Datensatz

Zusammenfassung

Catalysts based on Pd and Pd–Au nanoparticles supported on N-doped carbon nanotubes (N₂2O₂. 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.