The energies of formation and mobilities of Cu surface species on Cu and ZnO in methanol and water gas shift atmospheres studied by DFT Dominik Bjørn Rasmussen a , Ton V.W. Janssens a , Burcin Temel a,⇑ , Thomas Bligaard b,c , Berit Hinnemann a , Stig Helveg a , Jens Sehested a a Haldor Topsøe A/S, Nymøllevej 55, DK-2800 Kgs. Lyngby, Denmark b Center for Atomic-Scale Materials Design, Department of Physics, Technical University of Denmark, DK-2800 Lyngby, Denmark c SUNCAT Center for Interface Science and Catalysis, SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA article info Article history: Received 31 January 2012 Revised 10 May 2012 Accepted 3 July 2012 Available online 3 August 2012 Keywords: Methanol synthesis Water–gas shift Cu/ZnO catalysts Sintering Adsorbate–metal complex Density functional theory calculations Coalescence Ostwald ripening abstract Catalysts based on copper, such as the Cu/ZnO/Al 2 O 3 system are widely used for industrial scale methanol synthesis and the low temperature water gas shift reaction. A common characteristic of these catalysts is that they deactivate quite rapidly during operation and therefore understanding their deactivation by sintering is highly relevant. In this work, we study the nature of the species that are responsible for trans- port of the Cu metal in this catalyst type using density functional theory calculations within a chemical potential formalism. The stability and mobility of Cu–X (Cu, OH, CO, CH 3 O, HCOO) species are investi- gated in relevant synthesis gas compositions. The CuCO and Cu 2 HCOO species are identified to be pre- dominant for metal transport on Cu particles, which may contribute to sintering of Cu by particle migration and coalescence. Furthermore, transport of Cu on ZnO is found mostly to occur through CuCO species, which indicates that CuCO is an important species for Ostwald ripening in a Cu/ZnO catalyst. These results provide atomistic perspective on the diffusion of the species that may contribute to catalyst sintering, therefore lending a valuable foundation for future investigations of the stability of Cu catalysts. Ó 2012 Elsevier Inc. All rights reserved. 1. Introduction Methanol (MeOH) is one of the top 10 chemicals with a world pro- duction of 45 million metric tons in 2010 [1]. Most of the methanol is used to produce other chemicals, with formaldehyde and methyl tert-butyl ether (MTBE) being the most prominent ones. Methanol has been increasingly important for fuel applications, both as an alternative fuel to gasoline [1], and as a feedstock for the methanol- to-gasoline process providing an alternative route to synthetic fuel. The common production process for methanol is based on syn- thesis gas, consisting of mixtures of H 2 , CO, and CO 2 . In many parts of the world, the synthesis gas is produced from natural gas by steam reforming of methane followed by the water gas shift (WGS) reaction. The heterogeneously catalyzed production of methanol from synthesis gas, using ZnO stabilized by Cr 2 O 3 as a catalyst, was described for the first time in 1921 by Patart [2,3] and the process was commercialized by BASF in 1923 [4]. In the 1960s, the Cr 2 O 3 /ZnO catalysts were replaced by more active Cu- based catalysts including ZnO and Al 2 O 3 . Today, the Cu/ZnO/ Al 2 O 3 catalysts are commonly used for MeOH synthesis; these cat- alysts also show a good activity for low temperature WGS. Previously, much attention has been devoted to establish detailed structure–activity relationships. Particularly, the nature of the metal– support synergy in the Cu/ZnO system has been much disputed [5–12] and the debate reflects several effects depending on the reaction conditions, preparation procedures, and complex catalyst nanostructures [13]. Another issue of major technical concern is the stability of the Cu/ZnO/Al 2 O 3 -based catalysts since approximately 1/3 of the initial activity is lost [14] during the first 1000 h of opera- tion. The deactivation has mainly been attributed to the sintering of the Cu nanoparticles [15–17]. Sintering is a process depending on me- tal–support interaction, surface wetting, gas composition, and surface defects. However, good models that describe sintering of Cu in such catalysts are not presently available in part due to the complexity of the Cu/ZnO/Al 2 O 3 system. According to Tammann’s rule [18], thermal sintering of Cu crystallites should not occur for temperatures below 680 K, which is well above the temperature commonly applied in industrial methanol synthesis (470–570 K). Consequently, under methanol synthesis conditions, one would expect sintering of Cu oc- curs via another mechanism. Such a sintering mechanism is mediated by molecular complexes formed by adsorbates and Cu, under the temperatures and pressures applied in the methanol synthesis. The adsorbate-mediated sintering is illustrated experimentally by a strong dependence of the deactivation rate of methanol syn- thesis catalysts on the composition of the synthesis gas. Sun 0021-9517/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcat.2012.07.001 ⇑ Corresponding author. E-mail address: bute@topsoe.dk (B. Temel). Journal of Catalysis 293 (2012) 205–214 Contents lists available at SciVerse ScienceDirect Journal of Catalysis journal homepage: www.elsevier.com/locate/jcat