What is the primary mechanism by which Ni-based catalysts enhance the steam methane reforming reaction?

The primary mechanism by which Ni-based catalysts enhance the steam methane reforming (SMR) reaction is by providing active sites for adsorption and subsequent dissociation of reactant molecules (methane and steam). Adsorption refers to the process where molecules from a gas or liquid adhere to the surface of a solid. In SMR, methane (CH4) and steam (H2O) molecules are adsorbed onto the nickel (Ni) surface. The nickel atoms on the surface have unfilled electron orbitals, creating a favorable interaction with the reactant molecules. Once adsorbed, the Ni catalyst facilitates the breaking of chemical bonds within the methane and steam molecules, a process called dissociation. For example, methane dissociates into methyl radicals (CH3) and hydrogen atoms (H). Steam dissociates into hydroxyl radicals (OH) and hydrogen atoms. These adsorbed fragments are then more reactive and can readily combine to form the products, hydrogen (H2), carbon monoxide (CO), and carbon dioxide (CO2). The catalyst lowers the activation energy required for these reactions, allowing them to proceed at a much faster rate and at lower temperatures than would be possible without the catalyst. Finally, the product molecules desorb from the catalyst surface, freeing up the active sites for further reactions. Thus, Ni-based catalysts act by providing a surface for adsorption, facilitating bond breaking through dissociation, and lowering the activation energy of the reaction.