Infrared and TPD studies of nitrates adsorbed on Tb4O7, La2O3, BaO, and MgO/γ-Al2O3

Article abstract of DOI:10.1021/jp993894m

Catalytic approaches for NO removal in an oxidizing atmosphere are important to control NO emissions in coal-fired power plants and automobile industries. The adsorption of NO/O₂ on metal oxides (Tb₄O₇, La₂O₃, BaO, and MgO) supported on γ-Al₂O₃ and the decomposition of their adsorbates were studied using in situ IR spectroscopy coupled with temperature-programmed decomposition and TPD. NO/O₂ coadsorbed as bridging and chelating bidentate nitrates and monodentate nitrate on Tb₄O₇, La₂O₃, and BaO, and as bridging bidentate nitrate and monodentate nitrate on MgO via the reaction of adsorbed NO with adsorbed O₂ at 298 K. NO/O₂ coadsorbed as a chelating bidentate nitrate on BaO and MgO via the reaction of adsorbed NO with surface lattice O₂ at 523 K. The production of NO and O₂ as key products of chelating and bridging bidentate nitrate decomposition showed that the formation of these nitrates from NO and O₂ is basically reversible without involving NO₂ as an intermediate. Peak temperatures for Tb(NO₃)₃, La(NO₃)₃, Ba(NO₃)₂, and Mg(NO₃)₂ decomposition occurred between those for bridging and chelating nitrates. The difference in reactivity and stability of chelating and bridging bidentate nitrates on different oxides may offer a wide range of operating temperatures for NO_x storage. N₂ formation from decomposition of various forms of nitrates indicated the need to identify the step involved in the conversion of nitrate to N₂.

Full text of DOI:10.1021/jp993894m

J. Phys. Chem. B 2000, 104, 4673-4683
4673
Infrared and TPD Studies of Nitrates Adsorbed on Tb4O7, La2O3, BaO, and MgO/γ-Al2O3
Yawu Chi and Steven S. C. Chuang*
Department of Chemical Engineering, The UniVersity of Akron, Akron, Ohio 44325-3906
ReceiVed: NoVember 3, 1999; In Final Form: March 10, 2000
NO and O2 coadsorption on γ-Al2O3-supported Tb4O7, La2O3, BaO, and MgO has been investigated by in
situ infrared spectroscopy coupled with temperature-programmed decomposition and desorption. BaO/γ-
Al2O3 and MgO/γ-Al2O3 possess a higher NOx storage capability than Tb4O7/γ-Al2O3 and La2O3/γ-Al2O3.
NO/O2 coadsorbed on Tb4O7, La2O3, and BaO in the form of bridging bidentate, chelating bidentate, and
monodentate nitrates, and on MgO in the form of bridging bidentate and monodentate nitrates via the reaction
of adsorbed NO with adsorbed oxygen at 298 K. NO/O2 coadsorbed as a chelating bidentate nitrate on Tb4O7
and La2O3, and as a distinctive bridging bidentate nitrate on BaO and MgO via the reaction of adsorbed NO
with surface lattice oxygen at 523 K. These various forms of adsorbed nitrate differ in structure and reactivity
from Tb(NO3)3, La(NO3)3, Ba(NO3)2, and Mg(NO3)2, the precursor used to prepare metal oxides for NO/O2
coadsorption. Temperature-programmed desorption (TPD) of chelating bidentate nitrate on Tb4O7, La2O3,
and BaO produced primarily NO and O2, with maxima at 640 and 670 K, respectively. TPD of bridging
bidentate nitrate and monodentate nitrate on Tb4O7, La2O3, and BaO produced NO and O2 as major products
and N2 and N2O as minor products, at 320-500 K. Decomposition of bridging bidentate on MgO produced
NO as a major product and N2O as a minor product at a peak temperature of 690 K. Peak temperatures for
Tb(NO3)3, La(NO3)3, Ba(NO3)2, and Mg(NO3)2 decomposition occurred between those for bridging and
chelating nitrates. The difference in stability between chelating and bridging bidentate nitrates on various
metal oxides/γ-Al2O3 may provide a wide range of operating temperatures for NOx storage.
1. Introduction
The control of NO emissions to meet increasingly stringent
standards has been a major challenge for coal-fired power plants
and automobile industries. No catalysts for NO removal in an
oxidizing environment are found to have activities close to being
practical. Catalytic approaches for the removal of NO include
(1) the reaction of NO with CO over Rh-Pt and Pd-based
catalysts in the catalytic converter,1,2 (2) the reaction of NO
and CO on metal oxides,3,4 (3) the reaction of NO with
hydrocarbons on Cu- and Co-ZSM-55-9 and alkali-MgO
catalysts,10-12 (4) the selective catalytic reduction (SCR) of NO
with NH3,13,14 and (5) the direct decomposition of NO over
Cu-,15-19 Tb-,20 and Ba-based10-12,21-25 catalysts.
