Adsorption/desorption studies of NOx on well-mixed oxides derived from Co-Mg/Al hydrotalcite-like compounds

Article abstract of DOI:10.1021/jp056473f

Co_xMg_3-x/Al hydrotalcite-like compounds (where x = 0.0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0) were synthesized by the coprecipitation method and characterized by the XRD and TGA techniques. Incorporation of Co for x = 0.0-3.0 gradually decreased the transformation temperature of the hydrotalcites to the corresponding oxides from 444 to 246°C and also decreased the surface area from 162.7 to 21.6 m²/g upon calcination at 800°C for 4 h in air. The resultant oxide was generally composed of a poor MgO phase and a spinel phase, with more spinel phase at higher Co incorporation. The derived oxides were tested as the storage/reduction catalysts for NO_x adsorption/desorption. The storage capacity for NO _x was highly dependent on the catalyst composition and storage temperature. In general, more NO_x was stored at lower temperature (100°C) than that at higher temperature (300°C), and tertiary catalysts (x = 0.5-2.5) stored more NO_x than binary catalyst (x = 0.0 or 3.0). The catalytic conversion of NO to NO₂ and the catalytic decomposition of NO_x were observed on the tertiary catalysts during NO_x adsorption at 300°C, which was highly related to the loading of cobalt. The reducibility of catalysts was determined by TPR experiments, and the reduction of cobalt cations started at 150-200°C in H₂. In situ IR spectra of catalysts adsorbing NO_x revealed that the major NO_x species formed on the catalysts were various kinds of nitrites and nitrates, together with some forms of dimers, such as N₂O₂ ^2- and N₂O₄ (or NO⁺NO ₃^-). The storage/reduction mechanism and the function of Co in the mixed oxides are proposed and discussed on the basis of these observations.

Full text of DOI:10.1021/jp056473f

J. Phys. Chem. B 2006, 110, 4291-4300
4291
Adsorption/Desorption Studies of NOx on Well-Mixed Oxides Derived from Co-Mg/Al
Hydrotalcite-like Compounds
†
†
†
,†
,‡
Jun Jie Yu, Zheng Jiang, Ling Zhu, Zheng Ping Hao,* and Zhi Ping Xu*
Research Center for Eco-EnVironmental Sciences, Chinese Academy of Sciences, Beijing 100085, PR China,
and Australian Research Council (ARC) Centre for Functional Nanomaterials, School of Engineering,
The UniVersity of Queensland, Brisbane QLD 4072, Australia
ReceiVed: NoVember 8, 2005; In Final Form: December 22, 2005
Co
the coprecipitation method and characterized by the XRD and TGA techniques. Incorporation of Co for x )
.0-3.0 gradually decreased the transformation temperature of the hydrotalcites to the corresponding oxides
x
Mg3-x/Al hydrotalcite-like compounds (where x ) 0.0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0) were synthesized by
0
2
from 444 to 246 °C and also decreased the surface area from 162.7 to 21.6 m /g upon calcination at 800 °C
for 4 h in air. The resultant oxide was generally composed of a poor MgO phase and a spinel phase, with
more spinel phase at higher Co incorporation. The derived oxides were tested as the storage/reduction catalysts
for NO
and storage temperature. In general, more NO
temperature (300 °C), and tertiary catalysts (x ) 0.5-2.5) stored more NO
.0). The catalytic conversion of NO to NO and the catalytic decomposition of NO
tertiary catalysts during NO adsorption at 300 °C, which was highly related to the loading of cobalt. The
reducibility of catalysts was determined by TPR experiments, and the reduction of cobalt cations started at
50-200 °C in H . In situ IR spectra of catalysts adsorbing NO revealed that the major NO species formed
on the catalysts were various kinds of nitrites and nitrates, together with some forms of dimers, such as
x
adsorption/desorption. The storage capacity for NO
was stored at lower temperature (100 °C) than that at higher
than binary catalyst (x ) 0.0 or
were observed on the
x
was highly dependent on the catalyst composition
x
x
3
2
x
x
1
2
x
x
2-
+
-
N
2
O
2
and N
2 4 3
O (or NO NO ). The storage/reduction mechanism and the function of Co in the mixed
oxides are proposed and discussed on the basis of these observations.
Introduction
the catalyst. When the engine is switched to conditions with a
stoichiometric air-to-fuel ratio, i.e., rich-burn conditions, NOx
Various nitrogen oxides (NOx), generally produced in the
combustion of fossil fuels, are the major pollutants in air that
cause some environmental problems, such as photochemical
smog and acid rain, as well as some human diseases, such as
species are released and subsequently reduced by hydrocarbons
9,10
and carbon monoxide.
A model NOx storage/reduction
catalyst comprises three major components: a high-surface-area
support material (e.g., γ-Al2O3), a NOx storage component
containing alkali or alkaline earth metals (e.g., Ca, Sr, Ba, K,
1
asthma. Thus, there is a growing need to reduce emissions of
NOx from automobile combustion, mainly via catalytic decom-
position to environmentally friendly N2 and O2. In fact, Pt-Rh-
based three-way catalysts (TWCs) can effectively convert NOx
or Na), and a noble metal (e.g., Pt, Rh, or Pd) as the catalytic
9-12
redox component.
These storage/reduction catalysts are
efficient at removing NOx under lean-rich cycles in the absence
of SO2. Nevertheless, a problem is that SO2 generated in trace
amounts in the exhaust dramatically deactivates the storage/
1
,2
to N2 under conditions of a stoichiometric air-to-fuel ratio.
