Radius of curvature effect probed by transition metal substituted mesoporous molecular sieve materials.

摘要

Catalysis is both an exceptional intellectual challenge and critically important to energy production and utilization, to chemical manufacturing, and to the minimization of environmental impact of these processes. The scientific basis of catalyst preparation, the so called design of the catalyst, plays a key role in and lays the fundamental base of the development of catalysis. In this research, more quantitative synthesis of MCM-41 mesoporous molecular sieves was achieved based on extensive experience, trial-and error experiments, and statistical models.; The radius of curvature effect, as defined in this thesis, predicts a change in the thermodynamics surface tension of the pore wall as the pore size is varied. The change in surface tension will affect local stability at the surface relative to the bulk which might be reflected on a molecular level in bond angles, coordination unsaturation, stability of minority ion substitution, solid solubility, etc., or generally the surface structure which can affect the activity and/or selectivity of a catalytic site on the pore wall. The hypothesis of radius of curvature affect on catalytic behavior was tested.; It has been demonstrated that highly ordered and reproducible Co, V, and Ni-MCM-41 catalysts with nearly atomically dispersed metal ions has been prepared by a hydrothermal method. Experimental design and statistical models were built to analyze the MCM-41 synthesis. Modeling results show that our models are reasonably good at predicting the structure, pore diameter, and metal composition.; A strong effect of radius of curvature on the catalytic reaction and reduction stability exists when the methanol oxidation and hydrogen temperature programmed reduction were used as probes. The radius of curvature effect on the catalytic behavior is shown to be represented by a "volcano curve" where the turnover frequency increased with increasing pore diameter to a maximum and then decreased. The structural stability of catalysts affects the activity and how the vanadium is incorporated into the framework affects the selectivity, but it is probably the effect of radius of curvature on vanadium reducibility that overall controls reactivity and selectivity.; We have found that the reducibility of cobalt ions in the silica framework correlates strongly with the pore diameter of the MCM-41, with the cobalt incorporated in the smaller pore MCM-41 being more resistant to complete reduction. The size of the metallic Co clusters formed after different reducing treatments correlates linearly with the pore size, giving direct evidence for the effect of the radius of curvature on reducibility. It has been also demonstrated that the reducibility of nickel ions is not directly associated with the radius of curvature of the pore wall, due to the greater thermodynamic affinity of nickel ions for the reducing agent.