​Abstract:  Converting solar energy into useful chemical bonds via photocatalysis is a growing field aimed at addressing global challenges. The research disclosed describes heterogeneous photocatalysis as a nanophotoelectrochemical cell as photocatalysts enable both reduction and oxidation reactions using the local charge separation of photo-excited carriers. Herein, experimental and theoretical results of nanoscale electrolysis of water on the surface of CrOx/Pt/SrTiO3 showed that ohmic losses are negligible when the anode and cathode are within nanometer distances from each other. Additionally, increasing the photocatalytic rate of water splitting by increasing the light intensity demonstrated that pH gradients can still form at the nanoscale. These pH gradients can be minimized by the incorporation of buffers. Typically, photocatalysts decorated with noble-metal nanoparticles can be used for overall water splitting, but generally suffer from low yields due to the water-forming back reaction. The unwanted water-forming back reaction was successfully suppressed by coating Pt nanoparticles on the surface of SrTiO3 with a 2nm CrOx layer that block O2 gas from reaching the surface of the Pt nanoparticle. The back reaction can also be suppressed without the use of a protective layer material by changing the intrinsic nature of the Pt nanoparticle from a metallic state to an oxidized state. The Pt nanoparticles were able to maintain an oxidized state by reducing the particle size below 2 nm. Oxidized Pt particles are less likely to bind to H2, O2, and CO gas, unlike metallic Pt, thereby making it selective for hydrogen generation. Finally, CdS was found to be perform the direct trifluoromethylation of heteroarenes in a single step as opposed to the current multi-step synthetic procedures. The trifluoromethylation of organic compounds is relevant to the field of medicinal chemistry for the synthesis of pharmaceutical drugs. By improving overall water splitting via photocatalysis significantly, artificial photosynthesis may be achieved leading to a solution to the global energy security dilemma. By improving photoredox catalysis of organic compounds via photocatalysis, high value organic compounds (such as pharmaceuticals) can be synthesized more readily under milder conditions.

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• CONTACTLinda J. Sapolu
• EMAILLinda.Sapolu@kaust.edu.sa

Abstract:  Pincer compounds are organometallic complexes with intriguing tunable reactivities, thus applicable to a wide range of potential applications. In this work we explore their unique properties and reactivities, with a focus on the PN3P pincer platform, through spectroscopic and computational investigations.

We conducted a computational study on pincer complexes with stereogenic phosphine arms and found that an energy difference as high as 16.8 kcal/mol could exist between the highest and lowest energy conformer, and that high energy conformers have bulky groups adjacent, causing steric clash. The full set of conformers for reactant, transition state and product were evaluated in a reaction energy profile of a CO2 reduction by a pincer nickel hydride, and we found that this reaction could be found either favorable or unfavorable depending on the choice of conformer.
The basicity of three PN3P* nickel pincer complexes have been determined in THF using the spectroscopic vverlapping indicator method. The relative basicity was found to be tBu(PN3P*)NiH > cPe(PN3P*)NiH > tBu(PN3P*)NiCl. This concludes that the imine arms of the PN3P* nickel hydrides are more basic than PN3P* nickel chloride, and changing the alkyl group on the phosphine arms had a smaller impact on the basicity compared to changing from chloride to hydride ligand on the nickel metal center.

Finally, we explored the reactivity between a PN3P* rhodium carbonyl pincer complex and dioxygen, at room temperature in solution, and at 180-200 degrees Celsius in the solid state. The solution reaction affords oxidation on pyridine moeity on the ligand backbone. This reaction between the singlet PN3P* rhodium carbonyl complex and triplet dioxygen is found to be possible due to the ligands ability to transfer an electron to dioxygen, creating a superoxide radical anion and a ligand-centered radical cation. This reaction is an example of the pincer ligand being redox non-innocent. The solid state reaction was studied with in situ rhodium K-edge x-ray absorption spectroscopy and an isobestic point was observed, indicating a clean reaction and a significant change occuring at the rhodium metal center. In situ FTIR studies revealed that the oxidation on the ligand also occurs in the solid state, but additionally the PN3P rhodium carbonyl complex facilitates the reaction between dioxygen and carbon monoxide to produce CO2.

