Site Energy Distribution Research Articles

Site energy distribution of copper catalytic surfaces from volumetric data collected at various temperatures

Two catalysts prepared on a titanium silicate (ETS-10) matrix by loading 6 and 11 wt.% of copper were studied by employing nitrogen monoxide as adsorbate to probe the surface properties of copper centres. Volumetric isotherms of NO adsorption were collected at different temperatures (19–70°C). A thermodynamic model was applied to the isotherms in order to describe the behaviour of the NO–Cu system in terms of surface energy distribution of Cu sites. The higher loading copper sample had more energetic interaction with NO ( −171< Δ a H ( kJ/ mol) <−44 ), with higher heterogeneity, than the less loaded one ( −111< Δ a H ( kJ/ mol) <−41 ). The properties of the Cu sites were related with the activity measured in the SCR of NO with ethylene.

Copper Site Energy Distribution of de-NOx Catalysts Based on Titanosilicate (ETS-10)

This study deals with copper-based catalysts prepared on a crystalline titanosilicate (ETS-10) matrix. Two samples prepared by ionic exchange, obtained by depositing different amounts of copper (6 and 11 wt %, corresponding to partial and total exchange, respectively), were selected for this study. Nitrogen monoxide was chosen as adsorbate to probe the surface properties of copper sites in terms of NO−Cu interaction energy. Microcalorimetric analysis of NO adsorption gave a direct measure of the NO−Cu energy of interaction and a view of the energy distribution of the copper sites from the differential and integral adsorption heat curves as a function of NO coverage. Initial heats of adsorption were around 110−130 kJ·mol-1 and fell down to 30−40 kJ·mol-1 at higher NO coverage. Volumetric isotherms of NO adsorption were measured at different temperatures (19−70 °C). A thermodynamic model was applied to the isotherms, making it possible to describe the NO−Cu site interaction energy by means of a function giving the number of Cu sites vs adsorption enthalpy, ΔaH. The high-loading copper sample had more energetic interaction with NO (−171 < ΔaH/kJ·mol-1 < −44), with a broader distribution, than the low-loading sample (−111 < ΔaH/kJ·mol-1 < −41). Parameters of adsorption, such as adsorption constant, molar entropy of adsorption, and half-coverage temperature at unit pressure (To1/2), were obtained for each type of Cu site. The properties of the Cu sites were related with the activity measured in the reaction of selective catalytic reduction (SCR) of NO with ethylene.

Step potential analysis of cobalt oxide-based electrochromic devices

The characterization of electrochemical behavior of electrochromic intercalation device based on cobalt oxide thin film was carried out using the step potential excitation method. A method based on generating plots of current density as a function of passed charge has been applied for characterization of electrochromic cobalt oxide thin films using an aqueous KOH electrolyte. The device resistance and the intercalation capacity of the material are calculated. Dynamic built-in potential estimated from step potential experiment and plots of the built-in potential as function of the passed charge, V bi(Δ Q), are generated for intercalation process. The intercalation efficiency curve is obtained to confirm the nature of energy distribution of intercalation sites in electrochromic cobalt oxide.

A New Adsorption Isotherm for Heterogeneous Sorbents

An adsorption isotherm has been suggested for the intended description of the experimental isotherms for the adsorption of gases and vapours on heterogeneous surfaces. Two parameters of the suggested equation, which characterize the heterogeneity of a sorbent, were expressed in the form of explicit functions of temperature as well as of variance and the excess coefficient for the corresponding energy distribution of adsorption sites. The presence of such explicit expressions allowed the application of the suggested equation for the description of the dependence of the adsorption not only on pressure but also on temperature. The adsorption of N2 on TiO2 was considered as an example. It was shown that the suggested equation could be used as the thermal equation of state for a real adsorption system over quite wide ranges of temperature and pressure.

