Tin Oxide Gel Research Articles

Tin oxide gel shrinkage during CO 2 supercritical drying

Shrinking behavior of tin oxide gels during solvent exchange in CO 2 supercritical drying was investigated. The drying process consisted of two solvent-exchanging steps, including a first replacement of water by acetone and a second of acetone by liquid CO 2. Gels prepared from a basic sol with a ζ-potential of −75 mV exhibited extensive shrinkage during both steps, while those from an acidic hydrous precipitate of lower potential (| ζ-potential|<35 mV) shrank only during the second step. The shrinkage during the first step correlates with the ξ-potential reduction with increasing acetone concentration, while that during the second step for both gels can be attributed to osmotic compressive pressure that arises from an increase in the liquid–solid (gel skeleton) interfacial energy with increasing CO 2 concentration. Dramatically different aerogel microstructures were obtained by controlling the gel surface potential in conjunction with different combinations of solvent-evaporating and supercritical drying.

Tin oxide surfaces: XXII. Fourier transform infrared study of the thermal decomposition of organotin-substituted tin(IV) oxide gels

Organotin-substituted tin(IV) oxide gels have been obtained by the cohydrolysis of tin(IV) chloride with the appropriate organotin halide. On heating,

Tin oxide surfaces. Part 21.—Infrared study of the chemisorption of 2,2,2-trifluoroethanol, 1,1,1,3,3,3-hexafluoropropan-2-ol, 1,1,1-trifluoropropan-2-one, 1,1,1,3,3,3-hexafluoropropan-2-one and trifluoroacetic acid on tin(IV) oxide gel

The adsorption of trifluoroacetic acid, 1,1,1,3,3,3-hexafluoropropan-2-one, 1,1,1-trifluoropropan-2-one, 2,2,2-trifluoroethanol and 1,1,1,3,3,3-hexafluoropropan-2-ol on tin(IV) oxide gel has been studied by infrared spectroscopy. trifluoroacetic acid is adsorbed as both hydrogen-bonded molecular trifluoroacetic acid and surface trifluoroacetate, the former being totally converted into the latter by 533 K. The surface trifluoroacetate decarboxylates by 673 K to give surface trifluoromethoxide. Both of the ketones 1,1,1,3,3,3-hexafluoropropan-2-one and 1,1,1-trifluoropropan-2-one are chemisorbed as surface hemiacetal species by interaction with a surface hydroxyl group. These undergo subsequent thermal decomposition to give, respectively, surface trifluoroacetate and a mixture of surface trifluoroacetate and surface acetate. The two alcohols 2,2,2-trifluoroethanol and 1,1,1,3,3,3-hexafluoropropan-2-ol are chemisorbed as surface alkoxy species which thermally decompose to surface formate and surface trifluoromethoxide in the case of the former and additionally in the case of the latter, surface trifluoroacetate. The thermal stability of the surface carboxylates is in the order: surface acetate > surface trifluoroacetate > surface formate.

Tin oxide surfaces. Part 19.—Electron microscopy, X-ray diffraction, auger electron and electrical conductance studies of tin(IV) oxide gel

Physical and electrical properties of both unsintered and sintered tin(IV) oxide gel of high specific surface area and porosity are reported. Particle sizes range from 3 to ca. 5000 nm, the larger bodies apparently consisting of pressed agglomerations of smaller particles. The 3 :1 O : Sn atomic ratio determined at the surface of the discs was consistent with a fully hydroxylated oxide surface. A slight oxygen deficiency from the ideal SnO2 composition is observed at all depths. Sintering for extended periods at 1273 K increased the minimum particle size from 3 to ca. 40 nm without causing any appreciable change in morphology on the micron scale.The electrical conductance of the unsintered oxide in air as a function of temperature was found to be extremely complex and exhibits hysteresis dependent upon the history of the sample. Unsintered discs initially displayed a sigmoid relationship between the conductance G and temperature T, with G typically reaching a maximum at 420 ± 20 K, followed by a minimum at 520 ± 50 K, thereafter increasing monotonically. Subsequent G–T cycles produced a wide range of behaviour. Although in general, the G–T characteristic exhibited a more linear form, the rate of change with temperature varied greatly. The initial sigmoid curve was largely unaffected by strict control of the water content of the test atmosphere, and hysteresis effects generally persisted, despite the adoption of more rigorous experimental methods. The air conductance behaviour of presintered samples was typically less complex, with an increasing tendency towards a monotonically rising G–T characteristic with extended duration of the sintering treatment. Significant differences were observed between the electrical behaviour of the unsintered tin(IV) oxide gel samples and that generally exhibited by ‘commercial’ semiconductor gas sensors. However, these differences could be minimised by presintering of the oxide gel.

