Catalytic carbon oxidation over Ag/Al2O3

Article abstract of DOI:10.1016/j.jcat.2005.09.018

The potential of silver on alumina (Ag/Al₂O₃) catalyst for the oxidation of carbon particles in simulated lean combustion gas was explored. At low temperatures in the presence of NO_x, oxygen, and water, the silver of the catalyst is transformed into silver nitrate. On heating, finely dispersed silver metal is formed that catalyzes the oxidation of NO into NO₂. The carbon oxidation rates in loose mixtures of carbon black and nitrated Ag/Al₂O₃ catalyst were determined gravimetrically using a magnetic suspension balance and gas mixture comprising nitric oxide, oxygen, and water. The temperature for achieving carbon oxidation was lower with nitrated Ag/Al₂O₃ than with Pt/Al ₂O₃. The finely dispersed metallic state of the silver required for efficient NO oxidation can be regenerated in cycles involving cooling and heating in the presence of NO_x.

Full text of DOI:10.1016/j.jcat.2005.09.018

Journal of Catalysis 236 (2005) 172–175
Research Note
www.elsevier.com/locate/jcat
Catalytic carbon oxidation over Ag/Al2O3
Kenneth Villani, Roald Brosius, Johan A. Martens ∗
Centrum voor Oppervlaktechemie en Katalyse, K.U. Leuven, Kasteelpark Arenberg 23, B-3001 Leuven, Belgium
Received 30 June 2005; revised 8 September 2005; accepted 16 September 2005
Available online 17 October 2005
Abstract
The potential of silver on alumina (Ag/Al O ) catalyst for the oxidation of carbon particles in simulated lean combustion gas was explored.
2
3
At low temperatures in the presence of NO , oxygen, and water, the silver of the catalyst is transformed into silver nitrate. On heating, finely
x
dispersed silver metal is formed that catalyzes the oxidation of NO into NO . The carbon oxidation rates in loose mixtures of carbon black
2
and nitrated Ag/Al O catalyst were determined gravimetrically using a magnetic suspension balance and gas mixture comprising nitric oxide,
2
3
oxygen, and water. The temperature for achieving carbon oxidation was lower with nitrated Ag/Al O than with Pt/Al O . The finely dispersed
2
3
2 3
metallic state of the silver required for efficient NO oxidation can be regenerated in cycles involving cooling and heating in the presence of NO .
x
2005 Elsevier Inc. All rights reserved.
Keywords: Ag/Al O ; Carbon oxidation; Surface nitrates; Particulate trap regeneration
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3
1. Introduction
[12,16,20,21]. When exposed to NO-bearing gas mixtures on
heating, the alumina-supported silver nitrate decomposes into a
mixture of finely dispersed metallic silver particles and cationic
silver according to the following reaction stoichiometries [22]:
Diesel engines are energy-efficient power sources for vehi-
cles and are environmentally acceptable when provided with
efficient exhaust gas after-treatment systems for eliminating air
pollutants. A popular approach for removing soot from diesel
engine exhaust involves trapping the particles on a filter. Re-
generation of soot filters is a critical issue. In the continuously
regenerated particulate trap (CRT) [1–3], a platinum-based cat-
alyst positioned upstream of the filter catalyzes oxidation of NO
into the more powerful NO2 oxidant, such that the light-off tem-
perature of soot combustion is significantly decreased. In the
soot oxidation process, NO2 is reduced back to NO [4,5]. The
realization of an NO into NO2 oxidation function in the soot
filter itself presents a significant advantage, because it enables
multiple oxidation and reuse of the NOx molecules [6,7]. An
associated difficulty is the realization of efficient catalytic coat-
ings.
AgNO3 + NO → 2NO2 + Ag0
and
(1)
2AgNO3 + NO → 3NO2 + Ag2O.
(2)
Both of these reactions generate NO2. The transformation of
AgNO3 into Ag2O and Ag0 through the reaction of NO into
NO2 is thermodynamically favored at elevated temperatures
(Fig. 1). On the Ag/Al2O3 surface, reduced silver is present in
the form of partially charged clusters composed of up to eight
Ag atoms [23]. Reduced silver is an effective NO oxidation cat-
alyst, whereas cationic silver is inactive [12,24]. In this work
we report the unique activity of the finely dispersed silver metal
obtained through nitrate decomposition for oxidation of carbon
particles.
Ag/Al2O3 is known for its catalytic activity in selective cat-
alytic reduction of NOx with hydrocarbons [8–19]. Below ca.
250 ◦C, the Ag/Al2O3 catalyst exhibits low activity, because
of nitrate formation through reaction of the silver with NO2
2. Experimental
Ag/Al2O3 with 2 wt% Ag loading was prepared at Åbo
Akademi University by impregnation of a commercial Al2O3
support (LaRoche Industries). The support material was ground
*
Corresponding author.
E-mail address: johan.martens@biw.kuleuven.be (J.A. Martens).
0021-9517/$ – see front matter
doi:10.1016/j.jcat.2005.09.018
2005 Elsevier Inc. All rights reserved.

