Langmuir-Hinshelwood Mechanism Research Articles

Boosting Pt utilization via strategic Co Integration: Structural and mechanistic investigation of High-Performance Pt-Co alloy nanocatalysts for catalytic benzene combustion

Platinum (Pt)-based catalysts exhibit notable efficacy in reducing emissions of hazardous volatile organic compounds (VOCs), such as benzene. Nevertheless, the high cost and limited availability of Pt necessitate exploring reasonable strategies to optimize Pt utilization and enhance catalytic efficiency. Thus, pressing needs emerge to develop efficient Pt-based catalysts. This study investigates the catalytic combustion of benzene utilizing an antimony-doped tin oxide (ATO) supported platinum-cobalt (Pt-Co) alloy catalyst. The incorporation of cobalt into Pt nanoparticles leads to Co atoms occupying bulk sites in the Pt-Co alloy, elevating the surface exposure of Pt atoms and enriching Pt on the surface, thereby providing additional active sites for catalysis. Advanced characterization techniques provide critical insights into the morphology, size distribution, crystallinity and valence states of Pt upon strategic Co alloying. Pt-Co/ATO exhibits exceptional low-temperature activity, achieving 95 % benzene conversion at 170 °C, which is 30 °C lower than the 200 °C required for Pt/ATO. Pressure-dependent kinetic analysis reveals a Langmuir-Hinshelwood mechanism involving the co-adsorption of benzene and oxygen reactants on active Pt-Co alloy sites. This study contributes insights into tuning bimetallic alloy nanostructures to optimize noble metal utilization and catalytic performance for VOC abatement, establishing a foundation for advancing sustainable emissions control technology.

Direct coupling of CH4 and CO2 into acetic acid over CuPdO2/Al2O3 in the presence of H2O2

Direct conversion of methane and carbon dioxide into acetic acid is a dream way to address the two greenhouse gases, but is challenged by their chemical inertness. Cu and Pd are promising candidates both in CH4 and CO2 activation as well as the subsequent C-C coupling reactions at low temperature. Herein, we report that acetic acid can be produced by the direct coupling of CH4 and CO2 in the presence of H2O2 over Cu-Pd/Al2O3 catalysts at 50 °C. Results show that the acetic acid yield reaches 48.1 μmol·gcat−1·h−1 on the optimized reaction condition over Cu3.8-Pd1.6/Al2O3 catalyst, which is higher than that of either mono-Cu or mono-Pd catalyst. Extensive characterization indicates the higher activity of Cu3.8-Pd1.6/Al2O3 catalyst is attributed to the strong synergistic effect between Cu and Pd, which results in the formation of CuPdO2 phase, and thus promoting the electron transfer from Cu to Pd. DRIFTS experiments reveal that acetic acid is produced by the coupling of CH3* and COOH* over CuPdO2 active sites via Langmuir-Hinshelwood mechanism, where CH3* is originated from CH4 dehydrogenation step, and COOH* may formed by the activation of CO2 by H2O2. This study provides a new approach for the co-conversion of CH4 and CO2 into acetic acid at low temperature.

A density functional theory study of CO oxidation on single-atom Zn supported on PtX2 (X = S, Se, Te)

CO oxidation on Zn doped Platinum-based transition metal sulfides (Zn/PtX2, X = S, Se, Te) was systematically explored by density functional theory calculations. The geometric stability, electronic structure and catalytic properties of the three Zn/PtX2 catalysts were studied. CO oxidation on Zn/PtX2 along the Langmuir–Hinshelwood and Eley-Rideal mechanisms has been comparatively demonstrated. It is found that surface defect can stabilize Zn single atom, and the oxidation of CO on Zn/PtX2 is facilitated along the Langmuir–Hinshelwood mechanism. The dissociation of OOCO intermediate of Zn/PtSe2 catalyst is the rate-limiting step with an energy barrier of 0.27 eV. This work demonstrates that Zn single atoms on suitable substrate possess satisfied CO oxidation activity.