All these catalytic approaches involve (1) N-O bond
dissociation of adsorbed NO or precursor containing NO to form
adsorbed nitrogen and adsorbed oxygen; (2) combination of
adsorbed nitrogen with itself or with other N-containing species
to form gaseous nitrogen; (3) reaction of adsorbed oxygen with
reducing agents such as CO, NH3, and hydrocarbons; and/or
(4) desorption of oxygen. Removal of adsorbed oxygen is
required to complete the catalytic cycle of NO reduction. The
direct decomposition of NO to nitrogen (N2) and oxygen (O2)
appears to be an attractive approach; however, it suffers from
poisoning by the presence of excess oxygen. No NOx (i.e., NO
and NO2) removal catalysts for diesel/lean-burn engines are
known to have activities close to being practical. Although the
use of reducing agents effectively removes adsorbed oxygen, it
complicates the NO removal process. In addition, it has limited
the range of operating conditions, resulting in high capital and
Figure 1. Experimental approach: (a) temperature-programmed
decomposition of Tb(NO3)3/γ-Al2O3, (b) adsorption of NO and O2 on
Tb4O7/γ-Al2O3 at 298 K, and (c) temperature-programmed desorption
of NOx adsorbates on Tb4O7/γ-Al2O3 from 298 to 773 K.
* Author to whom all correspondence should be addressed. E-mail,
schuang@uakron.edu, tel, (330) 972-6993; fax, (330) 972-5856.
10.1021/jp993894m CCC: $19.00 © 2000 American Chemical Society
Published on Web 04/25/2000

4674 J. Phys. Chem. B, Vol. 104, No. 19, 2000
Chi and Chuang
Figure 2. IR spectra of (a) original nitrates and (b) nitrates/γ-Al2O3 at 298 K.
TABLE 1: Loading of Nitrates and Metal Oxides on
γ-Al2O3 and Particle Size
XRD analysis
wt% in MxOy
wt% in
species M(NO3)n
wt% in
MxOy
needed for
particle
monolayera crystal size (Å)
Tb
La
Ba
Mg
25.7
24.5
14.6
9.7
15.8 (Tb4O7)
15.0 (La2O3)
9.2 (BaO)
12.4
5.4
2.5
0.7
Tb4O7
BaO
105
198
1.7 (MgO)
a Estimated by packed monolayer model according to ref 47.
operating costs. The limiting operating temperature range is a
result of the interaction of competing reactions: NO reduction
and reductant oxidation. To avoid the direct oxidation of
reductant, the concept of NO storage and reduction has been
tested for removal of NO in an oxidizing environment. BaO-
CuO,21 Pt/Ba/Al2O3,22 binary oxides (Ba-Cu, Mn-Y, and Mn-
Zr),23 Pt-Ba/washcoat,24 and Cu-exchanged mordenite25 have
been tested for NOx storage. A better understanding of NO/O2
interaction with the NOx storage medium is needed to move
from the empirical material screening toward rational design.
The objective of this study is to investigate the adsorption of
NO/O2 on metal oxides and decomposition of their adsorbates.
Tb,20,26 La,27-30 Ba,10-12,24,31,32 and Mg10-12,32-34 oxides are
selected for this study due to their wide uses as promoters and
storage mediums in automobile and lean-burn NO removal
catalysts. This paper reports the results of an infrared study of
(1) NOx storage on metal oxides in an oxidizing environment
and (2) desorption behavior of adsorbed NOx on metal oxides.
Figure 3. X-ray diffraction patterns of decomposed nitrates/γ-Al2O3.
(NO3)2, and Mg(NO3)2 (Alfa Products) were prepared by
incipient wetness impregnation of γ-Al2O3 (Alfa Products;
surface area, 100 m2/g; pore size, 0.01-0.02 µm) with a metal
nitrate solution. The ratio of the volume of the solution to the
weight of γ-Al2O3 was about 1 cm3 to 2 g. The impregnated
samples were dried overnight in air at room temperature. The
loading of nitrate on γ-Al2O3 is listed in Table 1.
2. Experimental Section
2.1. Preparation of γ-Al2O3-Supported Nitrates and IR
Characterization. γ-Al2O3-supported Tb(NO3)3, La(NO3)3, Ba-

Infrared and TPD Studies of Nitrates
J. Phys. Chem. B, Vol. 104, No. 19, 2000 4675
Figure 4. MS profiles and in situ IR spectra during 0.08% NO and 2% O2 adsorption on Tb4O7/γ-Al2O3 at 298 (a, b) and 523 K (c, d).
Figure 5. In situ IR spectra during 0.08% NO and 2% O2 adsorption on La2O3/γ-Al2O3, BaO/γ-Al2O3, and MgO/γ-Al2O3 at 298 and 523 K.
TABLE 2: Amount of NO and O2 Adsorption on Various
The original metal nitrates and γ-Al2O3-supported nitrates
Metal Oxides/γ-Al2O3 at 298 and 523 K
(i.e., nitrates/γ-Al2O3) were mixed with KBr (Alfa Products,
at 298 K (µmol/g cata)
at 523 K (µmol/g cata)
KBr spectrograde, ultrapure) at a weight sample/KBr ratio of
1:100 and pressed into self-supporting disks for infrared
characterization at 298 K. The sample was mixed with KBr to
obtain high resolution of nitrate bands. The IR spectra of Tb-
(NO3)3‚6H2O, La(NO3)3‚6H2O, Ba(NO3)2, and Mg(NO3)2‚6H2O
were obtained by subtracting the KBr spectrum from each
specific nitrate/KBr spectrum; the IR spectra of Tb(NO3)3‚H2O/
γ-Al2O3, La(NO3)3‚H2O/γ-Al2O3, Ba(NO3)2‚H2O/γ-Al2O3, and
Mg(NO3)2‚H2O/γ-Al2O3 were obtained by subtracting the
γ-Al2O3/KBr spectrum from each specific nitrate/γ-Al2O3/KBr
spectrum.
