Pore Size Uniformity Research Articles

Properties of nanostructures obtained by anodization of aluminum in phosphoric acid at moderate potentials

The influence of the process duration, anodizing potential and methanol addition on the structural features of porous anodic alumina formed in a 0.3 M H3PO4 solutions by twostep self-organized anodizing was investigated for potentials ranging from 100 to 170 V. The structural features of porous structures including pore diameter and interpore distance were evaluated from FE-SEM top-view images for samples anodized in the presence and absence of methanol. For the highest studied anodizing time and methanol volume fraction, an excellent agreement between experimental values of the interpore distance and theoretical predictions was observed. The pore arrangement regularity was analyzed for various electrolyte compositions and anodizing potentials. It was found that the regularity ratio of porous alumina increases linearly with increasing anodizing potential and time. The addition of methanol improves the quality of nanostructures and especially better uniformity of pore sizes is observed in the presence of the highest studied methanol content.

Research on TiAl Alloy Porous Metal Flow Restrictors

For the restriction application, the uniformity of pore size and connectivity of pore channels are much more important for TiAl restrictor. TiAl porous materials have expansion phenomenon during reaction sintering for the Kirkendall Effect, and the pores were generated by its partial diffusion. In this paper, elemental powders blending and reaction sintering methods were adopted to prepare TiAl porous flow restrictor materials. The influence of particle size, the fraction of Ti and Al powder, pressing pressure, sintering temperature on the pore character of TiAl porous flow restrictor was studied. The results show that the maximum pore size and expansion coefficient increased with the increase of Al powder particle size, the porosity and permeability increased only when the Al powder particle size is larger than 400/+600 mesh. With the decrease of larger particle Al powder fraction, the porosity and permeability decreased, but the maximum pore size shows little change. With the increase of fraction of fine Al powder, the porosity and permeability increased to a certain value then decreased. With the increase of pressing pressure, maximum pore size, porosity and permeability decreased. When the sintering temperature is lower than 800C, the porosity and permeability increased with the increase of sintering temperature. At above 800C, the porosity and permeability decreased with the increase of sintering temperature. [doi:10.2320/matertrans.M2009120] (Received April 6, 2009; Accepted July 23, 2009; Published September 25, 2009)

Efficient photochemical water splitting and organic pollutant degradation by highly ordered TiO2 nanopore arrays

We synthesized highly ordered TiO2 nanopore arrays (TNPs) with controllable pore size and good uniformity by anodization at low temperature in fluorinated dimethyl sulfoxide (DMSO) solution with a post-sonication treatment. TNPs possess excellent separation and transport properties of photo-generated electron/hole pair and hence reveal enhanced photocurrent response and photochemical properties for water splitting and organic compound degradation. The TNPs present maximum photo-conversion efficiency for water splitting of 0.28% under AM1.5 irradiation. This value compares favorably with a maximum photo-conversion efficiency of 0.21% for TiO2 nanotube arrays (TNAs) under the same conditions. When illuminated with 1.0mWcm−2 UV light, the maximum photo-conversion efficiency can be increased to 22% for TNPs, which is 5% higher than TNAs. The kinetic constant of photoelectrocatalytic (PEC) degradation of methyl orange (MO) for TNPs is found to be 1.27 times as high as that for TNAs when biased at 0.5V. The visible light response of TNPs might be further improved by relevant surface modification technology.

Synthesis of titania–silica aerogel-like microspheres by a water-in-oil emulsion method via ambient pressure drying and their photocatalytic properties

Abstract Water-in-oil (W/O) emulsion system of Span 80–Tween 85/ n -heptane– n -butanol/titania and silica sols–formamide-1, 2-epoxypropane was manufactured. Wet titania–silica gel microspheres (TSGMs) were synthesized by the sol–gel process in the W/O emulsion system. Formamide as drying control chemical additive was introduced to improve the pore size uniformity of the wet gels, and 1, 2-epoxypropane was employed to reinforce the wet gel structure. Then the obtained TSGMs were dried directly at ambient pressure condition to prepare titania–silica aerogel-like microspheres (TSAMs). The resultant TSAMs were characterized by scanning electron microscope, transmission electron microscope, X-ray diffraction and Brunauer–Emmett–Teller method. The results indicate that TSAMs possess similar characters of those aerogels prepared through supercritical fluid drying. The TSAMs have mean diameters of around 100 μm, a bulk density of 0.3 g/cm 3 , the specific surface area of 415 m 2 /g and the pore size distribution between 2 and 35 nm with an average pore size of 15 nm. The TSAMs exhibit excellent photocatalytic activity for the degradation of methylene blue which acts as a simulated aqueous organic pollutant in waste water.

