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Steam methane reforming is an important industrial reaction for the conversion of methane with steam to synthesis gas, a mixture of carbon monoxide and hydrogen. Hydrogen is used in many applications, e.g. for hydrogenation purposes. It is also used for the production of bulk chemicals such as ammonia and in combination with carbon monoxide for the manufacture of methanol and other oxygenates. Fischer-Tropsch synthesis is rapidly becoming a more important source of clean transportation fuels and potentially of chemicals and is also based on the use of synthesis gas. The most frequently used metal for steam methane reforming (SMR) catalysts is nickel. Typical reaction conditions for SMR are 800-900 °C to obtain high conversion of this endothermic reaction. However, in a number of applications, e.g. in pre-reforming and especially in future membrane steam reformers, the use of noble metals with a higher catalytic activity may be interesting. Especially in separation-enhanced reformers, which are typically run at lower temperatures, it is important to maintain high catalytic activity by shifting to noble metals such as Ru or Rh. Noble metals also show a higher resistance against carbon formation, which is one of the primary causes of deactivation, especially under carbon-rich conditions as can occur in a separation-enhanced reformer. The main goal of separation-enhanced steam reformers is to obtain pure streams of H2 and CO2 (CO is converted by the water-gas shift reaction to CO2), the former to be used for electricity generation, the latter for storage. As such, this concept is a potential technology for carbon capture and sequestration. As cost is a crucial factor for electricity generation, it is important to decrease the catalyst cost as much as possible, especially when the use of noble metals is required. Thus, we explore here in detail the structure dependence of the SMR reaction for Rh nanoparticles with the aim to guide the design and synthesis of optimal steam reforming catalysts. To investigate structure sensitivity, we study the three candidate rate controlling elementary reaction steps in steam methane reforming, i.e. water dissociation (Chapter 3), CO formation (Chapter 4) and CH4 dissociation (Chapter 5). This is done by density functional theory (DFT), which is a state-of-the-art technique to compute activation barriers of periodic models of transition metal surfaces. The results of these three studies are used as input for a microkinetic simulation model, which aims to understand the importance of the various reaction mechanisms as well as to explain experimentally observed structure dependence (Chapter 6). Figure 1. Representation of two Rh surfaces models employed in the present work. A stepped surface (a) with (211) steps and (111) terraces. The reactivity of the edge atoms as occurring on nanoparticles is simulated using a nanorod model (b). In Chapter 3, the dissociation of water on planar and stepped surfaces (Figure 1a) and the role of oxygen adatoms herein were investigated. It is concluded that the activation of water is not influenced by the coordination number of the surface atoms involved. The activation energies are very close with a value of 63 kJ/mol for the planar Rh(111) and of 61 kJmol for the stepped Rh(221) surface. However, in the presence of a surface oxygen atom, which can act as a hydrogen acceptor, the barrier for the stepped surface (28 kJ/mol) is significantly lower than on the planar surface (53 kJ/mol). Despite this difference, a close inspection of the potential energy diagram indicates that the overall barrier in the latter cases is also around 60 kJ/mol, because one needs to account for the energy cost to bring the oxygen adatom in a favorable position. This process is endothermic because of the lateral interactions. A Brønsted-Evans-Polanyi type correlation of the activation barriers with the metal-hydroxyl bond shows that the transition state of water dissociation has a slightly late character. Another finding is that there are large compensation effect for the oxygen-assisted water dissociation mechanism. Chapter 4 compares three different reaction mechanisms for CO formation on planar Rh(111) and stepped Rh(211) surfaces (Figure 1a) starting from adsorbed CH and O. A general conclusion is that the direct mechanism via C+O recombination competes with the one going through a formyl intermediate (HCO), whereas the pathway via an alcoholate intermediate (COH) is unfavorable. On the planar surface the barrier for CH dissociation is ~100 kJ/mol, but the overall barrier for CO formation is 180 kJ/mol. The formation of the formyl intermediate on the planar surface has an activation barrier of 180 kJ/mol and its subsequent decomposition towards CO and H is easy. Overall barriers for these two processes are very similar. The presence of a step-edge site is favorable for formyl formation and decreases the barrier to 93 kJ/mol. Its influence of CH dissociation is minor and as a result the overall barrier for the formyl pathway is slightly preferred on the stepped surface. The consequences of these differences in reactivity are discussed for particle size dependence of the steam methane reforming reaction. Because this reaction is typically applied at relatively high temperature, the activation free energy of dissociative methane adsorption will be significantly higher than that of the surface recombination reactions that lead to CO formation. The particle size dependence observed experimentally, therefore, follows the changes in the activation of methane and this should be due to the increase of edge and corner sites with decreasing particle size. When C-O bond formation would have been rate controlling, a maximum in the rate of the methane steam reforming reaction as a function of decreasing particle size would have been predicted, because smaller particles will have fewer step-edge sites for CO recombination. Based on the metal-carbon and metal-oxygen binding energies the periodic trends of transition metals for the elementary reaction steps of the steam methane reforming reaction are compared. For highest catalytic performance both carbon and oxygen intermediates are required. The activation of methane and water can be related to the metal-carbon and metal-oxygen binding energies. Because of the requirement of optimal O coverage, the metal with the lowest barrier for methane activation (Ir) is not the metal with the highest reactivity in the methane steam reforming reaction (Ru). In Chapter 5, we compare the energetics of the dehydrogenation of CH4 to C on extended Rh(111) and Rh(211) surfaces and a planar (111) surface and the edge atoms that are shared between two (111) facets of a nanorod model (Figure 1b). We found that similar surfaces on the periodic and nanorod model have almost comparable adsorption and reaction energies. This is not the case for the adsorption of the carbon adatom, which adsorbs stronger on the (111) surface of the nanorod than on the periodic planar surface. This is attributed to the binding of the adsorbate to the more reactive edge atoms in the nanorod model. The reaction energies only differ slightly due to small geometrical differences. The