Ind. Eng. Chem. Res. 2004, 43, 4031-4037
4031
Al2O3-Coated Honeycomb Cordierite-Supported CuO for
Simultaneous SO2 and NO Removal from Flue Gas: Effect of Na2O
Additive
Qingya Liu, Zhenyu Liu,* Zhenping Zhu, Guoyong Xie, and Yanli Wang
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences,
Taiyuan, 030001, P.R. China
Effect of Na2O additive in an Al2O3-coated cordierite honeycomb ceramic-supported CuO catalyst
on simultaneous SO2 and NO removal activities was studied at 400 °C. Results show that SO2
removal activity increases with increasing sodium loading from 0 to 2.0 wt % but decreases
with further increasing sodium loading to 3.0 wt %, while NO removal activity monotonically
decreases with increasing sodium loading, from 99 to 65%, due to increased NH3 oxidation
activity. Mechanism of the effect of Na2O additive on SO2 removal activity is studied using N2
adsorption, scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), and X-ray
powder diffraction (XRD). Results show that Na2O additive (at sodium loadings of 1.0-2.0 wt
%) inhibits the formation of Al-Si-O species and improves the distribution of CuO, which limits
pore plugging and increases BET surface area of the catalyst. These behaviors account for the
promoting effect of Na2O additive on SO2 removal activity. However, with increasing sodium
loading from 2.0 to 3.0 wt %, more well-crystallized CuO are observed, which leads to a lower
utilization rate of copper and thus SO2 removal activity. Chemical analysis and XRD results
indicate that for the catalyst without Na2O additive, only CuSO4 is formed, whereas for the
catalysts containing Na2O, the adsorbed SO2 exist in three forms: CuSO4, NaCu2(SO4)2OH‚
H2O, and Na2SO4.
1. Introduction
SO2 and NOx, mainly from stationary sources, are
very harmful to the ecosystem and humans, and must
be removed before emission. Because of the simplicity
and better economics, combined or simultaneous removal of SO2 and NOx is advantageous.1 CuO supported
on pellet Al2O3 (CuO/Al2O3) has been widely studied for
this purpose because of its good capacity for SO2
adsorption and high activity for selective catalytic
reduction (SCR) of NO with NH3 in the presence of O2
in the temperature range of 300-400 °C.2-4 However,
a large volume of catalyst pellets may cause high flow
resistance and plugging by particulates in flue gas in
industrial application.5 It is reported that monolithic
catalysts not only can overcome these problems, but also
present other advantages over conventional pellet/
granular catalysts: uniform flow distribution, ease of
handling, attrition-free, etc.6,7 Therefore, developing
honeycomb-type CuO/Al2O3 catalyst for simultaneous
SO2 and NO removal presents technical merit.
Although molding pellet Al2O3 into monolith may
produce honeycomb CuO/Al2O3, the instability of γ-Al2O3
at temperatures higher than 700 °C makes it difficult
to calcine to yield sufficient mechanical strength. For
this reason, Al2O3 is usually coated on cordierite honeycomb ceramics, such as in preparation of the support
of three-way catalyst (known as TWC).8,9 However, our
earlier work10 found that loading of CuO directly on a
support of TWC yielded a catalyst of a low SO2 conversion. To improve the catalytic activities, it was found
* To whom correspondence should be addressed. Tel: +86351-413-4410. Fax: +86-351-405-0320. E-mail: zyliu@
sxicc.ac.cn.
that pretreatment of the cordierite with an acid solution
and addition of Na2O were important in catalyst preparation.
For CuO/Al2O3 catalyst, the promoting effect of sodium additive on SO2 removal was attributed to the
increased sulfation of Al2O3.11,12 However, this may not
be the case for the cordierite-supported CuO/Al2O3
because the properties of Al2O3, which is in situ synthesized on the cordierite using a sol-gel method and
subjected to repetitious calcinations, are still unclear.
Furthermore, the introduction of cordierite support may
complicate the effect of Na2O additive on SO2 removal.
Hence, the aim of this work is to make a further
contribution in these aspects.
