3. l. Catalyst selection
First, to select the precious metals, the perfor-
mance of NOx removal with H 2 in the presence of
an excess of oxygen was measured for the Pt, Rh, and Pd catalysts. When y-alumina was adopted as a
support, the Pt catalyst had the highest activity of
NOx removal in these catalysts. Next, in order to
improve the catalytic activity, Pt catalysts which
were prepared with several supports were tested.
The activity order of the catalyst supports of
catalyst was as follows:
ZSM-5 ~ mordenite > SiO~ > y-alumina According to our previous experiments, this order
of supports is similar to that of the catalyst for NOx
removal with HCs such as C3H 6. Judging from this
experimental result, a Pt/mordenite catalyst was se-
lected for the following experiments.
3.2. NO x remocal with H: in the presence of an
excess of oxygen
In order to investigate the catalytic reaction be-
havior, NOx conversion was observed at various
concentrations of NO and H4. Fig. 1 shows the NOx
conversions of the Pt/mordenite catalyst at a NO
concentration of 0.2% under various H 2 concentra-
tions as a function of temperature. NOx removal
activities of the catalysts were observed at tempera-
tures from 90 to 250°C. From Fig. 1, the relationship
of H~ concentration to NOx conversion at 120°C is
derived as shown in Fig. 2. NOx conversion in-
creased monotonously with increasing H~ concentra-
tion until the H2/NOx ratio reached around 2. But at
an NO concentration of 0.02%, as also shown in Fig.
2, NOx conversion increased with increasing H~
concentration until the H2/NO ratio became 6.
The relationship between temperature and NOx
conversion is almost exactly expressed by this equa-
tion. And this equation is similar to that for the NOx
removal with HCs such as C3H6, according to our
previous experiments. Therefore, it is suggested that
there may exist a same rate determining step on the
Pt catalyst between the catalytic NOx removals with
H 2 and with C3H 6 in the presence of an excess of
oxygen.