However, the general requirement of more fuel-efficient gasoline
engines due to limited fossil fuel resources on the earth drives
car manufacturers to develop lean-burn engines that combust
13
reduction catalysts, which, together with the high expense of
noble metals, demands the development of inexpensive and SO2-
tolerant storage/reduction catalysts with similar or better storage/
reduction performances.
3
fuel more efficiently under excess oxygen. The presence of
excess oxygen in the exhaust severely reduces the activity of
4
three-way catalysts for NOx decomposition. Therefore, catalysts
In sharp contrast to the abundant research on the improvement
that can effectively reduce the NOx amount in the presence of
excess oxygen have now been widely sought, among which NOx
storage/reduction (NSR) catalytic treatment seems a more
9-19
of the model storage/reduction catalyst Pt-Ba/Al2O3,
such
as introducing other components, including Fe, Ce, Cu, and Ni,
the search for other alternative catalysts seems to draw less
5
,6
promising approach to NOx removal in excess oxygen.
10,12,17,18,20-26
10,20-23
24
attention.
Perovskites
and zeolites are the
NOx storage/reduction technology is used in engines that
alternately operate under lean-burn and rich-burn conditions.
Under lean-burn conditions, NO is oxidized and is stored on
primary candidates that show potential as alternative NSR
catalysts. Recently, a type of well-mixed and well-dispersed
oxides derived from hydrotalcite-like compounds has received
considerable attention in the search for alternative NSR cata-
7
,8
1
2,17,18,25,26
lysts.
*
Corresponding authors. Tel.: +86-10-62849194 (Z.P.H.), 61-7-
3469973 (Z.P.X.). Fax: +86-10-62923564 (Z.P.H.), 61-7-33656074
Z.P.X.). E-mail: zpinghao@mail.rcees.ac.cn (Z.P.H.), zhipingx@
3
(
Hydrotalcite-like compounds (HTlcs), also known as anionic
clays or layered double hydroxides (LDHs), are multifunctional
materials that are widely used as adsorbents, ion exchangers,
base catalysts, and precursors of well-mixed oxides for various
cheque.uq.edu.au (Z.P.X.).
†
‡
Chinese Academy of Sciences.
The University of Queensland.
1
0.1021/jp056473f CCC: $33.50 © 2006 American Chemical Society
Published on Web 02/03/2006

4
292 J. Phys. Chem. B, Vol. 110, No. 9, 2006
Yu et al.
catalytic applications.27,28 HTlcs are a large family of mixed
hydroxides and can be chemically expressed by the general
In a typical measurement, 20-30 mg of HT sample was heated
in an Al2O3 crucible at a constant heating rate of 10 °C/min
from 25 to 1000 °C, with air purging at a flow rate of 30 mL/
min.
II
III
x+
n-
II
formula [M 1-xM x(OH)2] (A )x/n‚mH2O, where M repre-
III
sents any divalent metal cation, M any trivalent metal cation,
n-
27,28
and A an anion (inorganic or organic).
HTlcs can contain
The textural properties of calcined HTs (xCoMAO) were
analyzed by N2 adsorption/desorption at liquid nitrogen tem-
perature (77 K), using a Quantachrome NOVA-1200 gas
absorption analyzer. The specific surface area was calculated
with the BET equation, and the pore volume and pore size
distribution were obtained with the BJH method from the
adsorption isotherm.
metal cations of more than two types, which leaves vast
flexibility for the better selection of various cations in these
materials for NSR catalyst development. For example, in a
mixed oxide derived from an M-MgAl HT compound, Al2O3
acts as the support; MgO acts as a NOx storage component;
II
III
and M (M and/or M ) can act as the active component for
promoting storage of NOx, conversion of NOx to N2 and O2,
and reduction of NOx with carbon monoxide and hydrocarbons.
In fact, Cu-containing calcined MgAl HTlcs have been found
Temperature-programmed reduction (TPR) was performed for
all xCoMAO catalysts on a conventional TPR apparatus
equipped with a thermal conductivity detector (TCD). In general,
the sample (50 mg) was inserted in a quartz tube and sandwiched
between two quartz wool plugs. Prior to each TPR run, the oxide
catalyst was heated to 500 °C with O2 flushing (40 mL/min)
for 30 min and then cooled to room temperature under the
oxygen stream. After the flowing gas had been switched, N2
was introduced into the reactor at 50 mL/min for 1 h at room
temperature to purge away any residual oxygen. This pretreated
oxide catalyst was then heated to 900 °C at a ramp of 10 °C/
min and reduced in a reducing environment (5% H2 in N2 at a
flow rate of 50 mL/min). During the heating, hydrogen
consumption was monitored by the TCD.
1
7,18,29
to be active and selective catalysts of NOx removal.
Our
recent studies3 show that these HTlc-derived catalysts perform
well for NOx storage/reduction, especially at low temperatures.
Taking into account the fact that cobalt species are active
0,31
3
2,33
components for NOx removal and decomposition,
we
prepared a series of Co-Mg/Al mixed oxide catalysts from the
corresponding HTlc precursors upon calcination. These mixed
oxide catalysts were tested as storage/reduction catalysts of
mixed NO and NO2 gases at 100 and 300 °C. The storage
amount of NOx was very much related to the catalyst composi-
tion, and there should be an optimum NSR catalyst with a
careful tradeoff of the Co/Mg ratio in the precursor. The storage/
reduction mechanism of NOx on these catalysts and the effect
of cobalt in these processes were further proposed on the basis
of the in situ IR spectra and adsorption/desorption experimental
data.
