RESUMEN
Sol immobilization is used to produce bimetallic catalysts with higher activity to monometallic counterparts for a wide range of environmental and commercial catalytic transformations. Analysis of complementary surface characterization (XPS, Boehm's titration, and zeta potential measurements) was used to elucidate alterations in the surface functionality of two activated carbon supports during acid exposure. When considered in parallel to the experimentally determined electrostatic and conformational changes of the polymer surrounding the nanoparticles, an electrostatic model is proposed describing polymer protected nanoparticle deposition with several polymer-carbon support examples described. Consideration of the electrostatic interactions ensures full deposition of the polymer protected nanoparticles and at the same time influences the structure of the bimetallic nanoparticle immobilized on the support. The normalized activity of AuPd catalysts prepared with 133 ppm H2SO4 has a much higher activity for the direct synthesis of hydrogen peroxide compared to catalysts prepared in the absence of acid. Detailed characterization by XPS indicates that the surface becomes enriched in Au in the Au-Pd samples prepared with acid, suggesting an improved dispersion of smaller bimetallic nanoparticles, rich in Au, that are known to be highly active for the direct synthesis reaction. Subsequent microscopy measurements confirmed this hypothesis, with the acid addition catalysts having a mean particle size â¼2 nm smaller than the zero acid counterparts. The addition of acid did not result in a morphology change, and random alloyed bimetallic AuPd nanoparticles were observed in catalysts prepared by sol immobilization in the presence and absence of acid. This work shows that the deposition of polymer protected AuPd nanoparticles onto activated carbon is heavily influenced by the acid addition step in the sol immobilization process. The physicochemical properties of both the polymer and the activated carbon support should be considered when designing a bimetallic nanoparticle catalyst by sol immobilization to ensure the optimum performance of the final catalyst.
RESUMEN
The preparation of copper-based heterogeneous catalysts dedicated to the hydrogenation of CO2 to methanol typically relies on multi-step procedures carried out in batch. These steps are precisely tailored to introduce the active phase (Cu) and the promoters (e. g., zinc, gallium) onto a preformed support to maximize catalyst performance. However, each process step can be associated with the formation of waste and with the consumption of energy, thereby negatively impacting the environmental performance of the overall catalyst preparation procedure. Here, a direct and continuous production process is proposed for the synthesis of efficient catalysts for the CO2 -to-methanol reaction. Gallium- and zinc-promoted mesoporous Cu-SiO2 catalysts were prepared in one step by the aerosol-assisted sol-gel process. The catalysts consisted of spherical microparticles and featured high specific surface area and pore volume, with interconnected pores of about 6â nm. A strong promoting effect of Ga and Zn was highlighted, boosting the selectivity for methanol at the expense of CO. Upon calcination, it was shown that Cu species (initially trapped in the silica matrix) underwent a migration towards the catalyst surface and a progressive sintering. After optimization, the catalysts obtained via such direct, continuous, simple, and scalable route could compete with the best catalysts reported in the literature and obtained via multi-step approaches.