New insight into hydroxyl-mediated NH3 formation on the Rh-CeO2 catalyst surface during catalytic reduction of NO by CO

Article abstract of DOI:10.1016/S1872-2067(17)62856-0

Vibrational IR spectra and light-off investigations show that NH₃ forms via the “hydrogen down” reaction of adsorbed CO and NO with hydroxyl groups on a CeO₂ support during the catalytic reduction of NO by CO. The presence of water in the reaction stream results in a significant increase in NH₃ selectivity. This result is due to water-induced hydroxylation promoting NH₃ formation and the competitive adsorption of H₂O and NO at the same sites, which inhibits the reactivity of NO reduction by NH₃.

Full text of DOI:10.1016/S1872-2067(17)62856-0

Chinese Journal of Catalysis 38 (2017) 1399–1405
a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c h n j c
Article
New insight into hydroxyl‐mediated NH3 formation on the Rh‐CeO2
catalyst surface during catalytic reduction of NO by CO
Chengxiong Wanga,b, Wenzheng Xiab, Yunkun Zhao a,b,
*
a State Key Laboratory of Advanced Technologies for Comprehensive Utilization of Platinum Metals, Kunming Institute of Precious Metals, Kunming
650106, Yunnan, China
b State‐Local Joint Engineering Laboratory of Precious Metal Catalytic Technology and Application, Kunming Sino‐platinum Metals Catalysts Co. Ltd.,
Kunming 650106, Yunnan, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Vibrational IR spectra and light‐off investigations show that NH3 forms via the “hydrogen down”
reaction of adsorbed CO and NO with hydroxyl groups on a CeO2 support during the catalytic reduc‐
tion of NO by CO. The presence of water in the reaction stream results in a significant increase in
NH3 selectivity. This result is due to water‐induced hydroxylation promoting NH3 formation and the
competitive adsorption of H2O and NO at the same sites, which inhibits the reactivity of NO reduc‐
tion by NH3.
Received 20 March 2017
Accepted 4 May 2017
Published 5 August 2017
Keywords:
© 2017, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
NH3 formation
NO reduction
CO oxidation
In‐situ DRIFTS
Water gas shift
1. Introduction
metal‐support interfaces [4,5].
Few reports exists concerning the formation of NH3 during
The catalytic reduction of NO to N2 by CO has been widely
applied in emission control for automotive exhaust [1]. In re‐
cent years, CeO2 has generally been used as an oxygen buffer in
three‐way catalytic converter systems owing to its oxygen
storage/release capacity, undergoing redox processes involv‐
ing the Ce4+/Ce3+ couple, CeO2 ⇌ CeO2−x + (x/2)O2, that can
greatly promote CO oxidation [2]. Among transitional metals,
rhodium shows the strongest ability for specifically enhancing
NO reduction due to its ability to efficiently dissociate NO,
which is the main reason for Rh addition to commercial
three‐way catalysts [3,4]. Additionally, Rh/ceria interactions
improve both resistance to thermally induced catalyst sintering
and catalytic activity due to additional bifunctional sites at the
the catalytic reduction of NO by CO, but N2O selectivity has
been discussed [2,4,6,7], resulting in a misunderstanding of the
reaction mechanisms [8–11]. N2 selectivity is defined as ‘the
percentage of NO conversion to N2 (total reduced NO concen‐
tration minus double N2O concentration) in total NO conver‐
sion’ [7,12], indicating that only N2, N2O, and CO2 products are
taken into consideration during the catalytic reduction of NO by
CO on Rh catalyst. Dropsch et al. [13] suggested that CO could
react with hydroxyl groups on the support to release hydrogen
gas. Until now, NH3 detection in the effluent gas has not been
used to analyze N2 selectivity, resulting in an improper mecha‐
nistic and kinetic understanding of the NO/CO reaction on Rh
catalyst [11,14].
