Controlled Synthesis Of Monometallic Pd And Bimetallic Pd/M (M=Au, Rh) Nano-catalysts And Their Catalytic Properties

In the past twenty years, nanoscience and nanotechnology have obtained fast de-velopment and great progress, and the capital investment and R&D activities all overthe world have undergone an explosive growth. During this period, the "nano-mania"has gradually penetrated into every aspect of catalysis, and also created a new con-cept—nanocatalysis. After years of exploration and investigation, we have reached anagreement towards the ultimate goal of nanocatalysis, which is to produce catalystswith100%selectivity, extremely high activity, low energy consumption, and longlifetime. This can be achieved only by precisely controlling the size, shape, spatialdistribution, surface composition and electronic structure, and thermal and chemicalstability of the individual nano-components.The supported metal catalysts play an important role in heterogeneous catalysis.Among them, the Pd-containing catalysts are widely utilized in chemical industry, pe-troleum refining, organic synthesis and environmental protection due to its excellentperformance and much lower cost. Therefore, it is of great importance, scientific sig-nificance and practical value to reveal the influenting factors and functional mecha-nisms of supported Pd catalysts. However, the research on this aspect has still beenrarely reported. In recent years, the precise control in shape and size of Pd nanocrys-tals (NCs) has been achieved due to the tremendous progress in Pd NCs controlla-ble-synthesis chemistry, which provides necessary conditions to further investigate the"structure-performance" relationship of supported Pd catalysts. For the above reasons,the main purpose of this work is to obtain Pd or Pd-containing nanoparticles with dif-ferent characteristics (e.g., shape, size, surface composition and micro-structure) viathe controllable synthesis routes, study the mechanism of "shape effect","size effect"and "support effect" based on related characterization, reveal the "structure-property"relationship, and also provide a guide for the preparation of highly active and stablePd nanocatalysts. The research contents and results are as follows:Aspect1: The synthesis of Pd NCs and supported Pd catalysts(1) The optimized synthesis condition for cubic and octahedral Pd NCs were obtainedby conducting a series of controlled experiments. The products showed well-definedshapes, monodispersed sizes, good dispersion and high yield, which laid the founda-tion for the preparation of supported Pd catalysts.(2) In the synthesis process of Pd nano-polyhedra using the liquid chemical reductionmethod, the organic capping agents were usually used to prevent the agglomeration of Pd nanoparticles. However, the capping agents may inevitably cause impacts to thecatalytic evaluation results, while the traditional calcination treatments will destroythe initial morphology of Pd NCs. Therefore, we developed a "surface cleaning"method that could remove the capping agents and also preserve the particle morphol-ogy, which makes use of the solubility difference in diverse solvents.(3) By using the lab-developed "ultrasound-assisted membrane reduction"(UAMR)instrument, the adding and mixing modes of reductants or precipitants could be im-proved. Compared with the traditional process, the UAMR method could improve themonodispersity and phase purity of the products. In this paper, we have obtained thebimetallic AuPd nanoparticles and PdRh substituted perovskite catalysts via theUAMR method.Aspect2: Study of the "structure-property" relationship of supported Pd catalysts(1) The Pd NCs with different shapes were obtained by the controlled synthesismethod, including cubic, octahedral and spherical Pd NCs. Then the Pd/SiO₂catalystswere prepared to study the "shape effect" in CO oxidation. It was found that Pd octa-hedra and spheres that exposed Pd(111) facets showed higher activity compared withPd cubes that exposed Pd(100) facets. On the other hand, the Pt NCs with differentshapes were also obtained, and the evaluation results indicated a reverse trend that thePt cubes showed much better activity than Pt octahedra. The DRIFT experimentsshowed that the significant "shape effect" originated from the difference of CO ad-sorption strength over different metal facets. Based on the data reported previously,we found that Pt(100) and Pd(111) facets owned "appropriate" CO adsorption strength,which enabled the activation and timely desorption of CO. On the contrary, the COadsorption over Pd(100) was too weak to activate CO molecules, while the CO ad-sorption over Pt(111) was too strong to desorb, leading to the "CO inhibition" phe-nomenon. Therefore, the activity enhancement of metal catalysts could be achieved byadjusting the CO adsorption strength over NCs surface.