JOURNAL OF CATALYSIS 161, 143–155 (1996) ARTICLE NO. 0171 Ethylene Oxidation on Polycrystalline Platinum over Eight Orders of Magnitude in Ethylene Pressure: A Kinetic Study in the Viscous Pressure Regime U. Ackelid, L. Olsson, and L.-G. Petersson Laboratory of Applied Physics, Department of Physics and Measurement Technology, Link¨ oping University, S-581 83 Link¨ oping, Sweden Received April 17, 1995; revised December 7, 1995; accepted February 2, 1996 C 2 H 4 oxidation on polycrystalline Pt films and foils at T = 373– 473 K was studied with mass spectrometry in the pressure ranges 10 −6 –10 2 Torr C 2 H 4 and 0.3–1500 Torr O 2 (1 Torr = 133.3 Pa). A new, “spatially resolved gas sampling” method enabled true kinetic data to be collected in the viscous pressure regime. In situ measure- ments of surface hydrogen with a capacitance–voltage technique and ex situ characterization with Auger electron spectroscopy and atomic force microscopy were also performed. No structure sensi- tivity with respect to sample grain size could be seen. The reaction orders in C 2 H 4 and O 2 were +1 and −1 in oxygen excess, −2 and +3 in weak ethylene excess, and −0.5 and +1 in large ethylene excess, respectively. The apparent activation energy was between 30 and 75 kJ/mol for different reactant mixtures. The rate data could be qualitatively fitted to a simple Langmuir–Hinshelwood reaction scheme in the excess regimes, assuming competitive adsorption of C 2 H 4 and O 2 , abundant molecular desorption of the excess reactant, and a strong self-inhibition of C 2 H 4 adsorption. c  1996 Academic Press, Inc. INTRODUCTION Oxidation of hydrocarbons over platinum is one of the key processes in car exhaust conversion, and that is an obvious motive for studies of such reactions. Ethylene is a suitable test gas for several reasons: it is a fairly simple hydrocarbon, it interacts with metal surfaces even at moderate temperatures, and it gives few possible oxidation products. There are numerous investigations of ethylene interaction with clean (1–10) and oxygen-covered (8–10) Pt surfaces in ultrahigh vacuum (UHV) and also many studies of the C 2 H 4 + O 2 reaction near 1 atm (11–29). However, most of the latter have been done with supported Pt catalysts (12–16, 18, 21–24, 26) or with porous Pt films (17, 19, 20, 25); literature data on macroscopic Pt surfaces are scarce (11, 27–29). Table 1 presents data from isothermal kinetic studies at near-atmospheric pressures published throughout the past three decades. CO 2 and H 2 O are the chief reaction prod- ucts in all cases and partially oxidized species are explicitly reported only once (12). Positive first reaction order in ethy- lene in fuel-lean mixtures is also unanimously reported, but the order in ethylene excess and the dependence on oxygen pressure are not yet agreed upon. Both Rideal–Eley (R–E) and Langmuir–Hinshelwood (L–H) reaction mechanisms have been suggested. Two other interesting observations in Table 1 are the scatter of the activation energies and the vary narrow pressure ranges (maximum two orders of magnitude) used in all previous studies. There are also studies using other approaches than the “rate vs pressure at constant temperature” concept. Wolf et al. (21) used temperature and concentration program- ming techniques, FTIR spectroscopy, and computer sim- ulations to investigate the C 2 H 4 + O 2 reaction over sup- ported Pt/SiO 2 . Their results indicate that the reaction is of L–H type, but that the reaction pathways under fuel-lean and fuel-rich conditions differ significantly. In addition, this group have investigated self-sustained rate oscillations (22– 24), previously observed by others (15, 17, 25). Sheintuch et al. performed nonisothermal experiments with Pt/Al 2 O 3 (26) and Pt wires (27), which also indicate a L–H rate ex- pression. Kunimori et al. (28) detected syngas production (CO + H 2 ) in molecular beam experiments with a Pt foil at higher temperatures (>700 K) and lower pressures (≈10 −2 Torr). Wu and Phillips (29) found that the C 2 H 4 + O 2 re- action on Pt may induce catalytic etching and deposition of carbon; however, these phenomena were never seen at temperatures below 770 K. Despite the work done in the past, there are still lacking fundamental data for the C 2 H 4 + O 2 reaction on macro- scopic Pt surfaces and, consequently, macroscopic and sup- ported systems have never been systematically compared. A flat, continuous surface has some experimental advan- tages over a supported catalyst: it is easily examined by surface-sensitive techniques, it is often less prone to recon- struction, its surface area and temperature are easily de- termined, and its catalytic action is solely controlled by the metal properties. This communication describes a study of ethylene ox- idation over various polycrystalline platinum surfaces. In 143 0021-9517/96 $18.00 Copyright c  1996 by Academic Press, Inc. All rights of reproduction in any form reserved.