dissociation of CH was on all surfaces the reaction with the highest barrier. However, due to the contribution of the entropic loss during dissociative methane adsorption, this step is the most likely rate controlling step. In accordance with the earlier comparison of CO recombination on different surfaces, the barrier for CH4 dissociation is strongly structure sensitive. The rate is much higher for surfaces that contain low-coordinated surface atoms. A microkinetic model of SMR has been constructed and is described Chapter 6. This model is based on the elementary reaction steps and allows us to determined macroscopic properties such as the overall reaction rate, the rates of the individual elementary reaction steps, surface coverages and rate control parameters. The reaction rate constants are computed from the DFT computed barriers and entropic contributions. Based on this model, we conclude that for both planar and stepped surfaces, the rate of dissociative methane adsorption is rate limiting. The rate on the stepped surface is higher than on the planar. It becomes obvious that to maintain high activity a balance in the rates of dissociative adsorption of CH4 adsorption, dissociation of H2O and formation of CO is required. Although methane dissociation is rate limiting, it is necessary to also generate sufficient O adatoms to remove the carbon-containing surface intermediates. For the planar surface, the surface contains sufficient Oads to remove Cads under typical reaction conditions. However, because the structure dependence of water adsorption is less strong than for methane dissociation, it is found that at high temperature on the stepped surface the reaction rate becomes very low due to Oads depletion leading to Cads poisoning. The most important finding is that the experimentally observed structure dependence is consistent with the proposal of the dissociative methane adsorption on low coordinated surface atoms as the rate limiting step. However that may be, to maintain high activity one needs step-edge sites to remove Cads and Oads with low barrier. The step-edge density, however, is not critical as the activation free energy barrier for CO recombination on these sites is many times higher than the activation free energy barrier for dissociative CH4 adsorption. This work provides a complete picture on Rh-catalyzed steam methane reforming in which all potential rate controlling steps, namely CH4 and H2O dissociation and CO formation, have been studied with the same accuracy and methods. It is also clear that a microkinetic model is required to understand the exact implications of these reaction energy parameters. The present data only take into account lateral interactions between adsorbed CO. Moreover, the model has only assumed the presence of one type of sites. In this respect, it would be important to carry out similar simulations by kinetic Monte Carlo modeling with the advantage of (i) easily incorporating lateral interactions, (ii) diffusion, which may be important since the site for Cads generation is different from Cads removal and, related, (iii) the presence of different sites in one particle, their proportion depending on the particle size. Another research direction could be to use Brønsted-Evans-Polanyi relations to predict periodic trends as well as the behaviour of alloys. Coking due to carbon-carbon coupling reactions, likely occurring on terrace facets, is another topic of future interest.

The progress in the understanding of catalysis as a surface phenomenon in terms of molecular reactivity has been enormously driven by developments in surface science, computational catalysis and the possibility to synthesize nanosized objects. One of the most important challenges in the design of improved catalyst systems is to understand the relation between the structure and reactivity, most commonly referred to as structure sensitivity. Based on such understanding, it should become possible to design optimal catalysts in terms of activity, selectivity and stability for desired chemical transformations. Supported metal nanoparticles form an important class of heterogeneous catalysts with a wide variety of applications in the petrochemical and chemicals industry. Chapter 1 discusses the main aspects of structure sensitivity of support metal nanoparticles, which are surface topology and the nature of the rate limiting step, the formation of overlayers on the nanoparticles, the function of the support and deactivation. A brief introduction of the role of catalysis in the manufacture of hydrogen by reforming is given. Chapters 2 and 3 deal with the nature and stability of the actives sites of supported rhodium nanoparticles for steam reforming of methane, the principle reaction for the production of hydrogen and syngas. In particular, steam reforming of methane at low temperature (400-600 oC) was investigated as part of pre-combustion CO2 technology. Chapter 2 reports on the influence of the Rh nanoparticle size and the type of support on the catalytic performance in steam methane reforming with a view to identify the rate-controlling step. To this end, rhodium nanoparticle catalysts supported by zirconia, ceria, ceria-zirconia and silica were synthesized with the aim to have a set of supported Rh catalysts with particle sizes between 1-10 nm. These catalysts were extensively characterized by such techniques as H2-chemisorption, transmission electron microscopy and X-ray absorption spectroscopy to establish the nature and dispersion of the active Rh metal phase. The nanoparticle size was varied between 1 and 9 nm by careful choice of the metal loading, support and the pretreatment conditions. Particle growth was induced by reductive treatment at high temperature in the presence of steam and by using low surface area supports. An important finding was that the degree of Rh reduction during H2 activation depends strongly on the Rh nanoparticle size and the type of support. Very small Rh particles cannot be fully reduced, especially when ceria is the support. Reduction at 500 oC leaves a substantial part of the smallest Rh particles in the oxidic form. This fraction of oxidic Rh needs to be taken into account when determining the intrinsic rate of the supported Rh catalysts. By careful activity measurements it is found that the initial intrinsic surface atom based reaction rate of steam methane reforming at 500 oC increases linearly with Rh metal dispersion. Supported by kinetic data (first order dependence in methane, zero order in water), this implies that dissociative methane adsorption (C-H bond activation) is the rate-controlling step. This structure sensitivity can be explained by the increasing density of low-coordinated edge and corner metal atoms at the surface of Rh nanoparticles with decreasing particles size. This also implies that C-O recombination is not the rate-controlling step, even when the temperature is lowered to 400 oC. This implies that these particles contain sufficient step-edge sites to provide a facile reaction pathway for C-O recombination. In addition, it was fond that the intrinsic activity does not depend on the type of support. The support only affects the catalytic activity in steam methane reforming indirectly by influencing the