(2) The supported Pd catalysts usually deactivated in the high temperature environ-ment of industrial applications, while there are also several reports about the activa-tion behaviors after thermal treatments. To further investigate the evolution of cata-lytic property of supported Pd catalysts, the Pd nanocubes were used as "model" tomake Pd/SiO₂catalysts, and then calcined in different atmospheres (air or nitrogen)and temperature range of200-900oC, we found that the catalysts showed obvious ac-tivation behaviors. The conclusions based on the experiment results were as follows:the stability of shape and size of Pd NCs in N2atmosphere was much better than thatcalcined in air. In both atmoshpheres, Pd/SiO₂catalysts exhibited "activation effect"after being calcined at300oC. In air atmosphere, the activation originated from the formation of the PdO phase, while in N2, the activation was ascribed to the "latticeexpanding" phenomenon and shape transformation. Calcination at900oC caused adeterioration in activity to some extent, which was attributed to the sudden increase ofparticle size, suggesting that900oC was the critical temperature for the "deactivation"of Pd/SiO₂catalysts.(3) To systematically study the "support effect" of Pd catalysts, the Pd nanocubeswere used as "model" particles, and several metal oxides with different basicity andacidity were selected as the support material: CeO₂, SiO₂, TiO₂, Al2O₃and MgO. Itwas found that as the support basicity increased, the activity of Pd catalysts exhibitedan obvious enhancement, as shown in Pd/MgO. Based on the DRIFT results, the basicsupports acted more as the electron-donor, which increased the electronegativity of Pdparticle and strengthened the back-donation of metal electrons into the anti-bondingCO2δ*orbital, leading to the stronger CO adsorption. The effects above resulted inthe appropriate CO adsorption strength for Pd cubes and enhancement of CO oxida-tion activity. While Pd catalysts with neutral and acid supports showed poor activitythey are unable to strengthen the CO adsorption. Surprisingly, it was found a definitecorrelation between the acidity/basicity of the support and the CO adsorption peakwavenumber: the more basicity of the support, the lower of the peak wavenumber,and the larger of the red-shift behavior. The CO-DRIFT method was a good candidatefor the comparison on the support acidity/basicity as the complementary tool.(4) The influence of micro-structure and composition of bimetallic AuPd NCs to theCO oxidation activity was also studied. For the analysis of micro-structure, we foundthat the catalytic performance of AuPd NCs was mainly affected by the exposed at-oms. The core-shell structure Au@Pd and sub-cluster aggregated Au+Pd NCs thatexposed Pd atoms exhibited much better activity than the Pd@Au NCs that exposedAu atoms. For the analysis of compositions, we have synthesized Au100-xPdx(x=0,25,50,75,100) and found that the freshly prepared AuPd particles were all homogeneousalloy, and the CO oxidation activity showed a linear relationship with the surface Pdamount. After the calcination treatment, the AuPd NCs exhibited metal enrichment (orphase separation) to different extents: the AuPd NCs with ratio of Au:Pd=3:1and1:1showed as the surface Au-rich alloy, and the catalytic activity was close to monome-tallic Au catalysts. The AuPd NCs with ratio of Au:Pd=1:3transformed to Pd-rich al-loy, and the activity was close to monometallic Pd catalysts. The AuPd NCs in thiswork showed no obvious synergy effects, and the activity was mainly affected by thesurface atoms, which may be attributed to the large particle size of AuPd particles (>6nm). (5) Taking the exhaust gas purification as the application, the thermal stability ofPdRh substituted perovskite catalysts was studied. The PdRh substituted LaFeO₃catalysts were prepared by the UAMR method. It was found that the the crystallinestructure of LaFeO₃restricted the immigration of metal atoms, hence effectively pre-venting the aggregation and agglomeration of metal particles, leading to the enhancedthermal stability as compared with the traditional PdRh/Al2O₃. After calcination at900oC for8h, the particle size in perovskite was only1-2nm, while it was largerthan150nm in the Al2O₃-supported catalysts. On the other hand, the PdRh substitutedperovskite catalysts showed excellent TWC activity, especially for NOxelimination,while the HCxconversion still needs to be improved.The investigation and conclusions above provided valuable information for theunderstanding of the "structure-property" relationship of supported Pd catalysts, sup-plied important reference for revealing the functional mechanisms, and also contributeto the preparation of Pd-containing nanocatalysts with higher performance and stabil-ity.