(216b) Tap® and Bench-Scale Reactor Studies of Nox Storage and Reduction on Model Pt/Bao/Al2o3 and Pt/Al2o3

           NOx
storage and
reduction (NSR) is emerging as a potential NOx emission abatement
technology
for lean burn and diesel vehicles. There are two processes involved in
Lean NOx
trapping (LNR); in step one, NOx is adsorbed on the alkali earth oxide
site of
a bifunctional supported catalyst. In the second step, the nitrate is
released
by reduction of surface nitrates with intermittent hydrocarbon
injection for a
short time compared to trapping process. The reactions occurring on the
noble
metal catalyst are coupled with the storage process on the alkali earth
metal
catalyst. To improve trapping efficiency, the proximity of the noble
metal to
the alkaline earth metal particles plays a strong role. It is necessary
to
understand the roles played individually by the noble metal and the
storage function
to optimize the performance of catalyst. We employ Temporal Analysis of
Products (TAP®) to probe the transient kinetics of NSR. The TAP
method is well
suited for the NSR system because of the ability to conduct the NSR
process
under well characterized catalyst state and transport conditions. The
TAP
reactor has higher resolution compared to conventional flow reactors
since the
number of molecules introduced are small compared to the number of
active
catalyst sites.  Supporting experiments
are carried out in a bench-scale monolith reactor system.

            
The
TAP studies involve feeding pulses of reactants to model Pt/BaO/Al₂O₃
catalysts at ca. 10^-7 Torr total pressure. The product gas
emerging
from the catalyst bed is analyzed by a Quadrupole Mass Spectrometer
(QMS) in a
separate chamber maintained at ~10^-8 Torr of pressure.
Before each
experiment the catalyst is pre-reduced at 400 °C in H₂.
 NO or NO₂ were pulsed for a
prescribed period for several fixed catalyst temperatures in the range
of 300
to 450 ^oC.  The QMS monitored
effluent NO, N₂, O₂, and NO₂.  Feed pulse intensities were kept below ~5*10¹⁵
molecules to ensure Knudsen transport. 
“Pulse-probe” experiments are carried out in which a sequence of
a NO or
NO₂ “pulse” and H₂ “probe” was applied to
simulate the
complete NSR cycle.  After each run, the
temperature was increased to 400 ^oC in H₂ to
remove
surface O₂ or NOx, and to quantify the uptake of stored NOx
species. 

             Experiments
involving pulsing of NO over Pt/BaO/Al₂O₃
catalyst reveal
the decomposition of NO to N and O ad-species on Pt surface. The N₂
evolution steadily increases; goes through maxima and then decreases.
NO is
released after N₂ on NSR catalyst while the order is
reversed in
case of Pt/Al₂O₃ indicating that NO interacts
with the
BaO function of the NSR catalyst. These findings reveal the following
mechanism:

1:  NO + Pt ↔ NO-Pt;      

2:  NO-Pt
+ Pt ↔  N-Pt +
O-Pt;       

3:  2 N-Pt  →  N₂ + 2 Pt;

4:  O-Pt + BaO  ↔  BaO₂ + Pt;          

5:  NO + BaO₂  ↔  BaO₂-NO;

           
Pulsing
of NO₂ shows similar behavior with shorter induction period
that
coincided with the production of N₂. The ratio of NO
produced to NO₂
fed increases monotonically from 0 to 0.33 indicating the
disproportionation
reaction of NO₂. Based on these findings, the reaction
sequence is expanded
to include following steps:

6:  NO₂
+ Pt  ↔ NO₂-Pt                        

7:  NO₂-Pt
+ Pt  ↔  NO-Pt + O-Pt       

8:  NO₂
+ BaO  ↔  BaO₂-NO
      

9:  NO₂
+ BaO-NO₂   ↔  BaO-(NO₂)₂

10:NO₂
+ BaO-(NO₂)₂  ↔  BaO-(NO₃)₂
+ NO

   
        Reactions 8 – 10 summed up give
the disproportionation reaction giving the observed ratio of 0.33. As
the
number of NO₂ molecules introduced is increased, the N₂
maxima
shift towards the ordinate. The NO breakthrough curve also shifts
accordingly.

           In order to
elucidate
the role of Pt in the storage process, TAP and bench-scale experiments
are
carried out on a series of Pt/BaO/Al₂O₃ catalysts
with
different Pt and Ba loadings.  For
example, a comparison of NO and NO₂ pulsing experiments on
Pt/Al₂O₃
and Pt/BaO/Al₂O₃ provides important information
on the
kinetics of NOx uptake.  The promotional
effect of Pt on NOx uptake will be examined by comparing NOx uptake
under
different conditions of pre-exposure to oxygen.   

            Finally,
the
experiments will be complemented with reaction system modeling.  The TAP reactor is effectively simulated
because of its operation in Knudsen regime and the accompanying
microkinetic
modeling of the experiments with parameter estimation will be
presented.