* Corresponding author. Tel: +86‐871‐68393370; Fax: +86‐871‐68328841; E‐mail: ykzhao@sohu.com
This work was supported by the National Natural Science Foundation of China (21463015), the Provincial Applied Fundamental Research Program of
Yunnan (2014FA045), and the National High Technology Research and Development Program of China (863 Program, 2015AA034603).
DOI: 10.1016/S1872‐2067(17)62856‐0 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 38, No. 8, August 2017

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Chengxiong Wang et al. / Chinese Journal of Catalysis 38 (2017) 1399–1405
NH3 emission from automobiles is principally due to the
and/or 0.1% NO for transient experiments. The total flow rate
of the mixed gas stream in these experiments was 100 mL/min.
The catalytic activity and selectivity of the catalyst samples
were studied in the catalytic reduction of NO by CO in a con‐
tinuous flow fixed‐bed microreactor. Catalyst samples (500
mg) with an average diameter of 40–60 mesh were placed in a
stainless‐steel tube with an inner diameter of 5 mm to conduct
activity tests, and CO temperature‐programmed reduction
(CO‐TPR) and NO temperature‐programmed oxidation
(NO‐TPO) measurements. The reaction mixture gas, as shown
in Table 1, was supplied at a flow rate of 500 mL/min and the
space velocity was 60000 mL/(g·h). The temperature depend‐
ence of the reaction was determined in tempera‐
ture‐programmed mode by heating from ambient temperature
to 500 C at a heating rate of 10 C/min. The composition of the
effluent gas was analyzed online using a 2030DBG2EZKS13T
MultiGas FT‐IR Analyzer purchased from MKS Instruments, Inc.
The conversion rate of NO, X(NO), was calculated using
equation (1):
reduction of nitrogen oxides (NOx) by H2 generated from wa‐
ter‐gas shift and steam‐reforming reactions [15–17]. Recent
studies [18,19] have emphasized that NH3 is among the main
factors causing the formation of secondary inorganic aerosols
+
−
(NH4 , SO42−, and NO3 ). Therefore, an exact understanding of
the NH3 formation mechanism in the catalytic reduction of NO
to N2 by CO plays a significant role in effective NH3 emission
control for gasoline‐fueled vehicles under transient driving
conditions. This work focuses on determining the behavior of
NH3 generation and proposing a possible reaction mechanism
for NH3 formation during the catalytic reduction of NO by CO,
leading researchers to a more accurate analysis of light‐off test
results.
2. Experimental
CeO2 was synthesized using a precipitation method. A slight
excess of 25% NH4OH solution was added dropwise into the
requisite amount of aqueous solution of Ce(NO3)3·5H2O under
continuous stirring. The obtained gelatinous precipitate was
aged at 80 C for 3 h under vigorous stirring, filtered, and
washed with distilled water several times until no pH change
was observed. The precipitate was dried at 120 C overnight
and then calcined at 300 C in static air for 1 h. The resultant
powder was calcined at 550 C for 4 h under an air atmosphere.
Rh‐CeO2 catalyst was prepared by mixing the obtained ge‐
latinous precipitate with Rh nanoparticles (NPs), followed by
aging, drying, and calcinating using the same method outlined
for CeO2 preparation. For Rh NP synthesis, polyvinylpyrroli‐
done (PVP) and citric acid (CA) were added into 85 mL of
Rh(NO3)3 aqueous solution with a PVP monomer unit:CA:Rh
molar ratio of 5:4:1, and then placed in a 250‐mL three‐necked
flask. The mixture was preheated under a nitrogen atmosphere
and magnetic stirring to 60 C for 5 min and the pH was ad‐
justed to 3.0 by the addition of 10% NH4OH solution. The req‐
uisite borane tert‐butylamine (TBAB), which served as a re‐
ducing agent in the reaction, was dissolved in 15 mL of distilled
water and added to the above mixture with a TBAB:Rh molar
ratio of 5:1. The reaction proceeded at 60 C for 10 min.
