Hydrogen-Assisted Transformation of CO2 on Nickel: The Role of Formate and Carbon Monoxide

The nanoscale description of the reaction pathways and of the role of the intermediate species involved in a chemical process is a crucial milestone for tailoring more active, stable, and cheaper catalysts, thus providing "reaction engineering" capabilities. This level of insight has not been achieved yet for the catalytic hydrogenation of CO2 on Ni catalysts, a reaction of enormous environmental relevance. We present a thorough atomic-scale description of the mechanisms of this reaction, studied under controlled conditions on a model Ni catalyst, thus clarifying the long-standing debate on the actual reaction path followed by the reactants. Remarkably, formate, which is always observed under standard conditions, is found to be just a "dead-end" spectator molecule, formed via a Langmuir-Hinshelwood process, whereas the reaction proceeds through parallel Eley-Rideal channels, where hydrogen-assisted C-O bond cleavage in CO2 yields CO already at liquid nitrogen temperature.

Développement de réacteurs microstructurés pour la conversion du propane en produits valorisés

Olivier Wolfrath

Industry requires continuously new chemical processes to convert raw materials to valuable products limiting the formation of by-products. Since a few years, an increasing interest has been given to the intensification of chemical processes. This represents a new approach in chemical reaction engineering and calls for a new type of chemical reactors: the microstructured reactors. They are characterized by miniature structures whose components have a sub-millimeter size. The flow channels, for example, have a diameter from ten to several hundred microns. The distinctive feature of these structures is a very large surface/volume ratio, namely between 10'000 and 50'000 m2/m3. In these conditions, heat transfer coefficient is very high, up to 25 kW · m-2 · K-1. It is accepted that the residence time distribution in a reactor strongly influences the product selectivity and yield. In these thin channels, the RTD is particularly narrow due to a short radial diffusion time (ms), allowing a precise control of the residence time. Another advantage of these microstructures is an increase of the inherent safety due to a small amount of the compounds used. However, fabrication of microreactors, especially the introduction of active catalyst in microchannels remains a difficult operation. And integration of microsystems in conventional chemical processes is still a challenge. The aim of this work is first to develop a novel membrane reactor based on a new microstructured catalytic packing in the filamentous form and to apply it to the nonoxydative dehydrogenation of propane. A second objective is to develop a reactor with micro heat exchangers allowing an optimized control of homogeneous reactions. This reactor was tested with the partial oxidation of propene to propylene oxide. The microstructured catalyst above is made of thin and long catalytic filaments placed in parallel with a tubular reactor. This new kind of catalyst is based on silica fibres covered by a γ-alumina layer. The position in parallel of these filaments of 7 μm diameter forces the gases to flow in the fibres direction between the filaments. The hydraulic diameter is about 70 μm. Due to this laminar and regular flow, the pressure drop in the bed was divided by 5 compared to a conventional bed (100-160 μm spheres) and the residence time distribution was particularly narrower. Such novel packing brings new opportunities to the heterogeneous catalysis domain because it can be used for many other reactions improving highly the hydrodynamics and keeping an excellent contact with the reactants. The microstructured catalyst was installed in two concentric zones of a tubular reactor separated by a palladium-silver membrane permeable to hydrogen. This product was removed from the gas phase to shift the thermodynamic equilibrium, enhancing the propane conversion from 22% up to 30%. Another significant advantage in this reactor is the increase of propene selectivity up to 97% in comparison to conventional industrial processes reaching 80-90%. This difference is due to the diminution of hydrogenolysis and hydroisomerization reactions. The dehydrogenation is known to be endothermic. Oxidation of the hydrogen passing through the membrane produced heat for this reaction along the catalytic zone. Moreover, a periodic regeneration by air allowed recovering the activity of the catalyst deactivated by coke formed during dehydrogenation. Periodic reaction operations allowed producing continuously propene with a high selectivity, and regenerating simultaneously the catalyst. The second microstructured reactor was developed integrating two micro heat exchangers at the inlet and outlet of a macroscopic tube. The aim is to heat up very fast the gas and to cool it down as well. Without these exchangers, heating and cooling duration corresponded each to about 20% of the total residence time. The use of the microstructures allowed diminishing significantly these durations (4-5% each). Applied to partial oxidation of propene, this reactor reached a more precise control of reaction temperature and duration. The propylene oxide (PO) selectivity was not increased with this system but, due to the microstructures, it was possible to work at higher temperatures where the reaction becomes very fast, without reaching an oxygen conversion of 100%. Indeed, it is essential to avoid this situation where oligomers and coke form dividing by 2 the PO selectivity. So another new kind of reactor was developed suitable for any fast exothermic reaction whose progression has to be limited because of potential consecutive reactions reducing, for example, the selectivity of an intermediate desired compound.

