Nitrate Species Research Articles

Breaking the Selectivity Barrier: Reactive Oxygen Species Control in Photocatalytic Nitric Oxide Conversion

AbstractSemiconductor photocatalytic technology holds promise in efficiently reducing low concentrations of gaseous nitric oxide (NO). However, the suboptimal selectivity in NO removal, leading to the undesired production of NO2 byproducts, poses a challenge. In this study, a defective CdS/Na2Ti3O7 heterostructure is rationally designed with strong electronic interaction and intimate interface contact for promoting charge transfer kinetics. This design refines reactive oxygen species (ROS) generation, resulting in an impressive 81% NO elimination and 99.7% selectivity toward nitrates. Detailed mechanistic studies reveal an intriguing catalysis scenario in which reactant molecules are selectively adsorbed and activated at different sites. Anionic vacancies on the CdS/Na2Ti3O7 surface render the activation of molecular O2 to reactive superoxide radicals (O2 −) species. Furthermore, intrinsic surface basicity and O vacancy sites of the Na2Ti3O7 cocatalyst facilitate the capture and activation of acidic nitrogen oxides (NOx) molecules as nitrate species, contributing to enhanced catalytic activity and selectivity. As a result, anionic vacancies and basic sites over CdS/Na2Ti3O7 heterostructure synergistically regulate the NOx oxidation pathway, refining the products toward nitrate with remarkable selectivity. These insights guide the development of advanced photocatalytic systems for environmental remediation, highlighting the importance of managing ROS production for efficient pollutant removal.

Potassium and cobalt double-doped ferrierites as a new class of soot oxidation catalysts

Research efforts to develop an effective soot oxidation catalyst have intensified in recent years due to the increased awareness of the highly harmful effect of soot particles on the human health and the environment. Large-scale application of a developed catalyst calls for a cheap support material allowing for an effective dispersion and thus a decrease in the active phase loading. Zeolites, widely used in catalysis as active phase carriers, possess advantageous characteristics such as a uniform porous structure and large surface area. Furthermore, they are characterized by high availability, good price, and are environmentally friendly. Cobalt-based materials provide promising soot oxidation catalyst phases, and alkali doping can further enhance their activity. This study focuses on the optimization of the potassium loading in the potassium and cobalt double-doped ferrierites towards a new class of cheap and environmentally friendly soot combustion catalysts. The potassium doping of the ferrierite-supported cobalt spienel was optimized in the range of 0–14 wt%. The obtained materials were characterized in terms of their composition (XRF, XPS, XRD, Raman spectroscopy), morphology (SEM, TEM/EDX/FFT), and reducibility (H2-TPR), and tested in the process of soot oxidation in tight and loose contact modes. The strong promotional effect of potassium was discussed in terms of a significant modification of the catalyst electronic properties (work function studies) and supported by the potassium surface state studies by the alkali thermal desorption method (SR-TAD). It was found that the optimal potassium promoter dispersion leads to the lowest catalyst work function and its highest activity in soot combustion. The effect of nitric oxide was also investigated for selected catalysts. The opposite effect of NO on the activity of undoped and K-doped catalysts was discussed on the basis of in-depth spectroscopic studies. An enhanced formation of nitrate species was found on the surface of the K-containing catalyst which led to the stronger inhibiting effect of NO.

Unique κ-Ce2Zr2O8 Superstructure Promoting the NOx Adsorption-Selective Catalytic Reduction (AdSCR) Performance of the WO3/CeZrOx Catalyst.

Selective catalytic reduction of NOx by NH3 (NH3-SCR) for diesel emission control at low temperatures is still a great challenge due to the limit of the urea injection threshold and inferior SCR activity of state-of-the-art catalyst systems below 200 °C. Fabricating bifunctional catalysts with both low temperature NOx adsorption-storage capacity and medium-high temperature NOx reduction activity is an effective strategy to solve the issues mentioned above but is rarely investigated. Herein, the WO3/Ce0.68Zr0.32Ox (W/CZ) catalyst containing the κ-Ce2Zr2O8 pyrochlore structure was successfully developed by a simple H2 reduction method, not only showing superior NOx adsorption-storage ability below 180 °C but also exhibiting excellent NH3-SCR activity above 180 °C. The presence of the pyrochlore structure effectively increased the oxygen vacancies on the κ-Ce2Zr2O8-containing W/CZ catalyst with enhanced redox property, which significantly promoted the NOx adsorption-storage as active nitrate species below 180 °C. Upon NH3 introduction above 180 °C, the κ-Ce2Zr2O8-containing W/CZ catalyst showed greatly improved NOx reduction performance, suggesting that the pyrochlore structure played a vital role in improving the NOx adsorption-selective catalytic reduction (AdSCR) performance. This work provides a new perspective for designing bifunctional CeZrOx-based catalysts to efficiently control the NOx emissions from diesel engines during the cold-start process.