X(NO) = ([NO]in − [NO]out)/[NO]in×100%
(1)
where [NO]in and [NO]out are the concentrations of injected and
effluent NO gas, respectively. The conversion rate of CO, X(CO),
was calculated using the same method. The selectivities to N2O
and NH3, S(N2O) and S(NH3), respectively, were calculated us‐
ing equations (2) and (3), based on the concentrations of N2O
and NH3 products, respectively.
S(N2O) = 2[N2O]/([NO]in − [NO]out)×100%
S(NH3) = [NH3]/([NO]in − [NO]out)×100%
(2)
(3)
Activation energies were estimated for CO conversion using
the Arrhenius equation. About 200 mg of catalyst sample was
used in each measurement. The samples were first activated at
400 C in a CO/NO or CO/NO/H2O reaction mixture gas stream
at a total flow rate of 500 mL/min and then cooled to room
temperature. The composition of the effluent gas was analyzed
online at 10 C interval in the (5–20)% CO conversion temper‐
ature range. The specific reaction rate was defined as the total
number of moles of CO2 produced per second per mass of cata‐
lyst at a specific temperature.
3. Results and discussion
Fourier transform infrared (FT‐IR) measurements of the
solid samples were carried out in transmission mode on a Ni‐
colet iS10 FT‐IR spectrometer with a resolution of 2 cm−1 using
the KBr wafer technique and scanning 36 times. For wafer
preparation, catalyst sample (10 mg) was mixed with KBr (200
mg, Specpure) and pressed under a pressure of 20 MPa for
several minutes. The actual Rh loading amount was analyzed
on a PerkinElmer Optima 8000 ICP‐OES instrument.
In‐situ diffuse reflectance infrared Fourier transform spec‐
troscopy (DRIFTS) was performed on a Nicolet 6700 FT‐IR
spectrometer equipped with a mercury cadmium telluride
(MCT) detector. Each sample (100 mg) was pretreated at 500
C for 60 min in a N2 stream and then cooled to room tempera‐
ture. Before starting the experiment, background spectra were
recorded at 100 C intervals from 100 to 500 C in a N2 stream
at a heating rate of 10 C/min. Each corresponding spectrum
was measured in a reaction gas stream containing 0.1% CO
The catalytic reduction of NO by CO is an important model
reaction for simulating three‐way catalysis. In this work, model
CO/NO and CO/NO/H2O reactions were carried out using a
Table 1
Composition of reaction mixture gas stream for catalytic properties
test.
Reaction
CO‐TPR
NO‐TPO
CO/NO
CO/H2O
NO/H2O
CO/NO/H2O
NO/H2
CO (%)
0.1
0
0.1
0.1
0
NO (%) H2O (%)
H2 (%)
0
0
0
0
0
N2
0
0
0
0
5
5
5
0
balance
balance
balance
balance
balance
balance
balance
0.1
0.1
0
0.1
0.1
0.1
0.1
0
0
0.05a
a Total conversion of 0.1% CO via a water‐gas shift reaction would pro‐
duce 0.05% H2.

Chengxiong Wang et al. / Chinese Journal of Catalysis 38 (2017) 1399–1405
1401
100
90
80
70
60
50
40
30
20
10
0
(a)
CO/NO reaction on
Rh-CeO2
CO/NO/H2O reaction
on Rh-CeO2
CO/NO reaction on CeO2
Solid line: CO conversion
Dash line: NO conversion
100 150 200 250 300 350 400 450 500
Reaction temperature (C)
Fig. 1. (a) CO and NO conversion and (b) NH3 selectivity curves during catalytic reduction of NO by CO.
typical fixed‐bed flow reactor under a CO/NO stoichiometric
mixture. CO and NO conversion curves for the model reactions
are shown in Fig. 1(a). T50 values for CO and NO conversions in
the CO/NO reaction over Rh‐CeO2 catalyst with an actual Rh
loading amount of 0.296% were 152 and 156 °C (Table 2), re‐
spectively, which were significantly lower than those over the
CeO2 catalyst. This was further confirmed by the lower appar‐
ent activation energy (Ea) of 36 kJ/mol for the Rh‐CeO2 catalyst
(Fig. 2), compared with 105 kJ/mol for the CeO2 catalyst, which
explained the lack of intrinsic reactivity for base metals com‐
pared with precious metals [3,20]. For the Rh‐CeO2 catalyst,
water‐induced active inhibition of CO and NO conversions was
explained by the corresponding increases in T90 and Ea (up to
21 kJ/mol) compared with results under dry conditions.
