Reactivity of generated oxygen species from nitrous oxide over [Fe,Al]MFI catalysts for the direct oxidation of benzene to phenol

The catalytic performance of various steam-activated [Fe,Al]MFI catalysts in the direct oxidation of benzene to phenol using N₂O as oxidant is described. All [Fe,Al]MFI catalysts contain ca. 90% of iron in the high-spin Fe²⁺ state, independent of the iron concentration (0.075–0.6 wt.% iron). In the presence of N₂O at 623 K, most Fe²⁺ ions (>90%) were oxidized to Fe³⁺ ions as deduced from Mössbauer spectroscopy. In the presence of benzene, subsequent reduction of Fe³⁺ to Fe²⁺ takes place. However, not all of the oxidized Fe²⁺ to Fe³⁺ ions were able to selectively oxidize benzene to phenol. This indicates that only a fraction of iron is catalytically active. For [Fe,Al]MFI catalysts with relatively high iron concentration, most of the extra-framework iron species formed are inactive in the direct oxidation of benzene to phenol. Finally, a more detailed in situ Mössbauer study for one sample, i.e. [Fe,Al]MFI (1:8) catalyst, was performed to illustrate the reduction/oxidation properties of the different iron species formed after steam-treatment.     

Partial oxidation of light paraffins on supported superacid catalytic membranes

Superacid-supported catalytic membranes were found to be active and very selective in the partial oxidation of light paraffins (C₁–C₂) with H₂O₂ under mild conditions (T_R: 80–110°C; P_R: 1.4 bar) in a three-phase catalytic membrane reactor (3PCMR). Among different catalytic membranes investigated, Nafion-based ones showed the best performance in terms of both activity and selectivity. Addition of Fe²⁺ ions in the liquid phase enhances the reaction rate, however, a volcano-shaped trend between reaction rate and concentration of Fe²⁺ was observed. Reaction temperature drastically affects both reaction rate and product distribution. A reaction pathway based on the electrophilic hydroxylation of the C–H bond on superacid sites and subsequent reaction of the activated paraffin with OH radicals has been proposed.     

Effect of doping of TiO2 support with altervalent ions on physicochemical and catalytic properties in oxidative dehydrogenation of propane of vanadia–titania catalysts

Vanadia phase (one monolayer) was deposited on TiO₂ anatase doped with Ca²⁺, Al³⁺, Fe³⁺ and W⁶⁺ ions and the catalysts thus obtained (VMeTi) were characterized by XPS, work function technique, decomposition of isopropanol (a probe reaction for acido–basic properties) and tested in oxidative dehydrogenation of propane. The doping of the TiO₂ support modifies physicochemical and catalytic properties of the active vanadia phase with respect to the undoped TiO₂. The specific activity in the propane oxydehydrogenation decreases in the order: VFeTi>VWTi>VTi>VAlTi>VCaTi (3), whereas the selectivity to propene follows the sequence: VWTiVTi>VFeTi>VAlTi>VCaTi. This implies that the lower is the surface energy barrier for transfer of electrons from the catalyst to the reacting molecules the higher is the selectivity to the partial oxidation product. It is argued that owing to the decrease in this energy barrier the reoxidation step in the catalytic reaction, involving such a transfer: O₂+4e→2O²⁻ is fast, thus, preventing the presence of intermediate non-selective electrophilic oxygen species on the surface.     

Partial oxidation of propane on Nafion supported catalytic membranes

Nafion supported catalytic membranes were found to be effective in the partial oxidation of propane to oxygenates with H₂O₂ in the presence of Fe²⁺ under mild conditions. The influence of [Fe²⁺] and [H₂O₂] on the reaction rate and product distribution in the temperature range 80–110°C has been ascertained. A reaction pathway involving the electrophilic activation of propane on superacid sites and subsequent reaction of the activated propane molecules with OH radicals generated by Fe²⁺/H₂O₂ Fenton system is discussed.     