K. Villani et al. / Journal of Catalysis 236 (2005) 172–175
173
Fig. 1. Thermodynamic equilibrium of the conversion of AgNO (a) and NO
3
(b) into Ag O (c), Ag metal (d), and NO (e) at 0.1 MPa pressure (computed
2
2
®
using HSC Chemistry 4.0, Outokumpu).
to a particle size of <250 µm and mixed with a 0.022 M
AgNO3 solution, followed by drying and calcination at 550 ◦C
for 3 h [13]. The Ag/Al2O3 powder was compressed into pellets
with diameters of 0.25–0.5 mm. The formation and reactivity
of nitrates on Ag/Al2O3 catalyst were investigated in a fixed-
bed tubular quartz reactor. A total of 400 mg of catalyst pellets,
representing a bed volume of 0.6 ml, was held in position be-
tween two quartz wool plugs. Gases were fed from cylinders
and hydrated using a thermostatted saturator. Helium was used
as an inert gas. The contact time of the gas with the catalyst
on volume basis was 15,000 h−1. NO and NO2 concentrations
in the outlet of the reactor were analyzed using an internally
heated chemiluminescence detector (Ecophysics 700 EL ht).
The Ag/Al2O3 catalyst was loaded with nitrate in the follow-
ing manner. The catalyst was pretreated at 400 ◦C in a gas
stream comprising 6% oxygen and 12% water. The temperature
was decreased to 150 ◦C, and the gas mixture was spiked with
500 ppm NO2 for 2 h. Such treatment, according to our earlier
experience, leads to saturation of the catalyst with nitrates [22].
Pt/Al2O3 with 0.5 wt% Pt loading was prepared by the
incipient impregnation method using an aqueous solution of
Pt(NH3)4Cl2 ·H2O (Alfa Aesar). For impregnating 1 g of sup-
port, 9 mg of Pt(NH3)4Cl2 ·H2O was dissolved in about 1 ml
of water. The impregnated material was dried at 60 ◦C. Calci-
nation was done at 400 ◦C for 1 h under a stream of oxygen
gas. The catalyst was reduced under hydrogen flow at 400 ◦C
for 1 h.
Carbon oxidation was investigated gravimetrically using a
Rubotherm magnetic suspension balance. The carbon black
sample selected was Printex-U (Degussa AG), which has com-
bustion properties similar to those of diesel soot [25]. Catalysts
were loosely mixed with carbon, using a spatula to obtain re-
alistic contact conditions [26]. The catalyst:carbon ratio was
2:1 by weight. An aliquot of ethanol was added to obtain a
paste, which was dried at 60 ◦C for 2 h and crushed to obtain
small fragments. Then 270 mg of the catalyst/carbon mixture
was loaded into the suspended, perforated, stainless steel mini
basket. A heating rate of 5 ◦C/min and a total flow rate of
150 ml/min of 10% O2, 1000 ppm NO, and 5% H2O in he-
lium were used.
Fig. 2. NO, NO and NO outlet concentrations during temperature ramp-
x
2
ing experiment on nitrated Ag/Al O catalyst contacted with NO bearing gas
2
3
◦
mixture. Catalyst pretreated at 400 C in helium with 6% O and 12% H O;
saturated at 150 C in a gas mixture of 500 ppm NO , 6% O and 12% H O
in helium; temperature ramping at 5 C/min in helium with 1000 ppm NO, 6%
2
2
◦
2
2
2
◦
−1
O
and 12% H O; VHSV = 15,000 h
.
2
2
3. Results and discussion
The evolution of NO2 on heating of nitrated Ag/Al2O3 cata-
lyst in NO atmosphere was demonstrated as follows. The cata-
lyst temperature was raised at a rate of 5 ◦C/min, and 1000 ppm
NO was fed in the presence of oxygen and water. The evo-
lution of the NO, NO2, and NOx concentrations in the reac-
tor outlet are shown in Fig. 2. During the heating of nitrated
Ag/Al2O3 under an NO-bearing gas stream to 600 ◦C, the out-
let NO concentration was always below the inlet concentration
of 1000 ppm. Up to ca. 400 ◦C, there was a net release of NOx
from the catalyst stemming from nitrate elimination reactions
[Eqs. (1) and (2)]. Termination of these nitrate decomposition
reactions was marked by the outlet NOx concentration level re-
turning to the inlet value of 1000 ppm at around 400 ◦C. At
higher temperatures, NO oxidation into NO2 occurred. This
NO oxidation activity was attributed to metallic silver gener-
ated via silver nitrate decomposition according to Eq. (1). This
experiment demonstrates the reaction of the silver nitrate with
NO leading to NO2 formation according to Eqs. (1) and (2) to
400 ◦C and the NO oxidation activity of the catalyst setting in
at around 400 ◦C.
Nitrated Ag/Al2O3 catalyst was prepared in the quartz reac-
tor at 150 ◦C as described in the experiment of Fig. 2. The cata-
lyst was mixed with carbon, and a carbon oxidation experiment
was performed (Fig. 3, curve a). In the mixture of carbon and
nitrated Ag/Al2O3, carbon oxidation sets in at around 400 ◦C,

