Nanofibres de carbone sur filtre métallique comme support catalytique structuré

The main objectives of this work are 2-fold : The development of a novel type of composite material based on carbon nanofibers supported on sintered metal fibers filters (CNF/SMF) and to explore the use of this material as a catalytic support for a model reaction: selective hydrogenation of acetylene. The advantages of this novel support are: Due to carbon nanofibers, it combines a graphitic structure with a high specific surface area without microporosity. Supported carbon nanofibers can be used in a fixed bed reactor without very high pressure drop. Combination of two materials with high thermal conductivity gives a composite which keeps this property. It is suitable for highly exo/endothermic reactions, diminishing temperature gradients in the catalytic bed. The synthesis of carbon nanofibers on SMF filters was carried out by ethane catalytic decomposition at 655°C directly on metallic filters. An oxidation of SMF followed by a reduction in H2 create surface roughness, leading to a larger area for CNF nucleation. The CNF formation mechanism includes several steps: it starts with carbon deposition on metallic surface followed by its diffusion into the metal leading to carbides formation. When the limit of carbon solubility is attained, a layer of carbon is formed on the surface of metallic fibers. High tension within the bulk metal leads to carbide dissociation and by phase separation to graphite formation, which pushes the metallic particles through the carbon layer. Nickel, stainless steel an Inconel (an alloy of mainly nickel, iron and chromium) filters were used. The latter gave the highest uniformity of the carbon nanofibers layer showing an excellent mechanical stability. In order to control carbon deposition on the filters, a formal kinetic analysis of the decomposition reaction was undertaken. Partial orders of 1 for ethane, -2 for hydrogen and an apparent activation energy of 235 kJ/mol were obtained. The observed kinetics suggests strongly that the C – C bond scission is the rate determining step and not carbon diffusion into the metal which is often reported in the literature. The synthesis procedures like time on stream and reaction temperature were optimized: 1 hour of a mixture C2H6:H2:Ar (3:17:80) decomposition over SMFInconel at 655°C lead to ∼6% of carbon, leading to a CNF layer of 1 µm thickness covering the metallic fibers entirely. This material presents a high specific surface area (22.2 m2/g) and a low pressure drop for the gas flowing through. The carbon nanofibers have a diameter between 20 and 50 nm and a crystalline structure with platelets morphology where the graphene layers are stacked one above the other. Their surface is composed almost exclusively of carbon and hydrogen. Amongst the multiple possible applications of this novel composite material, its use as a catalyst support was assessed. Selective hydrogenation of acetylene was carried out over palladium supported on CNF/SMF and compared with commercial supports (ACF, EGF and AGF). The activated carbon fibers cloth (ACF) led to a Pd activity at least twice as E-type glass fibers tissue (EGF) or EGF covered by an alumina layer (AGF). Reaction kinetics was studied for Pd/ACF, leading to acetylene and hydrogen partial orders of -0.5 and 2.4, respectively and apparent activation energy of 50 kJ/mol. The negative partial order for C2H2 suggests high adsorption strength of this compound compared to other reagents on palladium. This particularity allows Pd to be selective. Hydrogenations are known to be structure sensitive. Therefore the catalyst activity for the model reaction depends on Pd particle size. It increases for large particle sizes. The control of this parameter was a priority. Variations of the palladium precursor, of the support and catalyst treatment were carried out. Micro-wave plasma was used since it is fast and has low energy consumption allowing chemically modifying the carbon support surface and activating catalyst after impregnation. This controls not only palladium sintering, but also decomposes tetraaminopalladate more efficiently, resulting in a catalyst activity of one order of magnitude higher. In order to control the palladium deposition on CNF/SMFInconel composite, it is important to have surface functional groups. The activation of carbon nanofibers was carried out with a 4 hours treatment in a boiling aqueous solution of hydrogen peroxide. This led to an amount on surface oxygen containing groups, per mass of carbon, higher than in ACF. Palladium particle size on the composite was controlled by support activation, the use of several metal precursors or different loadings or by a thermal treatment in argon at 500°C. Optimisation of the procedure allowed to obtain in a controlled way an average palladium particle sizes between 3.6 and 25 nm. The activity and selectivity towards ethylene of Pd/CNF/SMFInconel were observed to be higher compared to palladium supported on activated carbon, alumina or barite. Graphitic nature of carbon nanofibers, leading to strong metal-support interaction, was accounted as responsible for these improvements. The high thermal conductivity of the composite CNF/SMF ensured good heat transfer released due to the exothermicity of the reaction and allowed to work close to isothermal conditions in the catalytic bed. Finally it can be concluded, that for the first time carbon nanofibers synthesis was carried out in one step on sintered metal fibers filters, resulting in a novel structured composite material combining different specific properties and having promising potential applications.

