Catalytic combustion of hexane over transition metal modified zeolites NaX and CaA

Hexane deep oxidation was studied over NaX and CaA zeolites modified by ion exchange with transition metals (Mn²⁺, Co²⁺, Fe³⁺), the percentage of ion exchanged, determined by ICP-MS, varying between 39 and 98%. Parent and exchanged zeolites were characterized by X-ray diffraction (XRD), N₂ physisorption, temperature-programmed reduction (TPR), oxygen and ammonia temperature-programmed desorption (TPD) and inverse gas chromatography (IGC). Catalytic activities were evaluated through the recording of light-off curves in a pulsed microreactor, catalytic activity being correlated with physicochemical properties of the solids (crystallinity, surface acidity, adsorption properties and morphological parameters). As general trend, CaA zeolites are more active than NaX zeolites. Mn-exchanged CaA zeolite was the most active catalyst for hexane oxidation, whereas the addition of Fe to the zeolites leads to strong chemical and morphological changes in the parent zeolite.     

Catalytic oxidation of volatile organic compounds (VOCs) mixture (isopropanol/o-xylene) on zeolite catalysts

The catalytic oxidation of isopropanol and o-xylene alone and in mixture was investigated over basic zeolites (CsX and NaX) and an acidic zeolite (HY). For a given temperature, the conversion of VOCs mixture into CO₂ increases with the basicity of the zeolite. Results show that VOCs are oxidized through a basic mechanism over NaX, and through an acidic mechanism over HY. Over HY, the presence of isopropanol increases the oxidation of o-xylene, probably because of the formation of isopropyldimethylbenzenes obtained via an acidic mechanism. The addition of platinum over zeolites increases the rate of the VOCs oxidation, this behavior being more pronounced over PtHY maybe because of a higher Pt dispersion.     

Compositional Dependence of Absorption Spectra of Ti3+ in Silicate, Borate, and Phosphate Glasses

Absorption spectra of Ti³⁺ were measured for silicate, borate, and phosphate glasses doped with 0.5 mol% Ti₂O₃.The absorption coefficient at the peak wavelength of the ₂T₂→₂E transition of Ti³⁺ is used as a parameter showing the relative content of Ti³⁺ions in glass samples. The effect of glass composition on Ti³⁺/Ti⁴⁺ redox was studied. For multicomponent glasses, a basicity parameter calculated from glass composition is proposed in terms of coulomb force between the cation and the oxygen ion. The value of the absorption coefficient depends on basicity in silicate and borate glasses; however, it is independent of composition in phosphate glasses.     

Defective structure, oxygen mobility, oxygen storage capacity, and redox properties of RE-based (RE = Ce, Pr) solid solutions

Defective structures, surface textures, oxygen mobility, oxygen storage capacity (OSC), and redox properties of RE_0.6Zr_0.4O₂ and of RE_0.6Zr_0.4−xY_xO₂ (RE=Ce, Pr; x=0, 0.05) solid solutions have been investigated using X-ray diffraction (XRD), temperature-programmed desorption (TPD), temperature-programmed reduction (TPR), O₂−H₂ and O₂−CO titration, ¹⁸O/¹⁶O isotope exchange, CO pulsing reaction, and X-ray photoelectron spectroscopy (XPS) techniques. The effects of doping noble metal onto RE_0.6Zr_0.4−xY_xO₂ on oxygen mobility and surface oxygen activities have also been studied. Based on the experimental outcomes, we conclude that: (i) a Pr-based solid solution has better redox behavior than a Ce-based one; (ii) incorporation of yttrium ions in the lattices of CZ and PZ solid solutions could result in an enhancement in oxygen vacancy concentration, Ce⁴⁺/Ce³⁺ and Pr⁴⁺/Pr³⁺ redox properties, lattice oxygen mobility, and oxygen storage capacity; and (iii) doping the noble metal (Rh, Pt, and Pd) onto RE-based solid solution has positive effect on the properties concerned in this work.     

Potential rare-earth modified CeO2 catalysts for soot oxidation part II: Characterisation and catalytic activity with NO + O2