metal oxides
NO
O2
NO
O2
Tb4O7/γ-Al2O3
La2O3/γ-Al2O3
BaO/γ-Al2O3
MgO/γ-Al2O3
511
376
724
377
568
546
1171
32
91
125
213
226
34
57
160
<5
a Metal oxide/γ-Al2O3.
study of each metal nitrate/γ-Al2O3. The experimental system
consists of (1) the gas flow system with a 4-port switching valve
and mass flow controllers for NO, O2, and He; (2) the in situ
IR reactor cell with self-supporting oxide disks; and (3) the
analysis section with a Nicolet magna-IR 550 spectrometer for
2.2. Adsorption and Decomposition/Desorption Studies.
Figure 1 illustrates the experimental approach and sequence for

4676 J. Phys. Chem. B, Vol. 104, No. 19, 2000
Chi and Chuang
TABLE 3: IR Band Assignment of Adsorbates Produced from Flowing 0.08% NO and 2% O2 over Various Metal Oxides/
γ-Al2O3 at 298 K and 523 K
a
vibration mode and wavenumber (cm-1
)
-
bridging bidentate chelating bidentate monodentate
free NO3
NO2,as
M-(NO)+ O-(NO) O-(NO)- N2O2
temp (K)
2-
metal oxides NdO
NO2,as
NdO
NO2,as NO2,as NO2,sy
Tb4O7/Al2O3
1630
1630
1630
1600
1630
1630
1600
1630
1600
1630
1600
1630
1298
1282
1284
1570
1556
1562
1298
1282
1284
1470
1380
1370
1380
1928
298
523
298
La2O3/Al2O3
1450
1925
1722
1284
1291
1562
1541
1284
1291
1450
1458 1208
1375
1380
1722
1672
523
298
BaO/Al2O3
MgO/Al2O3
1766
1766
1291
1541
1291
1458
523
298
523
1458 1207
1458 1207
1380
1380
1678
1748b
1257
a Refs 16-34, 36-45. b Refs 34, 38.
Figure 6. Normalized product MS profiles of (a) thermal decomposition of Tb(NO3)3 on γ-Al2O3, (b) TPD of adsorbates on Tb4O7/γ-Al2O3 from
298 to 773 K, and (c) TPD of adsorbates on Tb4O7/γ-Al2O3 from 523 to 773 K.
recording IR spectra of adsorbed species and a Balzers QMG
112 quadruple mass spectrometer (MS) for the analysis of
reactant and product flows. The gaseous responses for He at
m/e ) 4, N2 at m/e ) 28, NO at m/e ) 30, O2 at m/e ) 32,
N2O at m/e ) 44, and NO2 at m/e ) 46 were monitored by the
MS. Contribution of N2O and NO2 to fragments at m/e ) 30
can be resolved from relative intensities of fragments and parent
ions of the calibrated N2O and NO2 pulse responses.
infrared beam path inside the infrared cell for temperature-
programmed decomposition (TPDE), NO/O2 adsorption, and
temperature-programmed desorption (TPD) studies.35 Addi-
tional nitrate/γ-Al2O3 disks with a total of 70 mg were broken
into flakes and placed in the vicinity of the nitrate/γ-Al2O3
disk to increase the quantity of decomposed products in the
reactor effluent, allowing accurate analysis of their composi-
tions.
To avoid complication from NOx adsorption on KBr, 30 mg
of fresh nitrate/γ-Al2O3 powder was pressed into a self-
supporting disk without mixing with KBr and placed in the
Metal nitrates/γ-Al2O3 underwent a series of studies: (1)
TPDE studies from 298 to 773 K at a heating rate of 10 K/min
in flowing He at 75 cm3/min, (2) adsorption of NO/O2 at 298

Infrared and TPD Studies of Nitrates
J. Phys. Chem. B, Vol. 104, No. 19, 2000 4677
TABLE 4: Amount of Decomposed and Desorbed Product of γ-Al2O3-Supported Nitrates and Adsorbed Nitrates Produced
from Flowing 0.08% NO/2% O2 during TPDE/TPD
amount of decomposed and desorbed product (µmol)
nitrates and
metal oxides
NOx storage
abilitya (%)
NO
N2
N2O NO2
O2
11
7.1
Ntotal
Ototal
N/O NO/O2
TPDE/TPD
Tb(NO3)3/Al2O3
Tb4O7/Al2O3
137
34
6.8 0.6
115
28
9.8 0.3
9.5
3
24
1.4
4
0.2
207
191
1.1
0.9
0.8
1.1
0.7
0.7
1.0
0.6
0.7
1.1
1.0
1.0
12.5
4.8
4.5
8.8
3.4
4.7
8.3
3.1
3.9
d
298-753 K (TPDE)
298-753 K (TPD)
523-753 K (TPD)
298-753 K (TPDE)
298-753 K (TPD)
523-753 K (TPD)
298-753 K (TPDE)
298-753 K (TPD)
523-753 K (TPD)
298-753 K (TPDE)
298-753 K (TPD)
523-753 K (TPD)
57.8
10.8
43 (96%)b
8 (100%)
173.5
31.9 (97%)
10.4 (95%)
112.3
50 (34%)c
9.8 (70%)
trace trace 1.5
La(NO3)3/Al2O3
La2O3/Al2O3
7
1.3
22
0.5
0.5
0.3
13
8.2
164
42.3
13.7
45.5 (35%)
14.0 (67%)
111.6
none trace 2.1
Ba(NO3)2/Al2O3
BaO/Al2O3
83
8
6
0.6
1.3
0.4
10
20
118.2
33.2
62
18
57
31
19
1.3
0.2
0.7
66.2 (97%) 103.4 (40%)
trace 0.2
4.6
18.6 (93%)
67.6
33.4 (95%)
20.7 (98%)
27.6 (55%)
63.4
33.4 (81%)
20.3 (97%)
Mg(NO3)2/Al2O3
MgO/Al2O3
4
1.2
0.8
0.3
none
none
none
87.8
54.5
none 0.8
none 0.7
d
d
a Storage ability ) total amount of N element of desorbed product during TPD (µmol/0.1 g cat)/total amount of metal cation in metal oxide/
γ-Al2O3 (µmol/0.1 g cat) × 100%. b Ratio of desorbed N to adsorbed NO in parentheses. c Ratio of desorbed O to adsorbed O. d No O2 was
produced.