Effect of hydrothermal treatment on the structure, stability and acidity of Al containing MCM-41 and MCM-48 synthesised at room temperature

The effect of hydrothermal treatment of the synthesis gel on the structure, hydrothermal and mechanical stabilities and acidity of MCM-41 and MCM-48 aluminosilicates synthesised at room temperature has been investigated by X-ray diffraction, nitrogen adsorption at 77 K and DRIFTS with pyridine as probe molecule. The influence of the Al content and pore size on the structure of the resulting treated Al-MCM-41 materials has also been studied. For all samples improvement of the structural ordering and increase of the pore size, was observed, with pore wall thickness remaining practically unchanged. For Al-MCM-48 an improvement of the pore size uniformity occurs during the treatment. Only a small loss of pore size uniformity occurred for Al-MCM-41 prepared with hexadecyltrimethylammonium bromide, but with samples prepared with tetra and octadecyltrimethylammonium bromide the treatment generated a bimodal pore size distribution. The pore volume increased (17%) in the case of Al-MCM-48 but decreased (5.5–14%) for Al-MCM-41, suggesting a decrease in surface roughness resulting from increase of the degree of condensation of the pore walls. Both treated and untreated samples presented relatively strong Brønsted sites and increase of the Lewis acidity was found to occur upon treatment. Treated samples were found to be more resistant to refluxing in boiling water and mechanical compaction, which was attributed to more polymerised pore walls, with Al-MCM-41 samples tested demonstrating higher stability than Al-MCM-48. However, the differences in stability of samples prepared with or without hydrothermal treatment were not significant. Both treated and untreated samples presented high hydrothermal stability. Although refluxing in boiling water lead to some loss of structural ordering, only a small decrease of pore volume (3–5.5% for Al-MCM-41 and 8-14% for Al-MCM-48) occurred, with practically no alterations in pore size and wall thickness. Ordered mesopore structure, with narrower pores and thicker walls, was still observed after compression at 590 MPa for most of the samples tested.

Photoprocesses in Mesoporous Silicas Prepared by a Supramolecular Templating Approach

AbstractFor Abstract see ChemInform Abstract in Full Text.

Noncircular pores on the surface of asymmetric polymer membranes: evidence of pore formation via spinodal demixing

To investigate the origin of noncircular pores in the selective layer of asymmetric poly(vinylidene fluoride) (PVDF), membranes cast by immersion precipitation, the mass transfer kinetics were controlled by casting membranes at different temperatures and casting solution concentrations. Interconnected surface pore morphologies were observed by scanning electron microscopy (SEM) for membranes cast at high concentration and/or temperature, supporting a mechanism of liquid–liquid demixing at the membrane surface by spinodal decomposition. The coarsening of this highly porous, interconnected morphology leads first to chain-like formations of discreet, noncircular pores and ultimately to scattered pores of circular shape and large size dispersity. Similar results for poly(sulfone) (PSF), membranes, which solidify via vitrification rather than crystallization, suggest a degree of universality in the mechanism of morphological development of asymmetric membrane surfaces. This new understanding of pore formation kinetics opens the possibility of tailoring membrane surfaces to achieve a high degree of porosity and pore size uniformity.

Preparation of the Porous Hydroxyapatite Ceramics by a Stereo-Mold Method

To improve the uniformity of pore size and its connectivity in porous hydroxyapatite (HA) ceramics, a new stereo-mold method was developed. The raw HA powder was prepared from the precipitation reaction between Ca(NO3)(2) and (NH4)(2)HPO4, then geleasting process was adapted to mold the HA slurry, for which a stereo-mold was designed and prepared by mechanical way using PE thread (0.33mm) to form the internal networks. During the sintering, the densification was mainly completed below 1150degreesC. In this temperature, the porous ceramics with very uniform pore size (200-300 mum) and high porosity (60-70%) was obtained.

