Effect of electro-chemical properties of sodium salts of various anions on their diffusional parameters in symmetrical cellulose acetate membranes

A relation was obtained between electro-chemical properties of sodium salts (NaCl, NaBr, and Na₂SO₄), and the thermodynamic property of permeability in symmetrical cellulose acetate membranes, the distribution coefficient K and the kinetic property, the overall diffusion coefficients D. K and D were obtained by the method we proposed using measured unsteady- and steady-state dialysis data. The K values increase with the increase of water content and are in the range of 10⁻² for sodium halides and 10⁻³ for Na₂SO₄. D is found to increase with the increase of the solute concentration, and the extrapolated values of D to zero concentration D⁽⁰⁾ are obtained as 0.015–0.03 μm²/s and increase with the increase of water content in the membrane. D can be divided into the concentration independent diffusion coefficients in the dense part of the membrane D_d and in the porous D_p, applying a two-part (perfect or dense and imperfect or porous) model of the membrane. Contrary to D_d, D_p increases with the increase of W_w and can be correlated as D_p,c = _d exp (γ × W_w). It is shown that the averaged D_d, _D increases with the increase of the quantity of the ionic mobility u of the solutes at infinite dilution divided by valence, and that the parameter γ increases with the increase of the ionic mobility u. The value of K increases slightly with the increase of water content and decreases with the increase of the Flory—Huggins parameter χ. The Flory—Huggins parameter χ is calculated from the measured values of distribution coefficients and data obtained from the literature. And it was found that the gradient of linear decrease of χ (λ_cation) depends on equivalent ionic conductivity of anion of salt, λ_an.     

Reactivity of titanosilicates-supported catalysts for the selective catalytic reduction of NO with NH3

The zeolites with MEL structure were synthesized via the hydrothermal method and the zeolites-supported catalysts, such as Cu²⁺, Ga³⁺, Co³⁺, Ce²⁺ and VO²⁺/zeolites, were prepared by the incipient wetness impregnation. The structures of the synthesized zeolites were characterized by techniques of XRD, FT-IR, SIMS, ²⁹Si and ²⁷Al MAS NMR. The selective catalytic reduction (SCR) of NO by ammonia was carried out with a glass reactor under a downstream flow. The synthesized TS-2 showed no significant DeNO_x activity, instead of catalyzing the ammonia oxidation at a high temperature. Furthermore, the catalytic activity of TS2 zeolite can be effectively modified and tuned up through incorporating second metal ion such as Fe³⁺, Co³⁺, and Al³⁺ into the framework (i.e., [Fe,Ti]Z11, [Co,Ti]Z11, and [Al,Ti]Z11). Among the synthesized bimetallosilicates, the [Fe,Ti]Z11 zeolite is the most active catalyst for the SCR DeNO_x with ammonia; the NO conversion and the N₂ yield reach around 80%. In addition, impregnating the metal ions on TS2 or bimetallosilicates is also a very effective way to improve the SCR DeNO_x activity. Ga³⁺/[Fe40,Ti40]Z11 and Co³⁺/[Fe40,Ti40]Z11 are the most active catalysts and show a potential for the practical applications.     

Active sites of Cu-ZSM-5 for the decomposition of acrylonitrile

The catalytic decomposition of acrylonitrile (AN) over Cu-ZSM-5 prepared with various Cu loadings was investigated. AN conversion, during which the nitrogen atoms in AN were mainly converted to N₂, increased as Cu loading increased. N₂ selectivities as high as 90–95% were attained. X-ray diffraction measurements (XRD) and temperature-programmed reduction by H₂ (H₂-TPR) showed the existence of bulk CuO in Cu-ZSM-5 with a Cu loading of 6.4 wt% and the existence of highly dispersed CuO in Cu-ZSM-5 with a Cu loading of 3.3 wt%. Electron spin resonance measurements revealed that Cu-ZSM-5 contains three forms of isolated Cu²⁺ ions (square-planar, square-pyramidal, and distorted square-pyramidal). The H₂-TPR results suggested that in Cu-ZSM-5 with a Cu loading of 2.9 wt% and below, Cu⁺ existed even after oxidizing pretreatment. The activity of AN decomposition over Cu/SiO₂ suggested that CuO could form N₂, but, independent of the CuO dispersion, nitrogen oxides (NO_x) were formed above 350 °C. Cu⁺ and the square-pyramidal and distorted square-pyramidal forms of Cu²⁺ showed low activity for AN decomposition. Temperature-programmed desorption of NH₃ suggested that N₂ formation from NH₃ proceeded on Cu²⁺, resulting in the formation of Cu⁺. The Cu⁺ ions were oxidized to Cu²⁺ at around 300 °C. Thus, high N₂ selectivity over Cu-ZSM-5 with a wide range of temperature was probably attained by the reaction over the square-planar Cu²⁺, which can be reversibly reduced and oxidized.     