AFeO3 (A=La, Nd, Sm) and LaFe1−xMgxO3 perovskites as methane combustion and CO oxidation catalysts: structural, redox and catalytic properties

Catalytic methane combustion and CO oxidation were investigated over AFeO₃ (A=La, Nd, Sm) and LaFe_1−xMg_xO₃ (x=0.1, 0.2, 0.3, 0.4, 0.5) perovskites prepared by citrate method and calcined at 1073 K. The catalysts were characterized by X-ray diffraction (XRD). Redox properties and the content of Fe⁴⁺ were derived from temperature programmed reduction (TPR). Specific surface areas (SA) of perovskites were in 2.3–9.7 m² g⁻¹ range. XRD analysis showed that LaFeO₃, NdFeO₃, SmFeO₃ and LaFe_1−xMg_xO₃ (x·0.3) are single phase perovskite-type oxides. Traces of La₂O₃, in addition to the perovskite phase, were detected in the LaFe_1−xMg_xO₃ catalysts with x=0.4 and 0.5. TPR gave evidence of the presence in AFeO₃ of a very small fraction of Fe⁴⁺ which reduces to Fe³⁺. The fraction of Fe⁴⁺ in the LaFe_1−xMg_xO₃ samples increased with increasing magnesium content up to x=0.2, then it remained nearly constant. Catalytic activity tests showed that all samples gave methane and CO complete conversion with 100% selectivity to CO₂ below 973 and 773 K, respectively. For the AFeO₃ materials the order of activity towards methane combustion is La>Nd>Sm, whereas the activity, per unit SA, of the LaFe_1−xMg_xO₃ catalysts decreases with the amount of Mg at least for the catalysts showing a single perovskite phase (x=0.3). Concerning the CO oxidation, the order of activity for the AFeO₃ materials is Nd>La>Sm, while the activity (per unit SA) of the LaFe_1−xMg_xO₃ catalysts decreases at high magnesium content.     

Anisotropic light absorption of the ferrite solid solutions Ca2(Fe1 − xAlx)O5

In the ferrite phase of the system Ca₂(Fe_{1 − x}Al_x)O₅ (x = 0 ≈ 0.67) Mg replaces Fe and Al up to 0.044 atom/formula irrespective of the composition x. In the presence of sufficient oxygen in the atmosphere tetravalent Fe ions are formed with resultant increase of both anisotropic light absorption and electric conductivity, due to a mechanism involving transfer of a positive electron hole from Fe⁴⁺ ions to Fe³⁺ ions in the vicinity. Coexistence of trivalent Y ions reduces this effect. Under low oxygen pressures the positive charge deficiency due to the replacement is compensated by creating oxygen defects; and the ferrite phase lightens with increase in yellowness.     

Heat-treatment of isomorphously substituted ZSM-5 (MFI) zeolites An ESR and Mössbauer spectroscopy and kinetic study

Fe-ZSM-5 zeolites comprise of three types (I, II and III) of iron (Fe³⁺) sites in both as synthesized (AS) and heat-treated states. Previous evidence could be confirmed by the assignment of ESR lines, registered in the X-band, stating: (i) type I framework Fe³⁺ sites (in T_h oxygen coordination), where charge compensation occurs mainly by Na⁺ ions and the crystal field (cf) approximates cubic symmetry, produce resonances at near to g = 2.00; (ii) type II and probably type III binuclear Fe...Fe dioxo- and oxo-bridges manifest themselves in producing slightly distorted surroundings of axial symmetry, and the relevant powder-averaged subspectrum is superimposed on the previous one in the g  2.45–1.98 interval. As the temperature of the heat-treatment (HT) is raised, type II and III sites will be annihilated with preference, under concomitant production of amorphous Fe₂O₃ possessing molecular dispersity. (iii) The weak ESR signal (2–4%) at g  4.27 is attributed to T_h coordinated framework sites in the surface layers (≈ 1 u.c. thick) under the influence of solid surface tension, giving rise to fully rhombic cf symmetry.

The size of the ejected Fe₂O₃ particles, as estimated from Mössbauer spectra (no hyperfine structure at 77 K) is less than 2.9 nm. The extremely large electron affinity of Fe³⁺ ions manifests itself in decreased ionicity (and unexpectedly dominant cubic symmetry), activity in biomimetic oxidations, autoreduction and spontaneous reoxidation etc.

