Unique catalytic properties of Pt and tungstophosphoric acid supported on MCM-41 for the reduction of NO(x) in the presence of water vapour

Article abstract of DOI:10.1039/a807758d

Pt and tungstophosphoric acid supported on MCM-41 type materials show a pronounced increase in the activity during the catalytic reduction of NO(x) with propene in the presence water vapour.

Full text of DOI:10.1039/a807758d

Unique catalytic properties of Pt and tungstophosphoric acid supported on
MCM-41 for the reduction of NOx in the presence of water vapour
Andreas Jentys,* Walter Schießer and Hannelore Vinek
Vienna University of Technology, Institute for Physical Chemistry, Getreidemarkt 9/156, A-1060 Wien, Austria.
E-mail: jentys@tuwien.ac.at; http://www.physchem.tuwien.ac.at/catalysis/
Received (in Cambridge, UK) 6th October 1998, Accepted 21st December 1998
Pt and tungstophosphoric acid supported on MCM-41 type
materials show a pronounced increase in the activity during
the catalytic reduction of NOx with propene in the presence
water vapour.
support improved the activity and the selectivity towards N2
formation compared to Pt supported on dense SiO2.6 After the
co-impregnation of MCM-41 with Pt and tungstophosphoric
acid the activity decreased to a level comparable to Pt/SiO2 and
Pt/Al2O3, while MCM-41 based catalysts containing tung-
stophosphoric acid only showed a significantly lower activity.
The main product of these reactions, however, was N2O, which
is characteristic for Pt group metals supported on oxides.13
Transition metals supported on zeolites, such as Fe/ZSM5,
show generally a higher N2 yield, but are only active above 650
K.14
The conversions of NOx over Pt/MCM-41 and Pt + HPW/
MCM-41 at 573 K as a function of the water vapour
concentration are compared in Fig. 1. At this temperature, the
activity and selectivity of the catalysts in the absence of water
vapour were almost identical. With increasing H2O vapour
concentration the activity of Pt + HPW/MCM-41 increased,
until a maximum was reached at 2.0 vol% H2O, where the NOx
conversion was 25% (relative %) higher compared to the water-
free reaction conditions. A further increase led to a decrease in
the activity; however, even at 8.5 vol% water vapour the activity
was 10% higher compared to the water-free reaction conditions.
In contrast, for Pt/MCM-41 a continuous decrease in the activity
with increasing H2O concentration was observed.
To combine the advantages of zeolites1–4 and oxide based
systems,5,6 we studied the potential of transition metal contain-
ing mesoporous molecular sieves with the MCM-41 type
structure for the catalytic reduction of NOx.7,8 As the acid sites
of these materials are only weakly acidic,9 we also impregnated
the catalysts with tungstophosphoric acid (H3PW12O40·6H2O)
in order to generate strongly acidic sites.10
Siliceous mesoporous molecular sieves with the MCM-418
type structure, synthesised with C16TMABr,11 were impreg-
nated with aqueous solutions of PtCl4 (Pt/MCM-41),12 of
H3PW12O40 (HPW/MCM-41) and of PtCl4 and H3PW12O40 (Pt
+ HPW/MCM-41). For comparison Pt/SiO2 (Degussa Aerosil
200) and Pt/g-Al2O3 (170 m2 g21), also prepared by impregna-
tion, were used. The metal and tungstophosphoric acid loading
of all catalysts investigated are summarised in Table 1.
The characterisation of the support (MCM-41) was carried
out by XRD, N2 BET and IR spectroscopy.9 The siliceous
MCM-41 type material showed four Bragg reflections (d-
spacing: 39 Å) and a sharp step in the N2 isotherm at p/p0 ≈ 0.4.
The dispersion of the metal, determined by volumetric H2
chemisorption after reduction for 2 h at 773 K in H2 (heating
rate 15 K min21), was 64% for the Pt/MCM-41 catalyst. The
BET surface areas of Pt/MCM-41 and Pt + HPW/MCM-41
were 1000 and 470 m2 g21, respectively.10
Catalytic properties were studied in a reaction system using a
chemiluminescence NOx analyser and a gas chromatograph
(TCD and FID) to determine the concentrations of the reactants
and products. The concentrations of the reactants were 1010
ppm NO, 1010 ppm C3H6, 4.9 vol% O2, 0–8 vol% H2O (balance
He). The reactions reported were carried out at 573 K and at a
space velocity of 11 000 h21 (w/F = 6 3 1022 g s cm23), the
activity reported was measured after 2 h on stream. Before the
reaction all catalysts were activated in He at 773 K for 1 h. TPD
indicated that at this temperature H2O was completely removed
from the Pt + HPW/MCM-41 catalyst.
The activity changed almost instantaneously with the addi-
tion of 2.5 vol% water vapour into the gas stream (see Fig. 2).
After switching back to water-free reaction conditions the
activity immediately returned to the initial value on Pt/MCM-
41. On Pt + HPW/MCM-41 it decreased to a constant level,
which was 12% above the initial activity, and only after an
additional heating for 1 h at 773 K in He could the initial activity
be restored.
In contrast to the published results,3,14,15 this is the first time
that a pronounced increase in catalytic activity during the
reduction of NOx with hydrocarbons in the presence of H2O
vapour has been observed. The activity of the Pt + HPW/MCM-
41 catalyst is significantly improved in the presence of up to
8.5 vol% water vapour compared to water free reaction
conditions, while the activity of Pt/MCM-41 was slightly
suppressed in the presence of water vapour.
A comparison of the activity and selectivity (determined at
the temperature of maximum NOx conversion) is given in Table
1. The use of siliceous mesoporous molecular sieves as the Pt
To elucidate the reason behind these unique catalytic
properties, the formation of acidic surface sites resulting from
the deposition of tungstophosphoric acid on MCM-41 was
Table 1 Composition and catalytic properties of the catalysts
Tungstophosphoric Max. NOx
acid (wt%) conversion (%) conversion/K
Temp. for max. NOx
Selectivity to
N2a (%)
Sample
Pt (wt%)
1.61
Pt/MCM-41
0
62
47
51
22
19
51
46
493
543
553
593
593
513
543
35
36
37
32
35
29
34
Pt + HPW/MCM-41 (0 vol% H2O) 1.61
Pt + HPW/MCM-41 (2 vol% H2O) 1.61
30
30
30
30
0
HPW/MCM-41 (0 vol% H2O)
HPW/MCM-41 (2 vol% H2O)
Pt/SiO2
0
0
1.61
1.62
Pt/Al2O3
0
a vs. formation of N2O (reported at the temperature at the highest NOx conversion).
Chem. Commun., 1999, 335–336
335

