Ba-doped vs. Sr-doped LaCoO3 perovskites as base catalyst in diesel exhaust purification

Advancements in low-temperature NH3-SCR of NOx using Ba-based catalysts: a critical review of preparation, mechanisms, and challenges

Presently, selective catalytic reduction (SCR), with either carbon monoxide, urea, hydrocarbons, hydrogen, or ammonia as a reductant, has become a nitrogen oxide (NOx) removal technology (NOx conversion) of many catalytic companies and diesel engine exhaust gas. Although, there exists a serious threat of low-temperature limitations. So far, certain scientists have shown that barium-based (Ba-based) catalysts have the potential to be highly effective at SCR of NOx at low temperatures when ammonia is used as the reducing agent. The process of NOx storage and reduction which alternate SCR is known as the Lean NOx trap. Herein, we give the condensed advancements and production of the catalysts that involve BaO in low-temperature NH3-SCR of NOx, the advantages of BaO catalysts compared to the recently hot electrocatalysis, the stability of BaO catalyst materials, and the condensed advancements and production of the catalysts that involve BaO in low-temperature NH3-SCR of NOx. These catalysts are viewed in the light of their preparation method, particulate, and posture in mixed oxides. Also, the characteristic features of Ba-based catalysts are carefully considered and briefed under the following areas: preparation method and precursor, crystallinity, calcination temperature, morphology, acid sites, the specific surface area for reaction, redox property, and activation energy of catalysts. More to these are the discussions on Eley-Rideal [E-R] and Langmuir-Hinshelwood [L-H] mechanisms, the H2O/SO2 and O2 permissiveness, and the NH3-SCR reaction mechanism over Ba-based catalysts highlighting their possible effects. Finally, we proposed the prospect and the likely future research plan for the low-temperature NH3-SCR of NOx.

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.

Study of Ce, Ca, Fe, and Mn-Doped LaCoO3 Perovskite Oxide for the Four-Way Purification of PM, NOx, CO, and HC from Diesel Engine Exhaust

Perovskite-type catalysts were widely used in the field of automobile exhaust purification due to their inherent physicochemical properties and excellent doping characteristics. A series of La_1−xM_xCo_1−yN_yO₃ (M = Ce, Ca; N = Fe, Mn) perovskite-type catalyst samples were prepared by sol-gel method for the four-way purification of PM, NO_x, CO, and HC emitted by diesel exhaust. The activity of catalyst samples was tested by simulation experiments and hydrogen temperature-programmed reduction (H₂-TPR). Catalyst samples were characterized by means of XRD, FT-IR, SEM, BET, and XPS analysis. The results demonstrated that the perovskite-type catalyst samples with a particle pore size of 3–5 μm can be prepared by sol-gel method. When A-site of LaCoO₃ perovskite-type oxide was doped by cerium ions, the catalyst samples produced small distortion. The doping of cerium ions to A-site was more conducive to the four-way purification of diesel exhaust than calcium ions. La_0.8Ce_0.2CoO₃ perovskite-type samples showed the best purification efficiency, and the purification efficiencies of PM, NO_x, CO, and HC were 90%, 85%, 94%, and 100%, respectively. When the B-site of La_0.8Ce_0.2CoO₃ perovskite was doped with iron ions, the purification efficiency of catalyst samples for PM and NO_x can be further enhanced. When the B-site of La_0.8Ce_0.2CoO₃ perovskite was doped with manganese ions, the purification efficiency of the catalyst samples for PM can be further enhanced. It can be seen from the simulation experiments that La_0.8Ce_0.2Co_0.7Fe_0.3O₃ perovskite was the best doping amount, and the purification efficiencies of PM, NO_x, CO, and HC were 95%, 92%, 94%, and 100%, respectively.

