Catalytic oxidation of soot over alkaline niobates

Article abstract of DOI:10.1016/j.jallcom.2012.10.015

The lack of studies in the current literature about the assessment of alkaline niobates as catalysts for soot oxidation has motivated this research. In this study, the synthesis, characterization and assessment of alkaline metal niobates as catalysts for soot combustion are reported. The solids MNbO ₃ (M = Li, Na, K, Rb) are synthesized by a citrate method, calcined at 450 °C, 550 °C, 650 °C, 750 °C, and characterized by AAS, N₂ adsorption, XRD, O₂-TPD, FTIR and SEM. All the alkaline niobates show catalytic activity for soot combustion, and the activity depends basically on the nature of the alkaline metal and the calcination temperature. The highest catalytic activity, expressed as the temperature at which combustion of carbon black occurs at the maximum rate, is shown by KNbO₃ calcined at 650 °C. At this calcination temperature, the catalytic activity follows an order dependent on the atomic number, namely: KNbO₃ > NaNbO₃ > LiNbO₃. The RbNbO₃ solid do not follow this trend presumably due to the perovskite structure was not reached. The highest catalytic activity shown by of KNbO₃, despite the lower apparent activation energy of NaNbO₃, stress the importance of the metal nature and suggests the hypothesis that K⁺ ions are the active sites for soot combustion. It must be pointed out that alkaline niobate subjected to consecutive soot combustion cycles does not show deactivation by metal loss, due to the stabilization of the alkaline metal inside the perovskite structure.

Full text of DOI:10.1016/j.jallcom.2012.10.015

Journal of Alloys and Compounds 551 (2013) 255–261
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
Catalytic oxidation of soot over alkaline niobates
a,
⇑
a
a
a
b
b
G. Pecchi , B. Cabrera , A. Buljan , E.J. Delgado , A.L. Gordon , R. Jimenez
a
Department of Physical Chemistry, Faculty of Chemical Sciences, Universidad de Concepcion, Concepcion, Chile
Department of Chemical Engineering, Faculty of Engineering, Universidad of Concepción, Concepción, Chile
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 23 July 2012
Received in revised form 24 September 2012
Accepted 4 October 2012
Available online 22 October 2012
The lack of studies in the current literature about the assessment of alkaline niobates as catalysts for soot
oxidation has motivated this research. In this study, the synthesis, characterization and assessment of
alkaline metal niobates as catalysts for soot combustion are reported. The solids MNbO
Rb) are synthesized by a citrate method, calcined at 450 °C, 550 °C, 650 °C, 750 °C, and characterized
by AAS, N adsorption, XRD, O -TPD, FTIR and SEM. All the alkaline niobates show catalytic activity for
3
(M = Li, Na, K,
2
2
soot combustion, and the activity depends basically on the nature of the alkaline metal and the calcina-
tion temperature. The highest catalytic activity, expressed as the temperature at which combustion of
Keywords:
Soot
Combustion
Alkaline niobates
Perovskite
carbon black occurs at the maximum rate, is shown by KNbO
perature, the catalytic activity follows an order dependent on the atomic number, namely: KNbO
bO > LiNbO . The RbNbO solid do not follow this trend presumably due to the perovskite structure was
not reached. The highest catalytic activity shown by of KNbO , despite the lower apparent activation
energy of NaNbO , stress the importance of the metal nature and suggests the hypothesis that K ions
3
calcined at 650 °C. At this calcination tem-
3
> NaN-
3
3
3
3
+
3
are the active sites for soot combustion. It must be pointed out that alkaline niobate subjected to consec-
utive soot combustion cycles does not show deactivation by metal loss, due to the stabilization of the
alkaline metal inside the perovskite structure.
Ó 2012 Elsevier B.V. All rights reserved.
1
. Introduction
oxidation mechanism via the melting process of the alkaline metal
[
5] has also been reported. The volatile nature of the alkali species,
Diesel exhaust gas is a major responsible for the air pollution
increases the activity of these materials, and is also responsible of
the loss of the active element during the catalytic process. It has
been reported that potassium impregnated catalyst degraded after
repeated thermal cycles due to the loss of potassium [6]. The addi-
tion of transition elements can inhibit this loss [7].
