Decrease In BET Surface Area Research Articles

Size Effects of Exchange Cation on the Pore Structure and Surface Fractality of Montmorillonite

Ca-montmorillonites were exchanged with both metal cations (manganese and copper) and organic cations (tetramethylammonium (TMA) and hexadecyltrimethylammonium (HDTMA)) to study the cation size effects on the pore structure and surface roughness of montmorillonite based on the classical and fractal analysis of their nitrogen adsorption isotherms. The surface fractal dimension D was calculated from their nitrogen isotherms with the aid of the recently proposed Neimark equation. The decrease of BET surface area of montmorillonite induced by the larger size of exchange cation was interpreted with both the coverture of some surface roughness (surface screening effect) and the inhibition of nitrogen molecule into some pores (pore blocking effect). The pore blocking effect was examined with the changes of mean pore size. Meanwhile, the D value was used to examine whether or not the surface screening effect existed. It was concluded that the combination of classical and fractal analyses of nitrogen isotherms may facilitate our understanding of the evolution of pore and surface structures of clay exchanged with different cations.

1,2-Diolates of titanium as suitable precursors for the preparation of photoactive high surface titania

1,2-Diolates of titanium of stoichiometric formula [Ti(OCHRCH 2O) 2] (R = −H, −CH 3, −CH 2CH 3) were prepared by reacting a titanium source (titanium(IV) isopropoxide (TIP) or titanium(IV) chloride) with an excess of the respective 1,2-diol. The diol was employed in a great excess; no other solvent was present in the synthesis procedure. The titanium diolates obtained have a white powder appearance and show a high degree of crystallinity as deduced from powder X-ray diffraction. It has been observed that their stability towards moisture is determined by the R group (R = −H, −CH 3, −CH 2CH 3). Hydrolysis of these titanium 1,2-diolates in water was a simple way to produce high surface porous titania with BET surface area up to 300 m 2 g −1. These as-synthesised titania contain anatase crystallites and show photocatalytic activity versus formaldehyde degradation prior to calcination. Nevertheless, the decomposition rate of formaldehyde increased notably when titania samples were annealed at 773 K. This thermal treatment increases their crystallinity at the expense of a BET surface area decrease down to 75 m 2 g −1 or less. The photocatalytic performance of the annealed samples is superior than that of commercial TiO 2 (Degussa P-25, Hombikat UV-100).

Physicochemical surface and catalystic properties of Cr 2O 3/Al 2O 3 system

Solid-solid interactions, surface and catalystic properties of Cr 2O 3/Al 2O 3 system were investigated using TG, DTA, XRD, nitrogen adsorption at −196°C and CO-oxidation by O 2 at 250–400°C. The amount of chromium oxide was varied between 4.3 and 42.7 wt.% Cr 2O 3. The results showed that heating of Al(OH) 3 pretreated with different amounts of CrO 3 at 400–900°C resulted in the formation of a mixture of γ-, k-,θ-aluminas, Cr 2O 5, CrO 2 and α-Cr 2O 3 phases. The rise in calcination temperature to 1000°C made the appearance of minute amounts of α-Al 2O 3 and Cr 3O 4 besides the other phases (α-Cr 2O 3 and θ-, k-Al 2O 3). The BET-surface area decreases by increasing the precalcination temperature in the range 400–1000°C. The mixed oxide adsorbents calcined at 400–600°C contain mainly narrow pores and turned to wide pores upon heating at 700–1000°C. The activation energy of sintering (Δ E s) decreases markedly from 21 to 7.6 kJ mol −1 by increasing the amount of Cr 2O 3 from 4.3 to 8.3 wt.% then increases progressively reaching to 33 kJ mol −1 in the presence of 42.7 wt.% Cr 2O 3. The catalystic activity increases as a function of Cr 2O 3 content. The curves relating the catalystic activity (reaction rate constant per unit surface area) and precalcination temperature exhibited a maximum at 700 or 800°C depending on the amount of Cr 2O 3 present. The rise in both Cr 2O 3 content and calcination temperature did not modify the mechanism of the catalystic reaction but changed the concentration of catalystically active constituents without altering their energetic nature.

