VOLUME 86, NUMBER 8 PHYSICAL REVIEW LETTERS 19 FEBRUARY 2001 Effect of Hydrogen Adsorption on the X-Ray Absorption Spectra of Small Pt Clusters A. L. Ankudinov and J. J. Rehr Department of Physics, University of Washington, Seattle, Washington 98195-1560 John Low and Simon R. Bare UOP LLC, Des Plaines, Illinois 60017 (Received 1 May 2000) Hydrogen adsorption on Pt 6 H n clusters leads to striking changes in the Pt L 2,3 x-ray absorption spectra. These effects are interpreted using a self-consistent real space Green’s function approach. Calculations show that they are due largely to changes in the atomic background contribution to x-ray absorption (i.e., atomic x-ray absorption fine structure) and to reduced Pt-Pt scattering at the edge, while Pt-H multiple scattering is relatively weak. The origin of both effects is traced to the change in the local Pt potential due to Pt-H bonding. DOI: 10.1103/PhysRevLett.86.1642 PACS numbers: 78.70.Dm, 82.65.+r, 82.80.Ej Although the dissociative adsorption of hydrogen is of paramount importance in many metal catalyzed hydro- carbon reactions, a detailed understanding of how chemi- sorbed hydrogen affects the reactivity of dispersed metal clusters is still not fully understood. Nevertheless, the electronic and structural information in x-ray absorption spectra (XAS) provides important clues [1–9]. Hydrogen adsorption on small Pt particles causes dramatic changes in the spectra at the Pt L 2,3 edges, as first observed by Lytle et al. [2]. In particular, at the Pt L 3 edge, the “white line” [10,11] (i.e., the most intense edge peak in Fig. 1) intensity decreases and the spectra broaden to higher energies as hydrogen is added. There have been many efforts to interpret these effects [1–9]. Most of these analyses are based on subtracting the Pt L 3 -edge spectra for catalysts with and without ad- sorbed hydrogen. This difference signal typically leads to a broad structure about 8 eV above the absorption edge. Sev- eral authors [3,7,8] attribute the structure to transitions to empty, Pt-H antibonding (s  ) states, while Iwasawa et al. [5] preferred a continuum resonance. Since the Pt clusters of interest are usually attached to a support, even the bound states of finite Pt clusters become scattering resonances due to cluster-support interactions. The calculations of Ham- mer and Nørskov [12] showed that the H-metal interac- tion leads to the creation of an s-d antibonding state just above the metal d band, which for Pt will be pushed above the Fermi level and hence remain empty. However, Reif- snyder et al. [7] claim that the observed antibonding level appears at higher energies relative to the Fermi level than calculated in Ref. [12]. Recently, Ramaker et al. [1] at- tempted to separate the electronic and geometric effects of H adsorption using a novel analysis involving experi- mental peak alignment and theoretical calculations using a high order multiple scattering code FEFF7 [13]. They at- tributed the difference signal to Pt-H multiple scattering (MS) between 3–20 eV above the edge. In addition they concluded that the Pt-H antibonding state produces a shape resonance 0–5 eV above the edge. In this Letter we reinvestigate the various interpretations of these effects using a self-consistent real space Green’s function approach. This approach is implemented in the code FEFF8 [14], which is an improvement on FEFF7. We demonstrate that this self-consistent approach can repro- duce the experimentally observed behavior of the XAS under the addition of hydrogen. An analysis of the re- sults allows us to make a definitive interpretation of the origin of the observed behavior. The key finding of this Letter is an interpretation of these effects as mainly due to changes in the local Pt scattering potentials induced by the H atoms, an interpretation which had not been pro- posed previously. In particular, this implies that the broad structure beyond the white line is primarily due to the phe- nomena of atomic x-ray absorption fine structure (AXAFS) [15 – 17]. We find that direct Pt-H MS is secondary in mag- nitude and concentrated closer to the edge. Our results for the local electronic density of states (DOS) on H and Pt atoms support the existence of Pt-H antibonding states in the continuum, as predicted in Ref. [12]; however, the lo- cal potential effect is larger. FIG. 1. Effect of adsorbed hydrogen on the XAS of octahedral Pt 6 : calculated spectra for Pt 6 H 0 (dotted line), Pt 6 H 2 (solid line), and Pt 6 H 8 (dash-dotted line) normalized at 50 eV above the edge. 1642 0031-9007 01  86(8)  1642(4)$15.00 © 2001 The American Physical Society