Among the traditional catalytic systems reported for soot com-
bustion such as noble metals [8], oxides and mixed oxide [9], alkali
based materials [10], recently perovskite-type mixed oxides has re-
ceived considerable attention for this purpose [11,12]. Perovskite-
with harmful hydrocarbons and particulate matter. The current
legislative restrictions for emission levels has stimulated the devel-
opment of new catalysts and technologies for the abatement of
soot particulates, a fine black to brown powder formed through
the incomplete combustion of diesel. Soot is in the category of air-
borne particulate matter, considered hazardous to the lungs and
health.
One of the solutions to avoid soot emissions is to trap in a filter
and then burn it. The temperature of exhaust gases of typical diesel
vehicles range between 150 °C and 400 °C, while the carbon com-
bustion occurs at 500–600 °C, therefore, the catalysts for soot com-
bustion must show high thermal stability to preserve their
catalytic activity.
The catalysts used for combustion and gasification of carbona-
ceous matter should have high mobility of surface species to
improve the contact between the catalyst and the soot. Alkali met-
als have been successfully used [1–3] due to the volatile nature of
their oxides. Also alkaline-earth metal oxide has been successfully
used [4]. The positive effect of nitrate in the low temperature soot
3
type oxides materials represented by the general formula ABO ,
where the A-site metal is usually a rare earth metal, and the B-site
metal is a first row transition metal. Due to the number of possible
combinations of metals for the A and B-sites, perovskites can be
tailored to create a wide family of catalysts, by varying either the
A-site and/or the B-site metal ion. Their well known thermal stabil-
ity in a broad range of oxygen partial pressures and resistance to
catalytic poisons has motivated their use as catalysts in catalytic
oxidation abatement. [13,14].
It has been reported that the activity of alkali based catalyst de-
pends on the atomic number of the metal [15,16], and that the pro-
moted effect of alkali metal in soot combustion catalyst increases
with the atomic number, i.e. K > Na > Li [9]. In light of the above
empirical evidence, we hypothesize that combining the advantages
of the perovskite-type structure and the alkaline metal properties,
⇑
Corresponding author. Tel.: +56 41 2204324; fax: +56 41 2245974.
E-mail address: gpecchi@udec.cl (G. Pecchi).
0
925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jallcom.2012.10.015

2
56
G. Pecchi et al. / Journal of Alloys and Compounds 551 (2013) 255–261
it would possible to obtain active catalysts for soot combustion. For
alkaline metals, the electroneutrality balance to meet the perov-
skite-type structure is satisfied with metals with stable oxidation
state of +5, such as niobium. Alkaline niobates, represented by
experiments of the solids, shown in Fig. 1 indicate a similar weight
loss for all the samples, with evolved masses corresponding mainly
to H
Traces of CO lower than 2% were also detected at the highest
evolved temperatures of CO . In Fig. 2 the DTA experiments show
2 2
O and CO , sign of the total combustion of the organic matter.
the MNbO
3
formula (M = Li, Na, K, Rb) as catalysts for soot oxida-
2
tion has not been reported so far.
a first weight loss of ꢁ20% corresponding to two sharp endother-
mic peaks below 200 °C, attributed to the loss of free and bound
water from the gel [20]. A second weight loss of about 60% ob-
served at 270 °C is attributed to the exothermic decomposition of
To a better enlighten the different reactivity of the alkaline met-
als and to identify the key-parameters involved in the catalytic oxi-
dation of soot on alkaline niobates with the perovskite structure, in
the present article Li, Na, K and Rb niobates has been prepared,
characterized and evaluated in the soot combustion. To assess
the role of the perovskite structure on the performance and ther-
mal stability, consecutive catalytic soot combustion cycles were
also evaluated.
2 2
the metal carboxylate gels and liberation of CO and H O; finally,
the weight loss between 400 °C and 500 °C is related to the decom-
position of residual organic groups [21]. The main differences
among the DTA profiles are the two exothermic peaks at 330 °C
and 500 °C for lithium niobate, while three exothermic peaks are
observed at ꢁ330 °C, ꢁ450 °C and ꢁ520 °C for the other niobates.
2 2
The evolution of CO and H O during decomposition is a confirma-
2
. Experimental
tion of the organic matter elimination. No additional weight losses
at temperatures above 800 °C, indicative of crystallization of these
compounds [22], also confirmed by XRD.
2
.1. Preparation
LiNbO
od using NbCl
ported by Wang et al. [17]. A molar amount of the alkaline acetate was dissolved in
distilled water at room temperature, and the stoichiometric amount of NbCl was
dissolved in H under constant stirring. Both solutions were mixed up and citric
acid was added under constant stirring. A molar ratio of citric acid to metal cations
of 3:1 was used. The pale-yellow solutions were heated up to 80 °C until the gels
were formed. The viscous gels were dried at 60 °C for 15 h and then thermally trea-
ted in air in an electric furnace at 450 °C, 550 °C, 650 °C or 750 °C for 10 h.