In the liquid phase oxidation by hydrogen peroxide of n-hexane and cyclohexane the heat-treated Ti-ZSM-5 samples exhibited both activity- and selectivity dependence on HT. On the basis of experiences drawn from the heat-treatments proposal is made for the structures of the defect site (producing the 960 cm⁻¹ IR signal) and the active centre of selective oxidation. The proposal seems to be in accord with the experimental observations (isotopic exchange, MAS-NMR behaviour etc.) published so far.     

Photodegradation catalyst screening by combinatorial methodology

In this work, a combinatorial methodology was developed for photodegradation catalyst screening. A fluorescence imaging detection system was designed for high throughput analysis, 1,6-hexamethylenediamine was used as the probe molecule for catalyst testing. The photodegradation activity of catalysts was evaluated by 1,6-hexamethylenediamine consumption during the photodegradation reaction. The methodology could provide reliable results. We found that pure TiO₂, ZrO₂, Nb₂O₅, MoO₃, and WO₃ did not show much activity for 1,6-hexamethylenediamine photodegradation under visible light. TiO₂ catalysts doped with different metal ions were tested. When TiO₂ was doped with Ta₂O₅, Nb₂O₅, V₂O₅, MoO₃, or WO₃, higher activity for photodegradation was observed. The doping of La³⁺, Ba²⁺, and Br⁻ to TiO₂ did not improve the catalytic activities. When doping TiO₂ with Mn²⁺, Cl⁻, Al³⁺, Cu²⁺, Fe³⁺, Na⁺, Mg²⁺, Li⁺, F⁻, Co²⁺, or K⁺, catalytic activity was lower than that of pure TiO₂. After elaborate catalysts screening, we discovered new catalysts, such as 50–70% TiO₂/0–20% WO₃/20–40% VO_2.5 and 20–30% TiO₂/30–50% MoO₃/40–60% VO_2.5 as well as 30% WO₃/20% ZrO₂/50% NbO_2.5 (synthesized from ZrCl₄, NbCl₅, and (NH₄)₅H₅[H₂(WO₄)₆]·H₂O in ethanol solution or suspension) and 60–70% WO₃/Nb₂O₅ (synthesized from WCl₆ and NbCl₅ in ethanol solution). We observed that the catalytic activity is sensitive to preparation methods and catalyst specific surface areas. When P123 (HO(CH₂CH₂O)₂₀(CH₂CH(CH₃)O)₇₀(CH₂CH₂O)₂₀H, designated EO₂₀PO₇₀EO₂₀) was used as template to synthesize mesoporous materials, the mesoporous catalysts showed higher activity than regular catalytic materials.     

On the mechanism of model diesel soot-O2 reaction catalysed by Pt-containing La-doped CeO2: A TAP study with isotopic O2

Pt supported on CeO₂ and 10 wt.% La³⁺-doped CeO₂ catalysts have been prepared, characterised and tested for soot oxidation by O₂ in TGA. The reaction mechanism has been studied in a TAP reactor with labelled O₂. Isotopic oxygen exchange between molecular O₂ and ‘O’ on the support/catalyst was observed and soot oxidation is being carried out by lattice oxygen. TAP studies further show that Pt improves O₂ adsorption and, therefore, 5 wt.% Pt-containing catalysts are more active for soot oxidation than the counterpart supports. In addition, CeO₂ doping by La³⁺ leads to an improved support, since La³⁺ stabilises the structure of CeO₂ when calcined at high temperature (1000 °C) and minimises sintering. In addition, La³⁺ improves the Ce⁴⁺/Ce³⁺ reduction as deduced from H₂-TPR experiments and favours oxygen mobility into the lattice. A synergetic effect of Pt and La³⁺ is observed, Pt-containing La³⁺-doped CeO₂ being the most active catalyst for soot oxidation by O₂ among the samples studied.     