K over metal oxide/γ-Al2O3 produced from TPDE, (3) TPD
study of NO/O2 adsorbates from 298 to 773 K at a heating
rate of 10 K/min in flowing He at 75 cm3/min, (4) adsorption
of NO/O2 at 523 K over metal oxide/γ-Al2O3, and (5) TPD
of NO/O2 adsorbates from 523 to 773 K in He flow. Figure
1a-c shows the typical IR spectra of nitrates as well as MS
profiles of gaseous species obtained during TPDE of Tb(NO3)3
on γ-Al2O3, NO/O2 adsorption on Tb4O7/γ-Al2O3, and TPD
studies. The metal oxides produced from TPDE of nitrates/γ-
Al2O3 were characterized by X-ray diffraction (XRD) using a
Philips Analytical, XRD B. V. diffractometer with Cu KR
radiation.
3. Results
3.1. IR Characterization of Metal Nitrates and Metal
Nitrates/γ-Al2O3. Figure 2a,b compares the IR spectra of metal
nitrates and metal nitrates/γ-Al2O3 at 298 K. All the nitrates
used in this study exhibit typical nitrate bands at around 1380
cm-1 (NO2 stretching, γ3), 1037 cm-1 (NO stretching, γ1), 814-
826 cm-1 (out-of-plane bending, γ2), 729-750 cm-1 (NO2
bending, γ4), and 1764-1778 cm-1 (γ1 + γ4).36-45 The IR
spectra of Tb(NO3)3‚5H2O, La(NO3)3‚6H2O, Ba(NO3)2, and Mg-
(NO3)2‚6H2O observed here agree well with those reported in
the literature.36,39 The bands at 3375 and 1640 cm-1 for Tb-
(NO3)3‚5H2O, La(NO3)3‚6H2O, and Mg(NO3)2‚6H2O can be
attributed to H2O. The absence of bands at around 3400 and
1630 cm-1 for anhydrous Ba(NO3)2 further confirms the H2O
assignment. Tb(NO3)3‚5H2O and La(NO3)3‚6H2O, the lanthanide
elements, exhibit the triplet in the 1290-1480 cm-1 region.
The γ-Al2O3 surface has a greater effect on Tb and La nitrates
than on Ba and Mg nitrates. Figure 2b shows that the contour
Figure 7. In situ IR spectra during TPD of adsorbates on Tb4O7/γ-
and wavenumber of IR bands for Ba and Mg nitrates are
modified by the γ-Al2O3 support, probably because of the
overlapping IR absorption of γ-Al2O3 and distortion of the
nitrate structure induced by the γ-Al2O3 surface. In contrast,
the triplet characteristics of free nitrate in the 1296-1475 cm-1
region decreased their intensities, and chelating nitrates at 1505-
1580 cm-1 emerged on La and Tb nitrates on γ-Al2O3. It is
interesting to observe that the nitrates of the elements in the
same group exhibit a similar vibration frequency, indicating the
similarity in their structures.
3.2. XRD Determination. Figure 3 shows the XRD patterns
of metal oxides/γ-Al2O3 produced from the thermal decomposi-
tion of nitrates/γ-Al2O3. Tb and Ba oxides gave distinct X-ray
diffraction patterns for Tb4O7 and BaO, whose crystallite sizes
were determined to be 117.2 and 220.8 Å, respectively, by the
Al2O3 from 298/523 to 773 K as well as IR spectra of Tb(NO3)3‚H2O/
γ-Al2O3 and Tb(NO3)3‚6H2O at 298 K.
Scherrer equation.46 The absence of XRD patterns for La and
Mg oxides suggests that these oxides were highly dispersed on
γ-Al2O3 with a crystallite size less than 30 Å. Those oxides
produced from nitrate decomposition were denoted as Tb4O7/
γ-Al2O3, La2O3/γ-Al2O3, BaO/γ-Al2O3, and MgO/γ-Al2O3 in
this study because they are the most abundant forms of oxides
on γ-Al2O3, as evidenced by XRD. Table 1 lists the loading of
metal nitrates and metal oxides, and the weight percentage
needed for achieving a monolayer dispersion47 of each metal
oxide on γ-Al2O3. Loading of La2O3, BaO, and MgO is
sufficiently high to achieve more than two layers of oxides on
the surface of γ-Al2O3. XRD results indicate that Tb4O7 and

4678 J. Phys. Chem. B, Vol. 104, No. 19, 2000
Chi and Chuang
Figure 8. Normalized product MS profiles of (a) thermal decomposition of La(NO3)3 on γ-Al2O3, (b) TPD of adsorbates on La2O3/γ-Al2O3 from
298 to 773 K, and (c) TPD of adsorbates on La2O3/γ-Al2O3 from 523 to 773 K.
