Synthesis and Characterization of Fe-doped Aluminosilicate Nanotubes with Enhanced Electron Conductive Properties

The goal of the protocol is to synthesize Fe-doped aluminosilicate nanotubes of the imogolite type with the formula (OH)3Al2-xFexO3SiOH. Doping with Fe ...

Politecnico di Torino

Variable temperature FT-IR spectroscopy (in the range of 298-380 K) is used to study the thermodynamics of formation of Ca(2+)...CO carbonyl species upon CO adsorption on the faujasite-type zeolite Ca-Y, and also the (temperature-dependent) isomerization equilibrium between carbonyl and isocarbonyl (Ca(2+)...OC) species. The standard enthalpy and entropy changes involved in formation of the monocarbonyl species resulted to be DeltaH(0)=-50.3 (+/-0.5) kJ mol(-1) and DeltaS(0)=-186 (+/-5) J mol(-1) K(-1), respectively. Isomerization of the (C-bonded) Ca(2+)...CO carbonyl to yield the (O-bonded) Ca(2+)...OC isocarbonyl involves an enthalpy change DeltaH(iso)(0)=+11.4 (+/-1.0) kJ mol(-1). These results are compared with previously reported data for the CO/Sr-Y system; and also, a brief analysis of enthalpy-entropy correlation for CO adsorption on zeolites and metal oxides is given.

Variable-temperature infrared spectroscopy studies on the thermodynamics of CO adsorption on the zeolite Ca-Y.

Adsorption of carbon dioxide on H-ZSM-5 zeolite (Si:Al=11.5:1) was studied by means of variable-temperature FT-IR spectroscopy, in the temperature range of 310-365 K. The adsorbed CO(2) molecules interact with the zeolite Brønsted-acid OH groups bringing about a characteristic red-shift of the O-H stretching band from 3610 cm(-1) to 3480 cm(-1). Simultaneously, the nu(3) mode of adsorbed CO(2) is observed at 2345 cm(-1). From the variation of integrated intensity of the IR absorption bands at both 3610 and 2345 cm(-1), upon changing temperature (and CO(2) equilibrium pressure), the standard adsorption enthalpy of CO(2) on H-ZSM-5 is DeltaH(0)=-31.2(+/-1) kJ mol(-1) and the corresponding entropy change is DeltaS(0)=-140(+/-10) J mol(-1) K(-1). These results are discussed in the context of available data for carbon dioxide adsorption on other protonic, and also alkali-metal exchanged, zeolites.

Thermodynamics of carbon dioxide adsorption on the protonic zeolite H-ZSM-5.

Interaction of gaseous ammonia with a NH(4)-ZSM-5 zeolite (Si/Al=11.5) was studied by means of infrared (IR) spectroscopy both at constant ambient temperature and in the temperature range 373-573 K. H-bonding of NH(3) molecules to the NH(4) (+) species takes place. The interaction is weak and reversible, resembling a solvation process. Spectral evidence shows that only one N--H moiety is actually available, indicating that ammonium ions are tricoordinated to the zeolite inner surface. H-bonded NH(3) has an absorption band at 1712 cm(-1), which grows with increasing pressure in two steps: a monosolvated ammonium species is initially formed, evolving to a disolvated species for pressures above 5 mbar. Coordination of the second NH(3) molecule takes place at the already coordinated NH(3) molecule and not at the ammonium cation. From the changes in intensity of the 1712 cm(-1) band with changing temperature under a moderate NH(3) equilibrium pressure, the calculated standard enthalpy and entropy of the monosolvation reaction were ΔH(0)=-34(±5) kJ mol(-1) and ΔS(0)=-88(±10) J mol(-1) K(-1), respectively. The enthalpy of the second solvation step was calculated from the corresponding equilibrium constant under the assumption of (nearly) the same entropy change for both solvation processes. In agreement with the overall picture, this enthalpy change is small (-15 kJ mol(-1) at the most). Since in a previous work (M. Armandi, B. Bonelli, I. Bottero, C. O. Areán, E. Garrone, J. Phys. Chem. C 2010, 114, 6658) the thermodynamic features of the reaction between bare Brønsted acid sites and NH(3) yielding NH(4) (+) species were determined, the data reported herein allow the study of the coexistence of different species in the NH(3)/H-ZSM-5 zeolite system: 1) unreacted acid Brønsted sites, 2) bare ammonium ions, and 3) variously solvated ammonium species. The relevant description is particularly simple when the overall average coverage is one molecule per site.

Ammonia-solvated ammonium species in the NH(4)-ZSM-5 zeolite.

Imogolite-like nanotubes have been synthesised in which SiCH(3) groups have been introduced in place of the SiOH groups that naturally occur at the inner surface of imogolite, an alumino-silicate with formula (OH)(3)Al(2)O(3)SiOH, forming nanotubes with inner and outer diameter of 1.0 and 2.0 nm, respectively. The new nanotubular material, composition (OH)(3)Al(2)O(3)SiCH(3), has both larger pores and higher specific surface area than unmodified imogolite: it forms as hollow cylinders 3.0 nm wide and several microns long, with a specific surface area of ca. 800 m(2) g(-1) and intriguing surface properties, due to hydrophobic groups inside the nanotubes and hydrophilic Al(OH)Al groups at their outer surface. Adsorption of methane at 30 °C has been studied in the pressure range between 5 and 35 bar on both the new material and unmodified imogolite: it resulted that the new material adsorption capacity is about 2.5 times larger than that of imogolite, in agreement with both its larger pore volume and the presence of a methylated surface. On account of these properties and of its novelty, the studied material has several potential technical applications, e.g. in the fields of gas chromatography and gas separation.

