Mechanism of CO + N2O reaction via transient CO3 2' species over crystalline Fe-substituted Lanthanum titanates

Article abstract of DOI:10.1021/jp102306u

Some newer mechanistic aspects investigated by in situ Fourier transform infrared (FTIR) in conjunction with catalytic activity under similar conditions over crystalline lanthanum titanates as a function of Fe substitution at the B-site for the CO + N₂O reaction are reported for the first time in the present communication. La₂Ti_2(1-x)Fe _2xO_7-?? (0.0 a?￥ x a?￥ 1.0) was synthesized by gel combustion where Fe³⁺ substitution effectively enhanced the conversion rates for N₂O reduction as compared to the pristine La₂Ti₂O₇ (LTOGC). Among all samples, maximum conversion over La₂Ti_0.8Fe_1.2O _7-?? [LF(0.6)GC] catalyst was observed. In situ FTIR results reveal that substitution-induced anionic vacancies/defects provide additional sites on the surface of LF(0.6)GC for CO chemisorptions, whereas a perfect stoichiometric lattice like LTOGC is devoid of such sites. Surface-adsorbed CO reacts with surface lattice oxygen in the case of nonstoichiometric LF(0.6)GC to produce carbonates (M-CO₃^2-) at a much lower temperature. The reaction proceeds via carbonate formation, leaving the catalytic surface oxygen deficient in LF(0.6)GC, and therefore facilitates the reduction of preadsorbed, N₂O [N₂O(g) + * a?? N₂ + *-O) by easily adsorbing the oxygen species (*-O) generated in N₂O reduction, which is subsequently driven away by adsorbed/gas phase CO, whereas in the case of LTOGC, progress of the reaction was sluggish in the absence of transient carbonate species. Dissociative chemisorptions of N₂O are not facilitated on stoichiometric oxygen excess titanate, as there is no vacancy in the surface to accommodate another oxygen atom. The redox mechanism via CO₃^2- species is proposed for CO + N ₂O reaction over La₂Ti_2(1-x)Fe _2xO_7-??, as against the associative mechanism observed in the unsubstituted sample, La₂Ti₂O₇, as suggested by in situ FTIR in conjunction with catalytic activity results. ? 2010 American Chemical Society.

Full text of DOI:10.1021/jp102306u

J. Phys. Chem. B 2010, 114, 6943–6953
6943
2
-
Mechanism of CO + N O Reaction via Transient CO Species over Crystalline
2
3
Fe-Substituted Lanthanum Titanates
Mrinal R. Pai, Atindra M. Banerjee, Krishnan Kartha, Rajesh V. Pai, Valmik S. Kamble,†
†
†
‡,§
⊥
,
†
and Shyamala R. Bharadwaj*
Chemistry DiVision, Fuel Chemistry DiVision, Bhabha Atomic Research Centre, Mumbai 400085, India, and
Department of Applied Chemistry, Cochin UniVersity of Science and Technology, Cochin 682022, India
ReceiVed: NoVember 13, 2009
Some newer mechanistic aspects investigated by in situ Fourier transform infrared (FTIR) in conjunction
with catalytic activity under similar conditions over crystalline lanthanum titanates as a function of Fe
substitution at the B-site for the CO + N
La 7-δ (0.0 e x e 1.0) was synthesized by gel combustion where Fe substitution effectively
enhanced the conversion rates for N O reduction as compared to the pristine La Ti (LTOGC). Among all
samples, maximum conversion over La 7-δ [LF(0.6)GC] catalyst was observed. In situ FTIR results
2
O reaction are reported for the first time in the present communication.
3
+
2
Ti2(1-x)Fe2xO
2
2
2 7
O
2
Ti0.8Fe1.2O
reveal that substitution-induced anionic vacancies/defects provide additional sites on the surface of LF(0.6)GC
for CO chemisorptions, whereas a perfect stoichiometric lattice like LTOGC is devoid of such sites. Surface-
adsorbed CO reacts with surface lattice oxygen in the case of nonstoichiometric LF(0.6)GC to produce
2
-
carbonates (M-CO
the catalytic surface oxygen deficient in LF(0.6)GC, and therefore facilitates the reduction of preadsorbed,
O [N O(g) + * f N + *-O) by easily adsorbing the oxygen species (*-O) generated in N O reduction,
which is subsequently driven away by adsorbed/gas phase CO, whereas in the case of LTOGC, progress of
the reaction was sluggish in the absence of transient carbonate species. Dissociative chemisorptions of N
are not facilitated on stoichiometric oxygen excess titanate, as there is no vacancy in the surface to accommodate
3
) at a much lower temperature. The reaction proceeds via carbonate formation, leaving
N
2
2
2
2
2
O
2
-
another oxygen atom. The redox mechanism via CO
La 7-δ, as against the associative mechanism observed in the unsubstituted sample, La
suggested by in situ FTIR in conjunction with catalytic activity results.
