Pd atom adsorption energy

Angle formed by plane of 3 atoms and the bond indicated by dash. Displacement of co-ordinated M atom from surface plane. Both atoms coordinate to a single Pd atom. Adsorption energy relative to gas phase ... 

Metals frequently used as catalysts are Fe, Ru, Pt, Pd, Ni, Ag, Cu, W, Mn, and Cr and some of their alloys and intermetallic compounds, such as Pt-Ir, Pt-Re, and Pt-Sn [5], These metals are applied as catalysts because of their ability to chemisorb atoms, given an important function of these metals is to atomize molecules, such as H2, 02, N2, and CO, and supply the produced atoms to other reactants and reaction intermediates [3], The heat of chemisorption in transition metals increases from right to left in the periodic table. Consequently, since the catalytic activity of metallic catalysts is connected with their ability to chemisorb atoms, the catalytic activity should increase from right to left [4], A Balandin volcano plot (see Figure 2.7) [3] indicates apeak of maximum catalytic activity for metals located in the middle of the periodic table. This effect occurs because of the action of two competing effects. On the one hand, the increase of the catalytic activity with the heat of chemisorption, and on the other the increase of the time of residence of a molecule on the surface because of the increase of the adsorption energy, decrease the catalytic activity since the desorption of these molecules is necessary to liberate the active sites and continue the catalytic process. As a result of the action of both effects, the catalytic activity has a peak (see Figure 2.7). 

Recent experiments determining the so-called chemicurrent [111] have provided some information on the importance of electron-hole pair excitation in adsorption processes. Using thin films deposited on n-type Si(l 1 1) as a Schottky diode device, the nonadiabatically generated electron-hole pairs upon both atomic and molecular chemisorption create the chemicurrent which can be measured [111, 112]. It has been estimated that for example in the NO adsorption on Ag one quarter of the adsorption energy is dissipated to electron-hole pairs. Adsorption-induced electron-hole pair creation has also been found for other metal substrates, such as Au, Pt, Pd, Cu, Ni and Fe, and even for semiconductors such as GaAs and Ge [112, 113]. 

To understand better the stability of the PdO species formed on the ideal and defective 111 surfaces of c-Zr02, we investigated the adsorption energies of the PdO entity in the different cases. From AEjds presented in Table 9 it is seen that the PdO entity is less stable than the Pd atom adsorbed in the identical adsorption sites ... 

In order to characterize the MgO sites where the Pd atoms are stabilized after deposition by soft-landing techniques, we used CO as a probe molecule [61]. The adsorption energy, Eb, of CO has been computed and compared with results form thermal desorption spectroscopy (TDS). The vibrational modes, (o, of the adsorbed CO molecules have been determined and compared with Fourier transform infrared (FTTR) spectra. From this comparison one can propose a more realistic hypothesis on the MgO defect sites where the Pd atoms are adsorbed. 

TABLE 8. Adsorption energies (kJ.mol ), optimized geometries, C-C and C-Pd Mulliken overlap populations (M.O.P.) and total adsorbate charges, Q, for C2H4 adsorbed on the short-bridge and top sites of the Pd(lOO) surface. No theoretical results of the adsorption of C2H4 on palladium surfaces have been found in the literature. ° The two values refer to the M.O.P. between each carbon atom and its nearest... 

In the previous work, the authors found that CO bonds more weakly to Pd monolayers supported on Ru(OOOl) than on Pd(lll). Remarkably, incorporating Cu atoms into the supported Pd monolayer was found to increase the CO adsorption energy on the Pd atoms by up to 12 kcal/mol, depending on the Cu concentration. In contrast, CO bonds slightly more strongly to Cu/Ru(0001), and incorporation of Pd into the Cu monolayer tends to decrease the CO adsorption energy on the Cu atoms by up to 6 kcal/mol. These effects are attributed to an increase in the back-donation to CO on these surfaces. 

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