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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. [ABSTRACT FROM AUTHOR]

The effect that introduction of cerium oxide into oxide systems based on chromium and silicon exerts on the catalytic properties of these systems in nonoxidative propane dehydrogenation in a flow-through system with a fixed catalyst bed was studied. The characteristics of the catalysts CrOx–SiO2 and CrOx–CeO2–SiO2, both containing 9 wt % CrOx assuming the Cr2O3 stoichiometry, were compared. The СеО2 content of the ternary system was 52 wt %. The catalysts were characterized by X-ray diffraction analysis, scanning electron microscopy, Raman spectroscopy, electron paramagnetic resonance, X-ray photoelectron spectroscopy, temperature-programmed reduction with hydrogen, and thermal analysis. In the initial period of the reaction, the catalyst containing CeO2 demonstrated higher propane conversion at 500 and 550°С and higher deactivation rate compared to CrOx–SiO2. Both catalysts can be regenerated by treatment in an air stream at 550°С for 30 min. Cerium dioxide favors the reduction of chromium in high oxidation states to form active reaction sites, Cr3+ ions, ensures higher dispersity of the chromium oxides, and favors partial oxidation of coke deposits in the initial period of the catalyst operation. [ABSTRACT FROM AUTHOR]

A comparison is made of CrOx–ZrO2–SiO2 catalysts (9 wt % chromium oxide based on Cr2O3); (Cr + Zr)/Si molar ratio of 0.8) synthesized using different orders of introducing components: (i) the simultaneous precipitation of all components, (ii) the deposition of CrOx on ZrO2–SiO2 via impregnation, and (iii) the co-precipitation of CrOx and ZrO2 on SiO2. The SiO2 precursors are TEOS in methods (i) and (ii), and SiO2 produced by calcination of rice husk in (iii). The catalysts are tested in the nonoxidative dehydrogenation of propane in a flow system with a fixed catalyst bed at 500–600°С. The co-precipitation of CrOx and ZrO2 ensures high efficiency of the catalysts. At 500 and 550°C, the most efficient catalyst is CrZr/SiO2 synthesized by depositing CrOx and ZrO2 on SiO2; at 600°C, the best on-stream behavior is exhibited by CrZrSi catalyst synthesized via the simultaneous precipitation of all components. SEM/EDX, XRD, H2-TPR, and Raman spectroscopy are used to show that in the catalysts synthesized via the co-precipitation of CrOx and ZrO2, these components (which form active sites) are uniformly distributed, have close contact, and are adequately dispersed, while Cr6+ is readily reduced to Cr3+ by the hydrogen contained in the reaction medium. [ABSTRACT FROM AUTHOR]

Catalysts of the general composition CrOx–ZrO2–SiO2 were prepared by two procedures: (1) one-step precipitation of all the components and (2) introduction of CrOx by impregnation of the ZrO2–SiO2 support. The CrOx content was varied from 4 to 9 wt % in terms of the Cr2O3 stoichiometry. The catalysts were characterized by X-ray diffraction analysis, Raman spectroscopy, X-ray photoelectron spectroscopy, temperature-programmed reduction with hydrogen, thermal analysis, and electron microscopy. The catalytic activity of the catalysts in nonoxidative propane dehydrogenation in a flow-through system with a fixed catalyst bed was compared. The one-step synthesis method ensures uniform distribution of chromium oxides in the catalyst. The presence of the silica precursor in the course of precipitation inhibits the Cr2O3 and ZrO2 crystallization and ensures close contact of coordination-unsaturated Zr4+ sites with Cr3+ sites on the surface; therefore, the catalysts prepared by the one-step method are more active. The low activity of the catalysts prepared by impregnation is caused by the formation of coarse Cr2O3 particles heterogeneously distributed on the surface. In the course of nonoxidative propane hydrogenation, the surface of the catalysts undergoes coking, which leads to a decrease in their activity. Short treatment in air at 550°С virtually fully restores the activity of the catalysts prepared by the one-step method. [ABSTRACT FROM AUTHOR]