National Synchrotron Light Source Symposium

The C-O stretching frequency (wCO) of CO adsorbed on Pt in PtRu alloys is used as a probe for analyzing the effect of alloying on bonding electronics. It has been difficult to reconcile the observations that alloying increases the Pt-C bond length, decreases the CO stretching frequency while the d-band center drops to lower energy. Using Density Functional Theory electronic and phonon property calculations on a library of spin optimized CO/Pt, and CO/PtRu clusters coupled with Mulliken/FEFF8 population analysis and FEFF8 local density of states we provide an explanation for all of the above observations based on asymmetric d-band dispersion that results from alloying Ru into Pt. There is electron transfer from the Ru d orbitals to the s-p band of the Pt alloy FCC lattice. Although the d-band vacancies lowers the d-band center, the d-band is energetically top heavy resulting in more back donation to the CO antibonding orbitals. We have extended our calculations to examine the CO coverage effect over the Pt surface. We observed that DFT calculated wCO increases in proportional to coverage as long as CO remains atop over the Pt surface (no CO tilt). In collaboration with BNL, CUNY and NIST we use the near-edge X-ray absorption fine structure (NEXAFS) technique to obtain carbon edge data to examine the effect of alloying on the filling of the 2* band of atop adsorbed CO on PtRu surfaces. Compositional arrays of Pt-Ru alloys are prepared, cleaned, and dosed with CO before partial electron yield NEXAFS data are collected from the adsorbed CO. The results of these experiments will be presented in the context of the current state of knowledge regarding the mechanism for CO adsorption on the Pt-Ru system.