Abatement of N2O in the selective catalytic reduction of NO(x) on platinum zeolite catalysts upon promotion with vanadium

Article abstract of DOI:10.1039/a906961e

The selectivity to N₂O during the selective catalytic reduction of NO(x) on platinum-containing zeolite catalysts is reduced by incorporation of vanadium into the catalyst.

Full text of DOI:10.1039/a906961e

Abatement of N2O in the selective catalytic reduction of NOx on platinum
zeolite catalysts upon promotion with vanadium
Yvonne Traa, Beate Burger and Jens Weitkamp*
Institute of Chemical Technology I, University of Stuttgart, 70550 Stuttgart, Germany.
E-mail: jens.weitkamp@po.uni-stuttgart.de
Received (in Cambridge, UK) 27th August 1999, Accepted 29th September 1999
The selectivity to N2O during the selective catalytic reduc-
tion of NOx on platinum-containing zeolite catalysts is
reduced by incorporation of vanadium into the catalyst.
The selective catalytic reduction of nitrogen oxides by hydro-
carbons is a promising process for the purification of exhaust
gases from diesel-powered engines. Since the average tem-
perature of diesel exhaust gases is well below 300 °C,1
platinum-containing catalysts have proven to be efficient for
this process owing to their low lightoff temperature. Another
important advantage of platinum-containing catalysts is that
they are comparatively insensitive to water in the exhaust gas.2
However, a major drawback of these catalysts is that large
portions of the nitrogen oxides are converted to nitrous oxide
rather than to nitrogen. Nitrous oxide is highly undesirable,
since it is a strong greenhouse gas and contributes to
stratospheric ozone destruction.
Fig. 1 Average selectivity to N2O in the selective catalytic reduction of NOx
with propene as a function of the molar ratio nV/(nPt + nV) of platinum- and/
or vanadium-containing H-ZSM-35 zeolites.7
In the past, several attempts were undertaken to overcome the
N2O problem with platinum-containing catalysts: Doumeki
et al. reported that the selectivity to N2 can be enhanced by
using organoplatinum complexes such as PtMe(OSiPh3)(cod)
instead of inorganic complexes as precursors for the preparation
of Pt/SiO2 or Pt/Al2O3 catalysts.3 However, the N2O selectivity
remained as high as 65%. Wunsch et al. found that, upon adding
Table 1 Catalytic results on platinum- and/or vanadium-containing H-
ZSM-35 samples
CeO2 to a Pt/ZSM-5 catalyst (mCeO /mPt ≈ 4), the maximum
Catalyst
XNO ,max (%)
DXNO ,max (%)
Tmax/°C
x
x
2
NOx conversion increased from 48 to 59% without a change in
temperature, whereas the selectivity to N2O decreased from 62
to 54%.4 More favourable results were reported by Seker and
Gulari who observed an N2O selectivity of only 17% on a Pt/
alumina catalyst not prepared by the usual impregnation
procedure but by a sol–gel process.5 These results seem to be
very promising, but further tests on the long-term and
hydrothermal stability of the catalyst have yet to come. A totally
different approach was followed by Burch and Ottery by
changing the reductant: On a Pt/Al2O3 catalyst, no detectable
amount of N2O was formed with toluene as reducing agent at
any temperature.6 However, with this method one would be
bound to toluene as reductant which is not a constituent of the
exhaust gas stream.
1.0V/H-ZSM-35a
41
—
19
32
21
26
40
—
303
215
205
212
196
177
165
2.6Pt–3.1V/H-ZSM-35 81
0.9Pt–1.0V/H-ZSM-35 89
3.0Pt–2.2V/H-ZSM-35 82
1.9Pt–1.0V/H-ZSM-35 90
2.6Pt–1.0V/H-ZSM-35 97
2.7Pt/H–ZSM-35
a
85
The numbers before the metals indicate the metal content in wt% in the
dry catalyst.
We followed still another approach: in order to tune the
activity and selectivity of platinum-containing catalysts we used
a second metal as promoter. As depicted in Fig. 1, the selectivity
to N2O (SN O) on platinum- and/or vanadium-containing H-
2
ZSM-35 zeolites decreases considerably with increasing vana-
dium content of the catalyst. By contrast, the maximal NOx
conversion (XNO ,max) at Tmax on Pt-V/H-ZSM-35 samples is
x
similar to that on catalysts containing platinum only (Table 1).
However, with increasing vanadium content of the samples, the
temperature at the maximal NOx conversion shifts to higher
values. An explanation for this effect could be that the platinum
is diluted by vanadium and, thus, the platinum clusters are
smaller, facilitating effective NOx reduction only at higher
temperatures.
On catalysts containing platinum and vanadium, oscillations
8
of the NOx concentration with large amplitudes (DXNO ,max
)
Fig. 2 NOx conversion on (a) 0.2 g 2.6Pt–1.0V/H-ZSM-35, Tcat = 183 °C
x
tend to occur [Fig. 2(a), cf. ref. 9]. Owing to these oscillations,
a precise determination of the selectivity to N2O (cf. Fig. 1)
and (b) 0.2 g 2.6Pt–1.0V/H-ZSM-35 diluted with 0.7 g quartz, Tcat.
=
190 °C.
Chem. Commun., 1999, 2187–2188
This journal is © The Royal Society of Chemistry 1999
2187

