Catalytic Dechlorination of Volatile Organic Carbons under Redox Conditions

Abstract

Chlorinated hydrocarbons, such as perchloroethylene (PCE) and trichloroethylene (TCE), are persistent environmental hazards, due to improper disposal, affecting groundwater sources located near a variety of industrial processes. As many chlorinated hydrocarbons are suspected carcinogens, there is great interest in developing inexpensive and environmentally sound technologies for the remediation of contaminated sites. Current efforts focus on the use of soil-vapor extraction (SVE) to pass gas phase contaminants through a granular activated carbon bed (GAC), which creates solid toxic waste, and possibly more harmful by-products during GAC regeneration. This research focuses on the use of hydrogen and short-chain alkanes, in combination with oxygen, to promote the conversion of PCE over a Pt/Rh three-way catalyst. The use of both of a hydrocarbon and oxygen creates mixed reducing-oxidizing (redox) conditions. Results indicate that redox conditions result in the complete removal of the target compound and produces primarily CO₂, H₂O and HCl. The process has proven to be most effective near stoichiometric conditions with respect to the reducing and oxidizing agents (2:1 for H₂:O₂ and 1:5 for propane:O₂). Residence times in the reactor are typically on the order of 0.1 to 0.5 seconds and catalyst surface temperatures range from 200 °C to 550 °C, with PCE conversion greater than 99% starting at 450 °C under slightly reducing (H₂:O₂ > 2) conditions. Laboratory results suggest that the catalytic mechanism is a multiple step surface reaction involving the three reactants (H₂/C₃H₈, O₂ and PCE). A mechanism based on Langmuir- Hinshelwood kinetics has been developed in an attempt to model the process. The role of the cerium oxide present in a three-way catalyst on the direct oxidation of perchloroethylene (PCE) has also been explored. Experiments have shown that in the absence of an external oxidizing agent, PCE can be converted over an alumina supported Pt/Rh catalyst. This work hypothesizes that the chlorine atoms in the adsorbed PCE interact with oxygen in oxidized cerium, CeO₂, reducing the cerium and replacing the oxygen atoms to create CeCl₃. This process begins at a catalyst surface temperature of 300 °C and reaches 100% conversion at 400 °C.