Pd overlayers

Figure 3.16 Volcano plot for the hydrogen evolution reaction (HER) for various pure metals and metal overlayers. Values are calculated at 1 barof H2 (298K) and at a surface hydrogen coverage of either 0.25 or 0.33 ML. The two curved lines correspond to the model (3.24), (3.25) transfer coefficients (not included in the indicated equations) of 0.5 and 1.0, respectively, have also been added to the model predictions in the figure. The current values for specific metals are taken from experimental data on polycrystalline pure metals, single-crystal pure metals, and single-crystal Pd overlayers on various substrates. Adapted from [Greeley et al., 2006a] see this reference for more details.

Figure 4.12. Electrochemically measured changes in the hydrogen adsorption energy for Pd overlayers on a number of metals shown as a function of the calculated shift of the d-band center. Adapted from Ref. [38].

Again the d band centers are found to describe changes in adsorption energies quite well [34-37]. This is illustrated in Figure 4.12, through the electrochemically determined variations in the hydrogen adsorption energy, for different Pd overlayers as a function of the calculated d band shifts [38]. 

In this paper we present results related to the atomic structure and catalytic properties of Pd overlayers on various substrates. A reaction has been chosen to test the catalytic properties of these systems, it is the 1,3-butadiene hydrogenation, a reaction for which Pd is known to be the best catalyst. In the following, after a short description of the experimental approach, the 1,3-butadiene hydrogenation reaction and the specific properties of Pd for this reaction will be presented. Then the reactivity of several Pd overlayers obtained either by surface segregation in Pd-based alloys or by atomic beam deposition on a metal will be investigated and discussed in terms of structure, composition related to surface segregation and surface stress. The influence of the surface orientation of the substrate will be discussed. 

A pseudomorphic Pd overlayer will have to sustain a large stress in compression on Ni and Cu, and in tension on Ag and Au. On the contrary one expects a quasi no stressed Pd overlayer on Pt. While less important, a similar effect will appear on the surface of alloys where Pd segregates in the top layer. This would be more pronounced on alloys of low Pd concentration exhibiting a very large surface segregation of Pd. 

For this system, Terada et al. [53] have observed by STM, at RT and near 1 ML, hexagonal moire superstructures which would be characteristic of an hexagonal arrangement of Pd adatoms aligned with a slight rotation with respect to the substrate axis and for which Pd-Pd atomic distances tend towards the known value for pure bulk palladium. This result differs from ours, which could indicate a limit of stability of the stressed Pd overlayer. 

Fig. 13. Compared catalytic activities for 1,-3-butadiene hydrogenation of Ni(llO), Pd(llO) and Pd overlayers on Ni(llO). Pd overlayers have been annealed at 500K. Reaction parameters RT, 10 torrs hydrogen and Ph2 / Pdiene = 10.

Badderley CJ, Ormerod RM, Stephenson AW, Lambert RM (1995) Surface-structure and reactivity in the cyclization of acetylene to benzene with Pd overlayers and Pd/Au surface alloys on Au(lll). J Phys Chem 99 5146... 

An example of producing a surface alloy from two miscible metals such as Au and Pd is shown in Fig. 3.3. In these experiments four monolayers (ML) of Pd were vapor deposited on a Au(lll) substrate. The LEIS results revealed that at hquid nitrogen temperature (Fig. 3.3b) the Pd overlayer is rather stable, but at room temperature a surface alloy forms spontaneously and Au can be resolved easily in LEIS spectra (Fig. 3.3c). The enrichment of the Au on the surface is a consequence of surface segregation, where the surface is enriched with the larger atom [50-53], similar to the PtjM alloys discussed earlier. 

The morphology of Pd overlayers in electrochemical environments has been recently examined by in situ STM for Au(M/)-Pd films [62, 63] and by a combination of FTIR and SXS for Pt(lll)-Pd films [55, 64, 65], For the former system, it is surprising that the in situ study of the morphology of Pd films on Auihkl) in an electrochemical environment was possible, because UFIV experiments revealed that at room temperature the arrangement/composition of surface atoms changes continuously due to the surface segregation of Au (Fig. 3.3). In contrast to published results, the preliminary in situ SXS studies by Lucas confirmed UHV observations (Fig. 3.3) that the Au(M/)-Pd system never reached structural/compositional equilibrium. 