3
, NaNbO
3 3 3
, KNbO and RbNbO powders were prepared by a citrate meth-
5
and the corresponding alkaline acetates, following the procedure re-
3.2. Specific area
5
2 2
O
The BET surface areas, reported in Table 1, show a weak depen-
dence on the calcination temperature for T > 450 °C. The larger SBET
areas were obtained for NaNbO at all of the studied calcinations
3
temperatures. The obtained values are in the range of previously
reported results for other lanthanum perovskites-type oxides
2
.2. Characterization
[
23,24]. The high SBET for Rb calcined at 450 °C is attributed to
the amorphous nature of the solid, as it was corroborated by
XRD. The following order of surface areas were obtained:
Differential thermal analysis/thermogravimetry (DTA/TG) experiments were
ꢀ
1
performed at a heating rate of 10 °C min up to 1000 °C in Netzsch 409 PC appara-
tus using an Al crucible in 11% O /He mixture and a constant sample weight of
0 mg was used for all experiments. Specific areas were calculated using the BET
2
O
3
2
4
method from the nitrogen adsorption isotherms at 77 K on a Micromeritics appara-
tus Model ASAP 2010. X-ray powder diffraction (XRD) patterns were obtained with
Ni-filtered CuK
1 2
radiation ( = 1.5418 Å), using a Rigaku diffractometer. O -TPD
experiments were performed in a TPR/TPD 2900 Micromeritics system with a ther-
mal conductivity detector. The samples were exposed to oxygen for 1 h at 600 °C,
followed by cooling to room temperature in the same atmosphere. After switching
the atmosphere to a helium flow, the sample was heated at a constant rate of
ꢀ
1
1
0 °C min and the desorbed gas was monitored with a thermal conductivity detec-
tor. Scanning electron microscopy (SEM) micrographs were obtained in a JEOL model
JSM 6380 LV. FTIR spectra were recorded in a Nicolet Magna-IR 550 instrument
equipped with a quartz sample holder with KBr windows. The niobates were finely
ground and mixed with KBr in an agate mortar, to obtain a niobate-KBr ratio of 1/
1
50. Absorption atomic spectroscopy (AAS) was performed using a Perkin Elmer
instrument model 3100 to determine the bulk alkaline content in the samples.
2.3. Catalytic activity
Fig. 1. TG profiles of alkaline niobates.
The catalytic activity of the niobates for soot combustion was studied by using
carbon black, CB, as a model of soot. The catalyst and carbon black mixture for the
assessment of the catalytic activity was prepared mixing 4 mg of CB and 16 mg of
catalyst in tight contact. The catalytic oxidation of soot was carried out in a thermo-
gravimetric apparatus (Netzch 409 PC) with 7.5 mg of the mixture heated in
ꢀ
1
ꢀ1
1
80 mL min flow of 12% O
2
/He at 10 °C min up to 800 °C. The temperature at
, was taken as the
which combustion occurs at the maximum rate, denoted as T
m
measure of the catalytic activity. The Stationary Point Method [18,19] was used
to calculate the apparent activation energy. Typically, samples of perovskite and
CB mixtures in tight contact were heated at four different heating rates in
ꢀ
1
1
2
80 mL min of 12% O /He in a thermogravimetric apparatus. In order to fully
appreciate the beneficial effect of the perovskite structure on the thermal stability
and catalytic performance of the niobates, four consecutive TPR experiments were
carried with the same catalyst.
3
. Results and discussion
3.1. Thermogravimetric and differential thermal analysis coupled with
mass spectrometry (TG/DTG-MS))
In the TG/DTG-MS experiments, the evolution of 14, 18, 28, 32,
4
4 and 46 amu were followed by mass spectrometry. The TG
Fig. 2. DTA profiles of alkaline niobates.

G. Pecchi et al. / Journal of Alloys and Compounds 551 (2013) 255–261
257
Table 1
stoichiometry RbNbO
at any of the studied temperatures. At
3
BET surface areas as a function of the calcination temperature.