Ceria (CeO₂) and rare-earth modified ceria (CeReO_x with Re = La³⁺, Pr^3+/4+, Sm³⁺, Y³⁺) supports and Pt impregnated supports are studied for the soot oxidation under a loose contact with the catalyst with the feed gas, containing NO + O₂. The catalysts are characterised by XRD, H₂-TPR, DRIFT and Raman spectroscopy. Among the single component oxides, CeO₂ is significantly more active compared with the other lanthanide oxides used in this study. Doping CeO₂ with Pr^3+/4+ and La³⁺ improved, however, the soot oxidation activity of the resulting solid solutions. This improvement is correlated with the surface area in the case of CeLaO_x and to the surface area and redox properties of CePrO_x catalyst. The NO conversion to NO₂ over these catalysts is responsible for the soot oxidation activity. If the activity per unit surface area is compared CePrO_x is the most active one. This indicates that though La³⁺ can stabilise the surface area of the catalyst in fact it decreases the soot oxidation activity of Ce⁴⁺. The lattice oxygen participates in NO conversion to NO₂ and the rate of this lattice oxygen transfer is much faster on CePrO_x. In general, the improvement of the soot oxidation is observed over the Pt impregnated CeO₂ and CeReO_x catalysts, and can be correlated to the presence of Pt°. The surface reduction of the supports in the presence of Pt occurred below 100 °C. The surface redox properties of the support in the Pt catalysts do not have a significant role in the NO to NO₂ conversion. In spite of the lower surface area, the Pt/CeYO_x and Pt/CeO₂ catalysts are found to be more active due to larger Pt crystal sizes. The presence of Pt also improved the CO conversion to CO₂ over these catalysts. The activation energy for the soot oxidation with NO + O₂ is found to be around 50 kJ/mol.     

Vanadium antimonate as a partial oxidation catalyst

The catalytic selective oxidation on vanadium antimonate oxides partially doped with titanium was studied. Solids with nominal composition VSbO₄, V_0.8SbTi_0.2O₄ and VSb_0.8Ti_0.2O₄ were prepared by solid-state reaction. The solids were characterised by sorptometry, X-ray diffraction (XRD), temperature programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS). Catalytic tests were carried out in a fixed-bed reactor using toluene and air as reactants. It was found that partial substitution of V or Sb by Ti produces changes in the unit-cell composition and the redox properties of the solids. A possible role for V and Sb in the reaction mechanism of partial oxidation was established. Vanadium sites (V⁴⁺) are involved in the incorporation of gaseous oxygen as lattice oxygen and hold the Sb sites in an oxidation state (Sb⁵⁺) high enough to insert the lattice oxygen in the hydrocarbon molecule. A relationship between activity and V⁴⁺/Sb⁵⁺ ratio of the unit-cell was found. The selectivity to benzaldehyde is correlated with the redox properties of the solids while the selectivity to total oxidation products could be interpreted by the site isolation theory.     

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.     

Mg改性L分子筛负载Pt重整催化剂的制备及其石脑油芳构化性能

为克服常规氧化铝型重整催化剂氯离子流失及对设备产生腐蚀等问题，通过离子交换法制备了Mg²⁺改性的L分子筛，采用浸渍法制备了不含氯离子的Pt/MgL重整催化剂，对分子筛载体进行了XRD、N₂吸附-脱附、NH₃-TPD和Py-IR等表征，并以硫含量0.50μg/mL工业精制石脑油为原料在固定床微反装置上评价了催化剂重整芳构化性能。结果表明，Mg²⁺离子交换对L分子筛的骨架结构没有破坏，Mg²⁺的存在提高了载体的酸量和酸强度，Mg²⁺改性的Pt/MgL催化剂重整芳构化性能明显提高，适当强酸性对L分子筛重整催化剂芳构化反应起到显著的促进作用。     

XAFS Investigation of Platinum Impurities in Phosphate Glasses

The coordination environments of Pt impurities in a ternary K-aluminophosphate (KAP) glass and commercial K,Mg-aluminophosphate (KMAP) laser glasses have been investigated by Pt L _III-edge X-ray absorption fine structure (XAFS) spectroscopy. Pt valence in the KAP glass depends on the melt preparation atmosphere. Pt⁴⁺ ions form in melts that are bubbled with oxygen, whereas metallic Pt particles form when these same samples are remelted in air. Residual chlorine in KMAP glasses has an effect on Pt bonding. In chlorine-free samples, Pt⁴⁺ ions are coordinated with ∼5.4 (8) oxygen atoms with an average distance of 2.02 (1) Å. For glasses with low chlorine contents (<200 ppm Cl), the Pt⁴⁺ ions have both O and Cl atoms in the first coordination shell. As the Cl concentration increases, the number of O nearest neighbors decreases and for Cl:Pt > 5, only Cl nearest neighbors are observed. Pt⁴⁺ ions in these latter glasses are coordinated by ∼5.5 (8) Cl atoms at an average distance of 2.27 (2) Å.     

