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Synthesis gas conversion to short olefins and aromatics using bifunctional catalysts has gained much attention in recent years. Here, we study the interaction between the components of bifunctional catalysts to design a more stable catalyst system. Mixing α-alumina supported iron (-carbide) promoted with sodium and sulfur with an H-ZSM-5 zeolite to convert synthesis gas to aromatics and short olefins we observed selectivity loss of the iron (-carbide) catalyst as well as the acid function. This was displayed by increasing methane and decreasing aromatics selectivity when the two individual catalysts were mixed in close proximity. We introduced different approaches to understand this selectivity related deactivation. Larger spatial separation of the iron and zeolite allowed a more stable system with constant methane and aromatics selectivity. Alternatively, iron supported on carbon nano tubes mixed with the zeolite in close proximity did not display selectivity related deactivation. We conclude that the selectivity loss was caused by migration of sodium ions that were used next to sulfur as promoters on the iron catalyst over the α-alumina support to the zeolite, which was supported by XPS model experiments. This migration seems hindered on carbon supported iron catalysts.

In the epoxidation of ethylene, catalyzed by supported Ag particles, the support plays not only a beneficial role, but can also negatively impact the selectivity to ethylene oxide. Especially high surface area supports, and supports containing acid groups, seem detrimental for the selectivity, which is attributed to secondary reactions on the support surface. We prepared Ag nanoparticles on supports with a wide range of specific surface areas and different chemical nature, but with similar Ag metal loading and particle size. Indeed, a strong dependence of selectivity on both the specific surface area, and the chemical nature of the oxide support was found, with α-Al 2 O 3 giving much more selective catalysts than SiO 2 . Furthermore, the conversion was an important factor in determining the selectivity, while on the other hand the support had no influence at all on the ethylene conversion. We described our experimental selectivities with a reaction model that used the rate constant for the secondary reaction of oxidation of ethylene oxide to carbon dioxide and water as the only variable parameter. The model was able to adequately describe the experimental results. It gave insight into the dependence of selectivity on conversion, how the secondary reaction depended on the chemical nature of the support, and how its rate scaled linearly with the support's specific surface area. Building on this insight we modified a SiO 2 support to passivate its OH-groups, which indeed yielded a 94% decrease in the rate of the secondary reaction, performing even better than α-Al 2 O 3 with similar specific surface area would. We have shown that the selectivity towards ethylene oxide is dependent on a metal specific, intrinsic selectivity, and the decrease in selectivity with conversion to be support dependent. This decrease is caused by the isomerization of ethylene oxide, which is found to be first order in ethyle

The effect of oxidative treatments on the depostion of cobalt nanocrystals (Co-NC) onto a support and subsequent ligand removal was investigated. Deposition of ϵ-cobalt NC led to extensive clustering of NC and low Fischer-Tropsch synthesis activity. Low-temperature oxidation of ϵ-cobalt NC resulted in a very uniform CoO-NC distribution and high activity whereas high-temperature oxidation to Co3O4 led to less uniform NC distributions and lower activity.

Currently, for the industrial ethylene epoxidation α-alumina supported silver catalysts are the only catalyst of choice. We demonstrate a novel method to produce these catalysts with different silver particle sizes, but without changing other key parameters that may affect the catalytic performance such as support specific surface area or metal precursor. α-Alumina was impregnated with a silver oxalate solution, and was subsequently dried and treated in different gas atmospheres and at different temperatures to tune the silver particle sizes in the range of 20–500 nm. Particles of 20 nm exhibited a lower turnover frequency than particles of 70 nm and larger, which exhibit a constant turnover frequency, in accordance with results in literature. However, the selectivity, when measured at constant conversion, was particle size independent. This is the first time that the effect of the particle size on the selectivity of ethylene epoxidation is reported at constant conversion. This was made possible by a new method of producing supported silver catalysts, which we expect that is also applicable for silver catalysts with other supports and for the preparation of other supported metal catalysts.

Colloidal synthesis routes have been recently used to fabricate heterogeneous catalysts with more controllable and homogeneous properties. Herein a method was developed to modify the surface composition of colloidal nanocrystal catalysts and to purposely introduce specific atoms via ligands and change the catalyst reactivity. Organic ligands adsorbed on the surface of iron oxide catalysts were exchanged with inorganic species such as Na2S, not only to provide an active surface but also to introduce controlled amounts of Na and S acting as promoters for the catalytic process. The catalyst composition was optimized for the Fischer-Tropsch direct conversion of synthesis gas into lower olefins. At industrially relevant conditions, these nanocrystal-based catalysts with controlled composition were more active, selective, and stable than catalysts with similar composition but synthesized using conventional methods, possibly due to their homogeneity of properties and synergic interaction of iron and promoters. (Chemical Equation Presented).

