Hydrodechlorination of Diclofenac in an Aqueous Solution over Pd/ZrO2 and Pd/ZrO2SiO2 Catalysts

Pd/ZrO₂ and Pd/ZrO₂SiO₂ catalysts prepared by wet impregnation and reduced with H₂ under mild (30°C, aqueous suspension) or harsh (320°C) conditions are compared in the hydrodechlorination of the microecotoxicant diclofenac (DCF) in an aqueous solution at 30°С. According to the TPR and XPS data, the addition of SiO₂ to the support reduces the degree of metal-support interaction and facilitates the reduction of palladium. Despite the lower Pd⁰ fraction, the Pd/ZrO₂ catalyst is more active in the batch reactor: after reduction at 320°С, it slightly, and after mild reduction, significantly (7 times), exceeds Pd/ZrO₂SiO₂ in catalytic activity. XRD and TEM show a wider size distribution of palladium nanoparticles in the Pd/ZrO₂ sample, while low-temperature N₂ adsorption, XPS, and TPR demonstrated better accessibility of palladium on the Pd/ZrO₂ surface due to the reduced decoration by the support components and increased pore size. These features explain the increased activity of Pd/ZrO₂. Testing in the flow system demonstrated higher DCF conversion in the presence of catalysts reduced at 320°C and higher stability of Pd/ZrO₂SiO₂ compared to Pd/ZrO₂. The stability is ensured by the increased reducibility of Pd²⁺ with H₂ and by the developed surface of Pd/ZrO₂SiO₂, which prevents deactivation under the action of HCl released in hydrodechlorination.