Chemical Kinetics and Optimization of Continuous Syngas Production in a Chemical Looping Scheme Over CexZr1-xO2 Supported Ni Catalysts

Author

Zoe C. Benedict, University of MaineFollow

Date of Award

Summer 8-18-2023

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science (MS)

Department

Mechanical Engineering

Advisor

Yingchao Yang

Second Committee Member

Rebecca Fushimi

Third Committee Member

Carl Tripp

Additional Committee Members

Sharmila Mukhopadhyay

Abstract

Carbon dioxide (CO₂) and methane (CH₄) are the two major greenhouse gases contributing to global climate change. One strategic pathway for repurposing them is by energy efficient chemical manufacturing of syngas – a combination of hydrogen (H₂) and carbon monoxide (CO) –a precursor to many hydrocarbon fuels and chemicals. Chemical looping continuous syngas production bypasses intense energy requirements associated with downstream purification steps of traditionally used methane reforming methods. Instead of cofeeding reactants, CH₄ and CO₂ are passed individually over a reduction-oxidation (redox) catalyst in half cycles. First, CH₄ is partially oxidized by lattice oxygen of an oxide catalyst to produce H₂ and CO. Subsequently, CO₂ is dissociated to replenish the lattice oxygen, further producing CO. These two half cycles can be tuned for better performance and are run cyclically for continuous syngas production.

In this work 5wt.%Ni/Ce_1-xZr_xO₂ (x = 0, 0.4, 0.625) catalysts were used in a chemical looping scheme to hone reaction and catalyst parameters, catalyst redox state, and half cycles to garner insight to tune for better reactions. Special emphasis was placed on utilizing coke, a side product from CH₄ decomposition, which can deactivate the catalyst if not oxidized off. An economic analysis of common catalysts for syngas production from CH₄ showed Ni/Ce_1-xZr_xO₂-based catalysts demonstrate inexpensive high performance. The stoichiometry of 5wt.%Ni/Ce_1-xZr_xO₂ (x = 0, 0.4, 0.625) catalysts was further optimized for continuous syngas production. 5wt.% Ni wet impregnated on coprecipitated Ce_1-xZr_xO₂ (x = 0, 0.4, 0.625) supports was characterized by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Chemical looping experiments in a fixed bed reactor using 25 sccm of gas flow comprising 10% CH₄ or CO₂ balanced with Ar indicated the catalyst composition 5wt.%Ni/Ce_0.6Zr_0.4O₂ best suited for this application. Carbon accumulations quantified by thermogravimetric analysis (TGA) were verified to be effectively removed by CO₂. Effect of CH₄ and CO₂ chemical looping conversion on 5wt.% Ni/Ce_0.6Zr_0.4O₂ structural and morphological features were investigated by XRD, electron microscopy, ex situ Raman, and transient reaction methods. Additionally, the role of initial catalyst redox state on reaction performance was explored. Although pre-reduction had minimal effect on multi-cycling performance, redox state impacted the initial cycles and carbon growth. Pre-reduction introduced oxygen vacancies that suppressed CH₄ total oxidation and hard-to-oxidize graphitic carbon forms, facilitating carbon removal in CO₂ half cycles. Both fresh and reduced samples formed multiwalled carbon nanotubes (MWCNTs). Carbon formation with time on stream of CH₄ was characterized to inform the appropriate half cycle timing, restricting the carbon accumulation only to easily oxidized carbons. Electron microscopy, ex-situ Raman spectroscopy, and TGA determined Ni-catalyzed MWCNTs became increasingly graphitic until full encapsulation deactivated nickel. Shorter half cycle times this and enhanced continuous syngas production.

These investigations demonstrate the potential opportunity of utilizing economic 5wt.% Ni/Ce_0.6Zr_0.4O₂ for sustainable repurposing of greenhouse gases (CH₄, CO₂) towards syngas production. Elucidating the underlying redox chemistry paves the path forward for designing the optimum chemical looping reaction conditions for energy-efficient sustainable chemical manufacturing. This work is thus an effort towards showcasing a potential process for decarbonized chemical manufacturing.

Recommended Citation

Benedict, Zoe C., "Chemical Kinetics and Optimization of Continuous Syngas Production in a Chemical Looping Scheme Over CexZr1-xO2 Supported Ni Catalysts" (2023). Electronic Theses and Dissertations. 3822.
https://digitalcommons.library.umaine.edu/etd/3822