Thermal NOx Adsorption/Desorption. Thermal NOx ad-
sorption experiments were carried out in a quartz flow reactor
(i.d. ) 8 mm and L ) 600 mm) using 1.0 g of Co-Mg/Al
oxide catalyst (20-40 mesh powder). Oxide catalyst was
pretreated in a gas flow of O2/N2 (8% O2 by volume) at 500 °C
Experimental Section
-
1
at a constant space velocity of ∼30 000 h for 1 h and then
cooled to the experimental temperature (100 or 300 °C). When
the temperature had stabilized at 100 or 300 °C, 1400 ppm NOx
Materials Preparation. Various CoxMg3-x/Al hydrotalcite-
like compounds with (Co2 + Mg )/Al molar ratio fixed at
+
2+
3+
3
.0 were prepared with a constant-pH coprecipitation method
(1300 ppm NO and 100 ppm NO2) and 8% O2 in N2 were
where x was set at 0.0, 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0 (denoted
as xCoMA-HT). Briefly, a mixed salt solution (100 mL) and a
mixed basic solution (100 mL) were simultaneously added
dropwise into 100 mL of doubly distilled water within 1 h at
constant pH (10 (0.5) under vigorous mechanical stirring. The
salt solution with a total metal concentration of 1.0 M contains
suitable amounts of Mg(NO3)2‚6H2O (>99%, Yili Company),
Co(NO3)2‚6H2O (>99%, Jinke Company), and Al(NO3)3‚9H2O
introduced at a rate of 500 mL/min for 30 min for thermal NOx
adsorption. Concentrations of NO, NO2, and NOx from the
reactor outlet were monitored by a Chemiluminescence NO-
NO2-NOx analyzer (model 42C High Level, Thermo Electron
Corporation).
After the thermal NOx adsorption, the flow gas was switched
-
1
to pure N2 (rate ) 500 mL/min, space velocity ≈ 30 000 h )
to flush the oxide catalyst for 20 min and remove the weakly
adsorbed species at the adsorption temperature. The oxide
catalyst was then cooled to 100 °C if the adsorption temperature
was 300 °C. The temperature programmed desorption (TPD)
was then conducted by heating the sample from 100 to 650 °C
at a ramp of 10 °C per min with N2 flowing at a rate of 500
mL/min. Concentrations of NO, NO2, and NOx from the reactor
outlet were monitored by a chemiluminescence NO-NO2-NOx
analyzer, and the adsorbed NOx amount was thus calculated.
(
>99%, Yili Company), and the basic solution contains NaOH
>96%, Beihua Company) and Na2CO3 (>99.8%, Beihua
(
-
2-
-
3+
Company) with [OH ]/[CO3 ] ) 16 and [OH ]/[Al ] ) 8.
Precipitates were aged in suspension at 60 °C for 4 h under
stirring in static air and then filtered and thoroughly washed
with doubly distilled water. The cake was dried at 70 °C for 12
h and again at 120 °C overnight. As-prepared HTlcs were
calcined at 800 °C for 4 h to derive corresponding Co-Mg/Al
mixed oxide catalysts, denoted as xCoMAO (x ) 0.0, 0.5, 1.0,
In Situ Infrared Monitoring. In situ FT-IR spectra were
1
.5, 2.0, 2.5, 3.0). The oxide catalysts were then crushed, sized
recorded on a Bruker Tensor 27 spectrometer in the range of
in 20-40 mesh for storage/reduction experiments, and kept in
a desiccator to avoid reconstruction of the hydrotalcite-like
structure.
Materials Characterization. The X-ray diffraction (XRD)
patterns of Co-Mg/Al HT materials (xCoMA-HTs) and the
derived mixed oxides (xCoMAO) were measured on a Rigaku
powder diffractometer (D/MAX-RB) using Cu KR radiation (λ)
-1
-1
6
00-4000 cm after 128 scans at a resolution of 4 cm . Self-
supporting pellets (∼50 mg, 20-40 mesh) were prepared from
the oxide catalysts and used directly in the IR flow cell. The
IR cell, made of stainless steel, contained a KBr window and
was connected to a vacuum apparatus with a residual pressure
-4
below 10 Pa. A K-type thermocouple was set in direct contact
with the IR flow cell to control the temperature. Prior to the
recording of an IR spectrum, the catalyst sample was pumped
off for 1 h at 350 °C to eliminate impure species on the sample
surface. After the sample had cooled to 100 or 300 °C under
vacuum, a spectrum of the treated sample was taken as the
0.15418 nm) in the 2θ range of 10-70° at a scanning rate of
4° per min. The tube voltage and current were set at 40 kV and
30 mA, respectively.
Thermal decomposition of Co-Mg/Al HTs (xCoMA-HTs)
was investigated with thermogravimetry (TG, Seteram, Labsys).

NOx Adsorption on Co-Mg/Al Hydrotalcite-like Compounds
J. Phys. Chem. B, Vol. 110, No. 9, 2006 4293
Figure 2. DTG profiles of catalysts xCoMAO in air.
for all HT samples are 0.306-0.309 and 2.29-2.37 nm,
respectively, in good agreement with the reported values.27 The
incorporation of Co into hydroxide layers seems to affect the
crystallinity of the HT crystallites. In particular, the characteristic
diffraction peaks of samples 2.0CoMA-HT and 2.5CoMA-HT
are much weaker and broader, probably because of the slight
incompatibility of Mg and Co in the ionic size (0.072 nm for
2
+
2+ 28
Mg vs 0.065 nm for Co ).