NO is partially reduced to N2O as the main product on the Ce
site below 250 °C, and N2O selectivity decreased with increas‐
ing temperature in the CO/NO reaction (Fig. 3). Notably, Fig.
1(b) shows that clear NH3 gas detection started at about 190 °C
and a maximum NH3 selectivity of 14.8% was obtained on the
CeO2 support at 268 °C in the H2O‐free reaction mixture. Fur‐
thermore, CO and NO conversions of 17.1% and 12.6%, respec‐
tively, were achieved at the same temperature. By contrast, N2O
selectivity strongly decreased when Rh nanoparticles were
supported because N2O was more readily formed and binded
less strongly. Therefore, N2O was more likely to desorb into the
gas phase on CeO2 surfaces than on Rh sites [11]. The Rh‐CeO2
catalyst also showed higher NH3 selectivity up to 236 °C, but
gave lower NH3 selectivity at higher temperatures due to a
rhodium‐mediated increase in reactivity.
Table 2
T50 and T90 values for CO and NO conversion in the catalytic reduction of
NO to N2 by CO.
CeO2
T50 a (°C) T90 b (°C)
Rh‐CeO2
T50 (°C) T90 (°C)
Reaction
CO/NO
Reactant
CO
NO
CO
NO
321
325
407
379
152
156
155
162
247
250
266
266
no data no data
no data no data
CO/NO/H2O
a Light‐off temperature for 50% conversion.
b Light‐off temperature for 90% conversion.
100
90
1.2x1020
1x1020
80
36 kJ/mol
70
57 kJ/mol
(1)
(2) (3)
60
50
40
30
20
10
0
8x1019
2x1019
(2)
(3)
105 kJ/mol
(1)
1x1019
100 150 200 250 300 350 400 450 500
1.9
2.0
2.1 2.4 2.5 2.6 2.7 2.8 2.9
1000/T (K–1)
Reaction temperature (C)
Fig. 3. N2O selectivity curves of the NO/CO reaction on (1) CeO2 surface ,
(2) Rh‐CeO2 surface, and (3) the NO/CO/H2O reaction on Rh‐CeO2 sur‐
face.
Fig. 2. Arrhenius curves of the NO/CO reaction on (1) CeO2 surface, (2)
Rh‐CeO2 surface, and (3) the NO/CO/H2O reaction on Rh‐CeO2 surface.

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Chengxiong Wang et al. / Chinese Journal of Catalysis 38 (2017) 1399–1405
Fig. 4(b) shows that the amount of NH3 product on the
100
90
80
70
60
50
40
30
20
10
0
Rh‐CeO2 catalyst increased in the following order: CO/NO/H2O
(0.0158% NH3 at 203 C) > CO/NO (0.0094% NH3 at 222 C) >
NO/H2 (0.004% NH3 at 240 C). There was no clear NH3 for‐
mation during the NO‐TPO and NO/H2O reactions at low tem‐
peratures. The results indicated that CO and NO might react
with hydroxyl groups on the surface of the CeO2 support to
release NH3 gas via a water‐gas shift reaction, comprising the
chemisorption and reaction of CO with hydroxyl species, fol‐
lowed by NO reduction, as described in equations (4) and (5)
[13,21,22].
CO TPR
CO/NO
CO/H2O
CO/NO/H2O
2OH + 2CO → CO2 + H2
2NOx + (2x+3)H2 → 2xH2O + 2NH3
(4)
(5)
Results concerning the influence of water on NH3 selectivity
are shown in Fig. 1(b). Two peaks for NH3 formation, centered
at 199 and 342 °C with NH3 selectivities of 25.2% and 20.9%,
respectively, were observed in the water mixed feed stream.
