A Novel Two-Step Hydrogen Cycled Methane Reforming Process
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A Novel Two-Step Hydrogen Cycled Methane Reforming Process Richard C. Breitkopf1, Yardlynne Smalley1, Zhong-Lin Wang2, Robert Snyder2, Michael Haluska2, and Andrew T. Hunt1 1 nGimat Co., Nanomiser® Division, Atlanta, GA 30341 2 Georgia Institute of Technology, Department of Materials Science and Engineering, Atlanta, GA 30332-0245 ABSTRACT: We have prepared novel transition metal fluorite materials for a 2-step hydrogen reformer process that generates low CO hydrogen for fuel cell applications. The nanopowder materials, which are converted to an oxygen vacancy rich form in the first step of the process using methane or any hydrocarbon fuel are subsequently brought back to their original state using water vapor while generating pure hydrogen in the process. We have observed large weight losses in TGA measurements below 500°C with our nanomaterials that we do not observe in similar powders with larger grain size that comprise an efficient low temperature first step. We have also observed decreases in lattice parameter in high temperature XRD measurements consistent with formation of a high concentration of oxygen vacancies. The process has produced fuel efficiencies as high as 65% with typical CO content below 30 ppm using methane fuel. Additionally, since we are using nanomaterials, we have enabled low temperature hydrogen generation with good cycling stability at 500°C. INTTRODUCTION Hydrogen production has been a difficult barrier toward practical fuel cell usage. Most reformate hydrogen contains high levels of carbon monoxide whose strong affinity for precious metal electrocatalysts (eg Pt, PtRu) precludes direct use in fuel cells without further processing. This can be avoided using a novel 2-step cycled hydrogen production method. In the efforts of Kang and Wang [1] a purer hydrogen product is made by initial production of hydrogen syngas and creation of oxygen vacancies in the ceria lattice while recovering these vacancies in a second low temperature step that generates hydrogen directly from water at low (375oC) temperatures. In our reported work, ceria nanocatalysts helped advance the performance of the doped ceria. We used a combustion chemical vapor condensation (CCVC) flame-based technique to produce nanopowders which both greatly improve surface area and hydrogen production kinetics and allowed for rapid synthesis and testing of different dopants at varying compositions. We use MxCe1-xO2-δ as an example (M is a transition metal) to illustrate the principle of the two-step separated reactions, where x is the amount of M doping and δ represents, if any, a minor oxygen deficiency. The step-one reaction is the oxidation of CH4 by using the lattice oxygen transferred from the oxide: yCH4 + MxCe1-xO2-δ → MxCe1-xO2-δ-y + yCO +2yH2
(1)
This step is the first half of the cycle and creates the necessary oxygen vacancies for the oxide. In the second step, the oxygen lost by the oxide is recovered from water vapor producing hydrogen:
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MxCe1-xO2-δ-y + yH2O → MxCe1-xO2-δ + yH2. (2) This
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