The XRD patterns of the derived mixed oxides (Figure 1B)
show the complete transformation from the hydrotalcite to the
oxide phase. It is obvious that the gradual replacement of Mg
with Co in the hydrotalcite structure leads to the gradual changes
in the phase composition. For sample 0.0CoMAO, i.e., Mg3Al
oxide, three peaks at 37°, 43°, and 62° are ascribed to the MgO
phase (JCPDS 43-1022, JCPDS 89-7746). The possible spinel
phase (MgAl2O4, JCPDS 86-2258), which is normally observ-
3
6
able at 1000 °C, was not detected. In contrast, for sample
.0CoMAO, i.e., Co3Al oxide, diffraction peaks with 2θ at
31°, 36°, 39°, 45°, 55°, 59°, and 65° were observed that are
only attributed to a spinel phase (Co2AlO4, JCPDS 38-0814;
3
∼
Figure 1. XRD patterns of (A) samples xCoMA-HT and (B) catalysts
xCoMAO, where the MgO phase is marked as “+” and spinel as “#”.
37
CoAl2O4, JCPDS 82-2246; Co3O4, JCPDS 74-2120) and could
II
III
be chemically expressed as Co 4/3Co 5/3AlO16/3. This formula
is supported by the TPR results (refer to section 3.2), suggesting
background at that temperature. Then, a mixture gas stream (total
flow 25 mL/min) containing 1300 ppm NO, 100 ppm NO2, and
II
III
that over half of Co ions are oxidized to Co during calcination
in static air. For all tertiary oxides, both MgO and spinel are
detected by XRD (Figure 1B). When more Co is incorporated
to replace Mg in the hydroxide layers, the derived oxide contains
more spinel and less MgO. For example, 0.5CoMAO is mainly
composed of a MgO phase with an unnoticeable spinel phase,
whereas samples 2.0CoMAO and 2.5CoMAO contain very
minor amounts of MgO phase. This observation suggests that
introduction of Co into the hydrotalcite promotes the formation
of the spinel phase in the oxide catalyst, which is believed to
affect the activity for NOx storage, conversion, and decomposi-
tion, as discussed shortly. In addition, the crystallite size of the
spinel phase (20-25 nm) is roughly 2-3 times larger than that
of the MgO phase (7-10 nm), as estimated from the peak width
8
% O2 in N2 was introduced for NOx adsorption at 100 or 300
°
C. Meanwhile, the IR spectra were sequentially recorded at
the time points of 1, 2, 5, 10, 20, 30, and 60 min.
Results and Discussion
Transformation of Hydrotalcites to Mixed Oxides. As-
prepared Co-containing Mg-Al hydrotalcites (xCoMA-HTs) are
identified by the X-ray diffraction (XRD) patterns, as shown
in Figure 1A. The peaks at 2θ ≈ 11°, 22°, and 35° correspond-
ing to the (003), (006), and (009) crystal planes, respectively,
indicate relatively well-formed crystalline layered structures with
rhombohedral symmetry (3R).2 On the other hand, the broad
diffraction peaks at ∼35°, 38°, and 46° attributable to the (012),
7,28
3
8
using the Debye-Scherrer equation.
(
3
015), and (018) crystal planes are characteristic of polytype
Figure 2 presents the weight loss rates of these HT compounds
during heating in air, revealing the transformation of these
xCoMA-HTs into the corresponding oxides. In general, the
thermal decomposition (weight loss) of the hydrotalcites consists
R1 hydrotalcites (JCPDS 22-700).28,34,35 The well-defined (110)
and (113) diffraction peaks reveal a quite good dispersion of
metal ions in the hydroxide layers. The cell parameters a and c

4294 J. Phys. Chem. B, Vol. 110, No. 9, 2006
Yu et al.
TABLE 1: Textual Properties and NO
x
Adsorption Data of
the Oxide Catalysts
SSA
(m /g)
pore volume pore size 100 °C 300 °C
2
3
sample
(cm /g)
(nm)
(mg/g) (mg/g)
0
0
1
1
2
2
3
.0CoMAO 162.7
.5CoMAO 157.8
0.47
0.44
0.30
0.24
0.22
0.13
0.13
11.6
11.1
14.2
17.2
21.0
24.8
23.7
4.15
6.65
5.99
5.22
5.91
5.61
4.10
2.57
4.14
4.70
4.96
5.90
5.90
3.43
.0CoMAO
.5CoMAO
.0CoMAO
.5CoMAO
.0CoMAO
83.5
55.3
41.2
21.6
21.6
of two steps. The first step occurs at 100-250 °C, and the
maximum rate of weight loss is found at 210-220 °C for all
HT samples, mainly ascribed to the loss of interlayer and
adsorbed water molecules.36 The second step of the weight loss
at 250-500 °C consists of dehydroxylation of interlayer
hydroxyl groups and decomposition of interlayer carbonate and
nitrate (if any), resulting in the collapse of the layered
structure.3 The corresponding differential thermal gravimetry
6,37
(DTG) peak moves to a lower temperature from sample
0
.0CoMA-HT to sample 3.0CoMA-HT, i.e., the HT structure
2
Figure 3. TPR profiles of catalysts xCoMAO in H .
is destabilized upon incorporation of cobalt into the hydroxide
layers. This is understandable given that Mg(OH)2 is thermally
hydrogen from readily accessing the cobalt ions and delays the
Co(II or III) reduction.
The low-temperature reduction below 600 °C involves three
small steps. The Co reduction starts around 150-200 °C and
continues to 300 °C at a relatively small magnitude. Between
more stable than Co(OH)23
7,39,40
and is also reflected by the
second peak temperatures of 0.0CoMA-HT (Mg3Al HT, 444
C) and 3.0CoMA-HT (Co3Al HT, 246 °C) in air (Figure 2). It
is known that the decomposition of carbonate takes place at
°
3
6
2
50-600 °C. However, the major weight loss in the second
step for all samples occurs only around the peak temperature
(20-30 °C). This could suggest that carbonate decomposition
3
00 and 450 °C, there is a reduction slightly stronger than the
former one (except for 0.5CoMAO). The major reduction below
00 °C occurs at 450-600 °C (except for 0.5- and 1.0CoMAO).
