Regeneration of Pd/TiO2 catalyst deactivated in reductive CCl4 transformations by the treatment with supercritical CO2, ozone in supercritical CO2 or oxygen plasma

Carbonaceous deposits formation was established as the primary reason of Pd/TiO2 catalyst deactivation during reductive processing of CCl4 to form hydrodechlorination and oligomerization products. Three methods of carbonaceous deposits elimination were tested: (1) extraction by supercritical CO2, (2) oxidation by ozone in supercritical CO2, and (3) low-temperature glow-discharge oxygen plasma treatment. Synchronic thermal analysis confirms effective carbonaceous deposits removal during regeneration by ozone or low temperature glow-discharge oxygen plasma; by XPS deep oxidation of surface Pd after oxidative treatment (by ozone or oxygen plasma) was found. Thus H2 reduction was proposed as the second step making possible full regeneration of initial catalytic activity of Pd/TiO2.

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► Carbonaceous deposits formation is the main reason of Pd/TiO2 deactivation during reductive transformations of CCl4 in H2 atmosphere, as it was found by STA and extraction.
► Efficiency of the oxidative regeneration methods decreases as follows: treatment by O3 in sc-CO2 > glow-discharge O2 plasma treatment ≥ extraction by sc-CO2.
► Ozone treatment causes destruction of condensed organic compounds to fragments, which are better extracted by sc-CO2.
► Deep oxidation of Pd during oxidative treatments (ozone + sc-CO2 or O2 plasma) was found by XPS. Hence to restore original catalytic activity oxidative stage of regeneration should be followed by reduction of active component.