Design Strategy of the MnOx Catalyst for SCR of NO with NH3: Mechanism of Lead Poisoning and Improvement Method.

The conventional Mn-based catalysts suffer from lead toxicity and require other transition-metal oxides to enhance their resistance in the selective catalytic reduction of NOx with ammonia (NH3-SCR). Herein, we found that the incorporation of inert silica into pure MnOx effectively improved the Pb resistance. The NOx conversion of the MnOx-SiO2-Pb catalyst was nearly 55% higher than that of the MnOx-Pb catalyst, exhibiting enhanced activity at lower temperatures (150-225 °C). To reveal the essential roles at the molecular level, the types and numbers of surface acidity, nitrate species, and catalytic cycle were established through experimental analysis and theoretical calculations of catalysts. The presence of PbCl2 occupied the active Mn sites, resulting in an obvious decline in the Brønsted acid sites (B-NH4+) and the oxidation performance, and the NH3-SCR cycle was energetically less favorable on the MnOx-Pb catalyst. Conversely, SiO2 played a crucial role in preserving the activity of Mn sites on the MnOx-SiO2-Pb catalyst by preferentially bonding with PbCl2, generating more active intermediates. Significantly, this work provided mechanistic insights into the role of SiO2 in regulating the surface acidity, oxidation performance, and stability of active Mn sites, which is helpful for the design of Mn-based catalysts with high Pb resistance for the NH3-SCR reaction.

Deep insights into the promotion role of Sm doping in the sulfur resistance of Fe2O3 catalyst for NH3-SCR: A combined experimental and DFT study

To investigate the inherent role of Sm doping in metal sulfation over the surface of Fe2O3 catalyst and reveal the reason for the SO2 tolerance enhancement of Sm/Fe2O3 catalyst at low temperatures (<320 °C), sulfated Fe2O3 and Sm/Fe2O3 catalysts were prepared by pretreating fresh catalysts in SO2+O2 atmosphere. Selective catalytic reduction (SCR) of NOx by NH3 activities and physicochemical characteristics of fresh and sulfated catalysts were studied, and the reaction mechanisms on sulfated catalysts analyzed. The minor shift of Fe signal to high value and larger shift of S signal to low value of sulfated Sm/Fe2O3 compared with sulfated Fe2O3 was observed with X-ray photoelectron spectroscopy (XPS), indicating that Sm modification could weaken the sulfation of active Fe sites. Consistent with the XPS results, density functional theory calculations showed that SO2 is more easily adsorbed on the Sm/Fe2O3 catalyst with the adsorption sites located at Sm atom and its neighboring Fe atoms. In-situ DRIFTS results showed that the SCR reaction over the sulfated Fe2O3 catalyst followed both L-H (the adsorbed NH3 species react with adsorbed nitrate species) and E-R (the adsorbed NH3 species react with gaseous NO) mechanisms at 250 °C, with the L-H route much faster, whereas only the L-H pathway occurred over sulfated Sm/Fe2O3 catalyst because of too strong affinity of NH3 on the sulfated sites to inhibit NH3 activation – this was also the reason for its enhanced Lewis and Brønsted acidity. More reactive nitrate species were formed on the sulfated Sm/Fe2O3 catalyst due to the existence of more un-sulfated Fe sites, making the Sm/Fe2O3 catalyst resistant to SO2 poisoning at relatively low temperatures.

