(544fp) PtOx and PdOx Formation during NO Oxidation on Diesel Oxidation Catalysts

1. Introduction

The catalytic oxidation of NO to NO₂
in a diesel oxidation catalyst (DOC) plays an important role in automotive
exhaust gas aftertreatment systems. The presence of NO₂ improves the
NO_x storage in lean NO_x traps (LNTs), NO₂/NO_x
ratio close to ½ is desired for maximum NO_x conversion
efficiency of selective catalytic reduction by NH₃ (SCR), and NO₂
also enables the low-temperature soot oxidation in diesel particulate filters
(DPFs) [1, 2]. Hence, it is desirable to control the NO oxidation activity of
DOC and maintain it at a proper level.

In several experimental studies [1,2,3,4], deactivation effects
during NO oxidation have been reported. Olsson et al. [1] observed a decrease
in NO conversion from 76% to 25% on a Pt/Al₂O₃ with
110gPt/ft³ at 300 ^oC within 3
hours. Also, inverse hysteresis of NO oxidation was reported during
temperature-ramps experiments with heating and subsequent cooling [2,3,4]. This
behavior can be attributed to the formation of platinum oxides (PtO_x), which are less active for the NO
oxidation compared to Pt [1]. The PtO_x
formation is induced not only by O₂, but also by NO₂, but
no systematic experiments comparing the individual contribution of these two oxidative species have been available. PtO_x decomposes above 400 ^oC
and it can be reduced at low temperatures by NH₃ or NO. Recently, Arvajova et al. reported that it is possible to reactivate
the catalyst and increase the NO₂ yield by CO and C₃H₆
pulses, while keeping overall lean composition of exhaust gas that is natural
for Diesel engines [4].

The aim of this work is to compare
the impact of O₂ and NO₂ on the PtO_x
and PdO_x formation during the NO oxidation
on Pt/Al₂O₃ and Pd/Al₂O₃
diesel oxidation catalysts and to develop the global kinetic model for the
above-mentioned processes on both types of catalysts.

2. Methods

Model diesel oxidation catalysts,
Pt/Al₂O₃ and Pd/Al₂O₃
coated on 400 cpsi cordierite monolith, were used for
the experimental study. Before each experiment, the catalyst was pretreated in
a reductive atmosphere of 2% H₂ in N₂ at 400^oC
for 15 min to reduce all Pt or Pd sites to the
metallic state. The experiments were carried out in a bench flow reactor with
defined mixtures of synthetic gases corresponding to Diesel exhaust and gas
hourly space velocity of 60 000 h^-1. Repeated heat-up and cool-down
temperature ramps with NO oxidation were performed in the range of 100–400 ^oC. Also, isothermal deactivation and
reactivation experiments with NO oxidation as probe reaction were performed at
150, 175 and 200 ^oC.

3. Results and
discussion

Fig.
1a shows the comparison between the experimental and simulated outlet NO₂
concentration during NO oxidation in subsequent heating and cooling temperature
ramps in the case of Pt/Al₂O₃ catalyst. At the beginning,
the catalytic surface is fully reduced so that the initial NO oxidation activity
during the first heat-up ramp is high. Much lower NO₂ concentration
is observed during the following cool-down ramp due to the formation of PtO_x species in the catalytic surface. The
catalyst activity during the second heat-up is still lower than during the
first one, indicating incomplete PtO_x
reduction by NO at low temperature. Similar experiments were then performed
with a reactivation period, consisting of short lean CO pulses applied at
constant temperature (180 °C) during the first cool-down period
(Fig. 1b). After the CO pulsation, PtO_x is
almost completely reduced and much higher NO₂ yield is observed in
both experiment and simulation.

Figure
1. Experimental and
simulated NO₂ concentration during NO oxidation subsequent
temperature ramps (feed: 250 ppm NO, 8% O₂, 8%CO₂, 8%H₂O)
(a) without and (b) with CO pulses (1000 ppm CO).

Fig. 2 shows isothermal
PtO_x formation experiments at 200^oC.
The Pt/Al₂O₃ oxidation can be induced either by O₂
(Fig. 1a) or NO₂ (Fig. 2b). Initially, the catalyst is fully reduced
so that during the first probe period of NO oxidation the highest NO₂
production is observed. The first probe period is followed by a deactivation
period during which Pt oxidation is induced by O₂ (Fig. 2a) or NO₂
(Fig. 2b). The second probe period of NO oxidation after the deactivation then
provides a lower NO₂ yield. The probe and deactivation periods are
alternated, revealing gradual decrease of NO₂ yield due to PtO_x formation, until steady state is achieved
after 25 000 s (ca. 7 h) of catalyst
operation. The PtO_x is partially reduced
during the lean CO pulsation at the end of the experiment with the NO₂
yield being near to that of the second probe period. Similar experiments are
performed with the Pd/Al₂O₃
catalyst and kinetic parameters are compared for both catalyst types. 

Figure 2. Isothermal experiment at 200^oC
with alternated probe periods of NO oxidation and deactivation periods with (a)
8% O₂ and (b) 250 ppm NO₂, followed by CO pulsation. Probe
feed: 250 ppm NO, 8% O₂, 7%CO₂, 7%H₂O + 1000
ppm CO pulses.

4.
Conclusions

NO₂ exhibits
a stronger effect on the Pt oxidation than O₂. However, due to much
higher oxygen concentration present in typical exhaust gas mixture, similar
overall rate and extent of deactivation is observed for both oxidizing agents.
The catalyst activity can be partly restored by CO or C₃H₆
pulses while keeping overall lean conditions. The developed kinetic model can
be used for the optimization of NO₂ yield in DOC during driving
cycles [5].

References

[1]     
L. Olsson, E. Fridell,
J. Catal. 210 (2002), 340–353.

[2]     
W. Hauptmann, M. Votsmeier,
J. Gieshoff, A. Drochner,
H. Vogel, Appl. Catal. B: Environ. 93 (2009), 22–29.

[3]     
K. Hauff, U. Tuttlies, G. Eigenberger, U. Nieken, Appl. Catal. B: Environ.
123-124 (2012), 107–116.

[4]     
A. Arvajova, P. Koci, V. Schmeißer, M. Weibel, Appl. Catal. B: Environ.
181 (2016), 644–650.

[5]   
 A. Arvajova, P. Koci, Chem. Eng. Sci.
158 (2017), 181–187.

Keywords: NO oxidation; reaction kinetics; catalyst
reactivation; mathematical modeling.