Tin oxide surfaces. Part 20.—Electrical properties of tin(IV) oxide gel: nature of the surface species controlling the electrical conductance in air as a function of temperature

The mechanisms controlling the electrical conductance of tin(IV) oxide gel have been investigated by examining the conductance–temperature characteristics of unsintered samples as well as samples sintered at 1273 K for various periods under atmospheres of both ambient (i.e. wet) air and dry O2(21 %)–N2(79 %) mixture. The air conductance–temperature characteristics of unsintered samples are sigmoid in form, the conductance rising between ca. 290 and 420 K and between 670 and 900 K. The majority of samples studied showed Arrhenius-type behaviour in both regions for both ambient air and dry O2–N2 as the test atmospheres. The mean values of the activation energy for unsintered materials are determined to be 0.41 ± 0.14 eV ( 650 K) under ambient air, and 0.14 ± 0.08 eV ( 650 K) under dry O2–N2. Repeated thermal cycling under a sealed atmosphere of dry O2–N2 generally produced no change in activation energy in either temperature region, although there were variations in turning-point positions and absolute conductance levels. Experimental curves for sintered materials become more linear as the length of presintering is increased, although in most cases there is still a region of significantly lower activation energy at a temperature (ca. 500–630 K) which correlates well with the minimum of the sigmoid curves found for unsintered material. The linear regions above and below this temperature have similar gradients, and there is a trend towards higher activation energies (ca. 1.7 eV) with increased length of sample presintering.The conductance mechanism is probably of the Schottky barrier type, where the magnitude of the barrier is modulated by the nature of the chemisorbed surface species, especially in samples with little or no presintering. With increasing degrees of sintering, however, the intergrain neck model may become more important. In unsintered material in the temperature range ambient ca. 420 K, surface hydroxyl groups are the major conductance-modulating species. Above 670 K in the unsintered material, and in the sintered material, the conductance is primarily modulated by surface O2– anions.

Tin oxide surfaces. Part 18.—Infrared study of the adsorption of very low levels (20–50 ppm) of carbon monoxide in air on to tin (IV) oxide gel

Three major types of surface species, unidentate carbonate, bidentate carbonate and carboxylate, are formed when tin(IV) oxide is exposed to dry air containing very low levels of CO(20–50 ppm). All three types appear to be formed immediately and simultaneously upon exposure to the CO–air mixtures, and the abundance of each continues to increase during the period of exposure. Little adsorption occurs at low oxide calcination temperatures when the surface retains a relatively high degree of hydroxylation, but abundances of the three surface species increase markedly at temperatures > 570 K, and appear to reach a maximum at calcination temperatures of ca. 590–610 K, declining thereafter. Although quite stable at lower temperatures, the surface bidentate carbonate and carboxylate slowly transform into surface unidentate carbonate at temperatures ca. 400 K under an atmosphere of dry air.

Tin oxide surfaces. Part 17.—An infrared and thermogravimetric analysis of the thermal dehydration of tin(IV) oxide gel

The thermal dehydration of high-surface-area (ca. 180 m2 g–1) tin(IV) oxide gel has been studied by thermogravimetric analysis and infrared spectroscopy in the temperature range 293–673 K. The thermogravimetricanalysis data show that two types of physisorbed water are present, one which is largely removed by evacuation at ambient temperature and a more strongly held type which is removed by evacuation at 373–423 K. Further mass loss occurring at higher temperatures is due to the condensation of hydroxyl groups on the surface of the oxide particles. The amount of physisorbed molecular water present on the freshly prepared gel corresponds to a ca. three-monolayer coverage, reducing to ca. 0.74 of a monolayer after evacuation at 295 K, and is totally removed after evacuation at 423 K. The surface concentration of hydroxyl groups is determined to be 19.8 nm–2 at 423–473 K, falling to 16.2 nm–2 at 573 K and 13.0 nm–2 at 673 K. Infrared spectra confirm that physisorbed molecular water [v(OH) 3800–2000 cm–1, δ(OH) 1640 cm–1] is removed by 423 K. Isolated and hydrogen-bonded surface hydroxyl groups give rise to stretching modes at 3640 cm–1(sharp) and 3500 cm–1(very broad), respectively. Surface hydroxyl deformation bands are found at 870, 940, 1175 and 1245 cm–1. The bands at 870 and 950 cm–1 are removed on evacuation by 473 and 523 K, respectively. The deformation bands at 1175 and 1245 cm–1 which remain at 523 K are assigned to isolated surface hydroxyl groups. The band at 770 cm–1, which increases in intensity during dehydration, is assigned to surface Sn—O—Sn linkages formed by the dehydroxylation process. The results are interpreted in terms of hydroxylated exposed [100], [101](both of which contain clusters of three hydroxyl groups attached to each surface tin atom) and [110](which contains both isolated and geminal pairs of hydroxyl groups attached to surface tin atoms) crystal planes of the rutile structure, the former predominating.