dispersion and the reduction degree of the metal phase. This result is to be expected when the dissociative methane adsorption is controlling the reaction rate. Chapter 3 explains in detail the deactivation of very small Rh nanoparticles as noted in the catalytic activity measurements: Rh nanoparticles smaller than 2.5 nm deactivate much stronger than larger ones. In general, catalysts can deactivate by formation of difficult to remove carbon species, sintering of the small metallic nanoparticles or oxidation of the active metal phase. The type and amount of coke was investigated by performing temperature-programmed oxidation on spent and intentionally coked catalysts. Such experiments showed that smaller particles give rise to more extensive coke formation, which is likely due to a lower rate of C-O recombination reactions that compete with C-C coupling coke forming reactions. Experiments at different steam-to-carbon (S/C) ratio, however, gave rise to different coke formation rates but similar rates of deactivation, which suggests that coke deposits is not the main cause of catalyst deactivation. By using in situ X-ray absorption spectroscopy measurements it was found that sintering is not occurring under the reaction conditions of steam methane reforming. However, these measurements clearly showed that very small particles oxidize during the reaction causing the deactivation. Larger particles are stable and retain their metallic character, which is essential for the steam methane reforming reaction. The active phase of Rh-based catalysts during CO oxidation is investigated in Chapter 4. It includes in situ X-ray absorption spectroscopic measurements and a thorough reaction kinetics study. The oxygen content of the Rh phase under catalytic conditions was determined by temperature-programmed surface reduction by CO. A clear trend between the increase in reaction rates with decreasing particle sizes was found, which can be attributed to the ease of oxidization of Rh particles below 2.5 nm under conditions of catalytic CO oxidation. These oxidized Rh nanoparticles are much more active with a difference of two orders of magnitude in comparison to the metallic Rh particles larger than 4 nm. The kinetic results provide convincing evidence that with the change of the particles size from larger than 4 nm to below 2.5 nm, the mechanism of CO oxidation completely changes. The susceptibility to oxide formation appears to be an intrinsic property of very small Rh particles. The support plays an important role in stabilizing these rhodium oxide species and the oxygen content of the rhodium oxide phase increases with the reducibility of the support. The support affects the dispersion of the metal oxide and thereby its CO oxidation activity. Although Rh is one of the most active catalysts for steam methane reforming, Ni is the preferred metal for commercial steam reforming. Experiments have shown that Ni-based catalysts may be sufficiently active and stable at temperatures as low as 600 oC for the use of membrane separation enhanced steam reformers. A matter of concern remains, however, the formation of carbon species, which initiates the deactivation of the metallic Ni phase, especially under the carbon-rich conditions at the end of a prospective membrane reactor. In the search for more active and stable Ni-based catalysts for steam methane reforming, the effect of three different additives, namely La, B and Rh, was compared in Chapter 5. These catalysts were investigated by TEM, TPR and X-ray absorption spectroscopy. The average Ni particle size was found to be between 4 and 10 nm. Promoters affected both the dispersion and the reducibility of Ni. Smaller particles were found to be more difficult to reduce than larger ones. The use of B gave catalysts with very small Ni particles. The degree of Ni reduction strongly increased by use of La and Rh promoters, whereas B strongly impeded Ni reduction. The initial intrinsic rate per surface metal atom was found to increase linearly with the Ni metal dispersion, suggesting that, similar to Rh, the rate is controlled by dissociative methane adsorption over low-coordinated surface atoms. The data showed that Rh and La acted as structural promoters to enhance activity. Catalysts modified by B showed a much higher activity of the Ni surface atoms. Catalyst stability was investigated by using feed compositions representing the inlet of the membrane reactor and the hydrogen lean reformate towards its outlet. Stability increases in the order La

Proton-exchange membrane fuel cells (PEMFC) are electrochemical devices that convert a fuel with the aid of oxygen directly into electrical energy with high efficiency without being limited by the Carnot cycle. With hydrogen as the preferred fuel, which can in principle be produced from renewable feedstocks, fuel cells may become important devices for electricity generation for stationary, mobile and portable applications. Commercial implementation of PEMFCs for mobile applications requires bringing down the current high costs of this technology. A major contributor is the catalyst cost and especially the ORR (oxygen reduction reaction) electrocatalysts because of high Pt loadings. Besides the rather slow rate of oxygen reduction, Pt catalysts also suffer from limited stability under PEMFC operating conditions. Deactivation of the electrocatalyst is primarily influenced by the loss in electrochemical surface area for Pt catalysts. Pt dissolution at high potential followed by particle sintering due to Oswald ripening, coalescence and particle migration characterize the surface area and mass activity loss. For alloys, the dissolution of the non-noble metal also contributes to deactivation of the catalysts. Additionally, carbon support corrosion also plays a role. This thesis addressed the issue of durability of carbon-supported Pt-based ORR catalysts. Specifically, the potential benefit of non-noble metal alloying (Co, Ni, Cu) on the ORR activity and stability of Pt catalysts is investigated. To learn about the intrinsic properties of such alloys the work involved studies of electrodeposited PtM layers followed by studies of carbon corrosion and the activity and stability of carbonsupported alloys. The main electrochemical technique was cyclic voltammetry at room temperature and 80 °C. The ORR activity and durability of unsupported Pt and PtM alloys with respect to non-noble mental dissolution and Pt surface area (ECSA) loss was discussed in Chapters 2 – 6. Unsupported Pt and PtM alloys were prepared through electrodeposition because of the ease of preparation of alloys with a wide compositional variety. In general, an enhancement in the ORR activity was achieved for all the alloys when compared to Pt after 15 CV scans. The ECSA loss was found to be more substantial in these first scans for the non-noble metal-rich alloys. Further potential cycling led to similar losses in the ECSA for Pt and the alloys. Regarding non-noble metal dissolution, Co and Ni were found to be more resistant towards dissolution than Cu during the initial stages of potential cycling. However, at the end of 1000 CV scans, the amount of non-noble metal in the catalyst layer was around 15 atom% irrespective of the alloying element