Design,Synthesis, Characterization And Catalytic Ammonia Decomposition Of Catalysts For Hydrogen Generation

In the context of globalization of energy crisis and severity of environmental pollution, the development of new efficient environment-friendly energy sources is of great importance and high emergency. Due to high efficiency, low weight and pollution, fuel cells have good potential application in many fields such as mobile generators, automobiles, mini electric motors, portable electronics, and military, aerospace, etc. Nevertheless, the biggest obstacles to commercial fuel cells are hydrogen production, storage, transportation and cost. The most effective way to solve these problems is on-board hydrogen generation by liquid fuels with high hydrogen density. Due to free COx, high energy density and hydrogen capacity, catalytic ammonia decomposition is promising as pure hydrogen carrier for fuel cells. However, the catalytic activity of ammonia decomposition is still low and could not meet the demand of on-board fuel cells. Therefore, the design and synthesis of highly efficient and stable ammonia catalyst is of great significance.In this paper, the activities of Ru catalysts supported on different carbons were studied. The relationship between the nature of carbon supports (including pore structure, surface property, and graphitization) and Ru dispersion on the supports was investigated. And the influence of properties of carbon supports as well as metal particle size on catalytic performance was studied in detail. The mechanism of ammonia decomposition was discussed primarily. On the basis of results above mentioned, mesoporous metal oxides were applied for catalytic decomposition of ammonia. A novel mesoporous chromium oxide was synthesized by solid thermal decomposition. The synthesis conditions of temperature, time, ratio of precursors and surfactants as well as surfactant species were studied for their influence on structure of the mesoporous chromium oxides. In addition, the catalytic activities of ammonia decomposition on Cr2O3 catalysts were discussed primarily.It is showed that the properties of CMK-3 mesoporous carbon molecular sieve synthesized by hard template method have great influence on Ru dispersion. Higher specific surface area and appropriate pore size distribution is advantageous for sufficient dispersion of Ru on the supports. The chemical treatment of carbon support produces significant changes in carbon surface chemistry and these in turn can have dramatic effects on both catalyst dispersion and catalytic activity. In conclusion, Ru state and particle size, as well as acid-base properties and surface properties of the supports are important factors for ammonia decomposition.Alternately, high surface area porous carbon blacks were synthesized by chemical vapor decomposition (CVD). The conditions such as reaction temperature and catalyst composition are crucial to the structure of the carbon blacks. Ru catalysts supported on carbon blacks showed distinctly higher activities of ammonia decomposition compared with the one supported on CMK-3. And different structure of carbon blacks as Ru supports showed obviously different catalytic activities for ammonia decomposition. And a parameter, Vnano, was introduced in this paper for characterization of the size of carbon crystallite, which explained quantitatively the influence of the structure of the carbon blacks on oxidation and ammonia decomposition activity.Further studies on the performance of Ru catalysts loaded on different carbons showed that pore structure of supports has influence on Ru dispersion and particle size, and has no evident influence on catalytic activities of ammonia decomposition. The electronic effect of supports and Ru particle size has notable influence on catalytic activity. The proper Ru particle size is 3.0-5.0nm.Mesoporous chromium oxides were successfully synthesized by solid thermal decomposition with citric acid, CTAB and P123 as structure directing agent. The pore structure and size can be tuned by the synthesis condition. The results of catalytic performance showed that chromium oxide has certain ammonia decomposition activity. The composition of catalyst changed to chromium nitride with ammonia decomposition, but the performance of catalyst showed no significant reduction, which means chromium nitride also is the active site of ammonia decomposition.