Impact of the Experimental Parameters on Catalytic Activity When Preparing Polymer Protected Bimetallic Nanoparticle Catalysts on Activated Carbon

Sol immobilization is used to produce bimetallic catalysts withhigher activity to monometallic counterparts for a wide range of environmentaland commercial catalytic transformations. Analysis of complementary surfacecharacterization (XPS, Boehm's titration, and zeta potential measurements) wasused to elucidate alterations in the surface functionality of two activated carbonsupports during acid exposure. When considered in parallel to the experimentallydetermined electrostatic and conformational changes of the polymer surroundingthe 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 protectednanoparticles and at the same time influences the structure of the bimetallic nanoparticle immobilized on the support. Thenormalized activity of AuPd catalysts prepared with 133 ppm H2SO4has a much higher activity for the direct synthesis of hydrogenperoxide compared to catalysts prepared in the absence of acid. Detailed characterization by XPS indicates that the surface becomesenriched in Au in the Au-Pd samples prepared with acid, suggesting an improved dispersion of smaller bimetallic nanoparticles, richin Au, that are known to be highly active for the direct synthesis reaction. Subsequent microscopy measurements confirmed thishypothesis, with the acid addition catalysts having a mean particle size similar to 2 nm smaller than the zero acid counterparts. The additionof acid did not result in a morphology change, and random alloyed bimetallic AuPd nanoparticles were observed in catalystsprepared by sol immobilization in the presence and absence of acid. This work shows that the deposition of polymer protected AuPdnanoparticles onto activated carbon is heavily influenced by the acid addition step in the sol immobilization process. Thephysicochemical properties of both the polymer and the activated carbon support should be considered when designing a bimetallicnanoparticle catalyst by sol immobilization to ensure the optimum performance of the final catalyst