Platinum crystallite size effects on oxide formation and reduction parameters—II

A detailed examination of the electrochemical formation and reduction of surface oxides on platinum crystallites shows a sensitivity for the oxidation rate at constant potential and the mechanism of the oxide reduction on the platinum crystallite size. In the crystallite regions under consideration, the percentage of Pt atoms located in the surfaces of the crystallites changes from 45% at 92 m²/g (30 Å dia.) to 4% at 10 m²/g (280 Å dia.) so that very small crystallites exhibit surface physical properties different from bulk metal surfaces. This is reflected in the changes in electrocatalytic behavior shown here. The oxide reduction mechanisms show two separate “Tafel” slopes of 30 mV and 55 mV, dependent on the oxide coverage. Transitions between the two mechanisms as a function of oxide coverage are linearly dependent upon the percentage of platinum atoms located in the surfaces of the crystallites. Electrocatalytic reaction parameters obtained at bulk platinum metal electrodes may not be applied to high surface area platinum electrocatalysts.     

Sintering of Low-Density Glasses: III, Effect of a Distribution of Pore Sizes

The sintering model presented in Part I is extended by considering a Gaussian distribution of pore sizes in the body. The effect of the breadth of the distribution on the densification kinetics is demonstrated; this effect is small except for very broad distributions. The greatest change occurs in the last few percents of shrinkage since the largest pores close relatively slowly. The range of pore sizes typically found in flame hydrolysis preforms or silica gel is narrow enough that the sintering kinetics are adequately described in terms of a single pore size. The specific surface area is shown to be independent of the breadth of the distribution.     

Mercerization of cellulose. III. Changes in crystallite sizes

When cellulose is mercerized slowly, the initial conversion is from the cellulose I crystal structure to that of Na–cellulose I. The conversion is a crystal-to-crystal phase transformation, without passage through an amorphous state. The analysis of crystallite sizes of cellulose I and Na-cellulose I during this transformation has shown that the process takes place in two steps. The first is a rapid step, occurring in approximately 1/7 (or less) of the total conversion time, resulting in a conversion ratio of ca. 65%. The measured crystallite sizes of both cellulose I and Na–cellulose I remain constant during this stage, at ～ 62 and ～ 35 Å, respectively. In the following slow step, complete conversion to Na–cellulose I takes place, and the crystallite size of cellulose I decreases steadily until disappearance. The crystallite size of Na–cellulose I increases steadily at the same time, to a final value of ～ 50 Å. Simultaneously, the unit cell parameters of cellulose I change significantly. The observation of a two-step conversion is consistent with a proposed mercerization mechanism in which the conversion is assumed to begin in the amorphous phase of the cellulose fiber.     

Platinum crystallite size considerations for electrocatalytic oxygen reduction—I

An examination of the activity contributions for atoms at corners, edges and planes of platinum electro-catalyst crystallites was carried out for the reduction of molecular oxygen in 1M H₂SO₄. Varying the mean crystallite diameters between 30 and 400 Å caused at least a 200-fold and 30-fold change in the surface densities of corner and edge sites, respectively. Deliberate sintering of the platinum crystallites caused crystallite growth, with a reduction of surface and lattice defects. The catalysts showed a uniform activity for oxygen reduction of 0·018 ± 0·003 mA/real cm² Pt at 900 mV (nhe) with a Tafel slope of 65 ± 5 mV at 50 and 70°C. Platinum atoms at corners, edges, kink sites or dislocations are not more active than atoms on the crystallite faces for this reaction.     

Dependence of the Lattice Parameter of Magnesium Oxide on Crystallite Size

Lattice parameters were measured on MgO specimens prepared in air between 450° and 1200°C. The lattice parameter, a , decreases with increase in preparation temperature, T _p, and with increase in crystallite size. A hydroxide layer is present on the MgO particles. If MgO is prepared in vacuum, a increases as T _p increases and as crystallite size, D , increases. It is concluded that a dilatant volume stress is imposed by the hydroxide layer. It is also shown that the oxide surface reactivity toward water is linked to the deviation from perfect order ensuing from low preparation temperatures.     