3
species is proposed for CO + N
2
O reaction over
2
Ti2(1-x)Fe2x
O
2
Ti , as
2 7
O
1
. Introduction
Concerning charge transfer involved in the processes of
nitrogen oxide decompositions, multiple oxidation states of
2
Dinitrogen oxide, more accurately nitrous oxide, N O,
catalytic metal cations have been found to be instrumental in
popularly known as laughing gas, poses a threat to our
environment, as it is a potent greenhouse gas with global
warming potential (GWP) 310 and 21 times of CO
19-22
many oxide systems,
such as perovskites where nonsto-
ichiometry and the valency of the metal ions can be controlled
2
and CH
4
,
14
in a regular way. Wang et al. reported the catalytic decomposi-
respectively, and is also involved in stratospheric ozone
tion of dinitrogen oxide (N
2
O) over perovskites, La2-xSr
(M ) Cr, Mn, Fe, Co, and Ni), and La
M ) Co, Ni, and Cu) mixed oxides and correlated the catalytic
x
CuO
4
destruction.1 The reduction of N
-3
O by CO is a much preferred
O levels emitted in the
2
(
(
x ) 0-1), LaMO
3
2
MO
4
and required reaction for lowering of N
environment, especially in processes where CO is already
present in the waste gas, particularly by automobile exhaust and
2
activity and the average oxidation number of copper (AON) as
2+
3+
observed for La2-xSr
x
CuO
4
and explained it by a Cu /Cu
, a perovskite, better known as
chemical processes like the production of oxalic acid (end of
redox mechanism. LaMnO
3
pipe solutions).1
-7
2
N O is also produced as an intermediate in
nonstoichiometric metal ion-deficient LaMnO3+δ, was found to
the NO/CO reaction1 involved in vehicle emission control.
-3
4,5
be less active than oxygen-deficient substituted mixed metal ion
Heterogeneous catalytic decomposition/reduction of N O pro-
vides an energy-efficient and suitable solution in many of the
anthropogenic emission sources. Various catalytic formulations
2
23
oxides for the N
2
O decomposition reaction. LaCoO
3
is
stoichiometric with a tendency to be oxygen-deficient and shows
a high catalytic activity for the N O and NO decomposition
reaction as reported previously. The catalytic decomposition
of N O on the oxygen excess, spinels, M (M )
2
8
-10
have been proposed thus far, including supported
and
and
24
3,5,11
8,12-17
unsupported metals,
pure and mixed oxides,
2
x 2 4
Co1-xCo O
zeolitic systems.2
,3,18
Kapteijn et al. have extensively reviewed
3
2+
2+
Ni and Mg , x ) 0.0-0.99) was also reported by Yan et
al. in the temperature range of 200-300 °C. The partial
3 4
replacement of Co by Ni or Mg in Co O spinel oxide
a variety of catalysts, such as metal oxides, mixed metal oxides
for N O decomposition.
16
2
2+
2+
2+
led to a significant rise in the catalytic activity as compared to
unsubstituted samples, which was correlated with ease in the
removal of adsorbed surface oxygen from partially substituted
*
To whom correspondence should be addressed. Tel: 91 22 25595100.
Fax: 9122 25505151. E-mail: shyamala@barc.gov.in.
†
Chemistry Division, Bhabha Atomic Research Centre.
Cochin University of Science and Technology.
Present address: Regional Research Laboratory (RRL), Industrial Estate
‡
samples, Ni0.74Co0.26Co
2
O
4
and Mg0.54Co0.46Co
2
O
4
. Belapurkar
et al.25 have also reported the role of oxygen vacancies in the
decomposition of N O over YBa Cu CuO oxide
§
P.O. Pappanamcode, Trivandrum - 695 019. Kerala, India.
⊥
Fuel Chemistry Division, Bhabha Atomic Research Centre.
2
2
3
O7-δ and Gd
2
4
1
0.1021/jp102306u  2010 American Chemical Society
Published on Web 05/05/2010

6
944 J. Phys. Chem. B, Vol. 114, No. 20, 2010
Pai et al.
TABLE 1: Identification of Phase in La
2
Ti2(1-x)Fe2x
O
7-δ Samples Synthesized by the Gel Combustion (GC) Route
abbreviation LM(x)R
M ) Ti/Fe, x ) 0.1-1.0)
no.
nominal composition
Fe content (x)
synthesis route
phase identification by XRD
(
1
2
3
4
5
6
7
La
La
La
La
La
La
2
2
2
2
2
2
Ti
2
O
7
0.0
0.1
0.2
0.4
0.6
0.8
1.0
LTOGC
GC
GC
GC
GC
GC
GC
GC
single phase, La
2
Ti
and LaFeO
and LaFeO
2 7
O
3
Ti1.8Fe0.2
Ti1.6Fe0.4
Ti1.2Fe0.8
Ti0.8Fe1.2
Ti0.4Fe1.6
O
O
O
O
O
7-δ
7-δ
7-δ
7-δ
7-δ
LF(0.1)GC
LF(0.2)GC
LF(0.4)GC
LF(0.6)GC
LF(0.8)GC
LFOGC
La
La
2
Ti
Ti
2
O
O
7
2
2
7
3
single phase LaFeO
single phase LaFeO
single phase LaFeO
single phase LaFeO
3
3
3
3
LaFeO
3
TABLE 2: Crystallite Size and N
the Scherrer Equation
2 2
-BET Surface Area of La Ti2(1-x)Fe2xO7-δ Samples as Calculated from XRD Line Width Using
no.
value of x
sample
particle size (nm)
surface area (m2g-1
)
pore size (Å)
1
3
4
5
6
7
.
0.0
0.2
0.4
0.6
0.8
1.0
LTOGC
60-62
14-16
15-17
40-42
44-46
77-80
12
14
68
47
9
∼400
∼300
∼20
LF(0.2)GC
LF(0.4)GC
LF(0.6)GC
LF(0.8)GC
LFOGC
.
.
.
