Nonporous Alumina Research Articles

Adsorption of salicylate and polyacrylate on mesoporous aluminas

An attempt has been made to assess the involvement of the molecular size of the adsorbate and the porosity of the adsorbent in the adsorption mechanisms of salicylic acid (SA) and polyacrylic acid (PAA) on two aluminas with different porous volume and on a non-porous alumina. This was achieved by adsorption isotherm and electrokinetic measurements. The adsorption mechanisms are different for the pore surface and the external surfaces. Salicylic acid is preferentially adsorbed on the walls of the mesopores with no effect on the charge of the external surface. When adsorption inside the pores is complete, it spreads over the external surface causing a decrease of the ζ potential. Polyacrylic acid cannot enter the mesopores and is adsorbed on the external surfaces; only a low adsorbed quantity of it is required to decrease the ζ potential. The structure of the adsorbed layer is discussed.

Adsorption of poly(methylmethacrylate) on an α alumina: evidence of formation of surface carboxylate bonds

Irreversible fixation of poly(methylmethacrylate) (PMMA) on the surface of non-porous alumina was examined by FTIR, XPS and NMR spectroscopies. The hydrolysis of side ester groups of PMMA is evidenced: PMMA bonds strongly to the alumina surface through ionic carboxylate groups.

Acidity and surface behaviour of alumina-boria catalysts studied by adsorption microcalorimetry of probe molecules

Acidity and basicity of alumina-boria catalysts supported on porous or non-porous alumina have been studied by adsorption microcalorimetry of probe molecules (ammonia, pyridine and sulphur dioxide). Despite decreasing in initial heats, the total acidity as determined by ammonia adsorption increased in number and strength as a function of percentage of boron oxide. Ammonia, as a strong base, was shown to cover all types sites from strong to weak acid sites. Pyridine, as a weaker probe, was shown to dose only the stronger sites of the samples which stay nearly constant after B203 coverage approaching the monolayer. The basic sites of the amphoteric alumina support are neutralized by 10 wt% of boron oxide on non-porous aJumina and 20 wt% of B203 on porous alumina. The catalytic activity for partial oxidation of ethane increased with acidity and reached a maximum constant value above 20 wt% of boron oxide.

FTIR study of bimetallic Pt-Sn/Al 2O 3 catalysts

A series of Pt-Sn/Al 2O 3 catalysts supported on Degussa nonporous alumina were characterized by infrared spectroscopy of adsorbed CO. The nominal platinum loading was kept constant in the catalysts at 1 wt% and the nominal amount of tin was varied from 0 to 5 wt%. At room temperature it was found that there was a general trend for the linearly adsorbed CO band to shift to lower frequencies with increasing amounts of tin in the catalyst, except for one of the catalysts (1.0 Pt-1.0 Sn/Al 2O 3) where a particle size effect seemed to be dominant. The amount of bridge-bonded CO species was very small and became undetectable in the case of large tin loadings (1 and 5 wt% Sn). Thermal desorption infrared spectroscopy was used to determine the vibration frequency of the isolated CO molecule and hence observe if there were any indications of electronic interactions between tin and platinum. For the monometallic 1.0 Pt/Al 2O 3 sample the singleton vibration frequency was 2041 cm −1. For all the Pt-Sn bimetallic samples it was found that the singleton vibration frequency lay within the range 2041 ± 6 cm −1. This suggests that the electronic effect of Sn on Pt in our Pt-Sn/Al 2O 3 catalysts is not very large.

A chemisorption and XPS study of bimetallic Pt-Sn/Al 2O 3 catalysts

Bimetallic Pt-Sn/Al 2O 3 catalysts with nominally 1 wt% Pt and varying tin contents (0–3.25 wt%) prepared by coimpregnation of nonporous Degussa alumina were characterized by chemisorption of H 2, O 2, and CO at room temperature. The surface compositions and oxidation states of the reduced catalysts were tracked by XPS. Addition of tin to Pt resulted in significant differences in the gas uptake characteristics of the three adsorbates. Both H 2 and CO showed an initial increase in gas uptake with addition of small amounts of tin, and then the chemisorbed amount tended to drop off with further addition of tin. In the case of O 2 adsorption, there was a steady increase in gas uptake with increasing tin content. XPS of the reduced catalysts showed that in alumina-supported samples most of the tin was in a valence state of either Sn(II) or Sn(IV). On the other hand, large amounts of zero-valent tin were found in a SiO 2-supported Pt-Sn catalyst which had been prepared from the same precursors and reduced under identical conditions. This supports the notion that interactions between the alumina support and tin prevent the complete reduction of tin.