The relevance of site energy distribution for the mixed alkali effect

The mixed alkali effect (MAE) can be explained as an interplay between network modifications induced by alkali ions of different size and an energy distribution of sites with a preferential occupancy of the low energy sites by the smaller cations. The underlying physical assumptions of this concept are discussed in the present paper in more detail and they are compared with results of Monte Carlo simulations of diffusion and the Anderson–Stuart model. It will be shown that the new concept is actually hidden in the Anderson–Stuart model. Applying the new concept to experimental data has been done so far with a box-type distribution of site energies. Besides the oversimplification, a major drawback of this distribution is that the diffusion coefficient of the smaller and minority cation is underestimated. It will be shown in the present study that by a different choice of distribution the concentration dependence of the diffusivity of the minority cations can be understood. In addition some results in quaternary glasses will be explained by a more general distribution function.

Physical and chemical properties of soil influence the sorption of the diketonitrile metabolite of RPA 201772

RPA 201772 is a preemergence herbicide that undergoes rapid conversion to a diketonitrile metabolite (DKN) in soil. The half-life of RPA 201772 is very short, but the half-life of DKN is much longer; hence, DKN remains for an extended time in soil. Sorption studies were conducted with five soils varying in physical and chemical properties using the batch equilibration technique. Analysis of 14C-ring–labeled DKN was performed using liquid scintillation counting, and sorption data were fitted to the Freundlich model. Isotherms of DKN were nonlinear in all the soils as depicted by the Freundlich exponent (n &lt; 1.0), indicating differential distribution of site energies for sorption. Multiple regression of the sorption constants against selected soil properties indicated that soil organic matter content was the best single determining factor of DKN sorption (r2 = 0.961) followed by soil pH (r2 = 0.947). The Freundlich sorption coefficient (KF) decreased in the following order Chelsea, MI &gt; Amherst, MA &gt; Moorhead, MN &gt; East Monroe, CO &gt; South Deerfield, MA. The organic matter content of the soils decreased in the same order. Clay content had a minimal effect on the sorption of DKN, whereas the sorption of DKN increased with an increase in organic matter content and a decrease in soil pH. There was an increase in the sorption of DKN with an increase in Ca2+ concentration of the soil solution, whereas the net sorption constant (Kd) was correlated to the organic matter content of the soils. The site energy distribution of DKN sorption was governed mainly by the organic matter content of the soils.

Simulation of photodesorption at the Photon Factory

A model of photodesorption which takes place in the electron storage ring and experimental apparatus in a photon beam line is presented. The model is constructed, based on the kinetics of adsorption, thermal desorption, photodesorption and the evacuation of gas phase molecules, taking account of the energy distribution of adsorption sites and the photon flux distribution on the chamber wall. The calculated results are compared with the experimental photodesorption yield obtained at the Photon Factory. The calculated decay curve of the photodesorption yield against the photon dose agrees well with the experimental ones.

ＭＡの応用 単体パラディウムのＢＭＡとその水素吸蔵特性

Nano-structured functional materials are expected to have superior properties to the coarse-grained, bulk materials. In the present paper, pure palladium is refined to nano-sized state to investigate the hydrogen absorption and desorption behavior. Using the bulk mechanical alloying, the palladium platelets can be successfully refined to nano-sized grains with the order of 10nm. In comparison of pressure composition isotherms between nano-sized and bulk palladium, the phase boundary of the Pd-H miscibility gap is significantly narrower for nano-Pd. While the hydrogen solubility at α (α+β) phase boundary is much increased, that at β/(α+β) phase boundary is drastically reduced by nano-structuring. This might be because of the broadening of the site energy distribution, induced by the intrinsic strains constraining the nano-grains. The broadening mechanism of site energy distribution has direct influence on the hydrogen absorption and desorption processes. The Curie temperature for hydrogen absorption and desorption is drastically reduced by 135K through the nanostructuring. The above precise discussion helps us to make nano-structured material design in the hydrogen storage alloys and compounds.