Ordered mesoporous carbon (CMK-3) with different surface modifications is applied as support for iron-based catalysts in the Fischer-Tropsch to olefins synthesis with and without sodium and sulfur promoters. Different concentrations of functional groups do not affect the size (3-5 nm) of iron particles in the fresh catalysts but iron(carbide) supported on nitrogen-enriched CMK-3 and a support with lower concentration of functional groups show higher catalytic activity under industrially relevant FTO conditions (340°C, 10 bar, H2/CO = 2) as compared to a support with oxygen-enriched surface. The addition of promoters leads to more noticeable enhancements of the catalytic activity (3-5 times higher) and the selectivity to C2-C4 olefins (~2 times higher) than surface functionalization of the support. Nitrogen surface functionalization and removal of surface groups before impregnation and calcination, however, further increase the activity of the catalysts in the presence of promoters. Confinement of the iron nanoparticles in the mesopores of CMK-3 restrict but not fully prevent particle growth and consequent decrease of activity under FTO conditions.

Colloidal synthesis routes have been recently used to fabricate heterogeneous catalysts with more controllable and homogeneous properties. Herein a method was developed to modify the surface composition of colloidal nanocrystal catalysts and to purposely introduce specific atoms via ligands and change the catalyst reactivity. Organic ligands adsorbed on the surface of iron oxide catalysts were exchanged with inorganic species such as Na2S, not only to provide an active surface but also to introduce controlled amounts of Na and S acting as promoters for the catalytic process. The catalyst composition was optimized for the Fischer-Tropsch direct conversion of synthesis gas into lower olefins. At industrially relevant conditions, these nanocrystal-based catalysts with controlled composition were more active, selective, and stable than catalysts with similar composition but synthesized using conventional methods, possibly due to their homogeneity of properties and synergic interaction of iron and promoters. (Chemical Equation Presented).

Ordered mesoporous carbon (CMK-3) with different surface modifications is applied as support for iron-based catalysts in the Fischer-Tropsch to olefins synthesis with and without sodium and sulfur promoters. Different concentrations of functional groups do not affect the size (3-5 nm) of iron particles in the fresh catalysts but iron(carbide) supported on nitrogen-enriched CMK-3 and a support with lower concentration of functional groups show higher catalytic activity under industrially relevant FTO conditions (340°C, 10 bar, H2/CO = 2) as compared to a support with oxygen-enriched surface. The addition of promoters leads to more noticeable enhancements of the catalytic activity (3-5 times higher) and the selectivity to C2-C4 olefins (~2 times higher) than surface functionalization of the support. Nitrogen surface functionalization and removal of surface groups before impregnation and calcination, however, further increase the activity of the catalysts in the presence of promoters. Confinement of the iron nanoparticles in the mesopores of CMK-3 restrict but not fully prevent particle growth and consequent decrease of activity under FTO conditions.

For decades it has been debated whether the conversion of synthesis gas to methanol over copper catalysts is sensitive or insensitive to the structure of the copper surface. Here we have systematically investigated the effect of the copper particle size in the range where changes in surface structure occur, that is, below 10 nm, for catalysts with and without zinc promotor at industrially relevant conditions for methanol synthesis. Regardless of the presence or absence of a zinc promotor in the form of zinc oxide or zinc silicate, the surface-specific activity decreases significantly for copper particles smaller than 8 nm, thus revealing structure sensitivity. In view of recent theoretical studies we propose that the methanol synthesis reaction takes place at copper surface sites with a unique configuration of atoms such as step-edge sites, which smaller particles cannot accommodate. 1

The Fischer–Tropsch to olefins (FTO) process is a method for the direct conversion of synthesis gas to lower C2–C4 olefins. Carbon-supported iron carbide nanoparticles are attractive catalysts for this reaction. The catalytic activity can be improved and undesired formation of alkanes can be suppressed by the addition of sodium and sulfur as promoters but the influence of their content and ratio remains poorly understood and the promoted catalysts often suffer from rapid deactivation due to particle growth. A series of carbon black-supported iron catalysts with similar iron content and nominal sodium/sulfur loadings of 1–30/0.5–5 wt% with respect to iron are prepared and characterized under FTO conditions at 1 and 10 bar syngas pressure to illuminate the influence of the promoter level on the catalytic properties. Iron particles and promoters undergo significant reorganization during FTO operation under industrially relevant conditions. Low sodium content (1–3 wt%) leads to a delay in iron carbide formation. Sodium contents of 15–30 wt% lead to rapid loss of catalytic activity due to the covering of the iron surface with promoters during particle growth under FTO operation. Higher activity and slower loss of activity are observed at low promoter contents (1–3 wt% sodium and 0.5–1 wt% sulfur) but a minimum amount of alkali is required to effectively suppress methane and C2–C4 paraffin formation. A reference catalyst support (carbide-derived carbon aerogel) shows that the optimum promoter level depends on iron particle size and support pore structure.