Unraveling distinct effects between CuOx and PtCu alloy sites in Pt−Cu bimetallic catalysts for CO oxidation at different temperatures

In situ exploration of the dynamic structure evolution of catalysts plays a key role in revealing reaction mechanisms and designing efficient catalysts. In this work, PtCu/MgO catalysts, synthesized via the co-impregnation method, outperforms monometallic Pt/MgO and Cu/MgO. Utilizing quasi/in-situ characterization techniques, it is discovered that there is an obvious structural evolution over PtCu/MgO from PtxCuyOz oxide cluster to PtCu alloy with surface CuOx species under different redox and CO oxidation reaction conditions. The synergistic effect between PtCu alloy and CuOx species enables good CO oxidation activity through the regulation of CO adsorption and O2 dissociation. At low temperatures, CO oxidation is predominantly catalyzed by surface CuOx species via the Mars-van Krevelen mechanism, in which CuOx can provide abundant active oxygen species. As the reaction temperature increases, both surface CuOx species and PtCu alloy collaborate to activate gaseous oxygen, facilitating CO oxidation mainly through the Langmuir-Hinshelwood mechanism.

Experimental and Computational Insights into the Catalytic Mechanism of Y1-xBaxCoO3-δ Perovskite Oxides with a Controlled Crystal Structure.

The crystal structure of Co-based perovskite oxides (ACoO3) can be controlled by adjusting the A-site elements. In this study, we synthesized Y1-xBaxCoO3-δ (x = 0, 0.5, and 1.0) via a coprecipitation method and investigated their CO oxidation performances. YCoO3 (x = 0; cubic perovskite oxide; Pbnm) shows a higher catalytic performance than Y0.5Ba0.5CoO2.72 (x = 0.5; A-site-ordered double perovskite oxide; P4/nmm), which exhibits high oxygen nonstoichiometric properties, and BaCoO3 (x = 1.0; hexagonal perovskite oxide; P63/mmc), which contains high-valent Co4+ species. To elucidate the reaction mechanism, we conducted isotopic experiments with CO and 18O2. The CO oxidation reaction on YCoO3 proceeds via the Langmuir-Hinshelwood mechanism, which is a surface reaction of CO and O2 gas that does not utilize lattice oxygen. Because of the significantly smaller specific surface area of YCoO3 compared with that of the reference Pt/Al2O3, the bulk features of the crystal structures affect the catalytic reaction. When density functional theory is applied, YCoO3 clearly exhibits semiconducting properties in the ground state with the diamagnetic t2g6eg0 states, which can translate to a magnetic t2g5eg1 configuration upon excitation by a relatively low energy of 0.64 eV. We propose that the unique nature of YCoO3 activates oxygen in the gas phase, thereby enabling the smooth oxidation of CO. This study demonstrates that the bulk properties originating from the crystal structure contribute to the catalytic activity and reaction mechanism.

Autocombustion Route Derived Zinc Ferrite Nanoparticles as Chemiresistive Sensor for Detection of Alcohol Vapors.

Prolonged exposure to alcohol vapors can have detrimental effects on human health, potentially leading to eye irritation, dizziness, and in some cases, damage to the nervous system. The present article aims to provide a comprehensive understanding on the synthesis and characterization of zinc ferrite (ZnFe2O4) nanoparticles, as well as their interactions with a range of alcohol vapors, including methanol, ethanol, n-propanol, and isopropanol. These alcohols differ in their molecular weight, boiling points, diffusivity, and other properties. The study reveals the semiconducting ZnFe2O4 nanoparticulate sensor's capability for reversible, repeatable, and sensitive detection of alcohol vapors. The sensor exhibits the highest response to ethanol within operating temperature range (225-300 °C). An attempt is made to establish a correlation between the properties of the target analytes and the observed sensing signals. Additionally, the response conductance transients of ZnFe2O4 under the exposure to the studied alcohol vapors are modeled based on the Langmuir-Hinshelwood adsorption mechanism. The characteristic time constants obtained from this modeling are justified with respect to the properties of the analytes.