4
50 °C the pattern is characteristic of an amorphous material
and at 550 °C the formation of Rb Nb 17 (JCPDS 33-1122) and
Rb Nb O (JCPDS 38-266) is detected. In order to try to
17ꢃ3H
get the stoichiometry RbNbO the sample was calcined at 850 °C,
Catalyst
S
BET, m2gꢀ1
50 °C
4
6
O
4
6
O
2
4
550 °C
650 °C
750 °C
3
LiNbO
NaNbO
KNbO
RbNbO
3
17
23
12
13
11
18
9
10
14
7
9
11
6
however, no new diffraction lines were detected, only sharper
and higher intensity of diffraction lines are observed as result of
the higher crystallinity of the mixed oxides. The crystallite size
determined according to Scherrer’s formula (d = K/Bcos) where d
is the average size of the particles, assuming particles are spherical,
3
3
3
8
8
7
K = 0.9,
c is the wavelength of X-ray radiation, B is the full width at
NaNbO
3
> LiNbO
3
> KNbO
3
3
ꢂ RbNbO , in agreement with results
half maximum of the diffracted peak and h is the angle of diffrac-
tion. The 2 values of 42.5° for Li and 58° for Na and K were chosen.
Table 2 shows larger grains and more crystalline phases when the
calcination temperature is increased, as expected [13].
reported in literature [25,26].
3.3. X-ray diffraction (XRD)
Since BET surface area and crystalline degree are properties that
vary inversely, it is expected that the lower SBET values should be
obtained in those perovskites with higher crystallite size.
A recent study on lanthanoid perovskites reported a mechanism
in which the soot particles is attacked by surface oxygen’s species
which migrate from the perovskite structure [12], finding which
stress the importance of this type of structure.
Fig. 3 shows the XRD patterns of the samples calcined at several
temperatures. For LiNbO the intensity and position of the pattern
is consistent with a rhombohedrical structure with hexagonal axis
JCPDS 20-631) and non perovskite-type structure is reported [27].
This crystalline phase, become more crystalline with calcination
temperature. For NaNbO the diffraction pattern is consistent with
an orthorhombic perovskite-type structure (JCPDS 82-606) [28].
The rough background of the diffractograms for samples calcined
at 450 °C and 550 °C indicates the presence of some impurities,
not detected at higher temperatures. The lowest calcination tem-
3
(
3
3.4. Fourier-transform IR spectroscopy (FTIR)
perature at which the stoichiometry KNbO
50 °C, although some amorphous phase is also found at this tem-
perature. Increasing the calcination temperature, the KNbO be-
came more crystalline, reaching the stoichiometry KNbO
3
can be obtained was
In this study, the FT-IR spectra were used as a complementary
technique, to identify the nature of the segregated species and to
verify the formation of the perovskite structure. Fig. 4 displays
5
3
ꢀ1
3
the region between 4000 and 500 cm of the samples calcined
orthorhombic structure (JCPDS 32-822) [29] at 650 °C. Further in-
crease of calcination temperature up to 750 °C leads to the a mix-
at 650 °C, a representative calcinations temperature. It can be ob-
ꢀ1
served an intense band at ꢁ600 cm , corresponding to an asym-
ture of KNbO
3
with complex oxides K
2
Nb16
O41 (JCPDS 14-287) and
metric enlargement of the B–O bond of the octahedral BO
structures [30]. The bands at 1620 and 3500 cm are associated
6
ꢀ1
K
3
Nb 19 (JCPDS 38-1499). For Rb, it was not possible to obtain the
7
O
Fig. 3. X-ray diffraction patterns of alkaline niobates: (a) Li, (b) Na, (c) K and (d) Rb.

2
58
G. Pecchi et al. / Journal of Alloys and Compounds 551 (2013) 255–261
Table 2
Crystallite size by XRD and particle size by SEM as a function of the calcination
temperature.
Catalyst
d, nm XRD
50 °C
d, nm SEM
650 °C
5
650 °C
750 °C
750 °C
LiNbO
NaNbO
3
33
15
15
30
40
18
17
33
48
26
26
60
138
83
75
150
103
101
200
3
KNbO
RbNbO
3
3
136
Fig. 5. Oxygen desorption profiles of alkaline niobates calcined at 650 °C.
Fig. 4. FTIR spectra of alkaline niobates calcined at 650 °C.
with the bending and stretching vibration modes of the adsorbed
water [30]. The cleanest surface structure is obtained for LiNbO
Fig. 6. Desorbed oxygen as a function of the calcination temperature. Li (e); Na
(h); K (4); Rb (s).
3
,
2
ꢀ
and the larger extent of segregated phases, represented as CO
3
groups in the solid, corresponds to the mixed oxides of rubidium.
