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10.2478/v10063-010-0001-7 ANNALES UNIVERSITATIS MARIAE CURIE-SKLODOWSKA LUBLIN POLONIA VOL. LXV, 1 SECTIO AA 2010 M. Bonarowska1, A. r bowata1 and Z. Karpi ski1,2,3 Institute of Physical Chemistry of PAS, ul. Kasprzaka 44/52, 01-224 Warszawa, Poland 2 Faculty of Mathematics and Natural Sciences School of Science, Cardinal Stefan Wyszy ski University, ul. Wóycickiego 1/3, 01-938 Warszawa, Poland 3 Corresponding author, tel. +48-22-343-336; fax: +48-22-343-33-33; e-mail: firstname.lastname@example.org The search for the most suitable hydrodechlorination s should consider both the C-Cl bond strength in a molecule subjected to reaction and the metal-chlorine bond, which should be neither too strong nor too weak. An improvement of Pd- and Pt-based s can be achieved by alloying with metals which bind chlorine even less strongly, e.g. with gold. Addition of platinum to palladium would also be beneficial because of metal-chloride bond energy considerations. Analogous effects occur in the hydrodechlorination of dichlorodifluoromethane and 1,2-dichloroethane, the molecules characterized by stronger carbon-chlorine bonds. 1. INTRODUCTION Recent interest in search for effective s for chlorine removal from harmful organic compounds resulted in a series of papers published in the period of last fifteen years. Catalytic hydrodechlorination (HdCl) offers an exceptional advantage over all oxidative (noncatalytic and catalytic) methods: the carbon skeleton of a chlorine-containing organic molecule is not irreversibly lost (i.e. This article is dedicated to Professor Tadeusz Borowiecki on the occasion of his th 65 birthday not converted to CO2), but several useful and less harmful products could be achieved in effect of catalytic transformation. In this paper we would like to review our results on hydrodechlorination of dichlorodifluoromethane, tetrachloromethane and 1,2-dichloroethane carried out over a number of platinum and palladium-containing systems. The main aim of this presentation is to show how the Sabatier principle  works for hydrodechlorination reactions and hydrodechlorination s. It is well known that the Sabatier principle, a qualitative concept in catalysis, states that the interactions between the and the reactant should be neither too strong nor too weak. If the interaction is too weak, the reactant fails to bind to the and no reaction takes place. On the other hand, if the interaction is too strong, the gets blocked by reactant or product. It is commonly accepted that the rate determining step of a large variety of hydrodechlorination reactions is the splitting of the first C-Cl bond  of a reacting molecule. Such bond splitting is easier when a binding of chlorine atom and metal surface is stronger. However, a metal-chlorine bond that is too strong would lead to blocking a metal surface with chloride species. In this respect the behavior of the three important catalytic metals, platinum, palladium and gold in hydrodechlorination of different chlorine-containing compounds is analyzed. It will be shown that the combination of two metals results in better catalytic performance, i.e. indicating a number of synergistic effects. 2. MATERIAL AND METHODS Preparation of silica-, alumina- and carbon-supported Pt, Pd, Pt-Pd, Pt-Au and Pd-Au was described in our previous publications [3-8]. In short, aqueous solutions of chloride-containing metal compounds (PdCl2, H2PtCl6 and NH4AuCl4) were used in incipient wetness (co)-impregnations. Some bimetallic s were prepared by direct redox reaction (Pd-Au , Pt-Pd ). Final pretreatment included reduction in H2/Ar flow, usually at 400oC for 3 h. Metal dispersion was assessed from CO or H2 chemisorption, verified by XRD, and, occasionally, by TEM. Palladium containing s were also investigated in the temperature-programmed (palladium) hydride decomposition [4,5]. Such experiments (combined with XRD data) allow us to estimate the degree of metal alloying in supported bimetallic s. The catalytic conversion of dichlorodifluoromethane, carbon tetrachloride and 1,2-dichloroethane was investigated using a glass flow reaction system [4-8]. After reduction, the s were cooled to the highest reaction temperature (for CCl2F2 180oC, CCl4 90oC and 1,2-C2H4Cl2 230oC) and contacted with the reaction mixture, i.e. with a flow of hydrogen + argon and selected chlorinecontaining compound provided from a saturator kept at 0oC (CCl4 and 1,2-C2H4Cl2) or supplied from a tank (CCl2F2). The mass of the used ranged between 0.1 and 0.4 g, depending on the reaction, in order to not exceed conversion levels beyond 10% (at steady state, for freshly reduced samples). In all kinetic runs, the activities of most s declined with time-on-stream. A typical run lasted ca. 24 h. 3. RESULTS AND DISCUSSION A number of previous results showed that palladium is a better than platinum for hydrodechlorination of dichlorodifluoromethane . On the other hand, platinum seems superior in hydrodechlorination of carbon tetrachloride [5,8], whereas gold is only slightly active in both hydrodechlorinations [4,6,7]. Simple consideration of the energies of C-Cl bond in three tested chlorinecontaining molecules (Table 1) leads to the following conclusions. First, the molecule of CCl4 should be more easily deprived of chlorine because the Cl-C bond energy is the lowest. Therefore, it is no surprise that such a respective catalytic reaction takes place at the temperature roughly 100oC lower than for analogous processes for the other chlorine-containing molecules (CCl2F2 and 1,2-C2H4Cl2) [4-8]. Second, the fact that Pt is better than Pd in catalytic hydrodechlorination of CCl4, whereas the opposite is true for hydrodechlorination of CCl2F2 and 1,2-C2H4Cl2 suggests that a weaker metal-Cl bond is less effective for dissociating Cl-C bond in these compounds. Gold is even less adequate for this process, so activation of Cl-C bond is difficult with this metal. The qualitative situation is presented in Figure 1. Tab. 1. Dissociation energies of ClC bonds in reactants used in this work. Investigated compound CCl4 CCl2F2 1,2-C2H4Cl2 Considered C-Cl bond Cl-CCl3 Cl-CClF2 Cl-CH2CH2Cl Cl-C bond dissociation energy, kJ/mol 305.9±7.5 346.0±13.4 348.1±9.6 Literature source Ref.  