Temperature programmed desorption of toluene, p-xylene, mesitylene and naphthalene on mesoporous high silica MCM-41 for characterizing its surface properties and measuring heats of adsorption

Temperature programmed desorption (TPD) of aromatic hydrocarbons (viz. toluene, p-xylene, mesitylene and naphthalene) on mesoporous high silica MCM-41 from 323 to 673 K at different linear heating rates (5, 10, 15 and 25 K min −1) has been investigated. TPD of toluene at different adsorbate loading (0.5 to 16.3 μmol g −1) has also been investigated. All the TPD curves have a single asymmetric peak. The heats of adsorption of aromatic hydrocarbons on the mesoporous material, estimated from the TPD peak maximum temperatures measured at different heating rates, was found to occur in the following order: toluene< p-xylene<mesitylene<naphthalene. This Δ H a increases with the decrease in the ionization potential of the hydrocarbons. The adsorption results from weak interactions between the π-electrons of aromatic hydrocarbons and the terminal poorly acidic silanol (Si–OH) groups. In the case of toluene TPD, the TPD peak maximum temperature increases with decreasing the adsorbate loading, indicating the presence of site energy distribution on the high silica MCM-41.

Comparative Study by Infrared Spectroscopy and Microcalorimetry of the CO Adsorption over Supported Palladium Catalysts

The adsorption of CO on Al2O3, ZrO2, ZrO2−SiO2, and ZrO2−La2O3 supported Pd catalysts was studied by adsorption microcalorimetry and infrared (IR) spectroscopy. Some interesting and new correlations between the results of microcalorimetry and IR spectroscopy have been found. The CO is adsorbed on palladium catalysts in three different modes: multibonded (3-fold), bridged (2-fold), both on Pd (111) and (100) planes, and linear (1-fold) adsorbed species. The corresponding differential adsorption heats lie in the field of high (210−170 kJ/mol), medium (140−120 kJ/mol), and low (95−60 kJ/mol) values, respectively. The nature of the support, the reduction temperature, and the pretreatment conditions affect the surface structure of the Pd catalysts, resulting in variations in the site energy distribution, i.e., changes in the fraction of sites adsorbing CO with specific heats of adsorption. Moreover, the CeO2 promoter addition weakens the adsorption strength of CO on palladium. Based on the exposed results, a ...

The mixed alkali effect as a consequence of network density and site energy distribution

By reviewing experimental findings on ion diffusion and electrical conductance in binary and ternary glasses it will be shown that three effects determine ion mobility. These are (i) the direct Coulomb interaction between alkali cations and non-bridging oxygen anions, (ii) a broad distribution of cation site energies arising from structural and/or Coulomb disorder and (iii) the network packing density as expressed by the molar volume per mole of oxygen. In ternary glasses with two network modifying oxides the site energy distribution leads to a competition for the low energy sites. By assuming that smaller cations succeed because they come closest to anions, their activation energies for diffusion have to be larger on the average. Thus substituting cations by bigger cations reduces the mobility of the smaller ones. The diffusivity of bigger cations is reduced as well, because they have to move through a network with a mesh size being reduced by the presence of the smaller cations. Describing these counteracting effects with two reasonable parameters allows one to explain the mixed alkali effect quantitatively for a variety of glasses. From a simple estimation of elastic energies one of the parameters can be calculated from the bulk modulus and, therefore, only one parameter which is the width of an energy distribution is left as a free fitting parameter to explain a large body of experimental results.

Adsorption of CO2 on Glass Fibers

Isotherms of adsorption of CO2 on two different glass fibers are measured at 158 and 185 K by the volumetric method. Heats of adsorption are determined. Experimental data are analyzed in terms of the Langmuir model of independent adsorption sites. Distributions of adsorption sites in free energy (not in energy) are determined at two temperatures, and their temperature dependence is discussed. This dependence makes it possible to obtain the entropy and energy distribution of adsorption sites and the relation of entropy to energy for a site. The latter enables one to assess the reliability of the distributions.

