Communications
SCR, reaction (2)], respectively. The promoting effect of
Our data further suggest that this new concept could be
applicable to both the current classes of SCR commercial
catalysts, namely, V-based systems and metal-exchanged
zeolites. Accordingly, the enhanced SCR reaction is suitable
to augment the crucial low-temperature DeNOx activity of
both SCR converters for Diesel vehicles and modern sta-
tionary SCR installations, for example, for energy-efficient
incinerators.
ammonium nitrate on the DeNOx activity compared to
standard SCR case was clearly apparent over the V-based
catalyst, too. Moreover, Figure 4 shows that the NOx removal
efficiency was limited by the substoichiometric NH4NO3 feed
content (< 500 ppm), increased with increasing NH4NO3 feed
concentration, and approached the optimal DeNOx activity
of the fast SCR reaction. The observed effect was in fact quite
similar to the well-known effect of increasing the NO2/NOx
feed ratio from 0 to 0.5.[4] Like over the Fe zeolite catalyst, the
enhancing effect of ammonium nitrate was most significant at Experimental Section
Gaseous NO/NH3/O2/N2 mixtures were fed to commercial Fe-ZSM-5
low temperatures, and progressively decreased with increased
temperature as total NOx and NH3 conversions were
approached.
and V2O5-WO3/TiO2 SCR catalysts in the shape of small 400 CPSI
honeycomb monoliths (about 5 cm3). In addition, aqueous solutions
of ammonium nitrate or nitric acid were dosed to the test reactor by
means of a peristaltic pump, vaporized, and mixed with the gaseous
feed upstream of the SCR catalyst. The concentrations of NO, NH3,
and NO2 at the reactor outlet were continuously monitored by a UV
analyzer (ABB Limas 11-HW).
In summary, the transient and stationary data presented
here confirm that enhanced SCR reaction (3) has superior
NO reduction efficiencies in the temperature range of 200–
3508C over both Fe-ZSM-5 and V2O5-WO3/TiO2 catalysts,
and is accompanied by total conversion both of the nitrate
additive and of ammonia when these are fed in substoichio-
metric amounts. Thus, this chemistry is also compatible with
limits on ammonia slip. We emphasize also the selectivity to
nitrogen associated with reactions (3) and (4): formation of
N2O was checked by a specific IR analyzer (ABB URAS 15)
in a few dedicated runs over a zeolite-based catalyst and
found to be comparable to that observed under fast SCR
conditions over the same catalyst (ca. 20–40 ppm).
Received: July 14, 2009
Published online: September 28, 2009
Keywords: exhaust control · heterogeneous catalysis ·
.
nitrogen oxides · selective catalytic reduction ·
sustainable chemistry
[1] a) N. de Nevers, Air Pollution Control Engineering, Mc Graw
Hill, New York, 2000; b) “Integrated Pollution Prevention Con-
trol (IPPC). Reference Document on Best Available Techniques
for Large Combustion Plants”, European Commission, July 2006
(http://ftp.jrc.es/eippcb/doc/lcp_bref_0706.pdf); c) U.S. Environ-
mental Protection Agency, Health Assessment Document for
Diesel Engine Exhaust (EPA/600/8-90/057F, May 2002).
[2] a) H. Bosch, F. Janssen, Catal. Today 1988, 2, 369 – 379; b) “Nitro-
gen Oxides Removal—E (Industrial Processes and Relevant
Engineering Issues”: P. Forzatti, L. Lietti, E. Tronconi in
Encyclopedia of Catalysis, Vol. 5 (Ed.: I. T. Horvath), Wiley,
New York, 2003, pp. 298 – 343, and references therein; c) “Mono-
lithic catalysts for NOx removal from stationary sources”: I. Nova,
A. Beretta, G. Groppi, L. Lietti, E. Tronconi, P. Forzatti in
Structured Catalysts and Reactors, 2nd ed. (Eds.: A. Cybulski, J. A.
Moulijn), Taylor & Francis, Boca Raton, 2006, chap. 6, pp. 171 –
214.
Injecting nitrates to reduce NOx is apparently paradox-
ical; nevertheless, our data prove that this is indeed effective
for NOx removal. The enhanced SCR DeNOx activity is in
fact similar to that observed in the well-known fast SCR
reaction, but in this case nitrate species in aqueous solution
rather than gaseous NO2 are introduced into the feed stream.
The role of nitrates as key intermediates in the fast SCR
reaction mechanism was recently proposed.[6] In view of the
well-known redox nature of the NH3 SCR catalytic cycle,[7] it
is believed that nitrogenous compounds with a high oxidation
state can play a strong promoting role in the rate-limiting
catalyst reoxidation step.[8] From a fundamental viewpoint,
the present results thus offer new perspectives for improved
understanding of SCR catalysis, as they establish a correlation
between the oxidizing activity of different NH3 SCR co-
reactants (oxygen, nitrogen dioxide, nitrates) and the asso-
ciated DeNOx activity. Rationalization of such an extended
SCR chemistry will likely motivate further significant
research in this area.
[3] A. Kato, S. Matsuda, T. Kamo, F. Nakajima, H. Kuroda, T. Narita,
40, 52 – 59; b) C. Ciardelli, I. Nova, E. Tronconi, B. Bandl-Konrad,
On the other hand, the present results also have practical
implications, as they may open new pathways for NOx
reduction at low temperatures, whereby the amount of
boosting agent added to the SCR reactor feed is no longer
determined by the activity of the upstream oxidation catalyst,
like in the case of NO2 for the fast SCR reaction, but can be
optimally controlled by dosing an aqueous solution of the
oxidizing additive by means of the same injection systems
currently employed for dosage of ammonia/urea solutions. A
single aqueous solution containing both the reducing agent
(urea) and the oxidizing additive (ammonium nitrate) could
be used for this purpose: commercial urea/ammonium nitrate
(UAN) solutions are available with suitable compositions.
[5] International Patent Application WO2008/126118, 23.10.2008.
[6] a) C. Ciardelli, I. Nova, E. Tronconi, D. Chatterjee, B. Konrad,
Nova, C. Ciardelli, E. Tronconi, D. Chatterjee, B. Bandl-Konrad,
[8] A. Grossale, I. Nova, E. Tronconi, D. Chatterjee, M. Weibel,
J. Catal. 2007, 245, 1 – 10.
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Angew. Chem. Int. Ed. 2009, 48, 8366 –8368