Revealing the mechanism of K-enhanced Cu-SSZ-13 catalysts against hydrothermal aging and P-poisoning for NOx reduction by NH3-SCR

Cu-SSZ-13 (Cu-Z) has been commercialized as an efficient catalyst for NOx removal in mobile sources. However, in practical applications, the catalyst faces problems of poisoning (K and P) and hydrothermal aging. Therefore, in this work, we investigated the effects of K on P poisoning and hydrothermal aging in Al-rich Cu2/Z (2 wt% Cu). The presence of 1 wt% K could not only improve the high-temperature activity, which could reach above 99% at 225–600 °C on the K1/Cu2/Z, but also enhanced the hydrothermal stability and the resistance of P poisoning. Combing with a series of characterizations, K was mainly present in the Brӧnsted acid sites in the CHA cage, which led to the transformation of part of Z-[Cu(OH)]+ to Z2-Cu2+, reducing dealumination and the formation of CuOx and CuAl2O4 species during hydrothermal aging. Moreover, the presence of K could inhibit the formation of Cu-PO3-/PO43- species by P poisoning of Z-[Cu(OH)]+ species. In-situ DRIFTS results showed that the catalysts followed both E-R and L-H mechanism at 150 ℃. K doping could promote the formation of nitrate species and thus favoring the L-H mechanism, which probably enhanced the NH3-SCR activity.

Structural Investigation of Technetium Dibutylphosphate Species Using X-ray Absorption Fine Structure Spectroscopy.

The speciation of Tc after the extraction of Tc(IV) from H2O and 1 M HNO3 by dibutylphosphoric acid (HDBP) in dodecane has been studied by X-ray absorption fine structure (XAFS) spectroscopy. Results show the formation of dimeric species with Tc2O2 and Tc2O units, and the formulas [Tc2O2(DBP·HDBP)4] (1) and [Tc2O(NO3)2(DBP)2(DBP·HDBP)2] (2) were, respectively, proposed for the species extracted from H2O and 1 M HNO3. The interatomic Tc-Tc distances found in the Tc2O2 and Tc2O units [2.55(3) and 3.57(4) Å, respectively] are similar to the ones found in Tc(IV) dinuclear species. It is likely that the speciation of Tc(IV) in dodecane is due to the extraction of a species with a Tc2O unit for (2) and to the redissolution of a Tc(IV)-DBP solid for (1). The XAFS results for (1) and (2) were compared to that obtained for the extraction of Tc(IV) with TBP/HDBP/dodecane from 0.5 M HNO3, (3) which highlight the formation of Tc mononuclear nitrate species {i.e., [Tc(NO3)3(DBP)] or [Tc(NO3)2(DBP·HDBP)]}. These results confirm the importance of the preparation and speciation of the Tc(IV) aqueous solutions prior to extraction and how much this influences and drives the final Tc speciation in organic extraction. These studies outline the complexity of Tc separation chemistry and provide insights into the behavior of Tc during the reprocessing of used nuclear fuel.

Remarkable N2 selectivity enhancement of NH3-SCO reactions over V0.5/Pt0.04/TiO2 catalyst through Cu-Er co-modification.

Pt-V bimetallic catalysts maybe promising substitutes to precious metal catalysts for selective catalytic oxidation (SCO) of NH3. But it remains a major challenge for Pt-V bimetallic catalysts to pursue high NH3 conversion rate and N2 selectivity simultaneously. In this work, both Cu and Er were adopted to modify V0.5/Pt0.04/TiO2 catalyst (denoted as V/PT), and the influences of Cu and Er doping amounts on NH3-SCO performance of V/PT catalysts were investigated systematically. The results indicated that the co-modification of Cu and Er imposed little influence on NH3 conversion efficiency, but significantly boosted N2 selectivity. Compared with other Cu-Er-modified V/PT catalysts, CEV/PT-4 catalyst exhibited outstanding NH3-SCO performance, which attained completely 100% NH3 conversion efficiency and > 90% N2 selectivity in the temperature range of 225-450°C. It was significantly superior to the NH3-SCO performance of most previously reported catalysts. The characterization results indicated that the adequate doping amounts of Cu and Er resulted in an obvious enhancement on redox property and surface acidity of CEV/PT-4 catalyst. It also led to abundant Pt0 and surface chemisorbed oxygen species on catalyst surface, which facilitated the oxidation of NH3 to NOx and enhanced i-SCR reactions. In situ DRIFTS results showed that -NH2 species on the surface of CEV/PT-4 catalyst could actively react with nitrate species to generate N2 and H2O.

