Applied Catalysis B: Environmental 144 (2014) 408–415
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Applied Catalysis B: Environmental
jo ur nal home p ag e: www.elsevier.com/locate/apcatb
Magnesia doped Ag/Al
2
O
3
– Sulfur tolerant catalyst for low
temperature HC-SCR of NO
x
Pavan M. More
a,b
, Neelam Jagtap
a
, Atul B. Kulal
a,b
,
Mohan K. Dongare
a,c
, Shubhangi B. Umbarkar
a,b,∗
a
Catalysis Division, CSIR-National Chemical Laboratory, Pune 411008, India
b
Academy of Scientific and Innovative Research, CSIR, Anusandhan Bhawan, New Delhi 110 001, India
c
Mojj Engineering Systems Ltd., 15-81/B, MIDC, Bhosari, Pune 411026, India
a r t i c l e i n f o
Article history:
Received 19 April 2013
Received in revised form 7 June 2013
Accepted 17 July 2013
Available online 26 July 2013
Keywords:
HC-SCR
NOx
Low temperature activity
Sulfur tolerance
a b s t r a c t
A series of magnesia doped Ag/Al
2
O
3
catalysts were prepared by modified impregnation method using
boehmite as alumina precursor. The prepared catalysts were characterized and tested for the SCR of NO
x
using propene as reductant under lean condition. Doping of magnesia improved the low temperature
catalytic activity for HC-SCR of NO
x
as well as sulfur tolerance. Maximum 98% NO conversion with 100%
selectivity for N
2
was obtained at 350
◦
C with 7% Mg doping to Ag/Al
2
O
3
. The improvement in low temper-
ature activity and the sulfur tolerance has been correlated to decreased acidity after addition of magnesia
to alumina support. In situ FTIR study showed that the deactivation in the presence of SO
2
was due to
the sulfation of silver and aluminum sites in Ag/Al
2
O
3
catalyst, however its formation was suppressed in
case of magnesia doped Ag/Al
2
O
3
improving its sulfur tolerance.
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
Automobiles with diesel engines are becoming more and more
popular because of their better fuel efficiency and lower CO
2
emission compared to their stoichiometric spark ignited gasoline
counterpart. The three way catalyst which simultaneously convert
NO
x
, CO and HCs into N
2
, CO
2
and H
2
O from the exhaust of stoichio-
metric spark ignited gasoline engines are not capable of reducing
NO
x
from the exhaust of lean burn engines because of excess of oxy-
gen [1]. Thus, removal of NO
x
from the exhaust of lean-burn diesel
engines is a major challenge. Therefore worldwide lots of efforts are
being taken both in academic and industrial laboratories to develop
suitable catalyst for the reduction of NO
x
under lean condition.
There are mainly two catalytic processes being investigated for the
removal of NO
x
under lean conditions viz.; NO
x
storage reduction
(NSR) developed by Toyota [2,3] and selective catalytic reduction
by NH
3
(urea) [4] or hydrocarbon as reductant (HC-SCR) [5]. In this
context, HC-SCR seems to be a promising catalytic process for the
removal of NO
x
under lean conditions since this method exploits
unburned hydrocarbons already present in the exhaust gas stream.
Precious metal-based catalysts (PGM), especially Pt based catalysts
∗
Corresponding author at: Catalysis Division, CSIR-National Chemical Laboratory,
Pune 411008, India. Tel.: +91 2025902044; fax: +91 2025902633.
E-mail addresses: sb.umbarkar@ncl.res.in, shubhangibuin@yahoo.com
(S.B. Umbarkar).
such as Pt/Al
2
O
3
exhibit the low temperature activity (lower than
300
◦
C) and are more resistant to water vapor and SO
x
poisoning
[6]. However Pt/Al
2
O
3
has two major disadvantages: low selectivity
for N
2
(substantial amount of N
2
O is formed) and narrow tempera-
ture window of operation for NO reduction [7,8]. Metal oxide-based
catalysts, for example Al
2
O
3
, TiO
2
, ZrO
2
, MgO and these oxides pro-
moted by various metals/metal oxides like Co, Bi, Ni, Cu, Fe, Sn,
Ga, In, Au, Ag have been reported to be active for HC-SCR [9–13].
Amongst these, Ag/Al
2
O
3
was found to be the most promising cata-
lyst for practical use since it exhibits high activity for NO reduction,
high selectivity for N
2
, moderate tolerance for sulfur and water
vapor, especially at higher temperatures [8]. However the major
drawback of this system is the low activity and poor sulfur tolerance
at low temperatures (573–673 K) due to the formation of silver and
aluminum sulfate [14]. Thus improvement in the low temperature
activity and sulfur tolerance in the temperature range 573–673 K
is still a major challenge for HC-SCR.
One way of improving the low temperature activity and the sul-
fur tolerance of Ag/Al
2
O
3
catalyst is by modification of the Al
2
O
3
support. Previously we have demonstrated modification of alumina
support by Si and Ti [15] for improved sulfur tolerance. Improve-
ment in catalytic activity of Ag/Al
2
O
3
by addition of magnesia to
-Al
2
O
3
has been reported for HC-SCR using various reductants
like propene or mixture of propene and hydrogen [16,17]. Improve-
ment in propene SCR activity of 3%Ag/Al
2
O
3
after addition of
magnesia has been reported by Anil Kumar et al. [16] and improve-
ment in the activity has been correlated to the MgAl
2
O
4
spinel
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http://dx.doi.org/10.1016/j.apcatb.2013.07.044