A common feature of H2-assisted HC-SCR over Ag/Al 2O3

Article abstract of DOI:10.1039/c3cy01033c

CH₄, C₂H₂, C₂H₄, C₃H₆, and C₃H₈ were selected as reductants for selective catalytic reduction (SCR) of NO_x over Ag/Al₂O₃. Activity measurement showed that NO_x reduction by hydrocarbons containing two- or three-carbon atoms was clearly promoted by H₂ over Ag/Al₂O₃ at low temperatures, while such enhancement did not occur in the case of CH ₄-SCR. Gas chromatography and gas chromatography coupled to a mass spectrometer analysis showed that the partial oxidation of hydrocarbons with more than one carbon atom was triggered at low temperatures by H₂ addition over Ag/Al₂O₃. On the surface of Ag/Al ₂O₃, in situ diffuse reflectance infrared Fourier transform spectra indicated that this enhancement mainly originated from the formation of reactive enolic species, which is a common feature of H ₂-assisted HC-SCR. the Partner Organisations 2014.

Full text of DOI:10.1039/c3cy01033c

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A common feature of H2-assisted HC–SCR over
Ag/Al2O3
Cite this: DOI: 10.1039/x0xx00000x
Yunbo Yu, Hong He*, Xiuli Zhang, and Hua Deng
CH4, C2H2, C2H4, C3H6, and C3H8 were selected as reductants for selective catalytic reduction
(SCR) of NOx over Ag/Al2O3. Activity measurement showed that NOx reduction by
hydrocarbons containing twoꢀ or threeꢀ carbon atoms was clearly promoted by H2 over
Ag/Al2O3 at low temperatures, while such enhancement did not occur in the case of CH4ꢀSCR.
Gas chromatography and gas chromatography coupled to a mass spectrometer analysis showed
that the partial oxidation of hydrocarbons having more than one carbon atom was triggered at
low temperatures by H2 addition over Ag/Al2O3. On the surface of Ag/Al2O3, in situ diffuse
reflectance infrared Fourier transform spectra indicated that this enhancement mainly
originated from the formation of reactive enolic species, which is a common feature of H2ꢀ
assisted HCꢀSCR.
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
respectively, while CO did not boost the HC–SCR activity.9,15,16
As a result, it was proposed that the promotion of H2 in HC–
1. Introduction
SCR is not attributed to structural changes of the active sites
but to a chemical effect, which was further announced by
Sazama et al. and Korhonen et al.17,18
Among the NOx reduction technologies being developed for
controlling diesel engine emission, selective catalytic reduction by
hydrocarbons (HC–SCR) has attracted much attention as a possible
alternative to the commercially used NH3/Urea–SCR.1ꢀ3 The
distinctive advantage of HC–SCR is that the onꢀboard fuel or
additive can be used as the reductant for NOx conversion, thus
reducing the cost involved in the infrastructure development for
reductant delivery. Hitherto, numerous catalysts such as zeolitic
oxide, base oxide/metal and noble metal catalysts have been found to
be effective for NOx reduction by hydrocarbons, among which
Ag/Al2O3 is known as one of the most effective catalysts even in the
presence of H2O and SO2.1ꢀ6 More importantly, the lowꢀtemperature
activity of Ag catalysts can be significantly boosted by H2 addition
both under laboratory conditions and on a fullꢀscale vehicle
equipped with a Ag/Al2O3 converter.7ꢀ8
It has been generally accepted that the partial oxidation of
hydrocarbons to active oxygenates, as an initial step of HC–
SCR, was triggered by H2 addition at low temperatures, and
thus enhancing NOx reduction.11,12,19,20 By using in situ
DRIFTS, Shibata et al. found that the addition of H2 results in
remarkable promotion of partial oxidation of C3H8 over
Ag/Al2O3, mainly to surface acetate, which was later also
reported by Bentrup and coꢀworkers.12,19 During the partial
oxidation of C3H6 over Ag/Al2O3, however, He and coꢀworkers
proposed that the presence of H2 promoted the formation of
enolic species (RCH=CHꢀOꢀ)ꢀM, particularly in the low
temperature range.20 The surface enolic species rather than
acetate on Ag/Al2O3, exhibited higher activity toward NO + O2
to produce the key intermediate of isocyanate (–NCO), and thus
contributed to the promotion effect of H2 on C3H6–SCR of NOx.
