(377h) CO Adsorption on Platinum and Cobalt: Site Preference and Coverage Effects

G. T. Kasun Kalhara Gunasooriya,^† Mark Saeys^†

^†Laboratory for Chemical Technology, Ghent University, Technologiepark 914, 9052 Gent, Belgium

Calculation of correct site preference, adsorption energies and distribution of the adsorbates forming stable phases remain important challenges for DFT.[1] In this presentation we demonstrate that van der Waals functionals[2] provide an accurate description for these challenges on platinum and cobalt catalyst surfaces. Platinum catalysts are widely used in CO oxidation in automotive exhausts and as anode catalysts for direct methanol fuel cells. Cobalt catalysts are the preferred choice for Fischer-Tropsch synthesis (FTS), the conversion of synthesis gas, a mixture of CO and H₂, to long-chain hydrocarbons.

At low CO coverage, CO adsorbs at the top sites on Pt(111) and Co(0001) facets. Low coverage CO vdW-DF adsorption energy agrees well with the experimental data and the site preference is correctly predicted as top site. PBE functional overestimates CO adsorption energy and favors hollow over top site as most stable configuration. Interestingly, low coverage CO BEEF-vdW adsorption energy at the top site is close to the respective vdW-DF adsorption energy. However, the BEEF-vdW indicates bridge site to be the most favorable over hollow and top sites on Pt(111) facet. With increase in CO coverage, CO adsorbs both on top and bridge sites on the Pt(111) and Co(0001) facets. The observation of change in site preferences with increase in CO coverages is remarkable. To elucidate the change in stabilities of top and bridge sites with increase in CO coverage, a Bader charge and Natural Bond Orbitals (NBO) analysis was conducted. Charge analysis indicate that, CO adsorption on top site is more sensitive to surface charge compared to bridge site. Bonding analysis indicate that increase in surface charge, increases Pauli repulsion between metal dz² states and the filled CO 5σ orbital. Thus indicating that charge is the essence in determining the preferred adsorption site. This effect was further experimentally observed on TiO₂supported Pt clusters.[3]

CO adsorption is often described by a Langmuir isotherm, sometimes accounting for a gradual decrease in the adsorption energy with coverage. Here, we show that CO adsorption on cobalt does not follow this typical behavior. Instead, adsorption on Co(0001) is dominated by two surface phases. At low pressures, the (√3×√3)R30°–CO structure is the stable phase, and CO forms (√3×√3)R30°–CO islands for coverages below 1/3 ML because of attractive CO–CO interactions. However, a similar attractive interaction was not observed on Pt or with PBE and RPBE functionals. Charge and bonding analysis indicate that, these attractive interactions are due to decrease in charge at the diagonal Co atoms, hence reducing the Pauli repulsion between the partially filled Co dz² state and the CO 5σ orbital. Increasing the pressure does not gradually increase the coverage beyond 1/3 ML. Instead, a transition to a high coverage (2√3×2√3)R30°–7CO surface structure occurs at 0.1 mbar at room temperature and at 21 bar at 500 K. First principle phase diagram calculations with the van der Waals functional confirm the stability of these two phases and show a true first-order phase transition between the two phases.[4,5] It is important to note that CO adsorption introduces five new low frequency modes, i.e., vibrations associated with frustrated rotation and translation, with frequencies in the range of 40 to 400 cm^−1. Those modes contribute significantly to the adsorption entropy and need to be computed accurately. For comparison, CO adsorption on Pt(111) was also studied, where a similar phase transition is not observed.

References

[1] Schimka, L., Harl, J., Stroppa, A., Grüneis, A., Marsman, M., Mittendorfer, F., Kresse, G. Nat. Mater. 2010, 9, 741

[2] Dion, M.; Rydberg, H.; Schroder, E.; Langreth, D. C.; Lundqvist, B. I. Phys. Rev. Lett. 2004, 92, 246401

[3] Gunasooriya, G. T. K. K., Seebauer, E. G., Saeys, M. ACS Catal.2017, 7, 1966

[4] Beitel, G. A.; Laskov, A.; Oosterbeek, H.; Kuipers, E. W. J. Phys. Chem. 1996, 100, 12494

[5] Gunasooriya, G.T.K.K.; van Bavel, A.P.; Kuipers, H.P.C.E.; Saeys, M. Surf. Sci. 2015, 642, L6