Enhanced activity of carbon-supported Pd–Pt catalysts in the hydrodechlorination of dichloromethane
Graphical abstract
Introduction
Air pollution has become a problem of growing interest and consequently stringent regulations have been developed for the emission of pollutants to the atmosphere. Dichloromethane (DCM) is a chlorinated volatile organic compound with high environmental impact, due to its toxicity and carcinogenic character and its contribution to the depletion of the ozone layer, the global warming and the formation of photochemical smog [1], [2], [3], [4]. Despite these harmful effects, its use in chemical and pharmaceutical industry is still widely extended (e.g., in paint strippers, in the manufacture of film coatings, in metal cleaning and finishing processes for the electronics industry, as a blowing agent in the manufacture of polyurethane foams, in bitumen testing, in the preparation of drugs, pharmaceuticals, decaffeinated teas and coffees, and hops for the brewing industry or as a post-harvest fumigant for grains and fruits) [5]. Its particular physical and chemical properties (high stability, low flammability, high volatility and high solvent capacity) make difficult its substitution in a number of processes. Hence, the development of suitable technologies for the removal of this toxic and similar contaminants in off-gas streams is necessary. Among these technologies catalytic hydrodechlorination (HDC) has a promising potential since it can operate under relatively mild conditions, transforming the organochlorinated compounds like DCM into less toxic products and it is effective within a wide range of pollutant concentrations allowing to control the reaction products [6], [7], [8]. Recently it has been also explored by our research group as an alternative technology for the production of valuable hydrocarbons with promising results [9]. On the opposite, thermal treatments-like incineration and catalytic combustion, which are the main current technologies for the removal of these pollutants, may lead to strongly hazardous byproducts such us dioxins, chloro-furans and phosgene [10], [11], [12].
The literature on HDC shows a growing number of studies where a wide diversity of catalysts are employed. The most commonly used are those based on noble metals [13], [14], [15] and among them, Pd and Pt catalysts have demonstrated high HDC activity, being highly selective to non-chlorinated products, specially the Pd ones due to the capacity of this metal to dissociate de H2 molecule, favouring the C–Cl hydrogenolysis [16], [17], [18]. In order to improve the efficiency and/or minimize the cost, those metals are commonly supported on a porous material. The interaction between the active phase and the support may affect the reaction by increasing the surface area, diminishing the metal sintering and/or improving the thermal and chemical stability of the catalyst, thus playing an important role in the catalytic activity. Activated carbon (AC) is frequently used as catalyst support due to its thermal stability and mechanical resistance, its high porosity and specific surface, which confers it a high adsorption capacity, and its good chemical resistance (except at high temperatures under oxidant atmosphere), as well as its relatively low price [19], [20], [21], [22].
In the last years, in order to improve their properties, an increasing number of catalysts whose active phases include more than one metal are being used for the sake of combining their respective abilities with better results in terms of behaviour and economy [23], [24], [25], [26], [27]. One of the main advantages observed in these bimetallic systems is an improved dispersion of the active phase [23], [28], [29]. Besides, the literature reports studies where bimetallic catalysts show higher activity than the homologous monometallics [30], [31] although there is a lack in understanding the real structure and morphology of these bimetallic materials, and the nature of the synergistic effects observed are not always comprehensively explained since the characteristics of those bimetallic systems are still not completely understood [32].
In previous studies [16], [17], [33], [34], [35], [36] the behaviour of four metallic phases (Pd, Pt, Rh and Ru) supported on activated carbon was analyzed in the gas-phase HDC of chloromethanes, finding significant differences in activity, selectivity and stability. Both the palladium and platinum catalysts (namely Pd/AC and Pt/AC) were highly selective to non-chlorinated products, especially Pd/AC. However, while Pd/AC showed a higher initial activity, it suffered an important deactivation upon time on stream [33], [34], whereas Pt/AC did not show signs of deactivation upon 26 days on stream [17], [33], [34], [37]. The aim of this work is to analyze the catalytic activity of Pd–Pt bimetallic catalysts supported on activated carbon in the HDC of DCM, in comparison with the behaviour shown by homologous monometallics. Even supported in materials different than activated carbon, this Pd–Pt combination has already been reported in the scientific literature for the hydrodechlorination of chlorinated compounds (CFC-12 [38], [39], [40], carbon tetrachloride [38] and 1,2-dichloroethane [38], [41]), finding it a promising catalyst.
Section snippets
Catalysts preparation
Two monometallic (Pd or Pt) and three bimetallic (Pd and Pt) catalysts supported on a commercial AC supplied by Merck (SBET ≈750 m2 g−1; Vmicro ≈0.30 cc g−1; bulk density ≈0.5 g cm−3; particle size: 0.25–0.50 mm) were prepared by co-impregnation, using aqueous solutions of PdCl2 and/or H2PtCl6 (Sigma–Aldrich) of the required concentrations to get catalysts with 0 to 1 wt.% Pd loading, balanced with Pt to obtain the same overall atomic concentration of active phase in all of them (86 μmol/g). The
Characterization of the catalysts
Table 2 summarizes the results of bulk metal content (by ICP-MS), porous structure (SBET and Vmicro), metal dispersion and metallic particle size (by TEM) of the catalysts prepared. The ICP-MS analysis confirm that the measured Pd and Pt contents lie close to the nominal values calculated from the amounts of the precursors used in each case. The 77 K N2 adsorption–desorption isotherms (not shown) approach Type 1 of the DBBT classification [48], [49], characteristic of microporous solids, as
Conclusions
The use of bimetallic catalysts of Pd and Pt in the gas-phase HDC of DCM results in a synergistic effect respect the catalytic activity when compared to the monometallic ones, that can be ascribed to a significant increase in metal dispersion, leading to metallic particles of sub-nanometre size (1.1, 0.7 and 0.6 nm for Pd–Pt (4:1), Pd–Pt (1:1) and Pd–Pt (1:3), respectively). These bimetallic systems transform DCM to non-chlorinated products more efficiently than the analogous monometallic Pd and
Acknowledgements
The authors gratefully acknowledge financial support from the Spanish Ministerio de Economía y Competitividad through the project CTM2011-28352. M. Martín Martínez acknowledges the Spanish Ministerio de Ciencia e Innovación and the European Social Fund for her research grant (BES 2009-016802).
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