Catalytic removal of gaseous pollutant NO using CO: Catalyst structure and reaction mechanism

Carbon monoxide (CO) has recently been considered an ideal reducing agent to replace NH3 in selective catalytic reduction of NOx (NH3-SCR). This shift is particularly relevant in diesel engines, coal-fired industry, the iron and steel industry, of which generate substantial amounts of CO due to incomplete combustion. Developing high-performance catalysts remain a critical challenge for commercializing this technology. The active sites on catalyst surface play a crucial role in the various microscopic reaction steps of this reaction. This work provides a comprehensive overview and insights into the reaction mechanism of active sites on transition metal- and noble metal-based catalysts, including the types of intermediates and active sites, as well as the conversion mechanism of active molecules or atoms. In addition, the effects of factors such as O2, SO2, and alkali metals, on NO reduction by CO were discussed, and the prospects for catalyst design are proposed. It is hoped to provide theoretical guidance for the rational design of efficient CO selective catalytic denitration materials based on the structure-activity relations.

Development of a cavity ring-down spectrometer toward multi-species composition

This work presents the development of a cavity ring-down spectrometer (CRDS) designed for the detection of several molecules relevant for air pollution, including the second overtone of ro-vibration transitions from CO at 1.58 µm and NO at 1.79 µm. A unique feature of this CRDS is the use of custom mirrors with a reflectivity of about 99.99% from 1.52 to 1.80 µm, enabling efficient laser coupling into the cavity while ensuring a minimum detectable absorbance of 1.1 × 10-10 cm-1 within an integration time of about 1.2 s. In this work, the successful implementation of the current CRDS is demonstrated in two different wavelength regions. At 1.79 µm, the transitions R17.5 and R4.5 of the second overtone of NO are detected. At 1.58 µm, carbon dioxide and water vapor from untreated ambient air are measured, serving as an example to investigate the suitability of a post-processing procedure for the determination of the molar fraction in a multi-species composition. This post-processing procedure has the benefit of being calibration-free and SI-traceable. Additionally, CRDS measurements of gas mixtures containing CO and CO2 are also shown. In the future, the advantages of the developed cavity ring-down spectrometer will be exploited in order to perform fundamental studies on the transport processes of heterogeneous catalysis by locally resolving the gas phase near a working catalytic surface. The possibility to cover a broad wavelength region with this CRDS opens up the opportunity to investigate different catalytic reactions, including CO oxidation and NO reduction.

Mesoporous LaCoO3 perovskite oxide with high catalytic performance for NOx storage and reduction

A mesoporous LaCoO₃ perovskite oxide (LaCoO₃-Meso) with three-dimensionally ordered helical interwoven structure was synthesized by a nano-casting method using KIT-6 as the hard template. The obtained LaCoO₃-Meso with high surface area was tested for its catalytic performance in the NO_x storage and reduction (NSR) reaction and compared with a sample synthesized by the conventional sol-gel method. The LaCoO₃-Meso showed a significant advantage for NO_x storage, with a NO_x storage capacity 2 times higher than the regular sample. LaCoO₃-Meso also exhibited improved NSR catalytic performance in the 150-450 °C temperature range, especially within 350-400 °C, where the NO_x conversion was raised for 40%. The results of X-ray photoelectron spectroscopy and X-ray absorption fine structure measurements suggested the presence of a high concentration of oxygen defects on the LaCoO₃-Meso surface. Further results provided by temperature programmed reduction and temperature programmed desorption indicated that the oxygen defects not only increase the amount of trapped NO_x, but also improve the low-temperature redox performance of the catalyst. The lower stability of NO_x species adsorbed on oxygen defects promotes the NO_x release step in the NSR reaction and benefits the regeneration of storage sites.

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.

