On the Nature of Selective Palladium-Based Nanoparticles on Nitrogen-Doped Carbon Nanotubes for the Direct Synthesis of H2O2

Catalysts based on Pd and Pd-Au nanoparticles supported on N-doped carbon nanotubes (N-CNTs) are studied in the direct synthesis of H2O2. The initial selectivity in H2O2 formation is rather high (>95%); however, there is a fast initial decrease during the first hour of time on stream. This was due to the initial presence of an organic capping agent (polyvinyl alcohol, which is used in the catalyst synthesis to obtain a high dispersion of metal particles). The removal of this capping agent during the reaction leads to a high mobility of metal nanoparticles. The high initial selectivity, when the capping agent is present, is due to small Pd terraces fully covered with chemisorbed O2 and limited H2 chemisorbed sites that consecutively hydrogenate the formed H2O2. The alloying of Pd with Au decreases the intrinsic reaction rate (per mg of Pd) and increases the selectivity in H2O2 formation, whereas Au alone is inactive. Au also has a minor effect on the consecutive conversion of H2O2 in both the decomposition and hydrogenolysis (in the presence of H2 only) reactions. These results suggest that Au does not block the unselective sites of H2O2 conversion but mainly creates isolated small terraces of Pd that can limit H2 chemisorption sites, which thus leads to higher selectivity to H2O2 under given reaction conditions.