Synthesis and characterization of hybrid organic/inorganic nanotubes of the imogolite type and their behaviour towards methane adsorption.

We report the first use of cobalt aluminophosphate (CoAPO5) as a water oxidation catalyst. A decrease in the overvoltage by about 0.2 V with respect to catalyst free FTO has been observed. Additionally, we show that CoAPO5, deposited on ITO or Pt, can also act as a photo-electro-catalyst, as it generates enhanced oxidation currents in the presence of light starting from a bias of +0.8 V vs. Ag/AgCl.

CoAPO5 as a water oxidation catalyst and a light sensitizer.

An IR spectroscopy study is reported on the nature and accessibility of external and internal surfaces of single-walled alumino-silicate nanotubes (NTs) of the imogolite type. NTs form bundles with hexagonal symmetry, in which three kinds of surfaces may be figured out: surface A is the inner surface of NTs; surface B is that between three aligned NTs in the hexagonal packing; and surface C arises from slit mesopores between bundles. Two materials were considered: proper imogolite (IMO, (OH)3Al2O3SiOH) and its methylated analogue, (Me-IMO, (OH)3Al2O3SiCH3). The chemical nature of the outer surface of NTs is the same in both materials, i.e. a curved gibbsite sheet with both Al-OH-Al and Al-O-Al groups and an amphoteric character. The inner surface is very hydrophilic in IMO NTs, lined by closely packed silanols, and hydrophobic in Me-IMO, all silanols being replaced by -SiCH3 groups. The change in chemical composition is accompanied by an increment in pore size, about 1.0 nm in IMO, and ca. 2.0 nm in Me-IMO, which implies a change in the accessibility of the B surface, not available to any molecule in IMO, and accessible in Me-IMO to small molecules like water, due to larger pores between NTs. Aluminol species at the B surface display an acidic nature, in contrast with that of the same species at surface C, because of a confinement effect.

Surface properties of alumino-silicate single-walled nanotubes of the imogolite type.

Samples of the activated microporous aluminophosphate Co-APO-5, featuring ca. 20% of Co(3+) cations, when immersed in water evolve molecular oxygen at room temperature in an endothermic process, without the need for either light or a sacrificial reactant. Successive drying of the sample at temperatures around 520 K releases molecular hydrogen, with recovery of the initial conditions. Several hydration-dehydration cycles may be performed without loss of activity, i.e. water is split in a thermal cycle under relatively mild conditions.

The behaviour of an old catalyst revisited in a wet environment: Co ions in APO-5 split water under mild conditions.

The hydrothermal synthesis of a nanosized cobalt doped aluminum phosphate CoAPO-5 (CoAPO-5-N) in a water-surfactant-organic solvent mixture (emulsion method) is reported, along with its physico-chemical characterization and comparison with a sample obtained by conventional synthesis (CoAPO-5-C). Both XRD (X-ray Diffraction) peak widths and FESEM (Field Emission Scanning Electron Microscopy) pictures of CoAPO-5-N are in agreement with a nanoscale structure, although the aggregation of nanoparticles occurred. EDX analysis shows a more homogeneous distribution of cobalt in CoAPO-5-N, not attainable by conventional synthesis. The specific surface area, as measured by nitrogen adsorption at 77 K, shows a limited increase in CoAPO-5-N (242 m(2) g(-1)) with respect to CoAPO-5-C (216 m(2) g(-1)), whereas the external surface area is almost tripled. Such a definite increase in the outer surface of CoAPO-5-N is also evidenced by the fourfold increase in the rate of a reaction only involving the exterior surface of particles, the light-driven oxidation of water by persulfate anions, as activated by the bulky Ru(bipy)3(2+) complex, unable to enter CoAPO-5 micropores. Two new features were also noted, adding to the knowledge of CoAPO-5 systems: (i) tetrahedral Co(3+) species may coordinate ammonia molecules, assuming an octahedral configuration, as determined by UV-vis spectroscopy; (ii) Co(2+) species in trigonal coordination occur, able to coordinate either CO molecules at a low temperature or ammonia (or water) at room temperature, as evidenced by IR and UV-vis spectroscopy, respectively.

Marco Armandi

Department of Applied Science and Technology,
Institute of Chemistry

Politecnico di Torino

Synthesis and Characterization of Fe-doped Aluminosilicate Nanotubes with Enhanced Electron Conductive Properties

Marco Armandi

.css-o250i3{font-size:18px;font-family:Open Sans,sans-serif;line-height:21.6px;}Department of Applied Science and Technology,Institute of Chemistry

Politecnico di Torino

Synthesis and Characterization of Fe-doped Aluminosilicate Nanotubes with Enhanced Electron Conductive Properties

Department of Applied Science and Technology,
Institute of Chemistry