Paper SS1-WeA8
An In Situ Investigation of Hyperactive States for CO Oxidation on Platinum Group Metal Catalysts

Wednesday, October 17, 2007, 4:00 pm, Room 608

The oxidation of carbon monoxide on platinum group metal catalysts has been studied for decades due to its technological importance in pollution control and fuel cells.^1-3 Ru catalysts were found to have dramatically different activities at ultrahigh vacuum compared with elevated pressure conditions. The formation of 1 monolayer (ML) surface oxide layer is believed to give rise to the much higher activity of Ru catalysts under high pressure conditions.⁴ On the other hand, at stoichiometric reaction conditions, similar activation energies and kinetic orders were found for supported Pd, Pt, Rh and Ir catalysts at high and low pressure conditions.³ Recent studies have shown that there are hyperactive states on Pd, Pt and Rh catalysts, with the CO₂ formation rate per metal atom site per second (turnover frequency, TOF) reaching into the thousand's, two orders of magnitude higher than the rate measured at stoichiometric reaction conditions. This hyperactive state takes place on an oxygen-rich surface at a well-defined O₂/CO ratio and specific temperature for each metal. The oxygen adsorption energies for Pt, Pd, Rh and Ru correlate directly with the O₂/CO ratio required to achieve the hyperactive catalytic state. PM-IRAS is used to investigate in situ the surface species under reaction conditions. The thickness and chemical nature of the surface oxygen layer giving rise to the hyperactive state is explored with XPS.

¹. T. Engel and G. Ertl, Adv. Catal. 28 (1979) 1
². J.E. Turner, B.C. Sales, and M.B. Maple, Surf. Sci. 109 (1981) 591
³. P.J. Berlowitz, C.H.F. Peden and D.W. Goodman, J. Phys. Chem. 92 (1988) 5213
⁴. C.H.F. Peden and D.W. Goodman, J. Phys. Chem. 90 (1986) 1360.