Comments on “Investigation of Ethylene Oxide on Clean and Oxygen-Covered Ag(110) Surfaces”

Abstract

This letter is a re-examination of the ethylene oxide (EO) temperature programmed desorption (tpd) peak shapes (reported in references 1 and 2) which were obtained by tpd after having adsorbed EO at 250 K on to Ag(110) and Ag(111) surfaces. In these papers, the peaks were deemed to originate from a unimolecular rearrangement of an adsorbed oxametallacycle, producing EO, which desorbed on formation. Consequently, on the basis of this thesis, the activation energy of the composite process of internal rearrangement of the oxametallacycle and desorption of EO was determined by solution of the first order Redhead equation [3] at the peak maximum temperature, using an assumed value of 10 13 s -1 for the desorption pre-exponential term. The conclusions of this re-examination are that the m/z = 29 desorption peaks shown in Fig. 1 derive from a second order surface reaction of an adsorbed O atom and adsorbed ethylene molecule and that this is the rate determining step in the desorption of EO. An important corollary of these conclusions is that an oxametallacycle is not involved in the ethylene epoxidation reaction coordinate as the papers suggest.Figure 1 shows the thermally induced desorption spectra of the m/z = 29 fragment (the CHO fragment) after having dosed ethylene oxide (EO) on to clean Ag(110) at 250 K for the following doses: (1) 10 L of EO, curve (a), (2) 5 L of EO, curve (b), (3) 0.5 L of EO, curve (c) and 0.1 L of EO, curve (d). They were reported in reference [1]. Lukaski and Barteau concluded in that paper that adsorption of EO at 250 K on to Ag(110) resulted in the opening of the epoxide ring, producing an adsorbed oxametallacycle [1].They concluded there that heating the adsorbate from 250 K caused the oxametallacycle to re-cyclise to EO which desorbed at a peak maximum temperature of 290 K for the lowest coverage of the adsorbate (curve d). The EO peak maximum temperature lowered sequentially to 275 K as the adsorbate coverage increased.The authors determined the activation energy for desorption of EO by solution of the first order Redhead equation, at a peak maximum temperature of 300 K, assuming a value of 10 13 s -1 for the pre-exponential term. The desorption activation energy so obtained was 18 k cal mol -1 , a value which the authors stated was consistent with density functional theory (DFT) calculations and with previous results on Ag(111) [1,2].The peak shapes shown in figure, however, 1 have the characteristics of a second order process in that: (1) they are symmetric about the peak maximum temperature and (2) the peak maximum temperature moves to lower temperatures with increasing coverage of the adsorbate [3]. The authors acknowledge that the m/z = 29 peaks shown in Fig. 1 are ''less consistent'' with a first order process than those reported for epoxybutene and styrene oxide derived oxametallacycles, but justify their first order kinetic analysis of them by citing the consistency of the energies derived from them with both DFT calculations and with the desorption energies derived from dosin...