The Effect of Co Incorporation on the CO Oxidation Activity of LaFe1−xCoxO3 Perovskites

Synthesis and Catalytic Performance of High-Entropy Rare-Earth Perovskite Nanofibers: (Y0.2La0.2Nd0.2Gd0.2Sm0.2)CoO3 in Low-Temperature Carbon Monoxide Oxidation

This study investigated the catalytic properties of low-temperature oxidation of carbon monoxide, focusing on (Y0.2La0.2Nd0.2Gd0.2Sm0.2)CoO3 synthesized via a glycothermal method using 1,4-butanediol and diethylene glycol at 250 °C. This synthesis route bypasses the energy-intensive sintering process at 1200 °C while maintaining a high-entropy single-phase structure. The synthesized material was characterized structurally and chemically by X-ray diffraction and SEM/EDX analyses. The material was shown to form nanofibers of (Y0.2La0.2Nd0.2Gd0.2Sm0.2)CoO3, thereby increasing the active surface area for catalytic reactions, and crystallize in the model Pbnm space group of distorted perovskite cell. Using a custom setup to investigate catalytic properties of (Y0.2La0.2Nd0.2Gd0.2Sm0.2)CoO3, the CO oxidation behavior of those high-entropy perovskite oxide was investigated, showing an overall conversion of 78% at 50 °C and 97% at 100 °C. These findings highlight the effective catalytic activity of nanofibers of (Y0.2La0.2Nd0.2Gd0.2Sm0.2)CoO3 under mild conditions and their versatility in various catalytic processes of robust CO neutralization. The incorporation of rare-earth elements into a high-entropy structure could impart unique catalytic properties, promoting a synergistic effect that enhances performance.

Role of La-based perovskite catalysts in environmental pollution remediation

Since the advent of the industrial revolution, there has been a constant need of efficient catalysts for abatement of industrial toxic pollutants. This phenomenon necessitated the development of eco-friendly, stable, and economically feasible catalytic materials like lanthanum-based perovskite-type oxides (PTOs) having well-defined crystal structure, excellent thermal, and structural stability, exceptional ionic conductivity, redox behavior, and high tunability. In this review, applicability of La-based PTOs in remediation of pollutants, including CO, NO x and VOCs was addressed. A framework for rationalizing reaction mechanism, substitution effect, preparation methods, support, and catalyst shape has been discussed. Furthermore, reactant conversion efficiencies of best PTOs have been compared with noble-metal catalysts for each application. The catalytic properties of the perovskites including electronic and structural properties have been extensively presented. We highlight that a robust understanding of electronic structure of PTOs will help develop perovskite catalysts for other environmental applications involving oxidation or redox reactions.

Effect of the Calcination Temperature of LaNiO3 on the Structural Properties and Reaction Performance of Catalysts in the Steam Reforming of Methane

The steam reforming of methane (SRM) reaction is a significant process for efficient syngas generation and for promising distributed hydrogen production. In this work, a series of LaNiO3 oxides were prepared using the Pechini method, calcined from 600 °C to 900 °C and tested for the SRM reaction. Fresh, reduced, and used samples were characterized using STA-MS-FTIR, in situ and ex situ XRD, N2 physical adsorption, H2-TPR, TEM, TPO, and Raman. The results show that LaNiO3 begins to crystallize at about 550 °C, and the increase in calcination temperature results in the following differences in the properties of the LaNiO3 samples: larger LaNiO3 grains, smaller specific surface area, higher reduction temperature, smaller Ni0 grains reduced from the bulk phase, and stronger metal–support interaction. The maximum CH4 conversion could be achieved over LaNiO3 calcinated at 800 °C. In addition, the effect of steam-to-carbon ratio (S/C) on the performance of the SRM reaction was studied, and a S/C of 1.5 was found to be optimal for CH4 conversion. Too strong a metal–support interaction and too much unreacted steam causes a loss of catalytic activity. Finally, it was also proved using TPO and Raman that an increase in calcination temperature improves the carbon deposition resistance of the catalyst.

