NO from Pt

Desorbed NO molecules due to laser irradiation cannot be detected by the REMPI method from this surface, except for a very small amount of the initial desorption. From RAIRS observations, on the other hand, the N-O stretching peak observed at 1717 cm-1 for saturated NO on clean Pt(l 1 1) is red-shifted by 3 cm-1, when NO is saturated on the surface after laser irradiation (shown in Fig. 13c), while the peak for NO coadsorbed with O atoms reveals a blue shift of 5 cm-1. These results show that the fee hollow species is desorbed as an O atom and the N atom remains as an adsorbate on the surface [57]. At X = 248 nm a similar result to that at X = 193 nm is observed by RAIRS, but the desorption cross section is much smaller, and at X = 354 nm the intensity reduction is not observed. Thus, the threshold energy for the O atom desorption from the fee hollow species is 5.0 eY [58]. 

Desorption from on-top species Buntin et al. [6] carried out observations on NO desorption from NO-saturated Pt(l 1 1) surfaces in detail, using X = 1064 (hoy = 1.17), 352 (2.33), and 355 nm (3.49 eY) at surface temperatures of 117 and 220 K. State-selective detection using the LIF method combined with the TOF measurement were used. In the present paragraph, the results observed at 220 K are described, because of the desorption from on-top species. Desorption occurs by the one-photon process, for which the desorption cross section is smaller at X = 1064 nm than those at X = 532 and 355 nm. Thus, the threshold energy for the NO desorption is regarded as 1.2 eV. 

Fukutani et al. [8] also observed NO desorption from on-top species at X = 193 nm. The results are very similar to those obtained by Buntin et al. at X = 532 and 355 nm. The decay of the desorption yield on this surface at X = 532 nm gives a desorption cross section of 1 x 10-22 cm2 [6]. A fit to the TOF spectrum by the non-Boltzmann form gives Tt = 910 K. Rotational energy distributions of 

NO (v = 0, 2 = 1/2 and 3/2) are measured with a probe laser beam-sample distance of 1.65 mm at a delay time of 3.5 pis (velocity of 0.47 km/s), and shown in Fig. 14, which reveals a non-Boltzmann distribution. Furthermore, the two spin-orbit states exhibit inversion population, the 2 = 3/2 state being more populated. Since the RAIRS spectmm shows that the peak intensity at 1700 cm-1 decreases with laser irradiation, it is concluded that desorption of the on-top species occurs. 

Desorption from hep hollow species The desorption from hep hollow species is observed from the surface shown in Fig. 9c. The decay curves of the desorption yield as a function of photon numbers at X = 193 nm observed by Fukutani et al. [8] consists of two components with cross sections of 1 x 10 18 and 1 x 10-20 cm2. The former may be obtained from NO adsorbed on minor defect sites. The REMPI spectra were observed for the slow component and the latter is assigned to be the desorption of the hep hollow species. 

---

More informative are the stochastic trajectory simulations run by Muhl-hausen et al. (M WT), on empirical interaction potential surfaces for scattering and desorption Although the major thrust was to understand the direct beam scattering results of NO/Ag(l 11), extension of these calculations allows for comparison to the desorption of NO from Pt(lll) Important insights derived from the NO/Ag(lll) calculations were ... 

Experiments were conducted in our laboratory to evaluate many of the dynamical expectations for rapid laser heating of metals. One of the aims of this work was to identify those population distributions which were characteristic of thermally activated desorption processes as opposed to desorption processes which were driven by nontbennal energy sources. Visible and near-infrared laser pulses of nominally 10 ns duration were used to heat the substrate in a nonspecific fashion. Initial experiments were performed by Burgess etal. for the laser-induced desorption of NO from Pt(foil). Operating with a chamber base pressure 2 x 10 torr and with the sample at 200 K, initial irradiation of a freshly cleaned and dosed sample resulted in a short time transient (i.e. heightened desorption yield) followed by nearly steady state LID signals. The desorption yields slowly decreased with time due to depletion of the adsorbate layer at the rate of ca. 10 monolayer... 

Figure 3.21. Rotational temperature 7 rot (defined as T in this chapter) in trapping-desorption scattering of NO from Pt(l 11) and covered Pt(lll) as a function of the surface temperature Ts. Open points are for Et = 80 meV and solid points are for Et = 220 meV. The straight line is for 7rot = Ts. From Ref. [206].

Figure 17 Angular distributions for direct scattering of preferentially oriented NO from Pt(l 11), presented in a polar plot. E = 0.18 eV, Ts = 573 K, i = 50°. The hnes through the angular distributions are drawn to guide the eye. The arrows indicate the angle of incidence and the specular angle. In case of N-end collisions less molecules are directly scattered. Molecules with are directly scattered after an N-end collision come off closer to the surface normal than molecules with an O-end collision. From Kuipers et al. [94].

