Research Journal of Recent Sciences ________________________________________________ ISSN 2277 - 2502
Vol. 1(ISC-2011), 152-159 (2012)
Res.J.Recent.Sci.

Carbon monoxide oxidation on LaCoO3 perovskite type catalysts prepared
by reactive grinding
Patel Femina and Patel Sanjay
Department of Chemical Engineering Institute of Technology, Nirma University, Ahmedabad-382481, INDIA

Available online at: www.isca.in
(Received 4th October 2011, revised 9th January 2012, accepted 25th January 2012)

Abstract
Perovskite oxides are used as promising three way catalysts for the removal of exhaust gases because of their low cost, thermal and
mechanical stability at relatively high temperature, great diversity and excellent redox properties The major traditional drawback
of perovskites is the low specific surface area (usually several m2/g) due to their preparation that involves a rather high
temperature (often as high as 8000C) to ensure the formation of the crystalline phase. This suppresses their activity and to some
degree limits their application. A new preparation method called reactive grinding was developed for the synthesis of perovskites at
room temperature via high-energy ball milling resulting in a relatively high surface area. Perovskite type mixed oxides LaCoO3
with high specific surface area was prepared by reactive grinding. These catalysts was characterized by X-ray diffraction (XRD),
Scanning electron microscope (SEM) - Energy dispersive X-ray spectroscopy (EDX or EDS) and BET surface analysis. The
formation of the perovskite structure has been shown by X-ray diffraction (XRD) for all samples. The catalytic performance of the
samples for carbon monoxide was evaluated. LaCoO3 found significantly more active than a reference sample prepared by
conventional synthesis method using amorphous citrate complexes. The activity per unit surface area was found to depend on
grinding conditions and calcinations temperature. These enhanced activities are associated with both rather high surface area and
high defect density reached by the reactive grinding synthesis method.
Keywords: Catalytic converter, perovskite, automotive emission, catalyst, citric acid method.

Introduction
Environmental, ecological and health concern result in
increasing stringent emissions norms of pollutant emission
from vehicle engine1,2. These regulations require the need for
more active and durable emission control system. A number
of alternative technologies like improvement in engine
design, fuel pretreatment, use of alternative fuels, fuel
additives, exhaust treatment or simply better tuning of the
combustion process etc. are being considered to reduce the
emission levels of the engine2.
Catalytic converters have been proved to be effective in
controlling harmful gaseous emissions (Carbon monoxide
(CO) to carbon dioxide (CO 2), hydrocarbons (HC) to CO2
and water vapor (H2O) and nitrogen oxides (NOx) to nitrogen
(N2) and oxygen (O2)) under normal working conditions for a
stochiometric air to fuel ratio 3-6. The technology to convert
simultaneously all three pollutants into innocuous materials
is referred to as three way catalysis or three way catalytic
converters (TWCs)7-10. The important role of automotive
catalysts in catalytic converter contain precious noble metals
palladium (Pd), platinum (Pt) and rhodium (Rh) is widely
recognized for the conversion of three pollutant emissions
such as CO into CO2, NOx to N2 and HC into CO2 and H2O
in engine exhaust gases (Car’s tail pipe)6-13.

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Due to their high activity and thermal stability, much
attention has been paid to perovskite-type oxides, of general
formula ABO3 (where A and B are usually rare earth,
alkaline earth, alkali and other large ions such as Pb +2, Bi+3
that fits in to the dodecahedral site of the framework (A
coordinated by 12 oxygen) and 3d, 4d and 5d transition metal
ions which occupy the octahedral sites (surrounded by six
oxygen atoms in octahedral coordination) respectively) as
catalysts for complete oxidation of CO in substitution of the
very active noble metals which are more expensive,
volatilization at high operating temperature, sublimation, do
not resist to operating temperatures exceeding 850 K,
sintering at high temperature and limited resources of noble
metal13-23.
Due to the great stability of the perovskite framework a large
number of metallic cations can occupy the A and the B sites
provided that the tolerance factor t [t = (rA + rO)/ √ 2(rB +
rO)] is in the range 0.8–1.0. Perovskite compounds can also
tolerate significant partial substitution (A and/or B with
metals (A’, B’ correspondingly) of different oxidation states)
and non-stoichiometry while still maintaining the perovskite
structure. Metal ions having different valence can replace
both A and B ions. This may generate a non-integral number
of oxygen atoms10,19.

