Heterogeneous Fe(3) single-cluster catalyst for ammonia synthesis via an associative mechanism

The current industrial ammonia synthesis relies on Haber–Bosch process that is initiated by the dissociative mechanism, in which the adsorbed N(2) dissociates directly, and thus is limited by Brønsted–Evans–Polanyi (BEP) relation. Here we propose a new strategy that an anchored Fe(3) cluster on the θ-Al(2)O(3)(010) surface as a heterogeneous catalyst for ammonia synthesis from first-principles theoretical study and microkinetic analysis. We have studied the whole catalytic mechanism for conversion of N(2) to NH(3) on Fe(3)/θ-Al(2)O(3)(010), and find that an associative mechanism, in which the adsorbed N(2) is first hydrogenated to NNH, dominates over the dissociative mechanism, which we attribute to the large spin polarization, low oxidation state of iron, and multi-step redox capability of Fe(3) cluster. The associative mechanism liberates the turnover frequency (TOF) for ammonia production from the limitation due to the BEP relation, and the calculated TOF on Fe(3)/θ-Al(2)O(3)(010) is comparable to Ru B5 site.