Browsing by Author "Heikkinen, Niko"

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  • An atomic layer deposition diffusion-reaction model for porous media with different particle geometries
    (2024-03-07) Heikkinen, Niko; Lehtonen, Juha; Puurunen, Riikka L.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    This work presents a diffusion-reaction model for atomic layer deposition (ALD), which has been adapted to describe radial direction reactant transport and adsorption kinetics in a porous particle. Specifically, we present the effect of three particle geometries: spherical, cylindrical and a slab in the diffusion-reaction model. The reactant diffusion propagates as a unidimensional front inside the slab particle, whereas with cylinder and spherical particles, the reactant diffusion approaches the particle centre from two and three dimensions, respectively. Due to additional reactant propagation dimensions, cylindrical and spherical particles require less exposure for full particle penetration. In addition to the particle geometry effect, a sensitivity analysis was used to compare the impact of the particles’ physical properties on the achieved penetration depth. The analysis evaluates properties, such as the combined porosity and tortuosity factor, mean pore diameter, specific surface area, pore volume, and particle radius. Furthermore, we address the impact of the reactant molar mass, growth-per-cycle (GPC), sticking probability, reactant exposure and deposition temperature on the simulated diffusion and surface coverage profiles. The diffusion-reaction model presented in this work is relevant for the design and optimization of ALD processes in porous media with different particle geometries.
  • Cobalt catalyst characterization and modification by atomic layer deposition for Fischer-Tropsch synthesis
    (2019-06-18) Eskelinen, Patrik
     Kemian tekniikan korkeakoulu | Master's thesis
    Fischer-Tropsch synthesis (FTS) is a heterogeneously catalyzed process which produces hydrocarbons from carbon monoxide and hydrogen. FTS is a promising stepping stone to more effectively use available gas resources both fossil and renewable, or to produce petroleum substitutes from industrial sidestreams and captured carbon dioxide converted to carbon monoxide. From the catalysts used in FTS, cobalt-based catalysts are of interest due to their higher activity, heavier product fractions, and natural selectivity towards paraffins as compared to iron-based catalysts. The tailoring of catalysts produced by commonly used impregnation methods is a means to develop more resilient and selective catalysts. Atomic layer deposition (ALD) is capable of coating very conformal layers in porous catalysts, becoming a useful tool in modifying supported catalysts very precisely. Top-coating by ALD alumina and promotion by ALD platinum are the modifications investigated in this thesis to improve resilience and applying a highly dispersed effective promoter. A roster of characterization methods including X-ray diffraction, electron microscopy, energy dispersive X-ray spectroscopy, X-ray fluorescence, among others are discussed and implemented in this work. The catalysts studied are composed of silicon doped gamma alumina supported cobalt catalysts with ~30 wt% Co and 0.1 wt% Pt for the unmodified sample and ALD alumina modified samples. A similar catalyst without Pt promotion is used as base for ALD Pt modified sample. Around a gram of each catalysts was ran in a micro-reactor from 40 to 70 h. The conversions, selectivity and productivity were quantified to determine the practical qualities of the catalysts. Average crystallite size was determined from XRD to observe possible sintering. ALD alumina showed the desired effect in reducing average particle size after the run when compared to unmodified catalyst from approx. 20 nm to as low as 14 nm for 40 ALD cycles. As an active component ALD alumina at 5 cycles increased activity and methanation, while at 20 cycles the properties were similar to the unmodified catalyst. ALD Pt coated catalyst showed product selec-tivity towards lighter products. The hydrogenating effect of platina was strongly present, demon-strating the effectiveness of only 5 ALD Pt cycles. This thesis gave some indication on the effectiveness of ALD modifications for Fischer-Tropsch cobalt catalysts and the synergy between some of the characterization methods used in FTS.
