AB INITIO MODELING OF FREE ENERGY PROFILES IN THERMALLY ACTIVATED PROCESSES

摘要

The quantitative modeling of many surface processes, such as diffusion or chemical reactions, requires accurate knowledge of free energy profiles. The need to go beyond the internal energy is especially important in entropy-controlled processes which may happen at both high (the thermally-activated regime) and low (the quantum tunneling regime) temperatures. We present results for a thermally-activated process, namely, the formation of the first intermediate in the methanol-to-gasoline process, catalyzed by acidic zeolites. At high temperatures of 700 K, the entropic contribution cannot be correctly evaluated in the harmonic approximation and we use ab initio thermodynamic integration within density functional theory. We find that, at reaction temperatures, the entropic contribution qualitatively alters the free energy profile. Different transition states are found from the internal energy and free energy profiles. The entropic contribution varies significantly along the reaction coordinate and is responsible for stabilizing the products and for lowering the energy barrier. An outlook is given for a proper treatment of entropically-controlled processes in both the thermally-activated and quantum regimes.