and the initial Pt:M ratio. The CV and XPS studies pointed to the formation of a Pt-enriched catalyst surface with the non-noble metals being in subsurface layers. In spite of having a similar catalyst surface, non-noble metal-rich alloys were found to be more stable towards potential cycling. In other words, the durability of the alloys at room temperature depends on the initial Pt:M ratio. Structural and elemental studies on the near-surface regions are necessary to understand these differences in more detail. The durability of the alloys studied at elevated temperature (Chapter 6) revealed that the PtM alloys maintained their enhanced ORR activity even after 1000 potential cycles. However, no difference in the ORR between the Pt-rich and non-noble metal rich alloys was found. Nevertheless, PtNi was found to be the most durable among the alloys followed by PtCo and PtCu. On the issue of non-noble metal dissolution, the alloys still retained about 15-20 atom% non-noble metals, even after extensive potential cycling. The investigation of the influence of chloride ions on the ORR activity and durability of Pt and PtNi alloys described in Chapter 3 shows that a chloride ion concentration as low as 5 ppm is sufficient to poison the catalyst and reduce the ORR activity by several orders of magnitude. However, among the catalysts studied in chloridecontaining electrolyte, Pt10Ni90 was found to be the most active one. Chloride ions, even in minute quantity, were found to accelerate Ni dissolution. To examine whether the enhanced durability of the unsupported alloys can in principle be useful for the development of actual fuel cell catalysts, Pt and PtM alloy nanoparticles were prepared on a carbon support and annealed at different temperatures (Chapter 7). The effect of particle size and the alloying element is discussed in this chapter. Non-noble metal rich alloys exhibited the highest activity at room temperature after initial dealloying. The electrocatalytic activities of the fresh alloys were found to be dependent on the particle size, alloying element and nonnoble metal concentration. Nonetheless, after 1000 potential cycles at 80 °C with almost complete dissolution of non-noble metal, the activities of the alloys were quite similar to that of Pt. Besides, the aged catalysts showed only a modest dependence on the particle size. Comparing electrodeposited and carbon supported alloys, it is noted that in both cases room temperature Cu dissolution is rapid as compared to Co and Ni. Also, an enhancement in the ORR activity was achieved for the alloys. However, unlike supported alloys, the electrodeposited layers were able to retain their enhanced activity after the durability tests. This could be related to the amount of non-noble metal retained: electrodeposited alloys retained about twice as much of the non-noble metal than the supported ones. To conclude with, the significance of PtM alloys as an alternative to Pt/C relies on how to retain a considerable amount of non-noble metal in the catalyst. The last part (Chapter 8) of this thesis deals with the electrochemical corrosion behavior of various commercial carbon supports at elevated potential (1.2 V) and temperature under potentiostatic conditions by employing on-line electrochemical mass spectrometer (OLEMS). The corrosion rate of the carbons decreased with time. The CVs revealed that the onset potential of carbon oxidation and CO2 evolution shifted towards higher values after the potential hold experiments again confirming the resistance of carbon towards corrosion. The carbon weight loss was found to be depending on their BET surface area. The BET-surface area normalized weigh loss is similar for all the carbons, which indicate that the corrosion behavior of these carbon supports is quite similar.

Zeolites are widely used as catalysts, especially in oil refining and the petrochemical industries. Nowadays the cracking of heavy oil feeds as well as the processing of larger (bio)molecules demands for improved catalysts that can overcome the pore size constraints and diffusion limitations of the conventional zeolite based microporous catalyst. The challenge is to develop new catalyst which have pores in the mesopore size region combined with the catalytic activity and stability of the conventional zeolite. A relatively easy method to increase the pore size from the micropore to the mesopore region can be found in the synthesis of the ordered mesoporous silicates. These materials can be made in a broad range of compositions with a variety of pore sizes. Nevertheless they are amorphous and therefore lack the typical features as stability and strong acidity found in crystalline zeolites. A set of SBA-15 type mesoporous silicates was synthesized with varying silicon to aluminium ratios in an attempt to increase the Brønsted acidity of the mesoporous silicate. Decreasing the silicon to aluminium ratio below 10 lead to a loss of the ordered pore structure. When the ratio falls below 5 it was not possible to obtain any ordered mesoporous structure despite the use of a single source molecular precursor or a different solvent to match hydrolysis rates. The aluminium structure and coordination in these SBA-15 catalysts resembles that of conventional amorphous silica alumina (ASA). This nature is reflected in the activity of the SBA-15 catalysts in the hydroisomerisation of n-heptane, where the activity for SBA-15 type catalysts is only marginally higher when compared to other ASA’s. The small difference in activity can be explained by the presence of an increased amount of weakly acidic hydroxyl groups on the surface of the SBA-15. Apparently ordered mesoporous aluminosilicates resemble the conventional ASA’s, and therefore lack stability and strong Brønsted acidity needed for catalysis. To systematically test the effect of mesoporosity on the catalytic activity of zeolites several synthetic routes including carbon black templating, desilication and templating with organosilanes were employed to synthesize Fe/ZSM-5 catalysts. The hierarchical Fe/ZSM-5 catalysts show similar crystallinities, which is substantially lower than the conventional microporous Fe/ZSM-5. Hierarchical catalysts prepared by either desilication or organosilane templating show an number of active Fe2+ centers comparable to the conventional catalyst. The carbon black templating method, which can be used to obtain high mesoporosity, is not conductive to the formation well dispersed iron and therefore shows an low active site density. Hierarchical Fe/ZSM-5 catalysts show an increased activity in the selective hydroxylation of benzene by nitrous oxide. These increased catalytic properties are a result of the hierarchical structure. First of all the open framework enhances diffusion, which results in a higher initial activity for the catalysts, secondly there is an increased accessibility of the microporous space. As a consequence of the latter the catalyst is less prone to deactivation as a smaller part of the microporous space becomes clogged upon formation of carbonaceous deposits. The best performance in terms of activity and stability is obtained for the organosilane templated catalysts, as these combine a high mesopore volume well integrated into the