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 O-2 to H-2 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 O-2 to H-2 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 O-2 with limited sites for H-2 chemisorption, and (ii) during the reaction, due to catalyst modifications, a change of available chemisorbed H-2 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 H-2 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 O-2 to H-2 ratio in the feed. The change is consistent with the model indicated above.