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Chemistry Letters Vol.37, No.12 (2008)
Low-temperature Hydrogen-selective Catalytic Reduction of NOx
on Pt/Sulfated-ZrO2 Catalysts under Excess Oxygen Conditions
Makoto Saito,1 Masahiro Itoh,1 Jun Iwamoto,2 and Ken-ichi Machidaꢀ1
1Center for Advanced Science and Innovation, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871
2Honda R&D Co., Ltd., Automobile R&D Center, 4630 Shimotakanezawa, Haga-machi, Haga-gun, Tochigi 321-3393
(Received September 1, 2008; CL-080826; E-mail: machida@casi.osaka-u.ac.jp)
Platinum catalysts supported on sulfated zirconia powders
Catalytic test reactions were performed on 0.1 g of plati-
num-loaded sulfated zirconia catalysts (denoted as Pt/s-
ZrO2(X), X represents the calcination temperature) individually
charged in a conventional fixed-bed quartz tube reactor (i.d.: ca.
7 mm) at 70–250 ꢁC. A stream of reaction gas (0.1 vol % NO,
0.4 vol % H2, 10 vol % O2, and balance of N2) was fed with a
flow rate of 30 mLꢄminꢃ1 and the outlet gas was introduced into
a gas cell equipped with KBr windows (optical length; 10 cm)
to analyze the gas composition by an FTIR apparatus (Perkin-
Elmer SPECTRUM 2000 with an MCT detector). Concentra-
tions of NO, N2O, and NO2 were determined using peak areas
of their absorbance peaking at 1911, 2212, and 1586 cmꢃ1, re-
spectively. The N2 yield based on NOx reduction was deter-
mined by substracting the concentration value of NO, N2O,
and NO2 in outlet gas from the inlet NO concentration. The
NOx (NO and NO2) conversion and N2 selectivity were deter-
mined as follows: NOx conversion, ([NO]inlet ꢃ [NOx]oultlet)=
highly promote the hydrogen-selective catalytic reduction (H2-
SCR) of NOx at 100 ꢁC with formation of ammonia intermediate
species derived from protons of the sulfonate groups fixed as
superacid sites on the surface, which are detected by in situ
DRIFTs measurements.
Selective catalytic reduction of NOx using hydrogen has at-
tracted much attention1–6 as a potent candidate for removing
NOx from exhaust gas containing excess amounts of oxygen
around 10 vol %, where the hydrogen is possibly supplied by
on-site producing systems from the gas components, viz. wa-
ter-gas-shift reaction, electrolysis of water, and so on. Although
the H2-SCR occurs at the lower temperature than HC-SCR7 or
CO-SCR,8 much N2O greenhouse gas usually accompanies as
a by-product.1 Some studies of such H2-SCR characteristics on
supported precious metal catalysts have noted that ammonia or
ammonium species are generated from NOx on Brønsted acid
sites of the catalysts via an intermediate of NH4NO3 with little
N2O formation.3,4 The formation of ammonia or ammonium spe-
cies and subsequent NOx reduction with them desirably takes
place on acidic support materials such as zeolites.5 Surface mod-
ifications of the support oxides with acidic functional groups
have been made to create strong solid acid sites, and among
them, especially sulfated zirconia is noteworthy as catalysts or
support material with its strong acidity (H0 ꢂ ꢃ16:04).9 In addi-
tion, it has good tolerance to the SO2 impurity contained in real
exhaust gases.7
In the present study, the H2-SCR for NOx on platinum cata-
lyst supported on sulfated zirconia powders was characterized
from the viewpoint of surface acidity. Furthermore, the reaction
intermediates were detected by in situ diffuse reflectance infra-
red Fourier transform spectroscopy (DRIFTS) measurements
to understand the reaction mechanism and evaluate the feasibil-
ity of this deNOx system at a practical industrial level.
Sulfated zirconia powders were prepared from tetragonal
ZrO2 (ZRO-3; supplied by the Committee of Reference Cata-
lysts, Catalysis Society of Japan) and reagent grade ammonium
sulfate by calcination at 700 or 800 ꢁC for 2 h in air. Then, 1 wt %
of platinum was loaded to them by an incipient wetness im-
pregnation method using a Pt(NO3)2(NH3)2 solution, followed
by drying at 120 ꢁC overnight and heating at 600 ꢁC for 2 h in
air. The conventional Pt/ZrO2 catalyst was prepared using the
same ZRO-3 powder and used as a reference. They were char-
acterized by a series of measurements for FTIR spectra (KBr
method; Jusco FT-IR/430), NH3-TPD (temperature-program-
med desorption) profiles, and BET surface area (about 80–
90 m2/g) values.
ð[NO]inletÞ ꢅ 100 (%); N2 selectivity, [N2]yield=ð[N2]yield
þ
[N2O]yieldÞ ꢅ 100 (%). In situ DRIFTs measurements were also
carried out using a ZnSe window-equipped diffusion reflection
cell (resolution; 4 cmꢃ1, 50 scans).
The NOx conversion and N2 selectivity of Pt/s-ZrO2(700 or
800) and Pt/ZrO2 are summarized in Table 1. The Pt/s-
ZrO2(700) exhibited the highest NOx conversion (80%) with
66% N2 selectivity at 100 ꢁC. The above-mentioned catalytic ac-
tivity maintained almost constant level at least for 8 h, and no
gaseous ammonia was detected in the present study. The higher
conversion and selectivity than values previously reported on Pt/
ZrO2 were attained at lower temperature (NOx conversion, 72%;
N2 selectivity, 57% at 110 ꢁC).2 Although the NOx conversion
fell at above 140 ꢁC, the N2 selectivity was enhanced up to
almost 100% with increasing reaction temperature. In the case
of Pt/s-ZrO2, the resultant NOx consisted of mainly NO at lower
temperature (<140 ꢁC), while NO:NO2 ratio in NOx residue
hardly changed on Pt/ZrO2 catalyst over the whole temperature
range. It implies that the NOx reduction proceeds via the oxida-
tive adsorption of NO molecules. The Pt/s-ZrO2(800) showed
Table 1. NOx conversion and N2 selectivity profiles of Pt/ZrO2
and Pt/s-ZrO2(700 or 800) catalysts
Reaction
Pt/ZrO2
NOx
conversion selectivity conversion selectivity conversion selectivity
Pt/s-ZrO2(700)
Pt/s-ZrO2(800)
temperature
/ꢁC
N2
NOx N2
NOx N2
/%
/%
/%
/%
/%
/%
70
100
140
175
210
250
0
17
71
41
17
8
—
49
68
46
42
69
27
80
68
44
23
10
55
66
92
93
97
100
54
86
64
31
17
15
22
61
83
86
86
78
Copyright Ó 2008 The Chemical Society of Japan