Carbide induced reconstruction of monatomic steps on Ni(1 1 1) – A density functional study M.P. Andersson, F. Abild-Pedersen * Center for Atomic-scale Materials Design, Department of Physics, Nano DTU, Technical University of Denmark, DK-2800 Lyngby, Denmark Received 16 August 2006; accepted for publication 25 October 2006 Available online 16 November 2006 Abstract We present density functional calculations for carbon adsorption at the two types of monatomic steps on a Ni(1 1 1) surface. We show that it is thermodynamically favourable to make a carbon induced clock-type reconstruction at the close-packed step with a [1 1 1] step geometry, which creates fourfold sites at the step-edge. It is furthermore possible to extend the carbide with the clock reconstructed geometry onto the upper terrace with a net energy gain compared to adsorption of carbon on unreconstructed close-packed steps or terrace sites on Ni(1 1 1). Our findings explain the fact that carbide islands start to grow preferentially on the close-packed steps as has been observed using scanning tunneling microscopy. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Density functional calculations; Nickel; Steps; Carbide; Clock reconstruction; Carbon; Carbon monoxide; Ethylene 1. Introduction Carbon induced reconstruction of nickel surfaces has been studied both experimentally [1–6] and theoretically [7–10]. This is unsurprising, since Ni is the industrial cata- lyst of choice for both steam reforming of methane [11] and the methanation reaction [12,13] and is also used for growing carbon nanotubes [14]. The study of carbon forma- tion on Ni and Ni-based catalysts are of primary impor- tance due to the severe difficulties it imposes on the catalyst. The carbon formed during reaction conditions may result in a complete encapsulation and hence the break-down of the catalyst particle entirely. Depending on substrate temperature, carbon may form either a carbide phase (400–600 K) or graphite phase (700 K) [15]. Exper- imental evidence have shown that both the Ni(1 0 0) and the Ni(1 1 1) surface undergoes a complicated reconstruction by carbon. The reconstruction on a Ni(1 0 0) surface is achieved by rotating a whole group of surface atoms surrounding the carbon atom clock- or anti-clockwise and on the Ni(1 1 1) surface by removing surface atoms to expand the adsorp- tion sites for carbon. Most of the work on carbon induced reconstruction of Ni surfaces concerns this particular type of reconstruction, called the p4g clock reconstruction [2]. In it the carbon has a fourfold coordination to its surround- ing Ni surface atoms, which as stated above implies that for the (1 1 1) facet some mass transport is necessary. Scanning tunnelling microscopy (STM) experiments show that when a Ni(1 1 1) surface is exposed to carbon monoxide at 400–600 K areas with fourfold symmetry (the p4g clock reconstruction [2]) start to grow at the close-packed step [5], and not at the open step [16]. No fourfold symmetric surface reconstruction was observed, when a Ni(977) surface was exposed to carbon monoxide at 350–600 K [16], and this surface is very densely stepped with the open type of steps. Results of STM experiments of carbide produced after exposure of a Ni(1 1 1) surface to ethylene have also been published [2,6,17] but here the re- sults differ. The older studies [2,6] report no preference for step nucleation, whereas newer results [17] show a pref- erence for nucleation of the carbide phase at the step. No mention to which of the two types of steps was more active was made. 0039-6028/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2006.10.036 * Corresponding author. E-mail address: abild@fysik.dtu.dk (F. Abild-Pedersen). www.elsevier.com/locate/susc Surface Science 601 (2007) 649–655