Supported ruthenium nanoparticles for supercritical water gasification - synthesis, activity and stability

Christopher Marc Hunston
EPFL, 2021
EPFL thesis

Abstract

Supercritical water gasification (SCWG) is a promising and versatile technology for the conversion of a variety of wet biomass streams into renewable natural gas. In this work, the focus was set on methane production with the help of an active and stable methanation catalyst. The supercritical water (SCW) environment can lead to rapid catalyst deactivation through different mechanisms, namely poisoning, coking, sintering or leaching. Although addressed, it remains challenging to disentangle them in SCWG. The goal of this thesis is to get a better understanding of the aforementioned deactivations pathways by trying to study them individually, in order to tailor an active and robust SCWG catalyst.The intrinsic gasification activity of carbon supports exhibiting different properties was assessed by feeding glycerol at 400°C and 30MPa. The negative effect of micropores was shown by significant coke deposition and surface area loss around 90%, compared to mesoporous carbons that only lost 20%. Carbon nanofibers (CNF) proved to be an interesting support showing very good stability and inertness. The synthesis of 5%Ru catalysts on selected supports confirmed these trends. Highly microporous Ru/C catalysts were less stable than the ones supported on carbon frameworks of more open pore structure. Ru/CNF seemed to be a promising SCWG catalyst, with an enhanced resistance towards coke formation. The completely open pore structure of CNF was found to be a crucial parameter.Ruthenium loss was investigated by measuring the its content in the process waters with ICP-MS. Ru was successfully quantified at levels close to thermodynamic models for Ru/AC and Ru/CNF of different loadings (0.01-0.2 µg L-1). Furthermore, the process parameters had no effect on the quantified Ru, showing that the steady-state loss represents a thermodynamically-driven leaching. Fluctuations in feed rate and pressure damaged the catalyst through friction and led higher Ru losses, mainly through the release of carbon domains containing Ru. Metal oxides were compared to AC by synthesising 2%Ru catalysts on alpha-Al2O3, ZrO2 and TiO2. They yielded 20-40 times higher Ru loss rates than for 2%Ru/AC and a significantly lower activity, confirming once again the superiority of Ru/C as SCWG catalyst. Ru losses were similar on larger rigs during glycerol and sewage sludge gasification, with values around 0.14-0.29 µg gRu-1 h-1.With Ru/CNF proving to be an interesting SCWG catalyst, more in-depth studies were performed on this system. Catalysts of different loadings (1-30%) were successfullysynthesised yielding very small Ru NPs (0.9-2.2 nm) with an easy synthesis method. Ru/CNF catalysts exhibited high gasification activities and improved stability compared to Ru/AC. The initial activity correlated with the Ru dispersion. However, 1%Ru/CNF catalysts underwent rapid deactivation compared to 5%Ru/CNF, showing the importance of site density to avoid rapid deactivation. The effect of surface Ru atom density was shown with a optimum in turnover frequency in the range 0.4-0.7 atomRu,sfc nm-2. For the first time, a particle size effect and more importantly a surface atom density effect was shown during SCWG of glycerol. Coke deposition was also investigated and observed at very high space velocities. The coke deposits could partially be removed by extraction in a mix of solvents, opening a potential new route for the regeneration of Ru/C catalysts.

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Christopher Marc Hunston
EPFL, 2021
EPFL thesis

Abstract

Official source

https://infoscience.epfl.ch/record/289341?ln=en

About this result

Related concepts (24)

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Related publications (31)

Related publications

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

Thermo-economic and environmental model of microalgae-to-SNG conversion process through hydrothermal gasification

Kevin Arrandel, François Maréchal, Alberto Mian, Adriano Viana Ensinas

Due to its high photosynthesis conversion efficiency, microalgae are currently considered as a serious feedstock for next-generation biofuels production. One possible conversion process is the production of synthetic natural gas (SNG) through catalytic hydrothermal gasification which uses a supercritical water-processing for the gasification and has the advantage to avoid drying of the biomass. The whole system includes four steps of production: microalgae cultivation, dewatering, gasification and product separation. In this paper the SNG production from microalgae is analyzed considering algal biomass cultivation system with open-ponds technology. To do so, a mathematical model based on mass and energy balances has been developed in order to represent microalgae growth process, and regarding culture temperature, solar-radiation incidence, CO2 and nutrients supply. The complete physical model of the biomass conversion system is developed including the dewatering system with settling ponds and centrifuges, catalytic hydrothermal gasification with salt separation unit and the SNG separation and purification system. An economic model is carried out for investment and operation cost of the process, from the biomass cultivation to the final product, including possibilities of nutrients, CO2 and water recovery. Finally, the environmental impact regarding the CO2 emission of this process and the avoided emission from the replacement of fossil natural gas is estimated in a complete life cycle analysis. A multi-objective optimization methodology is used to find the trade-off between the total cost of the system and the environmental impact that can be linked with the equipment size and the energy conversion efficiency.

