Ni-substituted LaMnO3 perovskites for ethanol oxidation

Article abstract of DOI:10.1039/c3ra46323k

The B-site substitution of LaMnO₃ perovskites by Ni was investigated under the oxidation of ethanol. Proper characterization techniques, including BET surface area measurement, XRD, FTIR, XPS, TPR, O₂-TPD, and FE-SEM experiments were performed to survey the physicochemical properties of perovskites. The results reveal that up to 25% of Mn can be replaced by Ni; beyond this limit, segregated NiO_x can be synthesized. Inserting Ni into the solid solution of perovskite yields unique bridging lattice oxygen sites (Ni-O-Mn) in Ni-doped LaMnO₃. Based on catalytic performance in ethanol oxidation, the Ni-O-Mn sites are likely to promote ethanol conversion and the oxidation of acetaldehyde to CO₂ at low reaction temperatures. The abatement of intermediates over Ni-O-Mn sites is hypothesized and a plausible reaction pathway is proposed. Moreover, the time on-stream testing revealed that the interaction between Ni and Mn is likely to enhance perovskite's thermal stability in ethanol oxidation.

Full text of DOI:10.1039/c3ra46323k

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PAPER
Ni-substituted LaMnO3 perovskites for ethanol
oxidation†
Cite this: RSC Adv., 2014, 4, 5329
*
Yi-Chen Hou, Ming-Wei Ding, Shih-Kang Liu, Shin-Kuan Wu and Yu-Chuan Lin
The B-site substitution of LaMnO3 perovskites by Ni was investigated under the oxidation of ethanol. Proper
characterization techniques, including BET surface area measurement, XRD, FTIR, XPS, TPR, O2-TPD, and
FE-SEM experiments were performed to survey the physicochemical properties of perovskites. The
results reveal that up to 25% of Mn can be replaced by Ni; beyond this limit, segregated NiOx can be
synthesized. Inserting Ni into the solid solution of perovskite yields unique bridging lattice oxygen sites
(Ni–O–Mn) in Ni-doped LaMnO3. Based on catalytic performance in ethanol oxidation, the Ni–O–Mn
sites are likely to promote ethanol conversion and the oxidation of acetaldehyde to CO2 at low reaction
temperatures. The abatement of intermediates over Ni–O–Mn sites is hypothesized and a plausible
reaction pathway is proposed. Moreover, the time on-stream testing revealed that the interaction
between Ni and Mn is likely to enhance perovskite's thermal stability in ethanol oxidation.
Received 1st November 2013
Accepted 11th December 2013
DOI: 10.1039/c3ra46323k
www.rsc.org/advances
successfully implemented trace platinum group metal (pgm)-
doped perovskites in their exhaust converter systems.8–11 The
Introduction
self-regeneration behavior of pgm-promoted perovskites under
oxidative and reductive conditions makes them more active and
durable than platinum-based catalysts. Our group has recently
reported on the effectiveness of similar perovskites in methanol
partial oxidation12,13 and combustion.14 The unique lattice
structure and oxygen nonstoichiometry of perovskite are
believed to be important in oxidation reactions.5
Ethanol can be derived from renewable energy crops and
lignocellulosic biomass. Using ethanol as a fuel additive is a
potential way to alleviate our fossil-fuel addiction. For example,
10% and 85% ethanol–gasoline blended fuels, i.e., E10 and E85,
are commercially available for drivers in the US. As demand for
energy continues to increase, the market growth for ethanol fuel
is inevitable.1 However, burning ethanol-containing fuels yields
different vehicle exhaust compounds from those produced by
burning fossil fuels.2 More volatile organic compounds (VOCs)
are generated using 10% and 15% ethanol–diesel blends in
existing engines when compared to certied diesel fuel.3
Unburned ethanol and its derivatives, such as acetaldehyde, a
carcinogen in humans,4 are harmful pollutants. Hence, the
development of suitable catalysts for the abatement of ethanol
and ethanol derivatives is urgently required.
Surprisingly, only a few studies of ethanol combustion using
perovskites are available. The pioneering study by the Hitachi
research group15 tested LnBO3 (Ln ¼ La, Pr, Sm, and Gd; B ¼ Fe,
Co and Ni) as ethanol sensors in ethanol oxidation. The short
response time at low ethanol concentration makes perovskite a
promising candidate in ethanol conversion. This inspires fol-
low-up studies to utilize various compositions and elements at
the A- or B-site position.16–22 Wang et al.17 and Najjar and Batis21
reported relatively high reactivities (low T50 and T95, where the
former denotes the temperature for 50% ethanol conversion, and
the latter is the temperature for 95% ethanol conversion) of their
catalysts in ethanol oxidation. Wang et al.17 used Ag-supported
La0.6Sr0.4MnO3 in both methanol and ethanol oxidations, and
reported that anchored Ag+ on the surface of perovskite not only
acts as a catalytic promoter, but also increases the surface
adsorbed oxygen-to-lattice oxygen (O2ꢀ2/Oꢀ) ratio; both factors
are essential in the conversion of alcohols. Najjar and Batis21
investigated gelling agents (glycine and citric acid) for LaMnO3
synthesis using a combustion method, and discovered that
LaMnO3 made by using glycine as the gelling agent had a high
thermal stability and activity in ethanol combustion. This is
attributed to the decrease of surface La/Mn ratio and the increase
of the adsorbed oxygen species on the perovskite's surface.