This behaviour was also obtained at 750 °C, 550 °C and 450 °C, in
line with XRD results.
3.6. Scanning electron microscopy (SEM)
The particle size and morphology of the alkaline niobates upon
calcination temperature were examined by scanning electron
microscopy (SEM). The importance of the calcination temperature
during the synthesis lies on the decomposition/combustion pro-
cess of the reacting precursors, which generate large amounts of
gaseous products determining in this way the solid morphology.
The formation of rough interfaces on the catalysts powder agglom-
erates will intensify the contact conditions between the catalysts
and the soot, increasing the catalytic activity of the solid. Perovsk-
ites-type oxides, having a size of the same order of magnitude of
the soot particulate, increases the specific contact number between
the soot capturated in the surface and the crystal constituting the
layer [32]. The micrographs of Li, Na, K and Rb niobates, at the
studied calcination temperatures, show homogeneous samples
without large agglomeration areas. Cubic particles are mainly ob-
served, even though the increasing presence of some spongy parti-
cles are detected at lower calcinations temperatures. These spongy
particles could be related with residual amorphous impurities
resulting from the decomposition of the organic matter and
responsible of the rough background in the XRD pattern [22].
3
.5. Oxygen-temperature programmed desorption (O
2
-TPD)
Fig. 5 displays the profiles for the samples calcined at 650 °C, a
representative calcination temperature. The large desorption peaks
located at ꢁ60 °C, ꢁ50 °C, ꢁ50 °C and ꢁ70 °C, for Li, Na, K and Rb,
respectively, are attributed to weakly physisorbed oxygen species.
The amount of oxygen desorbed for each niobate up to 200 °C cal-
culated from TPD profiles has been plotted as a function of the cal-
cination temperature and the results are displayed in Fig. 6. For
sodium and potassium niobates the amount of oxygen desorbed
reaches a maximum at about 550 °C, being that corresponding to
3 3 3
NaNbO the largest one. Considering that only KNbO and NaNbO
formed the perovskite structure [31], the importance of this type of
structure in the extent of desorbed oxygen is undoubted. On the
other hand, having in mind the importance of physisorbed oxygens
in combustion reaction, we can conjecture these niobates should
show higher catalytic activity in soot combustion than those corre-
sponding to lithium and rubidium.

G. Pecchi et al. / Journal of Alloys and Compounds 551 (2013) 255–261
259
Fig. 7. SEM images of alkaline niobates calcined at 650 °C.
Fig. 7 shows the obtained micrographs for the solids calcined at
50 °C. It can be observed that KNbO shows the well defined
6
3
shape. The mean particle sizes, evaluated from 5 micrographs
and 150 particles counted, are displayed in Table 2. As expected,
the grain sizes determined by XRD peak broadening and that from
SEM are different but follows the same trend. These differences are
attributed that SEM includes the grain boundary region, where
Scherer’s formula gives the crystallite size (i.e. length of coherence)
in a direction perpendicular to the incident X-ray beam. Moreover,
the size and shape of perovskites are of great importance due to
their strong structure-dependent physical properties [33]. The par-
3 3
ticle sizes of the NaNbO and KNbO perovskites are between 70
and 100 nm, the same order of magnitude as that of soot, and could
satisfy the requirement ensuring high contacting efficiency be-
tween the two counterparts and explain the higher catalytic activ-
ity of these two catalyst.
3.7. Catalytic activity
Fig. 8 shows the experimental curves obtained on the catalytic
oxidation of CB for the Li, Na and K niobates calcined at 650 °C. It
was also included the curve of the non catalized CB combustion,
that occurs at 657 °C. The curve for Rb niobate is not included be-
cause it shows additional weight loss due to non thermal stability.
Fig. 8. DTGA profiles for the catalytic combustion of carbon black by alkaline
niobates calcined at 650 °C: Li (e); Na (h); K (4); Rb (ꢄ).
