Low-temperature nitric oxide reduction over silver-substituted cobalt oxide spinels

Article abstract of DOI:10.1039/c5cy01643f

Catalytic reduction of NO by CO is performed over novel silver-substituted cobalt oxide nano-sized spinels, prepared via a citric acid-assisted sol-gel method. The catalysts are characterized by XRD, TEM, BET surface area measurements and CO chemisorption studies. Ag substitution in the cobalt oxide spinel lattice enhances CO chemisorption, hence the catalytic activity significantly. The prepared catalysts show excellent stability under the reaction conditions. The effect of moisture and oxygen dosage is studied for the reaction. The catalysts are found to be highly selective for N₂ over N₂O.

Full text of DOI:10.1039/c5cy01643f

View Article Online
View Journal
Catalysis
Science &
Technology
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: A. V. Salker and M.
S. Fal Desai, Catal. Sci. Technol., 2015, DOI: 10.1039/C5CY01643F.
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the Ethical guidelines still
apply. In no event shall the Royal Society of Chemistry be held
responsible for any errors or omissions in this Accepted Manuscript
or any consequences arising from the use of any information it
contains.
www.rsc.org/catalysis

Please do not adjust margins
Catalysis Science & Technology
Page 1 of 5
View Article Online
DOI: 10.1039/C5CY01643F
Catalysis Science & Technology
Communication
Low temperature nitric oxide reduction over silver substituted
spinel cobalt oxide.
Received 00th January 20xx,
Accepted 00th January 20xx
A. V. Salker and M. S. Fal Desai
Catalytic reduction of NO by CO is performed over novel silver substituted cobalt oxide nano sized spinels, prepared via
citric acid assisted sol-gel method. Catalysts are characterized by XRD, TEM, BET surface area measurements and CO
chemisorption studies. Ag substitution in spinel cobalt oxide lattice enhances CO chemisorption, hence catalytic activity
significantly. Prepared catalysts show excellent stability in the reaction conditions. Effect of moisture and oxygen dosage is
studied for the reaction. The catalysts are found to be highly selective for N2 over N2O.
DOI: 10.1039/x0xx00000x
www.rsc.org/
INTRODUCTION
Low temperature reduction of nitric oxide (NO) by carbon
monoxide (CO) is one of the challenging reactions being studied
by many research groups all over the world. Low temperature
reduction of NO by NH3 as reducing agent has been reported by
EXPERIMENTAL
AgxCo3-xO4 (x= 0.1, 0.2, 0.3) were synthesised by citric acid
assisted sol-gel method. Cobalt (II) nitrate hexa-hydrate (Sigma
Aldrich, ≥ 99.0) and silver nitrate (Sigma Aldrich, ≥ 99.0) were
dissolved in stoichiometric amount in water. To this solution,
citric acid (Thomas Baker, AR) in 1: 1 molar ratio (Co/citric acid)
was added and stirred for one hour at room temperature. The
solution was then evaporated at 120 ˚C which resulted in the
formation of a pink foamy mass. This was then ground using
mortar and pestle and calcined at 600 ˚C in air for 8 h.
2
many research groups.1, Unfortunately, NH3 has to be added
externally in such exhaust treatment systems, thus its practical
application is limited. CO is a very convenient reducing agent
which is present in the exhaust systems itself. Literature cites
very few reports on low temperature NO-CO reaction which has
been achieved using Cu and Au based catalyst.3 - 6
Cobalt spinel compounds are proven to be efficient in CO
oxidation at low temperatures.7,8 Co3O4 has a promising role in
low temperature CO oxidation due to its high surface oxygen
mobility. It is proven that CO oxidation by lattice oxygen is an
important step in nitric oxide reduction by CO.9 Thus, materials
like spinel cobalt oxide can be good candidates for low
temperature nitric oxide reduction. As per L. Wang et. al.,
pristine Co3O4 undergoes a phase change to CoO in the reaction
condition due to the lattice oxygen desorption.10 It is difficult to
reduce NO which is stable at higher temperatures (above 300
˚C) and therefore difficult to reduce it to N2.11 Thus, NO
reduction would be favored at lower temperatures.
Incorporation of precious metals such as Rh in cobalt spinel
lattice is found to be effective in stabilizing its structure and
phase.10 Since silver belongs to the Cu and Au family, which is
closely related to precious group metals, its incorporation in
cobalt oxide is studied for NO reduction. Preliminary studies are
carried out to understand the effect of moisture and oxygen on
the silver substituted catalysts.
The spinel phase of the catalysts powder samples was
identified using X-ray powder diffractometer (Rigaku Ultima IV)
with Cu-Kα source. TEM images were recorded with the help of
PHILIPS CM 200 electron microscope. Elementary studies were
performed to check the presence of silver using Energy
Dispersive X-ray spectroscopy on JEOL JSM 6360 LV scanning
electron microscope (SEM/EDS). BET surface area
measurements and CO chemisorptions were performed using
QUANTACHROME AUTOSORB IQ-MP-C surface area analyzer.
Prior to the BET analysis, samples were degassed at 120 ˚C for
three hours and nitrogen adsorption was performed at liquid
nitrogen temperatures. Chemisorption of CO was performed at
room temperature using 0.15 g of catalyst. Samples were
initially heated to 100 ˚C under N2 flow, cooled and CO was
passed at the flow rate of 60 ml min-1.
Redox reaction between NO and CO was performed in a
continuous flow, fixed bed glass reactor. About 1.5 g of the
catalyst was loaded in the glass reactor supported in between
the quartz wool. Moisture and adsorbed oxygen was removed
by passing stream of N2 flow at the rate of 20 ml h-1 at 100 ˚C for
15 min. Catalyst bed was cooled down at room temperature and
then CO was flushed over the catalyst at the rate of 4 ml min-1
Department of Chemistry, Goa University, Goa 403206, India.
Email: sav@unigoa.ac.in
This journal is © The Royal Society of Chemistry 20xx
Catal. Sci. Technol. 2015, 00, 1-3 | 1
Please do not adjust margins

