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Title:Desulfurization by reactive adsorption on oxide/metal composites
Author(s):Behl, Mayank
Director of Research:Jain, Prashant K.
Doctoral Committee Chair(s):Yang, Hong
Doctoral Committee Member(s):Jain, Prashant K.; Schroeder, Charles M.; Seebauer, Edmund G.
Department / Program:Chemical & Biomolecular Engr
Discipline:Chemical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Metal oxides
Materials chemistry
Solid-state reaction
Strong Metal-Metal Oxide Interaction
Organo-sulfur degradation
Brønsted acidity
Sour gas
High temperature desulfurization
incipient wet impregnation
Abstract:Nanostructured oxides – undoped, doped, composites and nano-heterostructures – having different morphologies and compositions were synthesized, characterized and tested under realistic operating conditions for their role as potential reactive adsorbents and catalysts in different environmental related applications. More specifically, different zinc oxide based nanostructures were studied for high and low temperature desulfurization applications. In addition, the focus was on moving away from the traditional practice of using oxides only as support materials. Oxides have traditionally been assigned the role of support materials in applications involving catalysis. As support materials, they are either considered as inert or they do occasionally participate indirectly along with more active “dispersed” metals by “strong metal-substrate interaction”. This thesis presents results from a set of experiments with nanostructured oxide where reactive nature of the bulk oxide core is essential to the overall reaction. Most of the experimental schemes that constitute this thesis go beyond the role of oxides in surface reactivity to cases where bulk solid state reactivity is important. In experiments studying reactive desulfurization, different synthetic techniques were used to extend zinc oxide’s temperature range by combining structural and compositional modifications. To increase sulfur capacity and uptake rate of ZnO-based sorbents at low/high temperatures, it is necessary to achieve a rapid surface reaction and complete bulk sulfidation of the oxide. To attain this goal, surface activity and diffusion through the ZnS product layer, must be accelerated. Nano-structuring of oxides, controlling the size and morphology together with the thoughtful compositional changes can help attain the goal. But bulk of the research has concentrated only on improving surface reactivity. In chapter 3, where developing reactive ZnO based adsorbents for high temperature syngas-desulfurization is one of the goals, it was experimentally demonstrated that a branched nanofibrous architecture combined with mixed metal oxide based composition (Zn-Ti-O) offers significantly enhanced sorbent bulk reactivity and recyclability, without needing to cycle temperature. Zn-Ti-O based nanofibrous mats offer rapid reaction rates and efficient material utilization of the oxide core by overcoming rate limiting transport resistance, which often affects conventional pellet-based sorbents. Problems of higher energy use, sluggish reactions, incomplete regeneration and progressive decline in activity are successfully mitigated. Higher reactivity enables regeneration to be carried out at a temperature that is identical to the sulfidation step, preventing damage and extra energy use caused by alternating temperatures. The efficient regeneration of the adsorbent is also aided by structural features such as the growth of hierarchical nanostructures (secondary nanorods) and preferential stabilization of a wurtzite phase in the sulfidation product. Such unique features are are attributed exclusively to high-aspect ratio fibrous nature of the sorbent used in this study. In the first half of the thesis (Chapter-2), application of reactive oxides to low-temperature desulfurization is studied. Here, role of strong metal – metal oxide interaction was found to be influential not only in accelerating the surface reactions, as previously observed, but it is seen to play a role in improving solid-state reactivity of the oxide as well. Based on the survey of open literature, this is the first time such phenomenon has been associated with solid state conversion. It was found that nanocomposites of gold-ZnO have more surface acidic sites for hydrolysis of thioacetamide (TAA). In addition, presence of gold has a long-range influence on the bulk chemistry of ZnO, making the oxide more reactive towards solid state transformations. Addition of gold to ZnO resulted in higher equilibrium concentration of defects in the bulk ZnO along with the formation of new acidic sites on surface. This reduced the overall energy barrier by around 20 kJ/mol. The results from this study suggest that the intrinsic oxide defects and the defects in oxides induced by presence of external metal, which traditionally had been thought to be only a localized phenomenon, can possibly play an important role in oxide’s overall solid-state reactivity.
Issue Date:2014-05-30
Rights Information:Copyright 2014 Mayank Behl
Date Available in IDEALS:2014-05-30
Date Deposited:2014-05

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