XPS studies of Pt—Sn naphtha reforming catalysts

Pt—Sn has been supported on a nonporous (110 m 2/g) alumina, a higher surface area (250 m 2/g) porous alumina and a silica (700 m 2/g). Both Sn (O) and Pt (O) have been observed in the reduced samples. Sn(O), and presumably alloy formation, occurs to a greater extent when platinum and tin are added as a complex dissolved in acetone as compared to either co-impregnation or co-precipitation. The ratio of Pt (O) to Sn (O) varies, at constant platinum loading, with the total amount of tin added in the preparation; this Pt:Sn ratio varies from about 0.4 to about 6 for the catalysts utilized in this study.

Alcohol dehydration over model nonporous alumina powder

The reactivity of alumina powders obtained by evaporation of aluminum in Ar/O 2 was investigated. These alumina powders contain single crystals of transitional alumina, 5–60 nm in diameter, with a surface area of 55 m 2/gm. The reactivity of the nonporous alumina was compared with that of Kaiser A-201 γ-alumina (220 m 2/gm) for the decomposition of ethanol, 1-propanol, and t-butanol. The product distributions on both aluminas were similar, but the nonporous alumina was more reactive than γ-alumina per unit surface area. The activation energy for the reaction was lower on the nonporous alumina. In view of these results, it appears that transitional alumina powders prepared by the evaporation method may provide good crystalline analogs to γ-alumina. These could be used as model alumina supports since the simple geometry of the primary particles facilitates examination by high-resolution TEM.

Influence of surface area and particle size of dispersed oxide on conductivities of lithium bromide composite electrolytes

Temperature dependent conductivities are reported for composites of LiBr and of LiBr·H 2O with a range of both non-porous and porous dispersed oxides. Enhanced conductivity is observed for both systems with high surface area aluminas. Porous aluminas with similar surface area to enhancing non-porous aluminas give large enhancements despite their much larger elementary particle size. Conductivity enhancements in LiBr·H 2O composites below the temperature characteristic of the α/sB phase transition of LiBr·H 2O prevent the conductivity loss characteristic of that phase at lower temperatures. No simple trend in conductivity enhancement with oxide mole fraction is transferable between oxides. A non-porous silica of similar surface area to enhancing aluminas does not give the same enhancements.

The behavior of model [formula omitted] catalysts in various chemical environments

The sintering behavior of silver crystallites supported on nonporous alumina substrates has been investigated by means of transmission electron microscopy. When the specimens were heated in O 2 at temperatures in the range 250–400 °C, the following phenomena were observed: crystallite migration and coalescence, disappearance of small and large particles, decrease in size of small crystallites, and appearance of a large number of small crystallites of circular or wormlike shape. A few possible explanations are suggested for the appearance of a large number of small particles. The behavior of silver particles heated in H 2 at temperatures in the range 300–400 °C was qualitatively similar to that observed in O 2. Sintering was, however, less severe, and the number of small particles that appeared was smaller. When heated in C 2H 4, the particles changed their shape from circular to ellipsoidal and, with few exceptions, remained almost unchanged in their locations, most probably as a result of carbon deposition on the surface and around the particles. When heated in a mixture of C 2H 4 and O 2, the particles exhibited milder sintering than in O 2, but acquired more irregular shapes, as during their heating in C 2H 4.

Simulation of the behavior of supported metal catalysts in real reaction atmospheres by means of model catalysts

Model catalysts formed of small crystallites of Ni, Co, or Fe supported on thin, electron transparent films of nonporous alumina were heated in chemical atmospheres having compositions in the range encountered in the steam-reforming reaction. Electron microscopy and electron diffraction were used to investigate the physical and chemical changes that occurred. For a better understanding of the phenomena involved, the effect of the mixture is compared to the effects caused by the individual components and by their combinations. It is shown that, depending upon the nature of the catalyst as well as upon the composition of the mixture, sintering or/and coking can occur much more rapidly in mixtures than in single-component atmospheres. This suggests that a cooperative action of the reaction components is the cause of the early deactivation of the steam-reforming catalysts. Various phenomena such as carbon deposition (as films or patches on the surface of the catalyst and on the substrate or as filaments), deformation of the crystallites, severe sintering, and permanent loss of metal to the gas stream were observed. Explanations are provided for the behavior of the Ni Al 2O 3 , Co Al 2O 3 , and Fe Al 2O 3 catalysts, in various atmospheres, by considering the competition between carbon deposition and its gasification, as well as the strength of the interactions between crystallites and substrate. Two kinds of filaments were observed: carbonaceous filaments and metal oxide filaments which probably contain some carbon. A thermodynamic condition for the formation of filamentous carbon is suggested. If the sum of the interfacial free energies between carbon and crystallite, and carbon and substrate is smaller than that between crystallite and substrate, then carbon could penetrate between crystallites and substrate and filaments would be generated. Indeed, carbonaceous filaments were identified only when the catalyst particles were present as metals. In such cases, the interfacial free energy between the metal crystallite and alumina is much greater than that between the oxidized metal and alumina and the above thermodynamic condition is more likely to be satisfied. Permanent loss of active metal to the gas stream occurred, either because of the disintegration of the crystallites caused by carbon precipitation inside the particles, and/or possibly because of the formation of volatile Carbonyls.

