Theoretical Study of the Stability of Carbene Intermediates Formed During the Hydrodechlorination Reaction of the CF x Cl4−x Family on the Pd(110) Surface

Luis Antonio M. M. Barbosa 0 1 Fabio H. Ribeiro 0 1 Gabor A. Somorjai 0 1 0 L. A. M. M. Barbosa F. H. Ribeiro (&) G. A. Somorjai School of Chemical Engineering, Purdue University , West Lafayette, IN 47907-2100, USA 1 L. A. M. M. Barbosa (&) F. H. Ribeiro G. A. Somorjai Schuit Institute of Catalysis, Eindhoven University of Technology , P.O. Box 513, 5600 MB Eindhoven, The Netherlands In the present work the stability of the species CCl2, CFCl, CF2 and CHF, which are produced during the hydrodechlorination reaction of the CFxCl4-x family, have been investigated on the Pd(110) surface by applying ab initio periodic Density Functional Theory. The most stable configuration for these carbenes on this surface is the short-bridge. Hollow positions have not been found as stationary points in most of the cases. For the chlorinated fragments, the optimisation of these hollow positions resulted in partial or full dechlorinated fragments. The most stable configuration for the carbenes (short-bridge) was compared to the least stable one (top) within different surface conditions in order to verify any change in this stability trend. Both geometries are equally affected by the surface modifications for most of the carbenes. The shortbridge is, however, more sensitive to the coverage increase in the CHF case. CHF has the strongest binding energy to the Pd(110) surface, whilst CF2 has the least one. The stability trend of CHF, CFCl and CF2 helped to better understand the selectivity of the hydrodechlorination reaction of the mono carbon CFC's, for example, the suggestion that CF2 is the most important intermediate on the hydrodechlorination of CF2Cl2 was confirmed by the calculations. 1 Introduction Chlorohydrocarbons and chlorofluorohydrocarbons are related to ozone layer destruction and groundwater contamination. Not surprisingly the handling and destruction of these molecules have become an important environmental issue in the past few years. The search of catalysts, which are able to dissociate the carbonchlorine bond, is strongly desired and necessary. In addition, the understanding of the structure and reactivity of the intermediates from the dissociation of the chlorofluorocomponds should help to increase the activity and selectivity of the catalyst for such transformation. The cleavage of the CCl bond has been studied by using different metal catalysts. The pure metals Pt, Pd, Cu and alloys combining Pt and Pd with Cu have been suggested to be excellent catalysts for the dechlorination reaction [133]. Within these studies the molecular size of the linear chlorocarbons was also well explored:C1 [5, 20, 24, 28, 30, 33], C2 [7, 21, 2527, 34, 31] and C3 [7]. It is well accepted in the literature that the chlorine atom leaves the molecule more easily than fluorine atom. The CCl bond dissociation becomes also more facile with the increase of the number of chlorine atoms in the molecule, being easier for the CCl2 group than for the CCl one [7, 11, 13, 21, 22, 25, 27]. Regarding the selectivity of the hydrodechlorination reaction on Pd it is higher for the formation of fully or partially dechlorinated products in the case of C2 fluorochlorocarbons [20, 21, 31, 35, 36] and for fully dehalogenated in the case C1 fluorochlorocarbons [20, 24, 30, 3739]. Fully dehalogenated or partially dechlorinated molecules are not desirable, because the target is to substitute chlorine atoms of the CFCs molecules by hydrogen atoms. The reaction selectivity is insensitive to the structure of the Pd surface, as demonstrated by Ribeiro et al. [36]. It seems, however, to be affected by dilution of Pd atoms on the catalyst surface after the introduction of an additional metal (Au, Pt or Ni) [24, 30, 40]. The CF2 carbene is considered the most important intermediary of the hydrodechlorination of CF2Cl2 on Pd catalysts [20, 24, 32, 3739]. It seems to be the key for this reaction selectivity. This reaction produces CF2H2 (83%) and methane (17%) [41]. However the selectivity of this reaction on Pd catalysts can be modified by the presence of chlorine on the catalyst [39]. The same authors also showed that F coadsorbed atoms were present in used catalysts but this atom did not influence the reaction kinetics. The hydrodechlorination of another CFC molecule (CFCl3) produces CFH3 and methane. The selectivity towards methane is almost twice higher than the one observed for the parent (CF2Cl2) compound [41]. It is clear that the key for this selectivity resides on intermeriaries produced during the dehalogenation reaction of this CFC: CFCl or CCl2. There are still some open questions regarding the selectivity of the hydrodechlorination reaction of the CFC molecules and certainly they can be answered by understanding the reactivity of the reaction intermediaries. In order to obtain more insights of the hydrodechlorination reaction the stability of CHF, CF2, CFCl and CCl2 carbenes on the Pd(110) surface have been investigated by means of periodical quantum chemical calculations. The analysis, at a molecular level, of the changes in the stability of these species upon different surface coverage offers an opportunity to confirm and to explain some of the current suggestions for the dechlorination mechanism on Pd catalysts. 2 Methods All geometry optimizations have been performed using the Vienna Ab-initio Simulation Package (VASP) [42, 43]. This code carries out periodic Density Functional calculations (DFT) using pseudopotentials and a plane wave basis set. The DFT was parameterized in the local-density approximation (LDA), with the exchange-correlation functional proposed by Perdew and Zunger [44] and corrected for nonlocality in the generalized gradient approximations (GGA) using the Perdew-Wang 91 functional [45]. The interaction between the core and electrons is described using the ultrasoft pseudopotentials introduced by Vanderbilt [46] and provided by Kresse and Hafner [47]. The Pd surface is modeled by a periodic five layer-slab with the carbene fragment adsorbed on one side of the slab. One slab is separated from its periodic image in the z direction by a vacuum space, which is equivalent to ten metallic layers. Each metallic layer is composed by 9 Pd atoms (3 9 3 structure). The two bottom layers have been maintained frozen at their bulk distances in all optimisations. In order to minimize the effect of stress that occurs due to the constraints in the slab model, the optimal bulk metalmetal distance was calculated. The calculated lattice parameter of 3.97A agrees well with experimental one of 3.92A [48]. In the slab model, these species are ordered over the bare surface in the following structure: (3 9 3) 1/9 ML. For some systems, the local coverage was higher than 1/9 ML due to the presence of extra adsorbed atoms (Cl, F or H). These systems have been also optimised with the same original unit-cell. The Brillouin-zone integrations have been performed on 3 9 2 9 1 Monkhorst-Pack grid of k- (...truncated)