Preparation and characterization of inexpensive heterogeneous catalysts for air pollution control: Two case studies

Relatively inexpensive heterogeneous catalysts for two reactions of great importance in air pollution control, NO reduction and VOC combustion, were prepared and characterized. Apart from their common practical goal and the frequent need for simultaneous removal of air pollutants, these reactions share a similar redox mechanism, in which the formulation of more effective catalysts requires an enhancement of oxygen transfer.

For NO reduction, supported catalysts were prepared by adding a metal (Cu, Co, K) using ion exchange (IE) and incipient wetness impregnation (IWI) to chars obtained from pyrolysis of a subbituminous coal. The effects of pyrolysis temperature, between 550 and 1000 °C, on selected catalyst characteristics (e.g., BET surface area, XRD spectrum, support reactivity in O₂) are reported. For IE catalysts, the surface area increased in the presence of the metals while the opposite occurred for IWI catalysts. For the Co-IE catalysts, the highest surface area was obtained at 700 °C. The XRD results showed that, except for Cu (which exhibited sharp Cu⁰ peaks), the catalysts may be highly dispersed (or amorphous) on the carbon surface. For the C–O₂ reaction the order of (re)activity was K  Co > Cu for IE catalysts and K > Cu > Co for IWI catalysts. For NO reduction the orders were K > Co > Cu (IE catalysts) and Cu > Co > K (IWI catalysts). In all cases the catalytic (re)activity for NO reduction was lower than that exhibited for the C–O₂ reaction. The K-IE and Cu-IWI catalysts appeared to be the most promising ones, although further improvements in catalytic activity will be desirable. Some surprising results regarding CO and CO₂ selectivity are also reported, especially for Co catalysts.

In VOC combustion, the effect of the nature of ion B (Fe and Ni) on the partial substitution of ion A (Ca for La) in ABO₃ perovskites (e.g., LaFeO₃ and LaNiO₃) and on their catalytic activity was studied. The perovskite-type oxides were characterized by means of surface area measurements, XRD, temperature-programmed desorption (TPD) and temperature-programmed reduction (TPR). The effect of partial substitution of La³⁺ by Ca²⁺ was more significant for the La_1−xCa_xFeO₃ perovskites. In this case, the electronic perturbation is compensated by an oxidation state increase of part of Fe³⁺ to Fe⁴⁺. The TPD results revealed that, at higher substitution levels, oxygen vacancies are also formed to preserve electroneutrality. For the La_1−xCa_xNiO₃ perovskites, the characterization results showed no evidence of large differences in electronic properties as calcium substitution increases. The La_1−xCa_xNiO₃ perovskites exhibited lower activity than the simple LaNiO₃ perovskite, whereas for the La_1−xCa_xFeO₃ substituted perovskites the most active catalyst (exhibiting the lowest ignition temperature) was obtained at the highest substitution level, La_0.6Ca_0.4FeO₃.

The performance of both groups of catalysts is briefly discussed in terms of redox processes, in which the interplay between oxygen transfer and electron transfer requires further elucidation for the improvement of catalytic activity.     

SrFe0.6Co0.4O3催化降解孔雀石绿的研究

具有复杂分子结构的三苯甲烷类染料孔雀石绿是一种典型的较难降解染料,是工业废水处理的难点之一。本文根据Goldschmidt半径容差规则法,设计了用于孔雀石绿降解的ABO₃型SrFe_(1-x)Co_xO₃催化剂,并选择出活性较高的SrFe_0.6Co_0.4O₃催化剂。通过XRD、SEM、BET吸附及XPS分析表明:该催化剂是纯净钙钛矿结构,颗粒形貌为无规则堆叠的“蜂窝”片状;吸附等温线没有明显的回滞环,说明没有“墨水瓶”类孔结构;XPS谱中,B位离子同时存在Fe²⁺/Fe³⁺和Co²⁺/Co³⁺ 4种价态离子,且反应前后,4种离子的分布比例有较大变化。根据实验结果,推测该催化反应机理为:催化剂B位Co³⁺与溶解氧形成活性氧[O₂]⁺和Co²⁺;活性氧[O₂]⁺完成氧化反应后其正电荷转移到B位Fe²⁺上形成Fe³⁺,Fe³⁺的正电荷可再转移到Co²⁺形成Co³⁺,完成催化过程的电荷转移与循环。     

