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“Ni-, Co- and Cu-TiO2 catalysts for the steam reforming of ethanol: how the preparation method affects catalytic performance”
Full text linkConcept
A set of Ni-, Co-, Cu-based catalysts supported over TiO2 for the steam reforming of ethanol were prepared by different procedures. For most of them the support was prepared by precipitation and calcined at 500°C or 800°C. Other samples were prepared by flame pyrolysis (FP), aiming at high metal dispersion coupled with synthesis at high temperature, to impart a strong metal-support interaction, besides high thermal resistance. The samples were characterised by AAS, XRD, N2 adsorption-desorption, TPR, SEM, TEM and FT-IR. Activity testing for the steam reforming of ethanol was performed after activation at 500°C or 800°C in H2 flow on a continuous micropilot plant, by feeding a 3:1 (mol/mol) mixture of water/ethanol at 500°C for 8 h-on-stream.
Motivations and Objectives
Ni-based catalysts raised much interest for the present application [1-3]. Very high activity has been observed at high reaction temperature (>600°C), but it would be interesting to operate under milder conditions, in order to lower the energy input to the process and to improve H2 productivity by favoring the water gas shift reaction. The major inconvenient for Ni-based samples is usually ascribed to coking, often due to the formation of carbon filaments over the active phase. This phenomenon is correlated to catalyst deactivation and appearance of by-products. Indeed, if at high reaction temperature with excess steam carbon may be gasified, at operation at 500°C the C balance, often much lower than 100% evidences coke accumulation.
Results and Discussion
H2 productivity at 500°C seemed firmly dependent on the calcination and on activation temperature of the catalyst when Ni was supported over TiO2 prepared by impregnation. When the sample was calcined and activated at 500°C, no H2 production was observed due to poor ethanol conversion, with selectivity mostly to acetaldehyde and poor carbon balance. By contrast, calcination of the sample at 800°C induced very high H2 productivity, higher C balance and negligible byproducts formation except some CH4 These results were attributed to the formation of a mixed oxide between Ni and the support during synthesis at high temperature (both for Impregnation and FP), which keeps the metal well dispersed in spite of the high activation temperature. This leads to very small Ni particles after activation, which demonstrated to be more active and, most of all, more stable towards coking. C balance was in general much higher for the Co- and Cu-containing samples than for the Ni-based ones. One may conclude that for most of these samples there is no additional coking on the catalyst due to the active phase. However, they proved in general less active than Ni-based ones when tested at 500°C.
References
[1] I. Rossetti, A. Gallo, V. Dal Santo, C.L. Bianchi, V. Nichele, M. Signoretto, E. Finocchio, G. Ramis, G. Garbarino, A. Di Michele, ChemCatChem, 5 (2013) 294.
[2] I. Rossetti, C. Biffi, C. Bianchi, V. Nichele, M. Signoretto, F. Menegazzo, E. Finocchio, G. Ramis, A. Di Michele, Appl. Catal. B, 117-118 (2012) 384.
[3] E. Finocchio, I. Rossetti, G. Ramis, Int. J. Hydrogen Energy, 38 (2013) 3213
Steam reforming of ethanol over Co and Cu based catalysts
No full textCo- and Cu-based catalysts prepared by flame pyrolysis (FP) technique are proposed as possible substitutes for Ni-based catalysts, very active for the Ethanol Steam Reforming reaction, but showing poor stability towards coke formation when operating at relatively low temperature. The FP method allowed to achieve a partial incorporation of the active phase into the support, leading to high dispersion and lower reducibility. The best results were achieved with 10wt% Co/SiO2, which led to higher activity, good C balance and low CO/CO2 ratio. This was ascribed to the high initial dispersion of Co into the silica matrix, which led to available Co particles well dispersed and stable
Fotoreforming di zuccheri per la produzione di idrogeno
No full textINTRODUCTION
The production of hydrogen through photoreforming of aqueous solutions of organic compounds is considered as a way to exploit solar energy storage in the form of hydrogen. The photocatalytic reforming occurs through the following general reaction:
which is promoted by a photocatalyst. In this work, we dealt with the use of different sugars, namely glucose, xylose and arabinose, as well as levulinic acid. They were used as examples of compounds that may be rather easily obtained from the hydrolysis of biomass.
Our attention was predominantly focused on the development of innovative reactors, possibly operating under unconventional conditions, with fine tuning of the operation parameters, rather than of materials properties.
EXPERIMENTAL/THEORETICAL STUDY
The selected photocatalysts were based on TiO2, since the focus was reactor optimization. The materials were prepared by flame spray pyrolysis and compared with commercial samples of nanostructured TiO2 P25 by Evonik. Different metals, such as Cu and Au, with loading ranging from 0.1 to 0.5 wt% were added as co-catalysts. The role of the metals was that of electron sinks, to inhibit the electron-hole recombination and they were also selected due to the formation of a plasmon resonance band which improves visible light absorption. The samples were characterized by N2 adsorption-desorption, X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and temperature programmed reduction/oxidation (TPR/TPO).
The photoreforming reaction was carried out in different prototypes of photoreactors specifically developed in our lab.
