Two-step, oxygen-free conversion of methane over supported NiB amorphous alloy catalysts

Abstract

NiB amorphous alloy and Ni catalysts supported on HMCM-22, HZSM-5, HY, γ-Al2O3 and SiO2 were prepared, respectively, by the chemical reduction method and the standard incipient wetness impregnation method. These catalysts were examined for catalytic performance in the two-step conversion of CH4 to produce hydrogen and higher hydrocarbons. All catalysts give similar methane conversion and yields of hydrogen and H-deficient carbon-containing species in step I. In the subsequent hydrogenation step (step II), they have similar carbon conversion, however, the yield of C2 and C3 hydrocarbons depends greatly on the nature of the metal particles and support acidity. Supported NiB amorphous alloy catalysts offer higher yield of C2 and C3 hydrocarbons than the corresponding Ni catalysts, due to their unique properties: nanoscale size, long-range disorder in structure, and electron-deficiency. Of the zeolite supported catalysts, HMCM-22 and HZSM-5 supported catalysts produce higher yield of C2 and C3 hydrocarbons than the zeolite HY supported catalyst because of stronger acidity of the supports. A NiB/HMCM-22 catalyst shows a rather slow deactivation during a multiple reaction cycles test. High temperature favors CH4 decomposition and H2 production in step I, but makes the subsequent hydrogenation of carbon formed from CH4 decomposition difficult. The nature of the carbons formed from CH4 decomposition was also studied by XPS and TEM combined with H2-TPSR.