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Res. J. Recent Sci.
Several methods are used to synthesis perovskites such as coprecipitation, sol-gel, citrate complexation, micro-emulsion
etc. which involves high temperatures (because of their
preparation method involving a high temperature heating
step as high as 8000C) to ensure the perovskite crystallization
is their poor specific surface areas (usually less than 2 m2/g)
severely limiting their practical application due to low
catalytic activity per unit mass4,8,19-25.
For example, Co-based perovskite catalysts prepared by the
conventional citrate complexation method have a surface
area of only 4–7 m2/g after calcination at 6000C for 6 h26.
The specific surface area observed on perovskites prepared
by these methods rarely exceeds 25 m2/g. A new preparation
method for perovskite can allow to avoid the thermal
treatment and the crystallize to proceed at nearly ambient
temperature. This technique uses high energy ball milling
(reactive grinding) resulting in relatively high surface
area4,18,19,25. Reactive grinding involve the synthesis of
materials by high-energy ball milling e.g. planetary mills,
vibratory mills, attritors and tumbling ball mills in which
elemental blends (or pre-alloyed powders, oxides, nitrides
etc.) are milled to achieve alloys or composite materials27.
The interaction between milling balls and powder particles
can be characterized by process like cold welding, plastics
deformation and further fragmentation of the particles (The
processing involves repeated cold welding, fracturing and
rewelding of powder particles in a high-energy ball mill).
Mechanical impact during the process reduces the precursors
crystallite size to nano scale provides a homogeneous
mixture and enhances the solid state diffusion. A wide
variety of perovskites with a crystallite size down to 10 nm
and a surface area varying between 4 and 100 m2/g have
been successfully synthesized for catalysis applications. The
resulting materials are in the form of porous and highly
agglomerated powders having a large amount of grain
boundaries. Doping of both grain boundaries and bulk
material could be performed during synthesis.
The synthesis is performed in a closed environment without
generating any waste28. Reactive grinding can be done easily
under solvent-free conditions and rapidly produces large
amounts of well-mixed nanocomposites29. Using various
grinding additives, high surface (measured after calcinations
at 473 K) perovskites such as LaCoO3 (>100m2/g), LaGaO3
(98.6 m2/g), LaCoxIn1−xO3 (>110 m2/g) or SrCoO3 (150 m2/g)
were prepared. The Que´bec firm, Nanox Inc. has installed a
demonstration unit of reactive grinding for the production of
perovskite with capacity of 15 t/year30.
In this work, perovskite type oxides (LaCoO3) was prepared
by reactive grinding and citrate method. These structures
were characterized by X-ray diffraction (XRD), Energy
dispersive X-ray spectroscopy (EDX or EDS) and BET

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surface analysis. The activity of the samples was also
evaluated in carbon monoxide oxidation reaction.

Material and Methods
Catalyst preparation Citrate method: The citrate
compound was prepared by complexation of the nitrate salts
with citric acid. A concentrated solution of the metal nitrates
was mixed with an aqueous solution of citric acid by fixing
at unity the molar ratio of citric acid to the metal cations.
Metal nitrates (La(NO3)3.6H2O and Co(NO3)2.6H2O were
first dissolved in distilled water (25 ml). Citric acid (10%
excess over the number of ionic equivalents of cations) was
separately dissolved in distilled water (25 ml) and added to
the precursor solution under vigorous stirring for 15 min.
Excess water was evaporated under slow stirring in oil bath
at 80 0C until a gel was obtained. The viscous gel was then
dried at 100 0C overnight in hot air oven. The obtained
spongy material was finely ground and calcined under air
atmosphere at 750 0C for 5 h.
Reactive grinding: As starting compounds, lanthanum
oxide (La2O3) (99.99% pure) and cobalt oxide Co3O4
(97.49%) of the analytic grade (AR) were used. LaCoO 3
sample was prepared from simple oxides by using
preliminary mechanochemical treatment (MCA) of the
stochiometric mixture of simple oxides with stainless steel
balls in the high energy ball mill i.e. planetary ball mill.
The La2O3 was first calcined at 873 K (600 0C) for 24 h in
order to transform any lanthanum hydroxide to lanthanum
oxide. Thus, 10 gm of prepurified lanthanum oxide and 5 gm
of cobalt oxide were mixed (La/Co atomic ratio equal to 1)
and introduced in a grinding jar of volume 250 cm 3 with 10
balls of 19 mm size, 10 balls of 14 mm size and 8 balls of 10
mm size. The jar and the balls are made with stainless steel
material. The jar was closed with a thick cover and sealed
with O-ring.
The balls and the powder were put inside the grinding jar of
planetary ball mill. On another side of the grinding jar balls
of the weight equal to the weight of the grinding material and
powder in the first grinding jar were added so that the weight
is balanced. Although milling proceeds at room temperature,
the numerous ball shocks within the jar slightly increases its
temperature. Thus the container was fan cooled and its wall
temperature is kept below 313 K. The milling atmosphere in
the jar could be controlled by replacing the seal by a filter
paper ring in order to let some air seep into the jar (oxidizing
atmosphere). In the sealed jar the oxygen in the trapped air is
rapidly consumed. The milling was carried out for 26 h as
per ball mill operating conditions mentioned in table - 1.
Prepared precursor was calcined at 600 0C in air atmosphere
for 5 h in muffle furnace.

