Supported a promising structured polymericmaterial of use in catalysis.

Supported metal nanoparticles on nanostructured polymericmatrices currently seem a valid alternative to the application ofmore common heterogeneous catalysts already employed inseveral processes also of industrial interest.1?4 Both inorganicand carbonaceous supports indeed show catalytic instability,which is often caused by the difficulty of exactly controllingeither the size distribution or the morphological properties ofthe metal particle.5,6 The pores inherently existing inside themacroscopic structure of certain polymers, on the other hand,can control the growth, and hence the sizes and shapes, of themetal nanoparticles.7 In this way, the catalytic efficiency ofmetal crystallites supported on porous polymeric materialscould be positively influenced.8,9 As an example, platinumnanoparticles supported on hyper-cross-linked polystyrene(HPS) have been shown to act as a very stable and activecatalyst for the wet air oxidation of phenol without anyformation of hazardous side byproducts.10 Noticeably, othercatalysts, such as unsupported Fe nanoparticles or SnO2/SiO2nanocomposite aggregates, when employed in the samereaction were considered inadequate because of the occurrenceof an intrinsic instability increased by leaching phenomena.The HPS matrix used in the catalytic example discussedabove was introduced as a new material more than 50 yearsago11 and soon became a promising structured polymericmaterial of use in catalysis. HPS polymers are characterized bysmall pores, leading to very high specific surface area.12 Theyare stable also at relatively high temperatures and can swell inalmost any liquid medium while their cavity sizes can be tosome extent modulated by simply varying the polymericsynthesis conditions. Because of these features, HPS matricesare primarily considered as peculiar organic polymeric materialsto be employed for supporting metal nanoparticles,13,14 thefeatures of which are of great interest in characterizing resultingcatalytic systems in both the industrial fine chemical and greenchemistry fields.10 In particular, it was demonstrated that HPScan be efficiently employed to control either the growth or theshaping of Co, Ru, Pd, and Pt nanoparticles13,15,16 while the useof HPS in catalysis has proven to be effective in the synthesis ofvitamin intermediates,7 selective hydrogenation of acetylenealcohols,17,18 and, most noteworthy, in oxidation of carbohydrates15,16and phenols.10Despite the great amount of work exploring the structuraland morphological properties as well as the catalytic activities ofthe HPS-supported metal nanoparticles, very little is knownregarding the particular interactions between the support andthe metal atoms, probably because of the extreme complexity ofthe HPS matrix, which has eluded a systematic and detailedcharacterization.In this work, we investigate at the atomistic level thenucleation of a palladium cluster inside a cavity of hyper-crosslinkedpolystyrene, monitoring the cluster growth until the