The conversion of di-sigma bonded ethylene to ethylidyne on Pt(111) monitored with sum frequency generation: evidence for an ethylidyne (or ethyl) intermediate Citation for published version (APA): Cremer, P., Stanners, C., Niemantsverdriet, J. W., Shen, Y. R., & Somorjai, G. A. (1995). The conversion of di- sigma bonded ethylene to ethylidyne on Pt(111) monitored with sum frequency generation: evidence for an ethylidyne (or ethyl) intermediate. Surface Science, 328(1-2), 111-118. https://doi.org/10.1016/0039- 6028(94)00820-5 DOI: 10.1016/0039-6028(94)00820-5 Document status and date: Published: 01/01/1995 Document Version: Publisher’s PDF, also known as Version of Record (includes final page, issue and volume numbers) Please check the document version of this publication: • A submitted manuscript is the version of the article upon submission and before peer-review. There can be important differences between the submitted version and the official published version of record. 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Shen b Gabor Somorjai a,, a Department of Chemistry, University of California, Materials Science Division, Lawrence Berkeley Laboratory, Berkeley, CA 94720, USA b Department of Physics, University of California, Materials Science Division, Lawrence Berkeley Laboratory, Berkeley, CA 94720, USA c Schuit Institute of Catalysis, Eindhoven University of Technology, 5600 MB Eindhoven, Netherlands Received 13 September 1994; accepted for publication 18 November 1994 Abstract The conversion of di-tr bonded ethylene (-CHz-CH2-) to ethylidyne (=CCH 3) on the Pt(lll) crystal surface was monitored with infrared-visible sum frequency generation (SFG) in the u(CH) frequency range. The conversion began around 255 K and involved a third stable species that is neither di-o- bonded ethylene nor ethylidyne. This species manifested itself as a peak at 2957 cm- l in the vibrational spectrum. Furthermore, we found the same species present during the conversion of vinyl iodide to ethylidyne on Pt(lll). The 2957 cm -1 feature has been assigned to the CH 3 asymmetric stretch of ethylidene and/or ethyl. Keywords: Ethylidyne; Sum frequency generation I. Introduction C-C axis perpendicular to the surface. Its structure and the relocation of platinum atoms around the site When ethylene is chemisorbed on the (111) crys- have been reliably determined by tensor LEED sur- tal face of platinum at temperatures in the range of face crystallography [5]. The transformation of di-o- 120-240 K, di-o- bonded ethylene (-CH2CH2-) bonded ethylene to ethylidyne is shown schemati- forms [1-3]. Annealing this species above 280 K cally in Fig. 1. The purpose of this paper is to causes ethylidyne (=---CCH 3) to form [4]. Ethylidyne investigate the mechanism of this transformation by is adsorbed in an fcc threefold hollow site with its spectroscopic identification of possible intermediate species. To this end we carried out vibrational spec- troscopy studies, using the technique of optical sum frequency generation (SFG) on the (111) crystal face * Corresponding author. E-mail: [email protected]; of platinum in the temperature range of 202-352 K. Fax: + 1 510 643 9668. 1 Present address, Silicon Video Corporation, 10460 Bubb Rd., Several different mechanisms involving the for- Cupertino, CA 95014, USA. mation of a variety of intermediates have been pro- 0039-6028/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0039-6028(94)00820-5 112 P. Cremer et al. / Surface Science 328 (1995) 111-118 SFG has been described in great detail in the literature [15-17]. Briefly, it is a second-order non- .... , ........ -+ .... &,,,r,, linear optical process in which an infrared beam is di- a ethylene ethylidyne combined with a visible beam to generate a sum- below 240{< above 270K frequency output. The process is only allowed (in the Fig. 1. Decomposition of ethylidyne on Pt(ll 1). dipole approximation) in a medium without inver- sion symmetry. The fcc lattice in the bulk of plat- posed in the literature. These intermediates include inum is centrosymmetric and, therefore, in the case ethyl groups (CH3-CH2-) [6], vinyl groups of an organic monolayer adsorbed on Pt(lll) the (CH2=CH-) [7,8], ethylidene (-CH-CH 3) [9,10], SFG signal may be dominated by the contribution and vinylidene (=C=CH2) [11,12]. Because direct from the interface between the metal and vacuum. spectroscopic evidence for any of these intermediates As the IR beam is tuned through vibrational reso- has been elusive, it has been widely assumed that the nances of surface species the effective surface non- rate limiting step for this reaction is either the break- linear susceptibility Xs~2) is resonantly enhanced. The ing of the first CH bond in the di-o" bonded species SFG signal is proportional to the absolute square of to form vinyl [7,8] or the isomerization of the ad- Xs~2) and, hence, a vibrational spectrum of the surface sorbed di-o- bonded species to ethylidene [9,10]. species is detected. Formally we can write: Although direct spectroscopic evidence has been ,)(S(2) = ANR'~'(2)_~_ )((2), lacking, some experimental and theoretical studies have pointed to the likelihood of a mechanism in- ,)((R2) : £ a q volving an ethylidene intermediate. Windham and q (.Oir -- O.)q -- iF' (1) Koel have shown spectroscopic evidence for ethyli- dene formation on Pt(lll) when ethylene is coad- where ann"{2), X[2), Aq, tOq, O.)ir, and Fq refer to the sorbed with 0.12 monolayers of potassium at 100 K nonresonant nonlinear susceptibility, resonant non- and then annealed to 300 K [10]. Evidence for an linear susceptibility, strength of the qth vibrational ethylidene species in the potassium promoted system mode, qth vibrational mode, infrared laser fre- has been further supported by Zhou et al. using quency, and the damping constant of the qth vibra- secondary ion mass spectroscopy (SIMS) [13]. Some tional mode respectively. theoretical evidence has also pointed to ethylidene as Our results indicate the presence of a third species a possible reaction intermediate. Carter and Koel present on the Pt(lll) surface during the conversion have made equilibrium thermodynamic estimates of of di-o" bonded ethylene to ethylidyne. This species the energetics of the transformation of di-o" bonded gave rise to a vibrational feature which was observed ethylene to ethylidyne [9]. These estimates support a only during the reaction and has been assigned to the mechanism involving ethylidene. However, it should CH 3 asymmetric stretch of ethylidene and/or ethyl. be noted that other theoretical studies did not support this conclusion [12,14]. In fact some experimental evidence points to vinyl intermediates [7,8]. 2. Experimental In the present work we utilize infrared-visible sum frequency generation to study the v(CH) por- All experiments were carried out in an ultra high tion of the vibrational spectrum (2700-3100 cm-1). vacuum chamber pumped with a turbo pump and an SFG is a powerful technique for surface vibrational ion pump, The chamber had a base pressure of less spectroscopy because it is both surface specific and than 1 X 10 10 Torr and was equipped with a Varian capable of providing resolution on the order of 10 retarding field analyzer (RFA) for Auger and LEED, cm 1 or less. The u(CH) portion of the vibrational a high resolution electron energy loss spectrometer spectrum is extremely valuable because of the fre- (HREELS), a VG SX300 mass spectrometer, and quent difficulty in interpreting the complex structure CaF 2 windows to allow the passage of infrared and often found in the 900-1500 cm ~ region of the visible laser beams. The Pt(l11) crystal used in this spectrum. experiment was spot welded onto the manipulator P. Cremer et al. / Surface Science 328 (1995) 111-118 113 onto the Pt(lll) sample.