In this paper, favorable sodium loadings are obtained
first, and then the mechanism of the effect of Na2O on
SO2 removal activity is elucidated with the aid of N2
adsorption, scanning electron microscopy (SEM), energydispersive X-ray (EDX), X-ray powder diffraction (XRD),
and chemical analysis.
2. Experimental Section
2.1. Preparation of Catalysts. The cordierite ceramic used in this work (marked as HC) is a commercial
product with a cell density of 200 cells per square inch
(cpsi) and a BET surface area of 0.7 m2/g. The preparation procedure includes three steps. Briefly, (1) HC was
pretreated in oxalic acid; (2) the resulting sample
(marked as CHC) was in situ coated with γ-Al2O3 5× to
obtain the substrate CHC-5Al; (3) CuO or CuO-Na2O
was introduced onto CHC-5Al by dip-impregnating with
a cupric nitrate solution or a mixed solution of cupric
nitrate and sodium nitrate to give a copper loading of
about 6.0 wt % and a sodium loading of 0-3.0 wt %.
10.1021/ie049942t CCC: $27.50 © 2004 American Chemical Society
Published on Web 06/15/2004
4032 Ind. Eng. Chem. Res., Vol. 43, No. 15, 2004
The final catalysts are named according to the metal
content. For example, Cu6Na1.5 refers to a catalyst with
6.0 wt % copper and 1.5 wt % sodium supported on
CHC-5Al (the name of the substrate is omitted in this
notation). It should be pointed out that the metal
loadings in this notation are estimated (the actual
values determined by ICP analysis are shown in Table
2).
2.2. Activity Test. The activity test was carried out
in a fixed-bed reactor of 22-mm diameter. A monolithic
catalyst sample (30 mm long and 20 mm diameter) was
fitted in the reactor and then heated to 400 °C under
an Ar flow. At steady state, a gas mixture containing
1960 ppm SO2, 500 ppm NO, 5.5% O2, 2.5% H2O, 500
ppm NH3, and balance Ar was introduced into the
reactor. In all the runs, the total flow rate was controlled
at 440 mL/min, which corresponded to a superficial
space velocity of 2800 h-1 (based on the monolith
volume). The concentrations of NO, SO2, and O2 in the
inlet and outlet of the reactor were simultaneously
measured on-line by a flue gas analyzer (KM9106).
2.3. Characterization Methods. BET surface area
was determined by nitrogen adsorption at 77 K using
an ASAP2000 volumetric adsorption analyzer (Micromeritics). The total pore volume (V) was calculated
from the amount of nitrogen adsorbed at P/P0 ) 0.98.
The average pore size was obtained by the formula of
4V/BET surface area.
XRD was performed on a Riguku D/Max 2500 system.
Diffraction patterns were recorded with Cu KR (λ )
0.1542 nm) radiation and the X-ray tube was operated
at 40 KV and 100 mA. Step scans were taken over a
range of 2θ from 5 to 85° at a speed of 8°/min.
SEM photographs were obtained on a JEOL JSM-35C
scanning microscope. Prior to SEM analysis, the samples
were embedded in epoxy resin, metallographically polished, and coated with a conductive layer of gold. The
magnification is marked in the photograph, such as
200× and 2000×. Energy-dispersive X-ray (EDX) analysis was performed on a combined system of LEO 438VP
scanning electron microscopy and a KEVEX SUPERDRY X-ray analyzer.
Total copper and sodium contents of the catalysts
were determined by dissolution of ground catalysts in
acid solutions and then analysis on an inductively
coupled plasma (ICP) optical emission spectroscopy
(Perkin-Elmer Optima 3300DV). Quantification of CuSO4 and Al2(SO4)3 in the catalysts consisted of dissolution of the samples in distilled water and analysis of
the dissolved metal by ICP.