The maximum NH3 selectivity at low temperature was at‐
tributed to OH groups, while NH3 was formed at high tempera‐
tures due to the reduction of NOx by H2, where CO reacts with
water to produce H2 and CO2 according to the water‐gas shift
reaction, as shown in equation (6). Compared with the CO‐TPR
results, CO conversion rates increased with increasing temper‐
ature above 230 C due to the water‐gas shift reaction, as
shown in Fig. 5. Theoretically, 0.05% H2 would be released
when a total conversion of 0.1% CO was achieved in the wa‐
ter‐gas shift reaction. The competitive reaction of H2O and NO
with CO during the CO/NO/H2O reaction might form H2 as a
product.
100 150 200 250 300 350 400 450 500
Reaction temperature (C)
Fig. 5. CO conversion curves on Rh‐CeO2 catalyst in tempera‐
ture‐programmed experiments using different gas mixtures.
of CeO2 to release NH3. The FT‐IR spectrum of pristine Rh‐CeO2
catalyst (Fig. 6) showed three main bands at 3693, 3650, and
3506 cm–1, which were assigned to Ce‐OH terminal hydroxyl
groups (type I), Ce2‐OH double‐bridging groups (type II), and
cerium oxyhydroxide impurities, respectively [24,25]. After the
CO/NO/H2O reaction, three main stretching vibrations of OH
groups were still detected, while the Ce‐OH vibration showed a
sharp decrease in intensity and shifted to higher wavenumber
(3704 cm–1). However, no band was observed in the range
3800–3400 cm–1 after CO/NO cycling. This was explained by
the interaction of adsorbed H2O with basic OH groups. Demou‐
lin et al. [22] suggested that hydroxylated surfaces resulted
from water vapor dissociatively chemisorbing to become hy‐
droxyl ions. Surface water molecules can also react with chem‐
isorbed CO species to form H+ and formate ions [26].
CO + H2O → H2 + CO2
(6)
As shown in Figs. 4 and 5, CO conversion was observed
during the CO‐TPR experiment, illustrating that CO indeed re‐
acted with active species on the catalyst surface. There was
little NO conversion and NH3 production under the NO/H2O
stream, which indicated that H2O inhibited NO adsorption due
to the competitive adsorption of H2O and NO at the same sites
[23]. This inhibition effect would reduce the reactivity of NO
reduction by NH3, leading to increased NH3 emission.
Surface species evolutions of CO and/or NO on pretreated
Rh‐CeO2 catalyst were also investigated using in‐situ DRIFTS
measurements. The stretching vibrations of hydroxyl groups
exhibited a significant wavenumber shift after CO adsorption at
100 °C, as shown in Fig. 7(a). This was mainly due to the pres‐
ence of surface hydroxyls on the CeO2 support, which (1) al‐
FT‐IR spectroscopy was used to further confirm that ad‐
sorbed CO and NO reacted with hydroxyl groups on the surface
0.027
100
NO TPO
CO/NO
(a)
90
(b)
0.024
80
70
60
50
NO/H2O
CO/NO/H2O
NO/H2
0.021
0.018
0.015
0.012
0.009
0.006
0.003
0.000
40
NO TPO
CO/NO
NO/H2O
CO/NO/H2O
NO/H2
30
20
10
0
-10
100 150 200 250 300 350 400 450 500
100 150 200 250 300 350 400 450 500
Reaction temperature (C)
Reaction temperature (C)
Fig. 4. (a) NO conversion and (b) NH3 formation curves on Rh‐CeO2 catalyst in temperature‐programmed experiments using different gas mixtures.

Chengxiong Wang et al. / Chinese Journal of Catalysis 38 (2017) 1399–1405
1403
(1594, 1568, and 1530 cm–1) were observed with increasing
temperature [23,30]. These results suggested that NO reacted
with OH groups to form nitrates and release NH3, explaining
NH3 formation and clear NO conversion during the NO‐TPO
experiment. In contrast, the peak at 3622 cm–1 started to ap‐
pear at a lower temperature (200 °C) and showed a higher
intensity at the same temperature when CO and NO were in‐
troduced simultaneously, illustrating that CO played an im‐
portant role in NH3 formation during the NO reaction. These
results were in good agreement with those in Fig. 4(b).
