Preparation Of High Dispersed Supported Palladium Nanocatalysts And Catalytic Performance

The catalytic activity of metal nanocrystals is strongly correlated withthe shape and size of metal nanoparticles, as well as the supporting material.The activity and selectivity of nanocatalysts can be tailored by controllingthe shape and size of nanocrystals in a catalyst. In some case, theshape–dependent effect of the metal nanocrystals can be versatility used toperform a superior property. Essentially, the shape can determine thespecific crystal faces exposed on the surface of nanocrystals, and thenumber of atoms located at the corners, edges, and defects that areintrinsically more active for a structure-sensitive reactionthe shape ofnanocrystals. Thus, an exquisite shape control of Pd nanoparticles is highlydesired for tailoring the catalytic properties and enhancement of catalyticperformance. The supporting material plays an important role in variouscatalytic applications. Based on practical considerations, metal particles located on the surface of suitable support can enhance the stability of thesupported catalyst and decrease the use of metal, increasing utilization ofthe catalyst.Spinel-type zinc aluminate （ZnAl₂O₄） has drawn considerableattention due to its unique properties, such as high surface areas, highthermal stability,high mechanical resistance, low temperature sinterability,low surface acidity, better diffusion and ductility. Especially, it is widelyused as catalyst supports in many catalytic reactions.Graphene, a single-atom-thick of carbon atoms packed into a densehoneycomb crystal structure, is a two-dimensional （2D） nanomaterial ofsp2hybridized carbon. Because of unique nanostructure and extraordinarycharacteristics such as unusual electronic properties, high surface area, andgood chemical stability, graphene can be a potential for a catalyst support.Spinel-type ZnAl₂O₄with high specific surface area and microporousframeworks was successfully synthesized by a facile one-step solvothermalmethod using methanol/water as solvent and hexamethylenetetramine asprecipitator, and used as a support for preparation of low loading Pdcatalyst. The supported Pd catalyst on microporous ZnAl₂O₄demonstratedexcellent catalytic activity and reused ability at least after five recycles fora series of Suzuki-Miyaura coupling reactions. The remarkableheterogeneous reactivity is attributed to the high dispersion of Pdnanoparticles on the ZnAl₂O₄and the strong interaction between the metal and support.Well-dispersed and uniform Pd nanocrystals with icosahedral, cubic,and spherical morphologies supported on graphene sheets were synthesizedthrough a ethylene glycol-water solution method with poly （vinylpyrrolidone）（PVP） serving as a stabilizer and then tested in thehydrogenations of2-methyl-3-butyn-2-ol （MBY）. It was observed that theactivity and selectivity of reaction were strongly affected by the shape ofPd nanoparticles present in the catalysts. As a result, compared with thesupported Pd nanocubes enclosed by six {100} facets, the Pd nanoparticlesicosahedra with only {111} facets exposed exhibited higher selectivitytoward2-methyl-3-buten-2-ol （MBE）. For the selective hydrogenation oftrans-cinnamaldehyde （CALD）, the supported Pd icosahedra exhibitedhigher selective hydrogenation of C=C band than the supported Pdnanocubes.