High performance of bulk Mo 2 N and Co 3 Mo 3 N catalysts for hydrogen production from ammonia: Role of citric acid to Mo molar ratio in preparation of high surface area nitride catalysts

Hydrogen production from ammonia decomposition was studied using a series of unsupported high surface area molybdenum nitride (Mo2N) and cobalt promoted molybdenum nitride (3%Co-Mo2N) catalysts prepared with citric acid (CA) as a chelating agent. To elucidate the influence of citric acid amount in preparation conditions on the structure and catalytic activity, we prepared catalysts with different citric acid to Mo molar ratios i.e. CA/Mo = 1, 2, 3 and 4. The catalytic activity was evaluated in the temperature range of 300–600 °C at atmospheric pressure. The catalytic activity of the tested samples has changed in the following order of CA/Mo atomic ratio of 1 < 2 < 3 > 4. Therefore, the catalyst prepared by using CA/Mo ratio = 3 showed the highest catalytic activity. BET, XRD, XPS, SEM and TEM-EDS techniques were been used to characterize the catalysts. The increased activity of Mo2N-3:1 and 3%Co-Mo2N-3:1 catalysts was due to increased surface area, decreased particle size and increased relative proportions of Mo2N and Co3Mo3N phases. The ammonia conversion for 3%Co-Mo2N catalyst was increased from 75 to 97% at 550 °C with the increase of CA/Mo ratio from 1 to 3. This enrichment of activity in 3%Co-Mo2N-3:1 catalyst is due to increased dispersion of Co3Mo3N microstructure on γ-Mo2N platelets confirmed by SEM and TEM results. No deactivation was observed for any catalysts investigated in this study for ammonia decomposition.