Previous research further established that enolic species play a
key role in the reduction of NOx by alcohols (such as ethanol
and butanol) over Ag/Al2O3,14,21ꢀ23 acetaldehyde over both
Ag/Al2O3 and Co/Al2O3,24,25 and acetylene over ZSMꢀ5.26
Interestingly, Taatjes and coꢀworkers also observed substantial
quantities of twoꢀ, threeꢀ, and fourꢀcarbon enols using
photoionization mass spectrometry (PIMS) of flames burning
representative compounds from modern fuel blends, indicating
that enols are common intermediates in hydrocarbon
oxidation.27 Herein, CH4, C2H2, C2H4, C3H6, and C3H8 were
selected as reductants for NOx reduction over Ag/Al2O3,
respectively. It was found that the formation of enolic species
was triggered by H2 addition during the oxidation of all the
employed hydrocarbons containing two or three carbons at low
temperatures, which may reveal the intrinsic mechanism
contributing to H2 assisted HC–SCR over Ag/Al2O3.
Up to date, however, the origination of the H2 effect is still
under debate, including the formation of Ag clusters and their
role in boosting HC–SCR, and chemical effect involving in
reactive species contributed to NOx reduction by
hydrocarbons.9ꢀ11 Using in situ Ultravioletꢀvisible spectroscopy
(UVꢀvis), extended xꢀray absorption fine structure (EXAFS),
and diffuse reflectance infrared Fourier transform spectroscopy
(DRIFTS) characterization, Satsuma and coꢀworkers proposed
that hydrogen would be indispensable for the formation of
δ+
Agn clusters, the presence of which is necessary for the
promotion of C3H8 oxidation, and thus contributed to NOx
reduction over Ag catalysts.11ꢀ13 The morphological
δ+
transformation of Ag+ to Agn was recently observed by Kim
et al. during H2 assisted C2H5OH + simulated diesel over
Ag/Al2O3, in the case of that highly active oxygen species was
created, enhancing the lowꢀtemperature deNOx activity of
Ag/Al2O3.14 On the other hand, the formation of oxidized Ag
clusters was also identified by Burch and coꢀworkers during
C8H18ꢀSCR in the presence of H2 and CO over Ag/Al2O3,
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DOI: 10.1039/C3CY01033C
NO/NO2/NOx analyzer (42C–HL, Thermo Environmental
Instrument Inc.).
2. Experimental
2.1. Catalyst preparation and characterization
2.3. In situ DRIFTS study
As described in our earlier papers, Ag/Al2O3 catalyst with silver
loading of 4wt% was prepared by an impregnation method,
immersing boehmite into an aqueous solution of silver nitrate.2,23
After impregnation, the excess water was removed in a rotary
evaporator at 333 K. And then the sample was calcined in air at 873
K for 3 h. The catalyst prepared by this method exhibited a specific
surface area of 239.8 m2/g. UVꢀvis analysis showed that Ag species
In situ DRIFTS spectra were recorded on a Nexus 670 FTꢀIR
(Thermo Nicolet), equipped with an in situ diffuse reflection
chamber and a high sensitivity MCT/A detector. In this case,
Ag/Al2O3 catalyst was finely ground and placed in ceramic
crucibles in the in situ chamber. Mass flow controllers and a
sample temperature controller were used to simulate the real
reaction conditions, such as mixture of gases, pressure and
sample temperature. Prior to recording each DRIFTS spectrum,
the sample was heated in situ in 10% O2/N2 flow at 823 K for 1
h, then cooled to the desired temperature to measure a reference
spectrum. All gas mixtures were fed at a flow rate of 300 mL
minꢀ1. All spectra were measured with a resolution of 4 cmꢀ1
and with an accumulation of 100 scans.