3. Results and Discussion
3.1. Effect of Sodium Loading on the Catalytic
Activities. To all the catalysts, the initial SO2 conversion is 100%. With increasing time on stream, SO2
conversion decreases due to transformation of copper
oxide to copper sulfates. To facilitate the discussion on
SO2 removal, two terms are introduced. The term t80 is
the time at which SO2 conversion decreases to 80%. The
term SC80 represents the amount of SO2 adsorbed on
the catalyst, in wt %, when the adsorption time reaches
t80, or in short, sulfur capacity at t80.
Figure 1a shows the effect of sodium loading on SO2
conversion. The behavior of Cu6 and Cu6Na0.5 is
similar, with t80 of about 55 min, indicating that
introduction of 0.5 wt % sodium additive into Cu6 has
little effect on SO2 removal activity. However, when
Figure 1. Effect of sodium loading on SO2 removal activity.
Experimental conditions: 1960 ppm SO2, 500 ppm NO, 5.5% O2,
2.5% H2O, 500 ppm NH3; total flow rate of 440 mL min-1; space
velocity of 2800 h-1; reaction temperature of 400 °C.
sodium loading is at 1.0 wt % or higher, an obvious
promoting effect of Na2O additive on SO2 removal
activity is observed. The t80 increases from 55 to 175
min with increasing sodium loading from 0.5 to 1.5 wt
%, and stays at 175 min for Cu6Na2. The t80 then
decreases to 165 min for Cu6Na2.5 and 85 min for
Cu6Na3. SC80 values of various catalysts are shown in
Figure 1b. It can be seen that Cu6 and Cu6Na0.5 exhibit
similar SC80 values of about 2.1 wt %, corresponding to
the equal t80. The SC80 values of Cu6Na1, Cu6Na1.5,
Cu6Na2, Cu6Na2.5, and Cu6Na3 are 5.0, 5.5, 6.8, 5.1,
and 3.0 wt %, respectively. Clearly, SC80 increases with
increasing sodium loading from 0.5 to 2.0 wt %, and then
decreases with further increasing sodium loading. It
should be pointed out that, because of a difference in
catalyst loading in the reactor, the same t80 for Cu6Na1.5
and Cu6Na2 yields different SC80. Anyway, the t80 and
SC80 data indicate that Na2O additive (1.0-3.0 wt %
sodium) largely enhances SO2 removal activity. However, it is interesting to note that the effect of Na2O
additive at sodium loadings of 0.5-2.0 wt % is different
from that at 2.0-3.0 wt %, as studied in the following.
Figure 2 shows NO conversion vs time on stream over
the catalysts of different sodium loadings, obtained in
the same experiments shown in Figure 1. For the
catalysts with sodium loading of 0, 0.5, 1, 1.5, 2, and
2.5 wt %, the steady-state NO conversions are 99, 95,
94, 92, 85, and 80%, respectively. However, within the
time period of the experiment (from 0 to t80), NO
conversion of Cu6Na3 does not reach a steady state,
with a maximum value of about 65%. These results
indicate that sodium additive has a negative effect on
NO conversion, as reported for V2O5/TiO2 catalyst,13 and
SCR activity monotonically decreases with increasing
Ind. Eng. Chem. Res., Vol. 43, No. 15, 2004 4033
Figure 2. Effect of sodium loading on NO removal activity
obtained in the same experiment in Figure 1.
Figure 4. Relationship of BET surface area and SO2 removal
activity.
Table 1. Physical Properties of Various Catalysts
Figure 3. Effect of sodium loading on NH3 oxidation at 400 °C.
sodium loading and a drastic decrease occurs between
sodium loadings of 2.0-3.0 wt %.
Our previous work on pellet CuO/Al2O3 catalyst14
showed that the inhibiting effect of sodium additive on
NO removal is from increased NH3 oxidation to form
NO. To understand the effect of sodium loading on the
extent of NH3 oxidation for the honeycomb catalyst, an
NH3 oxidation experiment was carried out in a stream
containing 550 ppm NH3 and 5.4 vol % O2 at 400 °C.