Perovskite (ABO3) is an effective catalyst for the removal of
CO, NOx, VOCs, and hydrocarbons.5,6 In some cases, perovskites
perform comparably to, or even better than, conventional
(precious metal-based) three-way catalysts, such as strontium-
containing perovskites in NOx abatement.7 Daihatsu Motors has
Department of Chemical Engineering & Materials Science, Chungli, Taoyuan, 32003,
Taiwan. E-mail: yclin@saturn.yzu.edu.tw; Fax: +886 3 4559373; Tel: +886 3 463
8800 ext. 3554
† Electronic supplementary information (ESI) available: The reduction properties,
C 1s core level spectra, and post-TPR XRD patterns of LaMn1ꢀyNiyO3, TPR proles
of physically mixed LaMnO3 and LaNiO3 particles, ethanol conversion and
product selectivity as functions of temperature of physically mixed LaMnO3
and LaNiO3 particles, the summary of XPS-derived Ni loading, amounts of
a- and b-oxygen, and T50 and T95 of tested LaMn1ꢀyNiyO3, the SEM images of
as-synthesized and used samples. See DOI: 10.1039/c3ra46323k
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This study investigates the use of Ni-incorporated LaMnO3 in dehydrated at 130 ꢁC for 30 min in a N2 stream (25 mL minꢀ1).
ethanol oxidation. LaMnO3 is known to have a wide range of All the spectra were obtained with an average of 16 scans and 4
oxidative (excess oxygen) and reductive (oxygen vacancy) non- cmꢀ1 resolution at room temperature.
stoichiometries.23 The non-stoichiometric oxygen content
X-ray photoelectron spectroscopy (XPS) was performed using
markedly affects various oxidation reactions, such as methane24 a Thermo Scientic K-Alpha system equipped with a 180ꢁ
and propane23 oxidations. This can be ascribed to the oxygen hemispherical sector analyzer and an Al monochromator (Al Ka
adsorption–activation step on catalyst surface and the X-ray sources, 1486.6 eV). The diameter of the X-ray spot was
enhancement of oxygen mobility.17 Partial substitution of Mn 400 mm. The C 1s signal (284.5 eV) of adventitious carbon was
with cations with a valence less than three (such as Cu) is used to correct the energy shi.
another way to improve the oxidation chemistry either in the
Single-point BET surface area measurement, temperature-
bulk or on the surface of LaMnO3-based perovskites.23 In this programmed reduction (TPR), and O2-temperature-pro-
work, Ni is chosen as the B-site dopant because of its bivalence grammed desorption (O2-TPD) were carried out using a
and reactivity in oxidative environments.25–27 Incorporating Ni Micromeritics apparatus model Autochem II 2920 with a
in LaMnO3 may have a synergistic effect on the conversion of thermal conductivity detector (TCD). Approximately 100 mg of
ethanol and its derivatives in oxidative environments. This sample was consumed per trial. Prior to each test, the sample
study attempts to explore the effect of Ni-substituted LaMnO3 was dehydrated at 150 ꢁC under an N2 stream (30 mL minꢀ1) for
perovskite in ethanol combustion. Physicochemical properties 30 min. In the BET test, a 30% N2/He stream was used for N2
of catalysts were examined, and the relationship between the Ni physisorption at ꢀ196 ꢁC. The specic surface area of each
content and the catalytic behaviors of LaMnO3-based perov- sample was estimated from the desorbed N2 area using the
skites was elucidated.
single-point BET equation. In TPR analysis, a 10% H2/Ar (30 mL
minꢀ1) stream was passed through the sample and the
ꢁ
ꢁ
temperature was raised from 50 to 900 C with a ramp of 5 C
minꢀ1. For O2-TPD, the sample was treated in a He stream
(30 mL minꢀ1) from 50 to 1000 ꢁC at 5 ꢁC minꢀ1 followed by an
Experimental
Catalyst synthesis
ꢁ
isotherm at 1000 C for half an hour.
Substituted LaMn1ꢀyNiyO3 (y ¼ 0.1, 0.25, and 0.4) and plain
LaMnO3 and LaNiO3 were synthesized by the Pechini
method.28,29 Metal nitrates precursors (La(NO3)3$6H2O (Merck,
99%), Mn(NO3)2$4H2O (Merck, 99%), and Ni(NO3)2$6H2O
(Merck, 99%)) were used during the synthesis of the respective
perovskites. These precursors were dissolved in water in the
desired stoichiometric ratios. The obtained solution was then
added dropwise to a 1 M citric acid solution, which contained
approximately four times as many citric acid ions in relation to
the metal ions of the precursors. The mixture was stirred and
The surface morphologies of as-synthesized and used
samples were determined using a eld-emission scanning
electron microscope (FE-SEM, JEOL JSM-6701F). The acquisi-
tion time per location was 10 min. The SEM accelerating voltage
was 10 kV. The scanned area was ꢂ4 mm ꢃ 3 mm. Both as-
synthesized and used catalysts were examined.
Activity evaluation
Catalytic performances were examined in a continuous xed-bed
system.30 Ethanol was injected into the system by a HPLC pump
(Jasco PU-2080) at a feeding rate of 3 ꢃ 10ꢀ3 mL minꢀ1. Oxygen
and nitrogen were regulated using mass ow controllers (Brooks
5850E). The molar composition of the feed was C2H5OH/O2/N2 ¼
1.7/18.1/80.2 and the gas hourly space velocity (GHSV) was
10 446 hꢀ1. About 10 mg of catalyst with a 40–80 mesh size was
used in each trial. To maintain an isothermal condition, the
catalyst was diluted with 150 mg SiO2 (40–80 mesh). SiO2 was
inert in each trial. Internal and external mass transfer limita-
tions were examined prior to the tests. The former was probed by
measuring the catalytic performances using two different cata-
ꢁ
heated to 80 C in a water bath. Aer stirring for an hour, an
equivalent molar ratio of ethylene glycol (Alfa Aesar, 99%) using
citric acid as the reference was poured into the solution. The
resulting solution was then stirred at 90 C for more than four
hours until a gel was formed. The gel was transferred to a
furnace, and dried at 80 C for 24 h. The remaining paste was
ꢁ
ꢁ
ꢁ
ꢁ
then calcined in air from 80 to 400 C at a heating rate of 1 C
minꢀ1, and kept isotherm for two hours. Finally, the precursors
were calcined from 400 to 900 C (3 ꢁC minꢀ1) for four hours.