(
6
might be catalyzed by the incorporated cobalt in some way. In
addition, for sample 3.0CoMA-HT, Co-OH is partially dehy-
droxylated in the first step39 as the two steps are partially
The peak temperature slightly increases from 510 to 570 °C
from sample 1.0CoMAO to 2.5CoMAO, probably because of
the difficulty in H2 approaching Co in larger spinel crystallites
in samples containing higher levels of Co.
II
overlapped (Figure 2). This, together with the oxidation of Co
III
to Co in air during the collapse facilitates the formation of
Because the final TPR compound for Co-containing oxide
catalysts should be Co(metal)/MgAl oxide, it is reasonable to
assign the high-temperature process to the reduction of bulky
the spinel phase and thermally destabilizes xCoMA-HT com-
pounds.41
II
42
The BET surface area and pore volume are listed in Table 1
for the oxide catalysts derived from the corresponding HTlcs
at 800 °C in air. Both the surface area and the pore volume of
these oxide catalysts decrease with increasing cobalt content,
as more spinel phase (big crystallites) is formed after calcination
Co cations dispersed in spinel and MgO. On this basis, we
can assign the process at 510-570 °C to the reduction of bulky
III
Co cations in the spinel and MgO phases. As suggested,
II
III
sample 3.0CoMAO has a formula of Co 4/3Co 5/3AlO16/3, so
its reduction processes could be
(
Figure 1B). On the contrary, the average pore size between
the crystallites increases. In particular, the pore size distribution
not shown here) indicates that all pores are over 2 nm, and
II
III
II
3
Co 4/3Co 5/3AlO16/3 f Co AlO f Co(metal)/AlO3/2
9/2
(
thus, gas molecule diffusion in these pores is not the rate-
determining step for adsorption and desorption.
and the H2 consumption ratio in these two stages should be
/3:6, i.e., 0.278. As calculated from the TPR profile, the real
5
TPR Analysis of xCoMAO Catalysts. The reduction profiles
of xCoMAO samples are shown in Figure 3. It is very clear
that the reduction behaviors of these catalysts are strongly related
to the Co content in the oxide catalysts. For 0.0CoMAO, i.e.,
Mg3Al oxide, no reduction peaks were detected until 900 °C,
as expected. The positive baseline over 100 °C might be caused
by the impurity in the experimental setup. For all other Co-
containing oxide catalysts, reduction of cobalt proceeds largely
in two stages, i.e., in the temperature ranges below and above
H consumption ratio at 400-590 and 590-900 °C is 0.302
2
II
for catalyst 3.0CoMAO. Bearing in mind that some Co is still
not reduced when the sample is heated to 900 °C (curve g in
Figure 3), the data agree well, supporting the assignments
proposed above. In addition, two more minor steps at 150-
300 and 300-450 °C could be assigned to reduction of surface
III
II
Co and Co on spinel and/or MgO, respectively, as these
36,43
cations are very easily accessed by H and readily reduced.
2
The total consumption of H in these two minor steps is about
2
6
00 °C. Obviously, the reduction over 600 °C predominates in
3.4% from the TPR profile of 3.0CoMAO, in acceptable
the TPR process, and the peak temperature decreases with
increasing Co content, e.g., from 2.0CoMAO (870 °C) to
agreement with the surface amount of Co (4.8%) as estimated
from the surface area in a previous report.
4
4
2
.5CoMAO (820 °C) to 3.0CoMAO (730 °C). Even though we
did not record data for samples 0.5-, 1.0-, and 1.5CoMAO below
00 °C, similar trends could be expected. This stabilization effect
NOx Adsorption on xCoMAO Catalysts. The adsorption
profiles of NOx on samples 0.0CoMAO, 1.5CoMAO, and
3.0CoMAO at 100 and 300 °C are presented in Figure 4. As
expected, NOx was adsorbed in the initial stage, as almost no
NOx was detected in the outlet stream. This adsorption lasted
9
of Mg on Co reduction is presumably attributed to the good
dispersion of Co in the MgAl oxide matrix that obstructs

NOx Adsorption on Co-Mg/Al Hydrotalcite-like Compounds
J. Phys. Chem. B, Vol. 110, No. 9, 2006 4295
Figure 4. NO adsorption on catalysts 0.0-, 1.5-, and 3.0CoMAO at (A-C) 100 and (D-F) 300 °C.
x
for 250 s, and then the NOx concentrations gradually recovered.
At 100 °C, the NO concentration recovered to 1200-1300 ppm
and the NO2 concentration to 90-100 ppm after 600 s for
catalysts 0.0CoMAO and 3.0CoMAO (Figure 4A-C). Under
the same conditions, the adsorption on catalyst 1.5CoMAO
continues to slowly trap NOx for 1500 s.