Crystallization of JBW, CAN, SOD and ABW type zeolite from transformation of meta-kaolin

The crystallization of low silica zeolites, including JBW, CAN, SOD and Li-ABW was investigated on the hydrothermal transformation of China meta-kaolin in alkaline medium at 473 K, and characterized with XRD, SEM and ²⁹Si MAS NMR. The significant influence of the inorganic additives on the products formed was observed. The crystallization process of JBW zeolite with high crystallinity from the transformation of zeolite A, which formed in a short period of the initial reaction stage, was observed in the reactant of meta-kaolin in NaOH solution. CAN and SOD zeolite crystallized in the reactant containing K⁺ and Na⁺ in the presence of additional nitrate and halide, respectively. A phase transformation of SOD into CAN took place during the synthesis. Li-ABW formed in the reactant with LiOH medium.     

Surface and bulk properties of Cu–ZSM-5 and Cu/Al2O3 solids during redox treatments. Correlation with the selective reduction of nitric oxide by hydrocarbons

This paper deals with the redox properties of Cu ions implanted in ZSM-5 and supported on Al₂O₃, catalysts active in the selective reduction of NO by hydrocarbons such as propane. Data on the reducibility of the Cu systems in various atmospheres (vacuum, CO, H₂, O₂) and on their DeNO_x activity are presented. The methods used to obtain informations on the surface and bulk transformations (and their link with catalytic behaviour) are complementary: UV–visible diffuse reflectance spectroscopy being useful to detect the presence of Cu²⁺ and Cu⁰, while Cu⁺ is detected indirectly by the analysis of the IR spectrum of CO bound selectively to this cation.

The main contributions to the previous knowledge are the following: it is possible to distinguish CO bound to isolated and non-isolated Cu⁺ ions; the isolated Cu²⁺ ions are reducible under vacuum without participation of organic impurities; the more active solids for the NO reduction into N₂ are characterized by the presence of isolated Cuⁿ⁺ ions beside the additional influence of the zeolitic framework; after the formation of Cu⁺ ions the redox cycles are reversible but, after the formation of Cu⁰, the reversibility or irreversibility of the redox cycles and the restoration of the SCR activity are function of the copper content; the activity decreases after agglomeration into bulk oxides; there is no formation of bulk CuO during the reaction and, with reducing and moderate oxidizing mixtures, part of the copper remains as cuprous ions.     

One-Step Synthesis of Dimethyl Ether from Syngas with Fe-Modified Zeolite ZSM-5 as Dehydration Catalyst

A number of Fe-containing ZSM-5 zeolites, such as HFeZSM-5 and HFeAlZSM-5 prepared by hydrothermal synthesis and Fe-modified ZSM-5 through solid-state ion-exchange, were adopted as methanol dehydration catalysts for syngas to dimethyl ether (STD) process. Their structures, acidic and basic properties were characterized by XRD, ESR, ICP-AES, TPD and FT-IR. Among these Fe-containing zeolites, the Fe-modified ZSM-5 displayed the highest dimethyl ether selectivity, least CO₂ production. Some correlations between catalytic performance and acidity and basicity of Fe-containing ZSM-5 zeolite were discussed.     

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.     

Phase Stability of Yttria-Stabilized Zirconia with Dissolved Cerium and Neptunium Oxides Under Oxidizing and Reducing Atmospheres

Phase stability and effects of sintering atmosphere on the crystalline structure of Np-doped yttria-stabilized zirconia (YSZ) were evaluated in comparison with those same properties and conditions for Ce-doped YSZ. Different sintering atmospheres for Ce-doped YSZ led to differences in phase formation through reduction of the dopant from Ce⁴⁺ to Ce³⁺. On the other hand, YSZ specimens containing up to 40 mol% Np formed only a fluorite-structure phase regardless of sintering atmosphere. Yttria-stabilized zirconia thus seems to accommodate Np within a wide range of concentrations and to have excellent phase stability under both oxidizing and reducing atmospheres.     

Redox State of Iron and Its Related Effects in the CaO–P2O5–Fe2O3 Glasses

Iron exists in Fe²⁺ and Fe³⁺ states in CaO–P₂O₅–Fe₂O₃ glasses and they impart characteristic optical absorption bands that allow analysis of relative proportion of the two species. This communication reports the redox states of glasses melted under air, argon, and oxygen atmospheres and relates them to the dissolution rates. The dissolution rate was found to be related to the redox state and it is lowered if the glass is melted under oxidizing atmospheres.     

