Decomposition of methane into carbon and hydrogen over Ni-Li/CaO catalysts

Abstract

Overdependence on fossil-based fuels and their effect on environment is a global concern by energy stake holders. Bulk of present day hydrogen comes from gasification of coal, steam reforming and partial oxidation of hydrocarbons. Steam reforming accounts for over 50% of world hydrogen production despite producing carbonaceous gases which are harmful to the environment and poisonous to both; proton exchange fuel cells and alkaline fuel cells. Natural gas is a preferred feed for hydrogen production, because it is abundantly available on earth. Catalytic decomposition of ammonia can produce clean hydrogen but ammonia itself is an air pollutant. Catalytic decomposition of methane into carbon and hydrogen is an attractive option to producing clean hydrogen because its products are carbon and hydrogen. In this work, five different catalysts comprising of varying quantities of nickel and lithium, supported on calcium oxide were synthesized by incipient wetness impregnation method and designated according to weight % as; 30%Ni/CaO, 37.5%Ni-12.5%Li/CaO, 25.0%Ni- 25.0%Li/CaO, 12.5%Ni-37.5%Li/CaO and 50%Li/CaO. The synthesized catalysts were characterized by (XRD, SEM, BET and TEM) and tested for methane decomposition. From the XRD patterns of the synthesized catalysts, distinct crystalline phases of CaO and NiO were positively identified in 50%Ni/CaO according to their reference JCPDS files. Introduction of Lithium hydroxides improved the crystalline structure of the Ni/CaO catalyst. SEM analyses of the catalyst material using Image-J software confirmed that all catalyst materials were nanoparticles ranging from 3.09-6.56nm. BET results confirmed that, all the catalysts are mesoporous with pore sizes ranging from 20.1nm to 45.3nm. Introduction of LiOH to Ni/CaO generates mesoporous structures by destructing the lattices of the CaO structure during the formation of Ni-Li/CaO species. Particle size distribution in TEM analyses revealed that, a higher nickel loading in the catalyst favours the formation of carbon nanotubes while higher lithium hydroxide loading favours the formation of carbon fibres (CF). Low yield of carbon fibres from methane decomposition on unsupported Ni catalyst in 50%Ni/CaO was attributed to the presence of large Ni particles with low index planes which were incapable of dissociating the unreactive methane molecule.

The aim of this work was to synthesize a catalyst for use in decomposition of methane into carbon and hydrogen, that addresses drawbacks of traditional solid metal catalysts such as sintering and coking. From the experimental results, 37.5%Ni-12.5%Li/CaO catalyst recorded 65.7% methane conversion and 38.3%hydrogen yield while 50%Ni/CaO recorded the lowest methane conversion of 60.2% and a hydrogen yield of 35.7% at 650℃. Outstanding performance of the 37.5%Ni-12.5%Li/CaO catalyst is attributed to the incorporation of lithium hydroxide which provided more catalyst active sites and a molten environment for proper dispersion of the nickel metal. The solid 50%Ni/CaO catalyst readily deactivated due to coking unlike the supported molten 37.5%Ni-12.5%Li/CaO catalyst in which methane decomposition reaction took place by both surface reaction and chemisorption.