Design, Synthesis And Catalytic Properties Of Gold Nanocatalysts For Selective Oxidation Of Hydrocarbons

Gold has always been recognized as a metal with special properties.More recently a new field of application of this extraordinary metal appears related to its characteristics as a catalytic material.Indeed,since the discovery in 1988 of its high catalytic activity in low temperature CO oxidation by Haruta,many studies and a constant growth of literature citations deal with gold.The discovery and the subsequent success of gold as a catalytically active metal were due to the discovery of suitable methods of obtaining finely dispersed nanoparticles.Gold is a metal with a relatively low melting point,therefore it is difficult to disperse gold,especially at the nanoscale,the useful dimension in catalysis.The research goals are mainly focus on the following aspects.One isthe trials devoted to obtaining highly active gold nanocatalysts for selective oxidation reactions.The second subject is to address the active site and elucidate the reIationship between the activity and the structure in gold catalysts.Our works are summarized as follows.Amine-functionalized siloxane oligomers were synthesized and used successfully to prepare colloidal Au particles smaller than 1 nm.Using nuclear magnetic resonance?NMR?to follow the interaction of gold tetrahydrothiophene chloride?Au?THT?Cl?with the functionalized siloxane,it was determined that the amine ligands displaced the THT ligand effectively.By comparison with other functionalized siloxane oligomers/compound,parameters such as density of ligating groups,oligomer steric barrier and reduction rates were found to be essential for the formation and stability of subnanometer Au particles.Without further treatment,the formed Au particles were active catalysts for the reduction of p-nitrobenzaldehyde by triethylsilane,forming an imine as the major coupling product.Deposition of the Au colloids onto silica,followed by thermal treatment to remove the organic groups resulted in subnanometer Au on silica,indicating this to be a promising method of fabricating subnanometer supported Au catalyst.Since the principles used here are general,this method should be applicable to the synthesis of subnanometer particles of other supported late transition metals.Interfacial sites between metal and support are often important catalytic sites.A mononuclear titanium siloxy complex III,Ti?acac?₂[OSiC₆H₅N?OCH₃?₂]₂ was synthesized to generate TiO_x units of different degrees of clustering to decorate Au nanoparticles to probe the importance of the interface between Au and TiO_x in the catalytic oxidation of propane in a stream of O₂and H₂.Two different methods of preparation were tested;one was to deposit III onto Au/SiO₂ and the other was to form III-covered Au nanoparticle first before deposition onto SiO₂.The former method generated more highly dispersed TiO_x units,while large domains of TiO₂ was formed with the latter method.The former samples with more highly dispersed TiO_x units generated acetone selectively in the oxidation of propane in a mixture with O₂ and H₂,whereas the latter samples formed both propene and acetone as major products.They were characterized with relevant spectroscopic techniques such as diffuse reflectance ultraviolet-visible spectroscopy?DR-UV-Vis?,X-ray photoelectron spectroscopy?XPS?and X-ray absorption spectroscopy?XAS?in order to understand these results and to establish the structure-activity relationship.The results confirmed the important role of both the Au-TiO_x interface and the extent of Ti isolation in the TiO_x phase in the reaction.The ability of Au catalysts to effect the challenging task of utilizing molecular oxygen for the selective epoxidation of cyclooctene is fascinating.Whereas supported nanometer-size Au particles are poorly active,solubilized atomic Au clusters,present in ng·mL^-1concentrations and stabilized by ligands derived from the oxidized hydrocarbon products,are active.They can be formed from various Au sources.They generate initiators and propagators to trigger the onset of the auto-oxidation reaction with an apparent turnover frequency of 440s^-1,and continue to generate additional initiators throughout the auto-oxidation cycle without direct participation in the cycle.Transmission electron microscope?TEM?and fluorescence spectroscopic characterization suggests that 7-8 atom clusters are effective catalytically.We further correlated the appearance of these atomic clusters with distinctive optical properties to the emergence of high catalytic activity.Extension of work based on these understandings led to the demonstration that these Au clusters are also effective in selective oxidation of cyclohexene,and that solubilized Pt clusters are also capable of generating initiators for cyclooctene epoxidation.