Selective catalytic reduction of NO by CO over spinel CuFe2O4 catalyst: The vital role of oxygen vacancies

Oxygen vacancies are widely employed to augment the catalytic reactivity in the selective catalytic reduction of NO by CO (CO-SCR) process. Herein, the specific promotion mechanism of oxygen vacancies was intensively explored through a series of CuFe2O4 catalysts via experiments and density functional theory (DFT) simulations. After the modification of oxygen vacancies, NO conversion was enhanced by 20 %-32 % and N2 selectivity was improved. Characterizations revealed that the redox performance, NO adsorption and activation capacity, active centers, and acidic sites of CuFe2O4 can be improved by enriched oxygen vacancies, contributing to the superior CO-SCR performance. DFT results identified that the adsorption energies of reactants were elevated and the intermediate reaction energy barriers were decreased with more oxygen vacancies. Furthermore, the CO-SCR reaction pathways with CuFe2O4 catalyst were proposed according to in-situ test results: it follows the Langmuir-Hinshelwood (LH) and Eley-Rideal (ER) mechanisms in the low-temperature range (50–250 ℃) and the Marse-van-Krevelen (Mvk) mechanism in the high-temperature range (≥250 ℃). Overall, this work highlights the bright prospects of oxygen vacancies on the modification of CO-SCR catalysts.

A single Mn atom supported by a boron-vacancy in a BN monolayer: An encouraging catalyst for CO oxidation

Single-atom catalysts (SACs) are known for their exceptional activity, primarily attributed to the abundance of low-coordinated metal atoms, which enable efficient activation of robust chemical bonds, including CO bonds. In this work, we explore the prospective utilization of a specific SAC, isolated Mn atoms supported on Boron nitrogen (Mn/BN), for CO oxidation. By analyzing the adsorption/co-adsorption energies of CO, O2, O2+CO, and 2CO, we create a set of screening criteria and conduct a comparative analysis of the chemical processes involved in CO oxidation. The Langmuir-Hinshelwood (LH), Eley-Rideal (ER), termolecular Langmuir-Hinshelwood (TLH) and termolecular Eley-Rideal (TER) mechanisms are investigated. Results indicate the ER and LH mechanisms feature pronouncedly lower barrier of 0.33 and 0.41 eV, respectively, for the rate-limiting step, than those of the TLH (0.72 eV) and TER (3.00 eV) mechanisms, suggesting that the ER and LH mechanisms are favorable at normal temperature. This study offers valuable insights that can inform future developments in the design of low-temperature CO oxidation processes utilizing SACs.

Termolecular Eley–Rideal pathway for catalytic oxidation of nitric oxide on [Pt2]0,± dimers using O2

AbstractA comprehensive density functional theory (DFT) investigation of the catalytic oxidation of NO to NO2 on neutral and charged [Pt2]0,± dimers has been considered employing M06L/def2TZVP level of theory. Single as well as co‐adsorption energies of NO and O2 molecules suggest that the traditional Langmuir Hinshelwood (LH) mechanism and less explored termolecular Eley–Rideal (TER) and termolecular Langmuir Hinshelwood (TLH) mechanisms, are suitable for a full catalytic reaction pathway in which two NO molecules are converted to two NO2 molecules, initiated by an activated O2 molecule. Activation barrier reveals that the TER mechanism is found to be more reliable in converting two NO molecules into two NO2 molecules on [Pt2]¯ dimer. In addition to shedding light on the intrinsic characteristics of Pt2 dimers, the study will serve as a benchmark for investigating the oxidation process of NO to NO2 utilizing models of termolecular chemical processes.

Improving Copper Active Site Speciation on Cu-Ce/SSZ-13 for Ammonia Oxidation via Si/Al Ratio Modulation.

