IMAGING OF ACETYLENE HETEROCYCLIZATION SITES ON THE SULFIDED PD(111) SURFACE

Scanning tunneling microscopy (STM) has been used to identify defects as heterocyclization reaction sites on a sulfided Pd(111) surface. The heterocyclization of acetylene (C2H2) to thiophene (C4H4S) occurs on sulfided Pd(111) surfaces that have been activated by heating to temperatures above 800 K. We have imaged the atomic scale structure of the surface before activation and found a sulfur overlayer with atoms arranged in broad, defect-free terraces having (square-root 7 x square-root 7)R19-degrees periodicity with respect to the substrate. Images of the surface after heating indicate that this activation process irreversibly generates monatomic height steps with a concentration that increases with annealing temperature. We have been able to show that the active sites for acetylene heterocyclization are associated with the steps by correlating the step density with the thiophene yield.