Catalytic Performances Of Pd Nanocrystals And Metal Oxide/Pd Nanocrystal Inverse Catalysts

Heterogeneous catalysis as an important part of catalytic science and technology plays a key role in modern energy utilization and environmental protection.Pd-based catalysis as a representative nanocatalysis has attracted considerable research interests in the field of heterogeneous catalysis.For the design and preparation of highly active and highly selective catalysts,the structure-activity relationship of catalysts is an inevitable research hotspot.Fundamental understanding the structure-activity relationship at the atomic and molecular levels has important guiding value on designing and synthesizing of desired catalysts in modern chemical industry.Recently,the synthesis of nanocrystals with uniform morphology,size,and structure are becoming more and more mature,which successfully breaks the limitation of surface science research caused by the "material gap" and "pressure gap" between single crystal model catalysts and powder catalysts.Based on the above ideas,we successfully synthesized Pd nanocrystals with different morphologies and sizes as well metal oxide/Pd inverse nanocrystals.The structure-activity relationship of Pd nanocatalysts was then systematically studied.Interestingly,we also found that the strong interaction between metal oxides and Pd in inverse nanocatalysts can effectively stabilize low valence metal oxides on metal nanocatalysts.The main achievements are as follows:1.Polyvinylpyrrolidone(PVP)capped cubic Pd(c-Pd)and octahedral Pd(o-Pd)nanocrystals(NCs)respectively enclosed with the {100} and {111} facets were successfully synthesized and have similar sizes of?8.0 nm.Additionnally,we also succeeded in synthesizing c-Pd nanocrystals with sizes of 12 nm and 19 nm,respectively(denoted as c-Pd-8 nm,o-Pd-7 nm,c-Pd-12 nm,c-Pd-19 nm).Next,we comparatively studied their electrocatalytic activities for methanol and ethanol in alkaline solutions.No significant difference was observed between intrinsic electrocatalytic activity of Pd cubes and octahedra of 7-8 nm,while intrinsic electrocatalytic activities of Pd cubes were observed to increase with the size.These observed electrocatalytic behaviors of Pd nanocrystals can be attributed to both exposed facets of Pd nanocrystals and size-dependent electronic structures of Pd nanocrystals resulting from the size-dependent PVP coverage and PVP-to-Pd charge transfer of Pd nanocrystals.The results demonstrate inevitable effects of capping ligands on surface structures and chemical properties of capped metal nanocrystals.2.Pd nanocrystals/SiO2 catalysts were synthesized via one-pot method.Al2O3 overlayers were then coated on Pd/SiO2 catalysts via ALD method.Thermal treatments can introduce pores within Al2O3 overlayers that makes embedded Pd particles accessible to reactants.Catalytic performances of Pd/SiO2-T and Al2O3/Pd/SiO2-T catalysts were then evaluated in successive cycles of methane combustion between 200 and 850?.Compared with Pd/SiO2-T catalysts,Al2O3/Pd/SiO2-T catalysts exhibited active and stable PdOxand Pd-PdOx structures to efficiently catalyze methane combustion between 200 and 850?.Different detection depths structural characterizations(XRD,XPS,DRIFT)combined with catalytic performance of methane combustion showed that the Al2O3 overlayers in Al2O3/Pd/SiO2 catalyst can promote the formation of surface PdOx species on Pd particles and stabilize Pd particles under the catalytic reaction conditions as harsh as CH4 combustion.27Al magic angle spinning NMR characterization results show that Al2O3 overlayers on Pd particles coated by ALD method have rich penta-coordinated Al3+ sites capable of strongly interacting with adjacent surface PdOx phases on supported Pd particles and stabilizing active PdOx-Pd structure.These results reveal the underlying mechanism for the significant stabilizing effect of ALD-coated Al2O3 overlayers on metal particles and provide a convenient strategy to explore stable and efficient supported Pd catalysts for methane combustion.3.A series of CeOx/Pd inverse catalysts with different Ce content were prepared by simple chemical precipitation method,where Ce(NH4)2(NO3)6 as cerium source.Ce2O3 particles with small sizes exclusively form on the c-Pd-8 nm nanocrystals at low Ce loadings via the charge transfer from Pd to Ce2O3,while larger-size CeO2-x particles with rich oxygen defects easily deposit on the c-Pd-19 nm nanocrystals with smaller specific surface area.Ce2O3 overlayers on c-Pd-8 nm nanocrystals resist to oxidation treatments up to 200?,while the electron interaction between metal oxides and metal weakened and disappeared due to the oxidation of Pd substrate at 300?,eventually generating CeO2-x particles with rich oxygen defects.H2-TPR results show that inverse catalysts have abundant interface sites compared with supported catalysts.A broad H2 production peak at high temperature was observed on the resulting CeO2-x/c-Pd-8nm sample(H2-TPR)after the oxidation at 300?,revealing that there is a reaction channel to generate H2 in this system stemming from the high temperature stable hydrogen species.The stabilization effect of low valence metal oxides by the electronic oxide-metal interaction can be easily extended to other metal oxide/metal inverse systems,which will open a new way to regulate oxide-metal interface interaction as well to design and synthesize a whole new family of ideal highly efficient metal-oxide catalysts.