Reaction Kinetics of Catalytic Dry Reforming of Methane on Spinel-Derived Nickel Catalyst at Low-Temperature Using an in-Situ Stagnation-Flow Reactor

Abstract

Dry reforming of methane (DRM) has been implemented to hydrocarbon-fueled ceramic fuel cells. This reduces their carbon footprint by utilizing carbon dioxide as a reactant. However, its application to low-temperature ceramic fuel cells has not been investigated, in which sluggish reforming kinetics and low durability induced by carbon coking should be resolved and overcome. In this study, DRM at low-temperature is examined by using spinel-derived nickel catalyst on aluminum oxide employed in a stagnation-flow reactor. With increasing calcination temperature, homogeneous mixture of nickel oxide and aluminum oxide show phase transition from mechanically mixed Ni/Al2O3 to chemically packed crystallite Ni/Al2O4 spinel structure. Due to their chemical structure, they have two different size of active nickel particle on aluminum oxide under reducing environment and show different catalytic activity. To elucidate the fundamental surface properties of fabricated catalysts, surface characterization technique such as XRD, TEM, BET, TPR, and CO-chemisorption are used. After surface characterization, their performance for DRM and reaction mechanism are examined by powder setup with gas chromatography and in-situ stagnation-flow reactor with mass spectrometer. The stagnation-flow reactor enables micro quartz-probe sampling of the compositional boundary layer in the vicinity of the catalyst surface. The vertical distance of micro quartz-probe from the catalytic surface is manipulated by a step-motor, and sampled gas are quantitatively analyzed with a quadrupole mass spectrometer. This configuration also makes it relatively simple to resolve numerically the complex reacting flow environment coupled with mass/momentum/energy transport phenomena. A numerical model can examine quantitatively the net production rate of each elementary step (48-step surface reaction mechanism) with varying operating conditions. Intrinsic reaction parameters such as a sticking coefficient of reactant adsorption, pre-exponential factor and activation energy of each reaction step are tuned by using experimental measurements. To determine the main reaction pathways and rate-determining steps, partial equilibrium, activation energy of C-H bond in methane and C-O bond in carbon dioxide, a sensitivity analysis with respect to reaction parameters is conducted. It is elucidated that the dry reforming of methane is pre-dominantly rate-determined by methane dehydrogenation steps. Especially, at low temperature region, an apparent activation energy of methane conversion increases sharply with the temperature, whereas that of carbon dioxide shows a trivial increase. The simulation results also show the dependence of thermal/fluidic effects on reaction kinetics including the catalyst temperature, reactant flow rates, mass/thermal diffusion in a porous catalyst layer. In this study, the detailed elementary reaction steps are developed, which enables predicting the effect of the reacting-flow environment on the overall reaction kinetics of dry reforming of methane.