Direct observation by controlled atmosphere electron microscopy of the changes in morphology of molybdenum oxide and sulfide supported on alumina and graphite

The changes in morphology of monolayer quantities of molybdenum oxide and sulfide phases supported on nonporous alumina and graphite thin films were studied by controlled atmosphere electron microscopy in oxygen and in H 2S H 2 gas mixtures. When molybdenum oxide was supported on alumina and heated in oxygen at temperatures up to 825 K, a strong oxide-support interaction was observed, resulting in a highly dispersed molybdenum phase. When supported on graphite, the support interaction was weaker and bulk molybdenum oxide formed that became mobile on the surface at temperatures higher than 930 K. On mixed supports containing both alumina and graphite regions, the mobile molybdenum oxide particles on graphite at 930 K were observed to disappear when they contacted alumina edges due to rapid spreading of the molybdenum oxide over the alumina surface. This behavior can be explained by the relatively low surface energies for MoO 3 and graphite compared to the high surface energy for Al 2O 3. The molybdenum oxide-support interaction on alumina was broken by sulfiding in 5% H 2S H 2 at ca. 750 K, with the formation of crystallites of MoS 2 located preferentially at grain boundaries on alumina. Larger particles were obtained at higher sulfidation temperatures. Reoxidation at temperatures higher than 645 K redispersed the crystallites due to spreading of molybdenum oxide over alumina. On the graphite support, a MoS 2 “rag phase” was formed from the MoO 3 crystallites upon treatment in 5% H 2S H 2 at 475 K, and molybdenum species did not spread over the graphite surface during reoxidation at temperatures up to 1100 K. Finally, sulfidation and reoxidation of a cobalt-promoted molybdenum/ alumina specimen showed behavior similar to that observed for the unpromoted samples.

Studies of the structure of molybdenum oxide and sulfide supported on thin films of alumina

The morphology of monolayer quantities of molybdenum oxide and sulfide phases supported on thin films of nonporous alumina was studied by X-ray photoelectron spectroscopy and conventional and scanning transmission electron microscopy. Following oxidation at 770 K, a strong oxide-support interaction was observed, resulting in a highly dispersed molybdenum phase for loadings below ca. 5 Mo atoms nm−2. The molybdenum oxide-support interaction was broken by sulfiding in H2SH2 at ca. 770 K, with the formation of faceted single crystallites of MoS2. The MoS2 crystallites were present as hexagonally shaped slabs with one truncated edge and they were bonded with their high-energy edge planes to the alumina surface. The edge planes contacting the gas phase were the 1010 set of planes, while the truncated edge bonded to the surface of alumina was the 2110 plane. After 910 K sulfidation, the particles became larger, and many were oriented with their basal planes parallel to the support. Reoxidation at 770 K redispersed the crystallites due to spreading of molybdenum oxide over alumina. These results can be explained by the formation of MoOAl linkages during oxidation and the breaking of these linkages during sulfiding. As these linkages that anchor the MoS2 crystallites to the support are broken, the MoS2 basal planes become oriented at angles close to 90 ° to the support surface. Upon more extensive sulfidation, the MoS2 crystallites became oriented with their basal planes parallel to the support when the MoOAl linkages at the edge plane are finally broken.

An investigation of surface area and porosity in solids by small-angle neutron scattering (SANS)

An investigation of surface area and porosity in solids by small-angle neutron scattering (SANS)

Surface changes induced in aluminas by coating with silica

Adsorption isotherms of nitrogen at -196 °C, of argon at -183 °C and of benzene and water at 35 °C were measured on alumina samples of various porosities before and after coating with a fixed amount of silica. The effects of the coating on the structure and surface properties of the aluminas were determined by differential thermal analysis, X-ray diffraction and pore structure analysis of the various samples calcined at different temperatures. The most obvious effects were as follows: coating microporous alumina with silica reduces the number of pores but does not significantly reduce their sizes; coating mesoporous alumina reduces the size but not the number of pores; coating non-porous alumina produces a porous layer on top of the non-porous substrate.