In Vitro Behavior of Silica‐Based Xerogels Intended as Controlled Release Carriers

We report on the in vitro behavior of room‐temperature‐prepared xerogels upon immersion in typical physiologic solutions. The effect of various parameters was studied: xerogel composition and particle size; solution composition; and immersion protocol. Three xerogel compositions were tested: a silica xerogel (S100); a Vancomycin–silica xerogel composite (S100V); and a calcium‐ and phosphorus‐containing silica xerogel (S70). The immersion was per‐formed integrally (i.e., without solution exchange) or dif‐ferentially (i.e., with solution exchange) to model the continuous flow of body fluids past implant surfaces. Four different solutions were used: tris(hydroxymethyl)amino‐methane hydrochloric acid buffer solution plus electrolytes (TE); TE supplemented with 3% H2O2; TE supplemented with 10% serum; and serum. Carbonated hydroxyapatite formed on all xerogels. The reaction was faster on S70 than on S100. The silica network fully dissolved for all xerogels in the differential experiment. S70 particles transformed to calcium‐ and phosphorus‐containing “shells” upon differ‐ential immersion. The presence of H2 O2 and serum proteins in the immersion solution slowed down the silicon dissolu‐tion of both xerogels. Xerogel dissolution was associated with decreased BET surface area and increased mean pore size. The data showed a linear relationship between the dissolution rate and the time‐dependent surface area. Re‐gardless of this relationship, our data also suggested that the rate‐limiting step of the dissolution during integral im‐mersion was the diffusion of the dissolved species to the bulk solution.

The effect of sulfur poisoning on methane oxidation over palladium supported on γ-alumina catalysts

The effect of sulfur on deactivation of palladium catalysts for methane oxidation has been investigated in this research. A PdO deactivation mechanism due to sulfur is proposed over the 100–400°C range. H 2S induced catalyst poisoning of PdO/γ–Al 2O 3 is attributed to the formation of aluminum sulfate. Catalyst activity decreases with increasing H 2S deposition and increasing temperature from 100 to 400°C. TGA results show that PdO promotes the reaction of H 2S with γ-Al 2O 3. Surface sulfite and sulfate groups are observed by FT-IR on poisoned catalysts and are believed to be the cause of a 25% decrease in BET surface area. Activation energy for methane oxidation of H 2S poisoned catalyst decreases due to the formation of Al 2(SO 4) 3 which causes the reaction to change from surface reaction control to pore-diffusion control. H 2 treatment at 600°C removes much of the sulfite and sulfate from the surface of poisoned catalysts and regenerates most of the fresh catalyst activity.

The fractal and percolation analysis of a polymeric Al 2O 3 gel

Fractal and percolation analysis have been performed on nitrogen adsorption isotherms of polymeric Al 2O 3 xerogel, prepared from a low-water non-aqueous solution of aluminum alkoxide. These xerogels showed a distinctively low pore connectivity compared to that reported in the literature on more conventional alumina gels. The alumina xerogel exhibited a surface fractal behavior in the scale range from nitrogen size to a few nanometers. The mean pore coordination number and the surface fractal dimension of the xerogel were rather stable upon calcination, until the appearance of the α-alumina crystalline phase, despite of a large decrease in BET surface area and pore volume.

Structural investigation of ormosils with potential as host photonic materials

Class II ormosils in the systems: methyl, propyl, vinyl and phenyl silicates have been structurally investigated for their suitability as photonic materials and this paper is a precis of many data. From visible and near-infrared spectroscopy these ormosils have: (i) intrinsic optical windows in the visible and at 1.06, 1.3 and 1.55 μ m and (ii) low residual water, despite being synthesised at only 60° C. The residual water can be expelled under vacuum. Solid state29 Si nuclear magnetic resonance spectroscopy shows that matrix condensation increases as the organic content of the matrix is increased; this is accompanied by a decrease in BET surface area.

Photocatalytic Degradation of Formic Acid via Metal-Supported Titania

TiO\d2 catalysts supported on stainless-steel plates were tested for the photocatalytic degradation of formic acid in an aqueous solution. The metal-supported TiO\d2 catalysts showed reaction activities comparable to the commonly used glass-supported catalysts. Preirradiation was needed to activate fully both of the supported catalysts and to obtain reproducible activity data. During the photodegradation reaction, the oxidized surface of the metallic support was reduced and bleached. The activity of the catalyst decreased with an increase in firing temperature, which implies a positive correlation of the activity with the decrease in BET surface area of the TiO\d2 catalyst. The apparent reaction rate, \IR\N\Da\N, increased as the thickness of the TiO\d2 film on the support increased. However, the specific reaction rate, \IR\N\Ds\N, calculated per unit of catalyst weight, decreased with an increase in the film thickness for the catalysts fired at temperatures of 300°C or less, and it was nearly constant irrespective of the film thickness for the catalysts fired at temperatures of 400°C or more. The X-ray diffraction patterns of the supported TiO\d2 catalysts that were fired at temperatures up to 600°C showed that all of the TiO\d2 films had only an anatase structure, and the particle size increased as the firing temperature increased.