∼20
7
systems, where stoichiometric compounds have shown much
poorer activity as compared to oxygen-deficient compounds.
To understand the fundamental processes occurring at the
2. Experimental Section
.1. Sample Preparation and Characterization. La
7-δ samples with nominal compositions of x ) 0.0, 0.2,
.4, 0.6, 0.8, and 1.0 were synthesized by gel combustion using
2
Fe2x
2
Ti2(1-x)
O
molecular level, the adsorption studies of N
extremely important. Several reports have been published on
the adsorption studies by in situ IR of N O and CO on various
oxides, metals, and supported metals, namely, MgO, Ir, Cu/
2
O and CO are
0
citric acid and calcined at 800 °C; this has been discussed in
2
40
detail in our earlier work. Samples were characterized by XRD,
2
6
27
2
N -BET, IR, SEM, M o¨ ssbaeur spectroscopy, and TPR. The
2
8
29
30
31-33
34
ZnO, SnO
2
, Cu/Al
2
O
3
, Pt/Al
2 3
O ,
2 3
Rh(111), Rh/Al O ,
results of these characterization techniques have been explained
and ZnO.35 The CO + N
O reaction reportedly proceeds either
2
40
in detail in our previous paper. Table 1 lists the abbreviations
by associative or by redox mechanism or by both, which are
discussed in detail in section 3.3.4 of this report. However, in
multimetal mixed oxide systems, in situ Fourier transform
infrared (FTIR) results are scantily reported, particularly as a
and phases identified in all substituted and unsubstituted
combustion-synthesized samples calcined at 800 °C. Henceforth,
in the present study, all of the samples will be referred to by
the abbreviation used in Table 1. The surface area, pore size
function of substitution for the CO + N
Pacultova et al.36 have reported the carbonate formation in the
CO + N O reaction and mentioned that its role was unclear
with an uncertain mechanism over mixed oxides, calcined
Co-Mn-Al hydrotalcite. It is obvious that the N O + CO
2
O reaction. Recently,
2
by N -BET, and crystallite size estimated by the Scherrer
2
equation using the XRD line width of La Ti2(1-x)Fe2xO7-δ samples
2
are mentioned in Table 2.
2
.2. Catalytic Activity. Temperature-dependent catalytic
activity of all La 7-δ samples was evaluated for the
CO + N O reaction in a flow through reactor under continuous
flow of reactant mixtures (CO:N O:He ) 2:2:16) at a feed rate
using 100 mg of sample in the range of 50-600
C, by holding for 1 h at each temperature. The effluent gas
2
2
Ti2(1-x)Fe2xO
reaction over a mixed metal oxide catalyst is very complex,
and exact elucidation of the reaction mechanism is very difficult.
More insights into this mechanism could be acquired by
experiments with isotopically labeled molecules or in situ FTIR
measurements or by providing transient pulse experiments. Here,
therefore, we have attempted to delineate the role of carbonate
formation in the mechanism of the CO + N O reaction over
2
Fe-substituted/unsubstituted lanthanum titanates by in situ FTIR
measurements.
2
2
-1
-1
of 18 L h
°
g
was analyzed by a gas chromatograph equipped with a Poropaq
Q column and thermal conductivity detector (TCD). The
catalytic conversions for the CO + N
2
O reaction were calculated
on the basis of product yield CO . To ascertain the stability
2
and reproducibility of every composition, the activity evaluation
over spent samples was repeated, and average conversion rates
were plotted. Prior to activity measurements, each sample was
Earlier, we have reported structure-activity correlations and
thermophysical properties of Th(VO
, In
catalysts.
preparation and characterization by X-ray diffraction (XRD),
-BET, FTIR, scanning electron microscopy (SEM), M o¨ ss-
bauer spectroscopy, and temperature-programmed reduction
TPR) profiles of Fe-substituted oxygen excess layered perov-
skites, lanthanum titanates, La 7-δ. The consequences
of iron substitution in the crystal lattice of lanthanum titanate,
rather than being dispersed for the N O reduction by CO and
3 4 3 2 x 5-
) , LaMnO , In Ti1-xFe O
δ
2 x
Ti1-xCr O5-δ (0.0 e x e 0.02), etc., mixed metal oxides
37-39
40
initially pretreated at 350 °C for 2 h under N flow.
We have dealt in detail elsewhere with the
2
2.3. In Situ FTIR. In situ infrared spectroscopy experiments
were performed using an indigenously developed high-temper-
N
2
ature, high-pressure, stainless steel cell equipped with two water-
cooled CaF
2
windows as explained previously.41 A schematic
(
of this cell is presented in Figure 1. A self-supporting sample
pellet (80 mg, 25 mm diameter, and 0.8 mm thickness) was
mounted in the IR cell, and the spectra were recorded in
transmission mode using a JASCO 610 FTIR spectrophotometer
in tandem with a DTGS detector. To achieve a good signal-to-
noise ratio, 300 scans were collected for recording each spectrum
2
Ti2(1-x)Fe2xO
2
also knowing the intermediates produced in the course of the
reaction, have yet to be verified. The present work thus deals
with in situ IR experiments carried out under CO alone, N
alone, and a CO + N O gas mixture to delineate the role of Fe
substitution at the B-site in enhancement of the catalytic activity
of substituted lanthanum titanate, La Ti , for the catalytic CO
O reaction.
