Ceria Supported Au Catalysts For Low-temperature CO Oxidation

Catalysis by gold has attracted significant attention in the last decades,since Haruta's discovery that these catalysts have ultrahigh catalytic activity in the low temperature CO oxidation and that the activity is even higher in humid atmosphere. The most attractive thing in gold catalysis is that the dramatic improvement of catalytic property brought by the nano-effect happens to a metal which has long been regarded as catalytically inert.Apart from the overwhelmingly important particle size effect of gold,the catalytic property also largely depends on the type and nature of the support.Generally,gold supported on reducible metal oxides is considered to be more active than that on the non-reducible oxides.The essential role of the reducible support is believed to stabilize the gold nanoparticles by strong gold-support interactions as well as to provide oxygen adsorption and activation sites.However,the activity may differ even in several orders of magnitude when using different preparing methods on a same support.The most common preparing method reported is the deposition- precipitation by NaOH(DP method).Ceria has been regarded as one of the most important components in many catalytic systems due to its remarkable redox properties.Of particular interest is that ceria can act as an oxygen buffer by releasing/uptaking oxygen through redox processes involving the Ce⁴⁺/Ce³⁺ couple,which plays an important role in many oxidation reactions.Nevertheless,until recently,the catalytic activities of Au/CeO₂ catalysts for CO oxidation were reported to be highly inconsistent or even contract with each other,indicating a great complexity.In this dissertation,we attempt to study the effects of support structure and surface property on the catalytic properties of Au/CeO₂ catalysts by using various preparing methods and characterizations.The main conclusions are described as follows:1.Commercial ceria VP Adnao 50 supported gold catalystsGold catalysts were prepared using commercial ceria VP adnano 50(provided by Degussa) as the support by the DP method.The effect of post-treatment,gold loading and storage was studied to optimize the preparing parameters and gain some general insights.The post-treatment under the feed gas of CO oxidation was found to be most favorable for high Au dispersion,leading to the optimum catalytic activity. The highest specific rate was gained in the catalyst with the lowest gold loading. Au/VP50 catalyst shows high durability in the storage test.2.Ceria pre-calcined in different temperatures supported gold catalystsA series of low-content Au/CeO₂ catalysts was prepared using the DP method in which the supports were subjected to calcination at increasing temperatures prior to Au loading.It is shown that the best performance was achieved for the gold deposited on 873 K-calcined supports,_apparently due to improved Au dispersion resulting from stabilization of cationic gold species in the thermally aged ceria supports.TEM results show good crystallinity as well as the exposed thermodynamically meta-stable {100} facets in 873 K and higher temperature pre-calcined ceria supports.The exposed thermodynamically meta-stable {100} facets was formed by truncating the sharp tips between the stable {111} facets.The effect of CeO₂-873 on the activity of gold catalyst can be explained in two aspects:a) {100} facets hold stronger interactions with gold,leading to higher gold dispersion on Au/CeO₂-873;b) {100} facets show better redox properties than {111} facets,providing more active oxygen species. Under high gas hourly space velocity(GHSV) conditions,complete CO conversion with specific rates up to 98.8 mol_CO·h^-1·g_Au^-1 can be achieved for the 873 K-calcined ceria supported gold catalysts and shows no deactivation in 6h.3.Ceria nanorad supported gold catalystsCeria nanorod with rod-like shape and surface was synthesized by hydrothermal method and was then used as support.The activity of Au/CeO₂-nanorod(Au-CR) catalysts was much higher than Au/CeO₂-nanoparticle(Au-CP) catalyst with the specific rate of 4.02 mol_CO·h^-1·g_Au^-1 at 278 K and the apparent activation energy of 15.9 kJ·mol^-1 comared with 0.15 mol_CO·h^-1·g_Au^-1 and 28.4 kJ·mol^-1,the data of Au-CP respectively.In consistent with the above studies,the characterization of XRD,TEM, H₂-TPR,XPS,UV-Vis and DRIFTS demonstrated the stronger support-metal strong interaction(SMSI) between the predominantly exposed {100} and{110} facets on the surface of ceria nanorod.In addition,the deposition of gold was found to modify the electronic structure of supports and enhance their redox properties.