crystal with an amount of active centers comparable to that of the conventional Fe/ZSM-5 catalyst. As the organosilane templated Fe/ZSM-5 catalyst outperforms the other hierarchical Fe/ZSM-5 catalysts in terms of activity and stability, the mechanism of formation of this hierarchical ZSM-5 zeolite was studied as function of time, temperature and template amount. It is shown that the crystallization of the hierarchical ZSM-5 is a two stage process. Firstly an amorphous highly mesoporous aluminosilicate phase is formed. In the second step this phase is consumed to form the final hierarchical zeolite, which consists of large globular particles. Under the reaction conditions tested, complete consumption of the amorphous phase is not possible. As a result an aluminium rich amorphous phase remains after the crystallization process and the Brønsted acidity of the hierarchical zeolite is low compared to conventional ZSM-5 with a similar Si/Al ratio. Increasing the synthesis temperature for the hierarchical zeolite leads to a more crystalline material, however this comes at the expense of the mesoporosity due to the limited stability of the organosilane mesoporogen at elevated temperatures. A decrease of the synthesis temperature on the other hand favors the formation of the mesoporous amorphous phase at the expense of the hierarchical zeolite. Addition of iron to the synthesis gel enhances the crystallization rate yielding hierarchical Fe/ZSM-5 zeolites, which show higher crystallinity as their hierarchical ZSM-5 analogues. The hierarchical ZSM-5 catalysts with various crystallinities were tested for their activity in the catalytic hydroconversion of n-heptane. Due to the low incorporation of aluminium in the framework the hierarchical ZSM-5, the catalysts show low activity compared to conventional ZSM-5. Where the hydroisomerisation is highly dependent on strong Brønsted acid sites, the selective hydroxylation of benzene depends on the presence of active iron centers and the structure of the catalyst. This is well reflected by the hierarchical Fe/ZSM-5 catalysts grown as a function of time. Where the initial activity for the hierarchical Fe/ZSM-5 catalyst crystallized only for a short time is lower compared to conventional Fe/ZSM-5, the overall phenol production is already higher due to the increased resistance towards deactivation. With increasing crystallization time for the hierarchical Fe/ZSM-5 both the initial activity and phenol production increase. As formation of the hierarchical Fe/ZSM-5 using an organosilane surfactant as the mesoporogen does lead to the formation of highly active catalysts, the mesopore structure appears random and decreases with crystallization time and temperature. Moreover part of the synthesis gel is not converted to a crystalline material and thus inactive in catalysis. To overcome these problems a dry gel conversion approach was used. Initial a amorphous hierarchical Fe,Al-silicate was grown a low temperature. At the given synthesis conditions surfactants comparable to the organosilane yield MCM-type mesoporous silicates, however only mesoporous silicates with a wormhole pore structure were obtained with the organosilane. Interestingly the "amorphous" Fe,Al-silicate synthesized using an organosilane surfactant as the mesoporogen shows already features of zeolitic ordering. Where the material is amorphous in XRD, the IR and Raman spectra show the presence of zeolitic building blocks. This remarkable feature is well reflected in the hydroxylation of benzene were the amorphous catalyst shows a relatively high activity compared to other mesoporous materials. To increase the activity in the benzene oxidation the as synthesized mesoporous Fe,Al-silicate was dry gel converted to a hierarchical zeolite. Initially the mesoporous framework is destroyed yielding a catalyst, which shows hardly any activity, however with increased dry gel conversion times a hierarchical zeolite grows. The iron on this hierarchical Fe/ZSM-5 zeolite is highly dispersed making the catalyst only moderately active in the catalytic decomposition of N2O. The activity for the selective hydroxylation of benzene on the other hand increases rapidly with increased dry gel conversion times as a result of the formation of well dispersed very small zeolitic domains in a mesoporous matrix. The higher activity of these hierarchical catalysts suggests that the conventional microporous Fe/ZSM-5 reference catalyst suffers from severe diffusion limitations due to its large coherent crystalline domain size. Clearly the decrease of the crystalline micropore domain size is beneficial in terms of longevity where it comes to deactivation due to clogging of the micropore space. In that light it seems worthwhile to develop a catalyst where the crystalline domains are as small as possible. Although small nanoblocks of several unit cells in dimension can be synthesized, their limited stability makes them unsuited for catalysis. Another way of decreasing the domain size to the limits is by using so called nano-sheets. In this case sheets are formed which have a thickness of exactly one unit cell, creating extremely short micropore path lengths along the straight channel of the MFI type zeolite. The effect of the open framework and short micropore channels on the selective hydroxylation of benzene in tremendous. The nanosheets have a higher activity and longevity than all the other Fe/ZSM-5 catalysts, despite the large amount of coke deposits on the catalysts. Physisorption shows that for the nanosheets, in contradiction to the conventional Fe/ZSM-5, the majority of the coke resides in the mesopores. Obviously the short micropore diffusion path lengths prevent secondary hydroxylation of the product phenol in the micropore and thus hamper the clogging of the micropores. The mesopores in the catalyst on the other hand open up the framework and provide a pathway to the micropores making them accessible for catalysis increasing the overall activity of the catalyst. The work in this thesis focuses on the synthesis of hierarchical zeolites and their application as catalysts. Although numerous hierarchical porous materials have become accessibile in the last two decades, there is still a need for improvement of tailoring their (pore) structure and reactivity. Whereas initially research has focussed on the synthesis of novel materials by trial and error, nowadays there is a shift towards rational design of hierarchical porous catalysts. It can be expected that in the near future rational design of templates to target specific zeolite pore topologies including mesopores will become possible. In this respect, it should be stressed that the number of templates accessible by straightforward organic synthesis methods is virtually endless. In order to select the most promising of these new templates thorough understanding of the templating-silicate interactions and zeolite growth are necessary. Therefore, the design of new hierarchical systems in the future will become an area involving the interplay of computer simulations, synthesis of templates and by applying them novel porous structures: ideally, we will be able to predict templates for synthesis of optimal zeolites for specific reactions by computer simulations.