Ref.  Ref.  Reaction rate, a.u. CCl2F2 CCl4 CCl2F2 CCl4 CCl4 CCl2F2 Au-Cl Pt-Cl Pd-Cl Metal-Cl bond energy Fig. 1. Suggested volcano-shaped relations between the hydrodechlorination activity of Au, Pt and Pd and metal-chloride bond. For explanation, see text. Figure 1 shows three volcano-shaped curves for Pd, Pt and Au, and suggested qualitative locations for hydrodechlorination of CCl4 and CCl2F2. As mentioned, HdCl of CCl2F2 requires higher energy for splitting the first Cl-C bond than in the case of CCl4, so platinum may not be too active in such a splitting, however for the analogous process with CCl4, this metal would be quite efficient. On the other hand, palladium which binds stronger chlorine atoms than platinum [12,13] should be more active in splitting a stronger Cl-C bond in the molecule of CCl2F2 (and that of 1,2-C2H4Cl2). However, in this case and even more drastically in HdCl of CCl4 hydrodechlorination, strongly-bound chloride species should block active sites of this metal. Therefore, the HdCl activity of palladium is situated on the decreasing branch of the volcano curve, irrespective of the reacted molecule. Gold binds chloride species much weaker than Pd and Pt, therefore its ability to hydrodechlorinate is poor. The situation depicted in a qualitative fashion in Figure 1 suggests that it is possible to search for synergistic effects in HdCl of three investigated Cl-containing compounds: CCl4, CCl2F2 and 1,2-C2H4Cl2. It is feasible that alloying with another element should either increase or decrease the metal-Cl bonding. For example, alloying Pt with Pd should be beneficial for HdCl of CCl2F2 and 1,2-C2Cl2H4. Similarly, alloying Pt with Au should decrease the metal-Cl bond energy, creating a situation which is more favorable for HdCl of CCl4. In both cases, the catalytic behavior of Pt should approach an expected maximum activity. Similar changes are anticipated for alloying palladium with gold. Figures 24 show that, indeed, the Sabatier's principle works well in such cases. Since the Cl-C bond energies in 1,2-dichloroethane and dichlorodifluoromethane are comparable (Table 1), the effect of adding some gold or platinum to palladium should be positive. Palladium itself, which is the best catalytic metal in HdCl of both organic compounds, is still "too strong" in binding chloride species. Therefore, an addition of less active metal, such as platinum or gold is very helpful. Figures 2 and 3 demonstrate the synergistic effects for the differently supported Pd-Au and Pd-Pt s. 3.0x10 1,2-C2H4Cl2 CCl2F2 Reaction rate, mol s gmetal 2.0x10 0.08 0.06 1.0x10 0.0 Pd 50%Au 75%Au Au Pd 20%Au 40%Au 50%Au Fig. 2. Catalytic activity of Pd-Au/SiO2 s in hydrodechlorination of 1,2-dichloroethane at 230oC and Pd-Au/C in hydrodechlorination of dichlorodifluoromethane at 180oC (adopted on r bowata  and Bonarowska et al. , respectively). 0.020 Pd-Pt/SiO2 0.016 Pd-Pt/Al2O3 100%Pd 5%Pt 20%Pt 100%Pt 100%Pd 10%Pt 20%Pt 50%Pt 100%Pt Fig. 3. Synergistic effect in hydrodechlorination of CCl2F2 on supported Pd-Pt/SiO2 s at 180oC (left side silica supported , right side alumina-supported s ). In this respect, application of Pt-Au s in hydrodechlorination of CCl2F2 (and 1,2-C2H4Cl2) does not seem very useful. Further weakening of metal-Cl bond by adding gold reduces the hydrodechlorination capability of platinum. Conversely, the beneficial effect could be achieved from alloying platinum with palladium (Figure 3). However, the catalytic performance of platinum in HdCl of CCl4 is greatly improved by adding gold (Figure 4). 4. CONCLUSIONS The selection of the most suitable HdCl metallic s depends on the strength of the C-Cl bond in a molecule subjected to reaction and on the metalchlorine bond, which should be not too strong and not too weak. A relatively weak C-Cl bond in CCl4 (~306 kJ/mol) does not require a high dechlorination potential, thus Pt is a better than Pd in CCl4 reaction. In addition, an improvement of Pt-based s can be achieved by alloying with metals which bind chlorine even less strongly than Pt (i.e. with Au). In contrast, Pd is a better than Pt for hydrodechlorination of a stronger C-Cl bond (~350 kJ/mol), present in CCl2F2 and 1,2-C2H4Cl2. However, a good performance of Pd can be improved further by alloying it with less active Pt (or Au), as a result of weakening of the metal-chlorine bond. 100%Pt 5%Au 10%Au 20%Au 30%Au 100%Au Fig. 4. Synergistic effect in CCl4 hydrodechlorination on Pt-Au/Al2O3 s at 90oC (based on Legawiec-Jarzyna  and ). Acknowledgments. This work was supported by the Polish Ministry of Science and Higher Education within Research Project N N204 161636. 5.
Annales UMCS, Chemia – de Gruyter
Published: Jan 1, 2010
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