Catalytic oxidation is a promising purification technique for ammonia (NH3) emission. However, high ignition temperatures and NOx peroxide generation limit its effectiveness due to a lack of active sites. Herein, the effects of Si/Al ratio (SAR) modulation on the speciation of copper active sites and the reaction mechanism at different acidic sites were investigated by loading CuO-CeO2 onto SSZ-13 with different SARs (Cu-Ce/SAR15, 20, and 30). Among them, Cu-Ce/SAR20 exhibits the lowest induction temperature (T20 = 180 °C) and the highest nitrogen selectivity (above 95%), attributing to a higher number of Cu2+ exchange sites. In situ IR spectroscopy and isotopic (18O2) transient response experiments indicate that more active Cu2+ in Cu-Ce/SAR20 provides sufficient Lewis acidic sites for NH3 adsorption and favors the stability of Si-OH-Al structures (Brønsted acid sites). NH3 adsorbed at Lewis acidic sites tends to form peroxide byproducts (NOx), while the NH4+ adsorbed at Brønsted acidic sites generates the key intermediate NH4NO2, which decomposes to N2 at high temperatures, thus enhancing nitrogen selectivity. The whole process mainly follows the Mars-van Krevelen (M-K) mechanism, with the Langmuir-Hinshelwood (L-H) mechanism playing a supporting role. Z2Cu2+ coordinates with adjacent Al atoms within the six-membered ring (6MR) and undergoes a slight deformation at high temperatures, facilitating the migration of the lattice oxygen. SAR plays a crucial role in local environmental speciation of reactive Cu2+, where the sufficient isolated Al provided in SAR20 pulls Cu2+ into the eight-membered ring (8MR), allowing it to come into contact with NH3 more readily.

Mechanism of heterogeneous reduction of NO over graphite-supported single-atom Fe catalyst: DFT study

The mechanism of nitrogen oxide (NO) reduction over graphite carbon-supported single-atom iron (Fe) catalyst (Fe/G) was investigated by density functional theory (DFT) and transition state theory (TST). The catalyst deactivation was also analyzed. The results revealed that the NO reduction, based on the Eley-Rideal (E-R) mechanism, underwent four stages including N2O formation and release as well as N2 formation and release. However, the NO reduction only involved two stages according to Langmuir-Hinshelwood (L-H) mechanism: N2 formation and release. Furthermore, for the E-R mechanism, the rate-controlling step was NO reduction, where a NO molecule was adsorbed on an Fe atom with an N, O-down structure with energy barrier of 15.5 kJ/mol, lower than that of other paths. Energy barrier analysis indicated that the energy barrier for the reduction of reactive oxygen species was higher than that for the formation of N2. Reactive oxygen species remaining on the surface of Fe atoms after NO decomposition inhibited the adsorption and reduction of NO, leading to catalyst deactivation due to the absence of active sites. The single-atom Fe species promoted the NO reduction. Kinetic analysis results suggested that, upon increasing the reaction temperature, the NO reduction rate increased more significantly than the reactive oxygen transfer rate.

Efficient green synthesis of n-amyl acetate in liquid phase over metal-organic framework catalysts

This investigation describes, for the first time, the efficient and selective liquid-phase green synthesis of n-amyl acetate over UiO-66 and NH2-UiO-66 metal-organic framework catalysts. By employing UiO-66 and NH2-UiO-66 solid acid catalysts, n-amyl acetate can be produced without the corrosive impact and separation problems associated with homogeneous catalysis by mineral acids. Thermal, structural, textural, and morphological properties of the catalysts were evaluated by TG-DTA, XRD, FTIR, Raman spectroscopy, BET-surface area, SEM and HRTEM. Their surface acidities were characterized by the dehydration of isopropanol and the chemical adsorption of suitable probs. The effects of reflux time, acid: alcohol molar ratio, and catalyst load were extensively studied. Under the optimized conditions, UiO-66, and NH2-UiO-66 catalysts, respectively offered 85 and 67 % conversions, and 100 % selectivity into n-amyl acetate. Activity of catalysts was well correlated with the total number of acidic sites, SBET and the kinetic rate constants of the esterification reaction. Analysis of the pre-adsorption studies of the reactants on the catalyst surface demonstrated that the Langmuir-Hinshelwood mechanism was followed in this reaction. The UiO-66 catalyst was regenerated numerous times with nearly the same activity and 100 % selectivity.

