(452c) Apparent Structure Sensitivity of the NO Oxidation Reaction on Pt Studied Using Model Catalysts

Introduction

                The NO oxidation reaction (NO +
½ O₂ ↔ NO₂) on platinum is the first step in the
NOx storage and reduction process (NSR), a technology currently being
commercialized for emissions reduction in diesel engines. In previous studies
in our group [1], the turnover rate (TOR) was shown to be a strong function of
platinum particle size with higher TOR occurring on catalysts with lower
dispersions indicating this reaction may be sensitive to the structure of the
Pt surface. Our studies suggest that the particle size effect can be attributed
to oxidation of small, <3nm, particles by NO₂ as oxides of Pt are
inactive for NO oxidation, however it is not clear if an underlying structure
sensitivity also exists. To further interrogate this apparent structure
sensitivity, the kinetics were measured on Pt(111), Pt(110), and Pt(100) which
represent the surfaces present on large, 3-10nm Pt particles, and also on the Pt(321)
surface which is similar to surfaces found on small, <3nm particles at the
same conditions used on supported catalysts. In-situ X-ray photoelectron spectroscopy (XPS) experiments
on Pt(111) were used to show that the most abundant
surface intermediate (MASI) [O*], is controlled by the ratio of NO to NO₂
and not O₂ which results in the kinetic inhibition of associative O₂
adsorption, the proposed rate determining step.

Materials and
Methods

                The kinetic study was performed in a custom
atmospheric pressure batch reactor system combined with an ultrahigh vacuum
(UHV) system which allows for in-vacuuo sample transfer. A FTIR spectrometer with a gas cell was used to measure NO and NO₂
concentrations in the batch reactor. The Pt single crystals were heated by
passing electrical current through them. A K-type thermocouple was spot welded
to the side of the crystal for temperature measurement. The UHV chamber
contains a sputter gun for sample cleaning, mass spectrometer, low energy
electron diffraction (LEED), and a cylindrical mirror analyzer (CMA) with
electron gun for Auger electron spectroscopy (AES). The TOR on Pt was
determined using numerical methods in order to account for the homogenous gas phase
reaction as well as approach to equilibrium. The resulting data set allowed us
to accurately determine apparent activation energies and reaction orders for
NO,  NO₂, and O₂ using
a simple power rate law.

In-situ
X-Ray Photoelectron Spectroscopy (XPS) experiments were performed at the
Ambient Pressure Photoemission Endstation, Beamline 9.3.2.1, at the Advanced Light Source (ALS), Lawrence
Berkeley National Labs (LBL). It contains a sputter gun for sample cleaning, a
temperature controlled sample stage, and a high pressure photoemission
spectrometer (HPPES) which consists of a differentially pumped electrostatic
lens system combined with a PHI hemispherical electron analyzer [2]. The HPPES
system allows pressures up to a few Torr in the analysis chamber during XP
spectra acquisition. Pt single crystals were heated using a ceramic button
heater in contact with the back side of the sample. The temperature was
measured using a K-type thermocouple spot welded to the side of the crystal.
The Pt surface was cleaned in accordance with procedures used in the kinetic
study which yield a clean, well annealed surface. Sample cleanliness was
checked by XPS throughout the experiments. 

Results and
Discussion

Preliminary
results from the kinetic study indicate that an underlying structure
sensitivity exists on the low index faces of Pt with the following TOR
relationship: Pt(100)>Pt(110)>Pt(111). Global
kinetics were also found to be similar to the kinetics
on supported Pt, showing the product NO₂ inhibits the forward rate
of NO oxidation. Ex-situ AES and XPS of the surfaces after quenching in the
reaction mixture indicate that none of the surfaces significantly oxidized
indicating that the TOR relationship is likely not due to Pt oxidation. LEED experiments
after reaction show that both Pt(110) and Pt(100)
re-construct to their bulk terminated (1x1) surface under reaction condition as
expected based on previous studies in UHV. Current work is on-going to study
the activity of the Pt(321) surface. Our hypothesis is
that this surface will be significantly less active because of the high
concentration of coordinatively unsaturated surface
Pt atoms which bind to oxygen more strongly and therefore help facilitate Pt
oxidation. We are also in the process of re-measuring the kinetics on the low
index faces because we have recently found a problem with our temperature
measurements and therefore the results presented here are preliminary.

In Situ XPS was
performed in order to determine the amount and chemical identity of Pt-bound
oxygen under reaction conditions on Pt(111) as well as
provide evidence for our proposed Langmuir-Hinshelwood  (L-H) mechanism. To determine the maximum oxygen
coverage under reaction conditions, the Pt(111) was
exposed to 0.275 torr NO₂ (362ppm NO₂) at 250°C which yielded an atomic oxygen coverage
of 0.7±0.1 ML. The error represents the standard deviation of five repeat
measurements. No significant surface nitrogen species and Pt oxide formation
were observed during the experiment which lasted about an hour. In a series of
experiments with NO and NO₂ and also with NO, NO₂, and O₂,
we will show that the ratio of NO:NO₂ controls the coverage of
oxygen on the surface, rather than the reactant O₂, and therefore
the availability of empty sites for adsorption of O₂, the proposed
rate determining step in our L-H mechanism. The experiments show why the
product NO₂ effectively inhibits the forward reaction rate by
poisoning the surface with oxygen as shown in our kinetic experiments.

According to Mulla and
co-workers [1], metallic Pt is the active state of Pt in the NO oxidation
reaction and the Pt oxide is inactive. Based on the kinetics and in-situ XPS results presented here along
with the work of Mulla, we believe that large platinum particles inhibit the
kinetics of oxide formation due to the closed-packed structure and remain
active whereas small particles which have a high concentration of coordinately
unsaturated surface Pt atoms are more easily oxidized due to an increased
oxygen binding energy and become inactive. Comparison of the rate on the low
index faces indicates that an underlying structure sensitivity also exists,
Current work is ongoing to confirm our preliminary results on the low index
faces, as well as measure the kinetics on the Pt(321)
surface.

References

1. .Mulla, S.S., Chen, N.,
Cumaranatunge L, Blau, G.E., Zemlyanov, D.Y., Delgass, W.N.,

Ribeiro, F.H., J.
Catal. 241, 389 (2006)

2. Ogletree, D.F., et al., A differentially pumped
electrostatic lens system for photoemission studies in the millibar range.
Review of Scientific Instruments, 2002. 73(11): p. 3872-3877.