BaO aggregated on the surface as particles. The absence of XRD
patterns for aluminate-containing mixed oxides suggests the lack
of extensive interaction/reaction between the supported oxides
and γ-Al2O3. The possibility of the formation of La, Mg, and
Ba aluminate can be ruled out because their formation occurred
at temperatures above 1000 K.48-51 It should be noted that
lanthanum is the structural promoter most commonly used to
stabilize Al2O3 against thermal sintering.51 Improvement of
hydrothermal stability of Al2O3 has also been observed with
MgO and BaO promoters.
mined due to the ill-defined oxide surface. Band assignment
here follows the reported classical works,44,45 previous litera-
ture,16-34,36-43 and IR spectra of nitrates and nitrates/γ-Al2O3
in Figure 2. The low intensity of the bands in the 1300-1450
cm-1 region for NO3 indicates that Tb(NO3)3 was not
-
reconstructed on Tb4O7/γ-Al2O3. Flowing NO/O2 at 523 K
produced (1) more N2 but less N2O than that at 298 K and (2)
O
a dominant Tb N-O at 1556 and 1282 cm-1. NO ap-
O
proached steady state faster at 523 K than at 298 K. The lack
of Tb-NO+ at 523 K may be due to the oxidization of adsorbed
O
NO+ to Tb N-O.
3.3. Adsorption of NO and O2 on Metal Oxides/γ-Al2O3.
Figure 4 shows the MS profiles of the reactor effluent and in
situ IR spectra of adsorbates produced from flowing 0.08% NO/
2% O2 in He over Tb4O7/γ-Al2O3 at 298 K (Figure 4a,b) and
523 K (Figure 4c,d), respectively. NO/O2 flow was introduced
into the reactor by a step-switch from He flow to NO/O2/He
flow. The use of a well-defined step switch allows observation
of adsorbate evolution and accurate determination of the amount
of NO adsorbed. The amount of NO adsorbed at 298 and 523
K corresponds to the shaded area in Figure 4a,c, respectively.
Table 2 lists the amount of NO adsorbed at 298 and 523 K on
Tb4O7/γ-Al2O3 and other oxides/γ-Al2O3 used in this study.
Exposure of Tb4O7/γ-Al2O3 to NO/O2/He flow at 298 K
produced (1) N2O and N2, as shown by the increase in N2O
O
Figure 5 compares the in situ IR spectra of adsorbates
produced from flowing NO/O2 over La2O3/γ-Al2O3, BaO/γ-
Al2O3, and MgO/γ-Al2O3 at 298 and 523 K. Band assignment
of these various adsorbates is summarized in Table 3. In general,
M-O
bridging bidentate nitrate (
> N-O) exhibits a NO
M-O
vibration band at 1600-1650 cm-1 and a weak NO2 asymmetric
vibration band at 1170-1225 cm-1. Chelating bidentate nitrate
O
(M
> N-O) gives a NO vibration band at 1500-1565
O
cm-1 and a weak NO2 asymmetric vibration band at 1260-
1300 cm-1. Monodentate nitrate (M-O-N < OO) shows a NO2
asymmetric vibration band at 1480-1530 cm-1 and a weak NO2
-
symmetric vibration band at 1250-1290 cm-1. Nitrate (NO3
)
displays a NO2 stretching band at around 1380 cm-1 44,45
IR
.
and N2 MS profiles in Figure 4a, and (2) bridging bidentate
spectra of adsorbates produced from La oxide resemble those
on Tb oxide despite the significant difference in their crystallite
size and in the stoichiometry of Tb4O7 and La2O3 (i.e., the
possible form of La oxide). Adsorbed nitrates on BaO and MgO
show distinct bridging bidentate nitrate bands at 1630 cm-1 and
N2O22- on MgO at 1748 and 1257 cm-1 34,52
. The formation of
bridging bidentate nitrate on MgO is significantly faster than
that of monodentate nitrate at 298 K. The IR intensity corre-
Tb-O
nitrate (
> N-O) at 1630 cm-1 and chelating bidentate
Tb-O
O
nitrate (Tb N-O) at 1570 cm-1, with their asymmetric NO2
O
stretching overlap at around 1298 cm-1; monodentate nitrate
O
(Tb-O-N < O) at 1470 cm-1, NO3 at 1380 cm-1 44,45
,
and
-
Tb-NO+ at 1928 cm-1. It should be noted that the broad IR
band in the region of 1380-1630 cm-1, which is the result of
multiple overlapping bands, could not be unambiguously deter-

Infrared and TPD Studies of Nitrates
J. Phys. Chem. B, Vol. 104, No. 19, 2000 4679
suggesting that these adsorbates are responsible for the formation
of NO, O2, N2O, N2, and NO2. The decrease in chelating
bidentate nitrate, which was produced from NO/O2 adsorption
at 523 K in Figure 7, corresponds to a symmetric NO, O2, and
N2 desorption profile with the N/O molar ratio of 0.8 centered
at 640 K in Figure 6c, indicating the conversion of this nitrate
to NO and O2. The small amount of desorbed O2 suggests that
the chelating bidentate nitrate decomposed primarily to NO,
leaving oxygen on the metal oxide surface. The NO desorption
centered at 640 K in Figure 6c has also been observed for NO
TPD from 298 to 773 K in Figure 6b. The NO desorption at
640 K in Figure 6b is indeed a result of decomposition of
chelating bidentate nitrate at 1556 cm-1 as shown in Figure 7.
