On the structure sensitivity in metal catalysis

D.A.J.M. Ligthart

Research output: ThesisPhd Thesis 1 (Research TU/e / Graduation TU/e)

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Abstract

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 <Rh <B. Deactivation of the catalysts is caused by insufficient removal of carbon species from the surface of Ni particles and the formation of stable, graphitic carbon deposits, most likely covering the surface of metal. This is substantially suppressed when the Ni particles are small. B is an excellent structural promoter to obtain small Ni particles, Rh stabilizes metallic Ni and La aids in the removal of some of the carbon deposits more effectively by gasification. Finally, the structure sensitivity of the most noble metal gold on nanostructured ceria nanocrystals was investigated in Chapter 6 by a careful study involving cyanide leaching, TEM, TPR, CO infrared spectroscopy and X-ray absorption spectroscopy. After deposition-precipitation and calcination of Au, these surfaces contained Au nanoparticles in the range 2-6 nm. For ceria nanorods, a substantial amount of gold was present as cations that replace Ce ions in the surface as follows from their first and second coordination shells of oxygen and cerium by EXAFS analysis after cyanide leaching. These cations were stable against cyanide leaching in contrast to Au nanoparticles. Upon reduction the isolated Au atoms formed finely dispersed metal clusters with a high activity in CO oxidation, the WGS reaction and 1,3-butadiene hydrogenation. By analogy with the very low activity of reduced Au nanoparticles on ceria nanocubes exposing the {100} surface plane, it was inferred that the Au nanoparticles on the ceria nanorod surface also have a low activity in such reactions. Although the finely dispersed Au clusters are thermally stable up to quite high temperature, the presence of Au nanoparticles resulted in their more facile agglomeration, especially in the presence of water (e.g. WGS conditions). For liquid phase alcohol oxidation, we found that metallic Au nanoparticles are the active sites. In the absence of a base, the O-H bond cleavage appeared to be rate controlling, while this shifts to C-H bond activation after addition of NaOH. In the latter case, the Au nanoparticles on the surface of ceria nanocubes were much more active than those on the surface of ceria nanorods.
Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • Chemical Engineering and Chemistry
Supervisors/Advisors
  • Hensen, Emiel J.M., Promotor
  • van Santen, Rutger, Promotor
Award date3 Nov 2011
Place of PublicationEindhoven
Publisher
Print ISBNs978-90-386-2780-9
DOIs
Publication statusPublished - 2011

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