UNDERSTANDING THE STRUCTURE-PERFORMANCE RELATIONSHIP OF HETEROGENEOUS CATALYSTS FOR ENERGY PRODUCTION

<p>The ever-increasing global energy demand and environmental concerns, hardly being met by the traditional fossil fuels, have stimulated the exploration of clean, sustainable, and economic energy resources. Among them, shale gas and hydrogen have been considered as promising alternatives to traditional fossil fuels, where shale gas has emerged as a significant bridge between conventional fossil fuels and renewables, and hydrogen is an ideal energy carrier due to its high energy density and zero carbon combustion byproducts. To date, the shale gas conversion and hydrogen production mostly rely on heterogeneous catalysis. To this end, the development of cost-efficient, active and stable catalysts is necessary for energy production reactions, such as the dehydrogenation, olefin oligomerization and hydrogen evolution reaction (HER), to step closer to produce the clean energy industrially and commercially. As we know, the surface structure of catalyst is of great importance in determining the catalytic performance. Precise determination of the surface structure of catalyst may lead to an in-depth understanding of the structure-performance relationships in heterogeneous catalysis, thus providing a deep insight to design more efficient and well-structured catalytic systems. Currently, intensive progresses have been made in boosting the catalytic performances regarding activity, selectivity, and stability. Nevertheless, there is still missing a deep understanding on the relationship of surface structure and catalytic performance due to the limitations of traditional characterization methods, especially for those ex-situ techniques. In this dissertation, a combination of advanced characterization methods, such as in-situ XAS, high-resolution STEM, in-situ IR, were employed to investigate the structure-activity relationship in three typical catalytic reactions including dehydrogenation, oligomerization and HER, which would enable the rational design and synthesis of highly efficient heterogeneous catalysts.</p> <p>In the first project, novel Pd-Bi bimetallic catalysts were synthesized from the conventional incipient wetness impregnation method with a high selectivity toward propane hydrogenation. The surface structure, composition, and properties of those Pd-Bi bimetallic catalysts were identified by a combined analysis of the X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), Fourier-transform infrared (FTIR) absorption spectroscopy of adsorbed CO, high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and energy dispersive X-ray analysis (EDS). The catalysts with a Pd-Bi intermetallic compound surface show a greatly enhanced selectivity > 95 % at 25% propane conversion, while that for monometallic Pd is approximately 44%. The superior performances could be attributed to the geometric isolation of the active sites in Pd-Bi intermetallic compound as well as the electronic effect due to the Pd-Bi bond formation.</p> <p>The second project focuses on investigating the new chemistry of a reported main group zinc single site catalysts with unexpected catalytic ability for olefin oligomerization. The Zn²⁺ single site catalysts supported on alumina were synthesized by strong electrostatic adsorption method. The X-ray absorption near edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) confirmed the unexpected ZnAl₂O₄ as well as the Zn²⁺ single site formed on the surface of Al₂O₃. To achieve a high olefin oligomerization conversion, the Zn²⁺ single site catalysts supported on alumina were tested in a range of high temperature (350 to 400 °C) and pressure (300 to 500 psi). At optimal condition (375 °C and 500 psi), ethylene conversion is~85%, strongly confirming the catalytic ability for ethylene oligomerization. Moreover, Zn²⁺ single site catalysts supported on alumina exhibits higher activity and higher selectivity toward high hydrocarbon products than the one supported on silica. The promoting effect could be ascribed to the ZnAl₂O₄ support effect and the electronic effect from the strong interaction between the alumina and metal species. </p> <p>In the last project, a novel facile and nontoxic ultra-fast laser shock method was reported for the first time for the synthesis of Rh-P/C HER catalysts. The obtained Rh-P/C catalysts exhibit high HER activity in terms of overpotential comparable to that of commercial Pt/C catalysts with even much lower Rh weight loading. The outstanding catalytic performance of Rh-P/C provides a new insight to synthesize P-rich TMPs in large-scale and cost-competitiveness toward HER. The <i>in situ</i> characterization of the Rh-P/C catalysts is will be conducted once the Advanced Photon Source at Argonne National Laboratory reopens from the pandemic.</p> Based on the combination of the advanced analysis, the structure-performance relationships were elucidated in these three projects, which provides a deep insight to rationally design more efficient catalytic systems targeting versatile catalytic processes.