“2011-iChEM讲座”第12讲:On the enigma of metal deposition and Electrocatalysis at nanostructures: can we build a bridge between theory and experiment?

报告题目1:On the enigma of metal deposition

报告题目2:Electrocatalysis at nanostructures: can we build a bridge between theory and experiment?

报  告  人: Prof. Wolfgang Schmickler and Prof. Elizabeth Santos
                  Institute of Theoretical Chemistry, Ulm University

时      间: 2017年5月22日(星期一)上午10:00-11:30

地      点: 卢嘉锡楼202报告厅

Prof. Wolfgang Schmickler 简历

Prof. Elizabeth Santos简历

Metal ions are strongly hydrated in electrolytes, and their deposition requires a loss of solvation energy of the order of 5 eV for monovalent , 16 - 20 eV for divalent, and event more for trivalent ions. In spite of this fact metal depositions is often fast. We have studied three processes as examples: The deposition of silver, copper, and zinc.The deposition of silver is one of the fastest electrochemical reactions, even though the reactant loses more than 5 eV of solvation energy when it is incorporated into the electrode. To explain this phenomenon, which is an example of the so-called enigma of metal deposition, we have investigated this reaction by a combination of molecular dynamics simulations, density functional theory, and a theory developed in our own group. Contrary to expectations, the Ag+ ion can get very close to the surface of a silver electrode without losing its energy of solvation. There it experiences a strong and long ranged interaction with the sp band of silver, which strongly catalyses the reaction. In accord with experimental data this results in an activation energy which is lower than that for the subsequent incorporation into a kink site.For both copper and zinc deposition, the reaction proceeds through two one-electron steps. The monovalent ions can get close to the electrode surface without losing hydration energy, while the divalent ions, which have a stronger solvation sheath, cannot.The 4s orbital of Cu interacts strongly with the sp band and more weakly with the d band of the copper surface, while the Zn4s orbital couples only to the sp band of Zn. At the equilibrium potential for the overall reaction, the energy of the intermediate Cu+ ion is only a little higher than that of the divalent ion, so that the first electron transfer can occur in an outer-sphere mode. In contrast, the energy of the Zn+ ion lies too high for a simple outer-sphere reaction to be favorable; in accord with experimental data this suggests that this step is af- fected by anions.

The investigation of promising materials for electrocatalysis using both experimental tools and theoretical models is a big challenge. Nowadays, the advances in the nanotechnology and in computational methods have facilitated the construction of bridges between these two approaches. The expansion of highly sensitive characterization techniques in surface science has allowed a detailed description at an atomic level, and the development of very sophisticated software applying the density functional theory (DFT) has provided realistic explanations. However, there are still some difficulties to achieve the right connections, particularly in electrochemical systems. Reactions occurring at these interfaces are affected by the presence of the solvent and ions, which also makes the characterization more difficult. The electrochemical potential plays a fundamental role in the driving forces of electron transfer and these aspects are not easy to be modelled only by DFT.We have selected some systems that are interesting for the electrocatalysis and discuss their behaviour in a framework of our own theory, which combines a model Hamiltonian with DFT calculations, molecular dynamics and Kinetic Monte Carlo to describe electron transfer reactions in an electrochemical environment. This method is not limited to single crystal surfaces, but the effect of different nanostructures such as steps, overlayers, and alloying can be incorporated as well. The advantage of our combined methodology is that it makes possible to calculate the effect of the electronic structure on the energy of activation, while DFT alone can give very useful information about thermodynamic processes. The other important aspect is that in our model is included the solvent reorganization, which plays a fundamental role in electrochemical reactions. We consider the hydrogen adsorption and evolution reactions as test for the investigations of the electrocatalytic properties, mainly focusing on the Volmer electrochemical step.



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