It can be observed that KNbO
lytic effect. Table 3 relates the values of T
temperature and the nature of alkaline metal. It can be concluded
that KNbO is the most active catalyst at any calcination tempera-
3
and NaNbO
3
show the higher cata-
Table 3
m
Apparent activation energy and temperature of maximum rate of combustion, T , as a
function of the calcination temperature.
m
with the calcination
E )
act (kJ mol 1
ꢀ
Catalyst
T
m
3
tures, except for the sample calcined at 450 °C, at which the perov-
450 °C
550 °C
650 °C
750 °C
skite structure is not reached. At the calcination temperatures of
LiNbO
NaNbO
3
547
591
562
564
610
641
586
630
636
523
460
612
570
539
504
561
104.2
60.9
85.5
–
6
50 °C and 750 °C the catalytic activity follows the order:
3
KNbO > NaNbO > LiNbO , i.e. the catalytic activity for CB combus-
3
3
3
KNbO
RbNbO
3
tion increases with the alkaline atomic number, in agreement with
results published elsewhere [9,16,30,34]. In order to obtain a better
correlation of the catalytic activity with the structural behaviour of
these materials, the values of the apparent activation energy are
also reported in Table 3. The obtained values in this study are much
lower than those reported for lanthanum perovskites-types oxides
3
Table 4
Temperature of maximum rate of combustion, T
m
, for diverse perovskite-type oxides.
References
Catalyst
NaNbO
KNbO
m
T , °C
[
11,34] suggesting the potential use of alkaline niobates as cata-
3
523
460
455
520
520
500
This study
This study
[2]
[34]
[34]
lysts for soot combustion. It is highly recognized the superiority
of potassium containing solids as catalysts in soot combustion,
3
LaCoO
LaMnO
LaFeO3
3
therefore, the lowest apparent activation energy for NaNbO
3
3
perovskite is not an expected result. However, this finding seems
to support the so-called compensatory effect stating that the high-
er reaction rate is due to an increase in the number of active sites
which in turn it is reflected in a larger pre-exponential factor [35].
This result suggests that the active sites for soot combustion are
LaCrO
3
[34]
+
directly related to K ions.
Table 4 their respective T
m
values for soot combustion. It is ob-
It has been reported significant differences in the mechanism
for catalytic soot combustion, from solid–solid reaction to struc-
tural and textural differences, evidencing the importance of the
contact conditions between soot and catalytic active sites for these
kind of reactions [36]. In order to compare the catalytic activity of
the niobates with reported lanthanoide perovskites, we show in
served that KNbO is one of the most active.
3
However, the applicability of a solid as catalyst depends not
only on the catalytic activity but on its thermal stability as well.
It is well known that potassium impregnated catalysts suffer
deactivation due to metal loss by volatilization during the process.
In order to assess the stability of the alkaline niobates, several

2
60
G. Pecchi et al. / Journal of Alloys and Compounds 551 (2013) 255–261
quently we conclude that no significant loss of K from the perov-
skite structure occurred. Table 5 summarizes the alkaline content
wt%) for nominal, fresh and used, after four combustion cycles,
(
samples. No significant decrease in the Na or K content are ob-
served, result that confirms the thermal stabilization of the alka-
line metal in the perovskite-type structure, in agreement with
other studies reported in literature [6].
4
. Conclusions
All the alkaline niobates show catalytic activity for soot com-
bustion. The observed catalytic activity depends mainly on the nat-
ure of the alkaline metal and the calcination temperature. At the
calcination temperature of 650 °C the catalytic activity follows an
atomic number dependent order, namely: KNbO
bO . The highest catalytic activity, expressed as the temperature
at which combustion of carbon black occurs at the maximum rate,
, is shown by KNbO calcined at 650 °C, despite the lower appar-
ent activation energy observed for NaNbO . This somewhat sur-
3 3
> NaNbO > LiN-
Fig. 9. Temperature of maximum velocity, T
cycles for the samples calcined at 650 °C: Li (e); Na (h); K (4); Rb (s).
m
vs consecutive soot combustion
3
T
m
3
3
prising result is an evidence of the participation of the so-called
compensatory effect stressing the importance of potassium in the
catalytic soot combustion. Additionally, no deactivation is ob-
served for the alkaline niobates upon consecutive combustion cy-
cles due to the stabilization of the alkaline metal inside the
perovskite structure avoiding volatilization of the metal. This study
opens interesting perspectives to be explored for future niobate
perovskite-type oxides catalyst development. It is worth to inves-
tigate the formation the
a-type weakly chemisorbed oxygen spe-
3
cies in non-stoichiometric of KNbO which should entail the
improvement of the catalytic activity.
Acknowledgements
The authors thank CONICYT (Fondecyt Grant 1090018) for
financial support.
References
Fig. 10. X-ray diffraction patterns of fresh and after four combustion cycles for
NaNbO and KNbO .
3 3
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Alkaline
Alkaline (wt.%)
Nominal
[
[
Fresh
After 4 cycles
NaNbO
KNbO
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Na
K
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21.7
13
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12
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