Gu, Hao; (2024) Time-resolved operando x-ray absorption spectroscopy study on dynamics of metal redox for heterogeneous catalysis. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The primary focus of traveling lies not in reaching the destination, but in appreciating the scenery encountered throughout the journey. Which is the same for the research of heterogeneous catalysis. In most of the studies on catalysts or catalysis reactions, massive efforts were made for steady-state investigation, which is like the research on starting and ending points. However, what happened during the journey actually determines catalytic pathways and catalytic kinetics which are the most important parts for understanding the mechanism of catalysis reactions. With the evolution of characterization techniques, high temporal resolution can be achieved for capturing the fast or even ultra-fast process during reactions. X-ray Absorption Spectroscopy (XAS) as its elementally selective and lenient testing requirements is becoming a crucial tool for electronic and local structure investigations. Different time resolution can be meet for different dynamics study. From half and full catalysis reaction kinetics (s), to intermediates capturing (s to ns) and even dynamics charge transfer (ps to fs), time- resolved XAS plays an indispensable role with different experimental set-up and light source facilities. In this thesis, techniques with time-resolved XAS are introduced including the operation mechanism and principles, experimental set-up and related researches. Brilliant studies using different temporal resolution XAS investigating different processes are also reviewed to show the importance of such characterization technique for rational designing and catalysis engineering. There are three main projects concluded using time-resolved XAS in this thesis, studying the kinetics of half and full catalysis reaction, intermediates during metal catalysts synthesis, and dynamics of charge transfer between metals and supports. In the first project, we obtained the VO formation and conversion kinetics by measuring the rate of the Ce4+ reduction and oxidation via operando energy dispersive Extended X-ray Absorption Fine Structure (EDE). The key findings are: 1) The rate of VO formation is 10 times faster than VO conversion; 2) VO formation rates are similar to CO oxidation rates, making it the crucial step in CO oxidation; 3) Pd and Cu act as catalysts for VO formation, enhancing its rate by 50 times at 250 °C by weakening metal-O bonding strength, resulting in reduced activation energies of 58.4 kJ/mol and 36.5 kJ/mol, respectively. Our approach to measuring and analyzing partial reaction rates during turnover is essential for chemical reactions. The second project is encouraged by the XFEL to study the charge transfer dynamics. With soft XFEL in Pohang Accelerator Laboratory, we employ ultra-rapid X-ray Absorption Spectroscopy XAS at the Cu L3-edge and Ce M5-edge to observe changes in the electronic structure caused by the photo-induced movement of electrons between 8 Ce and Cu through the Ce-O-Cu bond. The time frame for this charge transfer process is approximately 200-300 femtoseconds. When comparing the time scales of intraband excitation (1-2 picoseconds) and charge transfer, only a fraction of the excited electrons migrate to Cu. Nonetheless, this minor electron migration significantly modifies the temporary electronic configuration of the Cu atoms dispersed on the surface. This alteration provides an extra catalytic push for the reactants attached to the Cu sites. The third project focuses on presenting a combination of in situ X-ray absorption fine structure (XAFS) and in situ small angle X-ray scattering (SAXS) data during Pd nanoparticle formation to monitor chemical and physical changes simultaneously and determine intermediate Pd0 species with a time resolution of 500 milliseconds. Distinct stages of reduction, nucleation, growth, and reoxidation were observed in the growth profile of CTAC-capped porous Pd nanospheres, with Pd0-Cl identified as the growth species reaching peak concentrations of around 6.72 mM. Introduction of Pd seeds altered the growth dynamics, resulting in a peak Pd0-Cl concentration of 5.73 mM without a distinct nucleation stage, showing temperature-dependent formation rates and an activation energy of 22.59 kJ/mol for Pd nucleation in a population-balanced precipitated growth model. The use of in situ XAFS and SAXS could be extended to other transition metal systems for studying growth kinetics and designing nanoparticles effectively.

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