Saudi Cultural Missions Theses & Dissertations
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Item Restricted DENSITY FUNCTIONAL THEORY INVESTIGATIONS INTO METHANE ACTIVATION, OXIDATION, AND DIFFUSION ON IrO2(110), RuO2(110), AND MIXED METAL OXIDE MO2(110) (M=Ir & Ru)(Saudi Digital Library, 2023-12-01) Almarshad, Omar; Hibbitts, David; Weaver, JasonThe processes of methane adsorption and oxidation over late transition metal oxides of rutile type, as well as the adsorption and ensuing interaction of probabilistic species with the intermediate products of methane oxidation on the surfaces of iridium dioxide and ruthenium dioxide, and their various combinations, have been comprehensively examined utilizing the principles of density functional theory (DFT). Density functional theory calculations serve as a critical tool in this study, elucidating the stability of these mixed metal oxides, the energy dynamics of the corresponding reaction pathways, and the diffusion properties potentially influencing bifunctional reaction mechanisms. Further, DFT was employed to calculate reaction and activation-free energies pertinent to the methane reaction pathways on surfaces of IrO2(110) and mixed oxides. These computations provide valuable insights into the reaction mechanisms, critical structural properties, and the capacity of these pathways to function under steady-state catalytic conditions. This enhanced understanding permits the identification and mapping of the potential mechanisms that drive these reactions, contributing significantly to the body of knowledge on the structural properties that could determine the viability of these pathways under continuous catalytic conditions.40 0Item Restricted Reconstruction of Rhodium Clusters During CO Oxidation and Consequences on The Reaction Mechanism(2023) Albrahim, Malik; Ayman, KarimHeterogeneous catalysis plays a significant role in the chemical industry and the global economy. Most heterogeneous catalysts in the chemical industry and laboratory consist of supported metal nanoparticles, clusters and isolated (single) atoms. Understanding structure sensitivity and identifying the active site or sites are crucially essential for designing efficient catalysts. To determine the active sites of a catalyst for a particular chemical reaction, in-situ/operando spectroscopy, such as diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and X-ray absorption fine structure (XAFS) spectroscopy, is usually implemented as characterization tools. However, understanding the limitation of the characterization tools is crucial to eliminate misleading conclusions. Therefore, the main object of this work is not only to characterize the catalyst before and after the reaction but to investigate the reliability of the characterization tools as well as the stability of the metal clusters and single atoms during CO oxidation. Four main findings will be present in this work. First, a high-flux X-ray beam can induce structural change that leads to a reduction of the metal and agglomeration of metal clusters. This finding is very important since X-ray beam damage is uncommon for heterogeneous catalysis as for homogeneous catalysts and biological samples. In the study, the effect of high-flux X-ray on the Rh clusters and nanoparticles was highlighted along with providing mitigation strategies in order to reduce the damage caused by the high-flux X-ray beam. The second important finding is about the characterization of Rh clusters and nanoparticles during CO reduction treatment using DRIFTS. In this study, the integration of low-temperature CO oxidation kinetics as a characterization tool with DRIFTS, XAFS and scanning/transmission electron microscopy (STEM) was found to be necessary to improve the characterization of Rh single atoms. Implementing CO oxidation measurements at low temperatures can provide a rough estimation of the percentage of Rh single atoms. The third finding is related to the stability of Rh clusters upon exposure to CO, oxygen and CO oxidation at different temperatures. The study shows an unexpected dynamic structural change that the Rh cluster undergoes during exposure to oxygen even at room temperature in which the Rh clusters disperse to form Rh single atoms. This dispersion phenomenon was found to be size, gas environment and temperature dependent. For example, small clusters tend to disperse while large nanoparticles resist dispersion. additionally, increasing the temperature to ∼ 160 with CO and oxygen lead to an increase in the percentage of Rh single atoms. More importantly, the dispersed catalyst (Rh single atoms) exhibits higher CO oxidation activity than Rh nanoparticles by 350x. This finding can also be used for Rh single atoms synthesis for different oxide supports such as MgAl2O4 , TiO2 , and CeO2 . Finally, the fourth finding is about investigating the CO oxidation kinetics and mechanism. The kinetics of Rh single atoms differ from Rh nanoparticles. Implementing in-situ spectroscopy helps to identify the resting state of the Rh complex during CO oxidation which is Rh(CO)2 . By combining CO oxidation kinetics and in-situ spectroscopy, the plausible mechanism was suggested to be Eley-Rideal/Mars Van Krevelen mechanism.31 0