Designing Reactive Bridging O^2– at the Atomic Cu–O–Fe Site for Selective NH₃ Oxidation

Abstract

Surface oxidation chemistry involves the formation and breaking of metal−oxygen (M−O) bonds. Ideally, the M−O bonding strength determines the rate of oxygen absorption and dissociation. Here, we design reactive bridging O 2− species within the atomic Cu−O−Fe site to accelerate such oxidation chemistry. Using in situ X-ray absorption spectroscopy at the O K-edge and density functional theory calculations, it is found that such bridging O 2− has a lower antibonding orbital energy and thus weaker Cu− O/Fe−O strength. In selective NH 3 oxidation, the weak Cu−O/ Fe−O bond enables fast Cu redox for NH 3 conversion and direct NO adsorption via Cu−O−NO to promote N−N coupling toward N 2 .As a result, 99% N 2 selectivity at 100% conversion is achieved at 573 K, exceeding most of the reported results. This result suggests the importance to design, determine, and utilize the unique features of bridging O 2− in catalysis.

Unraveling the Lattice O Assisted Internal Selective Catalytic Reduction Mechanism on High N2 Selectivity of CuOx/PtCu Catalysts in NH3-SCO

“…In the i-SCR mechanism, nitrates, such as bidentate and monodentate nitrates, were regarded as major intermediates. 19,20,54,55 Herein, a hyponitrite ion (N 2 O 2 2− ) was observed over CuO x /PtCu catalysts by in situ DRIFTS, in good agreement with the bands observed by Liu et al 20 A reaction path named the O lat assisted i-SCR mechanism was then proposed by combining DFT calculations and in situ DRIFTS. First, NH 3 dehydrogenation is assisted by O lat and active O simultaneously.…”

“…The major difference between the TPSR process and NH 3 -SCO reaction was that O lat consumed by pre-absorbed ammonia could be replenished by oxygen rapidly during the NH 3 -SCO process. Thus, it was reasonable to speculate that O v generated from the O lat assisted NH 3 dehydrogenation process was beneficial to N 2 selectivity. ,,, In Figure S19, DFT calculations confirmed that O lat (lattice O) or O lat H in CuO x dominates the NH 3 dehydrogenation and activation rather than O* (active oxygen). The energy barrier of O lat * assisted NH 3 dehydrogenation was observed to be lower than O lat H*, which suggests that O lat H* assisted ammonia dehydrogenation has little tendency to occur.…”

“…Thus, it was reasonable to speculate that O v generated from the O lat assisted NH 3 dehydrogenation process was beneficial to N 2 selectivity. 19,20,54,55 In Figure S19 In situ DRIFTS of the reaction between O 2 and preabsorbed NH 3 was measured to investigate the reaction intermediates. As shown in Figure S21c, a new band at 2063 cm −1 was assigned to NO + species on the Pt/Al 2 O 3 IWI catalyst.…”

Cascade NH3 Oxidation and N2O Decomposition via Bifunctional Co and Cu Catalysts

“…Density functional theory (DFT) calculations support this hypothesis, showing the weakened O–O bond and the increased bond length of the oxygen molecule on the Co 3 O 4 (100) surface (Figure a) with an adsorption energy of −5.27 eV (Table S1). In comparison, the calculated results over the CuO (001) surface show physical O 2 adsorption with −0.13 eV adsorption energy …”

“…In comparison, the calculated results over the CuO (001) surface show physical O 2 adsorption with −0.13 eV adsorption energy. 45 Such an O 2 activation is observed in the O K edge near edge X-ray absorption fine structure (NEXAFS). After oxygen was introduced, the peak of gaseous oxygen in the O K edge (Figure S1, 530.6 eV) was not observed on the surface of Co 3 O 4 , while a peak at 530.1 eV was observed (Figure 1b).…”