Gold Nanoparticles Catalyzed Low-temperature CO Oxidation And Novel System For Green Reduction

For its high chemical inertness, gold has been hardly considered as a potential catalyst for a long time. Once Haruta et al. reported in1987that gold nanoparticles (<5nm) loaded on transition metal oxides displayed ultrahigh catalytic activities for low-temperature CO oxidation; it arouses the tremendous research interest of gold catalysis for about twenty years. For the simplicity, low-temperature CO oxidation was widely used to explore the origin of gold catalysis. What’s more, compared with Pt group metal (PGM) catalysts, supported gold catalysts have unique low-temperature catalytic performance in micromolecule-involved (including CO, H2O, H2-, HCOOH and formate etc.) organic reactions which have attracted tremendous interest for its potential application in green and sustainable synthese of fine chemicals. In this thesis, we try to systematically study the effect of treatment conditions on the microstructure of Au catalysts and its catalytic performance for CO oxidation, and explore its application in CO/H2O-and HCOOH-mediated selective reduction of nitro compounds.1. Hydroxylapatite supported gold nanoparticles for low temperature CO oxidationA series of gold supported on hydroxylapatite (HAP) catalysts were prepared by deposition-precipitation with urea to study the influence of calcination atmosphere, i.e., H2, He and O2, on the performance of the catalyst in low-temperature CO oxidation. Calcination atmosphere was found to have an important influence on the catalytic activity and stability of Au/HAP. The highest initial activity was obtained for Au/HAP-He obtained by inert He-calcination, which, however, suffered the most severe deactivation with time on stream. Calcination in oxidative O2environment resulted in the best stability and highest steady state activity among the three catalysts. TEM results revealed that inert He-calcination can produce the smallest Au nanoparticles over the HAP support, which was suggested to be responsible for the highest initial activity of Au/HAP-He. Based on the CO2-TPD and in situ DRIFTS studies, the superior stability of Au/HAP-O2can be attributed to a limited surface basicity in this material.2. Effect of pretreatment conditions on catalytic performance over Au/TiO2The microstructure of properties of Au/TiO2catalyst pretreated at low (200℃, H2-200) and high (500℃, H2-500) temperature under H2atmosphere in relation to their activities on low-temperature CO oxidation and decomposition of aqueous HCOOH were investigated. TEM images displayed that high reductive temperature didn’t cause notable sintering of gold nanoparticles, and the average gold particle sizes were3.5nm and3.6nm for H2-200and H2-500, respectively. Activity tests showed that CO oxidation activity of H2-500decreased, but the activity for HCOOH decomposition increased dramatically; however, low temperature reduction has a little depression on CO oxidation and increased its HCOOH decomposition activity slightly. XPS, CO adsorption DRIFTS and HRTEM images showed that gold nanoparticles were decorated by the partially reduced TiOx species, that is, the Au/TiO2has the classic strong metal-support interaction which is normal in PGM catalysts but has never been reported in gold catalysts. From the above data, it can be concluded that high-temperature-reduction-mediated SMSI decreased the amounts of low-coordinated Au sites, and tailored the properties of the interface between the gold particles and the support, thus showing different effects on the two kinds of reactions.As mentioned above, reductive pretreatment, regardless of the temperature, always depressed the CO oxidation activity. However, the study in last chapter demonstrated that O2treatment could improve the activity and stability of gold catalyst. Therefor, the microstructure of properties of Au/TiO2catalyst pretreated under systematic pretreatment conditions including temperatures and atmospheres (H2-200, O2-200, H2-500and02-500) in relation to their activities on low-temperature CO oxidation were investigated. TEM images indicated that neither low-temperature nor reductive pretreatments wouldn’t cause the sintering of gold particles, however, high-temperature (500℃) made the pronounced increase of gold particle size from3.2nm to5.5nm. Activity tests showed that the CO oxidation activity and stability of O2-200were all excellent, and02-500also had an outstanding stability but a very low activity; however, H2-200displayed the worst stability on CO oxidation. Nevertheless, the marked differences of activities and stabilities couldn’t be ascribed to the gold particles sizes and accumulation of surface carbonates over the catalysts during CO oxidation. Because O2-200had the most amount of surface carbonates species, and it also had the best catalytic performance. HRTEM results revealed that the shape of gold particles was sphere after O2/200℃pretreatment, but it was truncated octahedron after H2/200℃pretreatment. It can be concluded that low-temperature oxidative pretreatment induced the reconstruction of gold surface and exposed more low-coordinated gold sites for activation of CO which was confirmed by the much stronger CO adsorption band over O2-200than that over H2-200.3. Gold nanoparticles catalyzed reductive imination of nitroarenes and aldehydes using CO/H2O as a hydrogen sourceImine is a kind of very important compound for its wide usage in manufacture of fine chemicals. It is of great significance to explore cleaner and cheaper alternative procedures for imine synthesis for the non-environmentally benign of the traditional acid-catalyzed imination of aldehydes (ketone) with amines. Base on the outstanding performance in low-temperature water-gas shift reaction of gold catalysts, Au/TiO2can directly catalyzed reductive imination of nitroarenes and aldehydes using CO/H2O as a hydrogen source at25℃. From the model reaction between nitrobenzene and phenylaldehyde, solvent and the amount of water were found to be crucial for the imination reaction. Nitro compounds could be fully converted and the yields of imines were satisfactory when using one equivalent H2O in NEt3. This Au-CO/H2O catalytic reduction system displayed great generality for scope of the reaction. Structurally (Cl-, C=C, C=O) and electronically diverse nitro compounds could be catalyzed reductive imination with aldehydes, and the yields of related imnes were quite high, up to98%. For its high selectivity and mild reaction conditions, this novel nano Au-catalyzed reduction procedure provide a new approach for green synthesis of fine chenmicals.4. Au-catalyzed controlled reduction of nitro compounds using HCOOH as a versatile reagentFor the advantages of gold-catalyzed transfer hydrogenation and tandem reaction, we adopted the ready and cheap HCOOH as a hydrogen source. Firstly, at70℃, fixing the mol ratio of HCOOH to nitro compounds at3, Au/TiO2can quantitatively convert different substituted nitro compunds into related amines. If the mol ratio of HCOOH to nitro compounds is4, amides can be obtained by the condensation of in-situ derived amines and HCOOH; Using o-dinitro as a substrate and7equiv. of HCOOH, the derived2-amino-formanilide can form corresponding benzimidazole by intramolecular imination reaction. However, Pt group metal (PGM) catalysts scarcely show any activities for these reactions, which highlight the excellent performance of gold nanoparticls in green synthesis of fine chemicals.