CO2 hydrogenation reaction over pristine Fe, Co, Ni, Cu and Al2O3 supported Ru: Comparison and determination of the activation energies

Noris André Gallandat, Wen Luo, Emanuele Moioli, Robin Tobias Andreas Mutschler
ACADEMIC PRESS INC ELSEVIER SCIENCE, 2018
Article

Résumé

Fe, Co, Ni and Cu are the main non-noble industrially significant catalysts in the CO2 and CO gas phase hydrogenation reaction towards hydrocarbons and alcohols. These catalysts are typically supported on metal oxides such as SiO2, TiO2, Al2O3 and ZnO, in order to maximize the activity towards the desired reaction. The role of the supporting material is to stabilize the catalytic nanoparticles and to prevent sintering at the elevated reaction temperatures and pressures. The supporting phase can improve the reaction activity or even have a crucial role in the reaction, as is the case, e.g. for the Methanol synthesis over Cu based catalysts supported on ZnO. Studying the metals without a supporting oxide phase is of great importance for the fundamental understanding of the catalytic activity of the metal phase. Therefore, we investigated the pristine transition metals Fe, Co, Ni and Cu (diluted with silica glass beads to avoid sintering) towards their activity in the CO2 hydrogenation reaction and determined the activation energy. An Al2O3 supported Ruthenium catalyst with 0.5 mass percent of Ru loading was taken as reference system. It was found that Co, Ni and Ru/Al2O3 are mostly active in the Sabatier reaction, while Fe is active in the reverse water gas shift reaction. Cu as pristine metal shows no catalytic activity. C2+ hydrocarbons were formed on Co in low concentrations. For the calculation of the activation energy, the kinetically determined temperature range of the reaction is identified with a high resolution in time by means of a quantitative gas analysis method with an online mass spectrometer. The observation activation energy of the CO2 hydrogenation reaction was determined to be 50 kJ/mol over Fe, 77 kJ/mol over Co, 74 kJ/mol over Ni and 73 kJ/mol over the Ru/Al2O3 catalyst. This indicates similar reaction pathways over Co, Ni and Ru/Al2O3 and a different reaction mechanism on Fe. (C) 2018 Elsevier Inc. All rights reserved.

Official source

https://infoscience.epfl.ch/record/262373?ln=fr

À propos de ce résultat

Cette page est générée automatiquement et peut contenir des informations qui ne sont pas correctes, complètes, à jour ou pertinentes par rapport à votre recherche. Il en va de même pour toutes les autres pages de ce site. Veillez à vérifier les informations auprès des sources officielles de l'EPFL.

Concepts associés

Chargement

Publications associées

Chargement

Noris André Gallandat, Wen Luo, Emanuele Moioli, Robin Tobias Andreas Mutschler
ACADEMIC PRESS INC ELSEVIER SCIENCE, 2018
Article

Résumé

Official source

https://infoscience.epfl.ch/record/262373?ln=fr

À propos de ce résultat

Concepts associés (18)

Concepts associés

Chargement

Publications associées (36)

Chargement

Afficher plus

Publications associées

Chargement

Publications associées (36)

The aim of this thesis is to contribute to the development of analysis methods that enable an improved investigation of the CO2 hydrogenation reaction such as quantitative mass spectrometry and infrared thermography. Furthermore, the contribution is in the characterization of the activity of basic catalyst building elements such as Fe, Co, Ni, Cu and Ru. To achieve these aims, we have developed and build a gas-controlling and analysis system to carry out the experiments in a wide range of conditions. In the frame of setting up the instruments and working on the analysis scripts, a rapid and quantitative gas analysis method by means of mass spectrometry was developed. In a second step, the CO2 hydrogenation reaction over Fe, Co, Ni and Cu in their pristine form was investigated with a particular focus on the catalytic activity and the activation energies. An Al2O3 supported Ru catalyst was used as reference catalyst. We showed that the reaction goes through a similar rate limiting step on Co, Ni and Ru/Al2O3, while the reaction rates are significantly different. Pristine Co showed minor activity in the formation of C2+ hydrocarbons. Fe was mostly active in the Carbon Monoxide formation. Cu exhibited no catalytic activity without a supporting metal-oxide phase. Based on these findings, we carried out an in-depth kinetical analysis of the catalytic CO2 hydrogenation over pristine Ni, Co and Ru/Al2O3 was carried out. A reaction model describing the CO2 hydrogenation over a broad temperature range, taking kinetically, diffusional and thermodynamic limitations into account, was developed. Diffusional limitation occurs at temperatures where the reaction kinetics is already rapid and diffusion of educts -and products to -and from the catalyst and its pores becomes rate limiting before the thermodynamic limitations are approached. The turnover rate of the CO2 conversion depends critically on the partial pressure of CO2 in the educts gas stream: With a high CO2 partial pressure, the catalyst surface saturates and the reaction order approaches zero. Out of the unsupported pristine metals Fe shows the lowest activation energy, but also the lowest activity while Co showed the highest activity and the highest activation energy for the overall CO2 conversion. Therefore, we have synthesized bimetallic nanosized pristine Fe-Co catalysts to investigate the influence of alloying Fe to Co and how this influences the activation energies of the formation of methane and C2-C5 hydrocarbons. We found clear trends towards higher activation energies with increasing Fe content- attributed to the stronger binding energy of CO2 to Fe. Furthermore, a new experiment for thermography was built with the aim to visualization the exothermic reaction dynamics. The dedicated reaction cell with an IR transparent window is coupled with the mass spectrometer. With the rapid gas sampling, the gas composition can directly be linked to the evolution of the thermal signal, contributing to the deeper understanding of the reaction start-up. We found and could prove that a CO2 covered surface has a strong inhibiting effect on the reaction startup while an H2 covered surface leads to a linear acceleration of the reaction front in the form of a thermal runaway.