In-situ interzeolite transformation synthesis Cu-SSZ-13 from coal fly ash-derived zeolite for NH3-SCR reaction

Cu-SSZ-13 catalyst has excellent performance for removing NOx. In this study, the in-situ inter-zeolite transformation (IZT) method was proposed to prepare Cu-SSZ-13 catalyst. The coal fly ash-derived zeolite acted as a raw material rather than the chemical silicon and aluminum source. A series of catalysts were prepared using the in-situ IZT method with different ratios of the two template agents. The NO conversion of Cu-SSZ-13 catalysts was above 90 % within the temperature range of 200–700 °C, whereas the N2 selectivity was maintained at about 100 %. The as-synthesized catalysts had higher conversion rates for Si and Al, higher copper loading and more isolated Cu2+. The selective catalytic reduction (SCR) reaction mechanisms of Cu-SSZ-13 catalyst prepared using the in-situ IZT method followed both the Eley-Rideal (E-R) mechanism and the Langmuir-Hinshelwood (L-H) mechanism, though it was mainly based on the E-R reaction. In this study, the Cu-SSZ-13 catalysts were synthesized using coal fly ash based zeolite in one step. This study further improved the synthesis process route from coal based solid waste to SCR catalyst, shortened the preparation cycle, and improved catalyst performance.

A novel Cu-doped ZnO confined structure: Precisely preparation, and sensitization mechanism for ppb-level H2S gas detection

It is still highly desired and challengeable to design H2S sensor with high selectivity and sensitivity due to its toxicity and new application in biomarkers like monitoring halitosis. Here, we reported a novel confined structure as highly sensitive and selective H2S gas sensor. The sensing material was prepared with a HKUST-1 templated method by atomic layer deposition (ALD) ZnO decorated on HKUST-1 and calcination. The gas sensor exhibits a high gas sensitivity (Ra/Rg = 2.67 @ 2 ppm) with excellent selectivity to H2S. Furthermore, the obtained sensor shows ppb-level detection ability (Ra/Rg = 1.13 @ 0.1 ppm). Apart from releasing electrons by the oxidization of H2S on oxygen vacancies with adsorbed oxygen species O2− on ZnO surface (Langmuir-Hinshelwood mechanism), the Cu was proved to be the active sites, and would react with surficial S species. The suggestion was confirmed by operando XAS (X-ray adsorption spectrum) measurements combined with DFT calculations (Mars-van Krevelen mechanism). This work provides valuable insights for developing efficient sensing materials and understanding the sensing mechanism.

In situ DRIFTS study and mechanism analysis of selective catalytic reduction of NOx by NH3 over ions doping MnCeOx/GR catalyst

The present study demonstrates an innovative approach of doping MnCeOx/GR catalyst with three transition metal ions, namely Fe3+, Co3+, and Ni2+. Subsequently, the catalyst exhibiting superior performance was carefully selected. Meanwhile, the micro-reaction process of MnCeOx/GR and ion-doped MnCeOx/GR catalysts were established by in situ DRIFTS. The phenomena occurred in the experiment process were thoroughly examined through the application of various characteristic analysis, for instance, XRD, Raman, BET, XPS, H2-TPR, and NH3-TPD to analyze that Co3+ doping had a good improvement of activity and N2 selectivity on MnCeOx/GR catalyst at low temperature. This was evidenced by the activity test experiments, which revealed that CoMnCeOx/GR catalyst has the highest NO conversion within 150–225 °C temperature range, surpassing 90 %. Our findings showed that Co3+ doping not only enhanced redox capability and surface acidity, but also raised the relative Mn4+, Ce3+, and Oα content compared to MnCeOx/GR catalyst, which eventually contributed that the reactant molecules can be more effectively adsorbed onto the catalyst, thereby enhancing activation of NH3 and O2, thus improving efficiency of CoMnCeOx/GR catalyst. Additionally, it was proved that the Eley-Rideal and Langmuir-Hinshelwood mechanisms co-existed on CoMnCeOx/GR catalyst in a competitive way by in situ DRIFTS test.

