DOPANTS AND SURFACE EFFECTS ON Mo-BASED BIFUNCTIONAL CATALYST FOR METHANE DECOMPOSITION

Abstract

Specialized catalysts are required for high-yield carbon nanotube (CNT) and hydrogen productionfrom catalytic methane decomposition (CMD). Developing upgraded NiMo/MgO catalysts necessitates understanding of the dynamics of metal evolution and CNT growth. Here we evaluate the potential of several dopants (Gd, Ga, Bi, La) over NiMo/MgO and establish a new waterinduced metal exposure method in the catalyst preparation to boost methane activation and produce base growth CNTs. Catalysts doped with 0.2wt% Gd by incipient wet impregnation (IWI) showed higher carbon yields than other dopants. However, the method of dopant introduction impacted measured rates significantly after comparative analysis. It was determined that NiMo/MgO posttreatment, comprising of a water washing step and a second calcination, facilitated metal exposure that increased 30% of the initial catalyst activity. Upon treatment, carbon yields increased from 12.5 gC/gcat to 14.3 gC/gcat, generating a narrower CNT diameter distribution and more graphitic carbon after 3 hours on stream. Gd doping by this method showed a modest 5% increase on rate enhancement. Characterization by BET, XPS, XRD, TEM, and Raman revealed that water induced metal exposure increases the availability of Ni active sites on the surface by displacing Mo species during washing. Graphitic quality of CNTs increased by 10%, and average diameter shrank by 35%. Ni/Mo ratios were found to impact catalytic activity more than dopant addition. This work reveals an effective dopant impregnation method in catalyst preparation and provides insights into CNT growth understanding via surface phase interactions of treated and standard NiMo/MgO, demonstrating a synergistic effect between metal exposure and dopant addition.