Made of nitrogen and hydrogen, ammonia is an important carbon-free energy carrier and makes an appealing fuel since it burns without releasing carbon. The major issue lies in that NH₃-production is highly inefficient. According to some estimates, it contributes to ca. 2% of global fossil fuel use and to 1% of total annual global emissions of greenhouse gases through the release of more than 400 million tons of CO₂. Alternative approaches like the electrochemical ammonia synthesis are attractive, but the sluggish nitrogen reduction reaction (NRR) due to the high energy input to activate N₂ remains a significant challenge for NRR electrocatalysts. Transition metal nitride (TMN) is a promising material towards the energy related applications including catalysis and energy storage.¹ The surface N of TMN can be a solution to this with initial N vacancy formation via hydrogenation steps and ammonia formation. We have successfully deposited ZrN films using metal organic chemical vapor deposition (MOCVD) method with new single-source precursor and these deposited ZrN rapidly oxidizes when exposed to ambient conditions and forms amorphous ZrO_xN_y species on the surface.

In this presentation, we present our ab initio molecular dynamics (aiMD) simulation results to showcase the oxidation step of ZrN under ambient conditions. Two typical surface facets of ZrN (100) and ZrN (110) are selected to build the slab model of ZrN thin films. Three temperatures are applied to run the aiMD simulations: room temperature (295K) and MOCVD temperature (363K and 1023K). At room temperature and 363K, after structure relaxation from the final aiMD structure, we found a layer of ZrO_xN_y formation in the surface region. Interestingly, this surface ZrO_xN_y region can reduce any further oxidation into bulk ZrN and reach a self-limiting oxidation layer. This is in accordance with the contact angle analysis of deposited ZrN thin films. If we run aiMD at MOCVD temperature of 1023K, the relaxed structure after final aiMD structure shows the formation of ZrO_x on the surface. Since there is no evidence of ZrO₂ peak in the XRD pattern, the oxygen incorporation is excluded during the MOCVD deposition process.

We then present the theoretical results of NRR reaction pathway via the Mars-van-Krevelen (MvK) mechanism with introducing surface N vacancy and subsequent N₂ adsorption and N filling. With computing the reaction energy and activation barriers of key reaction steps on both ZrN and ZrO_xN_y/ZrN, we can find a clue on how surface N of TMN helps to activate N₂ and the role of small amount of surface oxygen on activating N₂.

These results can be useful to design and deposit ZrN acting as effective NRR electrocatalysts.