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Res. J. Recent Sci.
Table – 1
Ball mill operating conditions

Operating conditions
Material of jar
Stainless steel
Material of ball
Stainless steel
Speed ratio (Sunwheel and jar
1:-2
speed ratio)
jar speed (rpm)
300 (Clock wise)
Sunwheel speed(rpm)
150 (Anti clock wise)
19 mm (10), 14 mm
Ball diameter (mm)
(10), 10 mm (08)
Mass of powder in jar in (gm)
15
Catalyst characterization X-ray Diffraction: Phase
analysis, lattice parameters and particle sizes were
determined by X-ray diffraction (XRD) using PW1774
Spinner Diffractometer system XPERT-MPD operated at 40
kV and 30 mA with Ni-filtered Cu Kα radiation ( = 1.5406
Å). Spectra were recorded with step scans from 2 0 to 990 in
2θ angle and 1 s for each 0.050 step. Lattice parameters were
calculated from the reflections appearing in the 2θ = 2–990
range using the software program. The identification of the
crystal phases took place using the JCPDS (Joint committee
on Powder diffraction standards) data bank.
Particle sizes (D) were calculated by means of the Scherrer
equation D = Kλ/βcos θ after Warren’s correction for
instrumental broadening. K is a constant equal to 0.9, λ the
wavelength of the X-ray used, β the effective line width of
the observed X-ray reflection calculated by the expression β2
= B2 − b2 (where B is the full width at half maximum
(FWHM), b the instrumental broadening determined through
the FWHM of the X-ray reflection at 2θ ≈ 280 of crystalline
SiO2 with particles larger than 1000 Å, θ the diffraction
angle.
Energy dispersive X-ray spectroscopy (EDX or EDS):
Energy Dispersive of X-Ray (EDX) of samples was carried
out in JEOL made instrument JEM2100 model which has
attached detector OXFORD Instrument INCA X-SIGHT
model. EDX was used to investigate the morphology as well
as the elemental composition and distribution of all the
catalyst compositional analysis.
BET surface area: BET Surface area of the materials were
measured by nitrogen adsorption at the liquid nitrogen
temperature (-1960C) using a volumetric all glass apparatus.
The specific surface area of the materials calcined at 600 0C
prepared by reactive grinding and 7500C prepared by citrate
method for 5 h was determined from nitrogen adsorption
isotherms measured at -1960C using a Micromeritics ASAP
2020 instrument. Samples were degassed at 3000C under
vacuum (10-3 Pa) until complete removal of humidity (about
3-4 h) prior to adsorption–desorption experiments. Nitrogen

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adsorption measurements were performed up to a relative
pressure P/P0 = 1. The specific surface area was determined
from the linear part of the BET curve. The pore size
distribution was calculated from the desorption branch of N2
adsorption/desorption isotherms using the Barrett–Joyner–
Halenda (BJH) formula. Pore volume and average diameter
were also obtained from the pore size distribution curves
using the software.
Catalytic activity: The catalytic oxidation tests were carried
out in a stainless steel fixed bed reactor (I.D. 1.805 cm, O.D.
1.905 cm and L.50 cm) equipped with flow controllers and
heating system.
Before the activity tests, the catalyst in the bed was activated
by passing N2 (86 %) and O2 (14 %) at 490 N cm3 min−1 for
2 h at 500 0C below the calcinations temperature to remove
adsorbed moisture and cool with passing N2 at 420 N
cm3min-1 till bed temperature reach to 100 0C.
CO oxidation tests were carried out in a fixed bed of catalyst
particles (ca. 1 g) mixture previously added with 3 g of SiO 2
(0.5–1.5 mm granulate) in order to reduce the specific
pressure drop across the reactor and to prevent thermal
runaways placed between two ceramic blanket wool and
inserted into the reactor. The reactor was placed in a tubular
PID-regulated oven and the temperature was monitored with
a K type thermocouple positioned in correspondence to the
catalyst bed. The gaseous flow rates were measured by
rotameter and mixed at atmospheric pressure to obtain inlet
concentrations of 7.8 % CO, 13 % O2, N2 as balance with a
gas hourly space velocity (GHSV) of 32,000 Ncm3 g−1 h−1 for
reactive grinding and 7.9 % CO, 9.64 % O2, N2 as balance
with a gas hourly space velocity (GHSV) of 29,000 Ncm3
g−1 h−1 for citrate method. Reaction temperature was raised
from 473 to 873 K and product stream was analyzed by gas
chromatography (GC 2010 Model) using Shin Carbon ST
micropacked column and µTCD detector. The CO
conversion in activity tests was defined as:
XCO= (Ff,CO - Fp,CO)/ Ff,CO Where Ff,CO: Molar flow rate of
CO in feed stream, Fp,CO: Molar flow rate of CO in product
stream