Amorphous silica-alumina (ASA) is widely used as a solid acid catalyst or as a carrier for well-dispersed metal sulfide or metal catalysts. They are often involved in hydrocracking catalyst formulations, especially so when the aim is to produce middle distillates from heavy oil fractions. With increasing demand for diesel and kerosene balanced acidity in these catalysts to combine high conversion with high middle distillates selectivity is crucial. An important advantage of amorphous silica-alumina as an acidic catalyst is its open texture with substantial mesoporosity as compared to zeolites that typically suffer from diffusion limitations when bulky hydrocarbons need to be converted. Strongly acidic supports also cause excessive coke formation and overcracking of the feedstock resulting in lower middle distillate yields. Therefore, the use of ASA as the acidic component in bifunctional hydrocracking has become important. Although moderate acidity is important for ASA supports, accurate control of the acidity is hampered by a lack of understanding about the origin of acid sites in these materials and how they are formed. Regarding the former, the nature of the Brønsted acid sites (BAS) has not been unequivocally established. The more widely shared opinion is that the Brønsted acidity derives from tetrahedral Al3+ in the silica network, as initially proposed by Thomas and Tamele in the late 1940s. This proposal however has remained inconclusive. Alternative explanations for the acidity have been also been proposed. These include Lewis acidic Al ions substituting for protons of surface silanol groups and the higher acidity of silanol groups in the presence of neighbouring aluminium surface atoms The other reason that surface acidity of ASAs is understood to a much lesser extent than that of zeolites relates to the complex surface composition of these mixed oxides. ASAs are made by co-precipitation, co-gelation or grafting processes and in nearly all cases, the resulting materials contain a non-random distribution of aluminium in silica. The present project was thus undertaken with the aim of (i) synthesizing a set of ASA materials by as controlled a method as possible for use in catalytic activity studies and (ii) to learn about the genesis of Brønsted acid sites in ASAs and their strength. The synthesis method chosen was a well-defined variant of grafting, viz., homogeneous deposition-precipitation. The entire process of the deposition of aluminium on silica and subsequent calcination was followed by 27Al NMR spectroscopy . The study showed that the aluminium species in the dried precursors is a function of pH and starting aluminium concentration. At pH of 3 and at low aluminium concentration, the surface mainly consists of tetrahedral and octahedral aluminium species. Under these conditions an increase in pH gives mainly rise to tetrahedral aluminium species on the surface. This is attributed to the further condensation reaction occurring with the surface silanol groups. However, with increasing aluminium concentration, the deposition mechanism involves reaction of aluminium species in solution with species already grafted on the surface. This results in the formation of polymeric aluminium species. In addition, at higher aluminium concentrations some precipitation of aluminium hydroxide also occurs. When the dried precursors are then calcined, redistribution of the grafted aluminium species occurs, mainly with a small fraction of aluminium diffusing into the silica matrix thereby resulting in Brønsted acid sites. The formation of Brønsted acid sites upon calcination was also evidenced by n-alkane hydroconversion activity tests, which requires the presence of strong acid sites. From this systematic study the surface of amorphous silica- alumina could be described as consisting of three different species, namely a pure silica-alumina phase that originates from isolated aluminium grafted onto the silica surface, domains of aluminium oxide and a small fraction of aluminium in the silica network responsible for the strong Brønsted acidity. By following the selective H/D exchange of acidic hydroxyl groups in aluminosilicates by IR spectroscopy clear evidence was provided for the existence in ASAs of BAS comparable in strength to the bridging hydroxyl groups in zeolites. The method is able to distinguish various types and strengths of strong BAS in luminosilicates (zeolites, clays, ASAs) such as enhanced acidic sites in steam calcined faujasite zeolites. By carrying out the H/D exchange under conditions under which zeolites selectively exchange their bridging hydroxyl groups, weak bands were observed at 2630 and 2683 cm-1 in the deuteroxyl region of ASAs. By quantification it follows that the concentration of strong BAS in ASAs is 2-3 orders of magnitude lower than in zeolites. A number of techniques (CO IR, pyridine IR, alkylamine TPD, Cs+ and Cu(EDA)22+ exchange, 1H NMR and m-xylene isomerization) was used to validate the H/D exchange FTIR results and provide further insight into the heterogeneous surface composition of ASA. The results show that the surface contains both Brønsted and Lewis acid sites of varying acidity. The number of strong Brønsted acid sites of zeolitic strength is very low (

