Synthesis of Palladium-Rhodium bimetallic Nanoparticles for Formic Acid Dehydrogenation

Noble metals alloyed nanoparticles are fundamental for several essential industrial processes. Although academia is switching its attention on cheaper and abundant transition metals, industries continue to employ noble metal-based catalysts due to their higher catalytic performance. For these reasons a key challenge is to modify these catalysts, increasing their activity, selectivity and stability. In particular, palladium-based catalysts are widely employed in different types of heterogeneous catalytic reactions 1. The addition of a second metal to Pd can tune the catalytic properties of the material due to the formation of new electronic states (ligand effect) combined with the strain of the lattice (steric effects) 2. Rhodium also plays a fundamental role in many reactions such as formic acid hydrogenation3. Formic acid (FA) is a non-toxic compound obtained from catalytic conversion of biomass and it possesses a high hydrogen content (4.4 wt%). FA can be easily decomposed into CO2 and H2 (the desired reaction) and CO and H2O. Unfortunately, the formation of carbon monoxide is detrimental for the catalysts because of its affinity with metals. The selective decomposition of formic acid can act as an elegant way to store hydrogen and recycling CO2 to formic acid again 4. Ham and coworkers used theoretical calculation to demonstrate that the addition of Rh to Pd enhance the activity of pure Pd in the formic acid dehydrogenation, enhancing the selectivity to H2 and avoiding the formation of CO which acts as poison for Pd catalysts. The beneficial effect of introduction Rh to Pd was attributed to a contraction of surface Pd-Pd distance and the increase in the electron density in surface Pd atoms compared to pure Pd 5. Herein, we report 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 protective agent and NaBH4 as reducing agent, using carbon nanofibers as the support. . TEM shows that the average particle size of the Pd-Rh catalysts 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 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. Furthermore, monometallic Pd, Pd90Rh10 and Pd69Rh31 were tested in the muconic acid hydrogenation using formic acid as hydrogen donor, under mild conditions. The catalysts showed good activity towards the production of monounsaturated products but only when bimetallic catalysts were used, adipic acid was produced.