Progress on Noble Metal-Based Catalysts Dedicated to the Selective Catalytic Ammonia Oxidation into Nitrogen and Water Vapor (NH3-SCO)

A recent development for selective ammonia oxidation into nitrogen and water vapor (NH₃-SCO) over noble metal-based catalysts is covered in the mini-review. As ammonia (NH₃) can harm human health and the environment, it led to stringent regulations by environmental agencies around the world. With the enforcement of the Euro VI emission standards, in which a limitation for NH₃ emissions is proposed, NH₃ emissions are becoming more and more of a concern. Noble metal-based catalysts (i.e., in the metallic form, noble metals supported on metal oxides or ion-exchanged zeolites, etc.) were rapidly found to possess high catalytic activity for NH₃ oxidation at low temperatures. Thus, a comprehensive discussion of property-activity correlations of the noble-based catalysts, including Pt-, Pd-, Ag- and Au-, Ru-based catalysts is given. Furthermore, due to the relatively narrow operating temperature window of full NH₃ conversion, high selectivity to N₂O and NO_x as well as high costs of noble metal-based catalysts, recent developments are aimed at combining the advantages of noble metals and transition metals. Thus, also a brief overview is provided about the design of the bifunctional catalysts (i.e., as dual-layer catalysts, mixed form (mechanical mixture), hybrid catalysts having dual-layer and mixed catalysts, core-shell structure, etc.). Finally, the general conclusions together with a discussion of promising research directions are provided.     

Adsorption separation of carbon dioxide,methane, and nitrogen on Hβ and Na-exchanged β-zeolite

Adsorption isotherms of carbon dioxide （CO2）, methane （CH4）, and nitrogen （N2） on Hβand sodium exchanged β-zeolite （Naβ） were volumetrically measured at 273 and 303 K. The results show that all isotherms were of Brunauer type I and well correlated with Langmuir-Freundlich model. After sodium ions exchange, the adsorption amounts of three adsorbates increased, while the increase magnitude of CO2 adsorption capacity was much higher than that of CH4 and N2. The selectivities of CO2 over CH4 and CO2 over N2 enhanced after sodium exchange. Also, the initial heat of adsorption data implied a stronger interaction of CO2 molecules with Na＋ ions in Naβ . These results can be attributed to the larger electrostatic interaction of CO2 with extraframework cations in zeolites. However, Naβ showed a decrease in the selectivity of CH4 over N2, which can be ascribed to the moderate affinity of N2 with Naβ. The variation of isosteric heats of adsorption as a function of loading indicates that the adsorption of CO2 in Naβ presents an energetically heterogeneous profile. On the contrary, the adsorption of CH4 was found to be essentially homogeneous, which suggests the dispersion interaction between CH4 and lattice oxygen atoms, and such interaction does not depend on the exchangeable cations of zeolite.     

Studies on Water and Ammonia Programmed Thermodesorption of Mixed M(III)-vanadyl Phosphates

Hydrated M(III)-vanadyl phosphates (M (III)=Mn, Fe, Ga, Al) have been prepared and studied for water and ammonia adsorption properties by TG/DTA, NH₃ TPD, FTIR and XRD techniques. The compounds have the same tetragonal layered structure of VOPO₄ ⋅2H₂ O, but shorter interlayer distances. Ammonia adsorption leads to intercalation of large amounts (0.19–0.39 mol/mol) of base between the layers of the materials, without displacement of water. The ammoniated phases obtained from these compounds have interlayer distances shorter than that of the corresponding precursors. In this connection an interaction mechanism NH₃ -host is proposed. Treated at 450°C the materials adsorb ammonia only on the external surface because of the large decrease of the interlayer distance that prevents NH₃ from entering the interlayer space. All M(III)-vanadyl phosphates present a wide distribution of strength of ammonia adsorbing sites. This revised version was published online in July 2006 with corrections to the Cover Date.     

Thermally Induced Phase Transformation on 12-Tungstophosphoric Acid/ZrO2 Sol-Gel

Zirconium (IV)-n-butoxide and tungstophosphoric acid (WP) were co-gelled at pH 3, 5 and 7 with HCl acid, C₂H₄O₂ acid and NH₄OH, respectively. Pyridine adsorption bands at 1610 and 1442 cm^–1 corresponding to Lewis acidic sites were observed by FTIR spectroscopy. Acidity determined by ammonia thermodesorption shows values around 1100 mol of NH₃/g, which correspond to solids showing super acidity. It was found that the incorporation of WP to gelling zirconia delay the formation of tetragonal zirconia. Raman spectroscopy shows the stabilization of the Keggin structure on zirconia thermally treated at 400°C.     