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• CONTACTLinda J. Sapolu
• EMAILLinda.Sapolu@kaust.edu.sa

​Bio: Kai Lu is a Ph.D. candidate in the Earth Science and Engineering program. He graduated from University of Science and Technology of China as an undergraduate and achieved his master degree in KAUST. His research area includes seismic interferometry, seismic imaging, seismic field experiments and machine learning applications in seismic processing.

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• CONTACTKarema Alaseef
• EMAILKarema.alaseef@kaust.edu.sa

​Abstract:  We investigate experimentally the turbulent flow through a two-dimensional contraction. Using a water tunnel with an active grid we generate turbulence at Taylor microscale Reynolds number Re_lambda ~ 250 which is advected through a 2.5:1 contraction. Volumetric and time-resolved Tomo-PIV and Shake-The-Box velocity measurements are used to characterize the evolution of coherent vortical structures at three streamwise locations upstream of, and within the contraction. We confirm the conceptual picture of coherent large-scale vortices being stretched and aligned with the mean rate of strain. This alignment of the vortices with the tunnel centerline is stronger compared to the alignment of vorticity with the large-scale strain observed in numerical simulations of homogeneous turbulence. We judge this by the peak probability magnitudes of these alignments. This result is robust and independent of the grid rotation protocols. On the other hand, while the point-wise vorticity vector also, to a lesser extent, aligns with the mean strain, it principally remains aligned with the intermediate eigenvector of the local instantaneous strain-rate tensor, as is known in other turbulent flows. These results persist when the distance from the grid to the entrance of the contraction is doubled, showing that modest transverse inhomogeneities do not significantly affect these vortical-orientation results.

Biography:  Vivek is a PhD candidate in the Mechanical Engineering (ME) Program, PSE Division. He joined the High Speed Fluids Imaging Laboratory in 2015. He earned his bachelor’s degree in ME from Anna University Chennai, India in 2006; and master’s degree in Thermal and Fluids Engineering from Indian Institute of Technology Bombay (IITB), India in 2010. After completion of master’s, he worked as a faculty in the ME department at Amrita University Coimbatore, India. His current research is focused on tomographic measurements of turbulent flows.

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• CONTACTLinda J. Sapolu
• EMAILLinda.Sapolu@kaust.edu.sa

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Abstract: This thesis develops skeletonization methods for seismic inversion, seismic imaging, and machine learning to improve both their computational efficiency and accuracy. To obtain a good starting model for anisotropic full waveform inversion (FWI), the simultaneous inversion of anisotropic parameters vp0 and epsilon are initially performed using the wave-equation traveltime inversion (WT) method. Then a transmission+reflection wave-equation traveltime and waveform inversion (WTW) method is presented for a vertical transverse isotropic (VTI) medium where both traveltimes and waveforms are inverted for the velocity model.

The conventional FWI is sensitive to the amplitude mismatch between the recorded and predicted data. To mitigate this problem, multiscale phase inversion (MPI) is presented where the magnitude spectra of the predicted data are replaced by those of the observed data. Moreover, the data are integrated N times in the time domain to boost the low-frequency components. In this case, the skeletonized data are traces with the substituted magnitude spectra so that only the recorded phase data need to be inverted. 

I have developed a velocity-independent workflow for reconstructing a high-quality zero-offset reflection section from prestack data with a deblurring filter. This workflow constructs a migration image volume by prestack time migration using a series of constant-velocity models. A deblurring filter for each constant-velocity model is applied to each time-migration image to get a deblurred image volume. Ithis case the Hessian inverse is approximated by its skeletonized representation, also known as the deblurring operator. To preserve all events in the image volume, each deblurred image panel is demigrated and then summed over the velocity axis.

A fundamental step in aerial image georeferencing consists of determining the location of GPS control markers on the ground. The GPS marker has a unique hourglass shape and its color is dark. To take advantage of these features, superpixels are used as the skeletonized representations of the targets. Then a superpixel-based classification method is applied to the aerial images. The results show that this method quickly extracting the locations of GPS markers from aerial photographs.

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• CONTACTKarema Alaseef
• EMAILKarema.alaseef@kaust.edu.sa