The role of nickel in formation of iron aluminide: X. Wang, J.V. Wood (University of Nottingham, Nottingham, UK). Powder Metallurgy, vol 38, no 1, 1994, 59–65

The active catalysts for methane formation from the gas mixture of CO2 + 4H2 with almost 100% methane selectivity were prepared by reduction of the oxide mixture of NiO and ZrO2 prepared by calcination of aqueous ZrO2 sol with Sm(NO3)3 and Ni(NO3)2. The 50 at%Ni-50 at%(Zr-Sm oxide) catalyst consisting of 50 at%Ni-50 at%(Zr + Sm) with Zr/Sm = 5 calcined at 650 or 800 °C showed the highest activity for methanation. The active catalysts were Ni supported on tetragonal ZrO2, and the activity for methanation increased by an increase in inclusion of Sm3+ ions substituting Zr4+ ions in the tetragonal ZrO2 lattice as a result of an increase in calcination temperature. However, the increase in calcination temperature decreased BET surface area, metal dispersion and hydrogen uptake due to grain growth. Thus, the optimum calcination temperature existed.

Surface Areas and Porosities of Ices Used to Simulate Stratospheric Clouds

Low temperature ices formed by deposition from the vapor phase are used as laboratory simulations of stratospheric ice clouds. To obtain intrinsic reactivities of these ices, detailed information on their physical structure is required. Surface areas are obtained from BET analysis of gas adsorption isotherms. Bulk densities and porosities are determined photogrammetrically. Scanning electron microscopy is used to obtain particle sizes and to study the morphology of the ices at several temperatures. Water ice and HNO 3-H 2O ice films deposited from the vapor at temperatures below 200 K exhibit large BET surface areas and are highly porous. When these ices are annealed by warming above 200 K, a large decrease in BET surface area occurs without any large change in bulk density or particle size. For the annealed ices, external surface areas calculated from the observed particle sizes agree reasonably well with the BET areas. This is evidence that the annealed ices are composed of nonporous particles and that the porosity of these ices is due to interstices among the particles. For unannealed ices at 200 K, the BET surface areas are almost an order of magnitude higher than the external areas; this indicates that these ices are comprised of particles which have internal porosity. Pore size analysis of the unannealed ices yields internal or pore surface areas. At 200 K, the sum of the external and internal surface areas accounts for only about 30-40% of the BET area and suggests that part of the BET value may be due to pore filling and, thus, cannot be considered a true surface area in this case.

Catalyst deactivation during deep oxidation of chlorohydrocarbons

A commercial chromia-alumina catalyst was investigated for an extended period of time for the oxidation of two gas streams containing volatile organic compounds (VOCs) with compositions similar to those expected from groundwater air strippers. The first gas stream ( chlorinated stream ) contained 500 ppm of C 1 to C 2 chlorohydrocarbons, while the second stream (mixed stream) contained 450 ppm of C 5 to C 9 hydrocarbons plus 50 ppm of trichloroethylene. Catalyst deactivation was studied at constant conversion of the main reaction, achieved by increasing the temperature, when needed, to compensate for the loss of intrinsic catalyst activity. The catalytic activity and selectivity were found to be a function of the type of feed stream and the reactor configuration. Although the catalyst used for the oxidation of the chlorinated stream did not require any temperature increase when used fore 153 days-on-stream in a fixed bed reactor, the results suggest progressive loss in catalyst activity down the catalyst bed. A fluid bed reactor was found to be more effective than a fixed bed reactor in maintaining the catalyst activity for the oxidation of the chlorinated stream. The physical attrition of catalyst particles in the fluid bed reactor accompanied with the loss of chromium, via oxychloride formation, appears to be beneficial in maintaining the catalyst activity by constantly exposing the fresh catalyst surface. The catalyst used for mixed stream oxidation required a temperature increase of 33°C over 210 days-on-stream to maintain constant conversion. This decrease in catalytic activity was related to a decrease in BET surface area.