Crystal Structure and Properties of Gd1-xSrxCo1-yFeyO3-δ Oxides as Promising Materials for Catalytic and SOFC Application

A series of samples with the overall composition Gd1-xSrxCo1-yFeyO3-δ (x = 0.8; 0.9 and 0.1 ≤ y ≤ 0.9), which are promising materials for catalytic and SOFC application, was prepared by a glycerol nitrate technique. X-ray diffraction analysis allowed to describe Gd0.2Sr0.8Co1-yFeyO3-δ with 0.1 ≤ y ≤ 0.5 in a tetragonal 2ap × 2ap × 4ap superstructure (SG I4/mmm), while oxides with 0.6 ≤ y ≤ 0.9 exhibit cubic disordered perovskite structure (SG Pm-3m). All Gd0.1Sr0.9Fe1-yCoyO3-δ oxides within the composition range 0.1 ≤ y ≤ 0.9 possess the cubic perovskite structure (SG Pm-3m). The structural parameters were refined using the Rietveld full-profile method. The changes of oxygen content in Gd1-xSrxCo1-yFeyO3-δ versus temperature were determined by thermogravimetric analysis. The introduction of iron into the cobalt sublattice leads to a gradual increase in the unit cell parameters and unit cell volume, accompanied with increasing oxygen content. The temperature dependency of conductivity for Gd0.2Sr0.8Co0.3Fe0.7O3-δ exhibits a maximum (284 S/cm) at ≈600 K in air. The positive value of the Seebeck coefficient indicates predominant p-type conductivity in the Gd0.2Sr0.8Co0.3Fe0.7O3-δ complex oxide.

Approach to the Characterization of Monolithic Catalysts Based on La Perovskite-like Oxides and Their Application for VOC Oxidation under Simulated Indoor Environment Conditions

Catalysts are very important in controlling the pollutant emissions and are used for hundreds of chemical processes. Currently, noble metal-based catalysts are being replaced for other kinds of materials. In this study, three lanthanum-based perovskite-like oxides were synthesized (LaCo, LaCoMn, and LaMn) by the glycine-combustion method. The powder catalysts obtained were supported onto cordierite ceramic monoliths using an optimized washcoating methodology to obtain the subsequent monolithic catalysts (LaCo-S, LaCoMn-S, and LaMn-S). Sample characterization confirmed the formation of the perovskite-like phase in the powder materials as well as the presence of the perovskite phase after supporting it onto the monolithic structure. The XPS analysis showed a general decrease in lattice oxygen species for monolithic catalysts, mainly caused by the colloidal silica used as a binder agent during the washcoating process. Additionally, some variations in the oxidation state distribution for elements in Co-containing systems suggest a stronger interaction between cordierite and such catalysts. The catalytic activity results indicated that powder and monolithic catalysts were active for single-component VOC oxidation in the following order: 2-propanol > n-hexane ≅ mixture > toluene, and there was no evidence of loss of catalytic activity after supporting the catalysts. However, LaMn-S had a better catalytic performance for all VOC tested under dry conditions, achieving oxidation temperatures between 230–420 °C. The oxidation efficiency for the VOC mixture was strongly affected by the presence of moisture linking the oxidation efficiency at wet conditions to the VOC chemical nature. Additionally, for higher VOC concentrations, the catalyst efficiency decreased due to the limited number of active sites.

LaNiO3 Perovskite Synthesis through the EDTA–Citrate Complexing Method and Its Application to CO Oxidation

A series of LaNiO3 materials were synthesized by the EDTA–citrate complexing method, modifying different physicochemical conditions. The LaNiO3 samples were calcined between 600 and 800 °C and characterized by XRD, SEM, XPS, CO-TPD, TG, DT, and N2 adsorption. The results evidence that although all the samples presented the same crystal phase, LaNiO3 as expected, some microstructural and superficial features varied as a function of the calcination temperature. Then, LaNiO3 samples were tested as catalysts of the CO oxidation process, a reaction never thoroughly analyzed employing this material. The catalytic results showed that LaNiO3 samples calcined at temperatures of 600 and 700 °C reached complete CO conversions at ~240 °C, while the sample thermally treated at 800 °C only achieved a 100% of CO conversion at temperatures higher than 300 °C. DRIFTS and XRD were used for studying the reaction mechanism and the catalysts’ structural stability, respectively. Finally, the obtained results were compared with different Ni-containing materials used in the same catalytic process, establishing that LaNiO3 has adequate properties for the CO oxidation process.

