1.4.1 PARTICLE SIZE EFFECTS

Within nanocatalysis, the particle size is a well-documented key parameter influencing both activity and selectivity. This reflects the combination of quantum and geometric effects associated with the respective evolution of electronic properties from atomic like to delocalised bands, and shift­ing population of low to high coordination surface atoms, with increas­ing nanoparticle size and dimensionality. Kaneda et al. [130] hypothesised that the unique reactivity of 2060 atom Pd clusters supported on titania towards aromatic alcohol selox arose from a distribution of Pd0, Pd+ and Pd2+ surfaces sites, with n-bonding interactions between the phenyl group and Pd2+ species facilitating subsequent oxidative addition of the O-H bond by neighbouring Pd0 and eventual P-hydride elimination. Surface hydride was hypothesised to react with oxygen from a neighbouring Pd2O centre forming H2O and regenerating the metal site. Optimal activity for cinnamyl alcohol selox to cinnamaldehyde coincided with clusters pos­sessing the maximum fraction of Pd+ character.

Particle size dependency was also reported for the catalytic transforma­tion of benzyl alcohol over Pd nanoparticles dispersed on alumina, SiO2 and NaX zeolite supports [131, 132]. For Pd/NaX and Pd/SiO2-Al2O3, ben­zyl alcohol selox was fastest over particles between 3 and 5 nm, whereas geraniol and 2-octanol were structure-insensitive. Systematic studies of particle size effects in cinnamyl and crotyl alcohol selox over amorphous and mesostructured alumina and silica supports have likewise uncovered pronounced size effects in both initial selox rates and TOFs [133-136], which increase monotonically with shrinking nanoparticle diameters (even down to single atoms) [137]. HAADF-STEM analysis reveals atomically dispersed palladium exhibits maximal rates towards benzyl, cinnamyl and crotyl alcohols, with selectivities to their corresponding aldehydes >70 %. The origin of such size effects is revisited below. The use of colloidal Pd nanoclusters for aqueous phase alcohol selox is limited [138-140], where­in Pd aggregation and Pd black formation hinders catalytic performance. However, the successful stabilisation of 3.6 nm Pd nanoclusters is reported using an amphiphilic nonionic triblock copolymer, Pluronic P123; in the selective oxidation of benzyl alcohol, 100 % aldehyde selectivity and high selox rates are achievable, with high catalytic activity maintained with negligible sintering after 13 recycling reactions [141].