(2)
(3)
(1)
3704
3506
3500
3650
4. Conclusions
3693
3700
NH3 production in the catalytic reduction of NO to N2 by CO
under dry conditions was ascribed to NO reduction by molecu‐
lar hydrogen present in hydroxyl groups on the CeO2 surface.
In‐situ DRIFTS results suggested that the consumption of ceri‐
um oxyhydroxide impurities was directly accomplished by NO
reduction, whereas type I and II hydroxyls were consumed via
a water‐gas shift reaction followed by NOx reduction, which led
to NH3 generation. In the presence of water, water‐induced
hydroxylation could promote NH3 formation, whereas the
competitive adsorption of H2O and NO on the same sites would
inhibit the reactivity of NO reduction by NH3, leading to in‐
creased NH3 emission.
3800
3600
3400
3300
Wavenumber (cm–1)
Fig. 6. FT‐IR spectra of (1) Rh‐CeO2 catalyst, and used catalysts from (2)
CO/NO and (3) CO/NO/H2O reactions.
lows the migration of adsorbed CO on Rh across the CeO2 sur‐
face to isolated OH sites to form carbonates (1583, 1453, and
1402 cm–1), and (2) alters the coordination of chemisorbed CO
at the oxygen end, weakening the C–O bond followed by facili‐
tating CO dissociation during CO oxidation [24,25,27,28]. At
this temperature, the features at 2168, 2073, 2010, and 1760
cm–1 were attributed to Ce4+‐carbonyls, gem‐dicarbonyls, linear
carbonyls, and bridged carbonyls on Rh, respectively [27]. The
vibrational IR spectra of carbonyl species, and type I and II
hydroxyls, showed a decrease in intensity, which indicated the
consumption of adsorbed CO. The band at 3616 cm–1 was as‐
signed to O–H stretching vibrations of hydrogen carbonate
species formed by the interaction of CO with basic OH– ions
[10,28]. In summary, the reaction of type I and II hydroxyls
with adsorbed CO led to a weakening of their vibrations and
hydrogen carbonate formation.
Figs. 7(b) and (c) show in‐situ DRIFTS results for tempera‐
ture‐programmed experiments in NO and CO/NO streams, re‐
spectively. Under NO, the obvious consumption of three types
of hydroxyl groups was accomplished by the reaction with NO,
with a peak apparition appearing at 3622 cm–1 (when the tem‐
perature was raised to 300 °C). This was associated with N–H
stretching in coordinated ammonia from NH3 product adsorbed
on the oxide surface [29]. The consumption of adsorbed NO on
Rh (band at 1922 cm–1) and the formation of nitrates on CeO2
Acknowledgments
This work was supported by Xiaodong Wu (School of Mate‐
rials Science and Engineering, Tsinghua University) for in‐situ
DRIFTS characterization.
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Chengxiong Wang et al. / Chinese Journal of Catalysis 38 (2017) 1399–1405
Graphical Abstract
Chin. J. Catal., 2017, 38: 1399–1405 doi: 10.1016/S1872‐2067(17)62856‐0
New insight into hydroxyl‐mediated NH3 formation on the Rh‐CeO2 catalyst surface during catalytic reduction of NO by CO
Chengxiong Wang, Wenzheng Xia, Yunkun Zhao *
Kunming Institute of Precious Metals; Kunming Sino‐platinum Metals Catalysts Co. Ltd.
