Platinum- and Palladium-Based Single-Atom Alloy Catalysts for Selective Hydrogenation and Dehydrogenation Reactions
Liu, Jilei.
2018
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Abstract: Approximately 80% of all chemical, materials and fuel production in the modern industry use heterogeneous catalysts during some phases in the process. Platinum group metals, especially Pt or Pd are often used in heterogeneous catalysts but susceptible to unselective reactivity, deactivation, and poisoning. The industrial Pt and Pd catalysts, although very successful in many cases, are ba... read moresically "black boxes" without well controlled structural and compositional properties. Different additives are used to modify the metal nanoparticle catalysts on the support to form the multiple metallic catalysts with high precious metal loading. Some toxic poisons, such as Pb or V, are often used to improve the selectivity of these catalysts. Due to these drawbacks, a rational catalyst design approach to make Pt and Pd catalysts more efficient and cleaner is of great interest. The major challenges in Pt and Pd catalysts are their high price, unselective reactivity, carbon deposition and CO poisoning. This thesis addresses these problems with a novel single-atom alloy approach, where isolated Pt atoms, stabilized in Cu nanoparticle surface, are identified as the active sites for key elementary reactions. The Pt metal was predesigned with minimum atomic ensembles and loaded in the surface of cheaper metal nanoparticles to achieve the best possible selectivity, stability and reactivity and reduced concentration of precious metals. This class of single-atom alloy catalysts were applied in catalyzing industrially important selective hydrogenation and dehydrogenation reactions and thoroughly studied for their surface chemistry and structural and compositional properties. Pt-Cu single-atom alloy nanoparticle catalysts were prepared with the galvanic replacement method and supported on alumina and silica. With comprehensive characterization, the formation of single-atom alloy nanoparticles was for the first time demonstrated in the field. This class of catalysts showed excellent performance in selective hydrogenation of 1,3-butadiene, selective dehydrogenation of butane and CO tolerant hydrogen activation reactions. By depositing a small amount of Pt single-atoms in the surface of Cu nanoparticles, the reactivity in selective hydrogenation and dehydrogenation reactions is improved more than one order of magnitude. Notably, close to 100% selectivity to desired partial hydrogenation and dehydrogenation products are also achieved. The PtCu single-atom alloy catalysts are stable in reaction conditions without carbon deposition or sintering. In these processes, the single-atom Pt is efficient in catalyzing key elementary reactions including H2 dissociation and C-H activation, while the highly selective nature of Cu remains unaffected. The energy barriers of these elementary reaction steps are significantly lowered with the single-atom Pt sites. But single-atom Pt sites bind CO and the product molecules weakly compared to monometallic Pt nanoparticles, which results in CO tolerance and high selectivity. The suitability of the single-atom approach in PdAu catalysts was also studied. The PdAu single-atom alloy nanoparticle catalysts were successfully synthesized with sequential reduction methods and demonstrated to be highly selective, stable and reactive in partial hydrogenation of 1-hexyne in the liquid phase. This thesis work uses an integrated approach, including catalyst preparation, advanced characterization, catalyst evaluation under realistic conditions. Collaborative surface science information and DFT calculations complement the catalyst. This provides the platform materials that bridge the materials gap between catalysis and surface science studies. Moreover, it highlights a rational catalyst design approach to optimize the catalytic performance at the single-atom limit.
Thesis (Ph.D.)--Tufts University, 2018.
Submitted to the Dept. of Chemical and Biological Engineering.
Advisor: Maria Flytzani-Stephanopoulos.
Committee: Terry Haas, Matthew Panzer, Charles Sykes, and Frank Tsung.
Keyword: Chemical engineering.read less - ID:
- cr56nc497
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