Characterization of Gold and Copper Species in Nanoporous Materials
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Global warming and energy production from other sources than fossil fuels have the last years attracted more and more attention. Catalysis plays an important role in controlling the emission of greenhouse gases and other pollutants, as well as in clean up processes. Regarding energy production from more environmental friendly sources such as H2, catalysis is involved in most reactions. Catalysts in the form of nanoparticles (1-100 nm) deposited on two and three dimensional carrier systems represent a class of materials with interesting properties. Especially gold nanoparticles are well known as highly interesting catalysts for oxidation of carbon monoxide and hydrocarbons, hydrochloration of ethylene and reduction of nitric oxide. Copper is used in several industrial reactions, and several studies have also shown that copper materials are active catalysts for the reduction and decomposition of nitrogen oxide (NOx) pollutants, which are known to cause acid rain and formation of photochemical smog. Microporous (pore diameter < 2 nm) and mesoporous (pore diameter 2-50 nm) materials have been shown to exhibit interesting characteristics as carriers for nanoparticles. These materials exhibit regular pores and thus function as molecular sieves, and also have cluster-size defining properties. Microporous zeotypes are acidic, crystalline materials with a high surface area and are capable of ion exchange. In addition they exhibit shape selectivity during catalytic reactions. However, a major problem with microporous zeotypes is the internal diffusion limitations (mass transfer limitations) that arise during a catalytic reaction. This leads to poor utilization of the catalyst and the support and also coking and clogging of channels and cavities, affecting the catalyst activity, selectivity and lifetime. In that regard, the new class of hierarchical zeotypes with bimodal poresystems is highly interesting. These zeotypes are prepared to exhibit both micro and mesopores, thus reducing the diffusion limitations of micropores but still maintaining micro cavities. This thesis deals with the synthesis and characterization of two zeotypes; microporous SAPO-34 and hierarchical SAPO-34, as well as the ion exchange of these materials with gold and copper cationic precursors, the purpose being formation of bimetallic nanoparticles. The gold and copper cations are reduced in H2 causing formation of metal clusters, and the cluster growth is initially dictated by size limitations imposed by the pore aperture. The goal of the thesis is twofold. One important goal is to attempt to confirm the presence of mesopores in hierarchical SAPO-34 and establish characterization techniques able to distinguish hierarchical SAPO-34 with a bimodal poresystem from conventional microporous SAPO-34. Published results on the hierarchical SAPO-34 system are scarce, and current existing results, e.g. by Yang et al., indicate that hierarchical SAPO-34 exhibits properties similar to the conventional microporous SAPO-34, leaving distinguishing between the two systems somewhat challenging. The second goal of the thesis is to study the formation and the structure of gold and copper bimetallic nanoparticles. Since both copper and gold are exchanged into the zeotypic structures and reduced in H2, bimetallic interactions are expected. This is interesting, as bimetallic catalysts enable fine-tuning of the catalytic properties of a metal. A range of characterization techniques are applied to establish the presence of mesopores in the hierarchical system as differences can be discreet. An important technique is adsorption-desorption isotherms from N2 adsorption measurements, as the isotherms of microporous and mesoporous materials differ, and in addition mesoporous materials exhibit a hysteresis loop. Also a pore size distribution should reveal differences between a microporous and a mesoporous system. However, the adsorption-desorption isotherms of microporous SAPO-34 may also display a small hysteresis loop, and peaks in the mesoporous range of a pore size distribution may be present due to voids between the SAPO-34 particles. Another limitation concerning N2 adsorption and microporosity is filling of the micropores with adsorbate rather than the formation of an adsorbate monolayer. The main characterization technique for the study of the gold and copper nanoclusters is X-ray absorption spectroscopy (XAS), as small cluster sizes (< 4 nm) can be determined by this technique, a size range too low for detection by X-ray diffraction (XRD). XAS provide important information about bimetallic interactions, leaving it possible to prove the formation of gold and copper bimetallic particles and the structure of these. Also, when gold and copper precursors are ion exchanged into the zeotypic systems and subsequently reduced to form nanoparticles it is believed that XAS (more specifically, extended X-ray absorption fine structure (EXAFS)) is able to distinguish between the conventional and the hierarchical systems, by studying the growth limitations of gold and copper nanoclusters in these materials. As the nanoparticles are able to form in mesopores in hierarchical SAPO-34 a significant difference in cluster size depending on the poresystem could confirm the presence of mesopores and a bimodal poresystem in hierarchical SAPO-34, as bigger particle sizes would be observed for this material compared to the micropororus SAPO-34. Other characterization techniques such as X-ray diffraction (XRD), thermogravimetric analysis (TGA), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and inductively coupled plasma mass spectrometry (ICP-MS) are also employed in this thesis to characterize the hierarchical and the conventional SAPO-34 materials.