氧化铈-二氧化硅介孔材料制备及对铜离子的吸附性能

采用共沉淀法和沉淀浸渍法制备了纳米氧化铈-二氧化硅（CeO₂-SiO₂）介孔材料吸附剂,主要考察了其对水中铜离子（Cu²⁺）的吸附行为。通过X射线衍射（XRD）、扫描电镜（SEM）和氮吸附（BET）等手段对合成的介孔材料进行了性能表征,并通过静态吸附实验分析了溶液pH、溶液初始金属离子质量浓度、吸附剂用量、吸附时间等条件对介孔材料吸附Cu²⁺性能的影响。结果表明：共沉淀法制备的纳米CeO₂-SiO₂介孔材料对Cu²⁺的去除效果较沉淀浸渍法要好;当溶液pH=7.0时CeO₂-SiO₂介孔材料对Cu²⁺的吸附效果最好,20 min时基本达到吸附平衡;溶液初始Cu²⁺浓度增大Cu²⁺去除率降低,Cu²⁺累计吸附量增大;随着吸附剂用量增加Cu²⁺去除率增大,当CeO₂-SiO₂吸附剂用量为0.15 g/L时对Cu²⁺的去除率趋于稳定;CeO₂-SiO₂吸附剂对不同金属离子吸附性能由大到小的顺序为Cu²⁺、Fe²⁺、Mn²⁺,该吸附过程均符合准二级动力学模型。     

Cu-ZSM-5 zeolite highly active in reduction of NO with decane under water vapor presence: Comparison of decane, propane and propene by in situ FTIR

Selective catalytic reduction of NO_x (SCR-NO_x) with decane, and for comparison with propane and propene over Cu-ZSM-5 zeolite (Cu/Al 0.49, Si/Al 13.2) was investigated under presence and absence of water vapor. Decane behaves in SCR-NO_x like propene, i.e. the Cu-zeolite activity increased under increasing concentration of water vapor, as demonstrated by a shift of the NO_x–N₂ conversion to lower temperatures, in contrast to propane, where the NO_x–N₂ conversion is highly suppressed. In situ FTIR spectra of sorbed intermediates revealed similar spectral features for C₁₀H₂₂– and C₃H₆–SCR-NO_x, where –CH_x, R–NO₂, –NO₃⁻, Cu⁺–CO, –CN, –NCO and –NH species were found. On contrary, with propane –CH_x, R–NO₂, NO₃⁻, Cu⁺–CO represented prevailing species. A comparison of the in situ FTIR spectra (T–O–T and intermediate vibrations) recorded at pulses of propene and propane, moreover, under presence and absence of water vapor in the reaction mixture, revealed that the Cu²⁺–Cu⁺ redox cycle operates with the C₃H₆–SCR-NO_x reactions in both presence/absence of water vapor, while with C₃H₈–SCR-NO_x, the redox cycle is suppressed by water vapor. It is concluded that decane cracks to low-chain olefins and paraffins, the former ones, more reactive, preferably take part in SCR-NO_x. It is concluded that formation of olefinic compounds at C₁₀H₂₂–SCR-NO_x is decisive for the high activity in the presence of water vapor, while water molecules block propane activation. The increase in NO_x–N₂ conversion due to water vapor in C₁₀H₂₂–SCR-NO_x should be connected with the increased reactivity of intermediates. These are suggested to pass from R–NO_x → –CN → –NCO → NH₃; the latter reacts with another activated NO_x molecule to molecular nitrogen. The positive effect of water vapor on the NO_x–N₂ conversion is attributed to increased hydrolysis of –NCO intermediates.     