Structural, magnetic, and dielectric properties of solution combustion synthesized LaFeO3, LaFe0.9Mn0.1O3, and LaMnO3 perovskites

Nanocrystalline LaFeO3, LaFe0.9Mn0.1O3, and LaMnO3 perovskites have been synthesized by a novel solution combustion route, wherein oxalyl dihydrazide has been used as a fuel. These materials have been characterized by...

Sorption and Reaction of Biomass Derived HC Blends and Their Constituents on a Commercial Pt–Pd/Al2O3 Oxidation Catalyst

Within the Research Cluster of Excellence “The Fuel Science Center” at RWTH Aachen University, the production and application of new fuels from bio-based carbon feedstocks and CO2 with hydrogen from renewable electricity generation is being investigated. In this study, the storage and oxidation of ethanol, 1-butanol, 2-butanone, cyclopentanone, and cyclopentane as well as two blends thereof on a series production Pt–Pd/Al2O3 oxidation catalyst were investigated. Hydrocarbon (HC) storage and temperature-programmed surface reaction (TPSR) experiments were carried out to analyze their adsorption and desorption behavior. In addition, the individual HCs and both blends were investigated using Diffuse Reflectance Infrared Fourier Transform Spectroscopy (TP-DRIFTS). In general, all oxygenated HCs are adsorbed more strongly than cyclopentane due to their higher polarity. Interestingly, it could be observed that the two different blends [blend 1: ethanol (50 mol %), 2-butanone (21 mol %), cyclopentanone (14 mol %) and cyclopentane (15 mol %); blend 2: 1-butanol (45 mol %), ethanol (29 mol %) and cyclopentane (27 mol %)] exhibit a different storage behavior compared to the single hydrocarbons. It was shown that the presence of 1-butanol and cyclopentane in blend 2 strongly inhibits the oxidation of ethanol. As a result, the ethanol light-off temperature was increased by at least 100 K. A difference was also found in the storage behavior of cyclopentane. While no significant storage could be detected in the pure compound experiment, the experiments with both mixtures showed a larger amount stored. The presence of adsorbed species of the hydrocarbons and their corresponding reaction products has been demonstrated and gives an insight into the storage mechanism of blends.

Graphic Abstract

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.

Transition Metal B-Site Substitutions in LaAlO3 Perovskites Reorient Bio-Ethanol Conversion Reactions

LaAlO3 perovskites, as such and with 25% molar Al substitution by Cu, Co, or Ga, have been prepared by sol-gel methods and tested as heterogeneous catalysts in the gas-phase conversion of ethanol. LaAlO3 presented a significant acidic character, with high formation of ethylene by ethanol dehydration. B-site substitutions increased the basicity of the catalysts, favoring the dehydrogenation of ethanol to acetaldehyde. The most reducible Cu- and Co-substituted materials, characterized by easier formation of surface oxygen vacancies, promoted the self-condensation of acetaldehyde by the Tishchenko mechanism, with formation of acetone and odd-carbon number products. Aldol coupling of acetaldehyde, favored on pure and Ga-substituted LaAlO3, led to the formation of butadiene and hexadiene. The role of Ga insertion, favoring both dehydrogenation of ethylene and dehydration of higher alcohols, corresponds to an amphoteric character. The formation of olefins and diolefins on all catalysts suggests that LaAl-based materials present the most acidic character among La-perovskites.

Oxidation and Storage Mechanisms for Nitrogen Oxides on Variously Terminated (001) Surfaces of SrFeO3-δ and Sr3Fe2O7-δ Perovskites