Studies of the Hydrous Tin(IV) Oxide Ion Exchanger. V. The Sodium Isotopic Exchange Rate between the Exchanger in the Na+ Form and Aqueous Solutions

Abstract The sodium isotopic exchange rate between the hydrous tin(IV) oxide ion exchanger in the Na+ form and aqueous solutions was determined radiochemically. When the dried exchanger was used, the rate of the first step was controlled by the rate of the equilibration of water in the exchanger with the solutions, while the second step was found to be controlled by the diffusion of sodium ions in the particles (particle diffusion). When the exchanger had been sufficiently immersed in solutions in advance, the particle diffusion controlled the rate from the beginning. From the dependency of the rate of particle diffusion on the apparent particle size and the specific surface area of the exchanger, and on the sodium-ion concentrations in the solutions, the process controlling the rate is considered to be the diffusion in the primary particles, which are formed at the initial stage of precipitation and which aggregate to form the hydrous tin(IV) oxide gel. The diffusion constants and their activation energy (35±2 KJ mol−1) were also evaluated.

Tin oxide surfaces. Part 9.—Infrared study of the adsorption of CO, NO and CO + NO mixtures on tin(IV) oxide gels containing ion-exchanged CrIII, MnII, FeIII, CoII, NiII and CuII

Infrared transmission spectroscopy has been used to study the adsorption of CO, NO and CO + NO mixtures on tin(IV) oxide gels containing CrIII, MnII, FeIII, CoII, NiII and CuII as ion-exchanged cations. Exposure to CO results in the formation of linear physisorbed CO species exhibiting a single absorption band in the range 2200–2180 cm–1 for all the oxidised gels (except the FeIII exchanged sample). The increase in absorption frequency above that of the gas phase value (2143 cm–1) is rationalised by considering the strong electric field due to the transition metal ion, and it was concluded that the carbon monoxide is adsorbed perpendicular to the surface, probably via carbon, at a cationic transition metal site, except for CuII exchanged gel which was bonded via oxygen. Bands due to bidentate carbonate complexes associated with transition metal sites were also observed for the oxidised MnII, FeIII and CoII samples. In contrast, exposure of CO-reduced MnII, FeIII, CoII and NiII samples to CO + O2 mixtures resulted in the formation of unidentate carbonate complexes bound to transition metal ion sites. Nitric oxide is chemisorbed on all gels except the MnII-exchanged sample, but the nature of the chemisorbed species varies. All the samples catalysed the CO–NO reaction, and physisorbed CO2 was present in the MnII, FeIII, CoII and NiII samples as well as physisorbed N2O in the case of MnII. All samples showed spectra due to carbonate species consistent with a redox mechanism for the CO—NO reaction catalysed by these oxides. Additionally, those oxides with exchanged cations which adsorb NO from CO + NO mixtures (CoII, NiII and FeIII) show a marked selectivity for reduction of NO to N2O, whereas the CuII- exchanged gel, which preferentially adsorbs CO, exhibits a similar selectivity for reduction to N2.

Morphological studies of tin oxide gel

X-ray and electron diffraction patterns of tin oxide gel, with the gel samples heated to 400, 600, 800 and 1000° C, and tin oxide precipitate have been taken. Sharp diffraction rings prove, beyond doubt, the crystallic nature of all these samples. Electron transmission photographs of the samples have been taken, and from the photographs, the pore diameter and crystallite length have been measured. The pore diameters obtained thus have been compared with the values obtained by applying the relationshipd=4V/S whereV is the pore volume andS the surface area. The values are in quite good agreement. Thehkl planes of the sample have been calculated based upon the tetragonal equation.

The catalytic oxidation of carbon monoxide on tin(IV) oxide

The oxidation of carbon monoxide has been investigated on catalysts obtained by thermal activation of granular hydrous tin(IV) oxide gel in the temperature range 200–500 °C. These are active for this reaction at moderately low temperatures (<150 °C). Thermogravimetry and nitrogen adsorption have been used to investigate the water content and specific surface areas of the activated catalysts, and these parameters have been related to the initial catalytic activities. The partial deactivation during carbon monoxide oxidation at low temperatures, which is apparently accompanied by partial reduction of the catalyst surface, has been studied as a function of both the activation and reaction temperatures. The kinetics of the steady state reaction have been investigated at 180–210 °C on the 450 °C-activated catalyst, and the postulated mechanism involves adsorption of carbon monoxide, desorption of carbon dioxide and oxygen regeneration of the catalyst. The kinetics can be represented by the Langmuir-type rate equation, similar to that derived for the same reaction on both V 2O 5 and Fe 2O 3, and this permits evaluation of the experimental activation energies for the adsorption and desorption steps.