Zeolites modified with metal cations have been extensively studied during the last two decades because of their wide application in different technologically important fields such as catalysis, adsorption and gas separation. Contrary to the well-understood mechanisms of chemical reactions catalyzed by Brønsted acid sites in the hydrogen forms of zeolites, the nature of chemical reactivity, and related, the structure of the metal-containing ions in cation-exchanged zeolites remains the subject of intense debate. In this thesis, the chemical properties of zeolites modified with hard Lewis acids such as alkaline- and alkali-earth cations (Chapters 2 – 4) and with soft Lewis acids such as Zn-, Cd- and Ga-cations (Chapters 5 – 9) are discussed. Special attention is paid to the mechanism of chemical transformations promoted by such exchangeable species and, accordingly, their role in these processes. Low-silica zeolites modified with alkaline- and alkali-earth cations are rather inert materials. However, it has been experimentally found that they can efficiently promote photo-oxidation of unsaturated hydrocarbons with molecular oxygen. The details of this reactivity are not clear. Chapter 2 reports DFT calculations on the initial charge-transfer step for alkene photo-oxidation in zeolite Y modified with alkali-earth cations (Mg, Ca, and Sr). The photo-oxidation of 2,3-dimethyl-2-butene (DMB) with O2 has been used as a model reaction. It is predicted that the electrostatic field of the zeolite cavity plays only a minor role for the stabilization of the charge-transfer state, while the relative orientation and the distance between the adsorbed alkene and oxygen molecules are the critical factors. A high density and specific location of the exchangeable cations in the zeolite matrix determines a specific confinement of the adsorbed reagents in a suitable "pre-transition state" configuration. The optimum configuration of co-adsorbed DMB and O2 molecules is identified for CaY zeolite. A significantly lower activity of SrY and MgY in the photooxidation of 2,3-dimethyl-2-butene-2 in comparison with that of CaY is predicted. Another interesting property of low-silica zeolites modified with alkaline cations is their ability to promote N2O4 disproportionation under very mild conditions. Chapter 3 presents periodic DFT calculations on N2O4 disproportionation in Na-, K-, and Rb-exchanged lowsilica zeolite X. The disproportionation reaction results in rather polar NO+···NO3 – species, which are effectively stabilized by the cage of cation-exchanged zeolite. NO+ binds to the basic framework oxygens, and NO3 – anion coordinates to the exchangeable cations. Although the binding energy of NO+ ion to the zeolite is influenced by the basicity of the framework, the theoretical results show that the overall disproportionation reaction is mainly controlled by the interactions between the negatively charged nitro group and the extra-framework cations. The role of the interaction between the nitrosonium cation and basic sites of the zeolite is only of minor importance. The function of the microporous matrix is to facilitate the charge separation in a fashion similar to that of a polar solvent. It is concluded that steric properties of the zeolite cage, the cooperative effect of the extraframework cations as well as their mobility induced by adsorption are essential to form the optimum configuration of the active site for N2O4 disproportionation. Chapter 4 reports a combined infrared spectroscopic and computational study of light alkane adsorption to alkali-earth exchanged zeolite Y. Although these materials do not catalyze C–H or C–C bond cleavage, they can be successfully used as model adsorbents to investigate the factors influencing structural and electronic properties of the resulting adsorption complexes. The experimental IR spectra of the C–H stretching vibrations of the adsorbed hydrocarbons differ strongly for MgY and CaY zeolites. On the basis of ab initio MP2 and DFT calculations it is found that different geometries of the light alkane adsorption complexes are realized depending on the cation in the adsorption site. Topological analysis of the electron density distribution function in the framework of quantum theory of atoms in molecules is applied to investigate the bonding of the adsorption complexes. It is found that numerous van der Waals bonds between the H atoms of the alkane and basic oxygens of the zeolite are formed, when a hydrocarbon coordinates to Mg2+ ions. These intermolecular contacts significantly contribute to the overall adsorption energy, whereas they play only an indirect role in the adsorption of light alkanes on CaY. On the other hand, in the case of CaY the stabilization of alkanes in the electrostatic field of the partially shielded Ca2+ cation dominates the adsorption energy. It is concluded that the dominance of a particular type of intermolecular interactions is dependent on the properties of the adsorption site. The type of intermolecular interactions determines the final conformation of light alkanes adsorbed to the cation-exchanged zeolite Y. From the results in Chapters 2 – 4 an interesting effect is noted: although the smaller exchangeable cations are expected to bind molecules stronger and exhibit higher reactivity as compared to their larger counterparts because of the increased hardness of such cations, the calculations indicate that the properties of the metal ions stabilized in the zeolite matrix do not follow these trends. Indeed, when stabilized at zeolitic cation site, the larger ions are significantly coordinatively unsaturated. This leads to an enhancement of the adsorption properties of the larger cations in spite of their expected lower Lewis acidity. Molecular and dissociative adsorption of light alkanes on the more reactive high-silica zeolite ZSM-5 modified with zinc and cadmium is investigated in Chapter 5. Adsorption of ethane on coordinatively unsaturated soft Lewis acid sites (Zn2+ and Cd2+) in ZSM-5 zeolite results in stronger changes of the geometry and charge parameters of the adsorbed molecules as compared to the case of adsorption on MgY and CaY. It is found that the degree of the effective shielding of the exchangeable cations by the surrounding oxygen ions is an important factor that influences the perturbations of molecularly adsorbed ethane. The C2H6 binding energy does not apparently depend on the type of the cation (Zn or Cd), whereas the nature of the charge compensation of the cations is important. On the other hand, heterolytic dissociative adsorption is mainly controlled by the basicity of the proton-accepting oxygen-site (O-site) and the steric properties of the dissociation products, which determine their stability. As a result, no apparent correlation between the perturbations of the adsorbed molecules and their heterolytic dissociation is observed. Chapters 6 to 8 report cluster DFT calculations of the various potential reaction paths of catalytic dehydrogenation of light alkanes over