  • Effect of water and atomic layer deposited overcoating on cobalt-based Fischer-Tropsch catalyst
    (2020-10-20) Palo, Jasmiina
     Kemian tekniikan korkeakoulu | Master's thesis
    This master’s thesis studies the effect water and atomic layer deposited (ALD) overcoating on activity, selectivity, and stability of cobalt catalyst in Fischer-Tropsch (FT) synthesis. ALD is a thin-film deposition technique based on sequential, self-limiting gas-solid reactions, and can be used to design and synthesize heterogeneous catalysts. ALD can also be used to grow protective films over porous catalysts. These ALD overcoatings are found to enhance catalytic activity and selectivity, and can improve catalyst stability against deactivation in high-temperature processes and liquid-phase reactions. The literature part of the thesis presents different catalyst overcoating techniques and their successful examples, with an emphasis on ALD overcoatings. The principle of both ALD technique and FT synthesis are also presented. In addition, the effect of water on cobalt-based catalysts in FT synthesis is reviewed. In the experimental part, the effect of water and ALD overcoating on the performance of cobalt-based FT catalyst was studied. A Co-Pt-Si/γ-Al2O3 catalyst was synthesized by co-impregnation technique and was further overcoated with 0, 30, 35, and 40 cycles of alumina ALD. The reaction experiments were conducted in a fixed-bed reactor at 20 bar, 200 °C, and with H2/CO molar ratio of 2.0. The effect of water was tested by increasing the feed water concentration in a step-wise manner. Furthermore, one experiment without water addition was carried out with the non-ALD-overcoated catalyst and with the catalyst with 40 cycles of ALD to study the effect of overcoating in a long-term. The catalyst with 40 cycles of alumina was found to be more active and stable compared to the other catalysts. In the experiment without added water, the catalyst with 40 cycles of ALD maintained stable activity during 144 h on stream. In addition, the same catalyst showed improved stability at low water concentrations (20 mol-%) with only 2 % loss of catalytic activity, while other catalyst lost 19-27 % of their activity at similar conditions. At high water concentrations (30 mol-%), all catalysts suffered from permanent deactivation, and only a relatively small part of the catalysts’ activities were recovered after the water supply was turned off. All catalysts showed decreased methane selectivity and increased long-chain hydrocarbon selectivity upon water additions. ALD overcoating was found to enhance secondary hydrogenation of olefins, which was seen as decreased olefin-to-paraffin ratio. Moreover, the overcoating increased methane selectivity and decreased formation of long-chain hydrocarbons.
  • Kinetic modelling of isosynthesis
    (2023-12-12) Chen, Kristian
     Kemian tekniikan korkeakoulu | Master's thesis
    Isosynthesis is a catalytic process where CO and H2 are reacted over a specific catalyst, typically a ZrO2 catalyst, to produce a mixture of hydrocarbons. The process favours the formation of i-C4 hydrocarbons which are used in a variety of applications in the chemical industry. Since the discovery of the process, there has been a limited amount of research done on isosynthesis, with even less research into the kinetic modelling of the process. The aim of this work was to develop models for the isosynthesis process and calculate the kinetic parameters for said models through optimization. In the literature review part of the thesis the isosynthesis process is discussed from a modelling standpoint. This includes the derivation of the rate equations in heterogeneous catalysis and the numerical methods often used in kinetic modelling. In the numerical methods, the Nelder-Mead simplex method and Levenberg-Marquardt algorithm are explained, and both are later used in the applied part. In the applied part of the thesis, three models have been developed for the isosynthesis process. The first model contains all the significant products of the isosynthesis process, the second model contains only the i-C4 products, and the last model is like the first one, with slightly different rate equations. The rate equations for the last model were formed based on a proposed reaction mechanism pathway for iso-synthesis. The evaluated kinetic parameters for each model included the pre-exponential factors and the activation energies of each reaction. Each model and the resulting kinetic parameters were evaluated, and the last model was determined as the best, with the most reliable values for the kinetic parameters.
  • Modelling atomic layer deposition overcoating formation on a porous heterogeneous catalyst
    (2022-09-14) Heikkinen, Niko; Lehtonen, Juha; Keskiväli, Laura; Yim, Jihong; Shetty, Shwetha; Ge, Yanling; Reinikainen, Matti; Putkonen, Matti
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Atomic layer deposition (ALD) was used to deposit a protective overcoating (Al2O3) on an industrially relevant Co-based Fischer-Tropsch catalyst. A trimethylaluminium/water (TMA/H2O) ALD process was used to prepare ∼0.7-2.2 nm overcoatings on an incipient wetness impregnated Co-Pt/TiO2 catalyst. A diffusion-reaction differential equation model was used to predict precursor transport and the resulting deposited overcoating surface coverage inside a catalyst particle. The model was validated against transmission electron (TEM) and scanning electron (SEM) microscopy studies. The prepared model utilised catalyst physical properties and ALD process parameters to estimate achieved overcoating thickness for 20 and 30 deposition cycles (1.36 and 2.04 nm respectively). The TEM analysis supported these estimates, with 1.29 ± 0.16 and 2.15 ± 0.29 nm average layer thicknesses. In addition to layer thickness estimation, the model was used to predict overcoating penetration into the porous catalyst. The model estimated a penetration depth of ∼19 μm, and cross-sectional scanning electron microscopy supported the prediction with a deepest penetration of 15-18 μm. The model successfully estimated the deepest penetration, however, the microscopy study showed penetration depth fluctuation between 0-18 μm, having an average of 9.6 μm.
