Effect of Zn on performance of Ni/Al2O3–MgO catalyst for dry reforming of methane: enhancing activity and remarkably suppressing graphitization of carbonaceous deposit

Abstract

MgO–Al₂O₃-supported metallic Ni and bimetallic Ni–Zn catalysts (referred as Ni/MgAl and Ni_xZn/MgAl with the Ni/Zn atomic ratio of x, respectively) were prepared by the coprecipitation method, and their performance for methane dry reforming (DRM) was tested on an atmospheric quartz fixed-bed. The XRD, magnetic measurement, H₂-TPR, TEM/EDS and XPS results demonstrate that metallic Ni and Ni–Zn alloy form in Ni/MgAl and Ni_xZn/MgAl, respectively. With reducing the Ni/Zn atomic ratio, the amount of surface Ni sites decreases, while the CO₂ adsorption capacity and strength increase due to the presence of Zn. Ni_xZn/MgAl gives higher activity than Ni/MgAl for DRM, mainly due to the suppression of carbon deposition and graphitization on Ni–Zn alloy and the enhanced adsorption of CO₂. Under the condition of 800 °C, CH₄/CO₂ molar ratio of 1 and weight hourly space velocity of 20,000 mL g_cat⁻¹ h⁻¹, Ni/MgAl gives the CH₄ and CO₂ conversions of 76% and 80%, respectively. With decreasing Ni/Zn atomic ratio from 7 to 2, the CH₄ and CO₂ conversions on Ni_xZn/MgAl first increase and then decrease, and Ni₅Zn/MgAl has the highest activity with the CH₄ and CO₂ conversions of 81% and 87%, respectively. During the time on stream of 100 h at 800 °C, Ni₅Zn/MgAl exhibits better stability than Ni/MgAl, mainly ascribed to its less amount of filamentous graphite carbon. In all, the formation of Ni–Zn alloy enhances the catalyst activity and remarkably suppresses the graphitization of carbonaceous deposit.