A doping element improving the properties of catalysis: in situ Raman spectroscopy insights into Mn-doped NiMn layered double hydroxide for the urea oxidation reaction

Abstract

Elemental doping has been proved to be an efficient means to adjust the performance of Ni-based hydroxides for the electrochemical urea oxidation reaction. However, the mechanism by which elemental doping...

Reagent-adaptive active site switching on the IrOx/Ni(OH)2 catalyst

“…During the UOR on IrO x /Ni(OH) 2 , the conversion of Ni(OH) 2 to NiOOH begins at 1.35 V (Fig. 3(d)), well corresponding to the UOR onset potential of 1.32 V. A Raman band at 1004 cm À1 , for the symmetric C-N stretching vibration of urea, 16 is weakened with the increasing amount of Ni 3+ at high potentials, confirming that the Ni 3+ species is the UOR active species. When Ni 3+ is formed to drive the UOR, the Raman signal of Ir 4+ is invisible.…”

“…The Raman spectra at 1.45 V showed two new characteristic peaks at 475 and 554 cm −1 , which are assigned to the E g bending vibration and A 1g stretching vibration modes of Ni 3+ –O in NiO 6 units of NiOOH, respectively. 16 With the increasing applied potentials from 1.45 to 1.55 V, the two characteristic peaks of Ni 3+ –O increase and then decrease once Ir 5+ species occur at 1.55 V. 17 Given the big potential gap between Ni 2+ /Ni 3+ oxidation and water oxidation on both Ni(OH) 2 and IrO x /Ni(OH) 2 electrodes (Fig. 2(a)), we can conclude that the Ni 3+ species is unable to oxidize water, unlike the Ni δ + ( δ > 3) species.…”

Heteroatom-doped transition metal hydroxides in energy storage and conversion: a review

“…Wu et al 62 synthesized Mn doped Ni(OH) 2 for urea electrooxidation (UOR). They confirmed by Raman spectroscopy that Mn dopants promoted the formation of electroactive NiOOH at low potential and extended the Ni–O bond in NiOOH, resulting in increased phase structure disorder.…”

Evidence for Exclusive Direct Mechanism of Urea Electro-Oxidation Driven by In Situ-Generated Resilient Active Species on a Rare-Earth Nickelate

“…However, the formation and stabilization of active NiOOH species and its capability to drive UOR through the direct mechanism has not been properly understood because of the following reasons: (i) insufficient information for choosing an appropriate catalyst that can stabilize NiOOH species by preventing its subsequent reduction into Ni­(OH) 2 during UOR and (ii) minimal efforts in the scientific literature to identify the local oxidation state and environment of the active site of the UOR catalyst during operation. From several prominent previous works, it has been well established that the formation of NiOOH is more facile on the surface of Ni 3+ -rich compounds owing to the enhanced electrophilicity of adsorbed oxygen (Ni 3+ -O) which enables the more facile formation of NiOOH through nucleophilic attack by hydroxide ions. − Stevenson and group have demonstrated the UOR activity of the lanthanide perovskite, LaNiO 3 , and a series of La 2– x Sr x NiO 4+δ Ruddlesden–Popper catalysts and emphasized on the importance of inherently stabilized Ni 3+ ions in boosting UOR activity. , Several other catalysts rich in Ni 3+ -ions have been shown to exhibit enhanced UOR activity. ,− However, these studies do not have a primary objective of elucidating the correlation between the enduring presence of active NiOOH species and their consequential impact on the operating UOR mechanism.…”