Reaction Between Ethylene and Acetate Species on Clean and Oxygen-Covered Pd(100): Implications for the Vinyl Acetate Mo
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Reaction Between Ethylene and Acetate Species on Clean and Oxygen-Covered Pd(100): Implications for the Vinyl Acetate Monomer Formation Pathway Florencia Calaza • Zhenjun Li • Wilfred T. Tysoe
Received: 24 September 2010 / Accepted: 26 October 2010 / Published online: 11 November 2010 Ó Springer Science+Business Media, LLC 2010
Abstract The reaction between gas-phase ethylene and adsorbed acetate species on Pd(100)-p(2 9 2)-O and Pd(100)-c(2 9 2)-O surfaces is studied using infrared spectroscopy. It is found that acetate species are removed more rapidly by gas-phase ethylene on oxygen-covered Pd(100) than on Pd(111). However, in contrast to reaction on Pd(111), where vinyl acetate monomer (VAM) formation is detected by infrared spectroscopy, only CO is found on oxygen-covered Pd(100) surfaces. In the case of Pd(111), it has been shown that VAM is stabilized on the crowded, ethylidyne-covered surface. Since ethylidyne species do not form on Pd(100), any VAM that is formed can thermally decompose. The reaction shows an isotope effect when C2D4 is substituted for C2H4, indicating the hydrogen is involved in the rate-limiting step. Based on the surface chemistry found for VAM on a Au/Pd(111) alloy, where 30 to 40% ML of gold inhibits VAM decomposition, it is suggested that the VAM formation rate will increase on (100) alloy surfaces, while it will decrease at higher gold coverages since acetate formation is inhibited. Keywords Pd(100) Vinyl acetate monomer synthesis Ethylene Acetic acid Infrared spectroscopy
1 Introduction The palladium-catalyzed synthesis of vinyl acetate monomer (VAM) from acetic acid, ethylene and oxygen
F. Calaza Z. Li W. T. Tysoe (&) Laboratory for Surface Studies, Department of Chemistry and Biochemistry, University of Wisconsin-Milwaukee, Milwaukee, WI 53211, USA e-mail: [email protected]
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was discovered some 30 years ago [1] and it has been shown that Pd(111), Pd(100) and Au/Pd(111) and Au/ Pd(100) single crystals catalyze the reaction [2–4]. In particular, it is found that (100) alloy surfaces are more active than those with (111) orientation and that the VAM formation rate increases with gold coverage on the Au/ Pd(100) alloy, reaching a maximum at a palladium coverage of *0.1 ML on Au(100) [3]. This has been ascribed to the formation of a particular ensemble on the surface consisting of palladium atoms located on the opposite corners of the surface unit cell that are completely surrounded by gold. Scanning tunneling microscopy (STM) of Au/Pd(100) alloys [5] indicates that these ensembles predominate on the surface. Extensive work has been carried out to examine the reaction pathway for VAM synthesis on Pd(111) surfaces by titrating a saturated acetate overlayer with gas-phase ethylene and by monitoring the acetate removal kinetics in situ using infrared spectroscopy. It is found that the reaction proceeds on Pd(111) via the insertion of adsorbed ethylene into the O–Pd bond of the surface acetate species [6] to form an acetoxyethyl-palladium intermediate, which decomposes via a b
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