The outlet NOx concentration (monitored on-line by a
flue gas analyzer) was used to estimate the extent of
NH3 oxidation (NH3 to NO in a stoichiometric ratio of
1:1). Results in Figure 3 show that the outlet NO
concentration increases linearly with increasing sodium
loading, indicating an increase in NH3 oxidation. These
suggest that NH3 oxidation is also a main reason for
the monotonic decrease in SCR activity with increasing
sodium loading for the honeycomb catalyst.
On the basis of the above data, sodium loadings of
1.0-2.0 wt % may be proper compositions of the catalyst
for simultaneous SO2 and NO removal.
3.2. Physical Properties of Various Catalysts.
Generally, surface area of a catalyst is one of the main
factors influencing catalytic activities. To study the
effect of Na2O additive on SO2 removal, physical properties of the catalysts are analyzed. Table 1 indicates that
CHC-5Al, the substrate, has a BET surface area of 87.6
m2/g, and loading of 6.0 wt % copper decreases it to 42.9
m2/g (see Cu6). Accordingly, the total pore volume
decreases from 0.090 cm3/g for CHC-5Al to 0.043 cm3/g
for Cu6. These data suggest that impregnation of CuO
on the substrate results in pore plugging. However,
introduction of sodium additive to Cu6 increases its BET
surface area and pore volume to 52.5, 67.3, and 47.0
sample
BET surface area
m2/g
average pore size
nm
pore volume
cm3/g
CHC-5Al
Cu6
Cu6Na1
Cu6Na2
Cu6Na3
87.6
42.9
52.5
67.3
47.0
4.10
4.01
3.99
4.17
4.58
0.090
0.043
0.052
0.070
0.054
m2/g and 0.052, 0.070, and 0.054 cm3/g for Cu6Na1,
Cu6Na2, and Cu6Na3, respectively. This suggests that
sodium additive may prevent aggregation of some
species (CuO particles or others) and in turn result in
the lesser extent of pore plugging, which is similar to
that demonstrated by NaCl on calcined limestone15 and
ZnO on CuO/Al2O3 catalyst.16 However, it is worth
noting that increasing sodium loading from 2.0 to 3.0
wt % causes decreases in surface area and pore volume
and an increase in average pore size, suggesting the
increased pore plugging resulted from formation of large
or more particles in the impregnation.
To visually understand the relationship between SO2
removal activity and physical properties of the catalyst,
Figure 4 gives the curve of SC80 vs BET surface area.
As can be seen, SC80 increases roughly linearly with
increasing surface area, suggesting the importance of
catalyst surface area to SO2 removal.
3.3. XRD and SEM Analyses. The catalysts with
different Na2O loadings show different colors, black for
Cu6 and green for Cu6Na2, suggesting improved dispersion of CuO by Na2O additive. To understand the
distribution of CuO and changes of physical properties
caused by Na2O additive, Cu6, Cu6Na2, and Cu6Na3
were characterized by XRD and SEM, along with the
substrate CHC-5Al for comparison.
Figure 5 shows the XRD patterns of these catalysts.
Although CHC-5Al shows typical diffraction patterns of
cordierite ceramics, the slightly increased intensities of
peaks at 39, 45.5, and 66°, compared to the XRD pattern
of CHC (not shown here), suggest that γ-Al2O3 has
successfully synthesized onto the cordierite monolith.
With introduction of 6.0 wt % copper onto CHC-5Al, a
new strong peak at 35.6° appears and the intensity of
the peak at 38.8° increases (see Cu6), indicating the
existence of crystallized CuO (2θ ) 35.6 and 38.8°) in
Cu6 catalyst. However, with introduction of 2.0 wt %
sodium additive into Cu6 catalyst, the intensities of all
peaks decrease (see Cu6Na2), suggesting that some
changes take place. The visible peaks corresponding to
CuO suggest that they still exist in the congregated form
in Cu6Na2 catalyst. With increasing sodium loading to
4034 Ind. Eng. Chem. Res., Vol. 43, No. 15, 2004
Figure 5. XRD patterns of (a) CHC-5Al, (b) Cu6, (c) Cu6Na2,
and (d) Cu6Na3.