zinc- and gallium-exchanged high-silica zeolites. The mechanism of the catalytic reaction and the most probable active site are identified. In addition, an attempt is made to understand the factors, which determine the catalytic activity of different intrazeolite cationic species as well as the preference for a particular reaction path. The theoretical results form a basis for interpreting the experimental catalytic data. Catalytic dehydrogenation of ethane over various zinc species in Zn/ZSM-5 zeolite is investigated in Chapter 6. It is shown that isolated Zn2+ stabilized at the cation sites with distantly placed anionic [AlO2]– framework units are the most probable active species. A novel mechanism of ethane dehydrogenation is proposed. It involves decomposition of the products of dissociative ethane adsorption (Z–Zn2+-C2H5 –···H+Z–) via one-step desorption of ethylene and hydrogen. This path is strongly favored for the isolated Zn2+ sites as compared to the conventional mechanism involving consecutive desorption of the dehydrogenation products. Similar to the initial heterolytic C-H bond cleavage, the basicity of the O-sites is a determinative factor for the particular reaction mechanism. In the case of Ga-exchanged ZSM-5 zeolite (Chapter 7), univalent gallium cations are the most probable active sites for reduced catalysts. Hydrogenated extra-framework species decompose rapidly toward Ga+ cations during the catalytic reaction. Initial oxidative addition of C2H6 to Ga+, which has been observed experimentally before, is shown to proceed via an indirect route involving heterolytic C-H cleavage over the Lewis acid-base pair formed by the Ga+ cation and a framework oxygen anion. The direct route is strongly disfavored due to the electronic properties of univalent gallium. C2H4 and H2 desorption in one step closes the catalytic cycle. Although this reaction is reminiscent to that proposed for Zn/ZSM-5, it strongly differs in nature and is controlled by the properties of the Ga site. It has been observed experimentally that the catalytic activity of ZSM-5 zeolite predominantly containing Ga+ ions can be remarkably enhanced after selective oxidation with N2O. The higher activity of the resulting material has been attributed to formation of extra-framework GaO+ ions. However, a detailed investigation of various possible reaction paths over isolated gallyl ions in ZSM-5 zeolite (Chapter 8) shows that ethane interacts with these species stoichiometrically, because of the extremely low stability of these sites. Indeed, the unfavorable tridentate coordination of gallium along with the high basicity of the extra-framework terminal oxygen ion in GaO+ leads to a rapid heterolytic dissociation of C2H6 molecules. The resulting products are very stable, and the closure of the catalytic cycle is not likely to occur. It is concluded that the isolated gallyl ions cannot be considered as catalytically active sites for light alkane dehydrogenation. The very low stability of GaO+ species, on the other hand, can cause their oligomerization in the zeolite micropores, resulting in formation of various multinuclear cationic gallium-oxide clusters (Chapter 9). Periodic DFT calculations show that formation of cyclic Ga2O2 2+ dimers is strongly favored independently of the aluminum distribution in the high-silica zeolite. Moreover, oligomers with a higher degree of aggregation can be in principle formed in oxidized Ga-exchanged zeolites. The zeolite lattice plays the role of a chelating ligand which stabilizes the (GaO)n cationic cluster. Parallels between conventional coordination chemistry and chemistry of high-silica zeolites modified with gallium are drawn. It is shown that the location and stability of such cationic clusters is mainly controlled by the favorable geometrical environment of the Ga3+ ions, while the effect of the direct interaction with the framework anionic sites which compensates for the positive charge of the extra-framework species is less important. In spite of higher stability, binuclear sites are shown to be active for alkane activation. The lower basicity of the extra-framework oxygen ions provides a path for the closure of the catalytic cycle. However, these sites still tend to reduce upon light alkane dehydrogenation via water desorption, resulting in formation of less reactive reduced Ga-species. The mechanistic insight provided by the quantum-chemical calculations suggests that the reduction path can be suppressed by addition of water to the hydrocarbon feed. This would lead to an increased steady-state concentration of reactive oxygenated Ga-species in the catalyst. The experimental catalytic tests (Chapter 9) indeed show significant enhancement of the dehydrogenation activity of Ga+ sites in ZSM-5 upon water co-feeding. Continuous addition of water is required to maintain a high steady-state concentration of the reactive oxygenated extra-framework species in the zeolite and leads to high and stable activity of the catalyst. Thus, it is shown that the reactivity of low-silica zeolites modified with rather inert alkaline- and alkali-earth cations derives mainly from the properties of the confined space of the zeolite cages. The high density and the specific arrangement of the exchangeable cations in the microporous matrix lead to optimum configuration of the adsorbed reagents and consequently to their chemical activation. On the other hand, the active sites in highsilica zeolites modified with softer Zn, Cd, and Ga cations are rather local and usually directly involved in catalytic transformations of the reagents. The chemical reactivity of these system derives from the properties of the Lewis acid-base conjugate pair, which in turn are controlled by the topology of the zeolite cation site accommodating the extraframework species as well as by the type of charge compensation and the nature of the cation. Both the Lewis acidity of the extra-framework cationic species and the properties of the conjugate basic sites are important for their activity. An optimum must be found in the Lewis acid-base properties of the zeolite active site to achieve a high catalytic activity.