Fabrication of titanium-nickel-copper shape memory alloy powders by ball milling

In this study, porous Ti–50.8 at.% Ni shape memory alloys (SMAs) with tailored pore characteristics and porosities as well as adjustable mechanical properties were successfully prepared by using a novel pore-forming technique and powder sintering process. The results show that the use of the sieved NH4HCO3 particle (i.e., the space-holder) greatly facilitates the fabrication of porous NiTi alloys with more satisfactory pore features, improved pore size-uniformity and distribution, and thus significantly increased mechanical properties (including excellent linear superelasticity) than that using no-sieved NH4HCO3 powder. It is also shown that pore size of the space-holder engineered porous NiTi alloys increases correspondingly with the size of sieved NH4HCO3 particles added, and there exists a geometrical heredity effect of space-holder NH4HCO3 particles on pore shape and size of the porous NiTi alloys.

Preparation of Aluminum-Containing Mesoporous Silica Films

Aluminum-containing mesoporous silica films were synthesized by the solvent evaporation method using tetraethoxysilane, aluminum tri(sec-butoxide), and octadecyltrimethylammonium chloride as the silica source, the alumina source, and a supramolecular template, respectively. The Al/Si ratios were 0.004−0.031 in the present synthesis. The resulting films with the Al/Si ratios of 0.004, 0.013, and 0.022 were highly transparent in the wavelength region of 200−800 nm. The porous structures of the aluminum-containing films were characterized by X-ray diffraction, transmission electron microscopy, and nitrogen adsorption. Although the hexagonal pore arrangements were disordered to some extent by the addition of aluminum, the pore size uniformity was confirmed by the nitrogen adsorption isotherms. The cation exchange ability was evaluated by the adsorption of a cationic azobenzene derivative, p-(ω-dimethyl-hydroxyethylammonioethoxy)-azobenzene bromide, which was not adsorbed effectively on a siliceous mesoporous ...

Sequential Hydroboration–Alcoholysis and Epoxidation–Ring Opening Reactions of Vinyl Groups in Mesoporous Vinylsilica

We report the sequential transformation of vinyl groups into hydroborate and alcohol as well as vinyl into epoxide and diol functional groups in hexagonal mesoporous vinylsilica materials, denoted meso-vinyl-SiO2. The first transformation proceeds quantitatively through the hydroborylated derivative. After appropriate quenching, the hydroborylated materials are stable at ambient conditions and can undergo transformation into alcohols and various other functional groups. The pore volume and pore size uniformity were found not to be greatly affected by quenching of the hydroboranes with methanol, but they were reduced by quenching with water due to the deposition of boron-containing species in the channels. Complete conversion of hydroborylated materials to alcohol-functionalized materials required basic conditions and treatment time of several hours. Although this led to a significant structural shrinkage, decrease in pore volume, and decrease in ordering, there was no evidence of a partial collapse or removal of substantial parts of the pore walls under optimized synthesis conditions. This is the first successful conversion of organic groups of a functionalized ordered mesoporous silica host in alkaline solution, conditions known to be detrimental for silica frameworks. Epoxidation of the vinyl groups and subsequent conversion of the resulting epoxides into diols are also briefly described. The chemical transformation through epoxidation affords ordered mesoporous silica materials functionalized with potentially chiral organic groups, which could find applications in asymmetric catalysis and chiral separations. It was found that the epoxidation was slower than hydroboration and resulted in a lower degree of conversion. These two examples of hydroboration–alcoholysis and epoxidation–ring opening reactions of terminally bonded vinyl groups in meso-vinyl-SiO2 demonstrate the novel concept of sequential organic chemical transformations harbored inside the ordered channels of mesoporous organosilica materials.

Post-synthesis treatment of acid-made mesoporous silica materials by ammonia hydrothermal process

Ammonia hydrothermal treatment provides a novel and convenient method for refining the structural order and pore size uniformity of the acid-made mesoporous silica synthesized from different quaternary ammonium surfactants and acid sources. The post-synthesis treatment provokes transformation of silica wall–surfactant interactions within the mesostructure from the weaker hydrogen bonding (S+X−I0) to the stronger electrostatic (S+I−) interactions. The morphology of the mesoporous silica material remains unchanged after the ammonia hydrothermal treatment at 373 K. Such treatment resulted in an increase in both thermal and hydrothermal stability of the mesostructure. Whereas at higher treatment temperatures (>373 K), a marked increase in lattice and pore size, as well as an extrusion of silica nanotubes from both sides of the silica ropes, was observed. The extent of such silica nanotubes extrusion and lattice parameters is closely related to the hydrothermal conditions, such as temperature, ammonia concentration and aging time.

