Introduction
The NO oxidation reaction (NO + ˝ 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.