Characterisation of Catalysts Used in Wall Reactors for the Catalytic Dehydrogenation of Methylcyclohexane

The catalytically active coatings of tube wall reactors, consisting of an aluminium oxide washcoat and platinum, platinum/tin, or platinum/rhenium deposited by incipient wetness impregnation, are characterized using BET surface area measurements, CO chemisorption, X-ray photoelectron spectroscopy, plasma atomic emission spectroscopy, high resolution transmission electron microscopy (HRTEM), and extended X-ray absorption fine structure analysis (EXAFS). The mesoporous catalysts show a type-IV adsorption isotherm, yielding an average pore diameter of 9 nm, and a BET surface area between 178 and 197 m²/g. From the bulk and surface analysis results, an enrichment of tin or rhenium, respectively, is found to take place at the surface of the bimetallic catalysts. Very good agreement between the particle sizes estimated from EXAFS, CO-chemisorption, and HRTEM data is found for a low loading (1.5 wt%) platinum catalyst, leading to dispersion values between 75 and 80%, a cluster size of 30 atoms, and a particle diameter around 13 Å. Using the catalytic dehydrogenation of methylcyclohexane as a test reaction, it is demonstrated that a pretreatment at high reduction temperatures leads to a decrease in activity. EXAFS show that this effect is due to a stronger metal–support interaction, as indicated by the appearance of a Pt-O contribution at 2.25 Å in the coordination sphere of the surface platinum atoms.     

The effect of preparation variables on the dispersion of supported platinum catalysts

The effects of several preparation variables on the dispersion of supported platinum catalysts were examined in this study. Supported catalysts were prepared using two different platinum salts, four high surface area support materials, and two aqueous deposition techniques. The catalysts were characterised by hydrogen chemisorption, oxygen chemisorption, and the hydrogen titration reaction. In addition, X-ray diffraction measurements were conducted on all catalysts. Results from this study show that one of the platinum salts, chloroplatinic acid, always gave an equally or more highly dispersed catalyst than the other salt, tetraammine platinum (II) nitrate. The incipient wetness preparation technique produced equal or better dispersions than did the excess water method, with the possible exception of the silica-alumina support. The highest dispersions were attained with alumina, and the lowest with carbon. In some samples, a lack of good agreement existed between X-ray diffraction and chemisorption measurements of crystallite size which indicated broad or bimodal particle size distributions. However, these X-ray data were qualitatively helpful in understanding the unusual chemisorption behaviour of Pt/C catalysts, one of which had a dispersion higher than any previous reported Pt/C catalyst prepared by aqueous impregnation techniques. Experiments were also conducted which showed that either a 1 h or a 12 h treatment in hydrogen at 723 K was sufficient to reduce the platinum salt, and 1 h or 12 h evacuations at 698 K gave similar results. Finally, it was found that approximately one-half of the hydrogen monolayer on supported Pt could be removed by evacuating for 1 h at 300 K.     

Role of platinum surface morphology on hydrogen adsorption isotherms—IV

The changing characters of hydrogen quasi-equilibrium adsorption isotherms on high surface area platinum electrocatalyst crystallites in 1 M H₂SO₄ were examined as a function of the average crystallite sizes. On varying the platinum crystallite sizes (28–460 A dia), the relative concentrations of surface atoms identified with edges, vertices and crystallite faces are changed and are compared to the changing surface concentrations of the adsorbed hydrogen species. Model crystallite calculations assume a cubo-octahedral structure with (111) and (100) faces to give the lowest surface energy.The heterogeneity parameter, g, in the hydrogen quasi-equilibrium isotherms is an extrinsic factor, specifically associated with the adsorbed species and not an intrinsic function of the platinum crystallite surface. Also, interstitial B₅ sites are not important as specific hydrogen adsorption locations. Crystallite edge atoms (or Pt atoms with low coordination numbers) have been identified as having a specific adsorbed hydrogen species, θ_3a, and weakly and strongly bonded adsorbed hydrogen are associated with the (111) and (100) crystallite faces, respectively.     

Size‐Dependent Crystal Properties of Nanocuprite

Cuprite nanoparticles from 9 nm to 1 μm with narrow size distribution (±7%) are prepared by two different methods. The lattice parameter increases up to 0.2% as the crystallite size decreases to 9 nm from micron size. X‐ray Absorption Near Edge Spectroscope study of our copper oxide nanoparticles shows mostly Cu (I) with increasing concentration of Cu (II) as decreasing crystal sizes. The size effect in Cu(II)/Cu(I) ratio indicates that at smaller crystal size, the Cu₂O tends to be more oxidized at higher charge state with Cu(I)‐O bond. Thus, the lattice expansion can be explained by the presence of longer Cu (II)‐O bond than Cu (I)‐O bond and/or oxygen interstitials in nanocuprite.     

