High–performance catalysts are extremely required for controlling NH 3 emission via selective catalytic oxidation (SCO), and the anchoring structural feature of active sites is a key prerequisite for developing them. This study confirms the importance of hydroxyl groups on vacancy–deficient reducible oxides as active groups. On the one hand, spontaneous atomic dispersion of active metal Ir is promoted by the abundant terminal hydroxyl groups. On the other hand, Ir cations anchor on the TiO 2 surface through exchange with H + in Ti–OH groups, and thus occupy the Brönsted acid sites. The adsorption strength of NH 3 is another key factor affecting the reaction rate–determining step, namely NH 3 dehydrogenation , which occurs at a faster rate in the coordinated L–NH 3 rather than the ionic B–NH 4 + . Meanwhile, the coordinated L–NH 3 significantly avoids the competitive adsorption of water vapor in the NH 3 –SCO reaction by reducing the number of hydrogen bonding. The TOF of preferred 0.8Ir/TiO 2 sample is significantly higher than 0.2Ir/TiO 2 sample, although Ir is almost always atomic dispersed. Finally, NH 3 conversion is 85% in a wet circumstance (5% H 2 O) at 240 °C (GHSV = 85 000 h –1 ), with a N 2 selectivity of up to 65% on 0.8Ir/TiO 2 sample.
Constructing multiple nanosized heterogeneous structures with an in situ optimized coordination environment for dispersing and stabilizing noble metal single atoms (SAs) is a highly effective strategy for high-efficiency utilization of SAs and improving the electrocatalytic performances of the oxygen evolution reaction (OER). Herein, a multi-crystalline-amorphous heterogeneous composite (CoP2@CN–Ir(200)) composed of crystalline CoP2, amorphous CoOx and amorphous N-doped carbon was constructed for in situ loading of Ir SAs by cyclic voltammetry (CV) electrodeposition in an ionic liquid. By precisely controlling the electrodeposition cycles, the loading matrix of Ir SAs was simultaneously reconstructed and optimized. Some of the crystalline CoP2 is transformed into amorphous CoOx with activated high-valence Co; a coordination environment toward oxygenated species is built with more lattice oxygen and stronger adsorption ability of oxygen; the abundant heterointerfaces and the enhancement of the oxidation state disperse and stabilize Ir atoms in the form of OER-favorable high-valence Ir3+/Ir4+ by covalent Ir–O bonds (including Ir–O–Co bonds). As a result, abundant active sites with high intrinsic activity are exposed, and an accelerated charge transfer rate can be achieved, endowing CoP2@CN–Ir(200) with an outstanding electrocatalytic activity. Ultralow overpotentials of 189 and 300 mV at 10 and 100 mA cm−2, respectively, are achieved with a small Tafel slope of 36.9 mV dec−1, surpassing most reported remarkable Ir-related catalysts.
The metal-support interactions (MSI) could greatly determine the electronic properties of single-atom catalysts, thus affecting the catalytic performance. However, the typical approach to regulating MSI usually suffers from interference of the variation of supports or sacrificing the stability of catalysts. Here, we effectively regulate the site-specific MSI of Ir single atoms anchored on Ni layered double hydroxide through an electrochemical deposition strategy. Cathodic deposition drives Ir atoms to locate at three-fold facial center cubic hollow sites with strong MSI, while anodic deposition drives Ir atoms to deposit onto oxygen vacancy sites with weak MSI. The mass activity and intrinsic activity of Ir single-atom catalysts with strong MSI towards oxygen evolution reaction are 19.5 and 5.2 times that with weak MSI, respectively. Mechanism study reveals that the strong MSI between Ir atoms and the support stimulates the activity of Ir sites by inducing the switch of active sites from Ni sites to Ir sites and optimizes the adsorption strength of intermediates, thereby enhancing the activity.
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