Why silver is the unique catalyst for ethylene epoxidation M.O. Ozbek a,b , I. Onal a , R.A. van Santen b,⇑ a Chemical Engineering Department, Middle East Technical University, 06531 Ankara, Turkey b Chemical Engineering and Chemistry Department, Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands article info Article history: Available online 9 September 2011 Keywords: DFT Cu 2 O Ag 2 O Au 2 O Ethylene Ethylene oxide Oxide surface abstract The activities of Cu 2 O(001), Ag 2 O(001), and Au 2 O(0 0 1) surfaces for direct ethylene epoxidation and alternative paths for EO isomerization are studied. Among these three oxide surfaces, only Ag 2 O(0 0 1) surface enables direct path without a barrier. Au 2 O cannot regenerate surface oxygen, and overall reac- tion on Cu 2 O is endothermic. Furthermore, ring opening of ethylene oxide (EO) and subsequent acetalde- hyde (AA) formation on Cu 2 O is more favorable than EO desorption. Ethylene adsorption on an oxygen vacancy results in the oxametallacycle (OMC) formation, which causes AA formation and reduces EO selectivity. Cl adsorption removes these surface vacant sites and hence prevents the formation of the OMC intermediate. Ó 2011 Elsevier Inc. All rights reserved. 1. Introduction Silver catalyzed ethylene epoxidation to produce ethylene oxide (EO) is an industrially applied important heterogeneous catalytic process. The EO selectivity of the un-promoted metallic silver lays around 50% [1,2]. However, the industrial catalyst consists of silver particles supported on low surface alumina, with the addition of alkali compounds as promoters. During the process, ppm amounts of chlorine are added to the feed stream as a promoter in the form of chlorinated hydrocarbons, to enhance the EO selectivity. Today, the EO selectivity of the industrial catalyst lies in the range of 90% [3]. Scheme 1 shows the competing parallel (k 1 and k 2 ) and consec- utive (k 4 ) reactions catalyzed by silver [4]. Formation of acetalde- hyde (AA) acts as an intermediate for total combustion [5]. While the parallel reactions (k 1 and k 2 ) are uniquely catalyzed by Ag, EO isomerization (k 3 ) predominantly occurs on the alumina sup- port [5,6]. Earlier studies showed that atomic oxygen is the active species for the partial oxidation of ethylene [2,7–9]. Thus the cata- lyst should be able to dissociate the oxygen molecule, which can be easily achieved on the transition metals as well as by Ag and Cu, but not on Au. On metals that do not activate CH bonds, the surface oxygen has been found to act as a Lewis base that promotes C–H activation [2,10–14]. In the case of ethylene, it has been proposed that this initiates total combustion. Although there are many fun- damental studies devoted to understand the Ag epoxidation system, questions like why silver is the unique catalyst, what is the reaction mechanism for high EO selectivity, and what is the role of the Cl promoter are still in debate. There is a vast number of experimental [15–17] and theoretical [18–20] studies focusing mainly on model systems where silver is essentially metallic under low oxygen pressure regimes. These studies reported that selective (k 1 ) and non-selective (k 2 ) parallel reactions proceed through the oxametallacycle (OMC) intermedi- ate [18–25]. OMC forms on the catalyst surface through a well- defined Langmuir–Hinshelwood (L–H) mechanism in a reaction of surface adsorbed atomic oxygen (O s ) with C 2 H 4(ads) . The product selectivity depends on the relative barriers of the product forma- tion (E a EO and E a AA ) through the decomposition of the OMC interme- diate. Studies on the metallic silver surfaces have reported activation barriers for EO and AA formation, which are inconsistent with the high EO selectivity observed in the industrial process. However, the reported non-distinct values of E a EO and E a AA would ex- plain the 50% EO selectivity of the metallic silver. So far, silver is the only efficient catalyst for ethylene epoxida- tion. Other transition metal oxidation catalysts give complete combustion [26,27], because of ease of the CH activation. Unlike silver, the literature on the potential of the other metals for ethylene epoxidation is rather scarce. Two computational studies carried out for Cu(1 1 1) [25] and Au(1 1 1) [23] surfaces with 25% O-coverage reported that the epoxidation reaction proceeds through the OMC intermediate as in the case of metallic silver sur- faces. For the Au(1 1 1) surface, it was concluded that differences of the activation barriers on Au and Ag are comparable. The main dif- ference between these metallic surfaces relates to the ease of O 2 dissociation. On the other hand, the Cu(1 1 1) surface showed a lower activation barrier toward EO formation than that of AA. 0021-9517/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jcat.2011.08.004 ⇑ Corresponding author at: Chemical Engineering and Chemistry Department, Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands. E-mail addresses: m.o.ozbek@tue.nl (M.O. Ozbek), r.a.v.santen@tue.nl (R.A. van Santen). Journal of Catalysis 284 (2011) 230–235 Contents lists available at SciVerse ScienceDirect Journal of Catalysis journal homepage: www.elsevier.com/locate/jcat