δ+
were mainly present in the oxidized state (Ag+ and Agn clusters),
particularly as highly dispersed Ag+ ions, which was in agreement
with the results of XRD and TEM characterization.23
2.2. Activity tests
The inlet and outlet concentrations of hydrocarbons during their
oxidation over Ag/Al2O3 were analyzed by gas chromatography
(Agilent 6890N GC) equipped with Porapak Q column. During
this process, the products were also measured by gas
chromatography (Agilent 6890N GC) coupled to a mass
spectrometer (Agilent 5973N MS) with a columnꢀcontaining
HP–PLOT Q (Agilent 30 m × 0.32 mm, 20 ꢁm film). The
typical experiment conditions were as follows: 2571 ppm CH4
(or 2571 ppm C2H2, or 2571 ppm C2H4, or 1714 ppm C3H6, or
1714 ppm C3H8), 800 ppm NO (if used), 10 % O2, and N2 as
balance. The total gas flow rate was 2000 mL/min over 0.6 g
catalyst (W/F = 0.018 g s mL, GHSV = 50,000 hꢀ1). NOx
conversion was analyzed onꢀline by a chemiluminescence
3. Results
3.1. Activity tests with or without H2
We conducted a comparative study of the effect of H2 on the
oxidization of CH4, C2H2, C2H4, C3H6, and C3H8 over Ag/Al2O3
catalyst, respectively (Fig. 1A). It was evident that the addition
of 1% H2 promoted the catalytic oxidation of hydrocarbons
having more than one carbon atom in the temperature region of
423–773 K, while only a marginal effect was observed during
CH4 oxidation.
100
100
(B)
CH4
(A)
80
80
CH4
60
60
40
20
0
40
20
0
100
100
C2H2
80
C2H2
80
60
40
20
0
60
40
20
0
100
100
C2H4
80
C2H4
80
60
40
20
0
60
40
20
0
100
100
C3H6
C3H6
80
80
60
40
20
0
60
40
20
0
100
100
C3H8
C3H8
80
60
40
20
0
80
60
40
20
0
400
500
600
700
800
900
400
500
600
700
800
900
Temperature (K)
Temperature (K)
Fig. 1. (A) Conversion of hydrocarbons over Ag/Al2O3 during hydrocarbon oxidation with 1% H2 (●) and without H2 (■). (B) NOx
conversion during HCꢀSCR over Ag/Al2O3. Feed: 2571 ppm CH4 (or 2571 ppm C2H2, or 2571 ppm C2H4, or 1714 ppm C3H6, or
1714 ppm C3H8), 800 ppm NO (if used), 10% O2, N2 balance, total flow rate = 2000 mL/min, GHSV=50,000 hꢀ1.
The products in gas phase during the oxidation of different (CH3COCH3) were observed both in the presence and in the absence
hydrocarbons containing least two carbon atoms over Ag/Al2O3 were of H2 at temperatures below 673 K, together with the formation of
further measured by GC–MS. As for C2H2 oxidation (Fig. S1A and CO2, CO, and H2O. This result indicates that partial oxidation was
S1B), certain amount of acetaldehyde (CH3CHO) and acetone occurred over Ag/Al2O3. During the oxidation of C2H4 over
2 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C3CY01033C
Ag/Al2O3 (Fig. S1C and S1D), the final products of CO2, CO, and Ag/Al2O3, while it hardly changed the NOx conversion by CH4
H2O were also detected, while the partial oxidation products (Fig. 1B). Among the employed hydrocarbons, C3H6 was the
mentioned above were hardly observed in the gas phase even in the most active for NOx reduction in the presence of H2, exhibiting
presence of H2. At temperatures above 673 K, acetone was measured 90% NOx conversion even at a temperature as low as 490 K.
during the oxidation of C3H6 and C3H8 over Ag/Al2O3 (Fig. S1E and The most significant promotion effect of H2 was observed
S1G); after H2 was coꢀfed (Fig. S1F and S1H), this species was during the C3H8–SCR process, giving a T50 for NOx conversion
observed within the whole temperature range, together with the (temperature for 50% NOx conversion) of 480 K, 205 K lower
formation of acetaldehyde at low temperatures. These results clearly than that in the absence of H2 (Table 1). Within the whole
suggest that H2 promotes partial oxidation of hydrocarbons, in temperature region, NOx conversion by CH4 was lower than
particular for C3H6 and C3H8.
