466045 Synthesis of Porous Aggregates Made of Nitrogen-Containing Polymer Nanoparticles Incorporating Noble Metals for Catalysis
Nanoparticles of polyacrylonitrile, a polymer with repeating ternary-coordinated nitrogen atoms, have been produced by emulsion polymerization (particle diameter: 120-160 nm). These functional groups can undergo cyclization when thermally treated under specific conditions, forming a microporous structure inside the polymer matrix . Namely, a series of heat treatments with and without oxygen are required to induce cyclization reactions, needed to prevent polymer melting during the high temperature steps of the process. In addition, the removal of the non-carbonaceous part of the polymer by thermal degradation creates micropores and small mesopores inside the nanoparticles, with pore size ranging from 0.5 to 10 nm. Given the high sensitivity of the quality of the final material to the operating conditions of such treatments (temperature, gas flow and composition), they have to be selected very carefully. When copolymers are used (i.e., involving other monomers in addition to acrylonitrile), the extent of cyclization of neighboring cyanide groups is reduced, making the selection of the optimal conditions of the heat treatments even more difficult.
When considering these polymers as support for catalysts, wet impregnation is one of the most popular method to incorporate active species into the porous support. However, the loading might be incomplete as well as inhomogeneous and sintering of the catalytic species can be an important problem . To mitigate such issues, in this work metal precursors were incorporated during the polymerization before any heat treatment, thus achieving a very high metal loading within the nanoparticles. As a matter of fact, the presence of nitrogen was somehow “binding” the noble metal precursors within the structure, thus embedding them into the polymeric structure: together with the formation of graphite-like carbon network during the pyrolysis, this embedding at the atomic scale is expected to (i) provide high catalytic activity and (ii) prevent or at least hinder the sintering of active species during the final application. Then, the resulting metal-containing colloidal dispersion was destabilized in a controlled manner by salt addition under stagnant conditions to form a gel-like structure. After drying, the aggregated polymer exhibited a limited macroporosity due to empty space in between the non-porous nanoparticles. Then, the produced monoliths have been gently crushed and sieved down to solid particles in the size range from 560 to 180 μm, with pores of about 100 nm. Finally, the collected powders have been thermally treated as mentioned to create the micropores needed to access the metal sites: this way, metal reduction could be applied directly in gas phase, or through carbothermal reduction, taking advantage of the pyrolysis’s conditions, e.g. nitrogen atmosphere and high temperature .
In particular, Platinum-based precursors have been incorporated in polyacrylonitrile nanoparticles produced by miniemulsion polymerization. The experimental procedure involves the preparation of an oil-in-water emulsion by sonication of an aqueous phase containing surfactants and an oil-phase containing the monomer (acrylonitrile), an organic phase soluble initiator and the metal precursor, namely Platinum(II) acetylacetonate (Pt(acac)2). The formed microdroplets entrap the metal precursor and the following polymerization reaction prevents metal diffusion to the continuous phase. This technique was successful, since almost 100% of Platinum incorporation (measured by ICP-OES) was reached [4, 5]. In addition, TEM-EDX experiments showed a very homogeneous distribution of the Pt(II) within the nanoparticle.
The final thermal treatment of the metal-containing polymer is currently under investigation. We expect to reach high values of surface area, in the order of 700 m2/g, as was the case for polyacrylonitrile particles previously treated without incorporated metal. Similar to the approach used with single-site metal catalysts , the resulting materials will then be tested as atomic-sized metal catalyst for reduction reactions involving hydrogen.
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