K>
1K>1La>1Mg>2La>1Ce.
The obtained results are great in agreement with the literature. Better catalytic
activity of promoted catalysts due to surface rearrangement is stated by many
researchers [39–41]. The catalytic activity of un-modifed 5Ni/Al2O3 has been previ-
ously reported; the reported catalyst showed CO2 conversion of 32% and CO selec-
tivity less than 78% at 600 °C [42]. In this study we have CO2 conversion of about
35% and CO selectivity higher than 90% for the promoted catalysts at 600 °C. Lit-
erature reports emphasize the exceptional role of the supports with oxygen storage
ability and their efect on catalytic activity [16, 21]. These catalysts showed high CO
selectivity at low Ni loadings, but by increasing the Ni content the CO selectivity
decreased. The same results were reported for the Ni catalysts supported on SiO2
and Al2O3 [15, 17, 20]. Here we can conclude that for Ni based catalysts the size of
Ni particles and their dispersity plays a much more important role in CO selectivity.
Nickel species with high dispersion and small particle size are in favor of RWGS
reaction, where large and agglomerated nickel particles put forward the CO2 metha-
nation reaction [15]. Comparing the promoted Ni/Al2O3 catalysts with the reported
catalysts, these cost efective catalysts seem to be very promising catalyst for RWGS
reaction. Promoted Ni/Al2O3 catalysts exhibit high activity and CO selectivity for
1 3

Catalytic activity of rare earth and alkali metal promoted…
Fig. 8 a, b CO2 conversion and CO selectivity of alkali promoted catalysts (5Ni-M/Al2O3 (M=Mg, K)),
c, d CO2 conversion and CO selectivity of rare earth promoted catalysts (5Ni-M/Al2O3 (M=Ce, La)) at
diferent temperatures, H2:CO2 =1:1 (GHSV=1.5×104 ml g−1 h−1
)
the titled reaction; this can be accredited to the increase of Ni dispersion, concentra-
tion of surface active sites and CO2 adsorption by basic nature of promoters.
The obtained results in this research encourage the importance of Ni dispersity
and size in catalytic activity and CO selectivity in RWGS reaction. Addition of
promoters especially using co-impregnation technique improves Ni dispersity and
reducibility, as stated by Koo et al. [43]. The increase of Ni dispersity and reduc-
ibility of promoted catalysts were confrmed with TPR results (Fig. 4). The TPR
profles of promoted catalysts showed an additional peak at 380 °C related to well
disperse NiO species. The TPR results indicated increase of H2 consumption in
promoted catalysts. Smaller Ni particle size and higher dispersion of 5Ni promoted
catalysts were also confrmed by TEM images (Fig. 6). TEM images of 5Ni–2K
showed smaller Ni particles and higher dispersion in comparison with unpromoted
5Ni catalyst.
On the other hand, addition of a basic promoter enhances the CO2 adsorp-
tion and formation of formates. Formate dissociation mechanism is the leading
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A. Ranjbar et al.
Fig. 9 The long-term stability over 5Ni–2K and 5Ni–1La, reaction temperature: 600 °C, H2:CO2 =1:1
(GHSV=1.5×104 ml g−1 h−1
)
mechanism proposed for production of CO from RWGS reaction [44, 45]. The
possible mechanism over promoted catalysts is as follows; frstly, CO2 gas is
adsorbed on Ni, interfaces of Ni–K, while H2 is adsorbed and dissociated only
by nickel to H. Secondly, the hydrogen atoms on Ni associate with CO2 available
on the Ni surface or the CO2 adsorbed by on the interfaces of Ni–K to form for-
mates. Finally, by decomposition of formates CO is formed and hydrogen atoms
associate with OH to form H2O.
Chen et al. reported the increase of CO2 adsorption after impregnation of the cat-
alyst with K by using CO2-TPD analysis [12]. Addition of K to Cu/Al2O3 catalyst
increased the CO2 adsorption and CO selectivity up to 95.8% at pressures higher
than 10 Mpa [46]. Increase of the basicity of alumina support after addition of an
alkali or rare earth promoter (Mg and La) was also confrmed by Koo et al. [36].
High H2 adsorption puts forward the CO2 methanation reaction, while a decrease
in hydrogen coverage and increase in CO2 adsorption is in favor of RWGS reaction.
The long-term stability tests were performed on 1% La and 2% K, see Fig. 9.
Both catalysts performed great catalytic activity and CO selectivity during 50 h on
stream. One percent La showed about 1% decrease in CO2 conversion and CO selec-
tivity after 50 h, while 2% K exhibited about 3% decrease.
The SEM images of 5Ni–2K and 5Ni–1La after 15 h on stream and TEM image
of 5Ni–2K after 50 h on stream are shown in Fig. 10. The SEM images confrmed
stable catalytic structure after 15 h on stream (Fig. 10a, b). No sintering of particles
is noticed for both catalysts. The TEM image of spent 5Ni–2K showed no particular
changes (Fig. 10c) and stable catalyst structure is detected after 50 h on stream. The
spent catalyst showed only a small increase in Ni size after the stability test.
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Catalytic activity of rare earth and alkali metal promoted…
Fig. 10 images of spent catalysts; a, b SEM images of 5Ni–1La/Al2O3, 5Ni–2K/Al2O3 after 15 h on
stream and c TEM image of 5Ni–2K after 50 h on stream, reaction temperature: 600 °C, H2/CO2 =1,
GHSV=1.5×104 ml g−1 h−1
Conclusions
Five percent Ni/Al2O3 catalyst was impregnated with Ce, La, Mg or K and intro-
duced to increase the CO selectivity and catalytic activity of this catalyst. The pro-
moted Ni catalysts prepared by co-impregnation method showed mesoporous struc-
ture and great catalytic activity in reverse water gas shift reaction (RWGS). Addition
of rare earth metal (Ce, La) or alkali promoter (Mg, K) to 5Ni/Al2O3 reduced the
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A. Ranjbar et al.
CO2 conversion at lower temperatures but increased the CO selectivity up to 30%
for 2% K at 400 °C. At higher temperatures, CO2 conversion increased with addition
of promoters. Addition of an alkali or rare earth metal promoter by co-impregnation
technique enhanced the dispersity of Ni species on the support, besides increasing
the CO2 adsorption on the surface of catalyst. Addition of 1% La, 1% K and 2% K
increased the CO2 conversion of 5Ni/Al2O3 from 39.8% to 43.2%, 42.5% and 41.8%
at 700 °C, respectively. The long-term stability test over 5Ni–1La and 5Ni–2K
showed less than 3% decrease in catalytic activity after 50 h on stream.
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