酸性金属氧化物对锰铈氧化物催化剂脱硝温度窗口的调控作用

利用溶胶-凝胶法,采用三种酸性金属氧化物（氧化铌、氧化钨和氧化钼）对锰铈复合氧化物催化剂进行了改性. 测试了催化剂的氮氧化物选择性催化还原（SCR）活性,以筛选对应不同温度窗口的合适酸性氧化物改性剂. 同时评价了催化剂的NO氧化和NH₃氧化活性. 利用X射线衍射、BET比表面积测试、H₂程序升温还原、NH₃/NO_x程序升温脱附和NH₃/NO_x吸附红外光谱等手段对催化剂进行了表征. MnO_x-CeO₂催化剂表现出良好的低温（100-150 ℃）活性. 酸性金属氧化物的添加削弱了催化剂的氧化还原特性,从而抑制了NH₃的活化和NO₂辅助的快速SCR反应. 与此同时,相对高温（250-350 ℃）区NH₃的氧化也受到了抑制,B酸和L酸上的NH₃吸附得以增强. 因此,催化剂的SCR脱硝温度窗口向高温移动,改性效果Nb₂O₅ < WO₃ < MoO₃.     

Microcalorimetric Studies of the Surface Reactivity of Pollutant Gas Atmospheres on Supported SnO2 and Ga2O3 Catalysts

The surface properties of gallium oxide and tin dioxide supported on alumina or titania have been studied by adsorption microcalorimetry. The differential heats of adsorption of various pollutant adsorbates such as sulfur dioxide, nitrogen monoxide, nitrogen dioxide and also ammonia were measured on these catalytic surfaces. NH₃, SO₂, NO₂ are strongly adsorbed while NO is only physisorbed. The supported Ga₂O₃ samples show a slight decrease in acidity as probed by ammonia adsorption, compared to alumina or titania. The addition of SnO₂ decreases the number of strong acid sites but creates a few weak and medium strength acid sites on alumina and does not modify the acidity of titania. In all cases, the basicity, probed by SO₂ adsorption, is very strongly affected by the deposition of Ga₂O₃ or SnO₂. The differential heats of NO₂ adsorption remain nearly constant on all samples. The heats of adsorption are discussed as a function of the coverage and of the amount of guest oxide. This revised version was published online in August 2006 with corrections to the Cover Date.     

Ftir spectroscopic study of NH3 and no adsorption on alumina-supported manganese oxide catalyst

Adsorption and coadsorption of NH₃ and NO on alumina-supported manganese oxide catalyst prepared from manganese formate precursor has been studied by means of FTIR spectroscopy. Dedicated to the 65th birthday of Prof. Dr. H.D. Lutz (University of Siegen, Germany)     

Catalytic Performance of Spherical MCM-41 Modified with Copper and Iron as Catalysts of NH3-SCR Process

Spherical MCM-41 with various copper and iron loadings was prepared by surfactant directed co-condensation method. The obtained samples were characterized with respect to their structure (X-ray diffraction, XRD), texture (N₂ sorption), morphology (scanning electron microscopy, SEM), chemical composition (inductively coupled plasma optical emission spectrometry, ICP-OES), surface acidity (temperature programmed desorption of ammonia, NH₃-TPD), form, and aggregation of iron and copper species (diffuse reflectance UV-Vis spectroscopy, UV-Vis DRS) as well as their reducibility (temperature programmed reduction with hydrogen, H₂-TPR). The spherical MCM-41 samples modified with transition metals were tested as catalysts of selective catalytic reduction of NO with ammonia (NH₃-SCR). Copper containing catalysts presented high catalytic activity at low-temperature NH₃-SCR with a very high selectivity to nitrogen, which is desired reaction products. Similar results were obtained for iron containing catalysts, however in this case the loadings and forms of iron incorporated into silica samples very strongly influenced catalytic performance of the studied samples. The efficiency of the NH₃-SCR process at higher temperatures was significantly limited by the side reaction of direct ammonia oxidation. The reactivity of ammonia molecules chemisorbed on the catalysts surface in NO reduction (NH₃-SCR) and their selective oxidation (NH₃-SCO) was verified by temperature-programmed surface reactions.     

FTIR spectroscopy of NH3 and NO adsorption on copper-on-alumina catalysts

Adsorption and coadsorption of NH₃ and NO on copper-on-alumina catalysts prepared from formate precursors by calcination at 670 and 870 K was studied by means of FTIR spectroscopy. It has been established that the sample prepared at lower temperature exhibits a higher activity with respect to NO reduction with ammonia. Dedicated to the 65^th birthday of Prof. Dr. Chr. Balarew, Bulgarian Academy of Sciences, Sofia, Bulgaria     

FTIR Spectroscopic Study of NH3 and NH3 Adsorption on Alumina-Supported Mixed Copper-Manganese Oxide Catalysts

Adsorption and coadsorption of NH₃ and NO on alumina-supported mixed copper-manganese oxide catalysts prepared from formate precursors by calcination at 670 K were studied by means of in situ FTIR spectroscopy. This revised version was published online in June 2006 with corrections to the Cover Date.     