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K. Villani et al. / Journal of Catalysis 236 (2005) 172–175
for regeneration of the dispersed metallic state in exhaust gas
depending on composition and temperature.
A potential drawback of the Ag/Al2O3 system is its sensitiv-
ity to poisoning by sulfur compounds [28,29]. Further work is
needed to assess the sensitivity to sulfur.
4. Conclusions
Nitrated Ag/Al2O3 is an effective catalyst for oxidation of
carbon black in a gas mixture comprising NO along with oxy-
gen and water. This observation can be explained by the de-
composition of silver nitrates at around 400 ◦C, resulting in the
formation of very small metallic silver particles that catalyze
the oxidation of NO into NO2. Ag/Al2O3 shows higher catalytic
activity in carbon oxidation than Pt/Al2O3. Ag/Al2O3 catalyst
substantially decreases the temperature of NOx-mediated ox-
idation of carbon particles and potentially can be applied in
catalytic coatings of diesel particulate filters to facilitate regen-
eration.
Fig. 3. Carbon oxidation against temperature: (a) nitrated Ag/Al O ,
2
3
(b) Pt/Al O , (c) oxidized Ag/Al O , (d) carbon without catalyst, (e) ni-
2
3
2 3
trated Al O . Gas composition: 10% O , 1000 ppm NO and 5% H O in He;
2
3
2
2
−1
◦
VHSV = 15,000 h ; heating rate, 5 C/min.
as marked by the onset of weight loss. For comparison, carbon
combustion in the presence of nitrated Al2O3 support (Fig. 3,
curve e) occurred at similar temperatures as in the absence of
catalyst (Fig. 3, curve d), demonstrating that the presence of
silver is essential. In another comparative example, the carbon
was mixed with Ag/Al2O3 catalyst pretreated at 400 ◦C in ox-
idizing gas with 6% oxygen and 12% water. Oxidized silver
catalyst is much less active than nitrated Ag/Al2O3 catalyst
(Fig. 3, curve c). Nitrated Ag/Al2O3 oxidizes carbon at an even
lower temperature than the same alumina loaded with 0.5 wt%
Pt with 47% Pt dispersion (Fig. 3, curve b). The high activity
of nitrated Ag/Al2O3 in carbon oxidation can be explained by
the formation of small Ag0 clusters around 400 ◦C, which ox-
idize NO into the NO2 required for carbon oxidation (Fig. 1).
Bogdanchikova et al. [24] demonstrated that on Ag/Al2O3 cat-
alyst, reduced silver is the active phase in NO oxidation, and
that oxidized silver is inactive.
Acknowledgments
The authors acknowledge the European Union for the fi-
nancial support through the COMET project (contract num-
ber G3RD-CT-2002-00811). J.A.M. acknowledges the Flem-
ish government for supporting a concerted research action
on the fundamental understanding of catalytic sites (GOA).
The Ag/Al2O3 sample was a gift from Kalle Arve from Åbo
Akademi University in Finland.
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