Surface-catalyzed dehydrogenation and intermolecular C[sbnd]C bond formation at peripheral alkyl units on Cu(100) and Au(111)

Abstract

Hydrocarbon reactivity at surfaces has long been a topic of high interest for heterogeneous catalysis and is now gaining new importance for the development of surface nanostructures. To expand these structural libraries, molecules with larger functional groups are used on surfaces; these applications require the system to go to higher temperatures, necessitating studies of the reactivity and reaction pathways for alkyl type groups. We have designed and synthesized a prototypical molecule, 1,3,5-tris-(3,5-diethylphenyl)benzene (TDEPB), to examine reactivity on Cu(100) and Au(111) surfaces using high-resolution electron energy loss spectroscopy (HREELS) and scanning tunneling microscopy (STM). We report the dehydrogenation of ethyl groups in TDEPB at 450 K on Cu(100) and at 500 K on Au(111). For a structurally similar triphenylbenzene (TPB) molecule without the ethyl groups, dehydrogenation was only observed on the Cu(100) surface at 450 K. Desorption of TPB was observed from Au(111) at 500 K. For Au(111), it was thus the presence and reactivity of the ethyl groups that prevented complete desorption of the molecule. The reaction pathway for the ethyl groups was found to be different on Au(111) vs Cu(100), as two distinct steps were observed on Au(111). First, a dehydrogenation occurred at 500 K, followed by a structural change of the adsorbate at 550 K. The post-dehydrogenation structures on the two surfaces differ in the loss of the 750 cm−1 HREELS feature on the Au(111) while not on Cu(100) and in other spectral changes for TDEPB on Cu(100) at 650 K that were not observed for TDEPB on Au(111) or for TPB on either surface. These results demonstrate reaction pathways that may be encountered with alkyl-functionalized molecular building blocks on surfaces at elevated temperatures.