Highly Dispersed Vanadia Anchored on Protonated g-C3N4 as an Efficient and Selective Catalyst for the Hydroxylation of Benzene into Phenol

The direct hydroxylation of benzene is a green and economical-efficient alternative to the existing cumene process for phenol production. However, the undesired phenol selectivity at high benzene conversion hinders its wide application. Here, we develop a one-pot synthesis of protonated g-C₃N₄ supporting vanadia catalysts (V-pg-C₃N₄) for the efficient and selective hydroxylation of benzene. Characterizations suggest that protonating g-C₃N₄ in diluted HCl can boost the generation of amino groups (NH/NH₂) without changing the bulk structure. The content of surface amino groups, which determines the dispersion of vanadia, can be easily regulated by the amount of HCl added in the preparation. Increasing the content of surface amino groups benefits the dispersion of vanadia, which eventually leads to improved H₂O₂ activation and benzene hydroxylation. The optimal catalyst, V-pg-C₃N₄-0.46, achieves 60% benzene conversion and 99.7% phenol selectivity at 60 ^oC with H₂O₂ as the oxidant.     

CuFe-SAPO-34分子筛的结构和催化性能研究

采用一步水热合成法,通过调变初始凝胶中的硅铁比,制备系列铜铁含量不同的CuFe-SAPO-34催化剂用于NH₃选择性催化还原NO_x反应（NH₃-SCR）,并采用ICP、XRD、SEM、BET、H₂-TPR等方法对其结构进行表征.结果表明,CuFe-SAPO-34催化剂具有典型的CHA结构,Cu和Fe均处于分子筛载体的离子交换位.当初始凝胶SiO₂/Fe₂O₃=10时,Cu_2.5Fe_3.1-SAPO-34催化剂具有最大的比表面积和孔容.掺杂适量的Fe,可提高活性物种Cu²⁺的比例及其氧化还原性能,显著降低Cu物种的聚集程度.NH₃-SCR反应结果表明,Cu_2.5Fe_3.1-SAPO-34催化剂具有最宽的反应温度窗口.与Cu-SAPO-34相比,Fe的掺杂显著提高了其高温段的催化活性和低温抗水能力,提高了Cu-CHA催化剂在实际应用中的稳定性.     

Viscosity of aqueous NH4Cl and tracer diffusion coefficients of ions,water and non-electrolytes in aqueous NH4Cl,KI, and MgCl2 at 25°C

Viscosities for aqueous NH₄Cl and tracer diffusion coefficients for²²Na⁺,³⁶Cl^–, HTO, and CH₃OH, acetone and dimethylformamide (all¹⁴C-labelled) in NH₄Cl supporting electrolyte are reported for 25°, together with tracer diffusion coefficients for²²Na⁺,³⁶Cl^–, and¹⁴CH₃OH in 1M KI, and for¹⁴CH₃OH in 1M MgCl₂. The diffusion coefficient of HTO in NH₄Cl solutions is slightly larger, for most of the concentration range investigated (0.05 to 4.5 M), than the value for pure water and is almost unaffected by the supporting electrolyte up to about 4M. Similar behavior is shown by CH₃OH, acetone and dimethylformamide in NH₄Cl solutions. Onsager limiting law behavior is approached by Cl^– at NH₄Cl concentrations in the 0.05–0.1M region but at much lower concentrations by Na⁺.     

NH3-SCR脱硝催化剂抗硫性能的研究进展

氮氧化物(NO_x)是一种重要的大气污染物, 它造成严重的环境问题, 同时威胁人类健康. 以钢铁烧结烟气为代表的固定源和以柴油机尾气为代表的移动源是氮氧化物的主要来源. 氨气选择性催化还原法(NH₃-SCR)是目前最有效且应用最广泛的NO_x脱除技术. 然而, 无论是固定源还是移动源上NH₃-SCR催化剂, 都不可避免地会被SO₂毒化, 造成催化剂失活, 限制了NH₃-SCR技术的进一步应用. 因此, 研究NH₃-SCR催化剂的SO₂中毒机制以及提高催化剂的抗硫性能至关重要. 我们对固定源脱硝的金属氧化物和移动源上脱硝的Cu基分子筛这两类不同催化剂体系的SO₂中毒机制的研究进展进行了介绍, 并对这两种催化剂上提高抗硫性能改性方法的研究进展进行了评述, 为未来的研究提供了参考.     