A limitation to the application of SERS to electrochemical systems is the specificity of the enhancement effect to Ag, Au, and Cu. However, since the electromagnetic part of the enhancement is maintained over distances of several nanometers, it has been possible to coat a SERS active metal with a thin layer of another metal that is exposed to the adsorbing molecules and still obtain enhanced signals (69). For example, by constant-current deposition, it is possible to deposit pinhole-free layers of Pd on Au with a thickness corresponding to 3.5 monolayers and to study the adsorption of species on the Pd by SERS. The spectra of adsorbed benzene on such an electrode are shown in Figure 17.2.12 (84). The symmetric ring-breathing mode of benzene adsorbed on Pd appears at 950 cm shifted considerably from that found either for liquid benzene (992 cm ) or for benzene adsorbed on Au (975 cm ). Deuterated benzene (C D ) behaves similarly and shows the expected shift in the band to lower frequency. The attenuation of the enhancement effect with thickness of the Pd overlayer was reported to be only a factor of 4-5 for thicknesses of 3-30 monolayers. 

Fig. 7.30 Visible and near-infrared reflection and transmission spectra ofSOnm Mg-Ni, 60 nm Mg-Mn, 40 nm Mg-Fe and 40 nm Mg-Co films (Mg TM 6 1) with 7nm thick Pd overlayers on glass substrates in the...

Arenz et al. found that Pd/Pt(lll) has a higher ORR activity than Pt(lll) in alkaline solution, which is different from that in acid solution [66]. In alkaline solution, where only OH anions are present, the inhibition effect from the strong anion adsorption is much smaller than in acidic solution, resulting in a high ORR activity. Pd overlayers on Au(lOO) and Au(lll) also showed significant activity improvement with respect to an unmodified Au substrate in the alkaline solutions [67-69]. The very high activity of Pd in alkaline solutions is surprising and deserves further attention. 

Chapter 1 discusses the current status of electrocatalysts development for methanol and ethanol oxidation. Chapter 2 presents a systematic study of electrocatalysis of methanol oxidation on pure and Pt or Pd overlayer-modified tungsten carbide, which has similar catalytic behavior to Pt. Chapters 3 and 4 outline the understanding of formic acid oxidation mechanisms on Pt and non-Pt catalysts and recent development of advanced electrocatalysts for this reaction. The faster kinetics of the alcohol oxidation reaction in alkaline compared to acidic medium opens up the possibility of using less expensive metal catalysts. Chapters 5 and 6 discuss the applications of Pt and non-Pt-based catalysts for direct alcohol alkaline fuel cells. 

Eig. 40. Development of electrode potential E and in situ measured optical transmittance as a function of time for 55 nm Sm film (working electrode) covered Pd overlayer of thickness (i) 5 nm, (ii) 8 nm, and (iii) 11 nm, during galvanostatic hydrogen loading and unloading at current density 3.057 mA/cm in 1 M KOH (Kumar and... 

Fig. 42. Transmittance (i) and Reflectance (ii) spectra for 55 nm SmRx Alms covered with (a) 5.0 nm, (b) 8.0 nm, (c) 11.0 nm, and (d) 15.0 nm palladium over layers. The transmittance and reflectance spectra of a typical metallic samarium film covered with Pd overlayer of thickness 5.0 nm is shown in (e) (Kumar and Malhotra, 2004b).

Fig. 166. (i) Schematic diagram of the sample used. Y covered vanadium strips (1x10 mm ) of thickness 25-125 nm are deposited onto an a-Si02 substrate (15 x 10 x 0.53 mm ). The strips are partially covered with Pd overlayer. In the actual sample there are 11 strips, (ii) Optical image of sample loaded in hydrogen atmosphere (1 mbar, 473 K)... 

Fig. 169. Response of the sensing element in the presence of (i) 194 ppm and (ii) 10000 ppm hydrogen in air. A -absorption, B - desorption, (hi) Resistance versus time curve, for a 55 nm Sm him capped with Pd overlayer, on exposure to 10 000 ppm of CO2, H2S, argon plus CH4 and argon plus C2H5OH (Kumar et al., 2002).

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