Filtern
Dokumenttyp
Sprache
- Englisch (8)
Schlagworte
- Band shape analysis (2)
- Infrared spectroscopy (2)
- Surface layer (2)
- Confinement (1)
- Cyanobiphenyls (1)
- Liquid crystals (1)
- MCM-41 (1)
Composites containing 4-n-octyl-4?-cyanobiphenyl (8CB) either confined to nanopores of molecular sieves with very large pores or coating silica nanoparticles of aerosil type at high silica-to-8CB ratios are investigated by IR spectroscopy. Band shape analysis was performed in wavenumber regions in which the peaks due to CN stretching, CH stretching and CH out-of-plane vibrations appear. Some of molecules confined to molecular sieves show spectroscopic features characteristic to a bulk-like 8CB matter located in the centre of the pores or in the inter-grain space. Other features of the IR spectra are due to 8CB molecules located in the surface layer, mostly forming hydrogen bonds between their CN groups and surface OH groups. Another part of the 8CB molecules in the surface layer may also interact by ? electrons of the aromatic rings. Hydrogen bonding is less hindered for the molecules of the surface layers onto aerosil particles than inside pores of the molecular sieves. Comparison is also made with the case of composites based on molecular sieves with small pores.
This work presents some results obtained by confining octylcyanobiphenyl (8CB) to the pores of two SBA-15 type host materials: a silica SBA-15 (pores of 10.2 nm) and an aluminum containing AlSBA-15 (pores of 7.5 nm). The LC molecules were loaded inside the pores from a solution in acetone, in a percentage higher than 70% from the maximum possible loading. It was observed that the 8CB molecules confined to molecular sieves may preserve the phase transitions characteristic for the bulk LC provided that the confining pores are sufficiently large, as offered by SBA materials. However, present guest-host interactions play an important role in these phase transitions. Such a behavior is at variance with the 8CB confined to Controlled Porous Glass with the same pore dimensions, for which the bulk-like phase transitions can be always observed.
Selective Catalytic Reduction of Nitric Oxide by Ammonia over Egg-Shell MnOx/NaY Composite Catalysts
(2002)
A novel composite catalyst system for the selective catalytic reduction (SCR) of NOx by NH3 is described operating at temperatures lower than 470 K in the presence of water with NO conversions of 80100% at space velocities of 30,00050,000 h-1. The catalyst is prepared by egg-shell precipitation of MnO2 on the external surface of zeolite NaY. Structural and thermal stability of precipitated MnO2 as well as of the MnO2/NaY composite catalyst were characterized by N2 adsorption, X-ray diffraction, laser Raman spectroscopy, temperature-programmed reduction, and electron microscopy. MnO2 precipitated on zeolite NaY (15 wt% loading) retained its amorphous state up to calcination temperatures of 775 K. The zeolite component remained structurally intact. Calcination at higher temperatures destroyed the zeolite structure and transformed MnO2 into Mn3O4. DRIFT spectroscopic investigations revealed the presence of symmetric O=NON=O species formally corresponding to N2O3 on the composite catalyst after contact with NO. Catalytic measurements under integral flow conditions showed that the catalyst performance is associated with a close coupling of nitrite formation and its drain off from equilibria with NO/NO2 and nitrate by ammonia. Several results are in line with the diazotation mechanism, including NH3 protonation to NH4+, whereas prevailing Lewis acid sites should enable NH3 activation via amide species, thus leading to a parallel amide/nitrosamide SCR reaction route. The activity-temperature profile fulfills the requirements of a low-temperature NOx reduction catalyst for mobile diesel engines if an ammonia supply is implemented on board, e.g., by urea decomposition.
On the mobility of sodium ions and of confined liquid crystals in molecular sieves of faujasite type
(2002)
Broadband dielectric spectroscopy (10-2109 Hz) was employed to study in a rather large temperature interval the molecular dynamics of the composite systems containing a nematic liquid crystal (LC), 4-n-pentyl-4?-cyanobiphenyl, confined to NaY, a zeolite with faujasite structure. Thermal analysis was also performed in order to get information concerning the loading of the zeolite and the possible phase transitions. No phase transitions and no relaxation processes of the LC molecules inside the pores were observed. But remarkably, in the presence of these molecules the dielectric behavior of the faujasite host changes due to modifications appeared in the mobility of Na+ ions.
The influence of the aluminum contents of MCM-41 (Si/Al ratio varied between 2.7 and 69) on the coordination of Al, on the acidity, and on the catalytic properties is studied by 27Al MAS NMR, temperature programmed desorption of ammonia, and the conversion of acetone. Based on TPDA results, an assignment of the peaks of desorption of ammonia is proposed. With low Al contents, the concentration of strong Brønsted acid sites, which are attributed to tetrahedral aluminum, increases with growing Al amounts. At higher Al contents, however, the number of strong acid sites decreases again. Besides usual strong Brønsted sites, TPDA reveals the existence of weak Brønsted and Lewis sites and of Lewis sites of a high acidic strength. Separation between the two types of sites of weak acidity is incomplete.
In the temperature programmed decomposition of NH4-exchanged MCM-41 samples, strong Lewis acid sites temporarily re-adsorb ammonia generated by the thermal decomposition of the NH4+ ions located at the Brønsted sites. Ammonia re-adsorption at Lewis sites results in a shift of the maximum of the TPDA peak to a higher temperature. Calcination of samples leads to the formation of strong Lewis sites at the expense of Brønsted sites. To evaluate the acidity of Al-MCM-41, recording of decomposition profiles has been extended to Ga- and Fe-MCM-41.
Catalytic activity in the conversion of acetone reaches its maximum with the sample based on Al-MCM-41 with the molar Si/Al ratio of 6.85. Highly favored formation of isobutene points to a comparably high acidic strength of the active sites.