2
O
-1
at a resolution of 4 cm . Prior to its exposure to reactant gases,
2
-
4
the sample pellets were heated in a vacuum of about 10 Torr
for 4 h at a temperature of 300 °C. For IR experiments as a
function of temperature studies, the sample was exposed to a
2
2 7
O
+
N
2

Mechanism of CO + N2O Reaction
J. Phys. Chem. B, Vol. 114, No. 20, 2010 6945
Figure 1. Front and side view of a stainless steel cell for the in situ FTIR study of a catalyst surface during catalytic reactions. (A) Top flange with
sample heater block and (B) cell body: 1, rectangular stainless steel body; 2, cartridge heater block; 3 and 4, flanges; 5, sample wafer; 6, sample
holder block; 7, CaF window; 8 and 9, cooling water jackets; 10, manifold with gas inlet/outlet and evacuation facility; and 11, thermocouple to
2
measure sample temperature.
reactant gas mixture to maintain a final pressure of 100 mbar.
Unless mentioned otherwise, the final spectra were recorded
with an unexposed sample wafer as a reference, and the IR bands
due to unadsorbed gaseous spectra were subtracted wherever
applicable. The values given in the parentheses in some figures
represent the absorbance values of individual IR bands so as to
give an estimate of the relative intensity.
The samples used in the present study are La
2 7-δ
Ti2(1-x)Fe2xO
samples having x ) 0 and 0.6 and have been prepared by the
4
0
gel combustion method. Henceforth, in this paper, the above
samples will be referred to as LTOGC and LF(0.6)GC,
respectively. The samples chosen as LF(0.6)GC exhibited the
highest catalytic activity for the N
2
O + CO reaction in the
present study.40 So, the adsorption behavior of N
O + CO on
2
the surface of the host Fe-substituted and unsubstituted lantha-
num titanate samples was studied for delineating the role of Fe
doping in enhancement of activity. The gases used for adsorption
study are CO alone, N
1:1:17.
2 2
O alone, and a mixture of N O:CO:He
)
3
. Results and Discussion
.1. Characterization. We have dealt in detail40 elsewhere
with the preparation and characterization by XRD, N -BET,
FTIR, SEM, M o¨ ssbauer spectroscopy, and TPR profiles of Fe-
substituted oxygen excess lanthanum titanates, La
. Here, we are restating some of the characterization results
3
2
Figure 2. XRD patterns of La Ti2(1-x)Fe2xO7-δ samples synthesized by
citrate combustion calcined at 800 °C for different values of x, LTOGC
(a), LF(0.1)GC (b), LF(0.2)GC (c), LF(0.4)GC (d), LF(0.6)GC (e),
2
LF(0.8)GC (f), and LFOGC (g). *XRD peak at 2θ ) 32.17° is attributed
2
Ti2(1-x)Fe2x
O
7-
#
to perovskite LaFeO
La Ti phase.
3
, and XRD peak at 2θ ) 29.8° corresponds to
δ
2
2 7
O
for better understanding and establishing correlations in
structure-activity-in situ FTIR studies. Phases identified in all
substituted and unsubstituted combustion-synthesized samples
calcined at 800 °C are listed in Table 1. The XRD patterns of
samples (0.0 e x e 1.0) synthesized by gel combustion reaction
calcined at 800 °C are shown in Figure 2. The XRD pattern of
sample with x ) 0.0 (Figure 2a) matches with monoclinic
content more than 40%. The samples having x g 0.4 crystallizes
in a single rhombohedral LaFeO
3
phase (Figure 2d-g) having
unit cell parameters a ) 5.566 Å, b ) 7.854 Å, and c ) 5.56
3
Å, vol ) 242.81 Å (JC-PDS card no. 37-1493). The unsub-
2 2 7
stituted samples LTOGC and LFOGC are single phasic La Ti O
La
tion of lower-valent Fe in place of Ti decreased the extra
oxygen in the La Ti phase. This is manifested in the change
of layered perovskite to perovskite for the samples having a Fe
Ti O
2 2 7
(JC-PDS card no. 28-0517). As expected, the substitu-
3
and LaFeO , respectively, without any impurity phase. Samples
3
+
4+
with a Fe content g40% crystallized in single phase LaFeO
3
.
2
2
O
7
The in between (from 10-40%) substituted samples are
mixtures of two extreme phases. The room temperature M o¨ ss-

6
946 J. Phys. Chem. B, Vol. 114, No. 20, 2010
Pai et al.
2
Figure 3. Temperature-dependent activity of La Ti2(1-x)Fe2xO7-δ samples
for CO + N O reaction.
2
2
Ti2(1-x)Fe2xO
7-δ samples40 reveal distortion
bauer spectra of La
in symmetry in the LF(0.4)GC sample, which may arise due to
3+
anisotropic deformation of the environment around the Fe ions
4+
40
due to oxygen vacancies generated by substitution at Ti sites.
.2. Catalytic Activity. 3.2.1. CO + N O Reaction. The
temperature-dependent catalytic activity of La
samples synthesized by a gel combustion route calcined at
00 °C for the CO + N O reaction is exhibited in Figure 3.
Figure 4. In situ FTIR spectra of CO gas in the presence and absence
of La Ti O (LTOGC) and LF(0.6)GC samples recorded under ambient
2 2 7
3
2
and evacuated conditions. The subtraction spectrum: spectrum minus
the gaseous CO spectrum. A distinct extra band at 2056 and 2124 cm-
is attributed to the presence of carbonyl species of the type M-CO
2
Ti2(1-x)Fe2xO
3+δ
1
8
2
n
and M-(CO) type, respectively.