2013

Chemical and electrochemical promotion of supported rhodium catalyst

Olena Baranova

The chemical and electrochemical promotion of highly dispersed nanofilm Rh catalysts (dispersion: about 10 %, film thickness: 40 nm) has been investigated for the first time. To this end Rh metal was sputter-deposited, either on a purely ionic conductor (8 % Y2O3-stabilized ZrO2) or on a mixed ionic-electronic conductor (TiO2), the latter being a highly dispersed layer of TiO2 (4 µm) deposited on YSZ. These catalysts are designated as Rh/YSZ and Rh/TiO2/YSZ, respectively. It was established analytically that both in the Rh/YSZ and in the Rh/TiO2/YSZ system, the catalyst films have a nanoparticle-size grain structure. The Rh supported on titania is rather porous, exhibiting a higher dispersion and surface area than Rh on YSZ. Both after reduction (H2, T=400 °C) and after oxidation (O2, T=400 °C), Rh supported on TiO2 was found to be in a more highly reduced state than Rh on YSZ. After reducing treatment, the Rh/TiO2/YSZ samples contain a larger amount of weakly bonded oxygen, which can be attributed to oxygen "backspillover" from the TiO2. A major feature of this research was the electrochemical characterization of the oxygen/Rh/solid electrolyte three-phase boundary by steady-state polarization measurements and by impedance spectroscopy. These are powerful techniques for extracting experimental trends and details that are useful for an understanding of the electrochemical promotion principles. It was shown that the exchange current densities at the Rh/solid electrolyte interface are lower on account of the TiO2 layer. The exchange current densities are more than twice lower at Rh/TiO2(4 µm)/YSZ than at Rh/YSZ, demonstrating that the former interface is much more polarizable. The mechanism of oxygen exchange occurring close to equilibrium (O2/O2- couple) was investigated for the first time at Rh catalyst electrodes interfaced with solid electrolyte. The processes of cathodic oxygen reduction and anodic oxygen evolution are not symmetric, but they are similar in the two systems, Rh/YSZ and Rh/TiO2(4 µm)/YSZ. The cathodic process consists of three steps: dissociative adsorption of oxygen at the gas-exposed Rh surface, atomic oxygen diffusion to the electrochemical reaction sites (ERS), and a two-electron transfer to this oxygen on the ERS. The cathodic process is limited by interfacial diffusion of oxygen atoms from the gas-exposed metal surface to the ERS. The anodic process includes two steps: two-electron transfer reaction, which is the rate-determining step, and oxygen desorption to the gas phase. With data obtained from impedance spectroscopy at the equilibrium potential, it was possible to confirm the reaction scheme proposed. It was demonstrated that Rh nanofilm catalysts interfaced with YSZ or TiO2/YSZ can be electrochemically promoted for the reaction of ethylene oxidation. Small anodic currents cause periodic oscillations in catalytic rate and potential of the Rh/YSZ catalyst, while at Rh/TiO2/YSZ they give rise to a stable and reversible rate enhancement by up to a factor 80. The increase in ethylene oxidation rate is up to 2000 times larger than the electrochemical rate, I/2F, of O2- oxidation. The pronounced electrochemical promotion behavior that has been observed is due to the anodically controlled migration of O2- species from the electrolyte to the Rh/gas interface. At the Rh surface, these species destabilize the formation of rhodium surface oxide (Rh2O3). The existence of backspillover oxygen species has been confirmed by impedance measurements under positive applied potential. Another significant result for heterogeneous catalysis is the finding that thick as well as thin films of Rh/TiO2/YSZ catalyst are open to chemical promotion of ethylene oxidation and partial methane oxidation, ie, they offer a higher catalytic activity and stability than the Rh/YSZ catalysts. The modification of catalytic activity observed for Rh/TiO2/YSZ was attributed to either a "long-range" electronic-type SMSI mechanism or to a self-driven electrochemical promotion mechanism. In both cases, the ultimate cause of promotion are different work functions of catalyst and support. Equilibration of the work functions of two solids in contact induces surface charging, a migration of O2- ions to the catalyst/gas interface, and a weakening of the Rh-O chemisorptive bonds. It facilitates reduction of oxidized surface sites. Another important achievement of the present work was that of exploring and confirming the possibilities of current-assisted activation of Rh/TiO2/YSZ catalysts. In partial methane oxidation using close to stoichiometric gas compositions (CH4 : O2 = 2 : 1) at 550 °C, the inactive (oxidized) Rh/TiO2/YSZ catalyst was successfully activated by applied currents, either positive or negative. This phenomenon is an example of "permanent" electrochemical promotion furnishing a permanent rate enhancement ratio of γ = 11. The activation by negative currents is explained in terms of an electrochemical reduction of rhodium surface oxide, while the activation by positive currents can be explained by the mechanism of electrochemical promotion.

EPFL2005

Chemical and electrochemical promotion of supported rhodium catalyst

Olena Baranova

EPFL2005

EPFL2006