Catalytic hydrogenation reaction micro-kinetic model for dibenzyltoluene as liquid organic hydrogen carrier

The implementation of the liquid organic hydrogen carrier (LOHC) technology for efficient energy storage requires the development of a reliable kinetic model for both hydrogenation and dehydrogenation processes. In this research study, the catalytic hydrocarbon saturation for a dibenzyltoluene (DBT) mixture solution, containing dibenzylbenzene (DBB), dibenzylethylbenzene (DBEB) and impurities has been performed in the presence of Ru/Al2O3 particles. The influence of different reaction conditions, such as temperature, pressure, initial reactant concentration, catalyst amount and stirring speed has been examined. A measurement-based system micro-kinetics, based on the Langmuir–Hinshelwood mechanism with dissociative H2 surface adsorption, has been derived. H2 thermodynamic solubility equilibrium was defined through Henry's law. The adsorbing, desorption and reactivity of inert solvent molecules was not considered to be relevant. The mass transfer resistance over 1000 rpm stirring speed was negligible. Relative- and mean squared error of representation were 40.9% and 1.00×10−4, respectively. Expressions gave an excellent data prediction for the profile period trends with a relatively accurate estimation of H2 intermediates' rate selectivity, H2-covered area approximation and pathway rate-determining steps. Due to the lack of commercially available standard chemical compounds for quantitative analysis techniques, a novel experiment-based numerical calibration method was developed. Mean field (micro)kinetics represent an advancement in the mesoscale mechanistic understanding of physical interface phenomena. This also enables catalysis structure–activity relationships, unlocking the methodology for new LOHC reaching beyond traditional, such as ammonia, methanol and formate, which do not release H2 alone. Integrated multiscale simulations could include fluidics later on.

Effect of vanadium and tungsten loading order on the denitration performance of F-doped V2O5-WO3/TiO2 catalysts.

F-doped V2O5-WO3/TiO2 catalyst has been confirmed to have excellent denitration activity at low temperatures. Since the V2O5-WO3/TiO2 catalyst is a structure-sensitive catalyst, the loading order of V2O5 and WO3 may affect its denitration performance. In this paper, a series of F-doped V2O5-WO3/TiO2 catalysts with different V2O5 and WO3 loading orders were synthesized to investigate the effect of denitration performance at low temperatures. It was found that the loading orders led to significant gaps in denitration performance in the range of 120-240 °C. The results indicated loading WO3 first better utilized the oxygen vacancies on the TiF carrier promoting the generation of reduced vanadium species. In addition, loading WO3 first facilitated the dispersion of V2O5 thus enhanced the NH3 adsorption capacity of VWTiF. In situ DRIFT verified the rapid reaction between NO2, nitrate, and nitrite species and adsorbed NH3 over the VWTiF, confirming that the NH3 selective catalytic reduction (NH3-SCR) reaction over VWTiF at 240 °C proceeded by the Langmuir-Hinshelwood (L-H) mechanism. This research established the constitutive relationship between the loading order of V2O5 and WO3 and the denitration performance of the F-doped VWTi catalyst providing insights into the catalyst design process.