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Rh-CeO2催化剂表面CO还原NO反应中羟基介导NH3生成问题的探讨
王成雄a,b, 夏文正b, 赵云昆a,b,*
a昆明贵金属研究所稀贵金属综合利用新技术国家重点实验室, 云南昆明650106
b昆明贵研催化剂有限责任公司, 贵金属催化技术与应用国家地方联合工程实验室, 云南昆明650106
摘要: CO催化还原NO是发生在汽车尾气净化催化剂中的一个重要化学反应. CeO2容易发生氧化还原反应CeO2 ⇌ CeO2−x
+ (x/2)O2而具有氧储存/释放作用, 可以有效地促进CO氧化, 因而CeO2作为储氧材料和催化助剂被广泛应用于汽车催化剂
中. 在过渡金属元素中, 铑对NO的解离活性最高, 是目前汽车三效催化剂中最为重要的还原性活性组分. 目前, 有关
Rh-CeO2基催化剂表面CO还原NO的文献仅关注催化反应活性和N2O选择性, 对CO还原NO反应机理的理解还不够深入准
+
−
确, 无法为轻型汽油车NH3排放控制提供正确有用的理论基础. NH3排放至大气中会以NH4 形式与SO42−和NO3 离子结合,
导致二次颗粒物污染, 因此, 研究CO还原NO反应中NH3生成机理对轻型汽油车NH3排放控制具有非常重要的理论意义. 我

Chengxiong Wang et al. / Chinese Journal of Catalysis 38 (2017) 1399–1405
1405
们研究组强调了CO催化还原NO反应的表面羟基介导NH3生成问题, 并通过原位漫反射傅里叶变换红外光谱(in-situ
DRIFTS), 傅里叶变换红外光谱(FT-IR), 程序升温还原/氧化(TPR/TPO)等现代分析表征技术深入研究了CO还原NO反应机
理, 并首次提出了催化剂表面“羟基脱氢”反应的NH3生成机理.
研究发现, Rh-CeO2催化剂表面CO还原NO反应的NH3选择性最高可达9.7%, 其反应表观活化能仅为36 kJ/mol, in-situ
DRIFTS, FT-IR和NO-TPO测试结果表明, NH3的生成可归因于催化剂表面“羟基脱氢”反应, 即CO与催化剂表面端位羟基和
桥式羟基发生“水煤气转化”反应生成H2, 反应产生的H2还原NO生成NH3; CeO2中非骨架铈双羟基化形成的类氢氧化铈物
种则会直接与NO发生脱氢反应生成NH3, 但需要更高的反应温度. 值得注意的是, 当反应气中额外通入5%水蒸气时, 其反
应表观活化能提高了21 kJ/mol (同比增加58.3%), 更重要的是NH3选择性明显提高, 最高可达25.3%(同比增加160.8%),
FT-IR测试结果表明, 这是由于水蒸气作用促使催化剂表面羟基化, 表面活性氢源得以不断补充. 这从动力学角度促进了
端位羟基和桥式羟基的“水煤气转化”反应而提高NH3选择性. 同时, 对比NO/H2, CO/NO和CO/NO/H2O反应的NH3生成浓
度, 我们还发现, H2O分子与NO的竞争吸附会抑制未解离吸附的NH3进一步还原NO, 减少反应生成NH3的消耗, 促使更多生
成的NH3从催化剂表面脱附至气相中, 这也是水蒸气导致NH3选择性明显增加的重要原因. 以上结果清晰地表明了催化剂
表面“羟基脱氢”作用和水蒸气分子与NO的竞争吸附行为对CO还原NO反应中NH3生成的重要影响.
关键词: NH3生成; NO还原; CO氧化; 原位漫反射傅里叶变换红外光谱; 水煤气转化
收稿日期: 2017-03-20. 接受日期: 2017-05-04. 出版日期: 2017-08-05.
*通讯联系人. 电话: (0871)68393370; 传真: (0871)68328841; 电子信箱: ykzhao@sohu.com
基金来源: 国家自然科学基金(21463015); 云南省应用基础研究计划项目(2014FA045); 国家高技术研究发展计划项目(863计划,
2015AA034603).
本文的英文电子版由Elsevier出版社在ScienceDirect上出版(http://www.sciencedirect.com/science/journal/18722067).