Crystallite size and dispersion of platinum in an alumina matrix

A method for preparing thin sections of platinum on alumina reforming catalysts has been developed. The platinum particle size distribution and the degree of dispersion has been determined by transmission electron microscopy. There is agreement with the average crystallite size determined from X-ray diffraction line broadening. The morphology of the alumina matrix is also revealed.     

多晶X—射线衍射在催化研究中的应用

通过研究工作中的一些实例，介绍了ＸＲＤ在催化研究中的应用，它涉及物相分析、晶粒大小和晶格常数的测定等。用这些方法，研究了催化剂的活性组分、制备条件和制备过程的相变。     

The preparation of highly dispersed platinum on carbon

The preparation of highly dispersed platinum (Pt crystallite size less than 15 Å) catalyst on a carbon support, which was suitable for electrochemical studies, is described. A high degree of Pt dispersion was prepared only by the pyrolysis in the temperature range of 500–800°C of the calcium form of Amberlite IRC50 cation exchange resin in which platinum ions were adsorbed. The influence of the preparation conditions on the degree of platinum dispersion and on the properties of the dispersed Pt are described and the mechanism of the pyrolysis process is discussed.     

Preparation of metal-impregnated peat carbons and characterization of the platinum dispersion

A simple method for the preparation of carbon supported metals is described. The method involves liquid phase metal impregnation of untreated and ammoniated sphagnum peat followed by pýrolysis in the 500–800 C range and/or further activation. The characterization of the dispersion has been studied for the case of Pt-impregnated cokes. Moderate pyrolysis temperatures (540 C) result in a high degree of dispersion (Pt crystallite size less than 10 Å) as determined via H₂ chemisorption. The detailed preparation procedure as well as the effect of the pyrolysis temperature on the properties of the impregnated carbons are described.     

New insights into the structure of microporous membranes obtained using a new pore size evaluation method

The pore size distribution of the active pores of Millipore MF-VS and MF-VM microfiltration membranes, having nominal pore sizes of 250 Å and 500 Å, respectively, was investigated by a new evaluation method described earlier. The average values of the active pore sizes calculated from the pore size distributions obtained by this method were found to be systematically smaller than the nominal pore sizes indicated by the manufacturer. These differences are discussed in terms of the applicability and range of validity of the gas permeability method which is frequently used for evaluation of the average pore size of microfiltration membranes.     

Mercerization of cellulose. IV. Mechanism of mercerization and crystallite sizes

Using the Guinier plot, the 200 reflection profile of ramie cellulose I was resolved into two components. One component represented a crystallite size of ～ 20 Å, and the other component represented a crystallite size of ～ 61 Å. These components were followed during the conversion of cellulose I to Na–cellulose I, in order to estimate the contribution of the smaller crystallites to the early rapid stage of conversion. From the intensity ratio of the two components, it appears that during the first ～ 20% conversion the formation of Na–cellulose I occurs mainly in the oriented amorphous regions of the fiber. Following this, up to ～ 65% conversion the destruction and transformation of small crystallites takes place. The small average size of the ～ 20 Å crystallites, as well as their relatively high reactivity to alkali, appear to facilitate the conversion of the parallel-chain cellulose I structure to an antiparallel one.     

Effect of glycerol on structure–property relations in chitosan/poly(ethylene oxide) blended films investigated using wide‐angle X‐ray diffraction

Polymer blending is an effective method for providing desirable polymeric materials with properties useful for the packaging industry. In the study reported, blends of chitosan/poly(ethylene oxide) (PEO) were prepared in various weight ratios with and without glycerol. Line profile analysis of the X‐ray diffraction patterns of these blended films was carried out. Microstructural parameters such as crystallite size and lattice strain were determined using paracrystalline modelling of X‐ray data. These values were correlated with physico‐mechanical and optical properties of the chitosan/PEO blends with and without glycerol to understand the holistic behaviour of the blends. Two prominent Bragg reflections at 2θ ≈ 19° and 23° were observed in the wide‐angle X‐ray diffraction patterns of the glycerol‐based chitosan/PEO blended films of various ratios. Interruption of PEO crystallization with chitosan results in an amorphous polymer network and hence a reduction in crystallite size by almost 97.7%. For glycerol‐based blends, the crystallite size/area decreases to 94.4% of the virgin crystallite size. The X‐ray profile analysis supports the results for the physico‐mechanical properties of the blends. The results show that the addition of 20 wt% of glycerol results in an increase of the elongation at break by more than 150%, meaning that these chitosan/PEO films could be applied in flexible packaging. Copyright © 2010 Society of Chemical Industry     