H2O around 700 K. This reaction was only observed on
hydrated tungstophosphoric acid and, therefore, could be the
additional reaction pathway accounting for the improved
activity of the Pt + HPW/MCM-41 catalyst in the presence of
water vapour.
The reactions reported here however, were carried out at a
much lower temperature (573 K), where the NOx conversion
over the Pt-free HPW/MCM-41 catalyst was only 8%. Also, the
activity of the HPW/MCM-41 catalyst was not influenced by
the presence of water vapour. Therefore, we assume that the
direct reaction on tungstophosphoric acid is not the main route
contributing to the overall activity in these experiments and that
Pt and tungstophosphoric acid are essential to achieve the
additional activity observed in the presence of water vapour.
We propose two reaction pathways in order to explain the
additional activity observed on the Pt + HPW/MCM-41 catalyst
in the presence of water vapour: (i) NOH+, formed on the
hydrated tungstophosphoric acid, disproportionates and the
carbonaceous species formed on the Pt clusters react with the O
atoms to form CO2, while N recombines and desorbs as N2. This
reaction mechanism is similar to that described for C3H6/NO/O2
reactions, with an additionally proposed reaction pathway for
the formation of N2* and O2* on the surface of the hydrated
tungstophosphoric acid; (ii) in the other reaction pathway we
suppose that C3H6 adsorbs on the hydrated tungstophosphoric
acid. The CxHy species, formed upon adsorption on the
Brønsted acid sites, react with NO2* adsorbed on the metal.
The presence of highly acidic Brønsted sites on the hydrated
tungstophosphoric acid (see Fig. 3) generates additional
adsorption sites, which increases the local concentration of
hydrocarbons on the perimeter of the metal clusters and thus
give rise to the higher activity in the presence of water
vapour.
Fig. 1 Activity of (“) Pt/MCM-41 and (5) Pt + HPW/MCM-41 as a
function of water vapour concentration (573 K).
Fig. 2 Changes in activity of (“) Pt/MCM-41 and (5) Pt + HPW/MCM-41
during a stepwise change of the water vapour concentration between 0 and
2.5 vol% at 573 K.
investigated by following the adsorption of pyridine using IR
spectroscopy. The spectra shown in Fig. 3 were normalised to
the structural vibrations of MCM-41 between 2100 and 1770
cm21, the other experimental details are described in ref. 9.
After activation in vacuum ( ≈ 1026 mbar) at 773 K, pyridine
was adsorbed with a partial pressure of 1021 mbar at 423 K on
Pt + HPW/MCM-41 and on siliceous MCM-41. Subsequently,
the tungstophosphoric acid containing sample was hydrated by
the co-adsorption of 9 3 1021 mbar H2O. The bands at 1449
and 1600 cm21, present after adsorption of pyridine on MCM-
41 and on Pt + HPW/MCM-41, are assigned to hydrogen-
bonded pyridine formed on Lewis acid sites. Brønsted acid
sites, indicated by the bands at 1540 and 1614 cm21, were only
present on Pt + HPW/MCM-41. After the co-adsorption of H2O
the concentration of Brønsted acid sites on Pt + HPW/MCM-41
increased by about 75%, while the concentration of Lewis acid
sites was not affected by the presence of water vapour.
In both mechanisms proposed an additional pathway is
described, which is not present on Pt/MCM-41 or on dehydrated
Pt + HPW/MCM-41 catalysts. However, at the moment we can
not unequivocally decide which of the two routes described is
responsible for the catalytic effects observed.
The work was supported by the Fonds zur Förderung der
Wissenschaftlichen Forschung under project P10874 CHE and
¨
by the Osterreichische Nationalbank under project 7119.
Notes and references
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Chem. Commun., 1999, 335–336