Spectroscopic Link between Adsorption Site Occupation and Local Surface Chemical Reactivity

In this Letter we show that sequences of adsorbate-induced shifts of surface core level (SCL) x-ray photoelectron spectra contain profound information on surface changes of electronic structure and reactivity. Energy shifts and intensity changes of time-lapsed spectral components follow simple rules, from which adsorption sites are directly determined. Theoretical calculations rationalize the results for transition metal surfaces in terms of the energy shift of the $d$-band center of mass and this proves that adsorbate-induced SCL shifts provide a spectroscopic measure of local surface reactivity.

Figure 1

(a) Surface components of the $\mathrm{R}\mathrm{h}3d_{5/2}$ core level spectra for $\mathrm{O}/\mathrm{R}\mathrm{h}\left(100\right)$ at ${\Theta }_0=0$, 0.185, and 0.375 ML oxygen coverages. The gray component ${\mathrm{R}\mathrm{h}}_0$ originates from the clean surface Rh atoms, while ${\mathrm{R}\mathrm{h}}_{1,4}$ and ${\mathrm{R}\mathrm{h}}_{2,4}$ peaks originate from surface Rh atoms which bind to one and two oxygen atoms, respectively. (b) Oxygen coverage dependence of the three peaks' integrated areas, obtained by fitting a sequence of spectra (recorded in situ during the uptake experiment). The areas have been normalized to the initial clean peak area.

Figure 2

(a) Surface core level shifts induced by oxygen adsorption on Rh(111) and Rh(100) versus the Rh-O coordination $i/j$. (b) Core level shifts for rhodium atoms in different $C_{i,j}$ configurations, versus the calculated atom-projected shift of the $d$-band center. The shifts are measured from the corresponding clean surface peak.

Figure 3

Calculated NO adsorption energies versus experimental changes of surface core level binding energy for Rh(111) and Rh(100) surfaces with different amounts of preadsorbed oxygen.