  • Non-specific method for time-resolved fluorescence ion detection
    (2015-12-15) Heikkinen, Niko
     Kemian tekniikan korkeakoulu | Master's thesis
    Water in one form or another is present within countless manufacturing processes. In an oil extraction process, the volume of water is typically equal to the produced hydrocarbons and can even exceed the amount of extracted oil significantly. The large volumes of water, known as produced water, inflict problems. These problems are mainly related to dissolved ions that produced water is carrying. In oil production and several other industries, where pipelines are used to transport water, in suitable conditions, ions within the water will precipitate and form scale. Within a pipeline, scale may accumulate and disturb the liquid flow through the pipeline. In order to assess the scale potential, this thesis develops a fluorescence based analytical method for identifying components that can induce scale within produced water. In this thesis, the studied water samples are called produced water, which is an oil extraction by-product. This thesis examines the technique of time-resolved photoluminescence to develop a non-specific assay method for identifying several ions that may cause scale accumulation in production pipelines. The developed method was used to analyse both synthetic and field produced water samples. The results of this thesis indicate that thorough study of suitable assay components enables the possibility to create a measurement protocol to evaluate ion concentrations in a produced water sample. The quantification of ion concentrations allows the evaluation of scale potential in a production pipeline. This evaluation can be used to assess the need for treating chemical to prevent scale accumulation within the pipelines.
  • Ruthenium-based catalyst development for Fischer-Tropsch synthesis
    (2022-10-18) Singh, Abhinash
     Kemian tekniikan korkeakoulu | Master's thesis
    One of the most significant reactions in gas-to-liquid (GTL) technology is the Fischer-Tropsch synthesis (FTS), in which gaseous CO and H2 enter the reactor and liquid hydrocarbons exit. The primary goals of the thesis were to understand the current status of ruthenium-based FTS catalysts and to investigate the activity and selectivity of various Ru-based FTS catalysts. The literature part of the thesis focused on key parameters of catalyst design, including the use of different catalyst supports, promoters, active metal, metal particle size, metal loading, and preparation methods. In addition, the application of catalysts in microreactors is reviewed. In the experimental part, the activity and selectivity of different Ru-based catalysts were studied. Three 3 wt. % Ru-based catalysts on mesoporous silica support were prepared by vacuum sequential impregnation. Co and Zr promoters were investigated with metal loadings of 10 wt. % and 1.5 wt. %, respectively. Along with three prepared catalysts, one commercial Ru-based catalyst (5 wt. % Ru/Al2O3) was investigated in a fixed bed reactor at 20 bar, and H2/CO molar ratio of 2.0. The catalytic activity and product selectivity was studied at different temperatures and Gas Hourly Space Velocities (GHSV). Overall, the commercial catalyst was the most active catalyst. Among the prepared catalysts, the Ru-Co/SiO2 catalyst showed the highest activity while, the Ru/SiO2 catalyst showed the lowest activity at similar reaction conditions. The catalytic activity increased around 1.5 times with 10 °C rise in temperature. Overall, the product distribution is dependent on conversion levels. The unpromoted Ru catalyst, even at higher conversion level, showed lower methane selectivity, while the Ru-Co/SiO2 catalyst showed the highest methane selectivity. The Ru-Zr/SiO2 catalyst showed the lowest Olefin-to-Paraffin ratio, while the commercial catalyst showed the highest. The selectivity of long-chain hydrocarbons was highest for the Ru/SiO2 catalyst and the lowest for the Ru-Co/SiO2 catalyst. The Ru/SiO2 catalyst showed the highest probability of chain growth (α-value), while the commercial catalyst showed the lowest. The mesoporous silica support could have contributed to an increase in the α-value of all the prepared catalysts.