ꢁ
Characterization
Powder X-ray diffraction patterns were obtained using a Shi- lyst particle sizes (20–40 mesh and 40–80 mesh); the latter was
madzu Labx XRD-6000 with Cu Ka radiation (0.15418 nm). The tested by comparing two catalyst bed loadings, 10 mg catalyst
scan rate was set to 4ꢁ minꢀ1 and the 2q range was 20–80ꢁ. The mixed with 150 mg SiO2 and 15 mg catalyst mixed with 220 mg
Scherrer relation was employed to estimate the crystallite size of SiO2, with different ow rates but the same contact time (GHSV
each sample, based on the strongest peak from the (1 1 0) plane ¼ 10 446 hꢀ1). Both internal and external mass transfer resis-
of perovskite.
tances could be ignored because the differences of ethanol
Fourier transform infrared spectroscopy (FTIR) spectra were conversions in these trials were trivial (less than 3%).
recorded in the 400–4000 cmꢀ1 region with a Perkin-Elmer
The system outlet was connected to a gas chromatograph
Spectrum 100 Optica using the KBr pellet method. The catalyst (GC, SRI 8610) equipped with a TCD, a methanizer, and a ame
˚
powder was diluted to about 1 wt% within KBr. The mixture was ionization detector (FID). Two packed columns, 5 A molecular
then pulverized and pelletized. Prior to the test, the pellet was sieve and Porapak Q, were used to separate reactants and
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products. The detected compounds were ethanol, acetaldehyde,
CO2, CO, CH4, O2, and N2.
Nitrogen also served as the internal standard. All products
were measured relative to GC calibration standards. Catalytic
results were reported with carbon atom mass balances less than
10% errors. Three to four runs were made for each data point,
providing 95% condence intervals (error bars) for conversion
and selectivity. Ethanol conversion was calculated as moles of
ethanol reacted divided by moles of ethanol injected. Selectivity
of carbon-containing product was dened as 100 ꢃ (moles of
ethanol converted to product Xi)/(moles of ethanol converted).
The data of conversion or selectivity was interpolated with a B-
spline function to allow a better comparison.
Fig. 1 XRD patterns of LaMn1ꢀyNiyO3 perovskites. The diffraction of (1
1 0) plane is zoomed in to show the 2q angle shift with increasing Ni
loading.
Results and discussion
Table 1 lists the surface areas of the LaMn1ꢀyNiyO3 catalysts. All
catalysts displayed similar surface areas, ranging from 7.6 to
10.5 m2 gꢀ1. Fig. 1 shows the XRD patterns of LaMn1ꢀyNiyO3.
Those of LaMnO3 ([JCPDS: 35-1353]) and LaNiO3 ([JCPDS: 33-
0711]) were included for reference. All samples yielded the
characteristic peaks of ABO3-type perovskite phase. The main
diffraction peak of NiO ([JCPDS: 89-5881]) at 2q ¼ 43.3ꢁ was
observed on LaNiO3. It reveals the segregation of NiO particles
from perovskite solid solution. However, NiO was not identied
on Ni-substituted catalysts, possibly because no segregated NiO
was present or the NiO clusters were too small to be detected.
The crystallite size decreased from 15.1 to 13.0 nm with
increasing y values, suggesting the formation of disorder or
deformation in LaMnO3 perovskite by adding Ni.
proportion of segregation has been noted in the high substi-
tution region with the maximum shi value. LaMnO3 has an
orthorhombic unit cell. Substituting Mn with smaller Ni ions
destabilizes the MnO6 octahedra and enhances the rhombo-
hedral distortion (due to the Jahn–Teller effect).33 This struc-
tural phase transition of LaMn1ꢀyNiyO3 may lead to the
coexistence of multiple phases (e.g., La2NiMnO6 and LaNiO3) at
a high Ni-loading.32,33 Therefore, the complete insertion of Ni in
perovskite structure should occur at y < 0.25; NiOx phases
should segregate at y ¼ 0.25 and above.31
Fig. 3 shows the FTIR spectra of the LaMn1ꢀyNiyO3 catalysts.
Besides LaNiO3, all perovskites displayed an adsorption band at
approximately 600 cmꢀ1. This band is related to Mn–O
stretching in octahedral site.34 The band of Mn–O stretching
was shied from 610 to 601 cmꢀ1 (red shi) with an increase of
Ni loading. The red shi may be explained by the photon
response theory35 since Ni is heavier than Mn in the solid
solution. Therefore, the downward-shied wavenumber sug-
gested the presence of Ni–O–Mn bonding in Ni-containing
LaMnO3. Moreover, at y ¼ 0.25 and 0.4, a band at 411 cmꢀ1 and
a shoulder at 638 cmꢀ1 were observed. The former is the
stretching vibration of Ni–O bond;36,37 the latter, Ni–OH
bending.38 This is consistent with the XRD diffraction shis:
NiOx could be formed on the surface at y ¼ 0.25 and above. The
IR spectrum of LaNiO3 was not well resolved; a close inspection
within 400–600 cmꢀ1 was inserted in Fig. 3. The unclear vibra-
tional modes of LaNiO3 may be related to its low electrical
resistivity (10ꢀ1 U cm).39 At such a low resistivity, the residence
time (10ꢀ12 s) of charge carriers at a site can be comparable to
A close inspection of the XRD patterns showed that the 2q
angle of each peak increased slightly with increasing Ni
amount. This correlation is attributed to the decrease of inter-
planar distance by Ni substitution: the more Ni incorporated,
the more Mn3+ (low-spin 0.58 A, high-spin 0.65 A) could be
˚
˚
replaced by Ni3+ (low-spin 0.56 A, high-spin 0.60 A) ions,
resulting in a decrease of the unit cell volume.31 Blasco et al.32
have also observed a similar tendency regarding unit cell
volume with increasing Ni content in LaMn1ꢀyNiyO3
perovskites.
