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18 A series of size-controlled Pd/hydroxyapatite (HAp) catalysts ranging from single 19 sites (Pd clusters) to nanoparticles (d Pd : ~30 nm) was prepared and examined for the 20 H 2 O 2 synthesis directly from H 2 and O 2 . A Pd/HAp (d Pd : ~1.4 nm) showed a high 21 selectivity of 94% toward H 2 O 2 under mild conditions (283 K and atmospheric 22 pressure). The crystal phase, morphology, surface electronic states and coordination 23 number of Pd particles from atomic level to nano scale were characterized in detail 24 using multi-techniques such as X-rays diffraction, scanning transmission electron 25 microscopy and the extended X-rays absorption fine structure. Density functional 26 theory calculations indicated that Pd clusters in subnano size have the most effective 27 active sites for the selective activation of oxygen hydrogenation, thus resulting in the 28 high catalytic efficiency for H 2 O 2 synthesis. This work elucidates those smaller Pd 29 particles only in a proper size-range show the best catalytic performance for H 2 O 2 30 synthesis. 31 Keywords: size effects, subnano clusters, Pd/HAp catalysts, H 2 O 2 synthesis, 32 structure-performance relationship 33 3 1. Introduction 34The origin of size effects has long been a spotlight in the field of heterogeneous 35 catalysis [1][2][3][4][5][6]. Nowadays, many efforts have been contributed to the characterization 36 of active sites over catalysts, in which the size of metal particles is usually larger than 37 ca. 2.5 nm, a so-called magic size from non-metallic to metallic property for metal 38 particles [7, 8]. More recent studies have proven that active sites of practical metal 39 catalysts may contain a certain amount of single sites [9][10][11] or clusters [12], or to say, 40 the metal particles in the subnano size, which probably act as active components [13]. 41 Nevertheless, it is still a great challenge to understand the origin of particle size 42 effects, especially, to establish the structure-performance relationship of those 43 catalysts with subnano size. On the other hand, is the smaller size better for catalysis? 44 This question has intrigued a lot of debate on both academe and industry for the last 45 years, and it is of importance to the rational design and controllable synthesis of 46 industrial catalysts [14-17]. 47 4 formation of H 2 O 2 increase the production of undesirable water as well [25-30]. It is 56 worthy to emphasize that the selective hydrogenation of O 2 is also of great importance 57 to the development of fuel cells [31, 32]. 58 Our preliminary studies have revealed that the electronic structure of the surface Pd 59 atoms in a Pd/TiO 2 catalyst could be tuned by forming Pd-PdO ensembles, and the 60 maximum H 2 O 2 selectivity of ca. 60% was achieved without promoters [33]. With the 61 optimization of reaction conditions, such as pressure, solvent, and the addition of 62 promoters and acids, the selectivity could be improved to about 80% [26]. 63 Remarkable increase in H 2 O 2 selectivity could be obtained over Pd-M (M: Au, ...

Highly selective hydrogen peroxide (H O ) synthesis directly from H and O is a strongly desired reaction for green processes. Herein a highly efficient palladium-tellurium (Pd-Te/TiO ) catalyst with a selectivity of nearly 100 % toward H O under mild conditions (283 K, 0.1 MPa, and a semi-batch continuous flow reactor) is reported. The size of Pd particles was remarkably reduced from 2.1 nm to 1.4 nm with the addition of Te. The Te-modified Pd surface could significantly weaken the dissociative activation of O , leading to the non-dissociative hydrogenation of O . Density functional theory calculations illuminated the critical role of Te in the selective hydrogenation of O , in that the active sites composed of Pd and Te could significantly restrain side reactions. This work has made significant progress on the development of high-selectivity catalysts for the direct synthesis of H O at ambient pressure.