Crystalline BaTiO3 synthesis by ultrasound and microwave‐assisted sol–gel process

Highly crystalline BaTiO₃ particles were prepared by sol–gel process. Ultrasound and microwave techniques were used to facilitate the synthesis of BaTiO₃. BaTiO₃ samples were characterized by X‐ray diffraction (XRD), scanning electron microscopy (SEM), particle size analyser, and high temperature differential scanning calorimetry/thermogravimetric analysis (DSC/TGA). The results indicate that both barium and titanium sources and preparation techniques affected BaTiO₃ purity and crystallite size. The smallest particle and crystallite sizes were obtained by using BaCl₂ and (C₄H₉O)₄Ti and the crystallite size of BaTiO₃ was 8 ± 1 nm via MW technique.     

Effects of composition and crystallite size on the accuracy of the Rietveld method in determining lattice parameters of polytypes in multiphase SiC ceramics

The accuracy of the Rietveld method in determining the lattice parameters of polytypes in multiphase SiC ceramics was investigated as a function of the polytype composition and of the polytype crystallite size, using as benchmarks two sets of standard X-ray diffraction (XRD) patterns obtained by computer simulation. It was found that the Rietveld method is remarkably accurate, but also that the measurement of lattice parameters is sensitive to the polytype composition and to the polytype crystallite size. In particular, the accuracy of Rietveld-calculated lattice parameters for a given polytype not only decreases with increasing number of polytypes in the SiC ceramics, but also depends on which other polytypes are present. These two findings are interpreted based on the severity of the corresponding peak overlap phenomenon in the XRD patterns. In addition, the accuracy of the Rietveld analyses also decreases with decreasing crystallite size of the polytypes, which is due to the progressive loss of definition in the location of the peak maxima and to the greater severity of the peak overlap, both resulting from the corresponding peak broadening. This identification of the confidence limits of the Rietveld-calculated lattice parameters for a wide range of microstructural features may have important implications for the future characterization of SiC ceramics by XRD.     

Platinum crystallite size effects on the electrocatalytic oxidation and deposition of adsorbed hydrogen—III

Kinetic parameters for the oxidation and deposition of the two adsorbed hydrogen species on platinum in acid were examined using rapid linear potentiodynamic scans with good agreement between the theory and experimental data. Specific rate constants obtained from the magnitudes of the current peak potential shifts from quasi-equilibrium with potential scan rates were shown to be dependent upon the surface areas of the platinum electrocatalysts. It is postulated that the decrease in the specific rate constant with decreasing platinum crystallite size is caused by an increasing preponderance of surface platinum atoms as a fraction of the total atom content in the crystallites. Changes in the electronic, “metallic” or ionic—co-valent character of the surface platinum atoms are reflected in the changes of this specific rate constant and is an indication of the importance of the electronic structure due to the electrocatalyst sites in determining the electrocatalytic activity.     

Sol–gel derived nanoscale ThO2 using nonionic surfactant agents

In this research, two different types of sinterable ThO₂ nanopowders were synthesized by surfactant assisted sol–gel process using thorium nitrate as a starting material, and Triton X100 and Polysorbate 80, as two different structure directing agents. Both of the prepared gels were dried and characterized by thermogravimetry–differential scanning calorimetry (TGA–DSC) and X-ray diffraction (XRD). The dried gels were finally calcined and, the produced powders were subjected to XRD, Brunauer–Emett–Teller (BET), Barret–Joyner–Halenda (BJH) analyses and SEM studies. Detailed analyses show that thorium dioxide powders of different crystallite sizes, surface areas, pore volumes and sizes and sinterabilities were prepared using two different surfactants. This synthesizing route yielded the well crystallite nano-sized ThO₂ powders with relatively high surface area and good sinterability at low temperature of 1400 °C. The powder synthesized using Triton X100 surfactant exhibited higher surface area, pore volume, pore size and crystallite size. It contained some aggregated particles along with the smaller seeds. Furthermore, the pellet which is fabricated by powder synthesized using Triton X100 had a higher density and better grain growth, to some extent.