The combustion of methane on platinum—alumina fibre catalysts—I: Kinetics and mechanism

As a part of a study of a convective—diffusive type catalytic combustor, the oxidation of methane has been studied over platinum supported on porous and non-porous alumina fibre. New and used catalysts have been carefully characterised, and the kinetics of the reaction have been measured at temperatures above and below 815 K, where a change in the apparent activation energy was found to occur. Comparison with Langmuir—Hinshelwood models suggests that the mechanism involves the reaction between adsorbed methane and oxygen adsorbed or in the gas phase. This mechanism is supported by the characterisation studies, which also suggest that the formation of carbonaceous intermediates may affect the reaction selectivity. Permanent catalyst deactivation occurs as a result of the sintering of platinum or of alumina; the latter effect is accelerated in the presence of steam.

Comments on: "Porous Catalysts with No Diffusional Limitation Reaction Rates"

Comments on the article porous catalysts with no diffusional limitation reaction rates by S.K. Ihm and E. Ruckenstein cover the earlier experiments at the National Technical University of Athens in which films of pure or doped ..gamma..-Al/sub 2/O/sub 3/ (two modifications), ZnO, CuO, Fe/sub 2/O/sub 3/, and Fe/sub 3/O/sub 4/ prepared by anodic oxidation of the metals in 15Vertical Bar3< sulfuric acid showed better catalytic properties than the corresponding chemically prepared oxides in the reference reaction of formic acid dehydration, and some surface properties and details of the catalytic mechanism were established for such nonporous alumina films, which probably act by electron exchange as n-type semiconductors. A reply by E. Ruckenstein is included.

Surface properties and catalytic activity of pure and silica-coated aluminas

The surface properties of predominantly microporous, mesoporous, and nonporous alumina samples were studied and compared with samples modified by coating with a fixed amount of silica on their surfaces. The samples were characterized in terms of their specific surface areas, pore structure, and the chemistry of the surface, namely, surface acidity and surface OHs. An attempt was made to relate the activity toward cumene cracking with the chemistry as well as the predominant pore size. This study will hopefully reveal the role of the pore structure of alumina in determining its adsorption and catalytic activity and also the effect of doping with silica in modifying the surface properties of alumina.

Examination of Aluminum Copper Films during Anodic Oxidation: I . Corrosion Studies

Aluminum copper films in the range of 1–70 atom percent (a/o) Cu prepared by vapor deposition were examined in buffer solutions of pH 5.0, 6.0, and 9.2. Films with a low copper content (≤10 a/o) show similar corrosion properties as pure aluminum in weakly acid solutions. The formation of a layer of nonporous aluminum oxide leads to anodization up to a maximum potential of some 10V at which point the film is destroyed locally. For higher copper contents (≥40 a/o) the oxygen evolution cannot be prevented and only brittle noncontinuous oxide films are formed. These metal films have similar corrosion properties to those of pure copper. At the polarization curves obtained are closely related to those of pure copper.

Splitting of platinum crystallites supported on thin, nonporous alumina films

Direct observation in the transmission electron microscope of the behavior of metal crystallites supported on thin films of alumina is reported. Nonporous substrates of alumina have been prepared by anodizing thin aluminum foils in tartaric acid solutions. A sputtering technique was used to deposit platinum crystallites on the anodized aluminum foil. The remaining aluminum was dissolved by amalgamation. The specimens annealed in air at 600 °C showed strong sintering of the crystallites. Reheating these specimens at 500 °C in air lead, because of splitting, to a large decrease in the average size of the crystallites.

Specific and nonspecific interactions in the adsorption of argon, nitrogen, and water vapor on oxides

Adsorption isotherms of argon, nitrogen, and water vapor are reported on well-characterized nonporous aluminas and silicas, along with sorption isotherms of water vapor on various calcined chromium oxide gels. The argon and nitrogen isotherms are analyzed by means of the α s -method (a modification of the t-method of Lippens and de Boer). With low-area samples of alpha-alumina the argon α s -plots are linear in the multilayer region of the isotherm whereas the corresponding nitrogen α s -plots show marked departure from linearity. This, together with significant differences in the isosteric heats and in the BET C values, indicates that the nitrogen monolayer is localized to such an extent that the structure of the multilayer is significantly different from that formed on low-temperature aluminas and hydroxylated silicas. It is suggested that the nitrogen molecule occupies an average area of about 18 Å 2 in the localized monolayer instead of the usual value of 16.2 Å 2 in the close-packed monolayer. Self-consistent values of surface area are obtained if the molecular cross-sectional area of argon in the BET monolayer is also taken as 18 Å 2 for all the nonporous oxide and hydroxylated oxide surfaces. The water isotherms, expressed on a unit area basis, show a large variation from one type of solid to another. Five different mechanisms are identified for the sorption of water vapor, two of which involve penetration below the surface.