EPFL2019

Chargement

Mechanisms of the Heterogeneously Catalyzed Reaction of CO2 Hydrogenation on Transition Metal Surfaces

Kun Zhao

Renewable energy supply and energy storage in a closed materials cycle are the urgent global challenges of the 21st century. Carbon dioxide (CO2) hydrogenation over catalysts is a method to produce synthetic fuels from renewable energy in a CO2 neutral cycle. Numerous catalysts from pristine to novel materials have been investigated to achieve high CO2 conversion and selective hydrocarbon. Ahead of developing innovatively active catalyst with a high selectivity, it is crucial to understand the active sites and the related reaction mechanisms. Despite the fact that CO2 + H2 is a rather simple chemical equation, many reaction paths on a solid catalyst are possible. However, the proposed mechanisms from literatures have been controversial due to two main difficulties in 1) evidently identifying the reaction species, especially some vital transient species on the pathway of carbon hydrogenation and C-C coupling, and 2) proving the relation between surface structure of the catalysts and the reactivities. To address these difficulties, we built a new instrumental setup and developed an analysis program in order to investigate the catalyst surface and analyze the reaction mechanism in operando. We built up a diffuse reflectance infrared Fourier transform spectroscopy-mass spectroscopy-gas chromatography (DRIFTS-MS-GC) instrument for an operando study of the surface, gas, and liquid products using DRIFTS, MS, GC, and ex situ NMR. A bilevel evolutionary Gaussi-an fitting (BEGF) program was developed for dataset treatment including peak deconvolution and kinetic plot. Standard chemicals like carbonates, bicarbonates, formic acid, acetic acid, methanol, and ethanol, and isotopic spectra using 13CO2 and D2 in the reactions were em-ployed for supporting peak assignments. Based on these infrastructures, CO2 adsorption and hydrogenation reactions on pristine metals (Fe, Co, Ni, and Cu), metal hydride alloy (La-Ni4Cu), and metal-oxide (Co-CoO) were studied. We found out pure Fe, Co, Ni, Cu and La-Ni4Cu metals are not efficient for CO2 hydrogenation, and have no detectable adsorption spe-cies on the surface. High conversion (> 90%) of CO2 on Co-CoO surface emphasized the im-portance of oxide for CO2 chemisorption and for the observations of the surface species. The following investigation on oxide supported metal catalyst Ru/Al2O3 via an in situ control of the individual formation and hydrogenation of each adsorption species demonstrates that the oxide assists CO2 initial activation. CO2 methanation starts at the interface of the metal and the oxide and passes through the steps of CO2 ï® HCO3-* (and/or HCOO-* in CO2+H2 co-adsorption condition) ï® CO* ï® CH4. We further explored the relationship between met-al/(metal+oxide) ratio and their reactivities. We synthesized Co/(Co+CoO) catalysts in the DRIFTS-MS-GC instrument, and investigated the CO2 hydrogenation reaction on site. The results reveal that high concentration of the oxide in the catalyst boosts CO2 methanation, be-cause the CO2 binding is moderate and the adsorption is enhanced when the oxide concentra-tion increased. In short, we established equipment for an operando study of CO2 hydrogenation on catalyst surfaces, unraveled the surface reaction mechanisms and the active sites, developed a highly active catalyst, as well as initiated an efficient data analysis program.

EPFL2020

Chargement

Nanofibres de carbone sur filtre métallique comme support catalytique structuré

Pascal Tribolet

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.

EPFL2006

Chargement

Afficher plus

Nanofibres de carbone sur filtre métallique comme support catalytique structuré

Pascal Tribolet

EPFL2006

Mechanisms of the Heterogeneously Catalyzed Reaction of CO2 Hydrogenation on Transition Metal Surfaces

Kun Zhao

EPFL2020

EPFL2019