Methods and compositions for production of CO2-free hydrogen and carbon nanomaterials by methane decomposition

A method for producing hydrogen from methane, the method comprising:

• using a catalyst comprising (i) nickel and (ii) a second metal selected from copper or palladium to decompose methane into a product carbon and hydrogen at a reaction temperature ranging from 500° C. to 600° C. for a reaction time period, wherein the nickel and the second metal are present in amounts that provide a weight ratio of nickel to the second metal of 5:1 to 10:1 (nickel:second metal);
• combining the product carbon and the catalyst with nitric acid having a concentration of at least 2M to form a mixture;
• stirring the mixture at a temperature of at least 120° C., wherein the nitric acid causes metal ions or atoms from the catalyst to separate from the product carbon such that at least 86% of the total amount of metal ions or atoms are removed from the product carbon;
• removing the metal ions or atoms from the mixture;
• removing the product carbon from the mixture;
• and
• using from about 50 wt % to about 95 wt % of the product carbon to restart the method.