First-principles and experimental study on the inhibitory effect of ammonia on methane steam reforming over the Ni surface of SOFC anodes

Although ammonia significantly suppresses methane steam reforming on Ni-based SOFC anodes, thereby mitigating severe thermal gradients, the underlying mechanism remains unclear. Here, we combine density functional theory (DFT) calculations with experiments to elucidate the microscopic origin of this inhibition of methane reforming on Ni surfaces. DFT calculations evaluate the activation energies and reaction heats of key elementary steps, including methane dehydrogenation, ammonia decomposition, and competitive reactions involving CH*, NH x * , and OH* surface intermediates. The results reveal the following mechanisms of NH 3 -induced suppression: (i) NH 3 -derived surface species occupy Ni active sites and hinder CH 4 activation mainly through a site-blocking effect; (ii) hydrogen released from NH 3 promotes the conversion of C* to CH*, shifting the reaction network away from direct C–O coupling pathways toward CH-dominated intermediates; and (iii) NH x * species preferentially react with surface OH* to form H 2 O, depleting the OH* required for CH* oxidation via the CH* + OH* → CHOH* pathway. Because these competing processes have lower activation barriers and more favorable reaction energies than the corresponding methane steam reforming steps, they are more competitive at low temperatures, where methane reforming is kinetically and thermodynamically limited. This mechanistic picture accounts for the experimentally observed strong suppression of methane conversion.