The reactant mixture of CO:N O:He ) 2:2:16 was fed to the
2
catalyst at a rate of 18 L h- g . Partially Fe-substituted samples
1
-1
TABLE 3: Fundamental IR Bands of Gaseous CO and N
as Recorded in in Situ IR Cells
2
O
have shown better activity as compared unsubstituted samples.
Unsubstituted LTOGC and LFOGC with single phase La
2 2
Ti O7
cm-1
and LaFeO , respectively, have shown poor activity with a mere
3
state
CO
2
N O
∼
20 and ∼55% conversion at 400 °C, respectively, in curves a
gaseous
2170/2118
3498/3464
583/2547
238/2208
2197
2204
2217
2235
and c of Figure 3. LF(0.2)GC and LF(0.8)GC (curves b and d)
samples have shown 100% conversion at ∼475 and ∼450 °C,
respectively, higher than unsubstituted LTOSS sample with
2
2
chemisorbed 2056 (M-CO)
124 (M-COn)
1
00% conversion at above 550 °C. The LF(0.4)GC sample
2
2
-
resulted in a considerable rise in activity as shown in curve e
with 100% conversion at 425 °C in comparison to other samples.
Among all samples, LF(0.6)GC has shown a maximum activity
of 100% conversion at ∼395 °C with an onset at ∼275 °C for
1700-1000 (CO like species)
3
2
2
-
1230 (M-CO
1323 [M-CO
3
in LTOGC sample)
in LF(0.6)GC sample]
-
3
the CO + N
2
O f CO
2
+ N
2
reaction (Figure 3f). The light off
a number of overtones and combination bands in addition to
fundamental modes. Table 3 lists vibrational bands of gaseous
and chemisorbed CO and N O, observed in in situ FTIR spectral
2
measurements. It is pertinent to mention here that catalytic
activity evaluation and in situ FTIR were recorded for the CO
temperature of substituted LF(0.6)GC sample was at a much
lower temperature of 275 °C as compared to 400 °C in the case
of unsubstituted LTOGC. LFOGC and LF(0.8)GC samples have
shown similar activities.
T
50 and T100 suggest the temperatures at which 50 and 100%
+ N
2
O reaction under similar conditions in the absence of any
O + He).
conversion for the CO + N O reaction achieved oversubstituted
2
gaseous oxygen in feed gas (CO + N
2
and unsubstituted samples. Both T100 and T50 values are lowest
for LF(0.4)GC and LF(0.6)GC, suggesting that 40-60% Fe
substitution leads to catalytically active compositions for the
3.3.1. CO Adsorption. Transmission mode spectra of the
species resulting from adsorption of CO (100 Torr) over a disk
of LTOGC and LF(0.6)GC at ambient temperature are shown
in Figure 4. Prominent IR signals due to gaseous CO and various
2
CO + N O reaction. The difference in T50 and T100 values of
-
1
LF(0.6)GC sample is ∼28 °C (395 minus 365 °C from curve
overlapping signals in the 2000-2300 cm region due to
1
-
f), much lower as compared to unsubstituted pristine LTOGC
chemisorbed CO along with a shoulder at ∼2056 cm attributed
to stretching of CO in the M-CO bond over La Ti
(LTOGC), where M ) Ti , were observed in Figure 4. Unlike
the parent sample, in the case of Fe-substituted La
, (LF(0.6)GC) sample, there is a shift in the IR band at ∼2124
(
91 °C from curve a) in Figure 3. Thus, once reaction onsets,
2
2 7
O
4+
the rate increases steeply with a rise in temperature over
LF(0.6)GC, as evident from Figure 3.
2
Ti0.8Fe1.2O
7-
3
.3. In Situ FTIR. Linear molecules, CO and N
2
O (N-N-O),
δ
-
1
-1
conforming to 3N-5 (where N is the number of atoms in the
molecule), show a singlet and three doublets with P and R
branches as fundamental vibration modes, respectively, in their
cm (from 2118 cm , Figure 4), which is attributed to the
, where
formation of multicarbonyl species of the type M-(CO)
n g 2 and M can be Fe , Ti , anionic vacancy, or pores present
n
3+
4+
gaseous FTIR spectra. The N
2
O spectrum is also marked with
in the sample. In general, it is anticipated that the molecule

Mechanism of CO + N2O Reaction
J. Phys. Chem. B, Vol. 114, No. 20, 2010 6947
Figure 5. In situ IR spectra of adsorbed CO on LTOGC at different temperatures (A) and time (B).
will be bound more strongly to defect sites (which are
coordinative unsaturated) than to normal, terrace ones,4
resulting in a shift to higher vibration wavenumbers. The spectra
obtained after subtracting the gas phase CO are also shown,
2,43
-
1
where it is observed that there are residual bands at 2122 cm
due to multicarbonyl species in LF(0.6)GC and a sharp peak at
2
056 cm- in LTOGC. Evacuation of the in situ cell for 5 min
1
at room temperature, after the exposure of CO over LTOGC,
resulted in complete removal of gaseous CO, adsorbed CO, and
an extra band at 2056 cm- (Figure 4). This confirms the weak
adsorption of CO on LTOGC. On the contrary, strong chemi-
sorptions on LF(0.6)GC are confirmed by the presence of
1
-1
residual bands on its surface by the presence of 2124 cm peak
even after evacuation (Figure 4). When the LTO sample disk
is exposed to 100 mbar of CO gas, then the peaks that are
obtained as a function of temperature are shown in Figure 5A.
