Pd films

Although supported Pd catalysts have been the most extensively studied for butadiene hydrogenation, a number of other catalysts have also been the object of research studies. Some examples are Pd film catalysts, molybdenum sulfide, metal catalysts containing Fe, Co, Ni, Ru, Rh, Os, Ir, Pt, Cu, MgO, HCo(CN) on supports, and LaCoC Perovskite. There are many others (79—85). Studies on the weU-characteri2ed Mo(II) monomer and Mo(II) dimer on siUca carrier catalysts have shown wide variations not only in catalyst performance, but also of activation energies (86). 

Rates of oxygen uptake in the dynamometer-aged catalysts are consistent with published oxidation kinetics of l- m-thick Pd films. 

The design of the Pd-membrane reactor was based on the chip design of reactor [R 10]. The membrane is a composite of three layers, silicon nitride, silicon oxide and palladium. The first two layers are perforated and function as structural support for the latter. They serve also for electrical insulation of the Pd film from the integrated temperature-sensing and heater element. The latter is needed to set the temperature as one parameter that determines the hydrogen flow. 

Baldauf M, Kolb DM. 1996. Formic acid oxidation on ultrathin Pd films on Au(hkl) and Ft hkl) electrodes. J Phys Chem 100 11375-11381. 

In our recent work, we have used Pd deposited on t hkl) in order to characterize thin metal films [Arenz et al., 2002] and to test their catalytic activity (Fig. 8.16). We have employed both methodologies thermal evaporation in UHV and electrochemical deposition. For the Pd/Pt(l 11) system, in situ SXS measurements have been used to show that after the formation of 1 ML of pseudomorphic Pd film, three-dimensional pseudomorphic crystalline islands of pure Pd begin to aggregate. 

Arenz M, Stamenkovic V, Wandelt K, Ross PN, Markovic NM. 2002. CO adsorption and kinetics on well-characterized Pd films on Pt( 111) in alkaline solutions. Surf Sci 506 287-296. 

In this context, it is interesting to note that it has been claimed (56) that single-crystal Fe-Ni alloy films can be prepared by deposition on heated rock salt substrates in vacua of 10-3-10-4 Torr. Other workers (57) have found that the use of UHV permits single-crystal films of Fe-Ni to be formed (at deposition rates of 14 A/min) without the annealing necessary after deposition at 1(U5 Torr. Single-crystal Au-Pd films have also been prepared (58) and after quenching from 500°C gave an electron dif-... 

Fig. 20. Apparent activation energy, Ek, for formic acid decomposition over Pd-Au alloy films deposited and annealed at 450°C (O) pure Pd film deposited at — 196°C and annealed at 200°C ( ) (69).

Figure 10.6 (A) Desorption temperature of a 1 ML Pd film from different metal substrates. (B) Change in Pd(3d5/2) binding energy as a function of the metal substrate. The changes are referenced to the Pd(3d5/2) binding energy of Pd(100). (Reprinted from Rodriguez, J.A. and Goodman, D.W., Science, 257, 897-903, 1992. Copyright 1992. With permission from AAAS.)...

Kikuchi et al. (1989) Under the best operating conditions, the rate of hydrogen permeation through the Pd film was evaluated to be of the same order as the rate of production of hydrogen due to the reaction. 

Shore and coworkers [64] used a capillary reactor with a Pd thin film and microwave-assisted continuous-flow conditions for Suzttki-Miyara and Heck coupling reactions. The Pd film was prepared by passing Pd(OAc)2 solution into the 1150 pm eapillary at 150°C resulting in a highly porous catalyst composed of nanometer-size grains. 

ATR-IR spectra of PDI, CN-QH4-NC, adsorbed on a Pd film deposited on one face of a ZnSe crystal exhibit three v(N=C) peaks, at 2170, 2120 and 1960 cm" [26]. The 2120 cm" band is shifted only 8 cm" as compared with that for the free molecule and is assigned to the uncoordinated isocyanide group. The 2170cm" band is close to that observed for PDI adsorbed on Au film and was assigned an T) adsorphon mode. The 1960 cm" band is characteristic of an adsorption mode in which the N=C bond is weakened. The authors indicate two possible modes a bent T mode (B in Figure 13.1) or a mode (C in Figure 13.1) in which... 

For ethyne adsorbed on an evaporated Pd film, Ito et al. (74) reported infrared spectra similar to those described earlier for species formed from ethyne on Ni, with the modified wavenumbers of 3350/3220, 1880/1850, and 835/755 cm 1 for the pairs of perturbed ethyne species and 2870 and 2750 cm-1 for the saturated species. On wire-exploded Pd, Nash and De Sieno (73) reported a spectrum similar to that observed by Dunken et al. on Pd/Si02. 

The frequency of the linear CO is higher than that found for Pd films (j co = 2085 cm-1) (27) or for supported palladium (Pd/Si02), vco = 2060 cm-1) (28). The increase in frequency reported in this study is the result of the decrease of the backdonation from the d metal orbitals to the 7T orbital of CO. Y zeolites have very strong Lewis acid sites these sites should be able to decrease the electronic density of the palladium atoms bonded to CO. The decrease of the intensity of the band at 2100 cm-1 by increasing the hydrogen reduction temperature could be explained by the formation of agglomerates of palladium still in interaction with a Lewis acid site. 

Figure 6. A uger spectra of Pd film evaporated onto TiOt before and after annealing ( ( left 12 monolayers Pd deposited on TiOs (110) at room temperature, E, = 3.1 KeV (right) after 5 min anneal at 700°C)...

Related to the plasmon resonance physics is the micromirror optical sensor for hydrogen (Butler, 1991). Like gold and silver, palladium is a free-electron gas metal in which charge groupings such as phonons or plasmons are likely to occur. As we have seen already, palladium has a natural selectivity due to its sorption of monoatomic hydrogen. In that sensor, the reflectivity of the thin Pd film mirror mounted at the end of cladded optical fiber (Fig. 9.19) is modulated by absorption of hydrogen. 

A Pd film decoupler has also been constructed for amperometric detection of catecholamines. The Pd film has been thermally evaporated onto a plastic chip (without the use of the Cr or Ti adhesion underlayers). Owing to the fast diffusion of H2 on a Pd surface, gas bubbles will not form. Pd is able to absorb H2 produced at the cathode up to a Pd/H ratio of 0.6. This reduces one of the interferences to the EC signal, leading to an improvement of LOD to 0.29 pM dopamine [205,375]. With an optimal decoupler size of 500 pm, up to 6 h of operation was achieved with an electric field of 600 V/cm [375]. 

TEM micrographs of the structure of Ni and Pd films are shown in Figure 15.6. It can be seen that the arrangement of particles of all the three metals on the substrate is strongly disordered. At the same time, a close comparison of Figures. 15.5 and 15.6 shows that the structures formed by, e.g., Cu and Ni nanoparticles are somewhat different. For example, Cu nanoparticles combine into isolated islands composed of 3-6 particles, while Ni structures exhibit a tendency toward ordering of nanoparticles into chains containing up to 15-20 particles. 

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