Novel catalytic applications of carbon nanofibers on sintered metal fibers filters as structured supports

Marina Ruta

Supported metal catalysts are important from both an industrial and a scientific point of view. They are used, amongst others, in large-scale processes such as catalytic reforming, hydrotreating, polymerization reactions and hydrogenations. Often, these catalysts consist of nanosized metal particles deposited on a suitable support, which acts as an anchor for the active phase and, in several cases, contributes to improve the overall catalyst performances. The growth of carbon nanofibers on sintered metal fibers filters (CNF/SMF) by hydrocarbons catalytic decomposition results in a structured composite material presenting all the suitable characteristics for its application as catalyst support. The main objective of this thesis was to develop novel catalytic systems based on CNF/SMF as catalytic support for continuous gas-phase hydrogenations. At first, the synthesis of CNF/SMF was optimized to tailor the properties of the material for further applications as catalytic support. The use of ethylene as carbon precursor and high synthesis temperature (973K) provided high yields of well-ordered CNF with increased specific surface area (SSA), up to 516 m2/g. On the other hand, the synthesis of CNF by ethane decomposition, which is less reactive than ethylene, resulted in a support with higher permeability, minimizing the pressure drop and being more suitable in a continuous-flow reactor. After the synthesis, the CNF surface was activated by treatment with O3 or H2O2. A novel technique, which consist of XPS analysis after step-wise TPD in UHV conditions, was developed and applied for the surface characterization. It allowed to assess the nature of the functional groups created on the CNF surface. The O3-activation was found the most effective for increasing the acidity of the CNF/SMF surface, yielding a relatively high amount of carboxylic functional groups. These groups are important because involved in the chemical anchoring and stabilization of the active metal deposited on the support. Industrially, many hydrogenations are performed over supported metal catalyst. The majority of these reactions are known to be structure sensitive reactions, so the control of the metal particle size is crucial in order to study the catalyst activity and selectivity. We prepared monodisperse-sized Pd nanoparticles (8, 11 and 13 nm) via the reverse microemulsion method and deposited on CNF/SMF and activated-CNF/SMF for a two-fold study: the effect of the nanoparticles size and the effect of the support nature on the selective hydrogenation of acetylene. The antipathetic size dependence of the TOF disappeared at particle size bigger than 11 nm. The initial selectivity to ethylene (∼60%) was found size-independent. The structure-sensitivity relations have been discussed in terms of "geometric" and "electronic" nature of the size-effect and rationalized assuming a Pd-Cx phase formation which is known to be size-dependent. CNF/SMF supports with increased acidity diminished the formation of coke and changed the by-products distribution, diminishing the catalyst deactivation. Subsequently, CNF/SMF was applied to develop a new catalytic system: the Structured Supported Ionic Liquid Phase (SSILP) catalyst, embedding an active Rh complex. The selective gas-phase hydrogenation of 1,3-cyclohexadiene to cyclohexene was used as model reaction. The SSILP based on CNF/SMF supports showed a high turnover frequency, up to 250 h-1 and selectivity > 96%. Multiple advantages of supporting a homogeneous active phase, dissolved in IL, on CNF/SMF have been demonstrated: the thin layer of IL was homogeneously distributed over the mesoporous support, so mass transfer limitations could be avoided, furthermore the chemical inertness of the CNF prevented any interaction between the active phase dissolved in IL ensuring an efficient use of the Rh complex. The SSILP concept can be applied not only for the "heterogenization" of a transition metal catalyst, but also to obtain metal nanoparticles with monodispersed size via the reduction of a metal precursor dissolved in IL. In this case, IL provides a suitable environment for the nucleation and growth of the nanoparticles. By applying the SSILP concept to the selective hydrogenation of acetylene, the influence of IL on the stability of the nanoparticles was investigated. Moreover, the different solubilities of reactants and products in the IL phase allowed to improve the selectivity toward ethylene. Monodispersed Pd nanoparticles of 5 and 10 nm in SILP on CNF/SMF showed excellent long-term stability. The lower solubility of ethylene than acetylene in the IL had the beneficial effect of allowing high selectivity to ethylene up to 85%, due to the inhibition of its consecutive hydrogenation to ethane. In conclusion, a novel catalytic system for continuous-flow, gas-phase hydrogenations was conceived, taking advantage of the suitable characteristics of CNF/SMF supports. The SSILP concept, involving both homogeneous and heterogeneous active phases has been demonstrated, indicating a great potential and flexibility for continuous-flow industrial applications.