It is evident from Figure 5A that there is a slight decrease in
the intensity of the CO peak as the temperature increases from
room temperature to 200 °C with the appearance of a broad
and strong band at 1230 cm- at 200 °C. This band can be
attributed to the formation of surface carbonates, that is,
1
2
-
Figure 6. Presence of the CO
3
band even after evacuation of LTOGC
2-
M-CO
3
or bicarbonates type species, which remain present
after exposure to 100 mbar CO at 200 °C.
even after evacuation, as shown in Figure 6. The bands observed
-1
in 1700-1000 cm can be assigned to carbonate-like species,
CO(g) + * f *-CO
(a)
(b)
2
-
M-CO
catalysts.44 At 350 °C, the broad band vanishes completely, and
the CO peak appears; that is, the decomposition of carbonates
evolves CO with an increase in temperature. The CO peak at
50 °C is observed as a function of time in Figure 5B, where
it is observed that the CO peak increases in intensity with time
3
adsorbed as reported on Au-supported ZnO and TiO
2
2-
2-
-
*
-CO + 2*-O f *-CO3 + 2* + 2e
2
2
2
2
-
2-
3
*
-CO3 f CO (g) + *-O
(c)
2
2
along with the decrease in intensity of the CO peak, and no
carbonate peaks are observed. Thus, CO adsorbs at a surface
adsorption site, which at higher temperatures (200 °C) form
surface carbonates or bicarbonates, and then, at 350 °C, these
Under similar conditions when the LF(0.6)GC sample disk
is exposed to 100 mbars of CO, as the temperature is increased
-
1
to 100 °C, the carbonate peak appears at 1323 cm with a
4
5,46
carbonates decompose, producing CO
mechanism.
2
as per the following
decrease in intensity of adsorbed CO, but no CO is formed, as
2
shown in Figure 7A. This again indicates the formation of

6
948 J. Phys. Chem. B, Vol. 114, No. 20, 2010
Pai et al.
Figure 7. In situ FTIR spectra of 100 mbar of CO adsorbed on LF(0.6)GC as a function of temperature (A) and time (B).
2
-
surface carbonates or M-CO
3
type species. In the case of
3.3.2. N
2
O Adsorption. Let us now consider the effect of
LF(0.6)GC, the carbonate formation is observed at a much lower
temperature (100 °C) than LTO (200 °C), and also, the nature
exposure of 100 mbar of N
LF(0.6)GC. The transmission mode spectra of the species
resulting from adsorption of N O (100 Torr) as a function of
temperature over a disk of LTOGC and LF(0.6)GC are shown
in Figure 8A,B, respectively. The bands at 2112 and 2235 due
to P and R branches centered at 2223 cm correspond to the
antisymmetric NN stretching (ν mode) and 1274 and 1301 (P
and R) centered at 1285 cm correspond to symmetric NO
2
O alone on a pellet of LTOGC and
-
1
of carbonate is also different as the peak position (1230 cm )
and broadness also vary. Carbonate formation at lower tem-
perature in LF(0.6)GC is probably due to greater participation
of the lattice oxygen atoms on the surface in the substituted
samples. The substitution-induced nonstoichiometry generated
in the lattice makes the lattice oxygen more mobile, and thus,
a surface-adsorbed CO can react with surface lattice oxygen to
produce carbonates at a much lower temperature. In other words,
the lattice oxygen atoms on the surface must be reactive enough
to allow the adsorption and subsequent formation of carbonates
at these sites. Such sites are lacking in a perfect stoichiometric
lattice like LTOGC but are present in a defect structure43 like
LF(0.6)GC. The CO binds to two surface oxygen-forming
carbonates at the same time, reducing the cation, and this process
is also facilitated in the iron-substituted sample due to the
2
-
1
3
-
1
4
8-50
stretching vibrations (ν
LTOGC, it is seen that with temperature there is a decrease in
the peak intensity of N O. Because LTOGC is an oxygen excess
titanate, it is not expected that any dissociative chemisorptions
of N O can occur on its surface as any further oxidation of the
surface is impossible. Thus, the decrease in intensity of N
absorbance with an increase in temperature can be attributed to
desorption of the physisorbed N O with a rise in temperature.
On the contrary, on LF(0.6)GC, the N O absorbance values
remain mostly constant. Upon evacuation of the sample cell
after exposure to 100 mbar of N O, we observe from Figure
9B that there are residual bands at 2197 and 2204 cm on
1
mode) of the N
2
O molecule.
In
2
2
2
O
2
2
3
+
presence of more reducible cation Fe . It is also seen that the
carbonate peak appears at 1323 cm- at a much higher
wavenumber, which can be at an iron-substituted site. Thus, in
addition to increasing the oxygen mobility, Fe substitution has
resulted in the formation of additional surface sites for the
adsorption of CO. Subsequently, with a further increase in
1
2
-
1
-
1
LF(0.6)GC in addition to 2217 cm , whereas a broad band
centered at 2235 cm of negligible intensity is observed in the
spectrum on LTOGC (Figure 9A). Thus, on the Fe-doped
-
1
temperature to 300 °C, the CO
of carbonates increases, and after that, the intensity of carbonate
remains constant, but that of CO increases drastically at 350
C (Figure 7A). Another interesting feature is that the absor-
2
peak appears, and the intensity
sample, LF(0.6)GC, the adsorption of N
as compared to LTOGC when their spectra were compared after
evacuation of the IR cell. Thus, in LTOGC, N O is primarily
weakly adsorbed on the surface, which undergoes desorption
at higher temperatures, but Fe substitution in LF(0.6)GC
2
O is more strengthened
2
2
°
bance value of CO also decreases at this point. The effect of
time at 350 °C has also been observed and is shown in Figure
generates surface sites for chemisorptions of N
3.3.3. CO + N O Adsorption. The transmission mode spectra
of the species resulting from adsorption of N O + CO (100
Torr) as a function of temperature over a disk of LTOGC are
shown in Figure 10A. Both N O and CO get adsorbed on the
surface of LTOGC, as is evident from Figure 10A, where we
see a strong peak due to NN vibration of N O in comparison to
a weak band resulting from CO stretching, which is reflected
in the gas phase spectra as well. This difference in absorption
intensity of the two gases can be attributed to the higher
2
O.