15%, indicating that the C1 reductant was not active for the
The presence of H2 significantly promoted NOx reduction by SCR of NOx over Ag/Al2O3.
hydrocarbons containing at least two carbon atoms over
Table 1. Temperature required for the formation of surface enolic species (1633 cmꢀ1) and acetate (1464 cmꢀ1) with the strongest
intensity (Tmax) during hydrocarbon oxidation and temperature required for 50% NOx conversion (T50) during HC–SCR over
Ag/Al2O3.
Tmax for enolic species formation Tmax for acetate formation (K)
T50 for NOx conversion (K)
(K)
Without
H2
a
b
c
With H2
ꢂTmax
Without
H2
With H2
ꢂTmax
Without
H2
With H2
ꢂT50
C2H2
C2H4
C3H6
C3H8
623
673
723
773
573
523
523
523
50
673
773
773
773
623
673
573
573
50
649
705
647
684
560
594
477
479
89
150
200
250
100
200
200
111
170
205
a ꢂTmax = Tmax for enolic species formation without H2 – Tmax for enolic species formation with H2;
b ꢂTmax = Tmax for acetate formation without H2 – Tmax for acetate formation with H2;
c ꢂT50 = T50 for NOx conversion without H2 – T50 for NOx conversion with H2.
more significant than that of C2H4. In the former case, the peak
at 1633 cmꢀ1 due to enolic species exhibits the strongest
intensity within the temperature between 473–523 K, indicating
a high concentration on the surface of Ag/Al2O3.
3.2. In situ DRIFTS studies
To identify the origination of the H2 effect on the surface
species formed by oxidation of the different hydrocarbons, in
situ DRIFTS studies were performed (Fig. 2, see also Fig. S2).
As for CH4 oxidation over Ag/Al2O3, the peaks at 1595 and
1392 cmꢀ1 were observed which were due to νas (OCO) and νs
(OCO) of formate, respectively.17 By comparison with their
intensity, it indicates that the formation of formate was hardly
changed by H2 addition within the temperature of 473–773 K
(Fig. S2A and S2B). The peaks at 1550 and 1460 cmꢀ1 can be
assigned to surface carbonates (Fig. S2A and S2B).28 In the
presence of H2, the peak at 1651 cmꢀ1 assignable to adsorbed
water was observed at 473 K, the appearance of which was
detected over Ag/Al2O3 in flowing H2 + O2 at temperatures
between 423–523 K (Fig. S2K).
1415
1633
1573
0.2
1336
1464
1676
1392
1595
CH4 + O2
C2H2 + O2
C2H4 + O2
During the oxidation of C2H2 over Ag/Al2O3 (Fig. 2, see
also Fig. S2C and S2D), surface formate was also observed at
temperatures below 723 K. Meanwhile, the appearance of peaks
at 1573 and 1464 cmꢀ1 indicates the formation of surface
acetate.12,17,29,30 The peaks at 1633, 1415, and 1336 cmꢀ1 can be
assigned to surface enolic species.14, 21ꢀ25 Introduction of H2 into
the fed gas changes the intensity of peaks due to the acetate and
enolic species. Particularly at low temperatures, H2 promotes
the formation of surface enolic species over Ag/Al2O3. The
peak at 1676 cmꢀ1 can be assigned to the stretching vibration
model of C=O in acetone adsorbed on the surface,31 the
formation of which in the gas phase has been confirmed by
GC–MS analysis.
C3H6 + O2
C3H8 + O2
2200
2000
1800
1600
1400
1200
Wavenumbers (cm-1)
Fig. 2. In situ DRIFTS spectra of Ag/Al2O3 at steady state at
523 K during oxidation of hydrocarbons without H2 (dotted line)
and with H2 (solid line). The components of the feed gas are the
same as in Fig. 1, while at total flow rate = 300 mL/min.