Effect of calcination temperature of alumina on the adsorption/impregnation of Pd(II) compounds

A series of alumina supports derived from a common source material was subjected to a range of calcination temperatures and the results of this treatment on the adsorption of Pd(II) compounds were studied. Increasing calcination temperature lead to a decrease in BET surface area and produced a series of alumina samples carrying different inventories of hydroxyl groups. The surface chemistry of these hydroxyl groups was studied using mass and potentiometric titration techniques and from these data the protonation/deprotonation equilibrium constants for the surface hydroxyl groups were obtained. This allowed us to construct the pH-dependent surface charge distribution for each alumina sample. Cationic and anionic Pd(II) complexes were used to titrate the negative and positive surface charge sites. We find that the adsorption mechanism is primarily controlled by electrostatic effects in that saturation capacities of adsorbed complex showed a correlation with the surface charge density evaluated at the pH used for the adsorption/impregnation experiments.

The cumulative chemisorption of cyanogen chloride on a carbon-supported catalyst (whetlerite)

The kinetic chemisorption of cyanogen chloride (ClCN) on whetlerite was studied with constant flow parameters. There is a concurrent evolution of CO 2 during the removal of ClCN which is attributed to the reaction of the hydrolysis product, HCl, with the original carbonate content of the whetlerite. Two molecules of ClCN were retained and one molecule of CO 2 was formed. The slowly decreasing appearance of CO 2 after the cut-off of ClCN was attributed to the surface migration of HCl from the reaction site to the carbonate site. A scavenger for HCl added to the original whetlerite provided longer breakthrough times for ClCN. The overall decrease in BET surface area after the challenge is ascribed to the retention of nonvolatile hydrolysis products: cyanic acid (HOCN) and/or the trimer cyanuric acid.

Rate of reduction of nitric oxide with ammonia on coke catalysts activated with sulphuric acid

The rate of reduction of NO on activated coke catalysts was studied in detail as a function of gas composition, temperature and reaction time. The catalyst was activated at 400 °C using sulphuric acid in amounts 100–500 wt% of the catalyst. This procedure increased the N 2 formation rate by a factor of 1.5–2.6 compared with that for the original catalyst. The N 2 formation rate increased with increasing concentration of NO and O 2but was retarded by H 2O. The effect of NH 3 concentration was marginal because the adsorption of NH 3 was much stronger than that of NO. To simulate SO 2 poisoning, the catalyst was impregnated with different amounts of H 2SO 4. The decrease in BET surface area due to salt deposition was probably the main reason for catalyst deactivation.

Structural changes during the leaching of copper-based raney catalysts

Raney copper catalysts were produced by leaching pellets of two alloys containing 50 wt% Cu - 50% Al and 43 wt% Cu - 39 wt% Al - 18 wt% Zn in 20 wt% NaOH solutions. Changes in the surface area and pore structure were measured at temperatures from 1 to 50°C. It was found that prolonged contact of leached porous copper with caustic solution resulted in decreased BET surface area, increased pore diameters and increased copper crystallite sizes whilst the pore volume of the material remained constant. These changes, which have important consequences for Raney catalysts employed in fixed bed operation, occur through a dissolution reprecipitation process.

A surface spectroscopic study of [formula omitted] catalysts using ESCA, ISS, XRD, and Raman spectroscopy, I

Laser Raman spectroscopy, X-ray photoelectron spectroscopy, low-energy ion-scattering spectroscopy, and X-ray diffraction have been used to characterize a series of Coz.sbnd;Mo Al 2O 3 catalysts containing 15 wt% MoO 3 and 0 to 8 wt% CoO in their oxide, reduced, and sulfided forms. These data show that the catalyst surface contains CoMoO 4 and irreducible Co 2+ ions of tetrahedral symmetry when the CoO concentration is 0 to 6%. With 7 to 8% CoO, additional surface species includes Co 3O 4 crystallites on the γ-Al 2O 3 surface. Formation of Co 3O 4 coincides with an increased Mo reducibility and a decreased BET surface area. These results are compared to previously published data on Coz.sbnd;Mo Al 2O 3 and suggest that the state of dehydration-dehydroxylation of the Al 2O 3 surface before impregnation of Co and Mo affects their subsequent speciation. Autoclave studies investigating the hydrodesulfurization (HDS) and hydroconversion of coal using these catalysts are also reported. Significance of the surface speciation with respect to these activity studies is discussed.

Brass formation in a copper/zinc oxide CO shift catalyst

It is shown that thermal treatment of a copper/zinc oxide CO-shift catalyst in a reducing gas causes a decrease in BET surface area as well as partial reduction of zinc oxide followed by the formation of α-brass. The presence of zinc in the copper-rich phase has a very strong negative effect on catalyst activity. The amount of brass formed is in agreement with concentrations predicted from thermodynamic calculations and the rate of diffusion of zinc in the copper crystallites.