Theoretical Study of Water Gas Shift Reaction on α-Al2O3 (0001) Surface and Cu/α-Al2O3 (0001)

• Main Authors: Nur Pradani Damayanti, 如麗
• Other Authors: Jyh-Chiang Jiang
• Format: Others
• Language: en_US
• Online Access: http://ndltd.ncl.edu.tw/handle/37393858186712403209

碩士 === 國立臺灣科技大學 === 化學工程系 === 95 === Although a well established WGS industrial process has been developed, alternate catalysts are sought for WGS reaction in fuel cell application. Catalyst selection for the WGS reaction has, until recently, been based on trial-and-error screening of potential catalysts due to a lack of fundamental understanding of the catalyst’s functioning. Therefore the goal of this research is to study Al2O3 and Cu/Al2O3 as potential material for WGS catalyst system. DFT plane waved based using PAW pseudo potential and PW91 GGA has been applied to investigate surface, adsorption and reaction mechanism on five different systems:α-Al2O3(0001), Cu/α-Al2O3 (0001) 1/6 ML Cu, Cu/α-Al2O3 (0001) 4/3 ML Cu, Cu/α-Al2O3 (0001) with 3 H2O molecules, and Cu/Hydroxylated α-Al2O3 (0001). Redox, carboxyl and formate mechanism have been studied for WGS reaction in order to find the most favorable mechanism and the most favorable catalyst system that can be applied under industrial condition. Our results show Al terminated is the most stable surface termination and surface relaxation has significant effect to the 6 topmost layers. We observed α-Al2O3 (0001) is perturbed by Cu therefore enhances the interaction of CO and H2O with the surface. Carboxyl mechanism is the most favorable mechanism for WGS on α-Al2O3 (0001) however the high energy barrier indicates α-Al2O3 (0001) is potential support but not but not an excellent catalyst system for industrial purpose (Ebarrier industrial: 0.8-1.8 eV [42] ). Cu/α-Al2O3 (0001) at Cu coverage of 1/6 ML is potential catalyst system since WGS reaction on it has low barrier (acceptable in industrial range) via redox mechanism. Redox mechanism is more favorable to follow H2 route that means CO2 is formed after H2 formation. Moreover the increase of H2O coverage reduces the barrier of redox mechanism on Cu/α-Al2O3 (0001) 1/6 ML Cu, through proton transfer mechanism. Therefore we predict that H2O has autocatalytic properties in WGS reaction on Cu/α-Al2O3 at Cu coverage of 1/6 ML. The increasing coverage of Cu molecule as in Cu/α-Al2O3 4/3 ML Cu, does not give good enhancement for WGS reaction, since it shows dramatically high energy barrier for H2O dissociation, one of important step in WGS. However, Cu/α-Al2O3 4/3 ML Cu is a potential catalyst system for CO removal, since it exhibits low energy barrier for direct CO oxidation by O2 molecule. CO oxidation barrier is lower than CO oxidation on another system such as Au/TiO2, Pt and Ir [89-92]. Moreover we observed that direct CO oxidation by O2 molecule is more favorable than indirect CO oxidation via O2 dissociation. However, our result indicates that O2 will be easily dissociated on the Cu/��-Al2O3 4/3 ML for its low barrier. Cu/OH-α-Al2O3 (0001) is unfavorable catalyst system for WGS, since all of the three mechanisms have higher barrier on this system compare to another system in this research. Therefore we predict Cu should not be deposited on the hydroxilated surface. Neither Cu/α-Al2O3 1/6 ML Cu nor Cu/α-Al2O3 1/6 ML Cu with increasing of H2O coverage, shows that formate intermediate can be easily found. On the contrary, on α-Al2O3 (0001), energy barrier of formate intermediate formation is relatively low that indicate formate intermediate is easier to be found on the surface. However formate intermediate will lead into dead end mechanism in this system, for its high barrier.