(18f) Periodic DFT Study of the Effect of Uniform Electric Field On Chemisorption of Molecules On Pt(111) Surface

Imposed or intrinsic electrostatic fields can have a significant effect on the chemisorption of molecules at metal surfaces and thus are important in a number of physical systems related to electrochemistry and catalysis.  For example, electrochemical double layer at metal solution interface imposes a uniform electric field on the molecules adsorbed on the electrode surface in the electrochemical systems [1].  In heterogeneous catalysis chemisorbed molecules can experience strong local electric fields due to co-adsorbed ions and molecules [2] or metal support charge transfer [3].  In the present study periodic DFT calculations are used to study the effect of an homogeneous electric field applied perpendicular to a Pt(111) surface on the bond distances, binding energies and vibrational frequencies of different molecules at different adsorption sites (Figures 1 and 2).  The observed structural and energetic modifications can be understood in terms of modest field-induced charge transfer between charged metal surface and adsorbate and are well-described by classical first and second-order Stark models.  Zero-field dipole moment and polarizability of the molecule in chemisorbed state are the two parameters affecting the field dependence of chemisorption energy [4].

            The different molecules studied include CO, NO, ethylene, oxygen and other molecules of interest to heterogeneous catalysis. Particularly, the effect of electric field on adsorption energetics and vibrational properties of CO has been studied in detail.  Electronic differences between atop and fcc adsorption cause CO in these sites to respond differently to applied fields.  After correcting for the GGA site preference error, CO adsorption is predicted to shift from atop to fcc at potentials < ?0.19 V Å^?1 [Figure 2(left)].  The results are in qualitative agreement with previously reported cluster-based DFT models [5], but differ quantitatively due to difference in modeled coverage, surface relaxation, and finite size effects.  The calculated 44.4 cm^-1 V^-1 Å shift in C-O stretch frequency with electric field (Stark tuning rate) [Figure 2(right)] compares favorably with UHV experiments [6] but is significantly lower than the value obtained in electrochemical measurements[5], highlighting the importance of adsorbate environment on the magnitude of the tuning rate.  The calculated coverage dependence of the tuning rate is in good agreement with previous UHV experiments [6].  Experimental work is underway using nanofabricated nanowires and a new enhanced reflectance IR spectroscopy method to verify some of the theoretical predictions on supported catalysts. [1] Koper, M. T. M.; van Santen, R. A.; Wasileski, S. A.; Weaver, M. J. The Journal of Chemical Physics 2000, 113, 4392. [2] Pacchioni, G.; Lomas, J. R.; Illas, F. Journal of Molecular Catalysis. A, Chemical 1997, 119, 263. [3] Jochum, W.;
Eder, D.; Kaltenhauser, G.; Kramer, R. Topics in Catalysis 2007, 46, 49-55. [4] Deshlahra, P.; Wolf, E. E.; Schneider, W. F. Journal of Physical Chemistry A 2009, 113, 4125?4133.  [5] Wasileski, S. A.; Koper, M. T. M.; Weaver, M. J. Journal of Physical Chemistry B 2001, 105, 3518-3530. [6] Luo, J. S.; Tobin, R. G.; Lambert, D. K. Chemical Physics Letters 1993, 204, 445.

Figure 1: Effect of electric field on adsorption energy on nitric oxide (left) and ethylene (right) on Pt(111)

Figure 2: Effect of electric field on adsorption energy (left) and vibrational frequency (right) of carbon monoxide on Pt(111)