A BPP(Bloembergen-Purcell-Pound) model for nuclear spin relaxation dueto diffusion in disordered systems: combined barrier- andsite-energy disorder

Nuclear spin relaxation rates due to magnetic interactions betweendiffusing spins in ordered systems are often interpreted in terms ofan exponential correlation function (the BPP model). The commonextension of this to disordered systems is to average the relaxationrate for a jump rate with energy E over a distribution N(E) ofenergies in the disordered system. A more rigorous extension of theBPP model to disordered systems is described which incorporatessimultaneous barrier- and site-energy distributions. An efficientcomputational scheme is developed to evaluate the triple integralsrequired. Results are presented for the correlation functions andrelaxation rates for some typical values of parameters for both ofthe above extensions of the BPP model to disordered systems. Thereare significant differences between the results for the two models.The model presented here provides a means of analysing experimentalrelaxation data to deduce information about both barrier- andsite-energy distributions.

Comparison of isochronal and isothermal decays of Bragg gratings written through continuous-wave exposure of an unloaded germanosilicate fiber

We compare the results of thermally induced isochronal and isothermal decays of fiber Bragg gratings written through cw exposure of an unloaded germanosilicate fiber. We show that isochronal step decays can be used to predict isothermal decays, provided that some corrections are carried out to take into account a reversible change in grating reflectivity induced by the increase of the fiber temperature. The isochronal accelerated-aging method enables one to sample most of the initial distribution of trapped site energies in a fairly short time. Taking advantage of this property of the method, we show that the initial distribution for a weak grating is similar to those for stronger gratings. The consequences of this observation are discussed within the framework of the various reaction pathway model.

Hole Transport in Poly(p-Phenylene Vinylene)

The influence of side chains on hole transport in poly(p-phenylene vinylene) is examined as a function of temperature T and electrical field E by means of current–voltage experiments, and impedance spectroscopy which probes the transit time of injected carriers. The data are analyzed using a model for hopping in a Gaussian site-energy distribution. Energetic disorder predominantly governs the conductive properties of the PPV derivatives.

Sorption and desorption of the diketonitrile metabolite of isoxaflutole in soils

Isoxaflutole is a new pre-emergence corn herbicide that undergoes rapid conversion to a diketonitrile derivative (DKN) in soils. Sorption–desorption studies were conducted in five different soils varying in physical and chemical properties. A batch equilibration technique was used with total initial aqueous solution concentrations of DKN at 0.25, 0.75, 2.0, 8.0, 25, 75, 150, and 250 mg l −1. After the sorption process, two subsequent desorptions were conducted with an equilibration period of 7 days. A high correlation existed between the desorption coefficient, K Fd and the organic matter content of soils ( r 2=0.844 for the first desorption and r 2=0.861 for the second desorption), while the clay content did not greatly influence the desorption of DKN. Although the sorption of DKN was generally reversible, a sorption–desorption hysteresis was apparent in all soils. The site energy distribution curves emphasized the fact that DKN binds tightly to soils with higher organic matter content and greater proportion of DKN was retained by those soils

Hysteresis in a dipolar Ising model

Zero-temperature Monte Carlo simulations are performed to study the magnetic hysteresis in a three-dimensional model with only dipolar interactions among the Ising spins located inside a sphere on a cubic lattice. Relevant differences are found in the site energy distributions between the up and down spins during the magnetization process. Variations of the dipolar energy and the density of incomplete columns are also discussed.

Effects of Heat Treatment, Pre-Strain and Magnetic Field on the Formation of &amp;alpha;&amp;prime; Martensite in Fe&amp;ndash;25.5Ni&amp;ndash;4Cr and 304L Steels