˚
˚
Fig. 2 shows (1 1 0), (1 1 1), (2 0 0), and (2 1 1) diffraction
shis as functions of Ni content. The shi gradually increased
with Ni content from y ¼ 0 to y ¼ 0.25 then leveled off. Prov-
endier et al.26 and Pecchi et al.31 have observed the same trend of
XRD peak shi in the LaFe1ꢀyNiyO3 system. The shi has been
ascribed to the degree of Ni substitution, and a certain
Table 1 Surface area, mean crystallite size, and XPS-derived surface composition of LaMn1ꢀyNiyO3
LaMn1ꢀyNiyO3
SAA (m2 gꢀ1
)
dXRD (nm)
Mn/La
Ni/La
Ni/Mn
La/(Mn + Ni)
Ni/(Mn + Ni)
(Mn4+/Mn3+
)
Ox/Olatt
y ¼ 0
8.7
9.3
10.5
8.2
15.1
14.8
14.5
13.2
13.0
0.83
1.21
1.08
0.79
—
—
0
1.21
0.73
0.74
0.86
1.49
0
1.03
1.10
1.21
1.06
—
0.63
0.66
0.70
0.64
2.00
y ¼ 0.1
y ¼ 0.25
y ¼ 0.4
y ¼ 1
0.16
0.28
0.37
0.67
0.13
0.26
0.47
—
0.12
0.21
0.32
1
7.6
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Fig. 2 High angle shifts of the diffraction peaks as functions of Ni
substitution content.
Fig.
4
La 3d, Ni 2p, Mn 2p, and O 1s core level spectra of
LaMn1ꢀyNiyO3 perovskites.
presence of Mn2+ ions. The responses of Mn 2p3/2 and Mn 2p1/2
can be deconvoluted into four peaks. The responses at 641.5 eV
and 653.0 eV were assigned to Mn3+ cations, while the peaks at
642.9 eV and 654.5 eV were attributed to Mn4+ ions.42 To provide
quantitative analysis, the surface atomic ratios (see Table 1)
were estimated from the intensities of La 3d5/2, Ni 2p1/2, and Mn
2p3/2 peaks using the atomic sensitivity factors provided by
Fig. 3 FTIR spectra of LaMn1ꢀyNiyO3 perovskites.
the lattice vibrational period (10ꢀ13 s), at which a localized Wagner et al.46 The Mn4+/Mn3+ molar ratio was reported in
vibrational mode may not be specied.40 Two tiny bands at Table 1 as well.
428 cmꢀ1 and 502 cmꢀ1 were identied, in accord with an
The surface enrichment of La can be speculated for plain
earlier work.41 However, the assignment of these two responses LaMnO3 and LaNiO3 because the Mn/La and Ni/La ratios at
is unclear.
y ¼ 0 and 1 were all less than unity. However, this is not the case
Fig. 4 displays the XPS spectra of LaMn1ꢀyNiyO3 perovskites. for the y ¼ 0.1 to 0.4 samples: the La/(Mn + Ni) ratios were all
The La 3d spectra included two doublet peaks at 830 eV to less than unity. This suggests the surface of Ni-containing
840 eV and 850 eV to 860 eV, corresponding to La 3d5/2 and La LaMnO3 was enriched by Ni and Mn cations. To provide a clear
3d3/2 signals, respectively. Notably, the responses of La 3d3/2 view of the distribution of La, Mn, and Ni on the surface of
and Ni 2p3/2 overlap each other, and are difficult to be differ- perovskite, Fig. 5 displays measured surface atomic ratios as
entiated. A small hump at approximately 870 eV, which is functions of nominal surface atomic ratios of Mn/La, Ni/La, Ni/
associated with Ni 2p1/2, was identied in the spectrum of Mn, and Ni/(Mn + Ni). Apparently, the surface became Mn-
LaNiO3 (y ¼ 1). Multiple states of Ni (Ni3+/Ni2+) have been noted enriched for Ni-containing LaMnO3 because their experimental
in LaNiO3-based perovskites.42 The spin-orbit splitting of the La Mn/La values were all greater than nominal ones. The measured
3d level for each sample was close to 16.8 eV;42 moreover, the La Ni/La, Ni/Mn, and Ni/(Mn + Ni) all displayed a descending trend
3d5/2 and La 3d3/2 responses were close to the index peaks of with growing y value. Segregation of NiOx on perovskite's
pure La2O3,43 indicating that La ions are in a trivalent state. Mn surface at y ¼ 0.25 and above is a possible explanation. This is
2p3/2 and Mn 2p1/2 signals were located at approximately because segregated NiOx particles contained Ni cations in the
641.7 eV and 653.3 eV, respectively. Mn2+, Mn3+, and Mn4+ could bulk that cannot be detected by XPS, thereby lowering the
coexist in the Mn 2p3/2 region;44 however, the lack of a satellite surface atomic ratio for Ni with respect to La or Mn. The y ¼ 0.1
peak of Mn2+ (648.8 eV)45 eliminates the possibility of the sample had all its experimental Ni/La, Ni/Mn, and Ni/(Mn + Ni)
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carbon (284.5 eV),49 lanthanum carbonate (ꢂ286.2 eV),50 and
carbonyl group (ꢂ288.6 eV).49,51 This is in agreement with the
literature that lanthanum-based perovskites are oen contam-
inated by carbonate species.