7
B, where it is seen that the carbonate peak intensity remains
2
constant throughout 60 min, while that of CO decreases and
CO increases. Although the decrease of carbonate intensity was
less even with an increase in time, the decomposition of surface
2
2
2
4
7
carbonates does appear to have occurred. The amount of CO
2
formed is also much higher as noticed from the absorbance
values for LF(0.6)GC than LTOGC. This result is in agreement
with the catalytic activity evaluated on LTOGC and LF(0.6)GC
2
2
samples for the CO + O reaction.

Mechanism of CO + N2O Reaction
J. Phys. Chem. B, Vol. 114, No. 20, 2010 6949
2
Figure 8. Temperature-dependent in situ FTIR spectra of LTOGC (A) and LF(0.6)GC (B) upon exposure to 100 mbar of N O.
2
Figure 9. In situ FTIR spectra of 100 mbar of N O adsorbed on LTOGC (A) and LF(0.6)GC (B) at room temperature after evacuation in the
-
1
2
16-2260 cm region.
extinction coefficient for the NN stretching vibration of N
2
O.
of the adsorbed CO and N
for the increase in CO concentration even at so high temper-
atures. Thus, the CO + N O reaction proceeds very slowly on
the LTO surface. In contrast, in the case of LF(0.6)GC, the N
peak intensity decreases drastically with temperature along with
2
O with time, nor there is any evidence
The differences in chemisorptive behavior between the two
2
5
1,52
species have been discussed by de Paola et al.
the extent of N O adsorption/desorption and the rate of reaction
of CO + N O are definitely influenced by Fe doping. When
O as a
However,
2
2
2
O
2
-
1
the LTOGC disk is exposed to 100 mbar of CO + N
function of temperature, no change in peak position and intensity
2
the formation of carbonate at 200 °C at 1337 cm , as shown
in Figure 11A. The decrease in peak intensity of N O can be
attributed to dissociative chemisorptions of N O producing gas
phase N and adsorbed oxygen. Because the N molecule is IR
inactive, it is not seen in the spectrum. Considerable gas phase
CO formation is also observed from 300 °C (Figure 11A) over
the LF(0.6) sample. Thus, in the presence of N O, CO oxidation
proceeds with simultaneous N O reduction in both of the
2
is observed until 200 °C, and no carbonate or nitrate peaks are
2
observed. At 350 °C, a slight decrease in N
2
O peak intensity
is observed. The effect of
2
2
along with meager formation of CO
2
exposure of the gas mixture on LTOGC at 350 °C as a function
of time was also observed and is plotted in Figure 10B. It is
seen from Figure 10B that there is no change in peak intensity
2
2
2

6
950 J. Phys. Chem. B, Vol. 114, No. 20, 2010
Pai et al.
2
Figure 10. In situ FTIR spectra of 100 mbar of CO + N O adsorbed on LTOGC as a function of temperature (A) and time (B).
2
Figure 11. In situ FTIR spectra of 100 mbar of CO + N O adsorbed on LF(0.6)GC as a function of temperature (A) and time (B).
samples. In LF(0.6)GC, the carbonate formation assists the
reduction of preadsorbed N O, while in the case of LTO, the
absence of carbonate makes the reaction progress sluggishly as
inferred by the following results. In either of the samples, nitrate
results, a mechanism is proposed for reduction of N
2
O by CO
2-
2
via transient CO
next section.
3
species, which is explained in detail in the
3.3.4. Mechanism. The mechanism of the N
2
O decomposi-
formation is not observed, but the decrease in N
with temperature definitely suggests the dissosciative adsorption
of N O on the LF(0.6)GC surface. When the temperature is
increased from 200 to 300 °C in LF(0.6)GC (Figure 11A), then
2
O peak intensity
tion reaction (Scheme 1, eqs 1.1-1.4) is seen from published
kinetic studies,5
3,54,12
where it is interpreted that desorption of
2
adsorbed oxygen (eqs 1.3 and 1.4) is the rate-limiting step in
the decomposition.
2-
the CO
3
, N
2
O, and CO
2
peak absorbance values increase,
The surface-adsorbed oxygen (eq 1.3) could be removed more
easily in the presence of a reducing agent, for example, light
decrease, and increase, respectively. At an even higher tem-
hydrocarbons,55 NH
anisms of N
often published mechanism is “redox” (Scheme 2), when the
surface is oxidized by N O and consequently reduced by CO
with simultaneous formation of CO (eq 2.1-2.4). The “redox”
56
CO, and H
6,7
7
perature at 350 °C, the carbonate peak disappears, and moreover,
3
,
2
. Several different mech-
57,58
the CO
2
and the N
2
O peak intensities gradually increase and
2
O reduction with CO were proposed.
