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Critical Ensemble Required for Acetylene Cyclization on Pd(ll1): A Study of Steric Inhibition by Coadsorbed Oxygen R. Mark Ormerod and Richard M. Lambert* Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1 E W,U.K. (Received: March 9, 1992; In Final Form: May 28, 1992)
The kinetics of acetylene to benzene cyclization on Pd( 111) are strongly modified by the presence of coadsorbed atomic oxygen, the behavior of the system differing markedly from that observed when acetylene is coadsorbed with NO. When oxygen is preadsorbed, acetylene chemisorbs within the p(2X2)-0 mesh with unit sticking probability: at higher coverages chemisorbed oxygen suppresses overall cyclization activity. This is shown to be a geometrical or siteblocking effect, rather than a consequence of reaction between the coadsorbed oxgyen and either the reactant, acetylene, or the product, benzene. These observations are confirmed by reversing the order of adsorption of oxygen and acetylene: in this case,separate domains of oxygen and acetylene are formed. On the basis of these results, a simple model is put forward for the critical surface ensemble nececisary for benzene synthesis-namely 3CZHz molecules in 3-fold sites around a given Pd atom. This model predicts values for the maximum benzene yields both from the oxygen-saturated surface and from the clean surface, which are in very satisfactory agreement with experiment. The model is also in accord with all the other experimental facts.
1. Introduction Low temperature cyclization of acetylene to benzene over single-crystal Pd( 111) has been reported by several groups of workers.I4 This interesting reaction can be studied over an extremely wide range of pressure-from ultrahigh vacuum conditions up to 1 atm pressure? One remarkable feature is the way in which benzene evolution into the gas phase occurs in two stages during temperature programmed reaction: a low temperature pulse (-200 K) is followed by a high temperature pulse (-500 K). The structure and bonding of the adsorbed reactant, intermediate, and product species have been investigated by a variety of spectroscopic techniques including XPS, ARUPS, NEXAFS, and HREELSS-* In addition, kinetic methods, including isotope labeling, have provided important information*I2 about the reaction mechanism. These findings may be summarized as follows
2C2H2(flat-lying)