Fig. 6 presents the TPR proles of tested perovskites and
Table S1 (ESI†) lists the amounts of H2 consumed by them. The
TPR patterns of pure LaMnO3 and LaNiO3 included multiple
peaks. The reduction of LaMnO3 is complicated: Mn4+ and Mn3+
can coexist at the outset of TPR, and a full reduction of Mn ions
to metallic Mn is not possible.52,53 Therefore, the low-tempera-
ture responses (<400 ꢁC) are attributed to the reduction of Mn4+
to Mn3+; the high-temperature signals (>400 ꢁC) are referred to
the reduction of Mn3+ to Mn2+. Some reduction of Mn3+ to Mn2+
at low temperatures cannot be ruled out.54 The reduction of
LaNiO3 was similar to that in an earlier study:55 the reduction of
Ni3+ to Ni2+ (i.e., La2Ni2O5) at 347 ꢁC was followed by that of Ni2+
to metallic Ni at 469 ꢁC. The XRD patterns of post-TPR LaMnO3
and LaNiO3 (see Fig. S2 of ESI†) further support the afore-
mentioned assertions: the index peaks of La2O3 and MnO were
Fig. 5 Experimental Mn/La, Ni/La, Ni/Mn, and Ni/(Mn + Ni) atomic
ratios versus nominal ones.
ratios greater than nominal ones, indicating the presence of with the reduction of LaMnO3; those of La2O3 and Ni were
highly dispersed Ni on its surface. The Mn4+/Mn3+ molar ratios associated with the reduction of LaNiO3.
were close to 1, decreased followed the order as: y ¼ 0.25 (1.21) >
The rst reduction of LaMn1ꢀyNiyO3 occurred in the range of
y ¼ 0.1 (1.10) > y ¼ 0.4 (1.06) > y ¼ 0 (1.03). This is in accordance 250 ꢁC to 500 ꢁC, in which the LaMnO3 and LaNiO3 phases were
with Zhang et al.,42 who have reported similar values of LaMnO3 reduced. Hence, Mn and Ni ions were reduced concurrently in
at 0.975 and LaMn0.8Ni0.2O3 at 1.216. Apparently, Ni substitu- this range of temperatures. The second reduction of
tion in the B-site position can increase the composition of Mn4+ LaMn1ꢀyNiyO3 may be related to Mn and Ni ions since the
in LaMnO3 perovskites.
amount of H2 consumed in the high temperature range does
Combined signals were obtained from the O 1s state, sug- not increase in proportional to the extent of Ni replacement (see
gesting the presence of different oxygen species on the surface Table S1†).
layer. Lattice oxygen (e.g., La–O–Mn at 529.1 eV),13 (O2)ꢀ species
To provide a comparison, Fig. S3 (ESI†) presents the TPR
or low coordinated oxygen ions located at special site or results for physical mixtures of LaNiO3 and LaMnO3 with
domains of the surface (531.0 eV),47 and hydroxyl/carbonate LaNiO3-to-LaMnO3 ratios of 0.1, 0.25, and 0.4. In Fig. 6, Ni-
groups (532.3 eV)31 are believed to coexist on the surface of incorporated LaMnO3 exhibited two TPR responses at 200 ꢁC to
perovskite. Therefore, the O 1s spectra were deconvoluted based 500 ꢁC and 650 ꢁC to 850 ꢁC. The high-temperature responses of
on these three responses. As the Ni content increased, the BE of LaMn1ꢀyNiyO3 shied to higher temperatures than those of
lattice oxygen shied from 529.2 eV to 528.1 eV. Tabata et al.45 plain LaMnO3 and LaNiO3, especially for the y ¼ 0.1 sample.
observed a similar O 1s chemical shi in LaMn1ꢀyCuyO3 (y ¼ 0, Tailing of reduction response at 800 ꢁC and above could be
0.1, 0.2, 0.3, and 0.4). They proposed that more defects and
oxygen vacancies could be generated as the substitution of Cu
ions at the B-site increased. This effect alters the bond strength
of the surface lattice oxygen. Similarly, replacing Ni in LaMnO3
generates defects and/or segregated NiO particles, which
weakening the lattice oxygen bonds in perovskite. Therefore,
the BEs of lattice oxygen declined as the Ni content increased.
The molar ratio of (O2)ꢀ species or low coordinated oxygen ions
to lattice oxygen (Ox/Olatt) was presented in Table 1. The Ox/Olatt
ratio showed a same trend as that of (Mn4+/Mn3+), declined as: y
¼ 0.25 (0.70) > y ¼ 0.1 (0.66) > y ¼ 0.4 (0.64) > y ¼ 0 (0.63).
Accordingly, the more Mn4+ on the surface, the more (O2)ꢀ
species or low coordinated oxygen ions could exist.42 Plain
LaNiO3 displayed higher Ox/Olatt value (2.00) than the other
samples because La–Ni oxides are prone to form oxygen-de-
cient perovskites, which can be described as a superstructure
48
with a formula LanNinO3nꢀ1
. Hence, less lattice oxygens were
available on LaNiO3 than other perovskites.