Results and Discussion
Catalyst characterization: Figure - 1 illustrates X-ray
diffraction pattern of LaCoO3 prepared by reactive grinding
and citrate method. The comparison of these spectra with
JCPDS charts indicates that LaCoO3 sample prepared by
reactive grinding is essentially perovskite type mixed oxides
(JCPDS card 37-0804). Other phases, such as Co3O4 (JCPDS
card 43-1003) and La(OH)3 (JCPDS card 36-1481) were
detected in addition to the major ABO3 perovskite phase.

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Vol. 1 (ISC-2011), 152-159 (2012)
Res. J. Recent Sci.
The comparison of these spectra with JCPDS charts indicates
that Co based samples prepared by citrate method is
essentially perovskites type mixed oxides (JCPDC card 00025-1060, 00-006-0491). Phase formation is closely related
to the calcination temperature. At 750 0C, the catalyst sample
completely transformed lanthanum and cobalt nitrates into
LaCoO3 perovskite phase. No spinel phase La2CoO4 (that
normally tends to form at higher temperatures) was observed
in these sample.

achieved between the rate of welding which tends to increase
the average particle size and the rate of fracturing which
tends to decrease the average composite particle size.
Smaller particles can withstand deformation without
fracturing and tend to be welded into larger pieces with an
overall tendency to drive both very fine and very large
particles toward an intermediate size. Hence, the perovskite
samples prepared by reactive grinding after 26 h milling has
lower surface areas than the sample prepared by the citrate
method due to cold welding become predominate over
fracturing which forms agglomerates in reactive grinding
after milling of 26 h.

In figure - 1 (a) and (b), the XRD patterns of two LaCoO 3
samples prepared by reactive grinding and citrate method are
compared. They show some differences in the intensity of the
perovskite peaks at the same angles. This, in fact indicates
that citrate method has been more successful in forming
perovskite phase with a higher degree of crystallinity.
The crystallite sizes of prepared perovskites calculated by
Scherrer’s equation after Warren’s correction of instrumental
broadening are also reported in table - 2.

The citrate method has adsorption-desorption isotherms of
the catalysts have shown similar characteristics. BarettJoyner- Halendar (BJH) analysis showed that catalyst pores
were meso size and the average pore sizes were found to be
9.77 nm for citrate method and 16.94 nm for reactive
grinding methods respectively.

Surface area, pore size and pore volume are among the most
fundamentally important properties of a catalyst because they
determine the measure of its internal surface available to
accommodate active sites for high catalytic activity by
providing accessibility of the active sites to reactants and the
extent to which transport of products from the catalyst
surface to the bulk fluid. The results for the nitrogen
isothermal sorption at 77 K for perovskite- type mixed
oxides synthesized by reactive grinding after calcinations at
6000C for 5 h and synthesized by citrate method after
calcinations at 750 0C for 5 h are presented in figure - 2,
respectively. All the isothermal results show hysteresis loops
whose characteristics exhibit dependence on the structure of
the samples that are confirmed to be with a porous
morphology.

According to semi-quantitative EDX results, table - 3 shows
the relative atomic concentrations of La and Co for LaCoO3
prepared by reactive grinding. The values shown in table - 3
were calculated by the EDX software using an averaged
signal from several thousand particles. The compositions of
the catalysts intended and the EDX test results seem to be in
good agreement.
Catalytic oxidation of CO: The catalytic combustion tests
(W/F = 0.124 g/cm3 s-1) were performed with a gas mixture
containing over the catalyst samples prepared by the reactive
grinding and the citrate method as a function of temperature
Figure - 3 shows the effect of temperature on % CO
combustion for the catalyst prepared by reactive grinding and
citrate methods respectively. This figure show that those
sample exhibit large differences in terms of the catalytic
activity with the sample reactive grinding displays the
highest activity for CO conversions higher than 90 %. As
figure - 3 shows, 94 % CO combustion for this catalyst
sample was achieved only at 534 0C. In general, the citrate
sample shows lower CO conversion than the reactive
grinding at the same temperature.