乙二胺改性轻金属铝-金属有机骨架材料用于CO2/CH4分离

胺类分子在CO₂的捕获中可以起到选择性提升的作用,本文选择小尺寸的乙二胺分子对具有不饱和金属位点的轻金属铝基金属有机骨架（Al-MOFs）材料MIL-100Al进行改性,利用XRD、N₂吸附和FT-IR等对改性材料的结构进行表征,测试了不同浓度的乙二胺改性的MIL-100Al对CO₂和CH₄吸附性能。结果表明,与原始的MIL-100Al材料相比,改性后的材料对CO₂吸附量有明显提高,CH₄的吸附量却降低,从而进一步提高了材料的CO₂/CH₄吸附选择性,提升了吸附分离的效果。     

乙二胺改性轻金属铝-金属有机骨架材料用于CO_2/CH_4分离

胺类分子在CO_2的捕获中可以起到选择性提升的作用,本文选择小尺寸的乙二胺分子对具有不饱和金属位点的轻金属铝基金属有机骨架(Al-MOFs)材料MIL-100Al进行改性,利用XRD、N2吸附和FT-IR等对改性材料的结构进行表征,测试了不同浓度的乙二胺改性的MIL-100Al对CO_2和CH4吸附性能。结果表明,与原始的MIL-100Al材料相比,改性后的材料对CO_2吸附量有明显提高,CH4的吸附量却降低,从而进一步提高了材料的CO_2/CH4吸附选择性,提升了吸附分离的效果。     

TEA CO2 laser-induced reaction of CH3NO2 with CF2HCl: A mechanistic study

Dissociation of nitromethane has been observed when a mixture of CF₂HCl and CH₃NO₂ is irradiated using pulsed TEA CO₂ laser at 9R (24) line (1081 cm^-1), which is strongly absorbed by CF₂HCl but not by CH₃NO₂. Under low laser fluence conditions, only nitromethane dissociates, whereas at high fluence CF₂HCl also undergoes dissociation, showing that dissociation occurs via the vibrational energy transfer processes from the TEA CO₂ laser-excited CF₂HCl to CH₃NO₂. Time-resolved infrared fluorescence from vibrationally excited CF₂HCl and CH₃NO₂ molecules as well as UV absorption of CF₂ radicals are carried out to elucidate the dynamics of excitation/dissociation and the chemical reactions of the dissociation products.     

IR and Raman spectra of two layered aluminium phosphates Co(en)3Al3P4O16 · 3H2O and [NH4]3[Co(NH3)6]3[Al2(PO4)4]2 · 2H2O

The FT IR and FT Raman spectra of Co(en)₃Al₃P₄O₁₆ · 3H₂O (compound I) and [NH₄]₃[Co(NH₃)₆]₃[Al₂(PO₄)₄]₂ · 2H₂O (compound II) are recorded and analysed based on the vibrations of Co(en)₃³⁺, Co(NH₃)₆³⁺, NH₄⁺, Al---O---P, PO₃, PO₂ and H₂O. The observed splitting of bands indicate that the site symmetry and correlation field effects are appreciable in both the compounds. In compound I, the overtone of CH₂ deformation Fermi resonates with its symmetric stretching vibration. The NH₄ ion in compound II is not free to rotate in the crystalline lattice. Hydrogen bonding of different groups is also discussed.     

Influence of Pd precursors on the catalytic performance of Pd–H4SiW12O40/SiO2 in the direct oxidation of ethylene to acetic acid

The effects of palladium precursors (PdCl₂, (NH₄)₂PdCl₄, Pd(NH₃)₂Cl₂, Pd(NO₃)₂ and Pd(CH₃COO)₂) on the catalytic properties in the selective oxidation of ethylene to acetic acid have been investigated for 1.0 wt% Pd–30 wt% H₄SiW₁₂O₄₀/SiO₂. The structures of the catalysts were characterized using X-ray diffraction, N₂ adsorption, H₂-pulse chemical adsorption, infrared spectrometry of the adsorbed pyridine, H₂ temperature-programmed reduction and X-ray photoelectron spectroscopy. The present study demonstrates that the different palladium precursors can lead to the significant changes in the dispersion of palladium. It is found that Pd dispersion decreases as follows: PdCl₂ > (NH₄)₂PdCl₄ > Pd(NO₃)₂ > Pd(NH₃)₂Cl₂ > Pd(C₂H₃O₂)₂, which is nearly identical to the catalytic activity. This indicates that the dispersion of palladium plays an important role in the catalytic activity. Furthermore, density of Lewis (L) and Brönsted (B) acid sites are also strongly dependent on the palladium precursors. It is also demonstrated that an effective catalyst should possess a well combination of Brönsted acid sites with dispersion of palladium.     

Correlation of the position of the bond functions in H-X with electronegativity

S type Gaussian bond functions are optimized for HF, H₂O, NH₃, and CH₄. The optimization is carried out with respect to the exponent and position in the H-X bond. The position is found to correlate well with the electronegativity of Pauling and Allred-Rochow.