D-band vacancy

The shape of the edge itself examined by XANES (X-ray near-edge spectroscopy) can be employed to reveal information on d-band vacancy concentration vs. treatment. The oscillations at energies above the edge (EXAFS) ctui provide information on near-neighbor atom spacing tuid some limited information on the chemical environment. As we will show, the best way to use such tools is to use several at once, rather them only one. 

A study of the area of the L absorption edge resonance shows a correlation with chemical activity. This implies a correlation with the number of d-band vacancies. This occurred when Pt oxide was reduced by hydrogen or when the particle size was decreased (13). 

Early catalytic studies on Cu-Ni alloys were prompted by the suggestion of Dowden and Reynolds (107, 108) that d-band vacancies are... 

Gold forms a continuous series of solid solutions with palladium, and there is no evidence for the existence of a miscibility gap. Also, the catalytic properties of the component metals are very different, and for these reasons the Pd-Au alloys have been popular in studies of the electronic factor in catalysis. The well-known paper by Couper and Eley (127) remains the most clearly defined example of a correlation between catalytic activity and the filling of d-band vacancies. The apparent activation energy for the ortho-parahydrogen conversion over Pd-Au wires wras constant on Pd and the Pd-rich alloys, but increased abruptly at 60% Au, at which composition d-band vacancies were considered to be just filled. Subsequently, Eley, with various collaborators, has studied a number of other reactions over the same alloy wires, e.g., formic acid decomposition 128), CO oxidation 129), and N20 decomposition ISO). These results, and the extent to which they support the d-band theory, have been reviewed by Eley (1). We shall confine our attention here to the chemisorption of oxygen and the decomposition of formic acid, winch have been studied on Pd-Au alloy films. 

CO2 formation between pure Pd and alloys with a low Ag content corresponded with a large increase in E, 13.6 and 30.2 kcal/mole over pure palladium and the 62% Pd alloy, respectively, somewhat compensated by an increase in frequency factor. As the concentration of d-band vacancies falls to zero, E again decreased (13.9 kcal/mole at 43.5% Pd) andremained at low values. 

Figure 14. Effect of average particle size on the change in normalized d band vacancies (d band vacancies/% surface atoms) (filled circles) on going from 0.54 to 0.0 V vs RHE and on going from 0.54 to 0.84 V (open circles). (Adapted from ref 31 with permission. Copyright 1998 Elsevier Sequoia S.A., Lausanne.)...

The applied electrode potential has been shown to have an effect on both the XANES and EXAFS of PtRu catalysts. The variations of the Pt d band vacancy per atom, (/7j)t,s, with potential over the range 0.0—0.54 V vs RHE for both the poorly mixed 1 1 PtRu/C catalyst investigated by McBreen and Mukerjee ° and a well mixed 1 1 PtRu/C catalyst studied by Russell et al. were less than that for a pure Pt/C catalyst. McBreen and Mukerjee attributed this difference to a reduction in the adsorption of hydrogen on the Pt sites of the alloy catalyst. The results also provide evidence of an electronic effect upon alloying Pt with Ru. The effects on the Ru XANES were much less significant, but some evidence of a change to a higher oxidation state at potentials above 0.8 V was observed. ... 

Figure 29. Pt d band vacancy per atom obtained from XANES analysis at the Pt L3 and Lz edges for PtRu/C (filled circles) and PtMo/C (filled squares) as a function of the applied potential in 1 mol dm HCIO4 + 0.3 mol dm methanol. (Reproduced with permission from ref 103. Copyright 2002 Elsevier Sequoia S.A., Lausanne.)...

PtMo alloys are not as effective as PtRu for methanol, or ethanol, oxidation. As shown in Figure 29, the d band vacancy per Pt atom for the PtMo/C catalyst continues to increase until 0.6 V vs RHE, in contrast to the behavior of PtRu/C. ° The authors attribute this difference to the lack of removal of the Cl fragments from the particle surface by the oxy-hydroxides of Mo. However, the difference in the electrocatalytic activity of PtRu and PtMo catalysts may be attributed to ensemble effects as well as electronic effects. The former are not probed in the white line analysis presented by Mukerjee and co-workers. In the case of methanol oxidation, en-... 

Figure 30. Correlation of the oxygen reduction performance (log igoo mv) of Pt and Pt alloy electrocatalysts in a PEM fuel cell with Pt—Pt bond distance (filled circles) and the d band vacancy per atom (open circles) obtained from in situ XAS at the Pt L3 and L2 edges.(Reproduced with permission from ref 34. Copyright 1995 The Electrochemical Society, Inc.)...

XAS has been successfully employed in the characterization of a number of catalysts used in low temperature fuel cells. Analysis of the XANES region has enabled determination of the oxidation state of metal atoms in the catalyst or, in the case of Pt, the d band vacancy per atom, while analysis of the EXAFS has proved to be a valuable structural tool. However, the principal advantage of XAS is that it can be used in situ, in a flooded half-cell or true fuel cell environment. While the number of publications has been limited thus far, the increased availability of synchrotron radiation sources, improvements in beam lines brought about by the development of third generation sources, and the development of more readily used analysis software should increase the accessibility of the method. It is hoped that this review will enable the nonexpert to understand both the power and limitations of XAS in characterizing fuel cell electrocatalysts. 

Sample d Band vacancy No. of unfilled d states per Pt atom... 

The Liix and Ln edge white-line surface area of 5d metals is generally accepted to be a measure for the d-band occupancy, or better the d-band vacancy (28). Small reduced platinum and iridium clusters were reported to be electron deficient compared to the bulk metal (28). For a given particle size the whiteline surface area provides information on the relative degree of oxidation of the metal the higher the white-line surface area the higher the oxidation state of the metal. 

Figure 7. d-band vacancy (percentage of surface atoms) as a function of Pt particle size when the potential was changed from 0 to 0.54 V vs. RHE. 

A summary of some results for various anode electrocatalyst materials is given in Table III in relation to the d-electron configuration (357). It must be remarked that the linear dependence of several quantities on d-band vacancy, found by Arikado et al. in their CI2 evolution studies (357,434), will arise, in part, from the (probably doubtful) assumption that the d-band vacancy for alloys varies in linear proportion to the values for the elemental components, weighted by the composition ratio. A further factor is that the surface composition of alloys is rarely identical with that of the bulk (cf. Refs. 43 , 432). 

Rao et al. (433) also reported that a linear relationship exists between d-band vacancy and the coverage by adsorbed oxygen in an 02-saturated solution. Burke et al. (413) and Llopis and Vazquez (435) showed that the greater the d-band vacancy in the metal, the lower is the potential at which anodic film initiation is observed. The work of Arikado et al. (357, 407)... 

---