The most interesting observations in Figures 6 and 7 are that
(1) N2 was produced as a minor product and (2) the amount of
NO, O2, N2, and N2O desorbed from nitrates adsorbed on Tb4O7/
γ-Al2O3 is significantly less than the decomposition of the
original nitrate. It appears that metal oxide produced from Tb-
(NO3)3 decomposition has agglomerated and decreased the
number of surface metal cations available for adsorption.
Furthermore, each Tb in Tb(NO3)3 accommodates three (NO3)-,
while each Tb in bridging and chelating bidentate nitrates and
monodentate nitrate can accommodate only one (NO3)-.
Figures 8 and 9 show the product MS profiles and IR spectra
during TPDE of La(NO3)3‚H2O/γ-Al2O3 and subsequent TPD
studies on La2O3/γ-Al2O3. TPDE of La(NO3)3‚H2O/γ-Al2O3
exhibited IR and MS patterns similar to those of Tb(NO3)3‚
H2O/γ-Al2O3. N2 formation on La2O3/Al2O3 is less than that
on Tb4O7/γ-Al2O3.
Figures 10 and 11 show the MS profiles and IR spectra during
TPDE of Ba(NO3)2‚H2O/γ-Al2O3 and subsequent TPD studies
on BaO/γ-Al2O3. TPDE of Ba(NO3)2 on γ-Al2O3 exhibited
different MS profile patterns from those for Tb(NO3)3 and La-
(NO3)3, with only one O2 desorption peak at 725 K.
Although Ba(NO3)2/γ-Al2O3 and Mg(NO3)2/γ-Al2O3 exhibit
similar IR spectra, as shown in Figure 2, their TPDE and TPD/
IR profiles show dramatic differences, as shown in Figures 10-
13. The major difference is the absence of O2 during the TPDE
of Mg(NO3)2 and the TPD of NO/O2 adsorbates on MgO/γ-
Al2O3. The latter is due to the lack of O2 adsorption during
NO/O2 adsorption at 298 and 523 K. In addition, bridging
bidentate nitrate is centered at 1630 cm-1 on MgO and at 1600
cm-1 on BaO at 523 K. The former decomposed to NO and
N2O while the latter decomposed to NO and O2, reflecting their
differences in structure and reactivity.
Figure 9. In situ IR spectra during TPD of adsorbates on La2O3/γ-
Al2O3 from 298/523 to 773 K as well as IR spectra of La(NO3)3‚H2O/
γ-Al2O3 and La(NO3)3‚6H2O at 298 K.
sponds to the concentration of adsorbates, also reflecting the
number of specific adsorption sites for these adsorbates.
However, a lack of extinction coefficients for these various
forms of adsorbed NOx species does not allow use of these
adsorbate intensities to determine the number of adsorption
sites.
3.4. TPDE and TPD Studies. Figure 6 compares the
normalized product MS profiles during TPDE of Tb(NO3)3 on
γ-Al2O3 (Figure 6a) to those during TPD of adsorbates on
Tb4O7/γ-Al2O3 from 298 to 773 K (Figure 6b) and from 523 to
773 K (Figure 6c) in He flow. These adsorbates were produced
from NO/O2 adsorption at 298 and 523 K, respectively. Tb-
(NO3)3 began to decompose at 374 K, releasing NO, O2, N2O,
N2, and NO2 with a total amount of 185.5 µmol, a N/O ratio of
1.1, and a NO/O2 ratio of 12.5. The MS intensity profiles were
normalized with the calibration factors of each species so
that the shaded area under the MS profiles corresponds to the
product amount. Table 4 lists the amount of products produced
from TPDE of γ-Al2O3-supported nitrates and TPD of adsor-
bates on metal oxides. Because NO is the dominant product,
the ratio of N to O for all the products is near 1, indicating that
decomposition of nitrates leaves oxygen on the metal oxide
surface.
Figure 7 shows the in situ IR spectra of adsorbed NOx on
Tb4O7/γ-Al2O3 taken during TPD from 298 to 773 K and from
523 to 773 K, as well as the IR spectra of Tb(NO3)3‚5H2O and
Tb(NO3)3‚H2O/γ-Al2O3. The IR bands below 1200 cm-1 for
nitrates/γ-Al2O3 and adsorbed NOx on metal oxides/γ-Al2O3
were blocked by the γ-Al2O3 support and CaF2 windows.
Decrease in IR intensity with increasing temperature is indicative
of desorption/decomposition of adsorbates on Tb4O7/γ-Al2O3,
4. Discussion
4.1. Formation of Adsorbed Nitrate. Adsorption of NO and
O2 on Tb4O7, La2O3, BaO, and MgO proceeds through a
different pathway and pattern from that on noble metals. NO
adsorbs on the reduced noble metal through the N atom in the
form of M0-NO (where M0 is the reduced metal site).53-56 NO
stands either perpendicularly or titled to the surface, depending
on the nature of the surface sites. Coadsorption of NO and O2
usually leads to the formation of M+-NO where M+ is formed
by oxidation of M0. M+-NO has often been denoted as
M-NO+.53-56 Interestingly, this type of M-NO+ has been
observed on Tb4O7 and La2O3. The presence of M-NO+ on
Tb4O7 and La2O3 could be due to the nonstoichiometric nature
of Tb and La oxides where their cations are exposed for direct
NO adsorption.