Synergistic Effects of Co and Fe on the Oxygen Evolution Reaction Activity of LaCox Fe1-x O3

In a combined experimental and theoretical study we assess the role of Co incorporation on the OER activity of LaCox Fe1-x O3 . Phase pure perovskites were synthesized up to x = 0 . 300 in 0.025/0.050 steps. HAADF STEM and EDX analysis points towards FeO2 -terminated (001)-facets in LaFeO3 , in accordance with the stability diagram obtained from density functional theory calculations with a Hubbard U term (DFT+U). Linear sweep voltammetry conducted in a rotating disk electrode setup shows a reduction of the OER overpotential and a nonmonotonic trend with x, with double layer capacitance measurements indicating an intrinsic nature of activity. This is supported by DFT+U results that show reduced overpotentials for both Fe and Co reaction sites with the latter reaching values of 0.32-0.40 V, ∼0.3 V lower than for Fe. This correlates with a stronger reduction of the binding energy difference of the *O and *OH intermediates towards an optimum value of 1.6 eV for x = 0 . 250 , the OH deprotonation being the potential limiting step in most cases. Significant variations of the magnetic moments of both surface and subsurface Co and Fe during OER demonstrate that the beneficial effect is a result of a concerted action involving many surrounding ions, which extends the concept of the active site.

The Effect of Water on the 2-Propanol Oxidation Activity of Co-Substituted LaFe1- Cox O3 Perovskites

Perovskites are interesting oxidation catalysts due to their chemical flexibility enabling the tuning of several properties. In this work, we synthesized LaFe1-x Cox O3 catalysts by co-precipitation and thermal decomposition, characterized them thoroughly and studied their 2-propanol oxidation activity under dry and wet conditions to bridge the knowledge gap between gas and liquid phase reactions. Transient tests showed a highly active, unstable low-temperature (LT) reaction channel in conversion profiles and a stable, less-active high-temperature (HT) channel. Cobalt incorporation had a positive effect on the activity. The effect of water was negative on the LT channel, whereas the HT channel activity was boosted for x>0.15. The boost may originate from a slower deactivation rate of the Co3+ sites under wet conditions and a higher amount of hydroxide species on the surface comparing wet to dry feeds. Water addition resulted in a slower deactivation for Co-rich catalysts and higher activity in the HT channel state.

Dynamics of Reactive Oxygen Species on Cobalt-Containing Spinel Oxides in Cyclic CO Oxidation

Reactive oxygen species (ROS) are considered to be responsible for the high catalytic activity of transition metal oxides like Co3-xFexO4 in oxidation reactions, but the detailed influences of catalyst composition and morphology on the formation of these reactive oxygen species are not fully understood. In the presented study, Co3O4 spinels of different mesostructures, i.e., particle size, crystallinity, and specific surface area, are characterized by powder X-ray diffraction, scanning electron microscopy, and physisorption. The materials were tested in CO oxidation performed in consecutive runs and compared to a Co3-xFexO4 composition series with a similar mesostructure to study the effects of catalyst morphology and composition on ROS formation. In the first run, the CO conversion was observed to be dominated by the exposed surface area for the pure Co-spinels, while a negative effect of Fe content in the spinels was seen. In the following oxidation run, a U-shaped conversion curve was observed for materials with high surface area, which indicated the in situ formation of ROS on those materials that were responsible for the new activity at low temperature. This activation was not stable at the higher reaction temperature but was confirmed after temperature-programmed oxidation (TPO). However, no activation after the first run was observed for low-surface-area and highly crystalline materials, and the lowest surface-area material was not even activated after TPO. Among the catalyst series studied here, a correlation of small particle size and large surface area with the ability for ROS formation is presented, and the benefit of a nanoscaled catalyst is discussed. Despite the generally negative effect of Fe, the highest relative activation was observed at intermediate Fe contents suggesting that Fe may be involved in ROS formation.