The C 1s spectra were presented in Fig. S1 (ESI†). The C 1s
signals of all perovskites showed responses of adventitious Fig. 6 TPR profiles of LaMn1ꢀyNiyO3 perovskites.
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Table
catalysts
2
Oxygen desorption characterization of LaMn1ꢀyNiyO3
observed for the y ¼ 0.1 and 0.25 samples. This suggests an
existence of Ni–O–Mn interaction, which retards the reduction
process. The TPR proles of physically mixed LaNiO3 and
LaMnO3 further support the above-mentioned hypothesis: three
distinct peaks at 331, 466, and 731 ꢁC appear to be the merged
responses of LaNiO3 and LaMnO3. Notably, LaMn0.9N0.1O3
exhibited the highest reduction signal at 783 ꢁC, indicating that
the interaction within the solid solution of LaMn0.9N0.1O3 is the
strongest among all samples. Conversely, the weakest interac-
tion between Ni and Mn was found in LaMn0.6N0.4O3 since it
had the lowest high-temperature reduction response at 736 ꢁC.
Fig. 7 shows the O2-TPD proles. For clarity, the signal of
LaNiO3 was multiplied by a factor of 0.1. O2-TPD can be used to
estimate the lability of oxygen in tested perovskites. The oꢁutset
of desorption signal increased in the order of y ¼ 0 (82 C) <
O2-TPD
a
b
Catalyst
y ¼ 0
a-O2
b-O2
a-O2/b-O2
T (ꢁC) mmol/g cat. 131
686
69.8 (23.0%) 233.2 (77.0%) 0.30
y ¼ 0.1
T (ꢁC) mmol/g cat. 136
697
72.2 (28.1%) 184.5 (71.9%) 0.39
y ¼ 0.25 T (ꢁC) mmol/g cat. 205
928
68.1 (32.1%) 144.0 (67.9%) 0.47
y ¼ 0.4
y ¼ 1
T (ꢁC) mmol/g cat. 255
1000
24.8 (24.1%) 78.0 (75.9%)
0.32
T (ꢁC) mmol/g cat.
—
948
—
695.9 (100%)
0
a
Maximum temperature of oxygen desorption peak below 500 ꢁC.
Maximum temperature of oxygen desorption peak above 500 ꢁC.
ꢁ
ꢁ
ꢁ
b
y ¼ 0.1 (114 C) < y ¼ 0.25 (132 C) < y ¼ 0.4 (211 C) < y ¼ 1
(677 ꢁC), suggesting that more substituted Ni at B-site resulted
in lower oxygen lability in LaMn1ꢀyNiyO3. Desorbed oxygen is
ꢁ
LaMn3a+Mn O3+d (d ¼ (1 ꢀ a)/2). Divalent and trivalent Ni ions
4+
frequently categorized as a- and b-oxygen, using 500 C as the
1ꢀa
were present in LaNiO3, yielding the formula LaNi2b+Ni O3+l
3+
1ꢀb
demarcation. a-oxygen is adsorbed and near-surface oxygen,
while b-oxygen is lattice oxygen in the perovskite framework.56
Negligible a-oxygen desorption was detected in LaNiO3. Table 2
summarizes the amounts of desorbed oxygen and the temper-
atures of peak desorption of tested perovskites. The desorption
temperatures of a- and b-oxygen increased with Ni content for
LaMn1ꢀyNiyO3. Plain LaNiO3 exhibited a distinct peak at 948 ꢁC.
Ni-containing LaMnO3 perovskites had higher a-oxygen-to-b-
oxygen ratios than pure LaMnO3 and LaNiO3. The a-oxygen-to-
b-oxygen ratio followed the sequence of y ¼ 0.25 (0.47) > y ¼ 0.1
(0.39) > y ¼ 0.4 (0.32) > y ¼ 0 (0.30), in consistent with the trend
of the Ox/Olatt ratio observed by XPS. Among all catalysts,
LaMn0.9Ni0.1O3 released the most a-oxygen (72.2 mmol/g cat.);
LaNiO3 contained the highest b-oxygen (695.9 mmol/g cat.).
Ni substitution negatively affects oxygen lability/oxygen
availability in LaMnO3-based perovskites. This can be explained
by estimating the oxygen content from the charge balance.
According to the XPS results, trivalent and tetravalent Mn ions
could coexist in LaMnO3; therefore, its chemical formula is
(l ¼ ꢀ(1 ꢀ b)/2). Since d is positive and l is negative, LaMnO3
should host excess oxygen while oxygen vacancies are formed in
LaNiO3. Substituting Ni in LaMnO3 should satisfy the charge
balance condition, yielding an oxygen content between d and l.
The only exception is LaNiO3, which released the most oxygen
in O2-TPD. This nding is consistent with the XPS results con-
cerning the inhomogeneity of the surface and unstable Ni3+ in
perovskite, which yields weak lattice oxygen bonding in LaNiO3.
Still, doping Ni has a positive effect on a-oxygen content, which
is optimal at y ¼ 0.1.
Fig. 8 plots ethanol conversions of LaMn1ꢀyNiyO3 as func-
tions of temperature. Each catalyst yields an S-shaped curve
besides LaNiO3, which displays an abrupt increase of ethanol
conversion at 250 ꢁC. At temperatures below 200 ꢁC, all catalysts
converted less than 20% ethanol, except for LaMn0.9Ni0.1O3,
which achieved 21% conversion at 200 ꢁC. The T50 increased in
Fig. 8 Conversion of ethanol as a function of temperature over
Fig. 7 O2-TPD profiles of LaMn1ꢀyNiyO3 perovskites.
