| 1. |
- Creci, Simone, 1992, et al.
(författare)
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Acidity as Descriptor for Methanol Desorption in B-, Ga- and Ti-MFI Zeotypes
- 2021
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Ingår i: Catalysts. - : MDPI AG. - 2073-4344. ; 11:1, s. 1-12
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Tidskriftsartikel (refereegranskat)abstract
- The isomorphous substitution of Si with metals other than Al in zeotype frameworks allows for tuning the acidity of the zeotype and, therefore, to tailor the catalyst's properties as a function of the desired catalytic reaction. In this study, B, Ga, and Ti are incorporated in the MFI framework of silicalite samples and the following series of increasing acidity is observed: Ti-silicalite < B-silicalite < Ga-silicalite. It is also observed that the lower the acidity of the sample, the easier the methanol desorption from the zeotype surface. In the target reaction, namely the direct conversion of methane to methanol, methanol extraction is affected by the zeotype acidity. Therefore, the results shown in this study contribute to a more enriched knowledge of this reaction.
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| 2. |
- Creci, Simone, 1992, et al.
(författare)
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Methoxy ad-species in MFI zeotypes during methane exposure and methanol desorption followed by in situ IR spectroscopy
- 2021
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Ingår i: Catalysis Today. - : Elsevier BV. - 0920-5861. ; 369, s. 123-128
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Tidskriftsartikel (refereegranskat)abstract
- The formation and evolution of methoxy ad-species in MFI zeotypes after CH4 exposure, and during temperature programmed desorption of CH3OH have been investigated in situ with diffuse reflectance Fourier transform infrared spectroscopy. Fe and/or Al atoms have been incorporated in framework position prior to crystallization and the influence of the resulting acidity on CH4 activation and CH3OH desorption has been examined. The results show that the presence of Fe promotes CH4 activation and that methanol is more strongly bound to the zeotype in the presence of Al. Because CH4 activation and CH3OH extraction are two of the key steps in the direct conversion of methane to methanol, our results indicate that Al-free zeotypes containing Fe atoms pinpoint important catalyst design parameters needed for this reaction.
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| 3. |
- Martinelli, Anna, 1978, et al.
(författare)
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Local anisotropy in single crystals of zeotypes with the MFI framework structure evidenced by polarised Raman spectroscopy
- 2020
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Ingår i: Physical Chemistry Chemical Physics. - : Royal Society of Chemistry (RSC). - 1463-9084 .- 1463-9076. ; 22:3, s. 1640-1654
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Tidskriftsartikel (refereegranskat)abstract
- Polarised Raman spectroscopy is used to characterise the local structure in single crystals of zeotypes, namely silicalite-1 and ZSM-5, which share the MFI framework structure. Attributes favourable for applying polarised Raman spectroscopy are the orthogonal axes of these single crystals and their size, i.e. 10 to 30 micrometers in all three directions. We show that the intensity of certain vibrational modes in silicalite-1 depends on the polarisation of the incident light, reflecting the anisotropic character of the molecular bonds contributing to these vibrations. Using these observations, and by estimating the depolarisation ratio (rho) and the pseudo-order factor (f), we propose a more accurate assignment of the Raman active modes. More precisely, Raman intensities peaked at 294, 360, 383 and 472 cm(-1) are attributed to bending modes in 10-, 6-, 5- and 4-membered rings, respectively. In the region of stretching modes, the vibration at 832 cm(-1) is assigned to Si-O-Si bonds shared between 5-membered rings, which have an orientation parallel to the a-axis of the crystal. By virtue of having a strongly polarised character, the modes at 472 and 832 cm(-1) can be used as orientational indicators. The proposed assignment is supported by the good agreement between experimental and simulated polar plots, where Raman intensities are plotted as a function of the polarisation angle of the incident light. Finally, upon partial substitution of Si atoms by Al, the crystalline structure is maintained and almost no spectroscopic changes are observed. The only significant difference is the increased width of most vibrational modes, which is consistent with the local lower symmetry. This is also seen in the angular dependence of selected vibrational modes that compared to the case of pure silicalite-1 appear less polarised. In the Raman spectrum of ZSM-5 a new feature at 974 cm(-1) is observed, which we attribute to Al-OH stretching. In the high frequency range, the O-H stretching modes are observed which arise from the Si-O(H)-Al Bronsted acid sites. The intensity of the characteristic mode at 3611 cm(-1) reveals an anisotropic character as well, which is in line with previous findings from solid state NMR that Al atoms distribute nonrandomly within the MFI framework structure.
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| 4. |
- Nellessen, Alexander, 1994, et al.
(författare)
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Impact of Vanadium Loading and Thermal Aging on the Surface Properties of Titania-Supported Vanadium Oxide
- 2024
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Ingår i: Journal of Physical Chemistry C. - 1932-7447 .- 1932-7455. ; 128:7, s. 2894-2908
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Tidskriftsartikel (refereegranskat)abstract
- A series of VOx/TiO2 model catalysts with 0.5, 1.5, 2.0, 4.0, and 8 wt % V was prepared by incipient wetness impregnation (fresh) and then thermally treated at 580 °C for 100 h in static air (aged). Each catalyst was characterized with nitrogen physisorption, ammonia temperature-programmed desorption, X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, and in situ diffuse reflectance infrared Fourier transform spectroscopy upon adsorption of NH3 and NO. The fresh catalyst with 0.5 wt % V hosts monomeric VOx as the majority species. The fresh catalyst with 1.5 wt % V contains both mono- and polymeric VOx, whereof the latter becomes the majority species after aging. Polymeric VOx is the main species in the fresh 2 wt % V catalyst; however, upon aging, V2O5 is detected. The fresh 4 wt % V catalyst contains polymeric VOx and V2O5 species, whereas the fresh 8 wt % V catalyst has mostly crystalline V2O5. Upon aging, both of these catalysts show crystalline V2O5 as the majority species. Further, all catalysts expose V4+. Adsorption of NH3 reveals terminal and bridged hydroxyl groups as well as monomeric and polymeric Brønsted sites, which shift with increasing vanadium loading and aging to more bridged hydroxyls and polymeric Brønsted sites. As for the NO adsorption, the relative abundance of surface nitrates and NO2 changes with the increased vanadium favoring bridge-bound nitrates on crystalline V2O5. Vanadia appears to promote the morphological changes and phase transitions of titania. The NOx conversion during standard SCR conditions was measured in a chemical flow reactor, showing that high V loadings are beneficial for the low-temperature NOx conversion at the expense of low selectivity at higher temperatures and low efficiency after aging. On the contrary, catalysts with lower V loadings reveal an improved NOx conversion after aging. Normalization by V loading, V surface density, specific surface area, and ammonia uptake suggests polymeric VOx to be the most active species and that SSA and ammonia uptake are less important design parameters for stationary conditions.
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