Abstract
In this work, the synthesis of a stable supported Pd catalyst by aggregation of colloidal particles (CPs) of polyacrylonitrile (PAN) containing the noble metal is discussed. The metal is initially incorporated during the polymerization reaction (0.5 wt. %) and results to be well embedded inside the polymeric structure and anchored to the nitrogen atom of acrylonitrile. Moreover, aiming to provide free access towards the catalyst active sites, a KOH-activated heat treatment was performed on the PAN CP aggregates, thus creating pore sizes between 0.7 and 5 nm with surface area SBET = 1150 m2/g. After the harsh heat treatment, the amount of Pd remaining in the pyrolyzed polymer is 2.8 wt. %, as measured by Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES), which then corresponds to a total weight loss of 83% (assuming no loss of Pd). This result indicates a remarkable stability of Pd inside the polymer structure and that minimal Pd leaching can be expected during the catalyst use. X-ray absorption spectroscopic studies of pyrolyzed Pd-containing PAN and following extended X-ray absorption fine structure fitting showed no sign of bulk Pd(0), while a dominant proportion of Pd is present as dispersed Pd carbides. These results are consistent with the data obtained using X-ray absorption near edge structure. Finally, the catalyst was tested on a Suzuki coupling in the liquid phase and showed good activity, hence proving the accessibility of the active sites to reactants.
Highlights
At the frontier between heterogeneous and homogenous catalysis, metallic nanoparticles have found a significant role.[1]
Incorporation of palladium inside the PAN colloidal particles was performed during the polymerization, and this permits the complete embedding of the metal-precursor in the polymer matrix
The destabilization of the Pd-containing colloidal particles (CPs) led to the formation of a macroporous gel with the pore size of 1-2 μm among the particles.[33]
Summary
The support could play a role in the catalyst activity like in CO2-tomethanol conversion over zirconia-supported copper nanoparticles.[17] If chemical resistance is needed, carbon-based materials are preferred, whereas metal oxides are usually very resistant towards high temperature and pressure.[18,19,20] For targeting a possible industrial application, the support should exhibit high loading capacity with good and preferably tailored porosity, as well as an acceptable cost of production.
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