Please do not adjust margins
Catalysis Science & Technology
Page 2 of 5
View Article Online
DOI: 10.1039/C5CY01643F
Communication
Catalysis Science & Technology
for 10 min. Catalytic activity was measured as a function of
temperature using 5% CO and 5% NO in Argon at the rate of
5000 ml h-1. Controlled heating of the furnace was done using
temperature programmer at the rate of 2 ˚C min-1. Individual gas
flow rates were maintained by gas flow meters and precision
needle valves. Conversion of NO and CO was monitored by
employing an online Gas Chromatograph with Molecular sieve
13x and Porapak Q columns using TCD detector.
Stability test of catalysts in presence and absence of
moisture and oxygen was carried with the same feed gas
composition for 10 h. For the stability test in the presence of
moisture and oxygen, 2.5% moisture and 2.5% O2 were
introduced in the original feed gas composition.
RESULTS AND DISCUSSION
Figure 2. Chemisorption of CO at room temperature (25
˚C).
measurements carried out at liquid nitrogen temperature
indicates high surface area of about 106, 105 and 104 m2g-1 for
Ag0.1Co2.9O4, Ag0.2Co2.8O4 and Ag0.3Co2.7O4 respectively, whereas
pristine surface area of about 56 m2g-1 is observed for Co3O4.
Figure 2 shows the CO chemisorptions result performed at room
temperature. Enhancement in CO adsorption with increase in Ag
substitution is observed. This shows that electron rich elements
like Ag facilitate CO chemisorptions which increase linearly with
increased CO partial pressure.
Nitric oxide conversion plots are presented in Figure 3.
Results clearly indicate that silver substitution greatly enhances
the activity of NO reduction by CO as compared to the pristine
catalyst. 100% NO conversion was observed at 120 ˚C for
Ag0.3Co2.7O4 catalyst which is found to be the best among the
tested catalysts and is reported for the first time over such
catalyst system. 100% conversion (T100) for Ag0.2Co2.8O4 is
observed at 150 ˚C. Catalyst activity is tested for three catalytic
cycles without any regeneration process and the data is found
to be reproducible with no loss in activity. Formation of N2O in
lower concentration is observed during the activity test
(supplementary Table 1). It is a well-established fact that at low
temperatures selectivity for N2 over N2O is less due to the
partial reduction of NO in such a catalytic reaction. Increase in
Ag substitution improved the selectivity for N2, as evident from
the decrease or no N2O formation at low temperatures. CO
conversion is observed in line with NO conversion in all the
experiments. Higher amount of Ag substituted composition
Ag0.5Co2.5O4 (biphasic) is also tested for catalytic activity but
shows lower activity than Ag0.2Co2.8O4 catalyst.
Figure 1. a) XRD pattern of silver substituted catalyst
and b) TEM image of Ag0.2Co2.8O4.
Monophasic formation of Ag substituted spinel cobalt oxide is
seen and the XRD pattern matches with JCPDS card No. 00-042-
1467 (Figure 1a). The diffraction pattern shows peaks at 111
(~18.9˚), 220 (~31.3˚), 311 (~36.8˚), 222 (~38.56˚), 400 (~4.8˚),
511 (~59.3˚), 440 (~65.2˚) and 533 (~78.5˚) respectively, which
indicates the cubic phase formation. No extra reflection of Ag or
Ag2O is observed for doped catalysts. Broadening of the peaks is
attributed to the poor crystallinity and nano size particle nature
of the catalysts. An increase in lattice parameter was observed
with increase in silver substitution. Higher doping of Ag was
attempted (Ag0.5Co2.5O4) under same preparative conditions but
resulted in biphasic composition (shown in supplementary
Figure 1). Presence of silver is confirmed by SEM/EDS studies.
TEM images indicate the formation of spherical and plate like
nano particles ranging from 10 - 25 nm. Representative TEM
image of Ag0.2Co2.8O4 is given in Figure 1b. BET surface area
2 | Catal. Sci. Techno., 2015, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Please do not adjust margins
Catalysis Science & Technology
Page 3 of 5
View Article Online
DOI: 10.1039/C5CY01643F
Catalysis Science & Technology
Communication
chemical bond with CO and NO via back bonding.12 CO
adsorption studies show enhancement in CO adsorption with
