Polycrystalline Pd

Hydrogen in Metals, ed. G. Alefeld and J. Volkl, Topics in Applied Physics , Vols. 28 and 29, Springer Verlag, Berlin, 1978. 

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Figure 2. Non-steady state variation of Ot pressure with time during flashing of a stable polycrystalline Pd substrate. Initial OJCO = 1.5. The origin of the ordinate is defined as the Ot pressure at 300 K. Key O, observed 02 V, calculated on the...

Figure 53 shows relative rates of C02 formation under steady-state conditions that were recorded with various single-crystal surfaces of Pd as well as with a polycrystalline Pd wire (173). It must be noted that with these experiments no determination of the effective surface areas was performed so that no absolute turnover numbers per cm2 are obtained. Instead, the reaction rates were normalized to their respective maximum values. As can be seen from Fig. 53, all data points are close to a common line which indicates that, in fact, with this reaction the activity is influenced very little by the surface structure. As has been outlined in Section II, the adsorption of CO exhibits essentially quite similar behavior on single-crystal planes with varying orientation. Since the adsorption-desorption equilibrium of CO forms an important step in the overall kinetics of steady-state C02 formation, this effect forms at least a qualitative basis on which the structural insensitivity may be made plausible. 

It is interesting also to compare the results of the present experiment, which shows directly that a competitive mechanism occurs in the co-adsorption of NO and CO, with previous studies on several surfaces of the platinum group metals. On Pt(lll) and Pt(110), Lambert and Comrie (65) have inferred from thermal desorption data that gaseous CO displaces molecular NO from the surface and causes also a conversion between two thermal desorption states of molecular NO. Similarly, Campbell and White (55) report that adsorbed CO inhibits the oxidation of CO by NO at low temperature on polycrystalline Rh. They attribute this to the occupation of sites by CO which are required for NO adsorption and dissociation. Conrad et al. (66) have used UV-photoelectron spectroscopy to observe directly the displacement of molecular NO by gaseous CO from Pd(110) and polycrystalline Pd surfaces. Thus, it appears that adsorption of molecular CO and NO is competitive on these... 

The related volatile hexafluoroacetylacetonate, Pd(hfa cac)2, and several mixed allyl diketonates, including the liquid [Pd(C3H5)(Bu C(0)CH(CO)C3F7)], are used for chemical vapor deposition on surfaces (CVD) and subsequent reduction to Pd. The mechanism of formation of a Pd film on a copper surface by thermal decomposition of deposited Pd(hfacac)2 has been studied in detail. Reduction with H2 can also be used and it produces polycrystalline Pd. 

Fig. 36. Spectra for polycrystalline Pd electrode in 0.1 M NaC104 +25mM NaCN. The solid line is a combination of SNIF-TIRS and IRRAS spectra. Two SNIFTIRS spectra at -0.9 V (reference potential) and -1-0.7 V vs. Ag/AgCl are ratioed to obtain the spectrum shown. The dotted spectrum was obtained by a SNIFTIRS with s-polar-ized radiation. (After [124]). Reprinted by permission of Journal of Chemical Physics AIR...

While Pd is less expensive than Pt, the electrocatalytic activity of bulk polycrystalline Pd for ORR is at least five times lower than that of Pt, which prevents it from being used directly in fuel cells. Great efforts have been dedicated to improve the activity of Pd by surface modification and alloying. This chapter attempts to summarize the recent progress of electrocatalysts containing Pd for ORR. The development of Pd electrocatalysts for electrooxidation of hydrogen and small organic molecules is not discussed in this chapter but has been adequately covered in Chaps. 5 and 6 and recent reviews [5-7]. 

The metals of the Pt-group constitute a particular case. Their catalytic activity has long frustrated the determination of the pzc because of interference from adsorbed hydrogen and oxygen. Nevertheless, estimated values of pzc for polycrystalline Pt are included in all compilations in Table 26. However, after the publication by Frumkin and Petrii14 of a summary of pzc values forPt, Rh, Ir, and Pd, no further progress was made for about 20 years until recently UHV techniques of surface preparation have enabled pzc determinations using methods other than the traditional... 

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.

With such a definition, it was found that vfc at 445 K did not change within experimental error as the average Pd particle size, determined by transmission electron microscopy (TEM), was varied between 1.5 and 8.0 nm (Figure 1). Besides, this value of v was also the same as that reported for the ill face of a single crystal of Pd (2), the latter value being itself very much the same on other planes of Pd or on a polycrystalline wire (3). 

Underpotential deposition of Zn on R was seen as adsorption of Zvi ions in 1970, since is as high as 1V, a value that well exceeds the one expected from previous results on various metal combinations of underpotential deposition systems. Recently, Zn underpotential deposition on R was reported in acidic solutions, and then in alkaline solutions. Zn underpotential deposition on Pd was also found in acidic and alkaline solutions. Then cyclic voltammograms of underpotential deposition on three low-index, single-crystal Rs were observed in sulfuric and perchloric acid solutions, as shown in Figs. 27 and 28. " In comparing Figs. 27 and 28, Zn underpotential deposition on R seans to take place only on Pt(llO), but not on Pt(lll) and Pt(lOO). Therefore it can be said that a cyclic voltammogram on polycrystalline R was observed on a R(110)-like surface. 

The oxidation of CO by Oj over group VIII metal catalysts has been the subject of a large body of ultrahigh vacuum surface science and high pressure catalysis work due to its importance in pollution control. Currently, the removal of CO as CO2 from automobile exhaust is accomplished by catalytic converters which employ a supported Pt, Pd, and Rh catalyst. The importance of CO oxidation has led to numerous recent studies of the kinetics of this reaction on supported metal catalysts and transient kinetic studies on polycrystalline foils , which have sought to identify and quantify the parameters of the elementary mechanistic steps in CO oxidation. 

Detailed and fundamental studies of the mechanistic aspects of the electrochemical reduction and oxidation of nitric oxide were carried out by the Eindhoven group [82, 83] on a series of metals (Pt, Pd, Rh, Ru, Ir, and Au) both in the case of polycrystalline and well-defined single-crystal surfaces. It was found that the reduction process at all metals studied shows a high selectivity with respect to N2O production at high potentials while at low potentials the formation of NH3 is the dominant... 

The kinetic model developed from the detailed experimental data obtained with Pd(l 11) holds similarly for other single crystal planes as well as for the results from polycrystalline material. Without reviewing all the available measurements in detail, only two sets of data from the first study of this type with Pdf 110) will be mentioned (2). Figure 38 shows the variation of the steady-state rate of C02 formation with pco at constant oxygen pressure (Po2 = 1 x 10-7 Torr) and temperature (T= 520 K). For pco p0l, r increases linearly with the CO pressure while the simultaneously monitored LEED pattern indicates the presence of appreciable amounts of oxygen on the surface. Under these conditions case 2 is valid, whereby (15) correctly describes the observed kinetics. For pco > p0l the reaction rate becomes... 

FIGURE 10 Temperature dependence of the magnetic susceptibility x and the electrical resistance (inset) of a polycrystalline Y-Pd-B-C sample, indicating two superconducting phases with transition temperatures Tc of about 10 K and 23 K (Hossain et al., 1994). 

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