Novel Combination of Cerium and Nickel in Ce-Ni-MFI Catalysts in Low-Temperature Water Gas Shift Reaction

Abstract

Increasing demand for clean fuel has raised interest in the water gas shift reaction (WGSR), a well-established industrial technology that produces hydrogen as a clean, highly-efficient and recyclable fuel. This reaction is exothermic and reversible; therefore, catalysts are used to reduce the operating temperature and improve the performance of the process. The synergistic effect between nickel (Ni) and cerium (Ce) as catalysts in the WGSR is well-known. The cost of these metals is continuously increasing, as they are widely-used in industry, hence making the advent of novel low percentage and well dispersed nickel-cerium catalysts a necessity. In this research, novel and highly-efficient nickel-cerium catalysts were synthesized exhibiting several important features. First, low percentages of nickel and cerium (less than 3%) were incorporated in a crystalline silica framework (MFI). Second, the catalysts showed a high yield in the low-temperature WGSR (453 K – 593 K). Third, the catalysts showed high selectivity, hence the undesired produced methane was negligible. Last but not least, the catalysts benefit from simpler preparation, in which impregnation of metals, drying, and further calcination were not necessary. The produced solids were characterised by several techniques including x-ray diffraction (XRD), temperature-programmed reduction or oxidation (TPR-TPO), temperature-programmed desorption (TPD), scanning electron microscopy and energy dispersive X-ray analyzer (SEM/EDX), inductively-coupled plasma mass spectrometry (ICP-MS), and N2 physisorption. Ce-MFI solids were inactive at lower temperatures. However, Ni-MFI solids catalyzed the reaction, and more nickel led to higher activity. The synthesized Ce-Ni-MFI catalysts synergistically accelerated WGSR, and catalysts with similar mass percentage of cerium and nickel outperformed others. Furthermore, the maximum reduction temperature of these solids was reduced, confirming their higher performance in the WGSR. Moreover, the O-exchange capability of the solids was investigated using isotopic water H218O. Cerium was more active than nickel in water splitting. However, their combination outperformed the single metal solids in water splitting. Finally, to design a proper reactor for the WGSR using the proposed catalysts, kinetic expressions were determined to predict the reaction rate. Toward this goal, first, mass transfer limitations were eliminated. Then, several experiments were performed at different temperatures, and with different molar ratios of the reactants (CO and H2O). The effects of these changes on the reaction rate were analyzed, and the kinetic expression was obtained using an empirical approach, a microkinetic method, and an artificial neural networks technique. The calculated reaction rate was in good agreement with experimental results and the microkinetic study confirmed the cooperation of Ce and Ni for the dissociation of water.