Engineered bimetallic catalysts for energy and environment applications

  • Author / Creator
    Shen, Jing
  • Bimetallicity is known to provide synergistic effects and improve catalytic performance of monometallic counterparts in a variety of applications. Conventional impregnation methods for supported catalyst preparation do not allow for the control of bimetallic nanoparticle structure or size, which often leads to considerable difficulties when evaluating metal functions or possible synergism. The presented thesis involves the preparation of bimetallic catalysts with well-defined structures by controlled synthesis of nanoparticles in the presence of a steric stabilizer with the objective of gaining insight into bimetallic effects in selected catalytic reactions. Tightening environmental regulations as well as the increasing demand for premium fuels governed the choice of catalytic applications in this study. Bimetallic catalysts were developed and tested in: (1) low-pressure ring opening of indan as a model reaction in fuel upgrading, (2) hydrodesulfurization of 4,6-dimethyl-dibenzothiophene for the production of ultra-low sulfur fuels, and (3) methane combustion to reduce the greenhouse gas emissions from natural gas vehicles. Depending on the catalytic application and bimetallic system used, the fate of bimetallic catalysts in the reaction environment and catalytic consequences were found to follow different scenarios. The first demonstrated example (Pd−Ru catalysts for indan ring opening) showed that a certain bimetallic nanoparticle synthetic strategy allows developing the bimetallic system with such composition and properties that it can be used to replace rare and expensive iridium, known for its outstanding hydrogenolysis performance. A 3-fold and a 2-fold increase in the selectivities toward 2-ethyltoluene and n-propylbenzene, respectively, in indan ring opening were achieved by introducing Pd to Ru catalysts. The Ru−Pd system is envisioned as a suitable alternative to the Ir−Pt system for selective hydrogenolysis. The fate of structure-controlled systems used in a reductive atmosphere of indan hydrogenolysis was different under oxidative conditions. While Pd−Ru catalysts with different surface compositions revealed distinctively different behaviors in indan RO at 350 °C, in methane combustion up to 550 °C such structure control was unnecessary: Originally different structures demonstrated identical activity because of their structural transformation into one structure. The third scenario of bimetallic effects showed that significant improvements in catalytic behavior could be achieved without intrinsic alloyed nanostructures, even by a mere coexistence of two monometallic particles. The controlled synthesis of the nanoparticles, however, is still paramount. Traditional impregnation and colloidal techniques of bimetallic catalyst preparation yielded monometallic Pd particles on a binary NiAl2O4 support and Pd and Ni nanoparticles on the parent -Al2O3, respectively. The colloidal catalyst showed outstanding performance in wet methane combustion versus the conventional one. The catalyst is thus potentially valuable for natural gas catalytic combustion technologies because it dramatically decreased the required temperature for methane combustion with water presence in the feed versus monometallic Pd. The final addressed bimetallic effect included the improvement of sintering resistance upon metal alloying. The stability enhancement toward Pd sintering was achieved by alloying Ru to a Pd catalyst. The dispersion is responsible for the enhancement of the direct desulfurization (DDS) rate in the hydrodesulfurization of a refractory sulfur compound. A study of the Pd size effect on selectivities confirmed that DDS selectivity depends on Pd dispersions. Thus, for the majority of applications and catalytic systems, the synergistic effects between two metals were evaluated. Because the nanoparticles were synthesized in the presence of a polymeric stabilizer, its effect on the final catalytic performance was also addressed. The necessity for complete polymer removal depends on the nature of the active metal and also the catalyzed reaction involved. The activities of PVP-stabilized Ru and Ir nanoparticles in an indan RO were not affected by the residual polymer. To conclude, bimetallic catalysts with improved catalytic performance over conventional catalysts were developed for Pt- and Ir-free ring opening for fuel upgrading, low-temperature wet methane combustion for applications in natural gas vehicles, and ultra-deep hydrodesulfurization of a refractory sulphur compound. The study showed the advantages and limitations of the structure-controlled catalyst preparation, i.e., the ability to save rare and expensive catalysts by replacing them with alternative metals with similar catalytic behavior; the bimetallic nanoparticle stability and structural evolution under reaction conditions; and the effect of a nanoparticle stabilizer on the catalytic functions.

  • Subjects / Keywords
  • Graduation date
    Fall 2015
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.
  • Language
  • Institution
    University of Alberta
  • Degree level
  • Department
  • Specialization
    • Chemical Engineering
  • Supervisor / co-supervisor and their department(s)
  • Examining committee members and their departments
    • Sauvageau, Dominic (Chemical Engineering)
    • Zheng, Ying (Chemical Engineering, University of New Brunswick)
    • de Klerk, Arno (Chemical Engineering)
    • Semagina, Natalia (Chemical Engineering)
    • Li, Leijun (Material Engineering)