| AVS 62nd International Symposium & Exhibition | |
| In-Situ Spectroscopy and Microscopy Focus Topic | Monday Sessions |
| Session IS+AS+SS-MoM |
| Session: | Fundamental Studies of Surface Chemistry of Single Crystal and Nanomaterials under Reaction Conditions |
| Presenter: | Matthijs van Spronsen, Leiden University, Netherlands |
| Authors: | M.A. van Spronsen, Leiden University, Netherlands J.W.M. Frenken, Leiden University, Netherlands I.M.N. Groot, Leiden University, Netherlands |
| Correspondent: | Click to Email |
Platinum finds its main application as a car catalyst to control the emission of exhaust gases. Although automotive catalysis has been extensively investigated, challenges still exist. One of the challenges arises when increasing the oxygen/fuel ratio. Under oxygen-rich reaction conditions, much uncertainty exist about the structure of the active surface phase. This is even true for the Pt(111) surface, which is the facet lowest in energy and the simplest model catalysts available.
An early operando Scanning Tunneling Microscopy (STM) study [1] showed a stepwise increase in CO oxidation activity at oxygen-rich conditions. This increase concurred with a dramatic and instantaneous morphology change. From the STM images, the atomic structure could not be resolved, but roughening on a long length scale was observed. Under similar conditions, Surface X-ray Diffraction found the formation of thin, bulk-like α-PtO2 [2]. Surprisingly, a theoretical study concluded that this oxide is inert to CO oxidation [3].
With the high-pressure, high-temperature ReactorSTM [4], we studied the oxidation of Pt(111) both by exposing to O2 and to NO oxidation conditions.
Upon oxidation with O2 (1.0 bar, 423-523 K), we found a stable surface oxide consisting of triangles assembled in a ‘spoked-wheel’ superstructure. In addition, we found a second structure consisting of a lifted-row pattern. The two structures were coexisting on different regions on the surface. The lifted-row structure was becoming more predominant at higher O2 pressure. We propose that both oxides share the same building block, which are expanded Pt oxide rows.
After evacuation of the reactor, the ordered structures disappeared, although some remnants remained. The surface oxidation is a clear example of the pressure-gap effect. Furthermore, lower-temperature (291-323 K) experiments did not yield any ordered structure showing the dependence on atomic mobility.
Exposure of Pt(111) to NO and O2 or exposure to NO2 resulted in the formation of a mixture of small domains of both the spoked-wheel and the lifted-row structures.
The surface oxidation was accompanied with roughening of terraces. This is attributed to relaxation of adsorbate-induced stress on the surface. Identical roughness development was previously found under CO oxidation conditions [1]. Therefore, we argue that a surface oxide was also the relevant structure under CO oxidation conditions.
[1] Bobaru, PhD thesis, Leiden University, 2006
[2] Ackermann, PhD thesis, Leiden University, 2007
[3] Li & Hammer, Chem. Phys. Lettt., 409, 1, 2005
[4] Herbschleb, et al., Rev. Sci. Instrum., 85, 083703, 2014