Figure 6. SEM photograph of CHC-5Al.
3.0 wt %, the intensities of the peaks at 35.6 and 38.8°
increase again (see Cu6Na3) and become stronger than
those in Cu6 catalyst, suggesting that the particle size
of CuO in Cu6Na3 is much larger than that in other
catalysts and/or that the degree and the number of
crystalline CuO phase are increased. This observation
agrees with the report by El-Shobaky et al.17 which
showed increased intensities of CuO diffraction lines
with increasing amount of Na2O additive. It is worth
pointing out that no diffraction patterns corresponding
to Na2O (2θ ) 46.5, 32.3, and 27.3°) can be observed on
Cu6Na2 and Cu6Na3 catalysts, indicating the weakly
crystalline nature (amorphous) of Na2O additive.
To visually understand the particle size of CuO, SEM
photographs of the catalyst samples used for the XRD
study are shown in Figures 6-8 and 11. Figure 6
exhibits the morphology of CHC-5Al. Cracks can be
clearly observed, possibly due to the thick coating of
Al2O3 or the dilute sol used.18 Introduction of 6.0 wt %
copper into CHC-5Al results in the appearance of some
cotton-like species with sizes ranging from 13 to 20 µm
(see Figure 7). EDX analyses indicate that these species
are composed of O, Al, Si, and Cu (least). It is surprising
to see Si on the surface because Si is a component of
the cordierite which is supposed to be under the Al2O3
coating. This suggests that Al2O3 coating is very thin
or that Si may migrate to the surface in the Al2O3coating process. Comparing Figures 6 and 7, it can be
deduced that loading of CuO promotes the formation of
cotton-like Al-Si-O species. In addition, some wellcrystallized flat particles with sizes of about 3 µm are
visible on the cotton-like species. EDX results indicate
that these particles are composed of Cu and O, i.e., CuO.
The congregation of CuO is not a surprise. Strohmeier
et al.19 reported that the monolayer coverage of CuO
on an Al2O3 support with a surface area of 100 m2/g
resulted in a Cu loading of 4-5 wt %. On the basis of
this relation, the copper loading for a monolayer coverage on CHC-5Al (BET surface area of 87.6 m2/g) would
be about 4.0 wt %, which is much smaller than the CuO
loading used in this work. Because only the outer
surface of the CuO particle is easily available to react
with SO2, the congregated CuO particles will lead to a
low utilization rate of copper.
Figure 8 shows that with introduction of 2.0 wt %
sodium additive into Cu6 catalyst, the size of cottonlike species decreases to about 8 µm or below and many
thread-like materials radiating from the cotton-like
species are visible. The EDX result shown in Figure 9
indicates that the threads are composed of Si (most),
Al, O, and Cu (least), and no Na is detected. This
observation suggests that sodium additive may inhibit
the formation of congregated Al-Si-O species. SEM
studies on uncalcined Cu6 and Cu6Na2, shown in
Figure 10, suggest that this inhibiting effect of Na2O
additive occurs in the precipitation process, but cotton/
thread-like species are not formed in this process. The
behavior of Na2O additive must result in less pore
plugging and higher BET surface area of Cu6Na2 (see
Section 3.2). These in turn may improve the distribution
of CuO, as evidenced by XRD result. All of these
contribute to the enhanced copper utilization rate and
SO2 removal activity.
Figure 11 shows the morphology of Cu6Na3, which
is clearly different from that of Cu6Na2. The cottonlike species nearly disappear and two types of new
particles appear. One is octahedron shaped with sizes
of about 10 µm (see Figure 11b) and the other is a
compressed sphere with sizes of about 5 µm (see Figure
Figure 7. SEM photographs of Cu6 with magnifications of (a) 200 and (b) 2000.
Ind. Eng. Chem. Res., Vol. 43, No. 15, 2004 4035
Figure 8. SEM photographs of Cu6Na2 with magnifications of (a) 200 and (b) 2000.
Figure 9. EDX result of the threads in Figure 8.
Figure 10. SEM images of uncalcined Cu6 and Cu6Na2.
