科研之友
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摘要
An extended theoretical investigation of the electronic and interface properties of titania and alumina with and without supported platinum nanoparticles is presented and compared to recent experimental data with the aim to understand the mechanism of hydrogen activation, adsorption, and spillover. Thirteen-atom platinum particles on titania adopt a distinct different structure than on alumina, which results in distinct hydrogen coverages. Upon hydrogen adsorption, titania is reduced with creation of Ti(III) electronic trap states, strongly interacting with the surface adsorbed protons. The combined Ti(III)/proton migration rate is slower than the one of single surface protons, and it is not influenced by the presence of coadsorbates, such as water molecules. Hydrogen is instead heterolytically split on defect sites on alumina with the formation of a surface proton and a hydride moiety, bound to the particularly reactive surface tricoordinated aluminum site. The electronic structure of alumina is only marginally altered, without formation of defect states. The mobility of the hydride moiety is limited, in particular in the presence of coadsorbed water molecules, that compete for the adsorption sites. Modeling of hydrogen spillover from a platinum cluster to the metal oxide demonstrates that the spillover rate depends on the hydrogen partial pressure and on the thickness of the oxide acceptor layer. Kinetic Monte Carlo results confirm that hydrogen spreading on titania leads to a homogeneous coverage, while on alumina hydrogen can only be found up to a few nm from the platinum cluster because of kinetic competition between diffusion and desorption.
- 出版日期2017-8-24