Unlocking the Potential of Cu/Ti3C2Tx MXene Catalyst in Plasma Catalytic CO2 Hydrogenation

Plasma catalytic CO2 hydrogenation has emerged as a promising approach for converting CO2 into valuable chemicals such as CO, offering a potential solution to mitigate greenhouse gas emissions. In this study, we investigate the synergistic effect between plasma and a Cu/Ti3C2Tx MXene catalyst for CO2 hydrogenation via the reverse water gas shift (RWGS) reaction in a dielectric barrier discharge (DBD) reactor. Our findings reveal a significant enhancement in CO2 conversion (33.1%) and CO yield (31.9%) when using the Cu/Ti3C2Tx catalyst compared to plasma alone or with a Ti3C2Tx foam. Plasma treatment creates unsaturated Ti sites on the Ti3C2Tx surface, dramatically improving CO2 adsorption and activation, while Cu nanoparticles significantly enhances the activation and dissociation of H2 molecules on the catalyst surface. These surface adsorbed hydrogen species readily hydrogenate adsorbed CO2 molecules through the Langmuir-Hinshelwood mechanism. Furthermore, these surface hydrogen species can also participate in the conversion of gas-phase CO2 molecules through the Eley-Rideal pathway. This work sheds light on the synergistic plasma catalytic mechanism and provide insights for optimizing CO2 hydrogenation processes using MXene catalysts.

Mechanistic insights into the influence of preparation methods and Fe3+ doping on the low-temperature performance of MnCeOx catalyst for NH3-SCR reaction

Aiming to polish up the denitrification efficiency and N2 selectivity for MnCeOx catalyst at low temperature, an array of Fe3+-doped MnCeOx catalysts were synthesized through innovative approaches: the citric acid method (FeMnCeOx-CA) and CTAB-assisted template method (FeMnCeOx-ST). However, the catalysts prepared by CTAB-assisted template method (MnCeOx-ST and FeMnCeOx-ST catalysts) did not exhibit an excellent low-temperature activity compared with the catalysts made by citric acid method (MnCeOx-CA and FeMnCeOx-CA catalysts) below 300 °C. Therefore, an in-depth investigation of underlying impact of citric acid method and Fe3+ doping on MnCeOx catalyst was thoroughly implemented. We finally found that the main function of citric acid is to enhance the internal electron transfer of the catalyst by facilitating the mutual conversion between active components, particularly Mn4+/Mn3+, thereby improving the denitrification activity of FeMnCeOx-CA catalyst at low temperature. Further, the NH3-SCR performance of FeMnCeOx-CA exceed 90 % NOx conversion within 75–150 °C, and it can be reached 94 % NOx conversion at 125 °C, which was almost 10 % higher than that of MnCeOx-CA catalyst. Moreover, the N2O formation of FeMnCeOx-CA is significantly suppressed at temperatures ranging from 125-200 °C, exhibiting a higher N2 selectivity compared to MnCeOx-CA. Besides, solid solution structure of −Ce-O-Mn-O-Fe- formed after Fe3+ doping in MnCeOx catalyst strengthened surface acidity and redox ability over FeMnCeOx-CA catalyst, promoting NH3 adsorption and activation. On the other hand, the FeMnCeOx-CA catalyst exhibits the highest content of Ce3+ and Mn4+, facilitating oxygen vacancies formation and redox capacity, thereby promoting the key intermediates generation in the microscopic NH3-SCR reaction process. Finally, a reasonable Langmuir-Hinshelwood mechanism and detailed reaction pathways of NH3-SCR reaction existed on FeMnCeOx-CA catalyst were also proposed.

Mechanism of CO + NH3-coupled removal of NO from Fe/AC catalysts at medium and low temperatures

Catalysts with different mass ratios (Fe: AC = 7.5, 10.0, and 12.5) were prepared using the equal-volume impregnation method. The 12.5Fe/AC catalyst exhibited the highest NO removal activity and best N2 selectivity at any given temperature. Furthermore, NO removal was mediated by redox reactions between Fe2+ and Fe3+ and dominantly occurred at the Fe2+ active sites. As the loading amount of active metal (Fe) increased, the diffraction ring gradually diffused, and the number of Fe oxide crystals, number of oxygen-containing functional groups, and content of chemically adsorbed oxygen increased. Chemically adsorbed oxygen plays a major role in NO removal. The NO removal mechanism of the Fe/AC catalyst is dominated by physical adsorption and the Langmuir–Hinshelwood mechanism at low temperatures and by the Eley–Rideal mechanism at medium and low temperatures.