Formate, acetate, and enolic species also formed during
catalytic oxidation C2H4, C3H6, and C3H8 over Ag/Al2O3 (Fig. 2,
see also Fig. S2EꢀS2J). As for C2H4 and C3H8 oxidation, it
should be noted that surface acetate was more pronounced than
enolic species in the presence of H2 within temperatures range
of 523–773 K. During the oxidation of C3H6 over Ag/Al2O3, the
promotion effect of H2 on the formation of enolic species was
To clearly illustrate the changes of surface enolic species
and acetate triggered by the addition of H2 during the oxidation
of hydrocarbons, the spectra (Fig. S2C–2J) in the range of
1200–2000 cmꢀ1 were converted into Kubelka–Munk
function,32 and then fitted on the basis of the deconvoluted
curves (typical results for the deconvolution and curve fitting
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DOI: 10.1039/C3CY01033C
were presented in Fig. S3). After Kubelka–Munk conversion, Tmax for the two surface species originating from H2 addition
the integrated areas of peaks at 1633 (for enolic species) and can be calculated, as shown in Table 1 (hereafter denoted as
1464 cmꢀ1 (for acetate) in Fig. S2C–S2J are displayed as a ꢂTmax). Taking C2H2 oxidation as a example, H2 addition
function of temperature (Fig. 3). Based on these, one can obtain promoted the formation of enolic species at low temperatures,
the temperature at which the enolic species and acetate exhibit exhibiting the maximum area at the temperature of 573 K (Tmax
the strongest intensity (hereafter denoted as Tmax, see also Table = 573 K), 50 K lower than that in the absence of H2 (ꢀTmax = 50
1) during the oxidation of hydrocarbons. Then, the change of K).
300
250
200
150
100
50
15
12
9
Enolic species (H2)
Enolic species (H2)
(A) C2H2 oxidation
(B) C2H4 oxidation
Enolic species
Acetate (H2)
Enolic species
Acetate (H2)
Acetate
Acetate
6
3
0
0
400
450
500
550
600
650
700
750
800
400 450 500 550 600 650 700 750 800 850
Temperatuer (K)
Temperature (K)
60
50
40
30
20
10
0
25
Enolic species (H2)
Enolic species (H2)
(C) C3H6 oxidation
(D) C3H8 oxidation
Enolic species
Acetate (H2)
Enolic species
Acetate (H2)
20
15
10
5
Acetate
Acetate
0
400
450
500
550
600
650
700
750
800
400 450 500 550 600 650 700 750 800 850
Temperature (K)
Temperature (K)
Fig. 3. The integrated areas of the peaks due to enolic species (1633 cmꢀ1) and acetate (1464 cmꢀ1) during the oxidation of C2H2 (A), C2H4
(B), C3H6 (C), and C3H8 (D) at different temperatures, respectively.
the presence of H2 over Ag/Al2O3, although they had been
During the oxidation of C2H2, C2H4, C3H6, and C3H8 in the
considered to represent surface acrylate species.33ꢀ35
presence of H2, it should be noted that the Tmax values for enolic
species were always lower than those for acetate. These results
indicate that the stability of enolic species is lower than acetate,
250
250
providing the opportunity to react with Nꢀcontaining species at
low temperatures.8ꢀ11 Meanwhile, different hydrocarbons
200
exhibit different Tmax values for the formation of enolic species;
for a given hydrocarbon, the value of Tmax is dramatically
200
changed by the addition of H2. As a result, the Tmax value for
150
the enolic species formation and its change by H2 addition
reveal key clues that predict the intrinsic properties during
hydrocarbon oxidation, and also for HCꢀSCR over Ag/Al2O3.
Indeed, the Tmax for enolic species commonly appeared at the
lightꢀoff temperature range for NOx reduction in the presence
of H2 (usually denoted as T50, see also Table 1), possibly
indicating their key role in H2ꢀassisted HC–SCR. To further
elucidate this association, the relationship between the ꢂTmax
for enolic species and corresponding change of T50 for NOx
conversion (ꢂT50) during HC–SCR is shown graphically in Fig.
4. The stronger the promotion effect of H2 on the formation of
enolic species, the more significant the enhancement of H2 on
NOx conversion is, confirming the crucial role of enolic species
in H2ꢀassisted HC–SCR. Interestingly, the peaks herein
assigned to enolic species were also observed by other
researchers during C3H6–SCR and C8H18–SCR, particularly in
150
100
50
100
50
0
0
C3H6
C2H2
C2H4
C3H8
Fig. 4. Relationship between the ꢂTmax for enolic species
triggered by H2 addition and ꢂT50 for NOx conversion during
HC–SCR.
