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Steam Reforming of Lower Alkanes and the Water-Gas Shift Reaction

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Steam Reforming of Lower Alkanes and the Water-Gas Shift Reaction INVESTIGATION OF ACTIVE SITES AND REACTION NETWORKS IN CATALYTIC HYDROGEN PRODUCTION: STEAM REFORMING OF LOWER ALKANES AND THE WATER-GAS SHIFT REACTION DISSERTATION Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University By Sittichai Natesakhawat, M.S. * * * * * The Ohio State University 2005 Dissertation Committee: Approved by Professor Umit S. Ozkan, Adviser Professor Jeffrey J. Chalmers Professor David L. Tomasko _________________________ Professor Heather Davis Adviser Graduate Program in Chemical Engineering ABSTRACT Steam reforming (SR) of hydrocarbons and water-gas shift (WGS) are of importance in hydrogen generation technologies due to recent attention focusing on fuel cells. Current research efforts have focused on catalyst development to improve activity, selectivity, and stability under a realistic range of operating conditions. Although coke formation is a major concern for Ni-based catalysts when used for hydrocarbon steam reforming, their low cost and long-proven performance warrants further investigation. Moreover, hydrogen and fuel cell technologies will greatly benefit from innovative high- temperature shift (HTS) catalyst formulations that can overcome some serious drawbacks (i.e., low activity at low temperatures, sintering of magnetite (Fe3O4), a pyrophoric nature, and the harmful effects of Cr6+ on human health). In this Ph.D. study, lanthanide-promoted Ni/Al2O3 catalysts and Fe-based catalysts promoted with first row transition metals were synthesized by a modified sol-gel technique and a coprecipitation method, respectively. The effect of synthesis variables on the catalyst properties (i.e., BET surface area, reducibility, crystallite size, crystal structure, oxidation states, adsorption/desorption behavior, and surface intermediates during the reaction) and, in turn, on the catalytic performance in SR of lower alkanes and ii the WGS reaction has been investigated. Catalysts were characterized by BET surface area and pore size distribution measurements, temperature-programmed desorption/reduction/oxidation (TPD/TPR/TPO), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), static chemisorption, transmission electron microscopy (TEM), thermogravimetric analysis (TGA). The introduction of small amounts (2 wt%) of Ce and Yb in sol-gel Ni/Al2O3 catalysts improves SR activity significantly by enhancing Ni surface area, reducibility of Ni species, and Ni sintering resistance. In addition, the incorporation of Ce in Ni/Al2O3 catalysts results in the reduction of carbon dissolution/diffusion through nickel crystallites, thus preventing catalyst deactivation by coke formation. Formulations of highly active and Cr-free HTS catalysts can be achieved by adding both Al and Cu. The activity of Fe-based catalysts is strongly related to catalyst reducibility, magnetite crystallite size, and redox characteristics. More detailed characterization has provided additional insights into phase transformations, surface intermediates, and reaction/deactivation mechanisms, giving further evidence of the modification of the properties and performance of catalysts through optimized synthesis. iii Dedicated to my mother, Wannaporn Natesakhawat iv ACKNOWLEDGMENTS This dissertation is more than the product of the last four years and a half of my research. It represents contributions and influences from everyone who has taught me, inspired me, challenged me, or supported me throughout the process. First and foremost, I wish to thank my research adviser, Dr. Umit S. Ozkan, who not only guided my research but also served as a role model. This dissertation would not have been possible without her support and dedication. I was also fortunate enough to know and work with many past and present members of the Heterogeneous Catalysis Research Group. I would like to thank Dr. Okan Oktar, Dr. Xueqin Wang, Dr. Rick B. Watson, and Lingzhi Zhang for their help and stimulating discussion about research issues. From this group, I would like to especially thank Rick for his friendship and guidance. My biggest thanks go to my family especially my mother, Wannaporn Natesakhawat, for their love, support, and encouragement throughout not only the course of my Ph.D. studies, but throughout my entire life. Nothing in a simple paragraph can express the love and gratitude I have for them. I also wish to thank the Royal Thai Government for providing me financial support for my graduate studies. This research was supported by a grant from Honda Research Institute, USA Inc. and the Ohio Coal Development Office. v VITA September 8, 1973………………………..Born – Nakorn Pathom, Thailand 1994………………………………………B.E. Chemical Engineering, Kasetsart University 1998………………………………………M.S. Chemical Engineering, Colorado School of Mines 2000-present……………………………...Graduate Research Associate, The Ohio State University PUBLICATIONS Research Publication 1. J.G. Goodwin, Jr., S. Natesakhawat, A.A. Nikolopoulos, S.Y. Kim, “Etherification on Zeolites: MTBE Synthesis” Catal. Rev. 44 (2002) 287. 2. S. Natesakhawat, O. Oktar, U.S. Ozkan, “Lanthanide-Promoted Ni-Based Catalysts for Steam Reforming of Propane” Prepr. Pap.-Am. Chem. Soc., Div. Fuel Chem. 48 (2003) 852. FIELD OF STUDY Major Field: Chemical Engineering vi TABLE OF CONTENTS Page Abstract............................................................................................................................... ii Dedication.......................................................................................................................... iv Acknowledgments................................................................................................................v Vita..................................................................................................................................... vi List of Tables ..................................................................................................................... ix List of Figures.................................................................................................................... xi Chapters: 1. Introduction......................................................................................................................1 2. Literature review............................................................................................................10 2.1 Steam reforming of hydrocarbons ........................................................................10 2.1.1 Methane steam reforming ....................................................................11 2.1.2 Steam reforming of higher hydrocarbons ............................................18 2.1.3 Catalyst deactivation by coke formation..............................................25 2.2 The water-gas shift reaction..................................................................................31 2.2.1 Precious metal catalysts .......................................................................31 2.2.2 Non- precious metal catalysts ..............................................................33 3. Experimental..................................................................................................................42 3.1 Catalyst synthesis..................................................................................................42 3.1.1 Ni-based catalysts ...............................................................................42 3.1.2 Fe-based catalysts ...............................................................................43 3.2 Catalyst characterization.......................................................................................44 3.2.1 BET surface area measurement ..........................................................46 3.2.2 X-ray diffraction (XRD) .....................................................................46 3.2.3 Temperature-programmed desorption/reduction/oxidation (TPD/TPR/TPO) .................................................................................47 3.2.4 X-ray photoelectron spectroscopy (XPS) ...........................................50 3.2.5 Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) ......................................................................51 3.2.6 Chemisorption.....................................................................................53 vii 3.2.7 Transmission electron microscopy (TEM) .........................................54 3.2.8 Thermogravimetric analysis (TGA)....................................................55 3.3 Steady-state reactions............................................................................................56 4. Effect of lanthanide promotion on catalytic performance of sol-gel Ni/Al2O3 catalysts in steam reforming of propane....................................................................................62 4.1 Introduction...........................................................................................................62 4.2 Physical properties of sol-gel catalysts.................................................................64 4.3 Promotion effect on catalyst reducibility and nickel surface area ........................67 4.4 Reaction performance in propane steam reforming..............................................78 4.5 DRIFTS studies.....................................................................................................92
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