Hydrogenation by spillover of hydrogen from metal to acidic support or by reverse spillover of the reactant molecule from the support to the metal

摘要

For hydrogen spillover to be effective in catalysis, it must lead to a net transport of hydrogen atoms. The first prerequisite for this to happen is a thermodynamic driving force between the point of hydrogen atom creation by H_2 dissociation on the metal particle and the site where the hydrogen is used for hydrogenation. The second prerequisite is a means of transportation of the hydrogen atoms over the support surface. Proof of the existence of spillover came from experiments with reducible supports such as WEO_3. In a metal/WO_3 catalyst both prerequisites are met. The thermodynamic driving force is the reducibility of the support cations, and they provide at the same time the means of transportation by a redox transfer of the hydrogen atom between neighbouring tungsten cations. Scientists have tried to use the same explanation for systems in which the support metal cations cannot be reduced. Thus, hydrogen-deuterium exchange on a metal/SiO_2 catalyst is often quoted as an example of spillover as well. This, however, is a misunderstanding. There is no net hydrogen transport on an ideal metal/SiO_2 system, only hydrogen-deuterium exchange. Exchange between a deuterium atom on the metal surface and a hydrogen atom of a silanol group occurs at the metal-support interface by a bending over of a silanol group towards the metal surface. The diffusion of the deuterium atom over the silica surface, away from the metal particle, takes place by exchange of a Si-OD group with a Si-OH group. This is a water-assisted proton and deuteron jump process. The difference between the two examples is that in the case of the metal/WO_3 system there is a genuine transport of hydrogen atoms away from the metal particles. These hydrogen atoms turn into protons by donating an electron to the support surface cations, and the protons combine with the oxygen anions of the support to hydroxyl surface groups. In the case of the metal/SiO_2 system, however, hydrogen atoms cannot reduce the Si cations. As a result, there can only be exchange between OH and OD groups, but no net transport of hydrogen away from the metal particles. With the need to remove more sulfur-containing and aromatic molecules from diesel fuel, interest in metal catalysts supported on acidic carriers has increased.