RESULTS AND DISCUSSION
In one reactor configuration, an external 200 W lamp was used, with emission wavelengths centred around 365 nm. A first photoreactor was developed with internal capacity ca. 0.3 L, with big head space for gas collection and very efficient mixing of the suspension thanks to an optimized length/diameter ratio (L/D) ca. 2. A drawback was the poor irradiation efficiency of the suspension, which limited the overall productivity, irrespectively or the substrate. The same UVA radiation was tested also in a home designed photoreactor with an immersion lamp (75 or 150 W, coaxial with the reactor). Two reactor sizes were tested, ca. 200 ml or 1.5 L. A significant amount of H2 was obtained with very simple catalyst formulations, e.g. 11 mol kgcat-1 h-1 were obtained at 4 bar, 80 ̊C over commercial TiO2 samples and using glucose as hole scavenger. This result is very remarkable with respect to similar research in conventional photoreactors.
CONCLUSION
In the present work we developed different prototypes of photoreactors to accomplish hydrogen production from biomass derived organic compounds and for the photoreduction of CO2 for the regeneration of fuels and chemicals. Reactor modelling is in progress for both applications, including the optimization of radiation distribution in the photoreactor.
ACKNOWLEDGMENTS
Fondazione Cariplo / Regione Lombardia (UP-Unconventional Photoreactors) and MIUR (HERCULES - Heterogeneous robust catalysts to upgrade low value biomass streams) are gratefully acknowledged for financial support
State of supported rhodium nanoparticles for methane catalytic partial oxidation (CPO): FT-IR studies
No full text
Ni-based catalysts for the steam reforming of ethanol: surface acidity and catalytic activity
No full textConcept
A series of Ni-based catalysts supported on TiO2, SiO2 and ZrO2 (10 wt% Ni) were prepared by an innovative flame pyrolysis (FP) method to achieve proper thermal resistance and to tune metal dispersion. Every sample was characterised by various techniques, in among which infrared spectroscopy (FT-IR). The latter allowed to assess samples acidity and to define the nature of the Ni species present on catalyst surface. Catalytic activity for the steam reforming of ethanol was tested on a bench scale continuous plant under atmospheric pressure at different temperatures, i.e. 500, 625 and 750°C, with 3:1 mol/mol water/ethanol ratio.
Motivations and Objectives
The steam reforming of biofuels such as ethanol represents a hot research topic in the last few years. Different metals have been proposed as active phase, e.g. Ni, Co and Cu, to restrict the field to the less expensive non-noble metals. The most interesting results have been obtained with Co and Ni [1-3]. The latter seems very promising, though some drawbacks remain unsolved due to sintering and coking [1,4,5], especially when Ni particles are very dispersed [6].
From what above reported it seems that the thermal resistance of the sample is one of the key points for these catalytic materials and that another important feature is Ni dispersion and its interaction with the support. The aim of the work was then the design, the synthesis and the characterisation of heterogeneous catalysts to be used for the steam reforming of ethanol.
Results and Discussion
The FP technique proved an interesting method for the preparation of steam reforming catalysts, especially for use at high temperature (≥625°C). The titania supported catalyst showed higher activity and, above all, superior stability, with respect to similar samples prepared by coprecipitation. The advantages of the FP synthesis were less evident when dealing with silica and zirconia supported samples. Medium Lewis acidity due to exposed support ions was detected over the titania and zirconia based catalysts, whereas over the Ni/SiO2 catalyst, Lewis acidity could be induced by the metal phase itself. However, catalyst acidity did not seem really connected to coking. Based on H2 productivity and on the selected operating conditions, the best results were obtained with the silica-supported sample at 625°C, a temperature sufficient to achieve full ethanol conversion and 100% C balance. By contrast, coke formation was usually observed at 500°C, though in general not related to irreversible catalyst deactivation. An optimization of the reaction conditions is therefore required to further decrease the operating temperature, e.g. a higher water/ethanol ratio. Supposing to use such catalyst for the production of 7 Nm3/h of H2, suitable to feed a 5 kWel+5 kWth fuel cell, ca. 1.5 kg of catalyst would be needed, working at 625°C.
References
[1] V. A. Kirillov, V. D. Meshcheryakov, V. A. Sobyanin, V. D. Belyaev, Yu. I. Amosov, N. A. Kuzin, A. S. Bobrin, Theoretical Foundations of Chemical Engineering, 42 (2008) 1
[2] M. Benito, R. Padilla, A. Serrano-Lotina, L. Rodríguez, J.J. Brey, L. Daza, J. Power Sourc., 192 (2009) 158
[3] L.J.I. Coleman, W. Epling, R.R. Hudgins, E. Croiset, Appl. Catal. A: General, 363 (2009) 52
[4] A.J. Vizcaíno, A. Carrero, J.A. Calles, Int. J. Hydrogen Energy, 32 (2007) 1450
[5] J. Xuan, M.K.H. Leung, D.Y.C. Leung, M. Ni, Renewable and Sustainable Energy Reviews, 13 (2009) 1301
[6] S.Q. Chen, Y. Liu, Int. J. Hydrogen Energy, 34 (2009) 473
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