Abstract
Solar-light driven water splitting is a promising way for the sustainable production of molecular hydrogen, the latter representing an efficient carrier for energy storage and conversion into common liquid fuels. In search of novel catalysts for high-performance water splitting devices, Co-pyrphyrin (CoPy) has been recently synthesized and successfully used as a homogeneous water reduction catalyst. We investigate the adsorption of this molecule on the rutile TiO2(110) surface as a possible first step towards the design of a heterogeneous water reduction system. We find that the adsorption of the molecule is stabilized by the interaction of the cyano groups with the under-coordinated Ti centers present at the surface. This interaction induces the rehybridization of the molecular orbitals localized on the cyano groups and the realignment of the lowest unoccupied molecular states. Moreover, the highest occupied molecular orbital of CoPy@rutile(110) is localized on CoPy and the energy gap turns out to be significantly smaller than the gap of pristine rutile(110). This implies that direct or indirect injection of electrons from CoPy to the rutile(110) surface is in principle possible upon the absorption of light in the visible range. On the other hand, the electronic properties of the Co(ii) center are not modified by the adsorption, which suggests that CoPy and its derivatives may be used in water electrolysis for hydrogen production also in the adsorbed state.
Highlights
In the quest for renewable energy sources molecular hydrogen is one of the most favoured candidates as an energy carrier
The strongest adsorption is obtained for model-a, where the molecule is bent along the distance between the two NCN atoms (dCN) axes such that both the NCN atoms can sit exactly on top of Ti5c centers, while Co(II) coordinates two O2c
CoPy is bent along both axes, even if the curvature is more pronounced along dCN, reducing the dCN distance by about 1 Å with respect to the gas phase structure
Summary
In the quest for renewable energy sources molecular hydrogen is one of the most favoured candidates as an energy carrier. Several techniques can be employed for its production, such as water electrolysis,[3] steam and ethanol reforming,[4,5] partial oxidation of hydrocarbons,[6] dark fermentation,[7] and photo-chemical water reduction. The latter is of particular interest, because it emulates natural processes and needs only solar light as an energy source.[8]. Peters et al.[12] investigated the catalytic activity of
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