| The synthesis of fuel and/or high value-added chemicals from coal-derived syngas is considered as a promising alternative route in China to solve the problems about crude oil shortage. Presently, the production of methanol from coal-derived syngas has already been commercialized, so researchers are putting emphasis on the synthesis of oxygenates with longer chains (ethanol, acetaldehyde and acetic acid) from coal. CO hydrogenation is regarded as a structure-sensitive reaction, the acitivity and selectivity depend not only on macroscopic factors (e.g. active components, preparation methods and reaction conditions), but also on microscopic factors (e.g. particle size of catalysts, metal-promoter-support interactions). Recent works have found that Rh-based catalysts were active for CO hydrogenation to produce C2+oxygenates, while methane, as a low-value product, is still predominant in the products. One or more promoters, such as manganese oxide, have found to be highly effective to enhance the selectivity to C2+ oxygenates. Up to now, the promotional effects of MnOx on Rh is still debatable. Thus, this work mainly focused on the structure-performance relationships on Rh-MnOx/SiO2 catalysts. Results in this dissertation can be concluded as follows:(1) A series of Rh-MnOx/SiO2 catalysts have been prepared, characterized and tested. It was found that CO conversion increased with the addition of a certain amount of MnOx, but decreased with over-loadings. Furthermore, the addition of 0.2wt% Mn could greatly enhance the selectivity toward C2+ oxygenates. The morphologies of Rh nanoparticles were almost not affected by MnOx; while the chemical environment of Rh nanoparticles was changed remarkably. We assume that there is a strong interaction between Rh and MnOx, thus leads to the formation of a tilted CO adsorption mode at the Rh-MnOx interface. On the other hand, as electron withdrawing from Rh to MnOx, metallic Rh was partially oxidized to Rh+, which is responsible for the formation of tilted adsorbed CO and the enhancement of the dissociation of CO. However, the active sites might be blocked by adding excessive Mn, leading to a drop of catalyst activity. Nevertheless, the excessive amount of MnOx had less effects on the structure of Rh-MnOx interface and thus the selectivity of C2+ oxygenates did not rely on with the amount of MnOx.(2) The kinetics of CO hydrogenation on Rh-MnOx/SiO2 catalysts with different Mn loadings (0,0.4% and 4.0%) was studied. The effects of reaction temperature, pressure, space velocity, etc on the reaction rate were investigated. The apparent activation energies and reaction orders with respect to CO and H2 were acquired by fitting the kinetic data. It was also found that a small amount of MnOx could greatly increase the activation energies of hydrocarbons, while the activation energies of C2+oxygenates changed little. Interestingly, the amount of MnOx did not greatly influence the activation energies of products. These results indicated that the formation of hydrocarbons was suppressed and the selectivity to C2+ oxygenates was enhanced with the addition of MnOx.Meanwhile, the reaction orders of products with respect to H2 and CO over Rh-MnOx/SiO2 catalysts were much more complicated. For saturated hydrocarbons, the reaction orders with respect to H2 were always positive while the reaction orders with respect to CO were always negative. Furthermore, the reaction orders changed little with varying Mn loadings. For unsaturated hydrocarbons, the reaction orders with respect to H2 were always negative while all reaction orders with respect to CO were positive, unlikely affected by MnOx. These results indicated that high H2/CO ratios favoured the formation of saturated hydrocarbons while low H2/CO ratios favoured the formation of unsaturated hydrocarbons. For oxygenated products, the reaction orders of ethanol with respect to H2 increased in the present of a small amount of MnOx and then decreased with over-loading of MnOx, due to the enhancement of CH3CO* hydrogenation to ethanol in the presence of a small amount of MnOx. However, a part of active sites might be covered when Mn loadings reached 4.0%. In addition, the reaction orders of acetic acid with respect to H2 were negative and decreased with the addition of MnOx, because actic acid was produced from CH3CO* with surface OH over SiO2.Surface OH might react with H2 to form water under high H2 partical pressures, and thus led to the consumption of surface OH and consequently decreased the fromation rate of acetic acid. The decrease in the reaction orders with respect to H2 over Rh-MnOx catalysts compared with that over Rh catalyst might be related to the amount of surface OH on different catalysts. The reaction orders of C2+ oxygenates with respect to CO decreased in the presence of MnOx, this might be due to the creation of tiled CO adsorption sites that enhanced the adsorption of CO and decreased the reaction order with respect to CO.(3) Both hydrocarbons and oxygenated products followed the ASF distribution, and the chain growth probability varied with temperatures. The variation of chain growth probability with temperatures has proved that the reaction mechanism changed with temperatures. The chain growth probability of oxygenates decreased while that of hydrocarbons remained unchanged with addition of MnOx. This indicated that a second active site might exist over MnOx-promoted catalysts and the chain propagation over the catalysts was suppressed. With the combination of catalyst performance, characterization and the kinetics, the reaction mechanisms of CO hydrogenation over Rh/SiO2 and Rh-MnOx/SiO2 catalysts were proposed. For unpromoted Rh/SiO2 catalysts, all the elementary steps including H assisted CO dissociation, chain propagation, CO insertion and hydrogenation took place on metallic Rh sites and methane is the main product. For Rh-MnOx/SiO2 catalysts, the steps of H assisted CO dissociation, chain propagation and hydrogenation still took place on metallic Rh sites, but the production of acyl intermediate (CHxCO*) through CO insertion occurred on oxidized Rh+sites, increasing the selectivity to C2+oxygenates. On the other hand, the concentration of CHx species on Rh+was lowered due to the weak ability of CO dissociation on Rh+, and therefore chain propagation step was unable to take place on Rh+ sites. Thus. CHxCO* species could only be hydrogenated to produce short chain oxygenates and the selectivity to ethanol. acetaldehyde and acetic acid was greatly enhanced. In the meantime, CO conversion was singnificantly increased since the Rh-MnOx interface vielded extra sites for direct CO dissociation on Rh- MnOx/SiO2 catalysts. |
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