Photoassisted NO reduction with NH3 over TiO2 photocatalyst
Tsunehiro Tanaka,* Kentaro Teramura, Kyoko Arakaki and Takuzo Funabiki
Department of Molecular Engineering, Kyoto University, Kyoto 606-8501, Japan.
E-mail: tanaka@dcc.moleng.kyoto-u.ac.jp; Fax: (+81)75-753-5925
Received (in Cambridge, UK) 28th August 2002, Accepted 9th October 2002
First published as an Advance Article on the web 22nd October 2002
Photoassisted selective catalytic reduction of NO with
ammonia (photo-SCR) at low temperature over irradiated
TiO2 in a flow reactor was confirmed to proceed efficiently
and the adsorbed ammonia reacted with NO under irradia-
tion of TiO2.
The NO in the exhaust gas from a stationary emission source is
ordinarily removed by applying selective catalytic reduction
with ammonia (ammonia SCR) over V–Ti based oxide
catalysts.1 The ammonia SCR can be operated at relatively high
temperatures above 573 K. In the case of ‘dirty gas’ including
variable amounts of halogens and SOx, the ammonia SCR
process must be set downstream of de-halogen and de-SOx
processes. Through these processes, the gas temperature drops
Fig. 1 Outlet concentration of N2 and N2O in the SCR of NO with ammonia
to less than 473 K and re-heating the gas or heating catalyst bed
is needed to activate the ammonia SCR system. Therefore, the
development of an ammonia SCR system working at low
temperature is now strongly desired.2 In the present study, we
attempted to realise low temperature SCR with the aid of light3
and a photocatalyst.
at 323 K under irradiation.
a couple of experiments. In the first one NO and O2 was passed
in the first 90 min and then the feed gas was switched to NH3.
Neither N2 or N2O was detected in the outlet flow during the
whole reaction time, and NO was not detected after switching
the feed gas, suggesting that NO is very weakly adsorbed on
TiO2. In the second experiments NH3 and O2 was passed in the
first 90 min in the dark and then the feed was changed to NO and
O2 at the same time as irradiation was started. The result is
shown in Fig. 3. N2was evolved at a steady rate the moment that
the gas feed composition was changed and the irradiation was
started. The evolution rate then gradually decreased. This shows
that NH3 was firstly adsorbed and consumed by the contact with
NO. The total amount of formed N2 was determined to be 0.23
mmol g-cat21 by integrating the evolution rate of N2, and the
value was consistent with the amount of adsorbed ammonia
TiO2 photocatalyst, JRC-TIO-4, was kindly supplied by the
Reference Catalyst Committee of the Catalysis Society of
Japan. The reaction was carried out with a conventional fixed
bed flow reactor at atmospheric pressure. The catalyst bed
consists of two flat cells (50 3 15 3 1 mm3) connected in series
and TiO2 with matched grain size in 25 2 50 mesh was packed
in the catalyst bed. The amount of the TiO2 catalyst was 1.2 g
and volume of the catalyst bed was 1.5 ml in total. Light was
irradiated from a 300 W ultra-high pressure Xe lamp with
reflection by a cold mirror. Prior to reaction, the catalyst was
treated at 673 K by passing 5% O2 diluted with Ar at a flow rate
of 50 ml min21. The composition of the reaction gas was NO
1000 ppm, NH3 1000 ppm, O2 5%, and the balance Ar and the
flow rate was 100 ml min21 corresponding to the GHSV 4000
h21. The produced N2O and N2 were quantified by on-line gas
chromatography. NO2 was checked using a NOx meter and was
not detected at all. The temperature of the catalyst bed was held
to be 323 K. The origin of nitrogen atoms in the evolved N2 and
N2O was investigated by the reaction with 15NO and 14NH3 and
we found that all the nitrogen molecules were 15N14N and all the
nitrous oxide was 15N2O.
over TiO2 in equilibrium at 323 K, 0.24 mmol g-cat21
.
Evidently, NO reacts with adsorbed ammonia under irradiation.
The kinetic experiment carried out under differential conditions
in the pressure range 300 < p(NO), p(NH3) < 2000 ppm, and
the presence of excess O2 gave the result that the evolution rate
of N2 depends only on NO partial pressure; first order against
NO, and zeroth order against O2 and NH3. This strongly
suggests that the rate-determining step is the adsorption of NO
Fig. 1 shows the time course of the reaction. N2O was
produced only in trace amounts. The ammonia SCR is a down
hill reaction and therefore it proceeds in the dark at low
temperature at 20% conversion. Photoirradiation caused re-
markable enhancement of activity. When the reaction gas was
passed under irradiation, the evolution rate of N2 gradually
increased and attained a steady rate, ca. 80% conversion after
120 min irradiation. The induction period may be due to the
time for saturation of the adsorption equilibrium of the reactant
molecules. To examine this, in the first 90 min, the reaction gas
was passed in the dark and after 90 min, irradiation was started.
The result is shown in Fig. 2. In the dark, N2 was evolved at 20%
conversion as mentioned above, the conversion jumped re-
markably to the level of the steady rate as soon as the irradiation
was started. This clearly shows that the induction period is the
time for equilibrium adsorption of reactant molecules. In order
to find which reactant molecule is adsorbed first, we carried out
Fig. 2 Outlet concentration of N2 and N2O in the SCR of NO with ammonia
at 323 K.
2742
CHEM. COMMUN., 2002, 2742–2743
This journal is © The Royal Society of Chemistry 2002
Tanaka, Tsunehiro
