NOVELTY - A catalyst composition comprises an oxide of nickel (Ni) and magnesium (Mg). The molar ratio of Ni to Mg is 1:1-6.2:1. The % of Ni in the metallic state is 15-95% and the % of Ni in the active state is greater than 25%, based on the total number of metallic Ni sites.    USE - For generation of carbon monoxide free hydrogen and carbon by direct catalytic decomposition of methane.    ADVANTAGE - The catalyst exhibits improved performance and activity in the generation of carbon monoxide free hydrogen by catalytic decomposition of methane, as compared to the prior art catalysts having tendency for aggregation, lower temperature requirements and lesser amount of nickel in active state. The catalyst contains greater than 25% of the metallic nickel in active state, as compared to the prior art catalysts having not more than 25% nickel in the active state, hence exhibits improved catalytic performance. The catalysts produce the carbon oxide-free hydrogen and substantially pure carbon by decomposition of methane over a relatively low temperature range of 475-800 degrees C, with catalysts not including Cu and including Cu having a temperature requirement of 450-625, preferably 475-575, especially 500-525 degrees C; and 500-800, preferably 600-775, especially 750-760 degrees C. The temperature is higher than that required for the conventional catalysts (i.e. less than 650 degrees C, which also results in an extended lifetime of the catalyst with higher hydrogen and carbon yields even at higher reaction temperature i.e. greater than 700 degrees C. The catalyst can also be used for the reaction involving use of natural gas instead of methane, without leading to the production of CO or CO2 form ethane and other hydrocarbons in the natural gas. The carbon produced by the methane decomposition comprises graphite nanofiber with unique properties such as a surface area of (20-3000) m2/g, a crystallinity of (5-100)%, and interstices of (0.335-0.4) nm.    DETAILED DESCRIPTION - A catalyst composition (C1) comprises an oxide of nickel (Ni) and magnesium (Mg) of formula NixMgyO (I). The molar ratio of Ni to Mg is 1:1-6.2:1. The % of Ni in the metallic state and the active state is 15-95%, based on the total number of metallic Ni sites.    x = molar amount of Ni; and    y = molar amount of Mg.    INDEPENDENT CLAIMS are also included for:    (1) a catalyst composition (C2) comprising: an oxide of Ni, Mg and copper (Cu) of formula NixMgyCuzO (II). The molar ratio of Ni to Cu and Ni to Mg is 19:1-1:1 and 1.3:1-3.6:1, respectively. The % of Ni and Cu in the metallic state is 15-95%, and the % of Ni in the active state is greater than 25%; and    (2) preparation of (C1).    z = molar amount of Cu.