Date

2017

Document Type

Dissertation

Degree

Doctor of Philosophy

Department

Chemical Engineering

First Adviser

Israel E. Wachs

Abstract

Industrial hydrogen production from various resources is driven by the world’s advancement towards a hydrogen economy. The water-gas shift (WGS) catalytic reaction is an intermediate step between reforming of carbonaceous materials and hydrogen production. This catalytic reaction is important for hydrogen enrichment and carbon monoxide reduction. Copper-supported chromium-iron oxide (Fe3O4-Cr2O3-CuO) is the main catalyst system for the high temperature (HT) WGS redox reaction. Density functional theory calculations were carried out to investigate the role of the chromium and copper, which were demonstrated to act as textural and catalytic promoters, respectively, for the Fe3O4-Cr2O3-CuO catalyst system by in situ experimental studies at the atomic scale. The present study tries to explain the adsorption preferences of CO and H2O molecules on the Feoct2 termination of Fe3O4 (111) surface and the Feoct2 terminated surface that is modified by Cr substitution, adsorption of a Cu4 cluster and also the redox property of the surfaces. There is a minor effect of Cr on the dissociative adsorption of H2O, but no effect on CO adsorption indicating that Cr does not act as a chemical promoter. Copper promotion of the Feoct2 terminated structure with a supported Cu4 cluster facilitates CO adsorption at the new active sites present at the copper-iron oxide interface. Although Cr-Fe oxide-based catalysts are still the cornerstone for high temperature WGS reaction, the environmental and health issues arising from the presence of carcinogenic Cr6+ has motivated this investigation to look for alternative promoters to the toxic chromium. As indicated above from DFT calculations and the experimental literature, chromium functions as a structural promoter that prevents thermal sintering of iron oxide at elevated WGS reaction temperatures. In this study, zirconia-, niobia-, ceria-, and alumina-promoted supported copper/iron oxide catalysts were synthesized and investigated. The prepared catalysts were investigated with ex situ XRD, in situ Raman spectroscopy, High-Sensitivity Low Energy Ion Scattering (LEIS), CO TPR and flow BET surface area. The characterization studies revealed the interplay between the different components of this dynamic catalyst system. The promoted catalysts were found have significantly greater thermostability than the unpromoted iron oxide catalyst. The promoted catalysts without copper did not perform better than the unpromoted iron oxide catalyst for the WGS reaction. The ~3x higher catalytic activity of the supported 3Cu/8CeFe catalyst than the Cu-free 8CeFe catalyst may be due to a strong metal-support interaction between the metallic copper and the CeO2 nanoparticles and Fe3O4 surface. Al promoters provides better thermostability and higher activity. Potentially designed Cr-free catalyst is the combination of BET stability of Al promoter with redox activity of Ce promoter. This research study combines theoretical and experimental investigations under the partnership between Department of Chemical & Biomolecular Engineering of Lehigh University and the Department of Chemical Engineering of Middle East Technical University (Turkey).

Available for download on Sunday, September 01, 2019

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