CuOx and WOx modified CeO2-TiO2 with various morphologies for the selective catalytic reduction of NOx with NH3

CuOx and WOx modified CeO2-TiO2 (CeTi) with three various morphologies were fabricated and applied for selective catalytic reduction of NOx by NH3 (NH3-SCR). Catalytic performance showed that the loading of CuOx and WOx can enhance the low-temperature and high-temperature NH3-SCR activity of CuOx-WOx/CeO2-TiO2 (CuW/CT), respectively. CuOx-WOx/CeO2-TiO2 nanopolyhedrons (CuW/CT-NP), CuOx-WOx/CeO2-TiO2 nanorods (CuW/CT-NR) and CuOx-WOx/CeO2-TiO2 nanocubes (CuW/CT-NC) predominately exposed {111}, {110} and {100} crystal planes, respectively. The NH3-SCR performance indicated that CuW/CT-NP achieved the best NH3-SCR activity, excellent resistance to H2O&SO2 and the widest reaction temperature window with a full conversion temperature (T90) of 178 °C, much lower than that of CuW/CT-NR (210 °C) and CuW/CT-NC (273 °C). XPS, Raman, H2-TPR and NH3-TPD results confirmed that the redox and acidic properties of CuW/CT catalysts were closely related to the exposed facets of CeTi supports. The in situ DRIFTS results indicated that both NH3 and NOx adsorbed on the catalysts surface were active species. CuW/CT-NP exhibited stronger adsorption capacity for NOx and NH3 compared with CuW/CT-NR and CuW/CT-NC. A large number of active NH3 and nitrate species were formed on the surface of CuW/CT-NP, and the Langmuir Hinshelwood (L-H) pathway accelerated the NH3-SCR reaction, thereby enhancing the NH3-SCR performance.

Reinterpret the heterogeneous reaction of α-Fe2O3 and NO2 with 2D-COS: The role of SDS, UV and SO2

Heterogeneous reaction of mineral aerosols and atmospheric polluting gases play an important role in atmospheric chemistry. In this study, the reactions of NO2 with or without SO2 mixture gas on the surface of α-Fe2O3 particles under dry conditions were studied. The effects of sodium dodecyl sulfate (SDS) and the heterogeneous reaction under both dark and UV irradiation conditions were investigated. The infrared spectrum analyzed by the two-dimensional correlation spectroscopy (2D-COS) was used to obtain the products formation sequences. The results showed that UV irradiation can promote the production of nitrate. The 2D-COS analysis indicated SDS changed the sequence order of nitrate and nitrite species during reactions. In oxidation conditions, the final product of heterogeneous reaction of NO2 and α-Fe2O3 was monodentate nitrate. Only the heterogenous reaction of NO2 and α-Fe2O3 containing SDS (FOS) without UV light, the final product was bidentate nitrate. SDS was the catalysis agent supply and photoresist to the system. With surface active compounds, the environmental lifetime of heterogeneous reactions between trace gases and aerosols extends. Surfactants, ultraviolet light, and the types of gases involved in the reaction all have complex effects on the aerosol aging process. This study provided a reference for subsequent heterogeneous reaction studies and the formation of aerosols.

Effect of Ce and Mn on 3DOM La0.8Sr0.2MnO3 perovskite in soot combustion under different atmospheres

Fabrication of high-performance soot combustion catalyst and clarification the reaction mechanism has been of great importance but rarely achieved. Herein, a series of three-dimensionally ordered macroporous La0.8Sr0.2MnO3 perovskite were fabricated and further decorated by Ce and Mn. The well-dispersed CeO2 on the surface of perovskite could accelerate the adsorption and activation of gaseous oxygen, therefore promoting the oxidation process. The further introduction of Mn was conclusive to the storage and utilization NOx, which was beneficial for soot combustion with the assistance of NOx. According to the catalytic performance tests and DRIFTS, CeO2-loaded catalysts tended to follow the active oxygen species derived reaction, with lowest T50 of 460 °C for C1/LSM under 5 % O2/N2 atmosphere. While CeMnOx-loaded catalysts were inclined to follow the NOx-assisted catalytic reaction, and the generated nitrate species would directly participate in the reaction. The T50 of C1M2/LSM further decreased to 397 °C under 0.05 % NO + 5 % O2/N2 atmosphere. This work was not only devoted to design high performance catalysts for soot oxidation, but also to reveal the catalytic mechanism of these catalysts under different atmospheres.