Because of the ill-defined states of the surfaces of these
oxides, the elementary steps involved in the formation of nitrates
from NO/O2 adsorption are significantly more difficult to
elucidate than those on noble metals. NO/O2 coadsorption on

4680 J. Phys. Chem. B, Vol. 104, No. 19, 2000
Chi and Chuang
Figure 10. Normalized product MS profiles of (a) thermal decomposition of Ba(NO3)2 on γ-Al2O3, (b) TPD of adsorbates on BaO/γ-Al2O3 from
298 to 773 K, and (c) TPD of adsorbates on BaO/Al2O3 from 523 to 773 K.
these oxides produced three types of nitrates: bridging bidentate,
chelating bidentate, and monodentate nitrates. In addition, trace
amounts of N2 and N2O were produced, indicating the occur-
rence of N-O bond dissociation and N-N bond formation.
These two steps occurred to a very limited extent on the oxide
surface, as evidenced by low product formation rates. N-O bond
dissociation is expected to occur on the unsaturated coordination
sites where there are deficiencies in oxygen coordination.57-60
N-N bond formation, a facile process, could be immediately
followed by N-O dissociation. Coadsorption of NO and O2
allows O2 to compete for unsaturated coordination sites, further
decreasing the N2 and N2O formation.
adsorbed oxygen which serve as a Lewis base site. The overall
reaction may be written as:
O2 may adsorb on the oxide surface according to the
following sequence:61-63
The formation of chelating bidentate nitrate has been observed
on NO adsorption on Tb4O7.70 Results in Figure 4 show that
coadsorption of NO and O2 on Tb4O7 produced both bridging
and chelating bidentate nitrates at 298 K and formed only
chelating bidentate nitrates at 523 K. Similar observations have
been reported on oxidized chromia.71 The N-O single bond in
chelating bidentate nitrate appears to come from the lattice
oxygen of the oxide, whereas the oxygen in bridging bidentate
nitrate could result from adsorbed oxygen. The adsorbed oxygen
may be in the form of O- which locates in a configuration
favoring the bridging bidentate nitrate formation. This proposed
scheme explains the formation of a dominant chelating bidentate
band at 523 K and the formation of a bridging bidentate nitrate
at 298 K, where significantly more O2 is adsorbed than at 523
K as shown in Table 2. The absence of adsorbed nitro species
and NO2 in the product stream indicates either the lack of surface
sites available for the NO + O- reaction or the rapid conversion
of nitro/NO2 species to nitrates.
+e
+e
O2(g) S O2(ads) S O2- (ads) S O22- (ads) S
2O- (ads) +S2e 2O2- (lattice)
Electron spin resonance (ESR) studies of adsorbed oxygen
on TiO2, ZnO, Fe2O3, MgO, and NiO provide evidence to
support O2 formation.61,62,64-67 This type of oxygen is stable
-
-
at 298 K. Increasing temperature causes transformation of O2
to O2- (i.e., lattice oxygen).62,64,67-69 Thus, which types of
adsorbed oxygen predominate may depend on the type and state
of the surface and temperature. The metal cation may serve as
an electron donor, allowing electron transfer to oxygen and
increasing the oxidation state of the cation.68 The nonstoichio-
metric metal oxides, such as Tb4O7, and those oxides with redox
properties may allow oxidation of their cations.
Formation of bridging bidentate nitrate, chelating bidentate
nitrate, and monodentate nitrate from NO and O2 adsorption
on Tb4O7, La2O3, and BaO may involve different forms of

Infrared and TPD Studies of Nitrates
J. Phys. Chem. B, Vol. 104, No. 19, 2000 4681
About 80% of oxygen in chelating bidentate nitrate on Tb4O7
and La2O3 returned to the oxygen lattice form on the oxides.
Chelating bidentate nitrate appears to be the most stable form
of nitrate on Tb4O7, La2O3, and BaO. Bridging bidentate nitrate
and other forms of nitrates which were produced from NO/O2
adsorption decomposed at temperatures lower than chelating
bidentate nitrate, producing N2O, N2, NO2, NO, and O2 on
Tb4O7, La2O3, and BaO. Bridging bidentate nitrate, a major
nitrate, on MgO also produced N2O, N2, and NO2. Formation
of a small amount of N2 and N2O during TPDE/TPD of nitrates
suggests the occurrence of N-O bond dissociation and N-N
bond formation between these nitrate species.
Consistency in the TPD peak temperatures of NO and O2
for the NO/O2 adsorbates at 298 and 523 K suggests that the
nature of adsorption sites for the nitrate formation was not
affected by the heating cycle. Repeated NO/O2 adsorption and
TPDE studies on Pt-Tb/γ-Al2O375 show that the heating cycle
did not affect adsorption and desorption characteristics of
adsorbed nitrates. Further study is needed to determine the effect
of the heating cycle on the performance of these supported
oxides. It should be noted that the amount of O2 desorbed
(measured by TPD profile) is less than that of adsorbed
(measured by the step response profile), except for MgO/γ-
Al2O3, as shown in Table 4. This discrepancy may be attributed
to the readsorption of oxygen and the reaction of O2 with other
species in the mass spectrometer chamber. Underestimation of
O2 was also observed in the TPDE of nitrate/γ-Al2O3.
TPDE of the Tb(NO3)3, La(NO3)3, and Ba(NO3)2 produced
more N2O than TPD of the nitrates produced from NO/O2
adsorption, as shown in Table 4. This observation indicates that
these metal nitrates on γ-Al2O3 in the forms of mixed nitrates
(i.e., NO3-, bridging and chelating bidentate nitrates) produced
significantly more N2O than other forms of nitrates. The infrared
and mass spectrometer results reported here do not allow
elucidation of the elementary steps involved in the N2 and N2O
formation. Formation of N2 on Tb4O7 could be attributed to its
nonstoichiometric nature and redox property. The formation of
N2 from nitrate decomposition suggests the possibility of NO
decomposition in an oxidizing environment through nitrates, as
follows:
Figure 11. In situ IR spectra during TPD of adsorbates on BaO/γ-
Al2O3 from 298/523 to 773 K as well as IR spectra of Ba(NO3)2‚H2O/
γ-Al2O3 and Ba(NO3)2 at 298 K.
