Synthesis of palladium-rhodium bimetallic nanoparticles for formic acid dehydrogenation

Herein, we report for the first time the synthesis of preformed bimetallic Pd-Rh nanoparticles with different Pd:Rh ratios (nominal molar ratio: 80–20, 60–40, 40–60, 20–80) and the corresponding Pd and Rh monometallic ones by sol immobilization using polyvinyl alcohol (PVA) as protecting agent and NaBH4 as reducing agent, using carbon nanofibers with high graphitization degree (HHT) as the desired support. The synthesized catalysts were characterized by means of Transmission Electron Microscopy (TEM) and inductively coupled plasma optical emission spectroscopy (ICP-OES). TEM shows that the average particle size of the Pd-Rh nanoparticles is the range of 3–4 nm, with the presence of few large agglomerated nanoparticles. For bimetallic catalysts, EDX-STEM analysis of individual nanoparticles demonstrated the presence of random-alloyed nanoparticles even in all cases Rh content is lower than the nominal one (calculated Pd:Rh molar ratio: 90–10, 69–31, 49–51, 40–60). The catalytic performance of the Pd-Rh catalysts was evaluated in the liquid phase dehydrogenation of formic acid to H2. It was found that Pd-Rh molar ratio strongly influences the catalytic performance. Pd-rich catalysts were more active than Rh-rich ones, with the highest activity observed for Pd90:Rh10 (1792 h−1), whereas Pd69:Rh31 (921 h−1) resulted the most stable during recycling tests. Finally, Pd90:Rh10 was chosen as a representative sample for the liquid-phase hydrogenation of muconic acid using formic acid as hydrogen donor, showing good yield to adipic acid.