Dry reforming of methane on Ni-Fe-MgO catalysts: Influence of Fe on carbon-resistant property and kinetics

Abstract

Dry reforming of methane (DRM) converts two major greenhouse gases into syngas, a versatile chemical feedstock, requiring the identification of a robust catalyst. In this contribution, we explored the promoting effect of Fe in MgO supported Ni-Fe alloy catalysts in DRM. Towards this end, a series of catalysts with varied Fe/(Ni+Fe) ratios (0 to 0.17) and homogeneous metal distribution were prepared from a single FexNiyMg1-x-y(OH)2 (x = 0.00–0.07, y = 0.00–0.07) precursor derived from solvothermal synthesis. Catalytic evaluations at 760 °C and 800 °C showed that Fe passivated the activity of Ni surface while promoting the coke-tolerant property. Structural evolution and coke deposition of catalysts were extensively characterized by using XRD, SEM, TEM, HAADF-HRTEM, H2-TPR, CO2 (O2)-TGA and CO2-TPO techniques, manifesting that alloying of Fe with Ni had enhanced catalyst stability. Lowered amount of coke was found on spent Ni-Fe alloy catalysts with respect to pristine Ni catalyst, and Fe also changed the type of surface carbon from a refractory type to a soft one that can be readily gasified by CO2. Fe also increased the surface oxyphilicity of alloy particle surface via balancing the otherwise carbon affinity of Ni surface. Optimized catalytic performance was attained by FexNi0.07-xMg0.93O-R (x = 0.003–0.006) catalyst. Kinetic studies and coke gasification kinetic disclosed that Fe slowed down coke deposition rate and concomitantly accelerated surface coke gasification rate, without changing the essential reaction mechanism or kinetic features of pristine Ni catalyst. The promoting effect of Fe was found to be highly dependent on CO2 contents in the contacting atmosphere, as well as Fe/Ni ratios in the alloy. The stability of Ni-Fe alloy catalyst in DRM can be ascribed to combined effects from both ensemble size reduction and surface oxyphilicity change. The role Fe plays in enhancing DRM coke-resistant property is potentially useful for alike reforming catalyst and process design.