Y型分子筛的金属离子改性水热化学研究

采用密度泛函理论研究金属离子(La~(3+),Ce~(3+),Y~(3+),Ga~(3+),Cr~(3+),Zn~(2+),Cu~(2+))改性Y型分子筛的水热化学。结果表明,金属离子取代Y型分子筛骨架Al的能力较弱,以M(OH)(Z-1)+形式存在的金属离子能够进入分子筛β笼,且更倾向定位于β笼Ⅰ'位,增强分子筛Al和相邻骨架O原子之间的作用力,提高分子筛的结构稳定性。探讨制备金属离子改性Y型分子筛应考虑金属离子性能和金属离子与分子筛骨架元素的作用。     

Probing the basicity of zeolite frameworks with N2O4: A DFT approach

The basicity of alkali exchanged faujasite type zeolites is estimated on the basis of DFT calculations of the NO⁺ stretching frequencies of adsorbed N₂O₄. N₂O₄ is allowed to interact with the alkali cations and a range of molecules covering a wide basicity scale. An alkali nitrate salt is formed and NO⁺ interacts with the basic molecules. The resulting NO⁺ stretching frequencies decrease with increasing proton affinity and basicity of the basic molecules for each alkali cation. If experimental proton affinities and basicities of the model molecules are used, the data fall on two straight lines, which are almost parallel. The zeolitic basicities and proton affinities are obtained by interpolation of the experimental NO⁺ frequencies of alkali exchanged faujasites on the linear plots of NO⁺ frequencies against the experimental proton affinities and basicities. With theoretical proton affinities and basicities two non-parallel straight lines are obtained and three molecules are offset. The reason for this is unclear and needs more investigation. The NO⁺ stretching frequency is linearly correlated with the hardness of the alkali cations in zeolite X. For zeolite Y the same trend is observed, but the scatter of the data is too large to define it as linear.     

Deactivation of real three way catalysts by CePO4 formation

The contaminants present in a real three way catalysts (TWC) aged under real working conditions for ca. 30 000 km have been determined. The X-ray fluorescence (TXRF) and energy-dispersive X-ray analysis (EDS) showed that P, Ca, Zn, Pb, Cr, Ni, Fe, Cd and Cu are present in the used catalysts. Distribution within the washcoat was scrutinised by scanning electron microscopy (SEM-EDS) analysis. The more external region (10–15 μm) of the washcoat are very concentrated in Ca and P. CePO₄ was detected by X-ray diffraction (XRD). Other phases that can be formed between contaminants themselves and with the washcoat components were not detected by XRD. Temperature-programmed reduction (TPR) experiments show that an important fraction of the Ce present in the washcoat is unable to participate in Ce³⁺/Ce⁴⁺ redox pair under oxidising–reducing treatments. CePO₄ is proposed to explain the locking of the Ce³⁺/Ce⁴⁺ redox couple. Although other contaminants like Pb can be also poisoning the catalyst, the formation of CePO₄ can be invoked to explain the worse catalytic properties displayed by used sample. The locked Ce pair is unable of participating in the rapid Ce³⁺/Ce⁴⁺ redox couple required for correct oxygen storage capacity (OSC) properties and for the proper functioning of TWC.     

修饰的CuO/Fe2O3水煤气变换催化剂的性能测试与表征

ZrO₂是一种高熔点金属氧化物,同时具有弱酸性和弱碱性以及氧化性与还原性,具有p型半导体性质,易产生氧空穴,是理想的催化材料。通过添加不同质量分数的ZrO₂(0~5%) 作为助剂,采用分步沉淀法制备系列CuO/Fe₂O₃-ZrO₂催化剂,通过XRD、N₂物理吸附-脱附、H₂-TPR和CO₂-TPD等表征技术,考察ZrO₂助剂对CuO/Fe₂O₃水煤气变换催化剂催化性能的影响。结果表明,适量ZrO₂（质量分数1%）的添加,削弱了CuFe₂O₄中铜铁物种之间的协同作用,增加了催化剂中可被还原的铜物种的数量,形成较多的弱碱性位点,有利于增加活性中心铜的数量,具有较好的水煤气变换反应活性和热稳定性。     

A comparison on the catalytic activity of Zn1−xCoxFe2O4 (x = 0, 0.2, 0.5, 0.8 and 1.0)-type ferrospinels prepared via. a low temperature route for the alkylation of aniline and phenol using methanol as the alkylating agent

Depending on the variation of the Zn²⁺/Co²⁺ ratio in the Zn_1−xCo_xFe₂O₄ (x=0, 0.2, 0.5, 0.8 and 1.0)-type ferrospinels, the systems showed different activity trends for aniline and phenol methylation using methanol as the alkylating agent. An increase in Zn²⁺/Co²⁺ ratio increased the rate of N-monomethylation of aniline, whereas, a decrease in the ratio favored the rate of ortho methylation of phenol. An attempt has been made to interpret the observed trends based on the variation of surface acid–base properties of the catalyst surface with changes in the spinel composition. The efficiency of adsorption of aniline, phenol or methanol depends not only on the catalyst surface acid–base properties but also on the polarity of the adsorbing molecules. A controlled interplay of surface acid–base properties and polarity of the respective reacting molecules determines the efficiency of a particular reaction. In the case of aniline methylation, surface basicity plays a dominating role, whereas for phenol methylation surface acidity plays a dominating role.