Insight into SO2 poisoning and regeneration of one-pot synthesized Cu-SSZ-13 catalyst for selective reduction of NOx by NH3

The effects of SO₂ on an one-pot synthesized Cu-SSZ-13 catalyst for selective reduction of NO_x by NH₃ were examined. The addition of SO₂ inhibited NO_x conversion significantly below 300 ℃, while no effect on NO_x conversion was observed above 300 ℃. TGA, TPD, and XPS results showed that the deactivation was caused by the formation of (NH₄)₂SO₄, SO₂ chemisorption on the isolated Cu²⁺ ion sites, as well as the formation of CuSO₄-like species. Among them, the site-blocking effect of (NH₄)₂SO₄ on Cu²⁺ was the primary reason for deactivation. Fortunately, 89% of deNO_x activity of the poisoned catalyst was recovered after thermal treatment at 500 ℃ in air, where (NH₄)₂SO₄ was completely decomposed. The activity was further recovered with regeneration temperature increasing to 600 ℃, removing the adsorbed SO₂ on the Cu²⁺ sites. The regeneration at 600 ℃ could not recover the activity completely, because of the high stability of CuSO₄-like species.     

Selective catalytic reduction of nitric oxide by ethylene in the presence of oxygen over Cu ion-exchanged pillared clays

Cu²⁺ ion-exchanged pillared clays are substantially more active than Cu²⁺-ZSM-5 for selective catalytic reduction (SCR) of NO by hydrocarbons. More importantly, H₂O (or SO₂) has only mild effects on their activities. First results on Cu²⁺-exchanged TiO₂-pillared montmorillonite were reported by this laboratory (Yang and Li, Ref. [1]), that showed overall activities two to four times higher than Cu²⁺-ZSM-5.

A delaminated pillared clay was subjected to Cu²⁺ ion-exchange and studied for SCR by C₂H₄ in this work. The Cu²⁺ ion-exchanged delaminated Al₂O₃-pillared clay yielded substantially higher SCR rates than both Cu²⁺-exchanged TiO₂-pillared clay and Cu²⁺-ZSM-5 at temperatures above 400°C. The peak NO conversion was 90% at 550°C and at a space velocity of 15,000 h⁻¹ (with O₂ = 2%). The peak temperature decreased as the concentration of O₂ was increased. The macroporosity in the delaminated pillared clay was partially responsible for its higher peak temperatures (than that for laminated pillared clays). At 1000 ppm each for NO and C₂H₄, the NO conversion peaked at 2% O₂ for all temperatures. H₂O and SO₂ caused only mild deactivation, likely due to competitive adsorption (of SO₂ on Cu²⁺ sites and H₂O on acid sites). The high activity of Cu²⁺-exchanged Al₂O₃-pillared clay was due to a unique combination of the redox property of the Cu²⁺ sites and the strong Lewis acidity of the pillared clay. The suggested mechanism involved NO chemisorption (in the presence of O₂) on Cu²⁺OAl³⁺-on the pillars, and C₂H₄ activation on the Lewis acid sites to form an oxygenated species.     

In situ XPS investigations of Cu1−xNixZnAl-mixed metal oxide catalysts used in the oxidative steam reforming of bio-ethanol

A series of CuNiZnAl-multicomponent mixed metal oxide catalysts with various Cu/Ni ratios were prepared by the thermal decomposition of Cu_1−xNi_xZnAl-hydrotalcite-like precursors and tested for oxidative steam reforming of bio-ethanol. Dehydrogenation of EtOH to CH₃CHO is favored by Cu-rich catalyst. Introduction of Ni leads to CC bond rupture and producing CO, CO₂ and CH₄. H₂ yield (selectivity) varied between 2.6–3.0 mol/mol of ethanol converted (50–55%) for all catalysts at 300 °C. The above catalysts were subjected to in situ XPS studies to understand the nature of active species involved in the catalytic reaction. Core level and valence band XPS as well as Auger electron spectroscopy revealed the existence of Cu²⁺, Ni²⁺ and Zn²⁺ ions on calcined materials. Upon in situ reduction at reactions temperatures, the Cu²⁺ was fully reduced to Cu⁰, while Ni²⁺ and Zn²⁺ were partially reduced to Ni⁰ and Zn⁰, respectively. On reduction, the nature of ZnO on Cu-rich catalyst changes from crystalline to amorphous, relatively inert and highly stabilized electronically. Relative concentration of the Ni⁰ and Zn⁰ increases upon reduction with decreasing Cu-content. Valence band results demonstrated that the overlap between 3d bands of Cu and Ni was marginal on calcined materials, and no overlap due to metallic clusters formation after reduction. Nonetheless, the density of states at Fermi level increases dramatically for Ni-rich catalysts and likely this influences the product selectivity.     