Catalytic behavior of the Fe/Fe system in the electro-Fenton degradation of the antimicrobial chlorophene

The catalytic behavior of the Fe³⁺/Fe²⁺ system in the electro-Fenton degradation of the antimicrobial drug chlorophene has been studied considering four undivided electrolytic cells, where a Pt or boron-doped diamond (BDD) anode and a carbon felt or O₂-diffusion cathode have been used. Chlorophene electrolyses have been carried out at pH 3.0 under current control, with 0.05 M Na₂SO₄ as supporting electrolyte and Fe³⁺ as catalyst. In these processes the drug is oxidized with hydroxyl radical (OH) formed both at the anode from water oxidation and in the medium from electrochemically generated Fenton's reagent (Fe²⁺ + H₂O₂, both of them generated at the cathode). The catalytic behavior of the Fe³⁺/Fe²⁺ system mainly depends on the cathode tested. In the cells with an O₂-diffusion cathode, H₂O₂ is largely accumulated and the Fe³⁺ content remains practically unchanged. Under these conditions, the chlorophene decay is enhanced by increasing the initial Fe³⁺ concentration, because this leads to a higher quantity of Fe²⁺ regenerated at the cathode and, subsequently, to a greater OH production from Fenton's reaction. In contrast, when the carbon felt cathode is used, H₂O₂ is electrogenerated in small extent, whereas Fe²⁺ is largely accumulated because the regeneration of this ion from Fe³⁺ reduction at the cathode is much faster than its oxidation to Fe³⁺ at the anode. In this case, an Fe³⁺ concentration as low as 0.2 mM is required to obtain the maximum OH generation rate, yielding the quickest chlorophene removal. Chlorophene is poorly mineralized in the Pt/O₂ diffusion cell because the final Fe³⁺–oxalate complexes are difficult to oxidize with OH. These complexes are completely destroyed using a BDD anode at high current thanks to the great amount of OH generated on its surface. Total mineralization is also achieved in the Pt/carbon felt and BDD/carbon felt cells with 0.2 mM Fe³⁺, because oxalic acid and its Fe²⁺ complexes are directly oxidized with OH in the medium. Comparing the four cells, the highest oxidizing power regarding total mineralization is attained for the BDD/carbon felt cell at high current due to the simultaneous destruction of oxalic acid at the BDD surface and in the bulk solution.     

Fe2+/H2O2体系内各种自由基在氧化NO中的作用

Fe²⁺/H₂O₂体系可分解产生多种氧化性自由基, 主要包括O₂^-·、·OH和HO₂·。本文实验研究了O₂^-·、·OH及HO₂·在Fe²⁺/H₂O₂体系氧化NO气体过程中的作用。结果表明:在本实验条件下, O₂^-·对NO气体的氧化作用不明显;·OH及HO₂·是该体系氧化NO气体的主要活性物质, 其中·OH的氧化作用更大。加快自由基的生成速率可以增强Fe²⁺/H₂O₂体系对NO气体的氧化能力, 但O₂的生成速率同时加快。只有少量·OH及HO₂·参与NO的氧化, ·OH与HO₂·之间的快速反应是Fe²⁺/H₂O₂体系氧化NO过程中H₂O₂利用率低的主要原因。     

Comparative evaluation of Fe and TiO2 photoassisted processes in solar photocatalytic disinfection of water

