Low-Temperature Destruction of Carbon Tetrachloride over Lanthanide Oxide-Based Catalysts: From Destructive Adsorption to a Catalytic Reaction Cycle

Graphical Abstract

Catalytic destruction of CCl₄ in the presence of steam was investigated at 200–350 °C over a series of lanthanide (La, Ce, Pr, and Nd) and alkaline-earth metal (Mg, Ca, Sr, and Ba) oxide-based catalysts by kinetic experiments, various spectroscopic techniques and DFT calculations. For example, approach of a CCl₄ molecule to the surface of La₂O₃ results in the elongation of a CCl bond, and the Cl gets a partial negative charge, while the hydrocarbon fragment becomes partially positively charged (see picture).

Abstract

The catalytic destruction of carbon tetrachloride in the presence of steam, CCl₄ + 2 H₂O→4 HCl + CO₂, was investigated at 200–350 °C over a series of lanthanide (La, Ce, Pr and Nd) and alkaline-earth metal (Mg, Ca, Sr and Ba) oxide-based catalysts with kinetic experiments, Raman spectroscopy, X-ray photoelectron spectroscopy, IR spectroscopy, X-ray diffraction, and DFT calculations. This new catalytic reaction was achieved by combining destructive adsorption of CCl₄ on a basic oxide surface and concurrent dechlorination of the resulting partially chlorinated solid by steam. The combination of the two noncatalytic reactions into a catalytic cycle provided a rare opportunity in heterogeneous catalysis for studying the nature and extent of surface participation in the overall reaction chemistry. The reaction is proposed to proceed over a terminal lattice oxygen site with stepwise donation of chlorine atoms from the hydrocarbon to the surface and formation of the gas-phase intermediate COCl₂, which is readily readsorbed at the catalyst surface to form CO₂. In a second step, the active catalyst surface is regenerated by steam with formation of gas-phase HCl. Depending on the reaction conditions, the catalytic material was found to transform dynamically from the metal oxide state to the metal oxide chloride or metal chloride state due to the bulk diffusion of oxygen and chlorine atoms. A catalyst obtained from a 10 wt % La₂O₃/Al₂O₃ precursor exhibited the highest destruction rate: 0.289 g CCl₄ h⁻¹ g⁻¹ catalyst at 350 °C, which is higher than that of any other reported catalyst system.