自养硝化颗粒污泥吸附铜离子性能及吸附等温线

以自养硝化颗粒污泥作为吸附剂,研究了其对铜离子的吸附效果及最佳吸附工艺条件,并探索了其吸附等温线。通过单因素实验研究铜离子浓度、吸附时间、污泥浓度、搅拌速度和温度对吸附效果的影响,最终得出搅拌速度、吸附时间以及污泥浓度对吸附效果有明显影响。在此基础上,通过响应曲面法耦合出吸附的最佳工况点：时间（A）=2.50h,转速（B）=125r/min,污泥量（C）=5250mg/L。在此条件下研究了颗粒污泥的吸附特性及稳定性。结果表明,硝化颗粒污泥在重金属废水中表现出极强的耐受性与稳定性,不同浓度下颗粒化率均维持在93%以上。Langmuir与Freundlich方程动力学拟合结果显示：Langmuir等温方程的拟合度R²_Cu=0.999,表明硝化颗粒污泥对于Cu²⁺属于典型的单分子层吸附,且能描述最大吸附量为Q_max=15.02mg/g。Freundlich等温方程的拟合过程中,相关系数R²_Cu=0.969,1/n=0.1305,表明硝化颗粒污泥对Cu²⁺吸附能力较强;较高的拟合度也在一定程度上表明硝化颗粒污泥对Cu²⁺的吸附是一个复杂的物理化学过程。     

Elucidation of a preparation method for copper ion-exchanged ZSM-5 samples exhibiting extremely efficient N2-adsorption at room temperature: effect of counter ions in the exchange solution

The adsorption of N₂ on a copper ion-exchanged ZSM-5 sample (CuZSM-5) prepared by ion exchange using an aqueous solution of copper propionate, Cu(C₂H₅COO)₂, was examined at room temperature by measuring the FT-IR spectra, adsorption isotherms and heat of adsorption. This sample was found to be extremely efficient in terms of N₂ adsorption with regard to both the amount and the energy (i.e., heat) of adsorption, compared with samples prepared by a conventional ion-exchange method using an aqueous solution involving Cu²⁺ and simple counter ions, Cl⁻ or NO₃⁻. To clarify the specificity of the newly-prepared sample, the ion-exchange of ZSM-5 with Cu²⁺ was carried out by employing aqueous solutions involving Cu²⁺ and various types of counter ions [propionate (C₂H₅COO⁻), acetate (CH₃COO⁻), formate (HCOO⁻), chloride (Cl⁻) and nitrate (NO₃⁻) ions]. When the ion exchange was performed by using a Cu(C₂H₅COO)₂ or Cu(CH₃COO)₂ solution, the Cu²⁺ species with propionate or acetate ligand (in the monomer state) were ion-exchanged in ZSM-5, as confirmed by the DR, EPR and FT-IR spectra for CuZSM-5. In contrast, Cu²⁺ species were present in the form of aquo-complexes in samples prepared with other solutions. This distinct difference can be ascribed to the difference in the pK_a values of the counter ions; carboxylate ions, with a high pK_a value, are inclined to form a complex with Cu²⁺. Using this newly applied Cu(C₂H₅COO)₂ solution, the present ion-exchange method has the potential to develop new effective materials that possess the specific adsorption and catalytic properties of CuZSM-5.     

Preparation of Cu-ion-exchanged Fe-PILCs for the SCR of NO by propene

Cu-ion-exchanged iron-pillared interlayer clays (Fe-PILCs) were prepared under different pH conditions to analyze the influence on the distribution of the copper species over their structure, and on the catalytic performance for the selective catalytic reduction (SCR) of NO_x by propene. It was observed that for those samples prepared without pH control, the copper was as isolated Cu²⁺ ions. When the samples were prepared under acid pH, the catalytic activity decreased and an appreciable CO production was observed, likely due to the low amount of Cu²⁺ cations in those catalysts. Finally, for the samples prepared under alkaline pH, the copper was as Cu²⁺ ions and CuO clusters. Their catalytic tests showed the best results for the SCR of NO_x. The presence of CuO species led to an improvement in NO_x yield to N₂. With the catalytic tests and a study by in situ FTIR of SCR of NO, a reaction mechanism has been proposed, where the reaction intermediates are mainly acetates, organic nitro compounds and nitrous oxide species.     

Transport of K in PVC matrix membranes containing valinomycin

We report impedance measurements on PVC matrix membranes which contain KBPh₄ with varying proportions of valinomycin. In agreement with our earlier measurements the value of the bulk membrane resistance (R_b) is much larger in the presence of valinomycin, indicating that the mobility of K⁺ is greatly reduced by the valinomycin. R_b shows a linear variation with valinomycin/K⁺ ratio between 0 and 1, but it is invariant at higher valinomycin/K⁺ ratios. Thus there is no evidence for a special transport mechanism for K⁺ in these membranes.     