Lower C2-C4 olefins are important commodity chemicals usually produced by steam cracking of naphtha or fluid catalytic cracking of vacuum gas oil. The Fischer-Tropsch synthesis of lower olefins (FTO) with iron-based catalysts uses synthesis gas as an alternative feedstock. Nanostructured carbon materials are widely applied as supports for the iron nanoparticles due to their weak interaction with the metal species, facilitating the formation of catalytically active iron carbide. Numerous synthetic approaches towards carbon-supported FTO catalysts with various structures and properties have been published in recent years but structure-performance relationships remain poorly understood. We apply ordered mesoporous carbon (CMK-3) as a support material with well-defined pore structure to investigate the relationships between calcination/activation conditions and catalytic properties. After loading of iron and sodium/sulfur as the promoters, the structures and properties of the FTO catalysts are varied by using different calcination (300-1000°C) and activation (350 or 450°C) temperatures followed by FTO testing at 1 bar, 350°C, H2/CO = 1. Carbothermal reduction of iron oxides by the support material occurs at calcination temperatures of 800 or 1000°C, leading to a higher ratio of catalytically active iron(carbide) species but the catalytic activity remains low due to particle growth and blocking of the catalytically active sites with dense graphite layers. For the samples calcined at 300 and 500°C, the formation of non-blocked iron carbide can be enhanced by activation at higher temperatures, leading to higher catalytic activity. Olefin selectivities of ∼60%C in the formed hydrocarbons with methane of ∼10%C are achieved for all catalysts under FTO conditions at low CO conversion. The influence of the calcination temperature is further investigated under industrially relevant FTO conditions. Promoted CMK-3-supported, Technici Pool

Lower C2-C4 olefins are important commodity chemicals usually produced by steam cracking of naphtha or fluid catalytic cracking of vacuum gas oil. The Fischer-Tropsch synthesis of lower olefins (FTO) with iron-based catalysts uses synthesis gas as an alternative feedstock. Nanostructured carbon materials are widely applied as supports for the iron nanoparticles due to their weak interaction with the metal species, facilitating the formation of catalytically active iron carbide. Numerous synthetic approaches towards carbon-supported FTO catalysts with various structures and properties have been published in recent years but structure-performance relationships remain poorly understood. We apply ordered mesoporous carbon (CMK-3) as a support material with well-defined pore structure to investigate the relationships between calcination/activation conditions and catalytic properties. After loading of iron and sodium/sulfur as the promoters, the structures and properties of the FTO catalysts are varied by using different calcination (300-1000°C) and activation (350 or 450°C) temperatures followed by FTO testing at 1 bar, 350°C, H2/CO = 1. Carbothermal reduction of iron oxides by the support material occurs at calcination temperatures of 800 or 1000°C, leading to a higher ratio of catalytically active iron(carbide) species but the catalytic activity remains low due to particle growth and blocking of the catalytically active sites with dense graphite layers. For the samples calcined at 300 and 500°C, the formation of non-blocked iron carbide can be enhanced by activation at higher temperatures, leading to higher catalytic activity. Olefin selectivities of ∼60%C in the formed hydrocarbons with methane of ∼10%C are achieved for all catalysts under FTO conditions at low CO conversion. The influence of the calcination temperature is further investigated under industrially relevant FTO conditions. Promoted CMK-3-supported, RID/TS/Technici Pool

In the Fischer-Tropsch synthesis – the heterogeneously catalyzed conversion of syngas into hydrocarbon chains - transportation fuels can be produced from alternative sources to crude oil. In cobalt based Fischer-Tropsch catalysis the support material is generally oxidic in nature. A drawback of using oxidic supports is that they can form mixed compounds with cobalt that are not active in the FT catalysis and are only reducible at high temperatures. One way to overcome this problem is by the use of an inert support, e.g. carbon nanotubes (CNT). CNT exhibit a variety of interesting properties for support materials, including high mechanical strength, high thermal conductivity, and good chemical inertness in aggressive media. However, the well defined structures of the tubes would facilitate the study of the stability measures taken by means of surface roughening and heat treatment. The aim of this project was to prepare highly active cobalt on carbon nanotubes Fischer-Tropsch (FT) catalyst with low methane selectivity and a high C5+ selectivity, and to study the effects of surface roughening and heat treatment on the activity and stability of Co/CNT FT catalysts. Research presented in this thesis has shown the potential cobalt supported on CNT have in Fischer-Tropsch catalysis. With a relatively simple preparation route, highly active Fischer-Tropsch catalysts were prepared.