5334 | RSC Adv., 2014, 4, 5329–5338
LaMn1ꢀyNiyO3 perovskites.
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the order of LaMn0.9Ni0.1O3 (226 ꢁC) < LaMn0.75Ni0.25O3 (243 ꢁC) abatement because more than 40% acetaldehyde selectivity was
ꢁ
< LaMnO3 (249 ꢁC) < LaMn0.6Ni0.4O3 (255 ꢁC) < LaNiO3 (266 ꢁC); detected at 275 C.
ꢁ
ꢁ
the T95, LaMn0.9Ni0.1O3 (252 C) < LaMn0.75Ni0.25O3 (269 C) <
The oxidation activity and chemistry of alkane and alcohol
ꢁ
ꢁ
ꢁ
LaMnO3 (288 C) < LaMn0.6Ni0.4O3 (309 C) < LaNiO3 (314 C). are known to be strongly associated with the transport and
This indicates that LaMn0.9Ni0.1O3 is the most active catalyst in storage of oxygen.23,58 The above catalytic outcomes indicate
ethanol combustion while LaNiO3 is the least active. Ling et al.57 that adsorbed and near-surface oxygen (a-oxygen) are more
have observed a sudden increase of conversion at 250 ꢁC for effective than lattice oxygen (b-oxygen) in ethanol oxidation.
LaNiO3 in ethanol oxidation. By a conductivity test, surface Hence, the a-oxygen-lean LaNiO3 was the least active catalyst
reaction of chemisorbed ethanol and oxygen species was whereas the activities of a-oxygen-rich LaMn0.9Ni0.1O3 and
discovered to be promoted at 250 ꢁC. This leads to an increase of LaMn0.75Ni0.25O3 were enhanced. The high onset temperature
LaNiO3 conductivity due to oxygen consumption, and ethanol of O2-TPD and low concentration of a-oxygen of LaMn0.6Ni0.4O3
conversion can be greatly improved on partially reduced reected low reactivity, implying the critical role of a-oxygen in
surface. They also pointed out that the maximum conductivity ethanol oxidation. a-oxygen is known to act as an activated
change of LaNiO3 occurs in the range of 250 to 350 ꢁC, in oxygen species, which initiates the oxidation of chemisorbed
accordance with our conversion trend.
intermediates into carbon oxides and water.59 The role of
Fig. 9 shows acetaldehyde and carbon dioxide selectivities as a-oxygen in ethanol oxidation is as proposed elsewhere.17,21 To
functions of temperature. Negligible amounts of CO and CH4, provide a better comparison, Fig. S4 (ESI†) displays the varia-
both less than 1%, were detected in each catalyst, and are tions of XPS-derived Ni loading, amounts of a- and b-oxygen,
ignored for clarity. At temperatures below 225 ꢁC, the selectivity and T50 and T95 of tested LaMn1ꢀyNiyO3.
of acetaldehyde exceeded 80% while that of CO2 was less than
LaMn0.9Ni0.1O3 possesses the highest activity among all
20%. With increasing temperature, the selectivity of acetalde- catalysts, even greater than that of LaMn0.75Ni0.25O3, which has
ꢁ
hyde declined and that of CO2 increased. At 235 C, LaMn0.9- the highest Ox/Olatt ratio. This implies that besides a-oxygen
Ni0.1O3 generated about 38% acetaldehyde and 62% CO2. The effect, other factors should be considered for catalyst's efficacy
other catalysts produced more than 80% acetaldehyde and less in ethanol oxidation. Presumably, at a lower Ni loading, Ni can
than 20% CO2 at this temperature. At 250 ꢁC, LaMn0.75Ni0.25O3 be distributed more evenly in perovskite, generating more
yielded equal amounts of acetaldehyde (30%) and CO2 (70%) to Ni–O–Mn linkages on the surface. Increasing the Ni loading in
those of LaMn0.9Ni0.1O3. The remaining catalysts at 260 ꢁC and LaMnO3 produces segregated NiOx clusters on the surface of
above yielded less than 40% acetaldehyde and over 60% CO2. perovskite, reducing the amounts of Ni–O–Mn bonds. The
LaNiO3 seems to be the least effective in acetaldehyde bridging oxygen of Ni–O–Mn should possess an unique inter-
action, which is effective in promoting ethanol oxidation and
intermediate conversion to carbon dioxide. Similar bridging
sites have been noted on different perovskites (e.g., Ni2+–O–Ni3+
species of La2ꢀxSrxNiO4,60 Mn3+–O–Mn4+ bonds of
LaMn1ꢀyCuyO3,52 and Fe3+–O–Fe5+ pairs of SrFeO361). These sites
have been proposed to be effective in promoting charge transfer
and oxygen mobility of the catalysts in N2O decomposition60
and CH4 combustion.52,61
Fig. 10 displays the proposed reaction pathway of ethanol
conversion over LaMn1ꢀyNiyO3. The strongly basic O–La–O sites
are known to be active in ethanol activation;21 however, the
pathway for the possibility of ethanol chemisorption on La is
excluded when considering only the effect of B-site substitution
on LaMnO3. At the outset of the reaction, ethanol chemisorbs
on the surface, and forms an ethoxide at the metal site (Mn or
Ni) and a hydroxyl at the neighboring lattice oxygen site.
