Adsorption and decomposition of acetylene on planar and faceted Ir(210)

The adsorption and reaction of acetylene on both planar and faceted Ir(210) have been studied utilizing temperature-programmed desorption TPD), Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), and high-resolution electron energy loss spectroscopy (HREELS). Following adsorption of C2H2 at 300 K or 100 K, TPD data indicate that H-2 is the dominant desorption product, and that decomposition of adsorbed C2H2 occurs in a stepwise fashion. Multiple carbon-containing species are formed on Ir(210) upon adsorption of acetylene at high coverage, which are different from those formed at low coverage. Our HREELS results show that the dominant surface hydrocarbon species formed at high coverage are mainly acetylide and ethylidyne while acetylide dominates at low coverage. In contrast to reaction measurements on an Ir organometallic complex that catalyzes cyclization of C2H2 to C6H6, no evidence for the cyclization reaction is found on Ir(210). The results are compared with adsorption and decomposition of C6H6 on Ir(210); as for C2H2, the dominant desorption product is H2, but there are differences in the reaction sequence. In addition, evidence has been found in TPD measurements for structure sensitivity in decomposition of acetylene over the clean faceted Ir(210) surface versus the clean planar Ir(210) surface, which is attributed to nanometer scale structures on the faceted surface. The HREELS data give complementary information to TPD and AES results and provide insights into the reaction mechanisms for acetylene surface chemistry.