At 300 °C, the adsorption behavior of catalyst 0.0CoMAO
for NOx is similar to that at 100 °C, with a slightly higher NO2
concentration (180 ppm) and a slightly lower NO concentration
concentration from 1300 to 600 ppm after adsorption for about
350 s. This change in NO and NO2 concentrations indicates
the conversion of NO to NO2 during passage through the Co3-
Al oxide catalyst. A similar phenomenon was observed for all
of the other catalysts. Catalyst 1.5CoMAO, as an example,
gradually recovered the concentrations of NO, NO2, and NOx
to 700, 590, and 1290 ppm, respectively after 1200 s of
adsorption. The conversion from NO to NO2, i.e., the increase
in NO2 amount, is plotted in Figure 5 (2). It can be noted that
catalysts 0.0CoMAO and 0.5CoMAO (lower Co loadings)
increase the NO2 concentration by only 80-100 ppm while the
other catalysts increase the NO2 concentration by 400-800 ppm.
Another interesting observation is that the recovered NOx
concentration from catalyst 1.5CoMAO at 300 °C is always
(1200 ppm) in the later adsorption period. However, other
catalysts give much different gas compositions in the outlet
stream. For example, catalyst 3.0CoMAO (Co3Al oxide)
increases NO2 concentration from 100 ppm in the feed stream
to 800 ppm in the outlet stream while decreasing the NO

4296 J. Phys. Chem. B, Vol. 110, No. 9, 2006
Yu et al.
section. Obviously, the predominant desorbed species from
catalyst 0.0CoMAO is NO. Similarly, the desorption of NOx
from catalyst 3.0CoMAO that was adsorbed at 100 °C under-
went two similar steps, one at 255 °C and the other at 355 °C,
whereas these two steps occurred at much lower temperatures
and over a much narrower temperature range (Figure 6 and
Table 2), presumably because of the catalytic activity of cobalt
for NOx desorption. Interestingly, the NOx absorbed at 300 °C
on this catalyst desorbed only in the second step at about 360
°
C ([DeNOx(L)/DeNOx(H) ) 0.05, Table 2], suggesting that
there are mainly strongly bound NOx species, i.e., various
nitrates, on the catalyst surface, as addressed in the following
sections.
All other tertiary catalysts, represented by 1.5CoMAO as an
example, undergo a similar desorption process. However, the
two stages shift to lower temperatures by about 100 °C when
compared to those of catalyst 0.0CoMAO (Figure 6 and Table
2 x
Figure 5. NO increase and NO decomposition at 300 °C.
TABLE 2: NO
x
Desorption in Two Steps
temperature range (°C) DeNO
x
x
(L)/DeNO (H)
2
). Table 2 also indicates that the desorbed amount of NOx
sample
L
H
100 °C
300 °C
adsorbed at 100 °C in the first step is only about one-half or
less than that desorbed in the second step, suggesting that Co-
containing catalysts either prefer the strong binding with NOx
or convert the weakly bound nitrites to strongly bound nitrates.
This preference is more obvious for desorption of NOx adsorbed
at 300 °C as the desorbed NOx in the first step can be ignored,
meaning the nitrites formed on these catalysts are limited. In
addition, the desorbed amount of NO2 is comparable to that of
NO for these Co-containing catalysts, unlike the case where
NO is predominant in non-Co-containing catalyst 0.0CoMAO,
further suggesting that the major adsorbed NOx species are
nitrates.
0
0
1
1
2
2
3
.0CoMAO
.5CoMAO
.0CoMAO
.5CoMAO
.0CoMAO
.5CoMAO
.0CoMAO
100-435
100-290
100-280
100-280
100-280
100-330
100-295
435-650
290-600
280-600
280-600
280-600
330-650
295-550
2.37
0.54
0.46
0.42
0.15
0.53
0.55
0.65
0.01
0.02
0.15
0.01
0.05
0.05
lower than that in the feed stream (1400 ppm). This suggests
the partial decomposition of NO and/or NO2 to other kinds of
nitrogen compounds (such as N2O, N2) that were not detected
by the chemiluminescence NO-NO2-NOx analyzer, as well
as the continuous adsorption of NOx on the catalysts. The latter
seems not as likely because the adsorption is completed within
500 s at 100 °C for all catalysts. The decomposed amount of
NOx is significantly affected by the Co loading in the tertiary
catalysts, reaching a maximum of 300 ppm for catalyst
.5CoMAO, as shown in Figure 5 (0). In comparison, binary
catalysts (0.0CoMAO and 3.0CoMAO) decompose a limited
amount of NOx (about 10 ppm).
The adsorption (storage) data for NOx on xCoMAO samples
at 100 and 300 °C were calculated from the desorption profile
9
Species Formed on xCoMAO Catalysts. The storage/
decomposition mechanism of NOx on catalysts xCoMAO can
be revealed from the species formed during the adsorption/
desorption process. As described in the previous sections, NO
and NO2 are all adsorbed in the initial adsorption stage, and
then their concentrations in the outlet stream are recovered back
to a certain level, suggesting that the adsorption/desorption
process reaches a steady equilibrium. At 300 °C, in particular,
the NO2 concentration in the outlet stream (180-870 ppm) is
much higher than that in the feed (100 ppm), whereas the NO
concentration (500-1200 ppm) is much lower than that in the
feed (1300 ppm), indicating an apparent conversion of NO to
NO2. Moreover, the NOx total concentration in the outlet stream
from tertiary catalysts (0.5CoMAO-2.5CoMAO) is much lower
than the supplied 1400 ppm, revealing the decomposition of
NOx, probably to N2, O2, and N2O, on these catalysts. All of
these gaseous species could be evolved through the desorption
process, although N2, O2, and N2O were not directly determined
in this research.
1
2
(
refer to the next section) and are listed in Table 1. Note that,
at both temperatures, the storage capacity of NOx on three-
component catalysts xCoMAO (0 < x <3) is higher than that
on two-component catalysts xCoMAO (x ) 0 or 3). As expected,
the adsorption amount of NOx at the higher temperature (300
C) is less than that at the lower temperature (100 °C), except
for catalysts 2.0- and 2.5CoMAO.