Advances in Designing Efficient La-Based Perovskites for the NOx Storage and Reduction Process

To overcome the inherent challenge of NOx reduction in the net oxidizing environment of diesel engine exhaust, the NOx storage and reduction (NSR) concept was proposed in 1995, soon developed and commercialized as a promising DeNOx technique over the past two decades. Years of practice suggest that it is a tailor-made technique for light-duty diesel vehicles, with the advantage of being space saving, cost effective, and efficient in NOx abatement; however, the over-reliance of NSR catalysts on high loadings of Pt has always been the bottleneck for its wide application. There remains fervent interest in searching for efficient, economical, and durable alternatives. To date, La-based perovskites are the most explored promising candidate, showing prominent structural and thermal stability and redox property. The perovskite-type oxide structure enables the coupling of redox and storage centers with homogeneous distribution, which maximizes the contact area for NOx spillover and contributes to efficient NOx storage and reduction. Moreover, the wide range of possible cationic substitutions in perovskite generates great flexibility, yielding various formulations with interesting features desirable for the NSR process. Herein, this review provides an overview of the features and performances of La-based perovskite in NO oxidation, NOx storage, and NOx reduction, and in this way comprehensively evaluates its potential to substitute Pt and further improve the DeNOx efficiency of the current NSR catalyst. The fundamental structure–property relationships are summarized and highlighted to instruct rational catalyst design. The critical research needs and essential aspects in catalyst design, including poisoner resistance and catalyst sustainability, are finally addressed to inspire the future development of perovskite material for practical application.

Insights into the role of strontium in catalytic combustion of toluene over La1-xSrxCoO3 perovskite catalysts

A series of LaCoO₃ perovskite catalysts substituted by Sr in the A site (La_1-xSr_xCoO₃) were prepared via a facile sol-gel method. The catalytic activity of these perovskite catalysts for the deep oxidation of toluene was evaluated. It was found that Sr substitution significantly enhanced the redox properties, the concentration of oxygen vacancies, and surface Co³⁺ active species via an electron interaction between Sr and Co from the results of Raman spectroscopy, H₂-TPR (temperature programmed reduction), O₂-TPD (temperature programmed desorption) and XPS (X-ray photoelectron spectroscopy). Typically, La_0.82Sr_0.18CoO₃ (L_0.82S_0.18C) exhibited a superior catalytic performance among these samples owing to its best reducibility and highest number of active species. Kinetic analysis further revealed a higher reaction rate (5.1 × 10^-7 mol g^-1·s^-1 at 210 °C) and a lower apparent activation energy (69.1 kJ mol^-1) for toluene oxidation on the L_0.82S_0.18C sample in comparison to those on the others. In situ DRIFTS (diffuse reflectance infrared Fourier transform spectroscopy) confirmed the easy desorption of immediate products from the surface of the L_0.82S_0.18C sample, which could be responsible for its remarkable performance. These results could provide an efficient strategy for promoting the toluene oxidation through finely tuning the reducibility and surface active phase of the catalysts.

Reduced alkaline earth metal (Ca, Sr) substituted LaCoO3 catalysts for succinic acid conversion

Surface distribution and particle size play a key role in the catalytic activity of substituted La1−xAxCoO3 (A = Ca/Sr, x = 0.2–0.4) perovskites.

NOx Storage Performance at Low Temperature over Platinum Group Metal-Free SrTiO3-Based Material

Pt-based catalysts are commonly employed as NOx-trapping catalysts for automobiles, while perovskite oxides have received attention as Pt-free NOx-trapping catalysts. However, the NOx storage performance of perovskite catalysts is significantly inferior at low temperatures and with coexisting gases such as H2O, CO2, and SO2. This study demonstrates that NOx storage reactions proceed over redox site (Mn, Fe, and Co)-doped SrTiO3 perovskites. Among the examined catalysts, Mn-doped SrTiO3 exhibited the highest NOx storage capacity (NSC) and showed a high NSC even at a low temperature of 323 K. Moreover, the high NOx storage performance of Mn-doped SrTiO3 was retained in the presence of poisoning gases (H2O, CO2, and SO2). NO oxidation experiments revealed that the NSC of Co-doped SrTiO3 was dependent on the NO oxidation activity from NO to NO2 via lattice oxygen, which resulted in an inferior NSC at low temperatures. On the other hand, Mn-doped SrTiO3 successfully adsorbed NO molecules onto its surface at 323 K without the NO oxidation process using lattice oxygens. This unique adsorption behavior of Mn-doped SrTiO3 was concluded to be responsible for the high NSC in the presence of poisoning gases.