EPFL2001

Production d'hydrogène dans un réacteur microstructuré

Pierre Reuse

The aim of this work was to operate the steam reforming of methanol and the total oxidation of methanol in a microstructured two-passage reactor. The steam reforming enables the production of hydrogen at relatively low temperatures (250 °C) with only a small amount of carbon monoxide (~ 1%). After a cleaning step (CO preferential oxidation, for example), this hydrogen would be a good feed for a fuel cell to produce electricity for mobile applications. Steam reforming is an endothermic reaction. The energy needed to run this reaction is produced by the total oxidation, a strongly exothermic reaction. To allow an efficient heat transfer between these two reactions, a microstructured reactor was used. These structures, with dimensions in the micron range, are widely known to have far better heat transfer capacity than traditional heat exchangers. Furthermore, these structures have some unique features with respect to classical reactors. A narrow residence time distribution, close to that of a plug flow reactor, can be obtained, keeping selectivity high for complex reactions. Microstructured reactors also have small residence time, allowing very quick variations in the feed and experimental conditions. In chapter 4, the concepts relating to the channel coatings will be presented. A Cu/Zn/Al catalyst provided by Süd-Chemie was transformed into a suspension of nanoparticles in isopropanol by using an attrition grinding apparatus. A thin layer (~5 µm) of catalyst was formed on the microchannel walls. Several tests were performed to characterize the adhesion of this layer. The influence of the layer on the residence time distribution was also investigated; it was possible to maintain Bodenstein numbers as high as 50. Chapter 5 is focused on the steam reforming of methanol, the reaction that generates hydrogen. A kinetic analysis in differential and integral modes established that the rate of the reaction is inhibited by hydrogen, the main product of the reaction; the partial order is - 0.2. The influence of water on the rate of reaction is negligible (partial order of 0.1), and methanol has a positive partial order of 0.7. A kinetic constant of 5.57 · 10-6 mol0.4 · (m3)0.6 · gcat-1 · s-1 at 200 °C and an activation energy of 73.9 kJ · mol-1 complete the description of the kinetics of the steam reforming. Throughout the comparisons, we showed that the catalyst coated in the microchannels has an initial rate for the steam reforming 30% higher than the original catalyst in fixed bed. We can therefore conclude that the catalyst modification procedure does not diminish the catalyst activity. This was also shown by comparing our results with results available in the scientific literature for similar catalysts. CO2 selectivity remains high and constant then decreases as 90% conversion is approached. Working with an excess of water with respect to methanol contributes to keeping the selectivity high. A molar ratio of 1.2 is a good compromise between selectivity and increased energy costs due to the vaporization of the additional water. It's also beneficial to work at conversions below 90% to be in the high selectivity domain and to diminish the inhibiting influence of hydrogen. Finally, we showed that this catalyst deactivates over 100 hours at a temperature of 300 °C. A kinetic model for the deactivation at temperatures between 300 °C and 330 °C was established. In chapter 6 the total oxidation of methanol was studied. A cobalt based catalyst was developed to allow the synchronization of the two reactions temperatures. The minimum temperature for complete conversion for the total oxidation reaction must clearly not exceed an acceptable working temperature for the steam reforming catalyst (i.e. a temperature that results in an acceptable catalyst lifetime). A kinetic analysis determined partial orders for methanol and oxygen as 0.67, the kinetic constant at 230 °C as 5.45 · 10-3 mmol-0.33 · m4 · (gcat · s)-1 and the activation energy as 130 kJ · mol-1. The copper based catalyst used for the steam reforming of methanol was also active for the total oxidation of methanol. Unfortunately the minimal temperature to ensure a total conversion was not compatible with the working temperature of the steam reforming. In chapter 7, the results concerning the coupling of the two previously studied reactions are presented. The numerous fittings and cables necessary for the operation of the reactor didn't allow the reactor to be run in an autothermal mode. Therefore constant, not regulated, heating was used to compensate the thermal losses. By coupling the two reactions and by studying the dynamic response time of the reactor (temperature and conversion of the steam reforming reaction) at the time of stopping and starting the oxidation reaction, we've seen that the temperature increase was around +10 °C within 60 seconds and that the conversion for the steam reforming of the methanol fitted well with the temperature increase. This reactor also has good dynamics and responds quickly to feed variations. When the coupling mode was changed from co-current to counter-current we noticed a significant increase in conversion. Furthermore, selectivity in counter-current mode remained high, even for conversion approaching 100%.

EPFL2003

Investigation of the heterogeneously catalysed gas phase CO2 hydrogenation reaction: Development of analysis methods and reaction analysis on pristine metal catalysts

Robin Tobias Andreas Mutschler

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