The Ruddlesden-Popper (RP)-type layered perovskite is a candidate material for a new nitrogen oxide (NO_x) storage catalyst. Here, we investigate the adsorption and oxidation of NO_x on the (001) surfaces of RP-type oxide Sr₃Fe₂O_7-δ for all of the terminations by comparing to those of simple perovskite SrFeO_3-δ by the density functional theory (DFT) calculations. The possible (001) cleavages of Sr₃Fe₂O₇ generate two FeO₂- and three SrO-terminated surfaces, and the calculated surface energies indicated that the SrO-terminated surface generated by the cleavage at the rock salt layer is the most stable one. The oxygen of the FeO₂-terminated surfaces could be removed with significantly low energy because the process involves the favorable reduction of the Fe⁴⁺ site. Consequently, the surface oxygen at the FeO₂ site could easily oxidize adsorbed NO to NO₂ by the Mars-van Krevelen mechanism. The resulting oxygen vacancy in the surface would be filled easily with lattice oxygen in bulk. The oxidation of NO with adsorbed molecular O₂ was unfavorable by both the Langmuir-Hinshelwood and Eley-Rideal mechanisms because this process does not involve the reduction of the Fe⁴⁺ site. The oxygen of the SrO-terminated surfaces was tightly bound and acted as the adsorption site of NO and NO₂. An electron transfer strengthened the NO_x binding to the surface by forming nitrite (NO₂^-) or nitrate (NO₃^-) species. The DFT calculations revealed that the RP-type structure promoted NO_x oxidation and storage properties by forming active oxygen due to the Jahn-Teller distortion and by exposing SrO-terminated surfaces due to the cleavage at the rock salt layer.

Low-temperature NO oxidation using lattice oxygen in Fe-site substituted SrFeO3-δ

Improvement of the low-temperature activity for NO oxidation catalysts is a crucial issue to improve the NO_x storage performance in automotive catalysts. We have recently reported that the lattice oxygen species in SrFeO_3-δ (SFO) are reactive in the oxidation of NO to NO₂ at low temperatures. The oxidation of NO using lattice oxygen species is a powerful means to oxidize NO in such kinetically restricted temperature regions. This paper shows that Fe-site substitution of SFO with Mn or Co improves the properties of lattice oxygen such as the temperature and amount of oxygen release/storage, resulting in the enhancement of the activity for NO oxidation in a low-temperature range. In particular, NO oxidation on SrFe_0.8Mn_0.2O_3-δ is found to proceed even at extremely low temperatures <423 K. From oxygen release/storage profiles obtained by temperature-programmed reactions, Co doping into SFO increases the amount of released oxygen owing to the reducibility of the Co species and promotes the phase transformation to the brownmillerite phase. On the other hand, Mn doping does not increase the oxygen release amount and suppresses the phase transformation. However, it significantly decreases the oxygen migration barrier of SFO. Substitution with Mn renders the structure of SFO more robust and maintains the perovskite structure after the release of oxygen. Thus, the oxygen release properties are strongly dependent on the crystal structure change before and after oxygen release from the perovskite structure, which has a significant effect on NO oxidation and the NO_x storage performance.

Recyclable MFe2O4 (M = Mn, Zn, Cu, Ni, Co) coupled micro-nano bubbles for simultaneous catalytic oxidation to remove NO x and SO2 in flue gas

NO x can be efficiently removed by micro-nano bubbles coupling with Fe3+ and Mn2+, but the catalyst cannot be reused and the adsorption wastewater should be treated. This work developed a new technology that uses micro-nano bubbles and recyclable MFe2O4 to simultaneously remove NO x and SO2 from flue gas, and clarified the effectiveness and reaction mechanism. MFe2O4 (M = Mn, Zn, Cu, Ni and Co) prepared by a hydrothermal method was characterized. The results show that MFe2O4 can be activated to produce ˙OH which can accelerate the oxidation absorption of NO x . Compared with no catalyst, the NO x conversion rate increased from 32.85% to 83.88% in the NO x -SO2-MFe2O4-micro-nano bubble system, while the removal rate of SO2 can reach 100% at room temperature. The catalytic activities of MFe2O4 showed the following trend: CuFe2O4 > ZnFe2O4 > MnFe2O4 > CoFe2O4 > NiFe2O4. The results provide a new idea for the application of advanced oxidation processes in flue gas treatment.