The effect of oxygen concentration on the reduction of NO with propylene over CuO/γ-Al2O3

The effect of oxygen concentration on the pulse and steady-state selective catalytic reduction (SCR) of NO with C₃H₆ over CuO/γ-Al₂O₃ has been studied by infrared spectroscopy (IR) coupled with mass spectroscopy studies. IR studies revealed that the pulse SCR occurred via (i) the oxidation of Cu⁰/Cu⁺ to Cu²⁺ by NO and O₂, (ii) the co-adsorption of NO/NO₂/O₂ to produce Cu²⁺(NO₃⁻)₂, and (iii) the reaction of Cu²⁺(NO₃⁻)₂ with C₃H₆ to produce N₂, CO₂, and H₂O. Increasing the O₂/NO ratio from 25.0 to 83.4 promotes the formation of NO₂ from gas phase oxidation of NO, resulting in a reactant mixture of NO/NO₂/O₂. This reactant mixture allows the formation of Cu²⁺(NO₃⁻)₂ and its reaction with the C₃H₆ to occur at a higher rate with a higher selectivity toward N₂ than the low O₂/NO flow. Both the high and low O₂/NO steady-state SCR reactions follow the same pathway, proceeding via adsorbed C₃H₇---NO₂, C₃H₇---ONO, CH₃COO⁻, Cu⁰---CN, and Cu⁺---NCO intermediates toward N₂, CO₂, and H₂O products. High O₂ concentration in the high O₂/NO SCR accelerates both the formation and destruction of adsorbates, resulting in their intensities similar to the low O₂/NO SCR at 523–698 K. High O₂ concentration in the reactant mixture resulted in a higher rate of destruction of the intermediates than low O₂ concentration at temperatures above 723 K.     

One-electron reducibility of isolated copper oxide on alumina for selective NO–CO reaction

The H₂-TPR (temperature-programmed reduction) study was performed for supported copper oxide catalysts with low loading (0.5 wt% as copper). Among the various kinds of support materials (γ-Al₂O₃, TiO₂, ZrO₂, SiO₂, ZSM-5), alumina-supported copper oxide indicated a one-electron reduction behavior of Cu²⁺ into Cu⁺ ions in the presence of H₂. The reduction of the isolated Cu²⁺ species in a tetragonally distorted octahedral symmetry into the low coordinated Cu⁺ ions was identified by means of X-ray absorption spectroscopy (XANES and EXAFS). The isolated Cu⁺ ions hosted by γ-Al₂O₃ surface were prevented from further reduction into metallic Cu⁰ state under reducing condition with H₂ at 773 K. Less dispersed supported copper oxide species were easily reduced to Cu⁰ metal particles with H₂ at 573 K regardless of the kinds of support materials. It is suggested that the one-electron redox behavior of the isolated copper oxide species over γ-Al₂O₃ promotes the catalytic reduction of NO with CO in the presence of oxygen on the basis of redox-type mechanism between Cu²⁺ and Cu⁺ in atomically dispersed state.     

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.     

Einfluss der säurekonzentration auf die anodische auflosung des indiumamalgams

The effect of HClO₄ concentration on the rate of electrode processes on a stationary indium amalgam electrode in stirred In(ClO₄)₃ solutions at constant ionic strength (3 M NaClO₄) has been investigated. Alongside with the measurement of polarization curves, the exchange current and the true rate of the anodic process of amalgam dissolution (i_a at different potentials (φ) were determined by a radiochemical method. With an increase in the HClO₄ concentration, a sharp decrease in the exchange current is observed; at HClO₄ concentrations below 0.2 M at constant φ, the i_a values are inversely proportional to the HClO₄ concentration. The experimental results are expressed by the equation p] i_a = k[In][H⁺]⁻¹ exp (βφF|RT),

where [In] is the concentration of the indium amalgam. The rate of the cathodic process at constant φ also decreases with a decrease in pH. A hypothesis is advanced according to which partially hydrolysed In(H₂O)₅OH²⁺ ions, rather than In(H₂O)₆³⁺ which are predominant in the solution, participate in the electrode process. The OH⁻ ions, like some other anions, appear to be capable of facilitating the transfer of electrons between the electrode surface and the reacting particle. An analysis of the results obtained (and of data in the literature) shows that partially hydrolysed metal ions play an essential role in electrode processes.     