4 | J. Name., 2012, 00, 1-3
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The in situ DRIFTS studies were also performed during the
As for partial oxidation of C2H4, C3H6, and C3H8 in the
NOx reduction by different hydrocarbons (Fig. 5, see also Fig. absence of H2 over Ag/Al2O3 (Figs. 2 and 3), acetate was the
S4 and ESI). Similarly, H2 promoted the partial oxidation of mainly product. As a result, it is reasonable that these
C2H2, C2H4, C3H6, and C3H8 to produce enolic species (1633 reductants are not active for NOx reduction in the absence of
and 1336 cmꢀ1) and acetate (1573 and 1464 cmꢀ1) over H2 particularly at low temperatures (Fig. 1). It should be noted
Ag/Al2O3. In contrast, both the species were absent in the NOx that, H2 promoted the formation of enolic species during the
reduction by CH4, leaving large amounts of nitrates on the partial oxidation of C2H2, C2H4, C3H6, and C3H8 over Ag/Al2O3,
surface of Ag/Al2O3, exhibiting the feature frequencies of 1550 particularly in the low temperature range of 423–523 K (Figs. 2
and 1251 cmꢀ1 for monodentate nitrate, 1589 and 1306 cmꢀ1 for and 3). The stronger the promotion of enolic species formation,
bidentate nitrate, and 1612 cmꢀ1 for bridging nitrate.17,36 Taking the higher the activity for NOx reduction is (Fig. 4). As
the activity results into account (Fig. 1B), it may be the case suggested by GC–MS measurement, acetaldehyde was often
that a hydrocarbon containing at least two carbon atoms is produced during partial oxidation of hydrocarbons described
required for NOx reduction with high efficiency, even in the above. Previous studies have confirmed that the adsorption of
presence of H2. The peak for –NCO, as a key intermediate in acetaldehyde on the surface of Ag catalyst induced the
HC–SCR over Ag/Al2O3, was observed at 2235 cmꢀ1, the formation of enolic species.21,24 As a result, it is reasonable that
intensity of which was enhanced by H2 addition during NOx a higher concentration of enolic species was observed during
reduction by C2H4, C3H6, and C3H8.1ꢀ3,34,37 As for C2H2ꢀSCR, acetaldehyde was unambiguously detected in the gas phase (Fig.
however, a much stronger intensity –NCO peak was observed S1 and Fig. 2).
in the absence of H2, indicating that only a fraction of all the –
NCO observed by DRIFTS is reactive and able to serve as a
HCOO-
With or
CH4
potential intermediate for NOx reduction at
a
given
temperature.35
Without H2
-
NO + O2
NO2, NO3
1633
1573
1612
1336
0.2
1464
RHC=CH-O-
1306
1251
With H2
1589
2235
R-N
R-O
1550
-
C2H2, C2H4,
C3H6, C3H8
NO + O2
CH4-SCR
NO2, NO3
CH3COO-
C2H2-SCR
C2H4-SCR
Without H2
-
NO2, NO3
C3H6-SCR
C3H8-SCR
2000
Fig. 6. Proposed mechanism of H2ꢀassisted HC–SCR over
Ag/Al2O3.
Based on these findings, a universal pathway of H2ꢀ
promoted HC–SCR over Ag/Al2O3 was proposed as shown in
Fig. 6. The employed hydrocarbons containing at least two
carbon atoms is necessary for the formation of active enolic
species during their partial oxidation over Ag/Al2O3, which was
significantly enhanced by H2 addition. In this case, the
formation of enolic species and its further reaction to produce
N2 is the main pathway for NOx reduction with high efficiency
in the presence of H2. In our previous research,22 it was also
proposed that the oxygenated hydrocarbon such as alcohols
containing at least one CꢀC bond is prerequisite for enolic
species formation during their partial oxidation over Ag/Al2O3.
This intrinsic property has been further clarified during partial
oxidation of CH3OH, in which the enolic species was rarely
observed over Ag/Al2O3.40,41 Similarly result was also observed
in flowing CH4 + O2 over Ag/Al2O3. Thus, the pathway of CH4ꢀ
SCR even in the presence of H2 is quite different with others
presented here.
