| AVS 59th Annual International Symposium and Exhibition | |
| In Situ Microscopy and Spectroscopy Focus Topic | Tuesday Sessions |
| Session IS+AS+SS+EN-TuM |
| Session: | In Situ Spectroscopic Studies of Catalysis and Gas-Solid Reactions |
| Presenter: | J. Schnadt, Lund University, Sweden |
| Authors: | J. Knudsen, Lund University, Sweden Y. Monya, Keio University, Japan J. Schnadt, Lund University, Sweden M.A. Arman, Lund University, Sweden E. Grånäs, Lund University, Sweden H. Kondoh, Keio University, Japan J.N. Andersen, Lund University, Sweden |
| Correspondent: | Click to Email |
The platinum group metals are known to be excellent catalysts for the oxidation of carbon monoxide, and the reaction mechanisms over the surfaces of these metals have been studied for a long time. Nevertheless, only during recent years a new picture has emerged which suggests that the catalytically active phase often is formed first under reaction conditions – which implies realistic pressures rather than ultrahigh vacuum (UHV) – and that it is different from the adsorption structures known from UHV experiments. In the case of the Pt(111) surface a very oxygen-rich chemisorbed phase has been suggested as the catalytically active phase [1], whereas a surface oxide have been suggested for the Ru(0001) surface [2]. Thus, for each different surface different phases and mechanisms might be at play, and, moreover, the phase might depend quite strongly on the conditions (pressure and temperature) used.
With this in mind we have studied the CO oxidation reaction over the Ir(111) surface and the related adsorption systems of CO and oxygen on Ir(111) using a combination of in situ Ambient pressure x-ray photoelectron spectroscopy (APXPS) – carried out at the new APXPS instrument at the Swedish synchrotron radiation facility MAX IV Laboratory – and ex situ Scanning tunnelling microscopy (STM) and x-ray photoelectron spectroscopy (XPS) measurements performed in UHV.
A recent surface x-ray diffraction study reports different oxygen phases for the Ir(111) surface at oxygen pressures from 10-6 to 100 mbar – chemisorbed oxygen, a trilayer, a multilayer oxide, and a bulklike oxide [3]. Concentrating on pressures at around 1 mbar, we find a variety of oxygen-rich structures. The corresponding CO adsorption phase formed at 1 mbar CO pressure is an assembly of separated CO16 clusters with the CO molecules sitting in on-top sites [4].
The reactivity at 1 mbar total pressure (O2:CO ratio 9:1) and at different temperatures was studied by APXPS and simultaneous monitoring of the gas composition. We find that the phase with the highest activity for the oxidation of CO is a surface phase which contains both CO and oxygen. By comparing with the measured adsorption structures of oxygen we find that the oxygen structure is quite similar to the p(2x1)-O structure formed on Ir(111) under UHV conditions. This contrasts with was is found for other platinum group metals such as the Pt(111) surface [2], for which CO oxidation is favoured over oxygen rich phases.
[1] A. L. Gerrard, J. F. Weaver, J. Chem. Phys. 123 (2005) 224703.
[2] H. Over et al.,, Science 287 (2000) 1474.
[3] Y. B. He et al., J. Phys. Chem. 112 (2008) 11946.
[4] L.-M. Yang, S.-L. Yau, J. Phys. Chem. B. 104 (2000) 1769.