Implementation of New TPD Analysis Techniques in the Evaluation of Second Order Desorption Kinetics of Cyanogen from Cu(001)

Abstract

The interactions of cyanide species with a copper (001) surface were studied with temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). Adsorbed cyanide species (CN(a)) undergo recombinative desorption evolving molecular cyanogen (C(2)N(2)). As the adsorbed CN species charge upon adsorption, mutually repulsive dipolar interactions lead to a marked desorption energy reduction with increasing CN(a) coverages. Two new TPD analysis approaches were developed, which used only accurately discernible observables and which do not assume constant desorption energies, E(d), and pre-exponential values, ν. These two approaches demonstrated a linear variation of E(d) with instantaneous coverage. The first approach involved an analysis of the variations of desorption peak asymmetry with initial CN coverages. The second quantitative approach utilized only temperatures and intensities of TPD peaks, together with deduced surface coverages at the peak maxima, also as a function of initial surface coverages. Parameters derived from the latter approach were utilized as initial inputs for a comprehensive curve fit analysis technique. Excellent fits for all experimental desorption curves were produced in simulations. The curve fit analysis confirms that the activation energy of desorption of 170-180 kJ/mol at low coverage decreases by up to 14-15 kJ/mol at CN saturation.

Identification of On-Surface Reaction Mechanism by Targeted Metalation

“…Cyanogen (CN) 2 was not observed. 37 Note that the gradual monotonic increase of the H 2 TD signals (i.e. not the peaks) is due to a general increase of the partial pressure due to warming-up of the sample holder.…”

Identification of the c(10×6)-CN/Cu(001) surface structure

“…Later experimental studies [8] identified that the surface coverage of CN fragments Table 2 Nearest-neighbour distances between ion cores in the c(10 × 6)-CN/Cu(0 0 1) system shown in Fig. 2(a).…”

“…This allowed division of the unreconstructed c(10 × 6)-Cu(0 0 1) into a 'checkerboard' of 30 four-fold sites which were populated by 12 CN fragments in the optimal structure. The range of possible binding sites was reduced by invoking the experimentally observed c2 mm symmetry [7] of the system and the experimental observation that the CN fragments occupy equivalent binding positions [8]. The sequence was repeated using both the local density approximation (LDA) and the generalised gradient approximation (GGA) to describe the density functional.…”

“…Recent studies of cyanogen adsorption on Cu [6][7][8] have further illustrated the diversity of effects that are present on cyanated Cu surfaces, both in terms of the range and type of structures that can form and in terms of the level of interaction between the adsorbed CN fragments [6]. Experimental studies [7] of the adsorption of cyanogen on Cu(0 0 1) have shown a range of surface structures that may form depending on surface conditions.…”

“…Low temperature growth (with a substrate temperature of 163 K) results in the formation of a disordered surface whereas higher temperature growth (263 K) produces an ordered c2 mm structure with c(10 × 6) periodicity. Later experimental studies [8] have proposed a surface coverage  CN = 0.41 ML for the ordered c(10 × 6)-CN/Cu(0 0 1) system and has identified using X-ray photoelectron spectroscopy * Tel. : (+44) (0)115 848 3341.…”