°
NOx Desorption from xCoMAO Catalysts. The desorption
profiles of NOx from typical catalysts (0.0-, 1.5-, and 3.0Co-
MAO) with storage of NOx at 100 and 300 °C are shown in
Figure 6. In general, NOx desorption seems to follow a two-
step process in the temperature range of 100-650 °C. In the
case of the non-Co-containing catalyst (0.0CoMAO), the
desorption modes of NOx adsorbed at 100 and 300 °C are quite
similar, as can be seen from parts N and R of Figure 6 (0),
only with different magnitudes in the two steps peaked at 310-
On the other hand, the adsorbed species are indicated by the
in situ infrared spectra of the NOx-adsorbing catalysts. Briefly,
they are various nitrites and nitrates, as well as some other
nitrogen species, as summarized in Table 3. Their relative
amounts, i.e., the IR peak intensities, are dependent on the
adsorption time, the adsorption temperature, and the catalyst
composition. As shown in Figure 7A for catalyst 0.0CoMAO
(Mg3Al oxide) as an example, the strong peak, initially located
3
30 and 520-540 °C, respectively. The ratio of the amounts
-
1
-1
of NOx desorbed in these two steps [DeNOx(L)/DeNOx(H),
Table 2] is 2.37 at 100 °C and 0.65 at 300 °C. This could suggest
that NOx interacts with the catalyst in two different ways, where
the weaker interaction characterizes various types of nitrites
at 1246 cm and then slightly shifted to 1263 cm , is attributed
7,11,16,45
to bridging bidentate nitrite, due to the adsorption of NO.
The other major band, initially located at 1709 cm and later
split into a broad band at 1641-1688 cm , belongs to bridging
bidentate nitrate, attributed to NO2 adsorption.
-
1
-1
7
,11,16
(
corresponding to the desorption at 310-330 °C) and the
stronger one represents various types of nitrates (corresponding
to the desorption at 520-540 °C), as discussed in the following
Some other
species, such as monodentate nitrite, linear nitrite, and mono-
-1 7,11,16
dentate nitrate, which exhibit IR peaks at 1300-1500 cm ,

NOx Adsorption on Co-Mg/Al Hydrotalcite-like Compounds
J. Phys. Chem. B, Vol. 110, No. 9, 2006 4297
Figure 6. NO desorption profile on 0.0-, 1.5-, and 3.0CoMAO at (L-N) 100 and (O-R) 300 °C.
x
as well as possibly N2O2 (1400 cm-1),16 N2O (adsorbed, 2009
2
-
catalyst 1.5CoMAO (Figure 8B), monodentate nitrate (1276,
1480, and 1048 cm ) is the major species, with some other
-1
9
+
-1 46-48
-1
cm ), and NO (solid, 2332-2355 cm )
Table 3). The species adsorbed at 300 °C (Figure 7B) are quite
similar, only with a lower magnitude.
Similarly, for catalyst 1.5CoMAO (Co1.5Mg1.5Al oxides,
Figure 8A), the peak at 1649-1636 cm becomes stronger,
indicating more bridging bidentate nitrates formed than for
catalyst 0.0CoMAO. The initial peak at 1228-1240 cm due
to the adsorption of NO and assigned to bridging bidentate nitrite
shifts to 1308 cm as the strongest peak in the later adsorption
stage that could be assigned to monodentate nitrate (also
showing a peak at 1034 cm ),
are also present,
(
possible minor species such as bridging bidentate nitrite (1276
-
1
7,11,16
-1 46
cm ),
adsorbed N2O4 (1726-1732 cm ), chelating
-
1
+
bidentate nitrate (1553 and 1276 cm ), and NO (2343
-
1
-1 46-48
cm ).
The species adsorbed on catalyst 3.0CoMAO (Co3Al oxide)
are mainly various nitrates. For example, the adsorption of NOx
at 100 °C (Figure 9A) gives bridging bidentate [1628 (strongest),
-
1
-1
-1
-1
1196 cm ], monodentate (1429, 1345, 1011 cm ) and chelat-
-
1
7,11,16
ing bidentate (1511, 1290 cm ) nitrates.
The time-
-
1 7,11,16
suggesting a redox
dependent spectra (Figure 9A) clearly indicate that bridging
-
1
conversion from bridging bidentate nitrite to monodentate nitrate
in the presence of Co oxide. The other pronounced peak located
at 1470 cm belongs to the linear nitrite from adsorption of
bidentate nitrate (1628 cm ) is gradually formed and become
the major species in the final stage. However, this nitrate is
almost gone for 300 °C adsorption, with some monodentate
-
1
16
-1
NO in the presence of Co oxide. For adsorption at 300 °C on
(1445 and 1354 cm ) and chelating bidentate (1543-1583

4298 J. Phys. Chem. B, Vol. 110, No. 9, 2006
Yu et al.
TABLE 3: Characteristic Vibrations of Adsorbed NO
x
Species on xCoMAO Catalysts
-
1
cm ) nitrates remaining. The other major species corresponding
As cobalt is incorporated into the catalyst, similarly to Cu
and Pt, some adsorption reactions are catalytically accelerated,
-
1
to the IR peaks at 1741 and 1781 cm could be determined as
adsorbed N2O4 (or NO NO3 ), together with peaks at 2322
NO ) and 1300-1500 cm (NO3 ) that can be also noted in
the adsorption at 100 °C.