EPFL2008

Novel structured adsorber and oxidation catalysts for the removal of VOCs in low concentration

Kim Martin Nikolajsen

One possibility for the removal of volatile organic compounds (VOCs) is catalytic oxidation. For the removal of VOCs in low concentration (≲ 3000 ppmv for propane) it is not possible to sustain the required oxidation temperature over the catalyst without external heating. An approach to overcome this problem is to concentrate the VOC by adsorption, in a "two-step adsorption-incineration" process. This is an unsteady state process in which the VOC laden effluent gas is cleaned by passing it through an adsorber. Once the adsorber is saturated the VOC is desorbed by heating and purging. The concentrated VOC from the desorber can then be oxidized over the catalyst, producing enough heat to sustain the oxidation temperature. Conventional adsorbents have some drawbacks such as tailing during desorption due to the large particle size, high pressure drop and hot-spot formation. The objective of this work was to develop, characterize and test the adsorbent and catalyst, with low resistance to mass transfer resulting from thin films of active material, for the two-step adsorption-incineration process. Both the adsorbent and the oxidation catalysts were supported on sintered metal fibers (SMF). This is a novel, structured support offering several advantages over conventional randomly packed beds: Low pressure drop, due to the high porosity of the SMF, which is important where large flow rates and/or high quantities of gas must be treated. High thermal conductivity, due to the metallic nature, leading to smaller temperature gradients in the fixed-bed. A very high geometric surface area, due to the small fiber size. On the order of 200,000 m2/m3 with a fiber diameter of 20 µm. Furthermore, a mathematical model was developed to show the feasibility of the coupling of the two steps in the process. The adsorber was made from a thin, homogeneous film of MFI-type zeolite covering the fibers of the SMF. The film was grown by the seed-film method by hydrothermal treatment. A 3 µm film was obtained with 10 wt.% zeolite on the fiber after 24 h synthesis at 125 °C. This material had a specific surface area of 30 m2/g. The adsorption equilibrium of propane was described well by the Langmuir isotherm and the heat of adsorption was found to be ΔH0ads = - 43.1 kJ/mol. Isothermal breakthrough curves were measured as a function of temperature and film thickness. A mathematical model, comprised of a tanks-in-series model with a linear driving force (LDF) mass transfer description, successfully described the breakthrough behavior. The overall mass transfer was found to be solely due to diffusion in the zeolite film. However, the thin films show very low resistance to mass transfer leading to low internal concentration gradients and efficient utilization of the adsorbent. The pressure drop was measured and compared to that of a randomly packed fixed-bed of spheres. The equivalent diameter for a constant volumetric flow rate and superficial cross sectional area was dp = 180 µm, for a fiber diameter of df = 26 µm at 10 wt.% zeolite loading. The oxidation catalysts were all supported on SMF and cobalt oxide was used as the active phase. Three different groups of catalysts were developed. The support was modified by coating the fibers either with a thin film of fine powder (alumina or catalyst powder) or with a MFI-type zeolite film identical to the adsorbent. The third group was made from cobalt oxide impregnated directly on the oxidized fibers. The latter type of catalyst was found to be the most active during the screening experiments. The base support composition was varied (stainless steel (SS), FeCrAlloy and inconel) to investigate the support effect on the kinetics. The stainless steel and FeCrAlloy supports had similar activities, surpassing that of the inconel supported catalyst, which might be attributed to a Co3O4 surface enriched with iron from the support. Between the 1.1 wt.%Co3O4/SMFSS and 1.5 wt.%Co3O4/SMFFeCrAlloy catalyst the FeCrAlloy based had the highest normalized activity at low temperature (< 310 °C), low propane mole fraction (0.13 %) and high oxygen mole fraction (16.7 %). The chemical kinetics were determined from a novel and efficient experimental design. The apparent activation energy for this catalyst was found to be 87.5 ± 2.6 kJ/mol, with reaction orders in propane of 0.38 ± 0.04 and in oxygen of 0.30 ± 0.08. The catalysts were tested up to 350 °C. Mass transfer limitations were absent and the kinetics was described well with the power rate law as compared to the Mars-van Krevelen model. Catalysts based on Co3O4, supported directly on oxidized SMF, are very active catalysts, simple to prepare, withstand deactivation due to hot-spot formation and show great potential in comparison to the industrial reference catalyst (copper and manganese oxide supported on alumina). The adiabatic desorption process was investigated by mathematical modeling. Simply purging the adsorption bed with the hot exhaust gas from the reactor cannot result in the concentration of the VOC due to the high heat capacity of the fixed-bed. Hence, it will be necessary to use either a high heat carrier like steam, with the adverse effects of drying and separation of water and VOC, or an approach in which the desorber is heated prior to purging the bed, changing the adsorption equilibrium in favor of high gas concentrations. For the latter method, the minimum theoretical propane mole fraction for an autothermal process was found to be 350 ppmv to sustain a catalytic incinerator at 250 °C. Finally, it can be concluded that for the first time zeolite films on SMF were used as structured adsorbents, leading to very low internal mass transfer and increased process efficiency. Furthermore, the advantages of the SMF can be exploited in a catalytic oxidizer, which can be combined with the adsorber to annihilate VOCs in low concentrations. A novel method for the deposition of fine powder catalyst on SMF was also developed for the first time.