11c). EDX results show that both of them are CuO
particles, without Al or Si. The increased well-crystallized CuO phase was also observed on CuO/Al2O3
catalyst at sodium loading of 5 wt % or more,20 which
was attributed to the formation of a sodium aluminate
layer that hindered the thermal diffusion of Cu2+ into
the Al2O3 matrix.17 In any case, the more and larger
CuO particles of higher crystallinity in Cu6Na3, com-
pared to those in Cu6Na2, agrees with the increased
intensities of peaks corresponding to CuO in XRD
pattern, and may cause more pore plugging and thus a
decreased BET surface area and pore volume, as
indicated in Table 1. These behaviors in turn result in
a lower utilization rate of copper and SO2 removal
activity.
3.4. Characterization of Sulfates Formed on the
Catalysts. To further understand the reactive property
of Al2O3 coating synthesized in this work and the
promoting effect of sodium additive on SO2 removal
activity, the forms of sulfate formed on the catalysts
during SO2 removal were determined by XRD and
chemical analysis.
Figure 12 shows the XRD pattern of sulfated Cu6Na2,
along with that of a fresh one for comparison. It can be
seen that three new peaks at 13.4, 25.1, and 31.9°
appear in the XRD profile of the sulfated catalyst and
the intensities of peaks at 21.6, 26.3, 34.2, and 54.3°
increase. Peak-search indicates that (1) no diffractions
corresponding to Al2(SO4)3 (2θ ) 25.4, 15.7, and 20.4°)
can be observed on the sulfated catalyst, which provides
evidence for the limited formation of Al2(SO4)3; (2) there
are two kinds of sulfates, CuSO4 (2θ ) 21.2, 25.3, and
34.3°), usually observed in sulfated CuO/Al2O3 catalyst,
and NaCu2(SO4)2OH‚H2O (2θ ) 13.4, 26.3, 31.9, 35.5,
38.8, and 54.3°), never reported in the literature to our
knowledge. It is surprising that no Na2SO4 is detected
because it was believed to be a product of the reaction
between Na2O and SO2+H2O+O2.
To further identify the chemical forms of the sulfates,
chemical analyses were performed, and the results are
shown in Table 2. For sulfated Cu6 catalyst, the watersoluble copper and water-soluble alumina are 2.52 and
0 wt %, respectively. Because the atomic weight of Cu
(63.5) is very close to the molecular weight of SO2 (64),
the amount of SO2 associated with the Cu is also 2.52
wt %. This value is very close to the corresponding SC80
shown in Figure 1b, suggesting that most of the sulfates
formed on Cu6 catalyst are CuSO4 and little Al2(SO4)3
is formed. This is rather surprising because the formation of Al2(SO4)3 has been reported in the literature for
CuO/Al2O34 and sodium-doped Al2O321 catalysts. This
difference suggests that Al2O3 synthesized and supported on the cordierite ceramic in this work is different
from pellet Al2O3 generally used, possibly due to interactions between Al2O3 and SiO2, as evidenced by the
formation of cotton-like Al-Si-O species.
Table 2 also shows that the amount of water-soluble
copper increases with increasing sodium loading, from
2.52 wt % for sulfated Cu6 to 4.02, 4.98, and 6.14 wt %
for sulfated Cu6Na1, Cu6Na1.5, and Cu6Na2, respectively. These results clearly indicate the enhanced
copper utilization rate by sodium additive. It is worth
4036 Ind. Eng. Chem. Res., Vol. 43, No. 15, 2004
Figure 11. SEM photographs of Cu6Na3.
not more than 0.05 wt %, although it increases with
increasing sodium loading due to the promoting effect
of Na2O additive,21 it cannot account for the differences
between SC80 and the amount of SO2 linked to copper.
