Langmuir 1991, 7, 827-830
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Heterocyclization Reaction Sites on the Sulfided Pd(11 1) Surface Andrew J. Gellman Department of Chemistry, University of Illinois, Urbana, Illinois 61801 Received July 5, 1990. In Final Form: November 22, 1990
The sulfided Pd(ll1) surface is capable of cyclizing acetylene to both benzene and thiophene. We find that these reactions are sensitive to the temperature at which the sulfided surface has been prepared, occurringwith greatest yield on surfaces produced at high temperatures (>lo00 K). Study of CO adsorption on these surfaces using both high-resolutionelectron energy loss spectroscopy and desorption measurements reveals that heating the surface introduces defects at which CO can bind in bridging sites between exposed Pd atoms. This defect concentration, as determined by CO adsorption in bridging sites, is correlated with the amount of thiophene produced from acetylene. It appears that acetylene heterocyclization is a defectcatalyzed reaction and that on the sulfided Pd(ll1) surface we are able to control defect density. 1. Introduction The Pd(ll1) surface and the sulfided Pd(ll1) surface are catalysts for a number of interesting cyclization reactions that have been the subject of fairly intense study over the past few The clean Pd(ll1) surface will induce cyclotrimerization of adsorbed acetylene to benzene, while on the sulfided surface one can produce both benzene and thiophene. In a typical reaction producing thiophene the surface is first sulfided to near saturation coverage by repeated sequences of HzS adsorption and thermal decomposition. Adsorption of acetylene on the sulfided surface followed by heating results in the desorption of small quantities of thiophene (