Subsequent dehydration yields an acetaldehyde-derived inter-
mediate and an oxygen vacancy. At low temperatures, the
reduced surface is re-oxidized, and acetaldehyde is desorbed. At
high temperatures, the acetaldehyde intermediate may asso-
ciate with the surface bridging oxygen (Mn–O–Mn, Ni–O–Mn, or
Ni–O–Ni) to form an acetyl, which is then converted to carbon
oxides, water, and methane.62 Ethanol dehydrogenation to
acetaldehyde was excluded in Fig. 10 because negligible
conversion (less than 5%) of ethanol was identied without
feeding O2 in our system.
The catalytic results show that acetaldehyde selectivities are
nearly identical (above 80%) at temperatures below 225 ꢁC.
Fig. 9 Acetaldehyde and carbon dioxide selectivities as functions of
temperature over LaMn1ꢀyNiyO3 perovskites.
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Fig. 10 Proposed reaction pathway of ethanol oxidation.
Fig. 11 Conversion of ethanol as a function of time on-stream over
LaMn1ꢀyNiyO3 perovskites at 300 ꢁC.
Acetaldehyde is known to be derived through a redox cycle in
ethanol oxidation.21,63 Therefore, the redox properties for
LaMnO3, LaNiO3, and LaMn1ꢀyNiyO3 perovskites should be
similar. Carbon dioxide selectivities of these catalysts increase
differently with increasing temperature, implying changing
oxidation power in converting the chemisorbed intermediate
(herein acetyl species). In the proposed pathway, acetyl oxida-
tion proceeds on the bridging oxygen in Mn–O–Mn, Ni–O–Mn,
or Ni–O–Ni. The amount of CO2 increased abruptly over
LaMn0.9Ni0.1O3 at the lowest temperature (235 ꢁC) of any of the
tested perovskites, suggesting that its bridging oxygen was the
most strongly oxidizing. This is attributed to the unique Ni–O–
Mn interaction of LaMn0.9Ni0.1O3, as revealed by the TPR test.
To ascertain whether the Ni–O–Mn interaction in perovskite is
uniquely responsible for its reactivity and acetaldehyde oxida-
tion, catalytic tests of physically mixed LaNiO3 and LaMnO3 in
LaNiO3-to-LaMnO3 ratios of 0.1, 0.25, and 0.4 were conducted
(see Fig. S5 and S6 of ESI†). The T50 values of the physically
mixed samples were in the range of 245 ꢁC to 254 ꢁC – about 20
ꢁC higher than that of LaMn0.9Ni0.1O3 but close to those of
LaMn0.75Ni0.25O3 and LaMn0.6Ni0.4O3. However, the T95 values
of the physically mixed samples exceeded their respective
counterparts, ranging from 300 to 321 ꢁC. For all samples, below
250 ꢁC, acetaldehyde selectivity exceeded 80% and CO2 selec-
tivity was less than 20%. These results demonstrate that the
Ni–O–Mn interaction in Ni-doped LaMnO3 promotes the
conversion of ethanol and the oxidation of acetaldehyde to CO2.
Fig. 11 illustrates the evolution of time on-stream ethanol
could be observed within 20 h for other catalysts. Apparently,
LaMn0.9Ni0.1O3 has the highest thermal stability while LaNiO3
has the least among tested catalysts. Since coking is less likely to
occur under oxygen-rich environments, agglomeration of cata-
lyst particle and/or deconstruction of perovskite structure
should be responsible for catalyst deactivation.64 The SEM
micrographs of freshly prepared and used LaMn1ꢀyNiyO3 aer
on-stream testing (see Fig. S7 of ESI†) further supported this
claim: some welding and particle agglomeration could be
identied on used samples; however, no carbon deposition (e.g.,
carbon nanotubes) was specied. The high thermal stability of
LaMn0.9Ni0.1O3 may, again, ascribe to the unique interaction
between Ni and Mn, thereby suppressing the destruction of its
perovskite framework.
Conclusions
Doping Ni into LaMnO3 perovskite can substantially alter its
physicochemical properties, and thereby changing its catalytic
performances. A limited amount of Ni (less than 25%) can be
dispersed in LaMnO3 framework; adding more Ni forms
segregated NiOx clusters. Incorporating Ni generates Ni–O–Mn
sites, which are associated with a unique interaction, is
assumed to play a key role in enhancing catalytic activity,
acetaldehyde abatement, and stability in ethanol oxidation. A
plausible mechanism based on the surface bridging oxygen site
was thereby proposed. Future study should seek to clarify the
nature of the bridging oxygen site, possibly through computa-
tional chemistry such as density functional theory,65 to provide
a molecular-level understanding of perovskite catalyst design.
ꢁ
conversion over a 36 h period at 300 C. Initially, ethanol was
fully converted over LaMnO3, LaMn0.9Ni0.1O3, and LaMn0.75-
Ni0.25O3, while 93.7% and 91.9% conversions were achieved by
LaMn0.6Ni0.4O3 and LaNiO3, respectively. All catalysts suffered
of ageing but at different extents. Ethanol conversion decreased
from 100% to 96.2% for LaMnO3 (a decrease of ꢂ3.8%), 100% to
98.0% for LaMn0.9Ni0.1O3 (a decrease of ꢂ2.0%), 100% to 97.5%
Acknowledgements
for LaMn0.75Ni0.25O3 (a decrease of ꢂ2.5%), 93.7% to 89.7% for The authors acknowledge the National Science Council (Tai-
LaMn0.6Ni0.4O3 (a decrease of ꢂ4.4%), and 91.9% to 78.6% for wan) for the nancial support under Contract Number 102-
LaNiO3 (a decrease of ꢂ14.5%). Moreover, LaMn0.9Ni0.1O3 2221-E-155-060-MY2. The authors also thank Oscar Kerkenaar
remained its activity for more than 25 h, whereas deactivation for the English language editing.
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