Distinct effect of preparation methods on reaction efficiency of Mn-Ce/Al2O3 catalysts in low-temperature NH3-SCR

In this study, a series of Mn-Ce/Al2O3 catalysts was prepared by different methods of deposition-precipitation (MnCeAl-DP), impregnation (MnCeAl-IM) and citric acid (MnCeAl-CA), and the distinct effect of preparation methods on NOx removal performance at low temperature was explored. Results show that MnCeAl-DP exhibits not only the best activity but also the highest resistance against SO2/H2O. With the assistance of comprehensive characterizations from scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET), X-ray diffraction (XRD), H2-temperature programmed reduction (H2-TPR), NH3-temperature programmed desorption (NH3-TPD), and X-ray photoelectron spectroscopy (XPS), it is revealed that the MnCeAl-DP sample owns admired features of large surface area and pore volume, enriched Mn4+ and chemisorbed oxygen species originating from enhanced interaction between MnOx and CeO2, as well as improved adsorption capacity to NH3 and NO. All these factors contribute to activity enhancement. Further in-situ DRIFTS studies reveal that the improvement of NH3-SCR performance over MnCeAl-DP is related to the formation of abundant nitrate species, which is beneficial to the “NH4NO3” reaction pathway and thus enhances low-temperature activity.

The influence of partial substitution of Ce with K in CeMO3 (M = Mn, Fe, Co, Ni, Cu) perovskite catalysts on soot combustion performance

A series of Ce1−xKxMO3 (M = Mn, Fe, Co, Ni, Cu, and x = 0, 0.1, 0.2, 0.3, 0.4) perovskite catalysts were synthesized by citrate sol-gel method, in which A-site element Ce was partially substituted with K. The results showed that the catalytic performance could be significantly improved by partially substituting of Ce with K in CeCoO3 catalyst for soot combustion. Ce0.8K0.2CoO3 perovskite catalyst exhibited a superior catalytic activity, and its corresponding T10, T50, T90 and SmCO2 were 326, 383, 426 °C and 98.6%, respectively. The characterization results indicated that K+ had been successfully incorporated in the perovskite crystal structure. Partially substituting of Ce with K resulted in an increase in specific surface area. It also resulted in the formation of Co4+ in the perovskite catalysts together with abundant presence of oxygen vacancies on the surface. Moreover, it was of great significance to facilitate the activation of lattice oxygen species. In situ DRIFTS result showed that NO2 produced by thermal decomposition of monodentate nitrate species played a key role in soot combustion process.

Revealing the crystal-plane effects of CuO during the NH3-SCR over CuO/TiO2 catalysts

Copper-based (Cu-based) catalysts have garnered significant attention for their potential in ammonia-selective catalytic reduction (NH3-SCR) of nitrogen oxides (NOx). However, the widespread industrial application of Cu cation-exchanged chabazite catalysts has been hindered by poor hydrothermal stability, expensive raw materials, and complicated preparation. As a result, copper oxide-based (CuO-based) catalysts have emerged as ideal candidates due to their low cost and facile preparation. Nevertheless, the performance of NH3-SCR catalysts is a significantly influenced by the crystal plane effect, which has been scarcely investigated in Cu oxide-based NH3-SCR catalysts. In this study, CuO with exposed {001} and {111} facets were synthesized and combined with titanium dioxide (TiO2) support to construct CuO/TiO2 composite NH3-SCR catalysts. These catalysts displayed divergent NOx removal performance due to the crystal-plane effects of CuO. Specifically, the proportion of Cu+ and surface-adsorbed oxygen (Oα) in CuO(111)/TiO2 catalyst was higher than that of CuO(001)/TiO2 catalyst, which could facilitate the NH3-SCR reactions. The NH3-TPD, NO-TPD, and DFT calculations demonstrated the better NH3 and NO adsorption capability of CuO(111)/TiO2 catalyst than that of CuO(001)/TiO2 catalyst. Furthermore, the more -NH2 and nitrate species in CuO(111)/TiO2 catalyst resulted in better NOx removal efficiency than CuO(001)/TiO2 catalyst. This study provides fundamental insights on the crystal-plane effect of CuO, which is critical for optimizing Cu oxide-based NH3-SCR catalysts.