A lost of culturability of bacteria Escherichia coli K12 was observed after exposition to a solar simulator (UV–vis) in a laboratory batch photoreactor. The bacterial inactivation reactions have been carried out using titanium dioxide (TiO₂) P25 Degussa and FeCl₃ as catalysts. At the starting of the treatment, the suspensions were at their “natural” pH. An increase in the efficiency in the water disinfection was obtained when some advanced oxidation processes such as UV–vis/TiO₂, UV–vis/TiO₂/H₂O₂, UV–vis/Fe³⁺/H₂O₂, UV–vis/H₂O₂ were applied. The presence of H₂O₂ accelerates the rate of disinfection via TiO₂. The addition of Fe³⁺ (0.3 mg/l) to photocatalytic system decreases the time required for total disinfection (<1 CFU/ml), for TiO₂ concentrations ranging between 0.05 and 0.5 g/l. At TiO₂ concentrations higher than 0.5 g/l the addition of Fe³⁺ does not significantly increase the disinfection rate. The systems: Fenton (H₂O₂/Fe³⁺/dark), H₂O₂/dark, H₂O₂/TiO₂/dark showed low disinfection rate. The effective disinfection time (EDT₂₄) was reached after 60 and 30 min of illumination for the Fe³⁺ and TiO₂ photoassisted systems, respectively. EDT₂₄ was not reached for the system in the absence of catalyst (UV–vis). The effect on the bacterial inactivation of different mixture of chemical substance added to natural water was studied.     

Catalytic wet oxidation of H2S to sulfur on Fe/MgO catalyst

The room temperature wet catalytic oxidation was conducted in a batch reactor with Fe/MgO catalyst. Fe/MgO catalyst was prepared by the dissolution–precipitation method. XRD and temperature-programmed reductions (TPR) indicate that Fe oxide in the Fe/MgO is finely dispersed in the MgO support. The high H₂S removal capacities of Fe/MgO can be explained by the finely dispersed iron oxide MgO. The H₂S removal capacities of Fe/MgO are dependent on oxygen partial pressure (1.0 g H₂S/g_cat in air and 2.6 g H₂S/g_cat in oxygen). The valence state analysis of Fe/MgO catalyst suggests that the H₂S oxidation on Fe/MgO can occur by a redox couple reaction, reducing Fe³⁺ into Fe²⁺ by H₂S and oxidizing Fe²⁺ to Fe³⁺ by O₂.     

The kinetics of phenol decomposition under UV irradiation with and without H2O2 on TiO2, Fe–TiO2 and Fe–C–TiO2 photocatalysts

H₂O₂ used in the photo-Fenton reaction with iron catalyst can accelerate the oxidation of Fe²⁺ to Fe³⁺ under UV irradiation and in the dark (in the so called dark Fenton process). It was proved that conversion of phenol under UV irradiation in the presence of H₂O₂ predominantly produces highly hydrophilic products and catechol, which can accelerate the rate of phenol decomposition. However, while H₂O₂ under UV irradiation could decompose phenol to highly hydrophilic products and dihydroxybenzenes in a very short time, complete mineralization proceeded rather slowly. When H₂O₂ is used for phenol decomposition in the presence of TiO₂ and Fe–TiO₂, decrease of OH radicals formed on the surface of TiO₂ and Fe–TiO₂ has been observed and photodecomposition of phenol is slowed down. In case of phenol decomposition under UV irradiation on Fe–C–TiO₂ photocatalyst in the presence of H₂O₂, marked acceleration of the decomposition rate is observed due to the photo-Fenton reactions: Fe²⁺ is likely oxidized to Fe³⁺, which is then efficiently recycled to Fe²⁺ by the intermediate products formed during phenol decomposition, such as hydroquinone (HQ) and catechol.     

Characterization and catalytic properties of perovskites with nominal compositions La1-xSrxAl1-2yCuyRuyO3

Nine different metal oxide catalysts were prepared by impregnating alumina washcoats with water solutions containing La³⁺, Sr²⁺, Cu²⁺ and Ru³⁺ ions and calcining them at 900°C. The produced samples were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) studies combined with energy-dispersive spectroscopy (EDS) analysis, X-ray powder diffraction and specific surface area measurements. A perovskite phase of the nominal composition La_1-xSr_xAl_1-2yCu_yRu_yO₃ was found in all samples, in increasing amount in the samples with increasing contents of strontium and ruthenium. The catalysts were evaluated with respect to light-off temperatures and redox characteristics using two gas mixtures, one containing NO/CO/C₃H₆/O₂/N₂ and the other NO/CO/N₂. The light-off temperatures for nitric oxide reduction decreased from 534 to 333°C for the catalysts without and with strontium and ruthenium, respectively. In the presence of oxygen the conversion of nitric oxide declined rapidly under oxidative conditions whereas in absence of oxygen this decline was less pronounced and found to be linear over the entire redox interval studied. These studies suggest that the perovskite phase takes an active part in the conversion of nitric oxide and carbon monoxide to nitrogen and carbon dioxide.     