EPFL2007

Nitrous Oxide Abatement over Fe-Zeolite

Bryan Bromley

The use of zeolites for catalytic reactions is a field in continuous development. Since it was demonstrated that metal-zeolites are efficient catalysts for NOx abatement, decomposition of nitrous oxide (N2O) over Fe-containing zeolites has attracted great interest by the scientific community mainly due: N2O is the third most important gas contributing to global warming, its concentration in atmosphere is still on the rise and direct N2O decomposition to N2 and O2 is a sustainable alternative. N2O interaction with Fe-zeolite leads to the formation of "special" adsorbed oxygen (often called α-oxygen) with great potential for selective oxidation. Although the focus of a number of studies, the nature of the actives sites for the formation of this adsorbed oxygen is still unclear. The main objectives of this work are: 1) study the decomposition mechanism of N2O over isomorpously substituted Fe-ZSM-5 zeolite aiming on catalyst optimization and 2) optimization and further use of structured Fe-zeolite in novel micro-structured reactor. A number of refined transient kinetics methods were intensively used such as transient response, Temperature Programmed Desorption (TPD) and Temporal Analysis of Product (TAP). Density Functional Theory (DFT) and in situ IR spectroscopy studies were conducted in collaboration and our results support the following: In chapter 3, quantitative measurements have been carrier out to analyze the effect of temperature on the activation (under helium flow) and the irreversible deactivations that can happen during the pretreatment. The results obtained indicate that high temperature treatment (1273 K) results in the formation of a high amount of α-sites. The addition of a very low fraction of oxygen (2%) in the feed demonstrates that the catalyst is sensitive to its presence. Certain sites can be irreversibly destroyed, a result ascribed to the oxidation of active Fe(II) to Fe2O3 clusters which contain inactive Fe(III). Catalyst treatment at high temperature (1273 K) in He for up to 3 hours, also reduced irreversibly the concentration of α-sites. TPD measurements have shown desorption of oxygen at 3 different temperatures. Desorption at the lowest temperature (650 K) was assign to the α-oxygen recombination. The oxygen which desorbs at 700 K was associated to the oxygen from the deactivated sites. Oxygen which desorbs at the highest temperature (730 K) is originated from the surface NOx that desorbed as NO and O2. After a comparison of the concentration of the molecules accumulated, we propose that NO2 is the adsorbed specie. The mechanism of N2O decomposition on the binuclear oxo-hydroxo bridged extraframework iron core site [FeII(μ-O)(μ-OH)FeII]+ inside the ZSM-5 zeolite has been studied by combining theoretical and experimental approaches (see chapter 4). Rate parameters computed using standard statistical mechanics and transition state theory reveal that elementary catalytic steps involved into N2O decomposition are strongly dependent on the temperature. This theoretical result was contrasted with the experimentally observed steady state kinetics of the N2O decomposition and TPD experiments. As predicted by the theoretical study, the switch of the reaction order with respect to N2O pressure (from zero to one) occurs at around 800 K which confirms a change of the rate determining step from the α-oxygen recombination to α-oxygen formation. The proposed mechanism was also confirmed by O2-TD which confirmed the viability of the binuclear complex. The α-oxygen recombination at low temperature (

EPFL2010