The effects of solution heat treatment temperature, cooling rate from this temperature, pre-strain and re-heating on α' martensite formation were examined in Fe-25.5Ni-4Cr steel and 304L steels. The distribution of the potential energy of nucleation sites for α' martensite was also estimated using results obtained in magnetic fields of up to 20.70 MA/m. In Fe-25.5Ni-4Cr steel, solution heat treatment temperature and cooling rate from this temperature had almost no effect on the amount of α' martensite formed in magnetic fields over about 8 MA/m. In contrast, the amount of α' martensite formed in magnetic fields lower than about 8 MA/m increased as solution heat treatment temperature increased and as cooling rate decreased. This cooling rate effect in Fe-25.5Ni-4Cr steel is opposite to that in 304L steel, and the reason for this remains to be clarified. In Fe-25.5Ni-4Cr steel, re-heating almost did not affect the amount of α' martensite. From this result, it was expected that internal strain or lattice defects generated by water-quenching almost do not act as nucleation sites in this steel. In Fe-25.5Ni-4Cr steel, pre-strain of up to about 10% suppressed the formation of α' martensite and pre-strain over 10% enhanced the formation of α' martensite. Such a change of the amount of α' martensite with pre-strain was reduced as the magnetic field increased. In Fe-25.5Ni-4Cr steel, it was predicted that specimens that were solution heat treated at higher temperatures would have two peaks on the curve of the potential energy distribution of nucleation sites and that the small peak with a high potential energy would he easily annihilated by a small pre-strain. However this peak was not thought to be annihilated by re-heating. On the other hand, the large peak was expected to increase at larger pre-strain. This enhancing effect of pre-strain was not observed in 304L steel even in high magnetic fields. This supported the view that pre-strain suppresses the formation of α' martensite not through work hardening but through the annihilation of nucleation sites. In contrast, in Fe-25.5Ni-4Cr steel, some strain concentration induced by a large pre-strain was expected to generate nucleation sites. As mentioned above, clear differences were revealed in the effects of heat treatments and pre-strain on the formation of α' martensite between Fe-25.5Ni-4Cr steel and 304L steel. This shows that the nature of the nucleation sites and the procedure for the formation of α' martensite differ between these two kinds of steels.

Hydrogen absorption in Ti45Zr38-xNi17+x quasicrystals and measurements of the equilibrium vapor pressure

To evaluate their technological usefulness and to probe the local structure of the quasicrystal, p-c-T curves were measured for as-quenched Ti45Zr38Ni17 and Ti45Zr30Ni25 quasicrystal ribbons at 350°C with a computer-controlled apparatus. For both quasicrystals, the p-c-T curves do not exhibit a clear pressure plateau. Instead, the equilibrium vapor pressure remains low (<5 torr) below H/M ≈1; it increases sharply for increasing H/M. Such p-c-T curves may reflect structural features of the quasicrystal. Due to the formation of the crystal hydride phase during the p-c-T measurements, however, the site energy distribution in the quasicrystals could not be obtained. Instead, qualitative conclusions are presented. For Ti45Zr38Ni17 quasicrystals, estimates of the site energy distribution for hydrogen suggest a prominent site located between -0.2 eV and -0.15 eV with a possible broad, weakly binding second peak at higher energy. The site energy locations for hydrogen for Ti45Zr30Ni25 quasicrystals are similar. The Ti45Zr38Ni17 quasicrystals desorb most of the absorbed hydrogen at 650°C in one hour by continuous pumping without phase transformation, and do not become powder after three cycles.

Sorption hysteresis of pyrene on zeolite

Systematic studies were performed to investigate the conditions and the extent of irreversible sorption and sorption hysteresis of a hydrophobic compound on a mineral surface. Adsorption and desorption isotherms of pyrene on zeolite were established. Various solvents were used, such as surfactant solution, 2-propanol, 2-propanol/water (70/30) or a microemulsion. Different adsorption and desorption isotherms were found for pyrene physisorbed on the mineral surface. Dependence of sorption on the adsorbent particle density was observed. Kinetic experiments were carried out in order to determine desorption rates. The curves obtained by fluorescence spectroscopy were fitted to the Henry model (monoexponential fit) which can be obtained from the Langmuir kinetics and to modified models (biexponential fit). The latter fitted the curves perfectly. The dynamics of the equilibrated system were studied by radio-tracer experiments. After the preceding desorption process a portion of pyrene containing solution was replaced by an equal amount of radio-labelled pyrene solution of the same concentration. No exchange could be detected on the time scale of the desorption experiments. The results can be explained in terms of metastable equilibrium. They suggest energy distribution of adsorption sites and high activation energies for at least one of the processes (adsorption or desorption).