turns out to be difficult, because the N2 concentration fluctuates
considerably. Obviously, reaction, adsorption and desorption
take place simultaneously and cannot be separated more
properly, since the N2O and N2 concentrations change inde-
pendently. First attempts to reduce the amplitude of the
oscillations by diluting the catalyst with quartz sand are
promising: for a mechanical mixture of the catalyst with quartz,
the amplitude of the oscillations is strongly decreased, whereas
the average NOx conversion is even increased from 81 to 90%
[Fig. 2(b)].
The authors gratefully acknowledge financial support by
Deutsche Forschungsgemeinschaft, Fonds der Chemischen
Industrie and Max Buchner-Forschungsstiftung.
Notes and references
1 P. Zelenka, W. Cartellieri and P. Herzog, Appl. Catal. B, 1996, 10, 3.
2 H. K. Shin, H. Hirabayashi, H. Yahiro, M. Watanabe and M. Iwamoto,
Catal. Today, 1995, 26, 13.
3 R. Doumeki, M. Hatano, H. Kinoshita, E. Yamashita, M. Hirano, A.
Fukuoka and S. Komiya, Chem. Lett., 1999, 515.
These observations prompted us to look in some detail into
previously proposed reaction mechanisms. Burch et al. pro-
posed that, on platinum, NO molecules are adsorbed in addition
to N and O atoms resulting from NO dissociation.10 The
formation of N2 can then be accounted for by recombination of
two adsorbed N atoms, while the formation of N2O is best
interpreted in terms of the reaction of an adsorbed NO molecule
with an N atom. We speculate that, in our system, most platinum
atoms have vanadium atoms as next nearest neighbours. Owing
to the higher affinity of vanadium for oxygen, the dissociation
of NO could then be promoted resulting in a lower NO
concentration on the surface and, hence, a lower selectivity to
N2O.
In conclusion, vanadium as a promoter is able to reduce
considerably the selectivity to N2O during the selective catalytic
reduction of nitrogen oxides on platinum-containing zeolite
catalysts. If the oscillations occurring on Pt–V/H-zeolites do not
affect activity in real car exhausts or if the amplitudes can be
strongly reduced by coating the catalyst onto monoliths in much
the same way as this can be achieved by dilution with quartz, a
major obstacle for the use of platinum catalysts in diesel exhaust
gases could be overcome.
4 R. Wunsch, G. Gund, W. Weisweiler, B. Krutzsch, K. E. Haak, G.
Wenninger and F. Wirbeleit, SAE Trans., Sect. 4, No. 962 044, 1996,
1892.
5 E. Seker and E. Gulari, J. Catal., 1998, 179, 339.
6 R. Burch and D. Ottery, Appl. Catal. B, 1996, 9, L19.
7 The catalytic experiments were performed in a flow-type apparatus with
a fixed-bed reactor at atmospheric pressure. The gaseous components of
the model exhaust gas were premixed and afterwards saturated with
water vapour at 45 °C. Typically, the feed contained ca. 0.02 vol% NOx,
0.06 vol% C3H6, 0.03 vol% CO, 4 vol% CO2, 9 vol% O2 and 10 vol%
H2O in He at a flow rate of 150 cm3 min21. Tests with water
concentrations of 6.6 and 3.3 vol% revealed that the influence of water
on the average NOx conversion is small. The mass of the hydrated
catalyst amounted to 200 mg. The product stream was analysed for NOx
with a chemiluminescence detector; all other components were analysed
by capillary gas chromatography.
8 Amplitude of the oscillations as difference between maximum and
minimum NOx conversion.
9 Y. Traa, M. Breuninger, B. Burger and J. Weitkamp, Angew. Chem. Int.
Ed. Engl., 1997, 36, 2113.
10 R. Burch, P. J. Millington and A. P. Walker, Appl. Catal. B, 1994, 4,
65.
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Chem. Commun., 1999, 2187–2188