Peroxo salts as initiators of vinyl polymerization: Polymerization of acrolein initiated by potassium peroxodiphosphate

Kinetic studies were made on the polymerization of acrolein initated by potassium peroxodiphosphate (PP) in aqueous solution, in the presence and absence of Ag⁺ ions. The rate of polymerization was found to depend on [M]^3/2 (M = monomer) and was independent of both [PP] and [Ag⁺]. The overall activation energy was calculated to be 4.8 kcal mol⁻¹. A mechanism involving termination by PO^2-₄ radicals is proposed and discussed.     

Internal burning of pulverized semi-anthracite: the relation between particle structure and reactivity

Measurements have been made of structural properties of particles of pulverized semi-anthracite at various stages of combustion at temperatures in the range 1400–2200 K. Four size-graded fractions of the particles (mass-median sizes 78, 49, 22 and 6 μm) were burned at oxygen partial pressures of about 0.1 and 0.2 atm. The structural properties determined included specific surface areas, helium densities, X-ray diffraction patterns, and pore-volume distributions. The relation of these properties to the temperatures prevailing during reaction and to the degree of burn-off which took place showed that macropores (pores above 0.02 μm width) developed during reaction and that the micropore volume was reduced. The opening up of the macropores results from the combustion process, but the reduction in micropore volume is due mainly to the effect of the higher temperatures on the structure of the solid. The structural data are combined with kinetic data relating to the reaction of oxygen with the particles to determine values of the intrinsic reactivity coefficient R_s (the rate of carbon consumption per unit of total internal area per unit pressure of oxygen) under various conditions. R_s is independent of particle size and is given by the equation R_s(g/cm² s atm O₂) = 55 [−40 000/(RT_p)] where T_p is the temperature of the particles and R is in cal/mol K^*. Calculations of the intrinsic reactivity via two routes, treating the pores as having either a unimodal or bimodal distribution of sizes respectively, gave similar values of the reactivity. Values of R_s are used to predict, within a factor of two, the results of other workers' measurements of semi-anthracite combustion rates.     

Mechanical behaviour of partially stabilized zirconia crystals with terbia and ceria additives

Mechanical behaviour of partially stabilized zirconia crystals (PSZC) with terbia and ceria additives was investigated under bending and indentation conditions. Test specimens were oriented along the [010] direction and along the axis of crystal growth. The PSZC bending strength (σ_b) was dependent on the crystallographic orientation of the specimens. The specimen volume subjected to stress influenced the PSZC strength. The highest mechanical characteristics were measured for ceria-doped crystals (σ_b = 1.9 GPa, K_lc = 11.4 MPa m^1/2, E_d = 366 GPa). The failure process was studied on the Vickers indentation, with special emphasis put on the development and propagation of lateral cracks. Anisotropy of lateral cracks in the (100) plane associated with that of the elastic moduli was revealed. At the same time anisotropy of radial cracks and hardness was not found. A new version of the equation to evaluate the fracture toughness (K_c^v) on the Vickers indentation was derived. The K_c^v values calculated by this equation correspond to those (K_lc) obtained by an SENB method.     

The role of Bronstead acidity in poisoning the SCR-urea reaction over FeZSM-5 catalysts

Two series of FeZSM-5 catalysts prepared from Na⁺ and NH₄⁺ ZSM-5 precursors are studied in the selective reduction of NO_x using NH₃ and urea as reducing agents. All Fe-containing catalysts are active for NO_x reduction in the SCR-NH₃ reaction with ex-NH₄⁺ catalysts being more active than ex-Na⁺ materials and the activity depending (to a minor extent within each series of catalysts) upon [Fe]. Catalysts with Bronstead acid sites also show a small transient deNO_x activity at low temperatures. All catalysts are less active for the SCR-urea reaction but the ex-Na⁺ catalysts retain far more deNO_x activity than the ex-NH₄⁺ materials. NH₃ TPD shows that strongly binding Bronstead acid sites are present on the ex-NH₄⁺ materials and H⁺-treated parent zeolites while Urea TPD shows that the mode of decomposition of urea differs as a function of initial zeolite counter-ion. Urea TPSR shows that the reaction between adsorbed urea and gaseous NO/O₂ is related to [Fe]. It is proposed that the decreased activity of the ex-NH₄⁺ catalysts in the SCR-urea reaction is due to a less favourable mode of decomposition over these catalysts. Furthermore it is suggested that the Bronstead acidity plays some part in this less favoured decomposition.