increase in Ag substitution which is probably due to the back
bonding ability of Ag. Stronger CO adsorption result in
desorption of CO as CO2, since CO takes lattice oxygen as
explained by Mar’s - Van Krevelen mechanism which reduces
the oxide catalyst surface. NO has ability to re-oxidize the
catalyst surface by dissociative adsorption of NO which can
result in N2 and N2O formation. Selective N2 formation is
observed when NO dissociate completely over the oxide surface
and re-oxidize the reduced catalyst site. Whereas, partial
dissociation of NO results in N2O formation. Thorough
investigations are needed to explain the observed experimental
results. From the catalytic study, it is observed that Ag
incorporation improves the catalysts’ activity drastically may be
due to its high affinity for CO adsorption, small particle size and
higher surface area as compared to the pristine catalyst.
Figure 3. NO conversion over all the catalyst in feed
gas composition 5% NO, 5% CO in Ar at 5000 ml h-1.
CONCLUSIONS
Catalyst stability tests for Ag0.3Co2.7O4 at 120 ˚C in different
reaction environments are shown in Figure 4. As evident from
the figure, catalysts show excellent stability in the reaction test
conditions as there is no fall in the activity observed with
respect to time up to 10 h. Catalyst stability in presence of
moisture and oxygen is investigated. A marginal decrease in
activity due to the presence of moisture and O2 is observed.
Similar trend is also seen for Ag0.2Co2.8O4 catalyst
(Supplementary Figure 6). Moisture has no effect on selectivity
for N2. In the presence of oxygen, N2 selectivity is reduced due
to the N2O formation. Since O2 oxidizes both NO and CO
oxidation, complete reduction of NO to N2 becomes difficult.
Ag substituted catalysts were prepared by citric acid assisted
sol-gel method. Prepared catalysts were found to be 10 - 25 nm
in size. BET surface area measurements indicate catalysts to be
of high surface area in the range of 104 to 106 m2g-1. XRD and
SEM/EDS studies indicate that silver is incorporated in
appropriate concentration in spinel cobalt oxide but higher
concentration results in biphasic compound. Ag substitution
significantly improved the catalytic activity for NO-CO redox
reaction. CO chemisorption at room temperature showed an
enhancement in CO adsorption with silver substitution.
Substituted catalysts were found to be active at low
temperature. Among the tested catalysts Ag0.3Co2.7O4 gave
complete conversion of NO at 120 ˚C, which is novel among the
few best catalysts reported in the literature. Increase in Ag
substitution reduced N2O formation at low temperatures.
Catalysts also showed excellent reproducibility for NO reduction
reaction with same activity without regeneration. N2O
formation was not observed above 150 ˚C for Ag0.2Co2.8O4
indicating good selectivity for N2. Catalysts have showed
excellent stability for the NO-CO reaction for 10 h. Low level of
moisture did not show substantial decrease in the activity.
However, marginal fall in activity was observed with
introduction of oxygen in the catalytic system. It is concluded
that, Ag substituted spinel cobalt oxide are novel for NO-CO
redox reaction and highly active under various reaction
conditions studied.
Ag belong to the group of metals having outer electronic
configuration ns2 (n - 1)d9 (like Cu and Au) for which good
activity at low temperatures is reported. These metals are
closely related to platinum group of metals which are usually
found to be highly active for NO reduction due to their
electronegative nature which helps in strengthening the
Acknowledgements
Authors are grateful to CSIR - New Delhi, for financial
assistance under project proposal. Authors will also like to thank
the Director, NCAOR Goa for extending the SEM/EDS facilities.
Figure 4. Catalyst stability test of Ag0.3Co2.7O4 for NO
conversion in the reaction mixture for 10 h at 120 ˚C,
also in presence of moisture and O2 in Ar at flow rate of
5000 ml h-1.
References
1
G. Qi, R. T. Yang, Chem. Comm., 2003, 848-849.
This journal is © The Royal Society of Chemistry 2015
Catal. Sci. Technol., 2015, 00, 1-3 | 3
Please do not adjust margins

