Electro-Oxidation Reaction of Methanol over La_2–xSr_xNiO_4+δ Ruddlesden–Popper Oxides

Abstract

How materials’ crystalline structure influences the underlying electronic configuration, along with redox properties, and plays a pivotal role in electrocatalysis is an intriguing question. Here, solution combustion-synthesized La2–x Sr x NiO4+δ (x = 0–0.8) Ruddlesden–Popper (RP) oxides were explored for an electrocatalytic methanol oxidation reaction. Optimal doping of bivalent Sr2+ in the A site enabled the tetragonal distortion and oxidation of Ni2+ to Ni3+ that resulted ultimately in enhanced covalent hybridization of Ni 3d–O 2p with a closer proximity of the O 2p band to the Fermi level. The RP oxide La1.4Sr0.6NiO4+δ exhibited the highest methanol oxidation reactivity vis-à-vis the formation of HCO2H. The proposed mechanism over La1.4Sr0.6NiO4+δ considers a lattice oxygen-mediated methanol oxidation reaction, owing to Fermi-level “pinning” at the top of the O 2p band, which facilitated lattice oxygen atoms prone to oxidation. A high surface concentration of the key active species of Ni–OOH was observed to form during the methanol oxidation reaction with the help of lattice oxygen atoms and oxygen vacancies in La1.4Sr0.6NiO4+δ. The present study offers a uniquely comprehensive exploration of structural and surface properties of RP oxides toward methanol oxidation reactions.

Electro-oxidation Reaction of Methanol over Reducible Ce1–x–yNixSryO2−δ: A Mechanistic Probe of Participation of Lattice Oxygen

“…It was interesting to find out that there was a subtle increase in specific surface area with co-doping. The pore-size diameter estimated by the Barret–Joyner–Halenda model confirmed the presence of mesopores with an average pore diameter of ∼3 nm. , The activity of MOR can be greatly influenced by the ECSA. ,, The ECSA was calculated from the plot in Figure e obtained from chronoamperometric studies using the following Cottrell equation: Q = 2 nFAD 1/2 C o t 1/2 π –1/2 , where Q = charge in coulomb, F = Faraday constant, n = number of electrons being transferred, A = area of electrode in cm 2 , D = diffusion coefficient of [K 3 Fe­(CN) 6 ] (7.6 × 10 –6 cm 2 s –1 ), t = time (s), and C o = concentration of K 3 Fe­(CN) 6 in mol cm –3 . The pristine CeO 2 exhibited an ECSA of 0.82 cm 2 , while Ce 0.95 Ni 0.05 O 2−δ also showed a similar value of 0.83 cm 2 .…”

“…Moreover, the broadly distributed peak corresponding to O V in Ce 0.9 Ni 0.05 Sr 0.05 O 2−δ could be due to the presence of surface oxygen vacancies in the vicinity of Ce 3+ . , Post MOR, the O 1s spectra of the exhausted catalysts CeO 2 Ce 0.95 Ni 0.05 O 2−δ and Ce 0.9 Ni 0.05 Sr 0.05 O 2−δ are plotted in Figure b. Apparently, the deconvoluted O 1s spectra of Ce 0.95 Ni 0.05 O 2−δ and Ce 0.9 Ni 0.05 Sr 0.05 O 2−δ exhibited an extra peak at 533.2 eV corresponding to the surface-bound intermediate −OOH species formed during methanol oxidation in addition to the usual O L , O V , and O W peaks. , While the relative intensity of the peak corresponding to −OOH was highest in Ce 0.9 Ni 0.05 Sr 0.05 O 2−δ , it was not at all observed in the case of pure CeO 2 . A shift of −OOH to higher binding energy from Ce 0.95 Ni 0.05 O 2−δ to Ce 0.9 Ni 0.05 Sr 0.05 O 2−δ was also observed.…”

“…The pristine CeO 2, Ni-doped CeO 2 (Ce 1– x Ni x O 2−δ , where x = 0.03, 0.05 and 0.07), and Ni and Sr co-doped CeO 2 (Ce 1– x – y Ni x Sr y O 2−δ , where x = 0.05 and y = 0.03, 0.05, and 0.07) catalysts were synthesized by the low-temperature-initiated facile single-step solution combustion method. This self-propagating combustion method utilizes propellant chemistry, involving highly exothermic metathetical chemical reactions between the oxidizer and fuel. ,, Here, metal nitrates, Ce­(NO 2 ) 3 ·6H 2 O (Sigma-Aldrich, India), Ni­(NO 2 ) 3 ·6H 2 O (SD Fine Chem Limited, India), and Sr­(NO 3 ) 2 (Sigma-Aldrich, 99%) were taken as oxidizers, and glycine (Sigma-Aldrich, India) was used as the fuel. Due to the aqueous-phase homogeneous precursor, the solution combustion synthesis offers a precise and uniform formulation of the desired composition at the nanoscale.…”

The role of synthesis vis-à-vis the oxygen vacancies of Co3O4 in the oxygen evolution reaction

“…The smaller crystalline size, higher specific surface area and higher oxygen-ion vacancy concentration of Co 3 O 4 from solution combustion synthesis might be responsible for the higher ECSA value. 48…”

Tuning the Ni/Co Ratios and Surface Concentration of Reduced Molybdenum States for Enhanced Electrocatalytic Performance in Trimetallic Molybdates: OER, HER, and MOR Activity

“…The usage of methanol as a fuel potentially overcomes many problems in H 2 –polymer electrolyte membrane fuel-cell technology; however, low efficiency and activity in direct methanol fuel cells remain as key bottlenecks. − This is explicitly referred to two issues: (1) the generation of stable and poisonous CO (for electrode) and (2) a higher anode overpotential as compared to the H 2 –polymer electrolyte membrane fuel cell. , Their combined effect drastically hampers the overall fuel-cell efficiency, although the equilibrium potential of anode reaction is nearly same as that of the H 2 –polymer electrolyte membrane fuel cell. Many reports suggest that Pt displays higher catalytic activity for methanol oxidation reaction (MOR); nevertheless, high cost and CO poisoning limit their widespread applications. , Recently, the research in development of noble-metal-free electrocatalysts for MOR has been extensively carried out. − Particularly, non-noble-metal composites like hydroxides, oxides, carbides, nitrides, phosphides, and chalcogenides were also reported for electrocatalytic applications, thus making non-noble-metal transition compounds a budding choice for MOR activity. − …”