Regenerable nickel catalysts strengthened against H2S poisoning in dry reforming of methane

Abstract

In this study, alumina-supported bimetallic Ni-Cu and trimetallic Ni-Cu-Ce catalysts were synthesized to improve catalysts resistant to coke formation and sulfur poisoning for dry reforming of methane (DRM). The effects of parameters such as feed composition, synthesis method, and H₂S concentration using the catalyst with the best activity were also investigated. To determine the physical and chemical properties of the synthesized catalysts, XRD, N₂ adsorption–desorption, TGA-DTA, ICP-OES, SEM-EDX, XPS, and DRIFTS analyses were performed. XRD analysis showed that the fresh Ni-Cu catalysts have elemental nickel and γ-alumina phases in their structures. In addition to these structures, the CeO₂ crystal structure was determined for the Ni-Cu-Ce catalyst. Type IV isotherm with H1 hysteresis indicating uniform mesoporous structure was obtained with all the catalysts. The activities of the synthesized catalysts in DRM were performed in the presence of different concentrations of H₂S (2 ppm, 50 ppm, and 500 ppm) in a fixed bed reactor at 750 °C using a gas chromatography-equipped system. The alumina-supported 8Ni-3Cu-8Ce catalyst prepared by the impregnation method exhibited a higher and more stable activity comparing the bimetallic Ni-Cu catalyst in the presence of H₂S. Adding copper and cerium to the nickel catalyst has a curative effect on resistance to coke formation and sulfur poisoning. Excess CO₂ in the feed stream increased the H₂S poisoning resistance of the catalyst. To analyze the reactor exit stream in catalytic activity using different feed stream compositions such as H₂S+He, H₂S+CO₂+He, and H₂S+CO₂+CH₄+He, FTIR with a gas cell was used. The formation of carbonyl sulfide (COS) and H₂O, which occurs due to the possible reaction between CO₂ and H₂S, was observed. Regeneration studies showed that the catalyst could undergo regeneration with a low oxygen concentration (0.3 % O₂ in He). 8Ni-3Cu-8Ce@SGA, which gave 71 % CH₄ conversion in the first minute of the reaction test in the presence of 50 ppm H₂S, was regenerated after completely losing its activity at the end of 5 h. 66 % CH₄ conversion was achieved when tested again in the absence of H₂S (CH₄/CO₂/Ar:1/1/1). The 8Ni-3Cu-8Ce@SGA catalyst was deemed worthy of investigation for industrial applications.