Silicon membrane nanofilters from sacrificial oxide removal

Silicon micropore filter designs using a sacrificial oxide removal technique are described. These filters utilize surface and bulk micromachining for precise control of pore sizes in the tens of nanometers range. The semipermeable membrane of the sacrificial layer filters (SLFs) is typically composed of sandwiched p/sup +/ polysilicon/oxide/p/sup +/ silicon layers where the sacrificial oxide between the two silicon layers determines the pore size of the filter. The purpose of this paper is to address special design and fabrication considerations for control of pore size uniformity within 10%, adapting surface conditions for filtration fluxes of deionized water to fall in the range of 1 ml/cm/sup 2/h, and maximization of the structural response of SLFs. Fluorescent beads are used to analyze the pore size uniformity of fabricated filters, with achievements of four-log reduction in fluorescent bead concentration.

Efficient post-synthesis alumination of MCM-41 using aluminium chlorohydrate containing Al polycations

Aluminium chlorohydrate (ACH) which contains Al polycations such as the Al 13 7+ Keggin ion is an efficient source of Al for post-synthesis alumination of purely siliceous MCM-41 resulting in materials (herein designated AlMCM) with excellent structural integrity and enhanced acidity and catalytic activity. The amount of Al incorporated into the MCM-41 framework is dependent on the concentration of Al in the grafting solution; 27 Al MAS NMR confirms that a large proportion of the Al is inserted into tetrahedral positions within the framework. TEM and XPS indicate that there are no separate surface alumina phases. Alumination results in modest increases in the d 100 basal spacing. This is accompanied by much larger reductions in pore size with the result that the pore wall thickness increases with Al content. However the alumination has little or no effect on the long range structural ordering or pore size uniformity. Alumination generates Bronsted acid sites which increase in population as the Al content rises. The ACH-grafted AlMCM materials exhibit considerable catalytic activity for cumene cracking and are superior to AlCl 3 -grafted MCM-41 or ACH-grafted amorphous silica.

Monitoring of the structure of siliceous mesoporous molecular sieves tailored using different synthesis conditions

The hexagonally ordered siliceous mesoporous molecular sieve MCM-41 and the disordered mesoporous molecular sieve designated as KIT-1 were studied by means of the X-ray diffraction (XRD), transmission electron microscopy and low temperature nitrogen adsorption over a wide pressure range. It was shown that the combination of these techniques allows for a thorough characterization of novel mesoporous materials. A repeated pH adjustment happens to be crucial in the synthesis of MCM-41 samples of high pore size uniformity. An additional influence of the salt effect (i.e. the addition of Na 4EDTA during the hydrothermal crystallization) allows one to obtain MCM-41 materials of excellent porous structures and remarkably high thermal and hydrothermal stability. The latter features may be a result of the presence of relatively thick pore walls for those mesoporous molecular sieves. A disordered character of KIT-1, which was synthesized with an addition of Na 4EDTA to the initial synthesis gel, was supported by the fact that the sample exhibits a high pore size uniformity but yet a rather featureless XRD pattern. It was shown that comparative methods, such as the α s-plot, are very useful for analysis of mesoporous molecular sieves, allowing for an accurate calculation of the specific surface area of the samples, which exhibit condensation in mesopores in the pressure range used in the standard BET method. A method to calculate the pore diameter for MCM-41 samples from XRD and adsorption data was shown to be convenient and useful for an accurate pore size assessment.

Characterization of Large-Pore MCM-41 Molecular Sieves Obtained via Hydrothermal Restructuring

MCM-41 periodic mesoporous silicates were synthesized using cetyltrimethylammonium bromide, and their pore sizes were tailored by postsynthesis hydrothermal treatment method of Khushalani et al., allowing us to obtain large-pore MCM-41 samples with a high degree of structural ordering. It was shown that the pore size enlargement was accompanied by a significant improvement of pore size uniformity, a gradual decrease of the specific surface area, and the pore wall thickening. After a certain upper limit of pore size (i.e., about 6.5 nm) was reached, a further hydrothermal treatment led to samples of diminished quality. The structural uniformity decreases dramatically, i.e., the pore size distribution becomes broader and a small but noticeable amount of micropores develops. Moreover, the appearance of pore blocking effects (as inferred from the irreversibility of nitrogen adsorption−desorption isotherms) indicates that the pore geometry deviates significantly from its initial cylindrical shape characteristi...

