Effects Of Support And Additive On Activity Of Supported Noble Metal Catalysts:Theoretical And Experimental Study

Supported noble metal nano-catalysts play a vital role in chemical industry,energy,as well as environment.They have attracted considerable attention in the field of catalysis because of their unique volume effect,surface effect and quantum size effect.However,in nowadays,the design and development of new catalysts are mainly based on“try and error”method,it’s still a challenge to rationally design highly active and stable catalysts.The key to this problem is to deeply understanding the structure-activity relationship of the catalysts.Therefore,this paper focused on carbon materials and TiO₂ supported metal catalysts and their application in phenol hydrogenation and ORR reactions.Firstly,the effects of support and additive on the supported metal catalysts were studied,then tried to rationally design and prepare high efficient catalysts for phenol hydrogenation and ORR reaction.It’s expected that the regulation effects of support and additive on metal catalysts would be deeply understood,then rationally design high efficient catalysts,finally achieve the application of the catalysts in phenol hydrogenation and ORR reaction.The main results are as follows:1.The three-dimensional structure of Pt₁₄4 clusters is more stable than that of planar structure over TiO₂（110）surface,there is a strong interaction between TiO₂（110）surface and Pt clusters.CO is preferentially absorbed on the Pt atoms that away from the TiO₂（110）surface,the adsorbed CO could weaken the adsorption energies of the small molecules over the catalyst and induce the changes of the Pt cluster structures.From the phase diagram,it’s seen that in the CO environment at normal temperature and pressure,the surface of Pt clusters over TiO₂（110）tends to be completely covered by CO.While at a temperature of 600 K and a CO pressure of 10^-5 to 1 atm,the surface of the Pt clusters on Pt₁₄/TiO₂ is mostly occupied by CO,but also some Pt atoms are exposed.Over the Pt₁₄/TiO₂ catalysts with the low CO coverage,O₂ firstly dissociates into single O atoms,then combine with CO to generate CO₂.While under high CO coverage,CO will directly react with adsorbed O₂ by generating the OOCO^*intermediates,further dissociate into CO.2.Alkali–metal-promoted Pd/TiO₂ catalysts were prepared,and achieved a high phenol conversion of 99%and cyclohexanone selectivity of 99%at mild conditions of80°C and 0.06 MPa in water.Results of kinetic studies and TOF values suggested that the addition of alkali metals could promote phenol conversion without changing the selectivity of cyclohexanone.The stability of the catalyst is not pretty good,the main reason is the leach of the alkali metals.The addition of alkali metal could disperse the Pd nanoparticles and change the electronic structures of Pd atoms.The changes of electronic structures could facilitate the dissociation of H atoms of OH over phenol and desorption of the cyclohexanone.The mechanisms studies suggested that phenol hydrogenation occurs both along the direct hydrogenation pathway and the dissociation and hydrogenation pathway over Pd/TiO₂,and that the main pathway over alkali–metal-promoted Pd/TiO₂ catalysts is the dissociation and hydrogenation pathway.3.The nanopore over graphene could induce the changes of structures and enhance the binding energies of metal clusters,while avoiding the diffusion and agglomeration of metal clusters.The graphene with nanopore could change the electronic structures of metal clusters,and lead to a substantial downshift of the d-band center of metal clusters,further decrease the adsorption of CO.All binding energies,d-band centers,and adsorption energies show a linear relationship with changing the size of the nanopore:a bigger size of the nanopore corresponds to stronger bonding of metal clusters with graphene,lower downshift of the d-band center,and weaker CO adsorption.By using a suitable size nanopore,Pd clusters supported on graphene will have similar CO and O₂ adsorption abilities,thus leading to superior CO tolerance.4.The SiO₂@C binary supports and SiO₂ supports supported Pd catalysts were prepared.The introduction of a small amount of SiO₂ in Pd/C catalysts could effectively enhance the catalytic performance of ORR.When the mass ratio of SiO₂ to C is appropriate,Pd/SiO₂@C catalyst has better ORR catalytic performance than commercial Pt/C,but also has an excellent stability.The characterization results suggested that in the catalyst,the SiO₂ could act as electron donor while Pd nanoparticles receive electrons,which make Pd nanoparticles more electronegative.The prepared Pd/SiO₂ catalysts also have excellent ORR catalytic performance,better starting potential,half-wave potential,and electric current density than commercial Pt/C,as well as good performance of resistance to methanol poisoning.The stability of the catalyst is not pretty good because SiO₂ could react with the KOH solution.