Characterization and catalytic investigation of Fe-ZSM5 for urea-SCR

Fe-ZSM5 was prepared with high iron content by solid-state ion exchange and characterized by ICP-AES, BET surface measurements, TEM, UV–vis, EPR and DRIFT spectroscopy as well as supplementing catalytic tests in order to clear up its functionality in urea-SCR. Due to the over-exchange with iron small Fe₂O₃ particles were formed, identified by UV–vis, EPR and TEM measurements, which were proved to be not active for the SCR reaction. However, the oxidation of NO to NO₂ over Fe³⁺ ions in the catalyst was realized to be a pre-requisite for the SCR reaction and the rate-determining step. DRIFT investigations under SCR conditions showed adsorbates on Fe²⁺ up to 300 °C. The high SCR activity above 300 °C can be explained by the faster reoxidation of Fe²⁺ to Fe³⁺ sites at high temperatures. The observed inhibition of the SCR reaction by excess ammonia at low and intermediate temperatures can be explained in this context by the reducing properties of ammonia converting Fe³⁺ to Fe²⁺ or by preventing the reoxidation of Fe²⁺.     

Comparison of microwave and radiofrequency low-temperature ashing of coal using Fe Mössbauer spectroscopy of sulphur forms

Low-temperature ashing of coal was carried out by the normal radiofrequency method and a new microwave technique to compare the effects of the two procedures on the Fe-S compounds in coal. Results from ⁵⁷Fe Mössbauer spectroscopy and chemical analyses indicate that the microwave technique, like the normal LTA procedure, effectively ashes the coal without the oxidation of pyritic sulphur. Dehydration of iron sulphates and oxidation of Fe²⁺ to Fe³⁺ in sulphates occurs in both ashing procedures under the conditions tested, the sequence being FeS0₄.4H₂O → FeS0₄.H₂0→Fe³⁺. Dehydration is caused by sample heating rather than by reaction with the activated oxygen. The combination of the Mössbauer method and chemical analysis has allowed the determination of the recoilless fraction ratio f(sulphate)/f(pyrite) = 0.55 ±0.15, which puts the Mössbauer results on a more quantitative basis than before. This is thought to be the first quantitative work on coal and coal products that has accounted for differences in f-values for different Fe compounds.     

基于氮掺杂碳量子点的荧光微球制备和Fe3+检测

分别以尿素、氨水、二乙烯三胺、多乙烯多胺为氮源,绿色廉价的白菜为碳源,采用水热法合成氮掺杂的蓝色荧光碳量子点,结果表明多乙烯多胺氮掺杂碳量子点（NCDs）荧光量子产率最高为53.3%。然后将NCDs作为荧光探针应用于荧光微球制备和Fe³⁺检测方面,以三聚氰胺甲醛（MF）为载体,合成了氨基化MF荧光微球;基于Fe³⁺对NCDs良好的荧光猝灭效应,建立了一种荧光测定Fe³⁺的方法,并对NCDs和MF荧光微球的结构和性能进行表征。结果表明,NCDs的荧光性能得到了显著的改善;MF荧光微球单分散性好、荧光性能好且稳定,在生物医学领域方面有重要的应用价值;NCDs对Fe³⁺具有单一选择性,Fe³⁺浓度在0~2μmol/L内与NCDs的荧光猝灭程度呈良好的线性关系（R²=0.9945）,检出限为0.035μmol/L。将该体系应用于实际水样中Fe³⁺的测定,相对标准偏差（RSD,n=6）在1.42%~3.02%内,加标回收率在98.7%~104.5%之间。该体系对Fe³⁺检测灵敏性好、选择性高以及抗干扰性强,在离子分析检测方面有潜在的应用前景。