Nature of the Catalyst and Reaction Network

One common observation in patents and fundamental studies is that an acidic support promotes the selectivity. H2O2 is more stable in add medium, but a strong add could create problems of corrosion and further treatment of the solution. One of the motivations for the use of acidic supports was thus the reduction of the required concentration of inorganic acids in solution and hence minimization of 

There are no reliable data that indicate whether this conclusion is valid for all types of catalysts or is instead specific for the catalysts studied by Fierro et al. [91], because -SO3H functional groups also show other functions to which the effect on selectivity could be related. Fierro etal. [3], in their review, demonstrated that the high selectivity for H2O2 can be achieved in the presence of cationic Pd species, but such proof has not been reported in literature, because the reaction mechanism is complex and changes in the surface state of Pd as the H2O2 synthesis reaction occurs. Therefore, there are still no unequivocal conclusions on the real nature of the active and selective sites in direct H2O2 synthesis. 

The key aspects determining the selectivity in H2O2 formation and in turn catalyst productivity are  

Based on this discussion, it is possible to clarify some aspects of the literature interpretations on the nature of the active species. Lunsford and coworkers [49, 93] published many papers indicating that the active species is colloidal palladium, which implies the easy dissolution of Pd in solution. An analysis of patents clearly reveals that this is not the case for active catalysts and various patents explicitly indicate that there is no leaching of Pd. On the other hand, a colloid would be difficult, if not impossible, to manage in a commercial process and its recovery would be not viable at the very low concentrations of dissolved metal employed. In addition, the presence of even traces of Pd in commercial H2O2 could be extremely dangerous in terms of the possibility of explosion. Finally, if the solid is a simple reservoir for Pd going into solution, a deactivation is expected with time-on-stream in continuous operations. 

Finally, if the role of Au is essentially to act as a structural promoter for Pd, it is expected that the support influences this effect. The observation of Hutchings et al. [ 105 ] that Pd-Au/carboncatalysts showthe presenceofahomogeneous alloy (both Au and Pd on the surface) and improved reactivity with respect to Pd-Au/oxide (TiOz, AI2O3 and Fe203), where a core-shell structure is observed, is thus not surprising. This result evidences that Au has a structural effect more than an electronic effect [106]. 

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