Thus, in this review we present the desorption phenomena focused on the rotational and translational motions of desorbed molecules. That is, we describe the DIET process stimulated by ultraviolet (UV) and visible nanosecond pulsed lasers for adsorbed diatomic molecules of NO and CO from surfaces. Non-thermal laser-induced desorption of NO and CO from metal surfaces occurs via two schemes of DIET and DIMET (desorption induced by multiple electronic transitions). DIET is induced by nanosecond-pulsed lasers and has been observed in the following systems NO from Pt(0 0 1) [4, 5],... 

Figure 7 Coverage dependence of the TDS spectrum of NO from Pt(l 1 l)-NO adsorbed at 70 K [41]. A similar TDS spectrum from NO-saturated surface is shown in Fig. 5a of Ref. [6],...

The potential of NSR catalysts in the removal of NO from mobile sources has motivated in the last few years extensive investigations from both the academic and the industrial world, and several studies have been published in the open literature dealing with fundamental and practical aspects of LNT catalysts [4-53], However, the mechanisms that operate the NO adsorption and the respective subsequent reduction have not been completely clarified so far. It has been shown that under oxidizing conditions, NO are stored on the surface of a Ba-containing catalyst in various forms (surface nitrites/nitrates), whose precise nature is, however, still a matter of debate [9-29], Even less clear are the mechanisms, which are responsible for the reduction of stored NO when the A/F ratio is set to rich and the stored NO species are reduced over Pt to N2, ammonia, N20 or back to NO [11],... 

The common idea on the mechanisms governing the reduction of NO adsorbed species over LNT catalysts is that the regeneration process includes at first the release of NO, from the catalyst surface (i.e. from the alkali- or alkali-earth metal compound), followed by the reduction of the released NO to N2 or other products [11]. The reduction of the released NO in a rich environment is thought to occur according to the TWC chemistry and mechanisms in particular, it was suggested that NO is decomposed on reduced Pt sites [38], or that a direct reaction occurs between released NO species and the HC reductant molecules on the precious metal sites [39],... 

Zirconia cells similar to the ones employed in the present study, have been used i) by Mason et al (18) to electrochemically remove oxygen from Pt and Au catalysts used for NO decomposition. It was shown that electrochemical oxygen pumping causes a dramatic increase in the rate of NO decomposition (18,19), ii) by Farr and Vayenas to electrochemically oxidize ammonia and cogenerate NO and electrical energy (20,21), iii) by Vayenas et al (11,12,22,23) to study the mechanism of several metal catalyzed oxidations under open circuit (potentiometric) conditions. 

Fig. 5. Rotational temperatures ofNO desorbing from Pt(l 11). The data are representative of data published for (x) neat thermal desorption , ( +) thermal desorption in the presence of coadsorbed C0 ° (solid squares) and (solid triangles) trapping/desorption in molecular beam scattering, (open triangle) reaction limited desorption from NO-NHj complexes, (open circle) and (open square) NHj oxidation reactions. The solid line is for full accommodation. The dashed curve represents results for translational energy measurements in direct inelastic scattering ...

Alignment results reported for NO trapping/desorption from Pt(l 11). At high was positive implying a propensity for helicopter-type desorbing molecules. (Adapted from Re/ 29.)... 

Fig. 8. Laser-induced fluorescence detected times-of-flight for NO desorbed from Pt(foil) using 532 nm laser heating. The three traces were obtained while probing (top) NO (F, J—19.5 ,m° 662cm- ), (middle) NO (F J=12.5 , = 281cm- ) and (bottom) NO (F, J = 3.5, i , = 25cm" ). (Adapted from Rrf. 46.)...

The nanostructured thin-film electrode was first developed at 3M Company by Debe et al. [40] and Debe [41], who prepared thin films of oriented crystalline organic whiskers on which Ft had been deposited. The film was then transferred to the membrane surface using a decal method, and a nanostructured thin-film catalyst-coated membrane was formed as shown in Figure 2.10. Interestingly, both the nanostructured thin-film (NSTF) catalyst and the CL are nonconventional. The latter contains no carbon or additional ionomer and is 20-30 times thinner than the conventional dispersed Pt/ carbon-based CL. In addition, the CL was more durable than conventional CCMs made from Pt/C and Nation ionomer [40]. 

Figure 20. Structural parameters as a function of time extracted by fitting the data shown in Figure 20. (A) Data collected during the oxidation of the Pt/C electrode and (B) during the reduction long dashes, first shell O coordination number (no. of O atoms) short dashes, first shell Pt coordination number (no. of Pt atoms) solid line, absorption peak intensity (effectively white line intensity).(Reproduced with permission from ref 43. Copyright 1995 Elsevier Sequoia S.A., Lausanne.)...

Fig. 7.7 Percentage of photogenerated holes that contribute to anodic decomposition versus O2 evolution from naked (no catalyst), Pt-coated, and polymer-Pt coated n-CdS photoanode in 0.5 M Na2S04 solution (pH = 8.6) [14].

---