COMBINED THEORETICAL AND EXPERIMENTAL STUDY OF CO ADSORPTION AND REACTIVITY OVER PLATINUM AND COBALT

Abstract

The hydrogenation and oxidation of carbon monoxide (CO) over transition metal catalysts are arguably the most widely studied catalytic reactions, both experimentally and theoretically. The catalytic oxidation of CO over automotive exhausts catalysts and Fischer Tropsch synthesis to convert syngas to long chain hydrocarbons are some examples. Catalytic reactions start by adsorbing CO onto the catalyst surface. In this thesis, the adsorption of CO on cobalt and platinum terraces was studied using state-of-the-art density functional theory (DFT) using the rev-PBE functional and including non-local vdW-DF correlation for a wide range of coverages.

First, CO adsorption on hcp Co(0001) terraces was studied. Our approach, correctly predicts the preferred adsorption at the top as well as accurate adsorption energies, a long standing challenge for DFT. Moreover, only two stable surface structures or phases were found. All the other structures are significantly less stable. The stable structures are a low coverage (v3×v3)R30o¿CO phase and a high coverage (2v3×2v3)R30o-7CO phase, separated by a first order-phase transition.

The situation is very different on the well-studied Pt(111) surface. Many surface structures have comparable stabilities and even for a given coverage, many competing structures exist. About 20 structures were evaluated and for example different stable structures with long-range order were found for 0.5 ML {(4x2)-4CO or (v3×2)rect-2CO}, 0.6 ML {(v3×5)rect-6CO}, 0.67 ML {(v3×3)rect-4CO} based on the stability diagram.

Finally, to elucidate the gradual variation in the CO oxidation activity over sub-1 nm Pt clusters when the carrier density of the TiO2 support is gradually changed (known as the Schwab effect), the effect of charge transfer to Pt on the CO adsorption energy was studied in detail. Our calculations together with a natural bond orbital analysis demonstrate that charge injection into the Pt particles reduces the CO adsorption energy by increasing the Pauli repulsion between the filled CO 5s and the Pt dz2 orbitals, consistent with the trends in the measured reaction rates.