Study on carbon monoxide preferential oxidation over copper/ceria based catalysts for low temperature proton exchange membrane fuel cell system

Abstract

This thesis investigates about the carbon monoxide preferential oxidation (CO PROX) used in low temperature proton exchange membrane fuel cell (PEMFC) systems. Among the various fuel cell types, low temperature proton exchange membrane fuel cell systems offer several advantages, such as high power density levels, rapid start-up capability and good on-off cycle reliability. Due to these advantages, many academic studies have been conducted, and recently several companies have tried to introduce such systems into commercial applications. However, the proton exchange membrane fuel cell systems cannot penetrate the market due to its drawbacks such as the high price of platinum based catalysts, water management problems, complexities of the system and its vulnerability to carbon monoxide (CO) and sulfur compounds, which are contained in the hydrogen fuel stream. In particular, the carbon monoxide poisoning of the anodes in the fuel cell is a difficult technological problem. A hydrogen rich gas mixture is used as fuel for the proton exchange membrane fuel cell

normally, this mixture is produced by reforming fossil fuels, such as city gas, liquid propane gas, gasoline, and diesel. During the reforming of the fuels, large volumes of carbon monoxide are produced and must be eliminated before the fueling of proton exchange membrane fuel cell. Even little amount of carbon monoxide in the hydrogen mixture can poison the platinum anode and degrade the long term performance of the fuel cell. Therefore, proper carbon monoxide removal between the fuel processor and the proton exchange membrane fuel cell must be ensured.

Among the various carbon monoxide removal strategies, the carbon monoxide preferential oxidation is the most widely used due to its ease of application in other fuel processors, needlessness of special pressurizing or recycling equipment, and its minimal hydrogen consumption from the reformate flow. However, current catalysts for the carbon monoxide preferential oxidation, based on expensive precious metals such as ruthenium, platinum and gold, present wetting problem caused by steam due to their low operating temperatures, and present methanation problem consuming large amounts of hydrogen. Thus, it is necessary to develop a new catalyst that operates at high temperatures to prohibit the wetting problem, and prevents the side reaction, such as methanation.

In this thesis, recently identified copper/ceria based catalyst was considered as a new catalyst for carbon monoxide preferential oxidation. Various copper/ceria based catalysts are synthesized via lab-made techniques to find the composition of metal and support material that had better performance of carbon monoxide preferential oxidation without producing the side reaction. As a result, catalysts containing 4 to 10 weight percent copper on a pure ceria support were selected from the prepared samples as superior catalysts for carbon monoxide preferential oxidation. Through the physical and chemical characterization of the prepared catalysts, the importance of surface copper species for the carbon monoxide preferential oxidation is identified. The surface copper species as an active metal on the copper/ceria catalyst should be exist in proper amounts and valences for the carbon monoxide preferential oxidation reaction. To ensure the strong carbon monoxide preferential oxidation performance, the co-existence of Cu+- and Cu2+-containing species in the appropriate amounts on the surface of the catalyst was critical. Moreover, the insertion of gadolinium as a dopant within a ceria lattice had a negative effect on carbon monoxide preferential oxidation performance of the samples. Surface characterizations confirmed that gadolinium doping in the ceria lattice disrupted the formation of an appropriate ratio between the Cu+-and Cu2+-containing species on the surface and this disruption impeded the performance of carbon monoxide preferential oxidation of the prepared samples on the gadolinium doped ceria. Through an atomic binding energy calculations, it was confirmed that a surface copper atom was stabilized in a surface oxygen vacancy as a Cu+ state. Therefore, the doping of gadolinium could affect the ratio between the surface Cu+- and Cu2+-containing species by creating stable surface oxygen vacancies and Cu+ species, thus impeded the performance of carbon monoxide preferential oxidation of the samples. Moreover, the substitution of surface Ce3+ into Gd3+ might serve as another negative factor for the performance of carbon monoxide preferential oxidation. Even though the co-existence of copper species was intentionally restored, overall redox properties of the samples supported on gadolinium doped ceria were weaker than those of the catalysts supported on pure ceria. Consequently, the redox equilibrium between the surface copper and cerium was critical to the performance of carbon monoxide preferential oxidation.

The reaction kinetics of the developed catalyst were also investigated for the reactor engineering. The isothermal fixed bed reactor equipment was constructed with several flow controllers to mimic various compositions and flow rates of reactants. The carbon monoxide oxidation reaction rate for the developed catalyst was found in power law form. The reaction rate contained the activation energy of carbon monoxide oxidation and the reaction orders of different reactant species. Specifically, the reaction orders of the carbon dioxide and steam were found to take a negative value, exhibiting hindering effects for carbon monoxide preferential oxidation over copper/ceria catalyst.

Finally, simple computational fluid dynamic (CFD) simulations were conducted from the revealed reaction rate. The commercial computational fluid dynamic tool was used to create an isothermal reactor model. Several case studies that involved changing engineering variables e.g., the temperature, gas hourly space velocity (GHSV), the loading mass of the catalyst and the oxygen to carbon monoxide (O2/CO) ratio were conducted. The results of the case studies were used for the design of the carbon monoxide preferential oxidation reactor in a certain fuel cell system. Additionally, the developed copper/ceria catalyst exhibited strong carbon monoxide selectivity and stable carbon monoxide oxidation capacities when the oxygen to carbon monoxide ratio was increased in the reactor. Based on the simulation results, a dual carbon monoxide preferential oxidation reactor concept was proposed, using the developed copper/ceria catalysts and the excess oxygen strategy, for the construction of fuel processors for low temperature proton exchange membrane fuel cell systems.