Pretransition metal oxides such as MgO are very inert and
can neither be oxidized nor reduced easily.68,72 It is indeed not
surprising to observe the low O2 adsorption capability on MgO.
Although MgO lacks O2 adsorption capability, gaseous O2 can
exchange with O on the solid surface.59 The bridging bidentate
nitrate could be formed from NO adsorption coupled with
oxygen exchange.
BaO and MgO, the alkali earth elements, show distinct
differences in their TPDE and TPD/IR profiles. BaO is known
for its exceptional capability for NOx storage.24,31 Results in
Figure 5 show that NO and O2 coadsorb on BaO as bridging
bidentate, chelating bidentate, and monodentate nitrates at 298
and 523 K. Lunsford and co-workers10-12 reported that flowing
NO alone over defect-rich BaO/MgO produced nitrates, nitrites,
and Ba-nitro complexes; exposure of nitrite/nitro species to
oxygen led to the formation of NO3-; exposure of BaO2 to NO
produced nitro and NO3-. Our recent study73 on CuO/γ-Al2O3
showed that the successive oxidation of adsorbed nitrite and
nitro species produced various forms of nitrate during NO/O2
coadsorption. All of these studies reveal that nitrate is the final
form of the products on the oxide surface regardless of sequence
of NO/O2 adsorption or coadsorption. Excellent O2 adsorption
capability can be attributed to (1) its unsaturation sites and (2)
reaction as BaO + 1/2O2 (g) S BaO2.74 BaO2 has been produced
during NO decomposition on BaO/MgO.10-12
NO + O2 S nitrate w N2 + N2O + O2
Formation of N2, O2, and NO2 has been observed during the
decomposition of calcium nitrates.76 However, the amount of
N2 produced remained too low to develop a practical process
for NO removal. Isolation of the site and determination of the
steps involved in the product formation may lead to development
of an effective NO decomposition catalyst for removal of NO
in an oxidizing environment.
5. Conclusion
Infrared spectroscopy coupled with mass spectroscopy allows
determination of the form of NOx storage during NO/O2
adsorption and its behavior during TPDE/TPD. NO/O2 coad-
sorbed as bridging and chelating bidentate nitrates and mono-
dentate nitrate on Tb4O7, La2O3, and BaO, and as bridging
bidentate nitrate and monodentate nitrate on MgO via the
reaction of adsorbed NO with adsorbed oxygen at 298 K. NO/
O2 coadsorbed as a chelating bidentate nitrate on Tb4O7 and
La2O3, and as a distinctive bridging bidentate nitrate on BaO
and MgO via the reaction of adsorbed NO with surface lattice
oxygen at 523 K.
4.2. Nitrate Decomposition. TPDE/TPD of nitrates in Figures
7-13 shows that NO and O2 are the major decomposition
products. TPD from 523 to 773 K for chelating bidentate nitrate
(produced from NO/O2 adsorption at 523 K) on Tb4O7, La2O3,
and BaO gave primarily NO and O2 product at a ratio of NO/
O2 of 4.5-3.9, suggesting that reaction I is essentially reversible.

4682 J. Phys. Chem. B, Vol. 104, No. 19, 2000
Chi and Chuang
Figure 12. Normalized product MS profiles of (a) thermal decomposition of Mg(NO3)2 on γ-Al2O3, (b) TPD of adsorbates on MgO/γ-Al2O3 from
298 to 773 K, and (c) TPD of adsorbates on MgO/γ-Al2O3 from 523 to 773 K.
TPD of chelating bidentate nitrate on Tb4O7, La2O3, and BaO
produced primarily NO and O2, with maxima at 640 and 670
K, respectively. TPD of bridging bidentate nitrate and mono-
dentate nitrate on Tb4O7, La2O3, and BaO produced NO and
O2 as major products and N2 and N2O as minor products at
320-500 K. Decomposition of bridging bidentate on MgO
produced NO as a major product and N2O as a minor product
at a peak temperature of 690 K. Observation of NO and O2 as
major products of chelating and bridging bidentate nitrate
decomposition suggests that the formation of these nitrates from
NO and O2 is essentially reversible without involvement of NO2
as an intermediate. Peak temperatures for Tb(NO3)3, La(NO3)3,
Ba(NO3)2, and Mg(NO3)2 decomposition occurred between those
for bridging and chelating nitrates. The difference in reactivity
and stability of chelating and bridging bidentate nitrates on
various oxides may provide a wide range of operating temper-
atures for NOx storage. Formation of N2 from decomposition
of various forms of nitrates suggests the need to identify the
step involved in the conversion of nitrate to N2. Identification
of this step may guide the development of the oxide for
conversion of NO in an oxidizing environment to N2 and O2
via metal nitrates.
Acknowledgment. The authors gratefully acknowledge the
partial support of this research by the United States Department
of Energy under Grant DE-FG22-95PC955224 and the Ohio
Boards of Regents Research Challenge Grant. The assistance
from Bei Chen and Pisanu Toochinda in performing these
experiments is also appreciated.
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