+
- 46
leading to more NO adsorbed (Table 1). In particular, the
x
+
-1
-
(
following redox reactions probably take place much more
quickly than reactions 4 and 5
Effect of Cobalt on Adsorption/Desorption. As presented
previously, Mg3Al oxide catalyst can adsorb and store NOx as
various nitrites and nitrates that can be decomposed back to
NO and NO2 during the desorption.
3+
2-
2+
-
NO + Co -O f Co -NO
(9)
2
3+
2-
2+
-
NO + Co -O f Co -NO
(10)
2
3
(
a) adsorption
where surface Co3 (or Co ) acts as the oxidant that can
facilitate the adsorption of NO and NO2. As we found that
cobalt-containing oxide starts reduction in H2 stream at 150-
200 °C, so reactions 9 and 10 take place in NOx adsorption at
+
2+
NO + O f 2 NO - + NO
2
-
3
2
4
2
3
(
disproportionation) (1)
+
-
NO f N O f NO NO
2
2
4
3
3
00 °C. In contrast, the presence of Co can also facilitate the
(
disproportionation) (2)
decomposition of nitrites and nitrates. The catalytic activities
of cobalt oxide for adsorption and desorption seem to be similar
in magnitude, as we note that the NOx adsorption amount of
catalyst 3.0CoMAO (Co3Al oxide) is quite similar to that of
catalyst 0.0CoMAO (Mg3Al oxide) at 100-300 °C (Table 1).
However, the tertiary catalysts can take up much more NOx
NO + 2O f 2NO - + N O2
2
-
2-
2
2
(
disproportionation) (3)
NO + O + 2O f 4NO -
2
-
(4)
(5)
4
2
2
3
NO + O + 2O f 4NO -
2-
(
Table 1). It is suggested that higher NOx adsorption is due to
4
2 2
-
-
a migration process of NO3 and NO2 from Co to adjacent
Mg/Al to form relatively stable Mg/Al nitrates and nitrites.1
Therefore, a careful tradeoff between Co and Mg/Al in the oxide
structure and composition might lead to a powerful NOx
adsorbent catalyst, such as 2.5CoMAO, which shows a rather
high NOx adsorption at 300 °C that is comparable to that of
2,49
(
b) desorption
NO3- f 4NO + 2O + O2
2-
(6)
(7)
(8)
4
4
4
2
NO3- f 4NO + 2O + 3O2
2-
1
0
NO2- f 4NO + 2O + O2
2-
perovskites at the same temperature.
The presence of cobalt in the catalysts also causes a significant
conversion of NO to NO2 and a noticeable decomposition of
NOx at 300 °C, as shown in Figure 5. It is known that NOx
(mainly NO) is catalytically decomposed on transition metal
oxides, probably to N2 and O2.
When the adsorption/desorption reaches a steady state, the
apparent NO2 and NO concentrations do not change. The oxygen
produced in reactions 6-8 could coordinate the oxidation of
NO into NO2 and NO2 into NO3 , as shown in reactions 4
and 5.
-
-
9,12,17,30
The as-produced oxygen
can enhance the apparent conversion of NO to NO2 via (i)

NOx Adsorption on Co-Mg/Al Hydrotalcite-like Compounds
J. Phys. Chem. B, Vol. 110, No. 9, 2006 4299
Figure 7. In situ IR spectra of NO
at (A) 100 and (B) 300 °C.
x
adsorption on catalyst 0.0CoMAO
Figure 9. In situ IR spectra of NO
at (A) 100 and (B) 300 °C.
x
adsorption on catalyst 3.0CoMAO
Then, the nitrates decompose to give NO2. The overall sto-
ichiometric reaction is
4
NO f 2NO + N
(12)
2
2
This could account for the NO2 concentration increase by 50-
1
50 ppm. Most of the increase in the NO2 concentration from
-
NO is suggested to be due to the direct oxidation of NO2 to
NO3 by the supplied O2 on the Co oxide surface, as shown in
-
reaction 11. Other possible reactions that could account for the
conversion are
2
+
-
0
Co -(NO ) f Co + 2NO
2
(13)
(14)
2
2
2
+
-
2-
0
-
Co -NO + O f Co + NO
2
3
These reactions might also be responsible for the comparable
NO2 amounts desorbed from 100 to 650 °C in comparison to
NO for Co-containing catalysts adsorbing at 300 °C (Figure
6
1
O-R). In addition, these reactions could also take place during
00 °C adsorption and subsequent desoprtion, although the
degree could be very minimal and unnoticeable for some
reactions.
Conclusions
Co-Mg/Al hydrotalcite-like compounds were prepared via
a coprecipitation process and identified by their XRD patterns.
Their transformation to mixed oxides upon calcination was also
confirmed by XRD patterns and TG analysis. The more cobalt
incorporated, the more spinel phase formed, which influences
the NOx adsorption/decomposition. The surface areas of the
Figure 8. In situ IR spectra of NO
at (A) 100 and (B) 300 °C.
x
adsorption on catalyst 1.5CoMAO
reaction 5 to form more nitrites and/or (ii) reaction 11 to convert
more nitrites to nitrates
2
derived oxides (xCoMAO) decreased from 162.7 to 21.6 m /g
with increasing Co content from x ) 0.0 to x ) 3.0.
NO2- + O f 2NO3-
2
2
(11)

4300 J. Phys. Chem. B, Vol. 110, No. 9, 2006
Yu et al.
The investigation of the reduction behaviors of xCoMAO
catalysts indicates that the surface cobalt cations (Co and Co )
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(
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Acknowledgment. This work was supported by the National
Basic Research Program of China (2004CB719500), the key
project of Knowledge Innovation of Chinese Academy of
Sciences (KZCX3-SW-430), and a project of Natural Science
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