Heterocyclization of Acetylene to Thiophene at Defects on the Pd(111) Surface
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HETEROCYCLIZATION OF ACETYLENE TO THIOPHENE AT DEFECTS ON THE Pd(111) SURFACE ANDREW J. GELLMAN, Dept. of Chemistry, U. of Illinois, Urbana, IL 61801
ABSTRACT We have studied a novel surface reaction in which acetylene (C2H 2) cyclizes with sulfur atoms on the Pd(111) surface to form thiophene (C 4H4 S). The reaction occurs at defects in the (N/7 x-/7)R19° sulfur overlayer that is formed on this surface. The surfaces were prepared by decomposing H2S on the clean Pd(111) surface and annealing to temperatures in the range 600K to 1100K to produce defects in the sulfur overlayer. The reaction occurs to greatest extent on surfaces annealed to high temperatures and only on surfaces with coverages approaching saturation. The defects have been characterized by adsorption of CO using both vibrational spectroscopyand measurementsof desorption kinetics. On the sulfided surface CO is bound in a linear configuration to a single Pd atom. At defects in the sulfur overlayer CO is bound in a bridging site between two atoms, as on the clean surface. INTRODUCTION A wide variety of metal oxide and metal sulfide catalysts are thought to have active sites which are surface anion vacancies or defects [1]. While there has been a great deal of chemistry studied on well defined single crystalline metal surfaces prepared under ultra-high vacuum conditions there has been much less focussed on the role of defects. In the study of the surface chemistry of metals defects have been produced by using High Miller Index surfaces having high concentrations of steps and kinks. Preparation of well defined surface of metal oxides and sulfides is much more difficult in most cases than the preparation of a clean metal surface. Preparation of such surfaces with well defined defect concentrations is even more difficult. The Pd(111)/S surface which we describe in this work allows controlled formation of surface defects simply by heating. These surfaces have been observed to catalyze cyclization of adsorbed acetylene (C-2 H2 ) to thiophene (C 4H 4S) and we are able to show that this reaction occurs at these defects. The heterocyclization reaction involves cyclization of adsorbed acetylene with sulfur atoms on the sulfided Pd(111) surface to produce thiophene [2]. 2 C2H 2
-
*
C 4H 45
Pd(11l)/S This reaction is a variant on the cyclotrimerization reaction observed on the clean Pd(l11) surface which results in the formation of benzene from adsorbed acetylene [3,4].
Mat. Res. Soc. Symp. Proc. Vol. 209. 01991 Materials Research Society
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3 C 2H 2
?-
C 6H 6
Pd(111)
These reactions occur in a number of different environments ranging from high pressures to ultra-high vacuum and in molecular beam scattering from surfaces [3-6]. The mechanism is one in which acetylene first dimerizes to form a C4H 4 intermediate which can then cyclize with either additional acetylene or sulfur atoms to form benzene or thiophene. This mechanism has been proposed on the basis of similar cyclization reaction which have been observed following co-adsorption of cis-3,4-dichl
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