Synthesis Of Bimetallic Nanoparticles And Their Application To Control Selectivity In Solvent-free Benzyl Alcohol Oxidation

Bimetallic nanoparticles(BMNPs)with controllable morphology and composition have been intensively investigated over the last few decades due to their unique catalytic properties.The combination of properties associated with two distinct metals sometimes has a synergetic effect on catalytic performance.The addition of a second metal is an important approach for tailoring the electronic and geometric structures of MNPs to enhance their catalytic activity,selectivity and stability.In the field of nanocatalysis,it is of great importance to develop BMNPs reasonably and to understand and utilize their structure-activity relationships.In this thesis,we propose a method for solid-state synthesis of supported BMNPs,and successfully apply them in selective oxidation of benzyl alcohol in which the reaction selectivity is well-controlled.A confined solid-state seed-mediated strategy is introduced to synthesize BMNPs.Nitrate of nonprecious transition metal is first loaded onto mesoporous silica(EP-FDU-12)support by impregnation,and then colloidal noble metal NPs are deposited on the support as a seed for nonprecious transition metal growth.Under high-temperature reduction and calcination,BMNPs form via heteroatom migration.The formation of BMNPs was evidenced by characterizations such as STEM,XRD,and XPS.Controllable synthesis of Pd-based and Au-based BMNPs can be achieved through this method.Supported Pd-based catalysts have been recognized as one of the best catalysts in the oxidation of benzyl alcohol under solvent-free conditions owing to its excellent activity.However,it is found that the selectivity towards target product benzaldehyde is rather low over monometallic Pd catalyst and that toluene is the major byproduct.Although Pd-Au bimetallic catalyst can lower the selectivity towards toluene to some extent,a considerable amount of toluene is inevitably formed when it comes to a higher reaction temperature or conversion.In an attempt to control the selectivity,the as-synthesized Pd-based bimetallic catalysts were used in this reaction.Catalytic results showed Pd-Ni BMNPs were most effective in suppressing the toluene,thereby enhancing the selectivity towards benzaldehyde.At the isoconversion of 80%,toluene selectivity significantly decreased from 22.6%over monometallic Pd to 1.6%over bimetallic Pd1Ni20.Besides,Pd-Ni bimetallic catalysts showed excellent recyclability.Furthermore,we investigated the role of Ni in the bimetallic catalysts.Control experiments revealed that the addition of Ni could suppress toluene formation under anaerobic conditions.It suggests that Ni probably nullifies the sites for the disproportionation reaction and reduces the surface hydrogen.It can be attributed to the synergistic effect of the Pd-Ni BNMPs as verified by CO-DRIFT experiments and DFT calculations.Both ensemble effect and ligand effect weaken the dissociative adsorption and intermolecular interactions of benzyl alcohol species,which hinders the C-O bond cleavage.In addition,due to the oxophilicity of Ni,it can scavenge the adsorbed hydrogen through oxygen species.As the surface hydrogen concentration is reduced,it is more difficult for C-H bond formation,thus suppressing the formation of toluene.