Role of Feed Composition on the Performances of Pd-Based Catalysts for the Direct Synthesis of H₂O₂

The role of feed composition, in particular the O₂ to H₂ ratio and the concentration of CO₂, has been investigated on two Pd/N-CNT and PdAu/N-CNT catalysts. There is a significant influence on the catalytic behavior, but the effect is complex, and cannot be analyzed in terms of conventional kinetic approaches, due to the presence also of a change in the catalyst characteristics as a function of time on stream (in the fresh samples). Depending on the O₂ to H₂ feed and catalyst nature, the trend of productivity and selectivity varies differently with the time on stream. A simplified kinetic model has been developed which well accounts for the observed behavior. The model is based on the concepts that (i) the selective sites are associated to small Pd terraces covered by chemisorbed O₂ with limited sites for H₂ chemisorption, and (ii) during the reaction, due to catalyst modifications, a change of available chemisorbed H₂ for unselective parallel formation of H₂O and H₂O₂ hydrogenolysis occurs. The changes during time on stream are related to (i) the removal of PVA capping agent, with an initial increase of available Pd surface area leading also to an increase of the H₂ chemisorption sites for unselective parallel conversion and consecutive H₂O₂ hydrogenolysis, and (ii) for the longer times on stream aggregation of some Pd nanoparticles leading to some decrease in the available Pd surface area. The isolation of Pd ensembles by Au (PdAu/N-CNT) influences this trend of productivity and selectivity to H₂O₂ as a function of the O₂ to H₂ ratio in the feed. The change is consistent with the model indicated above.