DOI: 10.1002/cssc.201((will be completed by the editorial staff)) Dynamics of Pd on nanocarbon in the direct synthesis of H 2 O 2 Rosa Arrigo, *[a,b] Manfred E. Schuster, [a] Salvatore Abate, [c] Sabine Wrabetz, [a] Kazuhiko Amakawa, [a] Detre Teschner, [a] Maria Freni, [b] Gabriele Centi, [c] Siglinda Perathoner, [c] Michael Hävecker, [d] Robert Schlögl, [a,b] This work aims to clarify the nano-structural transformation accompanying the loss of activity and selectivity for H 2 O 2 synthesis of Pd and AuPd nanoparticles supported on N-functionalized carbon nanotubes (NCNTs). High resolution XPS allowed discriminating metallic Pd, electronically-modified metallic Pd hosting impurities and cationic Pd. This is paralleled by morphological heterogeneity observed by HRTEM where nanoparticles with 2 nm average size coexist with very small Pd clusters. The morphological distribution of Pd is modified after reaction through sintering and dissolution/re- deposition pathways. The loss of selectivity is correlated to the extent of these processes occurring as a result of the particle instability at the carbon surface. We assign beneficial activity in the selective hydrogenation of oxygen to the Pd clusters with a modified electronic structure as compared to Pd metal or Pd oxides. These beneficial species are formed and stabilized on carbons modified with N atoms in substitutional positions. The formation of larger metallic Pd particles not only reduces the number of active sites for the synthesis but enhances the activity for the deep hydrogenation to water. The structural instability of the active species is thus detrimental in a dual way. Minimizing the chance of sintering of Pd clusters by all means is thus the key to better performing catalysts. Introduction Recently H 2 O 2 faces a growing demand in the chemical industry. The conventional multistep synthetic process via anthraquinone presents the disadvantage of high production cost that renders it practicable only for large-scale production. Also, the safety and cost-related issues of the transportation of H 2 O 2 from the production site to the end-user facility represent disadvantages of the antraquinone process. The on-site realization of a cost effective direct synthesis of H 2 O 2 from H 2 and O 2 could expand considerably the application of H 2 O 2 . The Sumitono-Eni process for the synthesis of caprolactame and the Dow-BASF process for the synthesis of propene oxide are examples of oxidation process which aims to use H 2 O 2 produced on-site via a direct synthesis from H 2 and O 2 . [1,2] From the large variety of literature and patents on the topic, [3,4] it is evinced that effective catalytic systems for the direct synthesis of H 2 O 2 from H 2 /O 2 involve the use of supported Pd catalysts in a slurry of H 2 O/CH 3 OH and in the presence of H 2 SO 4 and halides to achieve good activity and selectivity to H 2 O 2 . [5-7] The major problems of the direct synthesis toward commercialization are related to the safety of the process as well as the selectivity. The apparent simplicity of the synthesis reaction is accompanied, in reality, by a complex network of thermodynamically favored deep hydrogenation reactions to H 2 O, as shown in Scheme 1. Important contributions to the development of selective catalysts for the H 2 O 2 direct synthesis were made by Hutchings and co-workers, [8-10] through the application of encapsulated Pd-nanoparticles obtained via sol- immobilization on C support, the introduction of C-supported Pd-Au nanoparticles and through the use of an acidic C support. However, for the industrialization of the process, the selectivity and the life time of these catalysts needs further improvement. [5] The shell-protected Pd-nanoparticles suffer instability, resulting in a rapid loss of the initial extraordinary high selectivity and activity through a quick transformation into an unprotected and thus unselective form. [8,11] The transient catalytic performance of the PVA-protected Pd nanoparticles is related to the peculiar dissolution of O, H, and C in the near surface region [12-17] that depends on the reactant gas phase chemical potential, [11-13] the fluid-dynamic conditions [11,18,19] and the morphological transformations of the nanoparticles following de-protection. [19-21] Scheme 1 illustrates how the desired selectivity of the hydrogenation reaction of oxygen depends on two chains of consecutive reactions. One chain is given by the ratio between dissociative oxygen activation and the abundance of hydrogen: more hydrogen helps forming faster the product but also leads to the thermodynamically more stable deep hydrogenation product. The other reaction chain is given by the ratio of desorption and re-adsorption of the desired product. It is obvious that a delicate optimization of the electronic structure of Pd will be critical. Reaction parameters but also solvents and kinetic site blockers as additives to the reaction system can be used for fine-tuning. The support can contribute through covalent interaction with Pd leading to wetting of particles as well as to strained systems.