Browsing by Author "Ghosh, Rajat-Subhra"
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Item Enhanced Selective Oxidation of NH3 in a Pt/Al2O3@CU/ZSM-5 Core Shell Catalyst(2020-12) Ghosh, Rajat-Subhra; Harold, Michael P.; Rimer, Jeffrey D.; Grabow, Lars C.; Brankovic, Stanko R.; Wang, DiThe ammonia slip catalyst (ASC) is an essential final step in the emission control system and involves the selective oxidation of NH3 to N2. The state-of-the-art ASC has a dual-layer architecture comprised of a Pt/Al2O3 (PGM) bottom layer and a metal (Fe, Cu)-exchanged zeolite (M-Z) top layer. The first part of the project deals with the challenges of reducing the PGM loading and ASC volume while enhancing low temperature activity. This is done by scaling down the dual-layer concept to the level of a single core-shell catalyst particle, Pt/Al2O3@Cu/ZSM-5, comprised of a PGM core and M-Z shell. The core-shell catalyst had an equivalent activity to that of a conventional Pt/Al2O3 catalyst containing 3-times higher Pt loading. The core-shell catalyst showed exceptional NH3 oxidation activity and N2 selectivity. The second part of the research is to investigate the NH3 oxidation kinetics over Pt/Al2O3 catalysts under atmospheric conditions for a wide range of NH3 concentration. We were able to show the non-monotonic reaction rate as a function of NH3 concentration for Pt/Al2O3 catalysts in presence of excess oxygen. We also showed the variation in reaction order from positive to negative order with zero order reaction at maximum reaction rate. Milling of Pt/Al2O3 resulted in enhanced NH3 oxidation activity. A microkinetic model with site-competition was developed which could predict the variation in reaction rate and order of the reaction. The final part of the research deals with the development of a working model for the core-shell catalyst with the microkinetics derived from the kinetic study over Pt/Al2O3 and along with multi-step SCR kinetic formulations to simulate the ASC performance of the Pt/Al2O3@Cu/ZSM-5 core-shell catalyst. Using the model we showed an optimized core-shell structure by investigating the ASC performance based on Pt loading and shell thickness. Finally, we discuss the scope of the future work based on our results and observations.Item Mesoscopic Protein Rich Phases in Ribonuclease a Solution(2016-12) Ghosh, Rajat-Subhra; Vekilov, Peter G.; Rimer, Jeffrey D.; Lubchenko, VassiliyDense liquid phases called clusters exist in protein solutions and act as a precursor to crystal nucleation. Clusters have a narrow size distribution and submicron size. These clusters are present in a minor fraction of total protein, not affecting the properties of the solution, but are essential precursors for the nucleation of ordered species. The experiments here are done with acidic protein solution of Ribonuclease A (RNase A). Here we studied the effect of ionic strength, urea and temperature with the help of Brownian microscopy (BM), static light scattering (SLS), and several biochemical assays. The cluster size distribution was measured for three days with BM and revealed a consistently narrow size distribution through this time period, indicating stability of clusters. The clusters respond to the change in protein concentration, this change is exhibited as decrease in cluster volume fraction as the protein concentration was increased, indicating these clusters are not irreversible aggregates. The intermolecular forces were varied by changing the ionic strength with NaCl, (NH4)2SO4 and CH3COONa and quantified by measuring second virial coefficient by SLS. The data revealed that intermolecular repulsion decreases the cluster size and cluster volume fraction. The hydrophobic interactions between the exposed hydrophobic residues due to partial unfolding and solvent was studied by the addition of urea, a chaotropic agent that destroys the shell of structured water around protein molecules. Elman’s assay was used to quantify the free thiols and the degree of unfolding was studied using ANS and ThT assays. The unfolding of protein by breaking disulphide bonds by heating showed an increase in cluster size and cluster volume fraction indicating the role of protein unfolding to aid the cluster formation. These experiments demonstrated the relevance of partial unfolding and intermolecular interactions in formation of clusters.