2200
1800
1600
1400
1200
Wavenumbers (cm-1)
Fig. 5. In situ DRIFTS spectra of Ag/Al2O3 at steady state at
523 K during the NOx reduction by different hydrocarbons
without H2 (dotted line) and with H2 (solid line).
4. Discussion
Previous studies mainly achieved by in situ DRIFTS, confirmed
that surface enolic species originating from the partial oxidation
of ethanol gave high activity for reaction with NO + O2 to
produce N2 via the formation of organoꢀnitrogen compounds,
which can be considered as follows: NO + O2 + C2H5OH
NOx + enolic species (acetate also formed) R–ONO + R–
NO2 –NCO + –CN
N2.20ꢀ23 By using PIMS analysis,
ꢀ adꢀ
ꢀ
ꢀ
ꢀ
ethenol in gas phase as an important intermediate during the
catalytic oxidation of ethanol over Ag/Al2O3 catalyst was
unambiguously identified,38,39 further confirming the formation
of surface enolic species during the NOx reduction by ethanol
over Ag/Al2O3. Acetate was also formed during partial
oxidation of ethanol, while it exhibits lower concentration and
low activity toward NO + O2 than enolic species, resulting in a
minor role in ethanolꢀSCR over Ag/Al2O3.
Sazama and Wichterlová found that the presence of
hydrogen peroxide enhanced substantially the activity of
Ag/Al2O3 for NOx reduction by decane.9 By using ESR
measurement, Shimizu et al. provided evidence for the in situ
ꢀ
generation of superoxide ions (O2 ) in H2 + O2 and H2ꢀassisted
C3H8ꢀSCR reactions over Ag/Al2O3.13 This active oxygen
species enhanced partial oxidation of hydrocarbon to acetate,
thus contributing to the “H2 effect”. More recently, Shimizu et
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VJieowuArrtnicalelONnlainme e
DOI: 10.1039/C3CY01033C
al. also proposed that the presence of H2 promoted activation of
O2 to OOH species over AgꢀMFI, the occurrence of which is
crucial for the partial oxidation of C3H8.11 In our case, it is
possible that reactive oxygen species mentioned above would
be produced in the presence of H2, and then triggered the
formation of enolic species during partial oxidation of
hydrocarbons having more than one carbon atom over
Ag/Al2O3.
It has been widely accepted that the structure of
hydrocarbons has a great influence on the activity of Ag/Al2O3
for NOx reduction.1,4 As for NO reduction by various alkanes
over Ag catalysts, Satsuma and coꢀworkers found that both the
reaction rates of NO and hydrocarbons had good correlations
with mean bond energy of the alkanes, which is an average of
all CꢀH and CꢀC bond energies in the hydrocarbons.4,42 Further
research achieved by in situ DRIFTS experiments revealed that
these correlations could be rationalized by the reaction rate of
the formation of surface oxygenated species such as acetate,
which was significantly promoted by the presence of H2.4,12 In
our cases, the intrinsic properties of the employed hydrocarbons,
such as carbon number, CꢀH and CꢀC bond energies, the
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promotion of H2 on enolic species formation.
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NOx reduction by hydrocarbons containing twoꢀ or threeꢀ
carbon atoms was clearly promoted by H2 at low temperatures, 22. Y.B. Yu, X.P. Song and H. He, J. Catal., 2010, 271, 343.
while this promotion effect of H2 did not occur during the CH4–
SCR over Ag/Al2O3. In situ DRIFTS spectra identified that the
formation of enolic species was triggered by H2 addition during
the oxidation of the hydrocarbons containing two or three
carbons at low temperatures, while the enolic species was
hardly observed during CH4 oxidation. The stronger the
promotion effect of H2 on enolic species formation, the more
significant the enhancement of NOx conversion is, confirming
the crucial role of enolic species in H2ꢀassisted HCꢀSCR, and
thus creating an effective pathway for NOx reduction.
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The authors thank the support from the National Natural
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National High Technology Research and Development Program
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DOI: 10.1039/C3CY01033C
Table of contents entry
The promotion effect of H2 on reactive enolic species formation is a common feature of H2-assisted HC-SCR over
Ag/Al2O3.
250
200
150
100
250
200
150
100
50
0
50
0
C H
C2H2
C2H4
C3H8
3
6