(308b) Effect of Pt Cluster Size on the Reaction of No and O2 to No2 on Supported and Model Catalysts

            The
oxidation of NO to NO₂ (NO+O₂ = NO₂) over a
supported noble metal component is an important step involved in the NOx storage/reduction
(NSR) process being developed for diesel engines.  In fact, NO to NO₂
oxidation is the first step that occurs in the cyclic NSR process.  It is also
a key step for soot removal.

            The reaction kinetics
of NO oxidation for two Pt/Al₂O₃ catalysts having
different Pt particle sizes were investigated.  The rate expression for the
forward reaction on a fresh catalyst having a Pt cluster size of 2.4 nm was r =
k[NO]^1.09±0.07[O₂]^0.86±0.06[NO₂]^-0.85±0.06,
where concentrations are in gas phase volume fraction, k = Exp(A-E_a/RT)
is the rate constant ( in s^-1) with an apparent activation energy (E_a)
of 81.8 kJ mol^-1±5 kJ mol^-1 and A is 15.1±1.03.  The
sintered catalyst, with an average particle size of 7 nm, had similar E_a
and apparent reaction orders with respect to NO and NO₂, but the
apparent O₂ order decreased to 0.7, compared to the fresh catalyst.  Thus,
the product NO₂ was found to inhibit the forward rate of NO
oxidation reaction on both catalysts.  

            The turnover rate (TOR),
defined as moles of NO reacted per second per mole of surface Pt, on the
sintered catalyst was found to be ca. 4 times higher than the TOR on the
fresh catalyst.  The oxygen uptake by Pt on these catalysts was quantified
using the CO titration method to determine the surface reactivity.  After NO oxidation
reaction reached steady state, both fresh and sintered catalysts showed oxygen
uptake of about 1.5 times the number of Pt surface atoms (Pt_S).  But
when these catalysts were exposed to high concentration of NO₂ at
room temperature, the oxygen uptake increased to the equivalent of about two
oxygen atoms per surface Pt and the NO oxidation TOR decreased by about 7 times
with respect to the original steady-state level for both catalysts.  Based on
the XPS measurements, the observed catalyst deactivation was attributed to an
increase in the bulk oxidation of the Pt particles towards PtO.  The O/Pt_S
values greater than one obtained in the CO titration suggest an increasing
penetration of oxygen into the interior of the Pt particles and an approach to
PtO stoichiometry.  To explain the higher rates on catalysts with larger
particle sizes, TEM measurements on the supported catalysts and rate
measurements on a Pt foil were made.  The TEM images of the two supported catalysts
showed that the sintering procedure used to increase the Pt particle size on
the fresh catalyst only broadened the particle size distribution, with less
than 20% of the particles on the sintered catalyst having sizes larger than the
average particle size seen on the fresh catalyst.  Preliminary results indicate
that the NO oxidation TOR on a Pt foil is about two orders of magnitude greater
than the rate seen on the sintered supported Pt catalyst.  The TEM and foil
results suggest that only particles larger than about 10 nm are responsible for
the observed NO oxidation activity.  As a consequence, most Pt clusters on a
supported catalyst with high dispersion are not active for NO oxidation.