Effect of trace impurities in perchloric acid on blank voltammetry of Pt(111)

Trace impurities of nitrate and sulphate species have been detected as the cause for the charge quench of existing voltammetric peaks and the appearance of new voltammetric peaks in the blank voltammetry of Pt(111) in high-purity perchloric acid. Cyclic voltammetry of Pt(111) in high concentrations of HClO4 (1 M) electrolyte display anomalous reduction peaks at 0.5 and 0.32 VRHE. The reduction of nitrates to nitric oxide, resulting in the reduction peak at 0.32 VRHE, has been detected using in situ Fourier transform infrared spectroscopy. The peak at 0.5 VRHE, previously assigned in the literature to specific adsorption of perchlorate anions catalytically dissociating to chloride anions, is consistent with sulphate desorption as elucidated by spiking HClO4 electrolyte with trace quantities (10 ppm) of sulphuric acid. The presence of kosmotropic sulphate anions was further confirmed by the positive onset potential shift of the sharp OHads butterfly peak at ∼0.8 VRHE with increasing concentrations of HClO4 (0.01–1 M). Although commonly overlooked, we show here the marked deleterious impact these trace impurities have on the blank voltammetry of Pt(111), and provide recommendations for minimising this impact.

Bifunctional bimetallic oxide nanowires for high-efficiency electrosynthesis of 2,5-furandicarboxylic acid and ammonia

It is an appealing avenue for electrosyntheis of high-valued chemicals at both anode and cathode by coupling 5-hydroxymethylfurfural (HMF) oxidation and nitrate reduction reactions simultaneously, while the development such bifunctional electrocatalysts is still in its infancy with dissatisfied selectivity and low yield rate. Here, we first report that Zn-doped Co3O4 nanowires array can be served as an efficient and robust dual-functional catalyst for HMF oxidation and nitrate reduction at ambient conditions. Specifically, the catalyst shows a faradaic efficiency of 91 % and a yield rate of 241.2 μmol h−1 cm−2 for 2,5-furandicarboxylic acid formation together with a high conversion of nearly 100 % at a potential of 1.40 V. It also displays good cycling stability. Besides, the catalyst is capable of catalyzing the reduction of nitrate to NH3, giving a maximal faradaic efficiency of 92 % and a peak NH3 yield rate of 4.65 mg h−1 cm−2 at a potential of –0.70 V. These results surpass those obtained using pristine Co3O4 and are comparable to those of state-of-the-art electrocatalysts. Moreover, the catalyst is further employed as the cathode catalyst to assemble a Zn–nitrate battery, giving a peak power density of 5.24 mW cm−2 and a high yield rate of 0.72 mg h−1 cm−2. Theoretical simulations further reveal that Zn-doping favors the adsorption and dissociation of nitrate and HMF species and reduces the energy barrier as well. Our work demonstrates the potential interest of Co3O4-based materials for the highly selective production of valuable feedstocks via ambient electrolysis.

Enhanced low-temperature performance of Al-rich Cu-SSZ-13 by Ce modification upon hydrothermal aging for NH3-SCR

3wt.% Cu/SSZ-13 (Cu3/Z) and Ce modified 3wt.% Cu/SSZ-13 (Ce-Cu3/Z) catalysts were prepared and then hydrothermally aged. The Ce-modified Ce-Cu3/Z catalyst revealed excellent NH3-SCR activity and hydrothermal stability. Combined with a series of characterizations, it was proved that the introduction of Ce could inhibit the dealumination, and prevent the aggregation of Cu ions to form CuOx and CuAl2O4 species upon hydrothermal aging, thus improving the hydrothermal stability of Ce-Cu3/Z. In addition, Ce could interact with Cu species to enhance the reducibility of catalyst, and the interaction between Ce and Cu species was strengthened after hydrothermal aging. Also, the presence of Ce species could enhance the NH3 and NOx adsorption capacity of the catalyst, resulting in higher NH3-SCR activity of Ce-Cu3/Z after hydrothermaly aged at 700 °C. In situ DRIFTS results showed that the NH3-SCR reaction followed both E-R and L-H mechanisms on the Cu3/Z-700 and Ce-Cu3/Z-700 catalysts at 150 °C. Ce doping could promote the formation of L-NH3 (NH3 species coordinated to Lewis acid sites) and nitrate species on the Ce-Cu3/Z-700 catalyst, as well as the involvement of L-NH3 in the reaction, which play a crucial role as the main active intermediates at low temperature.

