Low-temperature destruction of chlorinated hydrocarbons over lanthanide oxide based catalysts.

Abstract

Chlorinated hydrocarbons (CHCs) have widespread industrial applications as, for example, lubricants, cleaning solvents, heat-transfer fluids, and intermediates of pharmaceuticals, herbicides, and fungicides.[1] The increasing volume of CHCs released into the environment, together with the suspected toxicity and carcinogenic properties, have prompted researchers world-wide to find clean and effective routes for destroying CHCs. The method currently used most frequently is thermal incineration at temperatures higher than 1300 8C, so as to avoid formation of dioxins and polychlorinated biphenyls (PCBs).[2] The high incineration temperatures and related costs forced researchers to look for other solutions. As a result, a number of methods have been developed for the degradation of CHCs. These include for example, sonolysis,[3] electrochemical dehalogenation,[4] microbial systems,[5] destructive adsorption,[6] and catalytic transformation over supported metals, noble metals, and transition metal oxides.[7±10] Two main catalytic routes are reported in the literature. The first one is the (total) oxidation of CHCs at temperatures between 300 and 550 8C over supported noble metal catalysts (for example, Pt, Pd, and Au).[7] The essential drawback of this method is the deactivation of the catalyst by the decomposition products, such as Cl2 and HCl. This problem can be (partially) solved by using supported transition metal oxide catalysts (for example, V2O5 and Cr2O3), although the formation of volatile metal oxychlorides can still be a problem. Another possibility to avoid catalyst deactivation is the use of steam.[9] A second catalytic route is hydrodechlorination, in which CHCs are transformed in the presence of hydrogen into alkanes and HCl. Commonly used catalysts are supported Ni, Pd, and Pt.[8,10] Although this method has clear economic and environmental advantages, including the re-use of reaction products and the elimination of hazardous by-products (for example, Cl2 and COCl2), it is not very often used. The main reason is the very fast deactivation of the catalyst material as a result of chlorine poisoning and coke formation. Here, we report on a new and stable catalyst material, which destroys different CHCs, such as CCl4, in the presence of steam at temperatures between 250 and 350 8C. This destruction process can be written as CCl4 þ 2H2O!4HCl þ CO2, and no other products than HCl and CO2 are found in the effluent gas and condensate. The reaction is also not equilibrium limited. The catalyst itself is based on the use of lanthanide oxides, such as La2O3, Nd2O3, Pr2O3, and Ce2O3, which are supported on a high surface area support, preferably Al2O3. Figure 1 illustrates the catalytic performance of