Copper exchanged beta zeolites for the catalytic oxidation of ammonia
a
b
Teresa Curtin* and Sandra Lenihan
a
Materials and Surface Science Institute, University of Limerick, Limerick, Ireland.
E-mail: teresa.curtin@ul.ie; Fax: +353-61-213529; Tel: +353-61-202981
Department of Chemical and Environmental Sciences, University of Limerick, Limerick, Ireland
b
Received (in Cambridge, UK) 19th February 2003, Accepted 7th April 2003
First published as an Advance Article on the web 2nd May 2003
Copper exchanged on beta zeolites are extremely active and
selective for the catalytic oxidation of ammonia to nitrogen
and water and this activity correlates to the ease of reduction
of the copper species.
exchange. Temperature Programmed Reduction (TPR) using
5% H in N (heating rate 10 °C min ) was performed on all
2 2
2
1
catalysts and on some catalysts following exposure to water.
Ammonia oxidation testing was carried out in a continuous
flow system operated at ambient pressure. All catalysts (25 mg)
were pre-treated at 420 °C for 90 minutes in a flow of 20 ml
Ammonia is a well-known gaseous pollutant that has several
adverse effects on the atmosphere. For example, ammonia
emissions contribute to the acidification of the environment and
its precursors are responsible for the greenhouse effect and city
smog. Many techniques are available for the abatement of
ammonia such as adsorption, biological purification and
catalytic decomposition. Each has its disadvantages, for
example, in catalytic decomposition, ammonia decomposes in
the absence of oxygen into nitrogen and hydrogen using a
2
1
min helium before testing. The reactor was a quartz glass
vertical tube and the catalyst was kept in position by two plugs
of quartz wool. The temperature of the catalyst was monitored
by a thermocouple, which was outside the reactor close to the
catalyst bed. Typically, the gas composition used was 0.54%
3 2
NH , 8% O with a balance of helium. The gases leaving the
catalytic reactor were analysed using a Hewlett-Packard
HP5971A Mass Selective Detector. In some experiments, 1%
1
platinum–ceramic membrane catalyst. However, temperatures
2
H O was added to the reactant stream via a water saturator. All
of over 600 °C are required for this process. An alternative
method for ammonia destruction is the catalytic oxidation of
ammonia to nitrogen and water, which operates at lower
temperatures.
data were recorded between 5 and 12 hours on stream.
Fig. 1 presents ammonia conversion with temperature for the
prepared copper exchanged zeolites. For comparison a 4.3 wt.%
copper oxide on alumina catalyst is also presented as this was
7
The earliest reported work on the catalytic oxidation of
ammonia concerned a detailed study over unsupported metal
shown to be one of the most active catalysts tested to date.
Ammonia conversion increased with temperature for all
catalysts and ammonia conversion also increased with copper
content for the copper exchanged beta catalysts. Indeed, the
6.6Cu/beta catalyst proved to be extremely active, presenting
almost complete conversion even at temperatures as low as 250
°C. A promising feature of these catalysts was the very high
selectivity to nitrogen with little or no NO detected (see Table
2
oxides at temperatures between 160–370 °C. The products of
reaction observed were nitrogen, water and NO. Activity could
be improved by supporting the metal oxide and several
publications now exist showing that ammonia can be converted
to nitrogen and water in the presence of oxygen over metal
3
3 2
oxide supported catalysts. Examples include MoO /SiO ,
4
5
6,7
V
Fe
2
O
5
/TiO
/TiO
and NO are also formed in varying amounts.
2
and CuO/TiO
. For most of these publications by-products N
2
, NiO/Al
2
O
3
, CuO/Al
2
O
3
and
O
2
1). Low levels of N O were observed, however in comparison to
8
2
O
3
2
2
4.3CuO/Al , a marked improvement was observed in terms
2 3
O
of minimising formation of by-products. This high selectivity to
nitrogen and high activity makes the copper exchanged beta
catalyst one of the most active catalysts reported to date in the
literature for the oxidation of ammonia to nitrogen in the
presence of oxygen.
Introducing water vapour (1 vol%) into the reactant stream
resulted in a fall in activity for all catalysts. For example,
ammonia conversion fell to 5% from 25% with the introduction
of water for the 1.2Cu/beta catalyst. This represented a loss in
activity of 80%. However, this loss in activity was less severe
One of the first reports on the catalytic oxidation of ammonia
using exchanged zeolites was recorded by Golodets over a
series of cation exchanged Y zeolites.9 Copper exchanged
ZSM-5 was also reported as a potential catalyst for the oxidation
of ammonia and showed promising results.4 More recently,
both Cu and Fe exchanged on ZSM5 showed high activity for
,10
oxidation of ammonia with low levels of NO and N
2
O
produced.11
A feature of most catalysts presented to date is the dramatic
fall in activity when water vapour is introduced into the stream.
In general, this effect does not adversely affect the selectivity to
nitrogen.1
2,13
This work presents ammonia oxidation over a
series of copper exchanged beta zeolites. These catalysts have
not been reported before for the catalytic oxidation of ammonia.
The effect of water is also presented.
Copper exchanged beta zeolites (Si/Al = 25, Zeolyst
International) were prepared using a procedure outlined by
Iwamoto et al.14 The copper oxide impregnated on alumina
(
Rh oˆ ne-Poulenc, 134 m2
g ) catalyst was prepared by
21
conventional dry impregnation using copper nitrate. All pre-
pared catalysts were dried and then calcined at 450 °C for 5
hours in air. Following calcination, the catalysts were sieved to
a particle size 106–225 µm. The copper content of the prepared
catalysts was determined using atomic absorption (A.A.). The
catalyst nomenclature used throughout the paper includes the
copper content, the support and the method of preparation. For
example, 6.6Cu/beta(ex) indicates a catalyst containing 6.6
wt.% copper (measured by A.A.) on beta zeolite prepared by ion
Fig. 1 Ammonia conversion, with temperature, using the indicated copper
catalysts. Reaction conditions: 0.54% NH , 8% O , balance He; 25 mg
3
2
21
catalyst; W/F 0.015 g s ml
.
1
280
CHEM. COMMUN., 2003, 1280–1281
This journal is © The Royal Society of Chemistry 2003