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Chemistry Letters Vol.37, No.7 (2008)
Highly Selective NH3 Formation in a NO–CO–H2O Reaction over Pt/TiO2
Tetsuya Nanba,ꢀ Shouichi Masukawa, Junko Uchisawa, and Akira Obuchi
National Institute of Advanced Industrial Science and Technology (AIST),
Research Center for New Fuels and Vehicle Technology, 16-1 Onogawa, Tsukuba 305-8569
(Received April 9, 2008; CL-080367; E-mail: tty-namba@aist.go.jp)
Pt/TiO2 showed high selectivity toward NH3 formation
in the conversion of NO and CO in the presence of H2O, higher
than that observed for the reaction of NO with H2.
100
80
60
40
20
0
3000
2500
2000
1500
1000
500
A promising catalytic system for low-temperature nitrogen
oxides abatement, de-NOx, is selective catalytic reduction
(SCR) by H2, referred to herein as H2-SCR. Platinum catalysts
are known to exhibit activity at low temperatures in the presence
of excess oxygen.1 H2-SCR could be used in de-NOx of diesel
exhaust, but the practical application of H2-SCR is hindered be-
cause the H2 content in diesel exhaust is very low, and providing
an on-board hydrogen supply is not feasible in such cases. Re-
cently, Nakatsuji et al. proposed a de-NOx system that combines
several catalytic reactions and the switching of engine operation
between lean-burn and rich-burn conditions while simultaneous-
ly generating an on-board H2 supply.2 In this system, NOx is first
stored on platinum catalyst supports under lean-burn engine op-
eration conditions. The stored NOx then reacts with H2 generated
from the reaction of CO and H2O (i.e., the water–gas shift
(WGS) reaction: CO + H2O ! CO2 + H2) under rich-burn
operation conditions. In this study, we investigated the role of
the WGS reaction in the conversion of NO and CO in the pres-
ence of H2O, a reaction referred to herein as the NO–CO–H2O
reaction. We demonstrate that Pt/TiO2 exhibited high activity
and extremely high NH3 selectivity when used as a catalyst sup-
port in this reaction.
The catalysts used were 1 wt % Pt supported on transition-
metal oxides, and catalysts were prepared by means of the incip-
ient wetness method. H2PtCl6 (Kishida Chemical) was used as
the precursor of Pt. The oxides used as supports were TiO2 (P-
25; Nippon-Aerosil), ZrO2 (RSC-H; Daiichi-Kigenso), CeO2
(Nacalai Tesque), SiO2 (Wakogel C-100; Wako Pure Chemical
Industries), and Al2O3 (KHS-46; Sumitomo Chemical). After Pt
loading, the samples were calcined at 773 K for 4 h. Catalytic ac-
tivity was measured by a conventional plug flow reactor. The
weight of the catalysts was 0.1 g. To study the NO–CO–H2O re-
action, He feed gas containing 1100 ppm NO, 900–3100 ppm
0
300 350 400 450 500 550 600
Temperature/K
Figure 1. Temperature dependence of NO–CO–H2O reaction
over Pt/TiO2. Catalyst weight was 0.1 g. Feed gas composition
was 1100 ppm NO, 3100 ppm CO, and 1% H2O. Total flow rate
was 100 mLꢁminꢂ1. Symbols indicate NO ( ) and CO ( ) con-
versions, N2 ( ), N2O ( ), and NH3 ( ) selectivity, and H2 con-
centration (- - - -).
at the temperature at which NO conversion was more than
10%. The details of this analysis were described previously.3
Figure 1 shows the extent of NO and CO conversion over
Pt/TiO2 as a function of temperature. The selectivity toward
N2, N2O, and NH3 formation as a function of temperature and
the concentration of generated H2 are also plotted in Figure 1.
As shown in Figure 1, light-off temperature of NO and CO
conversion was at 400 K, and NO conversion reached 100%
at 483 K. It is noted that CO2 formation was equivalent to
the amount of CO converted. NH3 was formed with very high
selectivity, whereas the selectivities of N2 and N2O were lower
than 10% at all measured temperatures. Furthermore, H2 forma-
tion was observed at temperatures above 493 K, suggesting that
the WGS reaction occurred simultaneously with the conversion
of NO.
We confirmed that the conversion of NO and CO in the
absence of H2O (i.e., the NO–CO reaction) over Pt/TiO2 was
negligible at all temperatures investigated. CO conversions in
the WGS reaction in the absence of NO were 52, 86, and 98%
at 473, 523, and 573 K, respectively, and we confirmed that
the amount of H2 formed was equivalent to the amount of CO
CO, and 0 or 1% H2O was used at a flow rate of 100 mLꢁminꢂ1
.
Catalysts were treated in 10% H2 at 573 K for 1 h before they
were subjected to reactions. Product selectivity was calculated
Table 1. Activities of Pt/TiO2 for NO–CO–H2O and NH3 selectivity in NO–H2 reactiona
NO conversion/%
CO conc.
/ppm
CO conversion/%
423 K
NH3 selectivity/%b
423 K
373 K
423 K
473 K
373 K
473 K
373 K
473 K
3100
1900
900
3
7
17
17
75
75
90
100
100
2
8
33
16
83
100
76
100
100
— (71)
— (55)
82 (21)
94 (94)
90 (67)
45 (24)
99 (99)
77 (64)
17 (2)
aFeed: 1100 ppm NO, 900–3100 ppm CO, 1% H2O, Catalyst weight: 0.1 g, Flow rate: 100 mLꢁminꢂ1. bParenthetical values are NH3
selectivities for the NO–H2 reaction in which H2 and CO concentrations were equal.
Copyright Ó 2008 The Chemical Society of Japan