On polycrystalline Pt surface

Normal SNIFTIRS or FTIR Potential Step experiments also show a steady increase in coverage of COads with increasing potential and time (Fig. 18.7) [48-51]. Simple potential step experiments on polycrystalline Pt surfaces show clear peaks at 2050cm-1 and 1836cm-1, which can be... 

Peng et al. combined electrochemical surface-enhanced infrared spectroscopy (EC-SEIRAS) and DFT calculations to probe the Sb adatom enhancement mechanism on polycrystalline Pt surfaces [27]. The forward cyclic voltammogram in 0.1 M formic acid and 0.5 M H2SO4 showed a 2.7 x decrease in COads at potentials below 0.2 V versus RHE for a 0.6 mraiolayer (ML) Sb, with a tenfold current increase at 0.5 V in the forward scan. They concluded at coverages >0.25 ML that the [Sb] [Pt] dipole interacticHi enhances CH-down adsorption. This is consistent with Leiva et al. s work presented above. They additionally attributed this coverage dependence to a decrease in the Pt-COads bond strength with increased Sb coverage. 

Figure 6 shows that all of the chemisorbed oxygen is reactive. Here, a polycrystalline Pt surface at 370K is exposed to various amounts of O2 as indicated on the abscissa and, after termination of the O2 input, titrated with CO. As indicated by the four sets of dashed lines in the figure, all of the oxygen Auger signal vanishes. 

It is interesting to note that at this high temperature the frequency of the Pt-CO band did not shift appreciably as the CO coverage varied, in contrast to observations made on a (111)-oriented platinum ribbon (6) or on a polycrystalline Pt surface (34) at or below room temperature. 

Silver deposition on polycrystalline Pt electrodes at potentials positive to the equilibrium potential gave 2.5 atomic layers. Two binding types of Ag layers were found by anodic stripping the first Ag layer deposited on Pt, which seems to form an alloy of Ag-Pt, on which the second Ag deposition takes place in the Ag underpotential deposition region. STM images from the underpotential to the overpotential deposition region were observed for Cu underpotential deposition on Au(l 11) in sulfuric acid solution, where Cu underpotential deposition does not affect overpotential deposition, although the latter always takes place on the surface with Cu underpotential deposition and a metal. ... 

Models based on chemisorption and kinetic parameters determined in surface science studies have been successful at predicting most of the observed high pressure behavior. Recently Oh et al. have modeled CO oxidation by O2 or NO on Rh using mathematical models which correctly predict the absolute rates, activation energy, and partial pressure dependence. Similarly, studies by Schmidt and coworkers on CO + 62 on Rh(l 11) and CO + NO on polycrystalline Pt have demonstrated the applicability of steady-state measurements in UHV and relatively high (1 torr) pressures in determining reaction mechanisms and kinetic parameters. 

The first observations on the development of periodic modulations were made on Pt surfaces annealed in vacuum in the temperature range 920 to 1300C[9J. The 1-dimensional grating structures were made by mechanically ruling portions of gratings on polycrystalline Pt specimens. Annealing caused extensive... 

Fig. 41. Variation of the oxygen coverage with the CO pressure for the formation of C02 on a polycrystalline Pt surface under steady-state conditions (132).

Angular resolved measurements of the COz formed in the reaction have yielded rather surprising results. Palmer and Smith (196) have reported that the C02 formed on a Pt(l 11) surface is emitted in a peaked distribution that can be described by a cos6 a function where a is the emission direction measured from the surface normal. Becker et al. (190) have reported similar results for a polycrystalline Pt surface. They obtain angular distributions of... 

Of further interest is the apparent effect of surface geometry on the initial dissociation probability, S0, on the Pt surface. For Pt(l 00)-hex-R0.7°, S0 values on the order of 10-3 were observed in the investigations [83, 84], whereas experimental S0 values for Pt(l 00) [82] and Pt(l 1 1) [75, 79] were on the order of 10 1. These results illustrate the effect that surface reconstruction and geometry can have on the dissociation probability. This point may be of importance when considering the behavior of catalysts of a polycrystalline nature. 

The poisoning of Pt oxidation catalysts by phosphorus was also studied by Hegedus and Gumbleton107 and by Angele and co-workers.108-110 In a work by Dulcey and co-workers for the first time the thermal desorption of the PO radical was observed in the catalytic decomposition of DMMP on a polycrystalline Pt surface.111 Catalytic decomposition of Sarin using a Pt catalyst results initially in stoichiometric amounts of the oxidation products C02, HF, H20, and H3P04.106 The application of quantum-chemical methods to platinum and palladium surfaces has been quite limited because of the difficulty to treat large clusters of platinum and palladium atoms. 

Nonhomogeneous behavior was also predicted by Levine and Zou (343), who began with the oxidation/reduction model of Sales et al. (272) for CO/O2 on polycrystalline Pt and introduced CO surface diffusion. This model predicted one-dimensional, stable traveling waves on the Pt surface. Because no experimental measurements have been performed for traveling waves on polycrystalline catalysts at atmospheric pressures, no comparison to experimental data was possible. 

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