Sr-doping effects on La_2O_3 catalyst for oxidative coupling of methane

Density‐functional theory calculations were carried out to study the strontium(Sr)‐doping effect on methane activation over a lanthanum‐oxide(La2O3)catalyst for the oxidative coupling of methane(OCM)using the cluster model.Eight Sr‐doped La2O3cluster models were built from pure La2O3clusters that were used previously to model the La2O3catalyst.These form two distinct categories,namely,those without a radical character(LaSrO2(OH),La2SrO4,La3SrO5(OH),and La5SrO8(OH))and those with a radical character(LaSrO3,La2SrO4(OH),La3SrO6,and La5SrO9).The potential‐energy surface for CH4activation to form a CH3radical at different Sr-O and La-O pair sites on these Sr‐doped La2O3clusters was calculated to study the Sr‐doping effect on the OCM catalytic activity.CH4physisorption and chemisorption energies,and activation barriers,and CH3desorption energies were predicted.Compared with the pure La2O3clusters,in general,the Sr‐doped La2O3clusters are thermodynamically and kinetically more reactive with CH4.For the Sr‐doped La2O3clusters without the radical character,the Sr-O pair site is more reactive with CH4than the La-O pair site,although a direct release of the CH3radical is also highly endothermic as in the case of the pure La2O3clusters.In contrast,for the Sr‐doped La2O3clusters with a radical character,the activation of CH4at the oxygen radical site and the release of the CH3radical are much easier.Thus,our calculations suggest that the Sr dopant prompts the OCM catalytic activity of the La2O3catalyst by providing a highly active oxygen‐radical site and by strengthening the basicity of the M-O pair site,which leads to lower CH4activation energies and lower CH3desorption energies.

Impact of In^(3+) cations on structure and electromagnetic state of M-type hexaferrites

The solid solutions of In^(3+) doped M-type strontium hexaferrites were produced using a conventional solid-state reaction method,and Rietveld analysis of the neutron diffraction patterns was conducted.In^(3+) cations occupy octahedral (4f_(Ⅵ)and 12 k) and tetrahedral (4f_(Ⅳ)) positions (SG=P6_(3)/mmc(No.194)).The average particle size is 837–650 nm.Curie tempearature (T_(C)) of the compounds monotonically decreased down to~520 K with increasing x.A frustrated magnetic state was detected from ZFC and FC magnetizations.saturation magnetization (M_(s)) and effective magnetocrystalline anisotropy coefficient (k_(eff)) were determined using the law of approach to saturation.A real permittivity (ε″) maximum of~3.3 at~45.5 GHz and an imaginary permittivity (ε′) of~1.6 at~42.3 GHz were observed for x=0.1.A real permeability (μ′) maximum of~1.5 at~36.2 GHz was observed for x=0.Aμ″imaginary permeability maximum of~0.8 at~38.3 GHz was observed for x=0.1.The interpretation of the results is based on the type of dielectric polarization and the natural ferromagnetic resonance features.

A highly active and stable Sr_(2)Fe_(1.5)Mo(0.5)O_(6)‑δ‑Ce_(0.8)Sm_(0.2)O_(1.95)ceramic fuel electrode for efficient hydrogen production via a steam electrolyzer without safe gas

High temperature steam(H_(2)O)electrolysis via a solid oxide electrolysis cell is an efficient way to produce hydrogen(H_(2))because of its high energy conversion efficiency as well as simple and green process,especially when the electrolysis process is combined with integrated gasification fuel cell technology or derived by renewable energy.However,about 60%-70%of the electricity input is consumed to overcome the large oxygen potential gradient but not for electrolysis to split H_(2)O to produce H_(2)due to the addition of safe gas such as H_(2)in the fuel electrode.In this work,Sr_(2)Fe_(1.5)Mo_(0.5)O_(6)-δ-Ce_(0.8)Sm_(0.2)O_(1.95)(SFM-SDC)ceramic composite material has been developed as fuel electrode to avoid the use of safe gas,and the open circuit voltage(OCV)has been effectively lowered from 1030 to 78 mV when the feeding gas in the fuel electrode is shifted from 3%H_(2)O-97%H_(2)to 3%H_(2)O-97%N_(2),reasonably resulting in a significantly increased electrolysis efficiency.In addition,it is also demonstrated that the electrolysis current density is greatly enhanced by increasing the humidity in the fuel electrode and the working temperature.A considerable electrolysis current density of−0.54 A/cm^(2)is obtained at 800°C and 0.4 V for the symmetrical electrolyzer by exposing SFM-SDC fuel electrode to 23%H_(2)O-77%N_(2),and durability test at 800°C for 35 h demonstrates a relatively stable electrochemical performance for steam electrolysis under the same operation condition without safe gas and a constant electrolysis current density of−0.060 A/cm2.Our findings achieved in this work indicate that SFM-SDC is a highly promising fuel electrode for steam electrolysis.