The most
decrease, respectively, with time at 350 °C, as is observed from
the time-dependent curve (Figure 11B), indicating that the
reaction proceeds with time at 350 °C. On the basis of these
2
2

Mechanism of CO + N2O Reaction
SCHEME 1: Mechanism of N O Decomposition
J. Phys. Chem. B, Vol. 114, No. 20, 2010 6951
SCHEME 4: Mechanism Proposed for CO + N
2
2
O
2-
Reaction Proceeding via CO
3
Formation over
Fe-Substituted Lanthanum Titanate, LF(0.6)GC, Sample
SCHEME 2: Redox Mechanism
The replacement of Ti4 by Fe3+ has considerably modified
the surface properties of the oxides. On the basis of activity
data and the in situ FTIR spectra recorded for the LTOGC and
LF(0.6)GC samples, it is suggested that Fe substitution provides
+
2
a different pathway for the CO + N O reaction as compared to
unsubstituted lanthanum titanate. We have already discussed
that when the samples LTOGC and LF(0.6)GC are exposed to
CO alone, then the lattice oxygen atoms on the surface of
LF(0.6)GC are more reactive and react with an adsorbed CO
molecule, producing carbonates. The carbonate formation is
accompanied by the reduction of the cation, which is again
facilitated in the substituted sample by the presence of transition
3+
metal cation Fe , which is more reducible than any of the other
SCHEME 3: Associative Mechanism
cation-Ti4 or La . Furthermore, iron substitution provides
additional surface sites for the adsorption of CO. On the
substituted sample, CO is chemisorbed as compared to weakly
adsorbed CO in LTOGC. The catalytic activity results support
the in situ FTIR findings, where we find that the activity for
+
3+ 40
the CO + N
samples, LF(0.6)GC, as compared to unsubstituted LTOGC.
From the in situ FTIR studies for N O alone, we have seen that
O is primarily weakly adsorbed on the LTOGC surface, while
Fe substitution provides anionic vacant sites for the stronger
adsorption of N O but in neither case is any dissociative
chemisorptions of N O observed. On exposure to both of the
gases N O and CO simultaneously, we observe carbonate
2
O reaction is much higher in the substituted
2
N
2
2
2
2
formation in LF(0.6)GC but not in the stoichiometric LTOGC,
as a perfect stoichiometric lattice lacks reactive surface lattice
3+
oxygen atoms on the surface. Aliovalent Fe substitution in
place of Ti generates anionic vacancies, which results in
reaction could proceed by two ways: Adsorbed oxygen can react
4+
2
7
with adsorbed CO (eq 2.3) or with CO from the gas phase
increased lattice oxygen mobility and thus enhanced availability
2
8,29
according to the Eley-Rideal mechanism (eq 2.4).
The redox
2-
of lattice oxygen O on the surface. Thus, in LF(0.6)GC, a
mechanism was published over several oxides and supported
chemisorbed CO can react with surface lattice oxygen, produc-
ing surface carbonates (eq 4.2), which decompose, producing
metal oxides.2
9-36
The other mechanism is called “associative”
(
Scheme 3). The simultaneous oxidation and reduction are
CO
2
, and this process creates surface oxygen vacant sites for
O producing molecular N and
considered, and the reaction proceeds via species adsorbed on
the catalyst surface (eqs 3.1-3.4). The reaction can proceed
between both reactants in adsorbed form (eq 3.3) or between
dissociative chemisorptions of N
2
2
adsorbed oxygen (O*) (eq 4.3). The anionic vacancies present
in the lattice are responsible for the production of vacant surface
active sites for further oxygen adsorption (eq 4.3). Thus, in the
adsorbed CO and N
associative mechanism was described over Ag/Al , MgO,
NiO,61 and Fe
O /SiO . In some cases, both mechanisms take
2 3 2
2
O from the gas phase (eq 3.4). The
28
59,60
2
O
3
2
presence of CO, N O reduction on LF(0.6)GC proceeds via
62
surface activation for dissociative chemisorptions of N O by
2
part in the reaction, and contribution to the overall reaction rate
depends on the experimental conditions such as temperature and
the extent of catalyst reduction, or each mechanism takes place
at different sites. The combination of redox and associative
mechanisms was considered over calcined hydrotalcites and Fe-
silicalite. Apart from a simple reduction, the simultaneous
CO formation from carbonates. Then, either adsorbed CO or
2
gas phase CO forms CO using up the adsorbed oxygen (O*,
2
eq 4.4). So, a mechanism is proposed as shown in Scheme 4,
where the rate-determining step is CO formation via carbonates
2
(K ′, eq 4.2). As soon as a CO molecule is removed, producing
2
CO via carbonate, the catalytic surface becomes oxygen-
2
reduction and direct N
2
O decomposition can also proceed.
2
deficient, and dissociative chemisorptions of N O (eq 4.3)

6
952 J. Phys. Chem. B, Vol. 114, No. 20, 2010
Pai et al.
proceed spontaneously and the adsorbed oxygen generated in
the process is driven away by a gas phase CO or adsorbed CO.
Thus, in LF(0.6)GC, the carbonate formation assists the reduc-
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proposed mechanism is essentially a redox phenomenon, where
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catalytic activity data exactly corroborate the in situ FTIR results
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cannot activate the surface for any dissociative chemisorptions
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(
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(
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Investigations by in situ FTIR reveal that both the position and
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(
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