In the Fischer-Tropsch synthesis – the heterogeneously catalyzed conversion of syngas into hydrocarbon chains - transportation fuels can be produced from alternative sources to crude oil. In cobalt based Fischer-Tropsch catalysis the support material is generally oxidic in nature. A drawback of using oxidic supports is that they can form mixed compounds with cobalt that are not active in the FT catalysis and are only reducible at high temperatures. One way to overcome this problem is by the use of an inert support, e.g. carbon nanotubes (CNT). CNT exhibit a variety of interesting properties for support materials, including high mechanical strength, high thermal conductivity, and good chemical inertness in aggressive media. However, the well defined structures of the tubes would facilitate the study of the stability measures taken by means of surface roughening and heat treatment. The aim of this project was to prepare highly active cobalt on carbon nanotubes Fischer-Tropsch (FT) catalyst with low methane selectivity and a high C5+ selectivity, and to study the effects of surface roughening and heat treatment on the activity and stability of Co/CNT FT catalysts. Research presented in this thesis has shown the potential cobalt supported on CNT have in Fischer-Tropsch catalysis. With a relatively simple preparation route, highly active Fischer-Tropsch catalysts were prepared.

The reaction kinetics and reversibility for hydrogen sorption were investigated for supported Mg2Cu nanoparticles on carbon. A new preparation method is proposed to synthesize the supported alloy nanoparticles. The motivation of using a support is to separate the nanoparticles to prevent sintering at elevated temperatures. Supported nanocrystallites with an average size of 20 nm were obtained on porous graphite and larger particles ( 300 nm) on non-porous graphite by first deposition of metallic Cu species, using solution impregnation, followed by addition of molten Mg and hydrogenation. The temperature for hydrogen release of the 20 nm particles was much lower ( 150 C) than the micronsized material, and the reaction was reversible with the same improved kinetic performance after several hydrogen sorption cycles. The 20 nm Mg2Cu crystallites had a lower activation energy for the hydrogen desorption reaction compared to the bulk material (97 ( 9) and 128 ( 6) kJ mol 1 respectively). A desorption enthalpy of 66 ( 3) kJ mol 1 and an entropy value of 126 ( 10) J mol K 1 were found for this system. The use of a porous carbon support was beneficial for obtaining Mg2Cu nanoparticles, which improved the hydrogen sorption kinetics.

Controlling the nanoparticle distribution over a support is considered essential to arrive at more stable catalysts. By developing a novel freeze drying method, the nanoparticle distribution was successfully manipulated for the preparation of Co/SiO2 Fischer-Tropsch catalysts using a commercial silica-gel support. After loading the precursor via a solution impregnation or melt infiltration, differential scanning calorimetry was used to study the phase behavior of the confined cobalt nitrate precursor phases to ascertain suitable freeze-drying conditions. When a conventional drying treatment was utilized, catalysts showed inhomogeneous cobalt distributions, with 6 – 8 nm nanoparticles grouped in clusters of up to 400 nm. In contrast, by utilizing freeze-drying starting at liquid nitrogen temperatures, homogeneous distributions of 4 – 7 nm nanoparticles were obtained. Raising the temperature at which the freeze drying process takes place resulted in either uniform or strongly nonuniform nanoparticle distributions, depending on the specific conditions and precursor loading method. After reduction, all catalysts showed high activity for the Fischer- Tropsch reaction at 1 bar. The catalysts thus synthesized form an excellent platform for future studies of the stability under industrially relevant Fischer-Tropsch conditions.

Particle growth is a major deactivation mechanism for supported metal catalysts. This study reveals that the impact of pore size on catalyst stability is very sensitive to the nanoscale metal distribution. A set of ex-nitrate CuZn/SiO2 catalysts was synthesised using SiO2-gel supports (pore size 5-23 nm). The catalyst compositions were adjusted to attain series of catalysts with either constant pore volumetric (1.6 Cu nm-3) or surface (2.0 Cu nm-2) overall metal loading. The procedures of thermal decomposition of the metal nitrate precursors were adjusted to achieve <10 nm Cu particles displaying markedly different nanospatial distributions, i.e either gathered in high-metaldensity domains with small interparticle spacings or evenly distributed over the support with maximum interparticle spacings. Under industrially relevant methanol synthesis conditions, a strong increase of the deactivation rate with the support pore size is observed for catalysts with high-density domains of Cu particles. For these samples the local, nanoscale Cu surface loading is determined by pore size rather than by the overall metal content, as ascertained by HAADF-STEM/EDX. Conversely, Cu nanoparticles evenly spaced on the surface of the SiO2 carrier show improved stability, being the deactivation rate chiefly independent of the support pore size. The differences in catalyst stability are ascribed to the dominance of different particle growth mechanisms. Our study highlights the significance of local, nanoscale properties for rationalizing the relevance of structural parameters such as pore size for catalyst stability.