Structural and surface properties of siliceous and titanium-modified HMS molecular sieves

Siliceous and titanium-modified HMS molecular sieves were synthesized and characterized by means of X-ray diffraction, atomic absorption, diffuse reflectance UV visible spectroscopy and nitrogen adsorption at 77 K in a wide range of relative pressures. It was shown that the introduction of relatively small amounts of titanium does not lead to appreciable changes in the structure of the synthesized HMS materials. However, for higher titanium loadings, a significant decrease in the primary mesopore volume, the specific surface area and the pore size uniformity was observed. High pressure adsorption behavior of the HMS samples was compared to the adsorption properties of previously studied MCM-41 materials and the differences were attributed to differences in size and possibly shape of their particles. The primary mesopore sizes for the HMS materials were estimated by means of several methods and the results were compared and discussed. Low pressure adsorption behavior of the samples was also studied and it was shown that the incorporation of titanium in the HMS materials leads to a small but noticeable enhancement of nitrogen adsorption in the lowest pressure region. The nitrogen adsorption energy distributions were calculated and found to be similar for all samples. However, a small increase in the adsorption energy with increasing titanium content in the HMS samples was observed.

Image analysis for estimating ultimate oil recovery efficiency by waterflooding for two sandstone reservoirs

The objectives of this study are to consider the influence of pore structures obtained from image analysis on ultimate oil recovery and to incorporate the results of these analyses into generalized stochastic models for waterflooding performance predictions. Sandstone samples from a North American and a North Sea oil reservoir were used. The pore systems of the two reservoirs were quantified by image analysis of thin sections and a pattern recognition algorithm was used to classify the porosity objectively into pore types using size and shape information. The thin sections used in the image analysis were obtained from the ends of the core plugs used in the laboratory experiments to obtain recovery efficiency. Regression analysis was used to develop stochastic models to estimate recovery efficiency. The analyses of data indicate that increasing volume of well-connected pores and increase in pore size uniformity improve recovery efficiency. The models explain 72% and 88% of the variance in measured recovery efficiencies for samples with similar laboratory test procedures. The expected deviations between estimated and measured recovery efficiencies are 6.2% and 2.3%, respectively, for the two reservoirs. The analyses also indicate that relationships between initial water saturation and residual oil saturation are related to pore size distributions. Residual oil saturation correlates positively with initial water saturation and a direct relationship exists with degree of non-uniformity of the associated pore systems. The porosity classification method used in this study is shown to be useful for understanding the effects of pore structure on oil recovery by waterflooding under conditions of constant fluid properties and rock wettability. This technique could be used to evaluate recovery efficiency of large numbers of samples including samples too small for conventional core testing.

Porous plastic optical fiber sensor for ammonia measurement

A porous plastic optical fiber has been developed for use in chemical gas sensing. This porous plastic waveguide, which was made with copolymer materials, has an interconnective porous structure as well as uniformity of pore size. These sensors are based on in-line optical absorption within the porous plastic fiber core and have much greater sensitivities than sensors based on evanescent coupling to a surrounding medium. Furthermore, this fiber simultaneously exhibits very high gas permeability and liquid impermeability. This combination makes the fiber particularly suitable for gas concentration measurements in aqueous samples. An ammonia gas sensor has been tested to demonstrate the effectiveness of this porous plastic waveguide.

Porous structure and surface area of oxide, reduced and sulphided forms of alumina-supported cobalt molybdena catalyst

The porous structure and surface area of alumina-supported cobalt molybdena catalysts were examined as functions of their activation conditions. It was revealed that the process of reduction by hydrogen as well as simultaneous reduction — sulphiding by a hydrogen and hydrogen sulphide mixture — results in the disappearance of pores of larger diameters. This is caused by the formation of monomeric, dimeric and/or polymeric species on pore surfaces as a result of thermal activation in hydrogen and H 2-H 2S atmospheres. On the basis of the shapes of the hysteresis loops of nitrogen adsorption-desorption isotherms and on t plots of the samples investigated it was found that the dominant pores are those with diameters in the mesopore range. The shapes of the mesopores resemble tubular capillaries with diameters o of various size and/or tubular capillaries with short necks and wide sloping bodies. Some of the mesopores can be closed at one end. The uniformity of pore sizes appearing after the H 2-H 2S treatment provides good conditions for the diffusion of reactant molecules of large diameter. This is especially important for such catalytic reactions as depolymerization of asphaltenes, hydrodemetallization of metal porphyrins and other hydrotreatment processes.