This treatise is based on first-principles of the nano-scale properties of methanol synthesis on copper. We implement calculations using density functional theory within the framework of general gradient approximation. Initially, we gain insights into the mechanism of hydrogen dissociative adsorption on Cu(100) and find that the process is activated and the transition state involves an extended H... H bond. Hydrogens adsorb preferentially at surface hollows and upon saturation of these, penetrate lattice bulk and occupy interstitials. We proceed by monitoring the adsorption of atomic hydrogen on Cu(110) and find that the surface is likely to undergo reconstruction to (1x2) missing row Cu(110). We do not find threshold coverage for restructuring and suggest that the process initiates from zero-limit coverage. Bulk absorption is again found possible upon surface saturation. Moreover, we find that hydrogen dissolution on the reconstructed surface induces a contraction of the lattice, contrarily to the other surfaces examined. Based on this observation we add to the interpretation of experimentally observed hydrogen locking-in upon surface restructuring. Subsequently, we analyse the mechanism of carbon dioxide hydrogenation to formate. Three different possible routes are considered and the possibility of surface reconstruction taken into account. Reaction barriers are lower on the perfect structure, however results on the reconstructed surface agree better to experimental range. In all cases, transition states involve tilted formates. We also analyse the minimum energy pathway dependence on structural variables. We contribute to the understanding of the nature of interaction of several additional probable intermediates of methanol synthesis; namely dioxymethylene, formyl, formaldehyde and methoxy. The strongest bound species is dioxymethylene, closely followed by formate. We conclude that the active site of Cu(110) perfect and reconstructed surfaces are the top copper ridges in all cases examined. We explain the stabilisation of the adsorbents in terms of charge transfer from the top copper atoms to the bonding atoms of the species. Finally, we examine two different possible pathways to methanol synthesis from carbon dioxide and hydrogen. We consider seven different step reactions on each route. We infer that the most probable path involves dioxymethylene and that formyl is not likely to be a precursor to methanol. Based on our calculation of activation energies, we propose that the ratelimiting step is the hydrogenation of formate to dioxymethylene.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:505815 |
| Date | January 2006 |
| Creators | Nikolaidi, B. |
| Publisher | University of Manchester |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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