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 Bronsted-Evans-Polanyi (BEP) relation. Here we propose a new strategy that an anchored Fe-3 cluster on the theta-Al2O3(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 NH3 on Fe-3/theta-Al2O3(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/theta-Al2O3(010) is comparable to Ru B5 site.