Please do not adjust margins
Catalysis Science & Technology
Page 4 of 5
View Article Online
DOI: 10.1039/C5CY01643F
Communication
Catalysis Science & Technology
2
3
4
J. Zuo, Z. Chen, F. Wang, Y. Yu, L. Wang, X. Li, Ind. Eng. Chem.
Res.,2014, 53, 2647–2655.
I. Spassova, M. Khristova, D. Panayotov, D. Mehandjiev, J.
Catal., 1999, 185, 43-57.
J. Shu, W. Xia, Y. Zhang, T. Cheng and M. Gao, Chin. J. Chem.
Phys.,2008, 21,393-400.
5
6
A. Ueda, M. Haruta, Gold Bulletin., 1999, 32, 1-11.
G. Xianrui, L. Hao, L. Lichen, T. Changjin, G. Fei, D. Lin, J. Rare
Earth., 2014, 32, 139-145.
7
X. Xie, Y. Li, Z. Liu, M. Haruta, W. Shen, Nature lett., 2009,
458, 746-749.
8
9
J. Jasson, J. Catal., 2000, 194, 55-60.
J. Shan, Y. Zhu, S. Zhang, T. Zhu, S. Rouvimov, F.Tao, J. Phys.
Chem. C, 2013, 117, 8329–8335.
10 L. Wang, S. Zhang, Y. Zhu, A. Patlolla, J. Shan, H. Yoshida, S.
Takeda, A. I. Frenkel, F. Tao, ACS. Catal., 2013, , 1011-1019.
3
11 A. V. Salker, W. Weisweiler, Appl. Catal. A, 2000, 203, 221-
229.
12 C. D. Yazdi, A. L. Cooksy, A. M. Efstathiou, Surf. Sci., 602
2008, 602, 1858–1862.
4 | Catal. Sci. Techno., 2015, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 5 of 5
Catalysis Science & Technology
View Article Online
DOI: 10.1039/C5CY01643F
350x190mm (96 x 96 DPI)