The specific surface area (BET surface area), crystallite size,
pore size and pore volume of the samples synthesized by
reactive grinding after calcinations at 600 0C for 5 h and
synthesized by citrate method after calcinations at 750 0C for
5 h are listed in table - 2.
After milling for a certain length of time in reactive grinding,
steady-state equilibrium is attained when a balance is

Table - 2
Properties of Co-based catalysts
Sr.
NO.

Sample

Calcination T
(0C)

Crystallite Size
(nm)

LaCoO3 600
40-48
RG
LaCoO3 2
750
35-56
CT
Reactive grinding (RG): P+O: LaCoO3, CO3O4, La(OH)3
1

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Phases

Specific Surface
area (m2/g)

Pore diameter
(nm)

Pore volume
(cm3/g)

P+O

6.08

16.94

0.0258

P

8.78

9.77

0.0214

Citrate method (CT): P: LaCoO3

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Intensity (cps)

1000

500

0
20

40

60

80

40

60

80

Intensity (cps)

200
100
0
-100
-200
20

2-theta(deg)

(a)

(b)

Figure - 1
XRD patterns of LaCoO3 (a) prepared by reactive grinding (b) prepared by Citrate method

(a),

(b)

Figure - 2
N2 adsorption-desorption isotherm of LaCoO3 (a) prepared by reactive grinding (b) prepared by citrate method

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Res. J. Recent Sci.

Co

13.07

9.86

3.598

23.98

0.0609 19.98

nanoparticles which yields a looser agglomerates. The
resulting solid exhibits nanoparticle properties and these
agglomerates is the presence of a high density of nanoscale
grain boundaries. As oxygen mobility is usually higher in
grain boundaries, their occurrence is especially important for
the catalysis of redox reactions. Due to this higher oxygen
mobility, LaCOO3 prepared by reactive grinding displays the
highest activity for CO conversions compare to LaCoO 3
prepared by citrate method which has large surface area
compare to reactive grinding.

La
Total

61.61
100

19.73
100

8.476
15

56.50
100

0.0609 19.98
0.3047 100

Conclusion

Table - 3
Sample compositions determined by EDX for LaCoO3
prepared by reactive grinding

Element
O

Weight Atomic Weight Weight
gm Atom
%
%
gm
%
atom ic %
Experimental
Theoretical
25.32
70.40
2.926
19.50 0.1829 60.02

The temperature corresponding to 50% conversion of CO is
defined as the catalyst “light off” temperature and it is an
important parameter in catalytic reactions. The lower the
light off temperature, the more active the catalyst is. The
light off temperature for a catalyst prepared by reactive
grinding is 385 0C for CO oxidation.
The catalytic activity of the perovskite samples chiefly
depends on three factors: chemical composition, degree of
crystallinity and the crystals morphology (including particle
sizes, pore size distribution and specific surface area of the
perovskite catalyst). All these factors are affected by the
synthesis method and the specific synthesis operating
conditions.
100
90
CO conversion %

80

70
60
50
40
30

LaCoO3 Perovskite catalysts were prepared by reactive
grinding and citrate method. XRD tests showed the
existence of LaCoO3, Co3O4 and La(OH)3 phases for reactive
grinding and pure LaCOO3 for citrate method. BET surface
area measures indicated that LaCOO3 catalyst prepared by
citrate method has the largest surface area than prepared by
reactive grinding. Perovskites prepared at room temperature
by the reactive grinding technique yields solids which are
very active in the catalytic oxidation of carbon monoxide.
Compared to classical preparation procedures this increased
activity is due to the presence of a high density of nanoscale
grain boundaries. As oxygen mobility is usually higher in
grain boundaries, their occurrence is especially important for
the catalysis of redox reactions. Moreover, because reactive
grinding does not involve high temperatures, the solids
prepared by this technique have their surfaces essentially
covered with OH and therefore the calcination temperature is
a factor of their catalytic activity. Activity tests of the
catalysts shows that LaCoO3 has 86 % conversion for CO at
492 0C even it has surface area slight lower than citrate
method.

(a)

Acknowledgement

(b)

The work described above was fully supported by a research
grant from the Nirma University.

20
10

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