Journal of Catalysis 260 (2008) 380–383 Contents lists available at ScienceDirect Journal of Catalysis www.elsevier.com/locate/jcat Research Note Towards an understanding of promoter action in heterogeneously catalyzed ethene epoxidation: Why chlorine is the best halogen Daniel Torres a , Francesc Illas a,∗ , Richard M. Lambert b a Departament de Química Física & Institut de Química Teòrica i Computacional (IQTCUB), Universitat de Barcelona, C/Martí i Franquès 1, 08028 Barcelona, Spain b Departament of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB21EW, UK article info abstract Article history: Received 12 August 2008 Revised 17 October 2008 Accepted 19 October 2008 Available online 1 November 2008 Keywords: Ethene epoxidation Silver catalyst Halogen promoters Selectivity A detailed theoretical study is presented focusing on the role of halogens as promoters for the silver- catalyzed partial oxidation of ethylene to the epoxide (EO) and, in particular, aiming to understand why Cl is best. The study is based on periodic density functional theory calculations carried out for the reaction between ethene and atomic oxygen on the Ag(111) surface, taken as a model catalyst, in the presence of coadsorbed halogens. It is found that the presence of co-adsorbed halogens significantly decreases the energy barrier from the oxametallacycle intermediate (OMME) to EO relative to that from the same OMME intermediate to the undesired acetaldehyde (AC) product. However, co-adsorbed halogens are also found to increase the desorption energy of the reaction products. We present compelling evidence that the superior activity of Cl is due to the additional presence of subsurface Cl that favors EO formation with respect to AC but at the same time facilitates EO desorption, hampering further transformations. The results provide a signpost for the rational design of catalyst promoters.  2008 Elsevier Inc. All rights reserved. Trace amounts of foreign species are often incorporated into heterogeneous catalysts because they enhance activity, or selec- tivity, or both. Indeed, the viability of a number of strategically important large-scale processes is critically dependent on such so- called promoters. The silver-catalyzed partial oxidation of ethylene to the epoxide is a case in point. Here, the accidental discovery that chlorine greatly increases epoxidation selectivity from ∼50% to ∼80% has had a major impact on process economics; the as- sociated substantial reduction in CO 2 emission is also of no small consequence. Thus in terms of global production of ethene epox- ide, the superior selectivity of current generation catalysts is re- sponsible for a reduction of CO 2 emission amounting to 10 7 tons [1]. Nowadays, chlorine promotion is the universally and unques- tioningly adopted industrially, and, until relatively recently, no at- tention appears to have been paid to the other halogens [2]. We found experimentally that all four halogens are selectivity promot- ers, with Cl markedly the most effective. Thus a scientific basis for technological practice was established, and a simple hypothesis proposed to account for the observed behavior. However, this be- havior still awaits a detailed theoretical explanation, which is the subject of this Letter. First we briefly review the generally accepted two step molec- ular mechanism for ethene epoxidation (Scheme 1). * Corresponding author. E-mail address: francesc.illas@ub.edu (F. Illas). According to both theory [3–7] and recent experiments carried out under highly controlled conditions [8], the first step involves formation of an oxametallacycle (OMME) intermediate which may further react via one of two competitive pathways to form either ethene oxide (EO) or acetaldehyde (AC), herein defined as primary chemistry. AC formation leads to combustion and this is the key process, defined as secondary chemistry, that limits epoxidation se- lectivity. The key role of the OMME was recently emphasized by Barteau [9] in a perspective article highlighting the recent work of Klust and Madix [8] which provides clear experimental evi- dence that an oxametallacycle is also the common intermediate in styrene oxidation. Recent theoretical work has also provided further evidence that this is the molecular mechanism for ethene epoxidation on Au(111) [10] and to play an important role in the epoxidation on propene on Cu(111) and Ag(111) [11]. However, one must realize that desorption of the products may also sig- nificantly affect the selectivity as already pointed out by Loffreda et al. [12] for the catalytic hydrogenation of unsaturated aldehy- des on Pt(111). Therefore, the effect of co-adsorbed halogens on the elementary steps of the reaction mechanism in Scheme 1 as well as on the adsorption energies of reactants, intermediate and products needs to be investigated in detail. Note that desorption Scheme 1. Molecular mechanism for ethene epoxidation. 0021-9517/$ – see front matter  2008 Elsevier Inc. All rights reserved. doi:10.1016/j.jcat.2008.10.011