This leaves sodium as the only candidate to account for
SO2 adsorption, besides in the form of NaCu2(SO4)2OH‚
H2O. Table 2 shows that the water-soluble sodium in
the sulfated Cu6Na2 (S) is 1.24 wt %. Supposing all the
water-soluble sodium is from Na2SO4, with little formation of NaCu2(SO4)2OH‚H2O, then the overall amount
of SO2 adsorbed on Cu6Na2 (S) would yield a SC80 of
7.86 wt %, which is significantly higher than the SC80
shown in Figure 1b (6.83 wt %). This indicates that only
a fraction of sodium forms amorphous Na2SO4 upon
adsorption of SO2, which is not detectible by XRD.
4. Conclusions
Figure 12. XRD patterns of Cu6Na2 catalyst before and after
sulfation.
(1) SO2 removal activity increases with increasing
sodium loading from 0 to 2.0 wt % and then decreases
with further increasing sodium loading to 3.0 wt %,
while SCR activity for NO removal monotonically
decreases. Sodium loadings of 1.0-2.0 wt % in the
catalyst may be favorable for simultaneous SO2 and NO
removal. Under the conditions used, SC80 of 5.0 wt %
and a NO conversion of about 85% can be reached. (2)
The inhibiting effect of Na2O additive on NO removal
is due to increased NH3 oxidation to form NO. The
extent of NH3 oxidation increases linearly with increasing sodium loading. (3) Loading of CuO promotes the
formation of cotton-like Al-Si-O species, which causes
pore plugging and thus a decrease in surface area.
noting that both CuSO4 and NaCu2(SO4)2OH‚H2O,
determined by XRD, can yield water-soluble copper with
S/Cu atomic ratio of 1. This seems to suggest that the
amount of SO2 adsorbed on the catalyst can be calculated from water-soluble copper. However, all the values
of water-soluble copper, except 2.52 wt % for Cu6, are
smaller than SC80 of the corresponding catalysts (shown
in Figure 1b). This suggests that not all of the SO2
adsorbed is associated with copper in the presence of
sodium additive, and some of the adsorbed SO2 may be
linked to other species. Because the content of watersoluble alumina, in the form of Al2(SO4)3, is very small,
Table 2. Results of Chemical Analyses of Various Catalysts
a
samplea
total
copper (%)
total
sodium (%)
water-dissolved
copper (%)
water-dissolved
sodium (%)
water-dissolved
alumina (%)
Cu6(S)
Cu6Na1 (S)
Cu6Na1.5 (S)
Cu6Na2 (S)
Cu6Na2 (F)
6.69
6.34
6.49
6.40
6.40
1.00
1.51
2.04
2.04
2.52
4.02
4.98
6.14
0
0.88
1.00
1.24
0.70
0
0.01
0.03
0.05
0.54
Note that (F) represents the fresh catalyst; (S) represents the sulfated catalyst.
Ind. Eng. Chem. Res., Vol. 43, No. 15, 2004 4037
However, sodium additive, at sodium loadings of 2.0 wt
% or below, inhibits the formation of cotton-like species
and thus increases the BET surface area of the catalyst
and enlarges the pore size. Sodium additive also improves the distribution of CuO. All of these account for
the promoting effect of Na2O additive on SO2 removal.
(4) Addition of 3.0 wt % sodium causes completely
different behavior, i.e., a marked increase of wellcrystallized CuO phase. This behavior results in a
decreased BET surface area and pore volume, and a
lower utilization rate of copper and thus SO2 removal
activity. (5) For the catalyst without sodium additive,
all SO2 adsorbed is linked to copper, i.e., CuSO4.
However, for the catalysts containing sodium additive,
the SO2 adsorbed exists in three forms: CuSO4,
NaCu2(SO4)2OH‚H2O, and Na2SO4. Al2O3 coating synthesized in this work almost does not take part in SO2
adsorption to form Al2(SO4)3.
Acknowledgment
We gratefully acknowledge financial support from the
Natural science Foundation of China (20276078,
90210034), the National High Technology Research and
Development
Program
(The
863
Program,
2002AA529110), Chinese Academy of Sciences, and the
Shanxi Natural Science Foundation.
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Received for review January 17, 2004
Revised manuscript received April 30, 2004
Accepted May 12, 2004
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