Transformation of waste battery cathode material LiMn2O4 into efficient ultra-low temperature NH3-SCR catalyst: Proton exchange synergistic vanadium modification

It is essential to develop the catalyst for NH3-SCR with excellent performance at ultra-low temperature (≤150 °C), and resource recycling is another important part of environmental protection. Based on the principle of environmental friendliness, the LiMn2O4, one of the waste battery cathode materials, was successfully modified into a novel high-value catalyst for ultra-low temperature NH3-SCR through hydrogen ion exchange and two-dimensional vanadic oxide modification. The optimized LiMn2O4-0.5V-10H catalyst performed the best balance of NOx conversion and N2 selectivity, with activity reaching 96 % at 150 °C and N2 selectivity exceeding 70 % at ultra-low temperature. Due to the unique three-dimensional network structural characteristics of LiMn2O4 spinel, hydrogen exchange could exchange Li+ from the lattice and increase surface acidity; and a small amount of two-dimensional vanadic oxide loading could appropriately regulate redox ability and increase acidic sites. The in-situ DRIFTS results still showed that the L-H and E-R mechanisms coexisted during the reaction. Moreover, combining first-principles calculations and in-situ DRIFTS, the dual modification of H and V could enhance the adsorption of NH3 on the surface of LiMn2O4 but weaken the adsorption of NO, and promote the decomposition of nitrites while inhibit the formation of surface nitrate species, which was the core reason for the improvement of N2 selectivity. The modification mode in this work was simple and inexpensive, which provided a new idea for the high-value utilization of waste batteries and the design of NOx purification catalyst at ultra-low temperature.

Sintering resistance of Pt-based oxidation catalyst via constructing Pt/MnOx interface

It is a challenge to improve the catalytic activity and hydrothermal stability of platinum-based catalyst in concurrently oxidizing CO/C3H6/NO. The construction of Pt/MnOx interface over Pt-based diesel oxidation catalyst via overlayer deposition to form partial-embedded structure, inhibiting the aggregation of platinum nanoparticles in Ostwald Ripening process. Even after hydrothermal aging, the average platinum particle size is equal to that of the fresh catalyst via traditional impregnation method. The electronic-transferring from platinum to manganese results in a weaker CO adsorption ability on Pt species and an improved oxygen activation ability with the help of the redox cycle of Pt and MnOx, as well as the formation of more active nitrate species via in situ DRIFTs. Measurement on the activity shows the lower apparent activation energy, so that the light-off temperature of CO/C3H6/NO on the partial-embedded catalyst via overlayer deposition method decreases by around 20 °C and 40% NO maximum conversion still keeps after severely aging comparing with the traditional catalyst. The work enlightens us the importance of interface sites in stabilizing platinum atoms and creating oxidize active sites.

Unveiling the SO2 Resistance Mechanism of a Nanostructured SiO2(x)@Mn Catalyst for Low-Temperature NH3-SCR of NO.

Low N2 selectivity and SO2 resistance of Mn-based catalysts for removal of NOx at low temperatures by NH3-SCR (selective catalytic reduction) technology are the two main intractable problems. Herein, a novel core-shell-structured SiO2@Mn catalyst with greatly improved N2 selectivity and SO2 resistance was synthesized by using manganese carbonate tailings as raw materials. The specific surface area of the SiO2@Mn catalyst increased from 30.7 to 428.2 m2/g, resulting in a significant enhancement in NH3 adsorption capacity due to the interaction between Mn and Si. Moreover, the N2O formation mechanism, the anti-SO2 poisoning mechanism, and the SCR reaction mechanism were proposed. N2O originated from the reaction of NH3 with O2 and the SCR reaction, as well as from the reaction of NH3 with the chemical oxygen of the catalyst. Regarding improving the SO2 resistance, DFT calculations showed that SO2 was observed to preferentially adsorb onto the surface of SiO2, thus preventing the erosion of active sites. Adding amorphous SiO2 can transform the reaction mechanism from Langmuir-Hinshelwood (L-H) to Eley-Rideal (E-R) by adjusting the formation of nitrate species to produce gaseous NO2. This strategy is expected to assist in designing an effective Mn-based catalyst for low-temperature NH3-SCR of NO.